This is an edited repost from 2012 on the old blog.
It seems that there is some value a standardized GABA phantom for cross-platform testing etc... This recipe is also a reasonable place to start if you want a 'will-work-without-much-thought' option. Everything is available form Sigma Aldrich.
Container: 1 liter Nalgene bottle style 2125
Buffer: One PBS sachet P5368
GABA: 1.03 g (RMM 103.1) A2129
Edits from the original, also add:
Make up to 1 liter with deionized water. pH can be adjusted to 7.3 using NaOH/HCl (but to a first approximation does not need to be).