[student1010]

Hi everyone!

Here is the problem:

I have a set of sample of different (known) concentration 20ug/ml to 100ug/ml, in multiples of 20 and also a unknown solution (diluted 10X, 50X, 100X).

I plotted the standard curve using the absorbance values recorded from the known sample and used it as the basis to find the concentration of my unknown.

Should the values of the blank (DI water) i.e. coordinates (0,0) be added in the standard curve?

I noted that the absorbance for the unknown diluted 100X is drastically different from those in the other less diluted unknown samples and this absorbance value is even lesser than that of the lowest absorbance value of my known sample standards. i.e. the absorbance value is in between that of the blanks and the first standard.

Does this count as an extrapolation since the blank isn't exactly a recorded value?

[Arkcon]

Yes.  If you use this result that is outside the points of the standard curve, then you are extrapolating.

Now, using the blank as zero is controversial.  Some rare instances its done, but from a chemical point of view and a statistical point of view, and under basic quantitative chemistry concepts, more could be happening, in a blank, than you've accounted for in your curve.  Its kinda early in the morning for me to get a reference for you on the subject, but I'll fix that later.  But basically, when you go way lower, the solutions behave non-ideally.  So in my experience, you won't get good results.

[student1010]

Dear Arkcon

Thank you for taking time to reply to my question.

I was under the impression that the blank @ (0,0) should be included as part of the standard curve just like the rest of the absorbance results from the standards. If that is the case, I wouldn't consider the curve between my first standard and the blank as an extrapolation.

Now that I understand the controversy of using the blank as (0,0), I have my reservations about using the blank as a value on the graph. As such, the curve between my first standard and 0 would be an extrapolation and results from that area would then be in the "grey zone", which explains the deviation.

Once again, thank you for your reply! It was great *delete me*