Introduction
High-performance liquid chromatography (HPLC) is a separation technique that can be used for the analysis of organic molecules and ions. HPLC is based on mechanisms of adsorption, partition, ion exchange or size exclusion, depending on the type of stationary phase used. HPLC involves a solid stationary phase, normally packed inside a stainless-steel column, and a liquid mobile phase. Separation of the components of a solution results from the difference in the relative distribution ratios of the solutes between the two phases.
HPLC can be used to assess the purity and/or determine the content of many pharmaceutical substances. It can also be used to determine enantiomeric composition, using suitably modified mobile phases or chiral stationary phases.
Individual separation mechanisms of adsorption, partition, ion exchange and size exclusion rarely occur in isolation, since several principles act to a certain degree simultaneously.
Apparatus
The apparatus consists of a pumping system, an injector, a chromatographic column, stationary and mobile phases, connecting tubing and fittings, a detector, and a data collection device (computer, integrator or recorder).
Pumping system
HPLC pumping systems are required to deliver metered amounts of mobile phase at a constant flow rate. Pumping systems that deliver solvent from one or more reservoirs are available. Pressure fluctuations should be minimized, e.g. by passing the pressurized solvent through a pulse-dampening device. Tubing and connections should be capable of withstanding the pressures developed by the pumping system. Many HPLC pumps are fitted with a facility for "bleeding" the system of entrapped air bubbles.
Modern computer- or microprocessor-controlled pumping systems are capable of accurately delivering a mobile phase of either constant (isocratic elution) or varying (gradient elution) composition, according to a defined programme. In the case of gradient elution, solvent mixing can be achieved on either the low or high-pressure side of the pump(s).
Depending on the flow rate and composition of the mobile phase, operating pressures of up to 42000kPa (6000 psi) can be generated during routine analysis.
Injector
The sample solution is usually introduced into the flowing mobile phase at or near the head of the column using an injection system based on an injection valve design which can operate at high pressure. Such an injection system has a fixed-loop or a variable volume device which can be operated manually or by an auto-sampler. Partial filling of loops may lead to poorer injection volume precision.
Chromatographic column
Columns are usually made of polished stainless steel, are between 50 and 300mm long, and have an internal diameter of between 2 and 5mm. They are commonly filled with a stationary phase with a particle size of 5 - 10μm. Columns with internal diameters of less than 2 mm are often referred to as microbore columns. Ideally, the temperature of the mobile phase and the column should be kept constant during an analysis. Most separations are performed at ambient temperature, but columns may be heated to give better efficiency. Normally, columns should not be heated above 60°C because of the potential for stationary phase degradation or changes occurring to the composition of the mobile phase.
Stationary phases
Separation of pharmaceuticals is usually achieved by partition of compounds in the test solution between the mobile and the stationary phases. HPLC systems consisting of polar stationary phases and nonpolar mobile phases are described as normal-phase chromatography; those with nonpolar stationary phases and polar mobile phases are called reversed-phase chromatography.
There are many types of stationary phases used in HPLC including:
- unmodified silica, alumina, or porous graphite, used in normal-phase chromatography, where separation is based on differences in adsorption;
- a variety of chemically modified supports prepared from polymers, silica, or porous graphite, used in reverse-phase HPLC, where separation is based principally on partition of the molecules between the mobile phase and the stationary phase;
- resins or polymers with acid or basic groups, used in ion-exchange chromatography, where separation is based on competition between the ions to be separated and those in the mobile phase;
- porous silica or polymers, used in size-exclusion chromatography, where separation is based on the relative molecular mass of the molecules.
Most separations are based on partition mechanisms using chemically modified silica as the stationary phase and polar solvents as the mobile phase (reverse-phase HPLC). The surface of the support, e.g. the silanol groups of silica, is reacted with various silane reagents to produce covalently bonded silyl derivatives covering a varying number of active sites on the surface of the support. The nature of the bonded phase is an important parameter for determining the separation properties of the chromatographic system.
Commonly used bonded phases are shown below.
Common bonded phases
Octyl Si-(CH2)7-CH3 C8
Octadecyl Si-(CH2)17-CH3 C18
Phenyl Si-(CH2)3-C6H5 C6H5
Cyanopropy Si-(CH2)3-CN CN
Aminopropyl Si-(CH2)3-NH2 NH2
Diol Si-(CH2)3-OCH(OH)-CH2-OH
For the separation of enantiomers, special chemically modified stationary phases (chiral chromatography) are available, e.g. cyclodextrins, albumins, etc.
As a guide, silica-based reverse-phase columns are generally considered to be stable in mobile phases with an apparent pH in the range 2.0 - 8.0, but the column manufacturer's instructions should be consulted before using the column. Columns containing particles of polymeric materials such as styrene divinylbenzene copolymer are stable over a wider pH range.
Analysis using normal-phase HPLC with unmodified silica, porous graphite or polar chemically modified silica (e.g. cyanopropyl or diol) as the stationary phase and a nonpolar mobile phase is still employed in certain cases.
For analytical separations the particle size of the most commonly used stationary phases varies between 3μm and 10μm. The particles may be spherical or irregular, of different porosities and specific surface area. In the case of reversed-phase, the extent of bonding of the stationary phase is expressed as the carbon-loading. Furthermore, stationary phases may be "end-capped", i.e. the number of residual silanol groups is reduced by methylation. These parameters contribute to the chromatographic behaviour of a particular stationary phase. Tailing of peaks, particularly for basic substances, can occur when residual silanol groups are present.
Mobile phases
The choice of mobile phases is based on the desired retention behaviour and the physicochemical properties of the analyte.
For normal-phase HPLC using unmodified stationary phases, lipophilic solvents should be employed. The presence of water in the mobile phase must be avoided as this will reduce the efficiency of the stationary phase. In reverse-phase HPLC aqueous mobile phases, with and without organic modifiers, are used.
The mobile phase should be filtered through suitable membrane-type filters with a porosity of 0.45μm to remove mechanical particles. Multicomponent mobile phases should be prepared by measuring the required volumes (unless masses are specified) of the individual components, followed by manual or mechanical mixing. Alternatively, the
solvents may be delivered by the individual pumps or proportioning valves of the liquid chromatograph and mixed according to the desired proportion. Solvents are normally degassed by sparging with helium or by sonification before pumping to avoid the creation of gas bubbles in the detector cell.
If an ultraviolet detector is employed, the solvents used for the preparation of the mobile phase should be free of stabilizers and transparent at the wavelength of detection. Adjustment of the pH, if necessary, should be made using the aqueous component of the mobile phase and not the mixture. Buffers of high molarity should be avoided in the preparation of mobile phases. If buffers are used, the system should be rinsed with an adequate mixture of water and the organic modifier of the mobile phase to prevent crystallization of salts.
Mobile phases may contain other components, e.g. a counter-ion for ion-pair chromatography or a chiral selector for chiral chromatography using an achiral stationary phase.
Connecting tubing and fittings
The potential efficiency of an analytical column may never be achieved because of the design limitations of injectors and detectors. The connections between injector/column, column/detector, and/or detector/detector may compromise the overall efficiency of the system and any fittings should be of the "zero dead volume" (ZDV) type. It is recommended that minimum lengths of capillary tubing with a maximum internal diameter of 0.25 mm be used for these fittings to minimize band spreading.
Detectors
Ultraviolet/visible (UV/vis) absorption spectrometers are the most commonly used detectors for pharmaceutical analysis. In specific cases, fluorescence spectrophotometers, differential refractometers, electrochemical detectors, lightscattering detectors, mass spectrometers, or other special detectors may be used. Where an analyte possesses a chromophoric group that absorbs UV/vis radiation, the UV/vis detector is the most appropriate because of its sensitivity and stability. Such a detector is not suitable for detecting analytes with very weak chromophores.
A variant on the UV/vis type of detector, which is becoming increasingly popular because of its ability to furnish detailed spectral information, is the diode array spectrophotometer. This type of detector acquires absorbance data over a certain UV/vis range and can provide chromatograms at multiple, selectable wavelengths, together with spectra for the eluted peaks. In addition, the detector and accompanying computer programmes can be used to assess the spectral homogeneity of peaks, which may provide information on the chromatographic purity of the peaks.
This can be especially useful in method development and validation.
Enhanced sensitivity may be achieved in certain cases by using pre-column or post-column derivatization techniques.
(These techniques are to be avoided for the control of impurities.)
Data collection devices
Signals from the detector may be collected on chart recorders or electronic integrators that vary in complexity and in their ability to process, store, and reprocess chromatographic data. The data storage capacity of these devices is usually limited.
Modern data stations are computer based and have a large storage capacity to collect, process, and store data for possible subsequent reprocessing. Analytical reports can often be customized to the needs of the analyst.
Integration of peak areas and the setting of threshold levels are not normally problematic in an assay since the peak of the substance to be analysed should be free of interference. However, in a test for impurities, the selection of the peak area integrator parameters becomes very important, particularly when baseline separations are not always attainable. If baseline separations cannot be obtained, valley-to-valley integration should be employed.
HPLC allows limits to be set for individual impurities and for the sum of impurities, but there is a level at which peaks should not be integrated. This "disregard level" is set in relation to the area of the peak in the chromatogram of the prescribed reference solution and is usually equivalent to 0.05% of the substance being examined.
System suitability
The system suitability test represents an integral part of the method and is used to ensure the adequate performance of the chosen chromatographic system.
Efficiency, capacity factor, resolution factor, and symmetry factor are the parameters that are normally used in assessing the column performance; these terms are defined below. Factors that can affect chromatographic behaviour include mobile phase composition, temperature, ionic strength and apparent pH, flow rate, and column length, and stationary phase characteristics such as porosity, particle size and type, specific surface area, and, in the case of reversed-phase supports, the type of chemical modification, carbon loading, and end-capping.
Efficiency (N)
The efficiency of a chromatographic column is defined in terms of the number of theoretical plates (N) and can be calculated using the following formula:
where
tR = retention time or the baseline distance between the point of injection and the perpendicular dropped from the maximum of the peak of interest.
Wh = the width of the peak of interest determined at half peak height, measured in the same units as tR.
The number of theoretical plates can be expressed per metre (N'):
where
l = length of column in metres.
Capacity factor (mass distribution ratio, Dm)
The capacity factor or mass distribution ratio is defined as follows:
This factor determines the retention of a solute and can be calculated from the chromatogram using the following
formula:
where
tR = retention time of the solute
tM = retention time of an unretained component
A low Dm value indicates that the peak elutes close to the solvent front, which may compromise selectivity. A minimum Dm value of 1 is recommended for the peak of interest.
The retention time of the test substance can be varied, if necessary, by changing the relative proportion or composition of solvents in the mobile phase. Generally, an increase in the proportion of a more polar solvent will lead to a shorter retention time on a normal-phase column and a longer retention time on a reversed-phase column.
Resolution factor (Rs)
The resolution between two peaks of similar height in a chromatogram can be calculated using the following formula:
where
tR1 and tR2 = retention times or baseline distances between the point of injection and the perpendicular dropped from the maximum of each of the two peaks.
Wb1 and Wb2 = the respective peak widths determined at half peak height, measured in the same units as tR1 and tR2.
The value of Rs for a baseline separation between peaks of similar height should be at least 1.5.
Relative retention
The relative retention (r) is calculated as an estimate using the following formula:
Where
tR2 = retention time of the peak of interest
tR1 = retention time of the reference peak
tM = retention time of an unretained component
Symmetry factor (As)
The symmetry factor for a peak can be calculated using the following formula:
Where
Wx = peak width at 5% of peak height, measured from the baseline.
D = baseline distance between the perpendicular dropped from the peak maximum and the leading edge of the peak at 5% of the peak height, measured in the same units as Wx.
Values of As which are greater than 2 may lead to incorrect integration, resulting in erroneous quantitation. The main factors that influence peak symmetry depend upon retention, solvent effects, incompatibility of the solute with the mobile phase, or development of an excessive void at the inlet of the column. In reversed-phase chromatography, adsorption phenomena due to the presence of residual silanol groups in the stationary phase may lead to tailing (poor peak symmetry).
Repeatability
Unless otherwise stated in the "Assay" of the individual monograph, the relative standard deviation of peak areas or peak heights for a series of injections of reference solutions bracketing groups of test solutions should not exceed 2.0%.
In a "Related substances" test the relative standard deviation of peak areas or peak heights for three replicate injections of the reference solution should not exceed 5.0%, unless otherwise stated in the individual monograph.
In a series of injections the relative standard deviation of the retention time of the principal peak should not exceed 1.0%.
Recommended procedure
To equilibrate the column, allow the mobile phase to flow through the chromatographic system until the baseline is stable at the flow rate specified in the individual monograph (about 30 minutes). Prepare the prescribed test and reference solutions as directed. Inject the prescribed reference solution and, if necessary, adjust the detector and/or recorder response to produce an adequate peak size. For chart recorders and integrators this should be at least 50% of the full-scale deflection of the principal peak in the chromatograms obtained with the reference solution. Ensure that the criteria for system suitability are met.
The reference solution should be injected at the start, at regular intervals during, and at the end of a series of assays (e.g. every 2 - 4 samples). Both the reference and the test solutions should be injected in duplicate.
In determining the component composition of a complex mixture, a "normalization" procedure, based on the calculation of individual peak areas as a percentage of the total area of all the peaks, may be used where the relative response factors of the individual components are similar. The response factor is relative, being the response of the equal mass of one substance relative to that of another according to the conditions described in the test. For example, when an HPLC test with UV/vis detection is used for the control of impurities, the wavelength of detection should be such that the substance and its impurities have similar responses. If an impurity has a significantly different response (more than ±20%) from that of the substance being examined, the preferred manner of limiting this impurity is to use a reference substance of the impurity. If a reference substance is not available, the response factors of the potential impurities relative to those of the substance being examined are determined during method development.
Subsequently, the derived correction factors (i.e. the reciprocals of the response factors) are applied, if necessary, as described in the individual monograph.
If gradient elution is specified in the monograph, an instrument equipped with a special pumping system capable of delivering a mobile phase of continuously varied composition is needed. The mobile phase composition changes from the initial composition within a fixed period of time, as specified in the monograph. Where the mobile phase composition is varied at a linear gradient elution, perform a blank run to identify any interfering peaks by injecting the solvent specified for preparing the test solutions. Allow sufficient time for equilibration when the mobile phase is reset to the initial composition for the next injection.
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