Adjustment to the Chromatographic System in U.S. Pharmacopeia Monographs: Flexibility and Limitations -

Adjustment to the Chromatographic System in U.S. Pharmacopeia Monographs: Flexibility and Limitations

Oct 1, 2009
By: Ravi Ravichandran, Horacio N. Pappa
LCGC North America

The description of a chromatographic procedure in a US Pharmacopeia (USP) monograph contains necessary information that enables the procedure to be reproduced in a laboratory and typically includes system suitability requirements. System suitability is based upon the concept that "the equipment, electronics, analytical operations, and samples to be analyzed constitute an integral system that can be evaluated as such" (1), and it is used to demonstrate the system is stable and suitable for its intended use. In some instances, the system fails to meet those requirements, resulting in the need for corrective actions. The deviations can be temporary (short-term excursions caused by a particular factor or combinations of factors) or more permanent. The causes for such deviations might be assignable or might be of uncertain origin attributable to normal system variability.

Because this situation can arise, the System Suitability section of USP's General Chapter Chromatography <621> acknowledges that adjustments to operating conditions might be necessary to meet system suitability requirements and includes the maximum variation allowed for each chromatographic parameter. Users are allowed to implement these adjustments to bring the system into suitable performance without a full validation of the analytical procedure. The influence of the changes should be assessed by verification of the procedure under the adjusted chromatographic conditions. Chromatographers must demonstrate that the changes introduced will not affect the performance of the procedure adversely. If the allowed adjustments to the system are not successful and assistance from USP staff has not resolved the problem, the procedure might no longer be suitable for use with the article being tested. This conclusion could be a laboratory-specific issue (for example, a bad column) or it could be serious enough to warrant revision of the existing monograph. This article discusses the flexibility and limitations of the System Suitability section in the current version of <621> and provides some specific examples to help analysts understand how to take advantage of the flexibility that is allowed in this chapter.

Adjustments to the Chromatographic System

As mentioned, <621> allows a number of adjustments to the chromatographic system in response to issues with a procedure's system suitability. These are the maximum allowable variations unless the individual monograph directs otherwise.

Ratio of components in mobile phase: Although the replacement of any solvent in the mobile phase constitutes a change and not an adjustment, the amounts of the minor components (specified as 50% or less) in the mobile phase can be adjusted up to ±30% relative to the particular component. However, the change in any component cannot exceed ±10% absolute (that is, in relation to the total mobile phase). Adjustments can be made to one minor component in a ternary mixture.

pH of mobile phase: The pH of the aqueous buffer used in the preparation of the mobile phase can be adjusted to within ±0.2 units of the value or range specified.

Column temperature: The column temperature can be adjusted by as much as ±10 °C.

Concentration of salts in buffer: The concentration of the salts used in the preparation of the aqueous buffer for the mobile phase can be adjusted up to ±10% relative to the particular component, provided the permitted pH variation (see above) is met.

Wavelength of UV–vis detector: Deviations from the wavelengths specified in the method are not permitted.

Stationary phase:

Column length: The column length can be adjusted by as much as ±70%.
Column inner diameter: The column inner diameter can be adjusted, provided that the linear velocity is kept constant (see Flow Rate).

Flow rate: When column dimensions have been modified, the flow rate can be adjusted using the following formula:

where

F₁ is the flow rate indicated in the monograph, in milliliters per minute;

F₂ is the adjusted flow rate, in milliliters per minute;

l₁ is the length of the column indicated in the monograph;

l₂ is the length of the column used;

d₁ is the column inner diameter indicated in the monograph; and

d₂ is the internal diameter of the column used.

Additionally, the flow rate can be adjusted by ±50% (2).

Injection volume: The injection volume can be reduced as far as is consistent with accepted precision and detection limits, but no increase is permitted.

Particle size: Particle size can be reduced by as much as 50% but cannot be increased.

The degree of verification needed to implement these adjustments varies and depends upon the purpose of the procedure, the extent of the changes, and the performance characteristics that could be affected by the changes — see USP General Information Chapter Verification of Compendial Procedures <1226> (3). Meeting system suitability requirements might not be sufficient to demonstrate that the procedure has been verified successfully. Especially in older monographs, the requirements for the system suitability test might be weak, and other experiments might be needed to demonstrate acceptable system performance.

The following examples demonstrate the flexibility and limitations offered by the current version of <621>.

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