Analysis of low Z elements on Si wafer surfaces with undulator radiation induced total reflection X-ray fluorescence at the PTB beamline at BESSY II

Synchrotron radiation induced TXRF allows the nondestructive investigation of low Z contaminations on Si wafer surfaces at trace levels required by the semiconductor industry. The PTB (Physikalisch Technische Bundesanstalt) U180 undulator beamline at BESSY II, equipped with a plane grating monochromator ensuring an energy resolving power E/ΔE between 500 and 5000, can be operated either in wiggler mode for photon energies up to 1.7 keV to excite Al, Mg and Na efficiently, or in undulator mode, i.e. using one of the first odd U180 harmonics, to obtain intensive low energy radiation below 0.7 keV to excite carbon, nitrogen and oxygen. The specific feature of the beamline is its high spectral purity that allows for fundamental investigations. The TXRF wafer chamber of the Atominstitut was used for the experiments with a sidelooking Si(Li) detector with the wafer arranged vertically to take advantage of the linear polarization for background reduction. The energy dependence of the resonant Raman scattering, which is a limiter for the determination of Al at ultra trace levels excited with energies just below the Si absorption edge was studied as well as the influence of the incidence angle on the Raman peak. Droplet samples containing boron were measured and the detection limit of 3 ng determined. A single Carbon layer (5 nm) and a C–Ni–C multilayer sample on a Si wafer were characterized and it was shown that the thickness and density of these layers could be determined.     

Matrix effect on the detection limit and accuracy in total reflection X-ray fluorescence analysis of trace elements in environmental and biological samples

The effect of matrix contents on the detection limit of total reflection X-ray fluorescence analysis was experimentally investigated using a set of multielement standard solutions (500 ng/mL of each element) in variable concentrations of NH₄NO₃. It was found that high matrix concentration, i.e. 0.1–10% NH₄NO₃, had a strong effect on the detection limits for all investigated elements, whereas no effect was observed at lower matrix concentration, i.e. 0–0.1% NH₄NO₃.     

Trace element determination in thorium oxide using total reflection X-ray fluorescence spectrometry

Applicability of Total Reflection X-ray Fluorescence (TXRF) spectrometry for the determination of trace metals at concentration of µg/g level in thorium oxide was studied. The TXRF spectrometer was calibrated using a multielement standard solution and the method was validated by analyzing another multielement standard solution. Sample preparation conditions were optimized for the TXRF determinations of trace metals in thorium oxide. The elements K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Y, Ba and Pb present in thorium oxide standards were determined after dissolving them in HNO₃/HF mixture and separating the bulk matrix, thorium, by solvent extraction using tri-n-butyl phosphate (TBP) and tri-n-octyl phosphine oxide (TOPO) as extractants. A comparison of TXRF determined concentrations of trace elements Ca, V, Cr, Mn, Fe, Ni and Cu with the certified values shows that TXRF determined concentrations have an RSD of 20% (1 s for n = 4) and are within an agreement of 20% of the certified values in most of the cases.     

Analytical approaches for Hg determination in wastewater samples by means of total reflection X-ray fluorescence spectrometry

At present, there is a considerable interest in Hg monitoring in wastewater samples due to its widespread occurrence and the high toxicity of most of its compounds. Hg determination in water samples by means of total reflection X-ray fluorescence spectrometry (TXRF) entails some difficulties due to the high vapor pressure and low boiling point of this element that produce evaporation and loss of Hg from the surface of the reflector during the drying process, commonly used for sample preparation in TXRF analysis.The main goal of the present research was to develop a fast and simple chemical strategy to avoid Hg volatilization during the analysis of wastewater samples by TXRF spectrometry. Three different analytical procedures were tested for this purpose: (i) increasing the viscosity of the wastewater sample by adding a non-ionic surfactant (Triton^® X-114), (ii) Hg immobilization on the quartz reflectors using the extractant tri-isobutylphosphine (Cyanex 471X) and (iii) formation of a stable and non-volatile Hg complex into the wastewater sample. The best analytical strategy was found to be the formation of a Hg complex with thiourea (pH = 10) before the deposition of 10 μL of sample on the reflector for following TXRF analysis. Analytical figures of merit such as linearity, limits of detection, accuracy and precision were carefully evaluated. Finally, the developed methodology was applied for the determination of Hg in different types of wastewater samples (industrial effluents, municipal effluents from conventional systems and municipal effluents from constructed wetlands).     

Recent trends in total reflection X-ray fluorescence spectrometry for biological applications

This review is focused on the application of total reflection X-ray fluorescence (TXRF) spectrometry in the field of biological research. In the last decade, most papers were published by authors who applied laboratory-scale TXRF equipments. The application of synchrotron radiation as excitation source (SR-TXRF) shows a slowly increasing tendency. In the cited papers the micro-, trace and multielement capability of these TXRF techniques was demonstrated in the clinical and medical laboratory practice, as well as in various plant physiological studies. For speciation of elements in biological matrices, the TXRF was used as element specific detector following an off-line separation step (e.g., thin layer chromatography, high performance liquid chromatography), however, these off-line methods are not competitive with the on-line coupled HPLC-inductively coupled plasma mass spectrometry.     

Element determination in medieval soil samples by total reflection X-ray fluorescence analysis

An aqua regia extraction procedure for heavy metals in soils optimised for total reflection X-ray fluorescence analysis is presented. The procedure is applied to 92 soil samples of medieval layers from the city area of Dortmund. Sixteen elements (P, S, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Rb, Sr, Ag, Sn, and Pb) were used to characterise 17 sample sites. The results are projected onto the medieval urban structure of Dortmund. Two sites loaded with non-ferrous heavy metal could be detected and correlated with archaeological data. The efficiency and repeatability of the proposed extraction procedure is discussed.     

Determination of low atomic number elements at trace levels in uranium matrix using vacuum chamber total reflection X-ray fluorescence

Determinations of low atomic number elements Na, Mg and Al present at trace concentrations in uranium matrix were made by vacuum chamber total reflection X-ray fluorescence spectrometer for the first time. For this purpose, synthetic samples of uranium with known amounts of these low atomic number elements were prepared by mixing different volumes of their solutions with U solution of high purity. The concentrations of these elements in the samples were in the range of 100–300 μg/g with respect to uranium and 10–20 μg/mL in the solutions. Major matrix uranium was separated by solvent extraction with 30% solution of tri-n-butyl phosphate in dodecane. After the solvent extraction, aqueous phase containing trace elements was mixed with Sc internal standard and the samples were analyzed by vacuum chamber total reflection X-ray fluorescence spectrometer having a Cr Kα excitation source. The total reflection X-ray fluorescence results obtained, after blank corrections, indicated an average deviation of 14% from the calculated concentrations of these low atomic number elements on the basis of their preparation. However, the total reflection X-ray fluorescence determined concentration of Mg was exceptionally lower than the calculated concentration in two samples. These studies have shown that vacuum chamber total reflection X-ray fluorescence is a promising technique for the determination of low atomic number elements in uranium matrix after its separation.     

Construction of a windowless Si-anode X-ray tube for a more efficient excitation of low Z elements on Si-wafer surfaces in total reflection fluorescence analysis

To improve the achievable detection limits of low Z element with TXRF, a commercially available 2 kW X-ray tube (SEIFERT Type SF 60, Ahrensburg) with a 40 μm×8 mm fine focus has been modified. A windowless X-ray tube has been realized by removing the Be window out of the tube. The original Cu anode block has been changed to Al, because of sputtering reasons. A 4–6 μm thick pure silicon layer has been sputtered on the Al substrate. The geometry of the anode has been constructed in a specific way in order to optimize the photon flux of the X-ray beam concerning self-absorption and brilliance. Direct vacuum tight coupling to the measuring chamber and operation at 10⁻⁶ mbar vacuum was successfully shown. First measurements have been perfomed with a detector suitable for the detection of low energy photons in total reflection XRF geometry. Sodium has been analyzed on a Si-wafer surface and detection limits of 36 pg (corresponds to 3E9 atoms/cm²) have been achieved and are 10 times better than the detection limits for Na excited with a 1.3 kW Cr standard tube of 330 pg. With this developed X-ray tube the detection limits required by the Semiconductor industry for Si wafer surface contamination quality control are fulfilled.     

Analysis of low Z elements in various environmental samples with total reflection X-ray fluorescence (TXRF) spectrometry

Recently there is a growing interest in low Z elements such as carbon, oxygen up to sulphur and phosphorus in biological specimen. Total reflection X-ray fluorescence (TXRF) spectrometry is a suitable technique demanding only very small amounts of sample. On the other side, the detection of low Z elements is a critical point of this analytical technique. Besides other effects, self absorption may occur in the samples, because of the low energy of the fluorescence radiation. The calibration curves might be not linear any longer. To investigate this issue water samples and samples from human cerebrospinal fluid were used to examine absorption effects. The linearity of calibration curves in dependence of sample mass was investigated to verify the validity of the thin film approximation. The special requirements to the experimental setup for low Z energy dispersive fluorescence analysis were met by using the Atominstitute's TXRF vacuum chamber. This spectrometer is equipped with a Cr-anode X-ray tube, a multilayer monochromator and a SiLi detector with 30 mm² active area and with an ultrathin entrance window. Other object on this study are biofilms, living on all subaqueous surfaces, consisting of bacteria, algae and fungi embedded in their extracellular polymeric substances (EPS). Many trace elements from the water are bound in the biofilm. Thus, the biofilm is a useful indicator for polluting elements. For biomonitoring purposes not only the polluting elements but also the formation and growth rate of the biofilm are important. Biofilms were directly grown on TXRF reflectors. Their major elements and C-masses correlated to the cultivation time were investigated. These measured masses were related to the area seen by the detector, which was experimentally determined. Homogeneity of the biofilms was checked by measuring various sample positions on the reflectors.     

Determination of carbon in natural freshwater biofilms with total reflection X-ray fluorescence spectrometry

There is a growing interest in determination of low Z elements, i.e., carbon to phosphorus, in biological samples. Total reflection X-ray fluorescence spectrometry (TXRF) has been already established as suitable trace element analytical method with low sample demand and quite good quantification limits. Recently, the determinable element range was extended towards Z = 6 (carbon).Biofilms can be used for biomonioring purposes in the aquatic environment. Besides the trace metals, especially the determination of the carbon content is important for the better understanding of the early stage of biofilm formation. For this, an ATI low Z spectrometer equipped with Cr-anode X-ray tube, multilayer monochromator, vacuum chamber, and a Si(Li) detector with ultra thin window was used. Biofilms were grown on two different artificial supports (granite and plexiglass), freeze dried, suspended in high purity water and analyzed. As an internal standard the natural titanium content of the biofilms was used. The accuracy of the method was checked by total carbon measurement using a combusting carbon analyzer.     

Simple,fast, and low-contamination microwave-assisted digestion procedures for the determination of chemical elements in biological and environmental matrices by sector field ICP-MS

A simple and convenient method for the digestion of animal tissues, lichens, and plants for 33 metals measured by sector field inductively coupled plasma mass spectrometry (SF-ICP-MS) was described. Microwave-assisted acid digestions were performed at atmospheric pressure by means of a multi-samples rotor designed for processing a large number of samples at once in screw-capped disposable polystyrene liners. The digested samples were filled up to final volume directly in the polystyrene liners ready for elemental quantification. Seven certified reference materials, namely BCR 184 (bovine muscle), BCR 186 (pig kidney), DORM-2 (dogfish muscle), BCR 422 (cod muscle), BCR 62 (olive leaves), BCR 100 (beech leaves), and BCR 482 (lichen) were analysed to verify the accuracy of the method. The linearity range, limit of quantification, precision, and recovery by addition of non-certified elements were also assessed. All elements, with the exception of Hg in BCR 184 and As in BCR 186, were above the quantification limit and blank concentrations, and good agreement existed between found and target values in bovine muscle, pig kidney, and cod muscle. Significant deviations were observed for Al, Co, Cr, Mn, and Ni in dogfish muscle and for Ca, Cr, Fe, and Hg in lichens and plants. The proposed digestion procedure offers a low contamination risk, simplicity, speed, low cost, and applicability in routine analysis, and the SF-ICP-MS method allowed metals from a fraction of ng?g^?1 to hundreds of µg?g^?1 to be quantified in one analytical run.     

Comparison of digestion methods for ICP-MS determination of trace elements in fish tissues

A comparison is presented of six methods involving nitric acid in conjunction with other reagents to digest three certified marine biological samples (DOLT-3, DORM-3, IAEA-407) and a fish bone homogenate (prepared from Merluccius australis). An inductively coupled plasma-mass spectrometer with an octopole collision cell was used to determine up to 40 elements (Li, B, Na, Mg, Al, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Rb, Sr, Y, Ag, Cd, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Pb, Th, U) in the digests. These results were assessed in terms of their accuracy and precision, and a flow chart was developed to aid the selection of the optimal digestion method. Although none of the methods was found to give accurate determinations for all elements in the different reference materials, a relatively simple method involving nitric acid and hydrogen peroxide heated in a domestic microwave oven gave the most acceptable results.     

Trace elements determination in red and white wines using total-reflection X-ray fluorescence

Several wines produced in different regions from south of Brazil and available in markets in Rio de Janeiro were analyzed for their contents of elements such as: P, S, Cl, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Rb and Sr. Multi-element analysis was possible with simple sample preparation and subsequent analysis by total-reflection X-ray fluorescence using synchrotron radiation. The measurement was carried at the X-ray fluorescence beamline in the Synchrotron Light Source Laboratory in Campinas, Brazil. The levels of the various elements obtained were lower in the Brazilian wines than the values generally found in the literature. The present study indicates the capability of multi-element analysis for determining the contents of various elements present in wines coming from Brazil vineyards by using a simple, sensitive and precise method.     

Analysis of nutrition-relevant trace elements in human blood and serum by means of total reflection X-ray fluorescence (TXRF) spectroscopy

In clinical service laboratories, one of the most common analytical tasks with regard to inorganic traces is the determination of the nutrition-relevant elements Fe, Cu, Zn, and Se.     

Capability and limitations of the determination of sulfur in inorganic and biological matrices by total reflection X-ray fluorescence spectrometry

The capability and limitations of TXRF were explored for the determination of the light element sulfur in inorganic and biological samples. The recovery rates of sulfur were measured with tungsten L-radiation in the dry residues of various sulfates and sulfur containing amino acids with a sulfur concentration ranging from 0.5 to 20 mg/l. Furthermore, the surface topography of the dry residues of the samples was investigated by scanning electron microscopy (SEM) and their thickness profiles were recorded by an Alpha-Step. The results show that the reliable determination of sulfur in inorganic samples depends on the cation involved. Alkali sulfates tend to form bulky residues which disturb the determination of sulfur distinctly due to absorption effects of the soft sulfur K radiation. In this case the use of smoothing detergents like 1% HF, 1% malic acid and 2% hydrazine hydrate is necessary for the determinations with reasonable accuracy (tolerance range: ±10% of the recovery rate). The results for the biological samples measured agree well with the expected values. The investigations lead to the conclusion that TXRF combined with a proper sample preparation is well suited for the determination of sulfur in samples of various matrices in a wide range of concentrations.     

Direct elemental analysis of cancer cell lines by total reflection X-ray fluorescence

The elemental content of Cu, Fe and Zn in two human adenocarcinoma cell lines was investigated by total reflection X-ray fluorescence (TXRF) spectrometry. Cancer cells were sedimented directly to the quartz plates using a modified cytospin slide holder setup. Special glass stands and caps were also constructed to hold the quartz plates with the cells during the vapour-phase microwave assisted digestion. The method was validated by analysis of certified reference materials. The signal-to-noise ratio was optimized by washing the cells with different solutions. The technique was applied to the determination of Cu, Fe and Zn content of HT-29 and HCA-7 colorectal adenocarcinoma cell lines. Dry mass of the centrifuged cells were determined and the elemental analysis data reported for the two cell lines were referred either to cell numbers, to the total protein content or to the dry mass.     

Low volume microwave digestion and direct determination of selenium in biological samples by hydride generation-atomic fluorescence spectrometry

A low volume microwave digestion (LVMWD) procedure has been developed to have all forms of selenium (Se) compounds in biological samples decomposed to Se (IV) and allow total Se be directly determined by hydride generation-atomic fluorescence spectrometry (HG-AFS), or voltammetrically. Between 0.3 and 0.4 mL of mixed digestion reagents consisting of concentrated (15.4 M) HNO₃-(18.5 M) H₂SO₄ (v:v = 10:1) and >5 to <40 mg sample were found ideal systems with an optimized MW digestion program. Total Se in five certified reference materials was accurately determined. The results obtained by the conventional and LVMWD techniques agreed well. By avoiding pre-reduction step, this method, suitable for a wide range of biological samples, fully takes the advantages of HG-AFS or voltammetric techniques for their high sensitivity, high tolerance to matrix-related interference and accessibility in instrumentation. LVMWD not only enhances the sample output by 3 times and reduces the operational cost and acid wastes, but also makes the small sample analysis possible for many environmental and medical related research objectives. The digestion pathways of Se containing organic samples are also discussed based on the experimental results.     

Analysis of low Z elements on Si wafer surfaces with synchrotron radiation induced total reflection X-ray fluorescence at SSRL, Beamline 3-3: comparison of droplets with spin coated wafers

The unique properties of synchrotron radiation, such as high incident flux combined with low divergence, its linear polarization and energy tunability, make it an ideal excitation source for total reflection X-ray fluorescence (TXRF) spectroscopy in order to non-destructively detect trace impurities of transition metals on Si wafer surfaces. When used with a detector suitable for the determination of low energy radiation this technique can be extended to the detection of low-Z elements, such as Al, Na and Mg. Experiments have been performed at SSRL Beamline 3-3, a bending magnet beamline using monochromatic radiation from a double multilayer monochromator. The wafer was mounted vertically in front of the detector, which was aligned along the linear polarization vector of the incoming synchrotron radiation. This configuration allows the detector to accept a large solid angle as well as to take advantage of the reduced scattered X-ray intensity emitted in the direction of the linear polarization vector. A comparison between droplet samples and spin coated samples was done, in order to compare the capabilities of vapor phase decomposition (VPD-TXRF) with conventional SR-straight-TXRF. Detection limits in the range of 50 fg corresponding to 2E10 atoms/cm² have been obtained for Na. The spin coated samples, prepared from solutions containing an equal amount of Na, Mg and Al showed an unexpected result when performing a scan of the angle of incidence of the incoming X-rays suggesting a different adsorption behavior of the elements in a multielement solution on the wafer surface. The observation of this behavior is important because the spin coating technique is the standard method for the preparation of surface standards in semiconductor quality control. This effect could be characteristic of the Na, Mg, Al solution used, but the angle dependence of the fluorescence signal of a standard should always be investigated before using the standard for calibration of the apparatus and quantification.     

Comparison of sample preparation methods for the ICP-AES determination of minor and major elements in clarified apple juices

Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used for the determination of minor and major elements present in apple juices. Prior to ICP-AES measurement, samples were diluted with nitric acid or digested in a microwave assisted digestion system. The differences in the measured element concentrations after application of different types of sample preparation procedures are discussed. The direct measurement compared to closed microwave dissolution was found to be the best sample preparation procedure. Prior to the measurements the ICP-AES method was validated and optimized for the determination of elements in apple juices. For diluted apple juice samples the lowest limits of detection (LOD) were obtained for Ba and Cd (< 20 μg L^{− 1}), moderate ones for Cu, Mn, Ni, Fe, Ag, Ca, Cr, Zn, Mg, and Sr (20–100 μg L^{− 1}), and the highest LODs for K, Pb, Na, and Al (> 110 μg L^{− 1}). The results obtained for the repeatability (< 0.9%), the intermediate precision (< 4.5%), the day-to-day reproducibility (< 5.2%), and the overall uncertainty of measurement (approx. 4–7%) for all elements analyzed demonstrated the good applicability of the proposed method. Differences in major element content in fresh and commercial apple juice are discussed.