What Is a Liquid Chromatography Mass Spectrometer?

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  • Written By: E.A. Sanker
  • Edited By: Susan Barwick
  • Last Modified Date: 06 November 2019
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A liquid chromatography mass spectrometer (LC-MS) is a device used in analytical chemistry to determine the content of a sample substance. It does this by combining two different techniques: liquid chromatography and mass spectrometry. The first of these techniques, liquid chromatography, is used to separate the sample into its chemical components. Mass spectrometry is then used to gather information about the mass of each component.The liquid chromatography mass spectrometer can give highly sensitive readings and has uses in many different fields, including pharmaceutical research and forensic toxicology.

Liquid chromatography is the first technique employed by the liquid chromatography mass spectrometer. The sample solution, which is in a liquid phase, is drained through a column containing a stationary phase substance — for example, a porous solid. Since the different types of particles in the sample solution pass through the stationary phase at different rates, they separate into bands.

Separation in the column can occur in a number of ways. If the molecules of one substance are larger than the molecules of another, they take longer to drain through a porous material and therefore separate from the smaller molecules, which move faster. Chemical factors can also affect the rate of flow. For example, polar molecules — those that have a partial electric charge — may be attracted to the charge of the stationary phase and become “sticky,” flowing less quickly through the column.


The liquid chromatography mass spectrometer passes each band of separated material through an ultraviolet light and detects its absorbance. In other words, it records how much light, and which wavelengths of light, the substance absorbs. This information can often be used to identify a substance. In cases where more data is necessary, particles can be further analyzed using the mass spectrometer portion of the device.

The particles that have been separated by type the liquid chromatography phase are separated by mass in the mass spectrometry phase. First, the particles are ionized, or charged,by removing electrons, giving them a positive overall charge. A magnetic field is then used to “bend” the path of the charged particle as it passes through the device. Particles with greater mass are not as easily moved as lighter, smaller particles, so they are not deflected as far. The mass spectrometer electrically records the amount of each type of particle that reaches the detector successfully.

The advantage of the liquid chromatography mass spectrometer is that it can record, with great sensitivity, the type and mass of each chemical component of a sample. It is often used to analyze complex organic samples in pharmacological research and in proteomics, the study of proteins. The liquid chromatography mass spectrometer has even been refined for use in forensic toxicology, to analyze and identify samples that would otherwise be too fragile to study.


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