Mass spectrometer resolution

Figure 4.13 Theoretical molecular-ion region of the mass spectrum of a molecule of composition C35H48NgOnS at a mass spectrometer resolution of 1500.

The resolution of most mass spectrometers (the ability to separate ions of similar m/z values - see Section 3.3 above) in routine use is sufficient to allow the separation of the ions containing the individual isotopes if low-molecular-weight compounds (<1000 Da) are being studied. This is illustrated in Figure 4.13 which shows the molecular-ion region of a compound having the molecular formula C35H48N8O11S determined with a mass spectrometer resolution of 1500. The masses of the isotopes present in this molecule are shown in Table 4.2. 

If, however, we consider a protein of modest size, such as aprotin with a molecular formula of C284H432N84O79S7 at a similar mass spectrometer resolution, the molecular-ion region of its mass spectrum, shown in Figure 4.14, does not show the individual isotopic contributions, a resolution around 5000 being required for these to be evident (Figure 4.15). 

Figure 4.17 Theoretical mass spectrum of the 7+ charge state of aprotin (molecular weight, 6507) at a mass spectrometer resolution of 5000.

Photolytically degraded PBO was re-examined by GC-EIMS at a higher mass spectrometer resolution (5000) with PFK in the mass spectrometer source as an internal mass calibrant. This procedure permits the accurate mass measurement of molecular and key fragment ions and hence the determination of their elemental compositions and fragmentation pathways. This information, along with the molecular weight information (GC-CIMS) and the low-resolution GC-EIMS spectrum, allows a structure to be proposed for each of the degradation compounds observed. 

FIGURE 11-11 Mattauch-Herzog-type double-focusing mass spectrometer. Resolution >10 has been achieved with instruments based on this design. 

This discussion emphasizes that non-zero values for A" usually indicate potential uncertainties which are best avoided, if possible, by diagnosing the cause and taking remedial action, e.g., if A" > 0 a more selective analytical method such as different cleanup and chromatography and/or increased mass spectrometer resolution or, if A" < 0, changing columns and/or silylation to deactivate appropriate portions of the chromatographic train etc. However, if none of these remedies improve the situation, assumption of the appUcabdity of the calibration parameters B" and A" to analysis of sample extracts gives the working relation Equation [8.67]. 

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