Determination of ⁴⁰Ca⁺ in the Presence of ⁴⁰Ar⁺:  An Illustration of the Utility of Time-Gated Detection in Pulsed Glow Discharge Mass Spectrometry

Abstract

The glow discharge ionization source operated in the pulsed, or modulated, power mode affords a number of distinct advantages over its steady-state counterpart. It is well-known that pulsed plasma operation permits the application of higher instantaneous powers by allowing time for the sample to cool. This minimizes sample overheating while effecting higher sputtering yields and lower limits of detection. The presence of discrete time regimes affords the added advantage of temporal selectivity. Such selectivity allows the observation of analyte ions during a time regime in which their signal is at a maximum while that of electron ionized background species is declining. Significantly, time regimes are found when no background argon ion signals are observable but analyte ion signals remain. This means that discrimination against isobaric interferences arising from the discharge gas is possible. A prime example of the utility of this advantage arises in the determination of calcium with an argon glow discharge. Both the major argon and calcium isotopes are found at a nominal m/z of 40. Time-gated mass spectrometeric detection during the afterpeak time regime enables the ready determination of (40)Ca(+) in samples at the ppm level. A linear calibration curve is obtained that also demonstrates the elimination of the (40)Ar(+) signal from mass spectra obtained with either a dc or rf glow discharge ion source.

Pulsed radiofrequency glow discharge time of flight mass spectrometry for coated glass analysis

“…It has been previously described that discrimination against isobaric interferences arising from the discharge gas could be possible by selecting an appropriate time regime within the aerglow. 20 As collected in Fig. 1, the aerglow at m/z 40 ( 40 Ar + and 40 Ca + ) falls down earlier than for most sputtered analytes.…”

Millisecond pulsed radio frequency glow discharge time of flight mass spectrometry: Temporal and spatial variations in molecular energetics

“…This analytical advantage was demonstrated by determination of calcium with argon as discharge support gas, a task impractical for continuous plasmas because of the isobaric interference of 40 Ar ϩ [13]. However, the energetics of the pulsed plasma after the termination of the power can be lowered to the energy levels of metastable argon (11.55 and 11.62 eV), which is high enough to efficiently ionize calcium while too low to ionize argon and major background species such as H 2 O.…”

Characterization and applications of pulsed millisecond glow discharge sources

“…Illustrations of the analytical utility of pulsed glow discharge sources coupled with time-gated mass spectrometric detection include; 1) the temporal resolution between signals of ions arising from the discharge working gases and that from sputtered analyte 16,22 ; and 2) the determination of molecular weight and structure of organic molecules. 23 These examples rely on the temporal characteristics of the dynamic gas phase reactions that occur in these pulsed glow discharge sources.…”

“…20 advantage to be taken of these pulsed glow discharge sources. 13,17,22,36 It is well known that ionization in analytical glow discharge plasmas is dominated by two mechanisms; electron ionization and Penning ionization. 35 Upon plasma ignition, electrons accelerate away from the cathode surface and acquire kinetic energy as they approach the anodic end of the cathode darkspace.…”

“…Previous pulsed plasma research has demonstrated complete temporal discrimination of electron ionization and Penning ionization using time-gated techniques. 18,22 Current research has demonstrated that, by accessing these different temporal regions, concurrent molecular and structural information can be obtained from vapor introduced into a pulsed glow discharge ion source. 36,23 Appropriate timing of a laser sampling pulse to access each of these temporal regions will allow for solid molecular analysis, as well as elemental analysis.…”