The effect of different dopants on the sensitivity of ion mobility-mass spectrometry toward plant hormone auxin

Ion mobility spectrometry (IMS) equipped with a corona discharge (CD) ion source was used for measurement of three auxin plant hormones including indole-3-acetic acid (IAA), indole-3-propionic acid (IPA), and indole-3-butyric acid (IBA). The measurements were performed in both positive and negative polarities of the CD ion source. Dopant gases NH3, CCl4, and CHBr3 were used to modify the ionization mechanism. A time-of-flight mass spectrometer (TOFMS) orthogonal to the IMS cell was used for identification of the product ions.

In the positive polarity without dopants, auxins were ionized via protonation and three distinguished peaks were observed. Application of NH3 dopant resulted in two ionization channels, protonation and NH4+ attachment leading to peak overlapping. In the negative polarity two ionization reactions were operative, via deprotonation and O2- attachment. The separation of the monomer peaks was not achieved while the peaks of anionic dimers [2M-H]- were separated well. The best LOD (4 ng) was obtained in negative polarity with CCl4 dopant.