Nitrogen is an essential component of proteins, as well as many other biomolecules, making it an important analyte for applications involving the quantification of organic materials. Although there are many general spectroscopic methods that can be used to identify nitrogen, two main methods are used in instruments designed for total nitrogen (TN) determination: the Kjeldahl method and combustion analysis. The Kjeldahl method involves the decomposition of nitrogen-bearing compounds by sulfuric acid, with the final determination made through chemical means. In combustion analyzers, chemiluminescence or another spectroscopic method is used to quantify the nitrogen liberated by the oxidation process. In addition to nitrogen, many combustion analyzers can provide additional analyses with the inclusion of multiple detectors. These multielement analyzers measure some or all of the common biological elements: carbon, hydrogen, nitrogen, oxygen and sulfur. Of the two main methods, Kjeldahl is less expensive, but more time consuming and productive of chemical waste.
Both TN methods are widely used in the food industry as a surrogate for protein content, although the Kjeldahl method is practically synonymous with this kind of food analysis. Since they measure TN, the methods are susceptible to deception, as in the case of the pet food recalls of 2007. A pet food ingredient from Chinese sources had been adulterated with melamine in order to raise the protein level, leading to the deaths of numerous pets. Although there are calls to introduce more specific testing for protein analysis, any action will likely be slow and TN will continue to be widely used for these applications.
The other main application for TN analyzers is water analysis. In drinking water, nitrate content is regulated because high levels of the nitrate ion are associated with “blue baby” syndrome. Similarly, TN analyzers can monitor the amount of biological contamination in process water and wastewater. For water applications, and particularly for online monitoring in process environments, combustion analysis is preferred.
Automated Kjeldahl instruments relieve users from the inconvenience and labor associated with Kjeldahl analysis. The major manufacturers of Kjeldahl instruments, Büchi and FOSS, both have a strong focus on the food industry. For combustion instruments, there are a larger number of suppliers. LECO, which dominates all areas of elemental analysis, offers both a dedicated nitrogen/protein analyzer (FP-528) as well as variants of the multielement TruSpec series. Shimadzu offers standalone TN analyzers, as well as combined total organic carbon/TN analyzers. Other important suppliers include Analytik Jena, Danaher, Mitsubishi, PerkinElmer and Thermo Fisher Scientific. Including all of the multielement instrument variations, but excluding manual Kjeldahl analysis, the total market in 2007 for TN analysis was about $50 million.