Although ICP-MS offers a powerful multielement analytical method, the mass spectral interference due to argon often becomes a serious problem. For example,
40Ar
16O
+,
40Ar
35Cl
+ and
40Ar
2+ interfere with the determination of
56Fe
+,
75As
+ and
80Se
+, respectively. Therefore, we replaced argon by helium to develop an interference-free ICP-MS, and successfully detected iron and arsenic in aqueous solutions. Some weak background peaks, however, were still generated at
m/z 79∼82 and disturbed the determination of traces of bromine (
79Br
+) and selenium (
80Se
+). In the present work, it was found that these peaks originated from the copper interface by a secondary discharge. They were assigned to
63Cu
16O
+,
63Cu
16OH
+,
65Cu
16O
+ and
65Cu
16OH
+. The nickel interface caused more serious background at
m/z 50∼100. On the other hand, no spectral interference was observed over a wide
m/z range when the interface was constructed from aluminum. The optimized helium ICP-MS was used for the determination of traces of bromine and selenium in water. A 5 μl volume of a sample was placed and heated electrothermally on a tungsten filament. The resulting vapor was introduced into the helium plasma for a mass spectrometric analysis. The relative standard deviation of the signals (10 ng ml
-1,
n=10) was approximately 10%. The detection limits based on 3σ were 0.2 ng ml
-1 for Br and 0.09 ng ml
-1 for Se, which were lower compared with the conventional argon ICP-MS (Br 20 ng ml
-1, Se 0.25 ng ml
-1).
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