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Particle-Induced Desorption Mass Spectrometry with a Quadrupole Mass Spectrometer Kym F. Faull,* A. N. Tyler, H a r r y Sim, and J a c k D. Barchas Pasarow Analytical Neurochemical Facility, Nancy Pritzker Laboratory of Behavioral Neurochemistry, Department of Psychiatry and Behavioral Sciences, Stanford University School of Medicine, Stanford, California 94305
Ian J. Massey Syntex Research, 3401 Hillview Avenue, Palo Alto, California 94304
Christine N. Kenyon and Paul C. Goodley Hewlett-Packard Corporation, Scientific Instruments Division, 1601 California Avenue, Palo Alto, California 94304
John F. Mahoney and Julius Perel Phrasor Scientific Inc., 1536 Highland Avenue, Duarte, California 91010 Organic mass spectrometry has recently been revolutionized by the particle induced desorption (PID) technique in which a beam of atoms and/or ions is used to desorb charged molecules and fragments of molecules from a surface (1-12). Although the most spectacular applications of this technique have been with high molecular weight multifunctional compounds of biological origin (1-3, 7, 10) which in many cases have proven refractory with conventional ionization methods, the technique can also be used to obtain mass spectra of low molecular weight polar and thermally labile molecules (6,13, 14). In fact, the success with which the technique is being used for the analysis of a wide variety of organic compounds gives reason to believe that it will become a valuable ionization method which will find frequent use as a complementary technique to the electron impact (EI) and chemical ionization (CI) approaches conventionally used in mass spectrometry. One favorable feature of this technique is the relative ease with which most mass spectrometers can be retrofitted with a gun capable of supplying a beam of fast atoms and/or ions. Reports thus far have described the performance of magnetic sector (1-5, 7-10, 15-17) and quadrupole (13, 14, 18) mass spectrometers with Saddle Field ion guns. Here we describe the installation and performance of a quadrupole GC/MS instrument with a Capillaritron ion gun.
EXPERIMENTAL SECTION Reagents. Chemicals and reagents were of the highest purity available and solutions were prepared with doubly distilled deonized water. Naloxone hydrochloride, hexadecyltrimethylammonium p-toluenesulfonate, dimethyldidocosylammonium chloride, and fluoroalkyl quaternary ammonium salts were kindly donated by Endo Laboratories (Garden City, NY), Hexcel Chemicals (Zeeland, MI), Humco Chemicals (Memphis, TN), and E. I. du Pont de Nemours (Wilmington, DL), respectively. Source Design and Operation. A Capillaritron ion source equipped with an ion deflector, power supply, and gas control unit (Phrasor Scientific, Duarte, CA) was installed on a HewlettPackard 5985B quadrupole GC/MS instrument. The power supply and gas control unit provided independent control of the beam energy and current (19). By permanently mounting the gun on the vacant flange above the ion source and using minor modifications to the existing EI/CI source, which included drilling holes in the magnet, electron beam target, and ceramic target holder, there was no need to exchange sources when changing between the EI, CI, or PID modes. This arrangement allowed for rapid interconversion between the three ionization modes. More importantly, the modifications made to the source had no effect on performance of the instrument in either the E1 or CI modes. The barrel of the gun was fitted to a standard 2.75 in. diameter Conflat flange. The blank 8 in. Conflat top flange located directly above the ionization chamber was replaced with a flange modified with a 1.5 in, diameter hole. The barrel of the gun was inserted
through this hole, and a Viton or copper gasket was used to obtain a seal between the 2.75 in. diameter Conflat flange and the flange on the vacuum manifold (Figure 1). The xenon (99.995%, Matheson, Newark, CA) beam was directed into the ionization chamber through holes (0.079 in. diameter) in the magnet, electron beam target, and ceramic target holder of the ionization source. Samples were introduced into the ionization chamber on a stainless steel (303) tip (0.1 in. wide, 0.05 in. thick) attached to a direct insertion probe (DIP). The samples were confined to a circular cone-shaped depression or crater (0.09 in. diameter, 0.02-0.03 in. deep, approximate volume 2 wL)made in the surface of the stainless steel tip. With the DIP inserted, the crater aligned with the holes in the magnet and electron collector assembly and the Capillaritron nozzle was 3 in. above the crater surface. Desorbed secondary ions were mass analyzed in the quadrupole filter positioned perpendicular to the axis of the primary beam. Under operating conditions, the indicated source pressure in the torr. To prevent vacuum manifold was between 2 and 8 X excessive contamination of the quadrupole rod assembly, the analyzer manifold was held at 210 "C. To avoid premature evaporation of the liquid matrix from the probe tip, the source was kept either at 100 "C, and cooling water at 8 "C was circulated through the DIP, or the source was allowed to cool to 40-60 "C, in which case it was not necessary to run cooling water through the DIP. With either of these conditions, stable ion currents were obtained from glycerol droplets (1pL) for at least 10 min. Alignment of the primary ionizing beam with the holes in the magnet and electron collector assembly was checked by exposing a piece of UV-sensitive paper attached to the magnet, to a 30-9 pulse from the ion gun. The position of the exposed area on the paper indicated the orientation of the ion beam relative to the hole in the magnet. Some control of this alignment was achieved by tilting the Conflat flange containing the gun. The flange was tilted by varying the pressure on the Viton gasket. The alignment was checked, and when necessary corrected, during selected ion monitoring (SIM) by maximizing the intensity of desorbed ions. Sample Preparation and Data Collection. The liquid matrix (typically glycerol, 0.5-1.0 pL) was placed in the crater on the probe tip. The sample was then applied as a solid or solution (0.5-1.0 gL) to the surface of the matrix. The sample-matrix mixture was then introduced into the ionization chamber via the inlet vacuum lock. Spectra were collected via a data system using a low threshold intensity and a slow scan rate (56.25 daltons/s), corresonding t o 16 analog to digital measurements per datum point. Experience demonstrated that this produced the most consistent data for low intensity ions. Off-line spectrum averaging was also performed with the data system.
RESULTS AND DISCUSSION Characteristics of the Ion Gun and Deflector. Under the conditions employed, the gun produced a mixture of fast xenon atoms and ions in approximately equal abundance (19, 20). The ions were detected when there was no sample in the ionization chamber as a spectrum of signals ranging from m/z
0003-2700/84/0358-0308$01.50/00 1984 American Chemical Society
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128 to 136 corresponding to the seven most abundant xenon isotopes. The decline in intensity of the xenon ions with increasing voltage on the deflector plate showed that at maximum deflection voltage (2.2 kV) all the ions were deflected from the beam. Henceforth, the beam produced with maximum deflection voltage will be referred to as a fast atom beam. Angle of Incidence. The optimal angle for detection of molecular ions and fragment ions from hexadecyltrimethylammonium p-toluenesulfonate and leucine-enkephalin (Figure 2) was found to be approximately 64O, which is close to the angle reported in the literature for magnetic sector mass spectrometers equipped with Saddle Field ion guns (15-17). This angle was used in all subsequent experiments. For routine work the angle was conveniently optimized by rotating the DIP during SIM data collection to maximize the intensity of desorbed ions. Characteristics of the Primary Ionizing Beam. The combined effects of ion beam current and ion beam energy on secondary ion intensity were studied by using hexadecyltrimethylammonium p-toluenesulfonate with xenon beams of fast ions and atoms (zero ion deflection) and ex-
clusively fast atoms (maximum ion deflection). The results obtained with the 40 pA ion beam, shown in Figure 3, typify those obtained with 20,60, and 80 pA ion beams. At ion beam energies between 4 and 9 kV, the signal for the molecular ion was generally greater with the ion and atom beam. Background noise was generally greater with the ion and atom beam. Consequently, signal-to-noise ratios were slightly greater with the atom beam. With the ion and atom beam, maximal signal intensity was produced at 8,6,7, and 8 kV, with 20,40,60, and 80 pA beam currents, respectively. Raising the energy of the 40, 60, and 80 PA ion and atom beams beyond that necessary for maximal intensity resulted in a significant reduction in molecular ion peak height. On the other hand, the atom beams produced signal intensities which steadily increased as the energy of the beam was raised to a maximum at 9 kV. While the intensity maxima obtained for the molecular ion with the 20, 60, and 80 pA beams were similar, the intensity maxima obtained with the 40-wA beam was 30% higher. We are unable to explain why the 40-pA beams produced a molecular ion peak height which was consistenly greater than that produced by the other beams. However, there are at least three possible explanations for the decrease in molecular ion peak height observed with the 40,60, and 80 pA ion and atom beams at high beam voltages. Firstly, increased fragmentation could occur at the higher energy levels. Secondly, the surface layer of glycerol may become depleted of sample when the energetic beam is used. Thirdly, a slight misalignment between the ionizing beam and the holes in the magnet and electron collector assembly could become increasingly important as the energy of the beam is raised. In favor of this latter explanation is the decreased flight time of the ions and atoms which would result from increasing the voltage of the beam. This would reduce the opportunity for beam broadening by electrostatic repulsion between the positively charged ions and would produce a narrower beam with higher current density. Such a beam would be particularly prone to a slight misalignment in the ionization source. This possibility em-
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Flgure 5. Positive ion PID spectra obtained wlth aqueous solutions (1 bL) in glycerol matrices: (A) adenoslne (29 nM in 6 N HCI), (e) naloxone (HCI salt, 7.2 nM), (C) (-)-bicuculline methiodide (2.6 nu),(D) dimethyldidocosylammoniumchloride (696 pM). Xenon was the bombarding species (6 kV, 40 PA, zero ion deflection).
phasizes the requirement for accurate alignment of the source for maximal sensitivity with high voltage beams. Background Noise and Spectrum Averaging. Previous reports have already drawn attention to the high level of noise in the spectra produced by ion gun-quadrupole mass spectrometer combinations (11,13). Examination of the noise in the spectra generated by the Capillaritron ion gun-quadrupole mass spectrometer revealed that a very large part of it was random, suggesting that the signals were the result of either fast neutral species and/or fast ions, which escaped mass filtering, striking the electron multiplier. Spectrum averaging took advantage of the long-lived and stable secondary ion currents produced by the source and the rapid scanning capabilities of the quadrupole mass spectrometer, and resulted in a considerable improvement in spectral quality and enhancement of signal-to-noise ratios which was particularly dramatic in the high mass regions of the spectra (Figure 4). Calibration of the Mass Range and Optimization of Source Parameters. Initial experiments were carried out by using a mass calibration and source and lens voltages which were the same as those used in the E1 mode. In the range 0.5-35 nmol, this produced positive ion PID spectra of a
variety of organic compounds, including peptides (Figure 4, and spectra published in ref 20-22), adenine, adenosine 5'monophosphate, a-D-mannose 1-phosphate, naltrexone, Sadenosylmethionine (19) and adenosine, naloxone, and bicuculline methiodide (Figures 5A-C), which were qualitatively identical with PID spectra published in the literature for the same classes of compounds (1-3, 5, 8 10, 12, 23, 24). Aliphatic hydrocarbon quaternary ammonium salts proved useful for calibration of the mass range and tuning the source parameters in the PID mode. The advantage provided by these compounds was the presence of long-lived and exceptionally strong signals for the molecular ions which were clearly distinguishable above the background noise. For example, intense signals for the molecular ions were obtained from 100 pmol to 1 nmol of the halide salts of choline, acetylcholine, and tetradecyltrimethyl-, dimethyldocosylbenzyl-, dimethyldioctadecyl-, and dimethyldidocosylammonium (Figure 5D), and hexadecyltrimethylammonium p-toluenesulfonate at m / z 104, 146, 256, 444, 550, 663, and 284, respectively. Judicious choice of the hydrocarbon chain length provided standards which were suitable for calibration of any region of the mass range up to mJz 663. Although mixtures of fluoroalkyltrimethyl quaternary ammonium compounds (CF3(CF2)aCH2CHzSCH2CHzN+(CH3)3, n = 4-12) and fluoroalkylbetaines (CF3(CF2),CH2CH(OCOCH3)CH2N+(CH&CH2COO-, n = 3-8) gave similarly intense signals for the molecular ions, their shorter life, presumably attributable to increased volatility and/or more rapid desorption because of reduced hydrogen bonding with the matrix, precluded their use as convenient calibration standards. Tuning in the PID mode provided a rigorous check on mass calibration and mass peak width, and at the same time provided the opportunity for careful optimization of lens and source voltages for any particular region of the mass range. Spectra produced with the E1 tuning parameters were qualitatively identical with spectra produced for the same compounds with the PID tuning parameters. Such comparisons demonstrated only subtle differences in fragment ion to molecular ion ratios.
LITERATURE CITED (1) Barber, M.; Bordoll, R. S.;Sedgwlck, R. D.; Tyler, A. N. Nature (London)1981, 293, 270-275. (2) Williams, D. H.; Bradley, C.; BoJesen, G.; Santikarn, S.; Taylor, L. C.E. J . Am. Chem. SOC. 1981, 103, 5700-5704. (3) Morris, H. R.; Panlco, M.; Barber, M.; Bordoll, R. S.;Sedgwick, R. D.; Tyler, A. Blochem. Biophys. Res. Commun. 1981, 101, 623-631. (4) Barber, M.; Bordoli, R. S.;Sedgwlck, R. D.; Tyler, A. N.; BycroR, 8. W. Blochem. Biophys. Res. Commun. 1981, 101, 632-638. (5) Barber, M.; Bordoli, R. S.;Garner, G. V.; Gordon, D. B.; Sedgwlck, R. D.; Tetler, L. W.; Tyler, A. N. Biochem. J . 1981, 197, 401-404. (6) Surman, D. J.; Vlckerman, J. C. J. Chem. Res. Synop. 1981, 170-171.
Anal. Chem. 1984, 56,311-312 (7) Dell, A.; Morris, H. R.; Levin, M. D.; hecht, S. M. Biochem. Biophys. Res. Commun. 1981, 102,730-738. (8) Bradley, C. V.; Williams, D. H.; Hanley, M. R. Biochem. Biophys. Res. Commun. 1982, 104, 1223-1230. (9) Barber, M.; Bordoli, R. S.; Elliott, G. J.; Sedgwick, R. D.; Tyler, A. N. Anal. Chem. 1982, 54,645A-657A. (10) Williams, D. H.; Bradley, C. V.; Santikarn, S.; Bojesin, G. Biochem. J . 1982, 201, 105-117.. (11) Devienne, F. M.; Roustan, J.-C. Org. Mass. Spectrom. 1982, 17, 173-181
(12) Aberth, W.; Straub, K. M.; Burlingame, A. L. Anal. Chem. 1982, 54, 2029-2034. (13) Caprloli, M. C.; Beckner, C. p.; Smith, L. A. Biomed. Mass Spectrom. 1983, 10, 94-97. (14) Hunt, D. F.; Bone, W. M.; Shabanowitz, J.; Rhodes, J.; Ballard, J. M. Anal. Chem. 1981, 53, 1704-1706. (15) Martin, S. A,, Costello, C. E.; Biemann, K. Anal. Chem. 1982, 5 4 , 2362-2368. (16) Barber, M., Bordoli, R . S.; Sedgwick, R. D.; Tyler, A. N.; Green, E. N.; Parr, V. C.; Gower, J. L. Biomed. Mass Spectrom. 1982, 9 , 11-17. (17) Barber, M.; Bordoli, R. S.;Sedgwick, R. D.; Tetler, L. W. Org. Mass. Spectrom. 1981, 16,256-260.
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(18) Surman, D. J.; Vickerman, J. C. J . Chem. Soc., Chem. Commun. 1981, 324-325. (19) Mahoney, J. F.; Perel, J.; Goodley, P. C.; Kenyon, C. N.; Faull, K. Int. J . Mass Spectrom. ion Phys. 1983, 48,419-422. (20) Mahoney, J. F.; Goebel, D. M.; Perel, J.; Forrester, A T. 61'0med. Mass Spectrom. 1983, IO, 61-64. (21) Faull, K. F.;Barchas, J. D.;Kenyon, C. N.: Goodiey, P. C. Int. J . Mass Spectrom. Ion Phys. 1983, 46, 347-350. (22) Fauil, K. F.; Barchas, J. D.; Murray, S.; Haipern, E. Biomed. Mass. Spectrom ., in press. (23) Cotter, R. J.; Hansen, G. Anal. Chim. Acta 1982, 136, 135-142. (24) Unger, W. E.; Ryan, T. M.; Cooks, R . G. S I A , Surf. Interface Anal. 1981, 3, 12-15.
RECEIVED for review May 31, 1983. Accepted October 3, 1983. Partial support for this work was provided by an NIMH Neurosciences Progam-Prbject Grant (MH 23861) and an ONR Selected Research Opportunity Award (SRO001:N00014-79-C-0796).
Determination of Microgram Amounts of Selenium and Tellurium in Copper-Base Alloys by Atomic Absorption Spectrometry Michael Bedrossian
Armco, Inc., Research and Technology, 703 Curtis Street, Middletown, Ohio 45043 Selenium and tellurium are found in various sulfide ores and they are produced as a byproduct of metal refineries. Both selenium and tellurium are used in the refining of copper, in the manufacture of rubber, and in the electronics industry. Lately, selenium and tellurium have had important applications in the manufacture of steel and various alloys. Since certain properties of metals and alloys depend on the presence of selenium and tellurium, it is desirable to have a reliable analytical method for determining selenium and tellurium in coppebbase alloys below 100 ppm. An accurate method €or the determination of less than 50 ppm of tellurium in steels has been developed by using a solvent extraction procedure (1). Laboratory tests indicate that this procedure, with minor modifications, also is applicable to the determination of selenium and tellurium in nickel-base alloys. However, its application to copper-base alloys is limited. The most sensitive hydride evolution electrothermal atomic absorption spectrometric techniques are not reliable (2). In addition, the method is subject to serious interferences ( 3 , 4 ) . Tsukahara and Yamamoto (5) described the determination of tellurium in copper, lead, and selenium by flame atomic absorption. Albright et al. (6) used X-ray to determine selenium in copper-, nickel-, and iron-base alloys. After reduction to their elemental forms, iodide complexes of both selenium and tellurium can be extracted simultaneously from copper-base alloys by the use of trioctylphosphine oxide-methyl isobutyl ketone and then determined by flame atomic absorption. This present method is highly sensitive and allows the determination of 0.0002% selenium or tellurium. EXPERIMENTAL S E C T I O N Apparatus. A Perkin-Elmer Model 5000 atomic absorption spectrophotometer, equipped with an air-acetylene, single-slot burner head and Westinghouse electrodeless discharge lamps were used. Reagents. All chemicals used were ACS certified reagent grade quality. Standard selenium solution of lo00 mg/mL and 5tandai.d tellurium solution of 100 mg/mL were prepared from the pure metals. Working solutions of 25 ,ug/mL for both selenium and
tellurium were prepared monthly by appropriate dilution of the stock solutions in 10% SCl acid. Trioctylphosphine oxide-methyl isobutyl ketone (TOPO-MIBK) solution ( 5 % ) was prepared by dissolvihg 12.5 g of TOP0 in MIBK in a 250-mL volumetric flask. Procedure. A 2.0-g sample size was used for the determination of trace levels of selenium and tellurium (
