A PID Analyzers portable GC Model 312 was used to detect ppb levels of N2O in the atmosphere.

1. Analysis of ppb Levels of
Nitrous Oxide by Gas
Chromatography
Ch t h
Pittcon Paper # 460-7
p 460-
Dr. John N. Driscoll
PID Analyzers, LLC
Sandwich, MA
www.hnu.com
www hnu com

2. INTRODUCTION
Nitrous oxide (N2O) is produced in the soil by the decomposition
of nitrogen compounds. This results in an average atmospheric
concentration of approximately 0.25 ppm. It is also used in
dental offices and in hospital operating rooms during surgery
surgery.
Methods used for determination of N2O include infrared (IR)
spectroscopy and gas chromatography. The former method is
chromatography.
useful to just below 1 ppm although photoacoustic IR has a
detection limit of 30 ppb. The GC detectors used for N2O
ppb.
include the thermal conductivity detector (TCD) for high levels (>
100 ppm) and the electron capture detector (ECD) (100 ppm to
app. 0.1 ppm).
app. ppm).)
We will be evaluating a Far UV absorbance detector coupled to
a concentrator to detect low ppb levels of N2O

3. FUV Detector
Consists of a uv lamp (120 nm), a 1 cm
pathlength cell (50 uL) and a UV
photodiode that measures lamp intensity
As a sample enters the cell, the signal will
decrease in proportion to the absorption
coefficient of the species
Response- organic HC, fixed gases &
inorganic gases as shown in next slide

4. Chromatogram of HC with FUV Detector

5. Far UV
Detector

6. Photo of Model 301-B
301-

7. Equipment used
PA Model 301-B GC
301 B
Modified by adding a 6 port manual valve
with sample loops from 0 1 to 50 cc
0.1
Low ppm concentrations prepared by
dilution f
dil ti of gas standard (1000 ppm N2O) i
t d d in
zero air
10 M x 0.53mm PLOT Molesieve
Far UV detector (50 uL dead volume)

8. Actual vs Measured N2O by GC-FUV (ppm)
120
on
y = 1 0162x - 0 5833
1.0162x 0.5833
Measure N2O concentratio
100 2
R = 0.9987
80
60
ed
40
20
0
0 20 40 60 80 100 120
Actual N2O concentration
Detection limit- 500 ppb

9. Concentrator for N2O

10. Concentrator Parameters
Material : molecular sieve- 80/100 mesh
Desorb volume 10-50 cc
volume- 10 50
Desorb temperature: 150°C
Time
Ti to max.T – 4 seconds
T d
Can vary Tmax from 1 second to 4 min.
Cycled between runs to clean column
All parameters are under PC control

11. Chromatogram of 50 ppm
N2O with FUV Detector
10 M x 0.53 mm PLOT
Molesieve, 15 cc/min.
80C; 50 ppm N2O in
ambient air
bi t i

12. N2O 10 fold
concentration

13. 100 ppb N2O
25 fold
concentration

14. Calibration Curve
for Concentrator
ppb Levels of N2O
140
120 y = 1.0257x - 0.4667
ppb Measured
100 R2 = 0.9913
80
60
M
40
20
0
0 20 40 60 80 100 120 140
ppb Actual

15. SUMMARY
By coupling a FUV detector to a PLOT column, we
y p g
have been able to detect from 500 ppm down to 250
ppb of N2O.
This covers approximately the same range as the
ECD
We have evaluated a concentrator/ thermal desorber
That improves th d t ti of N2O t 5 ppb.
Th t i the detection f to b
The FUV (without a concentrator) is will provide a
range that would be adequate for workplace
g
monitoring of N2O.
The FUV with a concentrator is ideal for monitoring
N2O in ambient air