Gaseous Sample Dilution Device for the Gas Chromatograph Bernard E. Saltzman and Clarence A. Clemons, Laboratory of Engineering and Physical Sciences, Division of Air Pollution Robert A. Taft Sanitary Engineering Center, Public Health Service, U. S. Department of Health, Education, and Welfare, Cincinnati, Ohio 45226
to analyze Itions.gasesMany over a wide range of concentraof the ionization methods T IS OFTEN
SECESSARY
require special techniques for the measurement of detector response and many are not suited for measurements over extremely wide concentration ranges ( I ) . I n a recent study of the movement of air masses, a technique mas developed to analyze air samples for tracer gases over a range of concentrations as high as a millionfold. The analytical system (3) included a gas chromatograph with an electroncapture detector. The upper concentration limit of the analytical range of this system was only about 3 p.p.b., much below some of the higher concentrations that were sampled. il simple device was developed as an accessory to the gas sampling valve used on the instrument; this device provides a convenient means of diluting the gaseous samples quantitatively as much as 3000 to 1. The basis of the device, schematically illustrated in Figure 1, is a small aspirating system constructed by modifying a Beckman Model 4020 Atomizer-Burner assembly. I n this system diluent gas (usually nitrogen from a cylinder) is introduced through connection 3 or 4 into either the inner or outer annular space in the aspirator tip, depending upon the dilution desired. The gaseous sample was sucked through the central fine capillary tube in a reproducible manner to provide a quantitative dilution. (The device was not used as a burner.) The sample was contained in a plastic bag a t atmospheric pressure. The connection to the apparatus for the sample bag was a ball joint 1 (Figure 1) soldered to the bottom of the burner assembly. A vacuum source drew the sample through a side arm 2 on this joint at the rate of about 30 ml. per minute. Rapid flushing of the small dead volume of the connection minimized potential errors caused by adsorption on the tubing. This factor is important for rapid equilibration (2) to conserve time and diluent gas. To receive the diluted mixture, a '/*-inch 0.d. copper tube about 12 inches long (shown above 5, Figure 1) was assembled to a compression fitting soldered to the outer shell of the 800
ANALYTICAL CHEMISTRY
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Figure 1 . Schematic drawing of quantitative dilution device for gaseous samples 1.
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1 2/5 stainless steel ball-joint connection for sample b a g Vacuum connection for Rushing dead volume rapidly Dilution gas connection to inner nozzle Dilution gas connection to outer nozzle Tip of orpirotor 12/5 stoinless steel ball-joint connection to gas chromatograph sampling valve Waste outlet for excess diluted sample mixture
atomizer. A ball joint (not shown) connected to a gas sampling valve was mated to joint 6 to transfer the dilute mixture to the gas chromatograph. (When sample dilution is not required, the sample bag is connected directly to the gas sampling valve at the same point.) Another vacuum source drew the diluted sample from the stream through the calibrated loop of the sampling valve at 100 ml. per minute. Excess dilute mixture was vented into the room through the waste outlet 7 of the copper tube. The elbow bend in the tube was used to keep dust from settling into the fine orifices of the
atomizer. The ratio of flow rate of dilute sample drawn into the gas sampling valve to that vented was very small since the amount of diluent gas used was always in excess of 1 liter per minute. The dilution ratios obtained with this device, using nitrogen as the diluent gas are shown in Figure 2. The same ratios were obtained for all components in the gas mixtures. Other diluent gases may give different ratios because of differing gas viscosities and densities. These curves were obtained by measuring the peak heights obtained from known mixtures, converting the peak heights to concentrations, and then calculating the dilutions obtained. When this data was obtained, a 0.01inch i.d. capillary restriction ll/zinches long had been inserted just upstream from the outer annulus. Later, for greater accuracy the dilution gas pressure was measured with a 6-foot mercury manometer instead of a Uourdon gauge, without any flow restrictions other than the atomizer-burner filters supplied with the assembly. Other calibrations were made by flow measurements. These were more difficult to execute precisely, because great care was required to prevent leaks in the measuring system and to prevent even slight pressure changes due to the measurements; either would affect the dilution ratios. Sample flows mere measured conveniently by attaching a small graduated pipet with the tip cut off at ball joint 1, and closing connection 2 (Figure 1). The motion of a soap film past the graduations was then timed with a stopwatch. Precisely reproducible operation requires maintaining both the sample pressure and the pressure of the diluted mixture a t the exit of the copper tube very close to atmospheric pressure. This is easily accomplished with samples drawn from plastic bags. Figure 3 illustrates the suction measured at the sample connection with a water manometer (with no sample flow) under various operating conditions. Operation a t very low suctions was, of course, more subject to errors due to slight variations in sample pressure. Figure 4 gives the consumptions of diluent gas by the device a t various pressures. Because only a few seconds
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DILUTION GAS PRESSURE, psig
Figure 4. Consumption of dilution gas for various dilution gas pressures applied to either the inner or outer nozzle DILUTION GAS PRESSURE, p r i g
Figure 2. Sample dilution ratios obtained for various dilution gas pressures applied to either the inner or outer nozzle
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are required to reach steady conditions before injection of the diluted sample, a cylinder of gas suffices for many analyses. The accuracy which could be achieved by the dilution device waa determined to be better than 1%. &4series of 12 replicate samples without the flow dilution device indicated a standard deviation of the peak heights obtained from the gas chromatographic system of 0.6%. With the device, seven dilutions of each of two known samples yielded a standard deviation of l.2yo from the calibration plot. Thus the additional error attributable to the device appeared to be 0.6%. Achieving this precision requires maintaining all orifices free from dirt. Frequent calibration checks are advisable. The device should be broadly applicable for extending the range of gas chromatographic systems to higher gas concentrations. When the approximate sample concentrations are not known, a series of dilutions can be made rapidly. The system should make more convenient the use of newer types of detectors having limited dynamic ranges.
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LITERATURE CITED
(1) Lovelock, J. E., ANAL.CHEM.33, 162
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8 IO 20 DILUTION GAS PRESSURE, p s i g
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(1961). Saltzman, B. E., Ibid., p. 1100. (3) Saltzman, B. E., Coleman, A. I., Clemons, C. A., Ibid., 38, 753 (1966).
(2)
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Figure 3. Suction measured at sample inlet for various dilution gas pressures applied to either the inner or outer nozzle
Division of Water, Air, and Waste Chemistry, 150th National Meeting, ACS, Atlantic City, N. J., September 1965. Mention of commercial products does not constitute endorsement by the Public Health Service. VOL 38, NO. 6, MAY 1966
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