Anal. Chem. 1983,
55,967-968
methods of analysis should be utilized to unequivocally determine oxygen under these conditions. The permanganate modification (13) normally used to prevent ferrous iron interference may be a viable alternative for iodometric oxygen determination. However, the need to apply permanganate prior to fixing increases sample handling and is cumbersome in the field. The use of electrometric oxygen determinations in the well bore after flushing of stored water may be appropriate if interferences are known to be absent. Alternatively, careful sample collection followed by thermal conductivity/gas chromatography should prove to be a suitable analytical method when nitric oxide is present. ACKNOWLEDGMENT We thank Richard Zepp (USEPA-ERL, Athens, GA) for constructive discussions on this work and Pamela C. Beavers for help in preparation of the manuscript. Registry No. H20, 7732-18-5; 02,7782-44-7; NO, 10102-43-9. LITERATURE C I T E D (1) Mancy, K. H.; Westgarth, W. C. J.-Wafer Polluf. Control Fed. 1962, 34, 1037-1051. (2) Caritt, D. E.; Kanwisher, J. W. Anal. Chem. 1959, 3 1 , 5-9. (3) Atwood, D. K.; Kinard, W. F.; Barcelona, M. J.; Johnson, E. C. DeepSea Res. 1077, 2 4 , 311-313. (4) "National Handbook of Recommended Methods for Water Data Acquisition"; U.S. Geological Survey, Office of Water Data Coordina-
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tion, US. Department of the Interlor: Washington, DC, Jan 1980; Chapter 2, Ground Water, pp 2-108-2-114). Heaton, T. H. E.; Vogel, J. C. J . Hydro/. (Amsterdam) 1981, 50, 201-216. Alsterberg, G. Blochem. Zelf. 1925, 159,36. Carpenter, J. H. Llmnol. Oceanogr. 1965, 10, 141-143. Naymik, T. G.; Barcelona, M. J. Ground Wafer 1981, 19 (5),517426. Stumm, W.; Morgan, J. J. I n "Aquatic Chemistry"; Wlley: New York, 1981; Chapter 7, pp 418-503. Letey, J.; Valoras, N.; Focht, D. D.; Ryden, J. C. Sol/ Scl. A n . J . 1981, 45, 727-730. Jones, K. I n "Comprehensive Inorganlc Chemistry"; Bailar, J. C., Emeleus, H. J., Nyholm, R.. Eds.; Peraamon Press: Oxford, 1973: Chaoter 19, pp 147-388. Latimer, W. M. "The Oxidation States of the Elements and Their Potentials in Aqueous Solutions", 2nd ed.; Prentlce-Hall: New York, 1953: 92. "Stand& Methods for the Examinatlon of Water and Wastewater", 14th ed.; APHA-AWWA-WPCF, 1975 pp 448-449.
Michael J. Barcelona* E d w a r d E. Garske Section Head and Assistant Supportive Scientist Aquatic Chemistry Section-Water Survey Division Illinois Department of Energy and Natural Resources Box 5050, Station A Champaign, Illinois 61820 RECEIVED for review December 20, 1982. Accepted January 24, 1983.
AIDS FOR ANALYTICAL CHEMISTS Source-Mounted Cesium Ion Gun for Obtaining Liquid Matrix Secondary Ion Mass Spectra of Organics Charles N. McEwen Central Research1 & Development Department, E.
I. du Pont de Nemours &
The recently developed technique of fast atom bombardment (FAB) mass spectrometry of organic compounds in a liquid matrix has proven to be a powerful tool for easily obtaining long-lasting and intense mass spectra of a variety of compounds which are not amenable to conventional mass spectrometry ( I ). An important feature of the FAB technique is its adaptability to analytical mass spectrometers. Fast atom and ion guns are available commercially but require that an appropriately placed inlet port with a line-of-sight to the ion source be available for mounting the gun. Recently, a fast atom and ion gun developed here which mounts directly on the ion source inside the vacuum housing was reported ( 2 ) . While this approach greatly reduces the expense of modifying an instrument to obtain FAB spectra, it requires that the instrument have the pumping capacity to handle the gas loads normally associated with chemical ionization mass spectrometry. This report describes a source-mounted Cs+ ion gun which produces mass spectra essentially identical with those obtained by FAB using a liquid matrix (3). The Cs' ion gun also eliminates the need for a line-of-sight inlet port and has the additional advantages of low gas load (allowing insatallation on mass spectrometers with limited pumping capacity), close proximity to the sample (eliminating the need for focusing lenses), and simple construction from low cost materials commonly available in mass spectrometry laboratories. Chait and Stsinding ( 4 ) have obtained mass spectra from nonvolatile organic compounds deposited as a thin film on
Company, Experimental Station, Wilmingfon, Delaware 19898
a metal plate using low intensity Cs+ ion bombardment and time-of-flight mass spectrometry. Aberth et al. ( 5 ) recently reported the development of a source-mounted Cs+ ion gun. They also observed that Cs+ and Xeo bombardment of a liquid matrix produced very similar mass spectra and suggested the term LIQUID-SIMS replace the acronym FAB. EXPERIMENTAL SECTION The purpose of the experiment was to develop a small on-source alkali ion gun that was a simple to construct, easy to operate, and had a sufficiently intense alkali ion beam to produce spectra comparable in intensity to those obtained with a commerical fast atom bombardment source. Because we did not find literature references to alkali ion guns which met the criteria set for this experiment, several unique designs were tested. The simplest design involved placing a small amount of CsCl on the filament side of the shield plate of a conventional electron impact filament and biasing the filament 1-3 kV positive with respect to the ion source. Heating the filament to ca. 1000 O C produced a Cs+ ion beam of sufficient intensity to yield useful sputtered ion mass spectra. Although the spectra obtained were similar to FAB spectra, arcing and low intensity sputtered ion beams were problems. The most successful Cs+ ion gun utilized a short ceramic tube (l/s in. 0.d. X 1 in. long) with a standard 8-mil coiled tungsten filament wire spot welded to nickel (20 gauge) wire supports which were held in place with Sauereisen No. 8 ceramic cement (Sauereisen Cements Co., Pittsburgh, PA) as shown in Figure 1. The filament coil is recessed about 1mm inside the ceramic tube. Twenty-five microliters of a concentrated aqueous CsCl solution
0003-27O0/83/0355-0967$01.50/00 1983 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983 TO M S
VESPEL@ BRACKET
-1
-
1/8" I D x 3/4" CERAMIC TUBE ( I O N GUN HOLDER1
FILAMENT
FILAMENT
2 2MQ ALKALI
ION GUN
DETAIL
the VG Analytical FD ion source opposite a miniature onsource FAB gun which has been described by Rudat (2). Spectra could alternatively be taken by FAB or Cs+ SIMS by rotating the sample holder 180'. Samples of dyes, organometallic salts, and polypeptides were run in a glycerol matrix. Bradykinin, a nonapeptide, gave essentially an identical spectrum (over the mass range reported) to that reported by Barber et al. (7). Comparing the full spectrum of bradykinin obtained by the on-source Cst and FAB guns under conditions of equal MH+ ion intensity, it was found that the only differences in the spectra were less intense low mass ions observed with Cs+ bombardment and a medium intensity Cs+ ion at m/z 133. Similarily, the spectra of hexazinone (1) obtained by Cs+ SIMS and on-source FAB were nearly identical except that less intense low mass ions were again observed using Cs+ bombardment (see ref 2 for the FAB spectrum of hexazinone).
(OUTSIDE VACUUMl
Figure 1. Schematic of alkali ion source for liquid matrix SIMS.
was injected into the tube and evaporated to dryness before the tube was mounted onto the source with a Vespel fabricated bracket. For these experiments, the tube was mounted on a VG Analytical (Altrincham, England) ZAB-2F field desorption ion source so that the tip of the ceramic tube ion gun was ca. 1mm from the electron entrance hole (an earlier version was successfully mounted on the source of a Du Pont 21-110 mass spectrometer). Because the ceramic tube provides directionality to the Cs+ beam, it is important that the ion gun assembly be properly aligned with respect to the electron entrance hole of the source. A Cs+ ion beam was generated when CsCl vaporized and contacted the hot filament wire surface. The intensity of the ion beam is controlled by the rate of vaporization of CsCl which is turn is controlled by regulating the temperature of the filament (2-3 A at 12 V). The Cs+ions thus formed are accelerated toward the sample by biasing the filament 1-6 kV positive with respect to the ion source (high mass ions are favored at higher voltage). The sample was introduced into the path of the Cs+ ion beam in a glycerol matrix on a in. diameter steel probe tip mounted on the VG Analytical FD probe. The steel tip was cut and polished so that the Cs+ ion beam intersects it at an angle of ca. 70' from the surface normal as suggested by Barber et al. (6),and perpendicular to the mass spectrometer z axis. The only modifications of the FD source were replacing the FD extractor plate with one having a hole large enough for the steel tip to pass through and floating the plate at source potential. The filament is powered by a 12-V rechargeable battery with a 6 4 5 0 - W variable resistor in series with the filament to control the filament temperature. The current to the filament is monitored with a 10-A meter. [Cautionary note: The battery, resistors, and ammeter must be encased in a nonconductive box (we use a Lucite box) and the variable resistor is controlled by a Lucite acrylic resin rod to prevent electric shock.] A 2.2-MQ resistor is placed between the filament circuit and a power supply capable of +12 kV. The resistor reduces the possibility of arcs or discharges between the filament and the ion source (Figure 1). With the ion source at +6 kV and the filament of the Cs+ ion gun at +7 to +12 kV and powered to red heat, a cesium ion beam is produced which bombards the sample surface at 1-6 keV, and sputtered positive ions are observed. With the source at -6 kV and the filament at 0 to -3 kV, Cs+ ion bombardment allows observation of sputtered negative ions. The cesium ion gun has been operated for intervals of up to 3 weeks without aparent degradation and without unusual ion source contamination (the Cs+ ion gun was "on" only when a sample was present). The 12-V battery was recharged after each day's operation.
DISCUSSION For comparison purposes, the Cs+ ion gun was mounted on
These experiments, in agreement with those obtained by Aberth et al. (5), show that the charge of the bombarbment particle has little or no effect on the sputtering phenomena, a suggestion previously made by Magee (8)and others (9). On the other hand, the liquid matrix is critical for obtaining durable sputtered ion beams of good intensity. Additionally, the intensities of the higher mass ions relative to the low mass ions are greater for Cs+ than for ArO bombardment. In negative ion Cs+ SIMS the filament is grounded and the ion source (on a magnetic instrument) is operated a t several kilovolts negative potential. Under these conditions, and using a glycerol matrix, intense negative ion mass spectra were obtained for several dyes and polypeptides. However, the negative ion spectra obtained do not always show ions at every mass to aid determination of the mass value as is usually the case in the positive ion mode. Thus, either a mass marker or a data system for determining ion m / z ratios is highly desirable.
ACKNOWLEDGMENT The author thanks M. A. Rudat for helpful discussions and assistance in spectral comparisons and A. G. Bolinski for technical support. LITERATURE CITED (1) Barber, M.; Bardoli, R. S.; Elliot, G. J.; Sedgwick, R. D.; Tyler, A. N. Anal. Chem. 1982, 5 4 , 645A. (2) Rudat, M . A. Anal. Chem. 1982, 5 4 , 1917. (3) Rudat, M. A.; McEwen, C. N . Int. J . Mass Spectrom. Ion Phys. 1983, 4 6 , 351. (4) Chait, B. T.; Standing, K. G. I n t . J . Mass Spectrom. Ion Phys. 1981, 4 0 , 185. (5) Aberth, W.; Straub, K. M.; Burlingame, A. L. Anal. Chem. 1982, 5 4 , 2029. (6) Barber, M.; Bordoll, R. S.; Sedgwick, R. D.; Tyler, A. N . J . Chem. Soc., Chem. Commun. 1981, 7981, 325. (7) Barber, M.; Bordoli, R. S.; Sedgwick, R. D.; Tyler, A. N.; Whalley, E. T. Blomed. Mass Saectfom. 1981. 8 . 337. (8) Magee, C. W. I i . J . Mass Spectrom. Ion Phys., in press. (9) Garrlson, B. J.; Wlnograd, N. Sclence 1982, 216, 805. '
RECEIVED for review October 7, 1982. Accepted January 7, 1983. This work was reported at The 30th Annual Conference on Mass Spectrometry and Allied Topics, Honolulu, HI, June 6-11, 1982, paper WPB17. Vespel and Lucite are registered Du Pont trademarks.
