Sampling Gases from a Hostile Environment Andrew E. O’Keeffe Diuision of Chemistry and Physics, OfJice of Research and Monitoring, Environmental Protection Agency, National Encironmental Research Center, Research Triangle Park, N.C. 27711 ANALYSIS of gases that exist in an environment such as a flue, stack, or engine exhaust conduit can be a difficult assignment. Instrumentation is rarely available to accomplish the desired analysis under the conditions encountered, which typically include elevated temperature and the concurrent existence of high concentrations of gaseous, liquid, and solid interferences. The recent emergence ( I ) of instruments capable of measuring ambient levels of the gases with which we are presently concerned suggests very strongly that a sampling scheme should be devised that will reproducibly extract a sample from such an environment and deliver it to an appropriate ambient-level instrument for analysis. With the above end in view, and drawing upon our experience in accomplishing the quantitative transfer of minute amounts of pure gases (2-4, we have devised and demonstrated in the laboratory with equipment simulating stack conditions the following sampling scheme. Nitrogen containing 950 ppm of sulfur dioxide was passed through a 75-cm length of 1-cm i.d. quartz tubing. A flame photometric SO2 analyzer ( 5 ) (with associated pump) drew 200 rnl min-l of clean air through a 4-mm i.d. by 1-mm wall poly(tetrafluoroethy1ene) tube suspended along the axis of the quartz tube. A 10-cm segment of the assembly was maintained at 250 O C by enclosing it in a tubular combustion furnace. Under these conditions, the air sample delivered to the analyzer contained a constant and reproducible 0.9 ppm of SO2. Further, stepwise changes in the concentration of sulfur dioxide entering the quartz tube were followed, (1) R. K. Stevens and A. E. O’Keeffe, ANAL.CHEM.,42 (2), 143A
(1970). (2) A. E. O’Keeffe and G. C. Ortman, ibid.,38,760 (1966). (3) A . E. O’Keeffe, ibid., 39, 1047 (1967). (4) F. P. Scaringelli, A . E. O’Keeffe, E. Rosenberg, and J. P. Bell, ibid., 42,871 (1970). ( 5 ) S. S. Brody and J. E. Chaney, J . Gas Chromatogr.,4, 42 (1966).
A
c ’Ll
FILTER 7
4 A A
STACK Figure 1. Schematic of proposed stack-sampling device within a few seconds, by corresponding changes in the output signal. A suggested simple stack-sampling device, based on the laboratory demonstration described above, is shown in schematic form in Figure 1. Work recently completed in another laboratory and now being prepared for publication (6) will provide more detailed experimental data than I a m able to supply. (6) C. E. Rodes, Environmental Protection Agency, Technical Center, Box 12055, Research Triangle Park, N.C. 27711 (M.S. Thesis, North Carolina State University), unpublished work.
Volume Flowmeter for Gases of Variable Viscosity or Thermal Conductivity J. F. Parcher and C. L. Hussey Department of Chemistry, The Uniuersity of Mississippi, University, Miss. 38677 IN THE COURSE of a n investigation of frontal chromatography, it was necessary to measure the volume flow rate of a series of gases with continuously varying composition, for example, a binary mixture of n-hexane and helium with the composition varying from pure helium to pure n-hexane. The thermal conductivities of hexane and helium are 46 ( 1 ) and 368 cal/ sec-cm-’C x 10-6 (2), respectively, at 100 “C., while the viscosities are 84 ( I ) and 227 (2) micropoises. Thus the (1) “Handbook of Chemistry and Physics,” R. C. Weast, Ed., The Chemical Rubber Co., Cleveland, Ohio, 51st ed., 1970. (2) R. W. Gallant, “Physical Properties of Hydrocarbons,” Gulf
Publishing Co., Houston, Texas, 1968. 1102
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
thermal conductivity and viscosity of the gas mixture varied dramatically, and under normal chromatographic operating conditions-Le., constant inlet pressure and with a thermal conductivity detector-the flow rate and detector response also varied with the gas phase composition. The normal soap bubble flowmeter is not applicable under these conditions because of the presence of a component, which condenses out in water. Also, any hydrocarbon will interfere with the formation of the soap bubbles. The capillary and rotameter flowmeters cannot be used because of the variable viscosity of the gas. The other common type o f flowmeter is the mass flowmeter. However, this flowmeter is useful only for gases with constant thermal conductivity.
Both the capillary flowmeter and the mass flowmeter must be calibrated for each gas composition. The residence time of an elution peak in an empty chromatographic column can be used as a measure of the flow rate in situations where the other flowmeters are not applicable. The retention volume of a small sample of a n inert gas is given by Equation 1.
V,
=
L t ' F c dt
=
(1)
where V , is the retention volume in ml, F, is the corrected flow rate a t the outlet of the column, t7 is the residence time of the sample in the column, and J ; is the compressibility correction factor, For a given column a t a constant temperature, the retention volume is constant, independent of solute and/or mobile phase, and equal to the volume of the empty column, so that the flow rate, J: F,, is inversely proportional to the residence time, ir. Figure 1 is a plot of flowrate cs. litr for a small sample of nitrogen in a mixture of helium and hexane a t 65 ' C . The graph is linear with a slope equal to the volume of the empty chromatographic column, and a standard deviation of 1.091 X This experiment was done on a I-m X 'i'',(,-in.0.d. open tube and thus Jp = 1.00 throughout the flow rate range used in this study. This is a valid assumption for short. intermediate diameter, open tubes for the common range of flow rates used in chromatography-;.e., LIPto 150 m l h i n . In our laboratory we used a separate column and detector built directly into the thermostated detector compartment of a Beckman GC-45 gas chromatograph equipped with a thermal conductivity detector. The effluent from the normal Chromatograph detector was directed to a gas sampling valve; to one side of a Carle micro detector; then t o a short length of empty copper tubing and to the opposite side of the Carle detector. The Carle detector was powered by a separate electrometer and the resulting signal recorded o n a separate recorder. The gas sampling valve provided a small plug of inert gas for use as the sample. The gas sampling valve received inert gas from a storage tank, whose contents were allowed to bleed through the sample loop of the gas sampling valve at a constant rate. The sample size injected was determined by the size of the sample and was very reproducible. Actual flow rate measurements consisted of activating the gas sampling valve, and measuring the residence time of the peak from the retention time on the separate recorder. Once a calibration curve is prepared from corrected bubble flowmeter measurements and retention distances on pure carrier gas for a particular column and temperature, the flow rate for a
-1
t r (rnin.-') Figure 1. Calibration plot of flow rate cs. reciprocal time for an empty column
binary mixture of carrier gas and volatile solute can be found under the same conditions by consulting the graph o r by application of a calibration factor, since f7 is independent of the composition of the gas a t a constant flow rate. The thermal conductivity detector will respond to changes in the thermal conductivity of the effluent; however this does not affect the residence time of the eluted samples. Since the pressure drop across the empty column is low, changes in the gas phase viscosity have a negligible effect on the calibration factor. However, a t high flow rates o r for gases of greatly different viscosity, it may be necessary to recalibrate the instrument at the conditions encountered in the experiment.
RECEIVED for review April 29, 1971. Accepted November 4, 1971. This work was supported by a Fredrick Gardner Cottrell grant from the Research Corporation and Grant No. GP-27999 from the National Science Foundation.
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