Determination of soyasaponins in soy with LC-MS following structural unification by partial alkaline degradation

High-performance liquid chromatography coupled with electrospray ionization mass spectrometer was used to study the soyasaponins in soy. It was found that each soyasaponin belonging to group A existed mainly in their genuine acetylated forms. The partially to fully deacetylated structures coexisted in various proportions. Likewise, the soyasaponins belonging to group B in soy were detected as both 2,3-dihydro-2,5-dihydroxy-6-methyl-4-pyrone (DDMP) conjugated forms and non-DDMP forms. The structural diversity of soyasaponins hinders the separation and determination of the individual compounds in soy. In the present studies, the soyasaponins extracted from soy were treated with sodium hydroxide under mild conditions to cleave the acetyl groups from soyasaponins in group A as well as the DDMP from soyasaponins in group B, while the glycoside structures remained unaffected. By doing so, all soyasaponins originating from the same initial structures were unified into well-defined structures and then quantified individually using the selective ion recording of their [M-H](-) ions. The pure deacetyl and non-DDMP soyasaponins were used as the external standards. The quantification limits of soyasaponins in group A and group B were 1.74 and 1.89 ng injected on column with recovery rates of 94.1% +/- 4.2% and 96.9% +/- 2.9%, respectively.