Chemical Stability and Compatibility - Antimony and Molybdenum Troiubleshooting

Not sure where to start with this one! We are transitioning from an ICP to an ICPMS. During the initial stages we developed our method and ran everything pretty much the same as on the ICP. Then we noticed that the translation didn't always produce the results we needed. We have since tweaked out method but are having problems with Antimony and Molybdenum. Sb is a problem child for our wastewater samples and Mo is for our sludge samples. I do not think that this is an instrumentation issue but more so one of lack of understanding as well as issues with our matrix/digestion processes (our undigested calibration curve standards look great). Our standards are all made up with a matrix of 1% Nitric/0.5% HCl. When we digest our water samples we use 50 mL of sample and .5mL of HNO3 and .25 mL of HCl and heat for 3.5 hours then bring to a 50 mL volume. With sludge samples we use 5 g of solid and bring that to a 50 mL volume, add 3.0 mLs of HNO3 and heat for 4.5 hours then add 2.0 mL of H2O2 and heat an additional 30 minutes before cooling and bringing back up to 50 mL. If anyone has any suggestions on what we should to it would be greatly appreciated. I myself am new to the ICPMS world and can use any and all the help that I can get.