Applications of synchrotron radiation in forensic trace evidence analysis

Synchrotron radiation sources have proven to be highly beneficial in many fields of research for the characterization of materials. However, only a very limited proportion of studies have been conducted by the forensic science community. This is an area in which the analytical benefits provided by synchrotron sources could prove to be very important. This review summarises the applications found for synchrotron radiation in a forensic trace evidence context as well as other areas of research that strive for similar analytical scrutiny and/or are applied to similar sample materials. The benefits of synchrotron radiation are discussed in relation to common infrared, X-ray fluorescence, tomographic and briefly, X-ray diffraction and scattering techniques. In addition, X-ray absorption fine structure analysis (incorporating XANES and EXAFS) is highlighted as an area in which significant contributions into the characterization of materials can be obtained. The implications of increased spatial resolution on microheterogeneity are also considered and discussed.     

Beijing synchrotron radiation total-reflection X-ray fluorescence analysis facility and its applications on trace element study of cells

The feasibility of total-reflection X-ray fluorescence (TXRF) analysis excited by synchrotron radiation applied to trace element analysis of biological cells is investigated. The Beijing synchrotron radiation TXRF facility and the related experimental method are also described. The elemental minimum detection limits of some standard reference materials are determined. The elemental compositions of a cluster of small intestine cells of a small white mouse are given, and hence the average trace element contents of the single small intestine cell are also obtained. With this technique, the changes of some trace elements in the cells of lung and cervix cancer before and after apoptosis are also preliminarily studied.     

Instruments and methods for small-angle scattering with synchrotron radiation

Some aspects of small-angle X-ray scattering (SAXS) instrumentation for synchrotron radiation (SR) are considered below. The basic layout of instruments as well as some of the problems arising from the nature of the source and of the type of experiments are discussed. Further, a survey of the instruments available at major SR facilities is given.     

Applications of synchrotron radiation in materials analysis

The synchrotron radiation (SR) emitted by circulating high-energy electrons has extraordinary properties: The light is intensive and bright, it is tunable and highly collimated, and finally, it is linearly polarized. These exceptional properties have initiated a unique revival of many spectroscopies using electromagnetic radiation. The techniques of special concern for materials analysis which are treated in this article are: X-ray absorption, reflection, fluorescence, diffraction and topography. A number of examples will be given in order to illustrate the possibilities of these techniques when SR is used.On leave of absence from Institut für Festkörperforschung, KFA Jülich, D-5170 Jülich, Federal Republic of Germany     

Instrumentation for FT-IR reflection spectroscopy with synchrotron radiation

A versatile experimental set-up for infrared reflectance measurements with synchrotron radiation and its adaptation to a beamline is presented. Particular consideration is given to the collimation and the polarization of incident radiation. The performance is characterized with experimental results. Due to the high brilliance of the synchrotron radiation source, the irradiation of samples smaller than 1 mm(2) was found to be improved by more than one order of magnitude when compared to a globar.     

X-ray fluorescence spectrometry with synchrotron radiation

X-ray fluorescence spectrometry (x.r.f.) can be done through excitation with synchrotron radiation. This permits multi-element determinations in the trace region with improved detection limits compared to conventional x.r.f. Detection limits are evaluated and compared with theoretically calculated values. For a beam diameter of 0.5 mm and a sample of 1 mg cm^?2, absolute detection limits are between 0.1 and 0.4 pg. The dependence of the detection limit on the atomic number is reduced, when white synchrotron radiation is used for excitation instead of monochromatic radiation. The optimum of the limit of detection on the Z-scale can be shifted to higher atomic numbers and improved through filtration of the primary radiation by aluminium absorbers. Preparation of samples on different polymeric films is discussed in relation to blank values.     

Mid-IR synchrotron radiation for molecular specific detection in microchip-based analysis systems

Microstructures constructed from SU-8 polymer and produced on CaF₂ base plates have been developed for microchip-based analysis systems used to perform FTIR spectroscopic detection using mid-IR synchrotron radiation. The high brilliance of the synchrotron source enables measurements at spot sizes at the diffraction limit of mid-IR radiation. This corresponds to a spatial resolution of a few micrometers (5–20 m). These small measurement spots are useful for lab-on-a-chip devices, since their sizes are comparable to those of the structures usually used in these devices. Two different types of microchips are introduced here. The first chip was designed for time-resolved FTIR investigations of chemical reactions in solution. The second chip was designed for chip-based electrophoresis with IR detection on-chip. The results obtained prove the operational functionality of these chips, and indicate the potential of these new devices for further applications in (bio)analytical chemistry.     

Micro-heterogeneity study of trace elements in BCR CRM 680 by means of synchrotron micro-XRF

BCR CRM 680 is a newly released reference material (RM) with a polyolefinic matrix, doped with various metallic compounds in trace concentrations, to be used for calibration of multi-element trace analytical methods. The micro-heterogeneity of this new CRM was investigated to evaluate the suitability as a RM for trace-level micro analytical techniques. Synchrotron μ-XRF, a trace-level micro analytical method, allows to quantitatively measure the degree of heterogeneity of inorganic trace constituents in solid materials with a homogeneous matrix. The procedure for calculating the minimal sampling mass for homogeneous measurements was employed for BCR CRM 680. By repeating the entire procedure on several samples of the same material, the repeatability of the micro-heterogeneity characterization based on μ-XRF was investigated. Afterwards, the micro-heterogeneity results from the BCR CRM 680 material were used to evaluate the suitability of the micro-heterogeneity procedure by means of a Monte Carlo simulation model. Also the existence of nuggets in the polymer material is looked into.     

Micro-heterogeneity study of trace elements in BCR CRM 680 by means of synchrotron micro-XRF

BCR CRM 680 is a newly released reference material (RM) with a polyolefinic matrix, doped with various metallic compounds in trace concentrations, to be used for calibration of multi-element trace analytical methods. The micro-heterogeneity of this new CRM was investigated to evaluate the suitability as a RM for trace-level micro analytical techniques. Synchrotron micro-XRF, a trace-level micro analytical method, allows to quantitatively measure the degree of heterogeneity of inorganic trace constituents in solid materials with a homogeneous matrix. The procedure for calculating the minimal sampling mass for homogeneous measurements was employed for BCR CRM 680. By repeating the entire procedure on several samples of the same material, the repeatability of the micro-heterogeneity characterization based on micro-XRF was investigated. Afterwards, the micro-heterogeneity results from the BCR CRM 680 material were used to evaluate the suitability of the micro-heterogeneity procedure by means of a Monte Carlo simulation model. Also the existence of nuggets in the polymer material is looked into.     

Neutron activation analysis for bulk and trace elements in urine.

Problems in sampling urine for trace element analysis by neutron activation are systematically examined. Collection, storage, sample preparation and contamination hazards during irradiation are studied in detail. Three different sizes of urine samples are prepared for analysis, depending on the concentration and nuclear properties of the elements, and suitable multielement doped urine standards are used. As, Br, Ca, Cl, Co, Cr, Cs, Cu, Hg, I, K, Mg, Mn, Na, Rb, Sc and Zn are determined. The extreme care given to sample collection, use of “ultra-clean” vials, and work in a dust-free room, allows consistent values to be obtained over long periods of time. A literature review of the amounts of forty elements present in urine per day is also given.     

AZ-1518 Photoresist analysis with synchrotron radiation using high-resolution time-of-flight mass spectrometry

With the aim of identifying molecular modifications among photoresists unexposed and previously exposed to the ultraviolet light the photon stimulated ion desorption (PSID) technique was employed in the study of the AZ-1518 photoresist. Data acquisition was performed at the Brazilian Synchrotron Light Source (LNLS), during a single-bunch operation mode of the storage ring and using high-resolution time-of-flight mass spectrometry (TOF-MS) for ion analysis. PSID mass spectra on both photoresists (unexposed and exposed) were obtained following the S K-shell photoexcitation and desorption ion yield curves have been determined for the main fragments as a function of the photon energy. The AZ-1518 photoresists presented different PSID spectra, showing characteristic fragments. Most of the analyzed ions showed larger relative yields for the exposed photoresist. Fragments related to the photochemical decomposition of the photoresist could be clearly identified. These results showed that the PSID technique is adequate to investigate structural changes in molecular level in unexposed and exposed photoresists.     

A wavelength-dispersive arrangement for wafer analysis with total reflection X-ray fluorescence spectrometry using synchrotron radiation

Currently, the only apparent means to enhance the detection power of the TXRF technique would be to increase the intensity of the primary beam. Using synchrotron radiation, the most powerful X-ray source available, unfortunately, not only the fluorescence signal of the contaminant elements is increased, but also in equal measure, the intensities of the Si–K radiation from the wafer together with the scattered radiation. This results in an overloading of the energy-dispersive Si (Li) detector systems used hitherto, with the effect that the available primary intensity cannot be fully exploited. Wavelength-dispersive systems are free of such problems; they generate less detector background and can withstand higher count rates. Due to their small angle of acceptance, however, their detection efficiency is quite low. In this contribution we propose a wavelength-dispersive TXRF solution, which is optimized with regard to higher efficiency on the basis of large area multilayer mirrors in combination with a position-sensitive detector. The count rates in relation to energy-dispersive instruments and the energy resolution of the new system have been calculated using ray-tracing techniques.     

Extinction correction and synchrotron radiation

The primary extinction factor y_p is defined as the ratio of the integrated reflection from a coherently diffracting domain to the integrated kinematical reflection from the same domain. When y_p is larger than 0.5 it may be approximated by y_p = exp{−(αδ)2}, where α is about 0.5 andδ the average size of the coherent domain when measured in units of the extinction length A,δ = D/λ. Transfer equations are applied to symmetrical Laue diffraction, and the reflectivity per unit length, Σ(ε) is solved from the measured reflecting ratio as a function of the rocking angleε =θ− θ_B. Measurements with conventional x-ray sources are made on single crystal slabs of Be and Si using AgKΒ, MoKα₁ and CuKα radiation. The primary extinction factor y_p(ε) is solved from a point-by-point comparison of two measurements where the extinction length λ is changed by varying the polarization and/or wavelength of the x-ray beam. The results show that primary and secondary extinction are strongly correlated, and that the customary assumption of independent size and orientation distributions of crystal mosaics is unjustified. The structure factors for Be and Si show close agreement with other recent measurements and calculations. The limitations of the method are discussed in length, particularly the effects of beam divergences and incoherence of the rays in the crystal. It is concluded that under typical experimental conditions the requirements of the theory are met. Practical limitations arising from the use of characteristic wavelengths and unpolarized radiation prohibit the use of the full potential of the method. The properties of a synchrotron radiation source are compared with a conventional x-ray source, and it is demonstrated that the experimental limitations can be removed by the use of synchrotron radiation. A diffraction experiment with synchrotron radiation is outlined, as well as generalization of the method to small spherical crystals.     

Non-destructive depth profile analysis using synchrotron radiation excited XPS

We have used synchrotron radiation as excitation source in an X-ray photoelectron spectroscopy (XPS) experiment to analyse surface-near element depth profiles non-dectructively. By tuning the photon energy one can vary the kinetic energy of the photoelectrons and in turn the information depth of the measurement. To quantify the sample geometry (e.g. layer thicknesses) model calculations similar as for angle-resolved XPS (ARXPS) measurements are necessary. We have successfully applied this technique to several samples. We will show how to calculate the relative intensities of the peaks, using photoionization cross sections and an experimentally determined analyzer transmission function and the procedure to quantify the geometry for a model sample: natively oxidized Ta covered by carbon contamination. At Sn-doped indium oxide samples we found a sub-monolayer of segregated Sn at the surface which was expected from previous investigations.     

Analysis for trace elements with a slowpoke reactor

The SLOWPOKE reactor is very useful for neutron activation analysis and has been in operation at Dalhousie University for ten years. Improvements to the reactor are out-lined and flux evaluations after major changes are reported. Its use for the determination of trace elements in a broad variety of materials is described.