附件:
风之彩
第2楼2008/09/06
我先来吧。
谈谈分辨率。
原文:
Mass Resolution
A,How can mass resolution help you? If you have a co-elution of near isobaric compounds, high resolution can help you separate them, if the resolution is high enough. If you do MS2 it is important that you have the possibility of high resolution with ms1 and ms2 not only in the ms2 mode . You need the high resolution in ms1 for high resolution precursor ion selection. If you don’t have this capability you can get a mixed product ion spectrum that will lead you to the wrong conclusions or wrong quan and calibration curve results.
A.大意:质谱的分辩率在你分析分子量相近而且在色谱柱上共同洗脱的物质时就很重要了。如果你做二级质谱,MS1和MS2都需要高分辨。因为你需要高分辨率来帮你选择母离子。如果你的质谱没有这个能力,那么你测得的质谱图可能就是混合物,你的定量和标准曲线就会不准。
所以,这就和上面提到的内标里面有一句话“The labeled standard should be “far” enough away from the non-labeled to avoid signal contribution of the abundance of the natural isotopes to the signal of the internal standard.”的意思差不多,如果你用同位素做内标,而你的质谱分辨率如果不够高,那么你就要尽量使他们分得开些,不然你的定量会不准,因为你无法保证你测得的信号完全来自你的样品而非他的同位素。
B,Isobaric compounds are compounds with the same nominal mass but with a different molecular formula. E.g. N2 and CO both have the same nominal mass of 28 amu. With a low resolution instrument these two compounds can not be separated. Actually the exact mass of N2 is 28.006148 and the exact mass of CO is 27.99491464 the mass difference is 0.01123336.
B.大意:某些物质有相同的分子量但是他们的分子式不同。如N2 和 CO ,分子量都是28,如果质谱的分辩率低,那么质谱就不能区分它们。但是实际上N2 的精确分子量是28.006148,而CO 的是27.99491464 ,他们的分子量差0.01123336.
高分辨的质谱一般就是LC-TOF, Q-TOF,FT-MS等,用的毕竟少,而且也很少用来定量。我们定量还是QQQ多,现在还有线性离子阱等,这就是需要我们在设定分辩率参数时根据需要来选择。