Mass spectrometry signal intensity

Jeong, S. H., Borisov, O.V.,Yoo,J. H., Mao, X. L., and Russo, R. E. (1999). Effects of particle size distribution on inductively coupled plasma mass spectrometry signal intensity during laser ablation of glass samples. Anal. Chem. 71(22), 5123. 

Compared to other analytical techniques, NMR is quite insensitive. For molecules of the size of most drugs and natural products (100-600 Da), about a milligram of pure material is required, compared to less than 1 fig for mass spectrometry. The intensity of NMR signals is directly proportional to concentration, so NMR sees all and tells all, even... 

In Laser Ionization Mass Spectrometry (LIMS, also LAMMA, LAMMS, and LIMA), a vacuum-compatible solid sample is irradiated with short pulses ("10 ns) of ultraviolet laser light. The laser pulse vaporizes a microvolume of material, and a fraction of the vaporized species are ionized and accelerated into a time-of-flight mass spectrometer which measures the signal intensity of the mass-separated ions. The instrument acquires a complete mass spectrum, typically covering the range 0— 250 atomic mass units (amu), with each laser pulse. A survey analysis of the material is performed in this way. The relative intensities of the signals can be converted to concentrations with the use of appropriate standards, and quantitative or semi-quantitative analyses are possible with the use of such standards. 

From a mass spectrometry perspective, the pump must be pulse free, i.e. it must deliver the mobile phase at a constant flow rate. Pulsing of the flow causes the total ion current (TIC) trace (see Chapter 3) - the primary piece of information used for spectral analysis - to show increases in signal intensity when analytes are not being eluted and this makes interpretation more difficult. 

Quantitation using mass spectrometry is no different to quantitation using other techniques and, as discussed above in Section 2.5, involves the comparison of the intensity of a signal generated by an analyte in a sample to be determined with that obtained from standards containing known amounts/concentrations of that analyte. 

Different isotopes differ in their atomic masses. The intensities of the signals from different isotopic ions allow isotopic abundances to be determined with high accuracy. Mass spectrometry reveals that the isotopic abundances in elemental samples from different sources have slightly different values. Isotopic ratios vary because isotopes with different masses have slightly different properties for example, they move at slightly different speeds. These differences have tiny effects at the level of parts per ten thousand (0.0001). The effects are too small to appear as variations In the elemental molar masses. Nevertheless, high-precision mass spectrometry can measure relative abundances of isotopes to around 1 part in 100,000. 

Sugiura Y, Shimma S, Setou M. Thin sectioning improves the peak intensity and signal-to-noise ratio in direct tissue mass spectrometry. J. Mass. Spectrom. Soc. Jpn. 2006 54 45-48. 

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