Glass microfabricated nebulizer chip for mass spectrometry

A microfluidic nebulizer chip for mass spectrometry is presented. It is an all-glass device which consists of fusion bonded Pyrex wafers with embedded flow channels and a nozzle at the chip edge. A platinum heater is located on the wafer backside. Fabrication of the chip is detailed, especially glass deep etching, wafer bonding, and metal patterning. Various process combinations of bonding and metallization have been considered (anodic bonding vs. fusion bonding; heater inside/outside channel; metallization before/after bonding; platinum lift-off vs. etching). The chip vaporizes the liquid sample (0.1-10 microL min(-1)) and mixes it with a nebulizer gas (ca. 100 sccm N2). Operating temperatures can go up to 500 degrees C ensuring efficient vaporization. Thermal insulation of the glass ensures low temperatures at the far end of the chip, enabling easy interconnections.     

Distributed Jacobi eigen-beamforming for closed-loop MIMO systems

This work presents a novel eigen-beamforming technique suitable for tracking fading channels in multiple-input multiple-output (MIMO) systems employing low feedback rate, closed-loop feedback channels. The method uses a single unitary coordinate plane transformation to update the transmit antenna weights without requiring parameters that depend on fading rate, typically found in recursive methods. The computational complexity is fixed, in contrast with codebook solutions, whose operations count increases linearly with the codebook size. Simulations show that the new eigen-beamforming method reaches near optimal bit error probability performance in low mobility scenarios. Furthermore, by buffering the available feedback bits the proposed algorithm outperforms signed gradient methods for the same average feedback rate     

Blocking Behavior of Covalently Attached Anthraquinone Towards Solution‐Based Redox Probes

The electrochemical properties of glassy carbon (GC) electrodes modified with 9,10‐anthraquinone (AQ) have been investigated. Electrografting of GC surface was carried out from the solution of the AQ diazonium derivative. The blocking action of GC/AQ electrodes for Fe(CN)$\rm{{_{6}^{3-}}}$ and Ru(NH₃)$\rm{{_{6}^{3+}}}$ redox probes was studied using cyclic voltammetry (CV) and the rotating disk electrode (RDE) method. It was established that the extent of blocking was a function of AQ surface concentration. A peculiar behavior was observed at the potentials of AQ reduction.     

Capacity of Generalized UTRA FDD Closed-Loop Transmit Diversity Modes

Transmit diversity techniques have received a lot of attention recently, and open-loop and closed-loop downlink transmit diversity modes for two transmit antennae have been included into universal terrestrial radio access (UTRA) frequency division duplex (FDD) specification. Closed-loop modes provide larger system capacity than open-loop modes, but they need additional side information of the downlink channel in the transmitter. In FDD systems this requires a separate feedback channel. Quantization of channel state information (CSI) in closed-loop transmit diversity schemes decreases the performance when compared to a closed-loop system where the transmitter has access to complete CSI. In this paper, we analyze the effect of quantization of CSI and deduce approximate capacity formulae for closed-loop transmit diversity schemes that are generalizations of the closed-loop schemes included in UTRA FDD specification. Moreover, we calculate approximation error and show by simulations that our approximation is tight for flat Rayleigh fading environments with and without fast transmit power control.     

Integrated liquid chromatography-heated nebulizer microchip for mass spectrometry

A new integrated microchip for liquid chromatography-mass spectrometry (LC-MS) is presented. The chip is made from bonded silicon and glass wafers with structures for a packed LC column channel, a micropillar frit, a channel for optional optical detection, and a heated vaporizer section etched in silicon and platinum heater elements on the glass cover. LC eluent is vaporized and mixed with nebulizer gas in the vaporizer section and the vapor is sprayed out from the chip. Nonpolar and polar analytes can be efficiently ionized in the gas phase by atmospheric pressure photoionization (APPI) as demonstrated with polycyclic aromatic hydrocarbons (PAHs) and selective androgen receptor modulators (SARMs). This is not achievable with present LC-MS chips, since they are based on electrospray ionization, which is not able to ionize nonpolar compounds efficiently. The preliminary quantitative performance of the new chip was evaluated in terms of limit of detection (down to 5 ng mL⁻¹), linearity (r > 0.999), and repeatability of signal response (RSD = 2.6-4.0%) and retention time (RSD = 0.3-0.5%) using APPI for ionization and PAHs as standard compounds. Determination of fluorescent compounds is demonstrated by using laser-induced fluorescence (LIF) for detection in the optical detection channel before the vaporizer section.     

Determination of optical parameters of pulp suspensions by time-resolved detection of photoacoustic signals and total diffuse reflectance measurements

Time-resolved photoacoustics were used to measure the optical parameters of pulp suspensions for the first time. Reconstructing stress distribution along the direction of the incident laser light allows the effective attenuation coefficient of these suspensions to be determined. Simultaneously, the total diffuse reflectance of the suspensions was measured by the same laser source. Based on the effective attenuation coefficient and total diffuse reflectance, the absorption and reduced scattering coefficients of pulp suspensions can be calculated. In this study, three kinds of pulp suspensions with different kappa number (2, 13, and 16), a measure of lignin content in pulp fibers, were diluted with water to make samples with a consistency range from 1% to 5%, and studied at 355 nm wavelength. The results showed that the optical coefficients were approximately proportional to pulp consistency; on the other hand, the absorption coefficient was linearly correlated with kappa number, but the reduced scattering coefficient was not. Therefore, by determining its optical parameters, it is possible to extract the consistency and kappa number of an unknown pulp suspension.     

Feasibility of SU-8-based capillary electrophoresis-electrospray ionization mass spectrometry microfluidic chips for the analysis of human cell lysates

Monolithically integrated, polymer (SU-8) microchips comprising an electrophoretic separation unit, a sheath flow interface and an ESI emitter were developed to improve the speed and throughput of proteomics analyses. Validation of the microchip method was performed based on peptide mass fingerprinting and single peptide sequencing of selected protein standards. Rapid, yet reliable identification of four biologically important proteins (cytochrome C, β-lactoglobulin, ovalbumin and BSA) confirmed the applicability of the SU-8 microchips to ambitious proteomic applications and allowed their use in the analysis of human muscle cell lysates. The characteristic tryptic peptides were easily separated with plate numbers approaching 10(6), and with peak widths at half height as low as 0.6 s. The on-chip sheath flow interface was also exploited to the introduction of an internal mass calibrant along with the sheath liquid which enabled accurate mass measurements by high-resolution Q-TOF MS. Additionally, peptide structural characterization and protein identification based on MS/MS fragmentation data of a single tryptic peptide was obtained using an ion trap instrument. Protein sequence coverages exceeding 50% were routinely obtained without any pretreatment of the proteolytic samples and a typical total analysis time from sampling to detection was well below ten minutes. In conclusion, monolithically integrated, dead-volume-free, SU-8 microchips proved to be a promising platform for fast and reliable analysis of complex proteomic samples. Good analytical performance of the microchips was shown by performing both peptide mass fingerprinting of complex cell lysates and protein identification based on single peptide sequencing.     

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.     

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.