A. van Asten

Optimization and evaluation of the performance of arrangements for UV detection in high-resolution separations using fused-silica capillaries

Abstract The evaluation of arrangements for UV measurements in fused-silica capillaries as applied in, e.g., high-performance capillary electrophoresis, capillary liquid chromatography and hydrodynamic chromatography is described. The various designs are characterized with respect to sensitivity, noise, linearity, contribution to the peak width and sensitivity to refractive index effects. Some guidelines are formulated, based on theoretical and experimental observations, in order to maximize the performance. A cell with a focusing lens in front of the capillary resulted in a higher sensitivity and linear range than a cell with an adjustable aperture width. With the U-cell, having a design which is comparable to Z-shaped cells and having a longer longitudinal light path, a substantial increase in signal-to-noise ratio was achieved. Some applications of the UV cells are shown.

Temperature dependence of solvent viscosity, solvent thermal conductivity, and Soret coefficient in thermal field-flow fractionation.

Abstract In this work equations are presented to describe solute concentration profiles in thermal field-flow fractionation that account for the effect of the temperature drop across the solute zone on the thermal and ordinary diffusion coefficients. The influence of this effect, together with the effects of the temperature dependence of the solvent viscosity and solvent thermal conductivity, on the conversion of retention data into thermal diffusion data were studied. The systematic error made when the ratio of the thermal and ordinary diffusion coefficient (α/T) is assumed to be constant can be considerable and is larger for systems with low retention (e.g., for low molecular weight or small thermal diffusion). For the two systems studied (polystyrene in THF and in ethylbenzene), it was found that the temperature dependence of the solvent viscosity is of much greater importance than the temperature dependence of the solvent thermal conductivity. When all three parameters are considered to be independent...

Determination of the compositional heterogeneity of polydisperse polymer samples by the coupling of size-exclusion chromatography and thermal field-flow fractionation

Abstract An off-line coupling of size-exclusion chromatography (SEC) and thermal field-flow fractionation (ThFFF) was used successfully to cross-fractionate copolymers and polymer blends. Various fractions of different molecular mass were obtained from polydisperse polymer samples by SEC. Because the molecular diffusion coefficients were known, the ThFFF analysis of these fractions yielded directly thermal diffusion coefficients. As thermal diffusion is strongly affected by the chemical nature of the polymer, the chemical composition of polymer samples can be studied as function of the molecular mass with this technique. This is illustrated with the SEC-ThFFF cross-fractionation of a polystyrene sample blended with a polybutadiene and a polytetrahydrofuran standard, and of butadiene- and styrene-methylmethacrylate copolymers.

Characterization of thermal diffusion of polystyrene in binary mixtures of THF/dioxane and THF/cyclohexane

The thermal diffusion coefficient (Dτ) was determined for three polystyrene standards of different molecular masses in binary mixtures of tetrahydrofuran/dioxane and tetrahydrofuran/cyclohexane of various compositions. The Dτ values were obtained by combining retention data from thermal field-flow fractionation measurements with diffusion data from dynamic light scattering experiments. In agreement with earlier work of Schimpf and Giddings, the thermal diffusion coefficient was found to be virtually independent of the molecular mass of the polymers. In the binary mixtures of tetrahydrofuran and dioxane, both good solvents for polystyrene, the Dτ value was approximately equal to the average of the Dτ values in the pure solvents, weighted according to the mole fractions of the solvents in the mixture. However, for polystyrene in binary mixtures of tetrahydrofuran and cyclohexane this linear behavior of the thermal diffusion phenomenon was not observed. The addition of cyclohexane to tetrahydrofuran has initially only a minor effect on the molecular and thermal diffusion coefficients of the polystyrene standards. Because cyclohexane is a theta solvent for polystyrene, the preferential solvation of polystyrene by tetrahydrofuran could be an explanation for these results.

Use of thermal field flow fractionation for the fractionation of polybutadiene in various organic solvents

Abstract It is shown that thermal field flow fractionation (ThFFF) can be a valuable tool for the fractionation of polybutadiene (PB). The thermal diffusivity of four PB standards of different molecular mass was determined in six organic solvents. The results indicate that for this polymer species, the thermal diffusion coefficient varies significantly with the choice of the solvent but seems to be independent of the molecular mass. The highest retention for a given molecular mass was found with toluene and ethylbenzene as solvents. With respect to the speed of separation of polybutadiene, the best results were obtained with toluene. The influence of the temperature drop across the channel and the polymer concentration on retention was also studied. The use of an evaporative light- scattering detector made it possible to work with polymer sample concentrations as low as 0.1 mg/ml. The unique separation possibilities offered by ThFFF are demonstrated with the separation of polybutadiene, polyisoprene and polystyrene standards of the same molecular mass using tetrahydrofuran as solvent.

Study of thermal diffusion of polybutadiene and polytetrahydrofuran in various organic solvents

The thermal diffusion coefficient (Dτ) was determined for polybutadiene and polytetrahydrofuran (i.e., polyoxytetramethylene) standards of different molecular masses in various organic solvents. The combination of thermal field-flow fractionation retention data with diffusion data from dynamic light scattering or diffusion NMR experiments yielded the thermal diffusion coefficients. In agreement with earlier work on thermal diffusion of polymers in solution, the thermal diffusion coefficient was found to be independent of the molecular mass of the polymers. However, this work once more indicates that the Dτ value is strongly influenced by the chemical composition of both the polymer and the solvent.

Thermal field flow fractionation of polytetrahydrofuran

Abstract The possibility to fractionate polytetrahydrofuran (PTHF) samples effectively with Thermal Field Flow Fractionation has been investigated. Retention measurements of four standards of different molecular mass were performed in eight organic solvents. When methylethylketone or dioxane were used no retention was found. Highest retention for a given molecular mass was obtained with the solvents toluene and ethylbenzene. However, in ethyl acetate the highest separation speed was observed. Plate height measurements showed that both the thermal and molecular diffusion coefficients of the PTHF standards are very high in ethyl acetate. According to theory, this combination should result in a high separation speed. A baseline separation of two PTHF standards with molecular masses of 67 000 and 282 300 could be obtained in 8 minutes when ethyl acetate was used as the solvent.

Impurity profiling of trinitrotoluene using vacuum-outlet gas chromatography-mass spectrometry

In this work, a reliable and robust vacuum-outlet gas chromatography-mass spectrometry (GC-MS) method is introduced for the identification and quantification of impurities in trinitrotoluene (TNT). Vacuum-outlet GC-MS allows for short analysis times; the analysis of impurities in TNT was performed in 4min. This study shows that impurity profiling of TNT can be used to investigate relations between TNT samples encountered in forensic casework. A wide variety of TNT samples were analyzed with the developed method. Dinitrobenzene, dinitrotoluene, trinitrotoluene and amino-dinitrotoluene isomers were detected at very low levels (<1wt.%) by applying the MS in selected-ion monitoring (SIM) mode. Limits of detection ranged from 6ng/mL for 2,6-dinitrotoluene to 43ng/mL for 4-amino-2,6-dinitrotoluene. Major impurities in TNT were 2,4-dinitrotoluene and 2,3,4-trinitrotoluene. Impurity profiles based on seven compounds showed to be useful to TNT samples from different sources. Statistical analysis of these impurity profiles using likelihood ratios demonstrated the potential to investigate whether two questioned TNT samples encountered in forensic casework are from the same source.

Large area imaging of forensic evidence with MA-XRF

This study introduces the use of macroscopic X-ray fluorescence (MA-XRF) for the detection, classification and imaging of forensic traces over large object areas such as entire pieces of clothing and wall paneling. MA-XRF was sufficiently sensitive and selective to detect human biological traces like blood, semen, saliva, sweat and urine on fabric on the basis of Fe, Zn, K, Cl and Ca elemental signatures. With MA-XRF a new chemical contrast is introduced for human stain detection and this can provide a valuable alternative when the evidence item is challenging for conventional techniques. MA-XRF was also successfully employed for the chemical imaging and classification of gunshot residues (GSR). The full and non-invasive elemental mapping (Pb, Ba, Sr, K and Cl) of intact pieces of clothing allows for a detailed shooting incident reconstruction linking firearms and ammunition to point of impact and providing information on the shooting angle. In high resolution mode MA-XRF can even be used to provide information on the shooting order of different ammunition types. Finally, by using the surface penetration of X-rays we demonstrate that the lead signature of a bullet impact can be easily detected even if covered by multiple layers of wall paint or human blood.

P9: A closer look to the effect of external influences on cocaine in hair using imaging mass spectrometry

Introduction Forensic hair analysis is considered to be a standard method in identifying chronic drug users since it can give information about the time course of the substance abuse. Interpreting these results, one should be aware of all pitfalls. External contamination, decontamination protocols, cosmetic hair treatment,… They all might influence the analytical result of forensic hair analysis. Since the laborious standard extraction method used nowadays is inefficient in pointing out these pitfalls, there is an urgent need in new analysis methods that can help to unravel what is happening inside the hair. Imaging mass spectrometry is shown here as a very promising tool in studying the effect of external influences on the spatial distribution of cocaine in hair. Methods Matrix-assisted laser desorption/ionization mass spectrometric imaging (MALDI-MSI) and Metal Assisted Secondary Ion Mass spectrometry (MetA-SIMS) is used to visualize the effect of hydrogen peroxide treatment and different decontamination solvents on cocaine contaminated and users hair. Images are made on intact hair as well as cross and longitudinal sections. Cross sectioning is carried out on carboxymethylcellulose embedded hair using a microtome. Longitudinal sections are prepared using a self-designed cutting device without any use of embedding medium. Results After bleaching contaminated and users hair with hydrogen peroxide, images revealed a complete loss of detectability of cocaine (m/z 304 and 182) and its reaction products. Since all compounds of interest are found in the hydrogen peroxide and wash solution, these findings indicate that all evidence of cocaine use might be lost after a hair bleaching treatment. The comparison of different decontamination solvents shows that cocaine is most effectively removed using methanol or water. Remarkably, MALDI-TOF and SIMS images of cross and longitudinal sections of contaminated and cocaine users hair show that methanol and water also seem to influence the concentration of cocaine in the hair. SIMS images of unwashed hair are able to distinguish between external contaminated and users hair, making it very interesting as forensic evidence. Conclusion We show that imaging mass spectrometry is a very promising technique in studying the spatial distribution of drugs in hair. Being able to have a closer look to what is happening in the hair, the effect of external influences on drug incorporation and distribution can finally be solved. The presented study surprisingly shows the influence on drug distribution in the hair using certain decontamination solvents and also reveals the possibility of complete loss of forensic evidence in bleached hair.