FIG. 1.

Collision-induced dissociation mass spectra of GABA (A) and IS (B). An ESI in a positive ionization mode was used. Positive ESI generates mostly protonated ions ([M+H]⁺) from molecules containing a functional group with the potential for ionization. As an amino acid, GABA offers an ionizable amine function and can be protonated under the present conditions. The positively charged molecular ions m/z 104.1 and 110.2 were generated for GABA and IS, respectively. Using the product ion scan mode, several product ions were generated. Then, the most abundant and specific product ion was selected for MRM transition: m/z 104.1 [M+H]⁺→45.1 for GABA (A) and m/z 110.2 [M+H]⁺→93.2 for IS (B). GABA, γ-aminobutyric acid; IS, internal standard; MRM, multiple reaction monitoring; ESI, electrospray ionization.