ASTM E697-96(2006)
(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 E 260 or Practice E 1510 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 E 355. 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 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. For specific safety information, see Section .
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Designation:E697–96(Reapproved 2006)
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 Consumer Locations
CGAP-9 The Inert Gases: Argon, Nitrogen and Helium
1.1 This practice covers the use of an electron-capture
CGAV-7 Standard Method of Determining CylinderValve
detector (ECD) as the detection component of a gas chromato-
Outlet Connections for Industrial Gas Mixtures
graphic system.
CGAP-12 Safe Handling of Cryogenic Liquids
1.2 This practice is intended to describe the operation and
HB-3 Handbook of Compressed Gases
performance of the ECD as a guide for its use in a complete
2.3 Federal Standard:
chromatographic system.
Title 10, Code of Federal Regulations, Part 20
1.3 For general gas chromatographic procedures, Practice
E260 or Practice E1510 should be followed except where
3. Hazards
specificchangesarerecommendedinthispracticeforuseofan
3.1 Gas Handling Safety—Thesafehandlingofcompressed
ECD. For a definition of gas chromatography and its various
gases and cryogenic liquids for use in chromatography is the
terms, see Practice E355. These standards also describe the
responsibility of every laboratory. The Compressed GasAsso-
performance of the detector in terms which the analyst can use
ciation (CGA), a member group of specialty and bulk gas
to predict overall system performance when the detector is
suppliers, publishes the following guidelines to assist the
coupledtothecolumnandotherchromatographiccomponents.
laboratory chemist to establish a safe work environment.
1.4 This standard does not purport to address all of the
Applicable CGA publications include: CGAP-1, CGAG-5.4,
safety concerns, if any, associated with its use. It is the
CGAP-9, CGAV-7, CGAP-12, and HB-3.
responsibility of the user of this standard to establish appro-
3.2 The electron capture detector contains a radioactive
priate safety and health practices and determine the applica-
isotope that emits b-particles into the gas flowing through the
bility of regulatory limitations prior to use. For specific safety
detector. The gas effluent of the detector must be vented to a
information, see Section 3.
fumehoodtopreventpossibleradioactivecontaminationinthe
2. Referenced Documents laboratory. Venting must conform to Title 10, Code of Federal
2 Regulations, Part 20 and Appendix B.
2.1 ASTM Standards:
E260 Practice for Packed Column Gas Chromatography
4. Principles of Electron Capture Detection
E355 Practice for Gas Chromatography Terms and Rela-
4.1 The ECD is an ionizating detector comprising a source
tionships
of thermal electrons inside a reaction/detection chamber filled
E1510 Practice for Installing Fused Silica Open Tubular
with an appropriate reagent gas. In packed column GC the
Capillary Columns in Gas Chromatographs
carrier gas generally fullfills the requirements of the reagent
2.2 CGA Standards:
gas. In capillary column GC the make-up gas acts as the
CGAP-1 Safe Handling of Compressed Gases in Contain-
3 reagent gas and also sweeps the detector volume in order to
ers
pass column eluate efficiently through the detector. While the
CGAG-5.4 Standard for Hydrogen Piping Systems at
carrier/reagent gas flows through the chamber the device
detectsthosecompoundsenteringthechamberthatarecapable
of reacting with the thermal electrons to form negative ions.
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
These electron capturing reactions cause a decrease in the
Spectrography and is the direct responsibility of Subcommittee E13.19 on Chro-
matography.
concentration of free electrons in the chamber. The detector
Current edition approved March 1, 2006. Published March 2006. Originally
response is therefore a measure of the concentration and the
approved in 1979. Last previous edition approved in 2001 as E697–96(2001).
change in concentration of electrons (1-17).
DOI: 10.1520/E0697-96R06.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on AvailablefromU.S.GovernmentPrintingOfficeSuperintendentofDocuments,
the ASTM website. 732 N. Capitol St., NW, Mail Stop: SDE, Washington, DC 20401.
3 5
Available from Compressed Gas Association (CGA), 1725 Jefferson Davis The boldface numbers in parentheses refer to a list of references at the end of
Hwy., Suite 1004, Arlington, VA 22202-4102. this practice.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E697–96 (2006)
4.2 A radioactive source inside the detector provides a group promotes the ability of the molecule to attach free
sourceof b-rays,whichinturnionizethecarriergastoproduce electrons and also may stabilize the resultant negative
a source of electrons (18). A constant or intermittent negative
molecule-ion. Examples of a few electrophores are the halo-
potential,usuallylessthan100V,isappliedacrossthereaction
gens, sulfur, phosphorus, and nitro- and a-dicarbonyl groups
chamber to collect these electrons at the anode. This flow of“
(21-25).
secondary” electrons produces a background or “standing”
4.7 A compound could also have a high electron-capture
current and is measured by a suitable electrometer-amplifier
ratewithoutcontaininganobviouselectrophoreinitsstructure,
and recording system.
or its electron-capture rate could be much greater than that due
4.3 As sample components pass through the detector, they
totheknownelectrophorethatmightbepresent.Inthesecases
combine with electrons.This causes a decrease in the standing
certain structural features, which by themselves are only
current or an increase in frequency of potential pulses depend-
weakly electrophoric, are combined so as to give the molecule
ingonthemodeofECDoperation(see5.3).Themagnitudeof
its electrophoric character. A few examples of these are the
current reduction or frequency increase is a measure of the
quinones, cyclooctatetracene, 3,17-diketosteroids, o-phthalates
concentration and electron capture rate of the compound. The
and conjugated diketones (26-32).
ECD is unique among ionizing detectors because it is this loss
4.8 Enhanced response toward certain compounds has been
in electron concentration that is measured rather than an
increase in signal. reported after the addition of either oxygen or nitrous oxide to
the carrier gas. Oxygen doping can increase the response
4.4 The two major classifications of electron-capture reac-
toward CO , certain halogenated hydrocarbons, and polycyclic
tions in the ECD are the dissociative and nondissociative
mechanisms.
aromatic compounds (33). Small amounts of nitrous oxide can
4.4.1 In the dissociative-capture mechanism, the sample increase the response toward methane, carbon dioxide, and
moleculeABreactswiththeelectronanddissociatesintoafree
hydrogen.
−
radicalandanegativeion:AB+e→A+B .Thisdissociative
4.9 While it is true that the ECD is an extremely sensitive
electron-capture reaction is favored at high detector tempera-
detector capable of picogram and even femtogram levels of
tures. Thus, an increase in noncoulometric ECD response with
detection, its response characteristics vary tremendously from
increasing detector temperature is evidence of the dissociative
one chemical class to another. Furthermore, the response
electron-capture reaction for a compound. Naturally, detect-
characteristic for a specific solute of interest can also be
ability is increased at higher detector temperatures for those
enhanced or diminished depending on the detector’s operating
compounds which undergo dissociative mechanisms.
temperature (56) (see 4.4 and 5.5). The detector’s response
4.4.2 In the nondissociative reaction, the sample molecule
characteristic to a solute is also dependent on the choice of
AB reacts with the electron and forms a molecular negative
reagent gas and since the ECD is a concentration dependent
−
ion:AB+e→AB . The cross section for electron absorption
detector, it is also dependent on the total gas flow rate through
decreases with an increase in detector temperature in the case
the detector (see 5.5). These two parameters affect both the
of the nondissociative mechanism. Consequently, the nondis-
absolute sensitivity and the linear range an ECD has to a given
sociativereactionisfavoredatlowerdetectortemperaturesand
solute. It is prudent of the operator of the ECD to understand
the noncoulometric ECD response will decrease if the detector
the influence that each of the aforementioned parameters has
temperature is increased.
on the detection of a solute of interest and, to optimize the
4.4.3 Beside the two main types of electron capture reac-
parameters prior to final testing.
tions,resonanceelectronabsorptionprocessesarealsopossible
−
in the ECD (for example, AB+e=AB ). These resonance
5. Detector Construction
reactions are characterized when an electron absorbing com-
poundexhibitsalargeincreaseinabsorptioncrosssectionover
5.1 Geometry of the Detector Cell:
a narrow range of electron energies. This is an extremely
5.1.1 Three basic types of b-ray ionization-detector geom-
temperature sensitive reaction due to the reverse reaction
etriescanbeconsideredapplicableaselectron-capturedetector
whichisathermalelectrondeactivationreaction.Forsolutesin
cells: the parallel-plate design, the concentric-tube or coaxial-
this category a maximum detector temperature is reached at
tube design, and recessed electrode or asymmetric type (34-
whichhighertemperaturesdiminishtheresponsetotheanalyte
37). The latter could be considered a variation of the
(55).
concentric-tube design. Both the plane-plate geometry and
4.5 The ECD is very selective for those compounds that
concentric geometry are used almost exclusively for pulsed
have a high electron-capture rate and the principal use of the
operation.Although the asymmetric configuration is primarily
detector is for the measurement of trace quantities of these
−9
employed in the d-c operation of electron-capture detectors, a
materials, 10 g or less. Often, compounds can be derivatized
unique version of the asymmetric design (referred to as a
by suitable reagents to provide detection of very low levels by
displaced-coaxial-cylinder geometry) has been developed for
ECD (19, 20). For applications requiring less sensitivity, other
detectors are recommended. pulse-modulated operation.The optimum mode of operation is
usually different for each detector geometry and this must be
4.6 A compound with a high electron-capture rate often
considered, where necessary, in choosing certain operating
contains an electrophoric group, that is, a highly polar moiety
that provides an electron-deficient center in the molecule.This parameters.
E697–96 (2006)
5.1.2 In general, more efficient operation is achieved if the Sincethemaximumenergyofthe b-raysfromthe Niis0.067
detector is polarized such that the gas flow is counter to the MeVand Niisamoreeffectiveradiationsourcethantritium,
flow of electrons toward the anode. In this regard, the radio- the normal Ni activity is typically 10 to 15 mCi. An
active source should be placed at the cathode or as near to it as advantage of Ni is its ability to be heated to 350°C and the
possible.
concomitant decrease in detector contamination during chro-
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-
tion of the detector. Of course, both these variables can be
5.2.2 Although the energies and the practical source
significant at the extremes, and optimum values will also
strengths for these two radioactive isotopes are different, no
depend upon other parameters of operation. In the pulsed
significant differences in the results of operation need be
operational mode, the electrons within the cell must be able to
encountered. However, optimum interelectrode distance in the
reach the anode or collector electrode during the 0.1 to 1.0-µs
detector cell is generally greater for Ni than for tritium, that
voltage pulse. Generally, electrode distances of 0.5 to 1.0 cm
is, less than 2.5 mm for tritium and 10 mm for Ni. Thus,
are acceptable and can be used optimally by the proper choice
tritium sources have the potential of greater sensitivity for
ofoperatingconditions.Cellvolumeshouldbesmallenoughto
those compounds which undergo undissociative electron at-
maintain effective electron capture without encountering other
tachment because of tritium’s higher specific activity and its
types of electron reactions and also small enough so as not to
ability to be used in a smaller volume detector. Because low
3 63
lose any resolution that may have been achieved by high-
levels of radioactive Hor Ni are released to the laboratory
resolution chromatographic systems. Typical ECD cell vol-
environment, it is a wise safety precaution to vent electron-
umes range from approximately 2 to 0.3 cm . A detector cell
capture detectors by means of hood exhaust systems.
with a relatively low internal volume is particularly important
5.3 Operational Modes:
when the ECD is used with open tubular columns. In addition
5.3.1 Three operational modes are presently available with
to the preceding electrical and chromatographic requirements,
commercial electron-capture detectors: constant-dc-voltage
theelectrodedimensionsofthedetectorarealsodeterminedby
method (41), constant-frequency method, and the constant-
the range of the particular b-rays.
current method (42-47). Within each mode of operation, there
5.2 Radioactive Source:
lies the ability to optimize performance by selective adjust-
5.2.1 Many b-ray-emitting isotopes can be used as the
ments of various ECD operational parameters. This may
primary ionization source. The two most suitable are H
include,amongotherthings,notonlythechoiceofreagentgas
(tritium) (38, 39). and Ni (40).
to be used in the ECD (see 5.4) but also setting the detector’s
5.2.1.1 Tritium—This isotope is usually coated on 302 pulsetimeconstantontheelectrometertocorrespondtothegas
stainlesssteelorHas
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