HPLC method development column selection

SOURCE C. S. Young and R. J. Weigand, An Efficient Approach to Column Selection in HPLC Method Development," LCGC 2002, 20, 464. 

Figure 25-29 Separation of six compounds on (a) phenyl- and (b) Cle-silica columns with 3-p.m particle size using 35 65 (vol/vol) acelonitrile/0.2% aqueous trifluoroacetic acid. Column size 7 x 53 mm flow rate = 2.5 mL/min. [From C. S. Young and R. J. Weigand, "An Efficient Approach to Column Selection In HPLC Method Development." LCGC 2002,20.464. Courtesy Alltech Associates.]...

Moreover, once a particular column or columns that have provided the best selectivity are chosen, an automated method optimization may be performed. This would include employment of an integrated HPLC method development system such as AMDS/Drylab such that the gradient slope/temperature... 

The first step in method development is selecting an adequate HPLC mode for the particular sample. This choice depends on the character of the sample compounds, which can be either neutral (hydrophilic or lipophilic) or ionic, low-molecular (up to 2000 Da) or macromolecular (biopolymers or synthetic polymers). Many neutral compounds can be separated either by reversed-phase or by normal-phase chromatography, but a reversed-phase system without ionic additives to the aqueous-organic mobile phase is usually the best first choice. Strongly lipophilic samples often can be separated either by non-aqueous reversed-pha.se chromatography or by normal-phase chromatography. Positional isomers are usually better separated by normal-phase than by reversed-phase chromatography and the separation of optical isomers (enantiomers) requires either special chiral columns or addition of a chiral selector to the mobile phase. 

This chapter provides an overview of modern HPLC method development and discusses approaches for initial method development (column, detector, and mobile phase selection), method optimization to improve resolution, and emerging method development trends. The focus is on reversed-phase methods for quantitative analysis of small organic molecules since RPLC accounts for 60-80% of these applications. Several case studies on pharmaceutical impurity testing are presented to illustrate the method development process. For a detailed treatment of this subject and examples of other sample types, the reader is referred to the classic book on general HPLC method development by L. Snyder et al.1 and book chapters2,3 on pharmaceutical method development by H. Rasmussen et al. Other resources include computer-based training4 and training courses.5... 

The selectivity of reversed-phase liquid chromatography (RP-LC) columns is known to vary, even columns with the same ligand (e.g., Cjg). Column selectivity can also vary from batch to batch for columns claimed to be equivalent by the manufacturer. For different reasons, it is sometimes necessary to locate a replacement column for a given assay that will provide the same separation as the previous column. In other cases, as in HPLC method development, a column of very different selectivity may be needed - in order to separate peaks that overlap on the original column. For each of these situations, means for measuring and comparing column selectivity are required. Until recently, no such characterization of column selectivity was able to guarantee that two different columns can provide equivalent separation for any sample or separation conditions. 

Computer simulation refers to the use of a computer for predictions of separation as a function of experimental conditions. In most cases, computer simulation requires two or more experimental runs with a given sample, in order to calibrate the computer prior to making predictions. Computer simulation can be used in H PLC method development to select optimum final conditions for the separation of a given sample, so that fewer actual experiments are required and better HPLC methods result Ideally, computer simulation should (a) allow for the simultaneous variation of as many as two experimental conditions that affect separation selectivity, (b) be applicable for both isocratic and gradient elution, and (c) be able to predict the further effect on separation of changes in flow rate, column dimensions and particle size. 

The software has been equipped with a fimction such that it is not only able to conduct experiments with one column/organic modifier/buffer combination, but to automatically optimize a method by trying different column/organic modifier/ buffer combinations. The system provides imattended HPLC method development and performs autonomous development and optimization of isocratic and gradient methods for selection of the best variant column, pH value, solvent Two typical hardware configurations and other mixed combinations are supported by ChromSword standard and powerful. 

In the development of a SE-HPLC method the variables that may be manipulated and optimized are the column (matrix type, particle and pore size, and physical dimension), buffer system (type and ionic strength), pH, and solubility additives (e.g., organic solvents, detergents). Once a column and mobile phase system have been selected the system parameters of protein load (amount of material and volume) and flow rate should also be optimized. A beneficial approach to the development of a SE-HPLC method is to optimize the multiple variables by the use of statistical experimental design. Also, information about the physical and chemical properties such as pH or ionic strength, solubility, and especially conditions that promote aggregation can be applied to the development of a SE-HPLC assay. Typical problems encountered during the development of a SE-HPLC assay are protein insolubility and column stationary phase... 

The increased use of IV-methyl carbamate insecticides in agriculture demands the development of selective and sensitive analytical procedures to determine trace level residues of these compounds in crops and other food products. HPLC is the technique most widely used to circumvent heat sensitivity of these pesticides. However, HPLC with UV detection lacks the selectivity and sensitivity needed for their analysis. In the late 1970s and early 1980s, HPLC using post-column hydrolysis and derivatization was developed and refined with fluorescence detection to overcome these problems. The technique relies on the post-column hydrolysis of the carbamate moiety to methylamine with subsequent derivatization to a fluorescent isoindole product. This technique is currently the most widely used HPLC method for the determination of carbamates in water" and in fruits and vegetables." " ... 

Assessing the resources available for method development should also be done before beginning a project. The resources available include not only HPLCs, detectors, and columns, but also tools for sample preparation, data capture and analysis software, trained analysts, and especially samples representative of the ultimate analyte matrix. Also, it should be considered whether a fast, secondary method of analysis can be used to optimize sample preparation steps. Often, a simple colorimetric or fluorimetric assay, without separation, can be used for this purpose. A preliminary estimate of the required assay throughput will help to guide selection of methods. 

The same group reported in 1986 a sensitive and selective HPLC method employing CL detection utilizing immobilized enzymes for simultaneous determination of acetylcholine and choline [187], Both compounds were separated on a reversed-phase column, passed through an immobilized enzyme column (acetylcholine esterase and choline oxidase), and converted to hydrogen peroxide, which was subsequently detected by the PO-CL reaction. In this period, other advances in this area were carried out such as the combination of solid-state PO CL detection and postcolumn chemical reaction systems in LC [188] or the development of a new low-dispersion system for narrow-bore LC [189],... 

A simple and rapid RP-HPLC method was developed for the determination of retinoid in galenicals. Commercial preparations were diluted, filered and used for separation. Measurements were carried out in an ODS column (150 X 4.6 mm i.d. particle size 3 /xm). Solvents A and B were methanol-10 mM ammonium acetate (75 25, v/v) and methanol-THF (84 16, v/v), respectively. The flow rate was 0.8ml/min. Gradient conditions were 0-25 min, 0 per cent B 35 min, 100 per cent B, isocratic for 10 min. Typical chromatograms are shown in Fig. 2.37. The repeatability of peak area ranged between 0.48 -3.2 per cent for UV-DAD and 0.57 - 3.1 per cent for fluorescence detection. The reproducibility varied between 0.26 - 4.6 per cent. It was found that the method is precise, selective, sensitive and linear, therefore, it can be employed for the routine quality control of this class of drags [85],... 

The primary object of this book is to provide the HPLC practitioner with a handy guide to the use of HPLC for analyzing pharmaceutical compounds of interest. This means familiarizing the practitioner with the theory, instrumentation, regulations, and numerous applications of HPLC. This handbook provides practical guidelines using case studies on sample preparation, column or instrument selection, and summaries of best practices in method development and validation, as well as tricks... 

To apply a screening approach to proactive method development, analyses of selectivity samples under a variety of mobile phase conditions are conducted on different HPLC columns. HPLC columns should be as orthogonaT as possible and variations in solvent composition should be designed to maximize the probability of selectivity differences. Alternate separation techniques, such as ion exchange chromatography (IC), supercritical fluid chromatography (SFC), or capillary electrophoresis (CE) may also be used to obtain orthogonality. 

HPLC methods can usually be transferred without many modifications, since most commercially available HPLC instruments behave similarly. This is certainly true when the columns applied have a similar selectivity. One adaptation, sometimes needed, concerns the gradient profiles, because of different instrumental or pump dead-volumes. However, larger differences exist between CE instruments, e.g., in hydrodynamic injection procedures, in minimum capillary lengths, in capillary distances to the detector, in cooling mechanisms, and in the injected sample volumes. This makes CE method transfers more difficult. Since robustness tests are performed to avoid transfer problems, these tests seem even more important for CE method validation, than for HPLC method validation. However, in the literature, a robustness test only rarely is included in the validation process of a CE method, and usually only linearity, precision, accuracy, specificity, range, and/or limits of detection and quantification are evaluated. Robustness tests are described in references 20 and 59-92. Given the instrumental transfer problems for CE methods, a robustness test guaranteeing to some extent a successful transfer should include besides the instrument on which the method was developed at least one alternative instrument. 

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