High-precision trace gas measurements are required for a range of environmental research questions and for many industrial applications, such as greenhouse gas monitoring in the atmosphere. Cavity Ring-Down Spectroscopy (CRDS) is the leading technology for achieving the highest precision in measuring trace gas concentrations like carbon dioxide, methane, nitrous oxide, or carbon monoxide. However, hazardous, corrosive, and reactive gases come with additional challenges. 

One example of a reactive trace gas is ammonia (NH3). Ammonia is an air pollutant that is emitted by agriculture, industry, and traffic. The highest ammonia emissions are usually related to intensive farming: manure is produced as a by-product of livestock farming and then used as a nitrogen fertilizer in crop production. Intensive manure application and management is severely affecting the nitrogen cycle and leading to harmful levels of ammonia in agricultural soils, and to the formation of harmful aerosols in the atmosphere. Due to the rising ammonia emissions from agricultural activity and a rising awareness towards the various anthropogenic ammonia sources, there is a high need for ammonia measurements to monitor indoor and outdoor air quality, to quantify current emission sources and to develop reduction options. However, accurate ammonia measurements, at the parts per billion level, are challenging due to the high reactivity and its tendency to adsorb to surfaces, making it one of the most difficult trace gases to monitor. One of the main issues related to the high reactivity and adsorptivity is that it is very difficult to produce or store accurate standard gases. Picarro’s CRDS technology guarantees unmatched long-term stability, and the performance of the concentration analyzers only need to be validated on a monthly to yearly basis.

How do you validate a trace gas measurement if you cannot rely on the accurate concentration of your standard gas? 
To offer a straightforward solution to this challenge, Picarro has developed the method of surrogate gas validation for hazardous, corrosive, and reactive gases, such as ammonia, hydrogen chloride, hydrogen peroxide, hydrogen fluoride, and formaldehyde (see Table 1). The principle is that the accuracy and linearity of the analyzer can be validated using a surrogate gas, that is non-reactive, commercially available, and has an adsorption spectrum adjacent to the primary gas, such as ammonia. In the case of ammonia, carbon dioxide fulfills all necessary criteria. This means, the performance of the analyzer can be validated by measuring air standards with varying carbon dioxide concentrations instead of measuring ammonia standards. 

Table 1: Hazardous, corrosive, and reactive primary gases, and the corresponding surrogate gases that can be used to validate the performance of the CRDS analysers

The overall validation of a CRDS requires two steps:
1. Measuring the surrogate gas at different concentration levels to confirm the linearity of the analyzer.
2. Measuring a zero value, often by using an additional scrubbing agent, to verify if the zero value of the analyzer falls within an acceptable specification (typically a few parts per billion).

For more information, download the Rapid Analyzer Validation Using a Traceable Surrogate Gas Approach paper. 

By Dr. Magdalena Hofmann, Applications Scientist at Picarro Inc., the leading provider of CRDS-based solutions to measure greenhouse gas concentrations, trace gases and stable isotopes across many scientific and industrial applications.