Effective HPLC method development
Optimization of HPLC method development has been discussed extensively in many standard
textbooks. However, most of the discussions have been focused on the optimization of HPLC
conditions. This article will look at this topic from other perspectives. All critical steps in
method development will be summarized and a sequence of events that are required to develop the
method efficiently will be proposed. The steps will be discussed in the same order as they would
be investigated during the method development process. The rationale will be illustrated by
focusing on developing a stability-indicating HPLC-UV method for related substances
(impurities). The principles, however, will be applicable to most other HPLC methods.
In order to have an efficient method development process, the following three questions must be
1.1. What are the critical components for a HPLC method?
The 3 critical components for a HPLC method are: sample preparation, HPLC analysis
and standardization (calculations). During the preliminary method development stage,
all individual components should be investigated before the final method optimization.
This gives the scientist a chance to critically evaluate the method performance in each
component and streamline the final method optimization.
1.2. What should be the percentage of time spent on different steps of the method development?
The rest of the article will discuss the recommended sequence of events, and the
percentage of time that should be spent on each step in order to meet the method
development timeline. One common mistake is that most scientists focus too much
on the HPLC chromatographic conditions and neglect the other 2 components of
the method (i.e., sample preparation, standardization). The recommended timeline
would help scientists investigate different aspects of the method development and
allocate appropriate time in all steps.
1.3. How should a method development experiment be designed?
A properly designed method development experiment should consider the following
What sample should be used at each stage?
What should the scientists look for in these experiments?
What are the acceptance criteria?
The author will answer these questions in the following discussions.
2. Method Development Timeline
The following is a suggested method development timeline for a typical HPLC-UV
related substance method. The percentage of time spent on each stage is proposed to
ensure the scientist will allocate sufficient time to different steps. In this approach, the
three critical components for a HPLC method (sample preparation, HPLC analysis and
standardization) will first be investigated individually. Each of these steps will be
discussed in more detail in the following paragraphs.
Step 1: Define method objectives and understand the chemistry (10%)
Determine the goals for method development (e.g., what is the intended use of the method?), and
to understand the chemistry of the analytes and the drug product.
Step 2: Initial HPLC conditions (20%)
Develop preliminary HPLC conditions to achieve minimally acceptable separations. These
HPLC conditions will be used for all subsequent method development experiments.
Step 3: Sample preparation procedure (10%)
Develop a suitable sample preparation scheme for the drug product
Step 4: Standardization (10%)
Determine the appropriate standardization method and the use of relative response factors in
Step 5: Final method optimization/robustness (20%)
Identify the “weaknesses” of the method and optimize the method through experimental design.
Understand the method performance with different conditions, different instrument set ups and
Step 6: Method validation (30%)
Complete method validation according to ICH guidelines
3. Define Method Objectives
There is no absolute end to the method development process. The question is what is the
“acceptable method performance”? The acceptable method performance is determined by the
objectives set in this step. This is one of the most important considerations often overlooked by
scientists. In this section, the different end points (i.e., expectations) will be discussed in
descending order of significance.
For a related substance method, determining the “significant and relevant” related substances is
very critical. With limited experience with the drug product, a good way to determine the
significant related substances is to look at the degradation products observed during stress
testing. Significant degradation products observed during stress testing should be investigated in
the method development.
Based on the current ICH guidelines on specifications, the related substances method for active
pharmaceutical ingredients (API) should focus on both the API degradation products and
synthetic impurities, while the same method for drug products should focus only on the
degradation products. In general practice, unless there are any special toxicology concerns,
related substances below the limit of quantitation (LOQ) should not be reported and therefore
should not be investigated.
In this stage, relevant related substances should be separated into 2 groups:
3.1.1. Significant related substances: Linearity, accuracy and response factors should be
established for the significant related substances during the method validation. To limit the
workload during method development, usually 3 or less significant related substances should
be selected in a method.
3.1.2 Other related substances: These are potential degradation products that are not
significant in amount. The developed HPLC conditions only need to provide good
resolution for these related substances to show that they do not exist in significant levels.
3.2 Resolution (Rs)
A stability indicating method must resolve all significant degradation products from each other.
Typically the minimum requirement for baseline resolution is 1.5. This limit is valid only for 2
Gaussian-shape peaks of equal size. In actual method development, Rs = 2.0 should be used as a
minimum to account for day to day variability, non-ideal peak shapes and differences in peak
3.3 Limit of Quantitation (LOQ)
The desired method LOQ is related to the ICH reporting limits. If the corresponding ICH
reporting limit is 0.1%, the method LOQ should be 0.05% or less to ensure the results are
accurate up to one decimal place. However, it is of little value to develop a method with an LOQ
much below this level in standard practice because when the method is too sensitive, method
precision and accuracy are compromised.
3.4 Precision, Accuracy
Expectations for precision and accuracy should be determined on a case by case basis. For a
typical related substance method, the RSD of 6 replicates should be less than 10%. Accuracy
should be within 70 % to 130% of theory at the LOQ level.
3.5 Analysis time
A run time of about 5-10 minutes per injection is sufficient in most routine related substance
analyses. Unless the method is intended to support a high-volume assay, shortening the run time
further is not recommended as it may compromise the method performance in other aspects (e.g.,
specificity, precision and accuracy.)
3.6 Adaptability for Automation
For methods that are likely to be used in a high sample volume application, it is very important
for the method to be “automatable”. The manual sample preparation procedure should be easy to
perform. This will ensure the sample preparation can be automated in common sample
4. Understand the Chemistry
Similar to any other research project, a comprehensive literature search of the chemical and
physical properties of the analytes (and other structurally related compounds) is essential to
ensure the success of the project.
4.1 Chemical Properties
Most sample preparations involve the use of organic-aqueous and acid-base extraction
techniques. Therefore it is very helpful to understand the solubility and pKa of the analytes.
Solubility in different organic or aqueous solvents determines the best composition of the sample
solvent. pKa determines the pH in which the analyte will exist as a neutral or ionic species. This
information will facilitate an efficient sample extraction scheme and determine the optimum pH
in mobile phase to achieve good separations.
4.2 Potential Degradation Products
Subjecting the API or drug product to common stress conditions provides insight into the
stability of the analytes under different conditions. The common stress conditions include acidic
pH, basic pH, neutral pH, different temperature and humidity conditions, oxidation, reduction
and photo-degradation. These studies help to determine the significant related substances to
be used in method development, and to determine the sample solvent that gives the best sample
In addition, the structures of the analytes will indicate the potential active sites for degradation.
Knowledge from basic organic chemistry will help to predict the reactivity of the functional
groups. For example, some excipients are known to contain trace level of peroxide impurities.
If the analyte is susceptible to oxidation, these peroxide impurities could possibly produce
significant degradation products.
4.3 Sample Matrix
Physical (e.g., solubility) and chemical (e.g., UV activity, stability, pH effect) properties of the
sample matrix will help to design an appropriate sample preparation scheme. For example,
Hydroxypropyl Methylcellulose (HPMC) is known to absorb water to form a very viscous
solution, therefore it is essential to use mostly organic solvents in sample preparation.
5. Initial Method Conditions
The objective at this stage is to quickly develop HPLC conditions for subsequent method
development experiments. A common mistake is that scientists spend too much time at this
stage trying to get a perfect separation.
5.1 Preliminary HPLC Conditions
In order to develop preliminary HPLC conditions in a timely fashion, scientists should use
artificial mixtures of active pharmaceutical ingredients and related substances at relatively high
concentrations (e.g., 1-2% of related substance relative to API) to develop the preliminary HPLC
conditions. The concentration ratio between API and the related substances should be maintained
to ensure the chromatography represents that of a real sample. Alternatively, a highly stressed
sample (e.g., 5% degradation) can also be used at this stage. With the known composition and
high levels of degradation products in the sample, one can evaluate the chromatography to
determine whether there are adequate separations for all analytes. The high concentrations of
related substances are used to ensure all peaks will be detected.
Computer assisted method development can be very helpful in developing the preliminary HPLC
conditions quickly. Since the objective at this stage is to quickly develop HPLC conditions for
subsequent method development experiments, scientists should focus on the separation of the
significant related substances (section 3.1.1) instead of trying to achieve good resolution for all
related substances. These significant related substances should be baseline resolved from each
other with Rs > 2.0. After the preliminary method development, the HPLC conditions can be
further fine-tuned at a later stage (see section 8, method optimization/ robustness) to achieve the
required specificity for the other related substances.
5.2 Aged HPLC Column
An aged HPLC column should be used to develop the initial HPLC conditions. Usually it is
more difficult to achieve the required resolution with an aged column (e.g., column with about
200 injections). This will reflect the worst case scenario likely to be encountered in actual
method uses, and help the long-term method robustness.
In general, develop all methods with HPLC columns from the same vendor. The preferred brand
of HPLC column should be selected primarily based on the long term stability and lot to lot
6. Sample Preparation
6.1 Selection of Sample Solvent
This stage focuses on the selection of the sample solvent (for extraction) and the proper sample
preparation procedures. Investigate the effect of sample solvents of different % organic, pH,
extraction volume and extraction procedure on accuracy, precision, sensitivity (LOQ) and the
changes in the chromatography (e.g., peak shape, resolution). Whenever possible use the mobile
phase in the sample preparation. This will ensure that there will not be any compatibility issues
between the sample solution and the HPLC conditions.
6.1.1 Accuracy: To investigate the accuracy in sample preparation (i.e., extraction efficiency),
prepare a spiked solution by adding known amounts of related substances into a sample
matrix. Compare responses of the spike solutions and the neat standard solutions to assess the
recovery from the sample preparation. In this stage, since only one particular step is being
investigated (i.e., sample preparation), close to theoretical recovery should be observed at this
point (e.g., 90-110%).
6.1.2 Precision: Use the stressed sample to represent the worst case scenario and perform
replicate sample preparations from the same sample composite. Investigate the consistency of
the related substance profile (i.e., any missing peaks?) and the repeatability results from these
6.2 Another objective is to determine the sample concentration that gives an acceptable LOQ
(Signal to Noise ratio, S/N) in low level spike concentrations. The sample concentration should
be low enough to maintain linearity and precision, but high enough to achieve the desired LOQ.
For example, if the ICH reporting limit for this drug product is 0.1%, the LOQ of the method
should be less than 0.05% (i.e., desired LOQ, in %). By using spike sample solutions of very
diluted concentrations for the significant related substances, estimate the concentrations that give
a S/N of about 10 for the significant related substances. This estimated concentration is the
approximate LOQ concentration (i.e., estimated LOQ concentration, in mg/mL).
The following equation can be used to estimate the target sample concentration for the method:
Target sample concentration =
estimated LOQ concentration (mg/mL) x 1/desired LOQ (%) x 100%
7.1 Area % method
If the response of the active pharmaceutical ingredient is linear from LOQ to the nominal sample
concentration, use the % area approach where the related substance is reported as % area. This is
the most straightforward approach, and doesn’t require the preparation of standard solutions. It
also has the highest precision since preparation to preparation variation will not affect the results.
However, in order to ensure the concentration is linear within this range, the sample
concentration is usually limited and this will reduce the method sensitivity (i.e., increase LOQ).
In general, use this approach as long as the desired LOQ can be achieved.
7.2 External Standard method
Use the external standard method if the response of the active pharmaceutical ingredient is not
linear throughout the whole range, or the desired LOQ can not be achieved by the area %
method. The concentration of standard solution should be high enough to ensure the standard
solution can be prepared accurately and precisely on a routine basis, it should be low enough to
approximate the concentration of related substance in the sample solution. In general, the
standard concentration should correspond to about 5 % of related substances.
7.3 Wavelength Selection and Relative Response Factor
Generate the linearity plot of API and related substances at different wavelengths. At this point,
Photodiode Array Detector can be used to investigate the linearity of the active pharmaceutical
ingredient and related substances in the proposed concentration range. By comparing the
linearity slopes of the active pharmaceutical ingredient and the related substances, one can
estimate the relative response factors of the related substances at different wavelengths.
Disregard of whether Area % or External Standard approach is used, if the relative response
factors of some significant related substances are far from unity, a response factor correction
must be applied.
The optimum wavelength of detection is the wavelength that gives the highest sensitivity (lmax)
for the significant related substances and minimizes the difference in response factors between
those of the active pharmaceutical ingredient and the related substances.
After the optimum wavelength is determined, use a highly stressed sample (e.g., 5% degradation)
to verify that the selected wavelength will give the highest % related substance results.
7.4 Overall accuracy
A final check of the method performance is to determine the overall accuracy of the method.
Unlike the accuracy from sample preparation (section 6.1.1), which simply compares the
response of the analyte with and without spiking with matrix, the overall accuracy compares the
% related substances calculated from an accuracy solution with that of the theoretical value.
The accuracy solutions are the solutions spiked with known concentrations of related substances
and matrix. Since the extraction efficiency, choice of wavelength and the bias in standardization
influence the calculated related substance result, this is the best way to investigate the accuracy
of the method. Overall accuracy reflects the true accuracy of the method.
8. Method Optimization/ Robustness
After the individual components of the method are optimized, perform the final optimization of
the method to improve the accuracy, precision and LOQ. Use an experimental design approach
to determine the experimental factors that have significant impact on the method. This is very
important in determining what factors need to be investigated in the robustness testing during the
method validation (see section 9). To streamline the method optimization process, use Plackett
Burmann Design (or similar approach) to simultaneously determine the main effects of many
Some of the typical experimental factors that need to be investigated are:
HPLC conditions: % organic, pH, flow rate, temperature, wavelength, column age.
Sample preparation: % organic, pH, shaking/sonication, sample size, sample age.
Calculation/standardization: integration, wavelength, standard concentration, response
Typical responses that need to be investigated are:
Results: precision (%RSD), % related substance of significant related substances, total related
Chromatography: resolution, tailing factor, separation of all related substances (section 3.1.1 and
9. Method validation
Method validation should be treated as a “final verification” of the method performance and
should not be used as part of the method development. Some of the typical method validation
parameters should be studied thoroughly in the previous steps. In some cases, robustness can be
completed in the final method optimization before method validation. At this point, the
robustness experiments should be limited only to the most significant factors (usually less than 4
factors). In addition, unlike the final method optimization (see section 8), the experimental
factors should be varied within a narrow range to reflect normal day to day variation. During the
method validation, the purpose is to demonstrate that the method performance will not be
significantly impacted by slight variations of the method conditions.
9.2 Linearity, Accuracy, Response Factor
Linearity, accuracy and response factors should be established for the significant related
substances (section 3.1.1) during the method validation. In order to limit the workload of method
development, usually less than 3 significant related substances should be selected in a method.
Therefore, the other related substances (section 3.1.2) should not be included in these
9.3 System suitability criteria
It is advisable to run system suitability tests in these robustness experiments. During the
robustness testing of the method validation, critical method parameters such as mobile phase
composition and column temperature are varied to mimic the day-to-day variability. Therefore,
the system suitability results from these robustness experiments should reflect the expected
range. Consequently, the limits for system suitability tests can be estimated from these
All of the critical steps in method development have been summarized and prioritized. The steps
for method development are discussed in the same order as they would be investigated in the
actual method development process. These steps will ensure all critical method parameters are
optimized before the method validation.
In order to develop a HPLC method effectively, most of the effort should be spent in method
development and optimization as this will improve the final method performance. The method
validation, however, should be treated as an exercise to summarize or document the overall
method performance for its intended purpose.