Comparison of two low flow interfaces for measurement of mobilities and stability constants by affinity capillary electrophoresis-mass spectrometry.

Affinity capillary electrophoresis (ACE) is typically used for the determination of stability constant, Kst, of weak to moderately strong complexes. Sensitive detection such as mass spectrometry (MS) is required for extension of ACE methodology for estimation of Kst of stronger complexes. Consequently, an efficient interface for hyphenation of CE with MS detection is necessary. For evaluation of interfaces for electrospray ionization mass spectrometric (ESI/MS) detection in ACE conditions, potassium-crown ether complexation was used as model system. The effective mobilities of the crown ether ligands and the Kst of their potassium complexes were measured/determined by ACE-ESI/MS using two lab-made interfaces: (i) a sheathless porous tip CE-ESI/MS interface and (ii) a nano-sheath liquid flow CE-ESI/MS interface, and, in turn, compared with those obtained by ACE with UV spectrophotometric detection. Apparent stability constant of potassium-crown ether complexes in 60/40 (v/v) methanol/water mixed solvent, pH* 5.5, was about 1300 L/mol for dibenzo-18-crown-6, 1600 L/mol for benzo-18-crown-6 and 5200 L/mol for 18-crown-6 ligands, respectively. It was observed that electrophoretic mobilities from CE-MS experiments differ from reference values determined by UV detection by ∼7% depending on the CE-MS interface used. Good agreement of CE-MS and CE-UV data was achieved for nano-sheath liquid flow interface, in which the spray potential and the CE separation potential can be effectively decoupled. As for sheathless porous tip interface, a correction procedure involving a mobility marker has been proposed. It provides typically only ca. 1% difference of effective mobilities and Kst values obtained from CE-MS data as compared to those received by the reference ACE-UV method.

Study of solvent effects on the stability constant and ionic mobility of the dibenzo-18-crown-6 complex with potassium ion by affinity capillary electrophoresis.

The effect of solvent on the strength of noncovalent interactions and ionic mobility of the dibenzo-18-crown-6 complex with K+ in water/organic solvents was investigated by using affinity capillary electrophoresis. The proportion of organic solvent (methanol, ethanol, propan-2-ol, and acetonitrile) in the mixtures ranged from 0 to 100 vol.%. The stability constant, KKL , and actual ionic mobility of the dibenzo-18-crown-6-K+ complex were determined by the nonlinear regression analysis of the dependence of the effective electrophoretic mobility of dibenzo-18-crown-6 on the concentration of K+ (added as KCl) in the background electrolyte (25 mM lithium acetate, pH 5.5, in the above mixed hydro-organic solvents). Competitive interaction of the dibenzo-18-crown-6 with Li+ was observed and quantified in mixtures containing more than 60 vol.% of the organic solvent. However, the stability constant of the dibenzo-18-crown-6-Li+ complex was in all cases lower than 0.5 % of KKL . The log KKL increased approximately linearly in the range 1.62-4.98 with the increasing molar fraction of organic solvent in the above mixed solvents and with similar slopes for all four organic solvents used in this study. The ionic mobilities of the dibenzo-18-crown-6-K+ complex were in the range (6.1-43.4) × 10-9 m2 V-1 s-1 .

Determination of acid dissociation constants of triazole fungicides by pressure assisted capillary electrophoresis.

Pressure assisted capillary electrophoresis was applied to determination of acid dissociation constants (pKa) of six widely used triazole fungicides (cyproconazole, epoxiconazole, flusilazole, tebuconazole, penconazole and propiconazole) in aqueous medium. The pKa values were determined from the dependence of effective electrophoretic mobility of the triazole fungicides on p[H(+)] of the background electrolyte (BGE) using non-linear regression analysis. The p[H(+)] was used instead of pH to reflect the increased ionic strength of the strongly acidic BGEs (pH<1.75) as compared to the BGEs at pH equal to or greater than 1.75. Prior to the pKa calculation, the measured effective electrophoretic mobilities were corrected to the reference temperature (25°C) and constant ionic strength (25mM). The regression function was modified to allow the determination of pKa in the BGEs of varying ionic strength. The electrophoretic measurements showed that the above triazole fungicides are very weak bases - their pKa values were found to be in the range 1.05-1.97 and were in a good agreement with the values calculated by SPARC online pKa calculator.

Complexation between the fungicide tebuconazole and copper(II) probed by electrospray ionization mass spectrometry.

Electrospray ionization mass spectrometry (ESI-MS) is used to probe the complex formation between tebuconazole (1) and copper(II) salts, which both are commonly used fungicides in agriculture. Experiments with model solutions containing 1 and CuCl(2) reveal the initial formation of the copper(II) species [(1)CuCl](+) and [(1)(2)CuCl](+) which undergo reduction to the corresponding copper(I) ions [(1)Cu](+) and [(1)(2)Cu](+) under more drastic ionization conditions in the ESI source. In additional experiments, copper/tebuconazole complexes were also detected in samples made from soil solutions of various origin and different amount of mineralization. The direct sampling of such solutions via ESI-MS is thus potentially useful for understanding of the interactions between copper(II) salts and tebuconazole in environmental samples.

A new approach to study cadmium complexes with oxalic acid in soil solution.

This study presents a new analytical approach for the determination of heavy metals complexed to low-molecular-weight-organic acids in soil solutions, which combines the sensitivity of differential pulse anodic stripping voltammetry (DPASV) with the molecular insight gained by electrospray ionization mass spectrometry (ESI-MS). The combination of these analytical methods allows the investigation of such complexes in complex matrixes. On the voltammograms of the soil solutions, in addition to the expected complexes of oxalic acid with cadmium and lead, respectively, also peaks belonging to mixed complexes of cadmium, lead, and oxalic acid (OAH(2)) were observed. In order to verify the possible formation of complexes with OAH(2), aqueous solutions of OAH(2) with traces of Cd(II) were investigated as model systems. Signals corresponding to several distinct molecular complexes between cadmium and oxalic acid were detected in the model solutions using negative-ion ESI-MS, which follow the general formula [Cd(n)(X,Y)((2n+1))](-), where n is the number of cadmium atoms, X=Cl(-), and Y=OAH(-). Some of these complexes were also identified in the ESI mass spectra taken from the soil solutions.