Development of a sensitive detection method of cancer biomarkers in human serum (75%) using a quartz crystal microbalance sensor and nanoparticles amplification system

A simple and sensitive sensor method for cancer biomarkers [prostate specific antigen (PSA) and PSA-alpha 1-antichymotrypsin (ACT) complex] analysis was developed, to be applied directly with human serum (75%) by using antibody modified quartz crystal microbalance sensor and nanoparticles amplification system. A QCM sensor chip consisting of two sensing array enabling the measurement of an active and control binding events simultaneously on the sensor surface was used in this work. The performance of the assay and the sensor was first optimised and characterised in pure buffer conditions before applying to serum samples. Extensive interference to the QCM signal was observed upon the analysis of serum. Different buffer systems were then formulated and tested for the reduction of the non-specific binding of sera proteins on the sensor surface. A PBS buffer containing 200 μg mL⁻¹ BSA, 0.5 M NaCl, 500 μg mL⁻¹ dextran and 0.5% Tween 20, was then selected which eliminated the interfering signal by 98% and enabled the biomarker detection assay to be performed in 75% human serum. By using Au nanoparticles to enhance the QCM sensor signal, a limit of detection of 0.29 ng mL⁻¹ PSA and PSA-ACT complex (in 75% serum) with a linear dynamic detection range up to 150 ng mL⁻¹ was obtained. With the achieved detection limit in serum samples, the developed QCM assay shows a promising technology for cancer biomarker analysis in patient samples.     

Metal Ion‐Assisted Infrared Optical Sensor for Selective Determination of Tryptophan in Urine Samples

In this study, a simple infrared chemical sensor was developed for the selective detection of tryptophan in biological fluids. This sensor was capable of trapping tryptophan molecules through the formation of relatively stable metal ion complexes on the surface of the sensing element. A proline‐modified sensing phase was immobilized on the surface of the internal reflection element. With the assistance of appropriate metal ions, tryptophan molecules were selectively attracted nearby the evanescent field such that analytical signals were generated. Factors that affected the chemical equilibria in this detection system were examined including the species and concentration of metal ion, the pH of the sample solution, and the concentration of the chelating agent. Among the examined metal ions, nickel provided the best selectivity toward the detection of tryptophan as a result of its extremely high formation constant with tryptophan. Under the optimal conditions, the detected signals were related linearly (R² > 0.99) to concentrations of tryptophan up to 600 μM. Based on three times the baseline variation of blank samples, the detection limit was ca. 5 μM. From a study of possible interfering agents—metal ions and organic species—in the sample solution, the recoveries of tryptophan were greater than 95%.     

Voltammetric determination of Cu(II) in natural waters and human hair at a meso-2,3-dimercaptosuccinic acid self-assembled gold electrode

An electrochemical sensor for the detection of copper(II) ions is described using a meso-2,3-dimercaptosuccinic acid (DMSA) self-assembled gold electrode. First in ammonia buffer pH 8, copper(II) ions complex with self-assembled monolayer (SAM) via the free carboxyl groups on immobilized meso-2,3-dimercaptosuccinic acid (accumulation step). Then, the medium is exchanged to acetate buffer pH 4.6 and the complexed Cu(II) ions are reduced in negative potential of −0.3 V (reduction step). Following this, reduced coppers are oxidized and detected by differential pulse (DP) voltammetric scans from −0.3 to +0.7 V (stripping step). The effective parameters in sensor response were examined. The detection limit of copper(II) was 1.29 μg L⁻¹ and R.S.D. for 200 μg L⁻¹ was 1.06%. The calibration curve was linear for 3-225 μg L⁻¹ copper(II). The procedure was applied for determination of Cu(II) to natural waters and human hairs. The accuracy and precision of results were comparable to those obtained by flame atomic absorption spectroscopy (FAAS).     

An amperometric method for the determination of trace mercury(II) by formation of complexes with l-tyrosine

A selective and sensitive amperometric method of analysis has been developed for determination of the trace amounts of mercury in waters at a platinum electrode based on the effect of the presence of mercury ions on the current due to oxidation of l-tyrosine. A decrease of signal was observed due to the formation of a complex of tyrosine with the Hg(II) ion adsorbed on the electrode surface. Several parameters were varied, such as applied potential, pH and concentration of tyrosine. The calibration plot was linear in the range from 0.02 to 3 μmol l⁻¹ Hg(II) with r=0.997 and the detection limit (3σ) was 0.014 μmol l⁻¹; the relative standard deviation was 2.2%. The study of interferences from other metal ions revealed a good selectivity of this method towards mercury(II). The stoichiometry of the mercury-tyrosine complex was determined to be 1:2 and the formation constant 627±19. Formation of complexes with mercury ions was also demonstrated with several catechol compounds and other amino acids. The method was applied to the analysis of contaminated waters.     

Use of a solid sensing zone implemented with unsegmented flow analysis for simultaneous determination of thiabendazole and warfarin

For the first time, a solid sensing zone implemented with unsegmented flow analysis is described for the simultaneous determination of two pesticides, thiabendazole and warfarin. The system works as a simple and rapid spectrofluorimetric biparameter sensor. The sensor is based on the retention of the analytes on the sensing solid zone (octadecyl silane C₁₈ gel) placed in the detection zone itself into a quartz flow-cell. A temporary sequentiation in the arrival of the analytes to the sensing zone is achieved by on line separation using a pre-column of the same gel placed just before the flow cell. Thiabendazole is determined the first (using methanol 30% (v/v) as carrier/elution solution) because it passes through the pre-column while warfarin is strongly retained in it. Then, warfarin is conveniently eluted from the pre-column (using methanol 50% (v/v) as carrier/elution solution) the intrinsic fluorescence peak height measured at an excitation wavelength of 309 nm and an emission wavelength of 368 nm is used as analytical signal. Using a low sample volume (40 μl), the analytical signal showed a very good linearity in the range 10-800 ng ml⁻¹ and 2-40 μg ml⁻¹ with detection limits of 2.35 ng ml⁻¹ and 0.54 μg ml⁻¹ for thiabendazole and warfarin, respectively. The sensor was satisfactorily applied to the determination of these two analytes in pesticides and pharmaceutical preparations.     

Development of an optical chemical sensor based on 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol in Nafion for determination of nickel ion

An optical chemical sensor based on immobilization of 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (Br-PADAP) in Nafion membrane is described. The membranes were cast onto glass substrates and were used for the determination of nickel in aqueous solutions by spectrophotometry. The sensor system is highly transparent, mechanically stable and showed no evidence of reagent leaching. The influence of several parameters such as pH, ligand concentration, and type and concentration of regenerating solution were optimized. The sensor system showed good sensitivity in the range 0.5-20 μg ml⁻¹ with a detection limit of 0.3 μg ml⁻¹ Ni(II). The sensor has been incorporated into a home-made flow-through cell for determination of nickel in flowing streams with improved sensitivity, precision and detection limit. The calibration curve in the flow system was linear in the range 0.1-16 μg ml⁻¹ with a detection limit of 0.07 μg ml⁻¹. The sensor is easily regenerated by dilute nitric acid solution. The proposed method was successfully applied to the determination of nickel content in vegetable oil and chocolate samples and the results were compared with those obtained using atomic absorption spectrometry.     

Gel-surface enhanced fluorescence sensing system coupled to a continuous-flow assembly for simultaneous monitoring of benomyl and carbendazim

A novel, versatile and sensitive continuous-flow on-line solid phase fluorescence based system is proposed for the simultaneous determination of benomyl and carbendazim. The continuous-flow system is based on the on-line preconcentration and resolution of the pesticides on a solid sensing zone, followed by the sequential measure of their native fluorescence, monitored at 235/306 and 293/398 nm (λ_exc/λ_em for carbendazim and benomyl, respectively), and later desorption of these analytes (from the flow-through cell filled with C₁₈ silica gel) using aqueous methanol mixtures as carrier and eluent solutions.A double discrimination is used for the simultaneous monitoring of these analytes: (1) the usage of two pair of excitation/emision wavelengths, performed by the use of a multiwavelength fluorescence detection mode and (2) a temporary sequentiation in the arrival of the analytes to the sensing system by on-line separation due to the different kinetics showed by the analytes in the sorption-desorption process performed just in the solid support placed in the flow-through cell. Carbendazim is determined the first, because it shows a weaker retention in the C₁₈ bonded phase silica beads, while benomyl is strongly fixed. Then, benomyl is conveniently eluted from the flow-through sensing zone and its native fluorescence signal is measured (at 398 nm). The sensor was calibrated for two different injection volumes: 400 and 2000 μl. Using a 2000 μl sample volume, the analytical signal showed linearity in the range 0.050-1.0 and 0.020-0.50 μg ml⁻¹ with detection limits of 3.0 and 7.5 ng ml⁻¹ for carbendazim and benomyl, respectively, and R.S.D. values smaller than 2% for both analytes. A recovery study was performed on four different spiked environmental water samples at concentration levels from 0.05 to 0.35 μg ml⁻¹. The recovery percentage ranged from 97 to 104%, and from 98 to 104%, for benomyl and carbendazim, respectively.     

Design and evaluation of a thorium (IV) selective optode

A novel optical sensor has been proposed for sensitive determination of thorium (IV) ion in aqueous solutions. The thorium sensing membrane was prepared by incorporating 4-(p-nitrophenyl azo)-pyrocatechol (NAP) as ionophore in the plasticized PVC membrane containing tributyl phosphate (TBP) as plasticizer. The membrane responds to thorium ion by changing color reversibly from yellow to red-brown in glycine buffer solution at pH 3.5. The proposed sensor displays a linear range of 8.66 × 10⁻⁶-2.00 × 10⁻⁴ M with a limit of detection of 6 × 10⁻⁶ M. The response time of the optode was about 8.8-12.5 min, depending on the concentration of Th (IV) ions. The selectivity of optode to Th (IV) ions in glycine buffer is good. The sensor can readily be regenerated by exposure to a solution mixture of sodium fluoride and 5-sulfosalicylic acid (dihydrate) (0.01 M each). The optode is fully reversible. The proposed optode was applied to the determination of thorium (IV) in environmental water samples.     

Fiber optic-linear array detection spectrophotometry in combination with cloud point extraction for simultaneous preconcentration and determination of cobalt and nickel

A new combined method including fiber optic-linear array detection spectrophotometry (FO-LADS) and cloud point extraction (CPE) was developed using a cylindrical micro cell for simultaneous preconcentration and determination of different species. The CPE and FO-LADS methods have good matching conditions for combination because FO-LADS is suitable as a detection technique for the low volume of remained phase obtained after CPE. This combination was carried out using 50 μL cylindrical micro cell and then employed for simultaneous preconcentration and determination of cobalt and nickel.Cloud point extraction method was based on the chromogenic reaction of metal ions and 1-(2-pyridylazo)-2-naphthol (PAN) and then preconcentration of formed complexes using octylphenoxypolyethoxyethanol (Triton X-114). The remained phase after CPE was transferred into cylindrical micro cell and located at the cell holder of FO-LADS. The spectra of cobalt and nickel complexes were collected by FO-LADS and processed for ordinary and first derivative spectrophotometry.Optimization of different parameters was evaluated. Under optimum conditions, calibration curves were linear in the range of 0.6-30.0 and 0.1-15.0 μg L⁻¹ with detection limits of 0.2 and 0.04 μg L⁻¹ for Co and Ni respectively. The relative standard deviations (R.S.D.s) were lower than 4%. The obtained enhancement factors were 198 and 199 for cobalt and nickel, respectively.The proposed method was compared with the other methods and applied to the analysis of several real and spiked samples.     

High-performance liquid chromatographic method for determination of amino acids by precolumn derivatization with 4-chloro-3,5-dinitrobenzotrifluoride

This work presents an high-performance liquid chromatography method for the determination of amino acids after precolumn derivatization with 4-chloro-3,5-dinitrobenzotrifluoride (CNBF) which can readily react with both primary and secondary amines. The precolumn derivatization conditions, including the CNBF concentration, reaction pH, temperature and reaction time were investigated for method optimization. In pH 9.0 borate buffer, the reaction of amino acids with CNBF was carried out at 60 °C for 30 min, the optimized concentration of CNBF was 70 mmol L⁻¹ and the molar ratio of amino acids to CNBF was 1:5.25. The chromatographic separation of 19 amino acids derivatives was performed on a Kromasil ODS C₁₈ column (250 mm × 4.6 mm, 5 μm) with good reproducibility, and ultraviolet detection was applied at 260 nm. The mobile phase was a mixture of phase A (acetonitrile) and phase B (acetate buffer, acetonitrile, triethylamine; 82.8:17:0.2, pH 4.9), and the flow rate was 0.4 mL min⁻¹. The separation of all the labeled amino acids was achieved within 45 min at room temperature by gradient elution mode. The method linearity, calculated for each amino acid, had a correlation coefficient higher than 0.9979, in concentrations ranging from 9.60 to 3330.00 μmol L⁻¹. The detection limits of amino acids were 2.40-6.50 μmol L⁻¹, at a signal-to-noise ratio of 3. The proposed method was applied for the determination of amino acids in beer with recoveries of 97.0-103.9% and relative standard deviations of 2.62-4.22%, respectively. This method showed good accuracy and repeatability that can be used for the quantification of amino acids in real samples.     

Comparative study of graphene nanosheet- and multiwall carbon nanotube-based electrochemical sensor for the sensitive detection of cadmium

A novel nanocomposite was obtained through the controlled surface modification of graphene nanosheets (nanographene) with Nafion by ultrasonic oscillation. The composite was used as an ultrasensitive platform for the detection of cadmium ions (Cd²⁺) by differential pulse anodic stripping voltammetry (DPASV) analysis. The performance of the nanographene-based sensor was systematically compared with that of a multiwall carbon nanotube (MWCNT)-modified sensor. The results indicate that the nanographene-based sensor exhibits significant advantages over the MWCNT-based sensor in terms of repeatability, sensitivity and limit of detection (LOD). The nanographene-based sensor displayed superior analytical performance over a linear range of Cd²⁺ concentrations from 0.25 μg L⁻¹ to 5 μg L⁻¹, with a LOD of 3.5 ng L⁻¹. This sensor was also used to systematically screen for 6 types of chemicals, including sodium salts, magnesium salts and zinc salts. It was observed that the sensor could successfully differentiate cadmium ions from interferents (magnesium salts, zinc salts, etc.). The nanographene-based sensor was also demonstrated to be a promising and reliable tool for the rapid detection of cadmium existing in tap water and for the rapid on-site analysis of critical pollution levels of cadmium.     

Simultaneous determination of nickel(II) and cobalt(II) by square wave adsorptive stripping voltammetry on a rotating-disc bismuth-film electrode

This works reports the use of square-wave adsorptive stripping voltammetry (SWAdSV) for the simultaneous determination of Ni(II) and Co(II) on a rotating-disc bismuth-film electrode (BFE). The metal ions in the non-deoxygenated sample were complexed with dimethylglyoxime (DMG) and the complexes were accumulated by adsorption on the surface of the BFE. The stripping step was carried out by using a square-wave potential-time voltammetric excitation signal. Electrochemical cleaning of the bismuth film was employed, enabling the same bismuth film to be used for a series of measurements. The experimental variables (choice of the working electrode substrate, the presence of oxygen, the DMG concentration, the buffer concentration, the preconcentration potential, the accumulation time, the rotation speed and the SW parameters) as well as potential interferences were investigated and the figures of merit of the methods were established. Using the selected conditions, the 3σ limits of detection were 70 ng l⁻¹ for Co(II) and 100 ng l⁻¹ for Ni(II) (for 300 s of preconcentration) and the relative standard deviations were 2.3% for Co(II) and 3.9% for Ni(II) at the 2 μg l⁻¹ level (n = 8). Finally, the method was applied to the determination of nickel and cobalt in real samples with satisfactory results.     

Direct measurement of ammonia in simulated human breath using an inkjet-printed polyaniline nanoparticle sensor

A sensor fabricated from the inkjet-printed deposition of polyaniline nanoparticles onto a screen-printed silver interdigitated electrode was developed for the detection of ammonia in simulated human breath samples. Impedance analysis showed that exposure to ammonia gas could be measured at 962 Hz at which changes in resistance dominate due to the deprotonation of the polymer film. Sensors required minimal calibration and demonstrated excellent intra-electrode baseline drift (≤1.67%). Gases typically present in breath did not interfere with the sensor. Temperature and humidity were shown to have characteristic impedimetric and temporal effects on the sensor that could be distinguished from the response to ammonia. While impedance responses to ammonia could be detected from a single simulated breath, quantification was improved after the cumulative measurement of multiple breaths. The measurement of ammonia after 16 simulated breaths was linear in the range of 40–2175 ppbv (27–1514 μg m⁻³) (r² = 0.9963) with a theoretical limit of detection of 6.2 ppbv (4.1 μg m⁻³) (SN⁻¹ = 3).     

Optimized multilayer oxalate biosensor

The optimization of a biosensor prepared by the immobilization of oxalate oxidase (OOX) with a cross-linking agent onto a multilayer inorganic/organic modified electrode, is presented. A very thin Prussian Blue (PB) film covered by a self-doped polyaniline (SPAN) layer acts as very sensitive amperometric sensor for the H₂O₂ formed by the enzymatic reaction. The electrode allows the very reliable and sensitive oxalate detection in the 0.08 to 0.45 mmol l⁻¹ concentration range. The observed sensitivity was 131.3 μA mmol⁻¹ cm⁻² at the operation potential of 0.05 V versus Ag/AgCl in a succinate buffer solution (pH=3.8). The bilayer Prussian blue/SPAN leads to a very stable, sensitive and selective system that not only minimizes the interference caused by ascorbic and uric acids but also forms a very adherent sensing film that allows repetitive successive determinations.     

Amperometric sensor for nitrite using a glassy carbon electrode modified with alternating layers of iron(III) tetra-(N-methyl-4-pyridyl)-porphyrin and cobalt(II) tetrasulfonated phthalocyanine

The development of a highly sensitive amperometric sensor for nitrite using a glassy carbon electrode modified with alternated layers of iron(III) tetra-(N-methyl-4-pyridyl)-porphyrin (FeT4MPyP) and cobalt(II) tetrasulfonated phthalocyanine (CoTSPc) is described. The modified electrode showed an excellent catalytic activity and stability for the nitrite oxidation decreasing the peak potentials about 200 mV toward less positive values and presenting much higher peak currents than those obtained on the bare GC electrode. A linear response range of 0.2-8.6 μmol l⁻¹, with a sensitivity of 0.37 μA l μmol⁻¹ and detection limit of 0.04 μmol l⁻¹ were obtained with this sensor. The repeatability of the proposed sensor, evaluated in term of relative standard deviation, was verified to be 1.4% for 10 measurements of 0.2 μmol l⁻¹ nitrite solution. Interference caused by common ions has been investigated in simulated mixtures containing high concentration level of interfering ions and the sensor was found to be tolerant against these ions. The developed sensor was applied for the nitrite determination in water samples and the results were in agreement with those obtained by a comparative method described in the literature. The average recovery for these samples was 100.1 (±0.7)%.     

Photoelectrochemical determination of inorganic mercury in aqueous solutions

An analytical method using an optical probe in a photoelectrochemical cell for the sensitive and selective determination of aqueous Hg²⁺ is presented. A previously synthesized Hg²⁺ selective chemosensor, proven to be Hg²⁺ sensitive up to 2 μg L⁻¹, has been immobilized onto indium tin oxide (ITO) electrodes in a composite form with polyaniline. The coated ITO electrode was placed in a photoelectrochemical cell under closed circuit conditions in which the optical recognition of the chemosensor was converted to a measurable signal. A composite of the fluorescent chemosensor, Rhodamine 6G derivative (RS), and polyaniline (PANI) was immobilized on ITO glass plates and subjected to photovoltage measurements in the absence and presence of Hg²⁺. The optical responses of the coated electrode were used to determine the sensitivity and selectivity of the immobilized sensor to Hg²⁺ in the presence of background ions. The optical response of the PANI-dye coated electrode increased linearly with increasing Hg²⁺ concentration in the range 10-150 μg L⁻¹, with a detection limit of 6 μg L⁻¹.     

UV-induced carbonyl generation with formic acid for sensitive determination of nickel by atomic fluorescence spectrometry

UV-induced carbonyl generation with formic acid is used for gaseous sample introduction into an atomic fluorescence spectrometer for the determination of ultra-trace nickel. Compared with conventional carbonyl generation, no toxic gas CO is involved in this work, and volatile Ni(CO)₄ is generated with a single reagent formic acid under the irradiation of UV light (253.7 nm, 15 W). The reaction conditions, including reaction medium, UV irradiation time and reaction temperature, are optimized for the best signal. Under the optimized conditions, a limit of detection of 10 ng L⁻¹ for nickel is obtained without any analyte-pre-concentration, which is comparable to that using in situ trapping technique. Interferences from common transition metal ions, noble metal ions and mineral acids are also investigated. The proposed method is applied to the analysis of three certified reference materials and two organic acid samples for trace nickel, with analytical results in good agreement with certified values or those obtained by electrothermal atomic absorption spectrometry. This is a simple, fairly green and highly sensitive method for ultra-trace nickel determination.     

A new functionalized resin and its application in preconcentration system with multivariate optimization for nickel determination in food samples

In this work, Amberlite XAD-2 resin functionalized with 4,5-dihydroxy-1,3-benzenedisulfonic acid was synthesized, characterized and applied as a new packing material for an on-line system to nickel preconcentration. The method is based on the sorption of Ni(II) ions in a minicolumn containing the synthesized resin, posterior desorption using an acid solution and measurement of the nickel by spectrophotometry (PAR method). The optimization of the system was performed using factorial design and Doehlert matrix considering five variables: eluent concentration, PAR solution pH, sample flow rate, PAR solution concentration and sample pH. Signals were measured as peak height by using an instrument software. Using the experimental conditions defined in the optimization, the method allowed nickel determination with achieved sampling rate of 25 samples per hour, detection limit (3 s) of 2 μg l⁻¹ and precision (assessed as the relative standard deviation) of 8.2-2.6%, for nickel solutions of 10.0-200.0 μg l⁻¹ concentration, respectively. The experimental enrichment factor of the proposed system was 46, for 120 s preconcentration time. The proposed procedure was applied for nickel determination in food samples. Recoveries of spike additions (5 or 10 μg g⁻¹) to food samples were quantitative (94-110%).     

A gold@silica core–shell nanoparticle-based surface-enhanced Raman scattering biosensor for label-free glucose detection

The gold nanostar@silica core–shell nanoparticles conjugated with glucose oxidase (GOx) enzyme molecules have been developed as the surface-enhanced Raman scattering (SERS) biosensor for label-free detection of glucose. The surface-immobilized GOx enzyme catalyzes the oxidation of glucose, producing hydrogen peroxide. Under laser excitation, the produced H₂O₂ molecules near the Au nanostar@silica nanoparticles generate a strong SERS signal, which is used to measure the glucose concentration. The SERS signal of nanostar@silica∼GOx nanoparticle-based sensing assay shows the dynamic response to the glucose concentration range from 25 μM to 25 mM in the aqueous solution with the limit of detection of 16 μM. The sensing assay does not show any interference when glucose co-exists with both ascorbic acid and uric acid. The sensor can be applied to a saliva sample.     

A novel potentiometric biosensor for selective L-cysteine determination using L-cysteine-desulfhydrase producing Trichosporon jirovecii yeast cells coupled with sulfide electrode

Trichosporon jirovecii yeast cells are used for the first time as a source of l-cysteine desulfhydrase enzyme (EC 4.4.1.1) and incorporated in a biosensor for determining l-cysteine. The cells are grown under cadmium stress conditions to increase the expression level of the enzyme. The intact cells are immobilized on the membrane of a solid-state Ag₂S electrode to provide a simple l-cysteine responsive biosensor. Upon immersion of the sensor in l-cysteine containing solutions, l-cysteine undergoes enzymatic hydrolysis into pyruvate, ammonia and sulfide ion. The rate of sulfide ion formation is potentiometrically measured as a function of l-cysteine concentration. Under optimized conditions (phosphate buffer pH 7, temperature 37 ± 1 °C and actual weight of immobilized yeast cells 100 mg), a linear relationship between l-cysteine concentration and the initial rate of sulfide liberation (dE/dt) is obtained. The sensor response covers the concentration range of 0.2-150 mg L⁻¹ (1.7-1250 μmol L⁻¹) l-cysteine. Validation of the assay method according to the quality control/quality assurance standards (precision, accuracy, between-day variability, within-day reproducibility, range of measurements and lower limit of detection) reveals remarkable performance characteristics of the proposed biosensor. The sensor is satisfactorily utilized for determination of l-cysteine in some pharmaceutical formulations. The lower limit of detection is ∼1 μmol L⁻¹ and the accuracy and precision of the method are 97.5% and ±1.1%, respectively. Structurally similar sulfur containing compounds such as glutathione, cystine, methionine, and d-cysteine do no interfere.