Are blanks viewed as samples?

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In GMP analysis, are blank injections made between bracketing standards considered samples?

For example, a method states that duplicate injections of blank must be made before a bracketing standard and that up to 6 samples can be injected between brackets.

Does this mean I can inject 6 samples then 2 blanks and bracket, or 4 samples then 2 blanks and bracket?
6 standard injections are required by USP <621> if the %RSD acceptance criteria is greater than 2.0% as part of system suitability. Your SOP probably requires blank injections to ensure there are no interfering 'ghost' peaks from previous injections and to demonstrate the HPLC is suitable for analysis (related substances?).
I believe his question was about the definition of what a "sample" is, in regards to the specific method he's running.

CCfaron, please correct me if I am wrong but my understanding of your method says the sequence should be the following: Bracket standard injection | 6 sample injections | 2 blank injections | Bracket Standard injection.

The question is whether or not these 2 blank injections count towards the 6 sample injections.... in other words should you inject 4 sample preparations with 2 blanks in between brackets, or can you inject 6 samples and 2 blanks between brackets.

I would not consider a blank preparation to be the same as a sample preparation, it does not contain the active ingredients and as HPLC chemist stated, is most likely there to ensure there is no carryover from the sample injections into the bracket standard injection.
I'd agree: if asked to justify my decision, I'd argue that if the SOP intended that there be only 6 injections between brackets, it would/could/should have written "injections" rather than "samples".
ccfaron wrote:
In GMP analysis, are blank injections made between bracketing standards considered samples?

Short answer - no. It means 6 samples then 2 blanks and bracket.
I'd object :lol:
Step back for a moment and think about: What's the logic to limit the number of samples between bracketing standards? The sense is to have a specific scheme for recalibration. Think of it as the maximum time (!) which is allowed between reference injections. In this sense, of course blank injections would also count to the number of samples. Otherwise it would be perfectly allowed to have, let's say, 50 blank injections and 6 samples between your bracketing standards.

On the other hand, of course you can just take the method description literally: A sample is a sample and a blank is no sample. Just considering this method description, you would be perfectly safe to view the blanks NOT as samples.
Just another case why GMP is not the same as scientifically sound.

If I were in your situation, I'd look at the specific method: Are you confident (based on past experience) that the method (including samples and references) is stable enough to tolerate 6 samples plus blank injections between the reference injections? If yes, I'd run it like that.
as written, the method is potentially ambiguous, which is a bad thing, but I don't think it could be interpreted as permitting an enormous amount of blanks. It says you must run two blanks (I'd accept it doesn't say you mustn't run more) and you may run 6 samples. If the method or instrument were so unreliable that they couldn't manage more than 6 runs before needing recalibration, then it would have been correct to write that a calibration must be carried out after 6 injections (irrespective of what got injected), not after 6 samples.
Taking yet another step back, if you have a method or instrument that is so truly abysmal that it requires a fresh calibration curve after only 4 samples and 2 blanks, one has to wonder what purpose it could possibly be fit for.
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