October 15, 2018

Measuring nitrogen levels with the Kjeldahl method

By Jill Herrera, Product Manager

Helpful tips to effectively measure nitrogen levels using Kjeldahl analysis.

Nitrogen is everywhere. It’s not just in the air we breathe but involved in countless natural and manmade processes. So it isn’t surprising that many industries require precise tests to determine the concentration of elemental nitrogen.

In the food industry, for example, nitrogen testing helps determine the quantity of protein in food for quality control and labeling purposes. ISO standards that specify Kjeldahl analysis include the testing of general food products, cereals and pulses, and milk products.

Many wastewater treatment plants must also check nitrogen levels in water before discharge. Excess nitrogen in surface water can lead to eutrophication, the enrichment of surface water causing rel="noopener noreferrer" excessive growth of algae and plants. International standards, such as EU directive 91/271/EEC, regulate nitrogen discharge.

Nitrogen gas (N2) makes up around 80% of the atmosphere, but when analyzing solid or liquid samples, nitrogen is often present as ammonium (NH4+), nitrate (NO3-), nitrite (NO2-), and organic nitrogen. Which of these is measured depends on the specific application.

What is the Kjeldahl method?

Total Kjeldahl nitrogen is a commonly used and prescribed standard for nitrogen measurement in many industries. It measures the combined level of organic nitrogen and ammonium.

The analysis involves the following steps:

  1. Digesting a sample with concentrated sulfuric acid to convert organic nitrogen to ammonium.
  2. Adding sodium hydroxide after the sample has ‘cleared’ to liberate the nitrogen as ammonia gas (NH3).
  3. Leading the gas into a solution of boric acid, which converts the gas back to ammonium.
  4. Titration, for example with sulfuric acid, to determine the amount of ammonium—and therefore the total amount of organic nitrogen and ammonium—in the original sample.

As the method is standardized, much of it can be automated and analysis completed within a few minutes. This leaves sample collection and preparation as the most significant bottleneck and source of variability.

Collecting and preparing a sample

As with all analytical methods that rely on sampling, the validity of the measurement depends on whether the sample is representative of the bulk material, be it a foodstuff or a water sample. Collecting many, more representative samples helps maximize the reliability and reproducibility of nitrogen analysis.

Contamination, adsorption, and biological activity can all lead to changes in the measurable nitrogen in a sample. So, samples should be stored in a way that doesn’t affect their nitrogen content.

Take aqueous samples, for example. Adding a small amount of sulfuric acid directly after collection and storing at 4˚C improves sample preservation. When these samples contain solids, it’s necessary to homogenize them to maximize the reliability of measurement and digestion speed.

Filtration plays a vital part in preparing the samples for analysis. Filtering the sample after the acid digestion step removes any potential contaminants, leaving a clean sample that is unlikely to change composition. Disposable filter devices make this filtration step quick and easy.

Selecting a filtration device

In Kjeldahl sample preparation, filter selection depends on several factors, including chemical compatibility and the size and number of particulates in a sample. For example, passing a high-particulate sample through a small pore size filter can result in clogging and backpressure build-up. In these types of cases, a prefiltration step can improve the overall efficiency, ease, and effectiveness of filtration.

When filtering many samples every day, as is the case in environmental and food testing labs, syringeless filters can add a little convenience and save a lot of time. Syringeless filters are easy to use, suitable for difficult-to-filter samples, and reduce plastic waste by replacing multiple components with a single filtration unit.

GE Healthcare Life Sciences provides a wide range of filtration devices and filter papers for laboratory use. Our Whatman Autovial syringeless filters are well suited for Kjeldahl sample preparation due to their simple format and ease-of-use, both in the field and in the lab.

Our team of experts are familiar with a wide range of filtration applications and can advise on which filtration devices are most suitable for you. Contact your local GE Healthcare Life Sciences representative to discuss your needs, or try our filter selector to find your filter fast.