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 den itrification processes they can reveal. Most studies have employed a m ethodology involving the use of a hypobromite solution. This method ma y not be appropriate for many laboratories and contains substantial dr awbacks that warrant the consideration of another method. Off-line com bustion 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 q uantity of reagents for the combustion process can be made with no eff ect on results; and (3) off-line combustion is preferable for laborato ries 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.