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Their recollection was that the volume was within regulation. Since they were discussing whether it was ethical even if was strictly legal, I assume they read the regulation and deliberately stayed within limits. You don't need a lot of volume for l-carnitine, 1g would be a large dose compared with normal dietary intake (equivalent to about 1kg of beef steak). Injectable solutions are typically sold at 100mg/ml or 200mg/ml.
I thought the point was there was no record of the injection made by the U.K. Athletics doctor and therefore nobody knows what volume was injected.