Atmospheric Pressure Microwave Sample Preparation Procedure for the Combined Analysis of Total Phosphorus and Kjeldahl Nitrogen

Abstract

An atmospheric pressure microwave digestion method has been developed for the combined analysis of total phosphorus and Kjeldahl nitrogen in complex matrices. In comparison to the digestion steps in EPA Methods 365.4 (total phosphorus) and 351.x (Kjeldahl nitrogen), this method requires less time, eliminates the need for a catalyst, and reduces the toxicity of the waste significantly. It employs a microwave-assisted digestion step, using refluxing borosilicate glass vessels at atmospheric pressure. Traditionally, this method has a time-consuming sample preparation step and generates toxic waste through the use of heavy metal catalysts. These advantages are gained by the combination of a high boiling point acid (sulfuric acid) and the application of focused microwave irradiation, which enhances the digestion process by direct energy coupling. NIST standard reference materials 1572 (citrus leaves), 1577a (bovine liver), and 1566 (oyster tissue) and tryptophan were analyzed to validate the method. Phosphorus concentrations were determined by the colorimetric ascorbic acid method outlined in EPA Method 365.3. Kjeldahl nitrogen concentrations were determined using EPA Method 351.1. The results of the analyses showed good precision and are in excellent agreement with the NIST published values for both elements.

Microwave digestion method for phosphorus determination of fish tissue

“…This paper describes a third method utilizing microwave acid digestion and compares the results obtained with this method to those obtained using the ashing method. Microwave digestion has been used for other biological samples with good results (Nadkarni 1984;White and Douthit 1985;White 1988;Collins et al 1996). However, there are currently no published methods in the readily available literature that are not overly time-consumptive or that can be completed without specialized perchloric acid fume hoods.…”

Rapid Determination of Total Kjeldahl Nitrogen Using Microwave Digestion

“…Again, the operating temperature was not reported in their paper. Collins, Chalk, and Kingston [6] also reported similar results. The process required acid digestion first, at 275 • C to 310 • C for 11 min, and subsequently it was digested again, with the addition of hydrogen peroxide, at 320 • C for an additional 10 min.…”

Modern Strategies for Environmental Sample Preparation and Analysis

“…The sensitivity observed by the authors was compared with that obtained using the standard method, and similar results were verified. Collins et al (1996) developed a methodology for the combined analysis of total phosphorus and Kjeldahl nitrogen in complex matrices. They used a focussed-microwave oven to promote both dissolution of samples and nitrogen distillation.…”