impurities and rrf

[martinamarie]

Hi, I have another RRF question.

I have to validate a method for an impurity. The active is generally analysed at a different wavelength to the impurity, however, it can be seen at the same wavelength as impurity, to a much lesser extent.

In order to get a reasonable LOQ for the impurity in question I have increased the on column concentration for the sample solution compared to the previous method.

The active working standard concentration is 1% of the working sample solution concentration

I have to validate this method

I need to determine RRF as part of the validation. I planned to run active and impurity from LOQ-5 % of working sample solution concentration. and divide slopes

However, I was told that you can determine RRF using linearity plot and working standard at 100 % level. , and then another analyst determines RRF using impurity solution at 0.5 %. (specification limit)
Can anyone shed light on this, as I am not familiar with this approach.

Thanks

MartinaMarie.

[tom jupille]

The obvious first question is "does your lab have an SOP for determining RRF's?" If you do not, then stop what you are doing and get one established *now*. If you expect to get consistent results, you do not want to have each analyst make up his/her own definition and procedures!

The RRF is essentially the ratio of the slopes of the calibration lines for the two compounds. The catch is that you need to be in the linear range for both compounds. Either of the alternative approaches could work (and either could fail!) depending on there you are in the linear range.

[grzesiek]

that's a great response from tom
i see you're having fun in the lab - everybody does things his own way

you probably have already established in your validation response curves for the compounds, and as tom pointed the RRF is just their ratio - nothing magical here