Monday, 28 April 2014

TOTAL REFLECTION X-RAY FLUORESCENCE ANALYSIS

INTRODUCTION

Total reflection x-ray fluorescence (TXRF) analysis is a powerful analytical tool with respect to detectable elemental range, simplicity of quantification and detection limits. This includes the capacity to detect almost all elements of the periodic system, namely from boron to uranium. Even the highest-elements of the actinides can be detected. Quantitatively, the dynamic range covers several orders of magnitude, so ultra-trace element levels to major elemental concentrations can be determined. In terms of detection limits, the levels of femtogram absolute detectable masses under optimized excitation–detection conditions can be reached. Some of these features can be topped with additional properties such as rapid analysis time of a few seconds and simultaneous detection of the elements present. In some applications, non-destructiveness is of importance, e.g. while dealing with precious substances of cultural values from fine arts or also in cases of forensic investigations if only small amounts of sample are available. TXRF is an energy-dispersive XRF (EDXRF) technique, and excitation geometry with angles below the critical angle of total reflection is perfectly suited for these investigations.

The above statements emphasize the analytical power, and in addition to these arguments one can add the large number of applications that has led to the revival of x-ray fluorescence analysis for ultra-trace element analysis. The applications range from the interesting fields of medicine, techniques and environment to forensic, fine arts, extra-terrestrial samples and fundamental research. With new physical and technical ideas leading to modifications of the physical properties of the primary radiation, e.g. monoenergetic, linearly polarized, highly intense, or on the detector side high resolution, high counting capacity, large area or even arrays of detectors, new perspectives are opening up for TXRF

In Fig. 1 the experimental set-up of conventional EDXRF and TXRF is schematically shown. As TXRF is basically an energy-dispersive analytical technique, the main difference to conventional EDXRF is neither the source nor the detector but the geometry of excitation at small incidence angles below the critical angle of total reflection.
     Figure 1 Comparison between conventional (left) and total reflection mode of excitation (right).

THEORY

The theory of x-ray total reflection is based on the phenomenon that at an incident angle below the critical angle the narrow collimated primary beam is totally reflected. A beam gets reflected from a flat polished surface of any material at the same angle as the incident one and has almost the same intensity as the primary beam (total intensity is reflected), except for a small portion that is refracted and penetrates the reflecting medium. This evanescent wave loses intensity exponentially as it penetrates deeper into the medium. In Fig. 2, the fundamental formalism of x-ray total reflection is shown, based on the Fresnel formalism and the complex index of refraction for x-rays, which is given below. The index of refraction for x-rays differs only slightly from 1, which is described theoretically by the value of υ which is in the range of 10-5 . For many materials the angles involved are small, typically a few milli radians or a tenth of a degree.

      Figure 2 Sketch of the theoretical conditions for x-ray total


Applications to chemical analysis:

The incident radiation in TXRF is a fine, collimated, almost parallel beam with typical dimensions of 8 mm width and only 50 μm height. This narrow beam impinges at an angle below the critical angle of total reflection (1.8 mrad or 0.1 degrees in the case of a monoenergetic Mo K˛ radiation and Si reflector) on the surface of a flat polished material. that serves as the sample carrier. In the case of chemical analysis of different samples, such as from the environment, medicine or technical products, different procedures apply. The sample has to be transferred into the liquid form by chemical digestion procedures. A small volume of 1–20 μl of the dissolved sample is taken using a pipette. The acidic or aqueous solution is dropped at the center of the reflector and dried by infrared heating, on a hot plate or in vacuum.

Possible applications of TXRF.

Environment
Water:
sea, rain, pore water, river, mineral, spring water, drinking water, chemicals and deionized water
Air:
aerosols, vapour, air dust, airborne particles, fresh air
Soil:
sewage sludge, sediments
Plant material:
Algae, fine roots, cucumber plants, pollen.
Foodstuff:
fish, flour,  fruits, crab, mussel, mushrooms, nuts,
vegetables, wine, tea, soft drinks, onion
Drinks:
Coffee,  spirits and beverages, honey.
Medicine/biology
Body fluids:
blood, serum, urine, amniotic fluid,  cerebrospinal fluid
Tissue: hair, kidney, lung, liver, stomach, nails, colon
Various enzymes, polysaccharides, glucose, proteins,
cosmetics, biofilms, human bones
Industrial/technical
applications
Surface analysis:
Si wafer surfaces, GaAs wafer surfaces
Implanted ions:
Depth and profile variations
Thin films:
single layers, multilayers
Oil:
Crude oil,  fuel oil, grease, pure fuel oil, waste oil, petroleum, oil-shale ash, diesel
Chemicals:
Acids,  bases,salts, solvents
Fusion/fission research:
trans mutational elements in Al C Cu, iodine in
water
Mineralogy
Ores, rocks, minerals, rare earth elements, quartz,
mineral sands, diamond, crystals
Geological materials, bio-mineralisation
Fine
arts/archaeological/forensic
Pigments, paintings, varnish21

Bronzes, pottery, jewellery, manuscripts, Egyptian masks
Textile fibres, glass, cognac, dollar bills, gunshot residue, drugs, tapes, sperm, finger prints.



Sample preparation

The samples can be from many scientific disciplines and thus the physical state will be different. The best-suited samples for TXRF are in the liquid state—either in the form of aqueous or acidic solutions—so if solids or powders are to be analysed, these samples must be transferred into the liquid state. The presented procedures are typical but can of course be adapted to the sample type, and the world of chemistry is fully open to new ideas, given in detail in the respective literature. Sample preparation has been discussed by several authors and many publications deal with special methods. Even so, the complex subject leaves a lot of ideas open to get the sample prepared in a way that the elements present can be detected even if they are at the lowest concentrations. Pre-concentration methods or selective enrichment techniques by chelation or electrochemical methods can be introduced to ensure that adequate masses are present on the sample reflector in the range of absolute picograms from very low-concentration samples, e.g. sea water in which the salt extraction also is an important preparation step. Recently, the collection of fine dust and also aerosols directly on the sample carrier in a short time of a few minutes to several hours by multi-stage samplers and the direct analysis of the minute masses collected by TXRF were successfully proven in large-scale experimental series in different parts of the world at locations of interest.


INSTRUMENTATION IN TXRF

Various combinations of x-ray sources, spectral modification elements, reflector materials and detectors can be used to optimize excitation and detection conditions. Measurements can be performed in air, in a He atmosphere or in a vacuum chamber. Vacuum conditions are mandatory for the detection of light elements to avoid absorption, but it is also advantageous because the scattering of primary x-rays from air is avoided. The components of a TXRF spectrometer are shown in the below figure




        Figure 3 Components of a TXRF spectrometer

 Sources of x-rays for TXRF are mainly x-ray tubes, ranging from low power (50–75W) to high-power standing anodes of up to 3000W and finally rotating anodes up to 18 kW. The appropriate focus is a line focus with dimensions 8 or 12mm long and 40 μm wide. Everything is done to get a high flux of photons onto the sample. There is radiation from the ultimate source with best properties of having a naturally collimated beam characteristic, an extremely high flux and a linearly polarized beam—the SR. Even though it is difficult to get access to SR, the results achievable show the importance of this effective combination of the source and TXRF, from which ultimate low detection limits of a few femtogram have been achieved.


The beam from an x-ray tube is unpolarised and has a continuous spectral distribution of the characteristic radiation. In many cases, a monoenergetic excitation is the preferred one because the background is optimally low as only scattered photons of single energy are present and will appear as two lines elastically and Compton-scattered in the spectrum. The first low-cost approach to modify the spectrum was an optical flat in the beam path and taking advantage of the energy dependence of the critical angle to suppress the high-energy part of the bremsstrahlung spectrum. This leads to reduced background, in particular in the low-energy region, as the Compton edge produced by the backscattered high-energy photons disappears. Versions with two reflectors attached togetherwith a spacing of 50 mm lead to a double-reflector monochromator. Efficient monochromators are nowadays available using multilayers with reflectivities in the range of up to 80% of the characteristic radiation of the anode material of choice. Typically in use are combinations of layered structures made of W–C, Ni–C andMo–Si with a d spacing of 2–3 nm. Thus, they are adjusted to fulfill the Bragg equation and the full spectrum is modified to be monoenergetic. Using modern technology to bend these structures, it is possible to design x-ray optical components that focus the beam with a small divergence, thereby increasing the flux of photons in the focal spot which is designed to be at the sample position. The result is an increase in intensity on the sample several times more than that with the non-focussing flat multilayer. These optical components are commercially available and can be inserted in the beam path of the TXRF spectrometer to improve further detection limits. 



Quantitative TXRF

The conversion of the measured intensities into concentrations is one of the most important steps in analytical XRF. In the special case of TXRF, the complications are rather completely removed, as the approach for the thin-film sample can be applied, which leads to a simple and linear relation between the intensity, I, and concentration, C, of the element considered. The addition of an internal standard with known concentration leads to a simple quantification procedure, as follows:

  • Choose or add to the multi-element standard one element as internal standard, which is the reference for calibration of the spectrometer.
  • Establish the intensity vs concentration curve as the regression curve with reference to the internal standard to determine the sensitivity, Sstd/Si, from multi-element standard.
  • Add the internal standard of known concentration Cstd to the unknown sample.
  • Measure the intensity of element Ii and intensity of internal standard Istd.
  • Determine the concentration of unknown element Ci using the relation:



Advantages of TXRF


  • Double excitation by direct and reflected beams
  • Almost no penetration of the primary radiation into the substrate, resulting in low background Large solid angle, as the detector can be placed close to the reflector surface 
  • Large solid angle, as the detector can be placed close to the reflector surface
  • The consequent improved signal/background and improved detection limits
  • Very low detection limits: femtogram levels, picograms per gram concentrations, 108 atoms/cm2 of metal contamination detectable on wafer surfaces


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