1、Designation: D6968 03 (Reapproved 2015)Standard Test Method forSimultaneous Measurement of Sulfur Compounds andMinor Hydrocarbons in Natural Gas and Gaseous Fuels byGas Chromatography and Atomic Emission Detection1This standard is issued under the fixed designation D6968; the number immediately foll
2、owing 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 () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method
3、 is for the determination of volatilesulfur-containing compounds and minor hydrocarbons in gas-eous fuels including components with higher molar mass thanthat of propane in a high methane gas, by gas chromatography(GC) and atomic emission detection (AED). Hydrocarbonsinclude individual aliphatic com
4、ponents from C4to C6, aro-matic components and groups of hydrocarbons classifiedaccording to carbon numbers up to C12at least, such as C6-C7,C7-C8,C8-C9and C9-C10, etc. The detection range for sulfurand carbon containing compounds is approximately 20 to100 000 picograms (pg). This is roughly equival
5、ent to 0.04 to200 mg/m3sulfur or carbon based upon the analysis of a 0.25mL sample.1.2 This test method describes a GC-AED method employ-ing a specific capillary GC column as an illustration for naturalgas and other gaseous fuel containing low percentages ofethane and propane. Alternative GC columns
6、 and instrumentparameters may be used in this analysis optimized for differenttypes of gaseous fuel, provided that appropriate separation ofthe compounds of interest can be achieved.1.3 This test method does not intend to identify all indi-vidual sulfur species. Unknown sulfur compounds are mea-sure
7、d as mono-sulfur containing compounds. Total sulfurcontent of a sample can be found by summing up sulfur contentpresent in all sulfur species.1.4 This method is not a Detailed Hydrocarbon Analysis(DHA) method and does not intend to identify all individualhydrocarbon species. Aliphatic hydrocarbon co
8、mponentslighter than n-hexane, benzene, toluene, ethyl benzene, m,p-xylenes and o-xylene (BTEX) are generally separated andidentified individually. Higher molar mass hydrocarbons aredetermined as groups based on carbon number, excludingBTEX. The total carbon content of propane and higher molarmass c
9、omponents in a sample can be found by summing upcarbon content present in all species containing carbon.1.5 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.6 This standard does not purport to address all of thesafety concerns,
10、 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 Sampling Liquefied Petrol
11、eum (LP)Gases, Manual MethodD1945 Test Method for Analysis of Natural Gas by GasChromatographyD1946 Practice for Analysis of Reformed Gas by GasChromatographyD3609 Practice for Calibration Techniques Using Perme-ation TubesD4626 Practice for Calculation of Gas ChromatographicResponse FactorsD5287 Pr
12、actice for Automatic Sampling of Gaseous FuelsD5504 Test Method for Determination of Sulfur Compoundsin Natural Gas and Gaseous Fuels by Gas Chromatogra-phy and ChemiluminescenceD5623 Test Method for Sulfur Compounds in Light Petro-leum Liquids by Gas Chromatography and Sulfur Selec-tive DetectionD6
13、228 Test Method for Determination of Sulfur Compoundsin Natural Gas and Gaseous Fuels by Gas Chromatogra-phy and Flame Photometric DetectionE840 Practice for Using Flame Photometric Detectors in GasChromatography1This test method is under the jurisdiction ofASTM Committee D03 on GaseousFuels and is
14、the direct responsibility of Subcommittee D03.05 on Determination ofSpecial Constituents of Gaseous Fuels.Current edition approved June 1, 2015. Published July 2015. Originally approvedin 2003. Last previous edition approved in 2009 as D696803(2009). DOI:10.1520/D6968-03R15.2For referenced ASTM stan
15、dards, 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 Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, P
16、A 19428-2959. United States12.2 Other References:ISO 19739 Natural GasDetermination of Sulfur Com-pounds by Gas chromatography3GPA 2199 DeterminationSpecific Sulfur Compounds4“Improved Measurement of Sulfur and Nitrogen Com-pounds in Refinery Liquids Using Gas ChromatographyAtomic Emission Detection
17、,” Journal of Chromato-graphic Science, 36, No 9, September, 1998, p. 435.3. Terminology3.1 Abbreviations:3.1.1 Acommon abbreviation of hydrocarbon compounds isto designate the number of carbon atoms in the compound. Aprefix is used to indicate the carbon chain form, while asubscript suffix denotes
18、the number of carbon atoms (forexample, normal butane = n-C4; Iso-pentane = i-C5, aliphatichydrocarbons heavier than n-heptane but not heavier thann-octane = C7-C8).3.1.2 Sulfur compounds are commonly referred to by theirinitials (chemical or formula), for example, methyl mercaptan= MeSH, dimethyl s
19、ulfide = DMS; carbonyl sulfide = COS,di-t-butyl trisulfide = DtB-TS and tetrahydothiophene = THTor Thiophane.4. Summary of Test Method4.1 The sampling and analysis of gaseous sulfur compoundsis challenging due to the reactivity of these compounds.Samples should be collected and stored in containers
20、that arenon-reactive to sulfur compounds, such as thin silica-linedstainless steel vessels and Tedlar bags with polypropylenefittings or the equivalent. Sample containers should be filledand purged at least three times to ensure representativesampling. Laboratory equipment must also be inert, wellco
21、nditioned and passivated with a gas containing the sulfurcompounds of interest to ensure reliable results. Frequentcalibration using stable standards is required. Samples shouldbe analyzed as quickly as possible not beyond the provenstorage time after collection to minimize sample deterioration.If t
22、he stability of analyzed sulfur components is experimentallyproven, the time between collection and analysis may belengthened.4.2 A 0.25 mL sample of the fuel gas is injected into a gaschromatograph where it is passed through a 30 meter, 0.32 mmI.D., thick film, methyl silicone liquid phase, open tu
23、bularpartitioning column, or a column capable of separating thesame target sulfur and hydrocarbon components. A wider bore(0.53 mm I.D.) column may be used for better compoundseparation and/or for lower detection limits using a largerinjection volume.4.3 Atomic Emission DetectorsAll sulfur and carbo
24、n com-pounds can be detected by this technique. GC-AED hasrecently been developed for analysis of many elements, includ-ing sulfur and carbon. The AED uses a microwave inducedhelium plasma to disassociate molecules and atomize/exciteelements at high temperature (5000C). The characteristicemission li
25、nes from specific excited atoms are detected by aPhoto Diode Array detector (PDA). Sulfur emission is mea-sured at 181 nm. Carbon emission (193 and 179 nm) can bemonitored simultaneously. The amount of light emitted at eachwavelength is proportional to the concentration of sulfur orcarbon. Carbon an
26、d hydrogen emission can also be measured at498 and 486 nm, respectively, in a separate run using the sameGC procedure for additional elemental information. However,hydrogen response is not linear and a quadratic calibrationcurve must be constructed for hydrogen measurement. GC-AED offers a very high
27、 degree of selectivity and a widedynamic range for detection of various types of compound.The AED, just like the Sulfur Chemiluminescence Detector(SCD) employed in Test Method D5504 for sulfur analysis, hasthe advantage over other types of detector in that the elementalresponse is generally independ
28、ent of the structure of theassociated molecule containing the element of interest. It offersthe potential of using a single standard to calibrate theinstrument for determination of all sulfur and hydrocarboncomponents, diminishing the need of multiple standards thatmay not be commercially available
29、or that are prohibitivelyexpensive to prepare. The real-time simultaneous measurementof carbon and sulfur content by AED provides the elementalratio of carbon to sulfur for each sulfur compound, which alongwith retention time can be used to confirm the identity of sulfurcompounds. The elemental rati
30、o of carbon to hydrogen can beused to differentiate aromatic compounds from aliphatic com-pounds for identification and confirmation as well.4.4 Other DetectorsThis test method is written primarilyfor the atomic emission detector. The same GC method can beemployed with other detectors provided they
31、have sufficientsensitivity and response to all sulfur and hydrocarbon com-pounds of interest in the required measurement range. AFID-SCD combination detector may satisfy these criteria.5. Significance and Use5.1 Gaseous fuels, such as natural gas, petroleum gases andbio-gases, contain varying amount
32、s and types of sulfur com-pounds. They are generally odorous, corrosive to equipment,and can inhibit or destroy catalysts employed in gas process-ing. Their accurate measurement is essential to gas processing,operation and utilization, and may be of regulatory interest.5.2 Small amounts (typically,
33、1 to 4 ppmv) of sulfurodorants are added to natural gas and other fuel gases for safetypurposes. Some sulfur odorants can be reactive, and may beoxidized, forming more stable sulfur compounds having lowerodor thresholds. These gaseous fuels are analyzed for sulfurodorants to help in monitoring and t
34、o ensure appropriateodorant levels for public safety.5.3 This method offers a technique to determine individualsulfur species in gaseous fuel and the total sulfur content bycalculation.5.4 Gas chromatography is commonly and extensively usedto determine all components in gaseous fuels including fixed
35、3Available from International Organization for Standardization (ISO), 1, ch. dela Voie-Creuse, Case postale 56, CH-1211, Geneva 20, Switzerland, http:/www.iso.ch.4Available from Gas ProcessorsAssociation (GPA), 6526 E. 60th St., Tulsa, OK74145, http:/.D6968 03 (2015)2gas and organic components (Test
36、 Methods D1945 andD1946). Major components measured are often used for thedetermination of gas property, such as heating value andrelative density. Higher molar mass hydrocarbons are ofinterest even when present in small amounts because theirlarger impact on heating value, hydrocarbon dew point and
37、gasquality relating to gas operation, gas utilization and environ-mental impacts.6. Apparatus6.1 ChromatographAny gas chromatograph of standardmanufacture with hardware and software necessary for inter-facing to an atomic emission detector and for the intendedapplication and performance.6.1.1 Sample
38、 Inlet SystemGas samples are introduced tothe GC using an automated or manually operated non-reactivestainless steel gas sampling valve heated continuously at atemperature significantly (10C) above the temperature atwhich the gas was sampled to avoid sample condensation anddiscrimination. Inert tubi
39、ng made of non-permeable, non-sorbing and non-reactive materials, as short as possible andheat traced at the same temperature, should be employed fortransferring the sample from a sample container to the gassampling valve and to the GC inlet system. Silica-coated 316stainless steel (s.s.) tubing is
40、often employed. A fixed volume,0.25 mL, sampling loop made of the same non-reactivematerials is used to avoid possible decomposition or absorptionof reactive species. Other size fixed-volume sampling loopsmay be used for different concentration ranges. An on-columnor a split/splitless injection syst
41、em operated at the splitlessmode or at the split mode with a low split ratio may be usedwith capillary columns. One should avoid using a split linerwith a split ratio set to zero as a means of achieving splitlessinjection. A one-meter section of deactivated pre-columnattached to the front of the ana
42、lytical column is recommended.The inlet system must be well conditioned and evaluatedfrequently for compatibility with trace quantities of reactivesulfur compounds, such as tert-butyl mercaptan.6.1.2 Digital Pressure TransmitterA calibrated s.s.pressure/vacuum transducer with a digital readout may b
43、eequipped to allow sampling at different pressures to generatecalibration curves.6.1.3 Column Temperature ProgrammerThe chromato-graph must be capable of linear programmed temperatureoperation over a range of 30 to 250C, in programmed ratesettings of 0.1 to 30C/min. The programming rate must besuffi
44、ciently reproducible to obtain retention time repeatabilityof 0.05 min. (3 s) throughout the scope of this analysis.6.1.4 Carrier and Detector Gas ControlConstant flowcontrol of carrier and detector gases is critical for optimum andconsistent analytical performance. Control is best provided bythe us
45、e of pressure regulators and fixed flow restrictors. Thegas flow rate is measured by any appropriate means and therequired gas flow indicated by the use of a pressure gauge.Mass flow controllers, capable of maintaining gas flow con-stant to 6 1 % at the required flow rates can also be used. Thesuppl
46、y pressure of the gas delivered to the gas chromatographmust be at least 69 kPa (10 psig) greater than the regulated gasat the instrument to compensate for the system back pressure.In general, a supply pressure of 552 kPa (80 psig) is satisfac-tory.6.1.5 DetectorAn atomic emission detector calibrate
47、d inthe carbon and sulfur specific mode is used in this method.Other detectors capable of simultaneous measurement of sulfurand carbon as stated in 4.4 are not covered in this test method.The detector is set according to the manufacturers specifica-tions and tuned to the optimal sensitivity and sele
48、ctivity for theapplication.6.1.5.1 When sulfur and hydrocarbon compounds are de-composed in the high temperature AED zone they quantita-tively produce excited state atomic sulfur and carbon species.A diode array detector detects the light emitted from thesespecies as they relax to ground states. Car
49、bon containingcomponents are simultaneously detected at 179 and 193 nmwavelength for different sensitivity measurements extendingthe linear concentration range. Sulfur species are detected at181 nm with a high selectivity. The selectivity is normallybetter than 3104, by mass of sulfur to mass of carbon. Thedetector response is linear with respect to sulfur and carbonconcentrations. The dynamic range of this linear relationship isbetter than 1104.6.2 ColumnA30 m by 0.32 mm ID fused silica opentubular column containinga4mfilmthickness of bo
