ASTM D7493-14(2018)

Standard Test Method for Online Measurement of Sulfur Compounds in Natural Gas and Gaseous Fuels by Gas Chromatograph and Electrochemical Detection

SIGNIFICANCE AND USE
5.1 Gaseous fuels, such as natural gas, petroleum gases and bio-gases, contain sulfur compounds that are naturally occurring or that are added as odorants for safety purposes. These sulfur compounds are odorous, corrosive to equipment, and can inhibit or destroy catalysts employed in gas processing and other end uses. Their accurate continuous measurement is important to gas processing, operation and utilization, and is frequently of regulatory interest.  
5.2 Small amounts (typically, total of 4 to 6 ppmv) of sulfur odorants are added to natural gas and other fuel gases for safety purposes. Some sulfur odorants are reactive, and may be oxidized to form more stable sulfur compounds having lower odor thresholds which adversely impact the potential safety of the gas delivery systems and gas users. Gaseous fuels are analyzed for sulfur compounds and odorant levels to assist in pipeline integrity surveillance and to ensure appropriate odorant levels for public safety.  
5.3 This method offers an on-line technique to continuously identify and quantify individual target sulfur species in gaseous fuel with automatic calibration and validation.
SCOPE
1.1 This test method is for on-line measurement of volatile sulfur-containing compounds in gaseous fuels by gas chromatography (GC) and electrochemical (EC) detection. This test method is applicable to hydrogen sulfide, C1 to C4 mercaptans, sulfides and tetrahydrothiophene (THT).  
1.1.1 Carbonyl sulfide (COS) is not covered in this test method.  
1.1.2 The detection range for sulfur compounds is approximately from 0.1 to 100 ppmv (mL/m3) or 0.1 to 100 mg/m3. The detection range may vary depending on the sample injection volume, chromatographic peak separation and the sensitivity to the specific EC detector.  
1.2 This test method describes a GC-EC method employing packed GC columns and a specific detector for natural gas and other gaseous fuel composed of mainly light (C4 and smaller) hydrocarbons. Alternative GC columns, detector designs and instrument parameters may be used, provided that chromatographic separation, quality control and measurement objectives needed to comply with user, or regulator needs or both, are achieved.  
1.3 This test method does not intend to identify and measure all individual sulfur species, and is mainly employed for monitoring naturally occurring reduced sulfur compounds commonly found in natural gas and fuel gases or employed as an odorant in these gases.  
1.4 The test method is typically employed in repetitive or continuous on-line monitoring of sulfur components in natural gas and fuel gases using a single sulfur calibration standard. Need for a multipoint calibration curve or quality control procedures can be satisfied by making use of procedures delineated in Test Methods D5504, D5623, D6228, D6968, ISO 19739, or GPA 2199.  
1.5 The test method can be used for measurement of all sulfur compounds listed in Table 1 in air or other gaseous matrices, provided that no compounds that can interfere with the GC separation and electrochemical detection are present.  
1.6 This test method is written as a companion to Practices D5287, D7165 and D7166.  
1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.8 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.9 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Historical
• Publication Date: 30-Jun-2018
• ICS: 75.160.30 - Gaseous fuels
• Technical Committee: D03 - Gaseous Fuels
• Drafting Committee: D03.12 - On-Line/At-Line Analysis of Gaseous Fuels

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: D7493 − 14 (Reapproved 2018)
Standard Test Method for
Online Measurement of Sulfur Compounds in Natural Gas
and Gaseous Fuels by Gas Chromatograph and
Electrochemical Detection
This standard is issued under the fixed designation D7493; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope 1.5 The test method can be used for measurement of all
sulfur compounds listed in Table 1 in air or other gaseous
1.1 This test method is for on-line measurement of volatile
matrices, provided that no compounds that can interfere with
sulfur-containing compounds in gaseous fuels by gas chroma-
the GC separation and electrochemical detection are present.
tography (GC) and electrochemical (EC) detection. This test
method is applicable to hydrogen sulfide, C1 to C4 mercaptans, 1.6 This test method is written as a companion to Practices
sulfides and tetrahydrothiophene (THT). D5287, D7165 and D7166.
1.1.1 Carbonyl sulfide (COS) is not covered in this test
1.7 The values stated in SI units are to be regarded as
method.
standard. No other units of measurement are included in this
1.1.2 The detection range for sulfur compounds is approxi-
standard.
3 3
mately from 0.1 to 100 ppmv (mL/m ) or 0.1 to 100 mg/m .
1.8 This standard does not purport to address all of the
The detection range may vary depending on the sample
safety concerns, if any, associated with its use. It is the
injection volume, chromatographic peak separation and the
responsibility of the user of this standard to establish appro-
sensitivity to the specific EC detector.
priate safety, health, and environmental practices and deter-
1.2 This test method describes a GC-EC method employing
mine the applicability of regulatory limitations prior to use.
packed GC columns and a specific detector for natural gas and
1.9 This international standard was developed in accor-
other gaseous fuel composed of mainly light (C4 and smaller)
dance with internationally recognized principles on standard-
hydrocarbons. Alternative GC columns, detector designs and
ization established in the Decision on Principles for the
instrument parameters may be used, provided that chromato-
Development of International Standards, Guides and Recom-
graphic separation, quality control and measurement objectives
mendations issued by the World Trade Organization Technical
needed to comply with user, or regulator needs or both, are
Barriers to Trade (TBT) Committee.
achieved.
1.3 This test method does not intend to identify and measure
2. Referenced Documents
all individual sulfur species, and is mainly employed for
2.1 ASTM Standards:
monitoring naturally occurring reduced sulfur compounds
D3609 Practice for Calibration Techniques Using Perme-
commonly found in natural gas and fuel gases or employed as
ation Tubes
an odorant in these gases.
D4150 Terminology Relating to Gaseous Fuels
1.4 The test method is typically employed in repetitive or
D4626 Practice for Calculation of Gas Chromatographic
continuous on-line monitoring of sulfur components in natural
Response Factors
gas and fuel gases using a single sulfur calibration standard.
D5287 Practice for Automatic Sampling of Gaseous Fuels
Need for a multipoint calibration curve or quality control
D5504 Test Method for Determination of Sulfur Compounds
procedures can be satisfied by making use of procedures
in Natural Gas and Gaseous Fuels by Gas Chromatogra-
delineated in Test Methods D5504, D5623, D6228, D6968,
phy and Chemiluminescence
ISO 19739, or GPA 2199.
D5623 Test Method for Sulfur Compounds in Light Petro-
leum Liquids by Gas Chromatography and Sulfur Selec-
tive Detection
This test method is under the jurisdiction of ASTM Committee D03 on Gaseous
Fuels and is the direct responsibility of Subcommittee D03.12 on On-Line/At-Line
Analysis of Gaseous Fuels. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved July 1, 2018. Published July 2018. Originally approved contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
in 2008. Last previous edition approved in 2014 as D7493-14. DOI: 10.1520/ Standards volume information, refer to the standard’s Document Summary page on
D7493-14R18. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7493 − 14 (2018)
TABLE 1 Typical Retention Times of Sulfur Components of Different GC-ECD Runs
GC-EC instrument GC-EC #1 GC-EC #2 GC-EC #3
GC-Column and ⁄8 in. ID× 70 cm L, 1.6 mm ID× 1200 mm L, 4 mm ID× 400 mm L,
parameters N , 12 mL/min, 65 °C N , 100 mL/min, 20 °C N , 100 mL/min, 20 °C
2 2 2
Detector Size 5×20 mm 5×20 mm 30×25 mm
Sulfur Compound RT (sec.) RT (sec.) RT (sec.)
Hydrogen sulfide, H S 30 30 30
Methyl mercaptan (MeSH) 70 66 60
Ethyl mercaptan (EtSH) 105 150 80
Dimethyl sulfide (DMS) 120 200 80
i-Propyl mercaptan (IPM) 160 240 160
t-Butyl mercaptan (TBM) 220 342 240
n-Propyl mercaptan (NPM) 265 426 290
i-Butyl mercaptan (IBM) 440 . 560
n-Butyl mercaptan (NBM) 585 . .
A A
Thiophane (THT) 900 720 2100
A
The shorter GC column is employed for simultaneous accelerated analysis of THT.
D6228 Test Method for Determination of Sulfur Compounds separated by a GC column and measured by an EC detector.
in Natural Gas and Gaseous Fuels by Gas Chromatogra- The method requires periodic calibration using certified stan-
phy and Flame Photometric Detection dards. The test method conforms to the practices stated in
D6968 Test Method for Simultaneous Measurement of Sul- Practice D7165.
fur Compounds and Minor Hydrocarbons in Natural Gas
4.2 A fixed volume of the sample (normally 0.25 mL) is
and Gaseous Fuels by Gas Chromatography and Atomic
injected into a gas chromatograph operating isothermally
Emission Detection
where components are separated using two chromatographic
D7165 Practice for Gas Chromatograph Based On-line/At-
columns.
line Analysis for Sulfur Content of Gaseous Fuels
4.3 GC-separated sulfur compounds are determined using
D7166 Practice for Total Sulfur Analyzer Based On-line/At-
an electrochemical detector utilizing a chromic acid electro-
line for Sulfur Content of Gaseous Fuels
lyte.
2.2 ISO Standards:
ISO 19739 Natural gas – Determination of sulfur com-
5. Significance and Use
pounds by gas chromatography
4 5.1 Gaseous fuels, such as natural gas, petroleum gases and
2.3 GPA Standard
bio-gases, contain sulfur compounds that are naturally occur-
GPA 2199 Determination - Determination of Specific Sulfur
ring or that are added as odorants for safety purposes. These
Compounds by Capillary Gas Chromatography and Sulfur
sulfur compounds are odorous, corrosive to equipment, and can
Chemiluminescence Detection
inhibit or destroy catalysts employed in gas processing and
other end uses. Their accurate continuous measurement is
3. Terminology
important to gas processing, operation and utilization, and is
3.1 Common terminology used in this method are cited in
frequently of regulatory interest.
Terminology D4150. Sulfur compounds are commonly re-
5.2 Small amounts (typically, total of 4 to 6 ppmv) of sulfur
ferred by their initials (chemical or formula), for example,
odorants are added to natural gas and other fuel gases for safety
3.2 Abbreviations:
purposes. Some sulfur odorants are reactive, and may be
oxidized to form more stable sulfur compounds having lower
hydrogen sulfide = H S
methyl mercaptan = MeSH (MM) odor thresholds which adversely impact the potential safety of
ethyl mercaptan = EtSH (EM) the gas delivery systems and gas users. Gaseous fuels are
dimethyl sulfide = DMS
analyzed for sulfur compounds and odorant levels to assist in
i-Propyl mercaptan = IPM
pipeline integrity surveillance and to ensure appropriate odor-
n-Propyl mercaptan = NPM
ant levels for public safety.
t-Butyl mercaptan = TBM
5.3 This method offers an on-line technique to continuously
tetrahydrothiophene = THT or Thiophane
identify and quantify individual target sulfur species in gaseous
fuel with automatic calibration and validation.
4. Summary of Test Method
4.1 Gaseous fuel is directly sampled on-line for analysis of
6. Apparatus
sulfur compounds. Samples are introduced to the GC instru-
6.1 Chromatograph—Industrial gas chromatograph with an
ment through a sampling system. Sulfur compounds are
isothermal oven, automatic injection valve, and software nec-
essary for interfacing to a chromic acid electrochemical detec-
Available from International Organization for Standardization (ISO), 1, ch. de
tor and designed for the intended application. The GC system
la Voie-Creuse, Case postale 56, CH-1211, Geneva 20, Switzerland, http://
must be inert, well-conditioned and passivated with a gas
www.iso.ch.
4 containing the sulfur compounds of interest to ensure reliable
Available from Gas Processors Association (GPA), 6526 E. 60th St., Tulsa, OK
74145, http://www.gasprocessors.com. results.
D7493 − 14 (2018)
6.1.1 Sample Inlet System—The gas sample is introduced to by the use of pressure regulators and fixed flow restrictors. The
the GC by sample loop injection. An automated non-reactive gas flow rate is measured using a gas flow meter either
gas sampling valve is employed for a fixed sample loop volumetrically or based upon mass flow rates. Mass flow
injection. The sample injection port must be heated continu- controllers, capable of maintaining gas flow constant to 61 %
ously at a temperature significantly (~10 °C) above the tem- at the required flow rates should be used. The supply pressure
perature at which the gas was sampled to avoid sample of the gas delivered to the gas chromatograph must be at least
condensation and discrimination. Inert tubing made of non- 69 kPa (10 psi) greater than the regulated gas at the instrument
permeable, non-sorbing and non-reactive materials, as short as to compensate for system back pressure.
possible and heat traced at the same temperature, should be 6.1.4 Detector—An EC detector, whose operation is based
employed for transferring the sample from a sample source to upon the reduction/oxidation reaction between reduced sulfur
the gas sampling valve and to the GC inlet system. Silica- compounds and a solution of chromic acid (Fig. 1), is used in
coated 316 stainless steel (s.s.) and non-permeable polytetra- this method. The detector is set according to the manufacturer’s
fluoroethylene (PTFE) type tubing are often employed. Differ- specifications for this particular application. One EC detector is
ent size fixed-volume sample loops (0.25 to 10.0 mL) may be normally employed for measurement. A second column is
used to target multiple concentration ranges for components in employed for detection of late-eluting sulfur compounds, such
a gas, provided chromatographic separation and quality control as THT.
objectives are obtained. The same non-reactive materials are 6.1.4.1 The detector consists of a glass or methyl polymeth-
used for the sample loop to avoid possible decomposition or acrylate container. The electrodes, two pieces of platinum
absorption of reactive species. The sampling and GC inlet gauze grids, are arranged vertically in parallel, and are welded
system must be well conditioned and evaluated frequently for in a borosilicate glass tube. These grids are isolated from each
compatibility with trace quantities of reactive sulfur other and other conductive materials and connected to an
compounds, such as tert-butyl mercaptan. A programmable and amplifier for data acquisition.
computer-controlled multi-stream sample selector can be used 6.1.4.2 The electrolyte, a solution of chromium (VI) oxide
to sample fuel gases and calibration gases. in distilled or deionized water (100 g/L or 0.66 mole/L), is
6.1.2 Column Temperature—The gas chromatograph must contained in an acid-resistant vessel. A tube fitted with the
be capable of maintaining an isothermal temperature, normally electrodes is dipped into the solution such that the liquid is
at 65 °C, with temperature variation not exceeding 60.5 °C. retained by capillary action within the tube at a level approxi-
6.1.3 Carrier and Detector Gas Control—Constant flow mately midway between two grids.
control of carrier and detector gases is necessary for optimum 6.1.4.3 The gas flow from the GC column is discharged
and consistent analytical performance. Control is best provided through a narrow glass or PTFE tube (2 mm ID) immediately
FIG. 1 Typical Electrochemical Detection Cell
D7493 − 14 (2018)
above the upper grid center (normally 5 mm). Each sulfur non-mandatory protocol for compressed gas standards cited in
compound sequentially elutes and reacts with chromic acid. Appendix X1 of Test Method D5504 can be used to ensure the
Possible reaction mechanisms are illustrated as Eq 1 and Eq 2. quality of standards and to establish traceability to a National
The redox reaction occurs on the electrode surface, creating a Institute of Standards and Technology (NIST) or other national
potential difference between the two electrodes, thus causing a metrology institute (NMI) standard reference material.
current to be measured (using a low resistance measuring 7.1.2 Multiple sulfur gas standard mixes should be used as
circuit). For example, t-butyl mercaptan is oxidized to t-butyl recommended by a compressed gas standard manufacturer to
sulfoxide and chromium oxide (Eq 2). assure the long term stability of sulfur components. The
standard should by re-certified as per manufacturers’ recom-
2 CrO 12 R 2 SH→2 RS5 O1Cr O (1)
3 2 3
mendations or as needed for regulatory compliance.
where:
7.1.3 Compressed Gas Standard Delivery System—Pressure
R = organic moieties, such as CxHy regulators, gas lines and fittings must be inert, appropriate for
the delivery of sulfur gases and well passivated.
2 CrO 12 C H 2 SH→2 C H 2 SO1Cr O 1H O (2)
3 4 9 4 9 2 3 2
7.2 Sulfur Permeation Standards—Gaseous standards gen-
6.2 Column—1200 mm of 1.6 mm ID glass or PTFE tubing
erated from individual or a combination of certified permeation
packed with 150 to 180 um (80 to 100 mesh) Chromosorb W
tubes and devices at a constant temperature (60.1 °C) and a
support has been successfully used in performance of this test
constant flow rate can be used for calibrations. The standard
method. However, other columns that provide adequate reten-
concentration is calculated by mass loss at a fixed temperature
tion and resolution characteristics under
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D7493 − 14 D7493 − 14 (Reapproved 2018)
Standard Test Method for
Online Measurement of Sulfur Compounds in Natural Gas
and Gaseous Fuels by Gas Chromatograph and
Electrochemical Detection
This standard is issued under the fixed designation D7493; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This test method is for on-line measurement of volatile sulfur-containing compounds in gaseous fuels by gas
chromatography (GC) and electrochemical (EC) detection. This test method is applicable to hydrogen sulfide, C1 to C4
mercaptans, sulfides and tetrahydrothiophene (THT).
1.1.1 Carbonyl sulfide (COS) is not covered in this test method.
3 3
1.1.2 The detection range for sulfur compounds is approximately from 0.1 to 100 ppmv (mL/m ) or 0.1 to 100 mg/m . The
detection range may vary depending on the sample injection volume, chromatographic peak separation and the sensitivity to the
specific EC detector.
1.2 This test method describes a GC-EC method employing packed GC columns and a specific detector for natural gas and other
gaseous fuel composed of mainly light (C4 and smaller) hydrocarbons. Alternative GC columns, detector designs and instrument
parameters may be used, provided that chromatographic separation, quality control and measurement objectives needed to comply
with user, or regulator needs or both, are achieved.
1.3 This test method does not intend to identify and measure all individual sulfur species, and is mainly employed for
monitoring naturally occurring reduced sulfur compounds commonly found in natural gas and fuel gases or employed as an odorant
in these gases.
1.4 The test method is typically employed in repetitive or continuous on-line monitoring of sulfur components in natural gas
and fuel gases using a single sulfur calibration standard. Need for a multipoint calibration curve or quality control procedures can
be satisfied by making use of procedures delineated in Test Methods D5504, D5623, D6228, D6968, ISO 19739, or GPA 2199.
1.5 The test method can be used for measurement of all sulfur compounds listed in Table 1 in air or other gaseous matrices,
provided that no compounds that can interfere with the GC separation and electrochemical detection are present.
1.6 This test method is written as a companion to Practices D5287, D7165 and D7166.
1.7 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.8 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 safety, health, and healthenvironmental practices and determine the
applicability of regulatory limitations prior to use.
1.9 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D3609 Practice for Calibration Techniques Using Permeation Tubes
D4150 Terminology Relating to Gaseous Fuels
This test method is under the jurisdiction of ASTM Committee D03 on Gaseous Fuels and is the direct responsibility of Subcommittee D03.12 on On-Line/At-Line
Analysis of Gaseous Fuels.
Current edition approved June 1, 2014July 1, 2018. Published July 2014July 2018. Originally approved in 2008. Last previous edition approved in 2014 as D7493-08.-14.
DOI: 10.1520/D7493-14.10.1520/D7493-14R18.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D7493 − 14 (2018)
TABLE 1 Typical Retention Times of Sulfur Components of Different GC-ECD Runs
GC-EC instrument GC-EC #1 GC-EC #2 GC-EC #3
GC-Column and ⁄8 in. ID× 70 cm L, 1.6 mm ID× 1200 mm L, 4 mm ID× 400 mm L,
parameters N , 12 mL/min, 65 °C N , 100 mL/min, 20 °C N , 100 mL/min, 20 °C
2 2 2
Detector Size 5×20 mm 5×20 mm 30×25 mm
Sulfur Compound RT (sec.) RT (sec.) RT (sec.)
Hydrogen sulfide, H S 30 30 30
Methyl mercaptan (MeSH) 70 66 60
Ethyl mercaptan (EtSH) 105 150 80
Dimethyl sulfide (DMS) 120 200 80
i-Propyl mercaptan (IPM) 160 240 160
t-Butyl mercaptan (TBM) 220 342 240
n-Propyl mercaptan (NPM) 265 426 290
i-Butyl mercaptan (IBM) 440 . 560
n-Butyl mercaptan (NBM) 585 . .
A A
Thiophane (THT) 900 720 2100
A
The shorter GC column is employed for simultaneous accelerated analysis of THT.
D4626 Practice for Calculation of Gas Chromatographic Response Factors
D5287 Practice for Automatic Sampling of Gaseous Fuels
D5504 Test Method for Determination of Sulfur Compounds in Natural Gas and Gaseous Fuels by Gas Chromatography and
Chemiluminescence
D5623 Test Method for Sulfur Compounds in Light Petroleum Liquids by Gas Chromatography and Sulfur Selective Detection
D6228 Test Method for Determination of Sulfur Compounds in Natural Gas and Gaseous Fuels by Gas Chromatography and
Flame Photometric Detection
D6968 Test Method for Simultaneous Measurement of Sulfur Compounds and Minor Hydrocarbons in Natural Gas and Gaseous
Fuels by Gas Chromatography and Atomic Emission Detection
D7165 Practice for Gas Chromatograph Based On-line/At-line Analysis for Sulfur Content of Gaseous Fuels
D7166 Practice for Total Sulfur Analyzer Based On-line/At-line for Sulfur Content of Gaseous Fuels
2.2 ISO Standards:
ISO 19739 Natural gas – Determination of sulfur compounds by gas chromatography
2.3 GPA Standard
GPA 2199 Determination - Determination of Specific Sulfur Compounds by Capillary Gas Chromatography and Sulfur
Chemiluminescence Detection
3. Terminology
3.1 Common terminology used in this method are cited in Terminology D4150. Sulfur compounds are commonly referred by
their initials (chemical or formula), for example,
3.2 Abbreviations:
hydrogen sulfide = H S
methyl mercaptan = MeSH (MM)
ethyl mercaptan = EtSH (EM)
dimethyl sulfide = DMS
i-Propyl mercaptan = IPM
n-Propyl mercaptan = NPM
t-Butyl mercaptan = TBM
tetrahydrothiophene = THT or Thiophane
4. Summary of Test Method
4.1 Gaseous fuel is directly sampled on-line for analysis of sulfur compounds. Samples are introduced to the GC instrument
through a sampling system. Sulfur compounds are separated by a GC column and measured by an EC detector. The method
requires periodic calibration using certified standards. The test method conforms to the practices stated in Practice D7165.
4.2 A fixed volume of the sample (normally 0.25 mL) is injected into a gas chromatograph operating isothermally where
components are separated using two chromatographic columns.
4.3 GC-separated sulfur compounds are determined using an electrochemical detector utilizing a chromic acid electrolyte.
Available from International Organization for Standardization (ISO), 1, ch. de la Voie-Creuse, Case postale 56, CH-1211, Geneva 20, Switzerland, http://www.iso.ch.
Available from Gas Processors Association (GPA), 6526 E. 60th St., Tulsa, OK 74145, http://www.gasprocessors.com.
D7493 − 14 (2018)
5. Significance and Use
5.1 Gaseous fuels, such as natural gas, petroleum gases and bio-gases, contain sulfur compounds that are naturally occurring
or that are added as odorants for safety purposes. These sulfur compounds are odorous, corrosive to equipment, and can inhibit
or destroy catalysts employed in gas processing and other end uses. Their accurate continuous measurement is important to gas
processing, operation and utilization, and is frequently of regulatory interest.
5.2 Small amounts (typically, total of 4 to 6 ppmv) of sulfur odorants are added to natural gas and other fuel gases for safety
purposes. Some sulfur odorants are reactive, and may be oxidized to form more stable sulfur compounds having lower odor
thresholds which adversely impact the potential safety of the gas delivery systems and gas users. Gaseous fuels are analyzed for
sulfur compounds and odorant levels to assist in pipeline integrity surveillance and to ensure appropriate odorant levels for public
safety.
5.3 This method offers an on-line technique to continuously identify and quantify individual target sulfur species in gaseous fuel
with automatic calibration and validation.
6. Apparatus
6.1 Chromatograph—Industrial gas chromatograph with an isothermal oven, automatic injection valve, and software necessary
for interfacing to a chromic acid electrochemical detector and designed for the intended application. The GC system must be inert,
well-conditioned and passivated with a gas containing the sulfur compounds of interest to ensure reliable results.
6.1.1 Sample Inlet System—The gas sample is introduced to the GC by sample loop injection. An automated non-reactive gas
sampling valve is employed for a fixed sample loop injection. The sample injection port must be heated continuously at a
temperature significantly (~10 °C) above the temperature at which the gas was sampled to avoid sample condensation and
discrimination. Inert tubing made of non-permeable, non-sorbing and non-reactive materials, as short as possible and heat traced
at the same temperature, should be employed for transferring the sample from a sample source to the gas sampling valve and to
the GC inlet system. Silica-coated 316 stainless steel (s.s.) and non-permeable polytetrafluoroethylene (PTFE) type tubing are often
employed. Different size fixed-volume sample loops (0.25 to 10.0 mL) may be used to target multiple concentration ranges for
components in a gas, provided chromatographic separation and quality control objectives are obtained. The same non-reactive
materials are used for the sample loop to avoid possible decomposition or absorption of reactive species. The sampling and GC
inlet system must be well conditioned and evaluated frequently for compatibility with trace quantities of reactive sulfur
compounds, such as tert-butyl mercaptan. A programmable and computer-controlled multi-stream sample selector can be used to
sample fuel gases and calibration gases.
6.1.2 Column Temperature—The gas chromatograph must be capable of maintaining an isothermal temperature, normally at
65 °C, with temperature variation not exceeding 60.5 °C.
6.1.3 Carrier and Detector Gas Control—Constant flow control of carrier and detector gases is necessary for optimum and
consistent analytical performance. Control is best provided by the use of pressure regulators and fixed flow restrictors. The gas flow
rate is measured using a gas flow meter either volumetrically or based upon mass flow rates. Mass flow controllers, capable of
maintaining gas flow constant to 61 % at the required flow rates should be used. The supply pressure of the gas delivered to the
gas chromatograph must be at least 69 kPa (10 psi) greater than the regulated gas at the instrument to compensate for system back
pressure.
6.1.4 Detector—An EC detector, whose operation is based upon the reduction/oxidation reaction between reduced sulfur
compounds and a solution of chromic acid (Fig. 1), is used in this method. The detector is set according to the manufacturer’s
specifications for this particular application. One EC detector is normally employed for measurement. A second column is
employed for detection of late-eluting sulfur compounds, such as THT.
6.1.4.1 The detector consists of a glass or methyl polymethacrylate container. The electrodes, two pieces of platinum gauze
grids, are arranged vertically in parallel, and are welded in a borosilicate glass tube. These grids are isolated from each other and
other conductive materials and connected to an amplifier for data acquisition.
6.1.4.2 The electrolyte, a solution of chromium (VI) oxide in distilled or deionized water (100 g/L or 0.66 mole/L), is contained
in an acid-resistant vessel. A tube fitted with the electrodes is dipped into the solution such that the liquid is retained by capillary
action within the tube at a level approximately midway between two grids.
6.1.4.3 The gas flow from the GC column is discharged through a narrow glass or PTFE tube (2 mm ID) immediately above
the upper grid center (normally 5 mm). Each sulfur compound sequentially elutes and reacts with chromic acid. Possible reaction
mechanisms are illustrated as Eq 1 and Eq 2. The redox reaction occurs on the electrode surface, creating a potential difference
between the two electrodes, thus causing a current to be measured (using a low resistance measuring circuit). For example, t-butyl
mercaptan is oxidized to t-butyl sulfoxide and chromium oxide (Eq 2).
2 CrO 12 R 2 SH→2 RS 5 O1Cr O (1)
3 2 3
where:
R = organic moieties, such as CxHy
2 CrO 12 C H 2 SH→2 C H 2 SO1Cr O 1H O (2)
3 4 9 4 9 2 3 2
D7493 − 14 (2018)
FIG. 1 Typical Electrochemical Detection Cell
6.2 Column—1200 mm of 1.6 mm ID glass or PTFE tubing packed with 150 to 180 um (80 to 100 mesh) Chromosorb W support
has been successfully used in performance of this test method. However, other columns that provide adequate retention and
resolution characteristics under the experimental conditions as described in 8.1 can be used. A second GC column of the same ID
and phase, but of a shorter length, can be employed for faster measurement of late-eluting sulfur compounds such as THT. In this
case, two columns are connected to the GC injection system using a 10-port valve to direct sample flow through the appropriate
column and then onto the EC detector; thus, allowing measurement of low molecular weight sulfur gases and high molecular
weight sulfur gases such as THT from a single sample injection. The elution of high molecular weight sulfur gases such as THT
may also be accelerated by increased carrier gas flow rate after the elution of TBM. When samples may contain high boiling or
instrument damaging substances, a backflush column may be employed to remove these materials b
...