Control DNA preps for my mass-spec colleagues to test

I've got lots of GFAJ-1 cells in the fridge, so I think I'll make some DNA preps from them to send to my colleagues at Princeton for the initial mass-spec measurement of baseline levels of arsenic.  They've done the initial controls, determining that their setup's detection limit is about 1-^-7 - 10^-8 M arsenate, equivalent to replacement of about 0.01% of the phosphates in the DNA backbone with arsenate.  This is about 300-fold lower than the 4% replacement claimed by Wolfe-Simon et al.

The cells were grown in AML60 medium with either limiting or ample phosphate - I don't think it matters which.  I'll do one DNA prep with the limiting-phosphate cells.  As a control for carry-over arsenate contamination, I'll add sodium arsenate to the other culture (40 mM) and let them steep in it for a little while, then wash away the arsenate and do another DNA prep.  I'll also take some purified DNA, soak it in 40 mM arsenate for a little while, and then repurify it.  If my DNA purification methods are good, none of the DNAs should have detectable arsenic.