4 ACID “NEAR TOTAL” DIGESTION
This acid attack is the most vigorous digestion used in geochemistry analysis and uses hydrochloric, nitric, perchloric and hydrofluoric acids.
Even with this 4 Acid “near total” digestion, certain minerals (barite, gahnite, chromite, cassiterite, etc.) may only be partially dissolved or stable in solution. Other minerals including zircon, sphene and magnetite may not be totally dissolved. Most other silicates will be dissolved, however some elements will be erratically volatilized, including As, Sb, Cr, U and Au.
Near-Total digestion cannot be used to obtain accurate determinations of REE, Ta, Nb, As, Sb, Sn, Hg, Cr, Au and U.
A 0.25g sample is digested with four acids beginning with hydrofluoric, followed by a mixture of nitric and perchloric acids. This is then heated using precise programmer controlled heating in several ramping and holding cycles which takes the samples to incipient dryness. After incipient dryness is attained, samples are brought back into solution using aqua regia.
With this digestion, certain phases may be only partially solubilized. These phases include zircon, monazite, sphene, gahnite, chromite, cassiterite, rutile and barite. Ag greater than 100 ppm and Pb greater than 5000 ppm should be assayed as high levels may not be solubilized. Only sulphide sulfur will be solubilized.
The samples are then analyzed using an ICP. QC for the digestion is 14% for each batch, 5 method reagent blanks, 10 in-house controls, 10 samples duplicates, and 8 certified reference materials. An additional 13% QC is performed as part of the instrumental analysis to ensure quality in the areas of instrumental drift.
Elements and Detection Limits (ppm except where noted)
| Element | Detection Limit | Upper Limit |
|---|---|---|
| Ag | 0.1 | 100 |
| Al | 0.01% | 50% |
| As | 3 | 5,000 |
| Ba | 1 | 1,000 |
| Be | 0.05 | 10,000 |
| Bi | 1 | 10,000 |
| Ca | 0.01% | 70% |
| Cd | 0.3 | 2,000 |
| Co | 1 | 10,000 |
| Cr | 1 | 10,000 |
| Cu | 1 | 10,000 |
| Fe | 0.01% | 50% |
| Element | Detection Limit | Upper Limit |
|---|---|---|
| Ga | 1 | 10,000 |
| K | 0.01% | 10% |
| Li | 2 | 10,000 |
| Mg | 0.01% | 50% |
| Mn | 5 | 100,000 |
| Mo | 1 | 10,000 |
| Na | 0.01% | 10% |
| Ni | 1 | 10,000 |
| P | 0.001% | 10% |
| Pb | 2 | 5,000 |
| S+ | 0.01% | 20% |
| Element | Detection Limit | Upper Limit |
|---|---|---|
| Sb | 5 | 10,000 |
| Sr | 1 | 10,000 |
| Te | 2 | 10,000 |
| Ti | 0.01% | 10% |
| Tl | 5 | 10,000 |
| Y | 1 | 1000 |
| Sc | 0.5 | 10,000 |
| U | 5 | 10,000 |
| V | 0.5 | 10,000 |
| W | 2 | 10,000 |
| Zn | 2 | 10,000 |
| Zr | 3 | 10,000 |
Notes: Extraction of each element by Total Digestion is dependent on mineralogy.
+ Sulphide sulphur and soluble sulphates are extracted
Assays are recommended for values which exceed the upper limits.
Ultratrace 6 combines the Total Digestion (HF, HClO4, HNO3 and HCl) with analysis by ICP. Resistate minerals are not digested.
“Near Total” Digestion – ICP
A 0.25g sample is digested with four acids beginning with hydrofluoric, followed by a mixture of nitric and perchloric acids, heated using precise programmer controlled heating in several ramping and holding cycles which takes the samples to incipient dryness. After incipient dryness is attained, samples are brought back into solution using aqua regia.
With this digestion, certain phases may be only partially solubilized. These phases include zircon, monazite, sphene, gahnite, chromite, cassiterite, rutile and barite. Ag greater than 100 ppm and Pb greater than 5000 ppm should be assayed as high levels may not be solubilized. Only sulphide sulfur will be solubilized.
The samples are then analyzed using an ICP. QC for the digestion is 14% for each batch, 5 method reagent blanks, 10 in-house controls, 10 samples duplicates, and 8 certified reference materials. An additional 13% QC is performed as part of the instrumental analysis to ensure quality in the areas of instrumental drift.
“Near Total” Digestion – ICP-MS
Additional elements are determined by ICP-MS on the multi-acid digest solution above. The samples are diluted and analyzed on an ICP/MS. One blank is run for every 40 samples. In-house control is run every 20 samples. Digested standards are run every 80 samples. After every 15 samples, a digestion duplicate is analyzed. Instrument is recalibrated every 80 samples.
Elements and Detection Limits (ppm)
| Element | Detection Limit | Upper Limit | Reported By |
|---|---|---|---|
| Ag | 0.01 | 100 | ICP |
| Al | 0.01% | 50% | ICP |
| As | 0.1 | 5,000 | ICP/MS |
| Ba | 1 | 1,000 | ICP/MS |
| Be | 0.05 | 10,000 | ICP/MS |
| Bi | 0.02 | 10,000 | ICP/MS |
| Ca | 0.01% | 70% | ICP |
| Cd | 0.04 | 2,000 | ICP/MS |
| Ce | 0.01 | 10,000 | ICP/MS |
| Co | 0.1 | 10,000 | ICP/MS |
| Cr | 1 | 10,000 | ICP/MS |
| Cs | 0.03 | 100 | ICP/MS |
| Cu | 0.2 | 10,000 | ICP/MS |
| Dy | 0.05 | 5,000 | ICP/MS |
| Er | 0.03 | 1,000 | ICP/MS |
| Eu | 0.03 | 100 | ICP/MS |
| Fe | 0.01% | 50% | ICP |
| Ga | 0.05 | 10,000 | ICP/MS |
| Ge | 0.05 | 500 | ICP/MS |
| Gd | 0.05 | 5,000 | ICP/MS |
| Hf | 0.1 | 500 | ICP/MS |
| Ho | 0.01 | 1,000 | ICP/MS |
| In | 0.01 | 100 | ICP/MS |
| K | 0.01% | 10% | ICP |
| La | 0.1 | 5,000 | ICP/MS |
| Li | 0.2 | 10,000 | ICP/MS |
| Lu | 0.01 | 100 | ICP/MS |
| Mg | 0.01% | 50% | ICP |
| Mn | 5 | 100,000 | ICP |
| Mo | 0.05 | 10,000 | ICP/MS |
| Element | Detection Limit | Upper Limit | Reported By |
|---|---|---|---|
| Na | 0.01% | 10% | ICP |
| Nb | 0.1 | 500 | ICP/MS |
| Nd | 0.1 | 10,000 | ICP/MS |
| Ni | 0.4 | 10,000 | ICP/MS |
| P | 0.001% | 10% | ICP |
| Pb | 0.5 | 5,000 | ICP/MS |
| Pr | 0.03 | 1,000 | ICP/MS |
| Rb | 0.1 | 5,000 | ICP/MS |
| Re | 0.005 | 100 | ICP/MS |
| S+ | 0.01% | 20% | ICP |
| Sb | 0.1 | 10,000 | ICP/MS |
| Sc | 0.1 | 10,000 | ICP |
| Se | 0.3 | 1,000 | ICP/MS |
| Sm | 0.03 | 100 | ICP/MS |
| Sn | 0.2 | 5,000 | ICP/MS |
| Sr | 0.2 | 10,000 | ICP/MS |
| Ta | 0.05 | 1,000 | ICP/MS |
| Tb | 0.01 | 100 | ICP/MS |
| Te | 0.05 | 10,000 | ICP/MS |
| Th | 0.02 | 5,000 | ICP/MS |
| Ti | 0.001% | 10% | ICP/MS |
| Tm | 0.01 | 1,000 | ICP/MS |
| U | 0.1 | 10,000 | ICP/MS |
| V | 0.5 | 10,000 | ICP/MS |
| W | 0.1 | 10,000 | ICP/MS |
| Y | 0.1 | 1,000 | ICP/MS |
| Yb | 0.03 | 5,000 | ICP/MS |
| Zn | 2 | 10,000 | ICP/MS |
| Zr | 0.5 | 10,000 | ICP/MS |
Notes: Extraction of each element by Total Digestion is dependent on mineralogy.
Unaltered silicates and resistate minerals may not be dissolved.
If any elements are over-range, an 8-4 Acid ICP Assay can be performed
Assay packages for base metals using 4 acid digestion and ICP-OES.
8 – 4-Acid ICP Assay, Elements and Detection Limits
| Element | Detection Limit |
|---|---|
| Ag | 3 ppm |
| Cd | 0.003 % |
| Co | 0.003 % |
| Cu | 0.001 % |
| Li | 0.001 % |
| Mo | 0.003 % |
| Ni | 0.003 % |
| Pb | 0.003 % |
| Zn | 0.001 % |
Note: Extraction for Total Digestion is dependent on mineralogy
