You may extract ABA with ethyl acetate to be measured directly by HPLC. For GC-MS or LC-MS try to use a mixture of methanol:isopropanol:acetic acid (1% v/v final concentration) or methanol:isopropanol:formic acid(0.1% v/v final concentration).
You can try commercial ELISA kits, like the Agrisera kit (http://www.agrisera.com/en/artiklar/aba-elisa-quantitation-kit-2.html). However, from tissue samples, as leaves, you must obtain a clean sample, and one of the best methods is cleaning the samples through HPLC (as in the papers attached). If you don't clean the samples by extraction combined with HPLC or similar procedures, ELISA wouldn't work properly, as other compounds present in the samples can interfere with the ABA-antibody union.
I hope this will help you.
Regards.
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