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

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Abstract
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.