method to make accurate concentration and isotopic...

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

Melissa R. Palmer* and Edward Wahl

ASITA Conference

*[email protected]

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.

Presenter

Presentation Notes

Combined continuous and discrete samples – 1 analyzer does all Dissolved Organic Carbon & Dissolved Inorganic Carbon The d13C signature of CO2 and CH4 is frequently used in carbon cycle studies for a variety of applications, such as source attribution, studying biological processes, and partitioning remineralization and primary productivity in the water column. In particular, there is an interest in measure the isotopic composition of CO2 and CH4 from a variety of sources: ambient atmosphere, DOC and DIC, gases evolved from soil respiration and more. Some of these sample types are effectively unlimited in quantity (e.g., ambient atmospheric measurements), while others have a limited sample size. CRDS analyzers operate as continuous flow systems and as such require relatively large gas samples. The Small Sample Isotope Module was designed with sample size constraints in mind.

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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20mL Sample Volume

Zero Air

V1 V2

V4

Cal

gas


Sample Out

V3

V5

Sample

Normally Open

Normally Closed

Common

P

Pressure sensor


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20mL Sample Volume

Zero Air

V1 V2

V4

Cal

gas


Sample Out

V3

V5

Sample

Normally Open

Normally Closed

Common

P

Pressure sensor


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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

Presenter

Presentation Notes

The concentration is stable. Delta meets its precision spec. Everything looks correct, until the tank is measured directly by the analyzer and the concentration reading is almost 5% off.

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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Presenter

Presentation Notes

first injection of the DI procedure will last for 3minutes. This is ideal because after 3minutes, the outlet valve has closed and the SSIM pressure has reached equilibrium with the cavity (148torr).

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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 ‰

Presenter

Presentation Notes

Note the slight improvement in the isotopic value as well…

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.

Presenter

Presentation Notes

Higher concentrations, syringes and exetainers can be used but error will be worse due to dilution and worse repeatability

Thank you! Visit us at the Picarro booth or

email me at [email protected]

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SSIM Cleaning – Reducing Memory


20mL Sample Volume

Zero Air

V1 V2

V4

Cal

gas


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


20mL Sample Volume

Zero Air

V1 V2

V4

Cal

gas


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

17

Double Injection: SSIM Pressure

method to make accurate concentration and isotopic...

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