ANALYTICAL INSTRUMENTATION
ANALYZERS
SILICA ANALYZER:
In thermal power plants, silica content is measured in steam before turbine.silica analyzers are also used for anion exchanger effluent monitoring of mixed-bed exchangers.
A silica analyzer manufactured by Electronic Instruments Ltd(EIL), U.K. for continuous automatic stream monitoring works on the calorimetric analysis principle which is based upon the
well known molybdenum blue method.
Ammonium molybdate solution (pH7), sulphuricacid and a reducing solution are added to a metered volume of sample via separate measuring cylinders (to eliminate the precipitation of
molybdic acid).
The flow diagram of the analyzer. The inlet is at the top to prevent contamination by any solution from the previous analysis, and the cuvette is drained completely after every analysis cycle to
prevent the accumulation of gas or air bubbles in the measuring cuvette.
The analysing cycle takes twelve minutes and consists of two overlapping sequences. The first measures a chemical blank. The second is the actual quantitative determination.
An associated sequence timer controls the programme of operations.In the first sequence, ammonium molybdate solution, sulphuric acid and reduction solution are simultaneously added to
the mixing vessel. This solution is diluted with sample to a suitable volume and is then emptied
into the measuring cuvette where it is measured and then drained away.
In the second sequence, the reagents are added in the normal order; sample first then ammonium molybdate and sulphuric acid. The reduction solution is added five minutes later.
All materials coming in contact with analysis liquor are either made of plastic or of metal components coated with plastics.
The reason for the use of blank on each cycle is to give the analyzer long term stability by compensating for the effects of variables such as coloration of the sample or reagents,
temperature, or ageing of the lamp of photo cells.
The silicon photo-voltiac cells are conventionally illuminated by a common light source. They are connected in parallel opposition and the differential signal is fed into a very low input impedance
current amplifier. For the measurement of the blank solution, the Auto-Compensation Unit (ACU) driven by the current amplifier is connected to its internal motor potentiometer which
corrects the zero of the current amplifier.
For the measurement of silica concentration, this correction remains fixed and the output from the ACU is connected to the read-out unit. The ACU now controls a motor driven potentiometer in
the read-out unit, producing a current output to feed a meter or recorder. A scale length control
adjusts the gain of the amplifier.
Temperature compensation of the amplifier output, for changes in sample temperature, is provided by a thermistor located in the analyzer constant head unit.
The analyzer is normally supplied in a cubicle, complete with analyzer, measuring electronics, and manual facilities for testing and calibration. Reagents are contained in the rear of the cabinet and
these require regular replenishment. To assist in providing reliable and repeatable measurements,
the complete cabinet is temperature controlled by the use of the thermostas with electrical heaters.
SODIUM ANALYZER:
Sodium analyzers find applications in thermal power plants for determining sodium ion
concentration in boiler waters, monitoring carryover detection of condenser leaks and the
exhaustion of water treatment plant cation exchange units.
The sodium analyzer manufactured by Electronic Instruments Ltd(EIL), U.K. is based upon the use of a specific ion electrode.
In fact, ion selective electrode(ISE) is today considered to be one of the most powerful tools for specific analysis, i.e. to determine the specific constituents like cyanide fluoride, NH3 etc. in
sample; and is thus widely used for industrial water pollution monitoring.
ISE is an electrochemical sensor that develops an electric potential which can be related to the concentration of a specific ion in solution. The potential developed is actually proportional to the
logarithmic of the ion activity, which can differ greatly from ionic concentration in non-dilute
solutions. Where activity is a measure of the number of free (dissociated) ions in solution,
concentration includes both free and bound (undissociated) ions.
As the interest in this case is for concentration and not activity, the bound ions must first be liberated so that they can contribute to the measurement. As certain ions in the sample interfere
with the specific measurement either by forming bonds with the ions to be measured or by being
measured themselves it is essential that the sample be prepared properly before making any
measurement.
Sample is preferred by adding a reagent which forms bonds with the interfering ions, thereby either freeing the ions of interest, or preventing the interfering ions from entering into the
measurement.
If the concentration of interfering ions(like H and OH ions)be too high, then the adjuster solution will not be able to provide this over-riding ionic strength, and for such case sample pH must be
adjusted to within desired range.
The pH value of the sample is maintained in the flow cell by adding ammonia gas to the sample. A calomel reference electrode is used to complete the electrode pair across which a potential is
developed dependent on the sodium ion activity in the sample.
The potential is proportional to the logarithm of the sodium ion concentration, thus enabling low concentrations to be measured accurately and also provide high range capacity. The sequence of
operation is as follows. Sample water flows to a constant head tank to ensure a fresh sample and
it is pumped anaerobically. At a constant rate, into the flow cell, where it is equilibrated with
ammonia gas.
The ammonia gas is derived by pumping air through a 25% ammonia solution and passing the ammonia saturated air to the flow cell. The sample then flows past the measuring electrodes to a
drain outlet. The analyzer also includes a facility for automatic standardization.
The standardization sequences commences by activating a valve to stop the sample flow and to allow the standard sodium ion solution to be pumped into the flow cell. When the electrodes
have stabilized in the new solution the amplifier output is compared with a pre-set standard value
in the auto compensation unit, and any error is used to drive a servo potentiometer circuit, to
adjust the output to the correct value.
Cleaning of electrode at specified interval is essential for electrode longevity and accuracy. Some manufactures have automated the procedure of cleaning using solid state timers and achieving
electrode cleaning by mechanical (brush), spray nozzle, or acoustical means. In mechanical
system the brush strokes the probe 2-4 times in minute; in chemical method, clean fluid is
sprayed on membrane and in acoustical method, ultrasonic waves vibrate deposits off the spray
ISE based monitors can be used for measuring the following effluent parameters Ammonia
Chloride
Copper
Cyanide
Fluoride
Nitrate
Nitrite
Sulphide
Water hardness etc.
PH meter A pH meter is an electronic instrument measuring the pH (acidity or alkalinity) of a
liquid .
A typical pH meter consists of a special measuring probe (a glass electrode) connected to
an electronic meter that measures and displays the pH reading.
The pH probe measures pH as the activity of hydrogen ions surrounding a thin-walled
glass bulb at its tip. The probe produces a small voltage (about 0.06 volt per pH unit) that
is measured and displayed as pH units by the meter.
Calibration and use:-
For very precise work the pH meter should be calibrated before each measurement. For
normal use calibration should be performed at the beginning of each day. The reason for
this is that the glass electrode does not give a reproducible e.m.f. over longer periods of
time.
Calibration should be performed with at least two standard buffer solutions that span the
range of pH values to be measured.
For general purposes buffers at pH 4 and pH 10 are acceptable.
The calibration process correlates the voltage produced by the probe with the pH scale.
After each single measurement, the probe is rinsed with distilled water to remove any
traces of the solution being measured, blotted with a clean tissue to absorb any remaining
water which could dilute the sample and thus alter the reading, and then quickly
immersed in another solution.
Types of pH meters
1. Null Detection Type PH meter
2. Chopper Amplifier Type PH meter.
Null-Detector Type pH Meter:
Chopper Amplifier Type pH Meter:
Electrodes for pH measurement:
There are different types of electrodes for the measurement of pH. Such as
1. Hydrogen electrode
2. Glass electrode
3. Calomel electrode or Reference electrode
4. Silver/Silver chloride Reference electrode
Hydrogen electrode:
Glass electrode:
Calomel Electrode or Reference Electrode:
Silver/Silver Chloride Electrode:
CONDUCTIVITY METER
An electrical conductivity meter (EC meter) measures the electrical conductivity in a
solution. Commonly used in hydroponics, aquaculture and freshwater systems to monitor
the amount of nutrients, salts or impurities in the water.
An electrical conductivity meter
Principle of operation:
The common laboratory conductivity meters employ a potentiometric method and four
electrodes. Often, the electrodes are cylindrical and arranged concentrically. The
electrodes are usually made of platinum metal. An alternating current is applied to the
outer pair of the electrodes. The potential between the inner pair is measured.
Conductivity could in principle be determined using the distance between the electrodes
and their surface area using the Ohm's law but generally, for accuracy, a calibration is
employed using electrolytes of well-known conductivity.
Industrial conductivity probes often employ an inductive method, which has the
advantage that the fluid does not wet the electrical parts of the sensor. Here, two
inductively-coupled coils are used. One is the driving coil producing a magnetic field and
it is supplied with accurately-known voltage. The other forms a secondary coil of a
transformer. The liquid passing through a channel in the sensor forms one turn in the
secondary winding of the transformer. The induced current is the output of the sensor.
Temperature dependence: Electrical conductivity
The conductivity of a solution is highly temperature dependent, therefore it is important
to either use a temperature compensated instrument, or calibrate the instrument at the
same temperature as the solution being measured. Unlike metals, the conductivity of
common electrolytes typically increases with increasing temperature.
Over a limited temperature range, the way temperature affect conductivity of a solution
can be modeled linearly using the following formula:
where
T is the temperature of the sample,
Tcal is the calibration temperature,
T is the electrical conductivity at the temperature T, Tcal is the electrical conductivity at the calibration temperature Tcal, is the temperature compensation slope of the solution.
The temperature compensation slope for most naturally occurring waters is about 2 %/C,
however it can range between 1 to 3 %/C. The compensation slope for some common
water solutions are listed in the table below.
Aqueous solution at 25 C Concentration (mass percentage) (%/C)
HCl 10 1.56
KCl 10 1.88
H2SO4 50 1.93
NaCl 10 2.14
HF 1.5 7.20
HNO3 31 31
Conductivity factor:
conductivity factor (CF) of dissolvedsalts in a given solution is a measurement of
conductivity. Using the electrical conductivity between two electrodes in a water
solution, the level of dissolved solids in that solution can be measured. Measurements can
then be used to dose the solution with the necessary nutrients in the case of hydroponics.
Conductivity measurements are also used in ecology and environmental sciences to
assess the level of nutrients in lakes and rivers
Ultraviolet-visible spectroscopy or ultraviolet-visible
spectrophotometers (UV-Vis or UV/Vis):
It refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible
spectral region. This means it uses light in the visible and adjacent (near-UV and near-
infrared (NIR)) ranges. The absorption or reflectance in the visible range directly affects
the perceived color of the chemicals involved. In this region of the electromagnetic
spectrum, molecules undergo electronic transitions. This technique is complementary to
fluorescence spectroscopy, in that fluorescence deals with transitions from the excited
state to the ground state, while absorption measures transitions from the ground state to
the excited state.
UV/Vis spectroscopy is routinely used in the quantitative determination of solutions of
transition metal ions highly conjugatedorganic compounds, and biological
macromolecules.
Solutions of transition metal ions can be colored (i.e., absorb visible light)
because d electrons within the metal atoms can be excited from one electronic
state to another. The colour of metal ion solutions is strongly affected by the
presence of other species, such as certain anions or ligands. For instance, the
colour of a dilute solution of copper sulfate is a very light blue; adding ammonia
intensifies the colour and changes the wavelength of maximum absorption (max).
Organic compounds, especially those with a high degree of conjugation, also
absorb light in the UV or visible regions of the electromagnetic spectrum. The
solvents for these determinations are often water for water soluble compounds, or
ethanol for organic-soluble compounds. (Organic solvents may have significant
UV absorption; not all solvents are suitable for use in UV spectroscopy. Ethanol
absorbs very weakly at most wavelengths.) Solvent polarity and pH can affect the
absorption spectrum of an organic compound. Tyrosine, for example, increases in
absorption maxima and molar extinction coefficient when pH increases from 6 to
13 or when solvent polarity decreases.
While charge transfer complexes also give rise to colours, the colours are often
too intense to be used for quantitative measurement.
The Beer-Lambert law states that the absorbance of a solution is directly proportional to
the concentration of the absorbing species in the solution and the path length. Thus, for a
fixed path length, UV/Vis spectroscopy can be used to determine the concentration of the
absorber in a solution. It is necessary to know how quickly the absorbance changes with
concentration. This can be taken from references (tables of molar extinction coefficients),
or more accurately, determined from a calibration curve.
A UV/Vis spectrophotometer may be used as a detector for HPLC. The presence of an
analyte gives a response assumed to be proportional to the concentration. For accurate
results, the instrument's response to the analyte in the unknown should be compared with
the response to a standard; this is very similar to the use of calibration curves. The
response (e.g., peak height) for a particular concentration is known as the response factor.
The wavelengths of absorption peaks can be correlated with the types of bonds in a given
molecule and are valuable in determining the functional groups within a molecule. The
Woodward-Fieser rules, for instance, are a set of empirical observations used to predict
max, the wavelength of the most intense UV/Vis absorption, for conjugated organic
compounds such as dienes and ketones. The spectrum alone is not, however, a specific
test for any given sample. The nature of the solvent, the pH of the solution, temperature,
high electrolyte concentrations, and the presence of interfering substances can influence
the absorption spectrum. Experimental variations such as the slit width (effective
bandwidth) of the spectrophotometer will also alter the spectrum. To apply UV/Vis
spectroscopy to analysis, these variables must be controlled or accounted for in order to
identify the substances present.
Beer-Lambert law
The method is most often used in a quantitative way to determine concentrations of an
absorbing species in solution, using the Beer-Lambert law:
,
Where A is the measured absorbance, I0 is the intensity of the incident light at a given
wavelength, I is the transmitted intensity, L the path length through the sample, and c the
concentration of the absorbing species. For each species and wavelength, is a constant
known as the molar absorptivity or extinction coefficient. This constant is a fundamental
molecular property in a given solvent, at a particular temperature and pressure, and has
units of 1 / M * cm or often AU / M * cm.
The absorbance and extinction are sometimes defined in terms of the natural logarithm
instead of the base-10 logarithm.
The Beer-Lambert Law is useful for characterizing many compounds but does not hold as
a universal relationship for the concentration and absorption of all substances. A 2nd
order polynomial relationship between absorption and concentration is sometimes
encountered for very large, complex molecules such as organic dyes (Xylenol Orange or
Neutral Red, for example).
Ultraviolet-visible spectrophotometer
The instrument used in ultraviolet-visible spectroscopy is called a UV/Vis
spectrophotometer. It measures the intensity of light passing through a sample (I), and
compares it to the intensity of light before it passes through the sample (Io). The ratio I /
Iois called the transmittance, and is usually expressed as a percentage (%T). The
absorbance, A, is based on the transmittance:
A = log(%T / 100%)
The UV-visible spectrophotometer can also be configured to measure reflectance. In this
case, the spectrophotometer measures the intensity of light reflected from a sample (I),
and compares it to the intensity of light reflected from a reference material (Io)(such as a
white tile). The ratio I / Iois called the reflectance, and is usually expressed as a
percentage (%R).
The basic parts of a spectrophotometer are a light source, a holder for the sample, a
diffraction grating in a monochromator or a prism to separate the different wavelengths
of light, and a detector. The radiation source is often a Tungsten filament (300-2500 nm),
a deuterium arc lamp, which is continuous over the ultraviolet region (190-400 nm),
Xenon arc lamps, which is continuous from 160-2,000 nm; or more recently, light
emitting diodes (LED) [5]
for the visible wavelengths. The detector is typically a
photomultiplier tube, a photodiode, a photodiode array or a charge-coupled device(CCD).
Single photodiode detectors and photomultiplier tubes are used with scanning
monochromators, which filter the light so that only light of a single wavelength reaches
the detector at one time. The scanning monochromator moves the diffraction grating to
"step-through" each wavelength so that it's intensity may be measured as a function of
wavelength. Fixed monochromators are used with CCDs and photodiode arrays. As both
of these devices consist of many detectors grouped into one or two dimensional arrays,
they are able to collect light of different wavelengths on different pixels or groups of
pixels simultaneously.
Fig: Spectrophotometer
A spectrophotometer can be either single beam or double beam. In a single beam
instrumentall of the light passes through the sample cell. Io must be measured by
removing the sample. This was the earliest design, but is still in common use in both
teaching and industrial labs.
Diagram of
a single-beam UV/Vis spectrophotometer.
.
In a double-beam instrument, the light is split into two beams before it reaches the
sample. One beam is used as the reference; the other beam passes through the sample.
The reference beam intensity is taken as 100% Transmission (or 0 Absorbance), and the
measurement displayed is the ratio of the two beam intensities. Some double-beam
instruments have two detectors (photodiodes), and the sample and reference beam are
measured at the same time. In other instruments, the two beams pass through a beam
chopper, which blocks one beam at a time. The detector alternates between measuring the
sample beam and the reference beam in synchronism with the chopper. There may also
be one or more dark intervals in the chopper cycle. In this case the measured beam
intensities may be corrected by subtracting the intensity measured in the dark interval
before the ratio is taken.
Diagram of double beam UV/VIS Spectrophotometer.
Samples for UV/Vis spectrophotometry are most often liquids, although the absorbance
of gases and even of solids can also be measured. Samples are typically placed in a
transparent cell, known as a cuvette. Cuvettes are typically rectangular in shape,
commonly with an internal width of 1 cm. (This width becomes the path length, L, in the
Beer-Lambert law.) Test tubes can also be used as cuvettes in some instruments. The type
of sample container used must allow radiation to pass over the spectral region of interest.
The most widely applicable cuvettes are made of high quality fused silica or quartz glass
because these are transparent throughout the UV, visible and near infrared regions. Glass
and plastic cuvettes are also common, although glass and most plastics absorb in the UV,
which limits their usefulness to visible wavelengths.
Specialized instruments have also been made. These include attaching
spectrophotometers to telescopes to measure the spectra of astronomical features. UV-
visible microspectrophotometers consist of a UV-visible microscope integrated with a
UV-visible spectrophotometer. These are commonly used for measuring thin film
thickness in semiconductor manufacturing, materials science research, measuring the
energy content of coal and petroleum source rock, and in forensic laboratories for the
analysis of microscopic amounts of trace evidence as well as questioned documents.
A complete spectrum of the absorption at all wavelengths of interest can often be
produced directly by a more sophisticated spectrophotometer. In simpler instruments the
absorption is determined one wavelength at a time and then compiled into a spectrum by
the operator. By removing the concentration dependence, the extinction coefficient ()
can be determined as a function of wavelength.
IR spectrophotometry
Spectrophotometers designed for the main infrared region are quite different because of
the technical requirements of measurement in that region. One major factor is the type of
photo sensors that are available for different spectral regions, but infrared measurement is
also challenging because virtually everything emits IR light as thermal radiation,
especially at wavelengths beyond about 5 m.
Another complication is that quite a few materials such as glass and plastic absorb
infrared light, making it incompatible as an optical medium. Ideal optical materials are
salts, which do not absorb strongly. Samples for IR spectrophotometry may be smeared
between two discs of potassium bromide or ground with potassium bromide and pressed
into a pellet. Where aqueous solutions are to be measured, insoluble silver chloride is
used to construct the cell.
Sources
An inert solid is electrically heated to a temperature in the range 1500-2000 K. The
heated material will then emit infra red radiation.
The Nernst glower is a cylinder (1-2 mm diameter, approximately 20 mm long) of rare
earth oxides. Platinum wires are sealed to the ends, and a current passed through the
cylinder. The Nernst glower can reach temperatures of 2200 K.
The Globar source is a silicon carbide rod (5mm diameter, 50mm long) which is
electrically heated to about 1500 K. Water cooling of the electrical contacts is needed to
prevent arcing. The spectral output is comparable with the Nernst glower, execept at
The incandescent wire source is a tightly wound coil of nichrome wire, electrically
heated to 1100 K. It produces a lower intensity of radiation than the Nernst or Globar
sources, but has a longer working life.
Detectors
There are three categories of detector;
Thermal
Pyroelectric
Photo conducting
Thermocouples consist of a pair of junctions of different metals; for example, two pieces
of bismuth fused to either end of a piece of antimony. The potential difference (voltage)
between the junctions changes according to the difference in temperature between the
junctions
Pyroelectric detectors are made from a single crystalline wafer of a pyroelectric material,
such as triglycerine sulphate. The properties of a pyroelectric material are such that when
an electric field is applied across it, electric polarization occurs (this happens in any
dielectric material). In a pyroelectric material, when the field is removed, the polarization
persists. The degree of polarization is temperature dependant. So, by sandwiching the
pyroelectric material between two electrodes, a temperature dependant capacitor is made.
The heating effect of incident IR radiation causes a change in the capacitance of the
material. Pyroelectric detectors have a fast response time. They are used in most Fourier
transform IR instruments.
Photoelectric detectors such as the mercury cadmium telluride detector comprise a film
of semiconducting material deposited on a glass surface, sealed in an evacuated envelope.
Absorption of IR promotes nonconducting valence electrons to a higher, conducting,
state. The electrical resistance of the semiconductor decreases. These detectors have
better response characteristics than pyroelectric detectors and are used in FT-IR
instruments - particularly in GC - FT-IR.
Pneumatic detector
A pressure-sensitive detectorbased on the pressure increase of a gas. A special type is the
Golay cell where the pressure change is detected by observing the deflection off one of
the chamber walls.
Types of instrument
Dispersive infra red spectrophotometers
These are often double-beam recording instruments, employing diffraction gratings for
dispersion of radiation.
Radiation from the source is flicked between the reference and sample paths. Often, an
optical null system is used. This is when the detector only responds if the intensity of the
two beams is unequal. If the intensities are unequal, a light attenuator restores equality by
moving in or out of the reference beam. The recording pen is attached to this attenuator.
Fourier-transform spectrometers
Any waveform can be shown in one of two ways; either in frequency domain or time
domain.
Dispersive IR instruments operate in the frequency domain. There are, however,
advantages to be gained from measurement in the time domain followed by computer
transformation into the frequency domain.
If we wished to record a trace in the time domain, it could be possible to do so by
allowing radiation to fall on a detector and recording its response over time. In practice,
no detector can respond quickly enough (the radiation has a frequency greater than 1014
Hz). This problem can be solved by using interference to modulate the i.r. signal at a
detectable frequency. The Michelson interferometer is used to produce a new signal of a
much lower frequency which contains the same information as the original IR signal. The
output from the interferometer is an interferogram.
The Michelson interferometer
Radiation leaves the source and is split. Half is reflected to a stationary mirror and then
back to the splitter. This radiation has travelled a fixed distance. The other half of the
radiation from the source passes through the splitter and is reflected back by a movable
mirror. Therefore, the path length of this beam is variable. The two reflected beams
recombine at the splitter, and they interfere (e.g. for any one wavelength, interference
will be constructive if the difference in path lengths is an exact multiple of the
wavelength. If the difference in path lengths is half the wavelength then destructive
interference will result). If the movable mirror moves away from the beam splitter at a
constant speed, radiation reaching the detector goes through a steady sequence of maxima
and minima as the interference alternates between constructive and destructive phases.
If monochromatic IR radiation of frequency, f ( ir ) enters the interferometer, then the
output frequency, fm can be found by;
Wherev is the speed of mirror travel in mm/s
Because all wavelengths emitted by the source are present, the interferogram is extremely
complicated.
The moving mirror must travel smoothly; a frictionless bearing is used with
electromagnetic drive. The position of the mirror is measured by a laser shining on a
corner of the mirror. A simple sine wave interference pattern is produced. Each peak
indicates mirror travel of one half the wavelength of the laser. The accuracy of this
measurement system means that the IR frequency scale is accurate and precise.
In the FT-IR instrument, the sample is placed between the output of the interferometer
and the detector. The sample absorbs radiation of particular wavelengths. Therefore, the
interferogram contains the spectrum of the source minus the spectrum of the sample. An
interferogram of a reference (sample cell and solvent) is needed to obtain the spectrum of
the sample.
After an interferogram has been collected, a computer performs a Fast Fourier Transform,
which results in a frequency domain trace (i.e. intensity vs. wave number) that we all
know and love.
The detector used in an FT-IR instrument must respond quickly because intensity
changes are rapid (the moving mirror moves quickly). Pyroelectric detectors or liquid
nitrogen cooled photon detectors must be used. Thermal detectors are too slow.
To achieve a good signal to noise ratio, many interferograms are obtained and then
averaged. This can be done in less time than it would take a dispersive instrument to
record one scan.
Advantages of Fourier transform IR over dispersive IR;
Improved frequency resolution
Improved frequency reproducibility (older dispersive instruments must be
recalibrated for each session of use)
Higher energy throughput
Faster operation
Computer based (allowing storage of spectra and facilities for processing spectra)
Easily adapted for remote use (such as diverting the beam to pass through an
external cell and detector, as in GC - FT-IR)
GAS CHROMATOGRAPHY
Introduction
Gas chromatography - specifically gas-liquid chromatography - involves a sample being
vaporized and injected onto the head of the chromatographic column. The sample is
transported through the column by the flow of inert, gaseous mobile phase. The column
itself contains a liquid stationary phase which is adsorbed onto the surface of an inert
solid.
Have a look at this schematic diagram of a gas
Gas chromatography (GC), is a common type of chromatography used in analytic
chemistry for separating and analyzing compounds that can be vaporized without
decomposition. Typical uses of GC include testing the purity of a particular substance, or
separating the different components of a mixture (the relative amounts of such
components can also be determined). In some situations; GC may help in identifying a
compound. In preparative chromatography, GC can be used to prepare pure compounds
from a mixture.
In gas chromatography, the moving phase (or "mobile phase") is a carrier gas, usually
an inert gas such as helium or an unreactive gas such as nitrogen. The stationary phase is
a microscopic layer of liquid or polymer on an inert solid support, inside a piece
of glass or metal tubing called a column (a homage to the column used in distillation).
The instrument used to perform gas chromatography is called a gas chromatograph (or
"aerograph", "gas separator").
The gaseous compounds being analyzed interact with the walls of the column, which is
coated with different stationary phases. This causes each compound to elute at a different
time, known as the retention time of the compound. The comparison of retention times is
what gives GC its analytical usefulness.
Gas chromatography is in principle similar to column chromatography (as well as other
forms of chromatography, such as HPLC, TLC), but has several notable differences.
Firstly, the process of separating the compounds in a mixture is carried out between a
liquid stationary phase and a gas moving phase, whereas in column chromatography the
stationary phase is a solid and the moving phase is a liquid. (Hence the full name of the
procedure is "Gas-liquid chromatography", referring to the mobile and stationary phases,
respectively.) Secondly, the column through which the gas phase passes is located in
an oven where the temperature of the gas can be controlled, whereas column
chromatography (typically) has no such temperature control. Thirdly, the concentration of
a compound in the gas phase is solely a function of the vapor pressure of the gas.[1]
Gas chromatography is also similar to fractional distillation, since both processes separate
the components of a mixture primarily based on boiling point (or vapor pressure)
differences. However, fractional distillation is typically used to separate components of a
mixture on a large scale, whereas GC can be used on a much smaller scale (i.e. micro
scale). Gas chromatography is also sometimes known as vapor-phase
chromatography (VPC), or gas-liquid partition chromatography (GLPC). These
alternative names, as well as their respective abbreviations, are frequently found
in scientific literature. Strictly speaking,
The Modern Gas Chromatograph
The modern gas chromatograph is a fairly complex instrument mostly computer
controlled. The samples are mechanically injected, the analytical results are
automatically calculated and the results printed out, together with the pertinent
operating conditions in a standard format. However, the instrument has evolved
over many years although the majority of the added devices and techniques
were suggested or describe in the first three international symposia on gas
chromatography held in 1956, 1958 and 1960. These symposia, initially
organized by the 'British Institute of Petroleum' have been held every two years
ever since 1956 and the meetings have remained the major stimulus for
developing the technique and extending its capabilities. However, the majority
of the techniques and devices that have been incorporated in the modern
chromatograph, were described, reported, or discussed in the first triad of
symposia.
The layout of the modern gas chromatograph is shown as a block diagram in
figure 1.
Instrumental components
Carrier gas
The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium,
argon, and carbon dioxide. The choice of carrier gas is often dependent upon the type of
detector which is used. The carrier gas system also contains a molecular sieve to remove
water and other impurities.
Injection Devices
The basic injection devices that are used in chromatography, such as the
external loop valve, have been discussed in book 1. In gas chromatography two
basic types of sampling system are used, those suitable for packed columns and
those designed for open tubular columns. In addition, different sample injectors
are necessary that will be appropriate for alternative column configurations. It
must be stressed, however, that irrespective of the design of the associated
equipment, the precision and accuracy of a GC analysis will only be as good as
that provided by the sample injector. The sample injector is a very critical part
of the chromatographic equipment and needs to be well designed and well
maintained.
Sample injection port
For optimum column efficiency, the sample should not be too large, and should be
introduced onto the column as a "plug" of vapor - slow injection of large samples causes
band broadening and loss of resolution. The most common injection method is where a
micro syringe is used to inject sample through a rubber septum into a flash vaporizer port
higher than the boiling point of the least volatile component of the sample. For packed
columns, sample size ranges from tenths of a micro liter up to 20 micro liters. Capillary
columns, on the other hand, need much less sample, typically around 10-3
capillary GC, split/split less injection is used. Have a look at this diagram of a split/split
less injector;
The injector can be used in one of two modes; split or split less. The injector contains a
heated chamber containing a glass liner into which the sample is injected through the
septum. The carrier gas enters the chamber and can leave by three routes (when the
injector is in split mode). The sample vaporizes to form a mixture of carrier gas,
vaporized solvent and vaporized solutes. A proportion of this mixture passes onto the
column, but most exits through the split outlet. The septum purge outlet prevents septum
bleed components from enter the column.
Open Tubular Column Injection Systems
Due to the very small sample size that must be placed on narrow bore capillary
columns, a split injection system is necessary.
The basic difference between the two types of injection systems is that the
capillary column now projects into the glass liner and a portion of the carrier
gas sweeps past the column inlet to waste. As the sample passes the column
opening, a small fraction is split off and flows directly into the capillary
column, ipso facto this device is called a split injector. The split ratio is changed
by regulating the portion of the carrier gas that flows to waste which is achieved
by an adjustable flow resistance in the waste flow line. This device is only used
for small diameter capillary columns where the charge size is critical.
Columns
There are two general types of column, packed and capillary (also known as open
tubular). Packed columns contain a finely divided, inert, solid support material
(commonly based on diatomaceous earth) coated with liquid stationary phase. Most
packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 4mm.
Capillary columns have an internal diameter of a few tenths of a millimeter. They can be
one of two types; wall-coated open tubular (WCOT) or support-coated open
tubular (SCOT). Wall-coated columns consist of a capillary tube whose walls are coated
with liquid stationary phase. In support-coated columns, the inner wall of the capillary is
lined with a thin layer of support material such as diatomaceous earth, onto which the
stationary phase has been adsorbed. SCOT columns are generally less efficient than
WCOT columns. Both types of capillary column are more efficient than packed columns.
In 1979, a new type of WCOT column was devised - the Fused Silica Open
Tubular (FSOT) column;
These have much thinner walls than the glass capillary columns, and are given strength
by the polyimide coating. These columns are flexible and can be wound into coils. They
have the advantages of physical strength, flexibility and low reactivity.
The Packed GC Column
Packed columns are usually constructed from stainless steel or Pyrex glass.
Pyrex glass is favored when thermally labile materials are being separated such
as essential oils and flavor components. However, glass has pressure limitations
and for long packed columns, stainless steel columns are used as they can easily
tolerate the necessary elevated pressures. The sample must, of course, be
amenable to contact with hot metal surfaces. Short columns can be straight, and
installed vertically in the chromatograph. Longer columns can be U-shaped but
columns more than a meter long are usually coiled. Such columns can be
constructed of any practical length and relatively easily installed. Pyrex glass
columns are formed to the desired shape by coiling at about 700C and metal columns by bending at room temperature. Glass columns are sometimes treated
with an appropriate silanizing reagent to eliminate the surface hydroxyl groups
which can be catalytically active or produce asymmetric peaks. Stainless steel
columns are usually washed with dilute hydrochloric acid, then extensively with
water followed by methanol, acetone, methylene dichloride and n-hexane. This
washing procedure removes any corrosion products and traces of lubricating
agents used in the tube drawing process. The columns are then ready for
packing.
Column temperature
For precise work, column temperature must be controlled to within tenths of a degree.
The optimum column temperature is dependent upon the boiling point of the sample. As
a rule of thumb, a temperature slightly above the average boiling point of the sample
results in an elution time of 2 - 30 minutes. Minimal temperatures give good resolution,
but increase elution times. If a sample has a wide boiling range, then temperature
programming can be useful. The column temperature is increased (either continuously or
in steps) as separation proceeds.
Detectors
There are many detectors which can be used in gas chromatography. Different detectors
will give different types of selectivity. A non-selective detector responds to all
compounds except the carrier gas, a selective detector responds to a range of compounds
with a common physical or chemical property and a specific detector responds to a single
chemical compound. Detectors can also be grouped into concentration dependant
detectors and mass flow dependant detectors. The signal from a concentration dependant
detector is related to the concentration of solute in the detector, and does not usually
destroy the sample Dilution of with make-up gas will lower the detectors response. Mass
flow dependant detectors usually destroy the sample, and the signal is related to the rate
at which solute molecules enter the detector. The response of a mass flow dependant
detector is unaffected by make-up gas. Have a look at this tabular summary of common
GC detectors:
Detector Type Support
gases Selectivity Detectability
Dynamic
range
Flame
ionization
(FID)
Mass flow Hydrogen
and air Most organic cpds. 100 pg 10
7
Thermal
conductivity
(TCD)
Concentration Reference Universal 1 ng 107
Electron
capture
(ECD)
Concentration Make-up
Halides, nitrates,
nitriles, peroxides,
anhydrides,
organometallics
50 fg 105
Nitrogen-
phosphorus Mass flow
Hydrogen
and air Nitrogen, phosphorus 10 pg 10
6
Flame
photometric
(FPD)
Mass flow
Hydrogen
and air
possibly
oxygen
Sulphur, phosphorus,
tin, boron, arsenic,
germanium, selenium,
chromium
100 pg 103
Photo-
ionization
(PID)
Concentration Make-up
Aliphatics, aromatics,
ketones, esters,
aldehydes, amines,
heterocyclics,
organosulphurs, some
organometallics
2 pg 107
Hall
electrolytic
conductivity
Mass flow Hydrogen,
oxygen
Halide, nitrogen,
nitrosamine, sulphur
The effluent from the column is mixed with hydrogen and air, and ignited. Organic
compounds burning in the flame produce ions and electrons which can conduct electricity
through the flame. A large electrical potential is applied at the burner tip, and a collector
electrode is located above the flame. The current resulting from the pyrolysis of any
organic compounds is measured. FIDs are mass sensitive rather than concentration
sensitive; this gives the advantage that changes in mobile phase flow rate do not affect
the detector's response. The FID is a useful general detector for the analysis of organic
compounds; it has high sensitivity, a large linear response range, and low noise. It is also
robust and easy to use, but unfortunately, it destroys the sample
The Nitrogen Phosphorus Detector (NPD)
The nitrogen phosphorus detector (NPD), is a highly sensitive but specific
detector and evolved directly from the FID. It gives a strong
response to organic compounds containing nitrogen and/or phosphorus. Although
it appears to function in a very similar manner to the FID, in fact, it operates on
an entirely different principle. A diagram of an NP detector is shown in figure
24.
The Electron Capture Detector
The electron capture detector contains a low energy source which is used to
produce electrons for capturing by appropriate atoms. Although tritium
adsorbed into a silver foil has been used as the particle source, it is relatively
unstable at high temperatures, the Ni63
source was found to be preferable. The
detector can be used in two modes, either with a constant potential applied
across the cell (the DC mode) or with a pulsed potential across the cell (the
pulsed mode). In the DC mode, hydrogen or nitrogen can be used as the carrier
gas and a small potential (usually only a few volts) is applied across the cell that
is just sufficient to collect all the electrons available and provide a small
standing current. If an electron capturing molecule (for example a molecule
containing an halogen atom which has only seven electrons in its outer shell)
enters the cell, the electrons are captured by the molecule and the molecules
become charged. The mobility of the captured electrons is much smaller than
the free electrons and the electrode current falls dramatically. The DC mode of
detection, however, has some distinct disadvantages. The most serious objection
is that the electron energy varies with the applied potential. The electron
capturing properties of a molecule varies with the electron energy, so the
specific response of the detector will depend on the applied potential
Operating in the pulsed mode, a mixture of 10% methane in argon is employed
which changes the nature of the electron capturing environment. The electrons
generated by the radioactive source rapidly assume only thermal energy and, in
the absence of a collecting potential, exist at the source surface in an annular
region about 2 mm deep at room temperature and about 4 mm deep at 400C. A short period square wave pulse is applied to the electrode collecting the
electrons and producing a base current. The standing current, using 10%
methane in argon is about 10-8
amp with a noise level of about 5 x 10-12
amp.
The pulse wave form is shown in figure
Wave form of Electron Capture Detector Pulses
In the inactive period of the wave form, electrons having thermal energy only
will attached themselves readily to any electron capturing molecules present in
the cell with the consequent production of negatively charged ions. The
negative ions quickly recombine with the positive ions and thus
become unavailable for collection. Consequently the standing current measured
during the potential pulse will be reduced.
The period of the pulsed potential is adjusted such that relatively few of the
slow negatively charged molecules (molecules having captured electrons and
not neutralized by collision with positive ions) have time to reach the anode, but
the faster moving electrons are all collected. During the "off period" the
electrons re-establish equilibrium with the gas. The three operating variables are
the pulse duration, pulse frequency and pulse amplitude. By appropriate
adjustment of these parameters the current can be made to reflect the relative
mobilities of the different charged species in the cell and thus exercise some
discrimination between different electron capturing materials. A diagram of an
electron capture detector is shown in fig.
Electron Capture Detector
Gas Supplies
Gases for use with the gas chromatograph were originally all obtained from gas
tanks or gas cylinders. However, over the past decade the use of gas generators
have become more popular as it avoids having gases at high pressure in the
laboratory which is perceived by some as potentially dangerous. In addition, the
use of a hydrogen generator avoids the use of a cylinder of hydrogen at high
pressure which is also perceived by some as a serious fire hazard despite the
fact that they have been used in laboratories, quite safely for nearly a century.
Supplies from Gas Tanks
Gasses are stored in large cylindrical tanks fitted with reducing valves that are
set to supply the gas to the instrument at the recommended pressure defined by
the manufacturers. The cylinders are often situated outside and away from the
chromatograph for safety purposes and the gasses are passed to the
chromatograph through copper or stainless steel conduits at relatively low
pressure. The main disadvantage of gas tanks is their size and weight which
makes them difficult to move and replace.
Pure Air Generators.
Air generators require an air supply from air tanks or directly from the
laboratory compressed air supply. The Packard Zero Air Generator passes the
gas through a 0.5filter to remove oil and water and finally over a catalyst to
remove hydrocarbons. The hydrocarbon free air is then passed through a
0.01 cellulose fiber filter to remove any residual particulate matter that may be
present. The manufacturers claim the resulting air supply contains less than 0.1
ppm total hydrocarbons and delivers air at 125 psi at flow rates up to 2,500 cc
per min.
Preparative Gas Chromatography
Gas chromatography has not been used extensively for preparative work
although its counterpart, liquid chromatography, has been broadly used in the
pharmaceutical industry for the isolation and purification of physiologically
active substances. There are a number of unique problems associated with
preparative gas chromatography. Firstly, it is difficult to recycle the mobile
phase and thus large volume of gas are necessary. Secondly, the sample must be
fully vaporized onto the column to ensure radial distribution of the sample
across the column. Thirdly, the materials of interest are eluted largely in a very
dilute form from the column and therefore must be extracted or condensed from
the gas stream which is also difficult to achieve efficiently. Finally, the efficient
packing of large GC columns is difficult. Nevertheless, preparative GC has been
successfully used in a number of rather special applications; for example the
isolation of significant quantities of the trace components of essential oils for
organoleptic assessment.
The layout of a preparative gas chromatograph is shown in figure38
Figure 38 Layout of Preparative Gas Chromatograph
Air cannot normally be used as the mobile phase due to likely oxidation and so
either a gas tank or a gas (e.g., nitrogen) generator must be used. As the flow
rates can be large, more than one generator operating in parallel will often be
necessary. The sample is usually placed on to the column with a syringe pump
and rapidly vaporized in a suitable heater. Passing the gas in vapor form onto
the column helps evenly distribute the sample radially across the column. The
detector that is used must have specifications that are almost opposite to those
of an analytical detector. It should function well at high concentrations of
solute, have a generally low sensitivity, if in-line it must be non-destructive and
have minimum flow impedance. It need not have a particularly linear response.
The katharometer is one of the more popular detectors for preparative GC. The
column outlet is passed to a selection valve that diverts the eluent to its
appropriate collecting vessel. The collecting vessel may be cooled in ice, solid
carbon dioxide or if necessary liquid nitrogen (liquid nitrogen can only be used
if a low boiling gas such as helium is employed as the carrier gas). In some
cases the solutes contained in the eluent can be extracted into an appropriate
liquid or onto the surface of a suitable adsorbent. the desired fractions are then
recovered by distillation or desorption.
The Moving Bed Continuous Chromatography System
The concept of the moving bed extraction process was originally introduced for
hydrocarbon gas adsorption by Freund et al. (13) and was first applied to gas
liquid chromatography by Scott (14). A diagram of the moving bed system
suitable for GC was proposed by Scott and is shown in figure 39.
The feasibility of this process was established for a gas chromatographic
system, subsequently, its viability was also confirmed for liquid
chromatography which will be discussed in Book 19. The moving bed system
takes a continuous sample feed and operates in the following way. The
stationary phase, coated on a suitable support, is allowed to fall down a column
against and upward stream of carrier gas. In the original device of Scott, the
packing (dinonyl phthalate coated on brick dust) was contained in a hopper at
the top of the column and was taken off from the bottom the column by a
rotating disc feed table and returned to the hopper by a simple air-lift device.
Courtesy of Butterworths Scientific Publications Ltd. [Ref. 10]
Applications
Gas chromatography has a very wide field of application but its first and main
area of use is in the separation and analysis of multi component mixtures such
as essential oils, hydrocarbons and solvents. Intrinsically, with the use of the
flame ionization detector and the electron capture detector (which have very
high sensitivities) gas chromatography can quantitatively determine materials
present at very low concentrations. It follows, that the second most important
application area is in pollution studies, forensic work and general trace analysis.
Gasoline
Gasoline is a multicomponent mixture containing a large number of
hydrocarbons, many of which have very similar molecular weights and all are
almost exclusively dispersive in interactive character. The structures of many of
the hydrocarbons are also very similar and there are many isomers present. As a
consequence, due to their interactive similarity the separation factors between
individual components are very small.
It follows that columns of very high efficiency will be mandatory to achieve an
effective separation. It is clear that open tubular columns are ideal for this type
of separation problem. In fact, it would be impossible to separate the
components of gasoline efficiently with a packed column, even one that is 50 ft
long, and even if the inherent long analysis times could be tolerated. In addition
this type of separation demands the maximum number of theoretical plates and
therefore not only must open tubes be used but tubes of relatively small
diameter to produce the maximum number of theoretical plates. In fact, several
hundred thousand theoretical plates will be necessary and so the column must
also be very long. As has already been discussed, it is necessary to use small
radius open tubular columns with a split injection system. Furthermore, as a
result of the wide range of molecular weight of the components present,
gasoline has a relatively wide boiling range and so will require a temperature
program that will heat the column to 200 C or more. A thermally stable stationary phase must be employed. The individual gasoline components are
present over a wide concentration range and thus, for accurate quantitative
results, the linear dynamic range of the detector must also be large. These latter
demands mandate that the detector must be the FID.
Food and Beverage Products
Due to the likely contamination of food and beverage products with pesticides,
herbicides and many other materials that are considered a health risk, all such
products on sale today must be carefully assayed. There is extensive legislation
controlling the quality of all human foods and drinks, and offensives carry very
serious penalties. In addition, the condition of the food is also of great concern
to the food chemist, who will look for those trace materials that have been
established to indicate the onset of bacterial action, aging, rancidity or
decomposition. In addition, tests that identify the area or country in which the
food was processed or grown may also be needed. The source of many plants
(herbs and spices) can often be identified from the peak pattern of the
chromatograms obtained directly from headspace analysis. Similarly, unique
qualitative and quantitative patterns from a GC analysis will often help identify
the source of many alcoholic beverages.
Unfortunately, food analysis involves the separation and identification of very
complex mixtures and the difficulties are compounded by the fact that the
components are present at very low concentrations. Thus, gas chromatography
is the ideal (if not only) technique that can be used successfully in food and
beverage assays and tests.
The potential carcinogenity of the aromatic hydrocarbons make their separation
and analysis extremely important in environmental testing. However, the
aromatics can pose some serious separation problems (for example, the m-
and p-xylenes) due to the closely similar chemical structure and characteristics.
The xylene isomers differ in structure (although not optically active) have
similar spatial differences to pairs of enantiomers. It follows, chiral stationary
phases that separate enantiomers can also be used for separating spatial isomers
that are not necessarily optically active. Nevertheless, the separation ratios of
such isomeric pairs (even on cyclodextrin stationary phases) is still very small,
often in the 1.021.03 range. As a consequence, the use of high efficiency
capillary columns is essential, if reasonable analysis times are also to be
maintained.
Introduction
Liquid chromatography (LC) was the first type of chromatography to be discovered and, in the
form of liquid-solid chromatography (LSC), was originally used in the late 1890s by the Russian
botanist, Tswett (1), to separate and isolate various plant pigments. The colored bands he produced
on the adsorbent bed evoked the term chromatography (color writing) for this type of separation.
Initially the work of Tswett was not generally accepted, partly due to the original paper being in
Russian and thus, at that time, was not readily available to the majority of western chemists and
partly due to the condemnation of the method by Willstatter and Stoll (2) in 1913. Willstatter and
Stoll repeated Tswett's experiments without heeding his warning not to use too "aggressive
adsorbents as these would cause the chlorophylls to decompose. As a consequence, the
experiments of Willstatter et al. failed and their published results, rejecting the work of Tswett,
impeded the recognition of chromatography as a useful separation technique for nearly 20 years. In
the late 1930s and early 1940s Martin and Synge introduced a form of liquid-liquid
chromatography by supporting the stationary phase, in this case water, on silica gel in the form of a
packed bed and used it to separate some acetyl amino acids. They published their work in 1941 (3)
and in their paper recommended the replacement of the liquid mobile phase with a suitable gas,
which would accelerate the transfer between the two phases and provide more efficient
separations. Thus, the concept of gas chromatography was born. In the same paper in 1941, Martin
and Synge suggested the use of small particles and high pressures in LC to improve the separation
that proved to the critical factors that initiated the development of high performance liquid
chromatography.
The statement made by Martin in 1941 contains all the necessary conditions to realize
both the high efficiencies and the high resolution achieved by modern LC columns.
Despite his recommendations, however, it took nearly fifty years to bring his concepts to
fruition. Activity in the field of liquid chromatography was eclipsed in the 1950s by the
introduction of gas chromatography and serious attempts were not made to improve LC
techniques until the development of GC neared completion in the mid 1960s. The major
impediment to the development of LC was the lack of a high sensitive detector and it was
not until the refractive index detector was developed by A. Tiselius and D. Claesson (4)
in 1942 could the technique be effectively developed. Tswett's original LC consisted of a
vertical glass tube, a few centimetres in diameter and about 30 cm high, packed with the
adsorbent (calcium carbonate). The plant extract pigments were placed on the top of the
packing and the mobile phase carefully added to fill the tube. The solvent percolated
through the packing under gravity, developing the separation, which could be seen as
different colored bands at the wall of the tube. The simple apparatus of Tswett contained
all the essentials to achieve a chromatographic separation. The contemporary
chromatograph, however, is a very complex instrument operating at pressures up to
10,000 p.s.i providing flow rates ranging from a few microliters per minute to 10 or 20
ml/minute depending on the type of LC that is carried out. Modern detectors can detect
solutes at concentration levels of 1x10-9 g/ml and an analysis can be completed in a few
minutes with just a few micrograms of sample.
CO Monitoring:
Co monitoring consisting of two methods:
1. Chromatographic technique
2. Infrared method
Chromatographic Technique: --It's range is 0-200ppm
--Sensitivity 0.1ppm
Diagram:
Theory:
--Any series of copound which has high or regionalblevapour pressure can be supperated
by gas chromatograph.
--This is useful for supperation of Hydrogen,Carbon,Nytrogen and other organic
compounds.
--In this sample and air contains CO is passed through stripped column or pre-column
then heavy hydro carbon are retainetheir,other than CH4,CO.
--These are passed into the chromatographic column, and then toacatelic chamber and
here the CO is reduced to methene(CH4).Which is detected by flame ionization detector
from which peaks are obtain in which the 1st peak corresponds to CH4 and 2nd
corresponds to CO.
Advantages:
--Its an accurate.
--Highly Sensitive
--It can also measure CH4(Methene).
Disadvantages:
--It is expensive and complex.
Infrared Method: --This is uasally range of 0-25,50,100 ppm levels.
Diagram:
Advantages:
--Highly accurate and stable.
Disadvantages:
--It is Sensitive.
NOx Analyzers: If detection of nitric oxide is important to your research, have you thought of
detecting nitrite, which can bring you more insight? The detection of nitric oxide (NO) in
biological liquid samples is extremely difficult due to the transient half-life of NO
molecules. Some report the biological lifetime to be in the order of milliseconds. Once
you remove the sample from the animal or other source, it is almost impossible to directly
detect nitric oxide. Thus, detection of nitrite and nitrate, considered to be the major
metabolites of NO, can be used to determine the NO levels. Moreover, if you evaluate the
nitric oxide level with an in-vivo sensing procedure, it shows only one aspect of the nitric
oxide profile. Monitoring nitrite level has become increasingly important in recent years.
Studies have shown nitrite to be indirectly related to physiological activity and it is also a
signaling molecule.
The ENO-20s high sensitivity and specificity are accomplished with the combination of a dizao coupling method and chromatography. Nitrite and nitrate are
separated from other substances on a unique separation column and mobile phase. Nitrite
then reacts with a compound called Griess reagent and generates diazo compounds which
have a red color. The level of nitrite can be monitored with peak height or area with a
retention time of 4.5 min from the injection of the sample. Nitrate is reduced to nitrite on
a reduction column which reacts to the Griess reagent as well. The nitrate peak has an 8
min retention time. The level of diazo compound is measured by absorbance at 540 nm
using a visible detector. The separation column is robust as well as the entire ENO-20.
Normal lifetime of the column is at least 3 months with regular use. All separation and
detection technologies are provided by Eicom for the ENO-20 including mobile phase
ingredients, Greiss reagent, separation columns, and reduction columns. You can relax
and detect nitrite and nitrate with 10 nM sensitivity (0.1 pmol)
Nox analyzers are classified into three types:
They are
1. Laser opto acoustic spectroscopy
2. Chemiluminescence method
1. Laser opto acoustic spectroscopy:
Radiation source can be output from a laser, a monochromator furnishing
radiations in UV, IR, or a FT-IR spectrometer. All radiation must be pulsed at an
acoustical frequency 50-1200Hz. PA cell is filled with transparent gas often air or helium
and cell volume is kept small, less than 1cm3 in order to preserve the strength of the
acoustical signal.
Fig:1
One main advantage of opto acoustic spectroscopy is the ability to get information
about the depth in the sample of the absorption. The amount of the sample contributing to
the PA signal is proportional to the thermal diffusion depth. This thermal diffusion depth
, is inversely proportional to the modulation frequency f. Figure 3 shows a model
sample that has a thermally thin surface layer (thickness
relative to the light absorption and non- radiative decay, an absorption in the bulk will
have a phase lag between the time of absorption and the thermal signal. However, a
surface absorption should not have a phase lag since the heat doesn't have far to travel to
generate the detected pressure change in the transfer gas.
Fig 2
Advantages:
The sample does not have to be dissolved in some solvent or embedded in a solid
matrix, it is to be used as its.
Conventional absorption spectroscopy is based on excitation by electromagnetic
radiation with intensity I and the measurement of reflected or transmitted light
intensity I. Thus, the absorbencies derived indirectly from transmittance or
reflectance, whereas in PAS pressure waves are detected which are generated
directly by the absorbed energy.
PA signal is not influenced by the scattering particles.
PAS allows the determination of absorption coefficients over several orders of
magnitude. This analytical technique can be applied to the measurement of weak
absorption using PA cells with relatively small path lengths, allowing compact
and mobile set-ups
PA signal depends on the incident radiation power hence the sensitivity can be
tuned to desired range by choosing an appropriate radiation source (for example, a
lamp versus a laser).
PAS is useful for sample that are powered, amorphous or otherwise not
conductive to reflective or transmission form of optical spectroscopies.
2.Chemiluminescence method and Catalytic thermal decomposition:
Sample is reacted with oxygen using oxidation catalyst to transform the nitrogen
compounds into nitric monoxide (nitrogen and nitrogen dioxide do not transform to nitric
monoxide). When nitric monoxide is reacted with ozone, nitrogen dioxide in semi-stable
status is produced. When the nitrogen dioxide in semi-stable status changes into stable
nitrogen dioxide, it emits lights and the strength of the light is in proportional to the
concentration of nitric monoxide. Thus the total nitrogen concentration in the sample is
identified by measuring the strength of the light. An example of total nitrogen analyzer
using catalytic thermal decomposition and chemiluminescences method is illustrated below. Sample is weighed and then put into combustion tube, where the sample is heated
to a high temperature and oxidized with oxidation catalyst into nitric monoxide. After
oxidation, the sample is dehumidified with a dryer and fed into the detector, where the
sample is reacted with ozone as below.
NO + O3 --> NO2 + O2 + h
The strength of light of wavelength of 590 to 2500nm is measured.
Fig 3
(a).120C decomposition and UV absorptiometery:
Add alkali solution of potassium peroxodisulfate to the sample and heat the
mixture to approx. 120C to transform all the nitrogen compounds to nitric acid ions.
Control the pH of this solution to 2 to 3 using hydrochloric acid, and measure the
absorption of nitric acid ions in UV zone to obtain the total nitrogen concentration. An
example of total nitrogen analyzer using 120C decomposition and UV absorptiometer is
illustrated below. Sample is weighed and put into the sample water container. Specified
amount of potassium peroxodisulfate and sodium hydroxide solutions are added to the
sample, and the mixture is put into the thermal decomposition chamber. In the chamber,
the mixture is heated at 120C for 30 minutes to oxidize the nitrogen compounds into
nitric acid ions. The solution after the decomposition is put into reaction-cooling chamber
to cool off. The cooled solution is then adjusted to pH 2-3 by adding hydrochloric acid.
The absorbency of nitric acid ions in the solution is measured using UV absorptiometer at
a wavelength of 220nm.
Example of total nitrogen analyzer of 120C decomposition- UV
absorptiometery
Fig.4
(b) Photo oxidation decomposition and UV absorptiometery:
Add alkali solution of potassium peroxodisulfate to the sample and radiate
ultraviolet ray to transform all the nitrogen compounds to nitric acid ions. Control the pH
of this solution to 2 to 3 using hydrochloric acid, and measure the absorption of nitric
acid ions in UV zone to obtain the total nitrogen concentration. An example of total
nitrogen analyzer using photooxidation and UV absorptiometer is illustrated below.
Sample is weighed and put into the mixing chamber. Specified amount of potassium
peroxodisulfate and sodium hydroxide solutions are added to the sample, and the mixture
is put into the UV oxidation apparatus. In the chamber, the mixture is heated at 90C and
exposed to ultraviolet ray for 15 minutes to oxidize the nitrogen compounds into nitric
acid ions. The solution after the decomposition is weighed and then adjusted to pH 2-3 by
adding hydrochloric acid. The absorbency of nitric acid ions in the solution is measured
using UV absorptiometer at a wavelength of 220nm.
Example of total nitrogen analyzer of photooxidation decomposition- UV
absorptiometery:
Fig.5
Advantage of Chemiluminescences method:
Nitric oxide and ozone react in the gaseous phase to produce chemical
luminescence. It is possible to detect nitrite in the liquid phase using this procedure but it
is not as simple as using the ENO-20. What types of samples require the nitrite assay?
Usually the sample matrix is liquid; the ENO-20 is the perfect device for this. In the
process of reducing nitrite/nitrate, the system is heated which results in condensation on
the inner surface of the glassware after cooling down. The condensation dissolves the
nitrite/nitrate and will have an effect on carry-over when the condensation
evaporates during the next process. This is a common reason for variation in the detected
values of nitrite using the chemiluminescense procedure. As explained above, the ENO-
20 installed with an autosampler yields hands free analysis. You do not need to clean the
glassware of the detection system following the analysis of several samples or following
the sample conversion from liquid to gas.
H2S Analyzer Sample System
THE NEED
A critical measurement of natural gas quality is theconcentration of hydrogen sulfide
(H2S). Remarkably,thecommon technique for making this measurement, leadacetate tape,
is at least 70 years old. Users of this archaictechnology report general dissatisfaction with
it due to high maintenances, the cost and shelf life of tape cassettes, the need for reagents,
difficulty handling H2S overload conditions, the sensitivity of the technology to
ambientextremes, and, not least, used cassette disposal.
AMETEK Western Research has responded to the needfor a low maintenance, reliable,
and rugged H2S analyzer by developing the Model 933. The Model 933 uses
AMETEKWestern Researchs proprietary frontal elution chromatography sampling technique, combined with our exceptionally high resolution multi-wavelength UV optical
bench. The result is a unique low level H2S analyzer that is designed to operate
unattended for six months or more.
THE MEASUREMENT
AMETEK Western Research's unique sample conditioningsystem uses frontal elution
chromatography to separate andeliminate interfering species. This ensures an
accurateanalysis of the gas via direct-UV absorption spectroscopy.In sales gas
applications, H2S and COS are the first absorbingspecies to elute through the
chromatography column. Thesecompounds are analyzed photo metrically. The next
species toelute is methyl mercaptan, which is also measured, and finally, the interfering
ethyl mercaptan. Interference by ethyl mercaptan is prevented by terminating the sample
analysis before itelutes, and then switching to a fresh column so that theanalysis may
continue. Two columns are employed in 933.While one column is conditioning the gas
sample, the other isautomatically regenerated.
In normal operation, the 933 uses its analysis of the COS and methyl mercaptan
oncentrations to provide real timecompensation for the H2S measurement. Optionally,
the 933 can be configured to output concentration values for thesecompounds.
The Model 933 utilizes two onboard microprocessors that provide concentration
calculations, data processing,caliration, sophisticated self-diagnostics, and
columnswitching control.
SpectraSensors SS 2100 Laser
Based H2S Analyzershelp naturalgas processors, LNG plants, refineriesand chemical
plants ensure trace hydrogen sulfide in mixed hydrocarbongases quickly and reliably with
nohazardous waste or costly consumables
MINIMAL MAINTENANCE
Laser based technology is very reliablefor remote or hazardous installationsand requires
very little maintenance.SpectraSensors analyzers work by directmeasurement. There are
no chemical reactions to generate, no calibrationsneeded, and no results to interpret.
RELIABLE
Spectra Sensors analyzers work by direct measurement. By itsnature, the TDL based H2S detectionmethod is not susceptible to agingeffects, making its factory calibration
timeless constant. As a result,the factory calibration remainsstable over the life of the
analyzer.
Excellent measurement repeatability between instruments and as compared to lab
instruments makes the SS2100 H2S analyzer the ideal measurement tool for even the
most punishing of environments.
The analyzers lower detection limit is 1 ppm in natural gas and 2 ppm in mixed hydrocarbons with operating ranges from 0-10 ppmv to percent level and repeatability as
good as 500ppbv.
EASE OF OPERATION
Designed to be plug-and-play, the analyzers are remarkably easy to install; just connect
power, the data link, and the measured gas line and the analyzer begins working. Once set
up, measurements are performed in seconds.
Analog and digital communications and alarm options are available as well as manual or
automated H2S validation stream switching.
Hydrogen Sulfi de - Sulphur Dioxide Analyzer:
Product Specifications
Utilizing pulsed fluorescence technology the Model 450i H2S and SO2 analyzer operates
on the principle that H2S can be converted to SO2. As the SO2 molecules absorb
ultraviolet
(UV) light and become excited at one wavelength,the molecules then decay to a lower
energy state emitting UV light at a different wavelength. Specifically,
H2S -> SO2 SO2 +hv 1 -> SO2*-> SO2 +hv 2
The pulsing of the U.V. source lamp serves to increase the optical intensity whereby
aGreaterU.V. energy throughput and lower detectable SO2 concentration are realized.
Reflective band pass filters, as compared to commonly used transmission filters, are less
subject to photochemical degradation and are more selective in wavelength isolation.
This results in both increased detection specific city and long term stability.
This state-of-the-art gas analyzer also offers features such as an Ethernet port as well as fl
ash memory for increased data storage.Ethernet connectivity provides efficientremote
access, allowing the user to download measurement information directly from the
instrument without having to be on-site.
You can easily program short cut-keys to allow you to jump directly to frequently
accessed functions, menus or screens. The larger interface screen can display up to five
lines of measurement information while primary screen remains visible.
Principle of Operation:
Hydrogen Sulfide in Gaseous Fuels (Lead Acetate Reaction Rate Method).
When hydrogen sulfide contacts the sensing tape a brown stain appears. electronics
measure the rate of darkening over time and calculates the hydrogen sulfide
concentration.
A flow and pressure regulated filtered sample passes through a membrane humidifier and
into the samplechamber. A window in the sample chamber allows thegas to come in
contact with the sensing tape creatingconcentration dependent tain when hydrogen sulfide
is present.
In high concentration applications (over 20 ppm), there maybe a restricting aperture
behind the window. Various sizes of apertures match different measurement ranges
Analyzer Diagram
H2S Analyzer Sample System
Leave the sweep valve on the sample filter slightly open at all times. This will decrease
the likelihood of contamination.
If the analyzer requires cleaning on a regular basis, the sample point may have to be
relocated or additional sample conditioning be required. Please consult Envent
Engineering.
During start-up or plant upset situations, the H2S analyzer may become contaminated
with H2S scavenger solution.
This will cause the analyzer to read lower than the actual H2S concentration. This can be
determined at calibration. The analyzer will read low and require incremental increases in
the gain to maintain calibration. Please refer to factory calibration sheet for the default
gain factor.
The flowmeter should be inspected for liquids and to ensure the float moves freely.
The scavenger solution is water soluble and therefore is relatively easy to clean.
Dis-assembly of the pressure regulator and solenoids in the field is not advised.
Consult the factory if the regulator or solenoid appears contaminated.
Remove the filter element from the filter housing and discard.
Remove all sample system components and soak in cleaningsolution.
Ensure valves are fully open when cleaning. 3-way valves should be cleaned with handle in all positions.
Flush the sample components with fresh water.
Rinse with isopropyl alcohol
Blow dry with clean compressed air or fuel gas
If Tygon tubing (flexible tubing connecting humidifier) appears discoloured, replace with new tubing
Install new filter elements into filter housings .
Re-assemble Stainless Steel Tubing to analyzer according to analyzer drawing (refer to the back of the manual)
Once sample system has been re-assembled. Apply calibration gas to the analyzer.
Adjust gain to indicate value from calibration certificate.
Gains for streams should be +/- 2.00 from the factory calibration sheet or the last calibration. If the reading is not within range, then system may need further
cleaning. Please consult factory
Introduction to Nuclear Radiation Detection:
Nuclear radiation in any small doze is injurious to health. Nuclear radiation is invisible
and has no smell or sound. It is not possible to detect all kinds of nuclear radiation with
naked eyes or ears. There are several ways of detecting nuclear radiation. Nuclear
radiation consists of several particles and has the capacity to ionize particles. These
qualities are used in detecting these nuclear radiations.
Methods for Nuclear Radiation Detection
1. Geiger Muller Counter:
History:
Hans Geiger developed a device (that would later be called the "Geiger counter") in 1908
together with Ernest Rutherford. This counter was only capable of detecting alpha
particles. In 1928, Geiger and Walther Muller (a PhD student of Geiger) improved the
counter so that it could detect more types of ionizing radiation.
The current version of the "Geiger counter" is called the halogen counter. It was invented
in 1947 by Sidney H. Liebson (Phys. Rev. 72, 602608 (1947)). It has superseded the
earlier Geiger counter because of its much longer life. The devices also used a lower
operating voltage.
Description:
Geiger counters are used to detect ionizing radiation (usually beta particles and
gamma rays, but certain models can detect alpha particles).
Here the radiation is passed through the Geiger Muller tube. The radiations have
an ionizing effect. This property is used to detect radiation. In the Geiger Muller
tube an avalanche and a pulse are created, it can be amplified and counted to
identify the type of radiation.
GeigerMller tube (or GM tube)
GeigerMller tube in operation
GeigerMller tube output characteristics
A GeigerMuller tube (or GM tube) is the sensing element of a Geiger counter
instrument that can detect a single particle of ionizing radiation, and typically
produce an audible click for each.
A GeigerMuller tube consists of a tube filled with a low-pressure (~0.1 Atm)
inert gas such as helium, neon or argon (usually neon), in some cases in a Penning
mixture, and an organic vapor or a halogen gas.
The tube contains electrodes, between which there is a potential difference of
several hundred volts, but no current flowing. The walls of the tube are
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