Application of wavelet transform to background correction in inductively coupled plasma atomic emission spectrometry

A background correction method based on wavelet transform was devised and applied to inductively coupled plasma atomic emission spectrometry (ICP-AES). The proposed approach separated background from analyte signal according to their different frequencies. Compared with the analyte signal, the background has a low frequency. By removal of the components attributed to the signal, the background over the spectral window of the analyte line can be fitted through wavelet reconstruction. The results showed that the wavelet transform technique could handle all kinds of background and low signal-to-background ratio spectra, and required no prior knowledge about the sample composition, no selection of suitable background correction points, and no mathematical assumption of the background distribution. This technique performed as well as the conventional three-point background correction method for linear backgrounds, and provided better results than the latter for curved backgrounds. The proposed procedure was illustrated, by processing real spectra, to be an effective and practical tool for background correction in ICP-AES.     

Contributions to accuracy improvement of simultaneous ICP atomic emission spectrometry using multi-line measurements of analyte and internal standard elements Applications for the analysis of permalloy

For successful application of simultaneous ICP atomic emission spectrometry for major component determinations in multi-component materials the accuracy of the method has to be improved. As a contribution to solve this problem a combined procedure for multi-component standard sample preparation, optimum calibration and different variations of internal standard corrections is described. Variance-weighted multi-line calibrations give most accurate results. Internal standard corrections are effective, if the time-dependent spectral line intensity fluctuations of the standard and the analyte elements are well correlated. Their sensitivities against some responsible device parameter variations are investigated. On the basis of multi-line measurements of the analyte and internal standard elements a “group-selected internal standard correction” (GS-ISC) method is applied and results in relative errors of less than 1% even for extreme fluctuations of the raw intensities. For rapid routine determination methods of materials with variable element compositions the added line intensities of the internal standard element can be used to correct the added analyte line raw intensities (“intensity addition internal standard correction” (IA-ISC) method). These accuracy optimization procedures are applied for the analysis of the soft magnetic material permalloy using the internal standard element In.     

内标法在ICP－AES中的应用研究──Ⅰ．信号的相关性与精密度的改善

本文介绍了选择一系列具有不同物理性质的元素谱线来研究分析元素与内标元素间的内在联系。分别研究了背景之间、各谱线的信号与背景之间、分析信号与内标线信号之间的相关性。结果表明，只有当连续辐射背景较强时前两者才有较高的相关性。采用分析线信号与内标线的信号比可提高分析精密度，而内标元素的物理性质不必与分析元素的相匹配。同时详细分析了信号和背景波动的机制。     

Determination of traces of Ni, Co and Fe in Li(2)CO(3)/K(2)CO(3) melts by flame atomic absorption spectrometry

A flame atomic absorption spectrometry (AAS) method is described for the determination of trace levels of Ni, Co, and Fe in 62 mole percent (mol.%) Li(2)CO(3) and 38 (mol.%) K(2)CO(3) melts after dissolution of the sample in dilute nitric acid. A pneumatic nebulizer with a glass impact bead is used to sample introduction. The effect of the high salt loading of the solution on the analytical signals is minimized by optimization of acetylene flow-rate and height of observation above the burner head. Under the optimum conditions, the results of the analysis of synthetic sample solutions by aqueous standards calibration graphs well agree with those obtained by the method of standard additions. Recoveries ranged from 99 to 101% and the relative standard deviation is around 1%. Detection limits for Ni, Co and Fe in Li/K carbonate salts are similar to that in aqueous solutions, i.e. 0.5x10(-6) g analyte/g (Li(0.62), K(0.38))(2)CO(3). Furthermore, a background absorption is easily compensated by D(2) background correction, which does not affect the sensitivity. The proposed method is applied to the determination of cobalt in real melt samples.     

Evaluation of partial least-squares with second-order advantage for the multi-way spectroscopic analysis of complex biological samples in the presence of analyte-background interactions

The combination of unfolded partial least-squares (U-PLS) with residual bilinearization (RBL) has not been properly exploited to process experimental second-order spectroscopic information, although it is able to achieve the important second-order advantage. Among other desirable properties, the technique can handle incomplete calibration information, i.e., when only certain analyte concentrations are known in the training set. It can also cope with analyte spectral changes from sample to sample, due to its latent variable structure. In this work, U-PLS/RBL has been successfully applied to experimental fluorescence excitation-emission matrix data aimed at the quantitation of analytes in complex samples: these were the antibiotic tetracycline and the anti-inflammatory salicylate, in both cases in the presence of human serum, where significant analyte-background interactions occur. The interactions of the analyte with the serum proteins modify their spectral fluorescence properties, making it necessary to employ training sets of samples where the biological background is present, possibly causing analyte spectral changes from sample to sample. The predictive ability of the studied model has been compared with that of parallel factor analysis (PARAFAC), as regards test samples containing different sera, and also other pharmaceuticals which could act as potential interferents.     

The standard addition and successive dilution method for evaluation and verification of results in atomic-absorption analysis

A simple procedure for evaluation and verification of results obtained by atomic-absorption analysis is proposed. It allows estimation of the correct results in the presence of unknown strong interferents and does not require any preliminary information about the sample to be analysed. The concept of the procedure arises from theoretical considerations of various types of interference effects on the form of calibration curves. The procedure may be considered as a combination of the standard addition and successive dilution methods. Although the procedure was developed and tested for atomic-absorption analysis it seems to be applicable to all analytical techniques in which preliminary elimination of non-specific interference signals that are independent of the analyte concentration is possible.     

Absorption, concentration and determination of trace amounts of air pollutants by flow injection method coupled with a chromatomembrane cell system: application to nitrogen dioxide determination

The concentration distribution of an analyte in a chromatomembrane cell (CMC) was examined by using various air samples of different air pollutant (NO₂) concentrations and volumes, and the results obtained could be explained by a proposed principle of the concentration distribution of the analyte in the CMC. This principle was for the first time proved experimentally in the present study. On-line preconcentration and continuous determination of the air pollutant (NO₂) in air samples were realized by coupling a three-hole CMC with a flow injection analysis (FIA) system, where a triethanolamine (TEA) aqueous solution (2 g l⁻¹) was used as an absorbing solution for NO₂ in the air samples. A calibration method with standard nitrite aqueous solutions was developed for the determination of NO₂ in the air samples. Concentrations of NO₂ in indoor air and its diluted air samples were determined by the proposed CMC/FIA method. The volume of air sample necessary for the measurement was decreased to only 5 ml. The measuring time for one sample was about 5–6 min even when a 20 ml air sample was used.     

Progress in direct solid sampling analysis using line source and high-resolution continuum source electrothermal atomic absorption spectrometry

The literature about direct solid sample analysis of the past 10–15 years using electrothermal atomic absorption spectrometry has been reviewed. It was found that in the vast majority of publications aqueous standards were reported as having been used for calibration after careful program optimization. This means the frequently expressed claim that certified reference materials with a matrix composition and analyte content close to that of the sample have to be used for calibration in solid sample analysis is not confirmed in the more recent literature. There are obviously limitations, and there are examples in the literature where even calibration with certified reference materials did not lead to accurate results. In these cases the problem is typically associated with spectral interferences that cannot be corrected properly by the systems available for conventional line source atomic absorption spectrometry, including Zeeman-effect background correction. Using high-resolution continuum source atomic absorption spectrometry, spectral interferences become visible owing to the display of the spectral environment at both sides of the analytical line at high resolution, which makes program optimization straightforward. Any spectrally continuous background absorption is eliminated automatically, and even rapidly changing background absorption does not cause any artifacts, as measurement and correction of background absorption are truly simultaneous. Any kind of fine-structured background can be eliminated by “subtracting” reference spectra using a least-squares algorithm. Aqueous standards are used for calibration in all published applications of high-resolution continuum source atomic absorption spectrometry to direct solid sample analysis. This contribution is based on a presentation given at the Colloquium for Analytical Atomic Spectroscopy (CANAS ‘07) held March 18–21, 2007 in Constance, Germany.     

Quantitative line selection with the Kalman filter approach for inductively coupled plasma atomic emission spectrometry

With the Kalman filter approach, the true detection limit (i.e. the detection limit in the sample solution) only depends on the height of the total background signal, and is not determined by the uncertainties in interfering background signals. Based on the background equivalent interferent concentration, the true detection limit can be directly estimated in sample solutions containing various amounts of interferent. This eases the application of the true detection limit as quantitative criterion for line selection. Following this approach, quantitative line selection is demonstrated for Pb and Cd determinations in river sediments (Fe interferences).     

浓度同倍数递减离子选择电极标准加入法

本文报道了试液和标液浓度同倍数递减标准加入法。该法可测定几个体积均为V_x而浓度以C_x/N方式递减的试液(1≤N≤2),每个试液加等体积标液1次,在半对数算图上,各试液加入标液后的电位对原始试液电位差ΔE_N与浓度递减倍数N之间有线性关系,由交点N⁰解C_x=C_sV_s/[(N⁰-1)V_x+N⁰V_s]方程求得分析结果。该法不必测定电极的能斯特斜率。     

Use of tandem calibration for analyzing steels and alloys by inductively coupled plasma atomic emission spectrometry

A method is proposed for plotting calibration graphs by reference analyte solutions (tandem calibration) for the analysis of steels and alloys by inductively coupled plasma atomic emission spectrometry (ICP-AES) with spark ablation. The use of spectral lines of analytes and an internal standard with similar values of sums of ionization and excitation energy ensures an excellent repeatability and accuracy of the results of analysis using tandem calibration. A nebulizer chamber for the simultaneous introduction of solutions and solid sample aerosols into inductively coupled plasma (ICP) is designed. The optimal parameters of the introduction of sample aerosols and a method of plasma observation are chosen; the requirements for the choice of the analytical lines of analytes are proposed.     

Spectral interferences and background overcompensation in zeeman-corrected atomic absorption spectrometry : Part 1. The effect of iron on 30 elements and 49 element lines

The background compensation performance of a Zeeman corrector with the magnetic field acting on the graphite atomization cell was assessed for 30 elements and 49 element lines in an iron matrix. Two of the elements studied, gallium and zinc, are influenced by background overcompensation which introduces serious negative systematic errors. The overcompensation is due to the presence of iron lines close to the 287.4-nm gallium line and the 213.9-nm zinc line; when the magnetic field is on, the σ-components of the adjacent iron lines overlap at the position of the analyte line and a background, which is not present when the magnetic field is off, is recorded. When gallium and zinc are measured under the same conditions but with deuterium arc background correction, the adjacent iron lines cause positive systematic errors. These spectral interferences for gallium in the presence of iron can be avoided by doing the measurements at the 294.4-nm gallium line; the two lines have about the same sensitivity. When zinc is to be measured at the 213.9-nm line, with either type of background correction, the spectral interferences from iron can be avoided by careful selection of the graphite-furnace parameters. In addition to spectral interferences, iron also affects the sensitivity for both gallium and zinc.     

Towards minimization of chlorinated solvents consume in Fourier transform infrared spectroscopy determination of Propamocarb in pesticide formulations

A method has been developed for Fourier transform infrared (FTIR) spectroscopy determination of Propamocarb in emulsifiable pesticide concentrate formulations. Five microliter sample was directly injected without any pretreatment in a CHCl3 stream at 2 mL min(-1) into a closed system and the FTIR spectra of sample and standard solutions were obtained using a nominal resolution of 4 cm(-1) from 4000 to 900 cm(-1) spectral region and accumulating 2 scans per spectrum. Propamocarb determination was based on the measurement of flow injection analysis (FIA) recording height established from FTIR peak area measurements from 1713 to 1703 cm(-1) corrected using a baseline defined at 2000 cm(-1). The concentration of Propamocarb in samples was calculated by interpolation in an external calibration line obtained from several injections of 2 microL of a 47% (w/v) standard solution into the CHCl3 closed system. This procedure provided a limit of detection of 0.8% (w/v) in the original sample, a sensitivity of 0.3190 absorbance units mL mg(-1) for a path length of 0.11 mm and a relative standard deviation of 0.2% for five independent measurements at 0.74 mg mL(-1) concentration level. The maximum sampling frequency of the whole procedure was 34 h(-1) and the waste generation was reduced to only 2 mL of CHCl3 solution per sample and additional 2 mL for the whole calibration line.     

Correction parameters in X-ray fluorescence analysis applying the limit dilution method (LDM)

Summary Some corrective parameters: H, M, Y and Y/H are defined from the mathematical development justifying the Limit Dilution Method (LDM) in XRF analysis. These corrective parameters, for a given diluent and dilution, are characteristic of the analyte in each sample. The influence of these parameters on important phenomena such as the diluent effect is evaluated by a theoretical model. Using parameters Y, an algorithm is proposed which corrects the diluent effect and establishes linear functions of fluorescence intensity vs. analyte concentration. This paper also proposes a selection procedure for the optimum standard for a given problem using the Y/H parameter. The method is applied to the XRF analysis of mixtures of iron and zirconium oxides and these results show the corrective action of the suggested parameters and the appropriateness of the selection of the standards.     

Flow injection titration basing on the merging-zone technique

The proposed approach relies on titration of a sample that is gradually diluted in strictly controlled way in the flow injection system developed. On each step of sample dilution equal volumes of the sample and titrant solutions are simultaneously injected into two carrier streams and the zones are merged with each other. Then, they are mixed completely in the mixing chamber, merged with a stream of indicator and directed to a detector. It has been revealed that the method provides the results with accuracy better than ±3.3% (RE) and with mean repeatability lower than 1.0% (RSD). When the analyte concentration in a sample is too low to be determined directly, the procedure of titration with standard addition is exploited. The proposed approach has been successfully applied to the determination of total acidity in vinegars and magnesium and calcium in pharmaceutical products. The results obtained were comparable with those provided by the reference methods. The proposed procedure is characterized by low consumption of sample (usually less than 2 mL), titrant (about 3 mL) and indicator (about 0.6 mL). Average time of a single analysis is similar to time of traditional batch analysis.     

Use of iridium plus rhodium as permanent modifier to determine As, Cd and Pb in acids and ethanol by electrothermal atomic absorption spectrometry

The most severe interferences in atomic absorption spectrometry are caused by the presence of anions when they are in different concentrations in the samples and in the calibration solutions. The analyte addition technique or matrix matching calibration can be employed to minimize or compensate the non-spectral interferences, but they are time consuming or difficult to be carried out. The use of chemical modifiers usually allows higher pyrolysis temperatures and consequently the removal of components of the sample matrix, equalizing the analyte signal in the sample and in the calibration solution. In this work, a mixture of Ir and Rh is proposed as permanent modifier to determine As, Cd and Pb in diluted hydrochloric, sulfuric and phosphoric acids and in ethanol and methanol by electrothermal atomic absorption spectrometry (ET AAS) with calibration against 1% v/v nitric acid aqueous solutions. The performance of the proposed permanent modifier was compared to that of Pd plus Mg nitrates in solution. Better recoveries, low background levels and faster analysis were obtained with the permanent modifier. The permanent modifier was also successfully employed for the determination of As, Cd and Pb in different concentrations of sulfuric and hydrochloric acids. For the phosphoric acid, the proposed modifier was only efficient for acid concentrations up to 2% v/v for As and up to 5% v/v for Cd and Pb. The precision, expressed as the relative standard deviation (n=3), was lower than 10%, for all samples, including ethanol and methanol.     

在线固相萃取-超高效液相色谱-串联质谱法快速测定地表水中超痕量阿特拉津

建立了在线固相萃取-超高效液相色谱-串联质谱（on line SPE-UPLC-MS/MS）测定地表水中超痕量阿特拉津的方法。样品经滤膜过滤,HLB Direct Connect HP在线固相萃取小柱富集纯化,甲醇溶液洗脱,以Acquity BEH 130为分析柱,串联质谱进行检测,外标法定量。阿特拉津在1.0~5000 ng/L范围内线性关系良好,相关系数（r）为0.9989;该方法检出限为0.2 ng/L,阿特拉津的回收率为83.0%~105.1%,相对标准偏差为1.6%~5.3%（n=7）,满足超痕量分析测试的要求。该法灵敏度高,分析速度快,对于保障水环境安全、及时提供污染信息、有效应对环境应急突发事件具有十分重要的意义。     

试液进样量对ICP光源中稀土元素谱线的影响

研究了ICP光源中试液进样量对稀土元素谱线强度、背景强度、谱线强度测量的相对标准偏差,元素检出限,发光效率及试样相对利用率的影响。结果表明,降低试液进样量对稀土元素谱线强度有轻度影响,并 不影响测量的相对标准偏差和元素的检出发。降低试液进行量将提高发光效率及试样利用率,节省稀有和贵重样品用样量。     

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.     

New background correction method for liquid chromatography with diode array detection, infrared spectroscopic detection and Raman spectroscopic detection

A new method to eliminate the background spectrum (EBS) during analyte elution in column liquid chromatography (LC) coupled to spectroscopic techniques is proposed. This method takes into account the shape and also intensity differences of the background eluent spectrum. This allows the EBS method to make a better estimation of the background eluent spectrum during analyte elution. This is an advantage for quantification as well as for identification of analytes. The EBS method uses a two-step procedure. First, the baseline spectra are modeled using a limited number of principal components (PCs). Subsequently, an asymmetric least squares (asLS) regression method is applied using these principal components to correct the measured spectra during elution for the background contribution. The asymmetric least squares regression needs one parameter, the asymmetry factor p. This asymmetry factor determines relative weight of positive and negative residuals. Simulations are performed to test the EBS method in well-defined situations. The effect of spectral noise on the performance and the sensitivity of the EBS method for the value of the asymmetry factorp is tested. Two applications of the EBS method are discussed. In the first application, the goal is to extract the analyte spectrum from an LC-Raman analysis. In this case, the EBS method facilitates easy identification of unknown analytes using spectral libraries. In a second application, the EBS method is used for baseline correction in LC-diode array detection (DAD) analysis of polymeric standards during a gradient elution separation. It is shown that the EBS method yields a good baseline correction, without the need to perform a blank chromatographic run.