Calibration, blank determination and calculation procedures

As previously discussed, all nutrients are quantified by spectrophotometry. Calibration in the context of spectrophotometric analysis means comparison of the sample absorption after chemical reaction with the absorption of a standard (Le., an artificial sample) of known concentration, which has been treated in exactly the same manner. The reliability of calibrations decreases with increasing concentration differences between sample and standard. Consequently, the best calibration and thus the best analytical results are obtained with exactly matched sample and calibration ranges. Since the methods for calibration, blank determination and calculation of results follow the same procedures for all nutrients discussed here, the different steps to be taken simultaneously are outlined in the following. 

Starting the analysis with a new analytical setup requires a full range (initial) calibration including zero (blank) determination and linearity test. The operational calibration performed later on at regular intervals is a reduced two- or three-point calibration. 

The general calibration and calculation procedure consists of six steps. 

Prepare an adequate supply of zero water (ZW), Le., LNSW, ASW or a sodium chloride solution of medium sample salinity (see Section 10.2.2). If the expected variation of salinity is 2, salinity corrections are generally not required. 

Prepare a sequence of standards (minimum 5) from zero to a concentration slightly beyond the expected maximum sample concentration by pipetting 1-5 aliquots of a standard stock solution in pine water into calibrated flasks and fill up with ZW. Treat at least three sample aUquots per standard (including ZW as zero standard) as described for the respective analytical method and measure the absorbances (absolute or relative units). In manual analysis use ZW in the reference cell. A plot of the nominal standard (spike) concentrations versus absorbances (or the equivalent analogue signal, e.g., voltage, recorder pen deflection) should look like Fig. 10-4. 

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