ASTM D6968-03
(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 standard. The values stated in inch-pound units are for information only.
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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Designation:D6968–03
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 D 6968; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope mass components in a sample can be found by summing up
carbon content present in all species containing carbon.
1.1 This test method is for the determination of volatile
1.5 The values stated in SI units are standard. The values
sulfur-containing compounds and minor hydrocarbons in gas-
stated in inch-pound units are for information only.
eous fuels including components with higher molar mass than
1.6 This standard does not purport to address all of the
that of propane in a high methane gas, by gas chromatography
safety concerns, if any, associated with its use. It is the
(GC) and atomic emission detection (AED). Hydrocarbons
responsibility of the user of this standard to establish appro-
include individual aliphatic components from C to C , aro-
4 6
priate safety and health practices and determine the applica-
matic components and groups of hydrocarbons classified
bility of regulatory limitations prior to use.
according to carbon numbers up to C at least, such as C -C ,
12 6 7
C -C,C -C and C -C , etc. The detection range for sulfur
7 8 8 9 9 10
2. Referenced Documents
and carbon containing compounds is approximately 20 to
2.1 ASTM Standards:
100 000 picograms (pg). This is roughly equivalent to 0.04 to
D 1265 Practice for Sampling Liquefied Petroleum Gas
200 mg/m sulfur or carbon based upon the analysis of a 0.25
(Manual Method)
mL sample.
D 1945 Test Method for Analysis of Natural Gas by Gas
1.2 This test method describes a GC-AED method employ-
Chromatography
ing a specific capillary GC column as an illustration for natural
D 3609 Practice for Calibration Techniques Using Perme-
gas and other gaseous fuel containing low percentages of
ation Tubes
ethane and propane. Alternative GC columns and instrument
D 4626 Practice for Calculation of Gas Chromatographic
parameters may be used in this analysis optimized for different
Response Factors
types of gaseous fuel, provided that appropriate separation of
D 5287 Test Method of Automatic Sampling of Gaseous
the compounds of interest can be achieved.
Fuels
1.3 This test method does not intend to identify all indi-
D 5504 Test Method for Determination of Sulfur Com-
vidual sulfur species. Unknown sulfur compounds are mea-
pounds in Natural Gas and Gaseous Fuels by Gas Chro-
sured as mono-sulfur containing compounds. Total sulfur
matography and Chemiluminescence
contentofasamplecanbefoundbysummingupsulfurcontent
D 5623 Test Method for Sulfur Compound in Light Petro-
present in all sulfur species.
leum Liquids by Gas Chromatography and Sulfur Selec-
1.4 This method is not a Detailed Hydrocarbon Analysis
tive Detection
(DHA) method and does not intend to identify all individual
D 6228 Test Method for Determination of Sulfur Com-
hydrocarbon species. Aliphatic hydrocarbon components
pounds in Natural Gas and Gaseous Fuels by Gas Chro-
lighter than n-hexane, benzene, toluene, ethyl benzene, m,p-
matography and Flame Photometric Detection
xylenes and o-xylene (BTEX) are generally separated and
E 840 Practice for UsingAtomic Emission Detectors in Gas
identified individually. Higher molar mass hydrocarbons are
Chromatography
determined as groups based on carbon number, excluding
2.2 Other References:
BTEX. The total carbon content of propane and higher molar
1 2
ThistestmethodisunderthejurisdictionofASTMCommitteeD03onGaseous For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Fuels and is the direct responsibility of Subcommittee D03.05 on Determination of contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Special Constituents of Gaseous Fuels. Standards volume information, refer to the standard’s Document Summary page on
Current edition approved Oct. 1, 2003. Published November 2003. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
D6968–03
ISO 19739 Natural Gas—Determination of Sulfur Com- elements at high temperature (~5000°C). The characteristic
pounds by Gas chromatography emission lines from specific excited atoms are detected by a
GPA 2199 Determination—Specific Sulfur Compounds Photo Diode Array detector (PDA). Sulfur emission is mea-
“Improved Measurement of Sulfur and Nitrogen Com- sured at 181 nm. Carbon emission (193 and 179 nm) can be
poundsinRefineryLiquidsUsingGasChromatography— monitored simultaneously. The amount of light emitted at each
Atomic Emission Detection,” Journal of Chromato- wavelength is proportional to the concentration of sulfur or
graphic Science, 36, No 9, September, 1998, p. 435. carbon.Carbonandhydrogenemissioncanalsobemeasuredat
498 and 486 nm, respectively, in a separate run using the same
3. Terminology
GC procedure for additional elemental information. However,
hydrogen response is not linear and a quadratic calibration
3.1 Abbreviations:
curve must be constructed for hydrogen measurement. GC-
3.1.1 Acommonabbreviationofhydrocarboncompoundsis
AED offers a very high degree of selectivity and a wide
to designate the number of carbon atoms in the compound. A
dynamic range for detection of various types of compound.
prefix is used to indicate the carbon chain form, while a
The AED, just like the Sulfur Chemiluminescence Detector
subscript suffix denotes the number of carbon atoms (for
(SCD) employed in Test Method D 5504 for sulfur analysis,
example, normal butane = n-C ; Iso-pentane = i-C , aliphatic
4 5
has the advantage over other types of detector in that the
hydrocarbons heavier than n-heptane but not heavier than
elemental response is generally independent of the structure of
n-octane = C -C ).
7 8
the associated molecule containing the element of interest. It
3.1.2 Sulfur compounds are commonly referred to by their
offers the potential of using a single standard to calibrate the
initials (chemical or formula), for example, methyl mercaptan
instrument for determination of all sulfur and hydrocarbon
= MeSH, dimethyl sulfide = DMS; carbonyl sulfide = COS,
components, diminishing the need of multiple standards that
di-t-butyl trisulfide = DtB-TS and tetrahydothiophene = THT
may not be commercially available or that are prohibitively
or Thiophane.
expensivetoprepare.Thereal-timesimultaneousmeasurement
4. Summary of Test Method of carbon and sulfur content by AED provides the elemental
ratioofcarbontosulfurforeachsulfurcompound,whichalong
4.1 Thesamplingandanalysisofgaseoussulfurcompounds
withretentiontimecanbeusedtoconfirmtheidentityofsulfur
is challenging due to the reactivity of these compounds.
compounds. The elemental ratio of carbon to hydrogen can be
Samples should be collected and stored in containers that are
used to differentiate aromatic compounds from aliphatic com-
non-reactive to sulfur compounds, such as thin silica-lined
pounds for identification and confirmation as well.
stainless steel vessels and Tedlart bags with polypropylene
4.4 Other Detectors—This test method is written primarily
fittings or the equivalent. Sample containers should be filled
for the atomic emission detector. The same GC method can be
and purged at least three times to ensure representative
employed with other detectors provided they have sufficient
sampling. Laboratory equipment must also be inert, well
sensitivity and response to all sulfur and hydrocarbon com-
conditioned and passivated with a gas containing the sulfur
pounds of interest in the required measurement range. A
compounds of interest to ensure reliable results. Frequent
FID-SCD combination detector may satisfy these criteria.
calibration using stable standards is required. Samples should
be analyzed as quickly as possible not beyond the proven
5. Significance and Use
storage time after collection to minimize sample deterioration.
5.1 Gaseous fuels, such as natural gas, petroleum gases and
Ifthestabilityofanalyzedsulfurcomponentsisexperimentally
bio-gases, contain varying amounts and types of sulfur com-
proven, the time between collection and analysis may be
pounds. They are generally odorous, corrosive to equipment,
lengthened.
and can inhibit or destroy catalysts employed in gas process-
4.2 A 0.25 mL sample of the fuel gas is injected into a gas
ing. Their accurate measurement is essential to gas processing,
chromatograph where it is passed through a 30 meter, 0.32 mm
operation and utilization, and may be of regulatory interest.
I.D., thick film, methyl silicone liquid phase, open tubular
5.2 Small amounts (typically, 1 to 4 ppmv) of sulfur
partitioning column, or a column capable of separating the
odorantsareaddedtonaturalgasandotherfuelgasesforsafety
same target sulfur and hydrocarbon components.Awider bore
purposes. Some sulfur odorants can be reactive, and may be
(0.53 mm I.D.) column may be used for better compound
oxidized, forming more stable sulfur compounds having lower
separation and/or for lower detection limits using a larger
odor thresholds. These gaseous fuels are analyzed for sulfur
injection volume.
odorants to help in monitoring and to ensure appropriate
4.3 Atomic Emission Detectors—All sulfur and carbon
odorant levels for public safety.
compounds can be detected by this technique. GC-AED has
5.3 This method offers a technique to determine individual
recentlybeendevelopedforanalysisofmanyelements,includ-
sulfur species in gaseous fuel and the total sulfur content by
ing sulfur and carbon. The AED uses a microwave induced
calculation.
helium plasma to disassociate molecules and atomize/excite
5.4 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
Available from International Organization for Standardization (ISO), 1 rue de
D 1946). Major components measured are often used for the
Varembé, Case postale 56, CH-1211, Geneva 20, Switzerland.
determination of gas property, such as heating value and
Available from Gas ProcessorsAssociation (GPA), 6526 E. 60th St.,Tulsa, OK
74145. relative density. Higher molar mass hydrocarbons are of
D6968–03
interest even when present in small amounts because their Otherdetectorscapableofsimultaneousmeasurementofsulfur
larger impact on heating value, hydrocarbon dew point and gas and carbon as stated in 4.4 are not covered in this test method.
quality relating to gas operation, gas utilization and environ- The detector is set according to the manufacturer’s specifica-
mental impacts. tions and tuned to the optimal sensitivity and selectivity for the
application.
6. Apparatus
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 3310 , 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.The dynamic range of this linear relationship is
heat traced at the same temperature, should be employed for
better than 1310 .
transferring the sample from a sample container to the gas
6.2 Column—A 30 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. A fixed 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
6.3 Data Acquisition:
injection. A one-meter section of deactivated pre-column
6.3.1 The SRF should not exceed 10 % difference for all
attached to the front of the analytical column is recommended.
sulfur components. The CRF should not exceed 10 % differ-
The inlet system must be well conditioned and evaluated
ence for all hydrocarbon components as well. A multiple
frequently for compatibility with trace quantities of reactive
component calibration standard or a control standard or sample
sulfur compounds, such as tert-butyl mercaptan.
should be used daily to verify this. The day-to-day variation of
6.1.2 Digital Pressure Transmitter—A calibrated s.s.
F should not be greater than 5 %. The detector should be
n
pressure/vacuum transducer with a digital readout may be
maintained, flow rates readjusted to optimize the detector
equipped to allow sampling at different pressures to generate
performance, and the detector should be fully recalibrated for
calibration curves.
optimal sensitivity and linearity if F
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