Problem with atomic spectometry

[nickatomic]

i have a big problem while i m trying to measure Cr with flame with Aanalyst 200

i m following the guidelines of perkin elmer. i m using a rich yello flame 4.5/10.
i have made some standards 0.2, 1,2,4 with 0.2% nitric acid.
i m doing calibration and when i finish i have noticed that the same standards showing lower absorbances
why that is happening? is Cr unstable by time?
thank you

 

[chemisttree]

What concentrations are you measuring in your standards?

 

[nickatomic]

i was using 0.2 mg/lt to 4 mg/lt. i made a very good calibration but after i finished and measured again standards it was showing smaller absorbances and of course concentrations

 

[Gokul43201]

Could it be an inconsistency in the atomizer? What exactly is your sample, and how did you prepare it? Are you using a matrix modifier? The more detail you provide, the more like someone will be able to debug the problem.

 

[chemisttree]

i was using 0.2 mg/lt to 4 mg/lt. i made a very good calibration but after i finished and measured again standards it was showing smaller absorbances and of course concentrations

Were the samples still in the linear range? That is, could you obtain a 'new' calibration curve and judge it 'very good' as you did with the earlier analysis? If so, it could be instrument drift. This can occur for a variety of reasons. Did you take a background spectrum (blank) just prior to analyzing in both cases?
I think it is unlikely that the chromium is not stable or is precipitating out of solution but it is a very easy thing for you to check. Are you storing your standard solutions in plastic or glass? Was the glass cleaned with 6M nitric acid and baked out as a last step in its cleaning?

 

[nickatomic]

thank you for your replies
about matrix modifier. . i don't think that it needs because i m using flame - absorbion. i think that it is used only to furnace.
about the standard. i have made it from a comercial one. i don't think it has to do with dilutions or cleaning. the comercial one is in plastic bottle and i m having mine in glass that it is well cleaned with nitric acid
i m near the characteristic for yello rich flame when i begin the measurings. something is happening with the standard i think tha i can't explain. .

 

[nickatomic]

i want to add that for cu and Ni and pb i don't see that difference

 

[chemisttree]

When you analyze for Pb, Ni and Cu are you using the yellow rich flame? Could the soot in the yellow flame be the source of the variability? One time the flame is a little less yellow than the next perhaps?