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our transfer procedure is where this method stands out, and it should solve a major nuisance to NMR spectroscopists.
RESULTS AND DISCUSSION 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. The water peak is very noticeable and caused problems by obscuring resonances from the 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. The 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 NMR tube. We have found that water contamination is still very noticeable, although less than with no drying a t all. Thus,
ACKNOWLEDGMENT Les Licholls kindly supplied the NMR spectra of tocinamide.
(1)
LITERATURE CITED L. J. F. Nicholls, C. R. Jones, and W. A. Gibbons, Biochemistry, 16, 2248
(1977).
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 NIH (AM 18604) and the NSF (BMS-74, 23819). The Biochemistry 270 MHz Facility was made possible by grants from the Graduate School and The National Science Foundation (No. BMS 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 the most important developments in the area of mass spectrometry for chemical analysis in recent years have involved the use of ion sources in which the pressure is high relative t o that of the 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 the 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 the 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 the 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. The 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 the differentially pumped system because of the 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 the above methods, that is, simplicity and low cost along with significant pressure differentiation between critical and 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 G
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Table I. Pressure Observed in Two Regions of the Mass Spectrometer under Various Physical Conditions of the Vacuum Envelope Pressure, Torr Condition of vacuum envelope Region A Region B Quadruple housing collar 1.0 x 1.0 x NOT used 1.6 X 2.3 X Envelope curtain in place Curtain in place and 1.6 X 2.3 X 10.’ suadruDle ion aDerture closed
Figure 1. Atmospheric pressure ionization mass spectrometer with a curtain separating critical from noncritical regions of the vacuum envelope
quadruple rods behind the quadruple housing collar. The pressure in regions A and B was monitored by two ionization gauges (Leybold-Heraeus, Inc., Ionvic 2). The manufacturer’s pressure calibration was used. Accuracy was verified by interchanging three tubes and three controllers of the same type and obtaining identical pressure readings.
RESULTS AND DISCUSSION T h e measurements of pressure in regions A and B of the vacuum envelope under different test conditions are shown in Table I. By removing the disk and collar of the quadruple housing, the first condition tests the situation where no curtain separating regions A and B exists. As is expected, no difference in pressures in regions A and B is seen, both having a pressure of 1.0 x Torr. It will be shown later that this is about the magnitude of pressure expected given the source sampling aperature, the diffusion pump, baffle, and isolation valve of our system. I t is important to note that operation of quadruple rods and an electron multiplier at this pressure is not recommended by the manufacturer because of the likelihood of vacuum discharges causing damage to the rods and the premature death of the detector ( 5 ) . Replacing the quadruple housing collar results in an effective separation of regions A and B down to the diffusion pump throat, except for the small ion entrance aperture of the quadruple housing. The result is that the pressure in region A increases to 1.6 X lo4 Torr, but the pressure in region B decreases to 2.3 x lo-’ Torr. Since the quadruple rods and detector will also experience the pressure of region B, this simple alteration of the vacuum envelope has decreased the pressure in critical regions by a factor of 4. The successful operation of our quadruple rods and detectors for long periods of time may be a reasonable expectation a t this pressure of nitrogen ( 5 ) . For the sake of this study, the result of one further alteration of the vacuum system, blocking the ion entrance aperture of the quadruple housing, is shown in Table I. Here the regions A and B are entirely separated down to the diffusion pump throat. In region A, a pressure of 1.6 X Torr is observed and in region B, a pressure of 2.3 X Torr. Since these are the same values observed with the ion entrance aperture open, it is clear that the presence of a small hole the size of the ion entrance aperture (3 mm) has little effect on the differential pressures established in regions A and B. The results of Table I can be shown to be in harmony with known laws describing the behavior of flowing gases (6). The flow rate of nitrogen through the mass spectrometer is limited by the 20-pm aperture of the ion source. The conductance, C, of this aperature can be calculated from the equation for viscous flow of air a t 20 “C, C = 20 A L s-l, where the area, A , is expressed in cm’. The flow rate, Q = C X 2, where 1p
is the pressure differences across the aperture (about 643 Torr in Bozeman) is then calculated to be 4.0 x L Torr s? or 3.8 cm3 atm min-’. The pumping speed of our pump is reported to be 1400 L s-l by the manufacturer. For the valve and a baffle very similar to ours, the manufacturer of our valve and pump report a throughput efficiency of 50% for each device. Thus, the net pumping speed, S, of our stack will be about 1400 L s-l X 0.50 X 0.50 = 350 L sd. Thus, with no curtain separating regions A and B of our envelope, a pressure of P = Q/S = 4.0 X 10-2/350= 1.15 X Torr should be attained. This value is in good agreement with the result of the first test condition shown in Table I where a value of 1.0 x was observed. Perhaps the greatest uncertainty in our calculations results from the uncertainty in the size of the ion source aperture (&lo%). With the envelope curtain intact, the cross-sectional area of the pumping stack exposed to region A is reduced by about one-third the total area. Thus the pumping speed to region A should now be about 0.66 X 350 = 230 L s-l and a pressure in region A of P = Q/S = 4.0 X 10-‘/230 = 1.7 X Torr is predicted. A value of 1.6 x Torr was measured. To compute the pressure predicted in region B under this condition, gas flow from two directions, from the ion entrance lens of the quadruple housing and from the throat of the diffusion pump, must be considered. The conductance of the 3-mm ion entrance aperture can be calculated from the molecular flow equation for air a t 20 “C through an orifice, C = 12 A L s-l where the area is in units of cm’. The flow Torr = is then Q = C X 1P = 8.5 x lo-’ L s-’ X 1.6 X 1.35 X Torr L s-’. The pumping speed to region B will be 350 X 0.33 = 115 L s-l, and a pressure of P = Q/S= 1.17 X Torr is predicted for region B from flow through the ion entrance aperture alone. This value is an order of magnitude less than the value of 2.3 X lo-’ Torr observed in region B under this condition. It must be concluded, therefore, that flow through the ion entrance aperture plays a minor role in determining the pressure of region B. The other effect, flow to and from the diffusion pump throat can be easily perdicted. T h e pressure anywhere in a vacuum system having a single pump can never be lower than the pressure at the throat of the pump since this region experiences maximum pumping speed. Assuming t h a t region B is completely sealed from region A down to the throat of the diffusion pump, no net flow between B and the pump can occur, and the equilibrium pressure established in region B must be the same as the pressure a t the throat of the pump. Thus, to calculate the limit of achievable pressure in region B predicted from the equilibrium flow from the pump alone, we need only to calculate the pressure predicted at the diffusion pump throat. Torr. This This is P = Q/S = 4.0 X 10-2/1400 = 2.9 X value is much larger than the effect on region B predicted for flow through the ion entrance aperture and is, therefore, most important and a determining factor of the pressure in region B. T h e measured value of 2.3 X 10 Torr in region B is in good agreement with this prediction. The last condition tested, that of blocking the ion entrance aperture completely, conclusively illustrates that the pressure of region B is being
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limited to that a t the pump throat, since no effect of this change is reflected in the measured pressures. The advantages of this system for our application are obvious, and further interesting consequences might be predicted. For example, if additional baffling or trapping is desired in a vacuum system and if these are installed in a manner which maintains a curtain continuously down to the pump throat, it would still be possible to achieve the ultimate pressure of the diffusion pump throat or very close to it throughout the critical volume. In other words, the pressure achievable in a selected region of a vacuum system is limited only by the gas flow rate entering the entire envelope and the size of the pump evacuating it, and can be made independent of the baffles, traps, and isolation valves also used. Another potentially useful prediction is that a lower pressure in the critical volume might be obtained by using a larger diffusion pump even if the diameter of the pump exceeds that of the port and vacuum system to which it is attached. In a singly pumped system, one would gain little by this alteration because the net conductance of the pumping stack is limited by its smallest dimension. The major limitation of the vacuum design described here compared to a conventional differentially pumped system is, as has been shown, the ultimate vacuum achievable with it is inherently limited for a given gas flow rate and vacuum pump. As is possible with the truly differentially pumped system, the pressure of the critical volume cannot be con-
tinuously lowered by decreasing the size of the aperture connecting the two envelope volumes. Furthermore, for a given system, much lower pressures in the critical volumes can be obtained with a conventional differentially pumped system. In our application, for example, the pressure in region B could be reduced to about 1 X Torr by the proper addition of a second diffusion pump stack of modest size. Nevertheless, for many applications, the envelope design described here can offer satisfactory results a t a minimum of expense and instrumental complexity, and certainly provides a significant improvement in performance over the conventional singly-pumped vacuum envelope.
LITERATURE CITED (1) B. Munson, Anal. Chem., 43 (13), 28A (1971). (2) E. C. Horning, M. G. Horning, D. I. Carroll, I.Dzidic, and R. N. Stillwell, Anal. Chem., 45, 936 (1973). (3) M. McKeown and M. W.Siegel, Am. Lab., Nov. 1975. (4) E. P. Grimsrud and R. A. Rasmussen, Atmos. Environ., 9, 1010 (1975). (5) Extranuclear Laboratories, Inc., Instruction Manuals for Preamp counting head, model 032-3, and for particle multipliers, model 051, Oct. 1975. (6) S. Dushman, "Scientific Foundations of Vacuum Technique", 2nd ed., J. M. Lafferty, Ed., John Wiley and Sons, New York, N.Y., 1962, Chapter 2.
RECEIVED for review October 4, 1977. Accepted November 14, 1977. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society; to Research Corporation; and to the National Science Foundation for support of this research.
CORRECTION Chemically-Bonded Aminosilane Stationary Phase for the High-Performance Liquid Chromatographic Separation of Polynuclear Aromatic Compounds In Table I of this article by S. A. Wise, S.N. Cheder, H. S. Hertz, L. R. Hilpert, and W. E. May, Anal. Chem., 49, 2306
(1977), the logarithm of the retention index for pyrene on pBondapak NH2 should be 3.26 rather than 3.68.
