Electrochemical Detectors in HPLC and Ion Chromatography

Microdialysis-integrated HPLC system with dual-electrode detection using track-etched membrane electrodes for in vivo monitoring of dopamine dynamics

A capillary high-performance liquid chromatography (HPLC) system equipped with a dual-electrode detector utilizing track-etched membrane electrodes (TEMEs) was combined with a microdialysis sampling setup. The electrochemical detector benefits from the high electrolysis efficiency of TEMEs, allowing for calibration-free coulometric detection and simplifying data analysis to determine the dopamine recovery through a dialysis probe. Additionally, this system was used for in vivo monitoring of dopamine in the right striatum of a mouse brain. Temporal changes in dopamine levels, including an exponential decay immediately after the dialysis probe insertion and an excess release of dopamine induced by a high concentration of potassium ions, confirmed the system's proper operation. Furthermore, subsequent measurements following the intraperitoneal injection of mirtazapine showed no increase in dopamine levels in the right dorsal striatum. The dual-electrode system displayed characteristic dopamine detection behavior, with anodic and cathodic peak pairs indicative of reversible electrochemical reactions. This capability facilitated the identification of the dopamine peak within the complex chromatogram of the mouse brain dialysate. The consistency between dopamine collection efficiency from standard solutions and dialysate indicated the absence of interfering electroactive substances overlapping with the dopamine peak in the chromatogram. This integrated analysis system successfully tracked temporal fluctuations in dopamine concentration within the mouse brain.

Detection of polycyclic aromatic hydrocarbons using a high performance-single particle aerosol mass spectrometer

The real-time detection of the mixing states of polycyclic aromatic hydrocarbons (PAHs) and nitro-PAHs in ambient particles is of great significance for analyzing the source, aging process, and health effects of PAHs and nitro-PAHs; yet there is still few effective technology to achieve this type of detection. In this study, 11 types of PAH and nitro-PAH standard samples were analyzed using a high performance-single particle aerosol mass spectrometer (HP-SPAMS) in lab studies. The identification principles 'parent ions' and 'mass-to-charge (m/z) = 77' of each compound were obtained in this study. It was found that different laser energies did not affect the identification of the parent ions. The comparative experiments of ambient atmospheric particles, cooking and biomass burning emitted particles with and without the addition of PAHs were conducted and ruled out the interferences from primary and secondary organics on the identification of PAHs. Besides, the reliability of the characteristic ions extraction method was evaluated through the comparative study of similarity algorithm and deep learning algorithm. In addition, the real PAH-containing particles from vehicle exhaust emissions and ambient particles were also analyzed. This study improves the ability of single particle mass spectrometry technology to detect PAHs and nitro-PAHs, and HP-SPAMS was superior to SPAMS for detecting single particles containing PAHs and nitro-PAHs. This study provides support for subsequent ambient observations to identify the characteristic spectrum of single particles containing PAHs and nitro-PAHs.

Plasticized PVC Membrane Modified Electrodes: Voltammetry of Highly Hydrophobic Compounds

In the last 50 years, plasticized polyvinyl chloride (PVC) membranes have gained unique importance in chemical sensor development. Originally, these membranes separated two solutions in conventional ion-selective electrodes. Later, the same membranes were applied over a variety of supporting electrodes and used in both potentiometric and voltammetric measurements of ions and electrically charged molecules. The focus of this paper is to demonstrate the utility of the plasticized PVC membrane modified working electrode for the voltammetric measurement of highly lipophilic molecules. The plasticized PVC membrane prevents electrode fouling, extends the detection limit of the voltammetric methods to sub-micromolar concentrations, and minimizes interference by electrochemically active hydrophilic analytes.

Track-etched membrane-based dual-electrode coulometric detector for microbore/capillary high-performance liquid chromatography

The electrochemical flow cell containing track-etched microporous membrane electrodes was applied to a dual-electrode coulometric detector for microbore/capillary HPLC with a small injection volume and low eluent flow rate. The proposed flow cell with a 0.1-mm diameter inlet channel gave a detection volume of 0.08 nL per electrode, which was determined by the eluent flow through the electrode. For the dual-electrode detector, the calculated volume was 0.24 nL. The efficiency of electrooxidation of l-ascorbic acid increased as the flow rate decreased and was close to 100% when the flow rate was below 50 μL min^-1, which is a common flow rate in microbore or capillary liquid chromatography. Catecholamines, such as noradrenaline, adrenaline, and dopamine, were detected by total conversion with two-electron oxidation in the potential range from 0.8 to 1.0 V vs. Ag/AgCl after separation with a microbore column. These peaks were accompanied by corresponding cathodic peaks derived from quasi-stable electrooxidation products of the catecholamines. The detection limits of noradrenaline, adrenaline, and dopamine were 0.1, 0.1, and 0.2 μM, respectively. The RSD values for five replicate measurements of 5.0 μM of these compounds were 0.9%, 0.7%, and 1.5%, respectively. Coulometric detection was also demonstrated by determination of catecholamines in pharmaceuticals.

A New Quantification Method Using Electrochemical Mass Spectrometry

Mass spectrometry-based quantification method has advanced rapidly. In general, the methods for accurate quantification rely on the use of authentic target compounds or isotope-labeled compounds as standards, which might be not available or difficult to synthesize. To tackle this grand challenge, this paper presents a novel approach, based on electrochemistry (EC) combined with mass spectrometry (MS). In this approach, a target compound is allowed to undergo electrochemical oxidation and then subject to MS analysis. The oxidation current recorded from electrochemistry (EC) measurement provides information about the amount of the oxidized analyte, based on the Faraday's Law. On the other hand, the oxidation reaction yield can be determined from the analyte MS signal changes upon electrolysis. Therefore, the total amount of analyte can be determined. In combination with liquid chromatography (LC), the method can be applicable to mixture analysis. The striking strength of such a method for quantitation is that neither standard compound nor calibration curve is required. Various analyte molecules such as dopamine, norepinephrine, and rutin as well as peptide glutathione in low quantity were successfully quantified using our method with the quantification error ranging from - 2.6 to + 4.6%. Analyte in a complicated matrix (e.g., uric acid in urine) was also accurately measured. Graphical Abstract.

Periodically interrupted amperometry. A way of improving analytical performance of membrane coated electrodes

Amperometry is a powerful voltammetric measuring method. Its application is specially advantageous when used in combination with a separation step or with some other sample treatment method providing selectivity. The selectivity is often achieved by coating the amperometric working electrode surface with a membrane of special character. Size exclusion membrane, immobilized enzyme containing reaction layer, protecting dialysis membrane, perm selective ion exchange film etc can be mentioned here. In conventional amperometry the measuring potential is continuously applied, therefore in case of membrane coated electrodes the electrode process depletes the diffusion layer. In this work the performance of a new periodically interrupted amperometric (PIA) measuring program has been investigated in case of glucose enzyme sensor. The measuring program allowing time for reloading the diffusion layer provided higher current and therefore improved sensitivity and lower limit of detection.

Automated determination of levodopa and carbidopa in plasma by high-performance liquid chromatography-electrochemical detection using an on-line flow injection analysis sample pretreatment unit

An automated analytical procedure is described for the parallel determination of L-3,4-dihydroxyphenylalanine (levodopa, L-dopa, LD) and the analogous hydrazine compound carbidopa (CD) in dog plasma by ion-pair high-performance liquid chromatography with electrochemical detection (HPLC-ED). After deproteinization of the plasma samples with perchloric acid the catecholamines were extracted from the supernatant by adsorption on a small column filled with alumina. The extraction and redissolution were automatically performed in a flow injection analysis unit (FIA) coupled to the HPLC system. The performance of the whole system was tested on dog plasma samples including specimens taken after oral administration of the anti-Parkinsonism drug Duellin, which is a combination tablet of levodopa and carbidopa.

UV radiation protecting efficacy of cysteine derivatives, studies with UVA-induced binding of 8-MOP and CPZ to rat epidermal biomacromolecules in vivo

With the aim of optimizing the UV radiation protecting efficacy of N-acetylcysteine (NAC), the following topically applied cysteine derivatives were investigated: N-acetylcysteine ethylester (NACET), S-acetylcysteine ethylester (SACET), cysteine ethylester (CYSET), N,S-diacetylcysteinamide (SNACA), N,S-diacetylcysteine (SNAC) and N,S-diacetylcysteine ethylester (SNACET). As a measure for protection the inhibition of in vivo irreversible photobinding of the labelled phototoxic drugs chlorpromazine (CPZ) and 8-methoxypsoralen (8-MOP) to rat epidermal biomacromolecules was used. The duration of protection of the cysteine derivatives was shortened by S-acetylation, N-acetylation and carboxyl derivatization. Compounds with a free thiol group showed a long-lasting presence in the stratum corneum, probably by the formation of mixed disulphides with proteins. The intrinsic protecting efficacy with respect to the total epidermis increased in the order CYSET < SNACET,SNACA,SACET < NACET, SNAC,NAC. The results of this study are discussed in view of susceptibility to oxidation, epidermal bioavailability and metabolic activation. With respect to the viable epidermis we postulate that NACET and SNAC have the most promising properties as UV protective agents.