The SIM is an alternate and commonly used technique doing the Q-MS analysis. This technique lets the analyst monitor and discover specific m/z ions which are of interest to the investigator. This causes an increase in the sensitivity of the test as rather than scanning for m/z ions in a fixed time period, the analysts note m/z ions of their interest. If the ions are carefully selected and don’t have interfering background, then their sensitivity is quite improved.
The SIM is used quite commonly to confirm and quantify ions. The selected ions should be M+* or be a part of the fragment of this molecule. It is specified in different guidelines that the least number of ions which is monitored should range from 2 (AAFS/SOFT) to 3 (NCCLS/HHS-NLCP) ions. The guidelines also state that the intermolecular ratios should range within ±20% of the given standard reference. These methods should be validated so that other compounds should not be able to produce the same ion ratios and ion quantity in the given same time.
Internal standards (Deut rated) are commonly used while confirming the SIM method and its quantitation so that any variance that occurs between the samples can be sorted out during the analysis. Deut-rated compounds are those compounds in which hydrogen atoms are substituted with deuterium atoms which have neutron and proton both, thus being 1 mass unit heavier in weight as compared to the hydrogen atoms. Duet-rated compounds are able to fragment, ionize, extract and chromatograph ions in the same manner as the non duet-rated ones, thereby becoming the ideal internal standards. A typical duet-rated standard would have 3 molecules i.e. D3 but some an even have more molecules. For instance: methamphetamine-D14. The internal standard’s fragments have some or all deuterium molecules so that other molecules can be differentiated according to their weights. Quite typically, for SIM analysis, 2 ions of deut-rated standard are evaluated.
To confirm and quantify drugs and their metabolites, the commonly used mechanisms include GC-EI-Q-MS SIM techniques. Using instruments is more reliable and since, the techniques are reproducible, they are widely used in forensic toxicology now and in near future.