In the field of modern analytical chemistry, conventional chemical analysis is usually cumbersome and time-consuming. The use of chromatographic techniques can achieve effective separation of the mixture. In addition, mass detection has high sensitivity. The hyphenated technique of chromatography and mass spectrometry will be a good choice for mixture analysis. High performance liquid chromatography can achieve effective separation of mixtures, but often requires different mobile phases. And the mobile phases are all organic solvents, which have certain toxicity. Supercritical fluid chromatography generally uses carbon dioxide as the mobile phase, which is non-toxic. And by changing the temperature and pressure of the mobile phase carbon dioxide, the effective separation of the mixture can be achieved. Therefore, the supercritical fluid chromatography-mass spectrometry (SFC-MS) technology is an effective means to realize the separation of the mixture.
Supercritical fluid chromatography (SFC) refers to a chromatographic method that uses a supercritical fluid as the mobile phase and a solid adsorbent or an organic polymer bonded to the carrier (or capillary wall) as the stationary phase. Supercritical fluid refers to the fluid above the critical temperature and critical pressure. Its diffusion coefficient and viscosity are close to that of gas, so the mass transfer resistance of the solute is small, and the mixture can be separated quickly and efficiently. Its density and solubility are close to liquids, so substances with higher boiling points and poor thermal stability can be analyzed at lower temperatures. SFC has the advantages of gas chromatography (GC) and liquid chromatography (LC), and has good application prospects.
The common mobile phases of SFC include carbon dioxide, nitrous oxide, halogenated hydrocarbons, etc. in a supercritical state. Among them, carbon dioxide is the most commonly used. Because carbon dioxide is cheap and easy to obtain, non-toxic, recyclable, low critical temperature, moderate critical pressure, and can be miscible with various strong polar modifiers.
The chromatographic columns used in SFC include packed columns used in LC and capillary columns used in GC.
Gas chromatography and liquid chromatography detectors such as ultraviolet detector (UV), hydrogen flame ionization detector (FID), mass spectrometry detector (MS) can all be used as SFC detectors. The sensitivity of the UV detector is low, making the detection inaccurate. The hydrogen flame ionization detector does not allow the addition of modifiers in the mobile phase, which limits the application range of SFC. The mass spectrometer detector has very high sensitivity and is a more comprehensive universal detector. The hyphenation of MS and SFC has aroused widespread attention of researchers.
Mass spectrometry (MS) is a method that ionizes molecules into product ions and fragment ions, and separates them according to the mass-to-charge ratio (m/z), and is used for qualitative and quantitative analysis. Mass spectrometry is an important method for structural analysis of organic compounds. It can accurately determine the molecular weight of organics and provide molecular formula and other structural information. The detection sensitivity of mass spectrometry is much higher than other structural analysis methods such as infrared and nuclear magnetic.
SFC-MS is an efficient means of separation and detection. The earliest SFC-MS research began in the 1980s. In the initial development stage, researchers used capillary supercritical chromatography (cSFC) to couple to MS through electron ionization (EI) and chemical ionization (CI) under vacuum. cSFC-MS can analyze non-polar, high molecular weight compounds that were difficult to analyze in the past. After that, packed column supercritical chromatography (pSFC) showed good droplet formation ability in the process of using organic modifiers, and it was easy to evaporate the solvent in the ion source. Therefore, pSFC can be coupled with atmospheric pressure chemical ionization (APCI) sources and electrospray ionization (ESI) sources. pSFC-MS has a high-efficiency separation level and can perform separation and analysis of strongly polar compounds.
The scope of application of SFC-MS continues to expand with people's exploration, and has now penetrated into chemical, pharmaceutical, food, environmental, biological and other fields.