At Actlabs’ Assay Laboratory, the “true value” of a commodity is determined as accurately as possible with assays.
Assays provide quantitative determinations of elements in non-processed geological materials. Assays are usually required when samples are known or suspected to contain higher levels of metals.
Accuracy is typically in the 1-3% range as long as the analyte is greater than 100 times the detection limit of the method. For some elements more difficult to analyze, this may stretch to 5%. Analyses can be purely instrumental (INAA or Delayed Neutron Counting (DNC)) or can be produced by chemical treatment such as fusion or acid digestion with final analyses commonly by AA, XRF, ICP-OES, ICP-MS, titration or gravimetric methods.
Usually higher levels of dilution and use of internal standards help minimize matrix effects. All dilutions are done either volumetrically or gravimetrically to ensure minimal error.
This partial digestion uses a mixture of hydrochloric and nitric acids to dissolve sulphides, some oxides and some altered silicates. Generally base metals will be totally dissolved, but this depends on mineralogy (not silicate Ni or Zn in gahnite).
A 0.5g sample is digested in aqua regia and diluted volumetrically to 250 ml with 18 megaohm water. CANMET reference materials for the appropriate elements are digested the same way and are used as a verification standard(s). Samples are analyzed on a Varian Vista 735 ICP-OES or ICP-MS.
8 – AR – ICP-OES Elements and Detection Limits
Element Detection Limit Ag 3 ppm As 0.01 % Cd 0.003% Co 0.003% Cu 0.001% Fe 0.003 % Hg 0.001% Ni 0.003% Pb 0.003% Zn 0.001%
Note: Extraction for Aqua Regia is dependent on mineralogy.
This digestion is as total as an acid digestion can get, but will still not dissolve very resistate phases like chromite, barite, monazite, sphene, xenotime, etc. The method is NOT suitable for rare earth elements. This digestion is used for Cu, Ni, Co, Pb, Zn, Mo and Ag assays.
The digestion uses nitric, perchloric, hydrofluoric and hydrochloric acids with temperatures to 260°C. Assay packages for base metals using 4 acid digestions are then analyzed by ICP-OES.
8 – 4-Acid ICP-OES 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.
8-peroxide Fusion ICP
Samples are fused with sodium peroxide and undergo an acid dissolution. Samples are then analyzed by ICP-OES. Calibration is performed using five synthetic calibration standards. A set of (10-20) fused certified reference material is run with every batch of samples for calibration and quality control. Fused duplicates are run every 10 samples.
Element Detection Limit Al 0.01% As 0.01 - 10 % Be 0.001% Ca 0.01% Co 0.002% Cr 0.01% Cu 0.005 % Fe 0.05 % K 0.1 % Li 0.01 % Element Detection Limit Mg 0.01% Mn 0.01 % Ni 0.005% Pb 0.01 % S 0.01 % Sb 0.01 % Si 0.01 - 47% Ti 0.01 % W 0.005 % Zn 0.01 %
Samples are fused with sodium peroxide and undergo an acid dissolution. Samples are then analyzed by ICP-MS. Calibration is performed using five synthetic calibration standards. A set of (10-20) fused certified reference material is run with every batch of samples for calibration and quality control. Fused duplicates are run every 10 samples.
Elements and Detection Limits
Element Detection Limit Al 0.001 % As 0.001-10% Be 0.001% Bi 0.001% Ca 0.01% Co 0.001% Cr 0.01% Cs 0.001% Cu 0.001% Fe 0.05% Ga 0.001% Ge 0.001% In 0.001% K 0.1% Li 0.001% Mg 0.01% Mn 0.01% Mo 0.001 % Element Detection Limit Nb 0.001% Ni 0.001% Pb 0.001% Re 0.001% S 0.01% Sb 0.002% Se 0.001% Si 0.01 - 47 % Sn 0.001% Ta 0.001% Te 0.001% Th 0.001% Ti 0.01% Tl 0.001% U 0.001% W 0.001% Zn 0.001%
Total uranium can be determined by delayed neutron counting using an automated Delayed Neutron Counting system at a nuclear reactor. Samples are irradiated in series for a brief period, the samples are then transferred under computer control to a delayed neutron counter which is an array of six BF3 detectors and neutrons emitted from the fissioning U-235 is thermalized and counted. The principle advantage of this technique is it provides very rapid and accurate assays of uranium for high volumes of samples at a very low cost. The upper limit of this technique is 1% U. Results can be reported as U or U3O8. From 1 to 10% fusion XRF will provide the best quality assay results and above 10% titration is recommended for accuracy.
Three blanks and a minimum of two standards are run at the beginning and end of counting cycle to ensure proper setup. One blank and one standard are counted every 30 samples. Four standards of varying U concentration are rotated throughout the run. Duplicates are analyzed when sample is provided.
Assays provide quantitative determinations of elements in non-processed geological materials. Assays are usually required only when the client knows or suspects higher levels of metals in samples. Geochemical methods generally provide lower detection limits than assays. For lower levels, geochemical methods should be used. All assays are traceable to international reference standards. Prices listed in our fee schedule are for normal geologic materials and are not for metallurgical products. Metallurgical products such as heads and concentrates are handled separately to prevent contamination in the laboratory. These materials are charged at an additional fee.
8 Assay Products
Element Ga Ge Insolubles Fe2O3 La Pb PbO Li LOI – Loss On Ignition Hg MnO Moisture - H2O Mo - total Mo - oxide Mo - sulphide Ni Ni Sulphide Nb P2O5 Pt-Pd-Au K2O Re - Mo concentrates Element Rh Se SiO2 Ag Na2O Specific Gravity Specific Gravity wax encapsulation Sr Sulphur - infrared Sulphur - Gravimetric SO4 Ta Te Tl Th Sn TiO2 WO3 U3O8 V2O5 Zn