6.1. Precautions

6.1.1. When preparing CO₂ standards, an Ascarite II filter is used to remove CO₂ from the diluent air. Ascarite II contains sodium hydroxide and silica: Avoid contact with skin and mucous membranes. Wear gloves and safety glasses when handling Ascarite II.

6.1.2. The preparation of CO₂ standards should be performed in a hood. Avoid breathing in any toxic concentrations of CO₂.

6.2. Equipment

6.2.1. A gas chromatograph fitted with a 1-mL stainless steel gas sampling loop, sampling valves, and a TCD are used. Loops larger than 1-mL can also be used.

6.2.2. Standard preparation (8.11.):

Due to stability considerations, use only five-layer aluminum gas sampling bags for standard generation. Gas sampling bags composed of Tedlar or saran can be used for standards provided they are prepared and analyzed within 24 h.

6.2.3. Columns:

Chromosorb 102, 6 ft × 1/4 in. stainless steel, 80/100 mesh. Similar results were obtained using a 5 ft × 1/4 in. stainless steel, 80/100 mesh, Porapak QS column.

6.2.4. Data reduction:

An electronic integrator is used to calculate peak areas.

6.2.5. Standard generation:

Certified CO₂ standards can be used or standards can be prepared using any combination of: Calibrated gas-tight syringes or calibrated rotameters, mass flow controllers, or soap bubble flowmeters. A stopwatch is also necessary.

6.2.6. Gases:

1. If certified standards are not available, undiluted, bottled CO₂ (>99.8% purity) or pre-diluted CO₂ can be used to generate gas standards. If pure liquid CO₂ is used, a heating tape and variable transformer are necessary for regulator heating.
   
   
2. Filtered, compressed, CO₂-free air is used for dilutions. Ambient CO₂ is removed from the compressed air using an Ascarite II/Drierite in-line trap. (See Precautions in Section 6.1. before handling Ascarite) Other methods, such as slaked lime traps, can also be used for removal of ambient CO₂. A diagram of the Ascarite II trap and further information can be found in reference 8.11.
   
   
3. Helium (purified) is used as the carrier gas.

6.2.7. Additional accessories:

A personal sampling pump, with an inlet and outlet port and hose barbs, is used to load the gas sampling loop (loop loading can also be manually performed by squeezing the sampling bag).

6.3. Standard Preparation

6.3.1. Prepare standards by either calibrated syringe or metered injection of pure or diluted CO₂ into the dilution medium. Please see precautions in Section 6.1.2. before preparing.

6.3.2. Completely evacuate and flush the gas bags used for standard preparation with CO₂-free air (Note: The in-line trap with an Ascarite II/Drierite bed is used to remove any CO₂). Use a soap bubble flowmeter to measure air flow rates immediately before and after diluent air addition.

A standard dilution scheme using pure CO₂ is listed for 5-L gas bags:

ppm Standard	CO2 volume (mL)	Airvolume (mL)
Blank	0	4,000
1,248	5	4,000
2,494	10	4,000
4,975	20	4,000
9,901	20	2,000
19,608	40	2,000

Always prepare a blank standard from the compressed air to account for potential CO₂ contamination. Other dilution schemes using smaller or larger gas bags and gas volumes can be used. Standards should be prepared in concentrations that bracket the sample concentrations.

6.3.3. For concentrations other than those listed above, use the following equation when using pure or pre-diluted CO₂:

ppm CO2 =

(mL CO2)(1 × 106)(A) (mL diluent air + mL CO2)

where:

A

=

mole fraction or decimal per cent concentration of the pre-diluted mixture (i.e., for 1.93% CO2 stock standard, A = 0.0193. For pure CO2, A = 1).

6.3.4. If a metered generation of standards is desired, use a mass flow controller or calibrated rotameter to verify and control the CO₂ delivery rate from a gas cylinder. Use a soap bubble flowmeter immediately before and after the standard generation to verify the CO₂ flow rate. Meter a known amount of CO₂ into a bag already containing a known volume of CO₂-free air. Use a stopwatch to determine the volume of CO₂ delivered over time.

6.3.5. If using calibrated syringe injection, fill a gas sampling bag with concentrated CO₂ or use syringe extraction from an in-line gas cylinder septum. Most gas bags have injection ports or septa for gas syringe withdrawal or injection. Fill and flush a previously calibrated gas-tight syringe with pure CO₂. Then withdraw and inject the required volume of CO₂ into a gas bag already containing a measured amount of diluent air.

6.4. Analytical Procedure

6.4.1. Gas chromatograph conditions:

Helium carrier gas flow rate	15 to 25 mL/min
Reference gas flow rate	15 to 25 mL/min
Detector, manifold, and column temperature	20 to 25°C
Retention time	2 to 6 min

6.4.2. Sample and standard introduction:

1. Connect the outlet port of the personal sampling pump to the sampling loop via inert tubing.
   
   
2. Adjust the pump to give a suitable flow rate for sample loading from the bag to the sampling loop.
   
   
3. Connect a short piece of tubing from the inlet port of the pump to the sample bag. The bag valve is then turned counterclockwise to the open position and the pump is turned on.
   
   
4. After the sample is loaded into the loop, turn the pump off to allow the loop sample to return to atmospheric pressure. Wait 1 to 2 min for pressure equalization and then open the gas sampling valve. Carrier gas flow is now directed through the sampling loop to the column and detector. (Note: Samples and standards can be introduced into the loop without a pump by simply squeezing a sufficient amount of sample from the bag into the loop. The sampling bag must be released for loop sample pressure normalization before opening the gas sampling valve.)
   
   
5. Perform two determinations of each sample and standard.

6.4.3. If present in the sample, oxygen, nitrogen, carbon monoxide, and methane will elute before CO₂. Examples of integrated chromatograms of CO₂ and other common gases are shown in Figures 1 and F.