Chloromethane in EPA 524

[Bigbear]

I'm puzzled with what is going on on my lab regarding Chloromethane.
We recently changed our sampling procedure by sending out bottles containing 25mg ascorbic acid and ampoules containing 1:1 HCl. Prior to the change we did not dechlorinate sending out bottles containing 0.5ml 1:1 HCl. We had no issues with chloromethane.
Now we get 1-2 ppb chloromethane in our samples and none in the trip blanks.

I feel chloromethane is forming in the samples. Yesterday I sampled tap water in a restroom and saw the following:

No preservation no chloromethane
Ascorbic acid only no chloromethane
Ascorbic acid/ HCl about 0.9 ppb chloromethane
ampouled HCL trace chloromethane ( <0.5ppb)

Today I investigated if acidifying before all the ascorbic acid made any difference. This made no difference.
I'm tempted to just stop reporting chloromethane!

Any ideas?
Thanks

[Bigbear]

Update.

As hard as it is to accept I feel it is coming from my ampouled HCl. I did tests of tap water and AA using ampouled HCl and some HCl I preped in house and found chloromethane in the former but none in the later!
We had another lot of ampouled HCl in house that I tested and found
chloromethane at 0.06 ppb. My rl is 0.5 so I feel that the problem is solved.

I would have bet almost anything that the chloromethane was not coming from the acid!!!

[LindseyPyron]

Hey Big Bear,

Just an fyi. We have seen SO2 bleed from some Vocarb 3000 traps run quite high in the presence of Acid. Blank water seems clean, but the acidified water seems to strip the S02 out of the traps more efficiently.

Since the primary Ion of Chloromethane and SO2 is 64, you could be seeing this instead of Chloromethane. Just a thought.

[camino]

Hi,

Why no try with sodium thiosulphate as reductor agent?

If you didn't have this problem before adding ascorbic acid, maybe it could be the problem.

I hope it was useful.

best regard

[gpronger]

Lindsey,

Sorry, but you have your VOA compounds confused; 64 is chloroethane (CH3CH2Cl = 64 nominally) while chloromethane is 50 (CH2Cl).

Regarding the problem, I would not be surprised by the chloromethane as an artifact from the acid. Over the years, I've seen contaminants in VOA runs from unusual sources. The HCl in the ampoules could be a high end reagent, but the manufacturer of the HCl would never be checking it for trace chloromethane. Whether the source of the ampoules or isn't would be a good question. At one point we had been purchasing pre-preserved vials, and the samples were coming back loaded with bromoform. The manufacturer selling the vials as "certified" but their analysis was on the dry vial, not after they added preservative.

Greg

[Bigbear]

Good to know about the SO2 I've seen it once in a while and thought it was sample related. We have some sulphur wells in the state.
I went to ascorbic acid because I look for the gases, thiosulfate is for compounds that boil above 25C.