Analytical Methods (v.11, #12)

Front cover (1569-1569).

Inside front cover (1570-1570).

Contents list (1571-1576).

Revision of the International System of Units (Background paper) by Analytical Methods Committee AMCTB No 86 (1577-1579).
The International System of Units (SI) is the only globally agreed practical system of measurement units. Stemming from the Metre Convention of 1875, which established a permanent organisational structure for member governments to act in common accord on all matters relating to units of measurement, the SI was formalised in 1960 and defined by the ‘SI Brochure’. The foundation of the SI are the set of seven well defined base units: the metre, the kilogram, the second, the ampere, the kelvin, the mole, and the candela, from which all derived units (such as metres per second) are formed. On 16 November 2018 the 26th General Conference on Weights and Measures (CGPM) met, at an open meeting at the Palais des Congrès, Versailles, to discuss and vote on the re-definition of four of the SI’s seven base units: the mole, the ampere, the kelvin, and the kilogram. This change, effective from World Metrology Day (20 May) 2019, is perhaps the most fundamental change in the SI since its inception. For the first time the SI will be defined entirely in terms of fundamental physical constants, instead of requiring the maintenance of a physical artefact. This technical brief explains why this re-definition came about. No practical implications of the change are envisaged for analytical chemistry in the short term and improvements in measurement may take some time to realise.

A ratiometric two-photon fluorescent probe for the rapid detection of HClO in living systems by Li Wang; Mingguang Ren; Zihong Li; Lixuan Dai; Weiying Lin (1580-1584).
We have developed a ratiometric two-photon fluorescent probe: CR-HA. The CR-HA probe could be used for the determination of hypochlorous acid (HClO) in living systems. The result of fluorescence imaging showed that it could be used as a probe to image exogenous/endogenous HClO in living cells, and to detect HClO in living tissue and zebrafish.

Determination of urea concentration using urease-containing polyelectrolyte microcapsules by Aleksandr L. Kim; Egor V. Musin; Alexey V. Dubrovskii; Sergey A. Tikhonenko (1585-1590).
Nowadays, the enzymological method used in clinical diagnostics has several problems: a short lifetime of the enzyme sensor in solution, its susceptibility in the presence of proteolytic enzymes, and the impossibility of multiple uses. We proposed the modification of the standard method of urea detection, which will solve the problems mentioned above. The main idea of the proposed approach was encapsulation of urease into microcapsules obtained by the method of alternate adsorption of oppositely charged polyelectrolytes. The determination was conducted with the help of unfixed (microdiagnosticum) and fixed microcapsules on a solid surface (diagnostic plate). Multiple uses of microdiagnosticum and diagnostic plate have been shown. The results obtained exhibited deviations and did not exceed the value of 9.6%. We determined the urea concentration in the serum blood by the standard Berthelot method using a diagnostic plate and a microdiagnosticum for detection in the urine. The results obtained differed from the results obtained using a free enzyme by no more than 4.26% for diagnostic plates and 1.67% for microdiagnosticum. The diagnostic plate exhibited detectable concentrations in the range of 5–50 mmol L−1 and linearity up to 40 mmol L−1; moreover, deviation from linearity did not exceed 10%, and the coefficient of variation did not exceed 7.17. Microdiagnosticum exhibited detectable concentrations in the range of 5–55 mmol L−1 and linearity up to 50 mmol L−1; also, deviation from linearity did not exceed 7%, and the coefficient of variation did not exceed 2.85.

Bismuth nanoparticle (BiNP)/Nafion modified screen-printed electrodes were fabricated through screen printing and drop coating. Detection of lead (Pb2+) and cadmium (Cd2+) was performed via anodic stripping voltammetry (ASV). Optimum values of the deposition time (60 seconds) and scan rate (50 mV s−1) were obtained and utilized in the selection of the best BiNP modifier concentration of 1.0 mg. The fabricated electrodes were optimized by cyclic voltammetry, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The limit of detection was found to be 280 parts per trillion and 40.34 parts per billion (ppb) for lead and cadmium, respectively. Real samples of filtered tap water, unfiltered tap water, treated wastewater, and water obtained from Manila Bay were analyzed through ASV and atomic absorption spectroscopy (AAS). The Manila Bay water samples contained lead and cadmium levels far beyond safe limits set forth by the U.S. EPA and World Health Organization while trace amounts of lead (1–2 ppb) were detected in the other water samples. The treated wastewater contained the least amount of lead while the unfiltered tap water contained the highest concentration of lead. Nonetheless, these trace amounts are well within tolerance levels set forth by government agencies. The presence of heavy metals in all water samples was validated by AAS analysis.

Performance evaluation of enantioseparation materials based on chitosan isobutylurea derivatives by Gui-Hua Zhang; Shuang Liang; Sheng Tang; Wei Chen; Zheng-Wu Bai (1604-1612).
In order to enrich the types of chitosan-based chiral stationary phases and meanwhile discover new promising chiral separation materials for efficient enantioseparation, four chloro-substituted chitosan isobutylurea derivatives were prepared and proposed as chiral selectors in this work. The chitosan isobutylurea derivatives were characterized and were found to be successfully synthesized with expected structures. It was observed that the prepared chiral selectors exhibited a low swelling capacity in ethyl acetate, acetone or a mixed solution of n-hexane and a certain proportion of tetrahydrofuran, and the corresponding chiral stationary phases (CSPs) showed good solvent tolerance. These facts indicated that chiral separation of enantiomers could be performed under more types of chromatographic conditions with some “unusual organic solvents” as mobile phase additives. In addition, the prepared CSPs also possessed excellent chiral recognition and enantioseparation ability for the tested chiral samples. Compared with common coated-type CSPs derived from amylose and cellulose, the chitosan-based CSPs prepared in the present study exhibited more preferable enantioseparation performance for some analytes.

In this study, an enzyme-free and homogeneous fluorescent biosensing strategy is described for the detection of DNA based on an entropy-driven strand displacement reaction (ESDR) and DNAzyme. In the presence of target DNA, toehold-mediated strand displacement results in the circulation of target DNA and release of DNAzyme sequences from the three-stranded complex. Upon cyclic amplification, one target catalyzes the formation of Mg2+-dependent DNAzymes. The activated DNAzymes can catalyze the cyclic cleavage of fluorophore/quencher labeled DNA substrates, resulting in significantly amplified fluorescent signals. The P53 gene, one of the most intensively investigated genes due to its dysregulation in the majority of human cancers, was used as the model target DNA. Under the optimal experimental conditions, the fluorescent signal had a linear relationship with the logarithm of the DNA concentration in a wide range from 500 fM to 10 nM, and the detection limit was estimated to be 220 fM. In addition, the assay also possessed excellent detection specificity and could be applied to the analysis of DNA spiked plasma samples. Therefore, this strategy presents a new method for sensitive DNA assay and might become a potential alternative tool for biomolecule detection.

Polymer modified carbon fiber-microelectrodes and waveform modifications enhance neurotransmitter metabolite detection by Dilpreet Raju; Alexander Mendoza; Pauline Wonnenberg; Sanuja Mohanaraj; Mulugeta Sarbanes; Carly Truong; Alexander G. Zestos (1620-1630).
Carbon-fiber microelectrodes (CFMEs) have been used for several years for the detection of neurotransmitters such as dopamine. Dopamine is a fundamentally important neurotransmitter and is also metabolized at a subsecond timescale. Recently, several metabolites of dopamine have been shown to be physiologically important such as 3-methoxytyramine (3-MT), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA). Many of these neurotransmitter metabolites are currently only detected with microdialysis coupled with liquid chromatography with relatively low temporal and spatial resolution. Current electrochemical methods such as the dopamine waveform (scanning from −0.4 to 1.3 V at 400 V s−1) are utilized to electrostatically repel anions such as DOPAC and promote dopamine adsorption to the surface of the electrode. Moreover, polymer coatings such as Nafion have been shown to electrostatically repel anions such as 5-hydroxyindoleacetic acid (5-HIAA). In this study, we develop novel polymer and waveform modifications for enhanced DOPAC detection. Applying the DOPAC waveform (scanning from 0 to 1.3 V at 400 V s−1) enhances DOPAC detection significantly because it does not include the negative holding potential of the dopamine waveform. Moreover, positively charged cationic polymers such as polyethyleneimine (PEI) allow for the preconcentration of DOPAC to the surface of the carbon fiber through an electrostatic attraction. The limit of detection for DOPAC for PEI coated CFMEs with the DOPAC waveform applied is 58.2 ± 2 nM as opposed to 291 ± 10 nM for unmodified electrodes applying the dopamine waveform (n = 4). This work offers promise for the development of novel electrode materials and waveforms for the specific detection of several important biomolecules such as dopamine metabolite neurotransmitters.

Sensitive and portable electrochemical immunoassay for lipoprotein-associated phospholipase A2 using BSA-doped CaCO3 nanospheres to regulate pH readout by Wei Zhuang; Yining Li; Xiaoyuan Weng; Haixin Guo; Yongquan Zhang; Yating Yang; Chunmei Fan (1631-1638).
A simple and sensitive electrochemical immunoassay method was developed for the quantitative monitoring of lipoprotein-associated phospholipase A2 (Lp-PLA2) on a low-cost microtiter plate by using a portable handheld pH meter. Bovine serum albumin (BSA)-doped CaCO3 nanospheres were utilized to regulate the pH value of an acidic solution on the basis of classical chemical reactions. To construct such an immunosensing system, the synthesized BSA-CaCO3 nanospheres were initially conjugated covalently to an anti-Lp-PLA2 detection antibody, and then a sandwich-type immunoreaction was carried out on the anti-Lp-PLA2 capture antibody-coated microplate in the presence of the target analyte. Accompanying the sandwiched immunocomplexes, the labeled CaCO3 nanospheres were dissolved in the presence of acid (CaCO3 + 2H+→ Ca2+ + CO2 + H2O), thus resulting in the pH change of the detection solution. Introduction of CaCO3 nanospheres including numerous carbonate ions was expected to enhance the sensitivity of the electrochemical immunoassay. Under optimum conditions, BSA-CaCO3-based immunoassay displayed good pH responses for the determination of target Lp-PLA2 within the dynamic linear range from 0.1 ng mL−1 to 300 ng mL−1. The limit of experimental detection achieved was ∼78 pg mL−1 Lp-PLA2 with good coefficients of variation for the inter- and intra-assays. Also, this system gave good anti-interfering capacity toward other biomarkers and enzyme proteins. Importantly, our strategy was applied for the analysis of human serum specimens with satisfactory results in comparison with commercialized Lp-PLA2 ELISA kits.

A hydrogel sensor-based microfluidic platform for the quantitative and multiplexed detection of fertility markers for point-of-care immunoassays by Satish Kalme; Srinivasan Kandaswamy; Anusha Chandrasekharmath; Reeta Katiyar; Gokul Prasath Rajamanickam; Suraj Kumar; Dhananjaya Dendukuri (1639-1650).
We report a new microfluidics-based immunoassay platform based on 3D porous hydrogel particle sensors with planar rectangular geometries. Large numbers of sensors are made off-chip before being embedded into an injection molded microfluidic chip. This allows for both multiplexing and scalability of manufacturing processes. A specially designed adapter containing detection reagents and buffers is interfaced with a microfluidic chip allowing for an automated immunoassay to be performed. All antibody reagents required for the test are stored in a dried form on the chip, enabling stability for resource-limited settings. Fluorescence-based immunoassays are performed by a sequential flow of the sample, detection reagents and wash buffer over the sensors. A compact instrument that integrates microfluidic flow dispensing and pumping with highly sensitive array-based detection is used to conduct and read the assay. A multiplexed set of 4 important fertility-linked tests, beta-human chorionic gonadotropin (β-hCG), follicle stimulating hormone (FSH), luteinizing hormone (LH) and prolactin (PRL), that are commonly required at gynecology/IVF settings were developed and validated using discarded clinical samples. Detailed data on accuracy, precision, linearity, effects of interference and cross-reactivity have been studied, rendering the platform ready for clinical use.

Electrochemical sensing of H2O2 released from living cells based on AuPd alloy-modified PDA nanotubes by Guangli He; Fengli Gao; Wei Li; Pengwei Li; Xiaofan Zhang; Hang Yin; Baocheng Yang; Yibiao Liu; Shouren Zhang (1651-1656).
The highly efficient detection method for hydrogen peroxide (H2O2) has been attracting significant attention. In this study, AuPd alloy-modified polydopamine (AuPd-PDA) nanotubes were prepared, and an electrochemical nonenzymatic sensor based on the AuPd-PDA nanotubes was developed. A single AuPd alloy on the surface of PDA nanotubes consisted of many smaller nanoparticles of size ranging several nanometers, which greatly enhanced the catalytic activity. In addition, PDA nanotubes, as carriers of AuPd alloys, were used to amplify the signal and improve the sensitivity of H2O2 sensors. Owing to its synergistic effect and structural advantage, the H2O2 sensor based on AuPd-PDA nanotubes exhibited good sensing performances, including a high sensitivity of 314.2 μA mM−1 cm−2, a wide linear range from 1.0 μM to 11.22 mM and a low detection limit of 0.26 μM (S/N, 3 : 1). Moreover, the detection of H2O2 in HeLa and Raw 264.7 cells was also achieved, which indicated that the sensors based on AuPd-PDA nanotubes could be used for real-time monitoring of H2O2 in physiological and pathological processes in a clinic.

Simultaneous determination of 169 veterinary drugs in chicken eggs with EMR-Lipid clean-up using ultra-high performance liquid chromatography tandem mass spectrometry by Ping Luo; Xiaohong Liu; Fang Kong; Liangkai Chen; Qiang Wang; Wanyi Li; Sheng Wen; Lin Tang; Yonggang Li (1657-1662).
A multiresidue method for the simultaneous determination of 169 veterinary drugs in chicken egg was developed and validated according to the requirements of the China Conformity Assessment GB/T 27417-2017. The method was developed by extraction using acetonitrile with 5% formic acid and then using enhanced matrix removal-lipid in dispersive solid phase extraction and drying pouches for further clean-up with ultra-high performance liquid chromatography tandem mass spectrometry of an electrospray–ionization interface in positive ion mode. The limits of detection and quantification of the analytes were 0.01–3.81 μg kg−1 and 0.03–12.57 μg kg−1, respectively. For the analytes, the recoveries were 57–124%, and the relative standard deviations of all analytes ranged from 2% to 31%. Linearity was evaluated by performing matrix-matched calibration at 0.1, 0.2, 0.5, 1, 5, 10, 20, and 40 μg kg−1 levels. The evaluated method enabled the rapid, reliable, and robust quantification and identification of 169 veterinary drug residues and was successfully applied in real samples.

A simple, fast, cheap, and effective method named deep eutectic solvent-based emulsification liquid–liquid microextraction (DES-ELLME) was employed to extract thiophenols from water samples. DES was characterized by Fourier transform-infrared (FT-IR) spectroscopy, CHN elemental analysis, and proton and carbon nuclear magnetic resonance spectroscopies (1H-NMR and 13C-NMR). 50 μL DES was added to the aqueous sample solution and a uniform solution was formed immediately. After injection of 50 μL acetone into this solution, turbidity was observed. Then, the phase separation (DES rich phase/aqueous phase) was performed by centrifugation. DES rich phase was subjected to gas chromatography with a flame ionization detector (GC-FID). Significant factors including the volume of acetone and DES, ultrasonication time, and pH were combined using a desirability function (DF) and optimized through a Box–Behnken design. Calibration graphs were linear in the concentration range of 0.02–100 mg L−1 under optimum experimental conditions; the coefficients of correlation were more than 0.999. The limits of detection and quantification were in the ranges of 10–15 μg L−1 and 33–50 μg L−1, respectively. This method was used to determine target analytes in spiked water samples; the relative mean recoveries were 96 to 104%.

The technical development of liquid chromatography has provided the necessary sensitivity to characterise peptidoglycan samples. However, the analysis of large numbers of complex chromatographic data sets without the aid of a proper chemometric technique is a laborious task, carrying a high risk of losing important biochemical information. The present work describes the development of a simple analytical procedure using self-organising map (SOM) analysis to analyse the large number of complex chromatographic data sets from bacterial peptidoglycan samples. SOM analysis essentially maps the samples to a hexagonal sheet based on their compositional similarity, and thus provides an approach to classify the bacterial cell wall collection in an unsupervised manner. The utility of the proposed approach was successfully validated by analysing peptidoglycan samples belonging to the Alphaproteobacterium class. The classification results achieved with SOM analysis were found to correlate well with their relative similarity in peptidoglycan compositions. In summary, the SOM analysis-based analytical procedure is shown to be useful towards automatising the analyses of chromatographic data sets of peptidoglycan samples from bacterial collections.

The present study describes two stability-indicating HPLC methods with two different chromatographic approaches for the simultaneous estimation of bambuterol and its main degradation product, terbutaline in bulk and pharmaceuticals. Method I was carried out using a conventional particle-packed cyano column (150 mm × 4.6 mm, 5 μm particle size) and a micellar eluent composed of 0.1 M SDS, 15% n-propanol, and 0.3% triethylamine (pH 3.5), coupled with UV detection at 220 nm. Method II was based on the fused core shell technology, and the separation was carried out using a Kinetex C18 column (150 mm × 4.6 mm, 5 μm particle size) and an aqueous mobile phase composed of methanol and a 0.1 M phosphate buffer (45 : 55 v/v; pH 4), using fluorescence detection at λex = 275 nm and λem = 313 nm. The first method was rectilinear over the concentration range of 10–200 μg mL−1 for bambuterol and 5–100 μg mL−1 for terbutaline while the second method showed a good linearity in the range of 1–5 μg mL−1 for bambuterol and 0.1–1 μg mL−1 for terbutaline. The developed methods enabled the quantification of bambuterol and terbutaline in their dosage forms, including tablets and syrups and showed good results compared to the official methods. The practical and analytical comparisons between the two proposed methods is discussed.

A robust ultra-high performance liquid chromatography method with tandem mass spectrometry was established for the quantitative analysis of the major peanut allergens Ara h1 and Ara h2 in baked foodstuffs based on their signature peptides. Tryptic peptides DLAFPGSGEQVEK and NLPQQCGLR were selected as the target analytes for Ara h1 and Ara h2, respectively. Their corresponding isotope-labeled peptides DL*AFPGSGEQV*EK and NL*PQQCGL*R were synthesized and used as internal standards to overcome matrix effects during sample analysis. The Ara h1 or Ara h2 content values in samples were calculated according to the measured signature peptide of Ara h1 or Ara h2 and the calibration curve established using the hydrolysates of their corresponding proteins. The limits of quantification for Ara h1 and Ara h2 were 0.30 and 0.13 mg kg−1, respectively. The developed method had satisfactory accuracy, precision, and sensitivity according to in-house validation. The current method was successfully employed to measure the peanut allergens Ara h1 and Ara h2 in commercial baked foods, and could be considered a robust candidate method for promoting the quality control of commercial baked foods and providing accurate allergen warning information.

A rapid and sensitive colorimetric strategy for the determination of ascorbic acid (AA) is presented, using amino functionalized copper metal–organic framework nanoparticles (Cu-BDC-NH2 NPs) with effective intrinsic peroxidase-like properties. In our design, Cu-BDC-NH2 NPs catalyze the H2O2 oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) generating blue-colored oxidized TMB (ox-TMB) with a significant absorbance at 655 nm in the UV-vis spectrum. AA can reduce ox-TMB, inducing a dramatic blue color fading and a decrease in absorbance. Under the optimal experimental conditions, this proposed system presents a good linear relationship (0.5–60 μM) and the detection limit is 0.15 μM. Repeatability, high selectivity and practicability are also well-studied, proving that the sensing system based on Cu-BDC-NH2 NPs can be successfully applied for colorimetric detection of AA in human serum, food and pharmaceutical samples. The designed method can provide a highly selective and sensitive optical sensing system to detect AA and establish a new platform to develop extensive applications of metal–organic framework nanoparticles with enzyme-mimetic activity in colorimetric biosensing.

An environment-friendly device for rapid determination of chemical oxygen demand in waters based on ozone-induced chemiluminescence technology by Youquan Zhao; Junpeng Zhou; Yang Song; Zhende Li; Xiao Liu; Mimi Zhang; Hechao Zang; Xuan Cao; Chenxing Lv (1707-1714).
An environment-friendly device based on ozone-induced chemiluminescence (CL) technology was developed and optimized for the rapid measurement of chemical oxygen demand (COD) in waters. The purpose of this paper was to introduce a system that could be used in the field and has the characteristics of simple operation and avoiding the use of chemical reagents. Moreover, no more than 3 mL was introduced into the CL cell by using a syringe and mixed with ozone gas flow that could be produced online using an electrolytic ozone generator, which could lead to generating CL immediately and then the weak emission was focused into a photomultiplier tube through a specially designed optical system. Although the measurement procedure is simple, the method was sensitive as sharp peaks appeared within seconds for milligrams of COD, and the lower limit of detection was 0.1 mg L−1. The light intensity difference (ΔI) of the CL detection system between deionized water and real sample water was proportional to the COD value of the water sample. A complete analysis can be performed in 2 minutes including sample addition and signal processing. Compared with conventional methods, this method required a smaller volume of water sample (3 mL vs. 30 mL) and was less time-consuming (2 min vs. 2 h), and was free of chemical reagents. The correlation between the presented method and the conventional permanganate chemical oxygen demand (CODMn) for several real water samples showed a good determination coefficient R2 = 0.9947. Owing to its characteristics of rapid, simple, sufficient sensitivity and low cost, this method provides a promising tool for large-scale environmental assessment and industrial process control.

Back cover (1715-1716).