31. Determination of Partition Coefficients
31.1 The partition coefficients for the different gases being considered are listed in Table 6 at the analytical temperature of 70°C. They were established in the naphthenic Voltesso 35 oil with 12 % of aromatic carbon content (Test Method D 2140) and a negative gassing tendency (Test Method D 2300, Procedure B, H2 saturating gas). This set of coefficients may be used for measuring fresh or aged oil as well as oil of different compositions.14 However, oil with a different composition may have a slight effect on the value of some of these coefficients, which may lead to a decrease in the accuracy of the dissolved gas analysis. The determination of the K's in the oil being considered and their use for the calculation of the concentrations may ensure maximum accuracy.
31.2 Procedure - The equilibrium headspace-gas chromatography/phase ratio variation test method is used to determine the gas-liquid partition coefficients. This test method, performed with the system assembly described in Section 28, is based on the relationship between the partition coefficient and the phase ratio of the vial (ratio of volumes of headspace and sample phase). It comprises the following steps:
31.2.1 Successively collect at least ten 30-mL glass syringes from equipment where all the gases of interest have been detected in the oil. A 1-L dissolved-gas sample prepared in the oil in accordance with the procedure given in Annex A2 of Test Method D 3612 can also be used. In this case, ten aliquots of 30 mL are collected from the oil vessel using a 30-mL glass syringe. The oil samples should be equilibrated at the ambient laboratory temperature to proceed with the following steps.
31.2.2 Attach an 18G1 needle to the syringe plastic stopcock.
31.2.3 Using twenty-five purged vials, prepare five of each of the following volumes, 6, 8, 10, 12, and 14 mL, with the oil contained in the 30-mL syringes and then calculate the corresponding phase ratio (VG/VL). Introduce the oil samples into the vials using the procedure described in 30.4.2 and 30.4.3; note that for the oil volumes under 10 mL, there is no need to use a second needle for evacuating the overpressure. The precision is achieved on VL by weighing each vial before and after the introduction of the oil volume and by converting the weight into volume by using the density of the oil at the ambient laboratory temperature.
31.2.4 Place the vials in the headspace sampler and begin the analysis using the instrumental conditions given in Table 4.
31.2.5 Obtain the peak area of each gas under the different phase ratios. Plot the data in a graph with the reciprocal of the peak area as the Y-axis and the phase ratio as the X-axis.
31.2.6 From the regression analysis of these data, obtain the slope and intercept. From these values, calculate the partition coefficient of the substance: K = intercept/slope.
NOTE 16 - The highest accuracy on the K value is obtained when the correlation coefficient (R) of the regression line is better than 0.999. A statistical approach for rejecting any outlier can be used to achieve a correlation coefficient better than 0.999.
31.2.7 Use these partition coefficients to calculate the initial concentration of substance in the selected oil following the procedure given in Section 32.
