Integrative assessment of amino acid nitrogen isotopic composition in biological tissue samples determined by <scp>GC</scp>/C/<scp>IRMS</scp>, <scp>LC</scp> × <scp>EA</scp>/<scp>IRMS</scp>, and <scp>LC</scp> × <scp>GC</scp>/C/<scp>IRMS</scp>
Limnology and oceanography, methods