ASTM D4629 Test Method for Trace Nitrogen in Liquid Petroleum Hydrocarbons
ASTM D4629 Standard Test Method for Trace Nitrogen in Liquid Petroleum Hydrocarbons by Syringe/Inlet Oxidative Combustion and Chemiluminescence Detection
9. Assembly Apparatus
9.1 Assemble apparatus in accordance with manufacturer's instructions.
9.2 Adjust the gas flows and the pyrolysis temperature as recommended by the instrument manufacturer.
10. Calibration and Standardization
10.1 Prepare a series of calibration standards from the stock solution (see 6.6) covering the range of operation and consisting of nitrogen type and matrix similar to samples to be analyzed. There shall be a minimum of two calibration standards in addition to the solvent blank, used to generate the calibration curve.
10.2 Determine the volume or mass of the material to be analyzed by using one of the volumetric or gravimetric methods described below.
10.2.1 Volumetric measurement of the injected material is obtained by filling the syringe to the 80 % level, retracting the plunger so that the lower liquid meniscus falls on the 10 % scale mark, and recording the volume of liquid in the syringe. After the material has been injected, again retract the plunger so that the lower liquid meniscus falls on the 10 % scale mark and record the volume of liquid in the syringe. The difference between the two volume readings is the volume of material injected.
10.2.2 Alternatively, an automatic sampling and injection device may be used to volumetrically inject a reproducible volume of the material into the furnace.
10.2.3 Gravimetric measurement of the injected material is obtained by weighing the syringe before and after injection to determine the amount of material injected. This procedure provides greater precision than the volumetric procedure, provided a balance with a precision of at least ±0.01 mg is used.
10.3 To introduce the sample into the furnace, insert the syringe needle through the inlet septum up to the syringe barrel and inject the sample or standard at a uniform rate as specified by the instrument manufacturer (typically 0.2 to 1.0 µL/s). Rate of injection is dependent on such factors as viscosity, hydrocarbon type, and nitrogen concentration. Each user must adopt a method whereby a consistent and uniform injection rate is ensured. An automatic sampling and injection device may be used to introduce the material at a reproducible rate. If an automatic sampling and injection device is not being used, determine the quantity of material injected using either 10.2.1 (volumetric injection procedure) or 10.2.3 (gravimetric injection procedure).
NOTE 4 - For the most consistent injection rate and best analytical results, a constant rate injection unit or automatic sampling and injection device may be helpful. Coke formation at the outlet of the combustion tube may indicate too rapid of an injection rate. Consult manufacturer for recommendations.
NOTE 5 - With direct injection below 5 mg/kg of nitrogen, the needle septum blank may become increasingly important. Error due to this can be avoided by inserting the syringe needle into the hot inlet and allowing the needle-septum blank to dissipate before injecting the sample.
10.4 If a boat inlet system is used, then the material to be analyzed is injected into a quartz boat using one of the procedures described in 10.2.1, 10.2.2, or 10.2.3 and the quartz boat is moved into the hot portion of the combustion tube. Refer to the manufacturer's instructions for selecting the rate of boat movement into the furnace and boat residence time in the hot portion of the combustion tube.
10.5 Calibration curves shall be generated in one of the following manners depending on the capability of the instrumentation used.
10.5.1 For systems that use a microprocessor or computer system for data collection and calibration curve generation, the calibration curve shall be based on the linear regression of a minimum of three repeat measurements of each calibration standard.
10.5.2 For those detectors not equipped with a microprocessor or computer system for generating a calibration curve, construct a standard curve as follows. Repeat the determination of each calibration standard and the blank three times to determine the average net response for each. Construct a curve plot of detector response (integration counts) versus nanograms of nitrogen injected and apply the best straight line fit through the plotted data.
10.6 The response curve should be linear with a minimum R(2) of 0.999. The intercept should not be forced through zero. The calibration curve shall be checked each day that the instrument is used (see Section 13).
