A “full spectrum” analysis considers all the “peaks” within a spectrum. But, selective ion monitoring (SIM) only monitors selected peaks associated with a specific substance. Chemists assume that at a given retention time, a set of ions is characteristic of a certain compound. SIM is a fast and efficient analysis. SIM works best when the analyst has previous information about a sample or is only looking for a few specific substances. When the amount of information collected about the ions in a given gas chromatographic peak decreases, the sensitivity of the analysis increases. So, SIM analysis allows for a smaller quantity of a compound to be detected and measured. But the degree of certainty about the identity of that compound is reduced.