March 19, 2012 

MEMORANDUM 

SUBJECT: Data from the Evaluation of Instruments that Measure Nitrous Oxide (N2O) 

FROM: Chris Laroo, Environmental Scientist 

Assessment and Standards Division 

TO: Docket EPA-HQ-OAR-2010-0799 

This memorandum and attached file, “N2O Instrument Evaluation” shows the results of EPA’s 

evaluation of 4 technologies for N2O measurement, conducted during the fall of 2011 and winter 

of 2012. These technologies are the Gas Chromatograph Electron Capture Detector (GC-ECD), 

Fourier Transform Infrared Spectrometer (FTIR), Non-Dispersive Infrared (NDIR) analyzer, the 

Quantum Cascade Laser (QCL) analyzer, and scanning laser analyzer. These five technologies 

were evaluated for their accuracy in measuring light-duty vehicle N2O emissions at the dilute 

exhaust gas levels expected in meeting the 0.01 gram/mile N2O standard. This includes expected 

interference gas dilute exhaust concentrations of H2O in the 0 to 2% range, CO in the 80 to 300 

ppm range, CO2 in the 0.7% to 2% range, and CH4 in the 20 to 24 ppm range (where applicable). 

The target range for N2O in dilute exhaust gas ranged from 400 to 800 ppb, which is within the 

range of dilute exhaust gas concentrations expected for a vehicle complying with the light-duty 

vehicle N2O emission standard. 

GC-ECD 

The results indicate that the GC-ECD continues to be a robust, accurate measurement method for 

low level N2O analysis. 

FTIR 

The results show that the FTIR can be prone to high bias due to interference from CO and CO2, 

however steps can be taken (like increasing the gas cell size and path length) to minimize the 

interference contribution. 

NDIR 

While improvements have been made to the NDIR, the instrument evaluated was prone to 

interference when the interference gas concentrations strayed from the concentration at which 

the interference correction coefficient was generated. Thus interference could be controlled 

within a given range of CO, CO2, and N2O concentrations; but care must be taken ensure 

accurate measurement. 

QCL 

The QCL is the most recently developed instrument. Evaluation of the instrument shows that it 

is not susceptible to interference from CO, CO2, or water. The instrument has excellent low 

level accuracy and results from measurement with this instrument should be considered 
comparable to that from a GC-ECD. The quad gas version of this bench that measures NO, NO2, 

N2O, and NH3 is currently available for purchase. The single gas (N2O only) versions of the 

instruments that we evaluated from two different manufactures have just been brought to market. 

 Both manufacturer’s instrument are available for purchase. In both cases the single gas N2O 

bench is currently available for evaluation from each of the manufacturers. 

Scanning Laser 

The scanning laser spectrometer has also been recently developed for measurement of N2O at 

sub-1 ppm concentrations. Evaluation of the instrument shows that it is not susceptible to 

interference from CO, CO2, CH4, or water. The instrument has excellent low level accuracy and 

results from measurement with this instrument should be considered comparable to that from a 

GC-ECD and QCL. This instrument is commercially available from one supplier. 

Overall, the results indicate that methodologies are currently available to perform accurate low 

level (sub 1 ppm) measurement of N2O and that new instrumentation has been developed in 

response to EPA rulemakings to facilitate low level accurate measurement of N2O with no 

contribution from interference gasses. These instruments are currently available for purchase 

from the respective instrument suppliers. 

Attachment: “N2O Instrument Evaluation”