ASTM E697-96
(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: E 697 – 96
Standard Practice for
Use of Electron-Capture Detectors in Gas Chromatography
This standard is issued under the fixed designation E 697; 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 CGA P-12 Safe Handling of Cryogenic Liquids
HB-3 Handbook of Compressed Gases
1.1 This practice is intended to serve as a guide for the use
2.3 Federal Standard:
of an electron-capture detector (ECD) as the detection compo-
Title 10, Code of Federal Regulations, Part 20
nent of a gas chromatographic system.
1.2 This practice is intended to describe the operation and
3. Hazards
performance of the ECD as a guide for its use in a complete
3.1 Gas Handling Safety—The safe handling of compressed
chromatographic system.
gases and cryogenic liquids for use in chromatography is the
1.3 For general gas chromatographic procedures, Practice
responsibility of every laboratory. The Compressed Gas Asso-
E 260 or Practice E 1510 should be followed except where
ciation (CGA), a member group of specialty and bulk gas
specific changes are recommended in this practice for use of an
suppliers, publishes the following guidelines to assist the
ECD. For a definition of gas chromatography and its various
laboratory chemist to establish a safe work environment.
terms, see Practice E 355. These standards also describe the
Applicable CGA publications include: CGA P-1, CGA G-5.4,
performance of the detector in terms which the analyst can use
CGA P-9, CGA V-7, CGA P-12, and HB-3.
to predict overall system performance when the detector is
3.2 The electron capture detector contains a radioactive
coupled to the column and other chromatographic components.
isotope that emits b-particles into the gas flowing through the
1.4 This standard does not purport to address all of the
detector. The gas effluent of the detector must be vented to a
safety concerns, if any, associated with its use. It is the
fume hood to prevent possible radioactive contamination in the
responsibility of the user of this standard to establish appro-
laboratory. Venting must conform to Title 10, Code of Federal
priate safety and health practices and determine the applica-
Regulations, Part 20 and Appendix B.
bility of regulatory limitations prior to use. For specific safety
information, see Section 3.
4. Principles of Electron Capture Detection
2. Referenced Documents 4.1 The ECD is an ionizating detector comprising a source
of thermal electrons inside a reaction/detection chamber filled
2.1 ASTM Standards:
with an appropriate reagent gas. In packed column GC the
E 260 Practice for Packed Column Gas Chromatography
carrier gas generally fullfills the requirements of the reagent
E 355 Practice for Gas Chromatography Terms and Rela-
gas. In capillary column GC the make-up gas acts as the
tionships
reagent gas and also sweeps the detector volume in order to
E 1510 Practice for Installing Fused Silica Open Tubular
pass column eluate efficiently through the detector. While the
Capillary Columns in Gas Chromatographs
carrier/reagent gas flows through the chamber the device
2.2 CGA Standards:
detects those compounds entering the chamber that are capable
CGA P-1 Safe Handling of Compressed Gases in Contain-
3 of reacting with the thermal electrons to form negative ions.
ers
These electron capturing reactions cause a decrease in the
CGA G-5.4 Standard for Hydrogen Piping Systems at
3 concentration of free electrons in the chamber. The detector
Consumer Locations
3 response is therefore a measure of the concentration and the
CGA P-9 The Inert Gases: Argon, Nitrogen and Helium
change in concentration of electrons (1-17).
CGA V-7 Standard Method of Determining Cylinder Valve
3 4.2 A radioactive source inside the detector provides a
Outlet Connections for Industrial Gas Mixtures
source of b-rays, which in turn ionize the carrier gas to produce
a source of electrons (18). A constant or intermittent negative
potential, usually less than 100 V, is applied across the reaction
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
chamber to collect these electrons at the anode. This flow of“
Spectrography and is the direct responsibility of Subcommittee E13.19 on Chro-
matography.
Current edition approved April 10, 1996. Published September 1996. Originally
published as E 697 – 79. Last previous edition E 697 – 95. Available from Superintendent of Documents, Government Printing Office,
Annual Book of ASTM Standards, Vol 14.02. 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, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 697
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 rate without containing an obvious electrophore in its structure,
and recording system. or its electron-capture rate could be much greater than that due
to the known electrophore that might be present. In these cases
4.3 As sample components pass through the detector, they
certain structural features, which by themselves are only
combine with electrons. This causes a decrease in the standing
weakly electrophoric, are combined so as to give the molecule
current or an increase in frequency of potential pulses depend-
its electrophoric character. A few examples of these are the
ing on the mode of ECD operation (see 5.3). The magnitude of
quinones, cyclooctatetracene, 3,17-diketosteroids, o-phthalates
current reduction or frequency increase is a measure of the
and conjugated diketones (26-32).
concentration and electron capture rate of the compound. The
4.8 Enhanced response toward certain compounds has been
ECD is unique among ionizing detectors because it is this loss
reported after the addition of either oxygen or nitrous oxide to
in electron concentration that is measured rather than an
the carrier gas. Oxygen doping can increase the response
increase in signal.
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
molecule AB reacts with the electron and dissociates into a free
−
detector capable of picogram and even femtogram levels of
radical and a negative ion: AB + e →A+B . This dissociative
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
sociative reaction is favored at lower detector temperatures and
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.
5. Detector Construction
4.4.3 Beside the two main types of electron capture reac-
tions, resonance electron absorption processes are also possible
5.1 Geometry of the Detector Cell:
−
in the ECD (for example, AB+e=AB ). These resonance
5.1.1 Three basic types of b-ray ionization-detector geom-
reactions are characterized when an electron absorbing com-
etries can be considered applicable as electron-capture detector
pound exhibits a large increase in absorption cross section over
cells: the parallel-plate design, the concentric-tube or coaxial-
a narrow range of electron energies. This is an extremely
tube design, and recessed electrode or asymmetric type (34-
temperature sensitive reaction due to the reverse reaction
37). The latter could be considered a variation of the
which is a thermal electron deactivation reaction. For solutes in
concentric-tube design. Both the plane-plate geometry and
this category a maximum detector temperature is reached at
concentric geometry are used almost exclusively for pulsed
which higher temperatures diminish the response to the analyte
operation. Although the asymmetric configuration is primarily
(55).
employed in the d-c operation of electron-capture detectors, a
4.5 The ECD is very selective for those compounds that
unique version of the asymmetric design (referred to as a
have a high electron-capture rate and the principal use of the
displaced-coaxial-cylinder geometry) has been developed for
detector is for the measurement of trace quantities of these
pulse-modulated operation. The optimum mode of operation is
−9
materials, 10 g or less. Often, compounds can be derivatized
usually different for each detector geometry and this must be
by suitable reagents to provide detection of very low levels by
considered, where necessary, in choosing certain operating
ECD (19, 20). For applications requiring less sensitivity, other
parameters.
detectors are recommended.
5.1.2 In general, more efficient operation is achieved if the
4.6 A compound with a high electron-capture rate often detector is polarized such that the gas flow is counter to the
contains an electrophoric group, that is, a highly polar moiety flow of electrons toward the anode. In this regard, the radio-
that provides an electron-deficient center in the molecule. This active source should be placed at the cathode or as near to it as
group promotes the ability of the molecule to attach free possible.
electrons and also may stabilize the resultant negative 5.1.3 Other geometric factors that affect cell response and
molecule-ion. Examples of a few electrophores are the halo- operation are cell volume and electrode spacing, which may or
gens, sulfur, phosphorus, and nitro- and a-dicarbonyl groups may not be altered concurrently depending upon the construc-
(21-25). tion of the detector. Of course, both these variables can be
E 697
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
of operating conditions. Cell volume should be small enough to
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
the electrode dimensions of the detector are also determined by
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, among other things, not only the choice of reagent gas
(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 pulse time constant on the electrometer to correspond to the gas
stainless steel or Hastelloy C, which is a nickel-base alloy. 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
2 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
ECD response for the d-c mode is only linear over a narrow
3 3
Sc H and H as the occluded gas. The nominal source activity
...
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