Determination Of Nitrite In Seawater Research Articles

Feedback standard addition method coupled flow injection analysis – Validation by spectrophotometric determination of nitrite in seawater

The standard addition method is a powerful tool to eliminate matrix interferences and enables accurate determinations, but it is time-consuming and labor-intensive. This paper proposes a feedback standard addition method coupled flow injection analysis, FB-SAM/FIA, for the rapid and accurate determination of trace interest in seawater samples. This approach automatically prepares spiked samples by feeding back the peak height in the fiagram, thus significantly improving the sample throughput compared to the manual standard addition method. Spectrophotometric determination of NO2− with the Griess assay was used as an evaluation model. The proposed technique demonstrated a sample throughput of 3 samples/h, ranging from 0.0050 mg L−1 (LOD) to 0.125 mg L−1 NO2−. The accuracies of NO2− determination with FB-SAM/FIA and an absolute calibration method were tested using natural seawater and pseudo-seawater samples containing 0.040 mg L−1 NO2− in 0, 1, 2, 3, and 4 % sodium chloride media. With the absolute calibration method, the calculated concentration increased with an increase in salinity, reaching an overestimation of about 40 %. On the other hand, FB-SAM/FIA suppressed the errors in the range of −7.8 % to 1.3 % and applied successfully to the determination of NO2− in the natural seawater samples.

The performance of a new linear light path flow cell is compared with a liquid core waveguide and the linear cell is used for spectrophotometric determination of nitrite in sea water at nanomolar concentrations

The sensitivity of a spectrophotometric assay is enhanced either by increasing the concentration of the target molecules within the flow cell or by extending the length of the light path of the flow cell. Determination of nutrients at nanomolar concentrations in sea water has therefore been based either on the preconcentration of the analyte on a microcolumn from a large volume of sample followed by its elution into a conventional 1–2 cm flow cell, or by the use of Liquid Core Waveguide (LCW) with a light path as long as several meters. In order to evaluate the relative improvements of these different approaches to increasing sensitivity we have developed a preconcentration technique for the determination of nitrite in seawater using the Gries reaction and compare its sensitivity and precision to that of non preconcentration techniques using both LCW and Linear Light Path (LLP) cells of different lengths. In this work the performance of the LLP is investigated and compared with the performance of the coiled LCW flow cell. Next, the determination of nitrite, automated by programmable Flow Injection (pFI), was carried out by using LCW and LLP flow cells, as well as by using a 10 cm LLP flow cell together with the preconcentration step of nitrite on a microcolumn. The assay of nitrite in sub micromolar range was most efficiently performed by a combination of pFI with the LLP flow cell without the need for a preconcentration step. The determination was performed at a rate of 40 samples/hour with a Limit of Detection (LOD) = 0.6 nM N using a 50 cm long, and a LOD = 2.5 nM N using a 10 cm long, LLP flow cell. Analysis of sea water samples confirmed that salinity does not affect the sensitivity of the determination. At a much lower cost than LCW, the LLP flow cell can also be easily assembled from components usually at hand in a laboratory.

Sequential injection spectrophotometric determination of nanomolar nitrite in seawater by on-line preconcentration with HLB cartridge

The unstable state of nitrite results in its very low concentration in seawater, which is below the limit of detection (LOD) of conventional techniques of analysis. Some sensitivity-enhanced methods have been proposed for the determination of nitrite at nanomolar level to illustrate the role of nitrite in the marine nitrogen cycle. However, most of previous reports are not widely accepted, because of their complexity and cost equipment or intensive labor requirement. In this study, a simple automatic system for the determination of nanomolar level nitrite using on-line preconcentration with spectrophotometric detection was described. An Oasis HLB cartridge was adopted to quantitatively enrich the pink-colored azo compound, formed from nitrite via Griess reaction. The cartridge was rinsed with water and ethanol (volume fraction is 55%, the same below), in turn, then eluted by an eluent containing 50% ethanol and 0.25 M(mol/dm3) H2SO4, and determined at 543 nm with a 2 cm path-length flow cell. Under the optimized experimental conditions, the calibration curve showed a good linearity in the range of 1.4–85.7 nM, and the LOD (3σ) was estimated to be 0.5 nM. The relative standard deviations of 7 measurements were 4.0% and 1.0% for the samples spiked at 7.1 and 28.6 nM, respectively. The recoveries for the different natural water samples were between 92.2%–108.4%. Each HLB cartridge could be reused for at least 50 times. As compared with other SPE methods, the advantages of this method included the free of interference from salinity variation and less sample consuming. The results of the application of the proposed method to natural water showed good agreement with liquid waveguide capillary cell detection method.

Assessment of Equipment for the Determination of Nutrients in Marine Waters: A Case Study of the Microplate Technique

An essential prerequisite for quality assurance of the colorimetric determination of nutrients in seawater is the use of suitable photometric equipment. Based on a knowledge of the optical characteristics of a particular system and the absorption coefficient of the analyte, a statistical approach can be used to predict the limit of detection and the limit of quantitation for a given determinand. The microplate technique, widely used for bioassays, is applicable to colorimetric analysis in general, and its use for the determination of nutrients in seawater has been suggested. This paper reports a theoretical assessment of its capabilities in this context and a practical check on its performance, taking the determination of nitrite in seawater as typical. The conclusion is that short optical path length and insufficient repeatability of the absorbance measurement render it unsuitable for the determination of the low concentrations generally encountered in marine work, with the possible exception of nitrate. The perceived advantage of high-speed analysis is a secondary consideration in the overall process of determining nutrients, and the microplate technique's small scale of operation is a definite disadvantage as this increases the risk of exposure to contamination problems, in comparison with conventional techniques.

Determination of nitrite in sea-water by ion-exchanger colorimetry

A microdetermination method (at theμg/l level for nitrite) has been developed, based on ion-exchanger colorimetry. The nitrite is reacted with sulphanilamide andN-(1-naphthyl)ethylenediamine to form an azo dye which is then sorbed on Dowex 50W-X2. The resin phase absorbance at 550 and 800 nm is measured directly. The relative detection limit is 0.27μg/l and the distribution ratio is 2.6×104. The method has been applied to determination of nitrite in 500-ml sea-water samples and compared with a conventional technique.