A comparison of methods for nitrogen isotope analysis of groundwater

Nitrogen isotope studies of groundwater are now frequently undertaken because of the information concerning nitrogen sources and natural denitrification processes they can reveal. Most studies have employed a methodology involving the use of a hypobromite solution. This method may not be appropriate for many laboratories and contains substantial drawbacks that warrant the consideration of another method. Off-line combustion in sealed tubes is compared with the traditional hypobromite method. Principal results are that: (1) ammonium sulphate samples can be oven dried without fractionation; (2) a considerable reduction in quantity of reagents for the combustion process can be made with no effect on results; and (3) off-line combustion is preferable for laboratories using high-precision dual inlet isotope ratio mass spectrometry (IRMS) because hypobromite procedures are susceptible to problems such as air leaks and require considerable additional apparatus.