Anal. Chem. 2003, 75, 6478-6484
Ion Transport Diagnostics in a Microsecond Pulsed Grimm-Type Glow Discharge Time-of-Flight Mass Spectrometer Eric Oxley,‡ Chenglong Yang,§ Jian Liu,| and W. W. Harrison*,†
Department of Chemistry, University of Florida, P.O. Box 117200, Gainesville, Florida 32611-7200, LECO Corporation, 3000 Lakeview Avenue, St. Joseph, Michigan 49085-2396, INFICON, Inc., 6878 Santa Teresa Boulevard, San Jose, California 95119, and Department of Pharmacology, Medical University of South Carolina, 173 Ashley Avenue, BSB 303, Charleston, South Carolina 29425
A Grimm-type glow discharge ion source, operated in the microsecond pulsed mode, has been interfaced to a commercial time-of-flight mass spectrometer. Ion transport from the source to the mass spectrometer, an inherent limitation of a Grimm source and mass spectrometer combination, was evaluated. The primary discharge operating conditions found to influence transport efficiency were gas flow rate and source pressure. The configuration of the Grimm-type source also influenced ion transport, including use of a gas-directing sleeve device. The effect of transport efficiency was separated into two components: (1) total ion signal and (2) temporal resolution. The latter is an advantage afforded by use of a pulsed glow discharge source and time-of-flight spectrometer, which allows discrimination against interfering gaseous background ions by appropriate ion sampling time. Shown as an example is the identification of trace magnesium from potential background interference using an optimized source configuration based on this temporal resolution method. The glow discharge (GD) is a versatile source for the direct analysis of solid materials, and glow discharge mass spectrometry (GDMS) has become an established technique for applications that require elemental analysis.1,2 GD sources typically employ a diode configuration in which the sample is placed on the end of a direct insertion probe (DIP). The sample must assume a configuration that can be mounted on the DIP and contained within the source chamber. Problems attributed to the DIP configuration include sample placement, redeposition on source components, and thermal effects.3 An alternative GD format, the Grimm-type source, has found widespread use in glow discharge atomic emission spectrometry (GD-AES) owing to certain inherent advantages of a Grimm-type †
University of Florida. LECO Corp. § INFICON, Inc. | Medical University of South Carolina. (1) Coburn, J. W.; Kay, E. Appl. Phys. Lett. 1971, 18, 435. (2) Harrison, W. W.; Magee, C. W. Anal. Chem. 1974, 46, 461. (3) Glow Discharge Spectroscopies; Marcus, R. K., Ed.; Plenum Press: New York, 1993. ‡
6478 Analytical Chemistry, Vol. 75, No. 23, December 1, 2003
source over a DIP source. In the Grimm source, the sample is mounted externally, allowing easy sample interchange and good precision in sample placement. The Grimm source also allows depth profiling due to its well-defined planar sputtering on the sample surface.4 Despite its success as an atomic emission source, the Grimm source has found limited application for mass spectrometric analysis because of two primary limitations. First, unlike photons, which undergo self-transport to their optical detection systems, ions require assisted transport from the plasma to a mass spectrometer sampling orifice. Second, the mass spectrometer vacuum system provides an interface problem not encountered with optical spectrometers. While the vacuum problem can be alleviated with the addition of a slide valve,5 the transport requirement can prove to be a more intricate task. The transfer efficiency is crucial to the success of the analysis since it has a direct influence on detection limits and temporal separation of ions. Understanding the source parameters (i.e., operating conditions and source configurations) that influence ion transfer can allow more efficient analyses. The combination of a GD ion source and a time-of-flight mass spectrometer (TOFMS) has been described previously.5-9 In addition to allowing fast acquisition and high transmission, TOF affords collection of the entire mass spectrum with every pulse extraction. Because the TOF detects ions in a rapid pulse sequence, a pulsed ionization source or gated introduction of ions from a continuous source is required. The GD in the present research is operated in a microsecond pulsed power mode, providing certain advantages compared to a continuous direct current (dc) GD ion source.10-12 The application of high, short-term power produces enhanced sputtering, atomi(4) Oxley, E. S.; Yang, C. L.; Harrison, W. W. J. Anal. At. Spectrom. 2000, 15, 1241. (5) Yang, C. L.; Mohill, M.; Harrison, W. W. J. Anal. At. Spectrom. 2000, 15, 1255. (6) Steiner, R. E.; Lewis, C. L.; King, F. L. Anal. Chem. 1997, 69, 1715. (7) Heintz, M. J.; Myers, D. P.; Mahoney, P. P.; Li, G. Q.; Hieftje, G. M. Appl. Spectrosc. 1995, 49, 45. (8) Hang, W.; Yang, P. Y.; Wang, X. R.; Yang, C. L.; Su, Y. X.; Huang, B. L. Rapid. Commun. Mass Spectrom. 1994, 8, 590. (9) Harrison, W. W.; Hang, W. J. Anal. At. Spectrom. 1996, 11, 835. (10) Yan, X. M.; Hang, W.; Smith, B. W.; Winefordner, J. D.; Harrison, W. W. J. Anal. At. Spectrom. 1998, 13, 1033. (11) Hang, W.; Walden, W. O.; Harrison, W. W. Anal. Chem. 1996, 68, 1148. 10.1021/ac0346398 CCC: $25.00
© 2003 American Chemical Society Published on Web 10/31/2003
Figure 1. Schematic diagram of the microsecond pulsed Grimmtype glow discharge time-of-flight mass spectrometer system.
zation, excitation, and ionization, in addition to reduced sample heating. Inherently, the pulsed mode decouples the two types of discharge species: (1) gaseous components (e.g., argon, trace water vapor, etc.) and (2) sputtered cathode material, allowing each to be studied independently. This temporal phenomenon affords time-resolved analyses not possible using a dc mode.13 Time-resolved separation permits preferential detection of sample analyte ions while discriminating against background discharge gas ions. The combination of a microsecond pulsed Grimm-type glow discharge source with a time-of-flight mass spectrometer is described. Emphasis is placed on diagnosing and improving Grimm-type GD-TOFMS transport limitations. Overcoming these limitations opens new GDMS applications (e.g., depth profiling) that are more difficult using a DIP-type GD. Operating conditions and sampling configurations that play a significant role in ion transport efficiency are described. EXPERIMENTAL SECTION Grimm-Type Glow Discharge Ion Source. A schematic of the GD-TOFMS system is shown in Figure 1. The source is a Grimm-type GD ion source, with the cathode block and anode machined from brass. These two pieces are isolated from one another by a Macor plate. The end of the cylindrical anode (4mm i.d.) extends to approximately 0.25 mm from the sample surface. The sample is placed on the cathode block and sealed via an O-ring. Depending on the specific experiment, a flat sampling ring or a gas-directing sleeve made of stainless steel is mounted behind the anode piece. The skimmer cone, also machined of stainless steel, has an orifice diameter of 0.7 mm and a total sampling distance (i.e., sample-to-cone orifice) of approximately 13 mm. All source components except the cathode block are maintained at ground potential. A 3.9 L s-1 mechanical pump (Model: RV12, BOC Edwards, Wilmington, MA) is used as an auxiliary pump for the source. Ultra-high-purity argon (99.9999%) was used as the plasma gas (The BOC Group Inc., (12) Yang, C. L.; Ingeneri, K.; Harrison, W. W. J. Anal. At. Spectrom. 1999, 14, 693. (13) Harrison, W. W.; Yang, C. L.; Oxley, E. S. Anal. Chem. 2001, 73, 458A.
Murray Hill, NJ). Because of the low duty cycle of the pulsed source, our source does not need cooling techniques as required for dc sources.14 Plasma gas flow rate was controlled via a needle valve (Model SS-22RS4, Whitey Co., Highland Heights, OH) and measured with a 0-500 mL/min gas flowmeter (Model: GFM171, Aalborg, Orangeburg, NY). The source pressure was measured with a Pirani gauge (Model: APG-M-NW16, BOC Edwards, Wilmington, MA) and shown on a needle gauge display (Model: AGD, BOC Edwards). Installation of a butterfly valve (Model: Speedivalve, BOC Edwards) in the vacuum line of the first stage allowed independent control of flow rate and pressure. A high-voltage pulsed power supply (Model: M3k-20, Instrument Research Co., Hanover, MD) allowed changes of the pulse width and pulse frequency. Voltage and current profiles of the plasma were displayed on a 500-MHz digital oscilloscope (Model: TDS-724D, Tektronix, Inc., Beaverton, OR). The profiles allow monitoring of peak voltage, current, and plasma stability. The average voltage and current values were read directly from the discharge power supply. Time-of-Flight Mass Spectrometer. The mass spectrometer is a commercial time-of-flight instrument (Renaissance, LECO Corp., St. Joseph, MI) arranged in an axial configuration with a 1.0-m total flight path. The instrument was developed for inductively coupled plasma mass spectrometric (ICPMS) measurements, but was modified to accept an in-house designed Grimmtype GD source by removing the torch and machining a new source mount. The software (LECO Renaissance version 1.16, LECO Corp.) allows control of mass spectrometer conditions as well as real-time monitoring of the pressure within three successive vacuum stages. The software also controls isolation valves that protect the high-vacuum system during sample interchange, providing sample turnaround times of
