ABSTRACT
For cation separations with CNLSD, a suppressor is not needed to control the background (i.e., unlike the case with conductivity detection), and fairly high acid concentrations can be used, allowing rapid separations [50]. Reduction of metal ion contaminants in the mobile phase using scavenger columns and optimized numbers of diffusion screens improved background levels, allowing detection of a wide range of metal cations, including alkali and alkaline earth metals at 2-80-ng/mL levels [50] or low-molecular-weight aliphatic/ biogenic amines [51] at 8-20-ng/mL levels. Figure 14 displays a typical chromatogram for the separation of sub-f.lg/mL levels of biogenic amines. Detection of metal cations in rain water [50] and of biogenic amines in fish extracts [51] was demonstrated. For elution of transition metals, less volatile oxalic acid in the mobile required a larger number of diffusion screens, but 70-450-ng/mL LODs were still obtained. As indicated in Table 1, even volatile amines, such as trimethylamine, can also be detected with the addition of oxalic acid. With anion-exchange separation and the addition of ammonium-containing eluents to decrease their volatility, even anions could be detected with 60-ng/mL (Cl-) and 300-ng/mL (F-) LODs [50]. The level for Cl-with CNLSD is significantly lower than that obtained for ELSD (2.5 11g/mL [52,53].
