Estimation of limits of detection and determination in X-ray fluorescence analysis by the dependence of the relative standard deviation on analyte concentration

An experimental dependence of the relative standard deviation on analyte concentration of hyperbolic type, characterizing the precision of quantitative chemical analysis, was used to estimate the limits of detection and determination in the X-ray fluorescence analysis. A method is proposed for the determination of their values using the approximation of the experimental dependence of the relative standard deviation on the analyte concentration by a power function. The choice of the values of the relative standard deviation, being criteria for the estimation of these limits, is substantiated. A concept of the limits of detection and determination of an analytical procedure is formulated, according to which the limit of detection of an analytical procedure is an objective value depending only on the precision of determinations, and the limit of determination of an analytical procedure is a subjective value depending not only on the precision of determinations but also on the requirements to their limiting (admissible) accuracy. The limits of detection and determinations of an analytical procedure found by this approach completely characterize the possibilities of an analytical procedure in determining low concentrations of analytes. The proposed approach can be used for the estimation of the limits of detection and determination of analytical procedures and in other methods of chemical analysis with the hyperbolic dependence of the relative standard deviation on the analyte concentration.     

Simultaneous determination of glycols based on fluorescence anisotropy

Simultaneous determination of non-fluorescent glycols in mixtures without separation or chemical transformation steps is described. Two methods based in the measure of fluorescence anisotropy of a probe such as fluorescein dissolved in the analyte or analyte mixtures are described. In the first method, the anisotropy spectra of pure and mixtures of analytes are used to quantitative determination (if the fluorophor concentration is in a range where fluorescence intensity is proportional to concentration). In the second method, a calibration curve anisotropy-concentration based on the application of the Perrin equation is established. The methods presented here are capable of directly resolving binary mixtures of non-fluorescent glycols on the basis of differences on the fluorescence anisotropy of a fluorescence tracer. Best analytical performances were obtained by application of the method based on Perrin equation. This method is simple, rapid and allows the determination of mixtures of glycols with reasonable accuracy and precision. Detection limits are limited by the quantum yield and anisotropy values of the tracer in the solvents. Recovery values are related to the differences in anisotropy values of the tracer in the pure solvents. Mixtures of glycerine/ethylene glycol (GL/EG), ethylene glycol/1,2-propane diol (EG/1,2-PPD) and polyethylene glycol 400/1,2-propane diol (PEG 400/1,2-PPD) were analysed and recovery values are within 95-120% in the Perrin method. Relative standard deviation are in the range 1.3-2.9% and detection limits in the range 3.9-8.9%.     

Determination of perfluorochemicals in biological, environmental and food samples by an automated on-line solid phase extraction ultra high performance liquid chromatography tandem mass spectrometry method

A rapid on-line solid phase extraction ultra high performance liquid chromatography tandem mass spectrometry method was developed for the identification and quantitation of nine perfluorinated compounds in matrices of environmental, biological and food interest. Pre-treatment, solid phase extraction, chromatographic and mass detection conditions were optimised, in order to apply the whole methodology to the analysis of different matrices. Particular attention was devoted to the evaluation of matrix effect and the correlated phenomena of ion enhancement or suppression in mass spectrometry detection. LOD and LOQ range from 3 to 15ngL(-1) and from 10 to 50ngL(-1), respectively. Method detection limits (MDLs) were also calculated for each kind of matrix. The recovery, evaluated for each analyte, does not depend on analyte concentration in the explored concentration range: average Rˉ% values are always greater than 82.9%. In the whole, the results obtained for samples of river waters, blood serum, blood plasma, and fish confirm the ubiquitous presence of perfluorinated compounds, as recently denounced by many sources.     

An IUPAC-based approach to estimate the detection limit in co-extraction-based optical sensors for anions with sigmoidal response calibration curves

An approach based on IUPAC methodology to estimate the limit of detection of bulk optode-based analytical methods for anions has been developed. The traditional IUPAC methodology for calculating the detection limit was modified to be adapted to particular cases where the calibration curves have a sigmoidal profile. Starting from the different full theoretical models for every co-extraction mechanism of the analyte in the membrane in bulk optodes, several particular simplified models at low analyte concentration were obtained and validated. The slope of the calibration curve at low analyte concentration was calculated from the first derivative of the simplified equation and, subsequently, the detection limit was estimated. This fitted-for-purpose estimation strategy was applied to anion quantification for in-house bulk optode-based analytical methods, and the estimated limits of detection were compared with those obtained by applying classical geometrical methodology. This way of establishing the detection limit yields values that maintain their true statistical and probabilistic aspects. It can be easily applied to any analytical system which yields non-linear calibration curves at low analyte concentration.     

Quantitative X-RAy Diffraction Analysis of Four Types of Amphibolic Asbestos by the Silver Membrane Filter Method

Abstract

In this paper, the method that utilizes the X-ray diffraction technique combined with a procedure for X-ray absorption correction, already applied to chrysotile, was extended to the quantitative determination of microgram quantities of four types of amphibolic asbestos. These are: crocidolite, amosite, tremolite and anthophyllite. The effectiveness of the absorption correction procedure, based on the use of a silver membrane filter, was tested for crocidolite by preparing standards of this substance in three different matrices and comparing their diffractometric responses with those of pure analyte standards. These experimental data were also used to calculate the mass absorption coefficients of the matrices, which were compared with both the theoretical values and the values obtained in previous experiments where chrysotile was used as the analyte. The limits of detection of the four pure amphiboles, and crocidolite in matrix were established, and a detailed study of the most common interferences that disturb the analytical peaks of the four amphiboles, was performed.     

A glow discharge ion source with fourier transform ion cyclotron resonance mass spectrometric detection

A glow discharge (CD) ion source has been coupled to a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer using a four-element electrostatic lens to accelerate and focus ions generated external to the instrument’s high magnetic field into its analyzer cell. Like other CD mass spectrometers, GD-FT-ICR can provide a quantitative measure of bulk analyte concentration with good precision and accuracy. Although detection limits currently attainable are several orders of magnitude higher than the commercially available magnetic sector-based instrument, CD-FT-ICR holds promise for ultrahigh resolving power elemental mass analysis. Several schemes are proposed to lower the detection limits of the technique while still providing high enough resolution to resolve isobaric interferences.     

Construction and performance of a time-multiplex multiple-slit flame atomic fluorescence spectrometer for multi-element analysis

The design and performance characteristics of a new multi-element flame atomic fluorescence spectrometer are presented. Radiation from four hollow-cathode tubes is directed onto an unsheathed air—hydrogen flame. The resulting atomic fluorescence is viewed by a special monochromator with a separate exit slit for each element. The light exiting from all slits is directed to a single photomultiplier tube. The fluorescence signals from different elements are distinguished by a time multiplex approach. Single-element detection limits for ten elements and multi-element detection limits for four elements are presented. The degradation of detection limits by flame background emission noise and effect of flame composition on performance are discussed. Better than 1% precision is obtained for moderate analyte concentrations.     

Determination of carbendazim, thiabendazole and fuberidazole using a net analyte signal-based method

The net analyte signal (NAS)-based method HLA/GO, modification of the original hybrid linear analysis (HLA) method, has been used to determine carbendazim, fuberidazole and thiabendazole in water samples. This approach was used after a solid-phase extraction (SPE) step, using the native fluorescence emission spectra of real samples, previously standardized by piecewise direct standardization (PDS). The results obtained show that the modification of HLA performs in a similar way that partial least-squares method (PLS-1). The NAS concept was also used to calculate multivariate analytical figures of merit such as limit of detection, selectivity, sensitivity and analytical sensitivity (γ⁻¹). With this purpose, blanks of methanol and ternary mixtures, with the target analyte at low concentration and the other two ranging according to the calibration matrix, were used, with different results. Detection limits calculated in the last way are more realistic and show the influence of the other components in the sample. Selectivity for carbendazim is higher than the corresponding values for fuberidazole and thiabendazole, whereas sensitivity, as well as the values obtained for their detection limits, are lower for carbendazim, followed by thiabendazole and fuberidazole. Results obtained by modification of HLA vary in the same way that the ones obtained by PLS-1.     

Determination of benzo[a]pyrene tetrols by column-switching capillary liquid chromatography with fluorescence and micro-electrospray ionization mass spectrometric detection

The present work displays capillary liquid chromatographic column switching methodology tailored for determination of benzo[a]pyrene tetrol isomers in biological matrices using on-line fluorescence and micro-electrospray ionization mass spectrometric detection. A well-established off-line crude solid phase extraction procedure was used in order to make the method compatible with several biological matrices. The solid phase extraction eluates were evaporated to dryness, redissolved in 1.0 ml methanol:water (10:90, v/v), loaded onto a 0.32 mm I.D. x 40 mm 5 microm Kromasil C(18) pre-column for analyte enrichment and back-flushed elution onto a 0.30 mm I.D. x 150 mm 3.5 microm Kromasil C(18) analytical column. The samples were loaded with a flow rate of 50 microl min(-1) and the tetrols were separated at a flow rate of 4 microl min(-1) with an acetonitrile:10 mM ammonium acetate gradient from 10 to 90%. A sample loading flow rate up to 50 microl min(-1) was allowed. The fluorescence excitation and emission were set to 342 and 385 nm, respectively, while mass spectrometric detection of the benzo[a]pyrene tetrols was obtained by monitoring their [M - H](-) molecular ions at m/z 319. The method was validated over the concentration range 0.1-50 ng ml(-1) benzo[a]pyrene tetrols in a cell culture medium with 100 microl injection volume, fluorescence detection and the first eluting tetrol isomer as model compound, resulting in a correlation coefficient of 0.993. The within-assay (n= 6) and between-assay (n= 6) precisions were determined to 2.6-8.6% and 3.8-9.6%, respectively, and the recoveries were determined to 97.9-102.4% within the investigated concentration range. The mass limit of detection (by fluorescence) was 3 pg for all the tetrol isomers, corresponding to a concentration limit of detection of 30 pg ml(-1) cell culture medium. The corresponding mass spectrometric mass limits of detection were 4-10 pg, corresponding to concentration limits of detection of 40-100 pg ml(-1) cell culture medium.     

地球化学样品中钒检测方法的对比

地球化学样品中钒的测定方法主要有原子吸收光谱法、等离子体发射光谱法(ICP-AES)、电感耦合等离子体质谱法(ICP-MS)和X射线荧光光谱法.选用国家一级标样,分别用4种方法测定样品含量值,依次对方法检出限、准确度、精密度、加标回收率作比较.经对比,4种方法测定值与推荐值都基本吻合.等离子体发射光谱法检出限低,线性范围更宽,准确度与精密度更好,更适用于地球化学样品中钒批量的测试.     

Reduction of matrix effects in ICP-MS by optimizing settings of ion optics

The effect of ion optics settings on the degree of suppression of analyte signals due to the presence of different basic elements in a sample solution, such as manganese, copper, indium, lanthanum, hafnium, and lead, was investigated on an example of an XSeries II inductively coupled plasma mass spectrometer. The effect of atomic mass and the concentration of the matrix element on the optimum values of ion optics settings and the change in the analyte signals was studied. It was shown that a decrease in the potential of the extractor lens to ?300 V can significantly reduce the matrix effect. As an example, the results of determining impurity elements in solutions containing up to 1 g/L of lanthanum were obtained using standard settings of the spectrometer with a reduced voltage on the extractor. It was shown that the optimization of ion optics settings enables the use of more concentrated solutions, thus reducing the detection limits for elements by several times.     

Absolute and/or relative detection limits in laser-based analysis: the end justifies the means

The absolute limit of detection usually expresses the minimum amount of analyte detectable, while the relative limit of detection refers to the minimum concentration of analyte detectable. These concepts and their differences are obviously familiar to all analytical spectroscopists. Nevertheless, the two definitions are used liberally in the literature. For example, it is not uncommon to refer to exceptional sub-femtograms detection limits for a technique used to analyse ultratrace levels of an element in water and to a modest part per million detection limit of another technique used to characterise the microdistribution of an element in a sample mass of about one microgram. In this paper, an attempt is made to point out that the terms "ultratrace analysis" and "microanalysis" must refer to two conceptually different approaches and that there are cases in which one definition is more appropriate than the other. It is argued that, while there is no objection in reporting both detection limits when a single technique is evaluated, one has to be careful in choosing the most appropriate definition when different analytical techniques are compared.     

Detection of carbohydrates in capillary electrophoresis

This review focuses on recent developments in sensitive detection modes for carbohydrates after separation by capillary electrophoretic methods. To bring detection sensitivity for carbohydrates analysis in line with current methods in protein sequencing, concentration detection limits of 10⁻⁶ molar or better are requires. A discussion of mass detection limits and concentration detection limits is followed by an overview of detection modes for natural and labeled carbohydrates. Amperometric detection and UV and laser-induced fluorescence detection after reductive amination, in particular with 8-aminonaphthalene-1,3,6-trisulfonic acid (ANTS), are discussed in more detail. Finally, the paper outlines developments to be expected in the near future, focusing on the needs in glycobiology such as improved sensitivity and selectivity.     

Synchrotron radiation total reflection X-ray fluorescence and energy dispersive X-ray fluorescence analysis on AP1™ films applied to the analysis of trace elements in metal alloys for the construction of nuclear reactor core components: a comparison

Synchrotron radiation induced total reflection X-ray fluorescence and conventional 45° energy dispersive X-ray fluorescence analysis using a 150-nm-thick AP1™ film as sample carrier have been exploited for the elemental analysis of traces in alloys used for the construction of reactor core components of nuclear power plants. Both techniques are well suited for the analysis since they require a low amount of sample (μl), important on one hand because of the limited disposal and on the other hand because of its high specific activity. The methods provide a very low background due to the total reflection phenomenon in TXRF and the thin AP1™ film sample support, respectively. The employment of synchrotron radiation was necessary since there are no laboratory sources which can deliver a collimated beam of the energy and intensity needed to excite the K-shell of the rare earth elements, allowing the achievement of minimum detection limits relevant for the proposed purpose (ng/g range). Moreover, the linear polarization of synchrotron radiation combined with a side-looking detection geometry manages to reduce the scattering due to the remaining matrix of the analyzed samples. Detection limits for Nb and for some of the rare earth elements (pg range for absolute detection limits and ng–μg/g range for concentration detection limits) obtained with the two techniques are presented and the two approaches are compared.     

An X-ray refractive lens comprising two sections cut from a gramophone record for a portable total reflection X-ray fluorescence spectrometer

An X-ray refractive lens is assembled from two sections cut from a gramophone record. The refractive lens is placed in a portable total reflection X-ray fluorescence (TXRF) spectrometer, and it is used for collimation of the incident X-ray beams. A TXRF spectrum measured with the refractive lens is compared with that measured with a waveguide. Compared with the refractive lens, the waveguide enhances the intensities of the X-rays illuminating an analyte. Therefore, fluorescent X-ray intensities increase when using the waveguide. On the other hand, the vertical angular divergence of the incident X-ray beams is smaller when using the refractive lens, and the smaller angular divergence results in a reduction of the scattering of the incident X-rays from a sample holder. Therefore, the spectral background is reduced when using the refractive lens, resulting in an increase of the signal to background ratios of the fluorescent X-rays. Detection limits for 3d transition metals obtained with the refractive lens are sub-nanograms to a few nanograms, and the detection limits are similar to those obtained with the waveguide.     

Substitution–dilution method to correct the matrix effect in multi-element quantitative analysis by X-ray fluorescence

A mathematical model based on the dilution–addition method (DAM) for multi-elemental analysis using an X-ray fluorescence technique is proposed. The conditions for sample preparation do not require both the unknown and standard samples to be similar in composition and mineralogy, and the unknown sample is replaced quantitatively by the standard sample, hence the denomination substitution–dilution method (SDM). This method makes it possible to correct the matrix effect in multi-elemental quantitative analysis by X-ray fluorescence for each analyte. The proposed model presents hyperbolic behaviour of the experimental data when the X-ray fluorescence intensities are represented versus the substitution factor (h) for each analyte. After calculating the A/B parameter relations, which depend on the X-ray fluorescence intensity of each analyte (I_i^ns) and the substitution factor (h) and determining the analyte concentration in the multi-element standard sample (C_i^p), it is possible to calculate the analyte concentration in the multi-element unknown using an algorithm suggested for this purpose. This work studies the substitution–dilution phase proposed in the method, and the factors arising from incorporation of the standard and diluent are established according to the nature of the samples and the modifications. These factors make it possible to establish the experimental interval of analyte concentration, generally narrow, which corresponds to a section of the hyperbolic function which is so short that it can be accepted as linear. This linear model can be accepted for a wide variety of samples with a diluent/sample ratio greater than 10. The proposed linear method provides satisfactory results which are comparable to those calculated by applying the hyperbolic method. The proposed method (SDM) has been applied to two different types of matrices, a binary alloy (without diluent, using the hyperbolic model) and a geological sample (with diluent, using both hyperbolic and linear models). In all cases the results were satisfactory.     

Elemental analysis in solutions by radioisotope excited X-ray fluorescence

The problem of spectrochemical analysis of elements in solutions has been investigated for a number of elements across the periodic table using radioisotope-induced energy dispersive X-ray fluorescence method. In this study a low intensity¹⁰⁹Cd X-ray source was used for characteristic X-ray excitation. Experimental parameters such as saturation thickness and critical thickness have been studied to evaluate their role in this method. Minimum detection limits, sensitivities and the nature of concentration calibration at critical thickness have been obtained as a function of Z to find the limits of the method. Results obtained have been discussed in the light of theories and potential areas of applicability of the method have been indicated.     

A taylor series model to evaluate the interelemental effects in X-ray fluorescence analysis, applied to the iron-zirconium-diluent system

A semi-empirical model has been developed to quantify the interelemental effects in X-ray fluorescence analysis. The measured X-ray fluorescence intensity has been expressed as a function of the different fluorescence elements composing the sample. this complex function has become an operative function via a Taylor series development. An explication has been given for the significance of the different terms of the series. These terms respond to mathematical functions known as characteristic functions for each chemical system. A parameter (B) has been defined which makes it possible to quantify the influence of the interelemental effect as a function of the analyte concentration (C) and that of the concomitant elements. The proposed model has been applied to the Fe-Zr-diluent chemical system. Simple relationships between B vs C have been obtained independently considering iron or zirconium as analyte.     

Characterisation of antibodies and analytes by surface plasmon resonance for the optimisation of a competitive immunoassay based on energy transfer

The determination of binding constants using surface plasmon resonance (SPR) was introduced to optimise a competitive homogeneous fluorescence energy-transfer immunoassay (ETIA) before labelling. Steroids were chosen as model for the detection of three analytes estrone, estradiol and ethinylestradiol--by taking three polyclonal antibodies (anti estrone-, anti estradiol- and anti estrogen-antibodies) and the corresponding analyte derivatives used for the immunisation. The active concentration of the antibodies was determined before and after labelling. Inhibition curves were recorded using SPR for all possible combinations of analyte, antibody, and analyte derivatives. The experiments revealed that the active antibody concentration can be reduced to 30% whereas the antibody affinity is not affected by the labelling process. Limits of the use of SPR for determination of affinity constants in solution are discussed. All possible ETIA calibration for the quantification of estrone and estradiol was performed. The lower limits of detection for estrone (0.06 microg L(-1)) and estradiol (0.17 microg L(-1)) were reached with the anti-estrogen IgG and its derivative     

Application of nondestructive X-ray fluorescence analysis to determine the element composition of medicinal plants

A nondestructive X-ray fluorescence technique has been developed to determine Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, Br, Rb, Sr, Ba, and Pb in plants. The line element intensities were measured by an S4 Pioneer X-ray sequence spectrometer (Bruker AXS, Germany). The inversely proportional relationship was obtained between the analyte line intensity and mass of the plant, pressed on boric acid backing, for elements with an atomic number 11 < Z < 20. It was found that reduction of plant mass from 6 to 1 g leads to an increase in element determination sensitivity. The detection limits for 1 g of pressed plant were evaluated as μg/g: 5–20 (Na, Mg, Al); 1–4 (Si, P, S, Cl, K, Ca, Ti, Ba, Pb); 0.4-0.8 (Cr, Mn, Fe, Ni, Br, Cu, Zn, Rb and Sr). This technique was applied to determine the element composition of violets of Violaceae family, which are used in medicine.