[J.I.]

we're performing trace metal analysis of vegetables by standard addition method using AAS. our sample was digested using nitric acid. when we ran our sample in the AAS for analysis of copper, we didn't get a linear graph for three trials. but when we analyzed the sample for iron, we obtained a linear graph. what can be the possible reason for such results for copper? is the absorption lines of copper and aluminum near in terms of value? help please!

[chiralic]

Please, Could you post you data ? Are you check interferences?

Also, I see this paper on internet:
Determination of copper in tea leaves by flame atomic absorption spectrometry
http://images.katalogas.lt/maleidykla/che44/Che049_052.pdf

[J.I.]

I'm still going to get my absorbance readings next week, but here is the background:
when we finished digesting  our sample using nitric acid it was the end of the laboratory period so we covered the sample inside the erlenmeyer flask with a stopper covered in aluminum foil. the next laboratory period (two days later) the vapor from the acid reacted with the foil and formed salt like substance which might have contaminated our sample. when i checked the amount of copper in a commercial aluminum foil, it is only 0.02%, I assume such amount is insignificant. I already ruled out errors in dilution since we obtained desired results for iron analysis. thank you very much.

[Dude]

Essentially all elements are non-linear with repect to absorbance if the concentrations are outside of their "linear range".  Ideally, the element exceeds the linear range and a dilution is done to ensure that the concentration remains in the linear region.  I believe Perkin Elmer has an extensive listing of linear ranges.  Here is a brief one that I do not know the authenticity of:

http://www.chemistry.nmsu.edu/Instrumentation/800AAS_procd_temp.html

[J.I.]

If I obtained a non linear or scattered points on my graph, I can conclude that a mistake was done during the addition of standard because it went over the detection limit, meaning the solution is too concentrated with respect to the element of interest. Is that correct?

Finally, how will I say that in my report without our group looking bad? hehehe...
Thank you