Risto Kostiainen

Effect of solvent on dynamic range and sensitivity in pneumatically-assisted electrospray (ion spray) mass spectrometry

Mass-analyzed detector signal and spray current have been measured in pneumatically-assisted electrospray mass spectrometry. The sample was tetrabutylammonium bromide dissolved in water, methanol, acetonitrile, chloroform, dichloromethane or toluene. At low sample concentrations (less than or equal to 5 x 10(-6) M) the ion signal rose with increasing sample concentration. Above 10(-5) M the ion signal was fixed and independent of sample concentration, Comparison of signals with spray currents for tetrabutylammonium bromide at 2 x 10(-6) M in different solvents revealed a strong correlation between ion signal and spray current. Apparently, the abundance of the tetrabutylammonium ion at m/z 242 is fully controlled by the amount of charge on droplets, while other solvent properties such as volatility, surface tension and polarity do not play a role at low tetrabutylammonium bromide concentrations. Thus, water is a poor solvent for electrospray because it does not allow efficient droplet charging, not because it is less volatile and more difficult to spray than organic solvents. The ion signal at 2 x 10(-6) M tetrabutylammonium bromide in different solvents is highest for dichloromethane. At high sample concentrations (greater than or equal to 10(-5) M) the dependence of the ion signal on spray current is lost. It appears impossible to convert a high charge on droplets into sample ions. Creation of droplets having a surface fully covered with sample is assumed to be the cause of ion signal saturation. Increasing the sample concentration will only increase the number of ions inside a droplet. The number of sample ions at the surface escaping into the gas phase is fixed and independent of sample concentration above 10(-5) M.

Development of a membrane inlet mass spectrometric method for analysis of air samples

A sheet membrane inlet mass spectrometric (MIMS) method for the on-line analysis of volatile organic compounds (VOCs) in air at low μg/m3 levels was developed. The effects of the thickness of the membrane, the temperature of the membrane inlet and the flow rate of the sample on the responses and response times of some selected VOCs were studied in detail. Under optimised conditions the detection limits for the VOCs studied were 0.5–4 g m−3 and linear dynamic ranges were about four orders of magnitude. Response times of only a few seconds were recorded with a thin (25 μm) membrane. The developed MIMS method was applied to an analysis of air samples from an exhaust of a paintshop, where volatile organic solvents were used.

Accurate Mass Measurements of Some Glucuronide Derivatives by Electrospray Low Resolution Quadrupole Mass Spectrometry

The measurement of exact masses using negative-ion quadrupole low resolution electrospray mass spectrometry (ESMS) is demonstrated with p-nitrophenol, entacapone, mitecapone and 1-hydroxypyrene glucuronides. The results are compared with those obtained by high resolution fast-atom bombardment mass spectrometry (FAB-MS). The errors with ESMS between measured and calculated values ranged from −1.7 to 3.1 ppm (−0.8 to 1.2 milli-mass units (mmu). The standard deviations of the measurements were between ±1.1 and 4.8 ppm (±0.5 and 2.3 mmu). The errors with high resolution FAB-MS were between −2.5 and 3.2 ppm (−1.1 and 1.0 mmu). These results show that accurate mass measurements using low resolution quadrupole ESMS are comparable to those achievable in practice using high resolution instruments. A precondition for a successful measurement with low resolution mass spectrometry is that the sample ion be completely separated from background ions, since even very small levels of unresolved ions can interfere with the accuracy of mass analysis.

Fragmentation of protonated O,O-diethyl O-aryl phosphorothionates in tandem mass spectral analysis

The gas-phase ion chemistry of protonated O,O-diethyl O-aryl phosphorothionates was studied with tandem mass spectrometric and ab initio theoretical methods. Collision-activated dissociation (CAD) experiments were performed for the [M+H]+ ions on a triple quadrupole mass spectrometer. Various amounts of internal energy were deposited into the ions upon CAD by variation of the collision energy and collision gas pressure. In addition to isobutane, deuterated isobutane C4D10 also was used as reagent gas in chemical ionization. The daughter ions [M+H−C2H4]+ and [M+H−2C2H4]+ dominate the CAD spectra. These fragments arise via various pathways, each of which involves γ-proton migration. Formation of the terminal ions [M+H−2C2H4−H2O]+, [M+H−2C2H4−H2S]+, [ZPhOH2]+, [ZPhSH2]+, and [ZPhS]+ [Z = substituent(s) on the benzene ring] suggests that (1) the fragmenting [M+H]+ ions of O,O-diethyl O-aryl phosphorothionates have protons attached on the oxygen of an ethoxy group and on the oxygen of the phenoxy group; (2) thiono-thiolo rearrangement by aryl migration to sulfur occurs; (3) the fragmenting rear-ranged [M+H]+ ions have protons attached on the oxygen of an ethoxy group and on the sulfur of the thiophenoxy group. To get additional support for our interpretation of the mass spectrometric results, some characteristics of three protomers of O,O-diethyl O-phenyl phosphorothionate were investigated by carrying out ab initio molecular orbital calculations at the RHF/3–21G* level of theory.