Hello Uli, thank you for sharing your analytical question with the RG community. Before we theorise about possible physico-chemical effects, the obvious things should be ruled out first. Therefore, I would like to ask you the following questions:

Have you ruled out the possibility that a mistake was made in the preparation of the 50 ppb solution by preparing and measuring this solution again?

Did you prepare the 50 ppb solution by mixing equal volumes of eluent and 100 ppb solution? You write that you diluted the solution two times. Sometimes people make mistakes in thinking and dilute "1+2" (v/v) instead of 1:2. The peak area you determined for this solution surprisingly fits perfectly to a solution with 33 ppb (R² would be 0.998).

As suggested by Florian, first you should just rule out the possibility that the 50 ppb solution was actually a 33 ppb solution.

Dear Florian Morsbach and Luca Narduzzi

thank you so much for your advice and tips! this is my first time asking a question and I honestly did not expect to be answered very promptly

I see... I may make mistakes along the way while diluting the solution with the eluent. However, I already calculated the concentration from weighting, I took 5 grams of 100 ppb and diluted it until 10 grams (the total is now 15 g) to make 50 ppb. Is this way correct?

please do correct me if i am wrong

dear Florian Morsbach

now i see it! yes, it indeed becomes 33 ppb instead of 50 ppb! I understand after re-reading your answer

Thank you so much, kind sir! I can now correct my calculation from now on! so glad it has nothing to do with the ICP or HPLC !

Dear Uli Benyamin Sitompul

in this case I think that Florian is right. To have a concentration of 50 ppb you should have a total of 10 g.

the formula is Ci x Vi = Cf x Vf

Ci = initial concentration

Vi = initial volume (weight in your case)

Cf = final concentration

Vf = final volume (weight in your case)

so: (100ppb * 5g)/15g = 33,3333333 ppb

you can use this formula to calculate the exact amount of weight you should obtain to have a concentration of 50 ppb.

kind regards

dear Luca Narduzzi

I see, it was a mistake from the dilution all along.

thank you so much for explaining to me again!

best regards

I have another question. if i were to correct the above calculation, could I still use this calibration curve? (the r2 would then be improved, but the serial dilution of concentration would not be neat)

Dear Uli, you can absolutely use your calibration line because of the good linearity from my point of view. The linearity in the calibration range seems to be proven and there is no compelling reason to insert another calibration level between 33 and 100 ppm.

I understand. Thank you very much for your answer!