Abstract: The present study analyses a specific laser ablation cell, the High Efficiency Aerosol Dispersion (HEAD) cell (see J. Pisonero et al., J. Anal. At. Spectrom., 2006, 21, 922931), by means of computational fluid dynamics (CFD) simulations. However, this cell consists of different modular parts, therefore, the results are probably of interest for the further development of other ablation cells. In the HEAD cell, the ablation spot is positioned below an orifice in the ceiling of the sample chamber. The particle transport through this orifice has been analysed for a ceiling height of 0.8 mm. The critical velocity for the onset of particle losses was found to be independent on the ejection angle at the crater spot. The deceleration of the particles can be described as the stopping in an effectively steady gas. Particle losses were negligible in this modular part of the cell at the evaluated laser ablation conditions. The transport efficiency through the Venturi chamber was investigated for different sample gas flow rates. In this case, it was found that small particles were predominantly lost at low flow rates, the large particles at higher flow rates. Making use of the simulation results, it was possible to design a modification of the HEAD cell that results in extremely short calculated washout times. The simulations yielded a signal of less than 10 ms, which was produced by more than 99% of the introduced mass.

Abstract: The vibrational level populations of the electronic ground state of the nitrogen molecule have been calculated for typical glow discharge conditions in argonnitrogen mixtures with nitrogen concentrations between 0.1 and 1%. Stationary solutions of the master equations of the vibrational levels have been obtained using numerical methods. The main mechanisms responsible for the population and depopulation of the vibrational levels, and for the overall shape of the vibrational distribution function are pointed out. It has been found that vibrationvibration collisions play only a minor role and therefore the population of the vibrational levels is basically determined by the electron temperature.

Abstract: In this tutorial review paper, we illustrate how computer modeling can contribute to a better insight in inductively coupled plasma-mass spectrometry (ICP-MS). We start with a brief overview on previous efforts, studying the fundamentals of the ICP and ICP-MS, with main focus on previous modeling activities. Subsequently, we explain in detail the model that we developed in previous years, and we show typical calculation results, illustrating the plasma characteristics, gas flow patterns and the sample transport, evaporation and ionization. We also present the effect of various experimental parameters, such as operating conditions, geometrical aspects and sample characteristics, to illustrate how modeling can help to elucidate the optimal conditions for improved analytical performance.

Abstract: The laser ablation-induced plasma was used as a composition-con trolled source for ion implantation in Si crystals. Then, laser ablation in combination with inductively coupled plasma mass spectrometry was used for the elemental depth profiling of the implanted samples. Monte Carlo simulations permitted us to conclude that a depth resolution of tens of nm would be necessary to define the shape of the implantation profiles, as is obtained using XPS and RBS, whereas a hundred nm depth resolution is sufficient to determine the total implanted dose. The detection power of LA-ICP-MS would routinely allow rapid analytical control on the trace level implanted dose. Nevertheless, this technique is limited in terms of depth profiling resolution due to pulse mixing and signal tailing induced during the aerosol transport. Raw signal processing procedures were developed for the minimization of shapeline dispersion, deconvolution of pulse mixing and more appropriate assessment of the implanted profiles. Shapeline dispersion could be corrected for by determining the signal waning constant and implementing this information for a non-affine alibi transformation of the LA-ICP-MS signal traces. Pulse mixing deconvolution was attained with an algorithm that considered accumulated signal intensity due to pulse-on-pulse stacking, i.e., the latest pulse on top of all antecedent individual pulses' exponential tails proportionally.

Abstract: Scanning macro-X-ray fluorescence analysis (MA-XRF) is rapidly being established as a technique for the investigation of historical paintings. The elemental distribution images acquired by this method allow for the visualization of hidden paint layers and thus provide insight into the artist's creative process and the painting's conservation history. Due to the lack of a dedicated, commercially available instrument the application of the technique was limited to a few groups that constructed their own instruments. We present the first commercially available XRF scanner for paintings, consisting of an X-ray tube mounted with a Silicon-Drift (SD) detector on a motorized stage to be moved in front of a painting. The scanner is capable of imaging the distribution of the main constituents of surface and sub-surface paint layers in an area of 80 by 60 square centimeters with dwell times below 10 ms and a lateral resolution below 100 mu m. The scanner features for a broad range of elements between Ti (Z = 22) and Mo (Z = 42) a count rate of more than 1000 counts per second (cps)?mass percent and detection limits of 100 ppm for measurements of 1 s duration. Next to a presentation of spectrometric figures of merit, the value of the technique is illustrated through a case study of a painting by Rembrandt's student Govert Flinck (1615-1660).

Abstract: A combined Raman and electrochemical study of natural orpiment (As2S3), an arsenic sulfide pigment, was used to assess the quick formation of oxidized species such as arsenic oxide (As2O3) upon exposing the pigment to 405 nm or 532 nm monochromatic light while simultaneously recording the Raman spectra of the exposed sample. During this process, a distortion of the main band at 355 cm−1, associated with the stretching of the AsS3/2 pyramids of natural orpiment, was observed as well as an increased intensity of the 359 cm−1 band, corresponding to covalent AsAs bonds in natural orpiment. The distortion was accompanied by an overall decrease of the global Raman signal for natural orpiment, which could be explained by a loss in the crystal structure. The same phenomena were recorded in reference natural orpiment model paint samples stored for a long time together with verdigris (Cu(OH)2·(CH3COO)2·5H2O) and minium (Pb3O4) paints, the latter two appearing darkened on their sides closest to the orpiment sample as well as in several historical samples containing natural orpiment mixed with various blue pigments. By SEM-EDX and XRPD analysis, respectively on loose material and cast thin-sections of model paint samples, the darkening was identified as dark sulfide species such as chalcocite (Cu2S) and galena (PbS), suggesting the release of volatile sulfide or related species by the natural orpiment paint. XANES analyses of paint samples presenting AsAs bond increase indicated the presence of sulfur species most likely identified as organosulfur compounds formed upon the AsAs bond formation and explained the darkening of the Cu- and Pb-based pigments. To the best of our knowledge, this article reports for the first time the light-induced formation of AsAs bonds in natural orpiment used as an artists' pigment and objectively demonstrates the incompatibility between orpiment and (arsenic) sulfide-sensitive pigments.

Abstract: A combination of synchrotron radiation (SR) micro X-ray fluorescence (m-XRF) and XRF mode X-ray absorption near edge structure (XANES) measurements at the Cr K-edge already allowed us to establish that the photo-reduction of chromates to Cr(III) compounds is the cause of darkening of chrome yellow pigments (PbCr1-xSxO4, 0 <= x <= 0.8) in a number of paintings by Vincent van Gogh and in corresponding artificially aged paint models. A silicon drift detector (SDD) was employed to record the Cr-K XRF radiation in these X-ray micro beam-based measurements. However, in view of the limited count rate capabilities and collection solid angle of a single device, m-XRF and m-XANES employing single element SDDs (or similar) are primarily suited for collection of spectral data from individual points. Additionally, collection of XRF maps via point-by-point scanning with relatively long dwell times per point is possible but is usually confined to small areas. The development of the 384 silicon-diode array Maia XRF detector has provided valuable solutions in terms of data acquisition rate, allowing for full spectral (FS) XANES imaging in XRF mode, i.e., where spectroscopic information is available at each pixel in the scanned map. In this paper, the possibilities of SR Cr K-edge FS-XANES imaging in XRF mode using the Maia detector are examined as a new data collection strategy to study the speciation and distribution of alteration products of lead chromate-based pigments in painting materials. The results collected from two micro-samples taken from two Van Gogh paintings and an aged paint model show the possibility to perform FS-XANES imaging in practical time frames (from several minutes to a few hours) by scanning regions of sample sizes of the same order (more than 500 mm). The sensitivity and capabilities of FS-XANES imaging in providing representative chemical speciation information at the microscale (spatial resolution from similar to 2 to 0.6 mm) over the entire scanned area are demonstrated by the identification of Cr(OH) 3, Cr(III) sulfates and/or Cr(III) organometallic compounds in the corresponding phase maps, as alteration products. Comparable Cr-speciation results were obtained by performing equivalent higher spatial resolution SR m-XRF/single-point m-XANES analysis using a more conventional SDD from smaller regions of interest of each sample. Thus, large-area XRF mode FS-XANES imaging (Maia detector) is here proposed as a valuable and complementary data collection strategy in relation to “ zoomed-in” high-resolution m-XRF mapping and single-point m-XANES analysis (SDD).

Abstract: Scientific analyses of ancient glasses have been carried out for many years using elemental chemical analysis. However, it is known that the control of the redox conditions in the glass melt has a strong implication on the final hue of glass because it affects Fe2+/SFe. Therefore an increasing number of studies on the redox conditions have been published in recent years by means of synchrotron based Xray absorption spectroscopy. This is a technique which is not easily accessible and requires dedicated facilities. In this paper we describe an alternative approach by means of optical absorption spectroscopy. We synthesised 10 soda-lime-silica glasses with known redox conditions and iron concentration to calibrate the absorption at 1100 nm as a function of Fe2+ concentration. The linear extinction coefficient was also determined. These glasses were also studied by means of X-ray Absorption Near Edge Structure (XANES) spectroscopy. Electron paramagnetic resonance spectroscopy was additionally used as an ancillary method to verify the quality of our data. Furthermore 28 samples from real archaeological samples were analysed by XANES and optical spectroscopy as a case study. The Fe2+/SFe values obtained were compared and demonstrated that the two techniques were in good agreement with each other. Optical spectroscopy can be applied in situ with moderate sample preparation to determine the concentration of Fe2+. To investigate the redox conditions, especially as a first screening approach, this methodology is an important tool to take into consideration before applying more sophisticated techniques such as XANES, which is more elaborate and requires high-tech resources.