Possible amounts of oxygen in the carrier gas as compared to sample amounts

Summary The amount of oxygen which may be present in the gas volume corresponding to a peak is investigated and compared to the sample amounts. It is shown that even with high purity carrier gases one may have a significant excess of oxygen present at low sample amounts. Attention is drawn to the possibility of impurity build-up by diffusion e.g. through the septum.     

Thermally stimulated carrier transport due to stepwise sample heating

The article concerns non-isothermal dispersive carrier transport in an insulating solid with traps. The approximate solutions of transport equations derived previously are extended to the case of stepwise sample heating. The specific features of thermally stimulated currents (TSCs) can be attributed to non-linear dependence of the demarcation level on temperature. In particular, the initial TSC rise has a thermally activated character where activation energy equals to the demarcation energy at the end of previous heating cycle. The accuracy of the formulae describing TSCs is verified by Monte Carlo calculations for Gaussian trap distribution.     

Selection of carrier gas velocity in the analysis of a multicomponent sample

Summary The selection of the average linear carrier gas velocity in the analysis of a multicomponent mixture under isothermal and programmed-temperature conditions is discussed. It is shown that one shouls always select a velocity which is at least equal but preferably higher than the optimum average gas velocity of the earliest peak at the initial column temperature.These symbols essentially correspond to those specified by the British Standard [6], ASTM [7] and IUPAC [8] GC nomenclastures.     

Accurate calibration of TXRF using microdroplet samples

TXRF has been applied in combination with VPD to the analysis of trace impurities in the native oxide layer of Si wafer surfaces down to the range of 10⁸ atoms · cm^–2. Proper quantification of VPD/TXRF data requires calibration with microdroplet standard reference wafers. The precision of calibration function has been evaluated and found to allow quantification at a high level of 3 confidence with microdroplet standard reference.     

Modified field amplification sample injection for micellar electrokinetic chromatography of neutral compounds with amino-substituted cyclodextrin as carrier and 1-adamantanecarboxylate as displacer

A modified field amplification sample injection method was proposed and evaluated by using positively mono charged cyclodextrin (CD) as carrier and 1-adamantanecarboxylate as displacer for on-capillary preconcentration of neutral compounds and improvement of the concentration limit of detection in micellar electrokinetic chromatography. In modified sample injection mode a displacer plug was introduced before sample injection to reduce the length of the concentrated sample zone and increase the peak height by slowing down the forward movement of the neutral sample associated with beta-CD-NH2 and the backward movement of the neutral sample partitioned in the micelles of sodium dodecyl sulfate. Stability of the inclusion complexes formed between the carrier and the solute was found to be an important factor affecting stacking efficiency in both the conventional field amplification sample injection mode and the modified one. However, further enhancement of the stacking efficiency in the modified mode rested on the relative stability of the displacer-carrier complex to that of the solute-carrier complex. Practical limits to the stacking efficiencies in both modes were discussed as well.     

TXRF analysis of aged three way catalysts

A methodology for the quantitative analysis of the elements present in car exhaust catalysts by TXRF is described. Some of the most relevant catalytic components (Zr, Rh, Pd) and contaminant elements (P, Pb, Zn, Pt, Ca and Ni) on car Three Way Catalysts were quantified during vehicle aging. The study was conducted along the main axis of a Ford Focus 1.6i catalytic cartridge, aiming to obtain the axial profile of the elements retained or eliminated after more than 59,000 km. The analysis of a fresh Three Way Catalysts (0 km) was also conducted for comparative purposes. The existence of a strong differences for elemental concentration in the interphase between the two blocks of the catalytic cartridge has been quantitatively measured for first time. In addition, the behaviour of the contaminant absorption capacity of both catalytic blocks has been obtained. Finally, the loss of the contaminant elements Pb and Zn to the environment has been proven.     

A formula for the background in TXRF as a function of the incidence angle and substrate material

A simple formula has been developed that allows the calculation of the background contributed by the substrate in TXRF both exactly and in absolute terms. The scattered X-radiation from any surface is described as a function of the incidence angle and of the X-ray cross sections for scattering and total absorption of the substrate material. The formula has been confirmed experimentally for silicon, acryl and gold. Measured and calculated values agree within reasonable limits. The deviations between predicted and measured data decrease with increasing angle of incidence from up to 20% in the low angle regime down to a few percent beyond the critical angle of total reflection as a result of the decreasing influence of surface roughness.     

Accurate calibration of TXRF using microdroplet samples

TXRF has been applied in combination with VPD to the analysis of trace impurities in the native oxide layer of Si wafer surfaces down to the range of 10(8) atoms. cm(-2). Proper quantification of VPD/TXRF data requires calibration with microdroplet standard reference wafers. The precision of calibration function has been evaluated and found to allow quantification at a high level of 3 sigma confidence with microdroplet standard reference.     

Dextran as antioxidant's activity carrier

Two series of conjugates of dextran and antioxidants from the class of sterically hindered phenols were prepared. The conjugates were characterized by the substitution degree of glycoside units, solubility in different solvents, intrinsic viscosity. The investigation of radical scavenging activity (RSA) of conjugates was carried out in their reactions with two free radicals — 2,2-diphenyl-1-picrylhydrazyl (DPPH) and sodium salt of 2,2-diphenyl-1-picrylhydrazyl sulfonic acid (DPPH-salt). The usage of water soluble DPPH-salt enabled to estimate the RSA of the conjugates in water. It was shown that the rate constants of interaction of the DPPH-salt and the conjugates were 10–30 times higher than this value for low-molecular analogue of phenoxan. High RSA of the conjugates in water can be explained by large solvation shell formed due to high content of hydroxy groups in dextran.     

Elemental analysis of airborne dust samples with TXRF: Comparison of oxygen-plasma ashing on sapphire carriers and acid digestion for sample preparation

The concentrations of 13 elements (As, Ca, Co, Cd, Cr, Fe, Ga, Ni, Pb, Pt, Ti, V and Zn) were determined in air particulate matter using total reflection X-ray fluorescence spectrometry (TXRF). For silicon analysis synthetic sapphire was chosen as a new sample carrier material – it is silicon-free, resistant to oxygen-plasma, microwaves and concentrated acids. The dust samples were collected on cellulose acetate filters. The decomposition of the filters was carried out by oxidation in a microwave-generated low-pressure oxygen-plasma directly on the TXRF sample carriers. The recovery of the investigated elements was verified with the standard reference material SRM 1648 (urban particulate matter) and ranged from 90 to 97%. The oxygen-plasma method was compared with conventional sample preparation by acid digestion. Received: 9 April 1999 / Revised: 18 May 1999 / Accepted: 3 June 1999     

Elemental analysis of airborne dust samples with TXRF: Comparison of oxygen-plasma ashing on sapphire carriers and acid digestion for sample preparation

The concentrations of 13 elements (As, Ca, Co, Cd, Cr, Fe, Ga, Ni, Pb, Pt, Ti, V and Zn) were determined in air particulate matter using total reflection X-ray fluorescence spectrometry (TXRF). For silicon analysis synthetic sapphire was chosen as a new sample carrier material – it is silicon-free, resistant to oxygen-plasma, microwaves and concentrated acids. The dust samples were collected on cellulose acetate filters. The decomposition of the filters was carried out by oxidation in a microwave-generated low-pressure oxygen-plasma directly on the TXRF sample carriers. The recovery of the investigated elements was verified with the standard reference material SRM 1648 (urban particulate matter) and ranged from 90 to 97%. The oxygen-plasma method was compared with conventional sample preparation by acid digestion. Received: 9 April 1999 / Revised: 18 May 1999 / Accepted: 3 June 1999     

Determination of trace elements in blood samples by TXRF analysis

Trace elements were determined by TXRF in whole blood samples in a randomly non-occupational exposed population living in the Metropolitan Zone of the Mexico Valley (MZMV). Arithmetic and geometric means of S, Ca, Cu, Zn, Rb and Pb concentrations, were on the reported range values for non-occupational population in other countries, while those of K and Br were higher, possibly due to dietary habits and geographical or environmental factors. The noticeable decline in blood lead level (91%) should be positively perceived. As a part of the Quality Control Program, a certified IAEA-A13 was tested. This revised version was published online in August 2006 with corrections to the Cover Date.     

Optimization of TXRF measurements by variable incident angles

Variable incident angles in TXRF instrumentation open up new possibilities in the field of analytical quality assurance of TXRF measurements as well as the possibility of optimizing the measurement angle with respect to the sample carrier. Measurements on the same sample with different incident angles allow a check to be made on the behavior of the internal standard and the elements under investigation within the sample, which makes quantification more reliable, even for difficult samples. This is demonstrated on the example of standard reference material NIST 1633a comparing the relative fluorescence intensities of the elements K, Ti and Fe obtained from a sample prepared from a suspension and a digestion of the SRM material, respectively. Furthermore, it will be shown how the measurement conditions for different sample carrier materials such as quartz and acrylic glass can be optimized by measuring angular-dependent signal and background intensities.     

Carbon nanotubes as separation carrier in capillary electrophoresis

The utility and versatility of carboxylic single-walled carbon nanotubes (c-SWNT) in capillary electrophoresis (CE) is demonstrated, using as model solutes homologues and structural isomers. In the case of homologues of caffeine and theobromine, distinct changes in the electrophoretic parameters occur at a critical concentration of c-SWNT in the run buffer. It is suggested that the c-SWNT of a definite concentration could form a network in the run buffer as a pseudostationary phase on the basis of the unique tubule structure, providing a different separation from sodium dodecyl sulfate (SDS) micelles. In the case of structural isomers of catechol and hydroquinone, differing from the homologues, it is mainly attributable to the functional groups on the c-SWNT that have an effect on the electrophoretic behaviors by forming intermolecular hydrogen bonding with analytes. Furthermore, aggregated c-SWNT serve as anticonvective media and minimize solute diffusion contributing to zone broadening. The presence of charged c-SWNT suppressed the electrodiffusion and decreased the adsorption between capillary wall and solutes, which led to better peak shapes of isomers.     

Two trends of sample dispersion variation with carrier flow rate in a single flow-injection manifold

Two trends of sample dispersion variation with carrier flow rate in a single flow-injection manifold are completely revealed for the first time and an inflection point in the dispersion coefficient (D) vs. flow rate (q) curve is discovered. With the increase of the flow rate, the value of D increases before the inflection point but decreases after the inflection point. The value of the carrier flow rate at the inflection point (q_m) is independent of the sample injection volume, the tube length, the tube coil radius and the tube inner diameter. It is only affected by the substance diffusion coefficient (D_m) of the analysis. The value of q_m decreases as D_m increases. Therefore, the value of D_m for a sample can be estimated according to the D_m vs. q_m curve.     

Microtome sections of biomaterials for trace analyses by TXRF

Summary Different biomaterials were prepared as thin sections (up to 20 m thick, 8 mm in diameter) by means of a freezing microtome on a quartz-glass carrier and analyzed directly by total-reflection X-ray fluorescence (TXRF). They were originally available either as compact samples, as powder material or as solutions. About 20 elements with atomic numbers between 15 and 40 (P and Zr) and between 57 and 92 (La and U) were determined simultaneously with detection limits of some g/g. — The method was applied to the analysis of plant and animal foodstuff (nuts, mushrooms, shrimps) and of different organic tissue (human lung). The TXRF-spectra, like fingerprints, facilitate the easy characterization of samples and turn out to be very suitable for screening and monitoring tasks.

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Mikrotomschnitte von Biomaterialien zur Spurenanalyse durch TRFA

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Dedicated to Prof. Dr. W. Fresenius in gratitude on the occasion of his 75th birthday