Helium-Free Nitrogen/Protein Analysis with the Dumas Principle for High-Performance Results

N/protein analysis based on the Dumas principle depends upon quantitative conversion of analytes into properly-defined gaseous species at higher temperatures in the presence of appropriate reagents and elemental oxygen. Additional oxygen from the combustion phase is usually bound on metals such as tungsten or copper, referred to as reducing metals, before chromatographic separation of the combustion gases, allowing for detection of N2 using a thermal conductivity detector (TCD).

Helium-Free N/Protein Analyzers

  • rapid N exceed - carbon dioxide
  • rapid MAX N exceed – argon

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Using helium as an inert carrier gas for N/protein analyzers is well established and until lately served as the only option for Dumas analyzers. The reason for the lack of alternative methods mainly stemmed from confines of thermal conductivity detectors in managing the small conductivity differences between N2 and other inert gases. In the case of helium, this conductivity difference is big and easily recognizable. Unfortunately in the last few years, helium shortages have occurred more frequently causing prices to increase. To make matters worse and push prices upwards, U.S. policies on Federal Helium Reserves have changed together with the cost of extracting helium from natural gas, the most common helium source.

Keeping this in mind, Elementar has designed the next generation of future-proof N/protein analyzers utilizing proprietary EAS REGAINER® technology and use alternative carrier gases, such as argon for the rapid MAX N exceed and carbon dioxide for the rapid N exceed, without compromising analysis performance.

Helium-Free Nitrogen/Protein Analysis

rapid N exceed Using Carbon Dioxide as Carrier Gas

The rapid N exceed was introduced in 2014 and launched the innovative EAS REGAINER technology. This one-of-a-kind apparatus utilizes cost-efficient carbon dioxide as carrier gas, making the scrubbing of CO2 formed by the combustion process outmoded. A three-step gas drying design guarantees that only nitrogen is detected by the thermal conductivity detector.

To experimentally establish that carbon dioxide as carrier gas has no impact on the data quality, a series of measurements were conducted to confirm the protein content for a broad range of sample matrixes using carbon dioxide as carrier gas.

In a first set of experiments, different flour types were tested. The repeatability of the measurements is calculated in compliance with DIN EN ISO 16634-2 and is illustrated in Figure 1. The results reveal that the repeatability of the rapid N exceed measurements is well below 0.10%N, the mandatory repeatability according to the international standard. The average repeatability of all flour analyzes was 0.020%N, approximately 5x better than the required repeatability fixed by the standard.

In a second set of experiments, different animal feed samples were tested. Also for these samples the average repeatability of the rapid N exceed analyzes (0.028%N) was around 3x better than the mandatory repeatability according to DIN EN ISO 16634-1 (0.10%N).

The accuracy of the rapid N exceed analyzes is illustrated in Figure 2a, showing a comparison of the protein content of different sample matrixes (grains, dairy products, sausage, animal feed, yeast) examined on the rapid N exceed with carbon dioxide as carrier gas and the protein content established by the rapid MAX N exceed using helium as carrier gas. The protein contents analyzed on both instruments show an outstanding agreement with a calculated R2 value of 0.99.

Repeatability results for different flour types and various animal feed samples

Figure 1. Repeatability results for different flour types according to DIN EN ISO 16634-2 and various animal feed samples according to DIN EN ISO 16634-1 using the rapid N exceed with carbon dioxide as carrier gas.

Helium-Free Nitrogen/Protein Analysis

Comparison of the experimentally determined protein contents

Figure 2a. Comparison of the experimentally determined protein contents using the rapid N exceed with carbon dioxide as a carrier gas and the rapid MAX N exceed using helium as carrier gas on 20 different matrixes (see below).

RAPID N EXCEED
flour, yogurt, sausage, cheese, animal feed, yeast 300-350 gr.

Comparison of the experimentally determined protein contents

Figure 2b. Comparison of the experimentally determined protein contents using argon and helium as a carrier gas for the rapid MAX N exceed on 26 different sample matrixes (see below).

RAPID MAX N EXCEED
flour, animal feed, sausage, cheese, yeast 500 gr., yogurt 1000 gr., milk 1500 gr.

rapid MAX N exceed Using Argon as Carrier Gas

The rapid MAX N exceed uses the proprietary EAS REGAINER technology and is enhanced for high-throughput and a broad range of sample weights and types. It utilizes argon as carrier gas by default. The exclusive post-combustion technology ensures full digestion of even the most demanding species at larger sample weights.

The upright crucible design with robotic arm sample feeding guarantees ideal combustion for liquid or solid sample types without the requirement for change in hardware or costly liners. Carbon dioxide from the combustion procedure is quantitatively adsorbed on a gas-selective trapping column. The tandem operation of two trapping columns guarantees quick analysis times within five minutes. While one column adsorbs the carbon dioxide, the other column is heated to release carbon dioxide of the earlier run via a waste channel.

To experimentally prove that argon as carrier gas has no impact on the results, a series of measurements were conducted to define the protein content for 26 different sample matrixes (various types of milk, flour, sausages, yeast, cheese and animal feed). The results were compared with those attained with the rapid MAX N exceed using helium as a carrier gas and are illustrated in Figure 2b. The calculated R2 value of 0.99 reveals the exceptional agreement between the analyzes with both carrier gases, concluding that the gases are indistinguishable and readily interchangeable.

Moreover, the analyzes display an exceptional level of repeatability. As illustrated in Figure 3, the repeatability of the rapid MAX N exceed analyzes with argon as carrier gas achieved the required repeatability according to the international standards ISO 16634-2 and ISO 14891 by a factor of more than 4. The averaged repeatability for the analyzes of five different types of milk was 0.0032%N, while seven flour types produced an average repeatability of 0.019%N.

Repeatability of the analyzes of different flour types and of milk samples

Figure 3. Repeatability of the analyzes of different flour types according to DIN EN ISO 16634-2 and of milk samples according to DIN EN ISO 14891 using the rapid MAX N exceed with argon as carrier gas.

Summary

The presented experiments with the rapid N exceed and rapid MAX N exceed successfully show the use of carbon dioxide and argon as alternative carrier gases for N/protein analyzes using Elementar’s new EAS REGAINER technology.

Analyzes were conducted for a range of liquid and solid sample matrixes and even less homogeneous food/feed samples. Even for the most demanding species outstanding accuracy and precision was attained. Both instruments easily meet the mandatory repeatability in accordance with the majority of international food, feed and fertilizer standards such as ISO 14891, ISO 16634-1, ISO 16634-2, AOAC 99003, ICC 167, AOAC 99215, AOAC 99313, AOAC 99223, as well as various national standards, for example Lufa and DIN.

The use of the EAS REGAINER technology results in an approximately five-fold increase in lifetime of the reduction tube chemicals, boosting the instrument efficiency and decreasing costs for consumables and lab time.

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This information has been sourced, reviewed and adapted from materials provided by Elementar Analysensysteme GmbH.

For more information on this source, please visit Elementar Analysensysteme GmbH.

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