1、Designation: D7756 12Standard Test Method forResidues in Liquefied Petroleum (LP) Gases by GasChromatography with Liquid, On-Column Injection1This standard is issued under the fixed designation D7756; the number immediately following the designation indicates the year oforiginal adoption or, in the
2、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 the determination, by gaschromatography, of soluble hydrocarbon
3、 materials, sometimescalled “oily residue,” which can be present in LiquefiedPetroleum (LP) Gases and which are substantially less volatilethan the LPG product.1.2 This test method quantifies, in the range of 10 to 600mg/kg (ppm mass), the residue with a boiling point between174C and 522C (C10to C40
4、) in LPG. Higher boilingmaterials, or materials that adhere permanently to the chro-matographic column, will not be detected.1.3 UnitsThe values stated in SI units are to be regardedas standard. The values given in parentheses are for informa-tion only.1.4 This standard does not purport to address a
5、ll of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2D1265 Practice for
6、Sampling Liquefied Petroleum (LP)Gases, Manual MethodD1835 Specification for Liquefied Petroleum (LP) GasesD2158 Test Method for Residues in Liquefied Petroleum(LP) GasesD2163 Test Method for Analysis of Liquefied Petroleum(LP) Gases and Propene Concentrates by Gas Chromatog-raphyD2421 Practice for
7、Interconversion of Analysis of C5andLighter Hydrocarbons to Gas-Volume, Liquid-Volume, orMass BasisD2598 Practice for Calculation of Certain Physical Proper-ties of Liquefied Petroleum (LP) Gases from Composi-tional AnalysisD3700 Practice for Obtaining LPG Samples Using a Float-ing Piston CylinderD6
8、299 Practice for Applying Statistical Quality Assuranceand Control Charting Techniques to Evaluate AnalyticalMeasurement System PerformanceD6667 Test Method for Determination of Total VolatileSulfur in Gaseous Hydrocarbons and Liquefied PetroleumGases by Ultraviolet FluorescenceE355 Practice for Gas
9、 Chromatography Terms and Relation-shipsE594 Practice for Testing Flame Ionization Detectors Usedin Gas or Supercritical Fluid Chromatography3. Terminology3.1 Definitions of Terms Concerning ChromatographyThis test method makes reference to many common gaschromatographic procedures, terms, and relat
10、ionships. Detaileddefinitions of these can be found in Practices E355 and E594.3.2 Definitions of Terms Concerning Liquefied PetroleumGasesThis test method makes reference to the definitions ofliquefied petroleum gases as described in Specification D1835.3.3 Definitions of Terms Specific to This Sta
11、ndard:3.3.1 high pressure liquefied gas injector, nSample intro-duction device which injects liquefied gas samples underpressure and at room temperature directly onto the chromato-graphic column thereby maintaining the sample in liquid phaseduring the injection process.3.3.2 pressure station, nDevic
12、e that supplies high pres-sure nitrogen to a suitable sample cylinder and thereforemaintains sample in the liquid phase during the injectionprocedure.4. Summary of Test Method4.1 A sample cylinder of LPG is pressurized to 2500 kPa(363 psi) using nitrogen or helium.4.2 The injection system is flushed
13、 with LPG in liquid phaseat room temperature.1This test method is under the jurisdiction of ASTM Committee D02 onPetroleum Products and Lubricants and is the direct responsibility of SubcommitteeD02.H0 on Liquefied Petroleum Gas.Current edition approved Nov. 1, 2012. Published April 2013. Originally
14、approved in 2011. Last previous edition approved in 2011 as D775611.DOI:10.1520/D7756-12.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summ
15、ary page onthe ASTM website.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States14.3 After flushing, the injection device is routed to the GCinjector port and LPG (25 millise
16、conds activation time equiva-lent to 30 L) is introduced via a high pressure valve andneedle which is inserted into a large volume cold on-columninjector.4.4 The gas chromatograph is equipped with a solvent ventwhich routes most of the LPG light components out of theanalytical system and leaves behi
17、nd the components of interest.4.5 The oily residue to be determined is retained on apre-column.4.6 After venting the LPG, the flow from the pre-column isswitched to the analytical column and a temperature program isstarted.4.7 Oily residue contaminants are separated and identifiedbased on difference
18、s in boiling point temperature.4.8 Total residue is quantified using area summation ofcomponents corresponding to the expected range of C10to C40(174 to 522C).5. Significance and Use5.1 Control over the residue content as specified in Speci-fication D1835 is of considerable importance in end-useappl
19、ications of LPG. Oily residue in LPG is contaminationwhich can occur during production, transportation, or storage.5.2 This test method is quicker and much more sensitivethan manual methods, such as Test Method D2158, which isbased on evaporation of large sample volumes followed byvisual or gravimet
20、ric estimation of residue content.5.3 This test method provides enhanced sensitivity in mea-surements of heavier (oily) residues, with a quantification limitof 10 mg/kg total residue.5.4 This test method gives both quantitative results andinformation about contaminant composition such as boilingpoin
21、t range and fingerprint, which can be very useful in tracingthe source of a particular contaminant.6. Apparatus6.1 Gas Chromatograph (GC)Gas chromatographic in-strument equipped with a Large Volume Cold on-ColumnInjector (LVOCI), a linear temperature programmable columnoven, and a flame ionization d
22、etector (FID). The temperaturecontrol shall be capable of obtaining a retention time repeat-ability of 0.05 min (3 s) throughout the scope of this analysis.6.2 Data AcquisitionAny commercial integrator or com-puterized data acquisition system may be used for display ofthe chromatographic detector si
23、gnal and peak area integration.6.3 Solvent VentA controlled vent for venting the majorpart of the matrix.6.4 Retention GapUncoated stainless steel capillary. Suc-cessfully used columns and conditions are given in Table 1.6.5 Retaining Pre-ColumnA column with a polydimeth-ylsiloxane stationary phase.
24、 Successfully used columns andconditions are given in Table 1.6.6 Analytical ColumnA column with a polydimethylsi-loxane stationary phase. Successfully used columns and con-ditions are given in Table 1.6.7 Column CouplerCoupling DeviceSuitable for leak-free coupling of the retention gap to the retai
25、ning pre-column.(See Fig. 1 for a schematic overview of the couplings inside theGC oven and the couplings to the solvent vent valve.)6.8 Column SplitterSplitter suitable for leak-free couplingof the retaining pre-column to one side of the analytical columnand the deactivated capillary on the other s
26、ide. (See Fig. 1 fora schematic overview of the couplings inside the GC oven andthe couplings to the solvent vent valve.)TABLE 1 Typical Operating ConditionsOven program 35C for 3 min35 to 340C at 25C/min340C for 10 minInlet program Type: cool on-columnTemperature: 65C for 3 min55 to 340C at 25C/min
27、340C for 9 minDetector settings Air flow: 400 mL/minHydrogen flow: 40 mL/minMake up gas flow: 45 mL/minTemperature: 350CData rate: 20 HzColumn Retention gap: SulfinertAstainless steel capillary with inner diameter 0.53 mm andlength of 5 mRetaining pre-column: 3 m 100%Dimethylpolysiloxane: 0.53 mm, 2
28、.65 mAnalytical column: 100%Dimethylpolysiloxane 30 m, 0.32 mm, 0.25 mPressure station Sample flow: 2 mL/minNitrogen pressure: 2500 kPaNitrogen purge pressure: 500 kPaLiquefied GasInjectorInjection: 25 msASulfinert is a trademark of SilcoTek, 112 Benner Circle, Bellefonte, PA 16823, www.SilcoT.D7756
29、 1226.9 High Pressure Liquefied Gas InjectorA high pressurevalve directly connected to a needle which is inserted in theinjection port of the GC, after which the valve is triggered inorder to introduce a representative aliquot into the GC systemwithout sample discrimination. (See Fig. 2.)6.10 Pressu
30、re StationThis shall ensure a sample in liquidphase at a constant pressure. See Fig. 3 for a typical configu-ration.6.11 Typical Column OverviewSee Fig. 1.6.12 Typical Operating ConditionsSee Table 1.7. Reagents and Materials7.1 Mineral Oil in LPG Calibration MixtureCertifiedcalibration mixture with
31、 mineral oil in LPG. The concentrationof the mineral oil shall be close to the expected concentrationof the contamination in the LPG sample.7.2 Mineral Oil in Pentane Calibration MixturePrepare acalibration standard of mineral oil in pentane. Record theweighed value to the nearest milligram of miner
32、al oil andcalculate the concentration in mg/kg. The concentration of themineral oil shall be close to the expected concentration of thecontamination in the LPG sample.7.2.1 Standards that are prepared in pentane, normallyliquid at room temperature, should be stored in suitablecontainers under refrig
33、eration and transferred to sample cylin-ders prior to use. Alternatively, they may be stored in airtightcylinders.7.3 Mineral Oil or Local Hydrocarbon FractionBoilingpoint range approximately C10-C40. Alternatively, a well char-acterized local hydrocarbon fraction, within the range C10-C40,can be us
34、ed to provide quantitative and qualitative comparisonto the contaminant in the sample. Care should be taken toensure no significant fraction falls outside the C10-C40range.FIG. 1 Overview of the Couplings Inside the GC Oven and the Couplings to the Solvent Vent ValveFIG. 2 High Pressure ValveD7756 1
35、237.4 Validation Standard, Mineral Oil in PentanePrepare avalidation standard of mineral oil in pentane. Record the exactweighed value to the nearest milligram of mineral oil andcalculate the concentration in mg/kg. The concentration of themineral oil shall be close to the expected concentration of
36、thecontamination in the LPG sample.7.5 N-alkane Retention Time StandardMixture containingat least C10and C40in a concentration of (nominally) 5 mg/Leach, dissolved in pentane or heptane.7.6 SolventGC grade pentane.8. Hazards8.1 There is a significant fire hazard from LPG, and sincethe boiling point
37、of LPG can be as low as -41C, there is a riskof freezing “burns.” Take appropriate safety precautions toprevent ignition or fire, and wear suitable protective equipmentto protect against skin contact with LPG.8.2 An appropriate laboratory ventilation system shall beused.8.3 An appropriate waste line
38、 shall be installed. The pres-sure station and injector shall be connected to this line. Thewaste line should vent outside the building.8.4 Pressure station, cylinder, injector, and controller shallbe grounded appropriately.9. Preparation of Apparatus9.1 Gas ChromatographInstall and verify performan
39、ce inaccordance with the manufacturers instructions. Typical oper-ating conditions are shown in Table 1.A Sample cylinderB Sample line inC Injection deviceD Cool on column inletE Gas chromatographF Sample line outG RotometerH VaporizerI Waste systemP Pressure gaugeFIG. 3 Typical Configuration of a P
40、ressure StationD7756 1249.2 Pressure StationInstall in accordance with the manu-facturers instructions. Purge sample and check carefully forleaks.9.3 High Pressure Liquefied Gas InjectorInstall in accor-dance with the manufacturers instructions.9.4 Column ConfigurationInstall the columns as shown in
41、Fig. 1. Use low dead volume connections, and check for leaks.10. Calibration10.1 Perform a one point calibration at the startup of theinstrument, when the result of the validation sample fallsoutside the acceptable SQC limits in accordance with Section14 or after changes in the application hardware
42、or gas supply,or both.10.2 Run a blank run, without sample injection. Cycle theGC several times until the baseline is stable. A baseline isstable when the start and end signal (in pA) of two consecutiveblank runs are within 5%. An unstable baseline can be causedby a leak, detector gases, or by high
43、boiling point componentsor materials that have not yet eluted from the column. Thesignal height (in pA) at the end of an analysis of a calibration,validation, or sample shall be equal or higher than the blankbaseline. A signal higher than 5% could indicate a poorlyconditioned column or the elution o
44、f sample components witha boiling point higher than 522C. Refer to the datasheet of thecolumn for instructions on conditioning the column.10.3 Analyze the n-alkane retention time standard (7.6), andestablish the retention time for C10and C40. There should bebaseline separation between the solvent an
45、d the first normalalkane peak (C10). If the separation is not sufficient, adjust thetemperature program, re-establish the baseline, and then reana-lyze the retention time standard. An example is shown in Fig.4.10.4 Analyze the calibration mixture. The calibration mix-ture is either in LPG or in pent
46、ane (7.1 and 7.2).10.5 Integrate the oily residue by summing the area fromC10through C40.10.6 Determine the response factor by dividing the knownconcentration by the total area, and use this for the calculationof unknown samples under the assumption that all samplecomponents have the same response f
47、actor.10.7 Analyze the validation sample using the liquefied gasinjector. Analyze the validation sample once per day of usebefore the samples. Repeat the analysis when the result of thevalidation sample falls outside the acceptable SQC limits inaccordance with Section 14.11. Procedure11.1 Collect a
48、representative sample according to PracticeD1265 or D3700.11.2 Connect the sample cylinder to the pressure station andpressurize to approximately 2500 6 200 kPa (363 6 29 psi). Itis important to maintain and reproduce this pressure as closelyas possible to ensure sample size injection repeatability.
49、11.3 Open the cylinder at both sides and flush the sample forapproximately 3 min with a flow rate of about 5 mL/min.11.4 Inject sample (trigger pulse 25 ms at 2500 kPa,equivalent to approximately 30 L).FIG. 4 Chromatogram of C10through C40D7756 12511.5 Analyze each sample in duplicate. If the differencebetween the results of the two analyses is5%,perform anextra analysis and average the two closest results.11.6 Close the sample cylinder after injection and repeat11.3 for the next injection. When all analyses are finished,cl
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