I have great differences between duplicate digestions of sewage sludge, but only for some elements like Fe and P.
l use closed vessel microwave digestion with HNO3 and H2O2.
Usually the "failed" duplicates have a much lighter colour compared to the usual yellow solution and they have a fraction of the concentration for certain elements. Sometimes the same sample produces multiple failed duplicant, and sometimes none.
l checked the acid dispensers, the loss of mass during digestion and the microwaves, but no correlation so far.
l have trouble investigating the problem beacuse of its apparent randomness. l tried running a study on the effects of different digestion parameters with a sample that had previously produced the issue. But no major effect that is comparable to the main problem was discoverd.
Are you getting a clear solution after digestion?
In our Laboratory samples of sewage sludge are ashed, followed by digesting in aqua regia.
Recovery of P is usually higher in interlaboratory comparisons. The results of the ICP analysis are consistent with the WD-XRF method, especially for Ca and P.
Below I attached link to the article with the described method of P determination in a similar matrix.
http://ijrowa.khuisf.ac.ir/article_674495.html
Piotr Kucharski
We usually get a clear yellow solution afther filtration, but these failed digestions give a colourless clear solution. A dark sludge is separated during filtration and it is discarded. Failed digestons give a pale sludge.
l am less intrested in the phosphorus content, it is mostly just a simptom of the problem. But l regularly have to measure Fe with ICP-OES or Flame-AAS.
Thank you for your anwer.
How do you compare your usual samples and this particular sample? That might give a clue to you. Geography might affect
Previous experience with digestions (not of sewage sludge) has indicated that a small addition of HCl sometimes improves Fe recovery. I can't recall the effect on P. How well do you recover Fe and P from your reference material?
How homogeneous is your sample? Has it been thoroughly mixed prior to subsampling. That is often the biggest issue for duplicates.
How "great" a difference do you get between your duplicates? By the sound of it they are quite different, but if you are dealing with only a few ppm, or at the lower limit of detection for your method, perhaps you need to modify the procedure.
Plasmachem-L is a useful resource that might provide some answers: https://listserv.syr.edu/scripts/wa.exe?A0=PLASMACHEM-L
Matthew Wheal
l don't have this problem with reference materials.
The sample is dried and homogenised in a mortar before subsampling.
The results can be less than a third of the normal values. The samples are diluted to the middle of to calibration range.
Thank you for your answer.
Matthew Wheal, Piotr Kucharski, Udaya Jayasundara
l kept on gathering data on loss of mass during digestion (before opening the bombs). l might be close to finding the problem.
Do you measure this loss routienly?
What pecentage of loss do you find acceptable? EPA 3052 suggests
Please look at the following below links which may help you in your analysis.
Thanks
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