Anal. Chem. 1990, 62, 1547-1549
perature (8),and such changes are well-known for bulk alkanes (9,lO). Ordered alkyl conformations are comparable to liquid-crystal structures, and in fact the retention behavior of liquid-crystal phases in GC is remarkably similar to ordered polymeric CISphases in LC (I 1). A review of shape discrimination effects in liquid chromatography has been published (7)
In conclusion, the column cooler provides a compact, portable means of cooling LC columns below ambient conditions. The design allows for maximum flexibility in use and permits the device to be moved between instruments with little effort, as the need arises.
ACKNOWLEDGMENT We thank Richard Christensen for assistance in machining the column block assembly. LITERATURE CITED (1) Gilpin, R. K.; Sisco, W. R. J. Chromatogr. 1980, 794, 285-295. (2) Warren, F. v,; Bidlingmeyer, B, A. Anal, Chem, lg88, 6 0 , 2021-2824.
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Snyder, L. R.; Kirkland, J. J. Introduction to AMern LiquM Chromatography, 2nd ed.; Wlley-Interscience: New York, 1979. Sander, L. C.; Wise, S. A. Anal. Chem. 1989, 67, 1749-1754. Craft, N.; Sander, L. C.; Pierson, H. F. Submitted to J. Micronoh. Anal. Olsson, M.; Sander, L. C.; Wise, S. A., unpublished research. Sander, L. C.; Wise, S. A. LC-GC 1990, 8, 378-390. Sander, L. C.; Callis, J. B.; Fieid, L. R. Anal. Chem. 1983, 55, 1068- 1075. Snyder, R. G.; Maronceiii, M.;Qi, S. P.; Strauss, H. L. Science 1881, 274, 188-190. Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 79, 85-116. Wise, S. A.; Sander, L. C.: Chang, H.; Markides, K. E.; Lee, M. L. Chromatographia 1988, 25, 473-480.
RECEIVED for review January 12,1990. Accepted April 1,1990. Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
Effects of Ion Source Parameters on Ion Beam Energy in Mass Spectrometry Kuangnan Qian, Ani1 Shukla,* and Jean Futrell Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716
INTRODUCTION Tandem mass spectrometry (MS/MS) has become one of the most powerful gaseous-ion chemistry techniques in analytical and structural chemistry (1). For MS/MS studies most commercial mass spectrometers provide good mass resolution but relatively modest energy resolution, especially when primary ion currents are increased for sensitivity reasons. To obtain a good intensity ion beam under electron impact source conditions, strong repeller fields or strong field penetration are often used. The potential field inside an ion source may also be affected by space charge a t higher pressure and electron currents. This combination of circumstances both introduces broad energy distributions and causes the kinetic energy of an ion beam not to be equal to the nominal energy defined by the voltage difference between the source block and the final exit lens. In our ongoing studies (2, 3) of the fundamental reaction dynamics of colliiion-induced dissociation (CID) of polyatomic ions we measure the absolute kinetic energy of the ion beam many times during the course of an experimental series. In these measurements we have observed that the ion energy is dependent on the ion source chamber pressure and the nature of the gas. In this paper, we report a study of the effects of ion source parameters on ion energy and rationalize our results on the basis of a combination of field penetration and surface and space charge effects. Since our experiments have utilized an unmodified (commercial) ion source, we assume that these effects are ubiquitous and should be an important consideration in other experiments for which ion kinetic energy is a significant parameter.
EXPERIMENTAL SECTION The instrument has been described in detail in an earlier publication (4). Briefly, the ion beam is produced by a VG 7070E double-focusing mass spectrometer. The 3-keV ion beam is decelerated by a group of tube and rectangular deceleration lens to the desired energy at the point where it collides with a su-
personic beam of neutrals. The kinetic energy of the parent and fragment ions is measured by a hemispherical energy analyzer. A quadrupole mass filter is used for mass analysis. AU the voltages on the lenses and energy analyzer are referenced to the ion source voltage. Recently the entrance and exit slits on the energy analyzer were reduced to 3 mm X 7 mm each for improved resolution in both energy ( M / E = 0.02) and angle. Prior to installation in our instrument the energy analyzer was calibrated for measuring absolute ion kinetic energy in several ways. First, the geometry of the device defines ion energy quite accurately except for fringing fields at the ion entrance and exit slits of the analyzer. Computer simulations of three-dimensional ion trajectories provided us with a reliable estimate for these effects and an assessment of the transmission function for different slit sizes. Calibration using an ion source having a well-defined ion energy and energy distribution provided us with experimental confirmation of the theoretical predictions and basis for a correction to the theoretical equations. Finally during each experiment the energy analyzer calibration is checked independently by measuring the absolute energies of both the parent and the daughter ion formed by unimolecular metastable decay within the collision region of the tandem mass spectrometer. The precision with which the kinetic energy can be measured is 0.1 eV, and the absolute energy is determined with an estimated accuracy of 10.1 eV.
RESULTS AND DISCUSSION Figure 1 shows the effect of ion source pressure on the kinetic energy of argon, acetone, propane, and nitromethane primary ions. Data are plotted as the mean of the energy distribution as measured by the hemispherical energy analyzer after primary ions are decelerated to 50 eV versus the pressure reading from an ion gauge connected to the ion source vacuum chamber. The actual ion source pressure depends on the gas and is about 100 times greater than the plotted value. The ordinate is the maximum of the measured energy distribution minus the nominal energy defined by the potential difference between the ion source of VG 7070E mass spectrometer and the potential of the last plate of the deceleration lens. These
0003-2700/90/0382-1547$02.50/00 1990 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990
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experiments were carried out with zero repeller field in the ion source to minimize the spread in the energy distribution of the ions. The total ion current extracted from the source was also maximized at zero repeller, and this is a normal operating parameter for our experiments. Figure 1shows that the absolute ion energy of acetone and argon ions is equal to the nominal energy deduced from voltage difference between the ion source and the collision center in the pressure reading range (2-3) X lo4 mbar (1 mbar = 1.3333 Torr). In other words the absolute ion energy for ions leaving the source is equal to the nominal accelerating voltage. Propane ion energy equals the accelerating voltage at 2 X lo4 mbar. Below or above this range, ions have less or more kinetic energy than the nominal potential difference. The energy shift (the difference between real energy and nominal energy) changes rapidly at lower pressures but only slowly at higher pressures. However, the nitromethane ion behaves quite differently from the other two ions; specifically, its energy is about 0.5 eV lower than that of the other ions a t the same source pressure. Because of its low volatility, we did not extend our measurements for nitromethane to the higher pressure range investigated for other gases. The extrapolation to zero pressure in Figure 1to eliminate the effect of space charge on the potential of the region from which ions are extracted shows that the potential of this region is lower than that of the ion source block. This is caused by field penetration of the ion acceleration lens. We have confirmed this hypothesis by modeling our ion source as shown in Figure 2 using the SIMION program (5). The ion source has a 1-mm-wide exit slit. The beam centering plate, which is 2 mm from the source block, has 1.5 kV when the source potential is set at 3 kV. The potential fields inside the ion source calculated by using SIMIONare shown relative to source (and repeller) a t 3000 V with 1500 V on the centering plates. It can be seen that the potential at the circle that matches the electron beam profile where ions are produced is 1.8-3 V less than the source block potential. In the absence of space charge, the actual ion energy is therefore predicted by this figure to be below the nominal energy by 1.8-3 eV. The mean energy will be 3000-2.4 eV with an energy spread (fwfm) of 1.2 eV. Figure 3 shows the potential distribution at different repeller voltages as a function of distance within the ion source as calculated by using SIMION. It is obvious from this plot that
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the relatively high efficiency of this ion source results from its reliance on the field penetration for ion extraction. The variation of the shift in absolute ion energy with pressure demonstrated in Figure 1 indicates that ions and electrons inside the source also perturb the potential field. The curves shown in Figure 1 demonstrate that this space charge effect can shift the potential by as much as 3 V, a surprisingly large value. T o our knowledge, no one has previously suggested the possibility of such a large energy shift for conventional ion sources, although space charge has been used to trap the ions within the ion source for several milliseconds (7). The space charge effect can be described qualitatively as follows. In the absence of gas molecules the flux of electrons establishes a negative potential well surrounded by higher potential regions on all sides. At low gas pressure this potential well can serve as a trap for positive ions. As the gas pressure is increased, the resulting accumulation of ions in the beam causes a progressive neutralization of the negative space charge and further increase builds up a net positive space charge. The fact that ions are formed initially at thermal energy while the electrons are high-velocity particles causes positive space charge (6) to develop at relatively low gas pressure. Over most of the operating range of Figure 1positive space charge is the
ANALYTICAL CHEMISTRY, VOL. 62, NO. 14, JULY 15, 1990
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dominant factor determining ion energy. We also investigated the effect of repeller voltage a t fixed ion source pressure. These data are illustrated in Figure 4 for argon as a test gas. The voltage shift is zero for a repeller voltage of zero a t 2.8 X lo4 mbar. The ion current out of the source maximizes at 0 V repeller a t this pressure. With decreasing repeller voltage the energy shift drops off nearly linearly, becoming increasingly negative with negative repeller. At a repeller potential of -6 V ions are no longer extracted from the source. With a positive repeller voltage there is a positive energy shift but with a lower slope. Figure 5 demonstrates that the energy shift has only a weak dependence on ion source pressure at a positive repeller voltage. Contrast Figure 1 and 5 to see the dramatic effect of repeller field on space charge. Under these conditions the repeller field adds to the field gradient from field penetration, and the accumulation of positive ions necessary to generate positive space charge does not occur over the pressure range investigated. Space charge is a dominant effect for zero and negative repeller potentials and less significant for positive potentials. Ion energy distributions are narrower for zero and negative repeller, and ion extraction efficiency is similar to that for positive repeller voltages. Both are functions of pressure, as expected for space charge dominated operation. Although positive space charge is the major factor responsible for shifting the mean energy of ions extracted from the source, the results described in Figure 1 cannot be fully explained on the basis of space charge alone. Space charge for constant electron flux is a function of ionization cross section and ion mobility. Plotting data as a function of ion gauge reading in Figure 1 rather than actual source pressure automatically corrects for relative ionization cross sections. Ion mobility (6) is inversely related to square root of mass, and the curves are not ordered in this way. The curves for
Figure 5. Experimental energy shifts of the argon ion beam as a function of source pressure at constant repeller voltage.
argon, propane, and acetone group together, and nitromethane is an outlier offset by about 0.5 eV. This represents a shift in potential when this particular gas is introduced and probably results from the adsorption of this polar molecule on the electrode surfaces or from the relatively large number of anions that are formed in electron ionization of nitro compounds. The above results clearly demonstrate that ion source conditions play an important role in determining the actual energy of the ion beam. Ion energy depends on source pressure, repeller voltage, and the chemical nature of the gas used. The shift can be quite large, several electronvolts for the example illustrated here, and must be taken into account in interpreting the energetics of MS/MS. These effects will be especially significant in threshold energy measurements. Registry No. Propane, 74-98-6;nitromethane, 75-52-5;argon, 7440-37-1; acetone, 67-64-1.
LITERATURE CITED (1) Busch. K. L.; Glish, G. L.; McLuckey, S. A. Mass SpechomebylMass Spectrometry : Principles and Applkations of Tandem Mass Spectrometry; VCH: New York, 1989. (2) Shukla, A. K.; Qian. K.; Anderson, S. G.; Futrell, J. H. J . Am. SOC. Mass Spectrom. 1990. 1 , 6. (3) Qian, K.; Shukia, A. Futrell, J. J . Chem. Phys., in press. (4) Shukla, A. K.; Anderson, S.G.; Howard, S.L.; Sohlberg, K. W.; FutreII, J. H. Int. J . Mass Spectrom. Ion Processes 1988, 86, 61. (5) Dahl, D. A.; Delmore, J. E. The SIMION PClAT User's Manual Version 3 . 0 ; Idaho National Engineering Laboratory, E. G. and G. Idaho, Inc., 1987. (6) McDaniel, E. W. Collision Phenomena in Ionized Gases; Wiley: New York, 1964. (7) Lifshitz, C.; Gefen, S. Int. J . Mass Spectrom. Ion Processes 1980, 35, 31.
RECEIVED for review February 5, 1990. Accepted April 16, 1990. This work was supported by National Science Foundation, Grant No. CHE-8312069.
