I was LGF at Wellington for awhile, perhaps I can help a little.
NOTE: All my info is over nine months old, and the Embassy was upgrading as I left. I don't think any of this is OPSEC, but if so, feel free to delete (of course
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Now, as I recall, the atomiser (I'm sure you're meaning the same thing as the ITMS Itemiser here) could only pick up six or so differant type of explosives, RDX, Nitro, etc. I'm certain there wasn't a Hexogen on there, though it could fall into one of the categories on the ITMS. These were the old ITMS' though, so the new ones could pick them up.
Perhaps it's new type of explosive?