advantage. Momentary pressure surges which sometimes occur in the mass tube (maximum observed surge has been torr) during sample changeover do not affect the ultimate vacuum which is usually reestablished in 10 to 15 minutes. The Teflon “slip seal” deforms slightly when a rod is moved rapidly through the seal as demonstrated by helium leak detector tests. The leakage is not intolerable, however, and can he drastically reduced by further compressing the Teflon packing and translating the rod slowly. Recommended
rate of translation is less than one inch per second through a seal compressed so t,hat rod motion is initiated by about 7 pounds of pressure and maintained at thn recommended rate by less than 5 pounds. Valving operations during sample changeover are not discussed because t,hese should be obvious after study of Figure 1. LITERATURE CITED
( I ) Biernrtnn, Klaus, “Mass Spectrometry,’’ p. 33, hleGraw-Hill, New York, 1962.
(2) Cameron, A. E., Reu. Sci. Iml. 25, 1154 (1954). (3) Gohlke, R. S., Chem. Ind. (London) 1963,946. (4) Hill, H. C., Reed, R. I., J . Sci. Inel. 40,259 (1963). ( 5 ) Lynch, J. F., Wilson, J. M.,Bud&
kiewicz, Herbert, Djerassi, Carl, E1; erientia, XIX/4,411(1963). (6PReed, R. I., J . Chem. SOC. 1958, 3432. (7) Reed, R. I., Reid, W. K., Wilson, J. AI., “Advances in Mass Spectrometry”, R. M. Elliott, ed., Vol. 2, p. 416, Pergamon Press, London, 1963. (8) Svec, H. J., Junk, G.A,, J. Am. Chem. Soc. 86, 2278 (1964).
WORKperformed nt the Ames Laboratory of the U. S. Atomic Energy Commission.
Collection of Gas Chromatographic Fractions for Infrared Analysis R. A. Edwards’ and 1. S. Fagerson, Department of Food Science ond Technology, University of Massachusetts, Amherst, Mass.
of techniques have been A described (2-9) for the collection and subsequent infrared analysis of fracNUMBER
tions separated by gas chromatography. However, most of these are useful when the sample size or fraction is on the order of 50 pg. or more. A technique for samples of only a few micrograms in size has been described (11, but the present procedure may be simpler and more convenient for many applications. I n the course of our research on the identification of volatiles from heated fats, we have developed a simple and effective method of obtaining infrared spectra of chromatographic fractions representing less than 10 pg. of component.
approximately 1 inch from the end. .I Recton, Dickinson No. 606/L or similar adapter is silver soldered to this end of the tube to provide the male fitting for the needle assembly. This tube will make a snug sliding fit into one of the exit ports of the chromaton a p h . To prevent condensation of high boiling fractions prior to collection and to minimize fogging, the portion of the exit tube from the point of detector split to the needle fitting is wrapped with asbestos insulated Nichrome wire, and the temperature is controlled by a variable transformer. Temperatures up to 350’ C. can easily be obtained. On the emergence of a fraction, the needle assembly is attached at the
The ciillection device consists of a 1 S n c h 20-gauge hypodermic needle inserted through a sleeve-type rubber stopper I:Cat. No. 8826, A. H. Thomas Co., Phtiladelphia, Pa.) which then acta as II holder for powdered dry ice. The de! rice is attached to bhe exit port of the chromatographic column, and the eluted fraction is condenwd within tlle hypodermic needle. Althoiigh these fraction collectors are adaptable to other instruments we have used them in conjunction with a Perkin-Elmer Model 800 dual flame chromatograph typically operating with a 40 cc./min. flow rate of helium through a 6ft. ‘/.-inch column and a 1:4 detector-exit port split. In this case, the exit port is fitted with a stainless steel tube made from a 5-inch long 14-gauge special stainless steel hypodermic needle (No. LNR, Becton, Dickinsonand . . Co.,..Rutherford, . .. N.. J.), . l n e nub of the needle IS cut 08 and the needle is then bent at right angles
-
-.
Present address, Department of Food Technology, The University of New South Wales, Kensington, N.S.W., Australia. 1630
ANALYTICAL CHEMISTRY
fitting as shown in Figure 1. It has not been found necessary to provide additional support for the needle and rubber stopper which may be left hanging freely during collection. When the fraction has eluted, the needle assembly is removed, plugged, and stored in a Dewar flask containing chips of dry ice. For the recording of infrared spectra, the collected fractions are transferred to ultramicrocavity cells (Type D, 0.5mm. path, Barnes Engineering Co., Stamford, Conn.) in the following manner: The stopper containing dry ice is slipped off the needle, the plugs are removed, and the needle tip is positioned in the bottom of the cell cavity. We have found it most convenient to hold the cell and needle in a horizontal position a t this stage. Using a 10-~1.Hamilton syringe, which fits snugly inside the 20-gauge needle, approximately 3 pl. of CCl, or other suitable solvent is slowly injected into the upper portion of the needle. The cell and needle are then returned to the vertical position, and the finger is placed over the needle hub resulting in the movement of a slug of solvent along the needle and into the cell with the condensed fraction. The needle is removed, the microcell stop. pered, positioned in a cell holder, and the infrared spectrum recorded using a beam condenser. After use, the collecting needle and cavity cell are readily cleaned for reuse by attaching the barrel of a 0 . 5 . ~ ~ Tuberculin syringe to the needle and flushing the needle and cavity cell with several milliliters of solvent.
’.
Figure Needle assembly attached to g a s chirornatograph outlet
We have found the system simple to operate, effective for the collection of compounds over a wide boiling point range, and capable of providing meaningful spectra from fractions containing less than 10 pg. of material. For high boiling compounds, the
following modification permits obtaining spectra free of solvent, absorption. I n this case, the cell with solvent is transferred to a small desiccator, the cell cover is removed and the solvent allowed to evaporate until the meniscus can no longer be observed (15 minutes is usually sufficient). ~h~ cell is then removed and the spectrum Obtained as Previously described.
( 6 ) Hoffman, R. L., Silveira, A., ANAL. CHEM.36, 447 (1964). (,) Leggon, H. Ibid., 33, 1295 (1961). (8)Lohr, L. J., Kaier, R. J., Facts & Methods 2, 1 (1961). (9) Swoboda, P. A. T., Nature 199, 31
LITERATURE CITED
(1) Beroza, bl., Gas Chromatog. 2, 330 (1964). (2) C h a w s. s.3 BrobSt, K. Me, Ireland, (1962). C. E., Tai, H., ‘pectr*
w.,
(1963).
(3) Chang, S. S., Ireland, C. E. Tai, H., ANAL. CHEW 33, 479(1961). (4) Drew, C. M., Johnson, J. H., J. Chrom. 9,264 (1961). ( 5 ) Grasseli, J. G., Snavely, M. K., A p p l . Spectr. 16, 190 (1962).
THISinvestigation was supported in part by Public Health Service Research Grant EF-00099 from the Division of Environmental Engineering and Food Protect ion
Absolute Method for Measuring l o w Gas Flow Rates with High Accuracy F. W. Noble, Kenneth Abel, and P. W. Cook, Laboratory of Technical Development, National Heart Institute, National Institutes of Health, Bethesda, Md.
HE MOST commonly used equipment ‘items for measuring lorn gas flow rates are rotameters, soap film flowmeters, and devices which measure the pressure differential across a fixed orifice. Of these, only the soap film flowmeter is absolute-Le., does not require calibration against a primary standard. It has been shown (3) that the soap film flowmeter can, under carefully defined conditions, attain an accuracy of i1/4% for low flow rates of nonreactive, nonsoluble gases. The addition of conductometric ( I ) and photoelectric ( 2 ) relays to start and stop electrical timers can increase the accuracy slightly. An entirely theoretical design for a flowmeter similar in several respects to the one to be described has been published previously ( 4 ) . At the time of publication, the instrument had not been built so that it is impossible to compare the performance. Briefly, the device previously described is a continuous flowmeter which will indicate instantaneous as well as cumulative flow “on line.” The device to be described here is a discontinuous flowmeter which measures the average volume flow rate in an interval of time. This system is much simpler, less costly, and probably more reliable, although certainly not so flexible in application. The flowmeter described beloiv is absolute and, in addition, requires no vapor pressure correction, no operator attention, and can be used with any gas compatible with stainless steel and Teflon. The principle of operation can be seen from Figure 1. The flowing gas is connected to the unit by appropriate means. In the “R” (return) position, the gas passes through a normally open valve and is vented to atmospheric pressure. Closing the switch to the “M” (measure) position closes the valve thereby diverting the gas to a gas-tight, motor-driven syringe, and turns on the drive motor. A volume displacement switch actuates
2001 4
CLUTCH
NORMALLY OPEN SOLENOID VALVE-
TIMER
I ISV,6OU
Figure 1.
Overall system diagram
a magnetic clutch through a transistorized clutch driver to withdraw the syringe piston for each incremental accumulation of gas. The switch contacts are adjustable and will provide more than 1000 increments for full syringe travel a t a pressure differential of about 0.1 mm. H20. The time required for the piston to be withdrawn through a known fixed volume is elect,rically timed. Microswitch “A” provides a reproducible starting point for the piston while microswitch “BJ’shuts off the timer a t a reproducible terminal piston position. Microswitch “B” also shuts off the drive motor and opens the diversion valve. Opening of the diversion valve deactuates the clutch driver. With the switch in the “R” position, the motor is reversed and the clutch drive actuated. The piston is driven back
toward its starting position until halted by microswitch “A,” A rotameter is placed in series with the gas connection point and the diversion valve t o ensure that the flow rate does not exceed the capability of the drive mechanism. A full electrical schematic is shown in Figure 2. The displacement switch was made in ou shop. The diaphragm is aluminum, free diameter 1 inch, thickness 0.35 mil. The contact is stainless steel, threaded to allow adjustment of initial gap. The solenoid valve is stainless steel with a 3/32-in~h oriface operating on 115 volts, 60-cycle ax., Skinner Yo. V51D42126. The clutch is a Warner SFC-160,1-25512 with a 90-volt d.c. coil operated between 8 and 22 volts d.c. The motor is a Bodine No. 803PC035 Type KYC-23R8, 150 VOL. 37, NO. 12, NOVEMBER 1965
1631
