Gas chromatography–mass spectrometry
In selected ion monitoring (SIM), the instrument method focuses on certain ion fragments. Only those mass fragments are detected by the mass spectrometer.  The advantages of SIM are that the detection limit is lower since the instrument is only looking at a small number of fragments (e.g. three fragments) during each scan.  More scans can take place each second.  Since only a few mass fragments of interest are being monitored, matrix interferences are typically lower.  To improve the chances of reading a positive result correctly, the ion ratios of the various mass fragments are comparable to a known reference standard.