Multiple MRMs Improve Data Quality in LC/MS for Clinical Diagnostics: Highlights From ASMS 2018

Increasingly, liquid chromatography-mass spectrometry (LC/MS) is being used for clinical diagnostics since it provides qualitative and quantitative information. However, many small-molecule drugs have the same nominal mass. Plus, isomers are difficult to identify. 

The discriminating power of LC/MS can be improved by using LC/MS/MS with quadrupoles and collisional fragmentation at different energies. This produces fragment ions from breakup of the molecular ion. The fragments are also susceptible to fragmentation by the MS, which is called multiple reaction monitoring, or MRM.

Thus, isobaric compounds can produce isomeric intermediates, but the isomer often has unique fragmentation patterns that can be useful in qualitative identification. Traditionally, the compound has been identified based on the chromatographic retention time, molecular ion, and 2–3 primary fragments.

A poster from Shimadzu Corp. in Kyoto, Japan showed that recording 6–10 MRMs increased the discrimination power of LC/MS/MS significantly (Imoto, E.; Ashida, T. et al. “MRM Spectrum Mode and Library Searching for Enhanced Reporting Confidence in Forensic Toxicology,” ASMS Poster WP 261, ASMS 2018). The case in point involved a small library of 33 hypnotic drugs. Using standards, the library was filled out by retention time, formula, and MRMs at three collision energies (15, 30, and 45 V). This provided more MRMs. The authors report that 6–10 MRMs per analyte provided data permitting the unambiguous identification of each of the 33 target analytes in blood.

For quantitative analysis, whole blood was spiked with 33 drugs. Sample prep was a modified QuEChERS protocol covering the range from 0.5 to 1.0 ng/mL. Several of the compounds had low similarity scores based solely on their MS, but adding retention time brought the R2 to at least two 9s.

Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected]

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