Melissa R. Palmer * and Edward Wahl ASITA Conference * - - PowerPoint PPT Presentation

Method to make accurate concentration and isotopic measurements for small gas samples

Melissa R. Palmer* and Edward Wahl ASITA Conference

*mpalmer@picarro.com

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

• f, 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

• perates 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 ‰

12CO2 (ppm)

δ13C (‰)

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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10

Visible Improvement: Double Injection

12CO2 (ppm)

δ13C (‰)

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 ‰

330 340 350 360 370 380 390 17 17.5 18 18.5 19 19.5 20 920 940 960 980 1000 1020 9 9.2 9.4 9.6 9.8 10 350 360 370 380 390 1.5 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 mpalmer@picarro.com

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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