Laser applications to the study of non-volatile compounds in fruits

Laser applications to the study of non-volatile compounds in fruits 

One of the major analytical problems with fruit and vegetable samples is the detection and identification of non-volatile organic compounds present in low concentration levels, as happens for most of the phytoalexins produced by plants. Mass spectrometry is widely used in the analysis of such compounds, providing exact mass identification. However, the difficulty with their unequivocal identification and quantitative detection lies in their volatilization into the gas phase prior to injection into the analyser. This constitutes particular problems for thermally labile samples, as they rapidly decompose upon heating. To circumvent this difficulty a wide range of techniques have been applied for non-volatile compound analysis, including FAB (Fast Atom Bombardment), FD (Field Desorption), LD (Laser Desorption), PD (Plasma Desorption) and SIMS (Secondary Ion Mass Spectrometry). Further details of laser desorption can be found in Section 28.7. 

More recently, LD methods have been developed in which the volatilization and ionization steps are separated, providing higher sample sensitivity. While all 

The high selectivity of REMPI-TOFMS stems from the combination of the mass-selective detection with the resonant ionization process, i.e. the ionization is achieved by absorption of two or more laser photons through a resonant, intermediate state. This condition provides a second selectivity to the technique, namely laser wavelength-selective ionization. In addition, other clear advantages of REMPI-TOFMS are its great sensitivity and resolution, major ionization efficiency, easy control of the molecular fragmentation by the laser intensity and the possibility of simultaneous analysis of different components present in a matrix. 

As an example, we shall study here the application of a laser technique specially designed to perform fast and direct analysis of non-volatile compounds in fmit and vegetables, particularly rrans-resveratrol (3,5,4 -trihydroxystilbene) in grapes and vine leaves. The method is based on the combination of LD followed by REMPI and TOFMS detection, often identified by its sum of acronyms, i.e. LD-REMPI-TOFMS (e.g. Orea et al, 2001, and Orea and Gonzalez Urena, 2002) 

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