A Vacuum Lock for the Direct Insertion of Samples into a Mass Spectrometer Gregor A. Junk and Harry J. Svec, Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa
for introducing low vapor S pressure solids directly into the ionization chamber of a mass spectroYSTEMS
meter have been described in the literature (1-8). Also most commercial instruments are now equipped with more or less sophisticated vacuum locks for inserting solid samples as standard or optional accessories. All of these systems have the principal advantage of extending the variety of samples which can be handled, but each have one or more of the following disadvantages: All or part of the mass spectrometer must be vented to change samples. Greased components cause a high background at low mass numbers. Glass envelopes and/or probes are used. Precise, external control over the position of the sample in the ion chamber is not possible. Expensive components are used. The vacuum lock volume is large. The design, which is bulky and rigid, restricts use to a particular instrument. Described in this report is a direct insertion vacuum lock in which all of these disadvantages have been eliminated. The size of the lock is small and its fabrication from a commercial gate valve (Series 600, 5/8 inch diameter stainless steel, Goddard Industries Inc., Worchester, Mass.) is simple and flexible. The lock can be connected to almost any instrument available to the researcher with minimum down-time (one to two days) and low cost (under $200.00). The lock is shown ,in Figure 1 nttached to the metal vacuum housing of a General Electric mass spectrometer where it has given completely troublefree daily operation for the past two years. The lock is fabricated from the
gate valve by modifying only the removable end caps, A and B. A 9/32inch hole is drilled axially through both end caps. -4gland (-0.045 inch deep and 0.417 inch diameter) is machined into cap A leaving a small ridge (-0.010 inch high and wide) in the bottom of the gland. This ridge ensures a positive vacuum seal when the Teflon packing, F, is compressed. Disks for this packing are conveniently cut from '/i6-inch sheet with a cork borer of suitable size. The hole in the packing which forms the dynamic seal against the polished stainless steel sample rod, G, is made with a 0.250-inch punch or drill. Positive vacuum seals between the sample rod-Teflon surface and the gland ridgeTeflon surface are achieved simultaneously using the metal compression plate, C, which fits over the packing. A 1/4-inch copper vacuum line is hard soldered into the clearance hole drilled perpendicular to the axis through end cap A . -4metal ball valve, (Type 4354-316, Whitey Research Tool Co., Oakland, Calif.) D,in this line isolates the auxiliary pumping station from the vacuum lock volume when it is vented during the sample change-overs. This valve is also used for re-evacuating the 10-ml. internal volume of the vacuum lock. It takes less than three minutes t o reach the ultimate vacuum of torr. Modifications required on end cap, B, depend upon which access port is chosen to connect the lock to the mass spectrometer vacuum housing. In our application, the lock was attached to the mass spectrometer by the modification shown in Figure 1. Since the gate valve end caps are removable for machining and are available with standard pipe
thread or solder connections of any size, the connecting possibilities are great and the choice is arbitrary. For other instruments, suitable adaptation of the available mass spectrometer port and the valve end cap are required. It is important that a second seal, E , be made on the sliding metal shaft to isolate the two vacuum systems when the sample rod is in place. Scaled dimensions of this seal, as used in our application, are given in Figure 2. It is easily fabricated from Teflon rod with its outside diameter 0.003 inch larger than the hole into which it is to be fitted. The over-sized dimension ensures a positive friction fit over the entire outer surface of the seal when it is forced into place. ri slight taper, H , a t one end makes it convenient to start the seal into position. The inside tapers J and K help guide the sample rod through the seal. When the pressure differential between the mass spectrometer and the vacuum lock is not great (in our operation the mass spectrometer ultimate pressure is 5 x 10-5 torr and the vacuum lock is torr), this "plug seal" effectively isolates the two vacuum systems. No background ion currents are observed because of diffusion of oil, from the untrapped diffusion pump used in the auxiliary pumping station, past this seal. The sample rods are fabricated from 1/4-inch nonmagnetic stainless steel and are polished with 320- to 600-mesh silicon carbide grit in the form of waterproof abrasive paper (Tufbac Durite paper-Waterproof, Behr Manning Co., Troy, N. Y.). Better vacuum characteristics can be obtained by chrome plating carefully polished selected rods, but our experience indicates no real
MODIFIED GATE VALVE
BOX ION CHAMBER
INLET SYSTEM
Figure 1 . Vacuum lock attached to mass spectrometer housing. A, B, removable end caps; C, compression plate; D, valve; E, Teflon "plug seal"; F, Teflon "slip seal"; G, 1 /4-inch S.S.sample rod; H, replaceable Kel-F seats
L" 2
SCALE Figure 2. Cross "plug seal"
section of
VOL. 37, NO. 12, NOVEMBER 1965
Teflon
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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 t o 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
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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 t o 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
