Gibberellins are assigned A-numbers in order of discovery according to the system proposed by MacMillan and Takahashi (Nature 217, 170-171, 1968).
Numbers will be assigned to novel GAs for which evidence is presented for their natural occurrence and an unequivocal assignment of structure. Identification can be established by:
- Isolation of the pure compound and determination of structure from physical, primarily spectroscopic, properties.
- Comparison of the compound with a fully-characterised synthetic standard by GC-MS or equivalent method. Matches are based on KRI and m/z values with relative intensities from a full spectrum.
Retention indices can be obtained from capillary GC-MS to provide additional criteria for identification. To overcome variations in column temperature and flow-rate, the retention index procedure of Kovats(1958) is recommended.
A solution of each n-alkane from C16 to C36 is co-injected with the sample and the retention times of the n-alkanes peaks are recorded. The retention time of the peak of interest is also recorded.
The retention index is calculated with the following formulae.
Cx = (Rt(peak of interest) - Rt(preceding n-alkane) + Cn(preceding n-alkane)x ) / ( Rt(following n-alkane) - RT(preceding n-alkane) )
Cx × 100 = Retention index.
Reference: Kovats E (1958) Gas chromatographische Charakteriserung organischer Verbindungen.I.Retentions indices aliphatischer halogenide, alkohole, aldehyde und ketone Helv Chim Acta 41: 1915-1932