9. Sampling
9.1 Supply samples to the laboratory in high pressure cylinders coated internally with TFE-fluorocarbon, or otherwise specially treated to reduce or eliminate loss of COS due to reaction with the cylinder walls.
9.2 The sample cylinder and contents should be at room temperature prior to sampling to the chromatograph. Test samples as soon as possible after receipt.
NOTE 3 - Cooperative studies indicate that the measured value for COS will decrease with time.
9.3 Place the sample cylinder in a vertical position and use either of the following two techniques to obtain a vaporized sample from the container for introduction into the GC.
9.3.1 Connect the sample cylinder to the sampling valve on the chromatograph, using a minimum length of 316 ss tubing, so that sample is withdrawn from the bottom of the cylinder. Adjust the flow rate from the sample cylinder so that complete vaporization of the liquid occurs at the cylinder valve. A flow rate of 5 to 10 bubbles/s through a water bubbler placed at the sample vent is sufficient (see Note 4). Turn the sampling valve to the "flush" position and flush for approximately 15 s. Shut off the cylinder valve and allow the pressure to drop to atmospheric.
NOTE 4 - If the flow rate is too fast, warming of the valve can be required to avoid freezing and to ensure complete vaporization of the sample.
9.3.2 Alternatively, obtain a sample with a gas tight syringe. A convenient way to do this is to use flexible plastic tubing to connect the bottom of the sample cylinder to the water bubbler and then to pierce the tubing with the syringe needle after flow is established.
10. Preparation of Apparatus
10.1 Install in the GC according to the manufacturer's instructions any ofthe columns that meet the criteria in 6.3. Set the instrument conditions as follows:
10.1.1 Oven Temperature, as determined by column used,
10.1.2 Detector, 100 to 200°C, and
10.1.3 Injector, 100 to 150°C.
11. Calibration
11.1 Three methods of calibration are permitted. These are the Standard Sample Method (see 11.2), the Permeation Tube Method (see 11.3) and Blend Preparation Techniques (see 11.4). Obtain a calibration standard according to one of these methods, which are described below. Then follow the procedure in 12.1-12.4 and the calculations described in 11.5.
11.2 Standard Sample Method - Purchase a certified commercial calibration sample of 10 mg/kg COS in propylene, or other suitable matrix gas such as nitrogen, argon, or a propylene/argon mixture. If an inert gas is chosen, the user must ensure that the column is actually effecting a separation of COS and propylene. Establish a calibration curve with the standard sample using either a gas syringe or different size sample loops. For example, assume the normal sample size for the analysis is 1.0 mL and the calibration range to be established is 0.5 to 5 mg/kg of COS. Establish a calibration curve by injecting the volumes of a 10 mg/kg standard sample shown in the first column of the table below. The equivalent concentration of COS in a 1.0 mL sample would be that shown in the second column:
11.3 Permeation Tube Method - Refer to Practice D3609 for directions on using permeation tubes.
11.4 Blend Preparation Techniques - Techniques for the preparation and assay verification of calibration blends in the laboratory are described in Appendix X1 and Appendix X2. Also, a technique using a moving piston graduated cylinder apparatus, that is described in the calibration section of Test Method D 4468, can be used. However, some laboratories have found that the preparation of such blends is far from easy, and successful efforts require considerable knowledge and experience.
11.5 Quantitation - The flame photometric detector responds logarithmically to the mass of the sulfur present in the flame. Some GC/FPD systems are programmed to linearize logarithmic data, and with such systems the output can be correlated directly with the COS concentration, using a single point calibration. Calculate a calibration factor, F, in accordance with (Eq 1) below:
F = C/A
where:
F = calibration factor,
C = concentration, mg/kg, of COS in this test method, and
A = area (uVs) of the COS peak in this test method.
F will be used in (Eq 2) in 13.1.1. However, if a linearizer is not used, or if the data system does not have a provision to handle logarithmic output, use the method in 11.5.1 or the alternate in 11.5.2, below:
11.5.1 Calculate the nanogram (ng) amounts of sulfur, as described in Appendix X3, for each injection of the standard, and plot the natural logarithm (1n) of peak area versus the 1n (ng) of sulfur, as illustrated in Table 2 and Fig. 1. The plot should be a straight line.
11.5.2 Alternatively, plot the concentration of COS in mg/kg versus the square root of the peak area. This plot should also be a straight line.