Total Nitrogen Analysis: A New Perspective on TOC

Introduction

Monitoring the nitrogen content is of great importance in many industries for many reasons. In waters and wastewaters the most interesting forms of nitrogen, in order of decreasing oxidation state, are nitrate, nitrite, ammonia, and organic nitrogen. Organic nitrogen includes such natural materials as proteins, peptides, nucleic acids, urea, and numerous synthetic organic materials. In excessive amounts, nitrate contributes to the illness known as methemoglobinemia in infants. As a preventative measure, the current EPA-imposed maximum contaminant level (MCL) for nitrate in drinking water is 10 mg/L. Nitrite is an intermediate oxidation state of nitrogen both in the oxidation of ammonia to nitrate and the reduction of nitrate to nitrite. It can enter a water supply system through its use as a corrosion inhibitor in industrial process water. It is also a precursor in the formation of nitrosamines, which are known to be carcinogens. Ammonia is present naturally in surface and wastewaters. At some wastewater treatment plants, it is used as an additive to control residual chlorine. Analytically, ammonia and organic nitrogen can be determined together using a technique called Total Kjedahl Nitrogen (TKN). In the past, to arrive at an accurate assessment of total nitrogen, the analyst had to separately measure for all of the various nitrogen forms and then combine the results. Consequently, analyzing for total nitrogen proved to be a long and difficult process. What’s more, all of the EPA-approved methods for nitrogen analysis were extremely time-consuming. These methods were primarily wet chemical methods requiring extensive sample preparation in the form of digestions and/or titrations. Also, UV/VIS spectroscopy was typically used to measure for the various nitrogen forms (i.e. nitrate plus nitrite) in the wet chemical product. Thus, more time was consumed in waiting for color to fully develop in the absorbing species. As a result, the analysis time for separate nitrogen forms ranged from one to six hours depending upon the nitrogen form being determined and the number of samples being analyzed. Shimadzu understands the need for quick and accurate analysis of total nitrogen. Combined with our state-of-the-art TOC-V Visionary series Total Organic Carbon analyzer (TOC-V_CSH), Shimadzu’s Total Nitrogen Module, TNM-1, uses chemiluminescence to measure total nitrogen in less than four minutes! Similar to the TOC analyzer, the TNM-1 is a non-specific measurement of total nitrogen. A nitrogen-based sample is combusted to nitrogen monoxide and nitrogen dioxide. The nitrogen species are then reacted with ozone to form an excited state of nitrogen dioxide. Upon returning to ground state, light energy is emitted. Then, total nitrogen (TN) is measured using a chemiluminescence detector. To add, the combination of the TOC-V_CSH and the TNM-1 can simultaneously measure for TOC and TN.

September 6, 2001 GMT