Quantitative analysis of gibberellins by isotope dilution mass spectrometry: a comparison of the use of calibration curves, an isotope dilution fit program and arithmetical correction of isotope ratios

The quantitative analysis of gibberellin A1 (GA1) in apices of 19-day-old seedlings of Pisum sativum L. cultivar Alaska is described using isotope dilution mass spectrometry. Three methods are compared for determining the dilution of isotope in the internal standards, [17-H-2(2)]GA1 and [2beta-H-2(1)]GA1. Similar results were obtained with each method. The use of calibration curves gave the most precise determination with [H-2(2)]GA1 as internal standard irrespective of the amount added. The use of an isotope dilution fit program was more precise using [H-2(1)]GA1 as internal standard. A simpler arithmetical correction procedure gave consistently higher estimates with both internal standards. The relative merits of the methods are discussed and the requirements for reliable quantification by isotope dilution gas chromatography-mass spectrometry are presented.