ASTM D5307 Determination of Boiling Range Distribution of Crude Petroleum by Gas Chromatography
11.1 Baseline Compensation Analysis - To compensate for baseline drift and signal offset, subtract an area slice profile of a blank run from the sample run to obtain corrected area slices. This profile is obtained as follows:
11.1.1 After conditions have been set to meet performance requirements, program the column oven temperature upward to the maximum temperature to be used and hold for at least ten minutes.
11.1.2 Following a rigorously standardized schedule, cool the column to the selected starting temperature, and allow it to equilibrate at this temperature for at least 3 min. At the exact time set by the schedule, without injecting a sample, start the column temperature program.
11.1.3 Acquire the data in area slice mode (see 6.2.2), recording the area slices for each time interval from the start of the run until the end of the run. It is essential that all measurements be on the same time basis for the blank and sample runs.
11.1.4 Perform a blank analysis at least once each day analyses are performed.
NOTE 1 - A completely satisfactory baseline is difficult to obtain when compensation for column bleed is attempted with matched dual columns and detectors. In actual practice, the best compensation can be obtained by directly subtracting the area profile of the blank run derived from a single column.
NOTE 2 - Some commercially available gas chromatographs have the capability to make baseline corrections (from a stored blank analysis) directly on the detector signal. Further correction of area slices may not be required with such systems. However, if an electronic offset is added to the signal after baseline compensation, additional area slice correction may be required in the form of offset subtraction. Consult the specific instrumentation instructions to determine if an offset is applied to the signal.
11.2 Retention Time Versus Boiling Point Calibration:
11.2.1 Using the same conditions as for the blank run, and following the same rigorously standardized schedule (see 11.1), inject an appropriate aliquot of the calibration mixture (see 7.4) into the chromatograph. Record the data in such a manner that retention times and areas for each component are obtained (peak detection mode).
220.127.116.11 The volume of the calibration mixture injected must be selected to avoid distortion of any component peak shapes caused by overloading the sample capacity of the column. Distorted peaks will result in displacement of peak apexes (that is, erroneous retention times) and hence errors in boiling point determination. The column liquid phase loading has a direct bearing on acceptable sample size.
11.2.2 Plot the retention time of each peak versus the corresponding boiling point for that component, as shown in Fig. 2. Boiling points of n-paraffins are listed in Table 2. Tabulate these same data and save for later calculations.
11.2.3 The calibration curve should be essentially a linear plot of boiling point versus retention time. Since it is not practical to operate the column so as to completely eliminate curvature at the lower end of the curve, the calibration mixture must contain at least one n-paraffin with a boiling point equal to or lower than the IBP of the sample. Extrapolation of the curve at the upper end (to 538°C) is more accurate provided extrapolation is not made outside the temperature-programmed portion of the run. However, for best accuracy, calibration points should bracket the boiling range to be measured at both low and high ends. If normal paraffins can be unambiguously identified in the sample, these retention times may be used for calibration.
11.2.4 Perform a boiling point-retention time calibration at least once each day analyses are performed.
11.3 Sample Preparation:
11.3.1 Store very light samples to between 0 and 5°C. Allow the unopened sample to remain within this temperature range for at least 4 h (preferably overnight) before opening.
11.3.2 Shake or stir the sample to ensure homogeneity and pour out a small portion (approximately 100 mL) for subsequent weighing and analysis.
11.3.3 Heavy, viscous crude may require warming as well as stirring to ensure homogeneity.
11.3.4 Since water is not measured by the FID, a portion of the sample must be dried before the sample can be weighed. Add 2 to 3 g of drying agent, such as anhydrous calcium chloride, to a 50-mL vial and fill the vial about half full with sample. Cap the vial tightly and shake the vial vigorously. Allow the mixture to stand several minutes to allow the drying agent to settle out. By means of a disposable pipette, remove the dried oil layer for sample weighing and analysis.
11.3.5 Weigh at least 10 g of dried sample to the nearest 0.1 mg into a 25-mL vial.
11.3.6 Add approximately 1 g of internal standard mixture into the same vial. Determine the weight to the nearest 0.1 mg.
11.3.7 Dilute the mixture with an approximately equal volume of carbon disulfide.
11.3.8 Cap the vial tightly and shake the mixture vigorously for 3 min, or until the mixture is solubilized completely. Use this solution for the crude oil plus internal standard analysis (see 11.4.1).
11.3.9 In a second vial, dissolve approximately the same amount of dried sample as 11.3.5 with an approximately equal volume of carbon disulfide. Use this solution for the separate crude oil without internal standard analysis (see 11.4.4).
11.4 Sample Analysis:
11.4.1 Using the exact conditions that were used in the blank and calibration runs (see 11.1 and 11.2), and following the rigorously defined schedule (see 11.1), inject 1 µL of the diluted crude oil plus internal standard mixture into the chromatograph. Record the area slices of each time interval through the end of the run.
11.4.2 Continue the run until the retention time equivalent to a boiling point of 538°C (1000°F) is reached. Stop recording area slices under the chromatogram at this point.
11.4.3 To remove as much as possible of the heavy components remaining on the column, continue heating the column until the FID signal returns to baseline. The column temperature may be increased to speed this process.
11.4.4 Cool the column to the starting temperature. Use identical conditions as used in 11.4.1. Inject 1 µL of the crude oil sample without internal standard (see 11.3.9). Record the area slices of each time interval through the end of the run.
11.4.5 The sample plus internal standard analysis (see 11.4.1) and the sample only analysis (see 11.4.4) may be made in either order.
12.1 Area Corrections:
12.1.1 Obtain corrected area slices for both runs (see 11.4.1 and 11.4.4) by subtracting the corresponding area slice of the blank run profile (see 11.1) from each. (See Note 2.)
12.1.2 Sum the corrected area slices for both runs to obtain the cumulative corrected area at the end of each time interval during the run.
12.2 Theoretical Total Area (Refer to Fig. 3):
12.2.1 Based on retention times from the calibration chromatogram (see 11.2.1), select a retention time that is 5 % less than the retention time of n-C14, and another that is 5 % greater than the retention time of n-C17. These times define a segment of the chromatogram that includes the internal standard peaks. Record the total area within this segment from the chromatogram of the crude oil plus internal standard mixture (see 11.4.1) (area AIS from Fig. 3(a)). Also record the total area of the same segment from the chromatogram obtained from the crude oil only chromatogram (see 11.4.4) (area BIS from Fig.3(b)).
12.2.2 Record the total area of both chromatograms through the retention time equivalent to a boiling point of 538°C (1000°F).
12.2.3 Calculate the mass fraction (W) of the internal standard in the mixture of sample plus internal standard as follows:
W = I/(S + I)
I = mass of internal standard, g, and
S = mass of sample, g.
12.2.4 Calculate the ratio of areas (r) outside the internal standard segment and through the retention time equivalent to a boiling point of 538°C (1000°F) of the crude oil only chromatogram to the chromatogram from the mixture of internal standard plus crude as follows:
r = (B - BIS)/(A - AIS)
A = total area through 538°C (1000°F) of the crude plus internal standard mixture chromatogram,
AIS = total area of the internal standard segment of the crude plus internal standard mixture chromatogram,
B = total area through 538°C (1000°F) of the crude oil only chromatogram, and
BIS = total area of the internal standard segment of the crude oil only chromatogram.
12.2.5 Calculate the theoretical total area (T) for the crude oil only chromatogram (Area B + B', Fig. 3(b)) as follows:
T = [(AIS x r) - BIS][(1 - W)/W]
AIS = total area of the internal standard segment of the chromatogram of the sample plus internal standard mixture,
BIS = total area of the internal standard segment of the crude oil only chromatogram,
r = the ratio of areas outside the internal standard segment through 538°C (1000°F) for both chromatograms (see Eq 4), and
W = the mass fraction of internal standard in the mixture of crude sample plus internal standard (see Eq 3).
12.2.6 Calculate the percent residue (RES) above 538°C (1000°F) as follows:
RES = 100 - (B/T x 100)
B = total area through 538°C (1000°F) of the crude only chromatogram, and
T = theoretical total area of the crude oil only chromatogram (see Eq 5).
12.3 Calculation of Boiling Point Distribution:
12.3.1 Record the time at which the cumulative area at the beginning of the crude only chromatogram is equal to 0.5 % of the theoretical total area (T, From Eq 5). The temperature equivalent to this time is the IBP of the sample.
12.3.2 Multiply the corrected cumulative area at the end of each time interval by 100 and divide by the theoretical total area (T from Eq 5). This gives the percent of sample recovered at the end of each time interval.
12.3.3 Tabulate, in pairs, the cumulative percent recovered and the retention time at the end of each time interval.
12.3.4 Using linear interpolation where necessary, determine the time associated with each percent between 1 % and the percent eluted at the time equivalent to 538°C (1000°F).
12.3.5 For each 1 % and its associated retention time, determine the corresponding temperature from the table of boiling point-retention time calibration data (see 11.2.2).