Ionization Efficiency of Doubly Charged Ions Formed from Polyprotic Acids in Electrospray Negative Mode

Abstract

Abstract. The ability of polyprotic acids to give doubly charged ions in negative mode electrospray was studied and related to physicochemical properties of the acids via linear discriminant analysis (LDA). It was discovered that the compound has to be strongly acidic (low pK a1 and pK a2 ) and to have high hydrophobicity (logP ow ) to become multiply charged. Ability to give multiply charged ions in ESI/MS cannot be directly predicted from the solution phase acidities. Therefore, for the first time, a quantitative model to predict the charge state of the analyte in ESI/MS is proposed and validated for small anions. Also, a model to predict ionization efficiencies of these analytes was developed. Results indicate that acidity of the analyte, its octanol-water partition coefficient, and charge delocalization are important factors that influence ionization efficiencies as well as charge states of the analytes. The pH of the solvent was also found to be an important factor influencing the ionization efficiency of doubly charged ions.

IRMPD Spectroscopy of Bare Monodeprotonated Genistein, an Antioxidant Flavonoid

“…No multiply charged anions, such as [geni-2H] 2– , are observed, as expected in view of the known ESI-MS problematics in delivering multiply charged anions from relatively small polyprotic acids. 64 The fragmentation behavior of the mass-selected deprotonated ion, [geni-H] − , has been assayed by CID experiments.…”

Predicting ESI/MS Signal Change for Anions in Different Solvents

“…As analytes in training set benzoic acid (Reakhim), 3,5-dinitrobenzoic acid (Aldrich), 3-(trifluoromethyl)­benzoic acid (a kind gift from Prof. L. M. Yagupolskii, Institute of Organic Synthesis, Kiev), pentafluorophenol (Aldrich), 2,3,5,6-tetrafluorophenol (Aldrich), 4- tert -butylbenzoic acid (Aldrich), 4-(pentafluorophenyl)-2,3,5,6-tetrafluorophenol, pentakis­(trifluoromethyl)­phenol, 2-phenylphenol (TCI-EP), 4-phenylphenol (TCI-EP), 3-chlorobenzoic acid (Aldrich), 4-aminobenzoic acid (Aldrich), 3-aminobenzoic acid (Fluka), 4-hydroxybenzoic acid (Sigma-Aldrich), pentafluorobenzoic acid (Aldrich), 2-nitrophenol (Fluka), 3-nitrophenol (Fluka), 4-nitrophenol (Chemapol Nordic), 2,4-dinitrophenol (the same as in ref ), 2,5-dinitrophenol (Reakhim), 2,6-dinitrophenol (Reakhim), 3-chlorophenol (Reakhim), 2-bromophenol (a kind gift from Ivari Kaljurand), 3-bromophenol (Aldrich), 3-(dimethylamino)­benzoic acid (Merck), 4-isopropylbenzoic acid (a kind gift from Tullio Ilomets), 2- tert -buthylphenol (Merck), 2,4,6-tri- tert -buthylphenol (synthesized according to a standard procedure from phenol and isobutylene obtained from treatment of t -BuOH with H 2 SO 4 ), 2-isopropylphenol (ABCR GmbH&Co), 2-cyanophenol (Aldrich), 4-(phenylaso)­phenol (Aldrich), 3,5-diiodosalicylic acid (Aldrich), salicylic acid (Reakhim), 3-[(heptafluoropropyl)­sulphonyl]­benzoic acid (a kind gift from prof. L. M. Yagupolskii, Institute of Organic Synthesis, Kiev), 3-[(heptafluoropropyl)­sulphanyl]­benzoic acid (a kind gift from prof. L. M. Yagupolskii, Institute of Organic Synthesis, Kiev), 3-[tris­(trifluoromethyl)­methylsulfonyl]­benzoic acid (a kind gift from prof. L. M. Yagupolskii, Institute of Organic Synthesis, Kiev), 3-[(trifluoromethyl)­sulphanyl]­benzoic acid (a kind gift from prof. L. M. Yagupolskii, Institute of Organic Synthesis, Kiev), 3-[(trifluoromethyl)­sulphonyl]­benzoic acid (a kind gift from prof. L. M. Yagupolskii, Institute of Organic Synthesis, Kiev), 3-[tris­(trifluoromethyl)­methyl]­benzoic acid (a kind gift from prof. L. M. Yagupolskii, Institute of Organic Synthesis, Kiev), picric acid, pentabromophenol, pentachlorophenol, 2-hydroxy-1,3,5-tris­(2,2,2-trifluoroethyl)-1,3,5-benzenetrisulfonic acid ester (a kind gift from prof. L. M. Yagupolskii, Institute of Organic Synthesis, Kiev), 4-(4-nitrophenylaso)-1-napthol (Fluka), heptafluoronapht-2-ol, 2-hydroxy-1,3,5-tris­(2,2,3,3-tetrafluoropropyl)-1,3,5-benzenetrisulfonic acid ester, 2,4,6-trinitro-1,3-benzenediol (a kind gift from Tullio Ilomets), 3,5-dinitrosalicylic acid (Reakhim, purified by sublimation), 2,4-dinitrobenzoic acid (Lancaster), and 4-chlorophenol (Aldrich) were used.…”

“…The solvent composition has been found , to affect ESI ionization efficiency. Two significant effects have been observed: the effect of organic modifier content , and the effect of solvent buffer and pH. , …”

Influence of the amino acid composition on the ionization efficiencies of small peptides

“…Physicochemical parameters (aqueous pK a , charge delocalization parameter (WANS)) of amino acids were calculated with COSMO-RS 30 method in the same manner as in our previous publications. 31 The degree of ionization, α was calculated from the pK a values. The statistical significance of all the differences was determined with t test at 95% confidence level.…”