Determination of gold in rocks by neutron activation analysis using fire-assay preconcentration

Abstract

TWO RELIABLE TECHNlQUES for the determination Of gold in geologic materials are neutron activation analysis and classical fire assay. The high sensitivity of activation analysis has allowed the measurement of gold at the parts-per-billion concentration level in rocks (1-5). In conventional activation analysis, 0.1-to 1-gram samples and standards are irradiated with neutrons dissolved in the presence of gold carrier, and the carrier plus activated gold are separated from other radioactivity by radiochemical procedures. Counting of the lg8Au and measurement of the chemical yield of the carrier allows calculation of the amount of gold in the samples. Several factors contribute to the uncertainty in results obtained by this technique. The relatively small samples used may not be representative for gold (6, 7). In addition, the gold standards may not be exposed to exactly the same neutron flux as the samples, and the chemical exchange between lg8Au and the carrier may be incomplete. On the other hand, contamination of the carrier gold by gold from the reagents can be ignored, thus eliminating the necessity for a reagent gold blank.Bugbee (8) and Beamish (9) have discussed classical fire assay procedures. Variations of the classical technique have involved the estimation of the gold content of the silver bead by spectrography (IO) and atomic absorption (11), but the sensitivities are not sufficient to measure nanogram quantities of gold and therefore, gold cannot be determined in rocks a t the parts-per-billion concentration level. Although larger, more representative samples are used in fire assay, the possibility exists for incomplete recovery of gold in the fusion and cupellation steps. With low levels of gold, contamination by gold from the reagents necessitates carrying a blank through the procedure.It is a logical step to combine fire-assay preconcentration of gold with the measurement of gold by rapid instrumental neutron activation analysis. Washington and Holman (12) developed such a procedure in which they irradiated the final silver bead and determined lg8Au by gamma counting. Their detection limit, with a precision of 50%, was only 10 ppb gold because of the large amount of 253-day llomAg, generated during the neutron irradiation, and a sizable reagent gold blank.The technique we have developed involves irradiation of the lead button obtained by the fire-assay procedure prior t o the addition of silver. This approach provides a uniform, dilute, and reproducible matrix for the neutron irradiation of gold, reduces the counting background due to llornAg, and avoids the possibility of introducing gold contamination with the silver. After irradiation, silver is added, the lead button cupelled, and the 1g8Au counted instrumentally without further separations. The sources of error in this technique differ from those in conventional neutron activation analysis; therefore, if both methods give the same result for gold, confidence in the reliability of each is increased. EXPERIMENTAL

Determination of the noble metals in geological materials by neutron activation analysis

“…individual noble metals for counting (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19), with the disadvantages of length of time and potential losses during the radiochemical procedures.…”

The determination of gold in geologic materials by neutron-activation analysis using fire assay for the radiochemical separations

“…They found that the fireassay fusion will recover at least 98 percent of the gold in the samples. From radiotracer experiments Simon and Millard ( 1968) found that the slag retained from 0.3 to 1.5 percent of the gold and that the silver bead recovered between 97 and 99 percent ofthe gold. A study was made to de.termine the fire-assay recovery of gold tracer when gold carrier was added to give a gold bead after cupellation.…”

“…Shcherbakov and Perezhogin (1964), neutron-activation analysis with 8 radiochemical separation of gold. Simon and Millard (1968), neutron-activation analysis after fire-assay 9 preconcentration. Sarma, Sen, and Chowdhury (1965), emission spectrographic analysis on silver fire-assay bead.…”

“…Their detection limit was 10 ppb (parts per billion) gold on a 15-g sample because of the Ag 110m activity and the comparatively large reagent blank. Simon and Millard (1968) improved on this method by irradiating the lead button prior to cupellation with silver. Although this method has the advantage of using a 10-to 20-g sample for analysis, it is not applicable to routine analysis because only a few samples can be irradiated simultaneously and the fire-assay flux must be purified before assay to maintain a low blank.…”

Analysis of the polychlorinated biphenyl problem. Application of gas chromatography-mass spectrometry with computer controlled repetitive data acquisition from selected specific ions

“…To determine all of the noble metals in a diverse variety of geological materials, radiochemical separation procedures have had to be used with neutron activation analysis. These published methods have involved fire assay, cupellation, volatilization, ion exchange, solvent extraction, and final precipitation of individual noble metals for counting (6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19), with the disadvantages of length of time and potential losses during the radiochemical procedures.…”