Tsikas Dimitrios
Centre of Pharmacology and Toxicology, Hannover Medical School, Carl-Neuberg-Str. 1, D-30625, 30623 Hannover, Germany.
J Chromatogr B Analyt Technol Biomed Life Sci. 2017 Mar 15;1047:185-196. doi: 10.1016/j.jchromb.2016.06.018. Epub 2016 Jun 15.
GC-MS and GC-MS/MS of pentafluorobenzyl (PFB) ester trimethylsilyl (TMS) ether (PFB-TMS) derivatives of hydroxylated long-chain fatty acids including arachidonic acid metabolites, the eicosanoids, in the electron-capture negative-ion chemical ionization (ECNICI) mode are the most sensitive and accurate approaches to quantify carboxyl groups-containing compounds in complex biological fluids such as plasma and urine. Under ECNICI conditions, PFB-TMS derivatives of eicosanoids ionize to form very few ions, with the carboxylates [M-PFB] being typically the most intense. Less intense ions may be additionally formed by consecutive neutral loss (NL) of trimethylsilanol (TMSOH, 90Da) groups ([M-PFB-(TMSOH)]). By using [1,1-O]- and [1,ω-O]-eicosanoids, we studied ion processes following collisionally activated dissociation (CAD) of the precursor ions [M-PFB]. We found that CAD resulted in formation of product ions due to NL of a TMSOH (92Da) group in monocarboxylic and of a PFBOH (200Da) group in dicarboxylic eicosanoids. TMSOH NL implies an intra-molecular transfer of the TMS group from hydroxyl groups to their carboxylate anions [M-PFB]. From a mechanistic point of view, this rearrangement may explain formation of unique product ions in GC-MS/MS of eicosanoids under ECNICI conditions. From the quantitative point of view, quantification by GC-MS/MS of product ions due to [M-PFB-(TMSOH)] and [M-PFB-TMSOH-(TMSOH)]would reveal incorrect data, if [1,1-O]-eicosanoids are used as internal standards and if no correction for the O-loss is performed. In O-labelled dicarboxylic eicosanoids, such as the major urinary metabolite (MUM) of E prostaglandins, i.e., [1,ω-O]-PGE-MUM), no TMS ester/TMS ether rearrangement was observed. Yet, O-loss occurred upon CAD of [M-PFB] due to NL of PFBOH (200Da). In both cases the extent of O-loss needs to be determined and considered for accurate quantification of monocarboxylic acids such as 8-isoprostaglandin F (8-iso-PGF) and dicarboxylic eicosanoids such as PGE-MUM.
气相色谱 - 质谱联用（GC - MS）以及气相色谱 - 串联质谱联用（GC - MS/MS）对羟基化长链脂肪酸（包括花生四烯酸代谢物，即类二十烷酸）的五氟苄基（PFB）酯三甲基硅烷基（TMS）醚（PFB - TMS）衍生物进行分析时，在电子捕获负离子化学电离（ECNICI）模式下，是定量分析血浆和尿液等复杂生物流体中含羧基化合物最灵敏、准确的方法。在ECNICI条件下，类二十烷酸的PFB - TMS衍生物电离形成的离子很少，羧酸根离子[M - PFB]通常是最强的。强度较低的离子可能通过连续中性丢失三甲基硅醇（TMSOH，90Da）基团（[M - PFB - (TMSOH)]）额外形成。通过使用[1,1 - O] - 和[1,ω - O] - 类二十烷酸，我们研究了前体离子[M - PFB]碰撞激活解离（CAD）后的离子过程。我们发现，CAD导致单羧酸类中一个TMSOH（92Da）基团和二羧酸类二十烷酸中一个PFBOH（200Da）基团的中性丢失从而形成产物离子。TMSOH中性丢失意味着TMS基团从羟基向其羧酸根阴离子[M - PFB]进行分子内转移。从机理角度来看这种重排可以解释在ECNICI条件下类二十烷酸GC - MS/MS中独特产物离子的形成。从定量角度来看，如果将[1,1 - O] - 类二十烷酸用作内标且未对氧丢失进行校正，通过GC - MS/MS对因[M - PFB - (TMSOH)]和[M - PFB - TMSOH - (TMSOH)]产生的产物离子进行定量会得出错误数据。在氧标记的二羧酸类二十烷酸中，如E型前列腺素的主要尿液代谢物（MUM），即[1,ω - O] - PGE - MUM），未观察到TMS酯/TMS醚重排。然而，[M - PFB]在CAD过程中由于PFBOH（200Da）的中性丢失发生了氧丢失。在这两种情况下都需要确定并考虑氧丢失的程度，以便准确定量单羧酸如8 - 异前列腺素F（8 - iso - PGF）和二羧酸类二十烷酸如PGE - MUM。
