Liquid Chromatography-Mass Spectrometry (LC-MS)

Each analytical method has its strengths and limitations. The combination of analytical techniques can not only complement each other's strengths...

Liquid Chromatography-Mass Spectrometry (LC-MS)

Each analytical method has its strengths and limitations. The combination of analytical techniques can not only complement each other's strengths, but also has a synergistic effect to obtain certain functions that the two techniques do not have when used alone. Chromatography-mass spectrometry technology is one of the most important separation and analysis methods in modern times. Gas chromatography-mass spectrometry (GC-MS) developed earlier and the technology is more mature. The GC/MS is the earliest commercialized combination instrument, suitable for the analysis of small molecular weight, volatile, thermally stable, and vaporizable compounds. The spectrum obtained by electron bombardment (EI) can be compared with the standard library. Liquid chromatography-mass spectrometry (LC-MS) can separate and analyze large molecular weight, thermally unstable, non-volatile, and polar compounds. There is no commercial spectrum library to compare and query, you can only build your own library or analyze the spectrum by yourself. Therefore, LC-MS has a broader application prospect.

LC-MS is a conventional sample analysis technique, which uses liquid chromatography (LC) as the separation system and mass spectrometry (MS) as the detection system. The sample is separated from the mobile phase in the MS part, and after being ionized, the ion fragments are separated by mass by the MS mass analyzer, and the mass spectrum is obtained by the detector.LC-MS combines the high separation capability of LC with the high selectivity and high sensitivity of MS, making up for the shortcomings of traditional liquid phase detectors.

Advantages of LC-MS

  • Fast analysis time, large throughput, and wide analysis range: LC-MS has the characteristics of fast and large throughput, and it can detect almost all compounds. For example, dozens of test indicators during neonatal disease screening can get semi-quantitative results at one time.
  • Strong separation ability and low detection limit: Even if the analyzed mixture is not completely separated on the chromatogram, the characteristic ion mass chromatogram of the high-sensitivity MS can give their respective chromatograms for qualitative and quantitative analysis.
  • Strong specificity: LC-MS has strong specificity. For example, traditional laboratory methods can only detect the total amount of vitamin D, while using LC-MS, different types of vitamins such as VD2 and VD3 can be detected separately.
  • High degree of automation: HPLC-MS has a high degree of automation.

Classification of LC-MS

Commonly used LC-MS has two classification systems

  • According to the different ionization methods of mass spectrometry
  • The ionization methods of mass spectrometry include atmospheric pressure ionization (API), matrix-assisted laser desorption ionization (MALDI), and fast atom bombardment (FAB).

    API often uses quadrupole or ion trap mass analyzer, including electrospray ionization source (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI). ESI is suitable for the analysis of medium and high polarity compounds and is currently the most widely used ionization method in LC-MS. APCI is suitable for the analysis of low-polarity and medium-molecular-weight compounds. The range of compounds that APPI is suitable for analysis is similar to that of APCI and is a supplement to APCI.

    MALDI commonly used time of flight (TOF) as a mass analyzer, with high sensitivity and simple sample preparation, has been widely used for the analysis of proteins, peptides, nucleotides, polysaccharides and synthetic polymers.

    FAB has unique advantages for the analysis of thermally unstable and non-volatile compounds, especially for the analysis of peptides and proteins.

  • According to the mass analyzer of the mass spectrometer
  • Mass analyzers for mass spectrometry include quadrupoles, ion traps, time-of-flight and Fourier transform ion cyclotron resonance.

    The quadrupole mass analyzer makes the ions that meet specific conditions vibrate stably under the action of an alternating electric field, and reach the detector through the quadrupole. Unipolar quadrupole mass spectrometry is only used for primary mass spectrometry analysis to obtain molecular weight information of compounds. Triple quadrupole mass spectrometry can be used for secondary mass spectrometry analysis to obtain structural information such as the molecular weight and fragment ions of the compound.

    The ion trap mass analyzer has the function of multi-stage mass spectrometry, which is more advantageous for analyzing the structure of the compound, and its sensitivity is lower than that of the quadrupole mass analyzer.

    The time-of-flight mass analyzer realizes the separation according to the different flying speeds of ions with different mass-to-charge ratios, which causes the difference in drift time. Time-of-flight mass analyzers are often used in conjunction with MALDI, with fast scanning speed and wide analysis range.

    The Fourier Transform Ion Cyclotron Resonance Mass Analyzer has high resolution, accurate measurement results, and fast data acquisition. It can be connected with a variety of ionization methods and can be used for multi-stage mass spectrometry detection combined with MALDI and ESI. It can be used in biological sciences. Molecular research.

Application of LC-MS

With the improvement of LC-MS technology, LC-MS has been widely used in many fields such as medical research, food analysis and environmental analysis.

In medical research, it can be used for drug metabolism, pharmacokinetics, impurity analysis, and natural product analysis. In food analysis, it can be used to detect prohibited ingredients, harmful antibiotics, antibacterial drugs, hormones and pesticide residues. It can be used in the analysis of organic pollutants and pesticide residues in environmental analysis.

Reference

  1. Liu X.D., Liang Q.L., et al. The Application of Liquid Chromatography-mass Spectrometry in the Field of Medicine. Chinese Journal of Pharmaceutical Analysis, 2005, 1: 110-116.

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