تشخیص و تعیین نیترات و نیتریت: یک مقاله مرور منابع Detection and determination of nitrate and nitrite: a review

۱٫ Introduction Nitrate and nitrite are ubiquitous within environmental, food, industrial and physiological systems, and while our understanding of their role within such matrices has increased, a substantial degree of uncertainty and speculation remains. These ions have been profitably exploited throughout the ages to further the development of our various societies but there is no doubt that our affection for them has waned in recent years. Our incessant use of, and indeed reliance upon, these versatile agents combined with revelations of their potential toxicity have raised numerous concerns [1,2]. These problems have been widely recognised, and, as a consequence, statutory frameworks aimed at controlling their level within the wider environment and within food products have been imposed in most industrialised countries [3,4]. The restrictions placed upon the commercial utilisation of these ions have eased some of the apprehensions raised by the medical community but the establishment of adequate controls can only be achieved by fully exposing their influence on the various pathways that govern environmental and physiological well being. Our need and desire to monitor these ions are unquestionable, yet their ubiquity can pose a significant challenge to the analytical community. Few techniques possess sufficient generic applicability to enable their detection amongst the huge number of potential interferences that can be encountered within environmental, food, industrial and physiological samples. As such, a large number of protocols, encompassing almost all major analytical methodologies have been developed to overcome the peculiarities of the various matrices. The aim of this review has been to provide a representative survey of the scientific literature covering the detection of these important analytes and provide a concise appraisal of the relative merits of each approach. A number of excellent reviews have been compiled over recent years, but each has narrowed their remit to specific matrices or techniques [5–۱۰]. Through tabulating the various analytical parameters (detection limit, range, matrix, etc.) of each system and exposing their advantages and limitations, it was hoped that the specialism divides inherent within such a diverse subject such could be bridged. The decision to cover both nitrate and nitrite was made on the basis that their chemistries are inextricably linked, and as such, one is rarely found without the other. Indeed, their inter-conversion, particularly the chemical reduction of nitrate to the more reactive nitrite, features strongly within many of the reports and is often the only way in which the relatively inert nitrate ion can be detected. The simultaneous detection and speciation of these ions has, however, gained increasing interest as the checks and balances that mediate many environmental processes have become clearer. Some of the more common reaction pathways that form the backbone of the detection strategies assessed herein are shown in Fig. 1. ۲٫ Background perspective Nitrate and nitrite have become intertwined with domestic life, and it is effectively impossible to engage in everyday activities without encountering these ions or the products of their use. The chemical versatility of these agents has ensured their utilisation within a multitude of industrial processes ranging from the manufacture of fireworks to the production of the latest dyes. Their anti-microbial action has been recognised for centuries and still used for the preservation of meat produce (E249, E250, E251 and E252) today (UK Ministry of Agriculture, Fisheries and Food, http://www.maff.gov.uk). Despite the huge number of products that are reliant upon these ions, it is their association within environmental issues that has captured the interest of the public and a significant proportion of the scientific community. Inputs of nitrate and nitrite to the environment can occur through industrial and domestic combustion processes with gaseous NOx species converted to NO3 − through photochemical conversion within the atmosphere [11–۱۵]. The vast majority, however, arise from agricultural sources [16]. The misuse of inorganic fertilisers combined with the more general mismanagement of our natural resources has been suggested as resulting in the perturbation of both local and global nitrogen cycles [16,17]. The consequences of our environmental manipulations are as yet uncertain, and therefore monitoring the ecological fate of nitrate and nitrite has gained increasing importance. The high solubility and mobility of these ions within the soil and our continued reliance on inorganic fertiliser has led to reports that ‘‘run off’’ is a continual hazard wherever agricultural processes are in close proximity to surface water [18]. Eutrophication of lakes and more recently coastal outfalls are thought to result in the generation of algal blooms that wreak havoc with local ecological systems [19,20]. The contamination of edible shellfish and the occurrence of ‘‘red tides’’ of potentially toxic algae near tourist resorts can also impart a degree of economic misery to the afflicted communities [21]. The potential contamination of groundwater through the percolation of nitrates through natural aquifers presents the most immediate risk to health [1,2] and as such, the maximum permissible level for these ions in drinking water supplies is often levied and currently stands at 50 mg/l in the UK [3,4,22]. The two main threats to health that arise from the ingestion of these ions are reported as ‘‘blue baby’’ syndrome and gastric cancer [1,2,23,24]. In both cases, the principal protagonist is nitrite obtained directly from contaminated water supplies or derived from the reduction of nitrate by the multifarious bacterial colonies that reside within the mouth. Passage of nitrite into the bloodstream results in the irreversible conversion of hemogloblin to methemoglobin with oxygen uptake and transportation compromised [2]. This is particularly hazardous for infants, given their limited physical stature and the susceptibility of their neural development to impeded oxygen transport. A more contentious health concern is the possible formation of carcinogenic nitrosoamines within the acidic conditions of the stomach and their subsequent implication in the pathology of gastric cancer [1,25]. Upon reaching the stomach, nitrite is converted to nitrous acid, which can act as a powerful nitrosating agent. While nitrosoamines can be carcinogenic, conclusive evidence linking nitrite ingestion with the stomach cancer remains elusive [2].