I am developing method for testing heavy metal especially As, Cd, Pb, and Hg using AAS, for Hg using Vapour AAS. I dilute the ore using nitric acid and hydrochoric acid the put on hot plate. But when i compare my result with third party lab, my result is lower (lower around 40 ppm) than result from third party lab (using ICP-AES). Is there any best way to get maximal result?
Using vapour method by AAS IS good only for Hg and As not suitable for Pb and Cd.
Tropical Journal of Pharmaceutical Research February 2016; 15 (2): 427-434 ISSN: 1596-5996 (print); 1596-9827 (electronic) © Pharmacotherapy Group, Faculty of Pharmacy, University of Benin, Benin City, 300001 Nigeria. All rights reserved. Available online at http://www.tjpr.org http://dx.doi.org/10.4314/tjpr.v15i2.29
Purwati Purwati Sampling method. Is the same sample used by third party and yours? Ore is highly nonuniform in their metal content as far as sampling is concern. The best initial way is to send a known reference sample to the third party lab and see whether they get the same result. Just don't tell them the conc or the ref sample beforehand.
Hello Mr. Shukor,
The sample is the same, because i want to find the any result from my developing method using our new AAS. There is 4 samples that i have checked and compare with other trusted third party lab. But I never got the same or almost similar with their result. I think my dilution method is still can not give the maximal result.
When you state lower by 40 ppm, what is the relative size of this error? If it is 40 ppm in 100ppm (ie 40% low), that would be a problem. If it is 40 ppm in 4000 ppm (ie 1% low), maybe it isn't such an issue.
Some other points to consider:
- You are comparing AAS with ICP_AES. While they measure the same thing (elemental concentration), they do so with different degrees of sensitivity and precision.
- Have you checked that your methodology is sound? If your calibration is made with contaminated reagents (you don't state which elements are out of specification), you might be over-estimating the amount of e.g. Pb in the calibration, but then the same mass in the sample will be underestimated.
- You have also processed the ore differently from the third party lab, that could well have altered the concentrations in the digested sample (which is not necessarily the same as the original ore you started with, and also not the same subsample that the third-party laboratory used for their analysis).
- Do you have a standard reference material (of any sort, it does not need to be certified, just have known values), that you can measure in your lab by AAS and compare to the third-party ICPAES? If you can achieve recovery within the specified Measurement Uncertainty of the reference material, then the results are comparable. If not, there is a problem (under-extraction in the acid digestion or contamination from reagents are the first two options to consider).
Hi Mr. Mathew,
For example if third party result is 76 ppm then i got in my laboratory is only vary around 30-45 ppm.
I got difficulties in our developing existing method, especially to make sure that step is correct.
In that case, (50% recovery compared to other laboratory), your method appears inadequate OR their results are overestimates.
You will have to confirm by testing a known material as I suggested in the last point in my previous answer. Until you do that, you can't tell if their results are reading high, or your results are reading low.
Ask the lab what their quality control includes, and if they don't have a reference material, you will have to work with them to find one that you can both measure and see whether the results are similar or if they differ by a similar amount to your original samples.
Dear Mr. Mathew,
I tried to purchase a reference material for comparison, but not available in NIST, OREAS and Brammer.
The third party lab is in other country, and it is commercial lab that i dont think they want share about their testing.
That i have now in lab is only standard solution for making calibration curve ex. standard solution Arsenic 1000 ppm.
if it is there any solution from you, I really appreciate it.
Thank you.
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