Anal. Chem. 1982, 5 4 , 1917-1919
1917
(9) Kagevall, I.; Astrom, 0.; Cedergren, A. Ana/. Chim, Acta 1980, 114, 199-208.
(3) Verhoef, J. C.; Barendrecht, E. Anal. Chim. Acta 1977, 94, 395-403. (4) Scholz, E. Fresenlus’ Z Anal. Chem. 1981, 306, 394-396. (5) Scholz, E. Am. Lab. (Fairfield,Conn.) 1981, 13 (8), 89-91. (6) Verhoef, J. C.; Barendrecht, E. J . Electroanal. Chem. 1978, 7 1 , 305-315. (7) J. T. Baker Chemical Co., product information bulletlns 15 and 17. (8) Koupparis, M. A.; Walczrrk, K. M.; Malmstadt, H. V. J . Autom. Chem. 1980, 2 (2), 66-75.
RECEIVED for review November 25.1981. AcceDted June
14.
lgg2*Research partially supported by Research Grant PHS-5RO1 GM 21984.
Miniature Ion-Source-Mounted Fast Atom and Ion Gun for Fast Atom Bombardment and Secondary Ion Mlass Spectrometry Martln A. Rudat Central Research & Development Department, Experimental Station, E.
The recent development of fast atom boimbardment (FAB) mass spectrometry ( I ) has led to an explosive growth of research in the application of the new technique to previously intractable molecules. At the same time, commercial versions of fast atom sources have become available to the mass spectrometry community. All of the commercial sources thus far offered have required appropriately placed flanges on the vacuum housing, with a direct line-of-sight to the end of a probe tip where the sample resides in the ion source. The distance over which the beam must traverse causes broadening and therefore lower incident flux than if the atom gun were in the vicinity of the sample and also induces alignment problems. The flange requirement greatly increases the cost and conversion time for instruments not equipped with an appropriately located blank flange. The most direct way to deviate these problems is to attach the atom gun directly to the mass spectrometer ion source, close to the sample and within the vacuum chamber. This paper describes such a miniature fast atom source which has been attached to a commercial high-voltage field desorption ion source. The present design could be used as an ion beam source for magnetic or quadrupole “organic” or “inorganic” secondary ion mass spectrometry (SIMS) and as a quadrupole FAB source if a deflector plate is added.
EXPERIMENTAL SECTION The fast atom source essentially consists of a miniaturized Capillaritron ion source, the operation of which has been described (2), and is shown in Figure 1. Capillaritron ion source tips were obtained from Phrasor Scientific, Inc. (Duarte, CA); these consist of a l/g in. diameter stainless steel tubing base with a pin-sharp came attached to one end. In the tip of this cone is a 25 pm diameter hole, through which gas flows and is the point at which a microdischarge is maintained by a high voltage applied to the tip. In the standard Capillaritron ion source, a counterelectrode consisting of a 0.75-in. tube with an end plate having a l/g in. diameter exit hole provides a shielded volume and contributes to the beam focusing. The counterelectrode in the miniature device is a stainless steel washer, 10 mm o.d., to provide a sufficient planar electrode surface, with a 2-mm hole in the center. The washer is attached to a support and conduction wire which is Connected to ground through a 2.2-MQresistor. The Capillaritron tip is positioned just inside the plane of the inside surface of the washer and centered with respect to the hole. The counterelectrode is positioned 2-5 mm from the entrance hole of tlhe ion source to allow good pumping speed in this region. Support for the entire source is provided by an insulating bracket made of Vespel resin, through which the tubing end of the Capillaritron tip passes. This bracket is screwed to the side of the VG Analytical (Altrincham, England) ZAB-2F field desorption (FD) ion source. Gas is supplied through silicone tubing and a glass capillary discharge suppressor and connected to the CL gas inlet connection. 0003-2700/82/0354-1917$01.25/0
I. du Pont de Nemours & Company, Wilmington, Delaware 19898 Argon gas flow rates, as measured by a floating-element type flowmeter (Show Rate), can be in the range 2-6 cm3/min. The applied discharge high voltage can be in the range 3-10 kV, and the discharge can be operated at 1-100 pA or more current. Current draw from the high-voltage supply is measured by a meter protected by a 2.2-Mil resistor. The gas flow rate and applied high voltage determine the operating current for the discharge; for the results presented here, the conditions were approximately 4 cm3/min argon, source housing pressure 4 X torr, and 4-5 kV applied and 5-10 pA discharge current. The discharge current is apparently a good measure of the ion beam current but does not directly measure the neutral beam flux. Nonetheless, the observed FAB ion intensities are linearly related to the discharge current. Figure 2 shows the arrangement of the new atom source with respect to the sample probe and the slightly modified FD source. The sample probe tip is a solid 3-mm steel rod, cut and roughly polished at an angle of 20° on one end, with support pins spot welded to the other end. With the atom gun mounted on the side of the source and the probe tip entering from the back (as is normal for FD work), the atom beam strikes the tip surface at an angle of 70’ from the surface normal. The atom beam enters the source via an existing electron entrance hole, and the probe tip passes through an FD extractor plate with an enlarged opening. The extractor plate is connected to the ion source potential. Glycerol was placed on the probe tip and approximately a microgram of sample was mixed into it. A small bubble of the solution was evident on the probe tip.
DISCUSSION The Capillaritron source, although originally designed as an ion beam source for sputtering work, also produces a significant flux of energetic neutrals. This is substantiated by these experiments with the miniature source, in which the ion beam must be deflected by the net 1-2 kV potential of the ion source with respect to the beam. Even with this deflection, a substantial secondary ion signal is observed, which therefore must arise from energetic neutral particle bombardment. The intense photon yield from the Capillaritron ion source is a further indication that excited species are present and that charge neutralization is probably taking place. In the region just past the end of the tip, relatively high argon gas pressures exist (ca. 1 torr). The gas flow is in the same direction as the ion extraction, possibly increasing the probability of neutralization. Although the neutralized beam flux and energy are difficult to determine, it seems likely that a substantial energy spread exists, as is also likely with other atom beam sources. The operation of the miniature source in the voltage range below the mass spectrometer ion source potential prevents ions from striking the sample. Simply increasing the voltage above this value and adjusting the gas flow to lower flow rates results in similar discharge operation but ion bombardment 0 1982 American Chemical Society
1918
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982 FAST ATOM BOMBARDMENT SOURCE I--
I
4
0
171
3-10 kV
‘RODE M O UIT K JJ,
& 275
+I”+
Flgure 1. Cross sectional view of the miniature fast atom and ion source. The insulating mount holds the source in position on the side of the standard mass spectrometer ion source.
TO M.S.
T
50
150
rn/z
Flgure 3. FA6 spectrum of hexazinone in glycerol, with some NaCl added, recorded on a VG Data System at 3 sldecade. Glycerol ions have been suppressed by the presence of the sample (57, 75, 93, 185). Discharge voltage was 4.5 kV and 5 PA.
FOCUS PLATES
Flgure 2. Schematic diagram of the atom/ion source mounted on a VG ZAB-PF field desorption mass spectrometry ion source and the relationship to the sample. Probe tip bevel Is 20’.
(mixed with atom bombardment) of the sample. Thus, the design shown in Figure 1 is applicable to high voltage mass spectrometers for atom or ion bombardment and could be adapted to quadrupoles with the addition of an electrostatic deflector plate. For ion beam operation only, an alternative arrangement, in which the entrance hole into the mass spectrometer ion source is utilized as the counterelectrode (drawing several pA through the source supply), can be used, eliminating the separate counterelectrode. By use of the new fast atom source, rather intense FAB ion currents can be obtained quite easily, as has been reported in papers and lectures by a number of workers using commercial FAB sources. The signal is quite stable and readily lends itself to computerized data acquisition and high-resolution measurements. With slow scanning speeds, ions at every mass unit can be counted on oscillograph-recorded spectra. However, the new source provides sufficient ion intensity to record computerized spectra at low resolution with “normal” scan speeds, i.e., 3-5 s/decade, rather than the often reported 30-1000 s/decade. Ion current stability measurements were made by using the MH+ (939 ion from glycerol, which is the most intense ion in the glycerol spectrum. Stability was found to be excellent, at better than 0.7%. Figure 3 shows the computer-recorded spectrum of hexazinone (3-cyclohexyl-6-(dimethylamino)-l-methyl-s-triazine2,4(V1,3H)-dione),recorded at 3 sldecade. Note the virtually complete suppression of the glycerol spectrum (no subtraction has been performed on this spectrum), which is characteristic of many compounds analyzed with this technique and which is somewhat concentration dependent. Ions from Ultramark reference materials have been observed at m / z values above 1600 (at 6 kV source accelerating voltage), and collision-induceddissociation utilizing the reverse geometry of the ZAB-2F has been performed on a number of
1 2
4
6
DISCHARGE VOLTAGE ( k V )
Flgure 4. Effect of atom beam energy (discharge voltage) on absolute ion intensities in the FA6 spectrum of hexazinone in glycerol. All spectra were obtained from the same solution and at the same discharge current (beam flux). Note the reduction of fragment ion intensities with increasing beam energy.
ions from various compounds. The intense ion signals which allow such measurements are consistent with those reported by other workers and provide further evidence for the utility of FAB. The intensity of ions observed with the new source seems to be competitive with that obtained by using commercial sources. Although it seems to be generally believed that an atom beam of about 5 kV is required to obtain satisfactory FAB spectra, this is actually not the case. With the miniature FAI3 source, the discharge current can be adjusted by varying either the gas flow rate or the discharge voltage, allowing the production of similar discharge conditions over a wide range of beam energies. Spectra of very similar intensity can be obtained at beam energies of 2 keV and 6 keV when the discharge currents are the same. Qualitative variations in the spectra are apparent and provide justification for operating at higher energies. Figure 4 is an illustration of this effect, using hexazinone. At the lowest beam energy used in this experiment, 2.65 keV, the lowest mass fragment ions are the most intense ions in the spectrum. As the energy is increased, the absolute intensities of these ions decrease, while the intensities of the MH+ (253+)and 171’ ions increase. Note that the smaller fragment ions decrease in intensity more rapidly, so that the spectrum appearance changes from a low-mass skewed spectrum to a high-mass skewed spectrum from low to high energy (to yield a spectrum similar to Figure 3). This observation suggests that higher beam energies will yield FAB spectra which favor the higher mass ions.
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
The new source should find ready application in a wide variety of modern instruments equipped with pumps adequate to handle the gas flow, i.e., those equipped for chemical ionization. Although a field desorption ion source has been utilized in this laboratory, any standard ion source with the capability for introducing a liquid-carrying surface into the ion source volume and adequate space in the vacuum housing should be adaptable to the new fast atom source. An in-source probe tip can be used to hold the sample in the vicinity of the center of the ion source, beveled at a 20° angle to the incident atom beam, which can enter through an electron entrance hole or effluent entrance hole. Alignment of the miniature FAB source is essentially permanently ensured by
1919
the careful measurement for and machining of the insulating support. ACKNOWLEDGMENT The author thanks C. N. McEwen for helpful discussions and A. G. Bolinski for his technical support. LITERATURE CITED (1) Barber, M.;Bordoll, R. S.; Elliott, G. J.; Sedgwick, R. D.;Tyler, A. N. Anal. Chem. 1982, 54, 645A. (2) Mahoney, J. F.; Perel, J.; Forrester, A. T. Appl. Phys. Lett. 1981, 38, 320.
RECEIVED for review April 5, 1982. Accepted June 21, 1982. Vespel is a registered trademark of the Du Pont Company.
CORRECTIONS
Agglomerated Pellicular Anion-Exchange Columns for Ion Chromatography Timothy S . Stevens and Martin A. Langhorst (Anal. Chem. 1982, 54, !351-953).
There ie an unfortunate error on page 953, last sentence. The sentence should read: “A United States Patent application has, been filed on behalf of the authors covering the subject of this contribution and the technology has been licensed to the Dionex Corporation.”
Differential Pulse Voltammetric Study of Direct Electron Transfer in Glucose Oxidase Chemically Modified Graphite Electrodes Robert M. Ianniello, Thomas J. Lindsay, and Alexander M. Yacynych (Anal. Chem. 1982,54, 1098-1101). There are unfortunate errors in the third sentence of the Conclusions section (page 1101, lines 8-10). The correct sentence should read: “By behaving as a substitute for a chemical electron donor (e.g., substrate), the chemically modified electrode can be used as a medium for electron transfer for a variety of immobilized oxidoreducatase enzymes.”
