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Universität Duisburg-Essen Dr. Siddiqi Fakultät für Ingenieurwissenschaften Abteilung Maschinenbau Institut für Verbrennung und Gasdynamik Thermodynamik

ISE-Bachelor

CONCENTRATION MEASUREMENT

(THERE IS A SMALL PART IN GERMAN LANGUAGE)

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Concentration Measurement 1 Introduction 2 Important gas analysis methods

2.1 Orsat-Apparatus

2.2 Infra red spectroscopy

2.3 Paramagnetic oxygen analysis

2.4 Mass spectrometry

2.5 Gas chromatography

2.6 Thermal conductivity detector (TCD)

2.7 Flame ionization detector (FID) 3 Experiments for concentration measurements

3.1 Measurement of the concentration of Propane, Isobutane and n-Butane in gas mixture using a gas chromatograph

3.1.1 Basic principles 3.1.2 Experimental set up 3.1.3 Experimental procedure 3.1.4 Evaluation

3.2 Bestimmung der Raumanteile von CO2 und CO in einem Gasgemisch mit den Prüfröhrchen

3.2.1 Kurzbeschreibung 3.2.2 MAK Wert 3.2.3 Chemische Grundlagen 3.2.4 Versuchsaufbau 3.2.5 Versuchsablauf

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1 Introduction The general purpose of gas analysis is to measure the concentration of each component in a gas mixture. Both physical and chemical methods are applied in gas analysis. Each gas component is measured on the basis of different chemical/physical or physical principles of measurements. Such principles include:

a. Chemical reactions (e.g. Orsat Apparatus, Dräger test tubes) b. Absorption of radiation (e.g. infrared) c. Paramagnetism (this method measures oxygen concentration) d. Mass spectrometry e. Gas chromatography f. Thermal conductivity (requires separation using chromatography) g. Flame ionization (requires separation using chromatography)

2. Important gas analysis methods

2.1 ORSAT apparatus:

The Orsat apparatus, illustrated in Fig. 1, is generally used for analyzing gas mixtures (e.g. flue gases). The burette 4 is graduated in cubic centimeters up to 100, and is surrounded by a water jacket to prevent any change in temperature from affecting the density of the gas being analyzed.

It uses three pipettes (some use four pipettes for more accuracy), (1+1a),(2+2a),(3+3a), the first containing a solution of caustic potash (KOH) for the absorption of carbon dioxide, the second an alkaline solution of pyrogallol (1,2,3-trihydroxybenzene) for the absorption of oxygen, and the third an acid solution of cuprous chloride (Cu2Cl2) for absorbing the carbon monoxide. Each pipette contains a number of glass tubes, to which some of the solution clings, thus facilitating the absorption of the gas. In the pipette 3, copper wire is placed to re-energize the solution as it becomes weakened. The rear half of each pipette is fitted with a rubber bag, one of which is shown at 15, to protect the solution from the action of the air. The solution in each pipette should be drawn up to the mark on the capillary tube.

The gas is drawn into the burette through the valve 11. To discharge any air or gas in the apparatus, the valve is opened to the air and the bottle 9 is raised until the water in the burette reaches the 100 cubic centimeters mark. The valve 11 is then turned so as to close the air opening and allow gas to be drawn through, the bottle 9 being lowered for this purpose. The gas is drawn into the burette to a point below the zero mark, the valve 11 then being opened to the air and the excess gas expelled until the level of the water in 9 and in 4 are at the zero mark. This operation is necessary in order to obtain the zero reading at atmospheric pressure.

The apparatus should be carefully tested for leakage as well as all connections leading thereto.

Figure1 : Orsat apparatus.

Before taking a final sample for analysis, the burette 4 should be filled with gas and emptied once or twice, to make sure that all the apparatus is filled with the new gas. The valve 11 is then closed and the valve 14 in the pipette 1a is opened and the gas driven over into 1a by raising the bottle 9. The gas is drawn back into 4 by lowering 9 and when the solution in 1a

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has reached the mark in the capillary tube, the valve 14 is closed and a reading is taken on the burette, the level of the water in the bottle 9 being brought to the same level as the water in 4. The operation is repeated until a constant reading is obtained, the number of cubic centimeters being the percentage of CO2 in the gas mixture.

The gas is then driven over into the pipette 2a and a similar operation is carried out. The difference between the resulting reading and the first reading gives the percentage of oxygen in the gas mixture.

The next operation is to drive the gas into the pipette 3a.

The process must be carried out in the order named, as the pyrogallol solution will also absorb carbon dioxide, while the cuprous chloride solution will also absorb oxygen.

The analysis made by the Orsat apparatus is volumetric; if the analysis by weight is required, further calculations are to be done.

2.2 Absorption of radiation (Infra red spectroscopy)

The basis for the quantitative measurements using optical spectroscopy is provided by the Lambert-Beer law which relates the spectral response (absorbance) to the concentration.

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d

The absorbance of a measured absorption band is a function of the measurement wavelength, the thickness of the sample and the concentration of the absorbing species being measured. This is expressed as follows:

1 1 1A cλ λε= where 1A λ is the absorbance of species 1 at wavelength 1, λλ ε is the absorptivity of species 1 at wavelength 1,cλ is the concentration of the absorbing species 1, and d is the optical thickness of the sample or path length of the measurement cell. So from the absorbance measurements the concentration c of a particular component may be calculated. Infrared spectroscopy includes the methods that are based on the absorption (or reflection) of electromagnetic radiation with wavelengths in the range of 0.8 to 1000 μm. This spectral range has been divided into three groups: near infrared (NIR) (0.8 – 2.5 μm), mid infrared (MIR)(2.5 – 25 μ) and far infrared (FIR) (25 – 1000 μm). Of these three ranges the MIR region is the most accessible and the richest in providing structural information (molecular fingerprint). Infrared analyses are performed on dispersive (conventional) and Fourier-transform spectrometers. Besides the dispersive spectrophotometers many spectrometers have been designed for routine analysis (in laboratory as well as for on-site control) to quantify one or many compounds. Almost one hundred gases or volatile compounds can be quantified with the help of such photometers. The optical lay outs for two typical models are shown in Figure 2. The most widely used photometers are for the measurement of carbon monoxide (CO) [absorbance measurement at 2170 cm-1] and carbon dioxide (CO2) [absorbance measurement at 2350 cm-1] from car exhausts. In Figure 2a the light coming from the source S passes through an interface filter F depending on the wavelength to be measured. Cell C contains the sample while the reference cell R

contains a known concentration of the same type of gas to be quantified. Another cell A is filled with a non absorbing gas (N2). The cells A and R are placed in the optical path alternatively. By a comparison of the absorbance measured by the detector with and without the sample cell in the path the concentration of the concerned gas in the sample can be determined. In the other model (Figure 2b) the light beam from the source 1 travels through the measurement cell 2 before reaching the cells V1 and V2 which contains the gas component to be measured (e.g. CO or CO2). In this arrangement the sample absorbs a part of the radiation before the radiation reaches V1. The chambers V1 and V2 are so constructed that in the absence of any absorbance in the measurement cell (e.g. when the inert gas flows through it) the absorption in V1 and V2 are same and no pressure difference is observed by the pressure transducer. The beam chopper M is necessary to obtain a repetitive pulsed signal. This is the zero point adjustment. The intensity of the beam reaching V1 will be attenuated if the same gas is present in cell (as is the case it is the case when the sample flows through V) and this will be proportional to the concentration of the gas component to be measured.

Figure 2: Layout of some typical gas analysers.

2.3 Paramagnetic Oxygen Analyser

Oxygen has a relatively high magnetic susceptibility as compared to other gases such as nitrogen, helium, argon, etc. and displays a paramagnetic behaviour. The paramagnetic oxygen sensor consists of a cylindrical shaped container inside of which is placed a small glass dumbbell. The dumbbell is filled with an inert gas such as nitrogen and suspended on a taut platinum wire within a non-uniform magnetic field. The dumbbell is designed to move freely as it is suspended from the wire. When a sample gas containing oxygen is processed through the sensor, the oxygen molecules are attracted to the stronger of the two magnetic fields. This causes a displacement of the dumbbell which results in the dumbbell rotating. A precision optical system consisting of a light source, photodiode, and amplifier circuit is used to measure the degree of rotation of the dumbbell. In some paramagnetic oxygen sensor designs, an opposing current is applied to restore the dumbbell to its normal position. The

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current required to maintain the dumbbell in it normal state is directly proportional to the partial pressure of oxygen and is represented electronically in percent oxygen. There are design variations associated with the various manufacturers of magnetodynamic paramagnetic oxygen sensors. Also, other types of sensors have been developed that use the susceptibility of oxygen to a magnetic field which include the thermomagnetic or `magnetic wind' type and the magnetopneumatic sensor. In general, paramagnetic oxygen sensors offer very good response time characteristics and use no consumable parts, making sensor life, under normal conditions, quite good. It also offers excellent precision over a range of 1% to 100% oxygen. The magnetodynamic sensor is quite delicate and is sensitive to vibration and/or position. Due to the loss in measurement sensitivity, in general, the paramagnetic oxygen sensor is not recommended for trace oxygen measurements.

Figure 3: Layout of a paramagnetic analyser.

2.4 Mass spectrometry

Mass spectrometry (MS) is an analytical technique for the determination of the elemental composition of a sample. The MS principle consists of ionizing chemical compounds to generate charged molecules or molecule fragments and measurement of their mass to charge ratios. In a typical MS procedure:

1. a sample is loaded onto the MS instrument, and undergoes vaporization. 2. the components of the sample are ionized by one of a variety of methods (e.g., by

impacting them with an electron beam), which results in the formation of positively charged particles (ions)

3. the positive ions are then accelerated by a magnetic field 4. computation of the mass to charge ratio of the particles based on the details of motion

of the ions as they transit through electromagnetic fields, and 5. detection of the ions, which in step 4 were sorted according to m/z.

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MS instruments consist of three modules: an ion source, which can convert gas phase sample molecules into ions; a mass analyser, which sorts the ions by their masses by applying electromagnetic fields; and a detector, which measures the value of an indicator quantity and thus provides data for calculating the abundances of each ion present. The technique has both qualitative and quantitative uses. These include identifying unknown compounds, determining the isotopic composition of elements in a molecule, and determining the structure of a compound by observing its fragmentation. Other uses include quantifying the amount of a compound in a sample or studying the fundamentals of gas phase ion chemistry (the chemistry of ions and neutrals in a vacuum). MS is now in very common use in analytical laboratories that study physical, chemical, or biological properties of a great variety of compounds.

Figure 4: Schematic diagram of a mass spectrometer.

2.5 Gas chromatography

Chromatography is the collective term for a family of laboratory techniques for the separation of mixtures. It involves passing a mixture dissolved in a "mobile phase" through a stationary phase, which separates the component to be measured from other molecules in the mixture and allows it to be isolated. Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one which is the stationary (stationary phase) while the other (the mobile phase) moves in a definite direction. In chromatographic methods, separations results from differences in the distribution constants of the individual sample components between the two phases.

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In gas chromatography (GC) separations are achieved by distribution of a solute between an immobile solid or liquid stationary phase and a gas phase that percolates over the stationary phase. Sample molecules spend part of the time in the mobile phase and the other part in the stationary phase during the passage through the column. The time for an unretained solute to reach the detector from the point of injection is called the column dead time or the hold -up time(tM). The solute retention time (tR) is the time difference between sample injection and the detector sensing the maximum of the retained peak. The amount of time solute molecules spend in the stationary phase is called the adjusted retention time (tR’). tR= tR’+ tM Retention factor (or capacity factor) is more fundamentally important than the absolute retention time. This represents the ratio of the time spent by solute in the stationary phase to the time it spends in the mobile phase. k= tR’/ tM = (tR - tM)/ tM In gas chromatography it is usually more convenient to measure the retention factor, k, than the gas-liquid partition coefficient, K, which requires exact knowledge of the column phase ratio. The gas-liquid partition coefficient is the relevant free energy parameter for modeling retention but this is of no great consequence since the partition coefficient is related to the retention factor by the relationship K = bk where b is the column phase ratio (volume of gas phase/volume of stationary phase).

The main parts of a basic GC system are shown in Figure 5. One or more high purity gases are supplied to the GC. One of the gases (called the carrier gas) flows into the injector, through the column and then into the detector. A sample is introduced into the injector usually with a syringe or an exterior sampling device. The injector is usually heated to 150-250 °C which causes the volatile sample solutes to vaporize. The vaporized solutes are transported into the column by the carrier gas. The column is maintained in a temperature controlled oven. The solutes travel through the column at a rate primarily determined by their physical properties, and the temperature and composition of the column. The various solutes travel through the column at different rates. The fastest moving solute exits (elutes) the column first then is followed by the remaining solutes in corresponding order. As each solute elutes from the column, it enters the heated detector. An electronic signal is generated upon interaction of the solute with the detector. The size of the signal is recorded by a data system and is plotted against elapsed time to produce a chromatogram.

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Figure 5: A GC system.

The ideal chromatogram has closely spaced peaks with no overlap of the peaks. Any peaks that overlap are called co-eluting. The time and size of a peak are important in that they are used to identify and measure the amount of the compound in the sample. The size of the resulting peak corresponds to the amount of the compound in the sample. A larger peak is obtained as the concentration of the corresponding compound increases. If the column and all of operating conditions are kept the same, a given compound always travels through the column at the same rate. Thus, a compound can be identified by the time required for it to travel through the column (called the retention time). The identity of a compound cannot be determined solely by its retention time. A known amount of an authentic, pure sample of the compound has to be analyzed and its retention time and peak size determined. This value can be compared to the results from an unknown sample to determine whether the target compound is present (by comparing retention times) and its amount (by comparing peak sizes). If any of the peaks overlap, accurate measurement of these peaks is not possible. If two peaks have the same retention time, accurate identification is not possible. Thus, it is desirable to have no peak overlap or co-elution.

2.6 Thermal conductivity detector (TCD)

The TCD consists of an electrically heated filament in a temperature-controlled cell. Under normal conditions there is a stable heat flow from the filament to the detector body. When a component (analyte) elutes and the thermal conductivity of the column effluent is reduced, the filament heats up and changes resistance. This resistance change is often sensed by a Wheatstone bridge circuit which produces a measurable voltage change. The column effluent flows over one of the resistors while the reference flow is over a second resistor in the four-resistor circuit.

Figure 6: Schematic diagram of a TCD.

A schematic of a classic thermal conductivity detector design utilizing a wheatstone bridge circuit. The reference flow across resistor 3,4 (Luft,air) of the circuit compensates for drift

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due to flow or temperature fluctuations. Changes in the thermal conductivity of the column effluent flow across resistor 1,2 will result in a temperature change of the resistor and therefore a resistance change which can be measured as a signal.

Since all compounds, organic and inorganic, have a thermal conductivity different from helium, all compounds can be detected by this detector. The TCD is often called a universal detector because it responds to all compounds. Also, since the thermal conductivity of organic compounds are similar and very different from helium, a TCD will respond similarly to similar concentrations of analyte. Therefore the TCD can be used without calibration and the concentration of a sample component can be estimated by the ratio of the analyte peak area to all components (peaks) in the sample.

The TCD is a good general purpose detector for initial investigations with an unknown sample. Since the TCD is less sensitive than the flame ionization detector and has a larger dead volume it will not provide as good resolution as the FID. However, in combination with thick film columns and correspondingly larger sample volumes, the overall detection limit can be similar to that of an FID. In conclusion the TCD is not as sensitive as other detectors but it is non-specific and non-destructive.

The TCD is also used in the analysis of permanent gases (argon, oxygen, nitrogen, carbon dioxide) because it responds to all these pure substances unlike the FID which cannot detect compounds which do not contain carbon-hydrogen bonds.

2.7 Flame ionization detector (FID)

Flame ionization detector involves the detection of ions. The source of these ions is a small hydrogen-air flame. Sometimes hydrogen-oxygen flames are used due to an ability to increase detection sensitivity, however for most analysis, the use of compressed breathable air is sufficient. The resulting flame burns at such a temperature as to pyrolyse most organic compounds, producing positively charged ions and electrons.

The design of the Flame Ionization Detector varies from manufacturer but the principles are the same in any case. Most commonly, the FID is attached to a Gas Chromatography system.

The elute exits the GC column (A) and enters the FID detector’s oven (B). The oven is needed to make sure that as soon as the elute exits the column, it does not come out of the gaseous phase and deposit on the interface between the column and FID. This deposition would result in loss of effluent and errors in detection. As the elute travels up the FID, it is first mixed with the hydrogen fuel (C) and then with the oxidant (D). The effluent/fuel/oxidant mixture continues to travel up to the nozzle head where a positive bias voltage exists (E). This positive bias helps to repel the reduced carbon ions created by the flame (F) pyrolysing the elute. The ions are repelled up toward the collector plates (G) which are connected to a very sensitive ammeter, which detects the ions hitting the plates, then feeds that signal to an amplifier, integrator, and display system. The products of the flame are finally vented out of the detector through the exhaust port (J).

Figure 7: Schematic diagram of a FID.

The current measured corresponds roughly to the proportion of reduced carbon atoms in the flame. Specifically how the ions are produced is not necessarily understood, but the response of the detector is determined by the number of carbon atoms (ions) hitting the detector per unit time. This makes the detector sensitive to the mass rather than the concentration, which is useful because the response of the detector is not greatly affected by changes in the carrier gas flow rate.

FID is detecting oxidized carbon atoms in ion form. So in organic species that already have oxidized carbons via the presence of oxygen, a weaker signal is given when the sample enters the detector because the oxidized carbons are not ionized as effectively as compared to compounds solely of carbon and hydrogen. Functional groups such as carbonyl, alcohol, halogens, or amines are sources of these oxidized carbons, sometimes causing few if any ions. This points out one of the main drawbacks of using an FID to detect effluent as it comes off a gas chromatograph column. Another drawback is the sample is destroyed, making it impossible to use the sample for other measurements. For this reason the FID is typically the final detector or stage in a series of instruments.

Some of the benefits of a flame ionization detector are quite useful. FIDs are insensitive to H2O, CO2, CS2, SO2, CO, NOx, and noble gases because they are not able to be oxidized/ionized by the flame. This allows samples to be studied even if contaminated or if some leakage of ambient room gases occurs at the time of the injection. Additionally, it has the ability to determine when a sample will elute off the column with regards to the solvents used. Some detectors can be damaged if an effluent too concentrated is analyzed, making it necessary to turn it off to prevent damage.

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3 Experiments for concentration measurements 3.1 Mesurement of the concentration of Propane, Isobutane and n-Butane in a gas mixture using a gas chromatograph

A gas mixture (from a gas lighter) will be separated in its components with the help of a gas chromatograph and its composition will be determined.

3.1.1 Basic principles

Chromatography is a physical method of separation in which the components to be separated are distributed between two phases, one which is the stationary (stationary phase) while the other mobile phase(a gas) moves in a definite direction. In chromatographic methods, separations results from differences in the distribution constants of the individual sample components between the two phases. The various solutes (components) travel through the column at different rates. The fastest moving solute exits (elutes) the column first then is followed by the remaining solutes in corresponding order. Thus the components are separated. The concentration of each component is then measured using a suitable detector.

3.1.2 Experimental set up Components:

- Gas separation column - Glass jacket - Measuring probe - Control unit for gas chromatography

Figure 8: Chromatograph Type 36670.88 of Phywe company. The gas chromatograph consists of a separation column with a glass jacket. The separation column is a1.5 m long spiral shaped glass tubing having 4 mm diameter. The tubing end

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on a functional head. One end of the tubing serves as inlet for the carrier gas and the measuring probe is connected via a glass gas inlet tube (7 in Fig. 9)

Figure 9: A TCD detector is used as measuring probe.

The measuring probe model 36670.10 (Fig. 9) is used for the measurements. The details of this measuring probe and the control unit for gas chromatograph are given at the end under phywe-36670.99.

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Figure 10: Control unit for gas chromatography.

The control unit together with the measuring probe detects the components separated by gas chromatograph.

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3.1.3 Experimental procedure 1 ml of gas mixture is sucked in an injection gas syringe (1ml) [from gas lighter ampoule]. The gas is then injected into the apparatus with through the rubber seal via the needle of a gas syringe. Some pressure is applied to press the needle forward through the rubber seal up to the separation column inside the glass jacket, the gas is now injected in the separation column and then the needle is pulled back from the seal and the screw on the coupling ring. Care is to be taken that during this operation the needle is not separated from the syringe and falls inside the jacket. After multiple uses (test the tightness of the seal) the seal is to be replaced by a new one. The PC Program LabView will be started and it shows the measured data. The elution profile (see Figure 11) is displayed on PC screen. After all the 4 components are eluted (seen by the profile) and the voltage has dropped to the initial value the program can be stopped. 3.1.4 Evaluation There are 4 maxima in the elution profile. The first belongs to the part of the air which entered with the gas mixture. The other 3 are the components of the butane gas mixture (butane gas mixture from the lighter). The gas mixture was separated into 3 components. The first fraction is the smallest fraction and the third the largest. This is indicated by the elution profile. The two expected fractions belong to butane (normal butane b.p. -0.5°C, isobutane b.p. -11,7°C) are seen as fraction 2, (isobutane with lower b.p.) and fraction 3 (Normal butane with higher b.p.). Fraction 1 is an impurity. It is propane (b.p. -42,1 °C). This can be confirmed if pure propane (from the gas cylinder) is investigated in the apparatus in the way. The vol-% fractions of each component can be determined from the area under the concentration curve. The evaluation can be done with the help of a suitable computer program (e.g. Microsoft Excel).

Formula for the calculation of the area: Figure 11: Data evalaution with Microsoft Excel

=ABS((C2-$C$1)*(B2-B1)+0,5*(C2-

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3.2 Bestimmung der Raumanteile von CO2 und CO in einem Gasgemisch mit den Prüfröhrchen 3.2.1 Kurzbeschreibung

Für die Luftuntersuchung am Arbeitsplatz, die Bestimmung kleiner Konzentrationen giftiger Gase und Dämpfe in der Atemluft (MAK Wert) sowie für die technische Gasanalyse werden die Prüfröhrchen eingesetzt. Sie gehören heute zu den klassischen Messverfahren der Gasanalyse. Ein Prüfröhrchen ist ein Glasröhrchen, das eine chemische Substanz enthält, welche mit dem zu messenden Gas reagiert und eine Farbänderung herbeiführt. Die Röhrchen sind mit einer Skala versehen. Die Länge der Farbzone stellt ein Maß für die Konzentration des zu messenden Stoffes dar. 3.2.2 Der MAK Wert

Grenzwerte haben für den praktischen Arbeitsschutz eine zentrale Bedeutung. Die MAK-Werte (Maximale Arbeitsplatz-Konzentration) sind eine Beurteilungsgrundlage für die Bedenklichkeit oder Unbedenklichkeit der am Arbeitsplatz auftretenden Konzentrationen von Schadstoffen. Neben der Giftigkeit der eingeatmeten Stoffe werden bei der Festlegung der Werte noch andere Faktoren berücksichtigt, u.a. Ätzwirkung, Hautdurchdringungsvermögen, sensibilisierende und ernsthaft beeinträchtigende Eigenschaften. Der MAK-Wert ist die höchstzulässige Konzentration eines Arbeitsstoffes (Gas, Dampf oder Schwebstoff) in der Luft am Arbeitsplatz, die auch bei wiederholter und langfristiger, in der Regel achtstündiger Exposition im Allgemeinen die Gesundheit der Beschäftigten nicht beeinträchtigt und diese nicht unangemessen belästigt. Der MAK-Wert für Kohlenmonoxid beträgt 33mg/m³. Wir messen in diesem Versuch eine höhere Konzentration im Probengas. Da sich das Probengas aber im Raum verteilt, bleibt es im Ganzen unter dem MAK-Wert. 3.2.3 Chemische Grundlagen Die Grundlage der Prüfröhrchen sind chemische Reaktionen des zu messenden Stoffes (Gases) mit der chemisch-reagierenden Substanz der Füllschichten. Die Reaktionen sind mit einer Farbänderung verbunden. Die Farbänderung ist ein Maß für den Massenumsatz des reagierenden Gases mit dem Präparat (der chemisch-reagierenden Substanz) im Röhrchen. Meist gelingt es, diesem Stoffumsatz quantitativ in Form einer Färblängenanzeige darzustellen. Für Kohlenstoffmonoxid kommt die folgende Reaktion zur Anwendung: 5CO + I2O5 5CO2 + I2 ⎯⎯ →⎯ 42SOH

Die Umsetzung von Iodpentoxid zu Iod wird unter sauren Bedingungen mit Kohlenstoffmonoxid durchgeführt. Es ist grundsätzlich eine klassenselektive Reaktion zur

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Messung leicht oxidierbarer Stoffe. Die Selektivität läßt sich durch geeignete Vorschichten gezielt steigern. Die Messung von Kohlenstoffdioxid wird durch Oxidation von Hydrazinhydrat bei Anwesenheit von Kristallviolett als Redoxindikator durchgeführt: CO2 + N2H4 NH2-NH-COOH ⎯→⎯ Da die Konzentration von Kohlenstoffdioxid typischerweise im Vergleich zu anderen Schadstoffen höher ist, beeinträchtigt die Querempfindlichkeiten die Selektivität nicht. Unter Querempfindlichkeit versteht man die Eigenschaft eines Stoffes einen anderen im Messverfahren zu beeinflussen und so das Messergebnis zu verfälschen. 3.2.4 Versuchsaufbau

Das Meßsystem besteht aus einem Dräger-Röhrchen und einer Dräger-Gasspürpumpe. Jedes Dräger-Röhrchen enthält ein hochempfindliches Reagenzsystem, das immer dann präzise Messergebnisse ermöglicht, wenn die technischen Eigenschaften der verwendeten Gasspürpumpe auf die Reaktionskinetik des Reagenzsystems im Röhrchen exakt abgestimmt wird. Deshalb muss bei einer Gasspürpumpe das Fördervolumen und der zeitliche Ablauf des Volumenstromes (Saugcharakteristik), innerhalb geringer Toleranzen auf das Röhrchen abgestimmt sein. Diese Anforderungen sind in internationalen Prüfröhrchen-Standards (Normen) festgelegt, wonach die Verwendung von Prüfröhrchen mit einer dazu passenden Pumpe des gleichen Herstellers gefordert wird. Für die Messung von Momentankonzentrationen wird Dräger-Gaspürpumpe Modell 31 eingesetzt. Dräger-Gasspürpumpe Modell 31 ist eine handbetätigte Balgpumpe mit ca. 100 cm³ Hubvolumen, d.h. die Pumpe saugt pro Hub 100 cm³ an. Den Aufbau eines solchen Gerätes zeigt Abb. 20.

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Abb. 20: Die Balgpumpe und das Prüfrohrchen 3.2.5 Versuchsablauf

1. Das Gasgemisch aus der Prüfgasflasche mit einer Entnahmevorrichtung in einen Meßbeutel von 1 Liter Inhalt geblasen.

2. Die Gasspürpumpe auf Funktionsfähigkeit prüfen. Dichtigkeitsprüfung: • Pumpe mit ungeöffneten Röhrchen zusammen drücken. • Nach Freigabe der Pumpe darf sich die Position des Balges eine Minute lang nicht

ändern. Saugleistungsprüfung: • Nach Zusammendrücken der Pumpe muß sich der Balg schlagartig öffnen.

3. Beide Spitzen des Dräger-Röhrchens in der Abbrechöse (Abb21(a)) der Pumpe oder in der Abbrechhülse (Abb. 21(b)) abbrechen.

4. Dräger-Röhrchen in den Pumpenkopf dicht einsetzen, so daß der Pfeil zur Pumpe zeigt (Abb. 21(c)).

5. Pumpe in die Hand nehmen, wie Abb. 21(d) zeigt. 6. Die andere Seite des Röhrchens in den Meßbeutel einsetzen. 7. Balg bis zum Anschlag zusammendrücken (Abb. 21(e)). 8. Finger strecken. Saugvorgang läuft selbsttätig ab und ist beendet, wenn die Kette

gespannt ist. (Abb.21(f)). 9. Saugvorgang so oft wiederholen, wie es die Gebrauchsanweisung des Dräger-

Röhrchens vorschreibt. (Auf den Röhrchen ist ein n-Wert aufgedruckt. Dabei gibt n die Anzahl der notwendigen Pumpenhübe an. In unserem Fall ist n meist 1.)

10. Anzeige auswerten. Bei der Auswertung soll nach drei Fällen unterschieden werden: - die Farbanzeige endet rechtwinklig zur Röhrchen-Längsachse (Abb. 22(a)), - die Farbanzeige ist schräg zur Röhrchen-Längsachse (Abb. 22(b)), - die Farbanzeige verläuft nicht gleichmäßig (Abb22(c)).

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Abb. 21: Versuchsablauf

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Abb. 22: Drei unterschiedliche Fälle der Farbanzeige

Wenn die Farbanzeige rechtwinklig zur Röhrchen-Längsachse verläuft, kann die Konzentration direkt an der Skala abgelesen werden. Ist die Farbanzeige verzerrt (verläuft schräg zur Röhrchen-Längsachse), so ist eine lange und eine kurze Verfärbung zu erkennen. In diesem Fall wird aus den beiden Anzeigen der Mittelwert gebildet. Bei einer nicht einheitlich Verfärbung ist der Endpunkt dort abzulesen, wo eine noch schwache Verfärbung gerade sichtbar ist.

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1 PURPOSE AND CHARACTERISTIC FEATURESThe control unit for gas chromatography (article no.:36670.99) serves, together with the measuring probe for gaschromatography (article no.: 36670.10), for the detection ofseparated substances in the emergent flow of carrier gasfrom a gas chromatograph by means of the thermal conduc-tivity principle. The control unit not only supplies power to themeasuring probe, but also contains a measuring system withwhich the change in the resistance of the measuring probecan be detected.The most important part of the measuring probe is an NTCelement, which has a highest permissible temperature limit ofapproximately 110°C. To avoid exposure to mechanicalstress, the semiconductor and its fine lead wires are fused ina thin walled glass capillary tube, which is fitted in a glasstube with attached side tube. Power is supplied to the semi-conductor through a coaxial cable with BNC plug, with whichthe measuring probe is connected to the control unit.The measuring system in the control unit is based on theprinciple of a voltage compensated measuring amplifier. Withthe measuring probe connected, and after zero balancing,there is a voltage of 0 mV on the recorder output of the con-trol unit (see 2.3.4). Subsequent changes in the detector resi-stance effect a change in the measured value, which is pas-sed to the output of the control unit as a proportional voltagesignal.Zero balancing and measurement signals can be displayedby a measurement instrument connected to the recorder out-put of the control unit. Suitable instruments for this are a mul-timeter (e.g. analogue demonstration multimeter ADM 2,article no. 13820.00), a Yt recorder (e.g. Yt Recorder, 1 chan-nel, article no.: 11414.95), or an interface in connection witha PC (e.g. Cobra3 CHEM-UNIT, article no.: 12153.00).

36670.9936670.10

Control unit for gas chromatographyMeasuring probe for gas chromatography

136670.99/4902

Contents

1 PURPOSE AND CHARACTERISTIC FEATURES

2 HANDLING2.1 Functional and operating elements of the

control unit2.2 Functional and operating elements of the

measuring probe2.3 Putting into operation2.3.1 Electrical connection2.3.2 Connection of the measuring probe to the control unit2.3.3 Connection of a display instrument to the control unit2.3.4 Zero balancing the measuring bridge2.4 Notes on operation

3 RECORDING A CHROMATOGRAM

4 CHANGING THE FUSE

5 SPECIFICATIONS

6 NOTE ON THE GUARANTEE

7 RECOMMENDED ACCESSORIES7.1 Chromatograph7.2 Display instruments7.2.1 Analogue demonstration multimeters7.2.2 Recorders7.2.3 Interface systems7.3 Literature

PHYWE SYSTEME GMBHRobert-Bosch-Breite 10D-37079 Göttingen

Telefon (0551) 604-0Telefax (0551) 604-107

The instrument complieswith the correspondingEC guidelines.

Operating InstructionsFig. 1

2.2 Functional and operating elements of the measuring probe

7 Gas inlet tubeConnection of the measuring probe to the gas outlet ofa gas chromatograph is made via this glass gas inlettube. It has an outer diameter of 8 mm and the measu-ring probe is held inside of it.

8 Gas outlet tubeThe gas which flows into the measuring probe exits itvia this glass gas outlet tube, which also has an outsi-de diameter of 8 mm.

9 BNC plugThe measuring probe is connected to the control unitvia this BNC plug (see 2.1).

10 HandleA handle is fixed to the sensor for handling and holdingthe measuring probe.

236670.99/4902

2 HANDLING2.1 Functional and operating elements of the control

unitWith the exception of the on/off switch 1, all functional andoperating elements are on the front of the control unit

1 On/off switchThe on/off switch and the connecting socket for thesupply of power are to be found at the back of theinstrument.

2 Coarse balancing buttonPress button ("COARSE") for coarse balancing of themeasuring bridge.

3 Rotary knob for fine balancingRotary knob ("FINE") for fine balancing of the measu-ring bridge.

4 BNC socket for connection of the measuring probeInput ("IN") for connection of the measuring probe(article no.: 36670.10) for gas chromatography to thecontrol unit (see 2.2).

5 Recorder outputTwo 4 mm safety sockets with colour coding forconnection to a recorder, multimeter or interfacesystem for displaying measured values.

6 Control lampGreen light-emitting diode shows when the instrumentis switched on.

Fig. 2

2

4

5

3

61

Fig. 3

78

9

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approx. ±13 V at the recorder output. Carry out zero balan-cing in two steps. As first step, press the coarse balancingbutton 2 on the control unit. This effects an automatic elec-tronic coarse balancing which reduces the signal on therecorder output to a few millivolts, as can be seen on aconnected display instrument. As second step, compensatethis signal to reduce it to 0 mV by turning the fine balancingpotentiometer 3 in the one or other direction until around 0mV is actually displayed. Signals subsequently receivedduring the measurement will then be given out as differenceto this compensation at the recorder output.

2.4 Notes on operationThe high quality instrument which you have been suppliedwith fulfills all of the technical requirements which are sum-marized in the current European Community Guidelines. Ittherefore carries the CE mark.When operating the instrument, please pay attention to thefollowing points:

• Refer to the type plate for the fuse rating.

• Refer to the type plate for the mains voltage: permissiblevoltage/current values (+ 6-10%).

• The length of individual cables that are connected to thecontrol unit must not be longer than three meters.

• Electrostatic discharges and other electromagnetic phe-nomena can exert such an influence on the instrument,that it no longer operates within specifications:The following measures hinder or prevent disturbinginfluences:- Avoid fitted carpets.- Balance potentials.- Carry out experiments on a conductive, earthed

underlay.- Use screened cables.- Do not operate high frequency emitters

(radios, mobile phones) in the immediate vicinity.- After a blackout failure, carry out a "reset" with the

on/off switch.

• This instrument is only to be used under trained supervi-sion in housing, business or industrial areas (schools,universities, laboratories etc.).

3 RECORDING A CHROMATOGRAMTo record a chromatogram, connect the gas inlet tube of themeasuring probe for gas chromatography 7 to the gas outletopening of the gas chromatograph used (e.g. a Gas separa-tion column, article no.: 36670.00). If appropriate, connect aflow meter (e.g. a Soap bubble flow meter, article no.:36675.00) to the gas outlet tube of the measuring probe 8, toquantitatively determine the flow rate of the carrier gas. Theconnections of the measuring probe to the gas chromato-graph and to the flow meter must be gas-tight to ensure thatthe carrier gas from the chromatograph flows quantitativelythrough the probe. Leaks would result in measurementerrors. These connections are best made with so-called glassthreaded connectors of size GL 18/8 or with tight-fittingtubing. Connect the measuring probe to the control unit (see2.3.2) and connect a suitable display instrument to recorderoutput 5 (see 2.3.3). After arranging a continuous flow of car-rier gas, switch on the control unit.The control unit is generally ready to operate when switchedon. As the heat generated by the transformer it contains affec-ts the electronics, however, it should be switched on abouthalf an hour before the first measurement for optimal results.The conditions (temperature, flow rate of the carrier gas,separating material, ...) under which the mixture to be exami-

336670.99/4902

2.3 Putting into operation2.3.1 Electrical connectionPlug the connecting cable supplied with the control unit intothe socket in the back of the instrument and connect theother end to the AC mains. Check that the AC mains voltageis in accordance with the permissible voltage stated on thetype plate on the back of the instrument! The on/off switch isalso situated on the back of the instrument.

2.3.2 Connection of the measuring probe to the control unitWith the control unit still switched off, insert the BNC plug 9of the measuring probe into the BNC socket 4 of the controlunit and lock it in position by turning it.

2.3.3 Connection of a display instrument to the control unitVarious display instruments for recording chromatogramscan be connected via the two 4 mm sockets of recorder out-put 5. Connect the selected instrument to the control unit byusing two 4 mm connecting cables. When doing this, ensurecorrect polarity, as otherwise the signals will be falsely orien-ted. The yellow socket designates the positive terminal ofrecorder output 5 and the white one the earth connection.The following display instruments can be connected to recor-der output 5 for the presentation of measurement results:

• An analogue or digital multimeter (setting: voltage mea-surement): The peaks of the chromatogram are indicatedby the swing of the pointer or a change in the displayeddigital value.

• A Yt recorder: The signals for the chromatogram arerecorded over time by the vertical deflection of a penwhich writes on a horizontally moving sheet of paper.When one considers that, under some circumstances,the time interval between individual signals is very short,then a continuously self-recording Yt recorder is alwaysto be preferred. A further advantage over a multimeter isthe fact that the chromatogram printed by the recordercan be directly archived after the measurement, and sobe easily compared with other chromatograms.

• An interface system combined with a PC: The plotting ofthe chromatogram with such a system is analogous tothat with a Yt recorder. The difference is only that the sig-nals are not printed on paper, but are stored in digitalform and displayed on a PC screen. The advantage isthat the measurements are stored on a data carrier. Theycan be called up again at any time and, when appropria-te, be easily subjected to further evaluation.

2.3.4 Zero balancing the measuring bridgeThe sensor of the measuring probe for a gas chromatographis part of a circuit which works on the principle of a voltagecompensated measurement amplifier (see 1). As the resi-stance of the sensor varies greatly according to the ambientconditions (temperature and flow rate of the carrier gas; ...),the zero balance must as a rule be reset prior to the start ofeach new measurement.When the instrument is switched on, there is a voltage of

Important! It is imperative that you do not switch the con-trol unit on until carrier gas flows around the sensor of themeasuring probe. Should you do so, and so put the mea-suring probe into operation without a flow of carrier gas,heat conduction away from the sensor could be insuffi-cient with resulting damage to the NTC element.Further to this, the temperature of the carrier gas flowinginto the measuring probe must not exceed 100°C, againto avoid damage to the sensor by overheating.

7 RECOMMENDED ACCESSORIES7.1 ChromatographGas separation column Order no.: 36670.00Glass jacket Order no.: 02615.00Kieselguhr, 50 g Order no.: 31501.05Dinonyl phthalate, 100 ml Order no.: 31276.10Soap bubble flow meter Order no.: 36675.00Immersion thermostat A 100 Order no.: 46994.93Accessories set immersion

thermostat A 100 Order no.: 46994.02Bath for thermostat, 6 l Order no.: 08487.02Pressure cylinder, helium, 2 l Order no.: 41776.00Pressure reducing valve, helium Order no.: 33481.00Table stand for 2 l gas cylinder Order no.: 41774.00

7.2 Display instruments7.2.1 MultimetersAnalogue demonstration multimeter

ADM 1 Order no.: 13810.00Analogue demonstration multimeter

ADM 2 Order no.: 13820.00Multi range meter, analogue Order no.: 07028.01Digital multimeter Order no.: 07134.00

7.2.2 RecordersYt recorder, 1 channel Order no.: 11414.95Yt recorder, 2 channels Order no.: 11415.95

7.2.3 Interface systemsCobra3 CHEM-UNIT Order no.: 12153.00Cobra3 power supply Order no.: 12151.99Software Cobra3 CHEM-UNIT Order no.: 14520.61alternatively:Cobra3 BASIC-UNIT Order no.: 12150.00Cobra3 power supply Order no.: 12151.99Software Cobra3 universal recorder Order no.: 14504.61

7.3 LiteratureHandbook, glass jacket system Order no.: 01196.12

436670.99/4902

ned is to be gas chromatographically separated depend onthe properties of the mixture and the selected separatingcolumn. It is naturally not possible to go into details on theseparameters here. As a rule, the conditions under which sepa-rations are to be carried out are described in the appropriateliterature or in information provided by manufacturers ofseparating columns.

4 CHANGING THE FUSE

To change the fuse, first switch off the control unit, then unp-lug it from the alternating current mains connection. Onlyafter having done this, remove the rectangular fuse holderthat is integrated above the connecting plug of the instru-ment. Use a screwdriver or similar to open it, with the mainsconnecting cable removed from the instrument. Remove theold fuse, replace it with a new one and push the fuse holdercompletely back in again.

5 SPECIFICATIONSInput

BNC socket for connection to the measuring probe forthe gas chromatograph (article no.: 36670.10)

OutputRecorder output 4 mm sockets, Umax = ±13 V

Power supplyMains voltage 110 - 240 V~ / 50 - 60 HzPower consumption approx. 7 VAFuse (5 x 20) See the label on the instrumentfor the fuse rating

Dimensions of the casing 194 mm x 130 mm x 140 mm (W x H x D)

Weight 900 g

6 NOTE ON THE GUARANTEEWe guarantee the instrument supplied by us for a period oftwenty four months. This guarantee does not cover naturalwear nor damage resulting from improper handling.The manufacturer can only be held responsible for the func-tion and safety characteristics of the instrument, when main-tenance, repairs and changes to the instrument are only car-ried out by the manufacturer or by personnel who have beenexplicitly authorized by him to do so.

Caution! The fuse used as replacement must be of exac-tly the same rating as that given on the type plate of theinstrument. It is not permissible to use any other type offuse!

Caution! Do not change the fuse until the control unit hasbeen separated from the source of electricity by unplug-ging it from the AC mains connection!