Three replicate determinations of the signal for a standard solution of an analyte at a concentration of 10.0 ppm give values of 0.163, 0.157, and 0.161 (arbitrary units), respectively. The signal for a method blank was found to be 0.002. Calculate the concentration of analyte in a sample that gives a signal of 0.118.

So confused :S Can somebody explain this?

Question

Three replicate determinations of the signal for a standard solution of an analyte at a concentration of 10.0 ppm give values of 0.163, 0.157, and 0.161 (arbitrary units), respectively. The signal for a method blank was found to be 0.002. Calculate the concentration of analyte in a sample that gives a signal of 0.118.

So confused :S Can somebody explain this?

missmob · Answer

this is analytical chemistry right?

thomaster · Answer

yes

missmob · Answer

its hard

thomaster · Answer

If it was easy, I wouldn't ask for help :)

thomaster · Answer

I think i know how to calculate it.. but what to do with the different signal values. average them and subtract the blank? 
That way it would be:
AVG(0.163,0.157,0,161) = 0.160
subtract the blank = 0.160-0.002 = 0.158

10 ppm = 0.158
??  ppm = 0.118

$$\frac{10*0.118}{0.158}~=7.47~ ppm$$

thomaster · Answer

oh forgot to subtract the blank from the sample signal:
$$\frac{10*0.116}{0.158}~=7.34~ ppm$$

Can somebody confirm that i used the correct method?

thomaster · Answer

I ruled out the error from the average signal of the standard solution, not the signal of the sample. The correct answer is 7.33 so i wasn't that far away from it :)

chmvijay · Answer

yaaa i think you are right :) i just gone through it now :)

thomaster · Answer

great :) tnx