Hydride-trapping techniques for the speciation of arsenic

The determination of arsenic species by the trapping of volatile hydrides prior to atomization in the light path of an atomic absorption spectrometer is described and its operation in the measurement of arsenic species in the marine environment are discussed. Examples are drawn from the analysis of Tamar estuary water and sediment interstitial (pore) waters and from studies of the temporal variation of dimethylarsenic in coastal waters. Improvements in both the design and operation of the technique have resulted in enhanced performance. Baseline resolution of inorganic arsenic, monomethylarsenic and dimethylarsenic is now possible and trimethylarsine is resolved. Ultraviolet photolysis of arsenobetaine and arsenocholine gives partial conversion to trimethylarsine oxide. This can be employed in the qualitative appraisal of the presence of trimethylarsenic species. Current detection limits (3 sigma) for inorganic, mono- and di-methylarsenic lie in the range 19 to 61 pg absolute, giving 19–61 ng/1 concentration detection limits for 1 ml samples. This can be improved even further by using larger sample volumes. The properties of the analysis system when presented with various arsenic species are described. A ca. 10% loss of arsenite occurs in samples stored at —20 °C and immediate freezing of samples in liquid nitrogen is recommended.     

Capillary electrophoresis for the speciation of arsenic

Capillary zone electrophoresis (CZE) with on-line UV-detection was used for the determination of arsenite, arsenate, monomethylarsonic acid, dimethylarsinic acid, arsenobetaine and arsenocholine. The method is simple and rapid (<10 min) and allows the determination of six different arsenic species without sample pretreatment. Several instrumental parameters were studied to obtain the best performance (pH of buffer, injection mode, injection time, applied voltage). To determine the arsenic compounds, the instrument was used with a negative potential applied to the injection side of the capillary so that the anions can migrate towards the anode because of their own mobility and charge. The capillary wall was coated with an electro-osmotic flow modifier which reversed the electro-osmotic flow and thus increased also the overall migration of the anions towards the anode. The influence of high concentrations of matrix components such as NaCl, KNO₃ and NaNO₃, as well as the presence of acids such as HNO₃ and HCl was studied. CZE was used for the determination of the oxidation state of arsenic in percolate waters and in the leachate of solidified arsenic containing waste. The lowest detectable concentration was about 100 g/l. A comparison with the results obtained with hydride generation coupled to ICP-MS was made.     

Evaluation of atomic fluorescence spectrometry as a sensitive detection technique for arsenic speciation

The potential of coupling anion-exchange high-performance liquid chromatography, hydride generation and atomic fluorescence spectrometry (HPLC–HG–AFS) for arsenic speciation is considered. The effects of hydrochloric acid and sodium tetrahydroborate concentrations on signal-to-background ratio, as well as argon and hydrogen flow rates, were investigated. Detection limits for arsenite, dimethylarsinic acid (DMA), monomethylarsonic acid (MMA) and arsenate were 0.17, 0.45, 0.30 and 0.38 μg l⁻¹, respectively, using a 20-μl loop. Linearity ranges were 0.1–500 ng for As(III) and MMA (as arsenic), and 0.1–800 ng for DMA and As(V) (as arsenic). Arsenobetaine (AsB) was also determined by introducing an on-line photo-oxidation step after the chromatographic separation. In this case the limits of detection and linear ranges for the different species studied were similar to the values obtained previously for As(V). The technique was tested with a human urine reference material and a volunteer's sample. © 1998 John Wiley & Sons, Ltd.     

Improvement in arsenic speciation,using hydride generation and atomic absorption spectrophotometry

Improvements in arsenic speciation in sea-water using hydride generation, cold trapping and AAS are reported. Progressive addition of sodium tetrahydroborate and pH optimization are essential to obtain reliable results. The response factor for As^III, As^v, methylarsonic acid and dimethylarsinic acid is nearly the same when sulphuric acid is used for the hydride generation step.     

蜂蜜中葡萄糖、果糖和木糖的多元校正定量分析研究

根据单糖与邻甲苯胺加成缩合反应的显色特征,采用偏最小二乘法(PLS)辅助分光光度法对吸收光谱重叠较严重、加和性欠佳的葡萄糖、果糖和木糖三组分的模拟混合试样进行分析,对同时测定的条件进行了优化,建立了单糖多组分体系同时定量分析的多元校正方法,并用此方法对蜂蜜样品中上述单糖组分的含量进行了测定.时模拟混合试样,回收率分别为...     

Seasonal control of arsenic speciation in an estuarine ecosystem

Arsenic speciation in the Itchen estuary and Southampton Water (UK) has been shown to vary seasonally, with detectable (>0.02μg As dm^?3) dissolved arsenic(III) and methylated arsenic only being present from May to early October. This corresponds to the time period during which water temperatures exceed 12°C. For the remainder of the year, inorganic arsenic(V) was the only detectable species. At its peak, ca 30% of the dissolved arsenic was present as methylated forms with dimethylarsenic (DMAs) being the predominant bioarsenical. Significant quantities of monomethyl-arsenic (MMAs) and inorganic arsenic(III) were also present, however. The concentrations of the bioarsenical species varied with position in the estuary and generally increased with salinity. Measurements made during the period of peak algal activity implicated the highsalinity area of the estuary as the most probable region in which the methylated arsenicals are generated. At some sites, a distinct lag was observed between the appearance of dimethylarsenic and the detection of arsenic(III)and monomethylarsenic. Chlorophyll a concentration proved to be a poor predictor of the appearance of reduced and methylated arsenic in the water column. Possible sources of dissolved methylated arsenic are discussed.     

Using the L1 norm to select basis set vectors for multivariate calibration and calibration updating

With projection based calibration approaches, such as partial least squares (PLS) and principal component regression (PCR), the calibration space is spanned by respective basis vectors (latent vectors). Up to rank k basis vectors are formed where k ≤ min(m,n) with m and n denoting the number of calibration samples and measured variables. The user needs to decide how many and which respective basis vectors (tuning parameters). To avoid the second issue, basis vectors are selected top‐down starting with the first and sequentially adding until model criteria are satisfied. Ridge regression (RR) avoids the issues by using the full set of basis vectors. Another approach is to select a subset from the total available. The presented work develops a process based on the L₁ vector norm to select basis vectors. Specifically, the L₁ norm is used to select singular value decomposition (SVD) basis set vectors for PCR (LPCR). Because PCR, PLS, RR, and others can be expressed as linear combination of the SVD basis vectors, the focus is on selection and comparison using the SVD basis set. Results based on respective tuning parameter selections and weights applied to the SVD basis vectors for LPCR, top‐down PCR, correlation PCR (CPCR), PLS, and RR are compared for calibration and calibration updating using spectroscopic data sets. The methods are found to predict equivalently. In particular, the L₁ norm produces similar results to those obtained by the well‐studied CPCR process. Thus, the new method provides a different theoretical framework than CPCR for selecting basis vectors. Copyright © 2016 John Wiley & Sons, Ltd.     

Use of indirect multivariate calibration for quality control of agricultural products by near infrared spectroscopy

In the near infrared (NIR) region, (=800–2500 nm) of the spectrum, most organic molecules have weak but reproducibly measurable absorption bands. This phenomenon has been increasingly exploited for the rapid, quantitative analysis of major constituents of agricultural products. As there is strong spectral overlap and an interaction of constituents in NIR, the calibration of instrumentation has to be performed indirectly with a training set of samples applying multivariate methods. A software package for IBM compatible personal computers includes programs to select samples to be used for calibration, to compute multiple linear regression equations, to cross-validate regression equations and to detect unusual samples. These methods are applied to rapidly determine the oil content of over a thousand rapeseed samples of the harvest of 1986. A considerable saving of time, labour, and costs can be achieved, the agreement with the standard laboratory method is very satisfactory.     

Working methods: Complete extraction of arsenic species: a worthwhile goal?

Some issues regarding sample preparation for arsenic speciation analyses are briefly discussed. In particular, the use of a single set of extraction conditions for the many different arsenicals present in biological samples is questioned. Copyright © 2003 John Wiley & Sons, Ltd.     

A review of multivariate calibration methods applied to biomedical analysis

The determination of the contents of therapeutic drugs, metabolites and other important biomedical analytes in biological samples is usually performed by using high-performance liquid chromatography (HPLC). Modern multivariate calibration methods constitute an attractive alternative, even when they are applied to intrinsically unselective spectroscopic or electrochemical signals. First-order (i.e., vectorized) data are conveniently analyzed with classical chemometric tools such as partial least-squares (PLS). Certain analytical problems require more sophisticated models, such as artificial neural networks (ANNs), which are especially able to cope with non-linearities in the data structure. Finally, models based on the acquisition and processing of second- or higher-order data (i.e., matrices or higher dimensional data arrays) present the phenomenon known as “second-order advantage”, which permits quantitation of calibrated analytes in the presence of interferents. The latter models show immense potentialities in the field of biomedical analysis. Pertinent literature examples are reviewed.     

HPLC-ICP-MS method development to monitor arsenic speciation changes by human gut microbiota

Inorganic arsenic (iAs) has been classified as a type 1 carcinogen and has also been linked to several noncancerous health effects. Prior to 1995, the As^V methylation pathway was generally considered to be a detoxification pathway, but cellular and animal studies involving MMA^III (mono metyl arsonous acid) and DMA^III (dimethyl arsinous acid) have indicated that their toxicities meet or exceed that of iAs, suggesting an activation process. In addition, thiolated arsenic metabolites were observed in urine after oral exposure of inorganic arsenic in some studies, for which the toxicological profile was not yet fully characterized in human cells. Studies have revealed that microorganisms from the gut environment are important contributors to arsenic speciation changes. This presystemic metabolism necessitates the development of protocols that enable the detection of not only inorganic arsenic species, but also pentavalent and trivalent methylated, thiolated arsenicals in a gastrointestinal environment. We aim to study the biotransformation of arsenic (As) using a Simulator of the Human Intestinal Microbial Ecosystem (SHIME). To be able to analyze the arsenicals resulting from biotransformation reactions occurring in this system, a method using liquid chromatography hyphenated to an inductively coupled plasma mass spectrometer (HPLC‐ICP‐MS) was developed. A Hamilton PRP‐X100 anion exchange column was used. The method allowed separation, identification and quantification of As^III(arsenite), As^V(arsenate), DMA^V(dimethylarsinicacid), MMA^V(monomethylarsonicacid) and MMMTA (monomethylmonothioarsenate). Attempts to optimize the same method for also separating MMA^III and DMA^III did not succeed. These compounds could be successfully separated using a method based on the use of a Zorbax C₁₈ column. The properties of the column, buffer strength, pH and polar nature of mobile phase were monitored and changed to optimize the developed methods. Linearity, sensitivity, precision, accuracy and resolution of both methods were checked. The combination of the two methods allowed successful quantification of arsenic species in suspensions sampled in vitro from the SHIME reactor or in vivo from the human colon and feces. Copyright © 2011 John Wiley & Sons, Ltd.     

Tissue accumulation and distribution of arsenic compounds in three marine fish species: relationship to trophic position

In this study the accumulation and distribution of arsenic compounds in marine fish species in relation to their trophic position was investigated. Arsenic compounds were measured in eight tissues of mullet Mugil cephalus (detritivore), luderick Girella tricuspidata (herbivore) and tailor Pomatomus saltatrix (carnivore) by high performance liquid chromatography–inductively coupled plasma‐mass spectrometry. The majority of arsenic in tailor tissues, the pelagic carnivore, was present as arsenobetaine (86–94%). Mullet and luderick also contained high amounts of arsenobetaine in all tissues (62–98% and 59–100% respectively) except the intestines (20% and 24% respectively). Appreciable amounts of dimethylarsinic acid (1–39%), arsenate (2–38%), arsenite (1–9%) and trimethylarsine oxide (2–8%) were identified in mullet and luderick tissues. Small amounts of arsenocholine (1–3%), methylarsonic acid (1–3%) and tetramethylarsonium ion (1–2%) were found in some tissues of all three species. A phosphate arsenoriboside was identified in mullet intestine (4%) and from all tissues of luderick (1–6%) except muscle. Pelagic carnivore fish species are exposed mainly to arsenobetaine through their diet and accumulate the majority of arsenic in tissues as this compound. Detritivore and herbivore fish species also accumulate arsenobetaine from their diet, with quantities of other inorganic and organic arsenic compounds. These compounds may result from ingestion of food and sediment, degradation products (e.g. arsenobetaine to trimethylarsine oxide; arsenoribosides to dimethylarsinic acid), conversion (e.g. arsenate to dimethylarsinic acid and trimethylarsine oxide by bacterial action in digestive tissues) and/or in situ enzymatic activity in liver tissue. Copyright © 2001 John Wiley & Sons, Ltd.     

液相色谱-双通道原子荧光检测联用法同时测定砷和硒的形态

建立了一种利用高效液相色谱-双通道原子荧光检测联用同时进行砷和硒形态分析的方法。以10 mmol/L NH4H2PO4溶液(pH 5.6)(添加2.5%(体积分数)的甲醇)为流动相,在12 min内同时分离了三价砷(As(III))、一甲基砷(MMA)、二甲基砷(DMA)、五价砷(As(V))、硒代胱氨酸(SeCys)、硒代蛋氨酸(SeMet)和四价硒［Se(IV)］等化合物。As(III)、DMA、MMA、As(V)、SeCys、SeMet和Se(IV)的检出限分别为1,3,2,3,4,18和3 μg/L (进样量为200 μL),5次测定的相对标准偏差为1.9%～6.1%(As 100 μg/L, Se 300 μg/L)。应用该方法对人体尿样及硒酵母片中砷和硒的形态进行了分析,目标物在尿样中的加标回收率为83%～108%,在硒酵母片中的加标回收率为88%～105%。实验结果表明,该方法可用于尿样及药品中砷和硒形态的日常分析。该方法减少了样品的分析时间和试剂用量,降低了工作强度,提高了工作效率。     

Second-order multivariate calibration procedures applied to high-performance liquid chromatography coupled to fast-scanning fluorescence detection for the determination of fluoroquinolones

Different second-order multivariate calibration algorithms, namely parallel factor analysis (PARAFAC), N-dimensional partial least-squares (N-PLS) and multivariate curve resolution-alternating least-squares (MCR-ALS) have been compared for the analysis of four fluoroquinolones in aqueous solutions, including some human urine samples (additional four fluoroquinolones were simultaneously determined by univariate calibration). Data were measured in a short time with a chromatographic system operating in the isocratic mode. The detection system consisted of a fast-scanning spectrofluorimeter, which allows one to obtain second-order data matrices containing the fluorescence intensity as a function of retention time and emission wavelength. The developed approach enabled us to determine eight analytes, some of them with overlapped profiles, without the necessity of applying an elution gradient, and thus significantly reducing both the experimental time and complexity. The study was employed for the discussion of the scopes of the applied second-order chemometric tools. The quality of the proposed technique coupled to each of the evaluated algorithms was assessed on the basis of the figures of merit for the determination of fluoroquinolones in the analyzed water and urine samples. Univariate calibration of four analytes led to limits of detection in the range 20–40 ng mL⁻¹ and root mean square errors for the validation samples in the range 30–60 ng mL⁻¹ (corresponding to relative prediction errors of 3–8%). The ranges for second-order multivariate calibration (using PARAFAC and N-PLS) of the remaining four analytes were: limit of detection, 2–8 ng mL⁻¹, root mean square errors, 3–50 ng mL⁻¹ and relative prediction errors, 1–5%.     

Speciation of organic and inorganic arsenic by HPLC-HG-ICP

This paper deals with the application of high performance liquid chromatography (HPLC), hydride generation (HG) and inductively coupled plasma atomic emission spectrometry (ICP) to determine four species of arsenic: As(III), As(V), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA). The coupling conditions of HPLC-HG-ICP are given. Two anionic exchange columns (Nucleosil-5SB and Hamilton PRP X-100) are tested and the separation conditions optimized. Two acids (H₂SO₄ and HCl at different concentrations) are tested to obtain the hydrides. The proposed method is applied to determine four arsenic species in a synthetic matrix simulating a fish extract.     

Development of a method for speciation of inorganic arsenic in waters using solid phase extraction and electrothermal atomic absorption spectrometry

A solid phase extraction (SPE) procedure based on Amberlite IRA 900 resin was developed for speciation and separation of inorganic arsenic species (III, V) and total As in water samples. The As species and total As in eluent solutions were determined by electrothermal atomic absorption spectrometry (ETAAS) using Ni chemical modifier with 1200°C pyrolysis temperature. Experimental parameters such as pH value, sample volume, flow rate, volume and concentration of eluent solution for As(V) were optimised and 98.0 ± 1.9% recovery was found at pH 4.0. Experimental adsorption capacity of the resin for As(V) was investigated and 229.9 mg g^–1 was found. Under optimised experimental conditions, instrumental parameters such as limit of detection (LOD) and limit of quantification (LOQ) found were 0.126 and 0.420 µg L^–1, respectively. Interference effects of coexisting ions in the sample matrix on the recovery of As(V) were investigated. Concentration of As(III) was obtained by subtracting As(V) concentration found at pH 4.0 from total As(III + V) found at pH 8.0. The accuracy of the method proposed by using the resin was tested for analysing As species in a waste water standard reference material (SRM, CWW-TM-D) and spiked real water samples with recovery above 95%. The method proposed was also applied to the determinations of As species and total As in underground hot waters and tap water with relative error below 3%.     

Evaluation of extraction methods for arsenic speciation in polluted soil and rotten ore by HPLC-HG-AFS analysis

Three extraction systems including shaking, ultrasonic and microwave-assisted extraction were evaluated. Water and phosphate buffer were tested for the extraction of arsenic compounds in polluted soil, describing the water-soluble or plant-available fraction. The stabilities and recoveries of various arsenic species indicated that no obvious changes of species occurred during the extraction process. The raw extracts were cleaned up by C₁₈ cartridge before analysis. Having optimized the extraction conditions, the arsenic species in polluted soil and ore from the different pollution sources were extracted by microwave-assisted extraction with 0.5 M phosphate buffer as extractant. Arsenic species were quantitatively determined by high performance liquid chromatography on-line coupled with hydride generation atomic fluorescence spectrometry (HPLC-HG-AFS). As(III) and As(V) were the major arsenic species in the polluted soil samples resulting from irrigation by waste water. As^V was the only form found in the rotten ore sampled in mining area. During the extraction process, the recoveries of spiked As(III), As(V), DMA(V) and MMA(V) were 85.4 ± 7.2%, 80.2 ± 6.7%, 101.6 ± 6.7% and 98.8 ± 9.1%, respectively, showing that most water-soluble arsenic could be measured.     

Comparison of extraction procedures for arsenic speciation in environmental solid reference materials by high‐performance liquid chromatography–hydride generation‐atomic fluorescence spectroscopy

Water and ‘soft’ extractions (hydroxylammonium hydrochloride, ammonium oxalate and orthophosphoric acid) have been studied and applied to the determination of arsenic species (arsenite, arsenate, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)) in three environmental solid reference materials (river sediment, agricultural soil, sewage sludge) certified for their total arsenic content. The analytical method used was ion exchange liquid chromatography coupled on‐line to atomic fluorescence spectroscopy through hydride generation. Very low detection limits for arsenic were obtained, ranging from 0.02 to 0.04 mg kg^?1 for all species in all matrices studied. Orthophosphoric acid is the best extractant for sediment (mixed origin) and sludge samples (recent origin) but not for the old formation soil sample, from which arsenic is extracted well only by oxalate. Both inorganic forms (As(III) and As(V)) are significant in all samples, As(V) species being predominant. Moreover, organic forms are found in water extracts of all samples and are more important in the sludge sample. These organic forms are also present in the ‘soft’ extracts of sludge. Microwave‐assisted extraction appears to minimize the risk of a redox interconversion of inorganic arsenic forms. This study points out the necessity of combining direct and sequential extraction procedures to allow for initial arsenic speciation and to elucidate the different mineralogical phases–species associations. Copyright © 2002 John Wiley & Sons, Ltd.     

Study on simultaneous speciation of arsenic and antimony by HPLC-ICP-MS

A method was developed for the simultaneous speciation of arsenic and antimony with HPLC-ICP-MS using C30 reversed phase column. Eight kinds of arsenic compounds (As(III), As(V), monomethylarsonic acid (MMAA), dimethylarsinic acid (DMAA), arsenobetaine (AB), arsenocholine (AsC), trimethylarsine oxide (TMAO) and tetramethylarsonium (TeMA)), Sb(III) and Sb(V) were simultaneously separated by the special mobile phase containing ammonium tartrate. Especially for the species of organic As, a C30 column was better than a C18 column in the effect of separation. Limits of detection (LOD) for these elements were 0.2 ng ml⁻¹ for the species of each As, and 0.5 ng ml⁻¹ for the species of each Sb, when a 10 μl of sample was injected, respectively. The proposed method was applied to a hot spring water and a fish sample.     

Analysis of amino acids in complex samples by using voltammetry and multivariate calibration methods

A voltammetric method is proposed for the simultaneous determination of tryptophan, cysteine, and tyrosine using multivariate calibration techniques. Various electrodes and voltammetric techniques were explored to ascertain the optimum measurement strategy. Among them, differential pulse voltammetry (DPV) with a Pt electrode was selected as analytical technique since it provided a suitable compromise between sensitivity and reproducibility while allowing the oxidation peaks of the three compounds to be reasonably discriminated. The sensitivity of DPV with Pt electrode for Trp standards was 8.4×10⁻² A l mol⁻¹, the repeatability 3.7% and the detection limit below 10⁻⁷ M. The lack of full selectivity of the voltammetric data was overcome using multivariate calibration methods on the basis of the differences in the voltammetric waves of each compound. The accuracy of predictions was evaluated preliminarily from the analysis of three-component synthetic mixtures. Subsequently, this method was applied to the analysis of oxidizable amino acids in feed samples. Results obtained were in good concordance with those given by the standard method using an amino acid analyzer.