ASTM D7059 Standard Test Method for Determination of Methanol in Crude Oils by Multidimensional Gas Chromatography
8. Sampling
8.1 Ensure that the sample is representative of the crude oil from which it is taken. The use of multiple samples which are mixed or composite sampling is recommended when sampling from a large source of the crude oil. Follow the recommendations of Practice D4057, or its equivalent, when obtaining samples from bulk storage or pipelines. The use of epoxy-lined cans is recommended for storage or shipping, or both, of the sample.
8.1.1 Methanol content by this test method must be determined on a sample containing less than or equal to 0.1 % (v/v) water. All of the crude oil samples used in the cooperative study to develop the precision of this test method contained less than or equal to 0.1 % (v/v) water. Determine the water content using Test Method D4928, Test Method D4006, or equivalent.

NOTE 1 - The methanol content in crude oils is the combined methanol content in the crude oil and, if present, in the associated water. It is known that some crude oils are associated with high levels of water and in such cases, the water phase may extract a significant amount of the methanol and obscure the true total methanol content in the total sample determined by this test method.

8.2 Prior to analysis, allow the sample container as received to come to ambient temperature. Mix well the entire sample in the sealed sample container. Waxy or other solid crudes, or both, were not evaluated by this test method. Once mixed in the original sample container, immediately transfer the required aliquot for weighing to avoid potential segregation of the methanol from the crude oil. A poorly mixed sample may lead to erroneous results. Heating samples to attain homogeneity may result in the loss of methanol during sample transfers.

NOTE 2 - Waxy and solid samples were not evaluated by this test method. For such samples, it may be necessary to heat the sample to a temperature of 20 °C above the expected pour point in the original container. The sample is shaken vigorously to mix the sample. To avoid losses of the methanol, an aliquot for analysis may be transferred under slight pressure to a sealed vial by connecting a transfer tube from the sample container to the vial. The sealed vial is allowed to cool prior to the addition of the toluene solvent as described in Section 12.

9. Preparation of Apparatus and Establishment of Conditions
9.1 Assembly - Configure the GC system in a multidimensional configuration as described in 6.1. Configurations that were used successfully are described in 6.1 and may be used as a guideline. Connect the WCOT columns to the chromatographic system, including the multidimensional switching device, using low volume connectors and inert narrow bore tubing. It is important to minimize the volume of the chromatographic system that comes in contact with the sample; otherwise, peak broadening will occur.

9.2 This section gives details on how to set up two of the configurations described in 6.1. For other configurations, adjust cut times appropriately.
9.2.1 Configuration A - Adjust the operating conditions. Table 1 gives example conditions. Modifications to column lengths and so forth may require different operating conditions. Check the system for leaks before proceeding further.
9.2.1.1 Setting Cut Times - With the pre-column connected to the monitor FID, the transfer valve OFF, and the temperature and flow conditions as indicated in the method, inject the 1000 ppm calibration solution, and record the chromatogram. Identify the peak for methanol and the peak for 1-propanol. From this retention time data, set the methanol transfer valve time ON to 0.5 min before the methanol starts eluting, and the methanol transfer valve time OFF to 0.5 min after the methanol peak returns to baseline. Set the 1-propanol transfer valve time ON to 0.5 min before the 1-propanol starts eluting, and the 1-propanol transfer valve time OFF to 0.5 min after the 1-propanol peak returns to baseline. The transfer valve should turn on and off twice during the analysis. The times should be incorporated into the analysis method before calibration is begun. After setting the initial heart-cut times, reevaluate them, using the automated operating conditions that will be used for calibration and sample analysis, to ensure that no retention time shifts have occurred due to pressure or flow imbalances when the two columns are connected in series. If necessary, readjust the flows or pressures, and repeat the reevaluation.

9.2.1.2 To safeguard the PLOT column for oxygenates, avoid injecting crude oil samples until the valve times are properly optimized using calibration standards.

9.2.1.3 It is recommended that when all of the analyses are completed, the GC oven temperature be maintained at 250 °C and the pre-column carrier head pressure be maintained at 60 psi using the electronic pressure controller for at least several hours. This procedure conditions the PLOT column for oxygenates, which may trap carrier gas contaminants at the normal 40 °C starting temperature, and also elutes residual heavy hydrocarbons from the pre-column. Periodically, 25 cm can be cut off the front of the polydimethylsiloxane column to remove heavy nonvolatiles. The frequency can be determined from the analysis of quality control samples, by evaluating the SQC results and also from chromatographic performance, such as peak tailing and so forth. After cutting the polydimethylsiloxane column, reevaluate the heart-cut times and adjust, as necessary, to meet the specification in 9.2.1.1.

9.2.2 Configuration C - Adjust the operating conditions to those listed in Table 3, or equivalent. Check the system for leaks before proceeding further.
9.2.2.1 Setting the Backflush Time - It is essential that the appropriate backflush time be determined to prevent heavy contaminants from reaching the analytical column and potentially interfering with the determination of methanol. This test method restricts the carbon number range allowed to pass through to the analytical column to ~C10 -minus. The procedure used is as follows:
(1) Prepare a timing standard containing approximately 1000 ppm (m/m) of normal paraffins C9, C10, and C11 in toluene.
(2) Inject the timing standard into the GC in the foreflush position with no backflush.
(3) Analyze the resulting chromatogram and determine the position of each of the normal paraffin peaks.
(4) Starting at an arbitrary backflush time (30 s is a good starting point) reinject the sample and analyze the chromatogram for the presence of the normal paraffin peaks.
(5) If a significant amount of C10 and C11 are visible, reduce the backflush time in 2-s increments and repeat 9.2.2.1(4) until little or no C10 is visible. There should not be any significant decrease in the C9 peak area. Record this time as the backflush time. Conversely, if none of the peaks are visible, increase the backflush time in 2-s increments and repeat 9.2.2.1(4) until C10 is small or not visible.

NOTE 3 - Backflush time is dependent upon specific instrument operating conditions, such as flow rate and temperature.

9.2.3 Replacement of the inlet septum should be performed at least every 30 to 50 injections to reduce the possibility of leaks. Liner replacement should be performed regularly if system performance degrades (see Section 11). When using a split injector, contamination of the split vent line may occur with high boiling hydrocarbons. The injection port and split vent line should be inspected and cleaned as necessary after 150 to 200 injections or when performance deteriorates when quality control samples are analyzed.

9.3 Verification of Detectability - Inject a 2 ppm (m/m) solution of methanol in toluene and ensure that a signal/noise ratio of at least five is observed.

10. Calibration and Standardization
10.1 Identification - After the system's cut times are established, determine the retention time of the methanol and ofthe 1-propanol internal standard using a calibration solution.
10.1.1 To ensure minimum interference from hydrocarbons, it is strongly recommended that a crude oil devoid of methanol be chromatographed to determine the level ofany hydrocarbon interference.

10.2 Preparation of Calibration Standards - Prepare multiple calibration standards of methanol in the concentration range of interest, by mass, in accordance with Practice D4307.
10.2.1 Prepare a minimum of seven calibration standards spanning the range of approximately 5 ppm (m/m) to 1000 ppm (m/m) of methanol, and each containing 500 ppm (m/m) of the 1-propanol internal standard, in toluene. Standard concentrations should bracket the expected range of methanol concentrations. The standards may be prepared from a more concentrated stock solution of methanol in toluene (for example, 5000 µg/g, prepared by adding 0.5 g of methanol to 99.5 g of toluene). The 1-propanol internal standard stock solution may be prepared by adding 0.5 g of1-propanol to 99.5 g of toluene. Larger batches of standards may also be prepared proportionally. The calibration standard solutions should be stored in tightly sealed bottles in a dark place below 5 °C. Table 6 gives an example of the masses for preparation of working standards derived from the stock solution for a set of calibration standards 5, 25, 75, 125, 250, 500, and 1000 ppm (m/m). Wi and Wis in Table 6 represent the actual micrograms in each of the final calibration standards. Recalculate Wi and Wis to reflect the actual weights used:
Wi = actual µg in calibration standard of methanol = concentration of methanol stock solution (µg/g) x actual mass of methanol stock solution added to calibration standard (g)

Wis = actual µg in calibration standard of 1 - propanol internal standard = concentration of 1 - propanol stock solution (µg/g) x actual mass of 1 - propanol stock solution added to calibration standard (g)

10.2.1.1 Before preparing the standards, determine the purity of the methanol and make corrections for the impurities found. Ensure that the toluene and 1-propanol internal standard meet the methanol specification in 7.3 and 7.4.

NOTE 4 - The toluene and 1-propanol internal standard meet the methanol impurity specification ifa solution of 500 ppm ofthe 1-proponol internal standard prepared in toluene is analyzed, and the area of the methanol detected, if any, corresponds to less than the area observed for a 2 ppm methanol calibration standard solution containing an equivalent amount (500 ppm) of 1-propanol (see 10.2.1).

10.3 Standardization:
10.3.1 Analyze the calibration standards and establish the calibration curve for methanol. Plot the response ratio, rspi as the y-axis versus the amount ratio amti, as the x-axis to generate the calibration curve (see Fig. 5 for an example plot):
rspi = (Ai/Ais)
where:
Ai = area of methanol, and
Ais = area of 1-propanol internal standard.

amti = Wi/Wis
where:
Wi = mass of methanol in the calibration standard, and
Wis = mass of 1-propanol internal standard in the calibration standard.

10.3.2 Linear Least Squares Fit - For the calibration data set, obtain the linear least squares fit equation in the form:
rspi = (mi)(amti) + bi
where:
rspi = response ratio for methanol/1-propanol internal standard (y-axis),
mi = slope of linear equation for calibration curve,
amti = amount ratio for methanol/1-propanol internal standard (x-axis), and
bi = y-axis intercept.

10.3.3 Calculate the correlation r2 value for the calibration curve. The value r2 should be at least 0.99 or better. The correlation r2 may be calculated directly by the data system or can be obtained by using plotting software.

10.4 To test the value of the y-intercept, prepare a sample using only toluene instead of the crude oil sample as described in Section 12. The calculated result from this analysis must yield a methanol content of no greater than +/- 3 ppm methanol.

NOTE 5 - A single calibration curve approach has worked well in the inter-laboratory cooperative study. However, ifthe specifications in 10.3.3 and 10.4 cannot be attained using the described single curve calibration procedure, then it may be necessary to develop two calibration curves by using at least five calibration standards for each curve. For example, one curve may be used for concentrations <100 ppm and another for concentrations >100 ppm to attain the specifications in 10.3.3 and 10.4. In addition, if this dual calibration curve approach is used, then the requirements in Section 11 must also be attained for the check standard(s) corresponding to the respective calibration curve ranges. At least one participant in the cooperative study successfully reported results using the dual calibration approach.

11. Quality Control Standards
11.1 After the calibration has been completed, analyze two quality control check standards ofmethanol in crude oil(s). The check standards may be prepared in-house, obtained from crosscheck programs, or prepared by a commercial vendor in sealed ampoules, or a combination thereof. If sealed ampoules of commercial check standards are used, then the ampulization of the vials must include continuous stirring of the crude oil mixture containing the methanol during the vial filling process and follow Practice D6596. In cases where such commercial check standards are not available, prepare the check standards in-house using crude oils as described in 11.3. In-house and commercially prepared check standards using the above procedure were successfully prepared and tested using this test method.

11.2 The concentrations of methanol for the two check standards must be in the lower concentration range in one, for example, 20 ppm to 30 ppm, and in the upper concentration range in the other, for example, 600 ppm to 700 ppm. Analyze the quality control check samples as described in the sample preparation procedure (Section 12). The methanol concentration value obtained must agree within +/- 25 % (for example, methanol 20 ppm +/- 5 ppm) for the 20 ppm to 30 ppm sample and +/- 7 % (for example, methanol 650 ppm +/- 46 ppm) relative for the 600 ppm to 700 ppm sample. Ifthe individual values are outside the specified range, verify calibration and instrument parameters, accuracy of the preparation of quality control reference material, sample handling, and so forth. Do not analyze samples without meeting the check standard quality control specifications.
11.2.1 The upper level quality control check sample may be prepared by weighing approximately 70 mg of methanol into a 100 mL volumetric flask and diluting to the mark with a light crude oil containing <2 ppm methanol. The mass of the crude oil added is obtained and recorded. After shaking well, more dilute solutions may be prepared by gravimetrically diluting aliquots of the stock with crude oil containing <2 ppm methanol.

11.3 Bracket the samples with the quality control check standards. Analyze the check standards at least after every 5 samples analyzed. If the check samples do not meet the specifications in 11.2, results obtained for the samples analyzed immediately preceding the check samples are considered suspect and the samples must be rerun.