Nutrients working standards

Since certified reference materials for seawater nutrient analysis are currently unavailable, individual laboratories must prepare their own standard solutions for instrument calibration. Standard stock solutions are prepared at high concentrations (mM) so that they can be used for months without significant alterations in concentration. Working low-concentration standard solutions are unstable and need to be prepared daily by diluting stock solutions with distilled water or low-nutrient seawater. In this case, the accuracy of nutrient analysis at a given laboratory is highly dependent upon the accuracy of the daily preparation of the calibration solutions. 

Sampling sites are also referred to as station locations. For water column work, depth profiles are constructed from seawater samples collected at representative depths. Temperature and salinity are measured in situ with sensors. Remote-closing sampling bottles deployed from a hydrowire are used to collect water for later chemical analysis, either on the ship or in a land-based laboratory. The standard chemical measurements made on the water samples include nutrients (nitrate, phosphate, and silicate), dissolved O2, and total dissolved inorganic carbon (TDIC) concentrations. 

As an example, I will use the determinations of DOC and DON, partly because the discussions are very recent and partly because they are very familiar to me. The question of the accuracy of measurements of DOC in seawater has been disputed at least since the publications of Putter [62] and Krogh [63] the early work has been reviewed at length in an earher pubhcation [64]. While a variety of wet oxidation methods [65] were proposed for marine samples, the use of persulfate [35] provided the first real approach to a standard method. Persulfate oxidation, however, hke all purely chemical oxidations, was a batch process, and not easily automated. A number of workers proposed photo oxidation using ultraviolet (UV) hght [66-70], and automated analytical systems which produced data almost in real time were soon constructed [71-73]. Commercial units soon appeared, but many of the units in the field were jerry-built, constructed out of parts scavenged from discarded autoanalyzers formerly used for nutrient analysis. 

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