I have a serious problem with determination of potassium in soils and sediments. Samples are digested with two methods - aqua requia and microwave system. Obtained results are two times higher with aqua requia and three times higher with second method of digestion. I practiced on three different reference materials (soil and sediment). All other elements are correct. Of course I included blank samples into analysis. I have six years of experience with soil and sediment matrix on ICP-OES and for the first time I don't know what the solution could be. Is there anything special with determination of potassium on ICP-OES?
I think that you should calibrate of equipament.I think that you should calibrate the equipment. I think also that vassel from microwave could be contaminated.
If the reference material gives a good answer then make sure of the following:
1. Decontaminate all vessels using 50% nitric acid for 24 hours before digestion and analysis.
2. Ensure that the diluent is not contaminated - run a check on the diluent for dilution of your samples.
3. Ensure that digestion of sample is complete - no colloidal particles. In addition filter your samples using a 0.45 u filter mounted syringe immediately after digestion - do not leave it. Partial blockage of the nebuliser system could be one reason for your problem.
4. Insert replicates of your samples and international standards.
5. Ensure that there is a broad range of operational standards and include internal standards in your operational standards and samples
As Dr. Faroog Wahab mentioned: Are the matrix of your test samples and the standards similar? High temp. of ICP may ionized a lot of K, then if your samples have high Na (or even Rb), which would reduce K ionization differently from K in your standards, erroneous K readings may result.
Ok
1. Standard addition method gives the same results, but: should I add K to samples before digestion or after it?
2. I've checked others wavelenghts for K
3. vessels were washed exactly in 50% nitric acid
4. Value of K in blank samples does not havbe a major impact for result. For exaple:
concentration of solution is about 60 mg/L and in blank it is about 0,15 mg/L. Volume of sample is 50 mL and weight is 1g. I add 9ml of HCl and 3 mL of HNO3
5. Viscosity of standard is similar with samples
6. I used multielemnt standard and monoelement one.
I think that problem is relatedto matrix influence because microwave digestion i give "much much" bigger values. Thanks for your help. I will inform you about my subsequent attempts
You do not mention if your microwave method includes HF, I am assuming your aqua regia does not. If a significant portion of your K is in robust silicate minerals say feldspars rather than weathered minerals like clay, you will not extract all the K.
Hope this helps
regards
Simon Chenery
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