We've been digesting the same soil in a number of different ways and measuring the Cr content of the filtrate using an AAS. The results are highly variable, and only remotely reflected the levels of Cr the soil was spiked with.
How can this be the case?
I understood that after digestion all the Cr has been oxidised and then detected by the AAS. This does not seem to be so. Any reasons?
Did you check the purity of the chemicals you used for digestion? You also may consider calibration curve "rollover" with some instruments.
Dear Derk, please check for systematic and erroneous bias. In other words what is the variability per method compared to overall variability? In case variability per method is low it`s a method-related issue. Large non-systematic variabilities may are caused by contamination, inappropriate calibration, etc. (see comment sent by Thomas). BR Nils
On a first glance I would say it's something related to sample preparation. Check the following points:
- Are you using a standard method (ISO, EPA, etc)? If so, be aware of it's scope, quality control indicators, and follow it scrupulously.
- Is your sample properly dry?
- Is it homogenous? I mean in terms of particle size (narrow range) and mixture quality? Maybe you should consider smash it to the smallest possible size, mix it and sieve it.
Are the acidic conditions (amount of acid) of the final digested sample exactly the same as the standards?
If all that verifies, then consider checking your AAS. Although chromium is one of the most "well behaved" elements to analyse by AAS, it often generates parables instead of linear calibrations for the same ranges.
Finnaly, if you can access icp-ms, try comparing what you get from one and from the other.
Good luck!
did you measure total chromium or hexa valent chromium?do you use purchased standard solutions.Do you process Aqua regia digestions accordingly.If so, it is not problematic.I did it before but no problem.you can use another method EDXRF instrument.It is good for solid determination.
Sample uniformity/ homogeneity is a big factor in error analysis for trace metals. This was studied and a publication of error by the EPA a while ago. In my experience, digestion acid choice is a major contribution to error for chromium as well. Nitric acid, hydrochloride acid, and aqua regia gave different results. This was a study of heavy metals in a fertilizer matrix. As I recall the sample uniformity study was published as Gy Theory by the EPA. It was in the public domain. I’ll see if I can locate it and provide a link.
When I was testing Cr using AAS I found something interesting. Try making a very, very rich flame. The 'whiteish feather' filling the entire flame. Have it in a solution and then adjust the flame from lean to rich then very rich and watch the results. The sensitivity was greatly improved. Just something to try...
Thank you all for your replies. After looking into it a more I realise that the various digestion methods can result in differing concentrations measured as Sanford Siegel pointed out. Including all the other possibilities it will lead to various levels measured. It is just something I have not come across in many of the research papers. The digestion method is mentioned without much further comment even though it seems to be of some significance.
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