Preliminary investigation of electrodynamic charged droplet processing to couple capillary liquid chromatography with matrix-assisted laser desorption/ionization mass spectrometry

Charged droplet processing methodology, that utilizes electrodynamic levitation technology to control the trajectories of picoliter volume charged droplets and deliver them to a target plate at atmospheric pressure, has been developed. Termed wall-less sample preparation (WaSP), this methodology offers several features that could prove beneficial to the preparation of sample spots from separation column effluents for matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) analysis. These features include solute pre-concentration factors of 10(1) to 10(3) due to volatile solvent evaporation prior to droplet deposition onto the target plate, high spatial accuracy of the deposition position of each processed droplet (+/-5 microm), and the ability to prepare sample spots as small as 20 microm in diameter from a single droplet. Here a new mode of operation of this methodology is described and used as an offline post-column pre-concentrating interface between capillary liquid chromatography (capLC) and a target plate for offline MALDI-MS. Using a fraction from the capLC separation of peptides produced by the proteolytic digestion of the protein cytidine 5'-triphosphate:phosphocholine cytidylyltransferase, MALDI sample spots were prepared using the dried-droplet method, direct piezoelectric droplet dispensing, and the processing of piezo-dispensed droplets by WaSP. The sample spot morphology was investigated using light microscopy, and peptide ion abundances produced by MALDI were measured using time-of-flight (TOF) MS. The advantages of developing an online capLC/WaSP interface with MALDI-MS in the future are discussed along with some of the challenges that may be encountered in such an endeavor.     

Computer simulation investigation of the water-benzene interface in a broad range of thermodynamic States from ambient to supercritical conditions

The dependence of the properties of the water-benzene system on the thermodynamic conditions in a broad range of temperatures and pressures has been investigated by computer simulation methods. For this purpose, Monte Carlo simulations have been performed at 23 different thermodynamic states, ranging from ambient to supercritical conditions. The density profiles of the water and benzene molecules have been determined at each of the thermodynamic states investigated. Information on the dependence of the mutual solubility of the two components in each other as well as of the width of the interface on the temperature and pressure has been extracted from these profiles. The width of the interface has been found to increase with increasing temperature up to a certain point, where it diverges. The temperature of this divergence corresponds to the mixing of the two phases. The determination of the critical mixing temperature at various pressures allowed us to estimate the upper critical curve, separating the two-phase and one-phase liquid systems, of the phase diagram of the simulated water-benzene system. In analyzing the preferential orientation of the interfacial molecules relative to the interface, it has been found that the main orientational preference of the benzene molecules is to lie parallel with the plane of the interface, and the water molecules penetrated deepest into the benzene phase prefer to stay perpendicular to the interface, pointing by one of their O-H bonds almost straight toward the benzene phase, whereas the waters located at the aqueous side of the interface are preferentially aligned parallel with the interfacial plane. Although the strength of the observed orientational preferences decreases rapidly with increasing temperature, the preferred orientations themselves are found to be independent of the thermodynamic conditions. Remains of the orientational preferences of the molecules are found to be present up to temperatures as high as 650 K. The analysis of the relative orientation of the neighboring water-benzene pairs has revealed that the radius of the first hydration shell of the benzene molecules is independent of the thermodynamic conditions, even if the system consists of one single phase. It has been found that the nearest water neighbors of the benzene molecules are preferentially located above and below the benzene ring, whereas more distant water neighbors, belonging still to the first hydration shell, prefer to stay within the plane of the benzene molecule. In the two-phase systems the dipole vector of the nearest waters has been found to be preferentially perpendicular to the vector pointing from the center of the benzene molecule to the water O atom.     

The use of a highly sulfated cyclodextrin for the simultaneous chiral separation of amphetamine-type stimulants by capillary electrophoresis

We investigated the simultaneous chiral separation of nine amphetamine type stimulants (dl-norephedrine, dl-norpseudoephedrine, dl-ephedrine, dl-pseudoephedrine, dl-amphetamine, dl-methamphetamine, dl-methylenedioxyamphetamine (MDA), dl-methylenedioxymethamphetamine (MDMA), and dl-methylenedioxyethylamphetamine (MDEA)) by capillary electrophoresis using highly sulfated gamma-cyclodextrin (SU(XIII)-gamma-CD) as a chiral selector. Three different approaches using SU(XIII)-gamma-CD with 50 mM phosphate background electrolyte were designed; (I) high CD concentration (10 mM SU(XIII)-gamma-CD) at neutral pH (pH 7.0) in the normal polarity mode, (II) low CD concentration (1.0 mM) at low pH (pH 2.6) in the normal polarity mode and (III) high CD concentration at low pH (pH 2.6) in the reversed-polarity mode. In mode (II), the effects of adding three neutral CDs (beta-CD, dimethyl-beta-CD and hydroxypropyl-beta-CD) were also investigated. The best separation was obtained after optimizing mode (III) as follows: run buffer of 10 mM SU(XIII)-gamma-CD with 50 mM phosphate background electrolyte at pH 2.6, applied voltage of -12 kV and capillary temperature of 15 degrees C.     

Polarographic analysis of the reduction waves of Pt(II)-SATSC complex

A polarographic study of the Pt(II)-salicylaldehyde thiosemicarbazone complex in sodium perchlorate as supporting electrolyte is described. In addition to the reduction wave of the complex, a catalytic hydrogen wave is also recorded. The characteristics of this catalytic hydrogen wave are studied under different experimental conditions such as varying acid/complex/supporting electrolyte/surfactant concentrations and also with mercury droptime. Based on these, a probable mechanism for the electrode process has been postulated. Presented at the 12th Annual Symposium in Chemistry held at the Indian Institute of Technology, Madras in March 1987.     

Guanidinophosphazenes: design, synthesis, and basicity in THF and in the gas phase

A principle for creating a new generation of nonionic superbases is presented. It is based on attachment of tetraalkylguanidino, 1,3-dimethylimidazolidine-2-imino, or bis(tetraalkylguanidino)carbimino groups to the phosphorus atom of the iminophosphorane group using tetramethylguanidine or easily available 1,3-dimethylimidazolidine-2-imine. Seven new nonionic superbasic phosphazene bases, tetramethylguanidino-substituted at the P atom, have been synthesized. Their base strengths are established in tetrahydrofuran (THF) solution by means of spectrophotometric titration and compared with those of eight reference superbases designed specially for this study, P2- and P4-iminophosphoranes. The gas-phase basicities of several guanidino- and N',N',N',N'-tetramethylguanidino (tmg)-substituted phosphazenes and their cyclic analogues are calculated, and the crystal structures of (tmg)3P=N-t-Bu and (tmg)3P=N-t-Bu x HBF4 are determined. The enormous basicity-increasing effect of this principle is experimentally verified for the tetramethylguanidino groups in the THF medium: the basicity increase when moving from (dma)3P=N-t-Bu (pKalpha = 18.9) to (tmg)3P=N-t-Bu (pKalpha = 29.1) is 10 orders of magnitude. A significantly larger basicity increase (up to 20 powers of 10) is expected (based on the high-level density functional theory calculations) to accompany the similar gas-phase transfer between the (dma)3P=NH and (tmg)3P=NH bases. Far stronger basicities still are expected when, in the latter two compounds, all three dimethylamino (or tetramethylguanidino) fragments are replaced by methylated triguanide fragments, (tmg)2C=N-. The gas-phase basicity (around 300-310 kcal/mol) of the resulting base, [(tmg)2C=N-]3P=NH, having only one phosphorus atom, is predicted to exceed the basicity of (dma)3P=NH by more than 40 powers of 10 and to surpass also the basicity of the widely used commercial [(dma)3P=N]3P=N-t-Bu (t-BuP4) superbase.     

Mass spectral fragmentation mechanisms of some dibenzo[c,f][1,2] diazepines

The fragmentation mechanisms of 11H-dibenzo[c,f][1,2]diazepine (I), its 3,8-dichloro derivative (II), 3,8-dichlorodibenzo[c,f] [1,2]diazepin-11-one (III) and 3,8-dichloro-11H-dibenzo-[c,f][1,2]diazepin-N-oxide (IV) are discussed. The initial loss of molecular nitrogen is characteristic of I, II and III. Compound IV has a strong molecular ion, that competitively eliminates cither NO or Cl- and N₂O. The common radical ion, m/166 e present in the mass spectra of I, fluorene, 2-methyl-9,10-anthraquinone and 2-methylbenzo[c]cinnoline, appears to be formed in different states.     

Effects of steroidal carriers of alkylating agents on the phase transition in DPPC membrane bilayers

Several steroidal esters of alkylating agents have been synthesized and tested in vitro and in vivo in various experimental cancer types. 3β-Hydroxy-17α-aza-D-homo-5-androsten-7,17-dione-N,N-bis(2-chloroethyl) aminophenylacetate (I) is a highly active compound. DSC scans show differences between the alkylating agent alone and in conjugation with the steroidal part in the broadening and lowering of the phase transition of DPPC bilayers. These differences may in part explain the better pharmacokinetic profile and lower toxicity of conjugated congener I versus the alkylating agent alone.     

Reaction mechanism of deoxyribonucleotidase: a theoretical study

The reaction mechanism of human deoxyribonucleotidase (dN) is studied using high-level quantum-chemical methods. dN catalyzes the dephosphorylation of deoxyribonucleoside monophosphates (dNMPs) to their nucleoside form in human cells. Large quantum models are employed (99 atoms) based on a recent X-ray crystal structure. The calculations support the proposed mechanism in which Asp41 performs a nucleophilic attack on the phosphate to form a phospho-enzyme intermediate. Asp43 acts in the first step as an acid, protonating the leaving nucleoside, and in the second step as a base, deprotonating the lytic water. No pentacoordinated intermediates could be located.     

A combined μ-mercury reference electrode/Au counter-electrode system for microelectrochemical applications

Different types of mercury-based μ-reference electrodes (Hg/Hg₂SO₄/Na₂SO₄, Hg/Hg₂(CH₃COO)₂/NaCOOCH₃) have been developed following the concept of agar-based μ-reference electrodes. Mercury was electrochemically deposited onto a gold wire to form an amalgam. The corresponding mercury salt was formed electrochemically at the surface. This electrode can be inserted into a capillary that is filled with the electrolyte of interest. To simplify the handling of this μ-reference electrode, to reduce diffusion and to avoid leakage, the electrolyte was immobilised with agar. A 250-nm-thick gold layer on the outer surface of the capillary of the reference electrode served as counter-electrode. The electrochemical behaviour of reference electrodes and counter-electrodes were proven by micro-polarisation curves, electrochemical impedance spectroscopy, potential transients and cyclic voltammetry.     

Uncertainty sources in UV-Vis spectrophotometric measurement

An overview is given of the most important uncertainty sources that affect analytical UV-Vis spectrophotometric measurements. Altogether, eight uncertainty sources are discussed that are expected to have influence in chemical analysis. It is demonstrated that the well-known intrinsic (or “physical”) sources of uncertainty that originate from the instrument itself (repeatability of spectrophotometer reading, spectrophotometer drift, stray light, etc.) often have significantly lower contributions to the combined uncertainty of the result than the “chemical” sources of uncertainty that originate from the object under study (interference from the constituents of the matrix, decomposition of the photometric complex, etc.). Although selectivity of a photometric procedure is often considered more a validation topic than an uncertainty topic, it is very often important to include it also in the uncertainty budget.Usually the most difficult part of uncertainty estimation of a chemical measurement result is to evaluate the magnitude of the actual uncertainty components, especially the chemical ones. For most of the uncertainty sources discussed in this paper, approaches for their evaluation are given. A generic uncertainty budget for absorbance is presented. Electronic Supplementary Material Supplementary material is available for this article at