Hyphenated Techniques: GC-MS, LC-MS and Tandem Fragmentation
Joining a separation to a mass spectrometer gives every peak in the chromatogram its own spectrum. Selecting one ion and breaking it again gives a second dimension of evidence, which is what makes trace work in dirty samples possible.
What a learner can do afterwards
- Explains what an interface has to do to join a liquid separation to a vacuum instrument
- Distinguishes a total ion chromatogram from an extracted ion chromatogram and says when each is used
- Describes precursor selection and product scanning in a tandem experiment
- Explains why selected reaction monitoring gives such low detection limits in a complex matrix
1 · Read
Chromatography separates a mixture by making each component travel at its own speed. A mobile phase carries the sample through a stationary phase, and compounds that cling longer exit later. Each exit time marks one component, and the peak size measures its amount. Joining this separator to a mass spectrometer means every separated peak immediately yields its own spectrum.
Gas chromatography suits volatile compounds, while liquid chromatography handles fragile and polar ones. Each needs its own interface to the vacuum of the mass spectrometer: it must strip away the liquid or gas flow and deliver neutral molecules into the vacuum without destroying them.
Read the chromatogram two ways. The total ion signal shows everything at once, while the extracted ion signal shows one mass only, which reveals traces hidden under big peaks. Tandem operation adds a second stage: pick one precursor ion, break it apart, and scan the fragments. That pattern confirms identity even in dirty samples.
For trace work such as a pesticide in a food extract, watch only chosen precursor to fragment transitions. That selected reaction monitoring rejects noise, so detection limits drop sharply. The matrix stops interfering because two independent filters guard the result: retention time plus a characteristic transition, which matrix peaks almost never pass together.
Separate by travel speed, weigh each peak alone, and filter twice to beat the matrix.
2 · Watch
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.