Analysis of sulfonated compounds by reversed-phase ion-pair chromatography—Mass spectrometry with on-line removal of non-volatile tetrabutyl ammonium ion-pairing agents

Abstract

SummaryAn ion-pair reversed-phase high performance liquid chromatography method with tetrabutyl ammonium salts as ion-pairing agents and mass spectrometric detection has been developed for sulfonated compounds with special focus on structural identification.A cation exchange suppressor cartridge placed bef,,veen the UV detector and the ion source of the mass spectrometer completely removed the non-volatile ion-pairing agent resulting in excellent conditions for both electrospray ioniziation and atmospheric pressure chemical ioniziation. Peak broadening caused bythe dead volume of the suppressor is negligible.The application range of this method is demonstrated by the structural identification of individual compounds within a complex mixture of model substances consisting of sulfonated aromatics, textile dyes and cletergents with as many as four sulfonic acid groups, all largely differing from each other in their structural properties. The influence of the ion-pairing agent's counter ion on retention behaviour is also discussed.

Effects of ion‐pairing reagents on the electrospray signal suppression of sulphonated dyes and intermediates

“…High‐performance liquid chromatography/mass spectrometry (HPLC/MS) with atmospheric pressure ionization techniques is routinely applied for the analysis of various organic and organometallic compounds with wide ranges of polarities and molecular masses 1–3. From among the available ionization techniques, electrospray ionization (ESI) in the negative‐ion mode is the most suitable for (poly)sulphonic acids 4–17. Negative‐ion atmospheric pressure chemical ionization (APCI) can be also applied for the MS of some mono‐ and disulphonic acids,4–7 but the sensitivity is usually worse than with the ESI technique.…”

Ion‐pairing high‐performance liquid chromatography‐mass spectrometry of impurities and reduction products of sulphonated azodyes

“…In addition to the determination of the molecular masses and of the number of sulphonic acid groups, it can also yield additional structural information, especially with MS/ MS or MS n techniques [11, 14, 19 -22]. Negative-ion atmospheric pressure chemical ionisation (APCI) is restricted to mono-and disulphonic acids [7,10,13] and the sensitivity is lower than with ESI. The negative-ion thermospray ionisation [10,23] used in the early work before the introduction of ESI-MS is hardly applied any more.…”

“…Unfortunately, these additives are usually not volatile enough to be compatible with mass spectrometric detection. To prevent the tetraalkylammonium ions from entering the ion source of the mass spectrometer, an ion-suppressor column can be inserted between the UV and mass spectrometric detectors [7]; however, the additional extra-column volumes can impair chromatographic resolution, especially when micro-columns are used. Hence, HPLC/MS with volatile di-and trialkylammonium ion-pairing additives are often selected as a reasonable compromise between separation selectivity and mass-spectrometer performance for the analysis of dyes and intermediates [8 -12].…”

Separation of intermediates of iron-catalyzed dopamine oxidation reactions using reversed-phase ion-pairing chromatography coupled in tandem with UV–visible and ESI-MS detections

“…A problem inherent in coupling RP–IPC with MS is that non-volatile salts necessary for IPC could clog the LC–MS interface and suppress the analyte signals [10]. The on-line IPC trapping method was developed by Forngren and other groups for coupling RP–IPC with ESI–MS and successfully used for the separation of biopterin, sulphonated azo dyes, sulfonated compounds and other molecules [9,11,12]. To our knowledge, there has been no report on the use of RP–IPC coupled on-line with ESI–MS for detecting short-lived DA oxidation products.…”