October 31, 2011 

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. These 

technologies are the Gas Chromatograph Electron Capture Detector (GC-ECD), Fourier 

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

Quantum Cascade Laser (QCL) analyzer. These four 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, and CO2 in 

the 0.7% to 2% range. 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. 

 One manufacturer’s instrument is available for purchase while the other manufacturer’s 

instrument should be commercially available in Spring of 2012. In both cases the single gas N2O 

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

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. We expect this new instrumentation to fully penetrate the 

N2O measurement market in the next 6 months. 

Attachment: “N2O Instrument Evaluation” 
N2O Instrument Evaluation O Instrument Evaluation

10/31/11

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U.S. Environmental Protection Agency
GC-ECD Comparison to FTIR

Evaluation Results

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Overview

• GC-ECD and FTIR analyzed bags over 

phases of the FTP and HWFET for N2O

– Ambient measurement performed.

• Blended bag analysis of N2O using NDI

FTIR.

– Testing repeated, once on 5/9/11 and once 

5/31/11.

– Ambient analysis included.

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ARB Comparison of GC-ECD to F

• ARB analyzed bags over three phases o

FTP and HWFET for N

2O.

– Bag analysis on GC-ECD and FTIR meetin

SOP No. MDL 136.

– Testing was done on 3 vehicles no repeats Testing was done on

3 vehicles, no repeats

– Tests were performed at ATDC under EPA 

contract.

– FTIR compared very well to GC-ECD, whic

considered the gold standard.

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NDIR Comparison to FTIR

Evaluation Results

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Comparison of NDIR and FTIR

• Horiba loaned us one of their only AIA-721(LE

analyzers for evaluation.

• AVL new generation FTIR was also made ava

AVL new generation FTIR was also made ava

to EPA for evaluation.

• We just recently evaluated the analyzers on ba

consi ti f bl d f CO CO isting o

f blen

ds o

f CO, CO

2, wat N er, 

N

2O 

, a

• The purpose was to validate the analyzers’ ab

compensate for CO, CO

2, and water interferen

• Both the NDIR and FTIR analyzers performed 

however some questions regarding performanremain.

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Conclusion

• Current NDIR and FTIR technologies can have 

tendency to bias the measurement high due to interference contributions. interference contributions.

– Horiba NDIR can minimize interference effects by ta

representative CO2 concentration for factor generati

– Precision of NDIR CO correction remains an open is

(other possible contributing factors?).

– ARB FTIR data shows no bias, while AVL does (typ

0.75 ppm on average).

– Overall, interference contributions are well below the 

limit in 1065.375.

• GC-ECD does not have any measurement iss10/31/2011 U.S. Environmental Protection Agency 
Horiba MEXA-1100QL Evaluatio

Results

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Evaluation of Horiba MEXA-1100

• Horiba loan EPA a MEXA-1100QL quantum ca

laser based N

2O measurement instrument for 

evaluation.

• EPA GC-ECD is not yet operational, therefore 

1100QL performance was evaluated against bblends of CO CO blends of CO, CO

2, water N water, 

N

2O and N O, and

N

2.

• The evaluation was identical to pervious evalu

of NDIR and FTIR N

2O analyzers performed b

• The purpose was to validate the analyzers’ ab

compensate for CO, CO

2, and water interferen

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Evaluation of Horiba MEXA-1100

• The MEXA-1100QL performed with effe

no span drift over days of operation and 

minimal zero drift.

• The instrument consistently read ambie

b k dN ac

kgroun

d

N

2O 312 as

± 1 b pp

b.

• Interference check results:

– -9 ppb response on 15.19% CO

2

– 2 ppb response on 485 ppm CO

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0.6

Comparison of N2O Concentrations Across Different Bag Blends of CO2 a

0.5

1.461 % CO2194 8 ppmCO

1.948 % CO2292 2 ppmCO

TargetN2OConcentration = 0.391 ppm

H2OConcentration = 2.2%

0.30.4

ntraiton (ppm)

292.2 ppmCO 194.8 ppmCO

0.2

N O Conce 2n

0.00.1

Bag 5 Bag 6

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Calculated Blend Horiba MEXA-1100QL
0.8

Comparison of N2O Concentrations Across Different Bag Blends of CO2 an

TargetN OConcentration 0 587 ppm

0.60.7

0.97 % CO287.3 ppm CO

1.457 % CO2

116.5 ppmCO

0.777 % CO2

68.0 ppm CO

0 % CO2

193.9 ppmCO

1.457 % CO2

0 ppmCO

1.457 % CO2

193.6 ppmCO

1.457 % CO2

291.3 ppmCO

1.942 % CO

194.2 ppm

TargetN2OConcentration = 0.587 ppmH2OConcentration = 2.2%

0.40.5

entraiton (ppm)

0.20.3

N O Conc 2e

0.00.1

Bag 8 Bag 4 Bag 1 Bag 9 Bag 2 Bag 7 Bag 12 Bag 11 

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Bag 8 Bag 4 Bag 1 Bag 9 Bag 2 Bag 7 Bag 12 Bag 11 

Calculated Blend Horiba MEXA-1100QL
350

Comparison of CO Concentrations Across Different Bag Blends of N2O, CO

and CO

300350200250

iton (ppm)

100150

CO Concentra

0

50

B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 B 9 B 10 B 

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Bag 1 Bag 2 Bag 3 Bag 4 Bag 5 Bag 6 Bag 7 Bag 8 Bag 9 Bag 10 Bag 

Calculated Blend Horiba AIA-721 NDIR Analyzer
2 5

Comparison of CO2 Concentrations Across Different Bag Blends of N2O, CO

and CO

2.02.51.5

traiton (%)

1.0

CO

2 Concen

0.00.5

B 1 B 2 B 3 B 4 B 5 B 6 B 7 B 8 B 9 B 10 B 

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Bag 1 Bag 2 Bag 3 Bag 4 Bag 5 Bag 6 Bag 7 Bag 8 Bag 9 Bag 10 Bag 

Calculated Blend Horiba AIA-722 NDIR Analyzer
MEXA-1100QL Measurement Error vs. 

Divider Generated Concentrations

Target Generated Concentration Measured Concentration Error

Divider Generated Concentrations

5.056 5.077 0.41%

3.0336 3.0415 0.26%1.0112 0.9971 -1.41%0.9101 0.9023 -0.86%

0.809 0.8035 -0.68%

0.7078 0.7035 -0.61%0.6067 0.6081 0.23%0.5056 0.5093 0.73%0.4045 0.4075 0.74%0.3034 0.3088 1.75%0.2022 0.2054 1.56%0.1011 0.0983 -2.85%

0 0.0086 0.17%

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Conclusion

• The MEXA-1100QL performed very well and d

not appear to show any susceptibility to CO, C

water interference.

• Based on CO and CO

2 measurement error, we believe that the bulk of the associated N

2O measurement error is due to bag blending erro

( i f 5% f ll i ) (maximum error o

f 5%

for all species

).

• MEXA-1100QL out performs FTIR and NDIR 

previously tested by EPA.

• All i di ti th t th MEXA All indications are th

a

t the MEXA

-1100QL i 

1100QL is a 

accurate instrument for measurement of N

2O a

concentrations expected during light-duty chas

testing

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.
MAHA QCL Evaluation Results

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Evaluation of MAHA QCL Analyz

• Maschinenbau Haldenwang (MAHA) loaned E

MAHA QCL quantum cascade laser based N

2O 

measurement instrument for evaluation. measurement instrument for evaluation.

• EPA GC-ECD is not yet operational, therefore 

QCL performance was evaluated against bag of CO CO of CO, CO

2, water N water, 

N

2O and N O, and

N

2.

• The evaluation was identical to previous evalu

of QCL, NDIR, and FTIR N

2O analyzers perfor

EPA.

• The purpose was to validate the analyzers’ ab

compensate for CO, CO

2, and water interferen

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Evaluation of MAHA QCL

• The instrument had issues with the computer that 

controlled the zero and span functions (a complicaconverting the instrument from 220 VAC to 110 VA

– S h d t b t d ll Span 

h

a

d

to 

be en

tere

d manually.

– During the evaluation we lost the ability to rezero the 

instrument.

– Zero/s

p y an drift issues and the inabilit

y to calibrate out t

could be the cause of the negative bias seen throughotesting.

• The instrument read ambient background N

2O as 3

ppb drifting down to 305 ppb over the course of thppb, drifting down to 305 ppb over the course of th

• Interference check results:

– 4 ppb response on 15.19% CO

2

– -2 ppb res

ponse on 485 ppm CO

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pp p pp
0.9

Comparison of N2O Concentrations Across Different Bag Blends of CO2 a

T tN OC t ti 0 587

0.70.8

0.976 % CO287 8 ppm CO

1.457 % CO2116 5 ppmCO

0 % CO2195 4 ppmCO

1.467 % CO20 ppmCO

1.467 % CO2293 4 ppmCO

1.956 % CO2195 6 ppmCO

TargetN2OConcentration = 0.587 ppm

H2OConcentration = 2.2%

0.50.6

ntraiton (ppm)

87.8 ppm CON2ODiff ‐0.035

116.5 ppmCON2ODiff ‐0.010

195.4 ppmCON2ODiff ‐0.024

0 ppmCON2ODiff ‐0.032

293.4 ppmCON2ODiff ‐0.038

195.6 ppmCON2ODiff ‐0.017

0 20.30.4

N O Conce 2n

0.00.10.2

B 8 B 1 B 9 B 7 B 12 B 11 

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Bag 8 Bag 1 Bag 9 Bag 7 Bag 12 Bag 11 

Calculated Blend MAHA QCL
350

Comparison of CO Concentrations Across Different Bag Blends of N2O, C

and CO

300200250

iton (ppm)

100150

CO Concentra

0

50

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Bag 1 Bag 7 Bag 8 Bag 9 Bag 10 Bag 11 

Calculated Blend Horiba AIA-721 NDIR Analyzer
2.5

Comparison of CO2 Concentrations Across Different Bag Blends of N2O, C

and CO

2.01.5

ntraiton (%)

1.0

CO

2 Conce

n

0.00.5

Bag 1 Bag 7 Bag 8 Bag 9 Bag 10 Bag 11 

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Bag 1 Bag 7 Bag 8 Bag 9 Bag 10 Bag 11 

Calculated Blend Horiba AIA-722 NDIR Analyzer
Conclusion

• Th MAHA QCL f d ll d d The MAHA QCL performed well and does not appe

show any susceptibility to CO, CO2, or water interf

• Based on CO and CO2 measurement error (maxim

3%) we believe that some of the associated N 3%), we believe that some of the associated N2O measurement error is due to bag blending error.

• It should also be noted that the average 5% negati

on N2O measurement is influenced by the inability 

2 y y properly zero/span the instrument after the initial 

zero/span at the start of the testing.

– The negative drift is evident when looking at the increa

negative bias in the order the bags were read: negative bias in the order the bags were read: 7 8 9 

7, 8, 9, 

12, and 1.

– It is also noted in the decrease in ambient concentratio

the course of the testing: 320, 310, 305 ppb.

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Conclusion cont.

• Th MAHA QCL f FTIR d NDIR The MAHA QCL out performs FTIR and NDIR

previously tested by EPA.

• All indications are that if the instrument zero/sp

f ti ki l th MAHA QCL functions are working properly, the MAHA QCL

should be a viable, accurate instrument for measurement of N2O at the concentrations exp

during light during light-duty chassis testing duty chassis testing.

• EPA intends to re-evaluate this instrument afte

manufacturer has resolved issues regarding th

power configuration for use in North America power configuration for use in North America.

• We will update our results with this instrument 

our re-evaluation.

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