High mass resolution requirements

In the ToF-SIMS positive ion spectrum the peaks characteristic of LDPE and PEMA appear but as the MA concentration increases some of the hydrocarbon peaks (eg CH3+, C5H9+, C6Hn+, C7H13+), initially with the relative intensities expected from LDPE, increase in intensity and EMA-specific ions, eg, C2H302, are detected (high mass resolution required). Two of these ions, normalized to the nonspecific C2H3+ ion, are plotted as a function of bulk MA content in Figure 23. 

In the case of the third requirement, instrument manufacturers have developed highly specialized Magnetic Sector mass filter-based instruments and time-of-flight mass filter-based instruments. Quadrupole mass filter-based SIMS instruments are ineffective in this area because of their inability to provide the high mass resolution required. Fourier Transform Ion Cyclotron Resonance (FT-ICR) as well as Orbitrap mass filter-based instruments, on the other hand, show significant promise. Mass filters are discussed in Section 4.3.2.I. 

Increases in mass resolution are not only accortpanied by a loss in ion transmission efficiency and hence signal intensity (Figure 2.15), but also by a change in peak shape from flat topped to triangular (Figure 2.14(b)).Moreover, the extremely narrow peaks at high mass resolution require an absolutely stable mass calibration (especially for isotope ratio applications). 

The choice of mass spectrometer for a particular analysis depends on the namre of the sample and the desired results. For low detection limits, high mass resolution, or stigmatic imaging, a magnetic sector-based instrument should be used. The analysis of dielectric materials (in many cases) or a need for ultrahigh depth resolution requires the use of a quadrupole instrument. 

As noted earlier, the development of the dual cell (37), tandem quadrupole-FTMS (46, 47) and external ionization cell (48, 49) has facilitated the coupling of FTMS and chromatographic methods. Advances in interfacing separation techniques with FTMS will be important in the analysis of mixtures, especially where high mass resolution is required. For example, liquid chromatographic introduction of mixtures isolated from biological systems directly into an FTMS for analysis would eliminate the need for laborious sample clean up. 

As quadrupole ICP-MS (Q-ICP-MS) usually has a mass resolution of approximately 300 m/Am it is advantageous to employ high resolution (HR) sector field (SF)-ICP-MS since it is without argument better suited for eliminating spectral interferences (m/Am < 9,000). Additionally, its enhanced sensitivity yields detection limits improved by a factor of 50-100. But even HR-ICP-MS does not allow the separation of all occurring interferences. An overview of possible spectral interferences on Pd isotopes and the mass resolution required for... 

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