LC / MS analysis features
Liquid chromatography-mass spectrometry (LC-MS)
Liquid chromatography-mass spectrometry (HPLC-MS), also known as liquid chromatography-mass spectrometry technology, which uses liquid chromatography as a separation system, mass spectrometry for the detection system. The sample is separated from the mobile phase in the mass spectrum and is ionized. The mass fraction of the ion fragments is mass-separated by a mass spectrometer, and the mass spectrum is obtained by the detector. LC / MS reflects the complementary advantages of chromatography and mass spectrometry. Combining the advantages of chromatographic separation of complex samples with high selectivity, high sensitivity, and the ability to provide relative molecular mass and structural information, the advantages of LC / MS in drug analysis, Many areas such as food analysis and environmental analysis have been widely used.
LC / MS classification
There are two main classification systems commonly used in liquid chromatography and mass spectrometry. One is from the perspective of mass spectrometry ion source, including ESI, APCI, APPI and MALDI; the other is from mass spectrometry mass spectrometer, including quadrupole, ion trap, time of flight (TOF) and Fourier transform (MS). Most of ESI, APCI and APPI ion sources are combined with quadrupole and ion trap mass spectrometry and are the most widely used LC / MS instruments.
From the ion source point of view, ESI suitable for high polarity compounds, especially suitable for reversed-phase liquid chromatography and mass spectrometry, is currently the most widely used in LC / MS ionization; due to the development of pneumatic Auxiliary spray, the liquid phase flow rate can be increased to 1mL • min-1; through the formation of multi-charge ions, molecular weight analysis can be expanded to about 30,000, can be used for the analysis of biological macromolecules (such as low-quality protein). The advantage of ESI is that it is a concentration detector that can be used regardless of sample size. The μESI and nESI technologies developed in recent years are particularly suitable for the high sensitivity analysis of trace samples. APCI uses corona discharge to ionize the analytes in the gas phase, thus requiring the analyte to have a certain volatility. It is most suitable for middle and low-polarity, medium molecular weight compounds that are not easily multi-charged and have relatively simple spectral analysis. APPI is a technique that uses ionization of analytes in the gas phase at atmospheric pressure to ionize analytes. Its adaptation to APCI is complementary to APCI. MALDI is to add a sample to a matrix that strongly absorbs incident laser light and generate molecular or quasi-molecular ions for the sample by energy transfer. The usual practice is to formulate a sample of pmol into a matrix with a certain ratio of solution, A few microliters of this solution is placed on a stainless steel sample target and is evaporated to the mass ion source. The advantage of MALDI is that it is easy to measure high quality molecules in combination with TOF. Its high sensitivity and simple sample preparation have been widely used in the analysis of proteins, peptides, nucleotides, polysaccharides and synthetic polymers. However, due to the characteristics of MALDI itself, the application research of direct online and LC combination is still relatively few at present.

From the perspective of mass analyzer, quadrupole is in the role of alternating electric field, so that some of the ions that meet the requirements to reach the detector through the quadrupole; ion trap is the first ion concentration to the trap, by changing the electrical parameters The ions within the trap are released one by one to the detector. Single-stage quadrupole mass spectrometry is only used for the first-stage mass spectrometry analysis, while the third-stage quadrupole mass spectrometry can realize the second-stage mass spectrometry function. The first mass spectrometry gives the molecular weight of the compound, and the second mass spectrometry gives structural information such as the fragment ions of the compound by collision-induced dissociation (CID). Ion trap mass spectrometry with multi-level mass spectrometry function (generally can be done 5-11), especially with automatic multi-stage (Auto MS (n)) function, for the analysis of the compound structure is more favorable, reducing the requirements of the spectrum, However, the accuracy and resolution of the mass are less than those of the quadrupole mass analyzer. The triple quadrupole can also satisfy the general structural analysis function, but it is not suitable for the scanning analysis of a larger mass range due to the limitation of the scanning time. Quadrupole mass analyzers are typically 1-2 orders of magnitude more sensitive to ion traps for MS or MS / MS MRM and are therefore more suitable for quantification of trace or trace components analysis. Time-of-flight mass spectrometry is the use of different m / z ion flight speed is different, ion flight through the same path to reach the detector different time to obtain mass separation, it is often used in conjunction with MALDI, the advantage of scanning speed, wide range of quality analysis ; Fourier transform mass spectrometry is a new technology developed in recent ten years. Its working principle is different from the above-mentioned mass analyzers. This technique uses the Fast Fourier Transform method to convert the ion frequency signal to the mass spectrometer signal. The advantages of Fourier transform mass spectrometry Is high resolution, and sensitivity increases with the resolution.

LC / MS analysis features

In addition to its ability to analyze strongly polar, less volatile, thermally unstable compounds that can not be analyzed by gas chromatography-mass spectrometry (GC-MS), LC-MS offers the following advantages:
① wide range of analysis, MS can detect almost all compounds, easier to solve the problem of the analysis of thermally unstable compounds;
② separation ability, even if the mixture was not completely separated in the chromatographic analysis, but by MS's characteristic ion mass chromatograms can also be given their respective chromatograms for qualitative and quantitative;
③ qualitative analysis of the results are reliable, you can also give the molecular weight of each component and rich structural information;
④Low detection limit, MS with high sensitivity, through the selection of ions (SIM) detection, the detection capability can also be increased by more than an order of magnitude;
⑤ analysis time is fast, LC-MS column for the use of narrow-diameter column, shortening the analysis time and improve the separation effect;
⑧ high degree of automation, LC-MS has a high degree of automation.
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