I'm not too sure about your question. Wet weight and dry weight are already estimates of biomass. Ash weight however is most likely not.
The issue when estimating zooplankton biomass is that wet weight is not too reliable because water always stick to zooplankton artificially increasing biomasses. Dry weight might be better but if a lot of water was there, an important part of the biomass measured could be salt or other non-biological detritus (particularly if you work with marine samples).
This is why a biomass measure often computed is ash-free dry weight. It is dry weight - ash weight. Ashes are mostly composed of the mineral fraction. Therefore by removing their weight, you may obtain a more accurate biomass estimate. I hope this is what you were looking for?
Thank you so much Nicolas Djeghri . I was looking for the answer you gave. I have ash weight data too. That is why I wanted to know what would be the appropriate formula for biomass of zooplankton. You cleared my queries.