The resolution of high-resolution instruments can be changed by adjusting the width of the entrance and exit slits into the spectrometer. Typical HR-ICP-MS instruments have
resolving powers up to 10,000 and are typically operated at preset resolution settings for low, medium or high-resolution to make their operation easier for the user. As we can see from Table 1, the use of HR-ICP-MS will solve many, but not all interference problems.
High resolution instruments also have several limitations. First of all, they typically cost 2-3 times that of a quadrupole ICP-MS instrument. They are also more complex to operate and maintain. In addition, for every 10-fold increase in resolving power, there is a concomitant decrease in signal intensity. This may limit the actual detection capabilities if the concentration of the analyte of interest is very low. Finally, they are much slower than a quadrupole system. Due to the longer settling times required by the magnet when the voltages are adjusted for large mass jumps, HR-ICP-MS instruments typically are 4-5 times slower than a quadrupole instrument. This makes them unsuitable for the rapid, high-throughput, multielemental analyses that are routine in production-type laboratories. They are also not the instrument of choice for transient signal analysis, including those obtained using Laser Ablation techniques for elemental profiling or chromatographic separations as their scan speeds are too slow to look at more than 1-3 elements of similar mass in an analysis. As a result, this type of instrument is generally found in research institutions and in laboratories with highly specialized needs for a low number of samples.
A second type of HR-ICP-MS instrument is also available that uses multiple detectors – this type is called a Multi-Collector HR-ICP-MS or MC-ICP-MS. These instruments are generally designed and developed for the purpose of performing high-precision isotope ratio analyses. Since an array of 5-10 detectors can be positioned around the exit slit of a double-focusing system, the isotopes of a single element can generally all be determined simultaneously, leading to the technique’s high-precision. The disadvantage of this type of system is that the isotopes must all be in a narrow mass range (± 15-20% of the nominal mass) as the magnetic sector settings remained fixed while only the electric sector settings are scanned. This generally means that each elemental isotopic system must be measured in a separate analysis. This type of instrument is generally not suitable for routine multi-elemental analysis for major and minor constituents and is typically only used for performing isotope ratio measurements.
Once the ions have been separated by their mass-to-charge ratio, they must then be detected or counted by a suitable detector. The fundamental purpose of the detector is to translate the number of ions striking the detector into an electrical signal that can be measured and related to the number of atoms of that element in the sample via the use of calibration standards. Most detectors use a high negative voltage on the front surface of the detector to attract the positively charged ions to the detector. Once the ion hits the active surface of the detector, a number of electrons is released which then strike the next surface of the detector, amplifying the signal. In the past several years, the channel electron multiplier (CEM), which was used on earlier ICP-MS instruments, has been replaced with discrete dynode type detectors (see Figure 6). Discrete dynode detectors generally have wider linear dynamic ranges than CEMs, which is important in ICP-MS as the concentrations analyzed may vary from sub-ppt to high ppm. The discrete dynode
type detector can also be run in two modes, pulse-counting and analog, which further extends the instrument’s linear range and can be used to protect the detector from excessively high signals.