Is there a ‘matrix suppression effect’ under fast‐atom bombardment liquid secondary ion mass spectrometry of ionic surfactants in glycerol?

Abstract

Some features of a 'matrix suppression effect' caused by ionic surface-active compounds under fast-atom bombardment (FAB) liquid secondary ion mass spectrometry (LSIMS) are being revised. It is shown that abundant transfer of the glycerol matrix molecules to the gas phase does occur under FAB-LSIMS of ionic surfactants, contrary to popular belief. This process can be obscure because of the dependence of the charge state of the glycerol-containing cluster ions on the type of ionic surfactant. It is revealed that, while glycerol matrix signals are really completely suppressed in the positive ion mass spectra of cationic surfactants (decamethoxinum, aethonium), abundant deprotonated glycerol and glycerol-anion clusters are recorded in the negative ion mode. In the case of an anionic surfactant (sodium dodecyl sulfate), on the contrary, glycerol is completely suppressed in the negative ion mode, but is present in the protonated and cationized forms in the positive ion mass spectra. It is suggested that such patterns of positive and negative ion FAB-LSIMS spectra of ionic surfactants solutions reflect the structure and composition of the electric double layer formed at the vacuum-liquid interface by organic cations or anions and their counterions. Processes leading to the formation of the glycerol-containing ions preferentially of positive or negative charge are discussed. The most obvious of them is efficient binding of glycerol to inorganic counterions of the salts Cl S or Na R , which is confirmed by data from quantum chemical calculations. The high content of the counterions and relatively small content of glycerol in the sputtered zone may be responsible for the charge-selective suppression of neat glycerol clusters of opposite charge to the counterions. In the case of a mixture of cationic and anionic surfactants the substitution of inorganic counterions by organic ones was observed. The dependence of the exchange rate in the surface layer is not a linear function of the bulk solution concentration, and an effect of abrupt recharging of the surface can be registered. No both positively or negatively charged pure glycerol and glycerol-inorganic counterion clusters are recorded for the mixture. Correlations between the mass spectrometric observations and some phenomena of surface and colloid chemistry and physics are discussed.

‘Wet chemistry’ and crystallochemistry reasons for acidic matrix suppression by quaternary ammonium salts under matrix‐assisted laser desorption/ionization conditions

“…In the FAB mass spectra of surface-active decamethoxinum and 2,5-DHB solution in glycerol two characteristic groups of ions (with appropriate isotopic distribution) with comparable abundance corresponding to Cat 2þ Á Cl À (reported earlier in FAB/ SIMS studies on decamethoxinum 22,23 ) and Cat 2þ Á (DHB-H) À were recorded, pointing to the occurrence of partial anion exchange. It is important here that we probe not some separate 'ion pair' associates in the liquid, but a continuous two-dimensional surface layer formed by the cations and anions of the surface-active organic salt.…”

“…It is important here that we probe not some separate 'ion pair' associates in the liquid, but a continuous two-dimensional surface layer formed by the cations and anions of the surface-active organic salt. 23 Detection of the Cat 2þ Á (DHB-H) À ion is a firm confirmation of the binding of (DHB-H) À to Cat 2þ or, more precisely, of the incorporation of (DHB-H) À anions into the surface monolayer formed by the decamethoxinum surfactant (with a composition correlating roughly with the composition of micelles formed in solution). 23 Since FAB/SIMS sputtering affects mainly the surface layers of a sample, a 'competition for the surface' dependent on surface activity of the analytes causes the suppression effect in FAB/ SIMS.…”

“…23 Detection of the Cat 2þ Á (DHB-H) À ion is a firm confirmation of the binding of (DHB-H) À to Cat 2þ or, more precisely, of the incorporation of (DHB-H) À anions into the surface monolayer formed by the decamethoxinum surfactant (with a composition correlating roughly with the composition of micelles formed in solution). 23 Since FAB/SIMS sputtering affects mainly the surface layers of a sample, a 'competition for the surface' dependent on surface activity of the analytes causes the suppression effect in FAB/ SIMS. 24 Analyte compounds which are not incorporated into such a surface layer are 'invisible' in FAB spectra.…”

“…Complete glycerol (G) matrix suppression is observed in the positive ion mass spectra of decamethoxinum, while abundant production of [nG-H] À (n ¼ 1, 2) ions is seen in the negative ion mode. 23 In spite of the apparent similarity with the suppression of the matrix signals in only one charge state under MALDI conditions, the cluster of the decamethoxinum dication with glycerol matrix anion Cat 2þ Á (G-H) À is not detected in the FAB/SIMS spectra. At the same time the Cat 2þ Á (DHB-H) À cluster appears on the addition of DHB to the solution.…”

“…Since DHB is not a surfactant, it can be extracted into the surface layer occupied by the ionic surfactant only as a counterion in the surface electric double layer formed by the decamethoxinum organic dication and the original Cl À anions. 23 It is interesting that MSE shows up in relation to glycerol liquid matrix in the presence of surfactants under FAB/ SIMS conditions, 23,24 although its nature and mechanisms 23 are completely different from those of MSE in MALDI. Complete glycerol (G) matrix suppression is observed in the positive ion mass spectra of decamethoxinum, while abundant production of [nG-H] À (n ¼ 1, 2) ions is seen in the negative ion mode.…”