ASTM D6968-03(2009)
(Test Method)Standard Test Method for Simultaneous Measurement of Sulfur Compounds and Minor Hydrocarbons in Natural Gas and Gaseous Fuels by Gas Chromatography and Atomic Emission Detection
Standard Test Method for Simultaneous Measurement of Sulfur Compounds and Minor Hydrocarbons in Natural Gas and Gaseous Fuels by Gas Chromatography and Atomic Emission Detection
SIGNIFICANCE AND USE
Gaseous fuels, such as natural gas, petroleum gases and bio-gases, contain varying amounts and types of sulfur compounds. They are generally odorous, corrosive to equipment, and can inhibit or destroy catalysts employed in gas processing. Their accurate measurement is essential to gas processing, operation and utilization, and may be of regulatory interest.
Small amounts (typically, 1 to 4 ppmv) of sulfur odorants are added to natural gas and other fuel gases for safety purposes. Some sulfur odorants can be reactive, and may be oxidized, forming more stable sulfur compounds having lower odor thresholds. These gaseous fuels are analyzed for sulfur odorants to help in monitoring and to ensure appropriate odorant levels for public safety.
This method offers a technique to determine individual sulfur species in gaseous fuel and the total sulfur content by calculation.
Gas chromatography is commonly and extensively used to determine all components in gaseous fuels including fixed gas and organic components (Test Methods D 1945 and D 1946). Major components measured are often used for the determination of gas property, such as heating value and relative density. Higher molar mass hydrocarbons are of interest even when present in small amounts because their larger impact on heating value, hydrocarbon dew point and gas quality relating to gas operation, gas utilization and environmental impacts.
SCOPE
1.1 This test method is for the determination of volatile sulfur-containing compounds and minor hydrocarbons in gaseous fuels including components with higher molar mass than that of propane in a high methane gas, by gas chromatography (GC) and atomic emission detection (AED). Hydrocarbons include individual aliphatic components from C4 to C6, aromatic components and groups of hydrocarbons classified according to carbon numbers up to C12 at least, such as C6-C7, C7-C8, C8-C9 and C9-C10, etc. The detection range for sulfur and carbon containing compounds is approximately 20 to 100 000 picograms (pg). This is roughly equivalent to 0.04 to 200 mg/m3 sulfur or carbon based upon the analysis of a 0.25 mL sample.
1.2 This test method describes a GC-AED method employing a specific capillary GC column as an illustration for natural gas and other gaseous fuel containing low percentages of ethane and propane. Alternative GC columns and instrument parameters may be used in this analysis optimized for different types of gaseous fuel, provided that appropriate separation of the compounds of interest can be achieved.
1.3 This test method does not intend to identify all individual sulfur species. Unknown sulfur compounds are measured as mono-sulfur containing compounds. Total sulfur content of a sample can be found by summing up sulfur content present in all sulfur species.
1.4 This method is not a Detailed Hydrocarbon Analysis (DHA) method and does not intend to identify all individual hydrocarbon species. Aliphatic hydrocarbon components lighter than n-hexane, benzene, toluene, ethyl benzene, m,p-xylenes and o-xylene (BTEX) are generally separated and identified individually. Higher molar mass hydrocarbons are determined as groups based on carbon number, excluding BTEX. The total carbon content of propane and higher molar mass components in a sample can be found by summing up carbon content present in all species containing carbon.
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: D6968 − 03(Reapproved 2009)
Standard Test Method for
Simultaneous Measurement of Sulfur Compounds and
Minor Hydrocarbons in Natural Gas and Gaseous Fuels by
Gas Chromatography and Atomic Emission Detection
This standard is issued under the fixed designation D6968; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope BTEX. The total carbon content of propane and higher molar
mass components in a sample can be found by summing up
1.1 This test method is for the determination of volatile
carbon content present in all species containing carbon.
sulfur-containing compounds and minor hydrocarbons in gas-
eous fuels including components with higher molar mass than 1.5 The values stated in SI units are to be regarded as
that of propane in a high methane gas, by gas chromatography standard. No other units of measurement are included in this
(GC) and atomic emission detection (AED). Hydrocarbons standard.
include individual aliphatic components from C to C , aro-
4 6
1.6 This standard does not purport to address all of the
matic components and groups of hydrocarbons classified
safety concerns, if any, associated with its use. It is the
according to carbon numbers up to C at least, such as C -C ,
12 6 7
responsibility of the user of this standard to establish appro-
C -C,C -C and C -C , etc. The detection range for sulfur
7 8 8 9 9 10
priate safety and health practices and determine the applica-
and carbon containing compounds is approximately 20 to
bility of regulatory limitations prior to use.
100000 picograms (pg). This is roughly equivalent to 0.04 to
200 mg/m sulfur or carbon based upon the analysis of a 0.25 2. Referenced Documents
mL sample. 2
2.1 ASTM Standards:
1.2 This test method describes a GC-AED method employ- D1265Practice for Sampling Liquefied Petroleum (LP)
ingaspecificcapillaryGCcolumnasanillustrationfornatural Gases, Manual Method
gas and other gaseous fuel containing low percentages of D1945Test Method for Analysis of Natural Gas by Gas
ethane and propane. Alternative GC columns and instrument Chromatography
parametersmaybeusedinthisanalysisoptimizedfordifferent D1946Practice for Analysis of Reformed Gas by Gas
types of gaseous fuel, provided that appropriate separation of Chromatography
the compounds of interest can be achieved. D3609Practice for Calibration Techniques Using Perme-
ation Tubes
1.3 This test method does not intend to identify all indi-
D4626Practice for Calculation of Gas Chromatographic
vidual sulfur species. Unknown sulfur compounds are mea-
Response Factors
sured as mono-sulfur containing compounds. Total sulfur
D5287Practice for Automatic Sampling of Gaseous Fuels
contentofasamplecanbefoundbysummingupsulfurcontent
D5504TestMethodforDeterminationofSulfurCompounds
present in all sulfur species.
in Natural Gas and Gaseous Fuels by Gas Chromatogra-
1.4 This method is not a Detailed Hydrocarbon Analysis
phy and Chemiluminescence
(DHA) method and does not intend to identify all individual
D5623Test Method for Sulfur Compounds in Light Petro-
hydrocarbon species. Aliphatic hydrocarbon components
leum Liquids by Gas Chromatography and Sulfur Selec-
lighter than n-hexane, benzene, toluene, ethyl benzene, m,p-
tive Detection
xylenes and o-xylene (BTEX) are generally separated and
D6228TestMethodforDeterminationofSulfurCompounds
identified individually. Higher molar mass hydrocarbons are
in Natural Gas and Gaseous Fuels by Gas Chromatogra-
determined as groups based on carbon number, excluding
phy and Flame Photometric Detection
E840PracticeforUsingFlamePhotometricDetectorsinGas
Chromatography
ThistestmethodisunderthejurisdictionofASTMCommitteeD03onGaseous
Fuels and is the direct responsibility of Subcommittee D03.05 on Determination of
Special Constituents of Gaseous Fuels. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved June 1, 2009. Published September 2009. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 2003. Last previous edition approved in 2003 as D6968–03. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D6968-03R09. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6968 − 03 (2009)
2.2 Other References: ing sulfur and carbon. The AED uses a microwave induced
ISO 19739Natural Gas—Determination of Sulfur Com- helium plasma to disassociate molecules and atomize/excite
pounds by Gas chromatography elements at high temperature (~5000°C). The characteristic
GPA 2199Determination—Specific Sulfur Compounds emission lines from specific excited atoms are detected by a
“Improved Measurement of Sulfur and Nitrogen Com- Photo Diode Array detector (PDA). Sulfur emission is mea-
poundsinRefineryLiquidsUsingGasChromatography— sured at 181 nm. Carbon emission (193 and 179 nm) can be
Atomic Emission Detection,” Journal of Chromato- monitoredsimultaneously.Theamountoflightemittedateach
graphic Science, 36, No 9, September, 1998, p. 435. wavelength is proportional to the concentration of sulfur or
carbon.Carbonandhydrogenemissioncanalsobemeasuredat
3. Terminology
498 and 486 nm, respectively, in a separate run using the same
GC procedure for additional elemental information. However,
3.1 Abbreviations:
hydrogen response is not linear and a quadratic calibration
3.1.1 Acommonabbreviationofhydrocarboncompoundsis
curve must be constructed for hydrogen measurement. GC-
to designate the number of carbon atoms in the compound. A
AED offers a very high degree of selectivity and a wide
prefix is used to indicate the carbon chain form, while a
dynamic range for detection of various types of compound.
subscript suffix denotes the number of carbon atoms (for
The AED, just like the Sulfur Chemiluminescence Detector
example, normal butane = n-C ; Iso-pentane = i-C , aliphatic
4 5
(SCD)employedinTestMethodD5504forsulfuranalysis,has
hydrocarbons heavier than n-heptane but not heavier than
theadvantageoverothertypesofdetectorinthattheelemental
n-octane = C -C ).
7 8
response is generally independent of the structure of the
3.1.2 Sulfur compounds are commonly referred to by their
associatedmoleculecontainingtheelementofinterest.Itoffers
initials (chemical or formula), for example, methyl mercaptan
the potential of using a single standard to calibrate the
= MeSH, dimethyl sulfide = DMS; carbonyl sulfide = COS,
instrument for determination of all sulfur and hydrocarbon
di-t-butyl trisulfide = DtB-TS and tetrahydothiophene = THT
components, diminishing the need of multiple standards that
or Thiophane.
may not be commercially available or that are prohibitively
4. Summary of Test Method
expensivetoprepare.Thereal-timesimultaneousmeasurement
of carbon and sulfur content by AED provides the elemental
4.1 Thesamplingandanalysisofgaseoussulfurcompounds
ratioofcarbontosulfurforeachsulfurcompound,whichalong
is challenging due to the reactivity of these compounds.
withretentiontimecanbeusedtoconfirmtheidentityofsulfur
Samples should be collected and stored in containers that are
compounds. The elemental ratio of carbon to hydrogen can be
non-reactive to sulfur compounds, such as thin silica-lined
used to differentiate aromatic compounds from aliphatic com-
stainless steel vessels and Tedlar® bags with polypropylene
pounds for identification and confirmation as well.
fittings or the equivalent. Sample containers should be filled
and purged at least three times to ensure representative 4.4 Other Detectors—This test method is written primarily
sampling. Laboratory equipment must also be inert, well
for the atomic emission detector.The same GC method can be
conditioned and passivated with a gas containing the sulfur employed with other detectors provided they have sufficient
compounds of interest to ensure reliable results. Frequent
sensitivity and response to all sulfur and hydrocarbon com-
calibration using stable standards is required. Samples should pounds of interest in the required measurement range. A
be analyzed as quickly as possible not beyond the proven
FID-SCD combination detector may satisfy these criteria.
storage time after collection to minimize sample deterioration.
Ifthestabilityofanalyzedsulfurcomponentsisexperimentally
5. Significance and Use
proven, the time between collection and analysis may be
5.1 Gaseous fuels, such as natural gas, petroleum gases and
lengthened.
bio-gases, contain varying amounts and types of sulfur com-
4.2 A0.25 mL sample of the fuel gas is injected into a gas
pounds. They are generally odorous, corrosive to equipment,
chromatographwhereitispassedthrougha30meter,0.32mm
and can inhibit or destroy catalysts employed in gas process-
I.D., thick film, methyl silicone liquid phase, open tubular
ing.Their accurate measurement is essential to gas processing,
partitioning column, or a column capable of separating the
operation and utilization, and may be of regulatory interest.
same target sulfur and hydrocarbon components.Awider bore
5.2 Small amounts (typically, 1 to 4 ppmv) of sulfur
(0.53 mm I.D.) column may be used for better compound
odorantsareaddedtonaturalgasandotherfuelgasesforsafety
separation and/or for lower detection limits using a larger
purposes. Some sulfur odorants can be reactive, and may be
injection volume.
oxidized, forming more stable sulfur compounds having lower
4.3 Atomic Emission Detectors—Allsulfurandcarboncom-
odor thresholds. These gaseous fuels are analyzed for sulfur
pounds can be detected by this technique. GC-AED has
odorants to help in monitoring and to ensure appropriate
recentlybeendevelopedforanalysisofmanyelements,includ-
odorant levels for public safety.
5.3 This method offers a technique to determine individual
3 sulfur species in gaseous fuel and the total sulfur content by
Available from International Organization for Standardization (ISO), 1, ch. de
la Voie-Creuse, Case postale 56, CH-1211, Geneva 20, Switzerland, http:// calculation.
www.iso.ch.
5.4 Gas chromatography is commonly and extensively used
AvailablefromGasProcessorsAssociation(GPA),6526E.60thSt.,Tulsa,OK
74145, http://www.gasprocessors.com. to determine all components in gaseous fuels including fixed
D6968 − 03 (2009)
gas and organic components (Test Methods D1945 and at the instrument to compensate for the system back pressure.
D1946). Major components measured are often used for the In general, a supply pressure of 552 kPa (80 psig) is satisfac-
determination of gas property, such as heating value and tory.
relative density. Higher molar mass hydrocarbons are of 6.1.5 Detector—An atomic emission detector calibrated in
interest even when present in small amounts because their the carbon and sulfur specific mode is used in this method.
largerimpactonheatingvalue,hydrocarbondewpointandgas Otherdetectorscapableofsimultaneousmeasurementofsulfur
quality relating to gas operation, gas utilization and environ- and carbon as stated in 4.4 are not covered in this test method.
mental impacts. The detector is set according to the manufacturer’s specifica-
tionsandtunedtotheoptimalsensitivityandselectivityforthe
6. Apparatus application.
6.1.5.1 When sulfur and hydrocarbon compounds are de-
6.1 Chromatograph—Any gas chromatograph of standard
composed in the high temperature AED zone they quantita-
manufacture with hardware and software necessary for inter-
tively produce excited state atomic sulfur and carbon species.
facing to an atomic emission detector and for the intended
A diode array detector detects the light emitted from these
application and performance.
species as they relax to ground states. Carbon containing
6.1.1 Sample Inlet System—Gas samples are introduced to
components are simultaneously detected at 179 and 193 nm
the GC using an automated or manually operated non-reactive
wavelength for different sensitivity measurements extending
stainless steel gas sampling valve heated continuously at a
the linear concentration range. Sulfur species are detected at
temperature significantly (~10°C) above the temperature at
181 nm with a high selectivity. The selectivity is normally
which the gas was sampled to avoid sample condensation and
better than 3×10 , by mass of sulfur to mass of carbon. The
discrimination. Inert tubing made of non-permeable, non-
detector response is linear with respect to sulfur and carbon
sorbing and non-reactive materials, as short as possible and
concentrations.Thedynamicrangeofthislinearrelationshipis
heat traced at the same temperature, should be employed for
better than 1×10 .
transferring the sample from a sample container to the gas
6.2 Column—A30 m by 0.32 mm ID fused silica open
sampling valve and to the GC inlet system. Silica-coated 316
tubular column containinga4µmfilm thickness of bonded
stainless steel (s.s.) tubing is often employed.Afixed volume,
methyl silicone liquid phase is used.The column shall provide
0.25 mL, sampling loop made of the same non-reactive
adequate retention and resolution characteristics under the
materialsisusedtoavoidpossibledecompositionorabsorption
experimental conditions described in 7.3. Other columns that
of reactive species. Other size fixed-volume sampling loops
canprovideequivalentordesirableseparationcanbeemployed
may be used for different concentration ranges.An on-column
as well. For example, a 60 m by 0.53 mm ID column with a 5
or a split/splitless injection system operated at the splitless
µm film thickness of bonded methyl silicone liquid phase can
mode or at the split mode with a low split ratio may be used
be used with a larger sample volume injection for better
with capillary columns. One should avoid using a split liner
resolution and a lower detection limit when needed.
with a split ratio set to zero as a means of achieving splitless
injection. A one-meter section of deactivated pre-column
6.3 Data Acquisition:
attached to the front of the analytical column is recommended.
6.3.1 The SRF should not exceed 10% difference for all
The inlet system must be well conditioned and evaluated
sulfur components. The CRF should not exceed 10% differ-
frequently for compatibility with trace quantities of reactive
ence for all hydrocarbon components as well. A multiple
sulfur compounds, such as tert-butyl mercaptan.
componentcalibrationstandardoracontrolstandardorsample
6.1.2 Digital Pressure Transmitter—A calibrated s.s. should be used daily to verify this.The day-to-day va
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