Nutrients, blank determination

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. 

Colorimetric methods for the determination of DIP, nitrate, nitrite, ammonium, and silicate are well established, with both manual and automatic procedures well understood and able to provide adequate sensitivity for most purposes. These methods can readily be used at sea and in the future probably in situ. International intercalibration exercises have shown that many laboratories now have the analytical expertise to measure nutrient concentrations at ambient levels, but this is not true of all laboratories and the analysis still requires careful analytical procedures that recognize the importance of contamination control, blank correction, and the complications arising from the saltwater matrix. Although there have been a number of such intercalibration studies, there is no widely available standard reference material for nutrient analysis at present. 

The salinity error of a nutrient determination (A unc) is corrected by a linear correction equation including terms for the differences in the sample salinity S and the standard salinity Sstd which is used as the blank, wash and standard matrix. 

According to the general calibration procedures for nutrient determinations (see Section 10.2.4), blanks and standards have to undergo the same treatment as the samples. 

The use of non-zero water (e.g., LNSW) as ZW and for standard preparation requires determination of the nutrient content of the respective matrix and a determination of reagent blanks. Reagents in nutrient analysis do not absorb at the analytical spectrophotometer wavelength unless contaminated or old. Thus, a reagent blank is mostly caused by traces of the corresponding nutrient contained in chemicals or in the pure water. 

The offset concentration after subtraction of the reagent blank (C g) is the nutrient concentration Czero of the ZW. In the formula for the determination of sample concentrations use the nominal standard concentrations plus Cano for Ch and C. ... 

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