Achieving Rapid, Accurate, and Reliable Nitrogen Determination in Soils Using Dynamic Flash Combustion

Nitrogen is a necessary part of all living cells, proteins, enzymes , and metabolic processes involved in the synthesis and transfer of energy. The element is one of the most important mineral nutrients and is known to play a crucial role in a plant’s growth, reproduction, and survival. Nitrogen is present in chlorophyll, a green pigment that allows plants to capture energy from the sun and produce food for themselves in a process called photosynthesis. This vital nutrient facilitates rapid plant growth, increasing seed and fruit production and improving the quality of leaf and forage crops.

Although nitrogen is the most abundant element in the atmosphere, plants are unable to use it unless it is naturally processed in the soil or added using the commercial fertilizers that are popular among farmers and gardeners. However, despite its necessity in certain concentrations, deficiency or excess of nitrogen can negatively impact plant development. Overapplication of nitrogen-containing fertilizers can result in rapid, lush growth and a diminished root system. In extreme cases, too much nitrogen can cause burning of the leaf tissue and lead to plant death.

As a result, analysis of nitrogen content in soils is important both for the evaluation of organic matter and the calculation of the ideal fertilizer quantities required to maximize plant growth. Nitrogen content analysis is also necessary in order to determine the quality of various types of crops for feeding and processing, as well as for N-cycle and N-fixation monitoring in agricultural and environmental research.

Analytical techniques

For many years, the Kjeldahl method has been the worldwide standard for nitrogen analysis in a wide variety of materials. The method was introduced more than 100 years ago to facilitate the quantitative determination of nitrogen content in a variety of organic and inorganic substances, such as meat, feed, grain, wastewater, and soil. The method consists of five main steps: sample digestion in boiling sulfuric acid, neutralization with sodium hydroxide solution, distillation of the resulting ammonia gas into a trapping solution, titration with an acid solution, and determination of nitrogen content by calculation.

Despite the Kjeldahl method generating accurate and reproducible results, the technique is limited by a number of key disadvantages, including that it is a very time-consuming technique and requires more than 4 hr to complete a single analytical cycle. The method also demands constant operator interaction throughout the entire process, while the use of catalysts as well as strong, concentrated sulfuric acid at high temperature poses considerable health hazards to users of the method and generates harmful chemical waste.

The Dumas combustion method has been developed to overcome some of the limitations of the traditional Kjeldahl method. It is an easy-to-use, automated instrumental technique capable of rapidly measuring nitrogen concentration in soils without the use of toxic chemicals or catalysts. However, the technique requires a high initial investment and, as with the Kjeldahl method, does not offer measurements of the true protein content. In addition, the method uses a small sample size, making it difficult to obtain a representative sample.

As the demand increases for improved sample throughput, reduced operational costs, and a decrease in human error, it has become very important to have a simple and automatic technique that enables fast nitrogen analysis combined with excellent reproducibility. Adding a stage to the Dumas combustion method in which samples undergo dynamic flash combustion significantly increases the method’s safety, accuracy, speed, and dependability.

Benefits of dynamic flash combustion

The dynamic flash combustion method uses an innovative CO2 adsorber regenerating technology, which is automatically activated to adsorb the CO2 generated during the combustion. Contrary to conventional technologies, this self-cleaning filter does not need to be changed several times a week, saving time and money and enhancing the system’s autonomy while reducing instrument maintenance. The method also does not require sample digestion of toxic chemicals, eliminating health hazards.

The dynamic flash combustion method also uses a Peltier water elimination device and a safe, sensitive, and reliable thermal conductivity detector (TCD) that covers the quantitative nitrogen/protein determination from low ppm to high percentage concentrations. Compared to the Kjeldahl and Dumas combustion methods, the novel technique achieves a much higher sample throughput within 5–7 min, depending on the nature of the samples. The large sample size capacity of the method enables maximum accuracy and reliability of results, while reducing sample handling time and minimizing matrix effects. An experiment was performed to demonstrate the analytical capabilities of the dynamic flash combustion method.

Experimental

The Thermo Scientific FLASH 4000 elemental analyzer (Thermo Fisher Scientific, Cambridge, U.K.) based on the dynamic combustion of the sample was used to demonstrate the efficiency of the method for reliable nitrogen analysis in soils and plants. The samples were selected on the basis of their differing nature and nitrogen content, meaning that the combustion and the amount of oxygen required were completely different for each sample. The samples were weighed in a tin capsule and introduced into the combustion reactor via the Thermo Scientific MAS 4000 autosampler, together with the correct amount of oxygen, which was automatically calculated using the OxyTune® function of the Thermo Scientific Eager Xperience software platform on which the analyzer operates. Determining the exact amount of oxygen is necessary in order to ensure the complete combustion of the sample.

Following combustion, the produced gases were passed by a helium flow to a second reactor that was filled with copper. Water was trapped through a water condensation drainage device, while the CO2 was adsorbed by no-stop twin traps. Nitrogen was then passed through a GC column and detected by a thermal conductivity detector. The temperatures for the left tube, right tube, and oven were set at 950 °C, 840 °C, and 50 °C, respectively, and the carrier and reference flow were both 300 mL/min. A total of 500 mg of the ethylenediaminetetraacetic acid (EDTA) 9.59 %N standard were used.

Analyzer performance evaluation by WEPAL round-robin tests

The precision of the analyzer was evaluated through the participation in international round-robin tests conducted as part of the Wageningen Evaluating Programs for Analytical Laboratories (WEPAL) of the Wageningen University in The Netherlands. For soil samples, data were compared with the range accepted by WEPAL statistic studies, including all methods for nitrogen determination. For plant samples, the results were compared with the range accepted for both Kjeldahl and total nitrogen methods including the combustion method.

Results

The calibration of the system was performed with EDTA STD (9.59 %N) using K factor as the calibration method. The samples were analyzed as they were received by WEPAL without treatment, and the data obtained demonstrated the no-matrix effect in the determination of nitrogen, indicating complete combustion for all sample types.

Table 1 shows the reproducibility of 10 consecutive runs of soil WEPAL samples using a sample weight of about 1000 mg. Nitrogen data obtained were inside the range of nitrogen concentration approved by the WEPAL statistic studies.

Table 2 displays the reproducibility of 10 consecutive runs of plant WEPAL samples using a sample weight of about 1000 mg. Nitrogen data obtained were inside the range of nitrogen concentration approved by WEPAL statistic studies for both Kjeldahl and total nitrogen methods.

Table 3 shows the reproducibility of nitrogen determination in soils in a wide level of concentration. Samples were analyzed in triplicate, and the weight of sample used was 500–1500 mg, depending on the matrix.

Table 4 shows the reproducibility of nitrogen determination in plants in a wide level of concentrations. Samples were analyzed in triplicate, and the weight of sample used was 500–1500 mg, depending on the matrix.

Conclusion

The precise determination of nitrogen content in soils is necessary in order to get the maximum benefit from fertilization and facilitate rapid plant growth with minimal pollution hazard. Dynamic flash combustion is the ideal technique for analyzing nitrogen in soils and plants, with experimental data demonstrating excellent reproducibility and no memory effects when changing the type of sample, indicating the complete detection of this vital nutrient. The method is capable of analyzing nitrogen in a wide range of concentrations, from low to high, with optimal accuracy and without matrix effects.

Dr. Krotz is Product Specialist, OEA, and Dr. Giazzi is Product Manager, OEA, Thermo Fisher Scientific, Strada Rivoltana, 20090 Rodano, Milan, Italy; tel.: +39 02 95059336; fax: +39 02 95059276; e-mail: guido.giazzi@thermofisher.com.

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