30.1 Headspace Vial Volume Determination - Because of the importance of the VG/VL phase ratio factor in the concentration calculation (Eq 17), the following vial volume determination procedure must be applied to detect any variation from batch to batch and manufacturer to manufacturer.
30.1.1 Equilibrate at the ambient laboratory temperature about 20 vials from every batch and 500 mL of water and note the temperature. The vials should be preferably selected from the different packs received from a batch.
30.1.2 Determine the weight of each vial with an accuracy of 0.01 g. Fill each vial completely with water (the vial should be filled to the point where the water surface is flat and in line with the edge of the vial, that is, no meniscus), and reweigh the vial with an accuracy of 0.01 g.
30.1.3 Calculate the volume of each vial using the following equation:
30.1.4 Calculate the mean volume and the standard deviation over the twenty vials from every batch. Use the mean volume to determine precisely VG of the phase ratio used in Eq 17.
NOTE 13 - The analytical performance of Method C was established with a volume variation less than 0.7 % (%RSD over 20 vials) for vials from the same batch.
30.2 Headspace Vial Preparation:
30.2.1 Using an appropriate tool, seal a series of 20-mL vials using perforated aluminum caps fitted with a TFE-fluorocarbon-faced butyl septum. Ensure that the lined side is turned towards the inside of the vial and check that the vial is properly sealed by trying to turn the cap. If the cap is not tightly fixed, repeat the process. These vials will be used to perform 30.2.2 and 30.3.
30.2.2 Insert two 18G1 needles through the vial septum at different peripheral locations, one to be used as the inlet gas and the other as the outlet gas. Purge each vial with argon at a rate of about 2 L/min for at least 30 s. First remove the outlet needle and then the inlet needle. (This sequence allows pressure to build up inside the vial). These vials will be used in 30.4. (Warning - For safety consideration, the argon cylinder should be equipped with a two-stage regulator with a delivery pressure adjusted at 20.7 Kpa (3 psi) to limit the overpressure built up in the vial. The regulator should be purged with argon before proceeding with 30.2.2.)
30.3 Calibration - The calibration curves are obtained as follows:
30.3.1 Insert two 18G1 needles through the vial septum at different peripheral locations, one to be used as the calibration inlet gas and the other as the outlet gas. Purge one vial with each calibration gas mixture at a rate of about 2 L/min for at least 30 s. First remove the outlet needle and then the inlet needle. (This sequence allows pressure to build up inside the vial.) The vial pressure is then equilibrated at the ambient pressure by inserting a 26G 1/2 needle for a very short period of time (1 to 2 s). (Warning - For safety consideration, the calibrating gas cylinders should be equipped with a two-stage regulator with a delivery pressure adjusted at 20.7 Kpa (3 psi) to limit the overpressure built up in the vial. The regulator should be purged with calibrating gas mixture before proceeding with 30.3.1.)
30.3.2 Place the vials inside the headspace sampler and begin the analysis using the instrumental conditions given in Table 4. In this specific case, the sample equilibration time at 70°C with shaking could be set at 5 min.
30.3.3 Plot the calibration curves by selecting the appropriate commands in the data acquisition software or any statistical software.
30.3.4 Use a logbook to compile the values of the regression parameters (y = mx + b) and the correlation coefficients (R). The system should be recalibrated each day when analysis is being conducted or when the QA/QC program being used indicates an abnormal situation attributed to calibration.
NOTE 14 - Achieving calibration every working day ensures identical ambient atmospheric pressure and temperature conditions when filling both the calibration vials and the sample vials. When a QA/QC program is used, the day-to-day variations in the ambient atmospheric pressure and laboratory temperature are covered by the imposed limits of the control charts used for following the day-to-day performance of the system.
30.4 Analysis of Field Samples - Introduction of oil samples into vials is performed using glass syringes fitted with threeway plastic stopcocks. The stopcocks used in these procedures have the characteristic that the handle always points toward the closed port leaving the other two ports connected together. Rotation of the valve handle is restricted so that interconnection of all three ports is impossible. When a stainless steel bottle or a flexible-sided metal can is used, an aliquot test specimen should be transferred into a 30-mL glass syringe fitted with a three-way stopcock prior to applying the procedure. Any 30-mL glass syringe sample containing a large bubble should be rejected. For the syringes containing a small bubble (that is, ~0.05 mL), the operator should try to dissolve the bubble by compressing the plunger into the barrel and agitating the gas by tipping the syringe up and down. (Note that a hemispherical bubble of 0.05 mL has a diameter of 5.8 mm when in contact with the glass surface of a syringe.) In the event that it is not possible to dissolve a small bubble, the loss of a low solubility species may be estimated as indicated in the following note.
NOTE 15 - Prior to proceeding with 30.4.1, the volume of a small bubble is estimated by measuring the bubble's diameter and converting it into a volume by taking into account the hemispherical distortion that occurs when a bubble is in contact with a glass surface. After the analysis of the dissolved gases, the concentration of a species in a bubble is obtained by using Eq 17, where CG is the concentration of a species in a bubble in parts per million, K is the Ostwald solubility coefficient of the species (K, 25°C, 760 mm Hg listed in 9.5), VG the estimated volume of the bubble in millilitres, V the volume of oil in the syringe in millilitres, and CLo is the concentration of the species in the oil obtained from Section 32 in parts per million. The volume of a species in the bubble in microlitres is obtained by applying the concentration of the species in a bubble over the estimated volume of the bubble. The concentration of a species loss by the oil in parts per million is then obtained by applying the volume of a species in the bubble over the volume of oil contained in the syringe. The resulting value is added to the value obtained in Section 32.
30.4.1 Attach an 18G1 needle to the syringe stopcock.
30.4.2 Insert the needle through the septum at a peripheral location other than for 30.2.2 of a purged vial, which will release the argon overpressure.
30.4.3 Rotate the syringe valve handle a quarter turn and add about 5 mL of oil to the vial. Insert a second 26G 1/2 needle through the septum at a peripheral location other than 30.2.2 and 30.4.2 and fill the vial to about 10 mL. Remove the 26G needle and fill the vial to exactly 15-mL by reading the volume on the syringe barrel. Close the three-way valve by turning the handle back towards the syringe. This will allow the vial to equilibrate to atmospheric pressure through the side port of the valve. Finally, withdraw the syringe with its needle attached.
30.4.4 Place the vials in the headspace sampler and begin the analysis using the instrumental conditions given in Table 4.
30.4.5 Process the results by calculating the concentration levels in accordance with the procedure in Section 32.