Use of Activated Charcoal to Trap Gas Chromatographic Fractions for Mass Spectrometiric Analysis and to Introduce Volatile Compounds into the Mass Spectrometer J. N. Damico, N. P. 7Vong,l and J . A. Sphon Division of Food Chemiiitry, Food and Drug Administration, Washington, D.C. 20204
COMPLEX MIXTURES are now analyzed routinely by gas chromatography and mass spectrometry in many laboratories. Amy and coauthors ( I ) have detailed the advantages of analyzing individual trappec' fractions in lieu of the analysis of gas chromatographic effluents directly entering the ion source of a mass spectrometer. They also have discussed the shortcomings of the various trapping techniques currently used to collect individual fractions for subsequent mass spectrometric analysis and have described a general technique that eliminates many of these problems. In essence, the effluent Resent address, Dairy Products Laboratory, Agricultural Research Service, U. s. Department of Agriculture, Washington, D. C. 20250 (1) J. W. Amy, E. M. Chait, W. E. Baitinger, and F. W. McLafferty, ANAL.CHEM., 37, 1265 (1965).
fractions are trapped by a small capillary tube packed with 1 to 5 mg of the column packing material used for the gas chromatography, which has been conditioned under the usual column operating conditions. This technique utilizes a small capillary tube (0.070-inch i.d. x 1 inch) packed with approximately 20 mg of activated coconut charcoal, 70-100 mesh, and plugged with glass wool at each end to collect the effluent fractions. Charcoal has previously been used to trap gas chromatographic fractions but not in conjunction with mass spectrometric analysis (2, 3). The technique of Amy et al. ( I ) was used to introduce the sample into the mass spectrometer. The mass spectra shown were obtained with a Bendix time-of-flight mass (2) W. E. Harris, W. €3. McFadden, and R. G. McIntosh, Can.J . Chem., 39, 1784 (1961). (3) W. H. McFadden, R. G. McIntosh, and W. E. Harris, J. Phys. Chem., 64, 1076 (1960).
4 a4
99
h
127
h
142
Figure 1. Mass spectra of charcoal background (top), at 70' C, and 2-nonanone trapped on charcoal (bottom),at 70" C; mle 44, actual size VOL 39, NO. 8, JULY 1967
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Figure 2. Mass spectra of volatile compounds obtained on charcoal
spectrometer using the Bendix Model 843A direct inlet system and were recorded at the same sensitivities. Figure 1 (bottom) shows the spectrum of 2-nonanone trapped on charcoal from the effluent of 10 pg injected on a gas chromatographic column. The charcoal background (Figure 1, top spectrum) shows a very high ion current at mje 44 due to carbon dioxide. Attempts to prevent adsorption of carbon dioxide on the charcoal columns were unsuccessful. However, the charcoal background was reduced considerably by heating the charcoal columns with a Bunsen 1046
ANALYTICAL CHEMISTRY
burner, immediately placing them in a vial flushed with nitrogen, and storing them until use. A spectrum of the ketone was not obtained when attempts were made to trap approximately the same amount of material on 40 mg of Anakrom ABS (80-90 mesh) coated with 10% Carbowax 1540. The odor of the ketone could be detected at the exit of the column collection tube, whereas no odor was detected when the charcoal column was used. Obviously the capacity for retaining the compound is much greater on the charcoal. These two observations indicate that materials of
volatility similar to 2-nmanone are less efficiently trapped when the column packing is used as a trapping medium. There was no noticeable difference between the spectra of 2-nonanone obtained immediately after trapping and after being held in the trap 7 days at room temperature. Apparently such compounds are stable on charcoal and can be subsequently analyzed. Because of the slow rate of desorption from charcoal, highly volatile compounds can be introduced into the high vacuum of the mass spectrometer without rapid volatilization. The sample to be analyzed is added to the charcoal by inserting a capillary column packed with charcoal into the vapor phase and withdrawing a very small amount of vapor onto the charcoal column with a gas-tight syringe attached to the opposite end of the charcod column. Figure 2 shows examples of mass spectra of volatile compounds obtained in this manner at ambient temperature. If only qualitative information is required, there are two advantages of using a direct inlet in lieu of the conventional molecular leak inlet for obtaining mass spectra of volatile compounds; (1) samples can
be run more rapidly because very little pump-out time is required, and (2) sensitivity is greater because all of the sample is exposed to electron bombardment. Only a few samples have been selected to illustrate the advantages of charcoal as the trapping medium. In a study of trapping efficiency and desorption characteristics, three different lots of Fisher coconut charcoal (7Ck.100 mesh) and one lot of Analabs coconut charcoal (80-90 mesh) gave comparable results for 2-nmanone. A number of other types of compounds have been successfully trapped by this technique. In contrast, Cartwright and Heywood ( 4 ) found Molecular Sieve 5A to be a strong adsorbent only for short chain molecules. From our experience, GLC column packing materials would be poor adsorbents for many volatile compounds of interest.
RECEIVED for review December 22, 1966. Accepted April 18,1967. (4) M. Cartwight, and A. Heywood, Analyst, 91, 337 (1965).
Precision Picograrn Dispenser for Volatile Substances Andrew E. O'Keeffe arid Gordon C. Ortman Bureau of Disease Prevention and Environmental Control, National Center for Air Pollution Control, U.S . Department of Hedth, Education, and Welfare, Public Health Service, 4676 Columbia Parkway, Cincinnati, Ohio 45226
FORDISPENSING liquefiable gases at rates lower than can be attained by our original method ( I ) , the microbottle shown in Figure 1 has proved useful. The several steps of its construction and filling, as shown, are largely self-explanatory. Note that the glass envdope is drawn down to a size that forbids passage of the spherical bead (da > d ~and ) that a gastight fit is obtained between the glass envelope and the fluorinated ethylene propylene copolymer (FEP Teflon) insert by extruding the plastic member through the neck of the glass member (d3 > 4).The lowest emission rate that we have observed in such a microbottle has been 4 X g per sec (propene, room temperature), determined by flame ionization comparison against a gmvimetrically calibrated tube of our original design. For dispensing liquefiable gases incompatible with FEP Teflon (e.g. SFe; cf. Ref. I ) a similar microbottle having a polytetrafluoroethylene (TFE Teflon) permeation barrier has proved suitable. For dispensing k e d gsses (within which term we include those gases, such as acetylene, that cannot be handled safely as liquids at operating temperatures) a flat Teflon permeation membrane supported by a porous metal disk is connected directly to a cylinder 01' gas. Thickness and area of the membrane are selected 1.0 yield any desired flow rate. A small Bourdon gauge is permanently attached upstream of the device to give assurance that the infinite gas source has not been accidentally exhausted. This version is, of course,
(1) A. E. O'Keeffe and
(1966).
G. C. Ortman, ANAL. CHEM.,38, 760
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I PARTS SEAL HERE AFTfR ASSEMBLING PARIS
FORCE FIT B Y PULLING B ( L . 10 R.1 I H R U A. STOP A I POSITION SHOWN, FOR FILLING FROMGAS CYLINDER
-COOLlOCONDtNI€ OAS AIC I L l N D t l ~ R E S S U R E ~
If ASSEMBLING 8 FllllNG
J CLOSURf (L. TO R.1 IFFECIED TO LIMIT B Y PULLING I
111 FINISHED MICROBOTTLE
Figure 1. Construction and filling of low range permeation device incapable of providing a gravimetrically calibrated primary standard; indirect calibration will usually be necessary. RECEIVEDfor review January 16, 1967. Accepted May 1, 1967. Presented in part at 152nd Meeting, ACS, New York, N. Y.,September 14,1966; Division of Water, Air, and Waste Chemistry. Mention of commercial products does not constitute endorsement by the Public Health Service. VOL 39, NO. 8, JULY 1967
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