CHARACTERIZATION OF A RADIOFREQUENCY GLOW-DISCHARGE TIME-OF-FLIGHT MASS-SPECTROMETER

A radio-frequency glow discharge/time-of-flight mass spectrometer (RFG D-TOFMS) has been developed by simple modification of the interface to an ICP-TOFMS. The work described here evaluates the interface and ope rating conditions of the RFGD-TOFMS. The ion optics which focus ions t oward the entrance of the TOFMS are the same as those used in the orig inal ICP-TOFMS instrument. By means of pin-shaped brass samples of var ied lengths, the sample-skimmer distance in the RFGD-TOFMS has been op timized at 4 mm. The discharge pressure and power have been found to b e optimal at 50-60 W and 0.3 Torr, respectively. The application of sm all negative potentials to the skimmer cone (extraction orifice) was f ound to improve signals marginally. However, higher negative potential s reduced both signal levels and resolving power. The skimmer potentia l also affects the final kinetic energy of the ions before their extra ction 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 a re 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 i nstruments.