ASTM E697-96(2019)
(Practice)Standard Practice for Use of Electron-Capture Detectors in Gas Chromatography
Standard Practice for Use of Electron-Capture Detectors in Gas Chromatography
ABSTRACT
This practice describes the principles, construction details, operation and performance, and terms and relationships for the use of an electron-capture detector (ECD) as the detection component of a complete gas chromatographic system.
SCOPE
1.1 This practice covers the use of an electron-capture detector (ECD) as the detection component of a gas chromatographic system.
1.2 This practice is intended to describe the operation and performance of the ECD as a guide for its use in a complete chromatographic system.
1.3 For general gas chromatographic procedures, Practice E260 or Practice E1510 should be followed except where specific changes are recommended in this practice for use of an ECD. For a definition of gas chromatography and its various terms, see Practice E355. These standards also describe the performance of the detector in terms which the analyst can use to predict overall system performance when the detector is coupled to the column and other chromatographic components.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 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. For specific safety information, see Section 3.
1.6 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
Relations
Standards Content (Sample)
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.
Designation: E697 − 96 (Reapproved 2019)
Standard Practice for
Use of Electron-Capture Detectors in Gas Chromatography
This standard is issued under the fixed designation E697; 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 E355PracticeforGasChromatographyTermsandRelation-
ships
1.1 This practice covers the use of an electron-capture
E1510Practice for Installing Fused Silica Open Tubular
detector (ECD) as the detection component of a gas chromato-
Capillary Columns in Gas Chromatographs
graphic system.
2.2 CGA Standards:
1.2 This practice is intended to describe the operation and
CGAG-5.4Standard for Hydrogen Piping Systems at Con-
performance of the ECD as a guide for its use in a complete
sumer Locations
chromatographic system.
CGAP-1Standard for Safe Handling of Compressed Gases
1.3 For general gas chromatographic procedures, Practice
in Containers
E260 or Practice E1510 should be followed except where
CGAP-9The Inert Gases: Argon, Nitrogen and Helium
specificchangesarerecommendedinthispracticeforuseofan
CGAP-12 Safe Handling of Cryogenic Liquids
ECD. For a definition of gas chromatography and its various
CGAV-7Standard Method of Determining Cylinder Valve
terms, see Practice E355. These standards also describe the
Outlet Connections for Industrial Gas Mixtures
performance of the detector in terms which the analyst can use
HB-3Handbook of Compressed Gases
to predict overall system performance when the detector is
2.3 Federal Standard:
coupledtothecolumnandotherchromatographiccomponents.
Title 10Code of Federal Regulations, Part 20
1.4 The values stated in SI units are to be regarded as
3. Hazards
standard. No other units of measurement are included in this
3.1 Gas Handling Safety—The safe handling of compressed
standard.
gases and cryogenic liquids for use in chromatography is the
1.5 This standard does not purport to address all of the
responsibility of every laboratory. The Compressed GasAsso-
safety concerns, if any, associated with its use. It is the
ciation (CGA), a member group of specialty and bulk gas
responsibility of the user of this standard to establish appro-
suppliers, publishes the following guidelines to assist the
priate safety, health, and environmental practices and deter-
laboratory chemist to establish a safe work environment.
mine the applicability of regulatory limitations prior to use.
Applicable CGA publications include: CGAP-1, CGAG-5.4,
For specific safety information, see Section 3.
CGAP-9, CGAV-7, CGAP-12, and HB-3.
1.6 This international standard was developed in accor-
dance with internationally recognized principles on standard- 3.2 The electron capture detector contains a radioactive
ization established in the Decision on Principles for the isotope that emits β-particles into the gas flowing through the
Development of International Standards, Guides and Recom- detector. The gas effluent of the detector must be vented to a
mendations issued by the World Trade Organization Technical fumehoodtopreventpossibleradioactivecontaminationinthe
Barriers to Trade (TBT) Committee. laboratory. Venting must conform to Title 10, Part 20 and
Appendix B.
2. Referenced Documents
4. Principles of Electron Capture Detection
2.1 ASTM Standards:
E260Practice for Packed Column Gas Chromatography
4.1 The ECD is an ionizating detector comprising a source
of thermal electrons inside a reaction/detection chamber filled
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
with an appropriate reagent gas. In packed column GC the
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
carrier gas generally fullfills the requirements of the reagent
mittee E13.19 on Separation Science.
gas. In capillary column GC the make-up gas acts as the
Current edition approved Sept. 1, 2019. Published September 2019. Originally
approvedin1979.Lastpreviouseditionapprovedin2011asE697–96(2011).DOI:
10.1520/E0697–96R19.
2 3
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Available from Compressed Gas Association (CGA), 14501 George Carter
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Way, Suite 103, Chantilly, VA 20151, http://www.cganet.com.
Standards volume information, refer to the standard’s Document Summary page on Available from U.S. Government Publishing Office, 732 N. Capitol St., NW,
the ASTM website. Washington, DC 20401, http://www.gpo.gov.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E697 − 96 (2019)
reagent gas and also sweeps the detector volume in order to section over a narrow range of electron energies. This is an
pass column eluate efficiently through the detector. While the extremely temperature sensitive reaction due to the reverse
carrier/reagent gas flows through the chamber the device reaction which is a thermal electron deactivation reaction. For
detectsthosecompoundsenteringthechamberthatarecapable solutes in this category a maximum detector temperature is
of reacting with the thermal electrons to form negative ions. reached at which higher temperatures diminish the response to
These electron capturing reactions cause a decrease in the the analyte (19).
concentration of free electrons in the chamber. The detector
4.5 The ECD is very selective for those compounds that
response is therefore a measure of the concentration and the
have a high electron-capture rate and the principal use of the
change in concentration of electrons (1-17).
detector is for the measurement of trace quantities of these
−9
4.2 A radioactive source inside the detector provides a
materials, 10 g or less. Often, compounds can be derivatized
sourceofβ-rays,whichinturnionizethecarriergastoproduce
by suitable reagents to provide detection of very low levels by
a source of electrons (18). A constant or intermittent negative
ECD (20, 21). For applications requiring less sensitivity, other
potential,usuallylessthan100V,isappliedacrossthereaction
detectors are recommended.
chamber to collect these electrons at the anode. This flow of
4.6 A compound with a high electron-capture rate often
“secondary” electrons produces a background or “standing”
contains an electrophoric group, that is, a highly polar moiety
current and is measured by a suitable electrometer-amplifier
that provides an electron-deficient center in the molecule.This
and recording system.
group promotes the ability of the molecule to attach free
4.3 As sample components pass through the detector, they
electrons and also may stabilize the resultant negative
combine with electrons.This causes a decrease in the standing
molecule-ion. Examples of a few electrophores are the
current or an increase in frequency of potential pulses depend-
halogens, sulfur, phosphorus, and nitro- and α-dicarbonyl
ingonthemodeofECDoperation(see5.3).Themagnitudeof
groups (22-26).
current reduction or frequency increase is a measure of the
4.7 A compound could also have a high electron-capture
concentration and electron capture rate of the compound. The
ratewithoutcontaininganobviouselectrophoreinitsstructure,
ECD is unique among ionizing detectors because it is this loss
or its electron-capture rate could be much greater than that due
in electron concentration that is measured rather than an
totheknownelectrophorethatmightbepresent.Inthesecases
increase in signal.
certain structural features, which by themselves are only
4.4 The two major classifications of electron-capture reac-
weakly electrophoric, are combined so as to give the molecule
tions in the ECD are the dissociative and nondissociative
its electrophoric character. A few examples of these are the
mechanisms.
quinones, cyclooctatetracene, 3,17-diketosteroids, o-phthalates
4.4.1 In the dissociative-capture mechanism, the sample
and conjugated diketones (27-33).
moleculeABreactswiththeelectronanddissociatesintoafree
−
4.8 Enhanced response toward certain compounds has been
radicalandanegativeion:AB+e→A+B .Thisdissociative
reported after the addition of either oxygen or nitrous oxide to
electron-capture reaction is favored at high detector tempera-
the carrier gas. Oxygen doping can increase the response
tures. Thus, an increase in noncoulometric ECD response with
toward CO , certain halogenated hydrocarbons, and polycyclic
increasing detector temperature is evidence of the dissociative
aromatic compounds (34). Small amounts of nitrous oxide can
electron-capture reaction for a compound. Naturally, detect-
increase the response toward methane, carbon dioxide, and
ability is increased at higher detector temperatures for those
hydrogen.
compounds which undergo dissociative mechanisms.
4.4.2 In the nondissociative reaction, the sample molecule
4.9 While it is true that the ECD is an extremely sensitive
AB reacts with the electron and forms a molecular negative
detector capable of picogram and even femtogram levels of
−
ion:AB+e→AB . The cross section for electron absorption
detection, its response characteristics vary tremendously from
decreases with an increase in detector temperature in the case
one chemical class to another. Furthermore, the response
of the nondissociative mechanism. Consequently, the nondis-
characteristic for a specific solute of interest can also be
sociativereactionisfavoredatlowerdetectortemperaturesand
enhanced or diminished depending on the detector’s operating
the noncoulometric ECD response will decrease if the detector
temperature (35) (see 4.4 and 5.5). The detector’s response
temperature is increased.
characteristic to a solute is also dependent on the choice of
4.4.3 Beside the two main types of electron capture
reagent gas and since the ECD is a concentration dependent
reactions, resonance electron absorption processes are also
detector, it is also dependent on the total gas flow rate through
−
possible in the ECD (for example, AB+e=AB ). These
the detector (see 5.5). These two parameters affect both the
resonancereactionsarecharacterizedwhenanelectronabsorb-
absolute sensitivity and the linear range an ECD has to a given
ing compound exhibits a large increase in absorption cross
solute. It is prudent of the operator of the ECD to understand
the influence that each of the aforementioned parameters has
5 on the detection of a solute of interest and, to optimize the
The boldface numbers in parentheses refer to a list of references at the end of
this practice. parameters prior to final testing.
E697 − 96 (2019)
5. Detector Construction detector temperatures that are less than the maximum values
will lengthen the lifetimes of the tritiated sources by reducing
5.1 Geometry of the Detector Cell:
the tritium emanation rates. The newer scandium sources are
5.1.1 Three basic types of β-ray ionization-detector geom-
more effective at minimizing the contamination problems
etriescanbeconsideredapplicableaselectron-capturedetector
associated with electron-capture detectors because of their
cells: the parallel-plate design, the concentric-tube or coaxial-
capability for operation at 325°C. Furthermore, the tritiated-
tube design, and recessed electrode or asymmetric type (36-
scandium source displays a factor-of-three detectability in-
39). The latter could be considered a variation of the
crease for dissociative electron-capturing species, that is,
concentric-tube design. Both the plane-plate geometry and
halogenated molecules.Another advantage of scandium tritide
concentric geometry are used almost exclusively for pulsed
sources is their availability at much higher specific activities
operation.Although the asymmetric configuration is primarily
than nickel-63 sources; therefore, Sc H sources are smaller
employed in the d-c operation of electron-capture detectors, a
and permit the construction of detector cells with smaller
unique version of the asymmetric design (referred to as a
internal volumes. The maximum energy of the β-rays emitted
displaced-coaxial-cylinder geometry) has been developed for
by tritium is 0.018 MeV.
pulse-modulated operation.The optimum mode of operation is
5.2.1.2 Nickel-63 ( Ni)—This radioactive isotope is usu-
usually different for each detector geometry and this must be
ally either electroplated directly on a gold foil in the detector
considered, where necessary, in choosing certain operating
cell or is plated directly onto the interior of the cell block.
parameters.
Sincethemaximumenergyoftheβ-raysfromthe Niis0.067
5.1.2 In general, more efficient operation is achieved if the
MeVand Niisamoreeffectiveradiationsourcethantritium,
detector is polarized such that the gas flow is counter to the
the normal Ni activity is typically 10 to 15 mCi. An
flow of electrons toward the anode. In this regard, the radio-
advantage of Ni is its ability to be heated to 350°C and the
active source should be placed at the cathode or as near to it as
concomitant decrease in detector contamination during chro-
possible.
matographic operation.Another advantage of the high detector
5.1.3 Other geometric factors that affect cell response and
temperatures available with Ni is an enhanced sensitivity for
operation are cell volume and electrode spacing, which may or
compounds that undergo dissociative electron capture.
may not be altered concurrently depending upon the construc-
5.2.2 Although the energies and the practical source
tion of the detector. Of course, both these variables can be
strengths for these two radioactive isotopes are different, no
significant at the extremes, and optimum values will also
significant differences in the results of operation need be
depend upon other parameters of operation. In the pulsed
encountered. However, optimum interelectrode distance in the
operational mode, the electrons within the cell must be able to
detector cell is generally greater for Ni than for tritium, that
reach the anode or collector electrode during the 0.1 to 1.0 µs
is, less than 2.5 mm for tritium and 10 mm for Ni. Thus,
voltage pulse. Generally, electrode distances of 0.5 to 1.0 cm
tritium sources have the potential of greater sensitivity for
are acceptable and can be used optimally by the proper choice
those compounds which undergo undissociative electron at-
ofoperatingconditions.Cellvolumeshouldbesmallenoughto
tachment because of tritium’s
...
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