The grad student pointed out to me by email that I'd overlooked one big advantage of using CsCl gradients to clean up the DNA. He's not analyzing only the fractions that contain DNA, but all the fractions from the gradient. This allows him to detect where in the gradient any arsenic is, and thus lets him distinguish whether the arsenic is bound to the DNA or independent of it. So even a moderate level of arsenic contamination wouldn't be a problem.
6 minutes ago in Variety of Life