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Elemental Analysis of Soil Material
INTRODUCTION
In order to quantify the elemental composition of a soil, a
known weight of soil is
dissolved in a known volume of concentrated acid at high
temperature and pressure.
Solutions are then analyzed for elemental concentrations using
ICP spectrometry. To getcomplete dissolution of soil, 3 different
concentrated acids are used: HF (hydrofluoric
acid)-used to break silicate bonds, HNO3 (nitric acid)-used as
oxidizing agent and for
dissolution of metals, HCl (hydrochloric acid)-used for
dissolution of metals. Because of
the extremely dangerous nature of these compounds (especially
hydrofluoric acid, adeadly compound even with low doses of
contact), a special digestion instrument is
employed. The instrument allows single samples to be dissolved
in Teflon vessels at
temperatures up to 200 C and 200 psi. After digestion, the
solution is filtered, diluted to
a known volume and analyzed on the ICP.
EQUIPMENT
Before starting you will need to have:-12 clean 50 mL volumetric
plastic flasks
-12 clean digestion vessels AND lids. These MUST be soaked in
acid bath (1M
HCl) for 24 h before use.
-12 filter assemblies with 0.2 m filter paper and 12 syringes
(10 mL).
- 1 gram dry soil
PROCEDURE
1. Dry soil (preferably in oven at ~70 C) and grind to smallest
reasonable particle size
(0.1-100m, consistency of flour) using an agate mortar and
pestle. If possible, take
multiple subsamples from bulk soil mixture to minimize
variability.
2. Weigh soil into clean digestion vessel. This weight can be
from 0.1-0.25 g, dependingon the digestion conditions. RECORD the
weight of the soil used EXACTLY. This is
preferably done as weight by difference, i.e. (the weight of
soil & weigh boat) (mass ofweigh boat without soil). It is
crucial to have an exact weight of soil. Make sure to have
enough replicates in your samples to assure good statistics for
your samples. Usually in
every 12 samples, 1 sample is run in triplicate (to assure
reproducibility-precision), and 1
standard soil is run (example: Montana soil 2711, Mine Tailing
2709) to assure accuracy.Also, there will need to be at least 3
blank solutions (i.e. vessels without any soil, just the
acid mixture) run in order to make standard solutions.
3. Premix enough acid mixture for ALL digestion vessels. For
soil, the general recipe is:
6 mL HNO3, 2 mL HF and 1 mL HCl. If using all 12 digestion
vessels, premix enough
acid solution for 12 (actually mix enough for 13 vessels, just
so there is enough left over)vessels, and then add 9 mL acid
mixture to each reaction vessel.
4. Close and secure each vessel, checking them for leaks. Be
sure to use the proper
Teflon membrane (500 psi) with the yellow caps. Make sure to
tighten each vessel to theproper amount of torque using the
wrench.
5. Setup the control vessel. MAKE SURE the control vessel has
the SAME solution
volume and content as EVERY other vessel! This is imperative for
the users safety!
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6. Cook vessel. Maximum temperature = 200 C and maximum pressure
= 180 psi.
Standard ramp, and hold solution for 30 minutes at digest
conditions.7. Let cool to room temperature. Air out vessel (be
careful of high vapor pressure in
Teflon vessel due to volatile acids), and wash any liquid drops
stuck in lid into the
reaction vessel. Be sure to account for every drop of digest
solution!
8. Filter acid solution through .2um filter paper. Be sure acid
solution is cool to roomtemperature AND diluted at least 2 times to
keep filter membrane from rupturing. Make
sure to wash digestion vessel 3 times with DDI water, and wash
filter paper 3 times with
DDI water. Dilute solution to 50 mL, and run on ICP. Keep
solution out of contact with
glass, because leftover hydrofluoric acid will react with the
silicates.
