Ulrich Engel

A new low-power microwave plasma source using microstrip technology for atomic emission spectrometry

A new low-power, compact microwave-induced plasma source for applications in atomic emission spectrometry at atmospheric pressure using microstrip technology is described. The gas channel of about 1 mm2 is integrated in a fused silica dielectric wafer. The microstrip transmission lines are fabricated by sputtering and electro-plating. For example, a unit operates at an input power of 15 W with an argon gas flow of about 500 ml min-1 at atmospheric pressure. Rotational (OH) and excitation (Fe) temperatures of 650 K and 8000 K, respectively, were measured at these conditions. The emitted radiation can be taken up by an optical fibre positioned in the plasma-gas channel thus enabling an axial observation and coupling to a miniaturized spectrometer. The first devices showed an operation time of at least several hundred hours. Further investigations will lead to even smaller dimensions and lower power consumption and open the way for integrated microwave plasma sources with low detection limits as integrable parts of miniaturized total analytical systems applications.

Spatially resolved measurements and plasma tomography with respect to the rotational temperatures for a microwave plasma torch

To study the analyte evaporation and desolvation capacity of the microwave plasma torch (MPT), as a new source for atomic spectrometry, the radially-resolved rotational temperatures as a good approximation for the gas-kinetic temperatures were determined. An atmospheric pressure microwave discharge in argon at a frequency of 2.45 GHz at a power of 100 W and a gas flow rate of 0.6 l min–1 was studied. The procedure makes use of the rotational fine structure of the (A2Σ+→X2Πi) OH band at 306.4 nm and the temperatures were obtained from the slope of a Boltzmann plot. To obtain spatially-resolved intensities and for simultaneous detection of the different rotational lines, a charge coupled device (CCD) combinend with a Czerny–Turner monochromator was used. The image of the axially-symmetric plasma was rotated with the aid of a three-mirror arrangement by 90° and imaged onto the entrance slit. Radially-resolved intensities were calculated by means of an Abel inversion and measurements at different observation heights allowed complete tomography of the plasma. For the Abel inversion and temperature determination, an Interactive Data Language (IDL) program was developed, which computes the results in a short time and allows the presentation of the results as colour contour-plots. A mean temperature of about 3600 K with an error below 10% was found under the conditions mentioned above. Also the influence of power and water-loading of the carrier gas was investigated. Both were found to affect the temperature distribution but no significant changes in the mean temperature could be observed in the range 70–170 W and at a water-loading of between 0.6 and 9.0 mg min–1 of argon.

Trace element analysis of synthetic mono- and poly-crystalline CaF2 by ultraviolet laser ablation inductively coupled plasma mass spectrometry at 266 and 193 nm

Abstract The analytical figures of merit for ultraviolet laser ablation-inductively coupled plasma mass spectrometry (UV-LA-ICP-MS) at 266 nm with respect to the trace element analysis of high-purity, UV-transmitting alkaline earth halides are investigated and discussed. Ablation threshold energy density values and ablation rates for mono- and poly-crystalline CaF 2 samples were determined. Furthermore, Pb-, Rb-, Sr-, Ba- and Yb-specific analysis was performed. For these purposes, a pulsed Nd:YAG laser operated at the fourth harmonic of the fundamental wavelength (λ=266 nm) and a double-focusing sector field ICP-MS detector were employed. Depending on the background noise and isotope-specific sensitivity, the detection limits typically varied from 0.7 ng/g for Sr to 7 ng/g in the case of Pb. The concentrations were determined using a glass standard reference material (SRM NIST612). In order to demonstrate the sensitivity of the arrangement described, comparative measurements by means of a commercial ablation system consisting of an ArF excimer laser (λ=193 nm) and a quadrupole-type ICP-MS (ICP-QMS) instrument were carried out. The accuracy of both analyses was in good agreement, whereas ablation at 266 nm and detection using sector-field ICP-MS led to a sensitivity that was one order of magnitude above that obtained at 193 nm with ICP-QMS.

Direct solid atomic emission spectrometric analysis of metal samples by an argon microwave plasma torch coupled to spark ablation

Spark ablation has been combined to microwave plasma torch atomic emission spectrometry for the direct analysis of compact metallic samples. The material is ablated by a medium voltage spark (450 V, 370 Hz) in a point-to-plane configuration and swept into a 100-W, 2.45-GHz argon microwave discharge. The microwave plasma is observed end-on and the radiation analysed with a polychromator. The detection limits for Fe, Ni, Pb and Sn in brass, Cr, Cu, Ni, Mn, Mo, Si and V in steel and Cu, Fe, Mg, Mn, Si and Zn in aluminium with the microwave plasma torch in the case of measurements with a polychromator are in the μg/g range and by a factor of up to 20 higher than those obtained with spark ablation coupled to inductively coupled plasma atomic emission spectrometry using a high resolution sequential spectrometer. The stability of the emission signal depends on the element studied and relative standard deviations usually are between 0.5 and 3.5%. In the case of low-alloy steels, the linearity and the precision of the calibration could be improved by internal standardisation. Several elements (Cr, Cu, Ni, Si and V) could be determined in a steel sample (BAS SS 410/1) with high accuracy and precision.

Enclosed Inductively Coupled Plasma: Spatially Resolved Profiles of Rotational Temperatures and Analyte Atom Distribution

In order to investigate evaporation and desolvation capacities of the enclosed inductively coupled plasma discharge, rotational temperatures (which are commonly considered a good approximation of the heavy particle temperature or gas temperature) were determined as a function of their spatial distribution and their dependence on physical parameters such as gas flows (80–740 mL/min), moisture load, etc. The procedure utilizes the fine structure of the (A2Σ+ → X2Πi) OH band having its band head at 306.4 nm. The rotational temperatures were obtained from the slopes of their Boltzmann plots. Spatial resolution and simultaneous line detection was possible by using a charge-coupled device (CCD) camera in the focal plane of the removed exit slit of a Czerny–Turner monochromator. An interactive data language (IDL) program was developed to calculate the temperature distribution from the received CCD images. Results of the measurement show that the rotational temperatures are between 3750 and 4350 K. They further show the M-shaped spatial profile of analyte intensities and temperature. In the examined gas flow range (80–740 mL/min) the dependence on absolute gas flows and moisture load (5 mg/L) is negligible.