This method comes from a very reliable group: 

Thank you! A precisely suitable recent work!

We used a similar method to the article above along with a microwave digestion method for seed and leaf tissue respectively. See page 5 in the article below.

https://www.researchgate.net/publication/309556126_Root_morphology_and_seed_and_leaf_ionomic_traits_in_a_Brassica_napus_L_diversity_panel_show_wide_phenotypic_variation_and_are_characteristic_of_crop_habit 

Article Root morphology and seed and leaf ionomic traits in a Brassi...

Hi Rohan,

Using H2O2 and HNO3 is very common but you will always have a residue if silicates/silca are part of the plant structure. Hence the need for filtration. Microwave digestion are more effective in getting all inorganic ions into solution and also in destructing the organic parts through oxidation. We use a high pressure asher (HPA-S from Anton Paar) for even more effective digestions.

I would avoid the use of Mo as internal standard. We use either Rh or In, which cause no interference and are not essential elements.

TCM

I am prefer to dry ash the plant materials first and then dissolve the residue with HNO3, it is simple and easy to operate for large batch of samples. For high silica contain sample, HF can be used to dissolve the residue, and then evaporated to dryness. The acid digestion method always face the problem of some residues remained in the digested solution. Element spiked as internal standard depend on the elements you are going to measure, not have to be Mo. 

I agree with Xiaolin that HF should be used to get silicate/silca into solution but if you are not interested in the silica content I would try to avoid the use of HF (because it is a "mess" and actually only causes trouble). But if you use microwave or high pressure asher digestions you can leach most if not all elements of interest from the silicate/silica beside destroying all the organic matrix.

TCM

Dear Rohan,

I'd suggest microwave-assisted digestion (MAD) using diluted acid and H2O2. This helps to have less acid in the final solution: less dilution necessary = higher sensitivity. A typical procedure is 0.2 g of plant material + 1.4 mL HNO3 + 5.6 mL H2O + 3 mL H2O2. A typical MAD program for this procedure would be 15 min ramp to 200 oC, 15 min at 200 oC, and a 10 min cool down.

I believe you may not even need an internal standard. I'd suggest you use a certified reference material such as NIST 1573a (Tomato Leaves). You can digest this sample and analyze it by ICP OES using external standard calibration. If the results are adequate for the element of interest, you should be good to go with your sample. If you do not have access to the reference material, try some spike experiments. 

I prefer to totally digest the sample i.e to also digest the silicates in the sample. If the sample is a lot, then use pre-ashing of the material since it is organic based. If the sample is small, such as 0.5 g, then use perchloric acid first drop wise. Care should be taken as the sample might catch fire, hence drop wise. When the reaction has subsided, put in 10 mL of hydrofluoric acid and fume to near dryness on a hot plate. The experiment should take place in a FUME CUPBOARD WITH AN ATTACHED SCRUBBER. Then add concentrated nitric acid and heat to get everything in solution. The sample is then made up to volume so that the final solution is 3% with respect to nitric acid. This process ensures that only the sample is in solution and no addititves. The sample is then analysed using ICP-OES or ICP-MS depending on the concentration of analytes in the sample. Ensure that a blank is inserted and undergoes the same digestion procedure - minus the sample.

I hope this helps

Most important in getting reliable analytical results is the adjustment of the digestion method and the problem you are dealing with, and the lab equipment at hand. It makes quit a difference when dealing with ICP-OES or ICP-MS; I suppose you are dealing with ICP-OES. Also the type of element you are interested in makes quite a difference. In any case, the digestion should be well manageable, should yield reproducible results,, and the procedure should be achieved with basic lab equipment. There is general consensus that prior to analysis total sample destruction must be guaranteed. As already mentioned by Xiaolin Hou dry sample ashing with following acid digestion is the common method to achieve total destruction. In addition, this approach fits very well the analysis by ICP-OES. Depending on plant species and plant part the ashes are about 1 % by weight of fresh sample material and about 5 % in comparison to oven dried sample material (105 ºC). So the element concentration factor of 100 or 20, respectively, achieved by ashing favors the ease of ICP analyses when dealing with plant trace elements. As the devil is in the details, I hope some additional remarks will be helpful. The retrieval of about 0,2 g ashes requires about 4 g oven dried plant material, which after acid digestion of the ashes is recovered in 20 mL solution (preferably HNO3 1 N). When dealing with main elements (especially earth alkalines) much less material will do. After ashing procedure (in glazed porcelain crucibles; 550 ºC) I preferred to transfer (with final small acid rinsing) the ashes quantitatively to PFA vessels to do an aqua regia digestion (0.2 g ashes; after placing 5 mL distilled water in the vessel add 8 or 12 mL a.r.; do not forget blinds) at about 90 – 100 ºC (slow digestion); digest until dryness and add 20 mL HNO3 (1 N; in general, this volume is sufficient for ICP-OES analyses). Weigh vessel with solution and wait about 24 hours to allow for total extraction of elements from the dried digest; do not forget to stir at times. Before analysis weigh once more to factor the evaporation loss (when covering some condensates at the lid might blur the results as well). Whether or not you need a HF digestion depends on your questions.

Development of fumes during ashing in the muffle furnace must be avoided. Normally, fumes can be do not occur when using absolutely dry material (105 ºC) and slow heating rates (initial heating: 5 ºC/min; hold temperature at 250 ºC for 60 min; continue with 5 ºC/min up to 550 ºC; hold T for 120 min; cool down in exsiccator).  For some substances there might be fumes anyway; in this case the crucibles must be covered in the furnace and the fume deposits have to be rinsed from the lids to the PFA vessels.

There are other methods of ashing and digestion like wet ashing. This is even more of details and chances to contaminate the sample. When using ICP-MS you need much less sample. In case of ICP-MS an internal standard like Rh and/or Ir should be spiked; in ICP-OES an internal standard is of no help. Also a comparison with analyses of plant standard material would be a good idea; however, these materials are quite expensive (e.g. European standards: BCR-100 (beech leaves) or hay powder (BCR-129), about 200 Euro each). So, in case of ICP-OES I would prefer standard addition (quite annoying). Not to forget: when dealing with volatile elements like Hg (or some As or Se compounds) you have to use microwave digestion, by this following the recipes of the device supplier. Due to the high contamination sensitivity of Hg you should do this job in a special severed section of your lab, anyway.

Wish you good results

D. Zachmann

Amazing, insightful and detail replies by everyone. I am deeply thankful!

Important points have been highlighted. For example the method be reliable, replicable and depend on problem that one is studying. At the same time depend on plant parts to be studied and equipment at hand. I am providing the details below,

1) I am working on Soybean, on the question of "how soil carbon amendment affect nutrient availability to plants". In the pot culture and field plots, I take soil sample at 3 stages of Soybean growth and corresponding plant tissue sample. 

2) Study nutrients are: NO3, P, K, Ca, Mg, S, Zn, Cu, Fe, Mn, Mo, B (basically the 12 plant essential nutrients). 

3) Currently I am using leaf samples, third node from Top. I might consider digesting entire plant in case of pot culture esperiments.

4) Instruments: Yes, its ICP-OES. Microwave digestion facility is not available. Laboratory oven is available. Muffle furnace can be made available if necessary.  

I thank everyone again. 

Best Regards

Rohan