RF-pulsed glow discharge time-of-flight mass spectrometry for glass analysis: investigation of the ion source design.

Radiofrequency (RF) millisecond pulsed glow discharge (PGD) coupled to time-of-flight mass spectrometry (TOFMS) was investigated for direct elemental analysis of glass samples. Aiming at achieving highest elemental sensitivity, appropriate discrimination from polyatomics, and good crater shapes on glasses, a new Grimm-type GD chamber (termed from now "UNIOVI GD", designed and constructed in our laboratory) was coupled to TOFMS, and the results compared with those obtained with the former GD design (here denominated as "GD.1") of the initial RF-PGD-TOFMS prototype. The critical differences distinguishing the two GDs under scrutiny are the GD chamber thickness (15.5 mm for the GD.1 and 7 mm for the UNIOVI GD) and the "flow tube" which is inserted in the GD.1 and inexistent in UNIOVI GD. A pulse period of 4 ms and a duty cycle of 50% were selected for the PGD studies. In order to characterizing the UNIOVI GD, the effect of RF-PGD experimental conditions (pressure and power) on signal shapes along the pulse was first investigated. Then, the analytical performance achieved was compared with results obtained using the GD.1 chamber. Results show that the UNIOVI GD source induced less thermal stress on the glass specimens. Consequently higher GD power can be applied to the UNIOVI GD (up to at least 130 W), as compared to the GD.1 (maximum of 60 W), without glass sample breakdown. Also, better crater shapes on samples were obtained using the UNIOVI GD. Moreover, the detection limits attained with the UNIOVI GD (in the range of μg g(-1)) were in most cases about one order of magnitude better for the analytes investigated (B, Na, Al, Si, Mn, Ni, Co, Cu, Zn, As, Sr, Ag, In, Sn, Sb, Ba, Pb and Bi), than those observed using the GD.1.