method to make accurate concentration and isotopic...
TRANSCRIPT
Method to make accurate concentration and isotopic measurements for small gas samples
Melissa R. Palmer* and Edward Wahl
ASITA Conference
June 17th, 2014
δ13C of CO2 and CH4 for carbon cycle studies
With application to: – Atmospheric CO2 & CH4 (unlimited sample) – DOC & DIC (acidification to CO2) – Gases evolved from soil respiration – Dissolved gases sampled via headspace
equilibration methods – Laboratory-based microbial experiments
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Motivation: Use Picarro CRDS technology to measure concentrations of, and δ13C in, CO2 and CH4 for continuous monitoring or discrete samples using the same analyzer.
Measuring discrete samples of CO2 and CH4
Challenge: CRDS is easy-to-use, robust and field deployable, but it operates as a continuous flow system:
– Ideal for measuring ambient atmospheric conditions – More challenging for small volume samples
Objective: Evaluate an improved method using a Small Sample Isotope Module (SSIM) coupled to a continuous-flow Picarro G2201-i.
Instrumentation: G2201-i
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• Measurement technique: Cavity Ring-Down Spectroscopy • Analyzer data rate: 3-seconds for each species • Analyzer flow rate: ~ 25 sccm ~ 200 mL of gas for a 5-min measurement • Water concentration is measured and dry mol fraction reported
Molecule Specified Precision @ Ambient : 1-σ of 5 min averages
δ13C in CO2 < 0.16 ‰
[12CO2] 200 ppb + 0.05 % of reading
δ13C in CH4 < 1.15 ‰
[12CH4] 5 ppb + 0.05 % of reading Picarro G2201-i
Instrumentation: SSIM2
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Picarro SSIM
• Throughput: 9-12 min per replicate • Recommended sample size: 20 mL (ambient sample) per injection • SSIM aims to:
– Take advantage of improved precision with longer averaging time – Optimize sample volume and delivery to prevent dilution – Minimize leaks and dead volumes to prevent dilution and isotopic fractionation
Molecule Specified Precision with SSIM @ Ambient for 1 replicate:
δ13C in CO2 < 1.0 ‰
[12CO2] Results of this study
δ13C in CH4 < 1.6 ‰
[12CH4] Results of this study
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SSIM Sample Delivery
External Vacuum Pump
20mL Sample Volume
Zero Air
V1 V2
V4
Cal
gas
Valve attached to sample container
Sample Out
V3
V5
Sample
Normally Open
Normally Closed
Common
P
Pressure sensor
- Step 1: Pump Down Sample Loop -Step 2: Sample Delivered to Sample Loop -Step 3: Sample Delivered to Analyzer
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SSIM Sample Delivery
External Vacuum Pump
20mL Sample Volume
Zero Air
V1 V2
V4
Cal
gas
Valve attached to sample container
Sample Out
V3
V5
Sample
Normally Open
Normally Closed
Common
P
Pressure sensor
- Step 1: Pump Down Sample Loop -Step 2: Sample Delivered to Sample Loop -Step 3: Sample Delivered to Analyzer
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SSIM Sample Delivery
External Vacuum Pump
20mL Sample Volume
Zero Air
V1 V2
V4
Cal
gas
Valve attached to sample container
Sample Out
V3
V5
Sample
Normally Open
Normally Closed
Common
P
Pressure sensor
- Step 1: Pump Down Sample Loop -Step 2: Sample Delivered to Sample Loop -Step 3: Sample Delivered to Analyzer
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Current Methodology: Single injection
SSIM Single Injection from Tank
Direct Tank Measurement
∆(SSIM-Tank)
12CO2 (ppm) 366.3 ± 0.15 383.0 ± 0.17 ~ 17 ppm
δ13CO2 (‰) -68.55 ± 1.0 -67.33 ± 1.0 ~ 1 ‰
12CO
2 (pp
m)
δ13 C
(‰)
Sample injected
10 minutes
Improved methodology: Double injection
Approach: Reduce systematic dilution effect by injecting each sample twice. Analyze the second injection and improve precision of concentration measurement. Experimental design:
1. Directly measure three tanks of variable concentration and isotopic composition on the G2201-i (reference)
2. Analyze the tanks using the Single Inject (SI) method and determine accuracy over 25 injections.
3. Analyze the tanks using the new Double Inject (DI) method and determine accuracy over 25 injections.
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Visible Improvement: Double Injection 12
CO2 (
ppm
) δ1
3 C (‰
)
First injection
25 minutes
Second injection
SSIM First Injection
SSIM Second Injection
Direct Tank Measurement
∆(SSIM 2nd Inject-Tank)
12CO2 (ppm) 366.4 ± 0.15 383.2 ± 0.15 383.0 ± 0.17 ~ 0.2 ppm
δ13CO2 (‰) -68.54 ± 1.0 -67.3 ± 1.0 -67.3 ± 1.0 ~ 0.1 ‰
330 340 350 360 370 380 39017 17.5 18 18.5 19 19.5 20
920 940 960 980 1000 10209 9.2 9.4 9.6 9.8 10
350 360 370 380 3901.5 1.6 1.7 1.8
Single Injection vs. Double Injection: Concentration of 12CO2 and 12CH4
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Tank
3
Tank
2
Tank
1
High Range 12CH4 12CO2
- = Double Inject (DI) - = Single Inject (SI) - = Tank
92.9% 99.7%
93.5% 99.8%
94.2% 99.8%
94. 8% 99.8%
91.8% 99.8%
92.6% 99.8%
n = 25 injections for DI and SI on each tank % = average SSIM measurement / tank measurement (%)
100% 100%
100% 100%
100% 100%
-45 -40 -35 -30 -25 -20
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Single Injection vs. Double Injection: Concentration of δ13CO2 and δ13CH4
Tank
3
Tank
2
Tank
1
High Range δ13CH4 δ13CO2
- = Double Inject (DI) - = Single Inject (SI) - = Tank
n = 25 injections for DI and SI on each tank
1σ ~ 6 ‰
-42 -41 -40 -39 -38 -37 -36
-41.5 -41 -40.5 -40 -39.5 -39 -38.5 -38
1σ ~ 1 ‰
1σ ~ 0.5 ‰
-38 -37 -36 -35 -34
-41.4 -41.2 -41 -40.8 -40.6 -40.4 -40.2 -40 -39.8
-36.5 -36 -35.5 -35 -34.5 -34 -33.5 -33
1σ ~ 0.8 ‰
1σ ~ 0.4 ‰
1σ ~ 0.8 ‰
Conclusions and Recommendations
– Double injection conclusions: • Improves relative error of concentration measurements from 5% to < 0.5% error
• Has no effect on the relative error of the isotopic measurements, and performance remains high
– Recommended sample criteria: • Sample size ≥ 40 mL
• Sample concentration of CO2 between ambient and 2,000 ppm
• Sample concentration of CH4 between ambient and 500 ppm
• Sample container: tedlar bag, ss or glass flasks
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Note: this presentation reflects development work at Picarro, and does not represent a standard configuration of the SSIM2. This information should not be relied upon in making a purchasing decision. Please contact Picarro if you have additional questions.
Thank you! Visit us at the Picarro booth or
email me at [email protected]
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SSIM Cleaning – Reducing Memory
External Vacuum Pump
20mL Sample Volume
Zero Air
V1 V2
V4
Cal
gas
Valve attached to sample container
Sample Out
V3
V5
Sample
Normally Open
Normally Closed
Common
P
Pressure sensor
- Step 1: Pump Down Sample Loop -Step 2: Flush SSIM with ZA to the Analyzer
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SSIM Cleaning – Reducing Memory
External Vacuum Pump
20mL Sample Volume
Zero Air
V1 V2
V4
Cal
gas
Valve attached to sample container
Sample Out
V3
V5
Sample
Normally Open
Normally Closed
Common
P
Pressure sensor
- Step 1: Pump Down Sample Loop -Step 2: Flush SSIM with ZA to the Analyzer
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Double Injection: SSIM Pressure