Ion Kinetic Energy Determinations in Radio-Frequency Glow Discharge Mass Spectrometry

Radio-frequency glow discharge (rf-GD) sources produce an abundance of both atoms and ions. For the mass spectrometric application of the glow discharge technique, knowledge of the ion kinetic energies is required to optimize extraction and focusing of ions from the source region into the analyzer. This paper details kinetic energies experimentally determined with the use of the “retarding potential” method. For this study, the analyzer quadrupole of a double-quadrupole mass spectrometer was positively biased to act as a repeller. Ion kinetic energies (IKEs) determined for a variety of discharge and analyzer operating conditions ranged from 12.5 eV to 25.0 eV for 63Cu+. Kinetic energy measurements were confirmed from ion trajectory simulations and follow closely the experimental values for identical analyzer conditions and initial IKEs. Results of this study indicate that the conditions under which ions are formed (plasma conditions) affect IKEs and energy spreads to a greater extent than analyzer parameter variations. Different from atmospheric plasma sources, IKEs for rf-GD species do not vary as a function of ion mass/identity. Evidence is also given in support of a slight mass biasing owing to the transmission properties of double-quadrupole analyzers. The findings detailed herein demonstrate the effects of rf modulation on both ion kinetic energy values and distributions.

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Ion kinetic energy distributions and their relationship to fundamental plasma parameters in a radio frequency glow discharge source

Spectrochimica Acta Part B Atomic Spectroscopy ◽

10.1016/s0584-8547(97)00059-1 ◽

1997 ◽

Vol 52 (11) ◽

pp. 1627-1644 ◽

Cited By ~ 4

Author(s):

Steven J. Christopher ◽

R. Yuancai Ye ◽

Kenneth Marcus

Keyword(s):

Kinetic Energy ◽

Glow Discharge ◽

Radio Frequency ◽

Plasma Parameters ◽

Energy Distributions ◽

Radio Frequency Glow Discharge ◽

Discharge Source ◽

Ion Kinetic Energy ◽

Glow Discharge Source ◽

Kinetic Energy Distributions

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Characterization of a Radio-Frequency Glow Discharge/Time-of-Flight Mass Spectrometer

Applied Spectroscopy ◽

10.1366/0003702944028029 ◽

1994 ◽

Vol 48 (11) ◽

pp. 1337-1346 ◽

Cited By ~ 21

Author(s):

D. P. Myers ◽

M. J. Heintz ◽

P. P. Mahoney ◽

G. Li ◽

G. M. Hieftje

Keyword(s):

Kinetic Energy ◽

Glow Discharge ◽

Radio Frequency ◽

Mass Spectrometer ◽

Time Of Flight ◽

Detection Limits ◽

Operating Conditions ◽

Discharge Time ◽

Radio Frequency Glow Discharge ◽

Flight Mass Spectrometer

A radio-frequency glow discharge/time-of-flight mass spectrometer (RFGD-TOFMS) has been developed by simple modification of the interface to an ICP-TOFMS. The work described here evaluates the interface and operating conditions of the RFGD-TOFMS. The ion optics which focus ions toward the entrance of the TOFMS are the same as those used in the original ICP-TOFMS instrument. By means of pin-shaped brass samples of varied lengths, the sample-skimmer distance in the RFGD-TOFMS has been optimized at 4 mm. The discharge pressure and power have been found to be optimal at 50–60 W and 0.3 Torr, respectively. The application of small negative potentials to the skimmer cone (extraction orifice) was found to improve signals marginally. However, higher negative potentials reduced both signal levels and resolving power. The skimmer potential also affects the final kinetic energy of the ions before their extraction into the TOFMS. At 0.3 Torr all ions extracted for mass analysis have approximately the same kinetic energy, which can be estimated in the orthogonal TOFMS. Detection limits for several standard samples are at the single-ppm level, which is not unexpected; with the same ion-optical system, the current ICP-TOFMS also produces detection limits that are 2–3 orders of magnitude worse than those of many commercial instruments.

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