Sample Introduction System
for Gas Chromatography
H. M. Tenney and R. J. Harris, Esso Research Laboratories, Louisiana Division, Esso Standard Oil
chromatography has become one G of the most important analytical techniques. To exploit this technique AS
to its maximum, it is necessary to introduce small gaseous or liquid samples with high precision. In most of the work being reported from various laboratories (1, a), gases are introduced through some sort of bypass system. These can be satisfactory, but in their most convenient form they are not always available. Liquids usually are introduced (1, 3, 4) by means of a micrometer syringe through a rubber septum. The syringes now aiailable commercially will leak if operated against systems having appreciable pressures. Depressurizing a t the sample point can lead to erroneous measurement of retention time. Moreover, the presence of the rubber septum a t temperatures substantially lower than that of the column can be undesirable.
the O-ring. The pipet containing the sample is inserted into the tube until the handle forms a seal a t the O-ring. The cock is opened completely. The pipet is now under the pressure of the system, but no pressure drop is imposed across the capillary. The pipet is then inserted all the way through the cock until it comes in contact with the orifice seat. At this instant pressure drop is imposed across the capillary and the sample is swept into the column. Contact of a few seconds at the orifice is adequate. The pipet is then removed without disturbing the system flow.
Co., Baton Rouge,
La.
PERFORMANCE
Precision. To test the precision in the introduction of liquid samples, n-hexane was introduced 20 times using a pipet having a 0.00159-ml. capacity. This sample was injected into a PerkinElmer Fractometer system operating a t 25 pounds per square inch gage with a helium flow rate of 120 ml. per minute (1 atm., room temperature). The column was being operated a t 50" C. Using the measured peak heights, the 9570 confidence limits for the measured
CONSTRUCTION
The system described here is positive and simple, and operates with high precision both for liquids and for gases. It has none of the disadvantages referred to above. Figure 1 shows the design of this system. The sample is measured in a capillary pipet, n-hich is filled by capillary action or otherwise. The 1-01umes of the liquid pipets usually are in the range of 1 to 50 pl. Their exact size is readily determined from the n-eight of mercury which is required to fill them. The handle of the pipet is slightly larger than the pipet proper and forms a pressure tight seal with a neoprene O-ring. The tip of the pipet forms a reasonably good seat a t the orifice and its tip projects through the orifice. Satisfactory seats have been made of metal, Teflon, or neoprene. Pipets of all-glass construction suitable for gases have been used up t o about 1 ml. in size. Larger ones of metal are shown in Figure 1, in \\hich two metal capillary tubes lying parallel pass through the 0.25-inch tube serving as a handle to the outside end where a loop of any desired length is attached. Samples as large as 25 ml. have been successfully introduced with iirtually no air contamination.
Figure 1.
PROCEDURE
The operation is as follows: The metal cock (Republic Mfg. Co., Clev? land, Ohio, No. 309-GG-3/8-D) IS cracked slightly, allowing helium to flush air from the system to the outside past
Gas chromatographic inlet system
VOLUME
Figure 2.
PER CENT
Linear relationship of peak height to concentration
10 feet, 30% 1-octadecene on No. 545 Celite, 50" C, Pressure. 25 pounds per square inch gage Sample. 0.00442 ml. Column.
VOL. 29, NO. 2, FEBRUARY 1957
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Control Device for Precisely Regulating Flow of Air or Other Gas at low Rates
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W. H. McKINNEY, Koppers Co., Inc., Oil City, Pa. I-2 N RUNNING such tests as that for 2,000 8,000 2 oxidation of inhibited steam-turbine w K 1: oils (Am. SOC.Testing Materials, D 943) 6.000 a and the Swift stability test [Oil & Soap Y Y a a 22, 101-107 (1945); King, A. E., Rosw 1,000 4,000 2 n chen, H. L., Irwin, W. H., Ibid., 10,204207 (1933)) it is necessary to control the 2,000 flow of oxygen or air, respectively, a t fairly low rates for periods of time up to 0 2 months. This generally proves to be a 0 0.002 0.004 0.006 0.000 0.010 tedious task when using the normal SAMPLE VOLUME. M L . laboratory-type, single-stage, pressurereduction valve and flowmeter system. Figure 3. Effect of sample sizes on peak measurements A simple device can be easily fabricated which will provide a constant Column. 2 meters, Perkin-Elmer B, 50" C. Flow rate. 120 ml. per minute (1 atm., room temperature) flow of gas over periods of several Sample. n-Hexane months with very little attention. The heart of this device is an orifice nipple, shown in detail in the figure. This device is fabricated by soldering a 12-inch length of tubing, 0.01 inch in inside diameter, into a 1/4-inch extra peak heights were found to be A0.897, sample size itself. Here, goo linearity strong brass nipple. If a number of of the peak height. This is regarded as is found between sample size and peak these are to be used from one header, area. Broadening of the peak with highly satisfactory. Volatile liquids they should be matched to the tolerincrease in sample size results in a such as Zmethylbutane (isopentane) ances desired. This can be done readnonlinear peak height relationship. have been introduced equally well. ily by using a wet test meter to check To test the precision in the introducFor quantitative work, peak heights each nipple as it is made. The nipple tion of gas samples, propane was chosen. may be used with sample size held can be used to control flow rates over a It was introduced 27 times using a pipet constant. If areas under peaks are fairly wide range. It mill deliver apof 0.4ml. capacity. The column condiused, any size of sample can be chosen. proximately 13 liters per hour a t 30 tions were essentially the same as those pounds per square inch gage and 3 liters above. In this case the 95% conLITERATURE CITED per hour a t 7 pounds per square inch fidence limits for the measured peak heights were found to be =k0.78%,. gage. If rates lo-xer than 2 to 3 liters (1) Bradford, B. W., Harvey, D., Chalkper hour are required, a longer length linearity. Figure 2 shows that a lev. D. E.. J . Inst. Petroleum 41. 80of capillary tubing should be employed good linear relationship exists between 8 "(iQ55).' rather than pressures lorn-er than 5 concentration and peak height if the (2) Fredericks, E. M., Brooks. F. R., pounds per square inch gage. ANAL.CHEM.28, 297 (1956). size of the total sample remains con(3) Lichtenfels, D. H., Fleck, S. 8., The figure below shows how this stant. These data were obtained by Burow, F. H., Ibid., 27, 1510 device can be used to provide the flow using binary blends of n-hexane and (1955): of 3 liters per hour required by the cyclohexane. (4) Ray, N. H., J. A p p l . Chem. (London) turbine oil oxidation test. The use of 4 , 21, 82 (1954). Figure 3 s h o w the actual effect of two-stage pressure reduction with the orifice nipple manifold pressure carefully controlled is an essential part of this system. 10,000
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+ Flow control device for tank oxygen Brass nipple, '/4 inch, extra strong, 0.540-inch O.D. X 0.302inch I.D. B . 12 inches (approximately 20 turns) of 0.010-inch I.D. brass or copper capillary tubing (use '/s-inch mandrel for winding tubing) C. Silver solder D. Bore nipple to suit disk E. Brass disk, 6/16 inch O.D. X '/I6 inch I.D. X '/le inch thick F. '/a-inch American Standard pipe, threaded both ends G. Gas pressure reducing valve H. Low-pressure manifold, 7 pounds/sq. inch I. Shutoff volve J. Orifice nipple K . Line to test apparatus L. Cylinder pressure regulator, adjust to 15 pounds/sq. inch
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ANALYTICAL CHEMISTRY
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