Calibration of a Gas Sampling Valve for Gas Chromatography John E. Cuddeback,' Shelton R. Birch,* and William R. Burg Department of Environmental Health, University of Cincinnati, Cincinnati, Ohio 452 19
The multiport gas sampling valve is in widespread use as a sampling and injection device for chromatographic analysis of gas mixtures. Figure 1 diagrammatically illustrates a commercially available seven-port valve (Tracor Co., Austin, Texas). The valve is usually mounted directly on the gas chromatograph and has two functional positions: the sampling position and the analysis position. When the valve is operated in the sampling position, the sample loop is filled and the carrier gas by-passes the sample loop and goes directly to the chromatographic column. The sample loop is open through ports 2 and 5 when the valve is in the sample mode, thus permitting a continuous flow of sample through the loop. Moving the valve to the analysis mode then changes the carrier gas flow path so that the carrier sweeps the sample loop before entering the column. The volume of sample which is actually passed into the gas chromatograph is equal to the volume of the sample loop plus all of the connecting tubing and dead space in the valve. Mounting features, dead space, and sample loop size are variable because of custom mounting or design differences between instrument manufacturers. For these reasons, the sample loop volume stated by the manufacturer is not the actual sample volume. Thus, the true sample volume is an unknown and if needed must be determined. In many analytical situations, it is not necessary to determine the volume of the gas sampling system. This is true in qualitative analysis and quantitative analysis in which gaseous standards of known concentration as well as samples are taken through the gas sampling loop; for, in this case, a comparison is made between indentical, but unknown, volumes of the standard and the sample. The determination of the concentration of the analyte in the sample can then be made by comparison. However, in cases where a simple comparison cannot be made or in a case that requires knowledge of absolute response data, it is necessary to know the exact quantity of the standard or sample which is introduced into the instrument. A recent occurrence in this laboratory serves to illustrate the necessity of knowledge of sample volume introduced by a gas sampling valve: a standard sample of vinyl chloride in a solvent was compared to air samples of known vinyl chloride concentration. The air samples were introduced into the chromatograph by means of the gas sampling valve, whereas the solvent samples were introduced by syringe injection onto the same column. A comparison of response us. quantity for each of the samples required knowledge of the concentration of vinyl chloride per unit volume, and volume of respective samples introduced into the gas chromatograph. T o avoid the use of manometers or volumetric measurements, a common analytical technique, the method of standard additions, was applied in a novel way to determine the volume of the introduced sample in situ.
EXPERIMENTAL Apparatus. A Microtek Model Mt-160 (Tracor, Austin, Texas) gas chromatograph equipped with a flame ionizstion detector and a seven-port stainless steel gas sampling valve WAS used for analysis of air samples. The gas sampling valve was equipped with a 2-
' Author to whom correspondence should be addressed.
Present address, U.S.A.F. Environmental Healirh Laboratory, Kelly AFB, Texas.
INJECTION PORT
Figure 1. Gas sampling valve set up to accommodate gas or liquid samples. Permanent volume indicated by heavy line.
t
4 2000
/ i
-
, =/ I5
+PERMANENT
/*
8ooo
IO00
~
, 10
05
VOLUME rn 1
05
IO
15
I 20 24
SAMPLE LOOP VOLUME r n l 3
Figure 2. Permanent volume of the gas sampling valve and associated plumbing as determined by the method of standard additions
ml gas sample loop. The instrument was replumbed for sample introduction through a septum or by means of the gas sampling valve onto the same column. This modification was made because the vinyl chloride standards were gaseous while the vinyl chloride samples to be analyzed were in a carbon disulfide solution. A vinyl chloride permeation tube was made and calibrated gravimetrically as described previously for other gases (1 ). The vinyl chloride permeation tube was set up in a flow dilution system so that known concentrations of vinyl chloride in air could be made. The dynamic range of the system as operated was from 0.1- to 50.. ppm vinyl chloride in air. A portion of this air mixture was passed through the sample loop. Procedure. The stainless steel sample loop labeled 2 cubic centimeters, which was supplied by the manufacturer, was removed from the instrument. The sample loop was mounted under a 10-ml buret and the volume of water required to fill the loop was measured. The tube was dried and the operation repeated; an average of three trials was made and the loop was found to have a volume of 2.43 ml. Three more sample loops were made from short lengths of %-in. copper tubing and their volumes were determined as above to be 0.66, 0.32, and 0.22 ml. The flow dilution system was set up a t a fixed flow rate; it was not necessary to know the actual concentration of vinyl chloride, but the concentration had to remain constant during the experiment. With one of the sample loops in place, four samples of the air mixture were introduced into the gas chromatograph. The peak heights were averaged and multiplied by the attenuation factor. Each of the remaining three sample loops was carried through the same procedure. A least-squares regression line was fitted to the four points as shown in Figure 2 .
ANALYTICAL CHEMISTRY, VOL. 47, NO. 2 , FEBRUARY 1975
355
RESULTS AND DISCUSSION The application of the method of skandard additions provided an accurate and easy-to-use method for determining the total volume of the sample introduced into the chromatograph. The X-intercept as shown in Figure 2 gives the permanent volume of the gas samr,ling system and does not include the volume of the gas sampling loop. The only way this permanent volume would be altered, would be to change the plumbing from the sample valve to the sample loop. Thus, the permanent volume becomes part of the instrument specifications once the volume is measured. There are a number of points worth noting about the calibration procedure. There is no need to have a calibrated or known gas source. The only requirement is that the concentration remain constant throughout the experiment. A sample could be made up in a 'Pedlar bag or any other plastic bag in which sample losses would be insignificant in the time it took to run the sample through the four loops. For the method of standard additions to work, the sample concentration must be within the linear dynamic range of the particular detector being used. Thus, if the instrumentation is linear with an undiluted gas sample on the order of 4.0 to 1.0 ml, a dilution does not have to be made, and a gas source such as bottled gas or even air may be used as a constant level source. The gas must be compatible with the detector; thus, air or water vapor would not be suitable with a flame ionization detector. The volumes of the sample loops were determined by a standard method for measuring volumes and could just as easily have been measured by a number of other methods
such as weighing the amount of water a loop would hold or measuring the length and inner diameter. The buret method was chosen for speed and convenience and worked quite adequately. Reading errors and buret tolerances are such that a f 5 % error might be expected in measuring the smallest sample loop and a f0.5% error with the largest loop. The least-squares fit of the data points using the measured volumes gave a straight line with only 0.9% relative standard deviation of the slope. The principal advantages of this method for measuring gas sample valve plumbing are its ease and simplicity. The instrument does not have to be shut down, nor do any of the gas lines other than the sample loop have to be removed. The only equipment needed other than the chromatograph itself is a buret or balance and supply of gas of a constant composition. The necessity for an accurate calibration by the user can be seen from our nominal 2-ml sample loop which was actually 3.81 ml or 1.9 times the nominal value. LITERATURE CITED (1) E. E. Saltzman, W. R. Burg, and G. Ramaswamy, Environ. Sci. Techno/., 5, 1121 (1971).
RECEIVEDfor review August 23, 1974. Accepted October 18,974. This work was supported in part by the Center For The Study of The Human Environment under U.S. Public Health Service Grant ES 00159, and in part by the Environmental Protection Agency under Research Grant R800869.
Rapid Identification of Common Chelants by Anion-Exchange Thin Layer Chromatography Edward DL Fitzgerald ' Bet2 Laboratories, Trevose, Pa. 19047
A rapid, simple procedure is desirable to separate and identify chelants in commercial products. Thin layer chromatography provides a method. A few chelants have been reported separated ( I , 2 ) on silica gel layers. The only rapid TLC separation of many common chelants reported involves paper chromatography ( 3 ) .Since good separations of several chelants were reported on ion-exchange columns using formic acid eluant ( 4 ) ,an ion-exchange TLC separation was selected using formic acid. The method developed is faster than the paper chromatographic method and resolves NTA and EDTA more completely.
Table I . R , Values of Chelants Chelant
K,hr'-bis (2 -hydroxy ethy1)ethylenediamine-.V, *\"-diacetic acid (HEEDDA) iV,N-bis (2 -hydroxyethy1)glycine (DHEG) A'-2 -hydroxyethylethylenediamine S, S'-triacetic acid (HEEDTA) S - 2 -hydroxyethyliminodiacetic acid (HE IDA) Ethylenediaminetetraacetic acid
jol\-ent I
Solvent I1
Soh ent I11
1.0
1.0
1.0
1 .o
1.o
1.o
0.67
1.0
1.0
0.50
1.0
1.0
0.0
0.45
0.54
0.0
0.20
0.54
0.0
0.2-0.5
0.70
A \ " ,
EXPERIMENTAL Apparatus: Brinkmann Polygram cellulose 300 DEAE anionexchange thin layer sheets (chloride form) were eluted with 1.5M formic acid and air dried for twenty minutes before use. Reagents: The chelates were commercial grade obtained from Chem-Service, Inc., West Chester, Pa. Reagent grade Formic acid was used. Procedure. The 2% solutions of the chelants were spotted in 3g1 portions 2 cm from the bottom of the formic acid treated sheets. The 4- X 10-cm sheets were placed in 15- X 7-cm jars and eluted to a solvent front of 7 cm. The plates were removed, air dried for 30 minutes, and dried with a hot air gun for 30 minutes (to remove Present address, GAF Corp., Binghamton, N.Y. 13902. 356
(EDTA)
Nit rilot r iacet ic acid (NTA) Diethylenetriaminepentaacetic acid (DTPA)
ANALYTICAL CHEMISTRY, VOL. 47, NO. 2, FEBRUARY 1975
