Sample Digestion

Sample digestion is achieved by utilization of various acids and oxidizing agents.  Most samples may be digested with concentrated nitric acid (HNO₃).  However, many samples that contain refractory elements may and will resist treatment with the strongest acids or combination of acids, such aqua regia.  At our facility we engage various digestion protocols using a variety of acids combined with, among others, microwave digestion, hot-block digestion, and post furnace- fusion digestion.  For many samples there are set protocols provided by the Environmental Protection Agency (EPA), US Geological Survey, and various other agencies.  When required, our facility will conduct digestions in accordance with such protocols.  However, many submitted samples do not have set protocols and thus we rely on the expertise of our trained personnel.  Among others, a sample may be digested by either one or a combination of the following: nitric acid (HNO₃), hydrochloric acid (HCL), sulfuric acid (H₂SO₄), and hydrogen peroxide (H₂O₂).  Our facility does not use perchloric acid (HCLO₄) for digestion. 

Typical Digestion Procedure:

1)      Samples are placed in a pre-acid rinsed tubes (PP, LDPE, Teflon, etc.), carefully recording its volume or weight.

2)      To the sample, Optima grade nitric acid is added and the mixture is mixed by repetitive pipetting. The volume of nitric acid added depends on sample size.

3)      Samples are placed in a hot-block set to 90 degrees Centigrade for approximately two hours.

4)      When no further color change is seen and sample particulates are no longer visible, the sample is removed from the hot block and allowed to cool.

5)      The digestate is diluted to a final acid concentration that is similar to that of the calibration standards.

6)      This digestion protocol would take approximately 4 hours for 15 samples.