Fabrication of Internally Tapered Capillaries for Capillary Electrochromatography Electrospray Ionization Mass Spectrometry

Abstract

In this study, we report a novel procedure for fabricating internally tapered capillary columns suitable for the coupling of capillary electrochromatography (CEC) to electrospray mass spectrometry (ESI-MS). The internal tapers were prepared by slowly heating the capillary end in a methane/O2 flame. Due to continuous self-shrinking of the inner channel of the capillary, the inside diameter of the opening was reduced to 7-10 microm. The procedure is easy to handle, with no requirement for expensive equipment as well as elimination of problematic grinding of the tip. Several advantages of these new internal tapers, as compared to using externally tapered columns, are described. First, the problems of poor durability and tip breakage associated with external tapering were successfully overcome with the internal taper. A comparison of the online CEC/ESI-MS between external versus internal tapers showed that the latter provides enhanced electrospray stability, resulting in significantly lower short-term noise and very short-term noise values. In turn, the more rugged design of internal tapers allows performing CEC/MS utilizing a harsh polar organic mobile phase, which was not previously successful using an external taper due to higher operating current and electrospray arcing. Next, data on the reproducibility of the internally tapered CEC/MS column using warfarin and beta-blockers as model analytes are presented. For example, when comparing the reproducibility for separation of warfarin under reversed-phase conditions, the internal taper demonstrated superior intraday % RSD (1.6-3.4) as compared to the external taper intraday % RSD (5-6). Last, the applicability of performing quantitative CEC/MS with internally tapered capillaries is demonstrated for simultaneous enantioseparation of beta-blockers. Impressive quantitative results include good linearity of calibration curves (e.g., R2 = 0.9940-0.9988) and limit of detection as low as 30 nM. The sensitive detection of a minor impurity of one enantiomer at the 0.1% level in a major chiral entity buttresses the suitability of compliance with FDA guidelines.

Sensitive chiral analysis by CE: An update

“…CEC was also hyphenated to ESI-MS for the chiral analysis of drugs [47,48]. In CEC, the separation was carried out mainly in a packed capillary column (PCC).…”

“…Thus, Zheng and Shamsi [47] showed the feasibility of using a vancomycin CSP for the simultaneous enantioseparation of eight bblockers by CEC-ESI-MS. Internally tapered capillaries were employed to pack CEC-MS columns (see Fig. 8A), which were fabricated by a novel procedure described by the same research group [48]. A comparison between external and internal tapered capillaries for on-line CEC-ESI-MS showed that the latter provided enhanced electrospray stability, sensitivity and robustness resulting in significantly lower noise [48].…”

“…8A), which were fabricated by a novel procedure described by the same research group [48]. A comparison between external and internal tapered capillaries for on-line CEC-ESI-MS showed that the latter provided enhanced electrospray stability, sensitivity and robustness resulting in significantly lower noise [48]. Other problems associated with external tapering were successfully overcome with the internal taper, such as, the poor durability and tip breakage, as well as the possibility of using a harsh polar organic mobile phase, which was not previously successful using an external taper due to higher operating current and electrospray arcing.…”

“…Next, data on the reproducibility of this CEC-MS system using warfarin and bblockers as model analytes were presented. Last, the applicability of performing quantitative CEC-MS with these PPCs was demonstrated for the simultaneous enantioseparation of b-blockers achieving LODs , 3610 28 M [47,48]. Figure 8B shows the sensitive detection of one enantiomer (the pharmacological active S-propranolol) at 0.1% in R-propranolol, which is the limit above which impurities have to be identified according to ICH guidelines [1].…”

18 Coupling CE and microchip-based devices with mass spectrometry

“…Zheng and Shamsi demonstrated the possibility of using chiral CEC2ESI/MS with a commercially packed column for the determination of warfarin enantiomers in human plasma using coumachlor as an internal standard (IS). 286 Robustness of this chiral CEC capillary was recently improved by a novel procedure 287 and applied for the simultaneous enantioseparation of height b-blockers with multimodal CSP using different combinations of vancomycin and teicoplanin, as presented in Figure 5.…”