Papers by Christopher Pohl
Journal of chromatography. A, Jan 18, 2018
We discuss the reported capacities of available packed ion exchange columns and the different met... more We discuss the reported capacities of available packed ion exchange columns and the different methods used for their measurement. We outline basic considerations related to both packed and open tubular columns based on ion exchange latex particles. There is a large body of information covering the retention behavior of packed ion exchange columns based on ion exchange latex particles. We propose a parameter γ, which is the ion exchange capacity of a column (packed or open tubular) per unit liquid volume present in the column (including accessible volume within pores) and show that the retention factor for any given ion is directly related to γ. On this basis, if based on the same type of latex, the behavior of one type of column can be reasonably predicted from the known behavior of the other, even when the absolute capacities differ by more than 5 orders of magnitude.
Talanta, 2018
Carbonate and bicarbonate based eluents have been applied for ion analysis from the inception of ... more Carbonate and bicarbonate based eluents have been applied for ion analysis from the inception of ion chromatography. The product of suppression with carbonate and/or bicarbonate eluent is carbonic acid which is weakly dissociated and tends to outgas. While the act of suppression enhanced the signal for fully dissociated ions and lowered the background to a weakly dissociated level, the overall noise performance, however, varied depending on the suppression mechanism. Chemical suppression with a membrane suppressor yielded low noise performance with carbonate and/or bicarbonate eluents. Electrolytic suppression, on the other hand, resulted in a relatively higher noise with carbonate based eluents when compared to chemical suppression. In this work, we investigated the root cause of noise with electrolytic suppressors and carbonate based eluents. Further, a new electrolytic suppressor design based on a three-electrode design is discussed in this paper and provided low noise performanc...
Analytica chimica acta, Jan 13, 2018
With an enormous growth in the application of hydrophilic interaction liquid chromatography (HILI... more With an enormous growth in the application of hydrophilic interaction liquid chromatography (HILIC), there has also been significant progress in HILIC method development. HILIC is a chromatographic method that utilises hydro-organic mobile phases with a high organic content, and a hydrophilic stationary phase. It has been applied predominantly in the determination of small polar compounds. Theoretical studies in computer-aided modelling tools, most importantly the predictive, quantitative structure retention relationship (QSRR) modelling methods, have attracted the attention of researchers and these approaches greatly assist the method development process. This review focuses on the application of computer-aided modelling tools in understanding the retention mechanism, the classification of HILIC stationary phases, prediction of retention times in HILIC systems, optimisation of chromatographic conditions, and description of the interaction effects of the chromatographic factors in H...
Journal of chromatography. A, Jan 8, 2018
Quantitative Structure-Retention Relationships (QSRR) methodology combined with the Hydrophobic S... more Quantitative Structure-Retention Relationships (QSRR) methodology combined with the Hydrophobic Subtraction Model (HSM) have been utilized to accurately predict retention times for a selection of analytes on several different reversed phase liquid chromatography (RPLC) columns. This approach is designed to facilitate early prediction of co-elution of analytes, for example in pharmaceutical drug discovery applications where it is advantageous to predict whether impurities might be co-eluted with the active drug component. The QSRR model utilized VolSurf+ descriptors and a Partial Least Squares regression combined with a Genetic Algorithm (GA-PLS) to predict the solute coefficients in the HSM. It was found that only the hydrophobicity (η'H) term in the HSM was required to give the accuracy necessary to predict potential co-elution of analytes. Global QSRR models derived from all 148 compounds in the dataset were compared to QSRR models derived using a range of local modelling tech...
Journal of Chromatography A
Retention prediction for unknown compounds based on Quantitative Structure-Retention Relationship... more Retention prediction for unknown compounds based on Quantitative Structure-Retention Relationships (QSRR) can lead to rapid "scoping" method development in chromatography by simplifying the selection of chromatographic parameters. The use of retention factor ratio (or k-ratio) as a chromatographic similarity index can be a potent method to cluster similar compounds into a training set to generate an accurate predictive QSRR model provided that its limitation - that the method is impractical for retention prediction for unknown compounds - is successfully addressed. In this work, we propose a localised QSRR modelling approach with the aim of compensating the critical limitation in the otherwise successful k-ratio filter-based QSRR modelling. The approach is to combine a k-ratio filter with both Tanimoto similarity (TS) and a ΔlogP index (i.e., logP-Dual filter). QSRR models for two retention parameters (a and b) in the linear solvent strength (LSS) model in ion chromatography (IC), logk=a - blog[eluent], were generated for larger organic cations (molecular mass up to 506) on a Thermo Fisher Scientific CS17 column. The application of the developed logP-Dual filter resulted in the production of successful QSRR models for 50 organic cations out of 87 in the dataset. The predicted a- and b-values of the models were then applied to the LSS model to predict the corresponding retention times. External validation showed that QSRR models for a-, b- and t- values with excellent accuracy and predictability (Qof 0.96, 0.95, and 0.96, RMSEP of 0.06, 0.02, and 0.38min) were created successfully, and these models can be employed to speed up the "scoping" phase of method development in IC.
Journal of Chromatography A
An analysis and comparison of the use of four commonly used error measures (mean absolute error, ... more An analysis and comparison of the use of four commonly used error measures (mean absolute error, percentage mean absolute error, root mean square error, and percentage root mean square error) for evaluating the predictive ability of quantitative structure-retention relationships (QSRR) models is reported. These error measures are used for reporting errors in the prediction of retention time of external test analytes, that is, analytes not employed during model development. The error-based validation metrics were compared using a simple descriptive statistic, the sum of squared residuals (SSR) of outliers to the edge of an error window. The comparisons demonstrate that Percentage Root Mean Squared Error of Prediction (RMSEP) provides the best estimate of the predictive ability of a QSRR model, having the lowest SSR value of 20.43.
Journal of Chromatography A
The development of quantitative structure retention relationships (QSRR) having sufficient accura... more The development of quantitative structure retention relationships (QSRR) having sufficient accuracy to support high performance liquid chromatography (HPLC) method development is still a major issue. To tackle this challenge, this study presents a novel QSRR methodology to select a training set of compounds for QSRR modelling (i.e. to filter the database to identify the most appropriate compounds for the training set). This selection is based on a dual filtering strategy which combines Tanimoto similarity (TS) searching as the primary filter and retention time (t) similarity clustering as the secondary filter, using a database of pharmaceutical compound retention times collected over a wide range of hydrophilic interaction liquid chromatography (HILIC) systems. To employ tsimilarity filtering, correlation to a molecular descriptor is used as a measure of retention time. For the retention time of a compound to be modelled a relationship between experimental chromatographic data and various molecular descriptors is calculated using a genetic algorithm-partial least squares (GA-PLS) regression. The proposed dual-filtering-based QSRR model significantly improves the retention time predictability compared to the diverse, global, and TS-based QSRR models, with an average root mean square error in prediction (RMSEP) of 11.01% over five different HILIC stationary phases. The average CPU time for implementing the proposed approach is less than 10min, which makes it quite favorable for rapid method development in HILIC. In addition, interpretation of the molecular descriptors selected by this novel approach provided some insight into the HILIC mechanism.
Journal of chromatography. A, Jan 10, 2017
A microfluidic ion-suppression module for use in ion-exchange chromatography has been developed a... more A microfluidic ion-suppression module for use in ion-exchange chromatography has been developed and evaluated. The device consists of an ion-exchange membrane clamped between two polymer chips featuring a 200×100μm (width×depth) eluent channel (l=60mm), and a 300×150μm regenerant channel (60mm), respectively. The suppression efficacy using a Nafion membrane was compared with that of a styrene-sulfonate grafted fluorinated ethylene propylene (FEP) membrane. The latter was found to outperform Nafion in terms of lowest attainable background signal (suppression efficacy) and dynamic suppression range. Increasing the suppressor temperature or the sulfuric acid regenerant concentration led to an extension of the operational suppression range, this however at the cost of an increased background signal due to enhanced diffusion, inducing sulfate bleed. Under optimized operating conditions, the microfluidic suppressor provided a dynamic capacity of 0.35μEq./min, being compatible with gradien...
Journal of chromatography. A, Jan 24, 2017
Quantitative structure-retention relationship (QSRR) models are developed to predict the retentio... more Quantitative structure-retention relationship (QSRR) models are developed to predict the retention times of analytes on five hydrophilic interaction liquid chromatography (HILIC) stationary phases (bare silica, amine, amide, diol and zwitterionic), with a view to selecting the most suitable stationary phase(s) for the separation of these analytes. The study was conducted using six β-adrenergic agonists as target analytes. Molecular descriptors were calculated based only on chemical structures optimized using density functional theory. A genetic algorithm (GA) was then used to select the most relevant molecular descriptors and these were used to build a retention model for each stationary phase using partial least squares (PLS) regression. This model was then used to predict the retention of the test set of target analytes. This process created an optimized descriptor set which enhanced the reliability of the developed QSRR models. Finally, the QSRR models developed in the work were ...
Analytical and bioanalytical chemistry, Jan 9, 2017
Characterization of glycans present on glycoproteins has become of increasing importance due to t... more Characterization of glycans present on glycoproteins has become of increasing importance due to their biological implications, such as protein folding, immunogenicity, cell-cell adhesion, clearance, receptor interactions, etc. In this study, the resolving power of high-performance anion exchange chromatography with pulsed amperometric detection (HPAE-PAD) was applied to glycan separations and coupled to mass spectrometry to characterize native glycans released from different glycoproteins. A new, rapid workflow generates glycans from 200 μg of glycoprotein supporting reliable and reproducible annotation by mass spectrometry (MS). With the relatively high flow rate of HPAE-PAD, post-column splitting diverted 60% of the flow to a novel desalter, then to the mass spectrometer. The delay between PAD and MS detectors is consistent, and salt removal after the column supports MS. HPAE resolves sialylated (charged) glycans and their linkage and positional isomers very well; separations of n...
Journal of chromatography. A, Jan 24, 2017
Quantitative Structure-Retention Relationships (QSRRs) represent a popular technique to predict t... more Quantitative Structure-Retention Relationships (QSRRs) represent a popular technique to predict the retention times of analytes, based on molecular descriptors encoding the chemical structures of the analytes. The linear solvent strength (LSS) model relating the retention factor, k to the eluent concentration (log k=a-blog [eluent]), is a well-known and accurate retention model in ion chromatography (IC). In this work, QSRRs for inorganic and small organic anions were used to predict the regression parameters a and b in the LSS model (and hence retention times) for these analytes under a wide range of eluent conditions, based solely on their chemical structures. This approach was performed on retention data of inorganic and small organic anions from the "Virtual Column" software (Thermo Fisher Scientific). These retention data were recalibrated via a "porting" methodology on three columns (AS20, AS19, and AS11HC), prior to the QSRR modeling. This provided retenti...
Journal of chromatography. A, Jan 27, 2016
Quantitative Structure-Retention Relationships (QSRR) have the potential to speed up the screenin... more Quantitative Structure-Retention Relationships (QSRR) have the potential to speed up the screening phase of chromatographic method development as the initial exploratory experiments are replaced by prediction of analyte retention based solely on the structure of the molecule. The present study offers further proof-of-concept of localized QSRR modelling, in which the retention of any given compound is predicted using only the most chromatographically similar compounds in the available dataset. To this end, each compound in the dataset was sequentially removed from the database and individually utilized as a test analyte. In this study, we propose the retention factor k as the most relevant chromatographic similarity measure and compare it with the Tanimoto index, the most popular similarity measure based on chemical structure. Prediction error was reduced by up to 8 fold when QSRR was based only on chromatographically similar compounds rather than using the entire dataset. The study ...
Lc Gc North America, Feb 1, 2004
ABSTRACT The United States Environmental Protection Agency (EPA) Office of Water has determined t... more ABSTRACT The United States Environmental Protection Agency (EPA) Office of Water has determined that the use of hydroxide eluents in EPA Methods 300.0 and 300.1 is acceptable for compliance monitoring of common inorganic anions by ion chromatography under the Clean Water Act (CWA) and the Safe Drinking Water Act (SDWA). The authors evaluated the performance of a high-capacity anion-exchange column specifically designed for use with hydroxide eluents for fulfilling the requirements of EPA Method 300.0 Part A and compared the results with data generated with an anion-exchange column using carbonate eluents. The high-capacity anion-exchange column used with an on-line electrolytically generated potassium hydroxide eluent yielded improved calibration linearity (r2 > 0.999), improved retention-time stability (< 0.05% RSD for most anions), and comparable or better method detection limits.
The Application Notebook, Sep 1, 2008
Lc Gc North America, 2009
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Papers by Christopher Pohl