ASTM D7059 Standard Test Method for Determination of Methanol in Crude Oils by Multidimensional Gas Chromatography
6. Apparatus
6.1 Chromatograph - A multidimensional two-WCOT column gas chromatographic system, capable of adequately resolving methanol and the 1-propanol internal standard and of eliminating hydrocarbon and other interferences, is required for this analysis. Flow switching between the two specified WCOT columns may be accomplished by either using a valve or pneumatic (pressure) switching to redirect flows. The unwanted higher boiling hydrocarbons may be removed from the pre-column either by forward flush or backward flush. The system requires that carrier gas flow controllers must be capable of precise control for the typical pressures required. Such flow controllers are available on gas chromatographs. The precision of this test method was obtained using several instrument configurations described in 6.1.1-6.1.5. Other multidimensional configurations may be used, provided that they meet all of the requirements of this test method.
6.1.1 Configuration A - Cool-on-column injection (no backflush of pre-column) with two separate selective heartcuts for the methanol and 1-propanol internal standard. The chromatographic instrument can be operated at the approximate conditions given in Table 1 and Fig. 1. Figs. 2-5 give chromatograms and a calibration curve.
6.1.2 Configuration B - Heated split injection with a single heartcut of methanol, 1-propanol and several C9 minus hydrocarbons transferred to the PLOT column for oxygenates using a six-port valve. The pre-column, located in a separate auxiliary oven, is backflushed to a vent using the six-port valve. Table 2 and Fig. 6 give details of the configuration.
6.1.3 Configuration C - Heated split injection with a single heartcut for the methanol, 1-propanol and C9 minus hydrocarbons, followed by backflush of the pre-column through the injection port to remove the high boiling hydrocarbons retained on the pre-column. The chromatographic instrument can be operated at the approximate conditions given in Table 3 and Fig. 7. The pre-column is located in a separate auxiliary oven. Fig. 8 gives a chromatogram obtained with this system.
6.1.4 Configuration D - Direct PTV (no splitting) injection with a single heartcut for the methanol and the 1-propanol internal standard and several low boiling hydrocarbons, followed by backflush of the pre-column through the injector to a vent by pressure switching. Table 4 and Figs. 9 and 10 give flow configurations and operating conditions.
6.1.5 Configuration E - Split injection using pressure switching between the pre-column and the analytical column. After transfer to the analytical column, pressure is reduced at the inlet and the pre-column is backflushed to remove high boiling crude components. Table 5 and Fig. 11 give the flow configurations and operating conditions.
6.2 Detector - At least one, main flame ionization detector (FID) is required. In a two FID detector configuration, the optional second detector is used to monitor the pre-column elution and aids in setting heartcut or transfer times for the methanol and 1-propanol. The main detector's response, proportional to the concentrations ofthe methanol and 1-propanol, is recorded; the peak areas are measured; and the concentration of methanol is calculated with reference to the 1-propanol internal standard and a calibration curve.
6.3 Valve or Pneumatic (pressure) Switching, for systems using valve switching (for example, Configurations A, B in 6.1), a high temperature and low volume valve, located within the gas chromatographic column oven or in a separate oven, capable ofperforming the functions described in 6.1. The valve connections shall not contribute significantly to chromatographic deterioration. Refer to Fig. 2, Fig. 3, and Fig. 8 for compound peak performance, manufacturer's guidelines and examples given in 6.1 as a guide in assembling the GC system. For example, Valco Model No. A 4C10WP, 1.6-mm (1/16 -in.) fittings was used in Configuration A (6.1). Other equivalent valves may be used. Alternatively, pneumatic or pressure switching may be used instead of a valve (for example, Configurations C, D, and E in 6.1).
6.3.1 When using valve switching for heart-cutting, an automatic valve-switching device must be used to ensure repeatable switching times. Such a device must be synchronized with injection and data collection times. For the pressure switching (Configurations C, D, and E) approach, automatic precise and stable pressure control must be used to yield stable retention times.
6.4 Injection System - Several sample injection systems have been used successfully with the configurations in 6.1. Generally, splitless or direct injection types allow detection of lower levels (1 ppm or less) ofmethanol. The splitless or direct injection systems include: (A) a cool-on-column temperature programmable vaporizer, and appropriate autosampler to allow such injections (Configuration A in 6.1); and (B) PTV (Configuration D in 6.1). Alternatively, heated, splitting type injectors may be used with low split ratios (Configurations B, C, and E in 6.1). The splitting type of injection generally is applicable for concentrations greater than or equal to 2 ppm methanol. With splitting type injectors, the replaceable glass deactivated liner (single-taper style with deactivated glass wool at the bottom to retain unvaporized components) was used successfully (for example, as in Configuration C in 6.1).
6.4.1 Automatic microlitre syringe injectors must be used for introducing representative samples into the GC inlet. Cool-on-column automated injectors may need special syringe needles, and so forth. Follow the manufacturer's instructions.
6.5 Data Presentation or Calculation:
6.5.1 Recorder - A data system is used for recording and evaluating the chromatograms.
6.5.2 Integrator or Computer - A computer or electronic integrator is used to obtain peak areas.
6.6 Pre-column WCOT, containing a 5 µm film thickness of cross-linked polymethyldisiloxane, or equivalent, to meet all of the requirements in this test method. This column performs a pre-separation of the methanol and 1-propanol internal standard from hydrocarbons in the same boiling point range. The column shall perform at the same temperature as required for the column in 6.7, except if located in a separate auxiliary oven. The configurations in 6.1 give examples of columns used successfully.
6.7 Analytical Column, a PLOT column for oxygenates, 10 m long by 0.53 mm I.D., with a stationary phase coated onto a fused silica capillary or equivalent. At a minimum, the column shall have sufficient retention for methanol that it elutes after n-tridecane (retention index >1300) at 150 °C, and have sufficient efficiency and capacity to resolve methanol from 1-propanol to provide accurate quantitative results. Ensure that acetone does not interfere with the methanol analysis by analyzing a 1000 ppm mixture of acetone in crude oil containing the 1-propanol internal standard after the system has been optimized.
7. Reagents and Materials
7.1 Carrier Gas - Helium and ultrapure nitrogen have been used successfully. The minimum purity of the carrier gas used must be 99.95 mole %. Oxygen scrubbers are recommended to safeguard the WCOT columns.
7.2 Methanol, >99.9 % pure (Warning - Flammable; vapor harmful).
7.3 1-Propanol, >99.9 % pure, and containing <500 ppm methanol (Warning - Flammable; vapor harmful).
7.4 Toluene, >99.9 % pure, and containing <0.5 ppm methanol (Warning - Flammable; vapor harmful).
7.5 Quality Control Standards, containing known concentrations of methanol in crude oils (Warning - Flammable; vapor harmful).
