Dennis_Simpson

They might be apoptotic.  This measurement is used sometimes as a proxy for apoptosis.  Depending on what was done to the culture they could also arise from mitotic collapse or other non-disjunction type errors during mitosis.

I do use Linux but only for our databases and web hosting.  Analysis is almost entirely Windows.  I like Linux alot more than Windows but there just isn't much off the shelf science software available.

NAC is really easy to dissolve.  You are using it at a fairly high concentration so you could dissolve it in your growth media.  Did you mean to write mM and not uM for your working concentration?  When I have used this in the past I found that mM concentrations kill cells.

Cell biology is my area of expertise.  First, and this may sound strange, define what you want to mean by "dead".  Not as simple as it sounds.  Do you want dead to mean can't proceed through the cell cycle?  Do you want dead to mean de-polorized mitochrondiral membranes?  Do you want dead to mean that the cytoplasmic membrane is not intact?  In all these examples the cells will not form colonies but the  last is the only case that trypan will show dead.  These folks published a nice paper comparing different methods to measure death:  Weyermann J, Lochmann D, Zimmer A (2005). A practical note on the use of cytotoxicity assays. Int J Pharm 288: 369-376.  In a practical sense if your adhearant cells are floating then they are biologically dead.  Adhearent cells floating 4 to 6 hours (or longer) after treatment will never reattach and divide.  I am also not surprized that they look biochemically different than the ones still adhearent.  If at all possible a colony formation type assay will give the most unequivacal results.  Also, as noted by others, do not forget a vehical control sample.  Good luck.