The molecular weight of arsenic is 75, so 75 g/l is a 1 M solution, and 75 mg/l is 1 mM. I think I'd make a 1 M stock solution of sodium arsenate and, after somehow sterilizing it, use it to make culture media with up to 40 mM arsenate.
I found some US Environment Protection Agency limits online, but these are for industrial-scale work (e.g. how much arsenic for landfill) Sewage sludge can apparently contain 73 mg arsenic per kg; that's about 1 mM! The US EPA limit for drinking water is 10 ppb (10 ng/ml), which is about 0.13 µM if I've done the arithmetic correctly. That seems like a lot, but, according to this very detailed document, normal background arsenic concentrations in soil range from 1- 40 mg/kg, with a mean of about 5 mg/kg (that's about 70 µM). Our total intake in food and beverages per day ranges between 20 and 300 µg. Smokers get about 10 µg/day just from their cigarettes.
I got the attention of the university's chemical safety person by sending an email to their supervisor. Both of us are going to be away for the next week or so, and we plan to talk when we're back. To get things started I left a voicemail asking about the specific contamination limits - what level of arsenic requires cleanup-decontamination work, and what can be treated as 'normal background'? The preliminary response (by email) was
"...while we cannot talk about background level with respect to arsenate, any concentration above 0.01 mg/M3 of arsenate, will have a potential adverse health effect."Wait, what? We can't talk about background level? Why not? And, assuming 'M3' is cubic meter, cubic meter of what? Air? Water? What about contamination on a surface? A cubic meter is 10^6 ml, so 0.01 mg/M3 would be 0.01 ng/ml, 1000-fold lower than the EPA limit for drinking water. (The molecular weight of arsenate is about twice that of arsenic, but I'll neglect the difference for now.) OK, this EPA document says that 0.01 mg/m^3 is the maximum average level of airborne arsenic that a worker can be exposed to (averaged over an 8 hr workday).
My voicemail also asked how one could detect arsenic contamination? Is there a sensitive chemical test? If I were to miss cleaning up a drop of a spill and the dried up arsenic spreads around on people's shoes, how would we know? The email response said:
"If you have a concern that the area is contaminated, wipe samples can be sent to testing laboratory for analysis."But in the absence of any specification of what level of surface arsenic is considered 'contamination', how would I know whether to be concerned, or how to interpret the results? And how much does this analysis cost, what's the sensitivity, and what's the turnaround time? Maybe I'll be able to pry this information out of the safety person when we talk, but I suspect the real reason I'm getting such terse responses to my questions is that they don't know the answers.
This whole arsenic-safety business sucks. My experience so far suggests that the Safety Office is not going to be much help - I'm mostly going to have to dig out the information for myself and figure out how it applies to my experiment. Second, maybe there really aren't any guidelines for contamination in laboratory work. That would be consistent with the safety advice I got from my chemical engineering colleague, which was to be very careful to avoid personal exposure when handling the solid chemical (when making up solutions) and otherwise to just use normal care. I don't think they've been doing contamination checks.