METHANOL IN CRUDE OILS BY MULTI-DIMENSIONAL GAS CHROMATOGRAPHY: ASTM D7059
EXPLANATION
Methanol is used in production of crude oils to prevent formation of gas hydrates. The presence of residual methanol in crude oils can lead to costly problems in refinery operations. This method uses direct injection multidimensional gas chromatography in the concentration range of 15 - 900 ppm (m/m). The pooled limit of quantitation is 15 ppm (m/m).This test method is applicable only to crude oils containing less than or equal -0.1 % (v/v) water. This test method has not been tested with crude oils samples that are solid or waxy, or both, at ambient temperatures.
TEST SUMMARY
An internal standard, 1-propanol, is added to the sample which is then introduced into a gas chromatograph equipped with two columns and a flow switching system between the two columns. The sample first passes through the polydimethylsiloxane WCOT column that performs a preseparation of the methanol and 1-propanol and eliminates unwanted hydrocarbons. The methanol and 1-propanol are transferred to the analytical CP-Lowox column. While the methanol and 1-propanol are eluting from the CP-Lowox analytical column, auxiliary carrier gas is used to elute higher boiling crude oil hydrocarbons from the pre-column, either in the forward or backflush mode, to yield a stable baseline for the next analysis.
TEST PRECISION
Repeatability: 0.063(X + 60)
Reproducibility: 0.107(X + 60)
Where X is the mean value of replicates. The applicable range for above precision is 15 to 900 ppm (m/m).
The bias of this method cannot be determined because no material having an accepted reference value is currently available.
