Peak assignments for phosphorus-31 nuclear magnetic resonance spectroscopy in pH range 5-13 and their application in environmental samples

摘要

Phosphorus-31 nuclear magnetic resonance (NMR) spectroscopy has become popular for the characterization of P species in environmental samples.However,these are commonly made alkaline (pH > 13) to facilitate sample comparison and ease peak identification,but this may cause hydrolysis of some compounds.This study examined the chemical shift of known P compounds and supplemented this with published data to determine the viability of examining samples at their native pH,thereby minimizing sample disturbance.A ~(31)P NMR pH titration of known P compounds resulted in chemical shifts ranging from about -22 to 8 ppm in the pH range 5-13.Categorization and calculation of chemical shifts for over 100 naturally occurring compounds indicated that good distinction between orthophosphate diesters,orthophosphate monoesters,nucleotides,phosphonates,and phos-phagens was best at >=pH 7,but unlikely below this pH.Analysis of several water extracts of soil and dung,overland flow samples,and lake water indicated a wide variety of well-defined peaks that were assigned to orthophosphate,orthophosphate monoesters,orthophosphate diesters,pyrophos-phate,polyphosphate,or phosphonates.Changing the sample pH to >13 caused many species (such as phosphonates,orthophosphate diesters,and polyphosphates) to decrease either by hydrolysis or precipitation.Hence,it is recommended that samples be analysed at their native pH but,if poorly resolved,should have their pH raised to >=7.