Quantitation of a drug by gas chromatography-mass spectrometry (GC-MS) is typically performed in which mode?

Gas chromatography-mass spectrometry (GC-MS) is a powerful analytical technique used primarily for the quantification and identification of compounds, including drugs. In this context, use of selective ion monitoring is particularly advantageous. This technique allows the analyst to focus on specific ions (or fragments) that are characteristic of the target compound, which enhances sensitivity and specificity.

By monitoring only selected ions that are derived from the target analyte, the method becomes more efficient in differentiating the compound of interest from other substances present in the sample. This selective approach minimizes background noise and increases the reliability of the quantitation process, which is crucial for accurate drug analysis.

Total ion chromatography captures all ions generated during the mass spectrometric analysis, providing a complete ion profile of the sample but lacking the specificity needed for reliable quantitation of particular substances. Ion subtraction, while useful in certain contexts, isn't a standard mode for quantifying drugs in GC-MS. Selective reaction monitoring is more commonly associated with liquid chromatography-mass spectrometry (LC-MS) rather than with GC-MS, which further supports the use of selective ion monitoring as the correct mode for quantifying drugs in this specific analytical technique.