Validation of liquid chromatography mass spectrometry (LC-MS) methods
2.2. Selectivity: detector-side (MS) selectivity
What is so special about using MS as a detector in LC? MS adds a new dimension – m/z (mass/charge) – to the analytical signal. Without this additional dimension, we would just know whether some compound is eluting from the column or not. With the MS detector, the information will be more specific: whether the compound with a specific m/z is eluting from the column or not.
As an example, a total ion chromatogram (TIC) is presented in Figure 1. In this TIC, the intensities of all the ion abundances in the mass spectrum are summed, i.e. the discriminating power of a MS is not used. As a result, several bigger and smaller peaks, corresponding to different compounds, are observed in the chromatogram.
Figure 1. Total ion chromatogram (TIC) of Sudan I dye analysis.
When the additional provided by MS detection was employed, the chromatogram in Figure 2 was obtained. In the case of Figure 2, only the part of the MS signal corresponding to m/z 249 was monitored, resulting in an EIC – extracted ion chromatogram. As a result, chromatogram with just one peak, corresponding to Sudan I dye was obtained. This example demonstrates that MS can considerably enhance selectivity.
Figure 2. Extracted ion chromatogram (EIC) of Sudan I dye analysis. The m/z value 249 correspond to protonated Sudan I molecule [M+H]+.
What if compounds of identical or very close m/z are analysed? In this case there are three approaches. The best (and cheapest) would be the separation of the compounds chromatographically. If this is not possible, then tandem MS or high resolution MS can be used. In practical LC-MS analysis the first two approaches are routinely used: chromatographic separation is always attempted and as a rule all LC-MS analyses, at least in complex sample matrices, are carried out in tandem MS (also known as MS/MS or MS2) mode. High-resolution MS is seldom used in routine analysis.
