ASTM D6591 Standard Test Method for Determination of Aromatic Hydrocarbon Types in Middle Distillates - High Performance Liquid Chromatography Method with Refractive Index Detection
6. Apparatus
6.1 High Performance Liquid Chromatograph (HPLC) - Any HPLC capable of pumping the mobile phase at flow rates between 0.5 and 1.5 mL/min, with a precision better than 0.5 % and a pulsation of <1 % full scale deflection under the test conditions described in Section 9.

6.2 Sample Injection System, capable of injecting 10 µL (nominal) of sample solution with a repeatability better than 1 %.
6.2.1 An equal and constant volume of the calibration and sample solutions shall be injected into the chromatograph. Both manual and automatic sample injection systems (using either complete or partial filling of the sample loop) will, when used correctly, meet the repeatability requirements laid down in 6.2. When using the partial loop-filling mode, it is recommended that the injection volume should be less than half the total loop volume. For complete filling of the loop, best results are obtained by overfilling the loop at least six times.

NOTE 3 - The repeatability of the injection system may be checked by comparing peak areas from at least four injections of the system performance standard (see 9.3).

6.2.2 Sample and calibration injection volumes other than 10 µL (typically in the range from 3 to 20 µL) may be used, provided they meet the requirements laid down for injection repeatability (see 6.2), refractive index sensitivity and linearity (see 9.4.2 and 10.1.5), and column resolution (see 9.4.3).

6.3 Sample Filter, if required (see 10.2.1) - A microfilter of porosity 0.45 µm or less, which is chemically-inert towards hydrocarbon solvents, is recommended for the removal of particulate matter from the sample solutions.

6.4 Column System - Any stainless steel HPLC column(s) packed with an approved amino-bonded (or polar amino/cyano-bonded) silica stationary phase is suitable, provided it meets the resolution requirements laid down in 9.4.3. See Appendix X1 for guidance on the selection and use of suitable column systems.

6.5 HPLC Column Oven - Any suitable HPLC column oven (block heating or air circulating) capable of maintaining a constant temperature (+/- 1°C) within the range from 20 to 40°C.

NOTE 4 - The refractive index detector is sensitive to both sudden and gradual changes in the temperature of the eluent. All necessary precautions should be taken to establish constant temperature conditions throughout the liquid chromatograph system. The temperature should be optimized depending on the stationary phase.

NOTE 5 - Alternative forms of temperature control, for example, temperature-controlled laboratories, are permitted.

6.6 Backflush Valve - Any manual or automatic (air or electrically actuated) flow-switching valve designed for use in HPLC systems that is capable of operating at pressures up to 2 x 10(4) kPa.

6.7 Refractive Index Detector - Any refractive index detector may be used provided it is capable of being operated over the refractive index range from 1.3 to 1.6, meets the sensitivity requirement specified in 9.4.2, gives a linear response over the calibration range, and has a suitable output signal for the data system.

NOTE 6 - If the refractive index detector has a facility for independent temperature control, it is recommended that this be set at the same temperature as the column oven.

6.8 Computer or Computing Integrator - Any data system can be used provided it is compatible with the refractive index detector, has a minimum sampling rate of 1 Hz, and is capable of peak area and retention time measurement. The data system should also have minimum facilities for post-analysis data processing, such as baseline correction and reintegration. The ability to perform automatic peak detection and identification and to calculate sample concentrations from peak area measurements is recommended but not essential.

6.9 Volumetric Flasks, Grade B or better, of 10 and 100 mL capacity.

6.10 Analytical Balance, accurate to +/- 0.0001 g.

7. Reagents
7.1 Cyclohexane, > 99 % pure.

NOTE 7 - Cyclohexane may contain benzene as an impurity.

7.2 Heptane, HPLC Grade. For use as HPLC mobile phase. (Warning - Heptane is highly flammable and may cause irritation by inhalation, ingestion, or skin contact.)

NOTE 8 - Batch to batch variation of the solvent quality in terms of water content, viscosity, refractive index and purity could cause unpredictable column behavior. Drying and filtering the mobile phase could help to reduce the effect of the trace impurities in the solvent.

NOTE 9 - It is recommended practice to degas the HPLC mobile phase before use; this can be done conveniently, on-line, or off-line by helium sparging, vacuum degassing or ultrasonic agitation. A failure to de-gas the mobile phase may lead to negative peaks.

7.3 o-Xylene (1,2-Dimethylbenzene) , ≥ 98 % pure.

7.4 1-Methylnaphthalene, ≥ 98 % pure.

7.5 Phenanthrene, ≥ 98 % pure.

7.6 Dibenzothiophene, ≥ 95 % pure.

7.7 9-Methylanthracene, ≥ 95 % pure. (Warning - Gloves should be worn when handling aromatic compounds (for example, disposable vinyl gloves).)

NOTE 10 - Purity is determined by gas chromatography with flame ionization detection. The highest purity standards available should be used.