Development and evaluation of a graphite injector for inductively coupled plasma mass spectrometry

Desolvated solution aerosols are injected into the axial channel of an inductively coupled plasma (ICP) through a graphite tube inserted directly into the plasma. This hot injector constricts the stream of analyte and prevents it from widening excessively as it travels through the plasma. Thus, the sampling orifice for the mass spectrometer (MS) can be positioned several millimeters downstream from the tip of the initial radiation zone without substantial loss of analyte ion signal. The signal ratio for LaO'^/La* can be reduced to 0.05% with conventional desolvation or 0.01% with cryogenic that yield maximum metal ion signal. These values are greatly superior to those obtained with conventional torch injectors. ^Anal. Chem. 1995, 67, 1929-1934 ^Corresponding author 25 The graphite injector also improves sensitivity for atomic ions by factors of 1.5 to 15, with the best improvements seen for elements like As and Zn that have high ionization energies. This modification to the plasma does not compromise other analytical figures of merit such as rinse out time or the suitability of a single set of operating conditions for multielement analysis. Introduction ICP-MS is a sensitive and selective method for trace multielement analysis. The ability of the ICP to efficiently atomize the sample is one dimension of the overall selectivity of the technique. Although atomic ions (IvT) are the major species seen under normal operating conditions, other ions such as MO"^, ArM*, ArO'^, and hr^* hamper the determination of several important elements. Corrections based on isotope ratios can often be done for moderate cases of spectral interference,^'^ but these procedures become suspect when the analyte signal is small relative to that for the interferant. This paper focuses on a new method to remove most of the interference from MO* ions. A variety of such measures have been demonstrated for low-resolution instruments based on quadrupole mass analyzers. These schemes include mixed gas plasmas^'® and introduction of the sample in as dry a form as 26 possible, i.e., by desolvation,^'''^° electrothermal vaporization'^"^^ or laser ablation.^ A double focussing mass spectrometer can separate MO* from M* ions at the same nominal m/z value. In general, reducing the slit widths to attain the resolution required (6000 to 15000) also reduces the transmission by at least a factor of ten.^^*''^ Measures that reduce the level of MO* ions in the plasma would be valuable for either low or high-resolution experiments. The fundamental reasons for the persistence of MO* ions have been discussed by several groups.Suppose an aqueous solution is dispersed into droplets with a conventional nebulizer. The larger droplets, and much of the water vapor, are then removed by a cooled spray chamber. The spray chamber still transmits wet droplets in a range of sizes to the plasma. As these droplets leave the torch injector and traverse the plasma, they are first desolvated, then the solutes are progressively vaporized, atomized and ionized. The axial positions where these processes take place overlap. The smaller droplets desolvate, vaporize, etc. relatively quickly, while the larger droplets persist much farther dovmstream.^^'^' Furthermore, the stream of droplets, and the resulting clouds of atoms and ions, spread out as they travel further downstream. In ICP-MS, the sampler-skimmer combination transmits ions essentially from a small zone (of approximately the same diameter as the skimmer hole) that is