Determination of relative UV response factors for HPLC by use of a chemiluminescent nitrogen-specific detector

Online monitoring of small volume reactions using compact liquid chromatography instrumentation

A wide variety of analytical techniques have been employed for monitoring chemical reactions, with online instrumentation providing additional benefits compared to offline analysis. A challenge in the past for online monitoring has been placement of the monitoring instrumentation as close as possible to the reaction vessel to maximize sampling temporal resolution and preserve sample composition integrity. Furthermore, the ability to sample very small volumes from bench-scale reactions allows the use of small reaction vessels and conservation of expensive reagents. In this study, a compact capillary LC instrument was used for online monitoring of as small as 1 mL total volume of a chemical reaction mixture, with automated sampling of nL-scale volumes directly from the reaction vessel used for analysis. Analyses to demonstrate short term (~2 h) and long term (~ 50 h) reactions were conducted using tandem on-capillary ultraviolet absorbance followed by in-line MS detection or ultraviolet absorbance detection alone, respectively. For both short term and long term reactions (10 and 250 injections, respectively), sampling approaches using syringe pumps minimized the overall sample loss to ~0.2% of the total reaction volume.

A novel method for determining relative response factors using high-performance liquid chromatography with photodiode array detector and evaporative light scattering detection

OBJECTIVES

Because the purity of the two impurities reference standard (RS) of cefathiamidine is not easy to obtain, HPLC-PDA-ELSD were used to determin the RRFs of cefathiamidine impurities.

METHODS

Peak area correction was applied to calculate RRFs to eliminate the influence of different responses caused by the difference in pH between the two mobile phases of HPLC-PDA and HPLC-PDA-ELSD. The resulting RRF values have been verified by qNMR.

CONCLUSION

The new calcution method described in this article provides a reliable research idea for determintion the RRFs by HPLC-PDA-ELSD, especially when the purity of RS is unknown and the mobile phase of HPLC-PDA and HPLC-PDA-ELSD have difference. This method can be mutually verified with qNMR to ensure the accuracy of RRFs. It is also promingsing replace determination RRF by qNMR becausing economical, simple and low cost.

Reducing relative response factor variation using a multidetector approach for extractables and leachables (E&L) analysis to mitigate the need for uncertainty factors

Characterization of Extractables and Leachables (E&Ls) is an important aspect of product quality in important fields such as pharmaceuticals, medical devices and food contact materials. The main goal of an E&L study is identification and quantification of those species which may leach from packaging materials used to contain pharmaceuticals or which may leach directly out of a medical device or food contact material and thus may result in patient exposure. It is common practice to perform relative quantitation of extractables and leachables using surrogate standards due to the large diversity of species observed and the lack of available reference standards. A key problem in obtaining accurate E&L results arises due to response factor (RF) variation. Different compounds at the same concentration give different signal intensities and thus have different RF values. Two key aspects of study quality are affected by this problem. First, the evaluation of the number of compounds which are above the toxicologically relevant threshold (analytical evaluation threshold, (AET)) can be affected (RF Problem 1: AET Underreporting). Second, quantitative accuracy is affected which can reduce the reliability of the margin of safety (MOS) calculations which serves as the basis of the toxicological evaluation (RF Problem 2: Quantitative Error). RF databases have been the main solution proposed for solving these problems but do not reduce the underlying RF variation and lack the scope required to address quantitative error for compounds not contained in the database. In the absence of other solutions, large uncertainty factors (UF) have been applied in the AET calculations to account for RF Problem 1: AET Underreporting. These UF factors have been assigned values of 4 for GCMS and up to 10 for LCMS. Large uncertainty factors have a number of unintended negative consequences including the need for large amounts of sample concentration (>10X) prior to analysis resulting in potential compound loss or degradation and increased matrix effects. To overcome these problems, this publication demonstrates a multidetector approach using an HPLC system coupled with a Quadrupole Time of Flight Liquid Chromatography Mass Spectrometer (QTOF-LCMS), Charged Aerosol Detector (CAD) and an Ultraviolet-Visible Detector (UV) and a dual detection Gas Chromatography Mass Spectrometry (GCMS) system using a Polyarc Reactor system with Flame Ionization Detection (FID). Herein, it is demonstrated that this combination of methods (the multidetector approach) allowed detection and accurate surrogate standard quantitation of 217 unique extractables spanning a wide range of chemical properties (Mw, logP, pKa and boiling point). The combination of optimized detector selection with appropriate standard selection was verified to provide positive detection for 94% of the compounds at the AET level and a high level of quantitative accuracy (± 20% for 85% of the compounds and ±40% for 91% of the compounds) while using only a UF of 2. Unlike the RF database approach, the multidetector approach is not limited to only those compounds contained in the database but is applicable to the majority of extractables.

Analysis of unstable degradation impurities of a benzodiazepine and their quantification without isolation using multiple linear regression

This manuscript presents a novel methodology for calculating the relative response factors (RRFs) of unstable degradation impurities of molibresib (1). The degradation impurities were observed by HPLC during stress testing and were accompanied by large mass balance deficits. However, the impurities could not be isolated for traditional RRF determination due to their instability. The RRFs of two degradation impurities were determined without isolation by multiple linear regression analysis of HPLC-UV data. The results permitted accurate quantification of the degradants. The benefits and drawbacks of the approach are discussed, including suggested validation acceptance criteria.

Seeking universal detectors for analytical characterizations

It is highly desirable to have a universal detector that can detect all types of compounds and give a uniform response regardless of the physiochemical properties of the compounds. With such a universal detector, all components in a sample can be accurately quantified without the need for individual standards. This is especially needed for the characterization of unknowns and for non-targeted analysis, or for samples that have no isolated standards available for each component. Over the years, much effort has been put into seeking a universal detection technology. In this review, we discuss the commonly used detectors for analytical characterization, including UV, RI, ELSD, CAD, CLND, FID, VUV, MS, NMR, and hyphenated detection, with the focuses on the "universal" features of these detectors regarding the types of molecules they can detect and the uniformity of responses.

Determination of Relative Response Factors of Cefazolin Impurities by Quantitative NMR

The relative response factors (RRFs) of ten cefazolin impurities were determined by quantitative nuclear magnetic resonance (qNMR) and high-performance liquid chromatography (HPLC) equipped with an ultraviolet (UV) detector. The purities of these ten cefazolin impurities were successfully measured by qNMR for the purpose of RRFs determination by HPLC. The RRF values and their uncertainties determined by the two approaches are comparable. While the qNMR approach is effective and makes it easier to determine the RRFs for impurities, it also has the advantage of allowing the universal detection of protons without the limitations of common mass detectors. The use of qNMR provides a reliable and universal method for the RRF determination of impurities.

Stability study on an anti-cancer drug 4-(3,5-bis(2-chlorobenzylidene)-4-oxo-piperidine-1-yl)-4-oxo-2-butenoic acid (CLEFMA) using a stability-indicating HPLC method

CLEFMA, 4-(3,5-bis(2-chlorobenzylidene)-4-oxo-piperidine-1-yl)-4-oxo-2-butenoic acid, is a new chemical entity with anti-cancer and anti-inflammatory activities. Here, we report its stability in solution against stress conditions of exposure to acid/base, light, oxidant, high temperature, and plasma. The identity of the degradation products was ascertained by mass and proton nuclear magnetic resonance spectroscopy. To facilitate this study, we developed and validated a reverse phase high performance liquid chromatography method for detection of CLEFMA and its degradation. The method was linear over a range of 1–100 µg/mL; the accuracy and precision were within acceptable limits; it was stability-indicating as it successfully separated cis-/trans-isomers of CLEFMA as well as its degradation product. The major degradation product was produced from amide hydrolysis at maleic acid functionality caused by an acidic buffer, oxidant (3% hydrogen peroxide), or temperature stress (40–60 °C). The log k-pH profile showed that CLEFMA was most stable at neutral pH. In accelerated stability study we found that the shelf-life (T_90%) of CLEFMA at 25 °C and 4 °C was 45 days and 220 days, respectively. Upon exposure to UV-light (365 nm), the normally prevalent trans-CLEFMA attained cis-configuration. This isomerization also involved the maleic acid moiety. CLEFMA was stable in plasma from which it could be efficiently extracted by an acetonitrile precipitation method. These results indicate that CLEFMA is sensitive to hydrolytic cleavage at its maleic acid moiety, and it is recommended that its samples should be stored under refrigerated and light-free conditions, and under inert environment.

Expanding the analytical toolbox: pharmaceutical application of quantitative NMR

In response to the changing market pressures being applied to the pharmaceutical industry, a greater emphasis is being made to advance new drugs to market with minimal investment in early development stages. The use of quantitative NMR (q-NMR) has been shown to be a single point replacement for routine early development testing which previously combined elements of identity testing, chromatographic assay, moisture analysis, residual solvent analysis, and elemental analysis. This Feature will highlight the applications of q-NMR to early phase drug development testing and its efficient potency, solvent quantification, and relative response factor determinations.

Relative response factor determination of β-artemether degradants by a dry heat stress approach

During the stability evaluation of β-artemether containing finished drug products, a consistent and disproportional increase in the UV-peak areas of β-artemether degradation products, when compared to the peak area decline of β-artemether itself, was observed. This suggested that the response factors of the formed β-artemether degradants were significantly higher than β-artemether. Dry heat stressing of β-artemether powder, as a single compound, using different temperatures (125-150 °C), times (10-90 min) and environmental conditions (neutral, KMnO(4) and zinc), resulted in the formation of 17 degradants. The vast majority of degradants seen during the long-term and accelerated ICH stability study of the drug product, were also observed here. The obtained stress results allowed the calculation of the overall average relative response factor (RRF) of β-artemether degradants, i.e. 21.2, whereas the individual RRF values of the 9 most prominent selected degradants ranged from 4.9 to 42.4. Finally, Ames tests were performed on β-artemether as well as a representative stressed sample mixture, experimentally assessing their mutagenic properties. Both were found to be negative, suggesting no mutagenicity problems of the degradants at high concentrations. Our general approach and specific results solve the developmental quality issue of mass balance during stability studies and the related genotoxicity concerns of the key antimalarial drug β-artemether and its degradants.

Quantification of Drugs in Plasma Without Primary Reference Standards by Liquid Chromatography-Chemiluminescence Nitrogen Detection: Application to Tramadol Metabolite Ratios

Lack of availability of reference standards for drug metabolites, newly released drugs, and illicit drugs hinders the analysis of these substances in biologic samples. To counter this problem, an approach is presented here for quantitative drug analysis in plasma without primary reference standards by liquid chromatography-chemiluminescence nitrogen detection (LC-CLND). To demonstrate the feasibility of the method, metabolic ratios of the opioid drug tramadol were determined in the setting of a pharmacogenetic study. Four volunteers were given a single 100-mg oral dose of tramadol, and a blood sample was collected from each subject 1 hour later. Tramadol, O-desmethyltramadol, and nortramadol were determined in plasma by LC-CLND without reference standards and by a gas chromatography-mass spectrometry reference method. In contrast to previous CLND studies lacking an extraction step, a liquid-liquid extraction system was created for 5-mL plasma samples using n-butyl chloride-isopropyl alcohol (98 + 2) at pH 10. Extraction recovery estimation was based on model compounds chosen according to their similar physicochemical characteristics (retention time, pKa, logD). Instrument calibration was performed with a single secondary standard (caffeine) using the equimolar response of the detector to nitrogen. The mean differences between the results of the LC-CLND and gas chromatography-mass spectrometry methods for tramadol, O-desmethyltramadol, and nortramadol were 8%, 32%, and 19%, respectively. The sensitivity of LC-CLND was sufficient for therapeutic concentrations of tramadol and metabolites. A good correlation was obtained between genotype, expressed by the number of functional genes, and the plasma metabolite ratios. This experiment suggests that a recovery-corrected LC-CLND analysis produces sufficiently accurate results to be useful in a clinical context, particularly in instances in which reference standards are not readily accessible.

Structure-dependent response of a chemiluminescence nitrogen detector for organic compounds with adjacent nitrogen atoms connected by a single bond

High-throughput screening (HTS) of chemical libraries is indispensable for drug discovery research. However, the HTS data quality for lead discovery, lead optimization, and quantitative structure activity relationship studies has been severely compromised due to the uncertain compound concentrations in screening plates. In order to address this issue, we compared various high-throughput technologies for quantification of compounds in microtiter plate format without the need for authentic compounds as standards and identified the chemiluminescence nitrogen detector (CLND) as the method of choice at the present time. However, the structure dependence of this detector has not been well studied. A proposed rule suggested that the only exception to equimolar response is for compounds that contain adjacent nitrogen atoms. The response should be zero when the adjacent nitrogen atoms are connected by a double bond and 0.5 when they are connected by a single bond. In this investigation, we studied a broad range of compounds with isolated and adjacent nitrogen atoms. We confirmed that compounds with isolated nitrogen atoms produce an equimolar response with a 15-20% variation depending on structures and compounds with adjacent nitrogen atoms connected by a double bond giving nearly zero response. We discovered that the CLND response for compounds containing adjacent nitrogen atoms that are connected with a single bond is highly structure dependent. Substitutions on the nitrogen atoms or nearby in the molecule can increase the CLND response to approach a value higher than the predicted value 0.5 (maximal value 0.82/nitrogen atom). Without substitution, much lower values than predicted (minimal value 0.0-0.08/nitrogen atom) are obtained. Therefore, the prediction of response of 0.5/nitrogen atom for compounds with adjacent nitrogen atoms connected by a single bond should be abandoned. Compounds with similar structures should be used to generate calibration curves for quantification of this class of compounds.

Quantitative impurity profiling by principal component analysis of high-performance liquid chromatography–diode array detection data

Related organic impurities generally have approximately similar molar absorption coefficients (epsilon) due to their structural similarities. On the assumption that all peaks in an impurity profiling chromatogram have approximately the same maximum molar absorption coefficients (epsilon(max)) and the chromatogram contains one major peak and several much smaller ones, all of which are completely separated, integration of the summed score vectors from the principal component analysis (PCA) decomposition of high-performance liquid chromatography-diode array detection (HPLC-DAD) data will give areas that are quantitatively proportional to the actual content of the compounds. Due to the sequential nature of PCA, the first principal component (PC) will primarily be related to the main compound and all peaks showing a similar spectrum, while the second PC will be related to the impurities with a spectrum different from the main peak. Summing the two score vectors thus makes it possible to take account of different spectra in the score chromatogram, which make the method proposed give better quantitative estimates of the impurities than any single wavelength chromatogram. Multivariate curve resolution alternating least squares (MCR-ALS) is used for comparison. The results are presented for two examples of simulated HPLC-DAD data as well as for three examples of real HPLC-DAD data from impurity profiling. The results show that integration of the score chromatograms can handle differences in the unknown epsilon(max) of the peaks and take account of the different spectra of the impurity peaks, giving quantitative estimates of the content of the impurities that closely correspond to the reference values. The results obtained are also better than integration with the best possible separate wavelength. The method could be a straightforward approach to impurity profiling in order to obtain a good estimate of the content or relative response factors of small chromatographic impurity peaks without knowledge of their molar absorption coefficients and without any precalibration.

Photostability studies for micellar liquid chromatographic determination of nifedipine in serum and urine samples

Nifedipine is a photosensitive compound that is converted into its 4-(2-nitrophenyl) pyridine and 4-(2-nitrosophenyl) pyridine homologue. In order to obtain the most adequate conditions for handling nifedipine solutions in the analytical laboratory, a number of studies on the decomposition of this compound were performed. A simple micellar liquid chromatographic procedure was described to determine nifedipine in different biological matrices such as serum and urine, and to control its decomposition. To perform the analysis, nifedipine was dissolved in 0.1 m SDS at pH 3 and chromatographed using a mobile phase containing 0.125 m SDS-3% pentanol, pH 3 on a C18 column and UV detection at 235 nm. The chromatographic analysis time was 8 min. The response of the drug for both biological matrices was linear in the 1-100 microg/mL range, with r2>0.997 at all times. Repeatability, intermediate precision (CV, %) and limits of quantification and detection (ng/mL) were 0.19, 4.3, 104 and 31 in serum and 0.81, 2.1, 136 and 41 in urine. The method developed here does not show interferences or matrix effects produced by endogenous compounds. Micellar media and mobile phases have the advantage of stabilising the compounds, thus preventing photodegradation and allowing the direct injection of biological samples.

Toward Single-Calibrant Quantification in HPLC. A Comparison of Three Detection Strategies: Evaporative Light Scattering, Chemiluminescent Nitrogen, and Proton NMR

There is an urgent need for detection technologies that enable accurate and precise quantification of solutions containing small organic molecules in a manner that is rapid, cheap, non-labor-intensive, readily automated, and without a requirement for specific analyte standards. We provide a theoretical analysis that predicts that the logarithmic nature of the working domain of the evaporative light-scattering detector (ELSD) will normally bias toward underestimation of chromatographically resolved impurities, resulting in an overestimation of analyte purity. This analysis is confirmed by experiments with flow injection analysis (FIA) and gradient reversed-phase high performance liquid chromatography (RP-HPLC). Quantification is further compromised by the dependence of response parameters on the matrix composition and hence on the retention time of the analyte. Attempts were made to ameliorate these problems by using the response surface of a single compound to calibrate throughout the HPLC gradient. A chemiluminescent nitrogen detector (CLND) was also used in a similar manner, and the performance of the two techniques were compared against those of each other and that of a reference standard technique. A protocol for this purpose was developed using proton nuclear magnetic resonance (1H NMR) and the ERETIC method to enable quantification by integrating proton signals. The double-blind comparison exercise confirmed molar nitrogen CLND response to be sufficiently stable and robust across a methanol gradient to be used with a single external nitrogenous calibrant to quantify nitrogen-containing compounds of known molecular formula. The performance of HPLC-CLND was very similar to that of NMR, while that of HPLC-ELSD was seen to be significantly worse, showing it to be unsuitable for the purpose of single-calibrant quantification. We report details and experience of our use of RP-HPLC-CLND-MS to characterize and quantify small amounts of solutions of novel compounds at nominal levels of 10mM in microtiter plate (MTP) format.