Jeremy Ryan McFadden

Ammonium fluoride has been shown to improve sensitivity when using electrospray ionization (ESI) coupled with mass spectrometry (MS). Recent internal investigation furthered that claim, through the observation of improved sensitivity when analyzing steroid molecules. This work focuses on extending those observations to other small molecules to understand the impact ammonium fluoride has on detection sensitivity with optimized instrument conditions. Using conventional liquid chromatography ESI‐MS we investigated sensitivity differences between ammonium fluoride, formic acid, or ammonium hydroxide as mobile phase additives. Full source optimization was performed for nine compounds at three different organic concentrations (30%, 60%, or 90%) with formic acid, ammonium fluoride, and ammonium hydroxide adjustment. Optimization results were compiled to generate individual methods by compound, polarity, mobile phase, and organic concentration. Flow injection analysis was performed with fully optimized methods to compare compounds across different solvent systems under optimal conditions. Negative ESI data showed 2–22‐fold sensitivity improvements for all compounds with ammonium fluoride. Positive ESI data showed > 1–11‐fold improvement in sensitivity for four of seven compounds and no change for three of seven compounds with ammonium fluoride. Ammonium fluoride improved ESI⁻ sensitivity for all compounds studied when using optimized source conditions. Investigation with ESI⁺ analyses showed mixed results, with four of seven compounds showing improvement and others showing equivalency or slight loss in sensitivity, suggesting potential sensitivity gains for some analogs with ESI⁺.

Rationale Ammonium fluoride has been shown to improve sensitivity in electrospray ionization in mass spectral detection when used as a mobile phase additive. Internal investigation showed sensitivity improvement for steroid compounds with LCMS. Our work presented here investigates ammonium fluoride as a mobile phase additive in improving mass spectral ionization compared with other mobile phases under fully optimized instrument conditions. Methods For our research, we investigated ammonium fluoride in comparison with formic acid and ammonium hydroxide as mobile phase additives to measure sensitivity effects in electrospray ionization sensitivity. Full source optimization was performed for nine commercially available compounds at three different organic concentrations (30, 60, or 90%) with formic acid, ammonium fluoride, and ammonium hydroxide adjusted mobile phase sets. Optimization results were compiled to generate individual methods by compound, polarity, mobile phase additive, and organic concentration, respectively. These methods were used for final flow injection analysis to objectively compare compounds analyzed with the different solvent systems under optimal conditions for each mobile phase composition. Results Negative ESI data showed 2 – 22-fold sensitivity improvements for all seven compounds with ammonium fluoride. Positive ESI data showed > 1 - 11-fold improvement in sensitivity for four of seven compounds and equivalent (or slight drop in) sensitivity for three of seven compounds with ammonium fluoride adjustment. Acetaminophen showed the greatest sensitivity improvement in both polarities, with 7-22-fold (ESI-) and 4-11-fold (ESI+) improvements in sensitivity, respectively Conclusions With fully optimized source conditions there is good likelihood of improving MS sensitivity (ESI-) in using ammonium fluoride adjusted mobile phase. Investigation with ESI+ analyses showed mixed results, with four of seven compounds showing improvement and others showing equivalency or slight loss in sensitivity. Our investigation focused solely on sensitivity gains achievable in ESI-MS by source optimization and mobile phase additive selection by flow injection without chromatography.