质谱中的普遍电离技术

电离在质谱中起到中心作用，因为它将中性分子电离到空间和时间间隔的带电分子中基于它们的质量来充电（M / Z）比率。质谱分子具有几种可用于启动电离的可选选项，这取决于分子的分析要求和性质。简要概述了常见的电离技术，并且在本文中简要讨论了它们的益处。

电子撞击（EI）

电子撞击是最常用的硬离子化技术。它适用于相对较小的分子，其分子量低于600，其可以在没有分解的情况下容易地蒸发。气相分子进入电离室，其中在大约70eV的范围内具有电能的电子轰击它们足以敲出电极的电荷，导致相同质量（分子离子）的带正电荷的离子或具有较低质量的片段。碎片化用于提供母体分子的结构信息，但必须记住它在电子碰撞电离下可以不存在或非常少的数量。该技术仅限于非常稳定和挥发性的化合物。

化学电离（CI）

Chemical Ionization is a soft ionization technique which makes use of ion – molecule reactions to generate ions from neutral molecules. A reagent gas such as methane, isobutane or ammonia is ionized by electron impact and the resulting ions react with analyte molecules to produce analyte ions. The spectra are simpler in comparison with EI spectra.

Chemical ionization results in lower fragmentation and the reproducibility of spectra is lower. Library search becomes cumbersome as spectral reproducibility is dependent upon parameters such as reactant gas, pressure and reaction time. CI is a favoured technique for molecular weight confirmation and differentiation between isomers. The molecular weight range is almost similar to the EI technique.

Electrospray Ionization (ESI)

Electrospray involves formation of an aerosol of the liquid sample through a highly charged capillary under atmospheric pressure. Charged droplets are subjected to a cross flow of an inert
通过蒸发除去溶剂如氮气的去溶剂化气体。库仑排斥导致形成独立的带电分析物离子，并通过仪器的离子光学器件导致质量分析仪。

Electrospray is capable of producing multiple charged ions. It is suitable for polar molecules having high molecular weights and are thermally labile . Such molecules are ideal candidates for LC-MS applications.

Atmospheric Pressure Chemical Ionization (APCI) is similar to Electrospray technique.The liquid solution is rapidly heated in a stream of nitrogen between 300 – 450°C before emerging as a de-solvated aerosol droplets in the atmospheric pressure region. The technique is effective for molecules having lower polarities.

Atmospheric Pressure Photoionization (APPI) is another atmospheric pressure ionization technique which uses UV light source for ionization of the analyte molecules. It is less popular in comparison to the ESI or APCI and can be applied to moderately polar molecules such as polycyclic aromatics, steroids, etc.

矩阵辅助激光解吸电离（MALDI）

MALDI permits ionization of large thermally labile molecules and has potential scope of applications in life sciences. It is a soft ionization technique and is suitable for molecules having molecular weights up to 500,000 Da. The analyte is mixed with an excess of matrix compounds such as Sinapinic acid . It is commonly used for analysis of DNA, peptides and proteins. A small amount of analyte is mixed with the MALDI matrix and the solution is placed on a laser target which is irradiated from a laser source. The energy of the laser results in desorption of the analyte ions into the gaseous phase and are protonated or deprotonated in the hot plume of ablated gas to produce ionized species. This technique is commonly used with time of flight (TOF) or Fourier transform ion cyclotron (FTICR) systems compatible with pulsed ionization methods.

The scope of Mass Spectroscopy is virtually unlimited and role of hyphenated techniques will be taken up in subsequent articles.