Before revision:  Additionally, we used mass spectrometry to show that DNA purified from cells grown with limiting phosphate and abundant arsenate does not contain detectable arsenate.
After revision:  Additionally, we used mass spectrometry to show that DNA purified from cells grown with limiting phosphate and abundant arsenate does not exhibit the spontaneous hydrolysis expected of arsenate ester bonds. Furthermore, mass spectrometry showed that this DNA contains only trace amounts of free arsenate and no detectable covalently bound arsenate.