When you say 'higher', do you mean 1% every time, 10 parts per million, or 1-2 fold greater? It might be that the AA was a bit sensitive that day, or you diluted the ICP samples more than you intended.

To answer your question with a few clarification questions:

• By 'results', I gather you mean concentration of copper in the samples of cyanide compound?
• Did you analyse exactly the same solution on both instruments or were there different preparations for each?
• Was there any sort of preparation: acid digestion, dilution, internal standard, ionisation buffer?
• Did you run a known concentration standard in both instruments?
• What was your calibration range on each instrument? Did they overlap at all or was the ICP an order of magnitude lower than the AA?
• What is the uncertainty of measurement on your analyses? If the AA gives ±3% on a given concentration, and the ICP ±0.1%, perhaps there is no issue to be resolved.
• Could the glassware you used have been contaminated in some way? If you prepared the ICP sample first and re-used the glassware for the AA, there could be enough Cu coating the surface to "contaminate" the second aliquot. Was it used for Cu preparations before? High concentrations of any element will carry over unless extremely well washed with detergent and acid.

There are obviously numerous reasons why two instruments might give different results. If you can elaborate on your sample preparation and instrument performance, there might be a reasonable explanation for the difference you observed.

Thank you Matthew Wheal for the response.

I will get back to you with more clarifications.

Hi Matthew Wheal

I talked to experts and gathered more information about this issue. AA readings (copper concentrations) are higher than ICP by an average factor of ~15%.

All the samples were 10* diluted (based on a standard operating procedure) and a linear calibration line was obtained for both instruments. Sample preparation procedure (cyanide leaching in NaOH) was the same for both instruments.  We use Eu internal standard for icp and track its intensity throughout the analysis.

The only main difference is matrix of samples and standards for two instruments:

ICP copper standards (1 to 10 ppm Cu) are prepared in 4% nitric acid while samples have a cyanide, NaOH matrix. the matrix is not matching for standards and samples. This way exposure of ICP torch to Na was minimized (as much as possible).

For AA, standards (5 to 30 ppm Cu) have the same matrix as samples (cyanide, NaOH matrix).

I do not think there is cross contamination. We read blank several times between samples and all are zero/very close to zero.

For both ICP and AA, the uncertainty tolerance is 5%.

So, a 15% difference is worth considering - if it was 1.5%, I wouldn't have been worried.

The sodium content is probably the issue. ICP (OES or MS) is somewhat prone to matrix effects, despite what manufacturers claim.

Good luck!

Thank you Matthew Wheal

The SOP for sample preparation can not change so the dilution factors and sodium contents are constant. We did a copper analysis by ICP in a matching matrix for standards and samples. The ICP readings for reference solutions are very accurate. So, the similarity in the matrix (even in high sodium contents) is giving us good responses. Once the matrix is different readings are off

(lower than expected values).

I am curious to try different sodium content in copper solutions ( your #5 comment). The results will be very interesting and I will update you on that if we have the chance to try it.

Thank you again for your time.