In a conventional reverse phase SPE column, the organic moiety
(C8 or C18) is bonded to either a silica or polymeric skeleton.
The interactions are primarily hydrophobic ones. In typical
cation exchange columns, the interactions are based upon ion
exchange chemistry. GHB has been previously analyzed using
cation exchange chemistry [35]. In this published procedure, the
GHB is collected in the filtrate rather than on SPE column, and was
further isolated via a liquid–liquid extraction. In anion exchange
SPE, the organic moiety bonded to the silica backbone is a quaternary
ammonium material, which maintains its positive charge over
the whole pH range. The mechanism for sorbing the GHB is based
upon pH, at neutral pH, the carboxylic acid part of molecule is in a
negatively charged form, thus is strongly attracted to the sorbent
(being positively charged). The effect of lowering the pH (with 6%
acetic acid in methanol) allows the GHB/GHB-d6 to be eluted from
the sorbent by neutralizing its negative charge. Washing the SPE
sorbent with DI water and methanol has no effect except in removing
co-extracted material, thus does not impact upon recovery.