Desorption Electrospray Ionization Source Research Articles

Surface effects and electrochemical cell capacitance in desorption electrospray ionization

Time resolved measurements show that during a desorption electrospray ionization (DESI) experiment, the current initially rises sharply, followed by an exponential decrease to a relatively steady current. When the high voltage on the spray emitter is switched off, the current drops to negative values, suggesting that the direction of current flow in the equivalent DESI circuit is reversed. These data demonstrate that the DESI source behaves as a dc capacitor and that the addition of a surface between the sprayer and the counter electrode in DESI introduces a new electrically active element into the system. The charging and discharging behavior was observed using different surfaces and it could be seen both by making current measurements on a plate at the entrance to the mass spectrometer as well as by measuring ion current in the linear ion trap within the vacuum system of the mass spectrometer. The magnitude of the steady state current obtained without analyte present on the surface is different for different surface materials, and different capacitor time constants of the equivalent RC circuits were calculated for different DESI surfaces. The PTFE surface has by far the greatest time constant and is also able to produce the highest DESI currents. Surface properties play a crucial role in charge transfer during DESI in addition to the effects of the chemical properties of the analyte. It is suggested that surface energy (wettability) is an important factor controlling droplet behavior on the surface. The experimental data are correlated with critical surface tension values of different materials. It is proposed, based on the results presented, that super-hydrophobic materials with extremely high contact angles have the potential to be excellent DESI substrates. It is also demonstrated, using the example of the neurotransmitter dopamine, that the surface charge that develops during a DESI-MS experiment can cause electrochemical oxidation of the analyte.

Ambient Mass Spectrometry with a Handheld Mass Spectrometer at High Pressure

The first coupling of atmospheric pressure ionization methods, electrospray ionization (ESI) and desorption electrospray ionization (DESI), to a miniature hand-held mass spectrometer is reported. The instrument employs a rectilinear ion trap (RIT) mass analyzer and is battery-operated, hand-portable, and rugged (total system: 10 kg, 0.014 m(3), 75 W power consumption). The mass spectrometer was fitted with an atmospheric inlet, consisting of a 10 cm x 127 microm inner diameter stainless steel capillary tube which was used to introduce gas into the vacuum chamber at 13 mL/min. The operating pressure was 15 mTorr. Ions, generated by the atmospheric pressure ion source, were directed by the inlet along the axis of the ion trap, entering through an aperture in the dc-biased end plate, which was also operated as an ion gate. ESI and DESI sources were used to generate ions; ESI-MS analysis of an aqueous mixture of drugs yielded detection limits in the low parts-per-billion range. Signal response was linear over more than 3 orders of magnitude. Tandem mass spectrometry experiments were used to identify components of this mixture. ESI was also applied to the analysis of peptides and in this case multiply charged species were observed for compounds of molecular weight up to 1200 Da. Cocaine samples deposited or already present on different surfaces, including currency, were rapidly analyzed in situ by DESI. A geometry-independent version of the DESI ion source was also coupled to the miniature mass spectrometer. These results demonstrate that atmospheric pressure ionization can be implemented on simple portable mass spectrometry systems.

Non-Proximate Detection of Small and Large Molecules by Desorption Electrospray Ionization and Desorption Atmospheric Pressure Chemical Ionization Mass Spectrometry: Instrumentation and Applications in Forensics, Chemistry, and Biology

Ambient surfaces are examined by mass spectrometry at distances of up to 3 m from the instrument without any prior sample preparation. Non-proximate versions of the desorption electrospray ionization (DESI) and desorption atmospheric pressure chemical ionization experiments are shown to allow rapid, sensitive, and selective detection of trace amounts of active ingredients in pharmaceutical drug formulations, illicit drugs (methamphetamine, cocaine, and diacetylmorphine), organic salts, peptides, chemical warfare agent simulants, and other small organic compounds. Utilizing an ion transport tube to transport analyte ions to the mass spectrometer, nonproximate DESI allows one to collect high-quality, largely interference-free spectra with signal-to-noise (S/N) ratios of more than 100. High selectivity is achieved by tandem mass spectrometry and by reactive DESI, a variant experiment in which reagents added into the solvent spray allow bond-forming reactions with the analyte. Ion/molecule reactions were found to selectively suppress the response of mixture components other than the analyte of interest in nonproximate-DESI. Flexible ion transport tubing is also investigated, allowing performance similar to stainless steel tubing in the transport of ions from the sample to the mass spectrometer. Transfer tube temperature effects are examined. A multiple sprayer DESI source capable of analyzing a larger sample area was evaluated to decrease the sampling time and increase sample throughput. Low nanogram detection limits were obtained for the compounds studied from a wide variety of surfaces, even those present in complex matrixes.

Desorption Electrospray Ionization in a Small Pressure-Tight Enclosure

Desorption electrospray ionization (DESI) is made easier to implement, more robust, and exposure of the analyst to the sample is reduced by a novel DESI interface. This features a small, pressure-tight enclosure with fixed spatial relationships between the sprayer, surface, and sampling capillary. There is no loss in signal using this configuration, which provides improved safety and ease of use. Performance is largely independent of the geometrical configuration of the spray and inlet. Enclosed DESI sources with the incident and collection capillaries set at various angles were compared to the conventional optimized geometry open DESI source. Comparable results were obtained in all cases for typical proteins, peptides, and small molecules. With the spray and collection capillaries set parallel and at an angle perpendicular to the surface, the new DESI source allows for easy and direct analysis of the contents of standard 96-well plates. The effects of various operating parameters for the new source such as applied spray potential, liquid and gas flow rates, and enclosure material were investigated.

On-probe pyrolysis desorption electrospray ionization (DESI) mass spectrometry for the analysis of non-volatile pyrolysis products

An on-probe pyrolyzer has been constructed and interfaced with desorption electrospray ionization (DESI) mass spectrometry (MS) for the rapid analysis of non-volatile pyrolysis products. The detection and analysis of non-volatile pyrolysis products of peptides, proteins and the synthetic polymer poly(ethylene glycol) were demonstrated with this instrument. The on-probe pyrolyzer can be operated off-line or on-line with the DESI source and was interfaced with a tandem MS (MS/MS) instrument, which allowed for structure characterization of the non-volatile pyrolytic products. Advantages of this system are its simplicity and speed of analysis since the pyrolysis is performed in situ on the DESI source probe and hence, it avoids extraction steps and/or the use of matrices (e.g., as in MALDI–MS analyses).