Characteristic of direct current helium glow discharge mass spectrometry

We examined the basic characteristics of the mass analysis method using DC helium glow discharge. As a result of investigating the relative intensitie s of the ions generated, we found that the ratios of doubly charged ions (H e2+) and diatomic molecule ions (He-2(+)) to He+ were 100 : 0.4 : 170. Thes e ratios differed significantly from those in Ar glow discharge and thus in dicated the generation of excessive He-2(+). We also detected up to He-5(+) of polyatomic ions. SEM observations of the discharge traces of He glow di scharge and Ar glow discharge revealed that the trace of Ar glow discharge exhibited a rough and highly uneven surface as if the specimen had been pee led off. In contrast, trace of He glow discharge showed few irregularities, and specimen particles seemed to be removed as if a wall had collapsed. Fu rthermore, because He glow discharge produces a slower rate of spattering t han Ar glow discharge, it took a long time (120 min or more) in the prelimi nary discharge to remove the stains from the surface. We found that the sur face of the specimen must be cleaned in the preparation stage using a grind ing method equivalent to buff grinding with alumina paste. While investigat ing the spectrum interference, we measured C at m/z13, which revealed the e xistence of interference by cluster molecular ions of (CH+)-C-12-H-1 for up to 90 min after the start of discharge. The ion intensity ratios of light elements in He glow discharge were tens to hundreds of times greater than t hose in Ar glow discharge. The relative sensitivity factor of C (RSFC, Fe) obtained using a standard ferrous specimen was 0.263 in He glow discharge, but it was 3.309 in Ar glow discharge. We thus succeeded in achieving a hig h sensitivity to light elements.