liquid chromatography-mass spectrometry
/ LIK-wid kroh-muh-TOG-ruh-fee mass spek-TROM-eh-tree /
Imagine sorting a tangle of dissolved ingredients by gently rinsing them through a packed tube with liquid, so they leave one at a time, and then handing each one to a scale that also reads its barcode. That is the idea behind liquid chromatography-mass spectrometry: separate in liquid, then weigh and identify.
In detail, liquid chromatography (LC, usually high-performance HPLC) pushes a dissolved sample through a column in a liquid mobile phase, separating compounds by how strongly they cling to the packing. The flow then enters a mass spectrometer, most often through electrospray ionisation, which turns dissolved molecules into gas-phase ions gently enough to keep them intact, and the MS records each one's mass.
LC-MS is the workhorse for the large, polar, and heat-sensitive molecules that GC-MS cannot handle — drugs and their breakdown products, peptides, proteins, vitamins, pesticides in food. Its main caveats are that the liquid spray makes ionisation sensitive to the sample matrix (signals can be suppressed or enhanced by co-eluting junk), and the instruments are costly to buy and run.
A clinical lab measures a patient's blood level of an immunosuppressant drug on an LC-MS: the column separates the drug from blood components, electrospray ionises it, and the mass spectrometer quantifies it down to a few nanograms per millilitre to guide the dose.
Large, water-loving molecules that GC cannot vaporise are LC-MS territory.
LC-MS and GC-MS are complements, not rivals: the choice hinges on whether the analyte can be cleanly vaporised (GC-MS) or is large/polar/heat-sensitive and better kept in solution (LC-MS).