Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) instruments can seem complicated and it may be difficult to identify the specifications which lead to relevant performance benefits for the particular analytical task for which the instrument is intended.
This document presents some of the hardware incorporated into ICP-MS instruments and explains the relevance of these hardware features in real analytical situations.
Parameter: Sample Introduction System
Specification: The sample introduction system should comprise a low sample flow rate nebuliser (0.1 to 0.5mL/min), a chilled spray chamber and a wide-bore injector in the plasma torch.
Comment: These hardware features all act to reduce the sample matrix loading on the plasma, so maintaining a high plasma temperature. This in turn leads to increased sensitivity for elements having high first ionisation potentials, reduced levels of matrix polyatomic interferences and better tolerance to changes in matrix. Some ICP-MS systems operate at high sample flow rates and with plasma matrix tolerance compromised in the pursuit of higher sensitivity. For this reason, all sensitivity and interference level specifications should be demonstrated under a single set of operating conditions.
Parameter: Plasma RF Generator
Specification: The plasma RF generator should be an all solid state design, preferably operating at a frequency of 27.12MHz
Comment: Most modern ICP generators are based around solid state electronics. Valve-based generator designs are not only less reliable, they are also less stable and the regular replacement of the power valve adds considerably to system operating costs. 27.12MHz is acknowledged as the preferred frequency for ICP generators used in ICP-MS systems, as this frequency is known to give a higher plasma central channel temperature, so reducing interferences.
Parameter: Torchbox Position Control
Specification: The torchbox x, y and z-position should be under computer control using precise stepper motors with a minimum step size of 0.1mm. Flexible autotuning algorithms should be available to maximise sensitivity and minimise polyatomic ion formation.
Comment: Although most modern ICP-MS systems have stepper-motor control of torch-box position, some systems only have manual adjustment or do not permit routine adjustment of these parameters. This severely restricts the capability of the operator to make appropriate adjustments to optimize system parameters for a given application. More importantly, computer control and autotuning of torchbox position ensures that the small adjustments required after routine maintenance are carried out consistently, regardless of operator expertise.
Parameter: Sampling Interface
Specification: The sampling interface cones should be threaded for ease of fitting and removal and should be designed to resist clogging when analysing high levels of sample matrix. The cones should be constructed of Ni with an option for Pt tipped cones.
Comment: Screw-threaded cones are preferred, as they reduce the time and complexity of routine maintenance, as well as reducing the possibility of the cones seizing in position.
Parameter: Cool Plasma Operation
Specification: Where required, cool plasma operation should be achieved through the use of a grounded metal plate fixed in position between the load coil and plasma torch.
Comment: The use of cool plasma conditions is recognised as the most effective and simplest method for removal of plasma-based polyatomic interferences, to allow K, Ca and Fe to be measured at ppt and sub-ppt levels. Systems which do not utilize a grounded metal plate can only achieve this interference removal at very low plasma power, where matrix tolerance is compromised and sensitivity for elements with high first ionisation potentials is severely reduced. Use of a fixed metal plate ensures that interference removal can be achieved at high plasma powers (900W), so permitting the analysis of high matrix samples such as viscous acids, complex organic materials and mixed acid etch solutions. A moving metal plate is not acceptable, due to the poor positioning reproducibility of such devices.
Parameter: Ion Lens System
Specification: A compound ion lens system, which gives high transmission at all masses and offers flexible tuning options, is preferred. All ion lens settings should be under computer control and comprehensive auto-tuning routines must be available.
Comment: Most modern ICP-MS systems use an ion lens design which is much more efficient at transmitting heavy masses than light masses. As a consequence, most ICP-MS systems exhibit relatively poor sensitivity at low mass. A well designed compound ion lens system can transmit low mass ions equally efficiently, giving a "mass response curve" that is essentially flat. This improves signal to noise performance for light elements, makes semi-quantitative analysis more accurate and reduces mass bias. A compound ion lens system is preferred over a single lens with variable voltage settings, as the single lens design is known to suffer from severe drift when matrix contamination of the lens surface occurs.
Parameter: Quadrupole Mass Analyser
Specification: A high quality quadrupole is required, preferably with hyperbolic profile rods.
Comment: A large part of the cost of a high specification ICP-MS system is in the quadrupole mass analyser itself. High quality materials and complex manufacturing procedures are required to ensure the best transmission, peak shape and peak separation. Quadrupoles can be manufactured much more cheaply, either by the use of round metal rods or by using metal plated ceramic rods. These designs exhibit comparatively poorer performance, sometimes to the extent that the resolution of the quadrupole may need to be adjusted on a per-mass basis, to achieve adequate peak separation in cases where a low intensity peak must be measured next to a high intensity peak.
Parameter: Dual Mode Detector
Specification: A detector which operates in both pulse-counting and analog modes is required, to cover the range of concentrations typically measured by ICP-MS. The two signals should be acquired simultaneously, with no compromise to acquisition speed, whichever mode is selected.
Comment: While simultaneous dual mode detectors are accepted as the best method to extend the measurement range of ICP-MS from ppt to ppm levels, most such detectors have amplifier circuits that operate relatively slowly. As a consequence, the acquisition speed may be reduced when the analog mode is used, leading to compromises in the capability to measure rapid transient signals, such as those produced from laser ablation and certain types of chromatographic separations.
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