DFAA analysis

In practice, many difficulties of routine DFAA analysis in open ocean waters stem from the extremely low concentrations present. Great care must be taken to avoid contamination and to monitor procedural blanks. Assuming no special chromatographic problems exist, detection limits can also be affected by the sensitively of the florescence detector used. Simply replacing an old lamp may help substan-tiaUy, and new generations of florescence detectors (e.g., offered by Shimadzu, Agilent, Varian, etc.) claim substantial improvement in power and optics. It is also possible to use column pre-concentration to increase sensitivity (Lee and Bada, 1975). This approach could be extremely useful for investigation of minor DFAA components, such as D-enantiomers of DFAA (Lee and Bada, 1975, 1977). However, because most of the common DFAA components can be measured directly, pre-concentration approaches have not been widely used. 

DFAA and DCAA is the presence or absence of hydrolysis. In practice, however, the extremely low detection hmits needed for DFAA hmits the range of chromatography and detection options that can be used, while hydrolysis itself is problematic for some amino acids. For these reasons, DFAA and DCAA analysis are treated individually below. 

Despite the great interest in this class of compound and the numerous reports concerning the determination of total carbohydrates in seawater, surprisingly few determinations of individual monosaccharides have been made. Unlike the determination of free amino acids in seawater, no method for direct determination of free monosaccharides exists at present. Analogous to the developments in the detection of DFAA, the introduction of direct analysis techniques may change our concepts of the pattern of free sugars in seawater at a future date. 

Various eluent systems have been proposed for HPLC determination of DFAA. In most cases they are combinations of methanol with different buffers such as phosphate, dtrate or acetate. Mopper and Dawson (1986) discussed problems associated with phosphate buffers which often cause clogging of check valves. In our experience acetate buffers present no such problems in routine analysis. 

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