Detection limit calculations for the total reflection techniques of X-ray fluorescence analysis

In this work, theoretical calculations of detection limits for different total reflection techniques of X-ray fluorescence analysis are presented. Calculations include grazing incidence (TXRF) and gracing emission (GEXRF) conditions. These calculations are compared with detection limits calculated for conventional X-ray fluorescence (XRF). In order to compute detection limits, Shiraiwa and Fujino's model was used to calculate X-ray fluorescence intensities. This model makes certain assumptions and approximations to achieve the calculations, especially in the case of the geometrical conditions of the sample, and the incident and takeoff beams. Nevertheless, the calculated data of detection limits for conventional XRF and total-reflection XRF show a good agreement with previous results. The model proposed here allows us to analyze the different sources of background and the influence of the excitation geometry, which contribute to a better understanding of the physical processes involved in the XRF analysis by total reflection. Finally, a comparison between detection limits in total-reflection analysis at grazing incidence and at grazing emission is carried out. Here, a good agreement with the theoretical predictions of the Reciprocity Theorem is found, showing that, in theory, detection limits are similar for both techniques.     

Potential of Total Reflection and Grazing Incidence XRF for Contamination and Process Control in Semiconductor Fabrication

 The actual detection limits of total reflection X-ray fluorescence (TXRF) are determined and compared to those of destructive physical analytical methods like secondary ion mass spectrometry (SIMS) and chemical methods like vapour phase decomposition in combination with inductively coupled plasma-mass spectrometry (VPD-ICP-MS). The elements Ca, Ti, Cr, Fe, Cu were analyzed on a Si wafer with 10 nm thermal oxide using TXRF and VPD-ICP-MS. The deviation of the TXRF and the VPD-ICP-MS results is less than 30%. The thickness, composition and density of a Co/Ti two-layer stack were determined using angle dependent total reflection and grazing incidence X-ray fluorescence (A-TXRF). The obtained data were compared with X-ray reflectometry (XRR) and energy filtered transmission electron microscopy (EFTEM). The agreement between TEM and A-TXRF is excellent for the determination of the thickness of the metal layers. From these results we conclude, that A-TXRF permits the accurate determination of composition, thickness and density of thin metallic layers. The results are discussed regarding detection efficiency, acquisition time, accuracy and reproducibility.     

Performance of total reflection and grazing emission X-ray fluorescence spectrometry for the determination of trace metals in drinking water in relation to other analytical techniques

An intercomparison survey has been carried out in order to evaluate the performance of two related X-ray fluorescence techniques as compared to the achievements of several other analytical techniques applied for trace elements determination in drinking water. A relatively new technique, total reflection X-ray fluorescence (TXRF) and a novel related technique, grazing emission X-ray fluorescence (GEXRF) have been used for the analysis of a mineral water sample. The concentrations of the following elements have been determined: Na, Mg, K, Ca, Ni, Cu, Zn and Sr. The mineral water sample has also been analyzed by a number of other analytical techniques, routinely utilized in drinking water quality control. The analyses were performed in eleven laboratories which reported 286 individual determinations producing 75 laboratory means. From the obtained results, it can be concluded that the TXRF technique is suitable for a direct determination of heavy elements in drinking water (above potassium, Z = 19). This technique can compete with other analytical techniques routinely used in water quality monitoring. First results obtained with GEXRF spectrometry show that this technique can be successfully applied for the determination of low-Z elements in drinking water. However, results for sodium and magnesium were systematically too low, indicating that modifications of the quantification procedure may be required to improve the accuracy of determination for these light elements.     

Performance of total reflection and grazing emission X-ray fluorescence spectrometry for the determination of trace metals in drinking water in relation to other analytical techniques

An intercomparison survey has been carried out in order to evaluate the performance of two related X-ray fluorescence techniques as compared to the achievements of several other analytical techniques applied for trace elements determination in drinking water. A relatively new technique, total reflection X-ray fluorescence (TXRF) and a novel related technique, grazing emission X-ray fluorescence (GEXRF) have been used for the analysis of a mineral water sample. The concentrations of the following elements have been determined: Na, Mg, K, Ca, Ni, Cu, Zn and Sr. The mineral water sample has also been analyzed by a number of other analytical techniques, routinely utilized in drinking water quality control. The analyses were performed in eleven laboratories which reported 286 individual determinations producing 75 laboratory means. From the obtained results, it can be concluded that the TXRF technique is suitable for a direct determination of heavy elements in drinking water (above potassium, Z = 19). This technique can compete with other analytical techniques routinely used in water quality monitoring. First results obtained with GEXRF spectrometry show that this technique can be successfully applied for the determination of low-Z elements in drinking water. However, results for sodium and magnesium were systematically too low, indicating that modifications of the quantification procedure may be required to improve the accuracy of determination for these light elements. Received: 5 January 1998 / Revised: 17 February 1998 / Accepted: 18 February 1998     

Review on grazing incidence X-ray spectrometry and reflectometry

Grazing incidence X-ray techniques are now widely used for surface and thin film analysis. The present article overviews the recent advancement since 1993 of the grazing incidence X-ray spectrometry and reflectometry in both theoretical and experimental aspects. Every current topic related to the total reflection X-ray fluorescence spectrometry (TXRF) is described in detail through the introduction of numerous published works on the application in the various fields of the science and industrial technologies. Recent rapid growth in diffuse scattering at grazing incidence as well as in specular reflection is another important scope. The combined measurements of different grazing incidence X-ray techniques might be a future trend for realizing further advanced analysis of the surface and interfaces of materials.     

Comparison of conventional and total reflection excitation geometry for fluorescence X-ray absorption spectroscopy on droplet samples

X-ray absorption fine structure (XAFS) experiments in fluorescence mode have been performed in total reflection excitation geometry and conventional 45°/45° excitation/detection geometry for comparison. The experimental results have shown that XAFS measurements are feasible under normal total reflection X-ray fluorescence (TXRF) conditions, i.e. on droplet samples, with excitation in grazing incidence and using a TXRF experimental chamber. The application of the total reflection excitation geometry for XAFS measurements increases the sensitivity compared to the conventional geometry leading to lower accessible concentration ranges. However, XAFS under total reflection excitation condition fails for highly concentrated samples because of the self-absorption effect.     

Elemental distribution in breast tissue samples using X-ray fluorescence microtomography

Summary The X-ray fluorescence microtomography (XRFMT) is a non-destructive technique, based on the detection of X-ray fluorescence emitted by the elements in the sample, and it is used to complement other techniques for sample characterization. In this work some tissues of human breast (healthy tissue, benign tumor and malignant tumor) have been analyzed in order to verify the efficiency of the system in the determination of the elemental distribution in these types of samples. The experiments were performed at the X-ray fluorescence beam line (D09B-XRF) of the Brazilian Synchrotron Light Source (LNLS), Campinas, Brazil. A white beam was used for the excitation of the elements and an HPGe detector detected the fluorescence photons. The incident beam was monitored by an ionization chamber and a fast scintillator detector was used to detect the transmitted radiation. All the tomographies have been reconstructed using a filtered-back projection algorithm. In the tissues samples, the elements of higher concentration were Zn, Cu and Fe.     

Analysis of elemental concentration censored distributions in breast malignant and breast benign neoplasm tissues

The total reflection X-ray fluorescence method was applied to study the trace element concentrations in human breast malignant and breast benign neoplasm tissues taken from the women who were patients of Holycross Cancer Centre in Kielce (Poland). These investigations were mainly focused on the development of new possibilities of cancer diagnosis and therapy monitoring. This systematic comparative study was based on relatively large (∼ 100) population studied, namely 26 samples of breast malignant and 68 samples of breast benign neoplasm tissues. The concentrations, being in the range from a few ppb to 0.1%, were determined for thirteen elements (from P to Pb). The results were carefully analysed to investigate the concentration distribution of trace elements in the studied samples. The measurements of concentration of trace elements by total reflection X-ray fluorescence were limited, however, by the detection limit of the method. It was observed that for more than 50% of elements determined, the concentrations were not measured in all samples. These incomplete measurements were treated within the statistical concept called left-random censoring and for the estimation of the mean value and median of censored concentration distributions, the Kaplan–Meier estimator was used. For comparison of concentrations in two populations, the log-rank test was applied, which allows to compare the censored total reflection X-ray fluorescence data. Found statistically significant differences are discussed in more details. It is noted that described data analysis procedures should be the standard tool to analyze the censored concentrations of trace elements analysed by X-ray fluorescence methods.     

Synchrotron X-ray techniques for the investigation of structures and dynamics in interfacial systems

This review focuses on recent results obtained by synchrotron X-ray techniques applied to the characterization of interfacial systems, with main emphasis on flat interfaces and on colloidal systems. The techniques covered are, for structural determinations: X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD) and grazing incidence X-ray excited fluorescence (GIXF), while dynamics are investigated by X-ray photon correlation spectroscopy (XPCS) mainly in the grazing-incidence geometry (GIXPCS).The systems reviewed are, in order of growing complexity, floating Langmuir monolayers, supported films of lipids and proteins, polymeric films, buried interfaces, colloidal systems and gels formed by colloids either in 3D or in the form of 2D interfacial layers. Recent results are critically discussed, and some interesting directions of development are outlined, having also in mind new technical developments such as X-ray free electron laser sources and micro-focused synchrotron beamlines.     

Optimization of a glancing angle for simultaneous trace elemental analysis by using a portable total reflection X-ray fluorescence spectrometer

By using a portable total reflection X-ray fluorescence spectrometer with a 1 W X-ray tube, a specimen containing nanograms of Ca, Sc, Ti, V, Cr, Mn, Fe, and Ni is measured at several glancing angles of incident X-rays. Continuum X-rays are used as the excitation source. The intensities of the spectral background which degrades sensitivity to trace elements are decreased with a decrease of the glancing angle, and all these elements are detected at the glancing angle of 0.13° smaller than the critical angle for total reflection of the incident X-rays (0.20°). An optimum glancing angle for simultaneously detecting these trace elements is around 0.13°, and detection limits at 0.13° are sub-nanograms to ten nanograms.     

Applicability of a cut-off reflector for grazing incidence X-ray fluorescence analysis

The applicability of a cut-off reflector, instead of the commonly used multilayer reflector, for grazing incidence X-ray fluorescence (GI-XRF) analysis is demonstrated. Owing to the precise angular adjustment possible in the total reflection X-ray fluorescence (TXRF) spectrometer developed in house, it is possible to adjust the cut-off reflector so as to pass all X-ray energies up to Cu-K_α, eliminating Cu-K_β and higher X-ray energies emitted from a Cu target X-ray generator. The advantage of this technique is that one gets a higher flux of Cu-K_α radiation (>98%) compared to 80–90% from a good quality multilayer optics. Moreover, the same cut-off reflector, used at different grazing angles, serves the purpose for different primary beam energies. The suitability of such an arrangement for GI-XRF analysis for surface characterization has been demonstrated by analyzing a 50 ng aqueous residue of Fe on top of a float glass substrate. The GI-XRF results thus obtained are compared with those obtained using a multilayer monochromator in the primary beam as well as with theoretical calculations.     

Multilayer mirror as a substrate for total reflection X-ray fluorescence spectrometry

X-ray field intensity generated over a multilayer surface during a strong Bragg reflection condition has been used to analyze the particulate matter deposited on its surface, for the average particles size distribution and detection sensitivity of various elements. The elemental detection sensitivities achieved at Bragg reflection condition are compared to those obtained at incidence angles below critical angle, under total external reflection condition. The results obtained indicate that when big size particles (> 1 μm) are distributed over a large surface area, the observed fluorescence yields deteriorate by 15–18% in the total external reflection condition, due to strong sample absorption effects. In such a case, use of a multilayer mirror as a sample carrier and fluorescence excitation under Bragg reflection condition provides better fluorescence yield and hence improved detection sensitivity for an element.     

X-ray absorption spectral analysis with a 9 V battery X-ray generator.

X-ray absorption spectral (XAS) analysis is performed with a combination of a 9 V dry electric battery X-ray generator and a portable Si PIN X-ray detector. The calcium K edge (4.0 keV) in paper is measured with a grazing incidence geometry, which suppressed the artifact due to the Kalpha X-ray fluorescence peak at 3.5 keV. The 9 V dry battery X-ray emitter is useful for portable XAS measurements.     

Recent results of synchrotron radiation induced total reflection X-ray fluorescence analysis at HASYLAB,beamline L

At the Hamburger Synchrotronstrahlungslabor (HASYLAB), Beamline L, a vacuum chamber for synchrotron radiation-induced total reflection X-ray fluorescence analysis, is now available which can easily be installed using the adjustment components for microanalysis present at this beamline. The detector is now in the final version of a Vortex silicon drift detector with 50-mm² active area from Radiant Detector Technologies. With the Ni/C multilayer monochromator set to 17 keV extrapolated detection limits of 8 fg were obtained using the 50-mm² silicon drift detector with 1000 s live time on a sample containing 100 pg of Ni.Various applications are presented, especially of samples which are available in very small amounts: As synchrotron radiation-induced total reflection X-ray fluorescence analysis is much more sensitive than tube-excited total reflection X-ray fluorescence analysis, the sampling time of aerosol samples can be diminished, resulting in a more precise time resolution of atmospheric events. Aerosols, directly sampled on Si reflectors in an impactor were investigated. A further application was the determination of contamination elements in a slurry of high-purity Al₂O₃. No digestion is required; the sample is pipetted and dried before analysis. A comparison with laboratory total reflection X-ray fluorescence analysis showed the higher sensitivity of synchrotron radiation-induced total reflection X-ray fluorescence analysis, more contamination elements could be detected. Using the Si-111 crystal monochromator also available at beamline L, XANES measurements to determine the chemical state were performed. This is only possible with lower sensitivity as the flux transmitted by the crystal monochromator is about a factor of 100 lower than that transmitted by the multilayer monochromator. Preliminary results of X-ray absorption near-edge structure measurements for As in xylem sap from cucumber plants fed with As(III) and As(V) are reported. Detection limits of 170 ng/l of As in xylem sap were achieved.     

Monte Carlo simulation of X-ray fluorescence spectra: Part 4. Photon scattering at high X-ray energies

The photon scattering model of a Monte Carlo simulation code for synchrotron radiation X-ray fluorescence (SRXRF) spectrometers is evaluated at high X-ray energies (60–100 keV) by means of a series of validation experiments performed at Beamline BW5 of HASYLAB. Using monochromatic X-rays, Compton/Rayleigh multiple scattering experiments were performed on polypropylene, Al and Cu samples. Especially in the case of the first two matrices multiple Compton scattering occurs with high probability. This work demonstrates that the simulation model provides a reliable estimate of the spectral distribution of the multiply scattered linearly polarized photon beam as observed by an HPGe detector. Next to variations in sample composition and thickness, the ability of the code to simulate various detection geometries has also been verified. As an application of the code, the achievable detection limits of SRXRF for rare earth elements as obtained with white beam and monochromatic (80 keV) excitation are compared.     

High-energy-resolution grazing emission X-ray fluorescence applied to the characterization of thin Al films on Si

The grazing emission X-ray fluorescence (GEXRF) technique was applied to the analysis of different Al films, with nominal thicknesses in the range of 1 nm to 150 nm, on Si wafers. In GEXRF the sample volume from which the fluorescence intensity is detected is restricted to a near-surface region whose thickness can be tuned by varying the observation angle. This is possible because of the refraction of the fluorescence X-rays and the quite long emission paths within the probed sample. By recording the X-ray fluorescence signal for different shallow emission angles, defined relatively to the flat, smooth sample surface, the deposited Al surface layers of the different samples could be well characterized in terms of layer thickness, layer density, oxidation and surface roughness. The advantages offered by synchrotron radiation and the employed wavelength-dispersive detection setup were profited from. The GEXRF results retrieved were confirmed by complementary measurements. The experimental setup, the principles and advantages of GEXRF and the analysis of the recorded angular intensity profiles will be discussed in details.     

Analytical possibilities of different X-ray fluorescence systems for determination of trace elements in aqueous samples pre-concentrated with carbon nanotubes

This study was aimed to achieve improved instrumental sensitivity and detection limits for multielement determination of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Se, Pb and Cd in liquid samples by using different X-ray fluorescence (XRF) configurations (a benchtop energy-dispersive X-ray fluorescence spectrometer, a benchtop polarised energy-dispersive X-ray fluorescence spectrometer and a wavelength-dispersive X-ray fluorescence spectrometer).The preconcentration of metals from liquid solutions consisted on a solid-phase extraction using carbon nanotubes (CNTs) as solid sorbents. After the extraction step, the aqueous sample was filtered and CNTs with the absorbed elements were collected onto a filter paper which was directly analyzed by XRF.The calculated detection limits in all cases were in the low ng mL^− 1 range. Nevertheless, results obtained indicate the benefits, in terms of sensitivity, of using polarized X-ray sources using different secondary targets in comparison to conventional XRF systems, above all if Cd determination is required.The developed methodologies, using the aforementioned equipments, have been applied for multielement determination in water samples from an industrial area of Poland.     

Quantification in grazing-emission X-ray fluorescence spectrometry

In grazing-emission X-ray fluorescence (GEXRF) spectrometry wavelength-dispersive detection can be applied. Much softer radiation and hence lighter elements than in total-reflection X-ray (TXRF) spectrometry can thus be detected. We used simulations to investigate methods of quantification of GEXRF results involving soft characteristic radiation. From these studies, it is concluded that for ultra-thin layers, e.g. the sub-monolayer amounts encountered in semiconductor contamination analysis, calibration plots are linear. For thicker layers, quantification should be performed very carefully because of deviations from linearity due to absorption of radiation and to oscillations in the calibration curve. These oscillations are caused by interference of fluorescence radiation emitted directly towards the detector and radiation reflected at the sample–substrate interface. Suggestions for a judicious choice of measurement conditions are made and the benefits of internal standardisation are discussed.     

Depth profiles of Al impurities implanted in Si wafers determined by means of the high-resolution grazing emission X-ray fluorescence technique

The synchrotron radiation based high-resolution grazing emission X-ray fluorescence (GEXRF) technique was used to extract the distribution of Al ions implanted with a dose of 10¹⁶ atoms/cm² in Si wafers with energies ranging between 1 and 100 keV. The depth distributions of the implanted ions were deduced from the measured angular profiles of the Al-Kα X-ray fluorescence line with nanometer-scale precision. The experimental results were compared to theoretical predictions of the depth distributions resulting from ion implantation. A good agreement between experiment and theory was found which proved that the presented high-resolution grazing emission X-ray fluorescence technique is well suited to perform depth profiling measurements of impurities located within the extinction depth, provided the overall shape of the distribution can be assumed a priori.     

Development of a combined method to carry out a multielement analysis for environment preser vation

The paper discusses the possibilities of energy dispersive X-ray fluorescence spectroscopy and instrumental neutron activation analysis in the investigation of environmental pollutions. The X-ray fluorescence spectroscopy of samples was performed using the MECA-1044A analyzer (XR-500, LINC SYSTEMS, Great Britain) with a pulsed X-ray tube with a silver target as a source of excitation. Neutron activation analysis was carried out on the basis of two reactors: IBR-2 LNP JINR, Dubna, and TWR ITEP, Moscow. The optimal regimes of measurement for both methods as well as the detection limits corresponding to these regimes are given. To illustrate the perspectives of the methods, the ecological problem of the consequences of introducing into soil of the now available meliorator CaSO₄·2H₂O is solved in the paper. Data on absorption of trace elements in a wide range including rare-earth elements by agricultural plants are presented. This paper may be of interest for specialists in analytical chemistry and ecology.