The Investigation of GC-ESI/MS of TMS Derivative and the Development of Low EOF Hydrodynamic Flow Assisted Double Junction Interface

Autor: Ren-Yu Hsu, 許仁裕
Rok vydání: 2016
Druh dokumentu: 學位論文 ; thesis
Popis: 104
Gas chromatography (GC) and capillary electrophoresis (CE) are the common analytical techniques, both of them provide high separation efficiency and low sample consumption. The coupling of GC (or CE) with mass spectrometry (MS) provides a technique with high separation efficiency and high selectivity. Among the ionization techniques, electron ionization (EI) is the most commonly used ionization technique in GC/MS. Unlike EI, little fragmentation is observed under electrospray ionization (ESI). Therefore, the GC–ESI/MS combination provides a technique with high separation efficiency, high selectivity and abundant information on the molecular weight of the analyte. GC is an effective separation technique for volatile and nonpolar compounds. However, for polar analytes, derivatization is often required, because it can enhance the separation efficiency of polar compounds. Among the derivatization methods, trimethylsilylation is one of the most widely adopted approaches. A drawback of trimethylsilyl (TMS) derivatives is that they are sensitive to moisture. In aqueous environments, TMS derivatives may hydrolyze back to their native form. After GC separation, the TMS derivatives eluted from the GC column interact with the aqueous charged droplets during ESI analysis. The TMS derivatives may hydrolyze back to their native form and thus become more suitable for ESI analysis. In this work, several types of compounds (organic acids, phenols and beta-agonists) were trimethylsilylated before GC–ESI/MS analysis. The hydrolysis efficiency and application of GC-ESI/MS in TMS-derivatives were studied. Analyzing TMS-organic acids by GC–ESI/MS indicated that hydrolysis was incomplete. Both the TMS derivative ([M+TMS+H]+) and the hydrolysis product ([M-H]−) were detected. According to the results of polyprotic acids, the hydrolysis efficiency was lower with more TMS groups. Although the hydrolysis is incomplete, the detection of a TMS derivative and its hydrolysis product at the same retention time may facilitate compound confirmation. The analysis of TMS-phenols indicated that the hydrolysis efficiency was affected by the functional group attached to phenol. For example, partial hydrolysis was observed for methylphenols and nitrophenols. Both [M+TMS+H]+ and [M-H]− were detected. However, only the hydrolysis product, [M-H]−, was detected for CPs. This result suggested that TMS-CPs eluted from the GC were hydrolyzed back to their native form during the ESI process. To evaluate the limit of detection of GC-ESI/MS analysis of TMS-CPs, collision-induced dissociation (CID) was used. The LOD of TMS-CPs was estimated to be in the range of 0.25–5 ng/mL except for TMS-CPs with poor fragmentation efficiency. In GC–ESI/MS analysis of beta-agonists, only the protonated molecules of TMS-beta agonists were detected. The results suggested that the hydrolysis efficiency of TMS-beta agonists was considerably lower than that of TMS-organic acids and TMS-CPs. No hydrolysis product was detected even after the flow rate of the spray solvent was increased. The ESI mass spectra were dominated by protonated molecule signals with little fragmentation. Under CID, the LODs were estimated to be in the range of 0.5–10 ng/mL, although they did not hydrolyze back to their polar native form during ESI. Besides GC, CE is another separation technique with high separation efficiency. CE often involves adding nonvolatile additives or salts to improve the separation. However, nonvolatile additives are not suitable for ESI-MS, primarily because of their ion suppression effect. In this work, a hydrodynamic flow assisted double junction interface was used to prevent nonvolatile additives from entering the ESI source, to alleviate ion suppression caused by nonvolatile additives in CE-ESI/MS analysis. The hydrodynamic flow assisted double junction interface was fabricated with a liquid junction reservoir and conducting liquid reservoir and a short transfer column (1 cm). The hydrodynamic flow was introduced by a syringe into between the separation column and transfer column. By adjusting the hydrodynamic flow and electric field, the analytes, but not additive, were pushed to the ESI sprayer. To improve the versatility of the hydrodynamic flow assisted double junction interface, a conventional uncoated fused silica transfer column was replaced with a polyvinyl alcohol (PVA) column. Because of PVA-coated column has an extremely low electroosmotic flow (EOF), the velocity of an ion was determined by its electrophoretic mobility and the hydrodynamic flow applied to the interface. EOF no longer plays a role in the velocity of an ion. The migration behavior of the analyte and additive in transfer columns could be controlled by adjusting the hydrodynamic flow applied to the interface to alleviate the problem of ion suppression.
Databáze: Networked Digital Library of Theses & Dissertations
Externí odkaz: