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Instruments for Optical Spectrometry

• Optical spectroscopy encompasses regions of the electromagnetic spectrum that are not strictly within the visible range. However, the components of instruments used in the IR, visible, and UV are all very similar.

Instrument Components

• Spectroscopic instruments are usually constructed from the following components:

• A stable source of radiant energy• A wavelength selector that permits the selection and

isolation of a restricted wavelength region• One or more sample containers• A radiation detector or transducer that converts

radiant energy to a measurable signal – normally electrical

• A signal processor and readout

Instrument Components

• In emission instruments the source and sample are combined so that emission occurs from heating or electrical excitation of the sample.

• In absorption and fluorescence instruments the analyte is held in a sample container and an external radiant source is employed.

• In absorption instruments the source, sample, and detector are located along an axis. In fluorescence instruments the source is placed at an angle of 90° to the detector-sample axis.

Optical Materials

• The optical components of a spectrometer must be optically transparent in the wavelength region being employed for analysis.

• In IR studies, sodium chloride, potassium chloride, or silver chloride are used.

• In the visible region, borosilicate glass or plastics such as polystyrene are adequate.

• In the UV, quartz is used because glass absorbs too much radiation at these wavelengths.

Spectroscopic Sources• The source must provide a beam of sufficient intensity

in the wavelength region of interest to allow detection and measurement. It must be stable and not flicker so P and Po can be measured accurately. Some instruments measure these values simultaneously so stability is less of a factor.

• Both line sources and continuous sources are used in optical spectroscopy. The continuous source is made up of a whole range of emitting wavelengths that overlap while line sources emit discrete bands at one or two wavelengths.

Tungsten Filament Lamps

• Theses emit in the range 320 – 2500 nm so most of the emissions are in the IR.

• The lamp is useful for visible spectroscopy but not for UV because the lamp housing absorbs strongly at these wavelengths.

• The lamp should have a stable voltage supply so that its spectral characteristics remain constant – it tends to vary as its temperature changes.

Tungsten Halogen Lamps

• Tungsten halogen lamps are an improvement over tungsten filament lamps because they have a longer lifetime and their emission spectra extend into the UV.

• The lamp contains a small quantity of iodine within a quartz envelope that houses the filament.

• The lifetime of the tungsten halogen lamp is more than double that of the tungsten filament lamp.

• In the tungsten filament lamp, the filament eventually sublimes.

Tungsten Halogen Lamps

• In a tungsten iodine lamp the tungsten sublimes but reacts with the iodine to give WI2, which diffuses back to the filament where it decomposes and re-deposits tungsten.

Hydrogen and Deuterium Lamps

• These lamps generate true continua. The process for hydrogen may be shown as:

• H2 + E H2* H' + H'' + h where E is the energy required to excite the hydrogen molecule.

• The products, H' and H'' may have a variety of kinetic energies, which means that the photon emitted, h, will also vary in energy – thus the continuum.

Hydrogen and Deuterium Lamps

• The useful range of these lamps is from 160 – 375 nm. Deuterium is preferred to hydrogen because of its greater intensity.

• The emission is stimulated by generating an electrical arc in an envelope filled with the gas. A regulated power supply is necessary for constant lamp intensity.

Wavelength Selectors• Spectroscopic instruments need a wavelength selector

to provide a range of wavelengths to impinge on the sample. The narrower the range of wavelengths chosen by the wavelength selector, the more likely Beer’s law is to be followed. This also leads to improved sensitivity and selectivity.

• The closest type of radiation to a purely monochromatic source is a laser.

• With normal optical instruments the wavelength selector provides a range of wavelengths centred at a nominal wavelength.

Wavelength Selectors

• The range of wavelengths provided by the wavelength selector is known as the effective bandwidth or just bandwidth and is defined as the width of the band in wavelength units at half peak height. The ordinate in this plot is transmittance so the bandwidth is the range for 0.5T.


• Bandwidths vary enormously from one wavelength selector to another. For instance, a high quality monochromator for the visible region may have a bandwidth of a few tenths of a nanometre or less. An absorption filter operating in the same region may have a bandwidth of 200 nm or more.


• Generally two types of wavelength selector are used, filters or monochromators.

• Filters are rugged, inexpensive, and easy to maintain.

• Monochromators are costly but allow nominal wavelengths to be changed continuously during irradiation of the sample.

Radiation Filters

• A filter allows only a restricted band of wavelengths to pass through it.

• Two types of filter are used in spectroscopic instrumentation, interference filters and absorption filters.

• Interference filters allow a greater transmittance of radiation at the nominal wavelength than absorption filters.

Monochromators

• Two types of monochromators have been used to disperse multiwavelength radiation into its various components. These are prisms and diffraction gratings.

• Prisms are used infrequently nowadays because gratings can be made more cheaply. Gratings scratched on the surface of a mirror lead to more compact instrumentation.

Monochromators

• In a typical reflective grating monochromator, radiation from a source enters the monochromator by a narrow opening called a slit. The radiation is then collimated by a concave mirror so that a parallel beam strikes the grating. Angular dispersion from the grating then occurs at the reflective surface.

Monochromators

• Consider two wavelengths of radiation, 1 and 2, being dispersed by this grating. If 1 < 2 radiation of wavelength 1will be reflected from the grating at a sharper angle than that of wavelength 2. This is known as angular dispersion.

• After dispersion the two beams of radiation are focused by a concave mirror on to the focal plane of the instrument where they appear as two images. By rotating the grating either one of the images can be focused on the exit slit.

Gratings• Most gratings used in commercial instruments are replica

gratings obtained by making castings from a master grating. The master grating is prepared by scoring many parallel and closely spaced grooves on a flat polished surface with a sharp diamond tool. In UV-visible work the grating will have from 300-2000 grooves/mm.

• Preparation of the master grating is tedious and expensive. Replica gratings are prepared from a liquid resin casting process that preserves the optical accuracy of the original master grating. The surface of the replica is made reflective with a coating of aluminum, gold, or platinum.

Radiation Detectors and Transducers

• A detector is a device that indicates the existence of some physical phenomenon. Examples are the human eye, photographic film, the mercury level in a thermometer, etc.

• A transducer is a special type of detector that converts a physical phenomenon, e.g. light intensity, pH, mass, etc., into electrical signals that can be amplified, manipulated, and eventually turned into numbers that are related to the magnitude of the original signal.

Properties of Transducers

• The ideal electromagnetic radiation transducer responds rapidly to low levels of radiant energy over a broad wavelength range.

• The electrical signal it produces must be easily amplified and have a relatively low noise level.

• It is essential that the electrical signal produced by the transducer is directly proportional to the beam power, P.

• G = KP + K' • G is the electrical response of the detector in units of

current, resistance, or potential• K is a proportionality constant which measures the

sensitivity of the detector in terms of electrical response per unit of radiant power

• K' represents the dark current. This is a small constant response, present even when no radiation strikes the detector. When K' is reduced to a negligible quantity by electrical subtraction etc., G = KP

Properties of Transducers

Types of Transducers

• Two general types of transducers are used in optical spectroscopy. UV-visible transducers respond to light or photons

Photon Detectors

• Types of photon detectors that are used commercially are: phototubes, photomultiplier tubes, silicon photodiodes.

• Phototubes are one of the simpler types to understand because they work on the Einstein photo-emissive cathode principle.

Photomultiplier Tubes• This is similar in design to the phototube but is much

more sensitive.• The surface of the cathode is similar in composition to

that of the phototube. Electrons are emitted from its surface and accelerated towards the surface of another electrode called a dynode. Upon striking the dynode, each electron produces several additional electrons which are then accelerated towards another dynode. This process continues at a number of dynodes until 106 – 107 electrons have been produced for each photon.

• The cascade is collected at the anode where the resulting electric current is further amplified and measured.

Silicon Photodiodes

• These are transistor devices used in diode array detectors. In these devices, over 1000 can be fabricated side by side on a small silicon chip – each diode is about 0.02 mm thick.

• When one or more diode array detectors are placed along the focal plane of a monochromator, the intensity of all wavelengths can be measured simultaneously.

Signal Processors and Readouts

• The signal processor is an electronic device that amplifies the electrical signal from the detector. It may also subtract unwanted signals.

• Signal processors may display the signal in a number of ways including derivative and integrated peak areas, etc.

• Digital readout devices are most frequently found in modern instruments. However, chart recorders are used a great deal still.

Optical Instrument Design

• Optical spectroscopic instruments vary from the very sophisticated which are expensive and used for research, to the simple inexpensive variety which are used for routine chemical analysis. It is the nature of the components that determines the cost and eventual use.

Common Names of Some Optical Instruments

• A spectroscope is an instrument for visually identifying species excited in a flame or by other means. The instrument has a monochromator and an eyepiece in the focal plane to observe emission lines. It was used in early spectroscopic studies.

• A colorimeter is an instrument for absorption measurements in which the eye serves as the detector. Comparison standards are used to quantify the amount of analyze. This is a limited use of the term colorimeter which is now usually applied to any photometer used for absorption measurements in the visible region of the spectrum.


• A photometer is a photoelectric instrument that can be used for absorption, emission, or fluorescence measurements. Most colorimeters are photometers because they use filters for wavelength selection.


• A spectrograph records spectra on a photographic film or plate, placed along the focal plane of the monochromator. The spectra appear as a set of black images of the entrance slit. Spectrographs are usually used for qualitative analysis.

• A spectrometer is a monochromator equipped with a fixed slit at the focal plane. When the detection device is a phototransducer the instrument is known as a spectrophotometer.

Instrument Types

• Three general types of spectroscopic instruments are in general use.

• These are single beam, double beam in space, and double beam in time.

• All three types are used in absorption, emission, and fluorescence studies.

Single Beam Instruments

• In a single beam instrument used in absorption studies, a filter or monochromator is used to select the wavelength of radiation incident on the sample holder.

• Matched cells can be interposed alternately in the radiation beam which passes on to a detector, usually a photomultiplier tube.


• To operate a manual instrument the following adjustments are made:

• (a) The percent transmittance is corrected to zero by placing a shutter between the source and the detector and adjusting the meter until it reads zero.

• (b) solvent is placed in the radiation path, and the shutter opened. The percent transmittance reading of the meter is adjusted to 100 percent.


• A logarithmic scale can be used to determine absorbance; a linear scale is used to determine transmittance.

• Single beam instruments vary in complexity.

Spectrometers

Double Beam Instruments

• In a double beam instrument, two beams of radiation are formed from the same source by a V-shaped mirror called a ‘beam splitter’.

• One beam passes through the reference solution to a detector while the second simultaneously passes through a solution containing analyte to a second matched detector.


• The two outputs are amplified and their ratio (or log ratio) is determined electronically and displayed by the readout device.

Spectrometers


• A second type of double beam instrument utilizes a rotating sectored mirror to direct the radiation from the source, as a pulse, either through a reference cell or through the sample.

• The pulses of radiation are recombined by another sectored mirror before detection.

• Pulsed radiation is the basis of many modern research instruments.


• Double beam instruments compensate for anything but very short term fluctuations in radiant output from the source, as well as for any drift in the detector and amplifier.

• They also compensate for any wide variation in source intensity with wavelength.

• This design lends itself particularly well to the continuous recording of spectra. Most modern UV-visible recording instruments are based on this design.

Spectrometer

Time separated double beam

Molecular Absorption Spectroscopy

• Molecular optical spectroscopy is useful for both qualitative and quantitative measurements.

• UV-visible spectrophotometry is mainly used for quantitative work.

Ultraviolet and Visible Absorption Spectroscopy

• Molecules absorb a finite amount of energy to place the molecule in an upper electronic state. Various vibrational and rotational levels are occupied in this excited electronic state. The absorption spectrum should reflect this.

• In the gas phase, it is possible to observe absorption of energy to different vibrational and rotational energy levels. This is a resolved spectrum with fairly narrow lines.

Ultraviolet and Visible Absorption Spectroscopy

• In solution or in condensed phases the spectrum loses its resolution and becomes a broad band.

• This is because the molecules of absorbing species can bump into one another more easily, thus creating a variety of energies that cannot be resolved.

Absorption by Organic Compounds

• The wavelength at which organic molecules absorb UV-visible radiation depends to a large extent on how firmly bound the electrons are in the molecule.

• Electrons in single covalent bonds are tightly held between the atoms in the bond. Excitation of these electrons is only possible by using wavelengths of radiation below 180 nm.

• This is of no use for routine measurements because quartz and various atmospheric components absorb at these wavelengths.


• Electrons in double or triple bonds are more easily excited because they are more loosely held than single bond electrons. Unsaturated molecules have useful absorption spectra.

• The unsaturated organic functional groups that absorb UV-visible radiation are known as chromophores. They can serve as a very rough guide for identification purposes.


• Peak position is influenced by a number of factors such as:

• Solvent effects• Other structural details of the molecule, such as

conjugation• Molecules containing heteroatoms with lone pairs

of nonbonding electrons may also absorb in the UV-visible region of the spectrum and provide qualitative information on the molecule.

Absorbing species• Electronic transitions

and n electrons– d and f electrons– Charge transfer reactions

and n (non-bonding) electrons

Sigma and Pi orbitals

Electron transitions

Transitions

– UV photon required, high energy• Methane at 125 nm• Ethane at 135 nm

• n-> – Saturated compounds with unshared e-

• Absorption between 150 nm to 250 nm between 100 and 3000 L cm-1 mol-1

• Shifts to shorter wavelengths with polar solvents– Minimum accessibility

– Halogens, N, O, S

Transitions• n->,

– Organic compounds, wavelengths 200 to 700 nm

– Requires unsaturated groups• n->low (10 to 100)

– Shorter wavelengthshigher (1000 to 10000)

Absorption by Inorganic Species

• Typically visible absorption of visible radiation occurs with compounds of elements in the first and second row of the transition series. This is why these compounds are colored.

• The absorption involves transitions between filled and unfilled d-orbitals with energies that depend on the ligands attached to the elements.

• The position of the energy transition depends on the position of the element in the periodic table, its oxidation state, and the nature of the attached ligand.

Absorption by Inorganic Species

• The lanthanide elements behave somewhat differently in that the peaks are fairly narrow.

• This is because transitions are between electrons in f-orbitals. They are shielded from effects of the outer electrons and the energy levels are thus not distorted.

Qualitative Applications of UV-visible Spectroscopy

• UV-visible qualitative analysis is useful for determining the presence of certain gross structural features of molecules, e.g. unsaturation.

• However it needs to be supplemented by other spectroscopic methods such as IR and NMR to obtain detailed information on molecular structure.

Solvents• The most useful information is derived from spectra

obtained from dilute solutions or from a vapour of the analyte.

• Solvents should be of high purity and be transparent throughout the spectral range.

• Generally non-polar solvents distort spectra least. Polar solvents tend to obscure structure.

• Solvents tend to change the position of maximum absorbance so in any qualitative measurement of wavelength, the same solvent must be used for all samples.

Quantitative Applications

• This is one of the most important tools available for quantitative analysis.

• Important characteristics of the method are as follows:

• (1) Wide applicability. Most compounds will either absorb themselves or can be derivatized so that the derivatives absorb. UV-visible spectrophotometry is the single most important analytical technique for quantitative chemical analysis.


• (2) High sensitivity. Typically measurements can be made with detection limits as low as 105 mol/L. With procedural modifications these detection limits can be extended even further.

• (3) Moderate to high selectivity. This means that it is sometimes, but not too often, possible to make measurements on the analyte without prior separation from other absorbing and interfering compounds.


• (4) Excellent accuracy. Normally the precision of any given measurement will be within 1–5%. Errors are reduced to similar levels and can be made even lower by using special measuring techniques.

• (5) Ease and convenience. Spectrometers are not cheap, but are easy to use and lend themselves to automation.

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