Abstract: The invention comprises a method and apparatus for inductively-coupled plasma mass spectrometry which is particularly suitable for the determination of isotopic ratios.In order to improve the precision of such determinations, the nozzle-skimmer type interface of the mass spectrometer is arranged so that the ratio ##EQU1## is greater than 0.8 mB.sup.1/2, where d.sub.s is the diameter of the orifice in the skimming member, s is the spacing between the sampling member and the skiming member, and p.sub.1 is the pressure in the region between the sampling and skimming members. Although use of the invention slightly reduces the efficiency of the interface it results in more accurate isotopic ratio determinations.

Abstract: The invention comprises a mass spectrometer wherein a sample is ionized in a plasma (14), especially an inductively-coupled or microwave-induced plasma. Ions are sampled from the plasma (14) through an orifice (16) in a sampling member (15), a second orifice (37) in a hollow tapered member (19) and a third orifice (53) in a tubular electrode (43). The hollow tapered member (19) comprises a portion (35) both externally and internally tapered with an interior included angle greater than 60.degree., and preferably a shorter externally tapered portion (38) with an external included angle of less than 60.degree.. A tubular extraction electrode 43, preferably comprising a conical end-portion (47) , is disposed within the member (19) for efficiently transmitting the ions into a mass analyzer.