Sodium adduct formation efficiency in ESI source

Abstract

Formation of sodium adducts in electrospray (ESI) has been known for long time, but has not been used extensively in practice, and several important aspects of Na(+) adduct formation in ESI source have been almost unexplored: the ionization efficiency of different molecules via Na(+) adduct formation, its dependence on molecular structure and Na(+) ion concentration in solution, fragmentation behaviour of the adducts as well as the ruggedness (a prerequisite for wider practical use) of ionization via Na(+) adduct formation. In this work, we have developed a parameter describing sodium adducts formation efficiency (SAFE) of neutral molecules and have built a SAFE scale that ranges for over four orders of magnitude and contains 19 compounds. In general, oxygen bases have higher efficiency of Na(+) adducts formation than nitrogen bases because of the higher partial negative charge on oxygen atoms and competition from protonation in the case of nitrogen bases. Chelating ability strongly increases the Na(+) adduct formation efficiency. We show that not only protonation but also Na(+) adduct formation is a quantitative and reproducible process if relative measurements are performed.

Transferability of the Electrospray Ionization Efficiency Scale between Different Instruments

“…The mechanisms of ESI are complex and numerous studies [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] have been carried out for its elucidation. It has been shown that the electrospray ionization efficiency (IE) [16] of an analyte is influenced by its molecular structure as well as the solvent (including its pH and additives) and the MS system.…”

“…For a better understanding of the ESI process, IE scales, in both positive [7,9] and negative [8] mode, established for fixed solvent compositions are useful. In both modes, IE prediction models have been obtained [3,7,8,20].…”

“…The mass analyzers used most for electrospray ionization mechanism studies are ion-trap [3,4,[7][8][9][10] and triple quadrupole [2,6,19,20]. Only in few cases, direct comparison of the obtained results is possible because in most cases the studied compounds and solvent systems differ.…”

“…Two types of ESI sprayers were used: conventional (see Supplementary Figure S1 in the Supplementary Material) and in-house developed 3-R sprayer [7] (see Supplementary Figure S4). MS and ESI parameters used for the conventional sprayer were: nebulizer gas pressure 15 psi, drying gas flow rate 7 L/min, drying gas temperature 300°C, and only target mass (TM) was optimized (see reference [9]); for 3-R sprayer [21], nebulizer gas pressure 2 psi, drying gas flow rate 10 L/min, drying gas temperature 350°C, and inner capillary gas pressure 12 bar. For both setups the needle voltage was 3500 V.…”

“…Thus in logRIE calculations, the signals of the protonated forms and their fragmentation products are taken into account (the intensities of all thoe signals were added), but not those corresponding to sodium or ammonium adducts. The fragments are taken into account because it has been proven that they are formed from protonated ions [9]. The consistencies of both ionization efficiency scales-via protonation [7] and via sodium adduct formation [9] -demonstrate that these ionization pathways can be dealt with separately.…”