I am looking for alternative sample digestion protocols for dried herbs to be analyzed with AAS for presence of Pb, As and Cd. So far I have attempted acid digestion over hot plate (HNO3 and aqua regia), which did not yield satisfying results even with long processing times. Later I dried-ashed the samples at 600C in a muffle furnance, but I am worried this temperature is too high for some of the elements in question. Unfortunately dry ashing did not work for some of the woody herbal material, which did not dissolve completely, when subsequently treated with HNO3 and diluted with deionised water. Would appreciate remarks on how to improve the process or ideas for alternative sample preparation methods.
Malgorzata .... dry ashing of plant materials may result in the of some of the metal contents in the form volatile species usually found for the metals specified in your question . Thus the wet digestion is the most reliable procedure to transfer the metal compound in a plant sample into soluble material. To detect the reliability of the metal recovery, it is advisable to add some known quantities of the metals prior to the digestion.
There are a few areas to consider when digesting and preparing samples for testing.
1. Ashing and wet-digestion
Scientifically, it is not recommended dry ashing for elements like AS due to its volatility nature. But you can still do dry ashing for elements like Pb and Cd. All organic matter will be oxidized and converted to CO2 at the temperature of 550. I, therefore, recommend you to regulate the temperature in the muffle furnace during the ashing ( program- increase the temperature gradually with the time)
2. Wet-digestion
You may re-flux the samples with aqua-regia instead of hotplate method. This approach will help you obtain higher recovery, safe operation and complete digestion. If the samples are not properly digested in aqua-regia with re-flux method means it contains some non-digestible stuff like sand, stones.
3 Microwave-digestion
This method is good for all metals and ensure complete digestion. Try to adopt this method if you intend to publish your work.
4. Safety
Please note that it is extremely dangerous to use hot-plate method when working with aqua-regia. Use fume cupboard and ware appropriate PPE, if you still decide to go ahead with this technique.
5. Optimization of the recovery
It is recommended to work on complete digestion first. once you are satisfied with that add known amount of standard solution of the metals to calauclate the recovery. You then do trail and error method to improve the recovery (volume of aqua-regia, temperate, digestion time)
Microwave digestion with suprapure nitric acid is the best extraction method for these element. it will be accurate and safe.
The most reliable method is wet mineralization with oxidizing acids (HNO3 or HNO3 + HClO4), with the possible addition of fluorides (HF + H3BO3; HBF4; NH4F + H3BO3), in the case of higher levels of silicon in the samples, which is a common component of plant materials. The choice of a particular method of carrying out such mineralization depends on the preparative equipment that is available, the content of the elements to be determined in the samples, and the measuring techniques that are available. More detailed information is needed to give you specific advice.
Mineralizates containing higher concentrations of fluoride (g / L), even if complexed with boron, are corrosive (dissolve) to metallic titanium. Burners for the FAAS / FAES technique are often made of titanium. Stainless steel burners are relatively resistant to fluoride.
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