Resolution quadrupoles

Resolution Quadrupole instruments are not capable of achieving the high resolution that is common with double-focusing magnetic-sector instruments. In GC/MS analyses, a compromise is struck between sensitivity (ion transmission) and mass resolution. In the quadrupole instrument, the resolution is set to the lowest possible value commensurate with resolving peaks differing by 1 Dalton (unit resolution). 

These comments apply to the running of a low resolution quadrupole ICP-MS, capable of both solution and laser ablation analysis, since this is the most widespread form of the instrument. High resolution MC ICP-MS is, as they say, a whole other ball game ... 

As an alternative to commercial quadrupole based ICP-MS measurements at low mass resolution ( / 300), in 1996 Yiang and Douglas15 proposed a quadrupole ICP-MS system which allows a maximum mass resolution of 9000. At a mass resolution of 5000 the sensitivity was comparable to that of a commercial double-focusing sector field ICP-MS operated at the same mass resolution. Due to the very high continuum instrumental background of about lOOOcps no commercial high resolution quadrupole instrumentation with an inductively coupled plasma source exists and the development of high resolution quadrupole based ICP-MS has ceased. 

Structural information about toxaphene components can be obtained by gas chromatography coupled to electron ionization mass spectrometry (GC/EI-MS), positive ion chemical ionization mass spectrometry (GC/PICI-MS) or negative ion chemical ionization MS(GC/NICI-MS). Mass separation is performed by low resolution quadrupole or high resolution magnetic field instruments, by ion trap systems (GC/IT-MS), or by tandem mass spectrometry (MS/MS) offering a broad spectrum of possibihties. 

Date, A.R. Gray, A.L. Plasma source-mass spectrometry using an inductively coupled plasma and a high-resolution quadrupole mass filter. Analyst 1981, 106, 1255-1267. 

Unnatural Products Chemistry. The complete identification of unknown compounds that we have successfully resolved using PB/LC/MS will clearly require additional analytical information, such as provided via liquid chromatography ICP/MS (detecting nonmetals such as chlorine and sulfur), FT-IR, UV or proton and heteroatom NMR. This situation is analogous to that of a natural products chemist faced with making a complete structural assignment of an unknown compound isolated from some matrix such as seaweed instead of a leachate from a hazardous waste site. The natural products chemist would exploit the complete array of analytical instrumentation and not attempt identification based solely upon low resolution (quadrupole) mass spectrometry. 

Compounds with isobaric nominal masses can pose a problem in the analysis of libraries. For example, compounds Py-13 and Py-17 of library PYL-1 (see Section 17.7, Appendix) have the same nominal molecular mass. Using a low-resolution quadrupole mass spectrometer, there is no chance of differentiating between these compounds in the... 

Kostiainen R, Tuominen J, Luukkanen L, Taskinen J, Green BN. Accurate mass measurements of some glucuronide derivatives by electrospray low resolution quadrupole mass spectrometry. Rapid Commun Mass Spectrom 1997 11 283-285. 

However, an enhanced resolution quadrupole (Schoen 2001 Yang 2002) can in principle provide increased selectivity in quantitative analyses using SIM or MRM. An example in which this advantage is clearly evident is shown in Figure 6.12, where the SRM chromatogram obtained using a RP yielding a peak width (FWHM) of... 

Th for the precursor ion indicates interferences sufficiently serious that it is not clear where the analyte peak boundaries should be set in order to measure peak area in contrast, use of a peak width of 0.2 Th gave a much cleaner, well defined peak (Figure 6.12). This example is taken from an extensive examination (Jemal 2003a) of the performance of this enhanced resolution quadrupole system in the context of high throughput bioanalytical assays. While the increased selectivity was clearly established, it was also found that the enhanced resolution mode of operation requires considerably more attention... 

Selectivity, and its often necessary compromise with sensitivity, has previously been discussed mainly with respect to mass spectrometric detection (Section 6.2.1, also discussion of enhanced resolution quadrupoles in Section 6.4.2 especially Figure 6.12). However, for an integrated analytical method this is just one of the factors that determines the overall selectivity. The use of a control... 

ICP-MS is a good option for determining various radioisotopes with limits of detection comparable to radiometric methods in a shorter time, especially sensitive for the determination of isotopes with very long lives. However, low-resolution quadrupole ICP-MS is susceptible to isobaric interferences, e.g. " Pu/ " Am, Tc/ Ru, U/ Pu, molecular interferences, e.g. Pu/ UH, and spectral interferences, e.g. also high concentration of dissolved solids should be avoided in order... 

Table 2.45 Ion transmission for low and enhanced resolution quadrupoles at 0.2 Da mass peak width FWHM, 100% equals 0.7 Da FWHM (Jemal, 2003).

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