We tried using the microdist Lachat system with minimal success at my old lab (it seemed really cumbersome with the plastic tubes being so tough and the sample size so small that losing a drop or two became significant), then went back to using macrodist for cn analysis.  We were told that the micro method was only appropriate for aqueous samples and with the 6ml volume I would see any large particles being almost a semi-solid causing interference.  The sulfamic acid should take care of any N interference.  Have you tried running split samples with macro and micro dist methods? Dilution can be a great way to eliminate interference on the macro dist - were your results comparable and did you get good replicates?  Side issue-- I am curious about the lead acetate trap you mentioned, as sulfide was always a potential interference for us and I was told that if sulfide were present there would be no cn (neg interference).  Sorry I can't shed any light except to say that 4 experienced analysts ended up waving the white flag and hoping Lachat would eventually design something better   .

I meant to get in on this discussion last week, but better late than never. As you are aware, there are myriad sources of interference for cyanide analysis. The sulfamic acid treatment is the way to mitigate any nitrate/nitrite issues and can be done pre-distillation like you are doing, or can be added to the sample in the distillation tube with the larger systems. The Lachat tubes are a bit small to make that work well. For sulfides, most methods advise to treat prior to distillation. Standard methods says to precipitate with Pb(C₂H₃O₂)₂ or PbCO₃ while EPA 335.4 says to use CdCO₃. You test a drop of the sample on a strip of lead acetate paper that has been moistened with acetic acid buffers. If the drop turns brown/black you have sulfide present and proceed with precipitation. I can send you the full pretreatment procedure if you would like. Most of the other interferences relate to some form of organic species being in the sample with high heat and lowered pH. I suspect that because the Lachat system distillation time is only 30 minutes as opposed to 90-120 minutes the opportunity for these interferences is lessened. I know that if anyone is using ascorbic acid as a dechlorinating agent you will tend to see false positive cyanide hits, although these are reduced when using the MicroDist method, again most likely due  to the shortened reaction time. There is no good way to avoid the particulates issue with the Lachat tubes. My best suggestion is to homogenize the chunks as best as you can so that they aren't quite as large. Otherwise with such a tight diameter they can form a barrier at the surface that will prevent proper distillation from happening.

Thanks David and BWater,
I appreciate your discussion on this topic. I did not realize that there was such a difference between 335.2 and 335.4. I am in a lab that has been doing the cyanide test for several years (myself only for a year)and I assumed the method I was using was the proper one. Obviously, I still have quite a bit to learn. Based on my research so far, I can definitely see that cyanide is at best a tricky analyte to quantify in the "real world". Maybe the push for ever lower quantifiable limits will lead to the development of a more precise method.