Liquid chromatography tandem mass spectrometry (LC-MS/MS) combines the physical separation capabilities of liquid chromatography with the mass analysis capabilities of mass spectrophotometry. It has evolved into a widely used technology to perform routine tests in clinical laboratories. Historically, LC-MS/MS had been used primarily by research, pharmaceutical, or commercial laboratories. Advances in the technology, decreasing costs for basic systems, intelligible software, an increased number of published protocols and methods, and the release of Food and Drug Administration (FDA)-approved kits has enabled more clinical laboratories to pursue these instruments as viable clinical analyzers.
LC - MS/MS uses two mass filters arranged sequentially with a collision cell between them. The filters can be used in static or scanning mode to select a particular mass-to-charge ratio or range. In the middle collision cell, the precursor ions collide with gas molecules and are fragmented into smaller ions referred to as product ions. The resulting product ions are then separated and detected in a second stage of mass spectrometry.
Advantages of this methodology are that it has superior selectivity for many analytes since it recognizes them by at least two physical properties (their precursor and product ion mass). Clinical laboratories have also experienced situations in which manufacturers unexpectedly withdraw immunoassays from the market, leaving labs searching for alternate methods. LC - MS/MS also allows for versatility in enabling laboratories to offer new laboratory-developed tests for biomarkers or for newly approved medications before FDA-approved kits or immunoassays come on the market. In addition, the sensitivity of LC-MS/MS may allow lower limits of detection for some analytes compared to immunoassays and other methods. Cost is also another benefit; the system allows for identifying and quantifying several analytes of interest simultaneously which then lowers the cost per test.
One disadvantage of the system is that it requires a high level of technical expertise to develop and validate tests as well as troubleshoot the instruments. In addition, FDA’s recently released draft guidance document on regulating laboratory developed tests using this methodology could affect the majority of tests developed and performed on LC-MS/MS. Currently, laboratories develop and validate LC-MS/MS assays according to CLIA. Because these may be FDA-approved tests, they have additional analytical validation requirements including an evaluation of imprecision, accuracy, linearity, recovery, sensitivity, specificity, carry-over, ion-suppression, acceptable sample types/collection tubes/anticoagulants, storage/transport conditions, and the establishment of reference ranges. Determining the appropriate sample preparation, column choice, mobile phase, and selection of suitable internal standards are just a few of the many challenges faced during normal method development of an assay. Other issues are standardization. Variations between LC-MS/MS assays/ users can arise since LC-MS/MS assays are not standardized and commercial calibrators are not available at this time.
