1、Designation: D7419 07D7419 13Standard 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 Scope*1.1 This test method co
3、vers the 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 aromatics in the concentration range of 0.2 to 46 mass %.1
4、.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 successfully.6.9 Analytical Balanceaccurate to 60.0001 g.7. Reagents and Materials7.1 Heptane, HPLC grade. If necessary, dry solvent with molecula
5、r sieves and then filter before use.7.2 Dichloromethane, HPLC or UV grade. If necessary, dry solvent with molecular sieves and then filter before use.7.3 Octadecylbenzene, 97 % pure.7.4 Hexadecane, 98 % pure.8. Sampling8.1 Follow Practice D4057 or D4177, or a similar standard to obtain a representat
6、ive laboratory sample of the basestock. Mixwell before sampling.9. Preparation of Apparatus9.1 Set up the liquid chromatograph, injection system, columns, backflush valve, optional column oven, optional UV detector,refractive index detector and computing integrator in accordance with the manufacture
7、rs instructions and as depicted in Fig. 1.Insert the backflush valve so that the detector is always connected independently of the direction 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 roomt
8、emperature. To minimize drifts in signal, ensure that the ambient temperature is relatively constant during analysis and calibration.FIG. 1 Diagrammatic Representation of Liquid ChromatographD7419 1339.2 New commercial columns may be packed in water/methanol or other polar solvents. Before these col
9、umns 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 met, the column may be reactivated by flushing it with additionaldichloromethane. If the resolution s
10、till 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 performance when properlyconditioned. When not analyzing samples, column may be flushed with a low flow of
11、heptane such as 0.1 mL/min.9.2.1 Adjust the flow rate of the mobile phase to a constant 3.0 to 3.5 mL/min, and ensure the reference cell of the refractiveindex detector is full of mobile phase. Fill the reference cell as instructed by the manufacturer.9.2.2 To minimize drift, it is essential to make
12、 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 up for s
13、olvent 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 one tenth
14、 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 6 0.1 g) and octadecylbenzene (1.0 6 0.1 g)into a 10 mL volumetric flask and filling to the mark with heptane. For the preparation of standards, use the sam
15、e source for theheptane as that used for the mobile phase. Ensure that the octadecylbenzene is completely dissolved in the mixture, for example,by using an ultrasonic bath.9.3.2 When operating conditions are steady, as indicated by a stable horizontal baseline of the RI detector, inject 10 L of theS
16、PS 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. Calculate theresolution between hex
17、adecane and octadecylbenzene as follows:Resolution523t22t1!33y11y2!(1)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-height width of the octadecylbenzene pe
18、ak in minutes.If the resolution is less than five, 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 sufficientlyhigh quality. Finally, regene
19、rate 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 silicaFIG. 2 Chromatogram of System Performance Standard in Foreflush Mode for Determinatio
20、n of Resolution, Capacity Factor and Back-flush TimeD7419 134columns, the resolution is still less than 5 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 five is required.NOTE 1Resolution loss over
21、 time may occur if a heptane mobile phase 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 capacit
22、y factor, k, for octadecylbenzene from 9.3.2 as follows:Capacity Factor5k 5t22t1!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 Using the determined retention times of the hexad
23、ecane 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:t1 = retention time of hexadecane in minutes, andt2 = retention time of octadecylbenzene in minutes.9.4 Once the backflush time is d
24、etermined, 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 baseline shalldisplay as shown in Fig. 3, point B. This observation shall be made also for a
25、ll 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 backflush time.9.5 Check system precision as described in 12.412.3.NOTE 2If peak area precision is
26、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 vial and adding 5.00 g hexadecane.This makes a 0.2 mass % aromatics standard.9.6.1 Inject t
27、he 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 is not detected, recheck the instrument, making sure the RI detector meets the manufacturerssensitivity
28、 specifications. If necessary, increase the injection volume to 20 L and repeat all of the steps in Section 9. If the 20 Linjection 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 la
29、boratories.FIG. 3 Chromatogram of System Performance Standard in Backflush ModeD7419 1359.7 To perform the following step it will 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 % mix
30、ture of octadecylbenzene in hexadecane by weighing 0.01 g of octadecylbenzene in 10.0 g ofhexadecane and 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
31、 13 and ensure that the results are 0.15 mass %.10. Calibration10.1 Prepare five calibration standards (A, B, C, D, and E), in accordance with the concentrations given in Table 2, by weighing,to the nearest 0.0001 g, the appropriate materials into 10-mL volumetric flasks and making up to the mark wi
32、th heptane.10.2 When operating conditions are steady, as indicated by a stable 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.10.3 Repeat
33、 10.2 using calibration standards B, C, D, and E. For the low concentrations it may be necessary to draw thebaselines under the peaks manually if the baseline shows some drift. See Figs. 6-8.FIG. 4 Calculation of Signal/Noise Ratio for 0.2 Mass% Octadecylbenzene in Hexadecane. Peak Shown is that of
34、Octadecylbenzene Ob-tained in Foreflush Mode (Saturate Peak not Shown)FIG. 5 Chromatogram of a 0.1 mass % Octadecylbenzene in Hexadecane Minimal Response Test (9.7) in Backflush ModeD7419 13610.4 Plot concentration g/10 mL against area counts for hexadecane and octadecylbenzene. Calibration plots sh
35、all be linear andforced through the origin with a correlation coefficient r2 greater than 0.999. The slopes of 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.
36、 9 gives an example of calibration curves. A response factor of 1.75 (60.05) is required.1.67 to 1.80has been observed and recommended. The exact response factor may vary from system to system.11. Procedure11.1 Analysis of Samples for Total Aromatics Plus Total Polars Content:11.2 Weigh 0.5 g of sam
37、ple into a 1.5-mLautosampler vial, and add 0.5 mLheptane. Shake thoroughly to mix. If desired, largerquantities, such as with 2 g of sample, may be prepared in a larger container with a similar ratio of sample/solvent and then a wellmixed aliquot may be transferred to the autosampler vial or equival
38、ent.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 mL) or more dilute (0.25 g/0.75 mL) samplesolution as appropriate.11.2.2 Using operating cond
39、itions 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 time (see 9.3.5) to elutethe aromatics as a single sharp peak (see Fig. 3). When the analysi
40、s 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 Correctly identify the saturates peak and the aromatics peak. Figs. 10 and 11 show typical chromatogr
41、ams 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 saturates and aromatics becomes more critical as the concentration ofaromatics is reduced. The signal
42、 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 areas of the saturates and aromatics plus polars peaks by drawing a baseline from just before
43、 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 aromatics peak (C in Fig. 10) to a point on the chromatogram immediatelyafter the aromatics peak (D i
44、n 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 integrated. Visual inspection of the integrated baseline may be necessary and manual integration (dr
45、awing 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 recon
46、ditioning.12.2 Ensure that the response factor after calibration is 1.75 6 0.5.12.2 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 term precision in the foreflush mode by injecting theresolution mixture so
47、lution five times, and determine that the standard deviation of the areas are 0.6 relative % standarddeviations for the hexadecane and the octadecylbenzene components.12.4 After the instrument is placed into service, obtain a lube check sample with a total aromatics plus polar content of 8 to10 mass
48、 % and analyze it several times to begin establishing a laboratory intermediate precision SQC monitoring program, suchas described in Practice D6299. Ensure that the control limits for the min/max of this check sample fall between reproducibilityand repeatability values given in this test method.12.
49、5 It is strongly recommended that other check standards be used to cover higher and lower aromatics plus polars content.For example, if samples are analyzed at the 45 mass % aromatics plus polars level, then use a check sample at 45 mass % aromaticsTABLE 2 Concentrations of Calibration ComponentsCalibration Standard Hexadecane,g/10 mLOctadecylbenzene,g/10 mLA 0.01 0.01B 0.1 0.1C 1 0.5D 2 1E 5 3D7419 137plus polars level. For aromatics plus polars concentrations below 5 mass %, use a check sample in the range of 3 to 5 mass %.If available, the use of consens
