ASTM D2549 Separation of Representative Aromatics and Nonaromatics Fractions
ASTM D2549 Standard Test Method for Separation of Representative Aromatics and Nonaromatics Fractions of High-Boiling Oils by Elution Chromatography
8. Procedure
NOTE 3 - The procedural details differ depending on the initial boiling point of the sample. If the 5 % point is above 178°C (350°F), but below 232°C (450°F), use procedure described in Appendix X1. If above 232°C, continue as written depending on amount of sample to be analyzed. Instructions specific for 2-g samples are given in 8.4.1-8.4.13, and instructions specific for 10-g samples are given in 8.5.1 and 8.5.8.

8.1 Select the appropriate column, depending on whether 2 or 10 g of sample are to be analyzed. Clean the column with chromic-sulfuric acid (Warning - Causes severe burns), followed by distilled or demineralized water, acetone, and dry air or nitrogen.

8.2 Introduce a small plug of glass wool into the column, pressing it firmly into the lower end to prevent the flow of silica gel from the column.

8.3 Clamp the column in a vertical position. Add small increments of silica gel, while vibrating the column along its length, until the tightly packed silica gel extends to the lower mark on the chromatographic column. Continue to vibrate the column and add bauxite until the bauxite layer extends to the upper mark on the chromatographic column. Vibrate the column for an additional 3 min after filling is completed.

8.4 If 2 g of sample are to be analyzed, continue as in 8.4.1, otherwise continue as in 8.5.
8.4.1 If the sample is viscous, warm it with intermittent mixing or shaking until it is completely fluid. Transfer a representative sample (approximately 2 g) to a 25-mL weighing bottle or flask. Determine the weight of the sample to the nearest 1 mg by weighing the flask before and after sample transfer. Add 10 mL of n-pentane (Warning - Extremely flammable liquid), to the flask and dissolve the sample. If the sample does not dissolve completely in cold n-pentane, warm it in warm water or over a steam bath. If the sample does not dissolve in warm n-pentane, take a fresh sample and substitute cyclohexane for the n-pentane.

8.4.2 Add 10 mL of n-pentane to the top of the column to prewet the adsorbent. When the liquid level reaches the top of the bauxite bed, transfer the sample solution from the weighing flask to the top of the column. Rinse the flask with three successive 3-mL washes of n-pentane. Add each wash to the top of the column. Then rinse the walls of the column bulb with two 3-mL portions of n-pentane, allowing the liquid level to reach the top of the bauxite bed before adding the next portion. Finally add 35 mL of n-pentane to the column bulb.

8.4.3 Place a 50-mL graduate beneath the column to collect the eluate. The elution rate should be approximately 1 mL/min.

NOTE 4 - Gas pressure (Warning - Compressed gas) can be applied to the top of the column as necessary to maintain the elution rate at approximately 1 mL/min. If the correct pressure setting is known from previous runs, gas pressure may be applied after addition of the last increment of n-pentane. Otherwise, gas pressure should be applied when n-pentane begins to elute from the column and should be adjusted to give a flow rate of approximately 1 mL/min.

8.4.4 When the n-pentane level reaches the top of the bauxite bed, add 80 mL of diethyl ether (Warning - Extremely flammable). Connect the pressuring gas to the top of the column and adjust the pressure to maintain an elution rate of 1 to 2 mL/min.

8.4.5 Collect 50 mL of n-pentane eluate in the graduate. Rinse the tip of the column with 1 to 2 mL of n-pentane, adding this to the 50 mL in the graduate (Note 5). Label the 50-mL graduate as n-pentane eluate.

NOTE 5 - The n-pentane will have reached the adsorbent bed before the required volume of eluate has been collected in the 50-mL receiver. Continue collection in this receiver after the addition of ether until the proper volume has been collected before changing to the 100-mL graduate.

8.4.6 When the ether level reaches the top of the bauxite bed, release the gas pressure and add 100 mL of chloroform (Warning - Toxic. May be fatal if swallowed) to the top of the column. Reconnect the gas pressuring system and continue the elution. When 80 mL of eluate have been collected in the graduate, rinse the column tip with 1 mL of ether and add the rinse to the 100-mL graduate. Change the receiver to a 250-mL graduate. Label the 100-mL graduate as ether-eluted fraction.

8.4.7 When the chloroform level reaches the top of the bauxite bed, release the gas pressure and add 75 mL of ethyl alcohol (Warning - Flammable liquid). Reconnect the gas pressuring system and continue the elution until the alcohol level reaches the top of the bauxite bed. Release the gas pressure. Rinse the column tip with 1 mL of chloroform adding this to the graduate. Label the 250-mL graduate as chloroform-alcohol-eluted fraction.

8.4.8 Weigh a 100-mL inverted-rim beaker to the nearest 1 mg. Quantitatively transfer the n-pentane eluate to this beaker and allow the n-pentane to evaporate at room temperature. Cyclohexane, if used as the elution solvent, is evaporated on a steam bath. Evaporation is accelerated in both cases by directing a controlled stream of dry nitrogen downward onto the surface of the liquid.

8.4.9 When all the solvent appears to be evaporated, stop the nitrogen flow, allow the beaker to come to room temperature, and dry the outside of the beaker to remove any condensed moisture. Reweigh the beaker to the nearest 1 mg.

NOTE 6 - Complete solvent evaporation is indicated by a tendency of the oil to creep up the side of the beaker.

8.4.10 Repeat the evaporation step for 5-min periods until the weight loss between successive evaporations is less than 20 mg. Heat from a steam bath is generally required during the final evaporation steps to remove completely the elution solvent. The weight of the residue in the beaker is the quantity of the nonaromatics fraction.

8.4.11 Weigh a 250-mL inverted-rim beaker to the nearest 1 mg. Quantitatively transfer the chloroform-alcohol-eluted fraction to this beaker and evaporate on a steam bath with a controlled stream of dry nitrogen directed downward onto the surface of the liquid. When the solvent is evaporated, remove the beaker from the steam bath, cool to room temperature, and add quantitatively the ether-eluted fraction. Evaporate the ether at room temperature as described in 8.4.8-8.4.10. Determine the weight of the residue (aromatics fraction) to the nearest 1 mg.

8.4.12 The weight of the aromatics plus the nonaromatics fraction recovered must equal at least 95 % of the sample charged. If 95 % recovery is not obtained, repeat the test. Recoveries greater than 100 % indicates incomplete removal of solvent or the condensation of moisture in the beakers.

8.4.13 Transfer the aromatics and nonaromatics fractions into suitable size vials for storage pending further analysis.

8.5 If 10 g of sample are to be analyzed, continue as in 8.5.1.
8.5.1 Warm the sample with intermittent mixing or shaking until it is completely fluid. Transfer a representative sample (approximately 8 to 10 g) to a 50-mL weighing bottle or flask. Determine the weight of the sample to the nearest 1 mg by weighing the flask before and after sample transfer. Add 20 mL of n-pentane to the flask and dissolve the sample. If the sample does not dissolve completely in cold n-pentane, warm it in warm water or over a steam bath. If the sample does not dissolve in warm n-pentane, take a fresh sample and substitute cyclohexane for n-pentane.

8.5.2 Add 45 mL of n-pentane to the top of the prepacked large column to prewet the adsorbent. When the n-pentane level reaches the top of the bauxite bed, transfer the sample solution from the weighing flask to the top of the column. Rinse the flask with three successive 3-mL washes of n-pentane. Add each wash to the top of the column. Then rinse the walls of the column bulb with two 3-mL portions of n-pentane, allowing the level of each portion to reach the top of the bauxite bed before adding the next portion. Finally add 70 mL of n-pentane to the column bulb.

8.5.3 Place a 200-mL graduate beneath the column to collect the eluate. The elution rate should be approximately 3 mL/min.

NOTE 7 - Air or nitrogen pressure may be applied to the top of the column as necessary to accomplish and maintain a satisfactory elution rate. Three to five pounds of pressure generally is sufficient. If the correct pressure setting is known from previous runs, gas pressure can be applied after addition of the last increment of n-pentane. Otherwise, gas pressure should be applied when n-pentane begins to elute from the column and should be adjusted to give a flow rate of approximately 3 mL/min.

8.5.4 When the n-pentane level reaches the top of the bauxite bed, add 100 mL of diethyl ether. Connect the pressuring gas to the top of the column and adjust the pressure to maintain an elution rate of 3 to 5 mL/min.

8.5.5 Collect 130 mL of eluate in the graduate. Rinse the tip of the column with 1 to 2 mL of n-pentane, adding this to the 130 mL in the graduate. Change the receiver to a 100-mL graduate (Note 8). Label the 200-mL graduate as n-pentane eluate.

NOTE 8 - The n-pentane will have reached the absorbent bed before the required volume of eluate has been collected in the 200-mL receiver. Continue collection in this receiver after the addition of ether until the proper volume has been collected before changing to the 100-mL graduate.

8.5.6 When the ether level reaches the top of the bauxite bed, release the gas pressure and add 100 mL of chloroform to the top of the column. Reconnect the gas pressuring system and continue with the elution. When 100 mL of eluate have been collected in the graduate, rinse the column tip with 1 mL of ether and then change the receiver to a 500-mL graduate. Label the 100-mL graduate as ether-eluted fraction.

8.5.7 When the chloroform level reaches the top of the bauxite bed, release the gas pressure and add 175 mL of ethyl alcohol. Reconnect the gas pressuring systems and continue the elution until the alcohol level reaches the top of the bauxite bed. Release the gas pressure. Rinse the column tip with 1 mL of chloroform adding this to the graduate. Label the 500-mL graduate as chloroform-alcohol-eluted fraction.

8.5.8 Weigh a 250-mL inverted rim beaker to the nearest 1 mg. Quantitatively transfer the n-pentane eluate to this beaker and evaporate the solvent on a steam bath. Evaporation can be accelerated by directing a controlled stream of dry nitrogen downward onto the surface of the liquid. Complete the workup of the nonaromatics fraction as described in 8.4.9 and 8.4.10.

8.5.9 Weigh a 600-mL inverted rim beaker to the nearest 1 mg (Note 9). Complete the workup of the aromatics fraction as described in 8.4.11-8.4.13.

NOTE 9 - The 600-mL inverted-rim beakers from some sources can exceed the capacity of the standard analytical balance, in which case a 250-mL inverted rim beaker can be used, and the chloroform-alcohol-eluted fraction evaporated in increments.

9. Calculation
9.1 Calculate the percentage of the aromatics fraction and the nonaromatics fraction as follows:
Aromatics fraction, wt % = [A/(A + B)] x 100
Nonaromatics fraction, wt % = [B/(A + B)] x 100
where:
A = weight of aromatics fraction recovered, and
B = weight of nonaromatics fraction recovered.