Soil Analysis - Metals

Soil Analysis-Metals

The preparation method is critical to assessing the concentration of metals in a soil sample.  The following are questions to be considered when submitting a sample for analysis:
  • Is the sample uniform
  • Is the laboratory using the whole sample, or is the sample sieved (Particle Size Analysis) prior to analysis and only the fines are actually analyzed? 
  • What sieve size is used? 
  • Are the laboratories simply picking out a random portion of the sample for analysis if so what
All of these protocols will give widely differing results.

The soil is mixed, dried and crushed, and weighed out (usually 5g of sample), and then digested on a hot block with aqua regia (a 3:1 mix of concentrated hydrochloric and nitric acids) for up to two hours. The acid extract is filtered and then loaded into the autosampler rack on the ICP – OES, where it will be aspirated and pumped into the plasma (a very hot, ionised gas at 10,000o C).

Here, energy causes electron excitation, and then as the sample passes out of the plasma, this energy will be emitted as the electrons return to their ground state. The energy emitted will be at specific wavelengths, according to the metals present in the sample, and this energy is detected by the spectrophotometer.

One common myth is that this method provides a total metals result, but this is not the case, as many silicate minerals are not soluble in aqua regia, and therefore will not be included. Hydrofluoric acid would be required to dissolve the silicate matrices, but this presents significant health and safety handling problems, and is therefore not permitted in most laboratories. The aqua regia method is, however, now the industry standard, but should really be reported as ‘aqua regia soluble’.