GC - MS全扫描之间的区别是什么nd Selected Ion Monitoring?

In order to understand the difference between mass spectroscopic full scan and selected ion monitoring it is essential for you to have a clear concept of spectral scanning in optical spectroscopy as both have several commonalities.

Scanning in optical spectroscopy

Optical spectroscopy serves as a valuable tool for qualitative and quantitative estimation of constituents of light absorbing samples. The sample is taken in a fixed path length cell and irradiated with light covering the wavelengths of interest. Generally in the UV- Visible region a deuterium lamp serves as a light source for the UV region and a tungsten- halogen lamp for the visible region. A monochromator using a grating isolates the required wavelengths which irradiate the sample at a rate dependent on the scan rate selected.

所选波长范围内的完全扫描提供了几种反射样品中的光吸收物质的吸收峰。为了定量任何特定的物种，单色器被设置为特定波长，并且记录的吸光度强度以使用啤酒兰伯特的光吸收法达到浓度。

在质谱中扫描

在GC - MS分析中，由GC柱分离的混合物组分到达质谱仪的电离室。对分析物进行高能量电子束，这导致母体分子的电离和碎裂。所得的电离物质被LED进入四极质量分析仪中，该分析仪可以与光谱仪中的单色器进行比较。单色镜将特定波长与宽带光源分离，而Quadrupole基于其质量来解析离子物质以电荷比率（m / z）。

通过在四极杆的对角线相对的端部处施加DC电压和RF频率来实现质量分离。RF频率在范围内扫描，以允许特定的M / Z值片段通过四极杆，并聚焦在质量敏感探测器上。

Full scan monitoring

Full scan monitoring provides a qualitative picture of the composition of the sample. It involves scanning the mass range beginning at the smallest mass of fragment ions to the highest mass expected for the fragments. Typically the mass range selected would be from, say 50 to about 400, depending on what you would anticipate in your sample taking care to eliminate possible interferences from sample matrix and atmospheric gases. Too low mass settings could result in interference from nitrogen (m/z = 28) or carbon dioxide (m/z=44) present in the air. In case a very large mass range is selected the sensitivity would be lower as fewer scans per second would be performed. At the same time information on masses expected outside the expected mass fragments range would be of little use to you. In a nutshell it is necessary to run a full scan of your sample before you limit your search for identification and quantification of species of interest in the sample.

选择离子监测

The full scan mode is useful for qualitative detection, mass scouting and for studies on the fragmentation patterns of unknowns. On the other hand, selected ion monitoring helps in quantitative studies on the specific ions of interest in the spectra of compound (s) of your interest.

In the selected ion monitoring mode data is collected on only the selected masses of interest rather than over wide range of masses. This improves the specificity of the instrument resulting in 10 -100 fold increase in detection limits. This feature helps eliminate interferences arising due to complex sample matrices thereby contributing to the potential of mass spectroscopy as an analytical technique.