ASTM E697-96(2001)
(Practice)Standard Practice for Use of Electron-Capture Detectors in Gas Chromatography
Standard Practice for Use of Electron-Capture Detectors in Gas Chromatography
SCOPE
1.1 This practice is intended to serve as a guide for 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 E260or Practice E1510 should be followed except where specific changes are recommended herein for the use of an ECD. For 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 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 3.
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Designation:E697–96(Reapproved 2001)
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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope Outlet Connections for Industrial Gas Mixtures
CGAP-12 Safe Handling of Cryogenic Liquids
1.1 This practice is intended to serve as a guide for the use
HB-3 Handbook of Compressed Gases
of an electron-capture detector (ECD) as the detection compo-
2.3 Federal Standard:
nent of a gas chromatographic system.
Title 10, Code of Federal Regulations, Part 20
1.2 This practice is intended to describe the operation and
performance of the ECD as a guide for its use in a complete
3. Hazards
chromatographic system.
3.1 Gas Handling Safety—Thesafehandlingofcompressed
1.3 For general gas chromatographic procedures, Practice
gases and cryogenic liquids for use in chromatography is the
E260 or Practice E1510 should be followed except where
responsibility of every laboratory. The Compressed GasAsso-
specificchangesarerecommendedinthispracticeforuseofan
ciation (CGA), a member group of specialty and bulk gas
ECD. For a definition of gas chromatography and its various
suppliers, publishes the following guidelines to assist the
terms, see Practice E355. These standards also describe the
laboratory chemist to establish a safe work environment.
performance of the detector in terms which the analyst can use
Applicable CGA publications include: CGAP-1, CGAG-5.4,
to predict overall system performance when the detector is
CGAP-9, CGAV-7, CGAP-12, and HB-3.
coupledtothecolumnandotherchromatographiccomponents.
3.2 The electron capture detector contains a radioactive
1.4 This standard does not purport to address all of the
isotope that emits b-particles into the gas flowing through the
safety concerns, if any, associated with its use. It is the
detector. The gas effluent of the detector must be vented to a
responsibility of the user of this standard to establish appro-
fumehoodtopreventpossibleradioactivecontaminationinthe
priate safety and health practices and determine the applica-
laboratory. Venting must conform to Title 10, Code of Federal
bility of regulatory limitations prior to use. For specific safety
Regulations, Part 20 and Appendix B.
information, see Section 3.
4. Principles of Electron Capture Detection
2. Referenced Documents
4.1 The ECD is an ionizating detector comprising a source
2.1 ASTM Standards:
2 of thermal electrons inside a reaction/detection chamber filled
E260 Practice for Packed Column Gas Chromatography
with an appropriate reagent gas. In packed column GC the
E355 Practice for Gas Chromatography Terms and Rela-
2 carrier gas generally fullfills the requirements of the reagent
tionships
gas. In capillary column GC the make-up gas acts as the
E1510 Practice for Installing Fused Silica Open Tubular
2 reagent gas and also sweeps the detector volume in order to
Capillary Columns in Gas Chromatographs
pass column eluate efficiently through the detector. While the
2.2 CGA Standards:
carrier/reagent gas flows through the chamber the device
CGAP-1 Safe Handling of Compressed Gases in Contain-
3 detectsthosecompoundsenteringthechamberthatarecapable
ers
of reacting with the thermal electrons to form negative ions.
CGAG-5.4 Standard for Hydrogen Piping Systems at
3 These electron capturing reactions cause a decrease in the
Consumer Locations
3 concentration of free electrons in the chamber. The detector
CGAP-9 The Inert Gases: Argon, Nitrogen and Helium
response is therefore a measure of the concentration and the
CGAV-7 Standard Method of Determining CylinderValve
change in concentration of electrons (1-17).
4.2 A radioactive source inside the detector provides a
sourceof b-rays,whichinturnionizethecarriergastoproduce
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
Spectrography and is the direct responsibility of Subcommittee E13.19 on Chro-
matography.
Current edition approvedApril 10, 1996. Published September 1996. Originally
published as E697–79. Last previous edition E697–95. Available from Superintendent of Documents, Government Printing Office,
Annual Book of ASTM Standards, Vol 3.06. Washington, DC 20402.
3 5
Available from Compressed Gas Association, Inc., 1725 Jefferson Davis The boldface numbers in parentheses refer to a list of references at the end of
Highway, Arlington, VA 22202-4100. this practice.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E697
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
totheknownelectrophorethatmightbepresent.Inthesecases
4.3 As sample components pass through the detector, they
combine with electrons. This causes a decrease in the standing certain structural features, which by themselves are only
weakly electrophoric, are combined so as to give the molecule
current or an increase in frequency of potential pulses depend-
ing on the mode of ECD operation (see 5.3).The magnitude of 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 reported after the addition of either oxygen or nitrous oxide to
increase in signal.
the carrier gas. Oxygen doping can increase the response
toward CO , certain halogenated hydrocarbons, and polycyclic
4.4 The two major classifications of electron-capture reac-
aromatic compounds (33). Small amounts of nitrous oxide can
tions in the ECD are the dissociative and nondissociative
increase the response toward methane, carbon dioxide, and
mechanisms.
hydrogen.
4.4.1 In the dissociative-capture mechanism, the sample
4.9 While it is true that the ECD is an extremely sensitive
moleculeABreactswiththeelectronanddissociatesintoafree
−
detector capable of picogram and even femtogram levels of
radicalandanegativeion:AB+e→A+B .Thisdissociative
detection, its response characteristics vary tremendously from
electron-capture reaction is favored at high detector tempera-
one chemical class to another. Furthermore, the response
tures. Thus, an increase in noncoulometric ECD response with
characteristic for a specific solute of interest can also be
increasing detector temperature is evidence of the dissociative
enhanced or diminished depending on the detector’s operating
electron-capture reaction for a compound. Naturally, detect-
temperature (56) (see 4.4 and 5.5). The detector’s response
ability is increased at higher detector temperatures for those
characteristic to a solute is also dependent on the choice of
compounds which undergo dissociative mechanisms.
reagent gas and since the ECD is a concentration dependent
4.4.2 In the nondissociative reaction, the sample molecule
detector, it is also dependent on the total gas flow rate through
AB reacts with the electron and forms a molecular negative
−
the detector (see 5.5). These two parameters affect both the
ion:AB+e→AB . The cross section for electron absorption
absolute sensitivity and the linear range an ECD has to a given
decreases with an increase in detector temperature in the case
solute. It is prudent of the operator of the ECD to understand
of the nondissociative mechanism. Consequently, the nondis-
the influence that each of the aforementioned parameters has
sociativereactionisfavoredatlowerdetectortemperaturesand
on the detection of a solute of interest and, to optimize the
the noncoulometric ECD response will decrease if the detector
parameters prior to final testing.
temperature is increased.
4.4.3 Beside the two main types of electron capture reac-
5. Detector Construction
tions,resonanceelectronabsorptionprocessesarealsopossible
−
in the ECD (for example, AB+e=AB ). These resonance 5.1 Geometry of the Detector Cell:
reactions are characterized when an electron absorbing com-
5.1.1 Three basic types of b-ray ionization-detector geom-
poundexhibitsalargeincreaseinabsorptioncrosssectionover
etriescanbeconsideredapplicableaselectron-capturedetector
a narrow range of electron energies. This is an extremely
cells: the parallel-plate design, the concentric-tube or coaxial-
temperature sensitive reaction due to the reverse reaction
tube design, and recessed electrode or asymmetric type (34-
whichisathermalelectrondeactivationreaction.Forsolutesin
37). The latter could be considered a variation of the
this category a maximum detector temperature is reached at
concentric-tube design. Both the plane-plate geometry and
whichhighertemperaturesdiminishtheresponsetotheanalyte
concentric geometry are used almost exclusively for pulsed
(55).
operation.Although the asymmetric configuration is primarily
4.5 The ECD is very selective for those compounds that employed in the d-c operation of electron-capture detectors, a
have a high electron-capture rate and the principal use of the unique version of the asymmetric design (referred to as a
detector is for the measurement of trace quantities of these
displaced-coaxial-cylinder geometry) has been developed for
−9
materials, 10 g or less. Often, compounds can be derivatized pulse-modulated operation.The optimum mode of operation is
by suitable reagents to provide detection of very low levels by
usually different for each detector geometry and this must be
ECD (19, 20). For applications requiring less sensitivity, other considered, where necessary, in choosing certain operating
detectors are recommended.
parameters.
4.6 A compound with a high electron-capture rate often 5.1.2 In general, more efficient operation is achieved if the
contains an electrophoric group, that is, a highly polar moiety detector is polarized such that the gas flow is counter to the
that provides an electron-deficient center in the molecule.This flow of electrons toward the anode. In this regard, the radio-
group promotes the ability of the molecule to attach free active source should be placed at the cathode or as near to it as
electrons and also may stabilize the resultant negative possible.
E697
5.1.3 Other geometric factors that affect cell response and matographic operation.Another advantage of the high detector
operation are cell volume and electrode spacing, which may or temperatures available with Ni is an enhanced sensitivity for
may not be altered concurrently depending upon the construc-
compounds that undergo dissociative electron capture.
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
lose any resolution that may have been achieved by high- 3 63
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
63 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
stainlesssteelorHastelloyC,whichisanickel-basealloy.The
used.
tritium attached to the former foil material is in the form of Ti
5.3.1.1 DC-Voltage Method—A negative d-c voltage is
3H ; however, there is uncertainty concerning the exact means
applied to the cathode resulting in an increasing detector
of tritium attachment to the scandium (Sc) substrate of the
current with increasing voltage until saturation is reached.The
Hastelloy C foil. The proposed methods of attachment include
3 3
ECD response for the d-c mode is only linear over a narrow
Sc H and H astheoccludedga
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