Atom density decreases as the black fades to orange. (a) dc mode. The device consists of a negative cathode and a grounded anode immersed in a low-pressure rare gas, usually argon. A potential (V) breaks down the discharge fill gas, yielding Ar + that is attracted to the cathode-which, in this case, is the sample. The ions collisionally sputter neutral atoms from the surface into the adjacent GD plasma. As these sputtered atoms diffuse through the GD plasma, collisions with electrons, ions, and metastable atoms excite and ionize the sample atoms. (b) Pulsed mode operates on the same principles described for dc mode, but in short, repetitive dc pulses.
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.
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.
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