Unsihuay Daisy, Qiu Jiamin, Swaroop Sneha, Nagornov Konstantin O, Kozhinov Anton N, Tsybin Yury O, Kuang Shihuan, Laskin Julia
Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA.
Department of Animal Sciences, Purdue University, West Lafayette, IN 47907, USA.
Int J Mass Spectrom. 2020 Feb;448. doi: 10.1016/j.ijms.2019.116269. Epub 2019 Nov 27.
Nonpolar triglycerides (TGs) are rarely detected in mass spectrometry imaging (MSI) experiments unless they are abundant in the sample. Herein, we use nanospray desorption electrospray ionization (nano-DESI) to explore the role of the solvent composition and ionic dopants on the detection of TGs in a murine gastrocnemius muscle tissue used as a model system. We evaluated three solvent mixtures for their ability to extract nonpolar TG species: MeOH:HO 9:1 (v/v), MeOH:DCM 6:4 (v/v) and MeOH:AcN:tol 5:3.5:1.5 (v/v/v). We observe that TGs are mainly detected as [M+K] adducts and their extraction efficiency is improved using less polar solvents: MeOH:DCM and MeOH:AcN:tol. We also explore whether the ionization efficiency of TGs may be improved by doping the MeOH:AcN:tol solvent with ammonium formate (AF) and other ionic additives. However, the formation of [M+NH] adducts of TGs is less efficient than the formation of [M+K] adducts in the range of AF concentrations from 0.1 to 10 mM. Chemical derivatization using 100 μM of Girard T reagent predominately generates reaction products of phosphatidylcholine rather than TG species. Moreover, the presence of the Girard T reagent suppresses ion signals of all the species in the spectrum including TGs. Nano-DESI MSI experiments performed using MeOH:AcN:tol solvent enable imaging of TGs without any detectable adverse effect on signals of other lipids and metabolites. Specifically, 10 out of 14 TG species were detected exclusively using MeOH:AcN:tol and the sensitivity towards other TGs was improved by at least an order of magnitude. Although polyunsaturated TGs may be detected using both solvents, saturated and monounsaturated TGs are only detected using MeOH:AcN:tol. Our results provide a direct path for the improved detection of TGs in tissue imaging experiments using liquid-based ambient ionization techniques.
在质谱成像（MSI）实验中，除非样品中富含非极性甘油三酯（TGs），否则很少能检测到它们。在此，我们使用纳米喷雾解吸电喷雾电离（nano-DESI）来探究溶剂组成和离子掺杂剂对以小鼠腓肠肌组织为模型系统检测TGs的作用。我们评估了三种溶剂混合物提取非极性TG种类的能力：甲醇：水9:1（v/v）、甲醇：二氯甲烷6:4（v/v）和甲醇：乙腈：甲苯5:3.5:1.5（v/v/v）。我们观察到TGs主要以[M+K]加合物形式被检测到，并且使用极性较小的溶剂（甲醇：二氯甲烷和甲醇：乙腈：甲苯）可提高其提取效率。我们还探究了通过用甲酸铵（AF）和其他离子添加剂掺杂甲醇：乙腈：甲苯溶剂，是否可以提高TGs的电离效率。然而，在0.1至10 mM的AF浓度范围内，TGs的[M+NH]加合物的形成效率低于[M+K]加合物的形成效率。使用100 μM的吉拉德T试剂进行化学衍生化主要生成磷脂酰胆碱的反应产物，而非TG种类。此外，吉拉德T试剂的存在会抑制光谱中所有种类的离子信号，包括TGs。使用甲醇：乙腈：甲苯溶剂进行的nano-DESI MSI实验能够对TGs进行成像，而对其他脂质和代谢物的信号没有任何可检测到的不利影响。具体而言，14种TG种类中有10种仅使用甲醇：乙腈：甲苯被检测到，并且对其他TGs的灵敏度提高了至少一个数量级。虽然使用两种溶剂都可以检测到多不饱和TGs，但饱和和单不饱和TGs仅使用甲醇：乙腈：甲苯被检测到。我们的结果为使用基于液体的常压电离技术在组织成像实验中改进TGs的检测提供了一条直接途径。
