Question

Differentiate between total ion chromatogram (TIC) and extracted ion chromatogram (EIC). Why quantification of analytes is carried out through integration of EIC?

Selected ion monitoring (SIM) is often used in determination of analytes at trace levels, (very low concentrations) provide a brief explanation why one obtains lower detection lim-its with SIM than SCAN.

Discuss the need for rapid scan or MIM data acquisition with mass spectrometers when interfaced with chromatography techniques.

Answer 1

Total ion current (TIC) chromatogram gives the sum of the masses of all the ions (including those arising from the background) over the entire range of masses being detected. Extracted ion current (EIC) chromatogram gives all possible ions belonging to a particular m/z value representing a particular analyte of interest. EIC gives a more correct estimate of the m/z value of the analyte of interest since EIC peaks are less susceptible to being masked by ions having clos m/z values.

Quantitation of analytes is carried out through integration of EIC to avoid interferences from background and ions having masses close to the selected m/z values. EIC peaks are cleaner and easier to work with since they deal with only a particular analyte of interest.

Selected ion monitoring is more specific than EIC because only one peak corresponding to one analyte is obtained. EIC works with a range of ions over a selected small m/z range whereas SIM is more specific. Due to the high specificity of SIM mode, SIM mode can give lower detection limits than SCAN mode. The SCAN mode usually represents TIC and gives the sum of all ions in the sample including background interferences. What essentially happens in SCAN mode is that analytes having a lower concentration are masked by those having higher concentrations and hence an analyte having a low mass will possibly not be detected in SCAN mode.