Comparison of partial filling MEKC analyses of steroids with use of ESI‐MS and UV spectrophotometry

Until now, partial filling micellar EKC–ESI‐MS (PF‐MEKC–ESI‐MS) has seldom been applied for human biomolecules. In this study, determination of three electrically neutral endogenous steroid hormones, namely androstenedione, testosterone, and epitestosterone, by PF‐MEKC–ESI‐MS was investigated. Since ESI‐MS and ESI‐MS/MS behaviors of testosterone and epitestosterone proved to be nearly identical, efficient separation of the two compounds was required to obtain reliable identification. The chemical conditions as well as the instrumental parameters affecting the PF‐MEKC–ESI‐MS analysis were researched. In optimal conditions, ESI‐MS showed excellent selectivity, and all the steroids could be identified using SIM. LODs were 0.75–5 μg/mL. The results obtained by PF‐MEKC–ESI‐MS were compared with those obtained by corresponding PF‐MEKC–UV. PF‐MEKC–UV provided better resolution, repeatability, and more than ten‐fold higher sensitivity, in terms of LODs, than PF‐MEKC–ESI‐MS. The reasons for this were carefully examined. In comparison with PF‐MEKC–UV, PF‐MEKC–ESI‐MS suffered from greater band broadening owing to the sheath–liquid interface. Resolution was also decreased owing to the elevated capillary temperature. Finally, we discovered that in the analysis of electrically neutral compounds, in‐capillary sample concentration by micellar sweeping could be more efficiently utilized in PF‐MEKC–UV than in PF‐MEKC–ESI‐MS.