1、Designation: D7419 13D7419 18Standard Test Method forDetermination of Total Aromatics and Total Saturates inLube Basestocks by High Performance LiquidChromatography (HPLC) with Refractive Index Detection1This standard is issued under the fixed designation D7419; the number immediately following the
2、designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This test method covers t
3、he determination of total aromatics and total saturates in additive-free lube basestocks using highperformance liquid chromatography (HPLC) with refractive index (RI) detection. This test method is applicable to samplescontaining total saturates in the concentration range of 74.9 % to 100.0 % by mas
4、s and aromatics in the concentration range of 0.2to 46 mass %.0.0 % to 25.1 % by mass. The precision is expressed in terms of the total saturates.1.1.1 Polar compounds, if present, are combined with the total aromatics. Precision was determined for basestocks with polarscontent 1.5 mL have been used
5、 successfully.6.9 Analytical BalanceaccurateAccurate to 60.0001 g.7. Reagents and Materials7.1 Heptane, HPLC grade. If necessary, dry solvent with molecular sieves and then filter before use.7.2 Dichloromethane, HPLC or UV grade. If necessary, dry solvent with molecular sieves and then filter before
6、 use.7.3 Octadecylbenzene, 97 % 97 % pure.7.4 Hexadecane, 98 % 98 % pure.8. Sampling8.1 Follow Practice D4057 or D4177, or a similar standard to obtain a representative laboratory sample of the basestock. Mixwell before sampling.9. Preparation of Apparatus9.1 Set up the liquid chromatograph, injecti
7、on system, columns, backflush valve, optional column oven, optional UV detector,refractive index detector, and computing integrator in accordance with the manufacturers instructions and as depicted in Fig. 1.Insert the backflush valve so that the detector is always connected independently of the dir
8、ection of flow through the column (seeFig. 1). Maintain the sample injection valve at the same temperature as the sample solution; in most cases this will be at roomtemperature. To minimize drifts in signal, ensure that the ambient temperature is relatively constant during analysis and calibration.9
9、.2 New commercial columns may be packed in water/methanol or other polar solvents. Before these columns can be used, flushthem with dichloromethane followed with heptane before proceeding. Other suitable solvents that restore the required resolutionmay be used. If the resolution requirement is not m
10、et, the column may be reactivated by flushing it with additionaldichloromethane. If the resolution still cannot be attained, it may be necessary to replace the column or purchase an appropriatecolumn from other vendors. Si60 silica gel was found effective in yielding acceptable resolution and perfor
11、mance when properlyconditioned. When not analyzing samples, column may be flushed with a low flow of heptane such as 0.1 mL/min.9.2.1 Adjust the flow rate of the mobile phase to a constant 3.03.0 mLmin to 3.5 mL/min, and ensure the reference cell of therefractive index detector is full of mobile pha
12、se. Fill the reference cell as instructed by the manufacturer.TABLE 1 Examples of Operating Conditions Used in Cooperative StudiesLab A Lab B Lab CSilica Column Varian, 50 cm length by 7.7 mm i.d. 5 m Si60 Varian, 50 cm by 7.7 mm Si60 (CP28526) Phenomenex, 2 x Si60 (10 by 250 mm, 5 mCyano Column All
13、tech/YMC, 100 by 10 mm 10 m Waters/YMC, 100 by 12 mm 5 m YMC, 10 by 100 mm 5 mRI Detector Agilent 1200 Hewlett Packard RI, model HP1047A Shimadzu RID-10AHeptane Flow (mL/min) 3.5 mL/min 3.0 3.0Resolution 5 5-6 10.3Injected Volume (microlitres) 10 10 10D7419 1839.2.2 To minimize drift, it is essentia
14、l to make sure the reference cell of the RI detector is full of solvent. The best way toaccomplish this is either (1) to flush the mobile phase through the reference cell (then isolate the reference cell to preventevaporation of the solvent) immediately prior to analysis, or (2) to continuously make
15、 up for solvent evaporation by supplying asteady independent flow through the reference cell. The make-up flow is optimized so that reference and analytical cell mismatchdue to drying-out, temperature, or pressure gradients is minimized. Typically, this can be accomplished with a make-up flow setat
16、one tenth one-tenth of the analytical flow.9.3 Column Resolution and Capacity Factor:9.3.1 Prepare a system performance standard (SPS) by weighing hexadecane (1.0(1.0 g 6 0.1 g) and octadecylbenzene(1.0(1.0 g 6 0.1 g) into a 10 mL volumetric flask and filling to the mark with heptane. For the prepar
17、ation of standards, use thesame source for the heptane as that used for the mobile phase. Ensure that the octadecylbenzene is completely dissolved in themixture, for example, by using an ultrasonic bath.9.3.2 When operating conditions are steady, as indicated by a stable horizontal baseline of the R
18、I detector, inject 10 L of theSPS in the foreflush mode (backflush valve = OFF) and record the chromatogram using the data system. Fig. 2 gives an examplechromatogram of the SPS mixture.9.3.3 Ensure that the resolution between hexadecane and octadecylbenzene is five or greater as defined below. Calc
19、ulate theresolution between hexadecane and octadecylbenzene as follows:Resolution523t22t1!33y11y2!(1)where:where:t1 = retention time of the hexadecane peak in minutes,t2 = retention time of the octadecylbenzene peak in minutes,y1 = half-height width of the hexadecane peak in minutes, andy2 = half-he
20、ight width of the octadecylbenzene peak in minutes.If the resolution is less than five,5, verify that all system components are functioning correctly and that the chromatographic deadvolume has been minimized by using low dead volume connectors, tubing, etcetera. Ensure that the mobile phase is of s
21、ufficientlyhigh quality. Finally, regenerate or replace the column if necessary. The column may be regenerated by flushing withdichloromethane followed by heptane until the signal is relatively constant on the RI detector. If after regenerating the silicacolumns, the resolution is still less than 5
22、then replace the silica columns. Si60 was found to be an effective silica gel with properconditioning. For a proper analysis, a resolution of at least five5 is required.FIG. 1 Diagrammatic Representation of Liquid ChromatographD7419 184NOTE 1Resolution loss over time may occur if a heptane mobile ph
23、ase of low water content is not used. Use heptane as specified in this method.If necessary, dry the heptane with the addition of activated molecular sieves, such as MS 5A and then filter with at least 0.45 micron HPLC filter beforeuse.9.3.4 Calculate the capacity factor, k, for octadecylbenzene from
24、 9.3.2 as follows:Capacity Factor5k 5t22t1!t1!(2)where:where:t1 = retention time of the hexadecane peak in minutes,t2 = retention time of the octadecylbenzene peak in minutest1 = retention time of the hexadecane peak in minutes, andt2 = retention time of the octadecylbenzene peak in minutes.Ensure t
25、hat the capacity factor is 0.4.9.3.5 Using the determined retention times of the hexadecane and octadecylbenzene peaks in 9.3.2, calculate an approximateswitching valve backflush time, B, in seconds, using the following equation:B 5t110.13t22t1! (3)where:where:t1 = retention time of hexadecane in mi
26、nutes, andt2 = retention time of octadecylbenzene in minutes.9.4 Once the backflush time is determined, re-inject the SPS mixture with backflush in place and ensure that the backflush timeas observed as a signal marker on the chromatogram occurs at the base of the eluted saturate peak. The return to
27、 baseline shalldisplay as shown in Fig. 3, point B. This observation shall be made also for all actual lube samples analyzed. If necessary optimize,reconfirm the resolution and capacity factor, and recheck the backflush time. The use of the optional UV detector will simplifyoptimization of the backf
28、lush time.9.5 Check system precision as described in 12.3.NOTE 2If peak area precision is poor, verify that the injection system is working optimally and that the baseline is stable (minimal drift) andnoise-free.9.6 Prepare a detection limit standard (DLS) by weighing 0.01g octadecylbenzene into a v
29、ial and adding 5.00 g hexadecane.This makes a 0.2 mass % 0.2 % by mass aromatics standard.9.6.1 Inject the DLS in the foreflush mode and ensure that the octadecylbenzene is detected with a signal/noise (S/N) of at least8. Fig. 4 shows how to calculate the signal/noise.9.6.1.1 If the octadecylbenzene
30、 is not detected, recheck the instrument, making sure the RI detector meets the manufacturerssensitivity specifications. If necessary, increase the injection volume to 20 L and repeat all of the steps in Section 9. If the 20 LFIG. 2 Chromatogram of System Performance Standard in Foreflush Mode for D
31、etermination of Resolution, Capacity Factor, and Back-flush TimeD7419 185injection is successful in meeting all of the specifications in the test method, then use a 20 L injection for all analyses. In thecooperative study, 10 L was adequate for all laboratories.9.7 To perform the following step it w
32、ill be necessary to calibrate the system first as described in 10.1. Verify that a minimalresponse is obtained at low concentrations as follows:9.7.1 Prepare a 0.1 mass % 0.1 % by mass mixture of octadecylbenzene in hexadecane by weighing 0.01 g of octadecylbenzenein 10.0 g 10.0 g of hexadecane and
33、analyze as a sample in the backflush mode using the cut time determined in 9.3.5.9.7.2 Fig. 5 gives an example chromatogram response.9.7.3 Calculate the mass % of octadecylbenzene as described in Section 13 and ensure that the results are 0.15 mass %.0.15 %by mass.10. Calibration10.1 Prepare five ca
34、libration standards (A, B, C, D, and E),E) in accordance with the concentrations given in Table 2, byweighing, to the nearest 0.0001 g, the appropriate materials into 10-mL10 mL volumetric flasks and making up to the mark withheptane.10.2 When operating conditions are steady, as indicated by a stabl
35、e horizontal baseline, inject 10 L of calibration standard A.For the calibration, it is necessary to use the backflush mode. Record the chromatogram, and measure the peak areas forhexadecane and octadecylbenzene.FIG. 3 Chromatogram of System Performance Standard in Backflush ModeFIG. 4 Calculation o
36、f Signal/Noise Ratio for 0.2 Mass% % by Mass Octadecylbenzene in Hexadecane. Peak Shown is that of Octadecyl-benzene Obtained in Foreflush Mode (Saturate Peak not Shown)D7419 18610.3 Repeat 10.2 using calibration standards B, C, D, and E. For the low concentrations it may be necessary to draw thebas
37、elines under the peaks manually if the baseline shows some drift. See Figs. 6-8.10.4 Plot concentration g/10 mL against area counts for hexadecane and octadecylbenzene. Calibration plots shall be linear andforced through the origin with a correlation coefficient r2 greater than 0.999. The slopes of
38、the calibrations are used in thedetermination of response factors as described in Section 13. Computer software, such as Excel or a data system, may be used toestablish the calibration. Fig. 9 gives an example of calibration curves. A response factor of 1.67 to 1.80 has been observed andrecommended.
39、 The exact response factor may vary from system to system.FIG. 5 Chromatogram of a 0.1 mass % % by Mass Octadecylbenzene in Hexadecane Minimal Response Test (9.7) in Backflush ModeTABLE 2 Concentrations of Calibration ComponentsCalibration Standard Hexadecane,g/10 mLOctadecylbenzene,g/10 mLA 0.01 0.
40、01B 0.1 0.1C 1 0.5D 2 1E 5 3FIG. 6 Chromatogram of Standard A in Backflush ModeD7419 18711. Procedure11.1 Analysis of Samples for Total Aromatics Plus Total Polars Content:FIG. 7 Chromatogram of Standard B in Backflush ModeFIG. 8 Chromatogram of Standard E in Backflush modeModeFIG. 9 Example of Cali
41、bration Curves for Deriving Response FactorsD7419 18811.2 Weigh 0.5 g of sample into a 1.5-mL1.5 mLautosampler vial, and add 0.5 mLheptane. Shake thoroughly to mix. If desired,larger quantities, such as with 2 g of sample, may be prepared in a larger container with a similar ratio of sample/solvent
42、and thena well mixed well-mixed aliquot may be transferred to the autosampler vial or equivalent.11.2.1 For samples in which the concentration of the non-aromatic or aromatic hydrocarbon falls outside the calibration rangebased on absolute area, prepare a more concentrated (for example, 0.75 g/0.25
43、mL) or more dilute (0.25 g/0.75 mL) samplesolution as appropriate.11.2.2 Using operating conditions identical to those used for obtaining the calibration data (see 10.1), inject 10 Lof the samplesolution and start data collection. Actuate or turn on the backflush valve at the predetermined backflush
44、 time (see 9.3.5) to elutethe aromatics as a single sharp peak (see Fig. 3). When the analysis is finished, reverse the flow direction (turn off the switchingvalve) of the mobile phase, that is, return to foreflush, and allow the baseline to stabilize before injecting the next sample.11.2.3 Correctl
45、y identify the saturates peak and the aromatics peak. Figs. 10 and 11 show typical chromatograms for lubebasestocks.11.2.4 An optional UV detector at 254 nm may be used to monitor the overlap between the saturates and aromatic hydrocarbonswhen using a lube sample. The separation between the saturate
46、s and aromatics becomes more critical as the concentration ofaromatics is reduced. The signal of the UV detector during the elution of the saturates peak shall be insignificant when comparedto the signal of the aromatics peak. Fig. 12 gives an example of the use of the UV detector.11.2.5 Obtain the
47、areas of the saturates and aromatics plus polars peaks by drawing a baseline from just before the beginningof the saturates peak (A in Fig. 10) to a point on the chromatogram immediately before the backflush point (B in Fig. 10) and bydrawing a baseline from just before the beginning of the aromatic
48、s peak (C in Fig. 10) to a point on the chromatogram immediatelyafter the aromatics peak (D in Fig. 10).NOTE 3Particularly at low aromatic concentrations, the backflushed aromatics peak may exhibit a drifting baseline before and after the elution ofthe peak. Ensure that the peak is correctly integra
49、ted. Visual inspection of the integrated baseline may be necessary and manual integration (drawing thebaseline manually) may be required.12. Quality Control12.1 The quality control for this test method is summarized below:12.1.1 For each batch of samples, measure and record the column resolution as described in 9.3.3. Resolution may deteriorateas the column is used, and it may require reconditioning.12.2 Perform a detection limit evaluation as described in 9.6 for each batch.12.3 When the instrument is first placed into service, obtain a short te
