Nominal mass instruments

In the absence of compound-specific fragmentation to drive selectivity, metabolite detection on nominal mass instruments becomes significantly more difficult. Typically, significant ambiguity exists in nominal MS data due to the presence of endogenous interferences, isobaric with metabolites, which... 

Qualitatively, the spark source mass spectrum is relatively simple and easy to interpret. Most instrumentation has been designed to operate with a mass resolution Al/dM of about 1500. For example, at mass M= 60 a difference of 0.04 amu can be resolved. This is sufficient for the separation of most hydrocarbons from metals of the same nominal mass and for precise mass determinations to identify most species. Each exposure, as described earlier and shown in Figure 2, covers the mass range from Be to U, with the elemental isotopic patterns clearly resolved for positive identification. 

Electrostatic Analyzer In magnetic-sector instruments, an electrostatic sector can be incorporated either before or after the magnet to provide energy resolution and directional focusing of the ion beam. The resolution achievable in these double-focusing instruments is sufficient to separate ions having the same nominal mass (e.g., 28 Daltons) but with different chemical formula (e.g., N2 and CO). 

Note Mass accuracy is highly dependent on many parameters such as resolving power, scan rate, scanning method, signal-to-noise ratio of the peaks, peak shapes, overlap of isotopic peaks at same nominal mass, mass difference between adjacent reference peaks etc. An error of 5 mmu for routine applications is a conservative estimate and thus the experimental accurate mass should lie within this error range independent of the ionization method and the instrument used. [37] There is no reason that the correct (expected) composition has to be the composition with the smallest error. 

The standard mass analyzer of ICP-MS is still the quadrupole. He allows the resolution of nominal mass units clown to 0.2-0.5 mass units and is therefore a low-resolution device. The performance of all ICP-MS instruments is limited by the transmission of the interface and mass analyzer unit, the background count rate clue to photons and the remaining gas pressure and the background count rate caused by molecular ions or doubly charged ions. Typical quadrupole instruments offer instrumental background count rates of 10 cps, newer instruments with an off-axis quadrupole show less than 1 cps like high-resolution instruments. 

Lemire et al. (32) reported a quantitative method for the determination of IV-ethyl- and A-methyldi-ethanolamine, the hydrolysis products of HN-1 and HN-2, in urine using LC/ESI/MS/MS on a triple sector quadrupole instrument. The analytes were concentrated from urine by SPE on a strong cation exchanger. In order to obtain good peak shapes, 73% 3mM ammonium hydroxide (pH 10.5) - 27 % methanol was used as the mobile phase for LC. Isotope dilution ([13C]4-/V-Me and N-Et diethanolamines) was used to compensate for inherent variabilities. Detection was by MRM, monitoring the transition MH+ —> [MH-H20]+ for each analyte. The limits of detection were 0.4 ng/ml for V-ethyldiethanolaminc and 1 ng/ml for N-methyldiethanolamine. An interferent was present in urine with similar retention time and nominal mass characteristics as /V-mclhyldiclhanolaminc. 

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.

Isotopes of different elements that form ions of the same nominal mass-to-charge ratio are not resolved by the quadrupole mass spectrometer, and cause isobaric elemental interferences. Typically, ICP-MS instrument operating software will have all known isobaric interferences entered, and will perform necessary correction calculations automatically. However, increasing the spectrometer resolution power from m/Am=400 to 1000 minimises abundance sensitivity interferences. 

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