Theory of Analytical Chemistry

Pteridine determination in human serum with special emphasis on HPLC methods with fluorimetric detection

Conjugated and unconjugated pteridines and their derivatives are important cofactors in cellular metabolism. Hence, the amount of unconjugated pteridines in biological fluids has been found to be modified as a result of several disorders. It is necessary to note that while for the control of pteridines in urine samples there are numerous reference data, the literature referred to for the analysis of these analytes in serum/plasma is scarce. In biological fluids, pteridines can exist in different oxidation states, and these compounds can be classified into two groups according to: (a) oxidized or aromatic pteridines and (b) reduced pteridines. Oxidized pteridines yield a strong fluorescence signal, whereas reduced pteridines present a low quantum yield of fluorescence. In order to enable the analysis of the reduced forms, several preoxidation procedures to generate aromatic rings have been established. Also, stabilization of the reduced forms by the addition of reducing agents has been widely reported. The objective of this paper is to show possibilities and different approaches in the analysis of pteridines in serum samples. We have mainly focused on the description of the current situation in the application of high-performance chromatography methods with fluorimetric detection.

Interference-free spectrofluorometric quantification of aristolochic acid I and aristololactam I in five Chinese herbal medicines using chemical derivatization enhancement and second-order calibration methods

A rapid interference-free spectrofluorometric method combined with the excitation-emission matrix fluorescence and the second-order calibration methods based on the alternating penalty trilinear decomposition (APTLD) and the self-weighted alternating trilinear decomposition (SWATLD) algorithms, was proposed for the simultaneous determination of nephrotoxic aristolochic acid I (AA-I) and aristololactam I (AL-I) in five Chinese herbal medicines. The method was based on a chemical derivatization that converts the non-fluorescent AA-I to high-fluorescent AL-I, achieving a high sensitive and simultaneous quantification of the analytes. The variables of the derivatization reaction that conducted by using zinc powder in acetose methanol aqueous solution, were studied and optimized for best quantification results of AA-I and AL-I. The satisfactory results of AA-I and AL-I for the spiked recovery assay were achieved with average recoveries in the range of 100.4-103.8% and RMSEPs <0.78ngmL^-1, which validate the accuracy and reliability of the proposed method. The contents of AA-I and AL-I in five herbal medicines obtained from the proposed method were also in good accordance with those of the validated LC-MS/MS method. In light of high sensitive fluorescence detection, the limits of detection (LODs) of AA-I and AL-I for the proposed method compare favorably with that of the LC-MS/MS method, with the LODs <0.35 and 0.29ngmL^-1, respectively. The proposed strategy based on the APTLD and SWATLD algorithms by virtue of the "second-order advantage", can be considered as an attractive and green alternative for the quantification of AA-I and AL-I in complex herbal medicine matrices without any prior separations and clear-up processes.

Hitchhiker's Guide to Voltammetry: Acute and Chronic Electrodes for in Vivo Fast-Scan Cyclic Voltammetry

Fast-scan cyclic voltammetry (FSCV) has been used for over 20 years to study rapid neurotransmission in awake and behaving animals. These experiments were first carried out with carbon-fiber microelectrodes (CFMs) encased in borosilicate glass, which can be inserted into the brain through micromanipulators and guide cannulas. More recently, chronically implantable CFMs constructed with small diameter fused-silica have been introduced. These electrodes can be affixed in the brain with minimal tissue response, which permits longitudinal measurements of neurotransmission in single recording locations during behavior. Both electrode designs have been used to make novel discoveries in the fields of neurobiology, behavioral neuroscience, and psychopharmacology. The purpose of this Review is to address important considerations for the use of FSCV to study neurotransmitters in awake and behaving animals, with a focus on measurements of striatal dopamine. Common issues concerning experimental design, data collection, and calibration are addressed. When necessary, differences between the two methodologies (acute vs chronic recordings) are discussed. The topics raised in this Review are particularly important as the field moves beyond dopamine toward new neurochemicals and brain regions.

Simultaneous determination of umbelliferone and scopoletin in Tibetan medicine Saussurea laniceps and traditional Chinese medicine Radix angelicae pubescentis using excitation-emission matrix fluorescence coupled with second-order calibration method

A chemometrics-assisted excitation-emission matrix (EEM) fluorescence method is presented for simultaneous determination of umbelliferone and scopoletin in Tibetan medicine Saussurea laniceps (SL) and traditional Chinese medicine Radix angelicae pubescentis (RAP). Using the strategy of combining EEM fluorescence data with second-order calibration method based on the alternating trilinear decomposition (ATLD) algorithm, the simultaneous quantification of umbelliferone and scopoletin in the two different complex systems was achieved successfully, even in the presence of potential interferents. The pretreatment is simple due to the "second-order advantage" and the use of "mathematical separation" instead of awkward "physical or chemical separation". Satisfactory results have been achieved with the limits of detection (LODs) of umbelliferone and scopoletin being 0.06ngmL(-1) and 0.16ngmL(-1), respectively. The average spike recoveries of umbelliferone and scopoletin are 98.8±4.3% and 102.5±3.3%, respectively. Besides, HPLC-DAD method was used to further validate the presented strategy, and t-test indicates that prediction results of the two methods have no significant differences. Satisfactory experimental results imply that our method is fast, low-cost and sensitive when compared with HPLC-DAD method.

A road map for multi-way calibration models

A road map is proposed for the selection of a multi-way calibration model according to the data properties.

Combination of Liquid Chromatography with Multivariate Curve Resolution-Alternating Least-Squares (MCR-ALS) in the Quantitation of Polycyclic Aromatic Hydrocarbons Present in Paprika Samples

This work presents a strategy for quantitating polycyclic aromatic hydrocarbons (PAHs) in smoked paprika samples. For this, a liquid chromatographic method with fluorimetric detection (HPLC-FLD) was optimized. To resolve some interference co-eluting with the target analytes, the second-order multivariate curve resolution-alternating least-squares (MCR-ALS) algorithm has been employed combined with this liquid chromatographic method. Among the eight PAHs quantified (fluorene, phenanthrene, anthracene, pyrene, chrysene, benzo[a]anthracene, benzo[b]fluoranthene, and benzo[a]pyrene) by HPLC-FLD, only in the case of fluorene, pyrene, and benzo[b]fluoranthene was it necessary to apply the second-order algorithm for their resolution. Limits of detection and quantitation were between 0.015 and 0.45 mg/kg and between 0.15 and 1.5 mg/kg, respectively. Good recovery results (>80%) for paprika were obtained via the complete extraction procedure, consisting of an extraction from the matrix and the cleanup of the extract by means of silica cartridges. Higher concentrations of chrysene, benzo[a]anthracene, benzo[b]fluoranthene, and benzo[a]pyrene were found in the paprika samples, with respect to the maximal amounts allowed for other spices that are under European Regulation (EU) N° 2015/1933.

New frontiers in pharmaceutical analysis: A metabolomic approach to check batch compliance of complex products based on natural substances

Natural substances, particularly medicinal plants and their extracts, are still today intended as source for new Active Pharmaceutical Ingredients (APIs). Alternatively they can be validly employed to prepare medicines, food supplements or medical devices. The most adopted analytical approach used to verify quality of natural substances like medicinal plants is based still today on the traditional quantitative determination of marker compounds and/or active ingredients, besides the acquisition of a fingerprint by TLC, NIR, HPLC, GC. Here a new analytical approach based on untargeted metabolomic fingerprinting by means of Mass Spectrometry (MS) to verify the quality of grinTuss adulti syrup, a complex products based on medicinal plants, is proposed. Recently, untargeted metabolomic has been successfully applied to assess quality of natural substances, plant extracts, as well as corresponding formulated products, being the complexity a resource but not necessarily a limit. The untargeted metabolomic fingerprinting includes the monitoring of the main constituents, giving weighted relevance to the most abundant ones, but also considering minor components, that might be notable in view of an integrated - often synergistic - effect on the biological system. Two different years of production were investigated. The collected samples were analyzed by Flow Injection ElectroSpray Ionization Mass Spectrometry Analysis (FIA-ESI-MS) and a suitable data processing procedure was developed to transform the MS spectra into robust fingerprints. Multivariate Statistical Process Control (MSPC) was applied in order to obtain multivariate control charts that were validated to prove the effectiveness of the proposed method.

Second order advantage obtained by spectroelectrochemistry along with novel carbon nanotube modified mesh electrode: Application for determination of acetaminophen in Novafen samples

A matrix-augmentation multivariate curve resolution-alternating least-squares (MA-MCR-ALS) has been conducted on the spectroelectrochemical data of acetaminophen oxidation in order to quantify acetaminophen in Novafen capsule in the presence of unknown interferences. The experiments were carried out using new cheap mesh electrode, namely carbon nanotube modified mesh electrode (CNMME) as optically transparent thin layer electrode (OTTLE). For each sample, a second order spectroelectrochemical data was obtained and MA-MCR-ALS method was applied to analyze these data. Unlike full trilinear models such as PARAFAC, MCR-ALS is flexible in applying trilinearity constraint for each component, a fact which makes it manage deviations from trilinearity of data effectively. This method was employed in both spectral and kinetic augmentation mode of data under examining different trilinear components. However, spectral augmentation was the only setting which allows MA-MCR-ALS to solve the analytical problem achieving the second order advantage. Therefore, here the results of the augmentation in this mode have been described. In order to obtain the best analytical figures of merit in the analysis, different constraints were investigated. The results indicated the accuracy of the proposed method.

Solving signal instability to maintain the second-order advantage in the resolution and determination of multi-analytes in complex systems by modeling liquid chromatography-mass spectrometry data using alternating trilinear decomposition method assisted with piecewise direct standardization

The application of calibration transfer methods has been successful in combination with near-infrared spectroscopy or other tools for prediction of chemical composition. One of the developed methods that can provide accurate performances is the piecewise direct standardization (PDS) method, which in this paper is firstly applied to transfer from one day to another the second-order calibration model based on alternating trilinear decomposition (ATLD) method built for the interference-free resolution and determination of multi-analytes in complex systems by liquid chromatography-mass spectrometry (LC-MS) in full scan mode. This is an example of LC-MS analysis in which interferences have been found, making necessary the use of second-order calibration because of its capacity for modeling this phenomenon, which implies analytes of interest can be resolved and quantified even in the presence of overlapped peaks and unknown interferences. Once the second-order calibration model based on ATLD method was built, the calibration transfer was conducted to compensate for the signal instability of LC-MS instrument over time. This allows one to reduce the volume of the heavy works for complete recalibration which is necessary for later accurate determinations. The root-mean-square error of prediction (RMSEP) and average recovery were used to evaluate the performances of the proposed strategy. Results showed that the number of calibration samples used on the real LC-MS data was reduced by using the PDS method from 11 to 3 while producing comparable RMSEP values and recovery values that were statistically the same (F-test, 95% confidence level) to those obtained with 11 calibration samples. This methodology is in accordance with the highly recommended green analytical chemistry principles, since it can reduce the experimental efforts and cost with regard to the use of a new calibration model built in modified conditions.

Chemometrics-assisted determination of amiloride and triamterene in biological fluids with overlapped peaks and unknown interferences

Background: Amiloride (AMI) and triamterene (TRI) are both potassium-saving diuretics, which are ordinarily used as doping to enhance the performance of athletes in sports. For the similar structures and complex matrices existence, chromatography and extraction are commonly employed to realize the determination of AMI and TRI in biological fluids, which are very time-consuming and laborious. Results: A novel method is presented to simultaneous interference-free determination of AMI and TRI in complex biological fluids samples using excitation–emission matrix fluorescence coupled with second-order calibration method based on alternating normalization-weight error algorithm. Conclusion: The proposed method can obtain accurate qualitative and quantitative information of the analytes, even in the presence of the interference from complex biological fluids, which requires few prior purification and separation procedures.

Sensor Array Design for Complex Sensing Tasks

Chemical detection in complex environments presents numerous challenges for successful implementation. Arrays of sensors are often implemented for complex chemical sensing tasks, but systematic understanding of how individual sensor response characteristics contribute overall detection system performance remains elusive, with generalized strategies for design and optimization of these arrays rarely reported and even less commonly adopted by practitioners. This review focuses on the literature of nonspecific sensor array design and optimization strategies as well as related work that may inform future efforts in complex sensing with arrays.

A chemometric-assisted method for the simultaneous determination of malachite green and crystal violet in water based on absorbance-pH data generated by a homemade pH gradient apparatus

An attractive method of generating second-order data was developed by a dropping technique to generate pH gradient simultaneously coupled with diode-array spectrophotometer scanning. A homemade apparatus designed for the pH gradient. The method and the homemade apparatus were used to simultaneously determine malachite green (MG) and crystal violet (CV) in water samples. The absorbance-pH second-order data of MG or CV were obtained from the spectra of MG or CV in a series of pH values of HCl-KCl solution. The second-order data of mixtures containing MG and CV that coexisted with interferents were analyzed using multidimensional partial least-squares with residual bilinearization. The method and homemade apparatus were used to simultaneously determine MG and CV in fish farming water samples and in river ones with satisfactory results. The presented method and the homemade apparatus could serve as an alternative tool to handle some analysis problems.

Chemometrics-assisted cyclodextrin-enhanced excitation-emission fluorescence spectroscopy for the simultaneous green determination of bisphenol A and nonylphenol in plastics

The aim of this work was to quantify two relevant priority chemicals, bisphenol A (BPA) and 4-nonylphenol (NP), coupling the sensitivity of fluorescence in organized media and the selectivity of multivariate calibration, measuring excitation-emission fluorescence matrices in an aqueous methyl-β-cyclodextrin solution. The studied priority pollutants are two of the most frequently found xenoestrogens in the environment, and are therefore of public health concern.The data were successfully processed by applying unfolded partial least-squares coupled to residual bilinearization (U-PLS/RBL), which provided the required selectivity for overcoming the severe spectral overlapping among the analyte spectra and also those for the interferents present in real samples. A rigorous International Union of Pure and Applied Chemistry (IUPAC)-consistent approach was applied for the calculation of the limits of detection. Values in the ranges of 1-2 and 4-14 ng mL(-1) were obtained in validation samples for BPA and NP, respectively. On the other hand, low relative prediction errors between 3% and 8% were achieved. The proposed method was successfully applied to the determination of BPA and NP in different plastics. In positive samples, after an easy treatment with a small volume of ethanol at 35°C, concentrations were found to range from 26 to 199 ng g(-1) for BPA, and from 95 to 30,000 ng g(-1) for NP.

Second-order calibration for the determination of fatty acids in pomegranate seeds by vortex-assisted extraction-dispersive liquid–liquid micro-extraction and gas chromatography-mass spectrometry

Multivariate curve resolution-alternating least squares is proposed for the simultaneous analysis of 18 fatty acid methyl esters in a standard mixture and pomegranate seed sample using vortex-assisted extraction-dispersive liquid–liquid microextraction followed by GC-MS.

Second-order advantage maintenance with voltammetric data modeling for quantitation of ethiofencarb in the presence of interferences

This work presents the development of a method based on a voltammetric determination coupled to second-order data modeling with multivariate curve resolution-alternating least-square (MCR-ALS) and unfolded partial least squares regression followed by residual bilinearization (U-PLS/RBL) for the quantitation of the pesticide ethiofencarb in the presence of interferences in tap water. The determination of ethiofencarb by voltammetry is possible on the basis that this pesticide is hydrolysed in alkaline media giving rise to a compound which shows an oxidation peak at Ep=0.80 V. But some other pesticides belonging to the same family can be considered as interferences due to the high overlapping in their signals. The two such interferences are fenobucarb and bendiocarb, which have not been included in the calibration step. The possibility of second-order multivariate calibration was studied by using the hydrolysis time as the third variable, and MCR-ALS and U-PLS/RBL. Good results were obtained in this determination in spite of the high overlapping in voltammetric signals.