Total dissolved nitrogen analytical methods

Analytical Methods for Determination of Total Dissolved Nitrogen and Particulate Nitrogen... 

Analytical Chemical Data for Natural Waters. While elemental compositions of various natural waters usually can be determined with good reliability, analytical methods to distinguish between free and complex-bound species, oxidized and reduced forms, simple and polynuclear metal ion forms, and even between dissolved and colloidal or suspended phases are often lacking. Data on the nature and amounts of the individual substances which make up the total concentrations of organic material found in different natural waters are not yet extensive. These analytical deficiencies relate almost solely to the highly reactive, non-conservative elements—e.g., iron, manganese, phosphorus, carbon, nitrogen, aluminum, and other metal ions. 

PN is included in the TON, if unfiltered samples are digested by these analytical methods, and it might be determined as difference between TON and DON. However, a more accurate determination of PN requires the analysis of the particulate collected by filtration independently with respect to the determination of dissolved nitrogen forms, as PN often constitutes only a small fraction of the total nitrogen in the natural waters. The digestion of PN collected on filters can be achieved by KD and PO methods, but the most important technique used for this analysis is the HTO method (Figure 14.1). 

Analytical control followed the Standard Methods recommendations [ 4 ]. Metal ion concentrations were determined by a Perkin-Elmer Model 4000 Atomic Absorption Spectrophotometer with zinz lamp 303-6081, lead lamp 303-6039, and copper lamp 303-6024. Determination of dissolved sulfide was done by titrimetric iodine methods. Phenol determination was performed by distillation/ex-traction method. Total cyanide and ammonia nitrogen were detrmined by the respective distillation/titrimetric methods. 

The method of Thurnham etal. (1988) was modified for the quantification of plasma j3C and retinol. Plasma extracts were dissolved in 40 /il of dimeth-ylforamide and vortexed and then 210 pil of acetonitrile/methanol/chloro-form (47/47/6, v/v/v) was added. Reconstituted samples were vortexed and sonicated for 40 sec prior to being transferred to autosampler vials and sealed under nitrogen. The HPLC system consisted of a photodiode array detector (Waters 9%, Miliipore Corp., Milford, MA) with Millennium software, a Waters 717 plus autosampler, and a Hewlett-Packard Model 1050 pump. Analytes of interest were separated using acetonitrile/methanol/ chloroform (47/47/6, v/v/v), with 0.05 M of ammonium acetate and 1% triethylamine at a flow rate of 1.2 ml/min and a 4.6 X 15-cm Spherisorb ODS-2 column (LKB Instruments Ltd., Surrey, UK) maintained at 26°C using a column heater (Timberline Instruments Ltd., Boulder, CO). This analysis does not discriminate between C-enriched and nonenriched analytes, but rather measures the total concentration of each isotopomer. The retention times of retinol, retinyl acetate (internal standard), and /3-carotene were 2.1, 2.6, and 16.9 min, respectively. Plasma concentrations of retinol and /3C were calculated using a standard curve for each analyte and an internal standard to correct for volume recovery. 

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