Electrospray current

L. Tang and P. Kebarle, Effect of the conductivity of the electrosprayed solution on the electrospray current. Factors determining analyte sensitivity in electrospray mass spectrometry, A a/. Chem. 63 (1991), 2709-2715. 

Among the large variety of liquids that can be dispersed by electrospray, only a few have gained popularity in ESI. The most important selection criteria include solvent volatility, analyte solubility, and compatibility with the materials the liquid may come in contact with. Aqueous methanol solutions are often used because they ensure very stable electrospray operation. Acetonitrile is a widespread component of mobile phases in liquid chromatography (LC), thus very common electrospray solvent when coupling LC to mass spectrometry. Weak acids (e.g., acetic or formic acids) are often added to solutions to increase their conductivity, leading to larger electrospray currents and smaller droplet size distributions. Lower liquid flow rates also lead to smaller droplet size distributions. 

Doktycz, M.J., Hmst, G.B., Habibi-Goudarzi, S., McLuckey, S.A., Tang, K., Chen, C.H., Uziel, M., Jacobson, K.B., Woychik, R.P., and Buchanan, M.V. (1995) Analysis of polymerase chain reaction-amplified DNA products by mass spectrometry using matrix-assisted laser desorption and electrospray current status. Anal. Biochem., 230 (2), 205-214. 

One expects that the reaction with the lowest oxidation potential will dominate, and that the oxidation reaction will be dependent on the material present in the metal electrode, the solutes/ions present in the solution, and the nature of the solvent. Proof of the occurrence of an electrochemical oxidation at the metal capillary was provided by Blades et al. [16]. When a Zn spray capillary tip was used, release of Zn to the solution could be detected. Furthermore, the amount of Zn release to the solution per unit time when converted to coulomb charge per second was found to be equal to the measured electrospray current (J) in amperes (coulomb/s. Figure 1.1). Similar results were observed with stainless steel capillaries [16]. These were found to release Fe " " to the solution. These quantitative results provided the strongest evidence for the electrolysis mechanism. These oxidation reactions introduce ions which were not previously present in the solution (see Eq. (1.2)). However, they also provide an opportunity to generate reactive intermediates that can be studied by mass spectrometry. 

Ions introduced into the solution by inadvertent or deliberate electrolysis amount to very low concentrations. Taking the Zn capillary tip as example, a solution of 10 M NaCl in methanol at a flow rate Vf=20p,Lmin was found to lead to an electrospray current of 1.6 x 10 A. The Zn concentration produced by the Zn-tipped capillary evaluated from the current was 2.2 x 10 M. Assuming that the Na ion was the analyte ion, the concentration of the ions produced by the oxidation at the electrode is only 1 /5 of that of the analyte. It will be shown later that the electrospray current increases very slowly with the total electrolyte concentration. Therefore, ions produced by oxidation at the electrode may not be noticed in the mass spectrum at higher analyte or additive concentrations. 

Figure 1.7 (a) Total electrospray current (ampere) with increasing concentration of analyte morphine hydrochloride. Because of the presence of impurity ions (Na and NH ) at 10 M, /total remains constant up to the point where the analyte reaches concentrations above 10 M. (b) Analyte MorH ion intensity (corrected for mass-dependent ion transmission, Tm, of... 

The two previous examples demonstrate that high salt concentration has a deleterious effect on electrospray response. However, it is worth noting that the addition of smaller amounts of salt can actually improve electrospray response (even of analytes that are not detected as salt clusters) (Figure 2.11). The mechanisms responsible for this effect have been postulated to be increased electrospray current with increasing salt concentration (that effectively increases [Q ) and/or decreased thickness of the electrical double layer at the surface of electrospray droplets. ... 

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