The effect of laser wavelength and power density on the laser desorption mass spectrum of fulvic acid

Fulvic acid (FA) is a water-soluble component of natural organic matter whose environmental and organic geochemistry is routinely correlated to its molecular weight. In an attempt to examine the usefulness of laser desorption (LD) mass spectrometry to unambiguously determine this characteristic, laser wavelengths of 10.6 μm, 1.06 μm and 355 nm were evaluated using the four International Humic Substances Society FAs. Under the conditions employed, FA desorption and ionization were optimal when an infrared wavelength is used and thermal desorption conditions dominate. Mass distributions observed in the 10.6 and 1.06 μm FA positive-ion LD mass spectra were centered at 500–600 m/z, and ranged from 200 m/z to beyond 800 m/z. Lower m/z distributions were generally observed in the corresponding negative-ion spectra. Increasing power density, or use of the UV wavelength, resulted in extensive fragmentation of the FA component molecules. The positive-ion spectrum at 355 nm, or at a higher power density, consisted predominantly of low-mass fragments (<200 m/z). The corresponding negative-ion spectra displayed an ion distribution with greater m/z ratios, but the m/z distribution was still less than the negative-ion spectra obtained with an infrared wavelength. The complex pattern of mass distributions observed in the FA LD mass spectra reflect the inherent chemical complexity of these materials.