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Testing Method Performance
Feb 1, 2009
By: John W. Dolan
LCGC North America

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Often, one of the first signs of trouble with a liquid chromatography (LC) method is a failure to pass the system suitability requirements for the method. In fact, early warning of potential problems is one of the primary purposes of a system suitability test. As stated in the regulatory guidance (1), "The accuracy and precision of HPLC data collected begin with a well-behaved chromatographic system. The system suitability specifications and tests are parameters that provide assistance in achieving this purpose." Unfortunately, sometimes we lose sight of the purpose of a system suitability test and worry more about checking the boxes on a form than what the test is trying to do. I often am asked what system suitability requirements are mandated by regulation. This is an interesting question, because as far as I know, there are no firm requirements as to what parameters must be measured or what the minimum values of these parameters must be. The closest thing that I can find to requirements is a document called "Reviewer Guidance: Validation of Chromatographic Methods," (1) from the Food and Drug Administration's Center for Drug Evaluation and Research (FDA-CDER). This is a document intended to give CDER reviewers of LC methods some guidelines about what to look for in a "good" method. Or as the document states (1), "The purpose of this technical review guide is to present the issues to consider when evaluating chromatographic test methods from a regulatory perspective." Many workers feel that if their methods perform at least as well as the recommendations of this document, they will be safe from regulatory criticism. This may or may not be a well-reasoned response. This month's "LC Troubleshooting" discussion will center on the recommendations of the CDER document, especially in terms of what it means from a practical method performance standpoint.

The Recommendations


Table I: Minimum system suitability recommendations1
Table I summarizes four of the key parameters that can be used to evaluate method performance. These are standard measurements that most workers make on a routine basis. Let's review the definitions of each of these, then look at some examples.




Retention factor, k. The retention factor (also sometimes called capacity factor, k') is a measure of the distribution of the analyte between the mobile phase and the stationary phase in the column in isocratic (constant organic solvent) separations. It can be thought of as a way to measure retention in a manner that is independent of column dimensions and flow rate. Retention factor is calculated as

where tR is the retention time of the analyte of interest and t0 is the column dead-time ("solvent front"). As has been discussed in other "LC Troubleshooting" columns (for example, see reference 2), when developing a new method, we strive for 2 < k < 10, but usually will accept 1 < k < 20 for all the peaks of interest in a separation. Peaks with k < 1 can be compromised with real samples that contain large "garbage" peaks at t0. Methods with k > 2 also tend to be more robust to small changes in the chromatographic conditions.




Resolution, Rs. Resolution is the measurement of the separation of two peaks in a chromatogram and is a function of both the peak widths w and retention times:


Figure 1
where the subscripts refer to the first and second peaks. The peak width is determined by drawing tangents to the sides of the peak and measuring the distance between the tangents where they intersect the baseline. Baseline separation of a perfectly shaped Gaussian peak pair (a rarity in LC) is observed with Rs = 1.5 (see peaks 1 and 2 in Figure 1a).




Tailing factor, TF. Peak tailing is measured with the tailing factor in the pharmaceutical industry, and with a slightly different calculation called the asymmetry factor As for most nonpharmaceutical applications.

where AC is the width of the peak at 5% of its height and AB is the front half-width at the same height. (As is calculated as the back half-width divided by the front half-width at 10% of the peak height.) New columns with well-behaved test compounds will generate tailing factors of 0.9–1.2, so a little tailing is normal.

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Source: LCGC North America, 2/1/2009
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