Single instrumental mass discrimination

Internal correction has the advantage of simultaneous (MC instmments) or quasi-simultaneous (single-collector instruments) correction, but in this case obviously the measured isotope ratio cannot be corrected by using the same isotope ratio. In ICP-MS the mass discrimination for a limited mass range (< 10 mass units) only depends on the mass difference. Other dependencies have not yet been observed. Thus the mass discrimination for a certain isotope ratio in the first approximation can be calculated from a different isotope ratio close to the target one. In practice another isotope ratio of the same element with a constant universal value is used or the sample... 

Currently PCR and mass spectrometry are performed by two separate instruments. However, there is no reason why PCR followed by simple automated cleanup and mass spectrometry cannot be incorporated into a single integrated instrument. Essentially every configuration of the modern ESI mass spectrometer has been used successfully for the analysis of PCR products, from the highest to the lowest resolution involving. Fourier transform ion cyclotron resonance (FTICR), triple quadrupole, quadrupole-time of flight (Q-TOF), and ion trap.22-24 MS discriminates between two structurally related PCR products by MW difference. Mass accuracy is needed to differentiate the... 

Figure 6 Theoretical isotope distributions of single- and double-charged gra-macidin-S. (a) The theoretical distribution of the single-charged molecular ion with nominal mass resolution, (b) The double-charged molecular ion with nominal mass resolution, (c) Discrimination of the double-charged isotopes is possible with higher instrument resolution.

Quadrupole analyzers have unit resolution throughout their mass range i.e one mass is discriminated from the next, be that 28 from 29 or 2,000 from 2,001. This classifies quadrupoles as low-resolution instruments which are poorly suited to obtain accurate mass data. A consequence of the poor resolution is that two ions with the same nominal mass but with different empirical formulae, and therefore different exact masses (isobaric ions), will not be separated, but instead, their masses will be averaged into the mJz observed value. Furthermore, the peaks are broad (-1 mIz wide), and thus it is difficult to obtain precise measurements of the centroids. The mass range of quadrupoles extends to 4 kDa for singly charged ions. These analyzers are compatible with all ionization methods except MALDI (because of the mass range limitation). 

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