Fundamental Studies Related to Elemental Isotope Selection for Quantitative Analysis by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) with a Helium Collision Cell

Dec 01, 2012

Since the mid-1980s, ICP-MS instrumentation has been a rapid, sensitive, and robust platform for the elemental analysis of environmental water systems in both qualitative and quantitative determinations. Recent advances have included the use of collision cells inserted into the instrument geometry to increase the selectivity of the measurement by reducing polyatomic isobaric interferences via competing mechanisms described by the first principles of inelastic collisions. This study demonstrates a fundamental strategy to use the collision cell as a method development tool for isotope selection in the selected ion monitoring (SIM) operational mode for the quadrupole mass filter. The isotopic envelope of selenium (Se) was selected for this evaluation.

Inductively coupled plasma–mass spectrometry (ICP-MS) has proven utility in environmental monitoring with the advent of recent advances in commercial instrumentation (1,2). ICP-MS has demonstrated advantages over competing elemental analysis techniques, including better method detection limits, isotopic identification, and low level detection limits as an artifact of the unit resolution operation of modern quadrupole mass analyzers (3,4).

The ICP-MS instrument geometry consists of a region at atmospheric pressure where a sample is continuously introduced via a peristaltic pump with a flow rate on the order of microliters per minute to a nebulizer that creates a fine mist of aerosol droplets amenable to ionization by argon plasma (5). The nebulizer is housed in a Peltier-cooled spray chamber that is chilled to ~2 °C to minimize the formation of oxides. The driving force for the ionization mechanism is the presence of an induced voltage supplied by metallic radio frequency (RF) coils bound to a quartz torch. The ions are entrained in an RF field emitting from an orifice and pulled into a region under vacuum. Commercial design solutions for this step typically use an arduous off-axis path to separate neutral components of the mixture as a means of improving the signal-to-noise ratio (S/N). The vacuum regime is sequential in instrument segments, stepping down from atmospheric pressure (ionization region), to 10-3 Torr (focusing region), and to 10-5 Torr (analyzing region). It is generally accepted that the analyzing region is optimized to favor a mean free path in which ions are focused and accelerated in the z direction of the Cartesian plane when conceptually visualizing the hyperbolic field focusing the ion path through the quadrupole rods in terms of xyz coordinates (6).

This article focuses on the optimization of parameters related to the analyzing region under vacuum. More specifically, parameters related to the separation of polyatomic isobaric interferences that may systematically bias dissolved elemental measurements related to matrix effects of complex environmental water systems. For this reason, digestion procedures and related sample preparation methods will not be discussed. The readers of this work are referred to US Environmental Protention Agency (EPA) Methods 200.8, 1638, 3015, 3050, and 3051 for strategies related to sample preparation. The separation event of interest is that of the gas-phase collision events that occur on-line as a precursor to mass analysis. Because chemical noise is mass selective — and always additive, suggesting falsely high measured determinations (7–9) —strategic isotope selection is critical in minimizing systematic measurement bias.