Z. Anal. Chem. 282, 301-305 (1976) - �9 by Springer-Verlag 1976

Some New Aspects in Air Pollutants Analysis by Automatic Gas-Chromatography*

Herbert Ball

Siemens AG, Bereich Meg- und Prozel3technik, Postfach 21 1080, D-7500 Karlsruhe

Received May t 1, 1976

of Hydrocarbons

Einige neue Aspekte bei der Analyse yon KohIenwasserstoffverunreinigungen in Luft mit Hilfe automatischer Gas- Chromatographie. Immissionsmessungen von Kohlenwasserstoffspuren gewinnen immer mehr an Bedeutung. Die Messung der Kohlenwasserstoffe als Summe, wie gew6hnlich durchgeffihrt, ist ungeniigend, da organische Verbindungen unterschiedliche Wirkungscharakteristik aufweisen. Sie kann daher nur in Gebieten mit kleinen Kohlenwasserstoffverunreinigungen sinnvoll sein, vorausgesetzt, dab das ungeffihrliche Methan, welches iiberall in der Natur vorkommt, yon der Messung ausgeschlossen wird. Bei der toxikologischen Bewertung der Luft interessieren die Konzentrationen bestimmter Komponenten wie Athylen, Acetylen, Propylen, Butadien, Vinyl- chlorid, Benzol, Toluol oder Xylole. Die Konzentrationen dieser Komponenten k6nnen bei wenigen gg/m 3, d.h. im ppb-Bereich, liegen. Diese Werte k6nnen nur mit einer selektiven Mel3einrichtung erfal3t werden. Ein automatischer Gas-Chromatograph mit Speichersfiule, der die Forderung der selektiven Messung von Kohlenwasserstoffen in Luft im ppb-Bereich erfiillt, wird beschrieben. Als Applikationsbeispiel dieser Mel3ein- richtung wird die Messung von Ca - C6-Kohlenwasserstoffen (Paraffine, Olefine) und Aromaten (Benzol, Toluol, Athylbenzol, Xylol) erw~ihnt. Der Einsatz dieses Ger~ites in automatischen Luftmel3stationen wird derzeit pro- jektiert.

Summary. Ambient measurement of hydrocarbons is gaining in importance. The value of a total measurement of hydrocarbons, as it is usually performed, is slight since the characteristics of organic compounds are different. Total measurement of hydrocarbons can only be of interest in less polluted regions, provided that the harmless methane which occurs in relatively high concentrations in nature is not to be included in the measure- ment. In toxicological evaluation of air the concentrations of particular components (e.g. ethylene, toluene, acety- lene, propylene, butadiene, vinylchloride, benzene or xylene) are of interest and the concentrations of these components can lie at a few gg/m 3, i.e. in the ppb-range. These values can only be measured with a selective equip- ment. An automatic analyzer combined with a concentrating unit, which fully satisfies the requirements of selective measurement of hydrocarbons in air in the ppb-range, and as an example of the application of this equipment, the measurement of C a - C6 hydrocarbons (paraffins, olefins) and aromatics (benzene, toluene, ethylbenzene, xylene) are described. Complete measuring setups within automatic measuring stations are projected.

Best. von Kohlenwasserstoffen, Paraffinen, Olefinen, Aromaten in Luft; Chromatographie, Gas; automatisch, Speichers/iule.

In our highly industrialized society, various sources contribute to air pollution to various degrees. Air pollution from hydrocarbons is an important problem which is becoming more and more acute.

A large number of sources can be mentioned in this context, one of the most important being auto-

* Presented at the 6th Chemical Symposium on Recent Advances in the Analytical Chemistry of Pollutants, April 21-23, 1976; Vienna, Austria.

mobile traffic. Other local concentrations of hydro- carbon emissions are caused by specific emitters, such as

- plants for the manufacture and processing of chemical products

- plants for processing of crude oil and its reaction products

- plants for power generation - industrial and domestic heating systems.

302 Z. Anal. Chem., Band 282 (1976)

There are more or less differentiated possibilities for ambient measurements of hydrocarbons. As re- gards the measuring principle, all methods have only one common denominator, in as much as the de- tector used to detect the hydrocarbons is a flame ionization detector. The following systems can be distinguished:

1. Measurement of the total air pollution by hydro- carbons as the sum of CnHm.

2. Measurement of the sum CnHm without the con- tribution of the component methane which can be taken to be a natural and thus unavoidable quantity (which is, moreover, harmless).

3. Selective measurement of the individual hydro- carbon components.

The measurement method 1 - measurement of the harmful concentration as sum CnHm-provides the least amount of information on extent and harmfulness of the hydrocarbon content in the air. The deficiencies of this measuring method are as follows :

a) Depending on the chemical structure of the hydrocar- bons, the flame ionization detector used as detector indicates these hydrocarbons at different sensitivities. The result of the measurement can therefore be referred only to a reference component, such as methane.

b) The methane contained naturally in the air represents usually the predominant concentration proportion of the sum CnHm,

Items a) and b) above show that this measurement method is inadequate because it does not provide any information on the true concentration of dangerous and less dangerous hydrocarbon concentrations.

In the measurement method 2, the effect of the component methane on the result of measurement is eliminated. The amount of information provided by this type of measurement is thus greater than in the measurement method 1, but in this case as well there is no differentiation as to the danger of measured hydrocarbon pollution.

For these reasons, the detection of individual hydrocarbon components is the sole method to provide clear and distinct information on the extent and danger of the existing hydrocarbon pollution in the air, because organic compounds in air display different effects on human beings, animals, and plants. These effects undoubtedly depend first of all on the group of hydrocarbons to which the various components belong. The degree of danger increases f rom the saturated hydrocarbons, via olefins, chlorinated hydro- carbons, and on to the aromatic compounds. But even less reactive hydrocarbons, such as the group of paraffins, may participate in photochemical reac- tions in the presence of other harmful subs tances - such as ozone, NOx, a . o . - a n d under the effect of, solar irradiation. These photochemical reactions may result in the formation of toxic compounds, such as alde- hydes and peroxides.

Various states in the Federal Republic of Germany already have passed laws by which approval of re-

finery extensions and enlargements of petrochemical plants or new petrochemical plants is subject to a maximum air pollution by specific individual hydro- carbon components. Da ta on permissible concentra- tions of olefin components and aromatics, such as benzene, toluene, xylene, and ethyl benzene, will be found in these laws. Detection limits of a few I,tg/m 3 are required. The measurement networks under con- struction in these states are equipped with measuring instruments which detect these components and their concentrations in a fully automatic manner.

These levels can be monitored only with a measuring device which is selective in operation. Using com- ponents of already tried and tested process chromato- graphs, combined with a newly developed "concen- tration unit", an automatic analyzing unit has been designed to fully meet the requirements of selective measurement of hydrocarbons in air in the gg/m 3 range. The "concentration unit" makes it possible for components occurring only in slight traces in the gas under analysis to be concentrated in an enrichment column so that they can be sensed by the detectors used in gas-chromatography, for example by the FID.

Construction and Mode of Operation of the Automatic Hydrocarbon Analysis Unit

The automatic analysis unit is a compact construc- tion, consisting of a vertical arrangement of subas- semblies: a sample preparat ion device, a concentra- tion unit with cooling set and enrichment column, an analysis section, a programmer, and a "Kompenso- graph | potentiometric-type recorder. The unit is suitable either for stationary use or for installation on a trolley.

Sample Preparation Device (Fig. 1). The air to be analyzed is drawn in through a membrane filter by a pump and passed under pressure to the enrichment column. A regulator is provided to keep the pressure constant upstream of the enrichment column.

Retention Unit. The sample to be analyzed flows through the enrichment column, which is filled with an adsorptive material and has been cooled to -30~ The type of material used depends on the particular analysis problem. Once the traces of the components in the air have been concentrated in the column (collection time is 5 to 60 min), carrier gas is blown through it in the same direction as the air flow, the column remaining cold. This scavenging action causes all the non-retained components, in particular the main one, air, to be cleared from the tubings, whereas the retained components are not affected. The cold enrichment column is then connected into the flow-path to the separator column, the direction of flow being reversed. Immediately after this the heating stage starts, the concentrated components, because of the flow reversal, being eluted virtually simultaneously as the temperature 'rises rapidly. The water contained in the air is cleared completely from the enrichment column and the

H. Bail: Analysis of Hydrocarbons by Automatic Gas-Chromatography 303

Sample entering ~,

~ r e gauge

Pressure regulator[

~um~ filter

Fig. 1. Schematic sample preparation device

• Sample passed to enrichment column

piping during each heating stage. The components are then chromatographically separated, identified and recorded in the process chromatograph.

Analysis Section. The analysis section, P 180, contains the analyzer and equipment for carrier gas supply and tem- perature regulation. All normal separator column circuits can be set up in this section. Separator columns, changeover valves and a flame ionization detector are fitted on a perforated steel sheet, which can be exchanged. The analyzer can thus be prepared quickly for a new analysis project.

Programmer. The programmer controls not only the concentration unit but also the separator column circuits set up in the analysis section. A selector switch is used to preselect the type of chromatogram to be produced (full chromatogram or bar graph). The programmer also contains the mains unit for the detector. Models for the automatic detection of two and of ten components are available.

Potent&meter-type Recorder. The chromatograms are produced on the Kompensograph | potentiometer-type re- corder. In addition, electronic memory units for specific components can be used. They store the appropriate reading for the duration of the analysis so that it can be called up during this period by a computer, for example.

Typical Analysis

The apparatus described above permits de tec t ion- se- lect ively- of all hydrocarbons which accumulate in an enrichment column at - 3 0 ~ and which can be stripped at higher temperatures without having been decomposed. Splitting of the various components of the wide hydrocarbon spectrum is, of course, not possible in a single separator column system. Different systems of separator columns with specific applica- tions must be used to separate the individual com- ponents from specific groups of substances. By way of example, let us mention an application which makes it possible to measure C 2 - C 6 hydrocarbons (paraffins, olefins) and also aromatics (benzene, toluene, ethyl benzene, p + m xylene, o-xylene) in a single apparatus. The enrichment column is filled for this particular application with a hydrocarbon molecular sieve and Spherosil. The separator column system in the analyzer consists of two columns B and C which are connected in parallel and end up in an FID, and of a common precolumn A. In one analysis cycle, specimen is retained twice and dosed twice to two different combinations of separator columns. In the

analysis 1, the separator column A is connected down- stream of the separator column B in each program. The separator column B is filled with a material which ensures optimum separation of the C 2 - C 6 paraffins and olefins. All hydrocarbon components higher than C6 are retained in the precolumn A. This separator column is taken out of the gas flow to the detector at a specific moment of the analysis operation, and the components contained in the column are scavenged out. The purpose is to end the analysis with the com- ponent n-hexane.

In the analysis 2, there is the column combination A, C for each program. This combination of columns yields the C 2 - C 6 components more or less as a sum, but separates benzene, toluene, ethyl benzene, p + m- xylene, and o-xylene. The back-flush moment for column A has been chosen in this type of analysis so that o-xylene appears as last component in the chromatogram. The overall analysis time for both analyses is 30 rain, i.e., 15 rain per analysis. Specimen is collected for 15 rain as well. Owing to the specimen concentration, the measuring procedure is an integrat- ing method. The measured values obtained can thus already be termed quarter-hour averages (Fig. 2, 3).

Required Characteristics of Sample Preparation and Enrichment Column Filling Material

The materials used for the sample preparation device which come into contact with the sample must keep the wall absorption (memory effect) of the hydrocar- bon traces to be identified so low that it has no effect on the readings obtained.

In the example described above the sample prep- aration device meets these requirements; in other cases it has to be matched to the particular conditions. The adsorbent enrichment column filling is precisely matched to each different analysis. This prevents the occurrence of memory effects in the enrichment col- umn. The capacity of the retention column for the components to be measured is determined experi- mentally. It has to be such that none of the com- ponents emerge from the column when this holds sufficient of the sample to give the required concen- tration. It must also be adequate to guarantee en- richment of ten times the normal concentration. Maximum sample size, from which the hydrocarbons can be enriched, is 2 1 of air.

The automatic analysis unit was examined for memory effects in connection with the example de- scribed above.

When pure nitrogen was passed through the unit, which had previously been used for a number of days in the analysis of polluted outdoor air, the subsequent analysis gave a perfect zero line. The same result was

304 Z. Anal. Chem., Band 282 (1976)

Fig. 2. Analysis 1 : Low boiling hydrocarbons. Analysis time: 15 min. Measuring ranges 0 - i0 ppb and detection limits 0.5 ppb are possible

........... ~-U~ ~ ................................................................................... �9

, . ! . . . . . ! e !_s .......................................... i ...... " i : _ _

Fig. 3. Analysis 2: Aromatic compounds. Analysis time: 15 min. Measuring ranges 0-10 ppb and detection limits 0.5 ppb are possible

achieved by adding a filter of molecular sieve material in the sample intake.

Calibration

Equipment used for analyzing trace components can- not be calibrated by means of normal calibration gases, because these cannot be produced with sufficient accuracy or stored. A method for use with this ana- lysis unit has been worked out giving guaranteed calibration with only a minimum of sophistication.

A calibration mixture which can be produced accurately to within 1 ~o by volume per component is used. 10 lal of this are fed through a dosing valve into a sample of superpure N2. A calculation is carried out to relate the concentration of the calibration components to the sample size selected to give the appropriate enrichment (21 in the example above). The concentration measured then cor responds -as an average over the collection t i m e - t o a concentra- tion which is lower than that of the calibration gas by the ratio of 10 pl to 2 1, or a factor of 5• 10 .6 .

H. Ball: Analysis of Hydrocarbons by Automatic Gas-Chromatography 305

This method can be repeated with an accuracy of + 2 ~ of the maximum value of the measuring range.

Construction of a Complete Measuring Setup for Selective Hydrocarbon Measurements within an Automatic Measuring Station; Measured Data Processing

Using the apparatus described above, an extended measuring setup for the selective detection of hydro- carbons was designed for measuring stations within the air measuring system in North Rhine-Westphalia. The extension includes a hydrogen generator to gener- ate the required carrier gas, a compressed air unit for supplying control air to the changeover valves and combustion air to the FID of the chromatograph, a filter module for the treatment of auxiliary gas and waste gas, a test gas generator to generate defined concentrations of the components ethylene, propylene, benzene, and toluene in an air stream and to generate zero gas.

Measuring range full-scale values of 3 mg/m 3 are required for each of the components ethylene and propylene; the values for benzene are 1 mg/m 3, for toluene 6 mg/m 3, and the detection limit is 0.005 rag/ m 3 for each component.

The measured values are spread within the required dynamic range of 104 by means of a logarithmic amplifier operating according to the formula

wherein z = detected signal, y = original signal or its proportional image, a, b = external, specific param- eters. The slope of the logarithmic imaging curve is set by means of the parameter a. The parameter is selected in such a way that an output signal of 4 - 1 2 m A is available for concentrations between 0 and 600 gg/m 3 out of the available output signal of 0 - 20 mA with live zero at 4 mA, whereas the concen- trations up to the measuring range full-scale values

between 12 and 20 mA are available at lower resolu- tion. The spreading of the measured values makes it possible to detect average concentrations to be ex- pected with a high degree of accuracy, while measured values which are raised for a short period are not lost. The measured data are sent to a data processing terminal equipment within the measuring station. Transmission to a central computer is handled from there.

Important functions of the chromatograph, such as FID flame monitoring, cooling temperature of the enrichment column, heating current of the enrichment column during stripping, sample flow through the enrichment column, range of automatic zero correction, temperature of the analysis chamber, control air pressure, carrier gas pressure, are monitored con- tinuously and passed on to the computer as state signals in the form of floating contacts, or displayed as optical signals at the measuring setup. The calibra- tion of the measuring setup is automatically handled and checked by the computer once every day.

Conclusion

The method of specimen enrichment permits prac- tically continuous measurements which are not nor- mally possible with chromatographic equipment.

By a proper selection of combinations of separator columns, the measurements can be adapted to the requirements prevailing at the location of measure- ment.

The system described above for the selective detec- tion of hydrocarbons thus furnishes high-quality, reliable information on hydrocarbon ambient meas- urements, both as single system and in the extended version as measuring setup for automatic stations.

References

1. Ball, H., Mfiller, F. : Siemens Review 8, 344 (1975) 2. Siegel, O., Mfiller, F., Neuschwander, K. : Chromatogra-

phia 8, 399 (1974)