Standard Addition Plots Research Articles

A Double-layered Paper-based Analytical Device for Determination of Iron in Water Samples based on Standard Addition Method

A simple method for the determination of iron involved a novel paper-based analytical device (PAD) was developed. The PAD was composed of two layers. Each layer contained a circular hydrophilic reservoir (10 mm Ø) that was situated in a rectangular filter paper (25 Î 25 mm2). The hydrophobic area was created by painting the paper with a “waterproof” glue. The top and the bottom layers were assigned as “the filtration” and “the detection” platforms, respectively. The procedure was started by pipetting an aliquot of bathophenanthroline (Bphen) onto the hydrophilic zone of the bottom layer followed by spiking of standard solutions (0.1-0.5 mgL-1 Fe2+). The red complex was developed. Then, the top and the bottom layers were assembled by two-sided mounting tape. Later, a water sample was dropped onto the top layer, which removed (filtered) any suspended particles in the water sample. When the filtrate was exposed to the bottom layer, a further colored product formed. The bottom layer was removed and placed in a light-controlled box, and the optical image of the product was captured using a smartphone. Its intensity was evaluated through ImageJTM. Linear standard addition plots were obtained (r2 > 0.99). The PAD provided high precision (RSD < 6%) with good recovery (92.6-102%). It was applied to the analysis of drinking, tap, canal and river water samples without any prior filtration. The iron amounts were compared to the results obtained by the spectrophotometric method, and there was not significantly difference at 95% confidence (Paired-t test, n = 5 samples, tstat = 2.68, tcri = 2.78).

Development of a two-dimensional liquid chromatography-tandem mass-spectrometry method for the determination of vitamin D2 in mushrooms

Different foods, especially mushrooms, are a valuable source of vitamin D2. However, published concentrations in mushrooms show large variabilities. One reason for this is certainly the high biological variability caused by growth conditions, and another could also be found in the analytical methodology. Therefore, this study aimed to develop a sensitive and highly selective two-dimensional liquid chromatography mass spectrometry (LC–MS/MS) method for vitamin D2 analysis in mushrooms. After validation, the method was applied to four different mushroom species. The developed method with a one-step extraction procedure showed a limit of detection of 0.01 µg vitamin D2/g dry mass (DM), a limit of quantification of 0.05 µg vitamin D2/g DM, and recovery rates between 87.6 and 94.8%. The total run time including the re-equilibration of the columns for the next injection was 7.5 min. After adding increased concentrations of pure substance to Pleurotus ostreatus, Lentinula edodes, and brown and white button mushrooms the standard addition plot showed excellent correlation coefficients (R2) of > 0.9994. Mean vitamin D2 concentrations were observed at 0.122 ± 0.007, 0.074 ± 0.005, 0.099 ± 0.007, and 0.073 ± 0.005 µg/g DM. The coefficient of variation (CV) was between 5.1 and 7.6%. This well-optimized, sensitive LC–MS/MS method, with a fast and simple sample preparation and a short run time, can be applied to future studies especially in different mushroom species with variable growing conditions. This will improve our knowledge about the vitamin D2 content in mushrooms.Graphical abstract

Glomalin‐related soil protein in French temperate forest soils: interference in the Bradford assay caused by co‐extracted humic substances

SummaryThermostable soil protein, known as glomalin, is an important component of soil carbon stocks. Thought to originate from endomycorrhizal fungi, Glomales, this operationally‐defined fraction of soil organic matter contains proteins of diverse origin as well as non‐protein material, including humic substances. Accumulation results from the balance between production/release and subsequent degradation. Quantification of the protein is subject to uncertainty because of the co‐extraction of other components that interfere with the Bradford assay. We studied 10 topsoils from French temperate forests, taken from the national forest monitoring network (Renecofor). Two fractions were extracted, easily extractable (EE) at neutral pH and total extractable (T) at pH 8. Protein was quantified with the colorimetric Bradford method, either by direct calibration using bovine serum albumin (BSA) or by extrapolation of the standard addition plot of BSA. Solubilized organic matter was characterized by using absorbance at 465 and 665 nm and by three‐dimensional fluorescence excitation‐emission spectroscopy. Neither soil properties nor forest cover influenced glomalin‐related soil protein (GRSP) content. Direct assay gave the GRSPEE to be about 1 g kg−1 soil, and GRSPT in the range 3–10 g kg−1, accounting for about 2% of soil organic carbon and about 15% of soil nitrogen. Standard addition plots indicated a two to sixfold under‐estimation of protein in total extracts, caused by negative interference with the Bradford assay. The GRSPEE was correlated significantly with both estimates of GRSPT. Under‐estimation of GRSPT by direct assay was not related to the E4:E6 ratio but was correlated significantly with the intensity of absorbance at either 460 or 660 nm and with one of the fluorescence peaks. We conclude that GRSPEE is not necessarily more recent than GRSPT and that both fractions may be probes of protein content, but that absolute contents may be under‐estimated because of co‐extracted humic substances.

Development of a Combined Standard Additions/Internal Standards Method to Quantify Residual PEG in Ethoxylated Surfactants by MALDI TOFMS

Measuring the residual polyethylene glycol (PEG) in polyethylene oxide (PEO)-based surfactants is important to fully understanding the performance of these materials. Traditional methods of quantitating PEG in PEO-based surfactants can be time-consuming and struggle with low amounts or overlapping molecular mass distributions. This paper describes a matrix-assisted laser desorption/ionization (MALDI) mass spectrometry method developed to quantitate residual PEG in a series of ethoxylated surfactants. The technique addresses the difficulties faced in doing quantitative MALDI experiments by utilizing both internal standard and standard additions protocols. The method produces excellent straight line standard addition plots, and the quantitative results are verified using both a constructed standard and an independent traditional chromatographic separation.

Oligomer Content of α-Pinene Secondary Organic Aerosol

The quantity, extraction efficiency, and molecular composition of non-volatile oligomeric species in SOA generated by the reaction of α-pinene with ozone were studied. Two different methods of determining the total particulate mass in the reaction chamber were compared and found to be in good agreement when changes in the partitioning of semi-volatile compounds to the particle phase during measurement were properly handled. Almost all of the non-volatile organic carbon formed by the reaction was collected and recovered by extraction with organic solvents; recoveries with water extraction were somewhat lower. The identities of compounds extracted by the various solvents were determined using electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry. Over 80% of the peaks weighted by mass and intensity were the same in the spectra of samples obtained from different extraction solvents. Standard addition plots were used to determine the amounts of two commercially available monomer compounds in the SOA extracts. When the response factors for those compounds were applied to other monomers detected in the mass spectra, the weight percent of monomers was estimated to be slightly less than 50%, with the remaining mass (over 50%) assigned to oligomers. The oligomer content is sufficiently large that it should be taken into account when modeling the formation and properties of laboratory SOA.

Spectrophotometric determination of carminic acid in human plasma and fruit juices by second order calibration of the absorbance spectra–pH data matrices coupled with standard addition method

A simple analytical method based on the second-order calibration of the pH gradient spectrophotometric data was developed for assay of carminic acid (CA) in human plasma and orange juice over the concentration range of 1.5–14.0 μM. The multi-way data analysis method was coupled with standard addition to encounter the significant effects of plasma and juices matrices on the acid–base behavior and UV–vis. absorbance spectra of CA. Thus, the standard addition three-way calibration data of plasma or fruit juices samples were analyzed by parallel factor analysis (PARAFAC) and the concentration related scores were used to derive a standard addition plot such as one obtained in univariate standard addition method. The number of PARAFAC components was obtained utilizing different criteria such as core consistency and residual errors through pf-test implementation. The applicability of the proposed method was evaluated by analysis of human plasma and fruit juices spiked with different levels of standard CA solutions. The results confirmed the success of the proposed method in the analysis of pH gradient spectrophotometric data for determination of CA. The recoveries were between 86.7 and 106.7.

Application of Ratio Derivative Spectrophotometry for Simultaneous Determination of Naphazoline and Antazoline in Eye Drops

Ratio derivative spectrophotometrie method has been developed for the simultaneous determination of naphazoline (NAP) and antazoline (ANT) at micromolar levels in Britton Robinson buffer (pH 9) medium. In this method the overlapping spectra of naphazoline and antazoline were well resolved by making use of the first‐derivative of the ratios of their direct absorption spectra. The derivative ratio absorbances of naphazoline and antazoline were measured at 227.2 and 235 nm, respectively for their quantification. The method is simple, fast and does not require separation of naphazoline and antazoline. Another salient feature of the method is that simultaneous standard additions of both analytes permitted to resolve matrix effect and quantification at a unique standard addition plot. Naphazoline and antazoline were determined in the concentration range of 10‐150 μmol L-1 (NAP/ANT ratio varying from about 10 to 150) in the same aliquot with a precision and accuracy of about 1.7% and 1.8%, respectively. The recommended procedure was successfully applied for analysis of naphazoline and antazoline in eye drops.

Quantitative determination of wine polyfunctional mercaptans at nanogram per liter level by gas chromatography–negative ion mass spectrometric analysis of their pentafluorobenzyl derivatives

A fast method for the determination of aroma-powerful polyfunctional thiols at nanogram per liter level has been developed and applied to wine. A small volume of wine (6 mL) was extracted with 1.5 mL of benzene containing four internal standards. Pentafluorobenzyl derivatives of mercaptans were formed in the extract by adding small amounts (100 mg L −1) of pentafluorobenzyl bromide and a strong alkali: 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). After washing with a water:methanol (5:1) solution 0.5 M in HCl, 20 μL of the extract was directly injected into a gas chromatograph. Derivatives were detected by negative ion mass spectrometry. The method makes it possible to simultaneously determine 2-furfurylthiol (2-furanmethanethiol) (FFT), 4-mercapto-4-methyl-2-pentanone (MP), 3-mercaptohexylacetate (MHA) and 3-mercaptohexanol (MH). Inconsistent results were obtained for 2-methyl-3-furanthiol (MF). Detection limits were 0.5 ng L −1 (FFT), 0.1 ng L −1 (MP), 0.6 ng L −1 (MHA) and 7 ng L −1 (MH), well below the corresponding odor detection thresholds. Method repeatability (10% < RSD < 17%) and linearity (0.98 < R 2 < 0.999) were satisfactory. The linear range was more than 2 orders of magnitude wide, covering the natural range of occurrence of these compounds in wine, and the slopes of the standard addition plots from different wines were very similar. The different aspects of the method optimization are discussed.

Electrochemically Initiated Catalytic Oxidation of 5‐Thio‐2‐Nitrobenzoic Acid (TNBA) in the Presence of Thiols at a Boron Doped Diamond Electrode: Implications for Total Thiol Detection

AbstractThe electrochemical response of 5,5‐dithiobis(2‐nitrobenzoic acid) (DTNB) to increasing additions of thiol species has been examined at a boron doped diamond electrode. A reaction has been shown to occur with a range of biologically relevant thiols and proceeds via a CECC' process. A total thiol detection methodology has been developed showing that the sensitivities of the standard addition plots are independent of the individual thiol species added to the solution. The analytical utility of the reaction process has been assessed using chronoamperometry with the corresponding data producing detection limits of 5.7 μM, 4.4 μM and 5.8 μM for the detection of cysteine, homocysteine and glutathione respectively.

Electrothermal vaporization inductively coupled plasma-mass spectrometry for the determination of Cr, Cu, Cd, Hg and Pb in rice flour

Ultrasonic slurry sampling (USS)–electrothermal vaporization (ETV) dynamic reaction cell™ (DRC) inductively coupled plasma-mass spectrometry (ICP-MS) has been applied to determine Cr, Cu, Cd, Hg and Pb in rice samples. The influences of instrument operating conditions and slurry preparation on the ion signals were reported. Ascorbic acid was used as a modifier to enhance the ion signals. The background ions at the chromium masses were reduced in intensity significantly by using 0.4 ml min −1 NH 3 as reaction cell gas in the dynamic reaction cell while a q value of 0.6 was used. Since the sensitivities of Cr, Cu, Cd, Hg and Pb in rice flour slurry and aqueous solution were quite different, standard addition and isotope dilution methods were used for the determination of Cr, Cu, Cd, Hg and Pb in these rice samples. This method has been applied to the determination of Cr, Cu, Cd, Hg and Pb in NIST SRM 1568a rice flour reference material and two rice samples purchased from the market. The analytical results for the reference material agreed with the certified values. The results for the rice samples for which no reference values were available were also found to be in good agreement between the isotope dilution and standard addition methods. The method’s detection limits estimated from the standard addition plots were about 0.44, 1.7, 0.4, 0.53 and 0.69 ng g −1 for Cr, Cu, Cd, Hg and Pb, respectively, in the original rice flour.

Sono-Electroanalytical Determination of Lead in Saliva

Lead poisoning is recognized as a major environmental health risk and blood tests are used to assess occupational or domestic exposure to lead. Evidence has suggested that salivary lead concentration is closely related to recent lead exposure and may provide a useful and non-invasive indicator of lead status. Sono-electroanalysis has been shown to enhance detection of heavy metals in biological and environmental samples via the phenomena of acoustic streaming and cavitational shearing. This article therefore discusses the applicability of ultrasound enhanced square-wave anodic stripping voltammetry for the detection and determination of lead in saliva at an in situ plated Nafion coated mercury thin film glassy carbon electrode. Quantitative analysis of lead in artificial saliva from a realistic sputum volume, 220 μL, introduced to acetate buffer was shown to be possible. Employing an insonated preconcentration obviated the need for lengthy or degradative sample pretreatment by liberating the lead from the glycoproteins and other materials to which it binds in solution. Rapid experimental time was facilitated by short preconcentration periods of 120 s, due to the enhanced mass transport afforded by acoustic streaming. Quantitative depassivation of the electrode surface by cavitational shearing maintained the analytical signal throughout the experiment where under silent conditions the signal diminished to zero with time. Linear standard addition plots intersect the origin as predicted for samples uncontaminated with lead and exhibit R2 values between 0.988 and 0.994. The detection limit in the analyte is 0.25 μg L−1 which for a 220 μL sputum size corresponds to 38 μg L−1. With the onset of lead poisoning at a blood lead concentration of 100 μg L−1 this technique accesses biologically relevant lead levels.

Determination of sub-nanomolar concentrations of lead by anodic-stripping voltammetry at the silver electrode

The determination of lead at the nanomolar level by square-wave anodic-stripping voltammetry (SWASV) without removal of oxygen has been extended to the sub-nanomolar concentration range. The detection limit has been lowered with the use of a method of differences (SWASV_D) to 0.05 nM (10 ng l −1) at 60 s electrodeposition. In this method the analytical signal is the difference between the voltammogram of the sample and that obtained with no electrolysis, recorded sequentially. Measurements at the sub-nanomolar concentration level, including standard-addition plots, are performed in a rapid sequence enabled by the “Project” option of the GPES-μAutolab software. The repeatability of consecutive SWASV_D runs is good (0.5% at 4 μg l −1 for 30 s electrolysis; 2% at 0.4 μg l −1 for 60 s electrolysis; 5% at 0.06 μg l −1 for 60 s electrolysis). Hundreds of runs can be carried out without any pretreatment of the electrode. The high stability is attributed to a process that takes place during the electrodeposition step in a two-electrode cell: the silver counter/quasi-reference electrode generates silver ions that codeposit with lead at the Ag-RDE, thus ensuring a continuously renewed surface of the latter. The analysis of lead in rivers and drinking water has been performed. Surfactants distort the SWASV. In order to ensure surfactant-free solutions, the pretreatment of the samples included digestion with HNO 3 and H 2SO 4, evaporation to dryness and heating at 650°C. The detection limit is about 0.5 nM (0.1 μg l −1) and is limited by the purity of the reagents used in the digestion procedure.

Electrochemical Sample Pretreatment Coupled On-Line with ICP-MS: Analysis of Uranium Using an Anodically Conditioned Glassy Carbon Working Electrode

Uranium may be accumulated at an anodized glassy carbon (GC) electrode at −0.15 V and stripped at +1.15 V, allowing electrochemical preconcentration and matrix elimination on-line with ICP-MS detection. Analyte levels ranging from 20 μg/L to subnanogram per liter were determined in matrix-free standards with good linearity and precision. Detection limits (3sy/x/slope) determined from standard addition plots were 0.12 ng/L with 10-min accumulation time, permitting the minor isotopes of uranium to be determined with good precision at total uranium concentrations as low as 5.0 μg/L. High levels of salt matrix moderately suppressed uranium deposition efficiency, and therefore stripping peak height, but could be compensated for by extended accumulation times. Matrix elimination of up to 20 000 mg/L Na was complete with just a 90-s cell washout time. Uranium was quantified in a certified seawater matrix, NASS-4 (certified uranium level 2.68 ± 0.12 μg/L), at several levels of dilution: recoveries ranged from 10...

Determination of tantalum with hexafluorotantalate(V)-selective coated graphite electrode

A hexafluorotantalate(V)-selective coated-graphite electrode was prepared by coating a graphite rod with brilliant green-hexafluorotantalate(V) extract in 1-chloronaphthalene in a PVC matrix. Potential measurements were made against an HF-resistant plastic sleeve (Ag/AgCl) external reference electrode. The concentrations of sulfuric and hydrofluoric acids, for the optimum response of the electrode to hexafluorotantalate(V), were found to be 1M each in the test solutions. The electrode responded to hexafluorotantalate(V) in the linear range 5.0 × 10−6-5.0 × 10−3M, with a slope of -58 mV per decade and detection limit of 8.0 × 10−7M within 5–15s. The relative standard deviation for six determinations of 1.0 × 10−4M tantalum(V) was 2%. The life-time of the electrode was 60 days. The effects of forty diverse ions on the electrode response to the hexafluorotantalate(V) were studied and the electrode was found to be highly selective to hexafluorotantalate(V). Niobium, the element that commonly occurs with tantalum ores, showed a very low level of interference. The newly developed coated-graphite electrode has been applied to the determination of tantalum in tantalite-columbite ores and several synthetic matrices by direct, sample addition, standard addition, and Gran's plot potentiometric techniques with reasonable precision (2–4%) and accuracy.

Variability of the slopes of standard addition plots in the analysis of copper in seawater by anodic stripping voltammetry

AbstractCopper in seawater was analyzed with a ‘hybrid’ mercury film‐glassy carbon electrode using linear sweep, square wave and differential pulse anodic stripping voltammetry. The analyses were made in HCl (pH 2) and ethylenediamine (EN)‐spiked seawater (9 &lt; pH &lt; 10), with and without purging the sample of dissolved oxygen. The relative slopes of the standard addition plots were used to determine the effect of mode and media alteration on the results. Square‐wave voltammetry in EN‐altered seawater, without oxygen purging, was found to be not only the most practical technique, but also highly sensitive. The ‘chemistry’ of the oxidation reaction during the stripping step was found to affect the experimental results markedly. For this reason, the peak current is an experimentally dependent measure of the ion flux, both, into and out of the electrode.

A "Hybrid" Mercury Film Electrode for the Voltammetric Analysis of Copper and Lead in Acidified Seawater and Other Media

Plating conditions for preparing a consistently reproducible Hg film electrode on a glassy carbon substrate in acid media are evaluated. It is found that a hybrid electrode, i.e., preplated with Hg and then replated with Hg in situ with the sample, gives very reproducible results in the analysis of copper (and lead) in seawater. Consistently reproducible electrode performance allows for the calculation of a cell constant and prediction of the slopes of standard addition plots, useful parameters in the study of copper speciation in seawater.

Potentiometric determination of tanatalum with a hexafluorotantalate(V) -selective liquid membrane electrode

Brilliant Green-hexafluorotantalate(V) extract in nitrobenzene was used as electroactive material to prepare a hexafluorotantalate(V) -selective liquid membrane electrode with a PVC support. The electrode body was made from concentric polypropylene tubes which were joined by a cork. The liquid membrane electrode was connected with fluoride-selective electrode as internal reference and ground glass diaphragm ( Ag AgCl ) electrode as external reference electrode to obtain a complete cell assembly for the potential measurements. The concentrations of sulphuric and hydrofluoric acid, for the optimum response of the electrode to hexafluorotantalate(V), were found to be 1 M each in the test solutions. The electrode was found to respond to hexafluorotantalate(V) in the concentration range of 2.0 × 10 −6−1.0 × 10 −2 M tantalum(V) with a slope of − 59 mV per decade and detection limit of 3.7 × 10 −7 M tantalum(V) within 20–120 s. The effects of forty diverse ions on the electrode response to the hexafluorotantalate(V) have been studied and the electrode has been found to be highly selective to hexafluorotantalate(V). The newly developed liquid membrane electrode has been applied successfully to the determination of tantalum in tantalite-columbite ores by direct, sample addition, standard addition, and Gran's plot potentiometric techniques.

Extractive-spectrophotometric determination of amphetamine in urine samples with sodium 1,2-naphthoquinone 4-sulphonate

Sodium 1,2-naphthoquinone 4-sulphonate (NQS) was tested as a reagent for amphetamine in order to develop an extractive-spectrophotometric method for the drug in urine samples. The standard additions method showed the absence of proportional bias error whereas the Youden method and the two standard addition plots method showed the presence of a constant bias error [total Youden blank (TYB)]. Acceptable results were obtained by evaluating the TYB error or by using a placebo (urine sample from a normal subject). The dynamic range of concentrations was 1.4–50 mg 1 −1 in urine samples and the detection limit was 0.6 mg 1 −1 when 10 ml of urine sample were taken.

Determination of heavy metals in real samples by anodic stripping voltammetry with mercury microelectrodes : Part 1. Application to Wine

Differential-pulse anodic stripping voltammetry with a mercury microelectrode is used for the determination of zinc, cadmium, lead and copper in wine at its natural pH without pretreatment. The effects of the matrix on the stripping peaks are studied in detail by varying the concentration of the metals. Intermetallic (CuZn) interferences and the effects of oxygen are described. The results obtained for the labile metal contents varied from 2 μg l −1 for cadmium to 148 μg l −1 for zinc; standard addition plots were linear over about two orders of magnitude above these levels, demonstrating the negligible effect of organic matter. Acidification of the sample with hydrochloric acid to pH 1 allowed the total metal contents to be determined. The reliability of the method was tested by comparison with the results obtained with atomic absorption spectrometry; the differences were within 10–20%.

Determination of chloride, nitrate and calcium ions in sugar with ion selective‐electrodes

Chloride, nitrate, and calcium ion‐selective electrodes were tested and found satisfactory for the determination of the corresponding ions in highly refined white sugar, molasses and other impure sugar samples at different stages of sugar manufacturing or refining. The interferences by Br− and I− ions in Cl− determination were eliminated by oxidation with 3 M HNO3 and passing air to remove the free Br2 and I2 formed. The Cl− concentration was then determined with a chloride‐selective electrode against a calibration curve. Chloride should not be determined by ashing sugar, since this causes high losses of Cl−. It was found that NO3− and free Ca2+ ions could be determined directly in the sugar solution against corresponding calibration curves. Total calcium was determined after ashing of sugar samples. The ash was dissolved in 0.1 M HCl and passed through an anion exchange resin to remove PO3−4 and SiO2−3 ions, and Ca2+ was determined as before. Bound calcium was obtained by subtracting values of free from total calcium. Determination of bound calcium was useful to monitor the process of liming. For comparison, the three ions were also determined with acceptable precision without removing interfering ions using the method of standard addition and Gran's plot. The errors of determination for both direct and standard addition techniques were 3% for Cl− and NO−3 and 1–5% for Ca2+.