Concentration Calibration Range Research Articles

A Critical Review on the Development of Optical Sensors for the Determination of Heavy Metals in Water Samples. The Case of Mercury(II) Ion.

Recent publications are reviewed concerning the development of sensors for the determination of mercury in drinking water, based on spectroscopic methodologies. A critical analysis is made of the specific details and figures of merit of the developed protocols. Special emphasis is directed to the validation and applicability to real samples in the usual concentration range of mercury, considering the maximum allowed limits in drinking water established by international regulations. It was found that while most publications describe in detail the synthesis, structure, and physicochemical properties of the sensing phases, they do not follow the state of the art in the analytical developments. Recommendations are provided regarding the proper method development and validation, including the setting of the calibration concentration range, the correct estimation of the limits of detection and quantitation, the concentration levels to be set for producing spiked water samples, the number of real samples for adequate validation, the comparison of the developed method with a reference technique, and other analytical features which should be followed.

Desolvation-induced formation of recombinant camel serum albumin-based nanocomposite for glutathione colorimetric determination

Protein-based enzyme mimics hold advantages of easy preparation, desirable compatibility and superior catalytic performance. Taking advantage of the remarkable stability of Bactrian camel serum albumin (CSA) compared with its counterpart in human and bovine, here a novel hemin-loaded nanozyme system was prepared through the desolvation method using a recombinant CSA (rCSA). The obtained hemin-rCSA nanoparticles exhibited improved enzyme mimic activity with higher affinity to the substrates compared with bare hemin and horseradish peroxidase. Moreover, the particle size and morphology of hemin-rCSA NPs was adjustable by the optimization of ethanol addition during the desolvation process. With the efficient encapsulation of hemin in the protein nanocomposite, a colorimetric detection of glutathione with the linear calibration concentration range of 3.2–100 μM with an LOD of 0.667 μM was achieved within 8 min. The available camel-derived serum albumin of recombinant form of improved stability provides a robust and manipulable protein-based nanoplatform for the molecular design of nanozyme system with enormous biomedical potentials.

Sequential injection analysis Ferrozine spectrophotometry versus Luminol chemiluminescence for continuous online monitoring of the concentration and speciation of iron

We designed two sequential injection analysis systems (Luminol chemiluminescent and Ferrozine spectrophotometric) that are capable of monitoring the concentration of soluble iron(II) and reducible iron(III) in natural waters over extended periods (>120 h). The method based on Luminol chemiluminescence estimated the iron(II) concentration as the difference in signal with or without the addition of the strong iron(II) ligand, FerrozineTM (iron complexing reagent). Iron(II) in the sample without the iron complexing reagent induced a chemiluminescence signal whereas iron(II) in the presence of the iron complexing reagent did not induce a signal. The second method, based on FerrozineTM spectrophotometry, measured iron(II) and reducible iron(III) in a 100 cm path length liquid wave capillary cell. Both methods provided stability for >120 h and yielded similar analytical detection limits equal to ten standard deviations of the blank; 2.2 nM for Luminol chemiluminescence with a calibration concentration range of 0−12 nM, and 3.9 nM for FerrozineTM spectrophotometry with a calibration concentration range of 0−120 nM. The two chemical techniques do not appear to be directly comparable for practical monitoring purposes, and we suggest that Luminol-chemiluminescence is unable to measure strong ligand complexes of iron(II). Both methods used iron(II) as the analytical standard rather than a reduced iron(III) standard. The analysis based on the iron(II) standard was able to be performed without additional buffers and reducing reagents, which significantly minimized contamination.

Post-extraction disulfide bond cleavage for MS/MS quantification of collision-induced dissociation-resistant cystine-cyclized peptides and its application to the ultra-sensitive UPLC-MS/MS bioanalysis of octreotide in plasma

Disulfide-cyclized peptides, including the somatostatin receptor agonist octreotide, are usually resistant against collision-induced dissociation (CID) complicating their bioanalysis by MS/MS. Post-extraction reductive cleavage of the disulfide bridge of such peptides utilizing tris(2-carboxyethyl)phosphine generates the corresponding linear dithiol-peptides. This procedure enables monitoring of larger, specific fragments in CID which usually avoid matrix interference present for single amino acid iminium ions abundant in CID of the intact peptides. To assist formulation development for oral administration of the cystine-cyclized therapeutic peptide octreotide, we applied this methodology to the development of an ultra-sensitive UPLC-MS/MS assay for plasma octreotide and validated it according to FDA’s and EMA’s pertinent guidelines. Octreotide was extracted from plasma by fast and simple protein precipitation with acetonitrile and subsequently reduced to linear dithiol-octreotide for the monitoring of specific fragments in selected reaction monitoring. The calibrated concentration range of 10 (9.8 pM) to 20,000 pg mL−1 was linear with correlation coefficients > 0.99. Interday accuracy ranged between 100.0 and 108.9% with corresponding precision of <8.1%. The assay was successfully applied to the quantification of octreotide plasma concentrations after intravenous administration in beagle dogs. The presented procedure of disrupting the cyclic structure of cystine-cyclized peptides by reductive cleavage of the intramolecular disulfide bond enables the generation of more specific and intense fragments in CID can serve as a general methodology for the sensitive bioanalysis of cystine-bearing cyclic peptides.

Ultra-sensitive quantification of the therapeutic cyclic peptide bremelanotide utilizing UHPLC-MS/MS for evaluation of its oral plasma pharmacokinetics

Bremelanotide (Vyleesi®), a cyclic heptapeptide, was recently approved for the subcutaneous treatment of premenopausal hypoactive sexual desire disorder. To foster the development of alternative routes of administration, we aimed at determining the oral plasma pharmacokinetics of bremelanotide in beagle dogs. Therefore, we established a UHPLC-MS/MS assay with an LLOQ of 10 pg/mL (9.8 pM) using 100 μL of plasma and validated it according to the guidelines of the US Food and Drug Administration and the European Medicines Agency. Bremelanotide was isolated from plasma by protein precipitation and quantification was performed with positive heated ESI MS/MS in the SRM mode. The calibrated concentration range of 10−10,000 pg/mL was linear showing correlation coefficients > 0.99. In the calibrated range, interday and intraday accuracy ranged from 88.9–100.0 % with corresponding precision < 8 %. Accuracy at the LLOQ ranged from 93.6–100.8 % with corresponding precision < 11 %. Because of the validity of a dilution QC that showed accurate quantification of 10-fold diluted plasma samples (accuracy 99.4 %, precision < 6 %), the assay is suitable for bremelanotide quantification in its effective concentration range up to 100,000 pg/mL. The ultra-sensitive assay was applied to the quantification of bremelanotide plasma concentrations after oral administration to beagle dogs, which indicated minimal oral absorption.

Ultra-sensitive bioanalysis of the therapeutic peptide exenatide for accurate pharmacokinetic analyses at effective plasma concentrations utilizing UPLC-MS/MS

Exenatide is the first approved glucagon-like peptide 1 receptor agonist subcutaneously or intramuscularly injected for the treatment of type 2 diabetes mellitus. Typical therapeutic plasma concentrations are in the low pg/mL range, therefore requiring ultra-sensitive quantification. To enable the accurate evaluation of pharmacokinetic studies, we established a UPLC-MS/MS assay with a lower limit of quantification (LLOQ) of 5 pg/mL (1.2 pM) using 200 μL of plasma, validated according to FDA’s and EMA’s pertinent guidelines. Exenatide was isolated from plasma with solid phase extraction utilizing anion-exchange sorbent. Quantification was performed with positive electrospray ionization tandem mass spectrometry in the selected reaction monitoring mode. The calibrated concentration range of 5–10,000 pg/mL was linear showing correlation coefficients >0.99. Interday and intraday accuracy ranged from 97.5% to 105.4% with corresponding precision of <10.9%. Accuracy at the LLOQ ranged from 93.0% to 102.5% with corresponding precision of <15.9%. Because of the validity of a 10-fold dilution QC (accuracy 111.2%), the assay is suitable for exenatide quantification up to 100,000 pg/mL. The ultra-sensitive assay’s applicability was demonstrated by the quantification of exenatide plasma concentrations and pharmacokinetics after intravenous and nasal administration to beagle dogs.

Mathematical Modelling of a Non‐enzymatic Amperometric Electrochemical Biosensor for Cholesterol

AbstractThickness of the electro‐polymerized layer grown on a substrate and used as the recognition element for the analyte is critical to measuring the response of a biosensor, with high sensitivity and accuracy. However, it is difficult to control the thickness during synthesis. A mathematical model is developed in this study that considers thickness of the electro‐polymerized layer in simulating the electrochemical response of a non‐enzymatic biosensor for cholesterol in blood. The model includes transient kinetics and one‐dimensional diffusion of the analyte in the poly‐methyl orange (PMO) recognition layer electrochemically grown on the electrode. The governing partial differential equations resulting from the species conservation balances in the PMO layer are numerically solved. Time and spatial concentration profiles of the analyte in the PMO layer are determined. Model predictions are calibrated with the experimental data for different PMO thicknesses. Interestingly, model predictions show a linear response over the calibrated concentration range of cholesterol for all PMO layer thicknesses. Based on the chronoamperometry measurements, the model predictions for the cholesterol concentrations measured in the laboratory samples were also found to be remarkably accurate. This is the first mathematical model developed to understand the transport and kinetics of an analyte in the electro‐polymerized layer used as the recognition element of a non‐enzymatic biosensor.

An ultra-sensitive UHPLC-MS/MS assay for the quantification of orally administered vancomycin in plasma

Though marketed for over half a century, little is known about the pharmacokinetics of oral vancomycin except that its bioavailability is low, thus making accurate determination of plasma concentrations difficult. To quantify plasma concentrations of vancomycin after oral administration, we developed an ultra-sensitive UHPLC-MS/MS assay and validated it according to FDA´s and EMA´s pertinent guidelines. A fast and simple protein precipitation method followed by short UHPLC chromatography was developed for extraction and separation of vancomycin from plasma. Quantification was performed via heated positive electrospray tandem mass spectrometry with multiple reaction monitoring using deuterated internal standard. The assay was linear in the calibrated concentration range of 0.05–100 ng/mL showing correlation coefficients >0.997. Intraday and interday accuracy showed coefficients of variation <12% at the lower limit of quantification (LLOQ) of 0.05 ng/mL and <6% in the calibrated range while corresponding values for precision were <13% and <8%, respectively. With its high sensitivity, the assay allows for the accurate quantification of therapeutic plasma concentrations in the therapeutic range (up to 100 μg/mL) in 1000-fold diluted samples with a sample volume decreased down to 1 μL. The UHPLC-MS/MS assay was successfully used for the determination of trough plasma concentrations of two patients with Clostridium difficile infection receiving oral vancomycin therapy and its performance was compared to a commercial immunoassay for concentrations close to its LLOQ.

An ultrasensitive UPLC-MS/MS assay for the quantification of the therapeutic peptide liraglutide in plasma to assess the oral and nasal bioavailability in beagle dogs.

Aim: An ultrasensitive UPLC-MS/MS assay for liraglutide was developed and validated according to US FDA and EMA guidelines and applied to the quantification of plasma concentrations after intravenous, nasal and oral administration of liraglutide to beagle dogs. Results: Liraglutide isolation was performed with a combined protein precipitation and solid-phase extraction protocol. The calibrated concentration range of 0.1-200ng/ml was linear with correlation coefficients >0.998. Precise analysis was achieved through the utilization of an isotopically labeled internal standard. Absolute bioavailability of liraglutide after nasal and oral administration of liraglutide to beagle dogs was 0.03 and 0.006%, respectively. Conclusion: The assay matches the performance in sensitivity of the previously applied immunoassay and optimally covers the therapeutic range of liraglutide.

All-solid-state potentiometric sensor using single-walled carbon nanohorns as transducer

Single-walled carbon nanohorns (SWCNHs) was utilized as an ion-to-electron transducer to fabricate all-solid-state ion-selective electrodes (ISE) for the first time. To evaluate the performance of developed SWCNHs-based calcium ISE, various electrochemical experiments were carried out. A Nernstian response (29.69 ± 0.24 mV/decade, R2 = 0.9996) was acquired across a broad linear range between 10−6 and 10−2 M, and the detection limit of 10−6.1 M was achieved in the meantime. Additionally, water layer test and reversed chronopotentiometry were used to estimate the long-term and short-term stability of SWCNHs-based ISE, respectively, indicating SWCNHs is capable of enhancing potential stability on account of its hydrophobicity. Furthermore, a SWCNHs-based reference electrode (RE) was fabricated by directly coating a photo-polymerised reference membrane (RM) onto a SWCNHs modified electrode. Results demonstrated that the SWCNHs-based RE exhibited slight potential fluctuation for common ions with different charges over a wide concentration range of calibration. Based on these superior results, an integrated all-solid-state SWCNHs-based potentiometric sensing device combined SWCNHs-based ISE and SWCNHs-based RE was constructed for detecting calcium, which shows a Nernstian response with a slope of 27.14 mV/decade. This developed electrode in the aid of SWCNHs exhibits fast response as well as outstanding potential stability, making it extreme promising for routine sensing application.

Simultaneous analysis of allopurinol and oxypurinol using a validated liquid chromatography–tandem mass spectrometry method in human plasma

The present study describes a simple, reliable and reproducible liquid chromatography–tandem mass spectrometry method (LC–MS/MS) for the simultaneous determination of allopurinol and its active metabolite, oxypurinol in human plasma for a pharmacokinetic/bioequivalence study. After protein precipitation (PPT) of 100µL plasma sample with 1.0% formic acid in acetonitrile, the recovery of the analytes and allopurinol-d2 as an internal standard ranged from 85.36% to 91.20%. The analytes were separated on Hypersil Gold (150mm×4.6mm, 5µm) column using 0.1% formic acid-acetonitrile (98:2, v/v) as the mobile phase. Quantification was done using electrospray ionization in the positive mode. The calibration concentration range was established from 60.0 to 6000ng/mL for allopurinol and 80.0–8000ng/mL for oxypurinol. Matrix effect in human plasma, expressed as IS-normalized matrix factors ranged from 1.003 to 1.030 for both the analytes. The developed method was found suitable for a clinical study with 300mg allopurinol tablet formulation in healthy subjects.

An Electrochemical Biosensor with Uricase Immobilized on Functionalized Gold Coated Copper Wire Electrode for Urinary Uric Acid Assay

AbstractA new second generation uricase electrode for urinary uric acid determination has been developed by chemically binding both uricase and redox mediator to inexpensive copper wire through simple electrodeposition of gold on copper surface and subsequent functionalization of the gold with L‐methionine. During a 209‐day testing period, the overall electrode performance exhibits in average a low oxidation potential of 0.33 V, a response time of 5 s, a widest linear calibration concentration range (0–2.38 mM, r2&gt;0.9952), a sensitivity of 50 μA mM−1, and a detection limit of 2.4 μM. The measurement accuracy and precision for the determination of uric acid in human urine specimens were 85.6–95.5 % and 0.3–2.4 %, respectively. The developed uricase electrode is potential for clinical applications.

Manganese Oxide Nanoparticles/Reduced Graphene Oxide as Novel Electrochemical Platform for Immobilization of FAD and its Application as Highly Sensitive Persulfate Sensor

AbstractIn this study, manganese oxide nanoparticles/reduced graphene oxide(MnOxNPs/rGO) was used as support for strong immobilization of flavin adenine dinucleotide(FAD). A thin film of rGO cast on the electrode surface, followed by performing electrodeposition of MnOxNPs at applied constant potential of +1.4 V vs. Ag/AgCl for 200 s. Finally, FAD was electrodeposited onto the rGO/MnOxNPs film by potential cycling between 1.0 to −1.0 V in solution containing 1 mg ml−1 FAD. Electrochemical properties and catalytic activity of GCE/rGO‐MnOxNPs/FAD toward persulfate (S2O82−) reduction was investigated. Under optimized condition, the concentration calibration range, detection limit, and sensitivity were 0.1 μM–2 mM, 90 nM and 125.8 nA/μM, respectively, using hydrodynamic amperometry technique.

Development of surfactant assisted kinetic method for trace determination of thallium in environmental samples

A simple and highly selective surfactant assisted kinetic spectrophotometric method for the determination of thallium has been developed. The method is based on the quantitative oxidation of Tl(I) to Tl(III) using bromine water where the resulting Tl(III) liberated iodine from potassium iodide in acidic medium. The liberated iodine was subsequently reacted with I− ion to form tri-iodide ions (I3−). The tri-iodide ions formed were further reacted with cetylpyridinium cation (CP+) which produced a violet ion associate species having a λmax of 512nm at which reaction was monitored. The reaction variables such as time, temperature, reagent concentration and acidity were optimized for the indicator reaction to achieve maximum sensitivity. The linear regression calibration concentration range of 0.007–0.1μgmL−1 Tl(I) was established as the Beer's law was obeyed in this range. Sandell's sensitivity and molar absorptivity of the ion associate species in terms of thallium were determined to be 0.000133μgcm−2 and 1.99×106Lmol−1cm−1 respectively. The limit of detection was determined as 0.0029μgmL−1. The developed method has successfully been applied for the determination of thallium in different environmental samples with satisfactory results.

Novel electrochemical sensor based on graphene quantum dots/riboflavin nanocomposite for the detection of persulfate

A novel and sensitive electrochemical sensor based on graphene quantum dots/riboflavin modified glassy carbon (GC/GQDs/RF) electrode was constructed and utilized to determine persulfate (S2O82−). GQDs were expected to have the significant properties of graphene materials as well as new functions resulting from their quantum confinement and edge effects. The UV–vis, photoluminescence spectroscopy and transmission electron microscopy (TEM) techniques were used for characterizing the GQDs. The modified electrode showed stable and well-defined redox couples at a wide pH range (1–10), with surface confined characteristics. The electron transfer coefficient (α) and the heterogeneous electron transfer rate constant (ks) for redox reaction of RF were found to be 0.52 and 6.59s−1, respectively. The GC/GQDs/RF electrode exhibits an excellent electrocatalytic activity for a S2O82− reduction, with a measured kcat of ∼4.8×103M−1s−1. This catalytic reduction allows an amperometric detection of S2O82− at a potential of −0.1V with detection limit of 0.2μM, concentration calibration range of 1.0μM to 1mM and sensitivity of 4.7nAμM−1, respectively.

Electrochemical methods for determination of thymine in medical pipefish samples based on HPMαFP/Ppy/GCE-modified electrode

A sensitive electrochemical method was developed for the voltammetric determination of thymine at a composite film-modified electrode 1-phenyl-3-methyl-4-(2-furoyl)-5-pyrazolone (HPMαFP)/polypyrrole (Ppy)/glassy carbon electrode (GCE). The electrochemical parameters of thymine were investigated by cyclic voltammetry and differential pulse voltammetry. In pH = 7.4, one sensitive oxidation peak of thymine with E pa = 0.968 V was observed on the HPMαFP/PPy-modified electrode. The difference of peak potential (∆E pa) was 188 mV lower than that for bare GCE. Compared to the bare GCE and Ppy/GCE, the HPMαFP/Ppy/GCE-modified electrode showed an excellent electrocatalytic effect on the oxidation of thymine and displayed a shift of the oxidation potential in the negative direction with significant increase in the peak current. Under the optimum condition, the concentration calibration range and detection limit are 2 × 10−6–1 × 10−4 and 4.85 × 10−7 M for thymine. This developed method had been applied to the direct determination of thymine in medical pipefish samples with satisfactory results.

High sensitivity amperometric and voltammetric determination of persulfate with neutral red/nickel oxide nanowires modified carbon paste electrodes

Abstract A novel, highly sensitive sensor for detection of persulfate (S2O82−) is demonstrated by modification of carbon paste electrode (CPE) with nickel oxide nanowires (NiOx NWs) and neutral red (NR). Ni NWs are prepared by DC electrodeposition of Ni in the cylindrical pores of anodized aluminum oxide (AAO) templates. NWs are released, oxidized and immobilized with NR on the surface of CPE. Cyclic voltammetry studies demonstrate a pair of well-defined nearly reversible redox couples (E°′ = −0.2 V in a buffer solution pH 2.0) at a wide pH range of 1–12. The surface coverage and heterogeneous electron transfer rate constant of the adsorbed redox couple are 7.368 × 10−12 mol cm−2 and 7.9 s−1, respectively, indicating a high NR loading efficiency for NiOx NWs and efficient electron transfer between redox couple and the electrode. The modified CPE exhibits an excellent electrocatalytic activity for a S2O82− reduction, with a measured kcat of ∼7.14 × 103 M−1 s−1. This catalytic reduction allows an amperometric detection of S2O82− at a potential of −0.2 V with detection limit of 30 nM, concentration calibration range of 0.1 μM to 12 mM and a linear sensitivity of 647.33 nA μM−1, respectively.

Analysis of Calix[4]arenes Using Nonaqueous Capillary Electrophoresis

AbstractIn nonaqueous capillary electrophoresis (NACE), an organic solvent is used in place of an aqueous medium as the background solution to improve the solubility and selectivity for hydrophobic analytes. In this study, we employed NACE with UV detection for the analysis of eight calix[4]arenes. We examined the influence of several parameters—the buffer composition, the nonaqueous solvent‘s composition and proportion, and the concentration of the electrolyte of the nonaqueous buffer—on the efficiency of the electrophoretic separation. The separation was achieved through the analyte's different effective mobility via different degrees of deprotonation on the phenolic OH groups of the calix[4]arene. This deprotonation can further affect the analyte's ability to form a complex with the metal ion. The optimized background electrolyte (BGE), comprising a mixture of N‐methylformamide/acetonitrile (30:70, v/v) and 100 mM AcOH/20 mM NH4OAc, provided rapid (&lt;11 min) separation of the calix[4]arenes with good resolution. The relative standard deviations of the migration times for the eight analytes were all less than 1%. Within the calibration concentration range, the coefficients of determination (R2) were all greater than 0.9914. Thus, the present study demonstrated NACE can provide adequate separation for the analysis of calix[4]arenes.

Development of Polymeric Resin Ion‐exchanger Based Chloride Ion‐selective Electrode for Monitoring Chloride Ion in Environmental Water

AbstractA chloride ion‐selective electrode (ISE) membrane was developed by using a copolymeric ion‐exchanger resin (trimethyl ethenyl quaternary ammonium chloride polystyrene‐divinylbenzene copolymer resin, TMEQAC PSDVB), the ionophore ({μ‐[4,5‐Dimethyl‐3,6‐bis(dodecyloxy)‐1,2‐phenylene]}bis(mercury chloride), ETH9033), the plasticizer (bis(2‐ethylhexyl) sebacate, DOS), and the membrane substrate (polyvinylchloride, PVC). At 25 °C, the electrode exhibited an ideal Nernstian response of 59.2 mV/decade with the linear calibration concentration range from 1.0 × 10−4‐1.0 × 10−2 M (r2 = 0.9930). The limit of detection was 2.45 ppm (6.9 × 10−2 mM) and the measurement response time was less than 10 seconds. The working temperature range of electrode was 10‐45 °C. The working pH range for chloride ion measurement was 2.0‐11.0. Among the various anions examined in this work, only I−, SCN−, and MnO4− ions show significant interference to the electrode measurement. The chloride ISE can be used at least 72 days. The determination of chloride ion content in three kinds of environmental water sample with the electrode method was accurate (92‐95%) and precise (RSD &lt; 4.4%) and did not show significance difference from the high‐performance liquid chromatography method.

On-line flow injection analysis using gold particle modified carbon electrode amperometric detection for real-time determination of glucose in immobilized enzyme hydrolysate of waste bamboo chopsticks

Dual on-line flow injection analysis (FIA) systems were coupled to an immobilized enzyme reactor. The primary system was detected by amperometric enzyme electrode through a specifically designed three-electrode flow cell and a stopped-flow sampling and injection device to perform the successive real-time determination of glucose in the enzyme hydrolysate of waste bamboo chopsticks fibers. The secondary system was established via high-performance liquid chromatography-refractive index detection for comparison purpose. The novel enzyme electrode was made by chemically bonding both the redox mediator ferrocenecarboxaldehyde and glucose oxidase to a gold-particles modified pencil lead carbon electrode that possessed the widest linear standard calibration concentration range of 0–39.0 mM ( r 2 = 0.9956), a limit of detection of 7.8 μM, fast response time of 5 s, and a detection sensitivity of 2212 nA mM −1. The electrode was usable more than 50 days. The greatest amount of glucose produced from hydrolysis was 0.46 mM at 32 h. Interference from mannose and galactose was 7% and 5%, respectively, that was neglected by their small concentration in hydrolysate. The on-line FIA of amperometric enzyme electrode detection has good analytical precision (RSD 2.2–4.5%) and accuracy (94.4–97.7%) for the real-time glucose determination. The statistical analysis shows that no systematic error exists between the quantitative results of two FIA systems.