1、Designation: D 7419 07An American National StandardStandard 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 D 7419; the number imm
2、ediately following the 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 (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 Th
3、is test method covers the determination of totalaromatics and total saturates in additive-free lube basestocksusing high performance liquid chromatography (HPLC) withrefractive index (RI) detection. This test method is applicableto samples containing total aromatics in the concentrationrange of 0.2
4、to 46 mass %.1.1.1 Polar compounds, if present, are combined with thetotal aromatics. Precision was determined for basestocks withpolars content 1.5 mL have been used successfully.6.9 Analytical Balanceaccurate to 60.0001 g.7. Reagents and Materials7.1 Heptane, HPLC grade. If necessary, dry solvent
5、withmolecular sieves and then filter before use.7.2 Dichloromethane, HPLC or UV grade. If necessary, drysolvent with molecular sieves and then filter before use.7.3 Octadecylbenzene, $ 97 % pure.7.4 Hexadecane, $ 98 % pure.8. Sampling8.1 Follow Practice D 4057 or D 4177, or a similar standardto obta
6、in a representative laboratory sample of the basestock.Mix well before sampling.9. Preparation of Apparatus9.1 Set up the liquid chromatograph, injection system,columns, backflush valve, optional column oven, optional UVdetector, refractive index detector and computing integrator inaccordance with t
7、he manufacturers instructions and as de-picted in Fig. 1. Insert the backflush valve so that the detectoris always connected independently of the direction of flowthrough the column (see Fig. 1). Maintain the sample injectionvalve at the same temperature as the sample solution; in mostcases this wil
8、l be at room temperature. To minimize drifts insignal, ensure that the ambient temperature is relatively con-stant during analysis and calibration.9.2 New commercial columns may be packed in water/methanol or other polar solvents. Before these columns can beused flush them with dichloromethane follo
9、wed with heptanebefore proceeding. Other suitable solvents that restore therequired resolution may be used. If the resolution requirementis not met, the column may be reactivated by flushing it withadditional dichloromethane. If the resolution still cannot beattained it may be necessary to replace t
10、he column or purchasean appropriate column from other vendors. Si60 silica gel wasTABLE 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,
11、 5 mCyano Column Alltech/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 10D7419072found effective in y
12、ielding acceptable resolution and perfor-mance when properly conditioned. When not analyzingsamples, column may be flushed with a low flow of heptanesuch as 0.1 mL/min.9.2.1 Adjust the flow rate of the mobile phase to a constant3.0 to 3.5 mL/min, and ensure the reference cell of therefractive index
13、detector is full of mobile phase. Fill thereference cell as instructed by the manufacturer.9.2.2 To minimize drift, it is essential to make sure thereference cell of the RI detector is full of solvent. The best wayto accomplish this is either (1) to flush the mobile phasethrough the reference cell (
14、then isolate the reference cell toprevent evaporation of the solvent) immediately prior toanalysis, or (2) to continuously make up for solvent evapora-tion by supplying a steady independent flow through thereference cell. The make-up flow is optimized so that referenceand analytical cell mismatch du
15、e to drying-out, temperature, orpressure gradients is minimized. Typically, this can be accom-plished with a make-up flow set at one tenth of the analyticalflow.9.3 Column Resolution and Capacity Factor:9.3.1 Prepare a system performance standard (SPS) byweighing hexadecane (1.0 6 0.1 g) and octadec
16、ylbenzene (1.06 0.1 g) into a 10 mL volumetric flask and filling to the markwith heptane. For the preparation of standards, use the samesource for the heptane as that used for the mobile phase. Ensurethat the octadecylbenzene is completely dissolved in themixture, for example, by using an ultrasonic
17、 bath.9.3.2 When operating conditions are steady, as indicated bya stable horizontal baseline of the RI detector, inject 10 L ofthe SPS in the foreflush mode (backflush valve = OFF) andrecord the chromatogram using the data system. Fig. 2 gives anexample chromatogram of the SPS mixture.9.3.3 Ensure
18、that the resolution between hexadecane andoctadecylbenzene is five or greater as defined below. Calculatethe resolution between hexadecane and octadecylbenzene asfollows:Resolution 52 3 t22 t1!3 3 y11 y2!(1)where:t1= retention time of the hexadecane peak in minutes,t2= retention time of the octadecy
19、lbenzene peak in min-utes,y1= half-height width of the hexadecane peak in minutes,andy2= half-height width of the octadecylbenzene peak inminutes.If the resolution is less than five, verify that all systemcomponents are functioning correctly and that the chromato-graphic dead volume has been minimiz
20、ed by using low deadvolume connectors, tubing etcetera. Ensure that the mobilephase is of sufficiently high quality. Finally, regenerate orreplace the column if necessary. The column may be regener-ated by flushing with dichloromethane followed by heptaneuntil the signal is relatively constant on th
21、e RI detector. If afterregenerating the silica columns, the resolution is still less than5 then replace the silica columns. Si60 was found to be aneffective silica gel with proper conditioning. For a properanalysis, a resolution of at least five is required.NOTE 1Resolution loss over time may occur
22、if a heptane mobilephase of low water content is not used. Use heptane as specified in thismethod. If necessary, dry the heptane with the addition of activatedmolecular sieves, such as MS 5A and then filter with at least 0.45 micronHPLC filter before use.FIG. 1 Diagrammatic Representation of Liquid
23、ChromatographD74190739.3.4 Calculate the capacity factor, k, for octadecylbenzenefrom 9.3.2 as follows:Capacity Factor 5 k 5t2 t1!t1!(2)where:t1= retention time of the hexadecane peak in minutes,t2= retention time of the octadecylbenzene peak in minutesEnsure that the capacity factor is 0.4.9.3.5 Us
24、ing the determined retention times of the hexade-cane and octadecylbenzene peaks in 9.3.2 calculate an approxi-mate switching valve backflush time, B, in seconds, using thefollowing equation:B 5 t11 0.1 3 t2 t1! (3)where:t1= retention time of hexadecane in minutes, andt2= retention time of octadecyl
25、benzene in minutes.9.4 Once the backflush time is determined, re-inject the SPSmixture with backflush in place and ensure that the backflushtime as observed as a signal marker on the chromatogramoccurs at the base of the eluted saturate peak. The return tobaseline shall display as shown in Fig. 3, p
26、oint B. Thisobservation shall be made also for all actual lube samplesanalyzed. If necessary optimize, reconfirm the resolution andcapacity factor and recheck the backflush time. The use of theoptional UV detector will simplify optimization of the back-flush time.9.5 Check system precision as descri
27、bed in 12.4.NOTE 2If peak area precision is poor, verify that the injection systemis working optimally and that the baseline is stable (minimal drift) andnoise-free.9.6 Prepare a detection limit standard (DLS) by weighing0.01g octadecylbenzene into a vial and adding 5.00 g hexade-cane. This makes a
28、0.2 mass % aromatics standard.FIG. 2 Chromatogram of System Performance Standard in Foreflush Mode for Determination of Resolution, Capacity Factor andBackflush TimeFIG. 3 Chromatogram of System Performance Standard in Backflush ModeD74190749.6.1 Inject the DLS in the foreflush mode and ensure thatt
29、he octadecylbenzene is detected with a signal/noise (S/N) of atleast 8. Fig. 4 shows how to calculate the signal/noise.9.6.1.1 If the octadecylbenzene is not detected, recheck theinstrument, making sure the RI detector meets the manufactur-ers sensitivity specifications. If necessary, increase the i
30、njec-tion volume to 20 L and repeat all of the steps in Section 9.If the 20 L injection is successful in meeting all of thespecifications in the test method, then use a 20 L injection forall analyses. In the cooperative study, 10 L was adequate forall laboratories.9.7 To perform the following step i
31、t will be necessary tocalibrate the system first as described in 10.1. Verify that aminimal response is obtained at low concentrations as follows:9.7.1 Prepare a 0.1 mass % mixture of octadecylbenzene inhexadecane by weighing 0.01 g of octadecylbenzene in 10.0 gof hexadecane and analyze as a sample
32、in the backflush modeusing 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 de-scribed in 13 and ensure that the results are 0.15 mass %.10. Calibration10.1 Prepare five calibration standards (A, B, C, D, and E),in
33、 accordance with the concentrations given in Table 2,byweighing, to the nearest 0.0001 g, the appropriate materialsinto 10-mL volumetric flasks and making up to the mark withheptane.10.2 When operating conditions are steady, as indicated bya stable horizontal baseline, inject 10 Lof calibration stan
34、dardA. For the calibration, it is necessary to use the backflushmode. Record the chromatogram, and measure the peak areasfor hexadecane and octadecylbenzene.10.3 Repeat 10.2 using calibration standards B, C, D, and E.For the low concentrations it may be necessary to draw thebaselines under the peaks
35、 manually if the baseline shows somedrift. See Figs. 6-8.10.4 Plot concentration g/10 mL against area counts forhexadecane and octadecylbenzene. Calibration plots shall belinear and forced through the origin with a correlation coeffi-cient r2greater than 0.999. The slopes of the calibrations areused
36、 in the determination of response factors as described inSection 13. Computer software, such as Excel or a data systemmay be used to establish the calibration. Fig. 9 gives anexample of calibration curves. A response factor of 1.75(60.05) is required.11. Procedure11.1 Analysis of Samples for Total A
37、romatics Plus TotalPolars Content:11.2 Weigh 0.5 g of sample into a 1.5-mL autosampler vial,and add 0.5 mL heptane. Shake thoroughly to mix. If desired,larger quantities, such as with2gofsample, may be preparedin a larger container with a similar ratio of sample/solvent andthen a well mixed aliquot
38、may be transferred to the autosam-pler vial or equivalent.11.2.1 For samples in which the concentration of the non-aromatic or aromatic hydrocarbon falls outside the calibrationrange based on absolute area, prepare a more concentrated (forexample, 0.75 g/0.25 mL) or more dilute (0.25 g/0.75 mL)sampl
39、e solution as appropriate.11.2.2 Using operating conditions identical to those used forobtaining the calibration data (see 10.1), inject 10 L of thesample solution and start data collection.Actuate or turn on thebackflush valve at the predetermined backflush time (see 9.3.5)to elute the aromatics as
40、 a single sharp peak (see Fig. 3). Whenthe analysis is finished, reverse the flow direction (turn off theswitching valve) of the mobile phase, that is, return toforeflush, and allow the baseline to stabilize before injectingthe next sample.11.2.3 Correctly identify the saturates peak and the aromat-
41、ics peak. Figs. 10 and 11 show typical chromatograms for lubebasestocks.FIG. 4 Calculation of Signal/Noise Ratio for 0.2 Mass% Octadecylbenzene in Hexadecane. Peak Shown is that of OctadecylbenzeneObtained in Foreflush Mode (Saturate Peak not Shown)D741907511.2.4 An optional UV detector at 254 nm ma
42、y be used tomonitor the overlap between the saturates and aromatic hydro-carbons when using a lube sample. The separation between thesaturates and aromatics becomes more critical as the concen-tration of aromatics is reduced. The signal of the UV detectorduring the elution of the saturates peak shal
43、l be insignificantwhen compared to the signal of the aromatics peak. Fig. 12gives an example of the use of the UV detector.11.2.5 Obtain the areas of the saturates and aromatics pluspolars peaks by drawing a baseline from just before thebeginning of the saturates peak (A in Fig. 10) to a point on th
44、echromatogram immediately before the backflush point (B inFig. 10) and by drawing a baseline from just before thebeginning of the aromatics peak (C in Fig. 10) to a point on thechromatogram immediately after the aromatics peak (D in Fig.10).NOTE 3Particularly at low aromatic concentrations, the back
45、flushedaromatics peak may exhibit a drifting baseline before and after the elutionof the peak. Ensure that the peak is correctly integrated. Visual inspectionof the integrated baseline may be necessary and manual integration(drawing the baseline manually) may be required.12. Quality Control12.1 The
46、quality control for this test method is summarizedbelow:12.1.1 For each batch of samples, measure and record thecolumn resolution as described in 9.3.3. Resolution maydeteriorate as the column is used, and it may require recondi-tioning.12.2 Ensure that the response factor after calibration is 1.756
47、 0.5.12.3 Perform a detection limit evaluation as described in 9.6for each batch.12.4 When the instrument is first placed into service, obtaina short term precision in the foreflush mode by injecting theresolution mixture solution five times, and determine that thestandard deviation of the areas are
48、 0.6 relative % standarddeviations for the hexadecane and the octadecylbenzene com-ponents.12.5 After the instrument is placed into service, obtain alube check sample with a total aromatics plus polar content of8 to 10 mass % and analyze it several times to begin establish-ing a laboratory intermedi
49、ate precision SQC monitoring pro-gram, such as described in Practice D 6299. Ensure that thecontrol limits for the min/max of this check sample fallbetween reproducibility and repeatability values given in thistest method.12.6 It is strongly recommended that other check standardsbe used to cover higher and lower aromatics plus polarscontent. For example, if samples are analyzed at the 45 mass %aromatics plus polars level, then use a check sample at 45mass % aromatics plus polars level. For aromatics plus polarsconcentrations below 5 mass %, use a check sample in
