Vahid Majidi

Two simple interface designs for capillary electrophoresis–inductively coupled plasma mass spectrometry†

The design and implementation of two different interfaces for capillary electrophoresis–inductively coupled plasma mass spectrometry (CE–ICP-MS) are described. These interfaces will allow for on-line analysis of CE effluents with ICP-MS detection. One interface is based on a concentric tube nebulizer and the other on a standard cross-flow nebulizer. These systems were investigated in parallel and their performances, under various experimental conditions, were compared. Each interface possesses a unique set of advantages and shortcomings. Recognizing that typical sample flow rates for ICP-MS are of the order of ml min–1 and that the flow rates for CE are a few nl min–1, some difficulties in flow compatibility are encountered. Aspects discussed include interface considerations, flow compatibility and the influence of flow rates on the overall sensitivity. Several guidelines are provided for workers interested in implementing a CE–ICP-MS instrument for elemental speciation. The Cd detection limits in rabbit metallothionein were 2.36 and 0.21 µg ml–1 for the concentric and cross-flow nebulizers, respectively.

Spectral, spatial and temporal characteristics of a millisecond pulsed glow discharge: metastable argon atom production

Abstract Time resolved atomic emission, atomic absorbance, and laser-induced atomic fluorescence measurements of a millisecond pulsed glow discharge, made perpendicular to the insertion probe, provide temporal profiles of 1s 5 ( 3 P 2 ) and 1s 3 ( 3 P 0 ) metastable argon atom populations. Acquisition of these profiles at different spatial positions in the plasma provides data from which two-dimensional spatial plots of relative populations are constructed. Each map, the result of 368 individual pulse profiles, provides insight into the production of metastable argon atoms as a function of time and position within the plasma. During power application, intensities plateau after 3 ms as the plasma reaches a steady state condition. Metastable argon atoms are most abundant 1–2 mm above the cathode surface during this time. Excitation mechanisms such as electron excitation and fast atom/ion impact appear to dominate in this temporal regime. In contrast, argon ion–electron recombination dominates metastable formation after pulse termination. The relative population maximum for metastable argon atoms in the afterpeak shifts to 5–9 mm above the cathode surface. This shift should impact signals for analyte species generated by Penning processes in the plasma. Absorption and fluorescence measurements of the 3 P 2 (11.55 eV) and the 3 P 0 (11.72 eV) metastable argon atom states indicate possible differences in the populations of these two states between the plateau and afterpeak time regimes.

Monitoring the cellular response of Stichococcus bacillaris to exposure of several different metals using in vivo 31P NMR and other spectroscopic techniques

The cellular response of a green algae, Stichococcus bacillaris, to sudden exposure of Zn 2+ , Mn 2+ , Cd 2+ , and Cu 2+ was investigated. The algae adsorbed these metal cations on the cell wall initially, and the adsorption equilibrium was established within 3 min. Later, some of the adsorbed metals were either transported inside the algae or released into the solution. The transport of metals inside the algae was demonstrated by their interaction with intracellular polyphosphate. The release of the adsorbed metals into solution is probably the result of detoxification actions. The interaction of copper with algae was found to be unique in many ways (...)

Detection of Visible and Latent Fingerprints Using Micro-X-ray Fluorescence Elemental Imaging*

ABSTRACT: Using micro‐X‐ray fluorescence (MXRF), a novel means of detecting fingerprints was examined in which the prints were imaged based on their elemental composition. MXRF is a nondestructive technique. Although this method requires a priori knowledge about the approximate location of a print, it offers a new and complementary means for detecting fingerprints that are also left pristine for further analysis (including potential DNA extraction) or archiving purposes. Sebaceous fingerprints and those made after perspiring were detected based on elements such as potassium and chlorine present in the print residue. Unique prints were also detected including those containing lotion, saliva, banana, or sunscreen. This proof‐of‐concept study demonstrates the potential for visualizing fingerprints by MXRF on surfaces that can be problematic using current methods.

Spectral, spatial and temporal characterization of a millisecond pulsed glow discharge: copper analyte emission and ionization

Abstract Two-dimensional maps of the spatial distributions of excited and ionized sputtered copper atoms are presented for a millisecond pulsed argon glow discharge. These maps demonstrate the temporal as well as spatial dependence of different excitation and ionization processes over the pulse cycle. Transitions from the low energy electronic states for the atom, characterized by emission such as that at 324.75 nm (3.82→0.00 eV), dominate the plateau time regime at a distance of 2.5 mm from the cathode surface. These processes originate from the electron excitation of ground state copper atoms. Transitions from high-energy electronic states, such as that characterized by emission at 368.74 nm (7.16→3.82 eV), predominate during the afterpeak time regime at a distance of 5.0–6.0 mm from the cathode surface. This observation is consistent with the relaxation of highly excited copper atoms produced by electron recombination with copper ions during the afterpeak time regime. Analyses of afterpeak and plateau intensities for a series of copper emission lines indicate an electron excitation temperature equivalent to 5.78 eV at 0.8 torr and 1.5 W. Temporal profiles exhibit copper ion emission only during the plateau time regime.

Consideration of a millisecond pulsed glow discharge time-of-flight mass spectrometer for concurrent elemental and molecular analysis

The glow discharge ionization source operated in the pulsed (or modulated) power mode offers unique characteristics not available from its steady state counterpart. It has been well established that higher instantaneous power densities are obtainable without compromising the sample integrity when pulsed plasmas are implemented. This operating parameter affords higher sputter yields and lower limits of detection relative to the steady state plasmas. Of special interest are the discrete temporal regions associated with the modulated plasma. The presence of these time regimes offers temporal selectivity, allowing the collection of analytical data in a region where the contribution from background and contaminant species is minimized. These regions are characterized by strikingly different ionization mechanisms. Acquisition of data during each of these temporal regimes provides both molecular and elemental information. In this work the potential use of the pulsed glow discharge for collecting concurrent molecular and elemental information was explored. This task was accomplished using time-of-flight mass spectrometry (TOFMS). TOFMS has a significantly high throughput and duty cycle, making it ideally suited for rapid acquisition of spectra. This characteristic allows data acquisition during each of these temporal regions for each discharge pulse power cycle, affording concurrent elemental and molecular detection. p-Xylene was used as a test molecule for these studies.

Potential sources of error in capillary electrophoresis–inductively coupled plasma mass spectrometry for chemical speciation†

The distribution concentration of chemical species in a sample is dictated by the physical and chemical properties of the matrix. As such, when a sample is pre-treated, in any way, there is a potential for redistribution of homologous species. The extent of this analyte redistribution is determined by both thermodynamic properties of species (e.g., changes in concentrations of species according to their equilibrium expressions) and kinetic properties (e.g., the rate of the reactions compared with the duration of sample preparation and analysis). The redistributions of analyte species as a function of several experimental parameters (e.g., time, solution pH, injection methods and calibration methods) are illustrated in this paper. Whereas rabbit metallothionein protein showed a stability of more than a few days under certain storage conditions, coenzyme-B12 was rapidly degraded in less than 2 h. pH studies showed that the migration of free Cd2+ ions in rabbit metallothionein was not significantly affected unless the pH of the solution exceeds the solubility limit of the metal hydroxide. However, pH-sensitive compounds such as vitamin B12 showed significant changes in the migration time and analyte composition. The injection studies suggested that electrokinetic injection may produce biased results, in favor of species that have higher electrophoretic mobility. Hydrodynamic injection will produce a result that is more representative of the initial sample composition.

Error analysis for sampling of slurries: volumetric errors

The propagation of errors associated with the sampling of heterogeneous solutions consisting of small particles in a liquid media (e.g., slurries) is considered. Errors associated with uncertainties in the sample volume, the number of particles extracted in the sampling volume and the variation in the mass of the individual particles are considered in the analysis. A random Gaussian distribution for the error in the average mass of the particle and volume sampled is assumed. A relatively simple equation showing the relative standard deviation of the sampled mass is presented and examples of the graphs obtained are given. For samples containing large diameter particles, the errors resulting from the inability to pipette a fraction of a particle are dominant. In contrast, errors in the pipetted volume on particle size distribution are most significant for solutions containing small particles. The equations can be used with estimates of the particle size distribution to evaluate the extent of sample inhomogeneity. Alternatively, calculations can be used in making an a priori evaluation of the average particle radii, size distribution and sampling volume required to minimise sampling errors.

Inductively Coupled Plasma Mass Spectrometry for Elemental Speciation: Applications in the New Millennium

Abstract The current status of elemental speciation using inductively coupled plasma‐mass spectrometry (ICP‐MS) as the method of detection is examined in this paper. Manuscripts that describe specific speciation applications using on‐line hyphenation of gas chromatography (GC), liquid chromatography (LC), capillary electrophoresis (CE), and field flow fractionation (FFF) with ICP‐MS detection are presented. This review covers the application papers published between January 2000 and December 2003. Evaluation of the current literature shows that LC is the most prevalent separation technique for elemental speciation followed by CE, GC, and FFF, respectively.

Influence of discharge parameters on real-time chemical speciation for gas chromatography pulsed glow discharge plasma time-of-flight mass spectrometry

A millisecond pulsed glow discharge ionization source was employed for the determination of aromatic and chlorinated hydrocarbons. Chemical speciation employed four separate dimensions that incorporate analyte volatility, elemental constituents, chemical structure, and molecular weight. Analyses were performed in real time using a gas chromatograph that was interfaced to a pulsed glow discharge time-of-flight mass spectrometer. This instrument allowed for the sequential collection of the complete mass spectrum at three different time regimes occurring during the glow discharge pulse cycle. This report focuses on discharge parameters such as pressure and sampling distance that enhance or suppress different ionization processes. Increasing the ion source pressure resulted in a decrease in analyte ionization and sampling efficiency. Ions sampled in close proximity to the cathode surface demonstrated the best elemental sensitivity. Sampling ions at increasing distance, in the negative glow, produced structural information in the form of fragmentation patterns similar to conventional 70 eV electron impact spectra. Molecular weight information obtained after power termination was ascertained from M+ and MH+ ions. At very small distances, the most dominant molecular peak was the M+ ion produced by the Penning process. At increased sampling distances molecular peaks were dominated by MH+ ions; the ionization mechanism is attributed to proton transfer reactions from background species such as ArH+.