ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
resolution was readily regained by conditioning t h e column overnight at 400 "C with helium carrier gas flow.
LITERATURE CITED (1) M. Gruenfeid, Proceedings of Joint Conference on Prevention and Control of Oil Spills, Washington, D.C., March 13-15, 1973, pp 179-193. (2) E. B. Overton, J. Bracken, and J. L. Laseter, J . Chromatogr. Sci., 15, 169 (1977). (3) J. G. Pym, J. E. Ray, G. W. Smith, and E. V. Whitehead, Anal. Chem., 47, 1617 (1975). (4) J. G. Bendoraitis, 8. L. Brown, and L. S.Hepner, Anal. Chem., 34, 49 (1962). (5) I.R. Hills, G. W. Smith, and E. V. Whitehead, J . Inst. Pet., 56, 27 (1970). (6) J. D. Brooks, K. Goukl, and J. W. Smith, Nature(London),222, 257 (1969). (7) N. J. Bailey, A. M. Jobson, and M. A. Rogers, Chem. Geol., 11, 203 (1973). ( 8 ) W. W . Hanneman, C. F. Spencer, and J. F. Johnson, Anal. Chem., 32, 1386 (1960).
381
(9) J. M. Hunt, in vonder Borch et al., "Initial Reports of the Deep Sea Drilling Project", Vol. 22, U.S. Government Printing Office, Washington, D. C., 1974, pp 673-675. (IO) J. Han and M. Calvin, Geochim. Cosmochim. Acta, 33, 733 (1969). (1 1) 8. W. Jackson, R. W. Judges, and J. L. Powell, Environ. Sci. Techno/.. 9, 656 (1975). (12) W. E. Reed, Geochim. Cosmochim. Acta, 41, 237 (1977). (13) M. T. Murphy in "Organic Geochemistry", G. Eglinton and M. T. Murphy, Ed., Springer-Verlag, Berlin, 1969. (14) E. J. Gallegos, Anal. Chem., 43, 1151 (1971). (15) D. E. Anders and W. E. Robinson, Geochim. Cosmochim. Acta, 35, 661 (1971).
RECEIVED for review July 29, 1977. Accepted October 24, 1977.
Drying Small Amounts of Solvent for Use in Nuclear Magnetic Resonance Spectrometry Joseph B. Alper Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin-
Madison, Madison, Wisconsin 53 706
A major problem in t h e area of high field proton magnetic resonance (PMR) spectroscopy is the contamination of solvents with water (1). This problem is especially acute when using very small amounts of sample; the water signal can obscure sample peaks as well as cause computer-associated dynamic range problems when using time averaging techniques. We have developed a simple, fast, and efficient method of drying such solvents as dimethylsulfoxide (DMSO), acetone, and chloroform using a surplus gas chromatograph (GC), a broken 10-mL volumetric pipet, a 6-inch syringe needle, a Pyrex, Leur-lock syringe, Swagelok fittings and 48, molecular sieves. EXPERIMENTAL The column was constructed as follows and the end result is shown diagrammatically in Figure 1. The ends of a 10-mL volumetric pipet were cut and bent in such a manner as to allow a good fit in the GC oven: the end to be connected to the GC inlet was fitted with Swagelok fittings and the outlet end was joined to the syringe barrel. The barrel of the pipet was then packed with 4-A molecular sieves obtained from Altex Corp. A small hole ('/,-inch) was made in the back of the GC oven to accommodate the syringe needle, which is attached to the Leur-lock fitting of the syringe. The use of a syringe needle as the exit port allowed the dry solvent to be collected directly into a dry sample containing a NMR tube fitted with a septum cap. A small gauge needle is inserted through the cap during collection to allow for pressure relief. The column was then heated at 240 "C for 12 h, with a slight stream of dry nitrogen flowing during this time, to activate the molecular sieves. This procedure is also carried out after a day's use; we have found that after 9 months of daily use and overnight baking, the molecular sieves should be replaced. Samples are prepared in our laboratory by dissolving between 0.2-4.0 mg of a peptide in a small amount of water, lyophilizing in the NMR tube to give a dry, white solid. The NMR tube is then fitted with a suitable septum cap (Aldrich) and dry solvent was collected after fitting the NMR to the exit needle of the GC column. The GC oven is set at 100 "C for Me2SO-d6and 50 "C for both CCD13 and acetone-&; an injector temperature of 150 "C for Me2SO-d6and 100 "C for CDC13 and acetone-d, is used. Dry nitrogen was used as the carrier gas and the flow was such that 15 min after injection all solvent had been eluted from the column. Up to 4.0 mL of solvent has been dried at one time. 0003-2700/78/0350-0381$01 OO/O
Figure 1. Diagram of the apparatus used to dry small amounts of
solvents as described in this paper
t
I
I
4 . 5 4.1
I
,
I
3.7 3.3
I
I
2.9
Flgure 2. C, proton region of the NMR spectra of tocinamide in
MezSO-d6. T h e bottom spectrum was taken using Me,SO-d, taken from a sealed ampule. The top spectrum was taken using Me,SO-d, dried using t h e method outlined in this paper C 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 2, FEBRUARY 1978
RESULTS AND DISCUSSION
our transfer procedure is where this method stands out, and it should solve a major nuisance to N M R spectroscopists.
Figure 2 is the C, region of the NMR spectra of tocinamide ( I ) , a hexapeptide, in MezSO-d6which had been taken directly from a sealed ampule using a dry pipet; this was done in a dry glove bag. T h e water peak is very noticeable and caused problems by obscuring resonances from t h e glycine and cysteine residues. Figure 2 is the same region of a n identical sample only prepared using MezSO-d6taken directly off the above described GC column. T h e water peak is missing and all resonances are fully visible. Similar results have been obtained with chloroform and acetone. In all cases, residual water contamination is less than 50 pM as estimated by comparison of peak heights due to a known concentration of proton to the residual water peak, which is visible only upon extreme magnification. This procedure is much superior to storing solvents over molecular sieves and transferring them directly to the N M R tube. We have found t h a t water contamination is still very noticeable, although less than with no drying a t all. Thus,
ACKNOWLEDGMENT Les Licholls kindly supplied the N M R spectra of tocinamide.
LITERATURE CITED ( 1 ) L. J. F. Nicholls, C. R. Jones, and W. A. Gibbons, (1977).
Biochemistry, 16, 2248
RECEILTD for review September 12,1977. Accepted November 16. 1977. This research was supported by the College of Agricultural and Life Sciences of the University of Wisconsin by grants from the N I H (AM 18604) and the N S F (BMS-74, 23819). T h e Biochemistry 270 MHz Facility was made possible by grants from the Graduate School and The National Science Foundation (No. B M S 74-23826).
Vacuum Envelope for High Pressure Mass Spectrometry Applications Eric Grimsrud Department of Chemistry, Montana State University, Bozeman, Montana 59717
entire effluent from a gas chromatograph ( 4 ) .
Some of t h e most important developments in the area of mass spectrometry for chemical analysis in recent years have involved t h e use of ion sources in which the pressure is high relative t o t h a t of t h e conventional electron impact mass spectrometry technique. T h e now well-established and widely-used Chemical Ionization (1)method of generating mass spectra utilizes a n ion source pressure of about 7 Torr. More recently, ultrahigh detection capabilities have been demonstrated by t h e method of Atmospheric Pressure Ionization ( 2 ) mass spectrometry which requires a n ion source pressure of 7 atm. Because of the high gas flow rates inherent in these methods, perhaps the most significant modification required in the high pressure mass spectrometers results from t h e need of much greater pumping capacity on the vacuum envelope. T h e most simple and least costly way to do this modification is to install a single and relatively large pump on t h e vacuum envelope (2). Depending on the magnitude of gas flow, however, this approach may result in an inadequate vacuum, especially when baffles, traps, and isolation valves are added with the pump. T h e differentially pumped method (3),using two vacuum pumps in semi-isolated regions of the vacuum envelope, is generally recommended for high pressure mass spectrometers because lower pressures can be maintained in the critical volumes enclosing the mass analyzer and detector, while tolerating fairly high gas flow rates from the ion source into the first stage of the vacuum envelope. Of course, greater expense and complexity must be accepted with t h e differentially pumped system because of t h e additional pump and its associated baffles, traps, isolation valves, foreline pump, and fail-safe devices. We would like to report here an envelope design for high-pressure mass spectrometry methods which has some of the advantages of both of t h e above methods, that is, simplicity and low cost along with significant pressure differentiation between critical a n d noncritical volumes of the mass spectrometer envelope. Our application is for Atmospheric Pressure Ionization, but the principle could be applied equally well to a Chemical Ionization mass spectrometer or other applications of mass spectrometry in which high gas flow rates are inherent, such as an Electron Impact Ionization mass spectrometer which is sampling the 0003-2700/78/0350-0382$0 1.OO/O
EXPERIMENTAL Our mass spectrometer shown in Figure 1 is similar to the single-pump design of Horning et al. (2) for Atmospheric Pressure Ionization. Between the 6-in. diffusion pump (Edwards model E06) and the mass spectrometer envelope are a baffle and a 6-in. butterfly isolation valve. The baffle is homemade from straight stainless strips of 1.5 in. in width, soldered into the vacuum envelope each at a 45" angle. The ion source encloses a 1-cm3 volume, the walls of which are formed by a 63Ni-impregnated platinum foil. Most of the carrier gas entering the ion source is vented to the room air after passing through the cell. About 3.8 cm3 atm min-', however, passes into the vacuum envelope via a 20-pm aperture. This aperture is in the center of a nickel disk (Perforated Products, Inc.) of 25-pm thickness which forms the back wall of the ionization volume. The nickel disk is sealed between the ion source and the flange of the vacuum envelope with a gold O-ring. For these measurements, the ion source was maintained at room temperature. Within the vacuum envelope, a quadruple mass spectrometer (Extranuclear Laboratories, Inc.), including the quadruple rods and electron multiplier detector are mounted to the rear flange. The rods and detector are each housed in stainless steel cans which, except for the ion entrance aperture (3-mm diameter), was entirely closed when first received from the manufacturer. An effective curtain was fabricated to separate the volumes indicated as A and B in Figure 1 in the following manner. A stainless disk ('/,,-in.thick) was welded to the interior walls of the vacuum envelope and to the top of one of the baffle fins in the position shown. This disk has a hole of 5 in. in diameter centering on the quadruple axis. A piece of stainless plate, fitting from the bottom edge of the above-mentioned baffle fin to the axis of the butterfly baffle was welded to the fin and envelope walls. Another disk (5.5 in. in diameter) and an attached collar were made t o fit over the quadrupole housing as shown. The fit is such that the collar is snug, but easily movable. With the addition of this part to the quadruple housing, the separation of volumes A and B is caused by placement of the r e a flange and all of the components attached to it onto the mass spectrometer. As shown in Figure 1,with the butterfly valve open, the separation of volumes A and B extends to the throat of the diffusion pump. To ensure that the pressure near the quadrupole rods and the detector is that of region B, several holes were drilled into the jackets of the detector and the ,L
1978 American Chemical Society
