ASTM D5599 Determination of Oxygenates in Gasoline
ASTM D5599 Standard Test Method for Determination of Oxygenates in Gasoline by Gas Chromatography and Oxygen Selective Flame Ionization Detection
9. Preparation of Apparatus
9.1 Chromatograph and OFID - Place instrument and detector into operation in accordance with the manufacturer's instructions. Install the capillary column according to Practice E 1510. Adjust the operating conditions to provide for separation of all oxygenates of interest. Typical conditions used with the column specified in 7.3 are listed in Table 1.
9.2 System Performance - At the beginning of each day of operation, inject an oxygenate-free gasoline sample into the chromatograph to ensure minimum hydrocarbon response. If hydrocarbon response is detected, the OFID is not operating effectively and must be optimized according to the manufacturer's instructions before the sample can be analyzed.
10. Calibration and Standardization
10.1 Retention Time Identification - Determine the retention time of each oxygenate component by injecting small amounts either separately or in known mixtures. Table 2 gives typical retention times for the oxygenates eluting from a 60-m methyl silicone column temperature programmed according to conditions given in Table 1. A chromatogram of a blend of oxygenates is given in Fig. 2.
10.2 Preparation of Calibration Samples - The calibration samples are prepared gravimetrically in accordance with Practice D4307 by blending known weights of organic oxygenate compounds (such as listed in 8.2) with a known weight internal standard and diluting to a known weight with an oxygenate-free gasoline. The calibration samples should contain the same oxygenates (in similar concentrations) as are expected in the sample under test. Before preparing the standards, determine the purity of the oxygenate stocks and make corrections for the impurities found. Whenever possible, use stocks of at least 99.9 % purity. Correct for the purity of the components for water content determined by Test Method D1744 or Test Method E 1064. Quality control check standards may be prepared from the same oxygenate stocks and by the same analyst. Quality control check standards must be prepared from separate batches of the final diluted standards.
10.2.1 Tare a glass sample container and its PTFE-faced rubber septum sealing cap. Transfer a quantity of an oxygenate to the sample container and record the mass of the oxygenate to the nearest 0.1 mg. Repeat this process for any additional oxygenates of interest except the internal standard. Add oxygenate-free gasoline to dilute the oxygenates to the desired concentration. Record the mass of gasoline added to the nearest 0.1 mg, and determine and label the standard according to the mass % quantities of each oxygenate added. These standards are not to exceed 20 mass % for any individual pure component due to potential hydrocarbon breakthrough or loss, or both, of calibration linearity. To minimize evaporation of light components, chill all chemicals and gasoline used to make standards.
10.2.2 Tare the glass sample container, a PTFE-faced rubber septum sealing cap, and contents prepared in 10.2.1. Add a quantity of an internal standard (such as 1,2-–dimethoxyethane) and record its mass to the nearest 0.1 mg. The mass of the internal standard should be between 2 and 6 % of the mass of the calibration sample.
10.2.3 Ensure that the prepared standard is thoroughly mixed, and transfer approximately 2 mL of the solution to a vial compatible with the autosampler if such equipment is used.
10.2.4 At least five concentrations of each of the expected oxygenates should be prepared. The standards should be as equally spaced as possible within the range and may contain more than one oxygenate. A blank for zero concentration assessments must assessment shall also be included. Additional standards should be prepared for other oxygenates of concern.
NOTE 3 - If carryover is suspected to possibly occur, the blank should be run following a calibration sample containing high levels of oxygenates.
10.3 Standardization - Run the calibration samples and establish a calibration curve by performing a least-squares fit of the response ratios of the oxygenate standards to their amount ratios, as follows.
10.3.1 Calculate the response ratio ( rsps):
rsps = (As/Ai)
where:
As= peak area of the test oxygen compound in the calibration sample, and
Ai= peak area of the internal standard in the calibration sample.,
and the amount ratio (amts):
amts = (Ws/Wi)
where:
Ws = mass of the test oxygen compound in the calibration sample, g, and
Wi = mass of the internal standard in the calibration sample, g,
for each level of each oxygenate, s.
10.3.2 For each oxygenate, s, calibration data set, obtain the quadratic least-squares fit equation in the following form (forced through the origin):
rsps = (bo)(amts) + bl(amts)2
where:
rsps = response ratio for oxygenate s (y-axis),
bo = linear regression coefficient for oxygenate s,
amts = amount ratio for oxygenate s (x-axis), and
bl = quadratic regression coefficient.
10.3.3 Fig. 3 gives an example of a quadratic least-squares fit for MTBE and the resulting equation in the form of Eq5. Check the relative response factors on a mass % oxygen basis correlation r2 value for each value should be at least 0.99 or better.
