Characterisation of low molecular weight dissolved organic nitrogen by liquid chromatography-electrospray ionisation-mass spectrometry

P.K. Curtis-Jackson, G. Massé, M. Gledhill, M.F. Fitzsimons

Limnol. Oceanogr. Methods 7:52-63 (2009) | DOI: 10.4319/lom.2009.7.52

ABSTRACT: Low molecular weight (LMW; < 1 kDa) dissolved organic nitrogen (DON) is the dominant DON fraction in marine systems. However, existing methods for DON characterization exclude this fraction through size exclusion or destruction during sample hydrolysis. In this study, we developed a method for the extraction and analysis of LMW peptides in saline waters. The procedure involves a solid phase extraction (SPE) preconcentration step using a polystyrene-divinylbenzene sorbent, then elution and detection by liquid chromatographyelectrospray ionization-mass spectrometry (LC/ESI/MS). The procedure was tested with 1-L water samples containing peptides (10 ng L–1) ranging from 188.2-1946.0 Da. The analytes were characterized using “top-down” sequencing to confirm their structure, and the method was then successfully applied to saline water samples extracted from a mono-algal culture of Phaeodactylum tricornutum. This enabled a number of unknown analytes to be partially sequenced (complete sequencing should be possible using additional MSn scans and larger sample volumes). The method allows for the detection of peptides at the ng L–1 level and further preconcentration is possible. The SPE step allowed determination of peptides from saline water, a matrix incompatible with direct LC/MS analysis. The method can, therefore, be used in DON characterization studies.