Power function setting in charged aerosol detection for the linearization of detector response - optimization strategies and their application.

Charged aerosol detection (CAD) is an universal technique in liquid chromatography that is increasingly used for the quality control of drugs. Consequently, it has found its way into compendial monographs promoted by its simple and robust application. However, the response of CAD is inherently nonlinear due to its principle of function. Thus, easy and rapid linearization procedures, in particular regarding compendial applications, are highly desirable. One effective approach to linearize the detector's signal makes use of the built-in power function value (PFV) setting of the instrument. The PFV is basically a multiplication factor to the power law exponent of the equation describing the CAD's response, thereby altering the detector's signal output to optimize the quasi-linear range of the response curve. The experimental optimization of the PFV for a series of analytes is a time-consuming process, limiting the practicability of this approach. Here, two independent approaches for the determination of the optimal PFV based on an empirical model and a mathematical transformation in each case, are evaluated. Both approaches can be utilized to predict the optimal PFV for each analyte solely based on the experimental results of a series of calibration standards obtained at a single PFV. The approaches were applied to the HPLC-UV-CAD impurity analysis of the drug gabapentin to improve the observed nonlinear response of the impurities in the range of interest. The predicted optimal PFV of both approaches were in good agreement with the experimentally obtained optimal PFV of the analytes. As a result, the accuracy of the method was significantly improved when using the optimal PFV (90 - 105% versus 81 - 115% recovery rate for quantitation by either single-point calibration or linear regression) for the majority of the analytes. The final method with a PFV adjusted to 1.30 was validated with respect to ICH guideline Q2(R1).

Determination of tropane alkaloids by heart cutting reversed phase - Strong cation exchange two dimensional liquid chromatography.

Current Chinese Pharmacopoeia (ChP) standards apply liquid extraction combined with one dimensional liquid chromatography (1DLC) method for determining alkaloids in herbal medicines. The complex pretreatments lead to a low analytical efficiency and possible component loss. In this study, a heart cutting reversed phase - strong cation exchange two dimensional liquid chromatography (RP - SCX 2DLC) approach was optimized for simultaneously quantifying tropane alkaloids (anisodine, scopolamine and hyoscyamine) in herbal medicines and herbal medicine tablets without further treatment of the filtered extract. The chromatographic conditions were systematically optimized in terms of column type, mobile phase composition and flow rate. To improve peak capacity and obtain symmetric peak shape of alkaloids, a polar group embedded C18 column combined with chaotropic salts was used in the first dimension. To remove the disturbance of non-alkaloids, achieve unique selectivity and acquire symmetric peak shape of alkaloids, an SCX column combined with phosphate buffer was used in the second dimension. Method validation was performed in terms of linearity, precision (0.54-0.82%), recovery (94.1-105.2%), limit of detection (LOD) and limit of quantification (LOQ) of the three analytes varied between 0.067-0.115mgL-1 and 0.195-0.268mgL-1, respectively. The method demonstrated superiority over 1DLC method in respect of resolution (less alkaloid co-eluted), sample preparation (no pretreatment procedure) and transfer rate (minimum component loss). The optimized RP - SCX 2DLC approach was subsequently applied to quantify target alkaloids in five herbal medicines and herbal medicine tablets from three different manufactures. The results demonstrated that the developed heart cutting RP - SCX 2DLC approach represented a new, strategically significant methodology for the quality evaluation of tropane alkaloid in related herbal medicines that involve complex chemical matrix.

[A highly sensitive high performance liquid chromatography-electrochemical detection method for the determination of five phenolic compounds from Salvia miltiorrhiza].

The determination of antioxidants continues to be interested, since the oxidative damage is thought to be one of the main mechanisms involved in nearly all chronic renal pathologies. A highly sensitive high performance liquid chromatography-electrochemical detection (HPLC-ECD) method was developed for evaluating the antioxidant properties of Salvia miltiorrhiza (Dan Shen). The method was optimized with respect to selectivity and sensitivity. Chromatographic conditions, including mobile phase pH value, buffer concentration, buffer type, organic solvent type, gradient profile and flow rate, were systematically investigated. Low pH value (2.8), low buffer concentration (20 mmol/L NaH2PO4), a shallow water-acetonitrile gradient, and a flow rate of 0.2 mL/min were the determined optimal conditions for the quantitative analysis of aimed five antioxidants from 14 batches of Dan Shen samples. The described method provided a good recovery (>95%), a very wide linear range (up to 104 for all analytes), a good precision (RSDs<4.01%), and a high sensitivity (LOQ of caffeic acid, 1.5 µ g/L). Compared with UV detection, the described ECD method was also more effective for evaluating the antioxidant properties of Dan Shen, as it provided highly selective detection of electro-active antioxidants.

[High sensitive non-derivative determination of cyclovirobuxin D by high-performance liquid chromatography-electrochemical determination].

A high-performance liquid chromatography-electrochemical detection (HPLC-ECD) method was developed to determine cyclovirobuxin D (CVB-D) levels in tablets and human blood samples. A column with a positive charge-modified C18 stationary phase, C18HCE, was selected to analyze CVB-D, because it provided a sharper and more symmetric peak for CVB-D than conventional C18 stationary phase. Two types of working electrode materials, glassy carbon (GC) and boron-doped diamond (BDD), were evaluated. BDD was found to provide better sensitivity than GC owing to its lower background current and baseline noise. Utilizing the BDD electrode, C18HCE column, and optimized mobile phase composition, the developed HPLC-ECD method showed a much better sensitivity. The limit of detection and limit of quantification of the HPLC-ECD method for CVB-D were 0.198 and 0.297 µg/L, respectively. It was approximately 12727, 11481, and 2630 times more sensitive than ultraviolet (UV), evaporative light scattering detection, and charged aerosol detection, respectively. The sensitivity of the developed HPLC-ECD method was comparable or even better (16.8 times) than reported mass spectrometry (MS) methods for the determination of CVB-D. Additionally, it offered a much wider linear dynamic range (up to 4 orders of magnitude, 0.297-1891 µg/L) and was much less complicated than MS methods for determination of CVB-D. The developed HPLC-ECD method can be used for determination of CVB-D at both high and low concentrations. Good intra-day (relative standard deviation (RSD) of peak area<5.08%) and inter-day (RSD of peak area<5.57%) reproducibilities of the developed HPLC-ECD method were obtained even for a low mass concentration (59.1 µg/L) sample. After the optimized parameters were acquired, this method was applied to the quantitative analysis of CVB-D in CVB-D tablets and human blood samples. With a slight modification, the current HPLC-ECD method can also be applied to analyze many other basic compounds including basic drugs and environmental pollutants.

Determination of oxidized and reduced CoQ10 and CoQ9 in human plasma/serum using HPLC-ECD.

This chapter describes the use of reversed-phase HPLC with multichannel coulometric electrochemical detection for the routine, sensitive, and simultaneous measurement of oxidized and reduced CoQ10 and CoQ9 in human plasma and serum. Analytes are first resolved chromatographically prior to electrochemical detection using three serially placed flow-through coulometric sensors set for oxidation-reduction-re-oxidation. Such electrochemical manipulation of analytes not only improves selectivity and specificity (decreasing the likelihood of co-elution), but also leads to improved sensitivity and decreased noise. The method is completed in ,18 min, shows excellent linearity, good intra-day (% RSD = 1.2-2.3) and inter-day (% RSD 2.2-3.9) precision, and has a limit of detection to low pg levels (on column). This approach was used to measure oxidized and reduced CoQ10 and CoQ9 in 30 human plasma samples, and oxidized and reduced CoQ10 in 10 human serum samples (NIST Micronutrients Measurement Quality Assurance Program for fat-soluble vitamins).

Metabolomic applications of electrochemistry/mass spectrometry.

Analytical techniques used for multivariate analysis of endogenous metabolites in biological systems (e.g., metabolomics, metabonomics) must be capable of accurately and selectively monitoring many known and unknown molecules that span a diverse chemical spectrum and over extremely large dynamic concentration ranges. Mass spectrometric (MS) and electrochemical array (EC-Array) detection have been widely used for multi-component analysis with applicability to low-level (fmol) metabolites. Described here are practical considerations and results obtained with the combined use of EC-Array and MS for HPLC-based multivariate metabolomic analysis. Data presented include the study of changes in rat urinary metabolite profiles associated with xenobiotic toxin exposure analyzed by HPLC using water:acetonitrile binary gradient conditions and post-column flow splitting between EC-Array and MS detectors. Results show complementary quantitative and qualitative analysis and the ability to differentiate sample groups consistent with xenobiotic-induced histopathological changes. The potential applicability of this hyphenated technique for biomarker elucidation through measurement of redox active compounds that are commonly associated with disease pathology and xenobiotic toxicity is discussed. The use of EC reactor cells in series with MS is also presented as a means of producing likely metabolites to facilitate structural elucidation and confirmation.

Measurement of isoflavones using liquid chromatography with multi-channel coulometric electrochemical detection.

We evaluated the use of HPLC multi-channel coulometric electrochemical detection (HPLC-ECD) for the routine measurement of phytochemicals by analyzing standards and urine samples. Compounds were separated chromatographically and electrochemically with high sensitivity (10 ng/ml of isoflavones in urine). HPLC-ECD will enable better investigation into the relationship between lifestyle and the effects of phytochemicals on humans.