Milk digestion procedure for AAS analysis

I am working on a project of analyzing heavy metals in milk samples using flame AAS. So far I have only analyzed cadmium and the results of spiked and unspiked have large differences. I was wondering if something went wrong with my digestion procedure. The procedure is listed as followed:


* 2g of milk sample, add 5 ml of 68% HNO3 (spiked solution spiked with 1ml 100ppm of Cd stock solution)
* solution heated at 70 °C for 2 hours with hot plate until solution clears up
* 3ml of H2O2 added to solution and heated to effervescence and cool solution
* filter solution through filter paper
* dilute solution with minimum HNO3
* dilute with 5% HNO3 in 100ml volumetric flask
* filter with 0.7 nm syringe filter
* analyze with AAS


after the digestion of HNO3 and H2O2, there are some floating bits which I am not sure what are the compositions, which cannot be digested or dissolved even when I add extra acid. I suspect that filtering with the filter paper may have affected the metal content to be lower than expected. The spiked sample also do not show consistent readings. Glasswares have all been soaked in 5% HNO3 overnight.


I want to know is there anywhere I can improve my digestion procedure? Any relevant literature I can review? Thanks.


