Yes it is better than tri acid digestion. Which metal you want to analyze?

Thank you Seema and Rishikesh for your reply. The metals which I want to analyze are Pb, Cd, Ni, Cu , Zn, Fe and Mn. Do you know the percentage of recovery of the above metals while digesting with nitric acid- perchloric acid mixture?

I guess, Water treatments in soil mainly concerning the sequestration of arsenic, cadmium and lead. Different algae, fungai, and plant bio mass has been reported for phytoremidiation. The acid treatment for digestion of metal through a plants and algae depends of functional group moiety and growth of microbial content at specific pH conditions, or do you want chemical digestion for heavy metals settle as precipitate, it has different procedures.

I will recommend you to go for non-destructive analysis technique like XRF (Wavelength Dispersive or Energy dispersive) or PIXE, especially if you are planning for the above heavy metals in plants, and you required them to be quantified in ppm (parts per million) level. If you have to have go for other analytical tecniques after acid digestion like AAS or techniques based on inductively coupled plasma, you should consider the case of interference carefully. Anyway the above acid digestion method is standard, and you can go for it.

thank you Pranaba for your reply

Our lab is routinely Analyzed heavy metal from plant extract. However, I use microwave digestion method. Microwave digestion method is easy, robotics and has reproducible. For 625 mg of sample, we use 2.0 mL of conc. HCL, 7.0 mL of conc. HNO3 and 0.5 mL of conc. hydrogen peroxide, then after microwave digestion volume adjust to 50mL. You can try for acid digestion with same composition and digestion time might be 120 min at 60 to 80 degree Celsius.

Off course, analytical tools we use is ICP-OES

thank you Abzal Hossain

We routinely use nitric acid : perchloric acid digestions to dissolve plant material. The recoveries have about 95%-98%. We use 0.5 g ground plant material with 3-4 mL HNO3 and 2 mL of HClO4. We allow the plant acid mixture to pre-digest overnight at room temperature in a hood and then place it on a block at 125C. The nitric acid acid will evaporate after about 3-4 hours at which time we raise the temperature to 200C for 2 hours to allow the perchloric acid to further oxidize the plant material. This digest has been much more successful than just using nitric acid alone. There may be a tendency for potassium to form insoluble potassium perchlorate, however the digest can be heated slightly after bringing it to volume (usually 25 mL for a 1:50 dilution) for ICP analysis.

I have once used HNO3+HCLO3 for digestion of soil but the only problem associated with the combination is spurting at high heat. So if you want to use the method, I would advice you first digest with HNO3 for some minutes before you add HCLO3 in little quantity as per sample gram used.

Thank you Clement Ogunkunle . we are also following the same method. Can you tell me the reason for spurting when we are adding the combination of HNO3+HClO3 ?

Mr. Tharayil, spurting occurs when the temperature becomes too high and it is due to the presence of HCLO3. So allow HNO3 to finish the organic matter oxidation before adding HCLO3 in little quantity. This will reduce spurting.

Depends on the heavy metal u want to extract

In our lab we use a modified method with good recovveries for the majority of trace elements. We weight aorug 100mg of plant tiusse into teflon autoclaves. Then we add 7 ml of HNO3 and 1 ml of HCO4 (careful with this one). Autoclaves are heated ina oven at 110ºC for 3 hours and after cooling the autoclave content are transfered to volumetric 25mL flasks and volume complted with Milli-Q water. Recoveries usully varies between 91 and 108%.

We digested 0.3 g of plant materials with 9ml of Conc. Nitric and 2 drops of conc. HF and achieved a complete digestion with Microwave digestion procedure. Follow US EPA methods to digest samples with heavy metals. GOOD LUCK !!!!!!!!!

For most plant samples, digestion with nitric acid is usually sufficient, particularly if the sample is not very fibrous.

The most effective way is microwave-assisted digestion with nitric acid. It is fast, secure and clean, and you need really low sample quantity. And if your sample has not any inorganic, you dont need to filter the digested, you can directly inject it into the ICP or MS/ICP equipment (performing, of course, the needed dilutions)

Daniel Puyol, Darshani Wijayawardhana, and João Canário, can you give the references for such standard method. I have small quantity of plant sample (in some case o.01 g) that I want to analyze for Pb and Zn.

For a complete digestion, freeze dried the material, grind it into a ceramic mortar using small quantities of liquid nitrogen, place the powder in a ACV vessel and add HNO3 and small portions of H2O2. Once all the peroxide reacts, add a small amount of HF and complete the digestion in a closed microwave digester. Usefull also for Hg determination.

Here you have a reference, located at 2.8 chapter:

http://www.sciencedirect.com/science/article/pii/S0304389411008569

Thank you Maria Alvarez for your valuable reply. Unfortunately we don't have microwave digester. Can we follow the method which you had mentioned above without a microwave digester?

I think if using an ultrasonic bath at 60 C, keeping closed the containers and increasing the vol of HNO3 and the time of digestion. If you are not interested in Hg, the temperature could be higher.

you will be able to use microwave acid digestion in closed system you will be able use a little volumes of acids (HNO3 up to 5 mL), sample or CRM (up to 0.5 g) and can determine many elements present in high concentration or trace level.

In the absence of a microwave digestor, dry ashing with a muffle furnace is one strategy, followed by dissolution of the ash in aqua regia. My lab has used a variation on USEPA method 3050b, which is a nitric acid and peroxide wet ashing procedure. Make sure you use high quality reagents on include appropriate blanks to account for any background concentrations of your target elements.

Drawback of dry ashing in a muffle furnace is the total or partial loss of volatiles: As, Cd, Pb, Hg ...But it could be done the digestion in a muffle using closed teflon containers with steel housing employing also small quantities of acids

Very often, regular acid digestion on a good quality (e.g. graphite top with precise temperature control) hot block will give you same results as microwave digestion.

P.S. Sometimes is good to add a little bit of H2O2, but be very careful working with HF. If you can digest your sample without HF, don't use it.

I would suggest XRF which is a non-destructive technique and doesn't create any waste materials from the analysis.

Hi Dan!

In fact, XRF is a very nice and non-destructive method, but it is not a trace analysis tool (and often heavy metals in plants are at quite low concentration levels).

MW digestion: up to 2 g (weight is dependent on the sample) of sample+ 5 mL HNO3+ 1 mL conc. H2O2.If is necessary the digestion of silicate matrix the addition of HF is required. VV

@ Tom Sizmur thank you for your contribution, but can I use a hot plate instead of the block digester? if so what precautions can put into consideration.

@Dorcas, I think the only precaution for using hot plate is that the acid used in digestion must be carefully watched not to allow it to dry. There should be frequent addition of acid when dryness is suspected and spurting should be reduce or prevented by inserting glass funnel into the beaker especially when HCLO3 or HF is employed.

It is better to avoid the use of perchloric acid wherever you can, as it can lead to undesirable consequences if not handled properly and with the use of appropriate fumehood. We recently used MW digestion for plants and soil without using perchloric acid. This method may suit your need. I have attached for your interest.

Article Rapid Determination of Ultra-Trace Concentrations of Mercury...

Nitric acid either with HCl or perchloric acid is recommended. For more you can refer to APHA.

Use Lindsay and Norvell 1969 Method

Three well recognized digestion methods are based on a) wet digestion procedure with HNO3-HClO4-H2SO4 (very caustic) mixture; b) Dry ashing followed by treatment with HF using Pt crucibles; c) MW digestion using HNO3-HF-H2O2 using teflon tubes.

Thank you all for expressing your valuable suggestions. Can I use HNO3-HClO4 digestion mixture for Chromium? We are using spectrophotometer for chromium analysis.

Yes you can.

Yes, it is a good method

Megha, HNO3-HCLo4 digestion can be used to extract Cr. infact, I have used the method several times for that metal. The only caution I will flag is that you have to be very careful because HCLO4 could be explosive at times. There could be spurting when the temperature is too much, so to guard against that, you increase the temperature steadily and insert glass funnels into the container to prevent spurting or spitting. Best of luck

Thank you all.

To your first question I would say that

Basically digestion process will involve either of these methods

a) Wet Digestion (HCLO4 + HNO3 + H2SO4)

b) MicroWave digestion (HNO3 + HF) through teflon cylinders.

c) Dry ashing followed up by HF

You may chosse any of these as per your conveinence.

To your second question I would say that

Yes, HNO3 + HCLO4 digestion mixture is much appropriate.

You may refer Allen (1979) with newer editions for further details.

Thank you Rachna

Megha, your method is good, the other method is EPA 3052 for MW and used HNO3, HCl, HF, HClO4, if neccesary H2O2.

Yes, for plant materials, you may follow Allen (1989). The mixed acid digestion involves nitric acid, perchloric acid and sulphuric acid. the reference is as below:

Allen SE (1989). Chemical analysis of ecological materials (Ed. SE Allen). 2nd edition (completely revised). Blackwell Scientific Publications, Oxford, London. pp. 59-60.

Hope this would suffice for your query.

Thanks Anjan, for substantiating the reference.

most of plant samples contain high percentage with cellulose. sometime it is difficult to digest this kinds of sample. So the best way, the strong acid should be used for this purpose like the mixture of HNO3 and perchloric acid. Sometime i have used hydrogen peroxide to replace perchloric acid (with thorougly guided). Therefore, it also suggested to used non-destructive techniques like NAA or XRF methods.

Non-destructive techniques are more appropriate in the matrices like vegetables or fruits, etc. For plants or leaves, it may not be suitable.

What about the conventional dry ash method followed earlier ? It seems this method is sidelined because of the large amount of energy consumption, and probably out of the fear that a portion of the metals are lost during ashing process. Also probably the acid digestion methods appears to be convenient. However, the acid digestion methods causes lot of pungent fumes and acidic discharges to the environment. Has anybody accounted for the real losses during ashing method compared to the acid digestion methods ? And also on the efficacy of dry ashing method ?

Would like to know my peer group's experiences.

Only use perchloric with a temperature controlled block, otherwise the result could be nasty.

Leaf and stem tissue digestion with perchloric acid works well but it comes with a safety precaution, take it seriously.

Your method is good. But whatever the method , you'll have to validate it by measuring trace metals in certified material or reference material. If you have à recovry percentage ranged from 95% to 105% your method wil be valid.

Kenneth

Your comment is very well founded.

Of course, when the used a mixture of mineral acid (or a mineral acid with hydrogen peroxide), digestion of the sample is total and all the metals are in solution.

The question is whether that's the point, or maybe more at least partial speciation of heavy metals in plant material. If it is to be partial speciation, it extraction method with 5% sodium hypochlorite suitable for this purpose very well. Of course, not all the heavy metals formed easily complexes with proteins. Very good susceptibility, to the formation of such complexes, have transition metals of the groups in which the d orbitals are not completely filled by the electrons.

Kenneth

Sir your answer was appreciable

But in case of estimation of trace heavy metals in plant sample whether we go for hydrogen peroxide or sodium hypochlorite and in what percentage

You can go for standard chemical analysis given in APHA book.

After cleaning and washing the plants with deionized water we dried 30 g of each smple in a oven at 103-105°c and then we digested them with to procedure as below:

1- 5 gram of each sample was weighed carefully and 5 ml of concentrated nitric acid (Merck, 99.99%) was added to each sample. The sample, then, was heated up to 80°C until near dryness. The addition of acid and the process of heating were repeated two more times . An amount of water added to the residual material. The suspension was filtered (Whatman filter Merck, 0.45 μ m) and the filtrate was diluted by deionized water to a final volume of 50 ml.

2- 0.25-5 gr of each dried sample was weighed carefully and 4ml of concentrated sulfuric acid (Merck, 98%) was added to each sample.This suspension then transfered to a spesific container of digestion apparatus(model 23130-20)from HACH company and preheated in vacume at 440°C for 4 minute then added 10 ml of hydrojen peroxied and continued heating until a clear solution was obtained.Next, the solution was diluted by deionized water to a final volume of 50 ml.Finally, we used both of obtained solution by a Graphite furnace atomic absorption spectrometry (GFAAS, Model AAnalyst300) to determine heavy metals.

Dr. Abraham asked about experiences with dry ashing using muffle furnaces. We still use this approach to do large numbers of samples for routine micro- and macronutrients. Our results with CRMs are acceptable, although depending on the plant matrix, we sometimes have a problem with carbon residue and full recovery of copper. In such cases we will use an ashing aid (HNO3) and reheat the sample.

For some trace metals, like cadmium, you can have variable loss using the dry ash technique. We recently tested this on a large batch of tobacco samples and experienced losses between 40 - 60% for cadmium using the dry ash protocol, as compared to HNO3+peroxide. I do agree however, that a trade off in wet versus dry digestion techniques is proper handling of the acid fumes, especially if you do large numbers of analyses per year, such as our service center. Handling smoke from the dry combustion process is a lot easier than acid fumes, although the fire department may not necessarily agree with this assessment given the few times our smoke exhaust hood has failed.

The comments by Dr. Towe are note worthy in using NaOCl versus peroxide. My concern is that we try to avoid high chlorides in the final matrix due to chloride complexes when using ICP-MS. This would not be a problem with ICP-OES/AES. However, we have been looking at As, Se, and Cd in plant tissues where the only recourse is ICP-MS. With our collision cell design, chlorides are an issue. I would enjoy comments on whether the use of NaOCl has caused interference/baseline issues using ICP-MS with collision cell versus DRC models of instruments. We don't use perchlorate due to campus safety regulations regarding the need for a wash down hood and due to relative high levels of oils and fats we sometimes have in our sample matrices.

I used this method:

The acid digestion was carried out in a Microwave Digestion System (CEM). 10 ml of nitric acid concentrate 69.5 % w/v were added to 500 mg of dry matter. After

cooling, the extra acids were evaporated and the sample digest diluted with 10 ml

0.1 mol L-1 nitric acid and filtered through a PTFE membrane 0.45 μm and

measured by Varian ICP-MS.

To better succeed in the acid digestion (recovery) I was used to leave over night the samples with concentrated nitric acid before the microwave digestion.

Please conduct wet land construction method by identified potentate phytoremadation plants.

Di-Acid digestion

I followed microwave digestion and received good recovery. You can take application note from CEM web

This link provides some of the essential methods for plant analysis http://www.ncagr.gov/agronomi/pdffiles/sera368.pdf

Your digestion method is the effective and common one. The microwave digestion method can also be effectively used

I also used microwave digestion procedure and obtained a good results. This works for ppb level also.

Hi

Your methd is a good digesting method but also i used sulphuric acid +hydrogen peroxid  with digestion instrument (Hach company) under vacum and high temprature and obtained a good results.you can see standard methods for digestion methods

Please,I need a standard method for digesting animal tissues especially using Nitric acid and hydrogen peroxide.

you can consult this paper

It also depends on which metals need to be analyzed, since sulfuric acid does cause precipitation of some metals like Ag or Ba, etc. HF must be used if Si residues appear, likewhen digesting rice plants, so silica can be dissolved, or when Ti or Zr, residues appear too. this HF addition prevents the adsorption of heavy metals on these precipitates, for instance.

Usually when plant tissues are digested with sulfuric acid, leaving the plant sample in contact with the acid must be overnight, so the H2SO4 can start breaking down some of the structures of plant material.

Perchloric acid must be used with caution, since being a super oxidizing acid, a risk of explosion always arises.

HNO3 is the best option, with H2O2, and in case needed, HF too, in Teflon vessels by microwave digestion. If HF is used and Teflon vessels and MW oven are not available, Pt-crucibles must be used. Mineralizing afterwards with HCl is almost always recommended before measuring

plant material wet digestion with concentrated acid treatment using atomic absorption

We found that a nitric acid/perchloric acid digestion with the analysis by ICP provided excellent recovery of metals in standard reference materials. A perchloric hood is needed, however.