Solution stability

[mojo]

My company has traditionally measured standard solution stability by storing a portion of standard then analysing it the next day against a fershly prepared standard. I don't think this approach is correct as the freshly prepared standard may be at 98-102% of t's theoretical concentrartion. I would prefer to measure the response factor on day 1, then again on day 2; and not determine recovery using new stds. If this was done on the same system, do you think my alternative approach would be acceptable?

[danko]

Hi Mojo,
The response might change as well (most probably it will). So I don’t think your approach is valid either.
You should take advantage of some statistics. The simplest way of doing it is to make double/triple/multiple determinations (i.e. multiple preparations and injection) in order for you to find the variation due to preparation procedure, as well as injection volume in- accuracy and precision. All this should be done over several days or weeks or whatever time span you are interested in.
When you’ve collected the data, you’ll be able to find the error value which should be taken into account when evaluating the stability of your sample/standard solution.
Tip: As an extra precaution, you should bear in mind that compound instability should result in decreasing content/strength/potency or whatever you call it. I.e. you should be able to observe decreasing or no (in case of stability) trend over time. If you see increasing trend, you’re most probably having an evaporation problem.

Best Regards

[JGK]

My company has traditionally measured standard solution stability by storing a portion of standard then analysing it the next day against a fershly prepared standard. I don't think this approach is correct as the freshly prepared standard may be at 98-102% of t's theoretical concentrartion. I would prefer to measure the response factor on day 1, then again on day 2; and not determine recovery using new stds. If this was done on the same system, do you think my alternative approach would be acceptable?

We also use comparison of stored v fresh as our primary method for long term storagetesting (> 24 hour). For < 24 hour we use comparison vs time zero data. the reason we dont use it for longer periods is the risk of incorporating the daily instrument variations into the calculations.

What level of deviation are you willing to accept in the STDs and how is it determined.

On a personal Note: we have an acceptance criteria for reproducibility of STDs in a sequence CV <= 2% (consecutive replicate injections) and across a sequence (CV <= 3%) and usually set stabilty as additive of these two criteria so if the RF ratio is 95 - 105% the STD is stable.