Electrospray ionization mass spectrometry of terrestrial humic substances and their size fractions

Abstract

Electrospray ionization mass spectrometry (ESI-MS) was used to evaluate the average molecular mass of terrestrial humic substances, such as humic (HA) and fulvic (FA) acids from a soil, and humic acid from a lignite (NDL). Their ESI mass spectra, by direct infusion, gave average molecular masses comparable to those previously obtained for aquatic humic materials. The soil HA and FA were further separated in size-fractions by preparative high performance size exclusion chromatography (HPSEC) and analyzed with ESI-MS by both direct infusion and a further on-line analytical HPSEC. Unexpectedly, their average molecular mass was only slightly less than for the bulk sample and, despite different nominal molecular size, did not substantially vary among size-fractions. The values increased significantly (up to around 1200 Da) after on-line analytical HPSEC for the HA bulk sample, at both pH 8 and 4, and for the HA size-fractions when pH was reduced from 8 to 4. It was noticed that HA size-fractions at pH 8 were separated by on-line HPSEC in further peaks showing average masses which progressively increased with elution volume. Furthermore, when the HA and NDL bulk samples were sequentially ultracentrifuged at increasing rotational speed, their supernatants showed mass values which were larger than bulk samples and increased with rotational speed. These variations in mass values indicate that the electrospray ionization is dependent on the composition of the humic molecular mixtures and increases when their heterogeneity is progressively reduced. It is suggested that the dominance of hydrophobic compounds in humic supramolecular associations may inhibit the electrospray ionization of hydrophilic components. Our results show that ESI-MS is reasonably applicable to humic substances only after an extensive reduction of their chemical complexity.

Agricultural uses of plant biostimulants

“…In accordance with this theory, previous studies have suggested that humic substances (HS) have only an apparent high molecular size, which can be reversibly disrupted by treating humic solutions with low concentrations of mono, di, and tricarboxylic acids (Nardi et al 1988;Nardi et al 1996). More recently, numerous studies have shown that the amphiphilic properties of the organic acids in root exudates can dissociate HS into low molecular size and high molecular size (Nardi et al 1997(Nardi et al , 2000(Nardi et al , 2002Piccolo et al 2002;Piccolo and Spiteller 2003). This new interpretation support the hypothesis that the conformational behaviour of dissolved humus in the rhizosphere and therefore also the interaction of humic components with plant-root cells, may be controlled by the presence of root-exuded or microbereleased organic acids in the soil solution .…”

“…Interestingly, dissolved humic substances are common in freshwater bodies where they have been shown to interact with freshwater organisms, a topic reviewed by (Steinberg et al 2008). In soil, humic substances are reported to play key roles in various soil and plant functions (Berbara and García 2014) such as controlling nutrient availability, carbon and oxygen exchange between the soil and the atmosphere, and the transformation and transport of toxic chemicals (Piccolo and Spiteller 2003). In addition, humic substances in soils affect plant physiology and the composition and function of rhizosphere microorganisms (Varanini and Pinton 2001).…”

Caracterização química e espectroscópica de ácidos húmicos e fúlvicos isolados da camada superficial de latossolos brasileiros

“…As SH têm sido descritas genericamente como macromoléculas orgânicas heterogêneas, de coloração escura, resultado do produto do metabolismo de microrganismos (Aiken et al, 1985). Piccolo (2002) aplicou o conceito da química das supramoléculas para SH e, por meio de uma série de evidências cromatográficas (Piccolo et al, 1996) e espectroscópicas (Simpson, 2002;Piccolo & Spiteller, 2003), postulou uma nova concepção para a estrutura das SH. Moléculas orgânicas diversas, mas relativamente pequenas, formam agregados unidos entre si por ligações hidrogênio e interações hidrofóbicas fracas, formando um arranjamento supramolecular.…”

Effect of organic fertilization on yield and quality of rosemary (Rosmarinus officinalis L.) essential oil

“…Actually associated rhizosphere microorganisms are used frequently to solubilize compounds, to enhance element fixation, to promote growth through secondary metabolites or phytohormones induction, and to induce systematic resistance in plants of interest (Aseriet al, 2008). This activity is a product of microbial decomposition, humic substances (humic and fulvic acids) (Asli y Neumann, 2010) wich play an important role in soil, through nutrient availability, soil/atmosphere oxygen and carbon interchange and toxic chemical transport and transformation (Piccolo and Spiteller, 2003).…”