I know Analar is the best but its too expensive, so what is the other choice ??

I usually use the Sigma-Aldrich nitric acid ACS reagent, 70% 2.5L bottle and I dilute 2 fold to work with nitric acid 35%.

I feel that  are starting from the wrong point. You need to work out the lowest level of all the elements that would be of interest to you (which may be the instrumental limits of detection available to you) and  look for a blank level (originating from the acid) that is at least a factor of 5- 10 lower  (taking into account the mass of sample and the lowest volume of acid you can use). From that set of data seek out a commercial product that has a specification that fits the bill. This may be an AnalaR product or it may be a more expensive grade. If you  still cannot find an acid that is suitable, and if you are confident that you are capable of working in conditions clean enough to benefit from it, you might consider an in house purification step though these days this is rarely necessary.

Dear Geoffery Marchal, Thank you very much, I found it in sigma catalog as you mentioned.

Dear Alan P Newman, Thank you very much for your advises; I have to check some details before starting my measurements. 

Reagent grade acid is not a good choice for metal analysis because of the metal levels it can contain. I would assume that bacteria can have pretty low levels of metal. Therefore, I would recommend that you at the very least use a trace metal trade acid and use the purest water such as deionized distilled water for your digestions and to make your standards. I also like Alan Newman's approach which will provide you with a good idea of your instrumental capabilities.

Jackie Schroder makes a good point which I neglected to add myself. The water quality is  also very important.

i think HF digestion is a best way as been done by many researchers

I also agree with Jackie Schroder.

I never measured bacteria samples, but for other tissues I always used HNO3 60% Pro Analysis, about 20 ml per gr of tissue and after the digestion I diluted in 2% acidic water (HNO3 in MilliQ water) 10 ml maximum each sample.

Dear Jackie Schroder and Cátia Cardoso 

Thank you very much. 

Dear Karzan Hawrami, I think HF is important for sand and clay samples not biological samples. 

The use of an ultrapure acid is very important in the trace analysis but equally important is the technique of mineralization. As the dr. Newman pointed out, you should use the minimum possible amount of acid in function of the mass of the sample. For this purpose, the most reliable technique is the microwave system at high pressure which allows to obtain good results with minimal amounts of HNO3 and the possible addition of a few drops of H2O2 30% w / v

The bacteria might have low level of heavy metal and surely you should have control containing only nitric acid and water. This will give you an idea of your heavy metal background as already mentioned before.

Water is also a very good point. I use deionized water followed by a second purification with MilliQ system.

If the level of heavy metal in bacteria is too low compare to your background (heavy metal in nitric acid and water), you may possibly increase density of bacteria by centrifugation of a larger volume of liquid media. I get very good result with 200 to 300 mg of dry matter.

Your sample having low level of heavy metal, so the reagents should be trace metal grade/ supra pure type. The reagent water should also should have 18,3 M-Ohms.

Dear Abdelrasheed

 Because concentration of heavy metals in your samples are very trace I would recommend that you must be use a supra pure type  of acid and use the purest water such as deionized  water for your digestions

Dear Mohamed

Most of the AR or reagent grade nitric acid are found to contain Pb, Cd, Cu, Hg in traces. I recommend you to use Emparta or Supra Pure Merck nitric acid  for digestion of bacterial sample. It is also recommended to use Supra Pure nitric acid for digestion before analysing Pb, Cd, Cu and Hg with the help of ICP MS/OES as bacterial sample contained tracess of said metals. 

We use AR (sigma) for trace metal analysis in biological samples (fish). But if you want more purified reagent use the ultra pure or extra pure grade, but the price will be nearly 10 times higher. Or unless you can take GR nitric and purified it in your lab (refer CEM web, they have acid purifier)

I do agree with microwave technique as well as minimum usage of HNO3. Sometimes for digestion of tissues H2SO4 would be better. The AR Grade or GR Grade will do as far as you prepare blank solutions to check out the amount of impurities added to the actual sample through use of acids.

Hi, look at this article. It might help you.

Biologia 67/5: 827—836, 2012Section Cellular Molecular BiologyDOI: 10.2478/s11756-012-0099-5Isolation and characterization of heavy metal tolerantGram-positive bacteria with bioremedial properties from municipalwaste rich soil of Kestopur canal (Kolkata), West Bengal, IndiaKamala Gupta1, Chitrita Chatterjee & Bhaskar Gupta.

Use SUPRAPUFR quality always. If not you must konew the real concentation of each mertal in ech batch of acid you use.