Atmospheric pressure thermospray ionization using a heated microchip nebulizer

When a standard atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI) ion source is used without applying the corona discharge or photoirradiation, atmospheric pressure thermospray ionization (APTSI) of various compounds can be achieved. Although largely ignored, this phenomenon has recently gained interest as an alternative ionization technique. In this study, this technique is performed for the first time on a miniaturized scale using a microchip nebulizer. Sample ionization with the presented microchip‐APTSI (µAPTSI) is achieved by applying only heat and gas flow to a nebulizer chip, without any other methods to promote gas‐phase ionization. To evaluate the performance of the described µAPTSI setup, ionization efficiency for a set of test compounds was monitored as the microchip positioning, temperature, nebulizer gas flow rate, sample solution composition, and solvent flow rate were varied. The µAPTSI mass spectra of the test compounds were also compared to those obtained with ESI and APCI. The µAPTSI produces ESI‐like spectra with low background noise, favoring the formation of protonated or deprotonated molecules of compounds that are ionizable in solution. Multiple charging of peptides without in‐source fragmentation was also observed. Unlike ESI, however, the µAPTSI source can tolerate the presence of mobile phase additives like trifluoroacetic acid (TFA) without significant ion suppression. The µAPTSI source can be used with standard mass spectrometer ion source hardware, being a unique alternative to the present interfacing techniques. Copyright © 2009 John Wiley & Sons, Ltd.     

Chargeability of ethanol–petrol biofuels

Chargeability of ethanol–petrol biofuels during refuelling has been studied in real dispenser environment in order to assess safety risks due to fuel charging at fuel filling stations. Two biofuel blends were studied: E10 containing 10 vol-% of ethanol and 90 vol-% of petrol, and E85 containing about 85 vol-% of ethanol and 15 vol-% of petrol. Charging of standard 95 Octane petrol was studied as a reference. The results show that the charging of E85 is negligible and no charge will be accumulated as long as the fuel dispenser system is properly grounded. In the case of refuelling with E10, charge is accumulated but the level of total charge is still so low that no real electrostatic ignition hazards exist due to fuel charging at filling stations as long as the system is properly grounded. Electrostatic ignition hazards due to fuel charging are real only for standard petrol fuel.     

Surface assisted laser desorption/ionization on two-layered amorphous silicon coated hybrid nanostructures

Matrix-free laser desorption/ionization was studied on two-layered sample plates consisting of a substrate and a thin film coating. The effect of the substrate material was studied by depositing thin films of amorphous silicon on top of silicon, silica, polymeric photoresist SU-8, and an inorganic-organic hybrid. Des-arg⁹-bradykinin signal intensity was used to evaluate the sample plates. Silica and hybrid substrates were found to give superior signals compared with silicon and SU-8 because of thermal insulation and compatibility with amorphous silicon deposition process. The effect of surface topography was studied by growing amorphous silicon on hybrid micro- and nanostructures, as well as planar hybrid. Compared with planar sample plates, micro- and nanostructures gave weaker and stronger signals, respectively. Different coating materials were tested by growing different thin film coatings on the same substrate. Good signals were obtained from titania and amorphous silicon coated sample plates, but not from alumina coated, silicon nitride coated, or uncoated sample plates. Overall, the strongest signals were obtained from oxygen plasma treated and amorphous silicon coated inorganic-organic hybrid, which was tested for peptide-, protein-, and drug molecule analysis. Peptides and drugs were analyzed with little interference at low masses, subfemtomole detection levels were achieved for des-arg⁹-bradykinin, and the sample plates were also suitable for ionization of small proteins.     

Capillary liquid chromatography-microchip atmospheric pressure chemical ionization-mass spectrometry

A miniaturized nebulizer chip for capillary liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (capillary LC-microchip APCI-MS) is presented. The APCI chip consists of two wafers, a silicon wafer and a Pyrex glass wafer. The silicon wafer has a DRIE etched through-wafer nebulizer gas inlet, an edge capillary insertion channel, a stopper, a vaporizer channel and a nozzle. The platinum heater electrode and pads for electrical connection were patterned on to the Pyrex glass wafer. The two wafers were joined by anodic bonding, creating a microchip version of an APCI-source. The sample inlet capillary from an LC column is directly connected to the vaporizer channel of the APCI chip. The etched nozzle in the microchip forms a narrow sample plume, which is ionized by an external corona needle, and the formed ions are analyzed by a mass spectrometer. The nebulizer chip enables for the first time the use of low flow rate separation techniques with APCI-MS. The performance of capillary LC-microchip APCI-MS was tested with selected neurosteroids. The capillary LC-microchip APCI-MS provides quantitative repeatability and good linearity. The limits of detection (LOD) with a signal-to-noise ratio (S/N) of 3 in MS/MS mode for the selected neurosteroids were 20-1000 fmol (10-500 nmol l(-1)). LODs (S/N = 3) with commercial macro APCI with the same compounds using the same MS were about 10 times higher. Fast heat transfer allows the use of the optimized temperature for each compound during an LC run. The microchip APCI-source provides a convenient and easy method to combine capillary LC to any API-MS equipped with an APCI source. The advantages and potentials of the microchip APCI also make it a very attractive interface in microfluidic APCI-MS.     

New surfaces for desorption electrospray ionization mass spectrometry: porous silicon and ultra-thin layer chromatography plates

The performance of nanoporous silicon (pSi) and ultra-thin layer chromatography (UTLC) plates as surfaces for desorption electrospray ionization (DESI) was compared with that of polymethyl methacrylate (PMMA) and polytetrafluoroethylene (PTFE), both popular surfaces in previous DESI studies. The limits of detection (LODs) and other analytical characteristics for six different test compounds were determined using all four surfaces. The LODs for the compounds were in the fmol-pmol (pg-ng) range. The LODs with the pSi surface were further improved for each of the compounds when heat was applied to the surface during sample application which gave LODs as low as or lower than those achieved with PMMA and PTFE. The UTLC plates were successfully used as a rapid means of chromatographic separation prior to DESI-MS analysis. Another advantage achieved using the newer pSi and UTLC surfaces was increased speed of analysis, associated with drying of solution-phase samples. This took place immediately at the UTLC surface and it could be achieved rapidly by gently heating the pSi surface. The presence of salts in the sample did not cause suppression of the analyte signal with any of the surfaces.     

Desorption electrospray ionization mass spectrometry for the analysis of pharmaceuticals and metabolites

The performance of desorption electrospray ionization (DESI) in the analysis of a group of pharmaceuticals and their glucuronic acid conjugates is reported. The suitability of different sprayer solvents and different surfaces was examined. In the positive ion mode, water/methanol/trifluoroacetic acid performed best, whereas, in the negative ion mode, water/methanol/ammonium hydroxide was found to be the most suitable spray solvent. Of the surfaces investigated, polymethylmethacrylate (PMMA) was found to give the best performance in terms of sensitivity. Spray solution flow rate and the distance of the sprayer tip from the surface were also found to have significant effects on the signal intensity. Analytes with basic groups efficiently formed the corresponding protonated molecules in the positive ion mode, whereas acidic analytes, such as the glucuronic acid conjugates, formed intense signals due to the deprotonated molecules in the negative ion mode. Ionization of neutral compounds was less efficient and in many cases it was achieved through adduct formation with simple anions or cations.     

Minimum proton affinity for efficient ionization with atmospheric pressure desorption/ionization on silicon mass spectrometry

We performed a systematic study using a set of compounds with different proton affinities (PAs) on the ionization in atmospheric pressure desorption/ionization on silicon mass spectrometry (AP-DIOS-MS). The compounds studied included various aromatic molecules of different sizes. The PAs of these compounds were calculated using ab initio and hybrid density functional theory calculations at the B3LYP/6-31G(d) level of theory. We observed that only compounds with relatively high PAs above a threshold value of 920-950 kJ/mol were efficiently ionized as protonated molecules under AP-DIOS conditions and produced very clean mass spectra.     

Novel water-soluble quaternary piperazine derivatives of chitosan: synthesis and characterization

Novel synthesis methods for the preparation of quaternary piperazine derivatives of chitosan were developed. Quaternary ammonium moiety can be selectively inserted into either one or both of the piperazine nitrogens, yielding structurally uniform chitosan derivative structures. Water-soluble end products were thoroughly characterized with FT-IR, 1H NMR, 13C NMR and 2D 1H-13C HSQC NMR. The molecular weights of the end products were determined by GPC with triple detection.     

Sequence dependence of DNA translocation through a nanopore

We investigate the dynamics of DNA translocation through a nanopore using 2D Langevin dynamics simulations, focusing on the dependence of the translocation dynamics on the details of DNA sequences. The DNA molecules studied in this work are built from two types of bases A and C, which have been shown previously to have different interactions with the pore. We study DNA with repeating blocks A(n)C(n) for various values of n and find that the translocation time depends strongly on the block length 2n as well as on the orientation of which base enters the pore first. Thus, we demonstrate that the measurement of translocation dynamics of DNA through a nanopore can yield detailed information about its structure. We have also found that the periodicity of the block sequences is contained in the periodicity of the residence time of the individual nucleotides inside the pore.