ASTM D3120 test method for Trace Quantities of Sulfur in Light Liquid Petroleum Hydrocarbons by Oxidative Microcoulometry
10. Preparation of Apparatus
10.1 Carefully insert the quartz pyrolysis tube into the furnace, and connect the oxygen and inert carrier gas lines.
10.2 Connect the boat drive system or syringe drive system to the pyrolysis tube.
10.3 Perform a leak check according to the manufacturer's instructions.
10.4 Assemble the titration cell, and add all required solutions, including cell electrolyte, according to the manufacturer's instructions.
10.5 Connect the titration cell to the apparatus according to the manufacturer's instructions.
10.6 Adjust the flow of gases and set the furnace temperatures and instrumental parameters according to the manufacturer's instructions.
10.7 If using a boat inlet system, pre-bake the sample boats used for the analysis per the manufacturer's instructions.
11. Calibration and Standardization
11.1 Following the manufacturer's recommended procedures, set the operational parameters of the apparatus for internal calibration. If the apparatus is not so equipped, analyze the solvent blank and calibration standards, record their values as µg or ng of sulfur, and manually generate a calibration curve.
11.2 Calibration Standards - Select the desired calibration range from Table 2. Prepare the appropriate calibration standards for the range selected as outlined in Table 3. The calibration standards concentrations are calculated based on the formula below:
where:
A = µg-S/mL of working or stock solution.
B = mL of working or stock solution.
NOTE 9 - A calibration curve with three standards is recommended. It is recommended to use standards covering the range of sulfur concentrations expected in the samples being analyzed. Calibration curves with one or two standards can be used if it can be shown the calibration curve is linear over the concentration range being measured and the standards meet the performance criteria of this test method. Larger series of standards can also be used if it can be shown the calibration curve is linear over the concentration range being measured and the standards meet the performance criteria of this test method.
NOTE 10 - The use of calibration standards with different concentrations is possible provided they cover the expected concentration range for the samples being analyzed and meet the performance criteria of this test method.
11.3 Select the appropriate syringe based on the recommended sample size in Table 4 for the concentration of standards used to calibrate the apparatus.
11.4 Flush the microlitre syringe several times with the sample prior to analysis. If bubbles are present in the liquid column, flush the syringe and withdraw a new sample.
11.5 The sample size can be determined volumetrically or by mass with a syringe. The sample size should be 80% or less of the syringe's capacity.
11.5.1 Volumetric Measurement - Obtain the volume of injected material by filling the syringe to the selected level, retracting the plunger so the lower meniscus falls on the 10% scale mark, and recording the volume of liquid in the syringe. After the sample has been injected, retract the plunger again so the lower liquid meniscus falls on the 10% scale mark, and record the volume of liquid in the syringe. The difference between the two readings is the volume of sample injected.
NOTE 11 - An automatic sampling and injection device can be used in place of the procedure described for the manual syringe injection.
11.5.2 Mass Measurement - The sample syringe may be weighed before and after the injection to determine the amount of sample injected. This technique provides greater precision than the volume delivery method, provided a balance with a precision of +/-0.01 mg is used.
11.5.3 Once the appropriate sample size has been measured into the microlitre syringe, promptly and quantitatively deliver the sample into the apparatus. Again, there are two alternative techniques available.
11.5.4 Boat Inlet System - Add the sample by slowly discharging quantitatively the contents of the syringe into a sample boat containing quartz wool, being careful to include the last drop from the tip of the syringe needle. Remove the syringe and immediately start the analysis. The apparatus' baseline should remain stable until the boat approaches the furnace and volatilization of sample begins (see Note 12). The program of the inlet system should keep the boat completely in the oxidation zone of the furnace a time (residence time) sufficient to totally oxidize the sample before withdrawing the sample boat out of the furnace (see Note 13). Allow at least 2 min for cooling before the next sample injection after the boat has reached the fully retracted position (see Note 14).
NOTE 12 - It is recommended, if necessary, to reduce the boat speed, or briefly pause the boat, or both, as it is introduced into the oxidation zone of the furnace to ensure complete sample combustion and avoid overloading the combustion capacity of the system.
NOTE 13 - The residence time required to completely oxidize any remaining residue in the boat is dependent on the sample matrix. A 120-s residence time is usually sufficient. Increase the residence time, if necessary, to ensure complete oxidation of the sample.
NOTE 14 - The level of boat cooling required prior to sample injection is directly related to the volatility and size of sample. The use of a boat cooling device, an increase in boat cooling time, or both, can be used to control the vaporization of sample until the boat approaches the furnace.
11.5.4.1 Reduce the boat drive introduction rate, sample size or both if coke or soot is observed on the exit end of the pyrolysis tube. Increase the residence time for the boat in the oxidation zone of the furnace if coke or soot is observed on the boat after the analysis is finished.
11.5.5 Syringe Inlet System - For direct injection, carefully insert the syringe needle into the inlet of the combustion tube and attach the syringe to the driving mechanism. Allow time for the sample residues to be volatilized from the needle (needle blank). Once the baseline has been reestablished, immediately start the analysis. Remove the syringe once the sample has been syringe once the sample has been completely injected and the analysis is finished.
NOTE 15 - To avoid overloading the combustion capacity of the system, injection rates between 0.5-1.0 µL/s are recommended.
11.5.5.1 Reduce the sample size, rate of injection, or both, for the direct injection the test sample into the furnace if coke or soot is observed on the exit end of the pyrolysis tube.
11.6 Calibrate the instrument using one of the following two techniques:
11.6.1 Automatic Internal Calibration - Measure the calibration standards and solvent blank using one of the procedures described in 11.3 - 11.5.5. Analyze the solvent blank and each standard a minimum of three times. Calibrate the apparatus per the manufacturer's instructions. Use either a first or second order linear regression and use this calibration curve to determine the amount of sulfur in the unknown sample. The system's performance shall be checked each day of use. (See Section 12.)
11.6.2 Manual Calibration Curve - Measure the solvent blank and calibration standards using one of the procedures described in 11.3 - 11.5.5. Analyze the solvent blank and each calibration standard a minimum of three times and record their absolute values in either µg or ng sulfur. Subtract the averaged blank value from each calibration standard and construct a curve plotting the mass of sulfur measured for each calibration standard (y-axis) verses the theoretical mass of sulfur injected. Perform a first or second order linear regression and use the calibration curve to determine the amount of sulfur in the unknown sample. The system's performance shall be checked each day of use. (See Section 12.)
NOTE 16 - Not all of the sulfur in the sample exits the oxidation zone of the furnace as titratable SO2. In the strongly oxidative conditions of the pyrolysis tube, some of the sulfur is also converted to SO3, which does not react with the titrant. Accordingly, sulfur standards appropriate to the sample's boiling range and sulfur type is recommended to guarantee adequate calibration. Recoveries less than 75% are to be considered suspect. If recoveries below 75% are observed, it is recommended the operator check the coulometric system, measuring parameters, and operating techniques are in proper order. If the apparatus is being operated properly, recoveries between 75 to 90% are to be expected.
11.7 If the fraction of sulfur converted to SO2(recovery factors) drops below 75% of the standard solutions, prepare fresh standards to check if the standards have been altered. Procedural details should be reviewed if a low recovery factors persist.
11.8 Cleaning and Recalibration - Clean any coked or sooted parts per the manufacturer's instructions. Assemble and leak check the apparatus per the manufacturer's instructions after any cleaning or adjustment. Repeat instrument calibration prior to reanalysis of test samples.
12. Quality Assurance (QA)
12.1 Confirm the performance of the instrument of the test procedure by analyzing a quality control (QC) sample after each calibration and at least each day of use thereafter.
12.2 When QC/QA protocols are already established in the testing facility, these can be used when they confirm the reliability of the test result.
12.3 When there is not QC/QA protocol established in the testing facility, Appendix X1 can be used as the QC/QA system.