There are 2 "old timer" methods that are used in gold mill assay labs for accurate assay of high grade caustic (cyanide) solutions such as found in electo-winning cells.
The first is a "Spike" method where 75 grams of flux and 1/2 tsp.flour is mixed in crucible, a cavity is made with a spatula in flux and 5 or 10 ml of solution is pipetted into cavity. Cavity is covered with flux, inquarted with 4 times expected gold content and sample is fused, cupelled, parted and weighed as normal fire assay.
Second method is "Test Lead" method where 50 to 100 ml of solution is measured into a 500 ml beaker and diluted with DI water to 300 ml. 8 to 10 grams of granulated test lead is sprinkled into beaker and beaker placed on hot plate under fume hood. 10 ml of concentrated Hydrochloric acid is added and solution brought to a boil for 15 minutes.
Beaker is then cooled and solution carefully decanted leaving the lead on bottom of beaker. Beaker is placed on hot plate to thoroughly dry granulated lead. A 4 inch (8 gram) piece of lead foil is formed into a cone and the granulated lead is brushed into the cone and foil is folded and compressed into a tight cube, cupelled, parted and weighed as
regular fire assay sample. This method also works extremely well with very low grade gold solutions such as mill tailing solutions. A volume of up to 750 ml can be used in a 1000 ml beaker to accurately assay solutions as low as 0.03 g/tonne as the resulting fillet will be large enough to weigh gravimetrically.