LC-MS Detection

The figure below provides a block diagram of a typical liquid chromatography-mass spectrometer (LS–MS) instrument. As illustrated here, a three component mixture is injected onto the HPLC. When component A elutes from the column, it enters the MS ion source and ionizes to form the parent ion, A+, and several fragment ions, x+, y+, and z+. The ions enter the mass analyzer, which separates them by their mass-to-charge ratio, providing the mass spectrum shown at the detector.


There are several options for monitoring the chromatogram when using a mass spectrometer as the detector. The most common method is to continuously scan the entire mass spectrum and report the total signal for all ions reaching the detector during each scan. This total ion scan provides universal detection for all analytes. As seen in the following chromatogram for the determination of riboflavin in urine, however, we can achieve some degree of selectivity by monitoring only specific mass-to-charge ratios, a process called selective-ion monitoring.


For this chromatogram, an initial parent ion with an m/z ratio of 377 enters a second mass spectrometer where it undergoes additional ionization; the resulting fragment ion with an m/z ratio of 243 provides the signal. The selectivity of this detector is evident when you compare the LC-MS chromatogram to those obtained using a fluorescence detector or an absorbance detector.


Data provided by Jason Schultz, Jonna Berry, Kaelene Lundstrom, and Dwight Stoll, Department of Chemistry, Gustavus Adolphus College.

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