Chromatography Forum

It depends in your RSD. For example for the USP. If the RSD is lower than 2.0 you need 5 injections. If your RSD is greater than 2.0 you will need 6 injections as minimum. If you need better system suitability you could decrease your RSD and decrease your number of injections.

Why for precision studies only 6 injections should be given? & for system Suitability only 5 injections ? What is the logic behind the digit 5 and 6?

Naah not entirely correct, the table in USP and Ph Eur has another factor that is the spec limit for drug substance assay and if I recall it is clearly stated that it applies to drug substances only.(typical drug product spec limit is 95-105%). Personally I prefer to set a company policy that for drug substances we do x, drug product perhaps Y...to keep things simple.

Anything is simpliest than USP or EP but in pharmaceutical analysis we do not have choice.

It depends in your RSD. For example for the USP. If the RSD is lower than 2.0 you need 5 injections. If your RSD is greater than 2.0 you will need 6 injections as minimum. If you need better system suitability you could decrease your RSD and decrease your number of injections.

Why for precision studies only 6 injections should be given? & for system Suitability only 5 injections ? What is the logic behind the digit 5 and 6?

Naah not entirely correct, the table in USP and Ph Eur has another factor that is the spec limit for drug substance assay and if I recall it is clearly stated that it applies to drug substances only.(typical drug product spec limit is 95-105%). Personally I prefer to set a company policy that for drug substances we do x, drug product perhaps Y...to keep things simple.

Agreeing with everybody. But just wanted to point something out. It is a huge difference between 2% and 2,0%. With Ph. Eur the precent value of repatability is expressed with this equation:

Sr(%)max = (K*B*sqrt(n)) / t90%,n-1

K = constant (0.349), obtained from the expression in which represents the required percentage relative standard deviation after 6 injections for B = 1.0;
B = upper limit given in the definition of the individual monograph minus 100 per cent;
n = number of replicate injections of the reference solution (3 ≤ n ≤ 6);
t90%,n−1 = Student’s t at the 90 per cent probability level (double sided) with n−1 degrees of freedom.

So for example if you limits for assay of substance are 95-105%, so the max repatability limit is 1,83% with n=5, which rounding up to 2%. For n=6 the max rep limit is 2,12%, also rounding up to 2%

What should be the value of RSD of two replicate injection of sample

For just two values, there is no RSD. Mathematically, a standard deviation begins to exist beginning from three values.