In short, require calculation of:
1. short-term precision as the relative standard deviation percentage, RSD%, for 5 or more repeated injections of the system suitability sample, > 1%
2. column efficiency as plates, > 2000
3. capacity factor, >2
4. resolution of the analyte peak from its nearest neighbor, >2
5. tailing. < 2
Deciding on system suitability should look to chromatographers both inside and outside your system. That is, conformance of a separation to a set of parameters should ensure the quality of work done under the specified chromatographic conditions, but don’t drive yourself or your operators crazy with needless sample prep, calculation, or report. For example, the system suitability sample will have specified concentrations, you know the operators will have to measure retention time and peak width to calculate separation efficiency, and the unretained peak elution time to calculate capacity. Ask for the number of plates and capacity factor. Don’t ask to have the retention times, and peak width reported. Make reporting short and easy.
Along the line of easy, construct the concentration of the system suitability sample to give proof of the accuracy of the upcoming separations of unknowns. The concentration should depend on the type of analysis the run will experience. I.e., for an acceptance, release, stability, or impurities from degradation methods make the concentration of the analyte near the nominal value, and for trace method make the concentration about five to ten times the limit of detection. These concentrations will produce chromatograms that allow quick comparison to ones from unknowns so correct functioning of each separation shows clearly.
1. Short-term precision. The requirement for 5 or more runs serves to indicate the equipment is running correctly. (The other calculated parameters indicate more long-term changes that could interfere with accuracy.) I usually see these runs made at the beginning of a series of assays. No one wants to run overnight and find in the morning that the system was not working correctly. This practice is not enough to prove valid results for the assays. The chromatograph could fail after the precision runs. At the very least, run system suitability after the assays, better run system suitability every 10 to 15 assays.
2. Column efficiency. The number of plates decreases as the column ages. At some stage accuracy and or precision of the assay will fail. Reporting plates will help the operator monitor column health.
3. Capacity factor. If the peak is moving about the system has a big problem requiring attention to restore it to correct performance. This parameter rarely falls out of tolerance and reporting it merely shows the operator’s attention to getting good separations. Reporting the data, (retention times or retention volumes of the void volume and analyte peaks,) will aid the laboratory when changing the method slightly, e.g., new column type, or different equipment. Correct changes will preserve capacity factor but perhaps not retention times like going from a 5m packing to a 1.7m packing and suitable length changes.
4. Resolution. Resolution calculation provides the most sensitive indication of system change. Calculation of one system suitability run suffices if the peak heights of the other system suitability runs remain the same.
5. Tailing. Increase in tailing indicates a deteriorating column. A specified for the system suitability value greater than about 1.2 indicates a non optimized separation that can be expected to have problems when put to use, or a very difficult separation and desperation of the method developer. Require the operation to select the most tailing run peak for calculation. If the operator has difficulty telling which peak tails the most, it doesn’t matter which peak he chooses.
Separation of the analyte from a possible impurity shows the column retains its selectivity. Hopefully your method has a resolution of 3 or more. The usually requirement, 2, allows quantitation without problems. As the column ages the resolution usually decreases, (from peak broadening.) Give yourself some space and specify a resolution of one less than you think you can get easily, i.e., if your separation has resolution of 3, specify that the system suitability resolution must be 2 or greater. (If you are asked to write a system suitable specification for a separation with resolution less than 2, do all you can to bounce the method back to the method developers. A little more effort on method development will save a lot of headaches when the method goes into use.)
Tailing indicates a method going sour or a sour method. Tailing less than 1.5 indicates the method usually can be used without problem, more than 2 usually causes quantitation errors. Most separations of a single analyte, can have tailing less than 1.2, small, not easy to measure. Methods that separate compounds with widely differing properties like acids and bases may have tailing that must be lived with. Otherwise solvent composition and or column selection will almost always reduce tailing to a small factor. Specify 1.5 to make it easy on the operators, but train them to prepare to change to a new column when tailing starts to increase. The quantitation will be O.K. until the system suitability parameter fails out of spec, but don’t push it. The system suitability run should insure all the samples run in between two good suitability runs will be accurate.
If you wish to limit the system suitability calculations, capacity factor would be first to go. It serves mostly to allow transfer of the method without so many traumas. Secondly efficiency could go. It provides the basis for resolution. I would leave though because including the value is so traditional that its absence would elicit questions that required long explanations.
When I goggled your topic, I found lots of good suggestions for system suitability samples. I liked and older paper from the Center for Drug Evaluation and Research (CDER.) Goggling “Reviewer Guidance 'Validation of Chromatographic Methods’” should bring it up. I know the writer has good facts for what he wrote, as they support my opinions. J You make like later guidances especially if you are young, as they will be expressed in current jargon. Good chromatography, however, has not changed and to prove you have good chromatography, you will have to show compliance to acceptable system suitability criteria.
The FDA drug group may have been involved in strict requirements for testing the longest, so literature involving drug assays may help you get grounded in what system suitability samples span in requirements. Remember though when looking up FDA approved assays, the FDA doesn’t have time to peer review the methods and run them in their laboratories. As you get experience you will find many defects individually in the methods, but overall, a review a methods in use will give you a good feel for what your method will require for a system suitability sample and calculations.
Irgendwann Theorie richtig ist und Praxis ist falsch , manchmal Praxis ist richtig und Theorie ist falsch , wenn sie damit einverstanden sind sie wahrscheinlich beide falsch
