On‐line measurements of δ 15 N in biological fluids by a modified continuous‐flow elemental analyzer with an isotope‐ratio mass spectrometer

A modified continuous‐flow elemental analyzer coupled to an isotope‐ratio mass spectrometer (modified EA‐IRMS) was tested for on‐line δ 15 N measurement on urea solution and biological fluids (e.g. urine). The elemental analyzer configuration was adapted by adding a U‐shaped cold trap and an X‐pattern four‐way valve for on‐line trapping/venting of water from the liquid samples. Results indicate that the δ 15 N ratios show little variation (standard deviation (SD) = 0.05‰) with a sample size above the equivalent N yield of 0.2 mg urea (0.092 mg N) when the mass spectrometer conditions were carefully optimized. By contrast, a significant logarithmic decrease in δ 15 N with sample size was observed but this can be offset by applying a linearity correction or blank correction when the sample size is between equivalent N yields of 0.05 and 0.2 mg urea. The blank corrected δ 15 N ratios give an overall precision of ∼0.16‰ whereas the average precision for δ 15 N corrected using combined linearity and shift correction is 0.05‰. The relatively large variation in blank corrected δ 15 N values may be attributed to the variability of the blank δ 15 N in the sequence. Therefore, the blank correction should be carefully performed in routine measurements. As a result, the linearity range of a modified EA‐IRMS can be extended to a minimum sample size of 0.023 mg N. In addition, the reproducibility of the new system is good, as indicated by the precision (<0.2‰) for a set of standards and unknowns. The data show that fluids containing nitrogen can be successfully analyzed in the modified EA‐IRMS. Copyright © 2008 John Wiley & Sons, Ltd.