Purpose and Background

For solution ICP-MS, analytes must be completely dissolved for accurate analysis of solutions by ICP-OES and ICP-MS. Undissolved analytes will not be accurately measured if the samples are not digested. The goal of digestion is to dissolve the analytes and to decompose solids while avoiding loss or contamination of the sample. For example, digestion with nitric acid oxidizes organic matter to CO2 and NO while forming soluble nitrates with most elements, which can then be analyzed. For metals that do not dissolve in nitric acid (e.g., tin and platinum group elements), hydrochloric acid or aqua regia may be required. All acids used should be trace metal grade or better and sold/stored in plastic bottles. Deionized (DI) water should be used for all solutions. Glass should be avoided wherever possible (it contains metals that can leach out in acidic environments). Plastics (polypropylene or PTFE) are recommended. During digestion, do not cap tightly or the tubes may explode.

The methods provided in the next sections are commonly used digestion methods. However, we always encourage you to read the literature in your field and determine the best digestion method for your research questions.

Acid-Leaching Plastic Sample Tubes

Metal-free plastic sample tubes can be obtained from many scientific companies. Alternatively, regular plastic sample tubes can be acid-leached to achieve low background levels. Tubes are filled with 2% nitric acid and heated for 24 h at 60°C. The tubes are rinsed 3 times with DI water and stored with DI water. Before use, the water is discarded and sample tubes are dried.

Water Samples

Samples should be collected in acid-cleaned metal-free containers (see above). Keep in mind that the containers may leach analytes into your sample. Choose accordingly. Samples with high salinity (seawater or biological buffers) cannot be analyzed undiluted because of the high total dissolved solids. Please let us know if your sample contains high salinity. Process a sample of DI water following the same procedure as the samples and submit as a method blank.

For the analysis of dissolved analytes, the sample should be filtered through a 0.45 µm membrane filter. Acidify the filtrate with trace-metal grade nitric acid to pH<2 (but no greater than 10% acid by volume). For the analysis of total recoverable elements in aqueous samples, do not filter. Acidify with trace-metal grade nitric acid to pH <2 (but no greater than 10% acid by volume).

Cell Pellets

Cells should be washed to remove any metal-containing media or drug. Process 2 samples of DI water (with same mass as cell pellet) following the same procedure as the samples and submit as a method blank.

Sample tubes are pre-weighed. Washed cells in sample tubes are weighed and then digested with 0.1 mL of concentrated, trace-metal grade nitric acid. The samples are heated for 2 hours at 90°C. The tubes are allowed to cool and 0.05 mL of trace-metal grade hydrogen peroxide (30%) is added. The samples are heated for an additional hour at 90°C. The sample is diluted to a final acid concentration of 2-5% with DI water and re-weighed.

Proteins

If your samples denature or are unstable at low pH they will need to be digested. Process 2 samples of DI water (with same volume as protein) following the same procedure as the samples and submit as a method blank.

An aliquot of protein solution is transferred to a pre-weighed tube and weighed.  0.5 mL of concentrated, trace-metal grade nitric acid is added and allowed to react at room temperature overnight. The sample is then heated for 2 hours at 90°C. The sample is diluted to a final acid concentration of 2-5% with DI water and re-weighed.

Nanoparticle Biological Samples

For most samples, nitric acid digestion is sufficient. For platinum group metals (e.g. gold and platinum), digest sample with aqua regia. Process 2 samples of DI water (with same volume as sample) following the same procedure as the samples and submit as a method blank.

Weigh sample (0.2 – 0.5 g) into a pre-cleaned, pre-weighed sample tube. Add 0.5 mL of concentrated trace-metal grade nitric acid (or aqua regia). Heat the sample at 90°C for 2 hours. If particulates remain, add 0.25 mL trace-metal grade hydrogen peroxide and heat for an additional hour. Repeat until no precipitate remains. Dilute with deionized water to final acid strength of 2-5% and re-weigh.

Trace elements in plant material

Process 2 blank samples following the same procedure as the samples and submit as a method blank. Process 2 certified reference materials (of similar composition to your sample) following the same procedure as the samples and submit. Weigh 0.2 g of sample into a pre-cleaned, pre-weighed 50 mL plastic tube. Add 1 mL of trace-metal grade nitric acid and loosely cap (to prevent the tube from exploding). Heat the sample at 90°C for 2 hours. Remove the sample from heat and allow the sample to cool. Add 0.5 mL of hydrogen peroxide and loosely cap. Heat the sample for an additional hour. If particulates remain, add 0.5 mL of hydrogen peroxide and heat for an additional hour; repeat this step until no particulates are visible. Dilute to a final acid concentration of 2-5% with DI water and re-weigh.

Trace elements in soils and sediment

NOTE: We do not perform total digestion or analyze total digestates of these matrices (i.e. we do not digest with HF and do not accept samples that have been digested with hydrofluoric acid). Process 2 blank samples following the same procedure as the samples and submit as a method blank. Process 2 certified reference materials (of similar composition to your sample) following the same procedure as the samples and submit.

Sample should be dried and homogenized. Weigh 0.25 g of sample into a pre-cleaned, pre-weighed 50 mL plastic tube. Add 5 mL of nitric acid. Loosely cap the tube to allow gasses to escape without exploding the tube. Let the sample react at room temperature for a minimum of 1 hour. Heat samples at 90°C for 30 minutes. Allow the sample to cool, add an additional 5 mL of nitric acid and heat for 30 minutes; repeat until no brown fumes are emitted from the sample. Heat the sample (still loosely capped) until all but 5 mL have evaporated. Add 2 mL of DI water and 3 mL of trace-metal grade hydrogen peroxide. Heat until effervescence subsides. With the tube still capped, heat the sample until 5 mL remains. Dilute to a final acid concentration of 2-5% with DI water and re-weigh.