ASTM E697-96(2011)
(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 and health practices and determine the applicability of regulatory limitations prior to use. For specific safety information, see Section 3.
General Information
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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation: E697 − 96 (Reapproved 2011)
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 2.2 CGA Standards:
CGAG-5.4Standard for Hydrogen Piping Systems at Con-
1.1 This practice covers the use of an electron-capture
sumer Locations
detector (ECD) as the detection component of a gas chromato-
CGAP-1SafeHandlingofCompressedGasesinContainers
graphic system.
CGAP-9The Inert Gases: Argon, Nitrogen and Helium
1.2 This practice is intended to describe the operation and
CGAP-12 Safe Handling of Cryogenic Liquids
performance of the ECD as a guide for its use in a complete
CGAV-7Standard Method of Determining Cylinder Valve
chromatographic system.
Outlet Connections for Industrial Gas Mixtures
1.3 For general gas chromatographic procedures, Practice HB-3Handbook of Compressed Gases
E260 or Practice E1510 should be followed except where 2.3 Federal Standard:
specificchangesarerecommendedinthispracticeforuseofan Title 10Code of Federal Regulations, Part 20
ECD. For a definition of gas chromatography and its various
3. Hazards
terms, see Practice E355. These standards also describe the
performance of the detector in terms which the analyst can use
3.1 Gas Handling Safety—The safe handling of compressed
to predict overall system performance when the detector is
gases and cryogenic liquids for use in chromatography is the
coupledtothecolumnandotherchromatographiccomponents.
responsibility of every laboratory. The Compressed GasAsso-
ciation (CGA), a member group of specialty and bulk gas
1.4 The values stated in SI units are to be regarded as
suppliers, publishes the following guidelines to assist the
standard. No other units of measurement are included in this
laboratory chemist to establish a safe work environment.
standard.
Applicable CGA publications include: CGAP-1, CGAG-5.4,
1.5 This standard does not purport to address all of the
CGAP-9, CGAV-7, CGAP-12, and HB-3.
safety concerns, if any, associated with its use. It is the
3.2 The electron capture detector contains a radioactive
responsibility of the user of this standard to establish appro-
isotope that emits β-particles into the gas flowing through the
priate safety and health practices and determine the applica-
detector. The gas effluent of the detector must be vented to a
bility of regulatory limitations prior to use. For specific safety
fumehoodtopreventpossibleradioactivecontaminationinthe
information, see Section 3.
laboratory. Venting must conform to Title 10, Part 20 and
2. Referenced Documents
Appendix B.
2.1 ASTM Standards:
4. Principles of Electron Capture Detection
E260Practice for Packed Column Gas Chromatography
4.1 The ECD is an ionizating detector comprising a source
E355PracticeforGasChromatographyTermsandRelation-
ships of thermal electrons inside a reaction/detection chamber filled
with an appropriate reagent gas. In packed column GC the
E1510Practice for Installing Fused Silica Open Tubular
Capillary Columns in Gas Chromatographs carrier gas generally fullfills the requirements of the reagent
gas. In capillary column GC the make-up gas acts as the
reagent gas and also sweeps the detector volume in order to
pass column eluate efficiently through the detector. While the
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
carrier/reagent gas flows through the chamber the device
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
detectsthosecompoundsenteringthechamberthatarecapable
mittee E13.19 on Separation Science.
Current edition approved Nov. 1, 2011. Published December 2011. Originally
approved in 1979. Last previous edition approved in 2006 as E697–96(2006).
DOI: 10.1520/E0697-96R11. Available from Compressed Gas Association (CGA), 4221 Walney Rd., 5th
For referenced ASTM standards, visit the ASTM website, www.astm.org, or Floor, Chantilly, VA 20151-2923, http://www.cganet.com.
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM AvailablefromU.S.GovernmentPrintingOfficeSuperintendentofDocuments,
Standards volume information, refer to the standard’s Document Summary page on 732 N. Capitol St., NW, Mail Stop: SDE, Washington, DC 20401, http://
the ASTM website. www.access.gpo.gov.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E697 − 96 (2011)
of reacting with the thermal electrons to form negative ions. 4.5 The ECD is very selective for those compounds that
These electron capturing reactions cause a decrease in the have a high electron-capture rate and the principal use of the
concentration of free electrons in the chamber. The detector detector is for the measurement of trace quantities of these
−9
response is therefore a measure of the concentration and the materials, 10 g or less. Often, compounds can be derivatized
change in concentration of electrons (1-17). by suitable reagents to provide detection of very low levels by
ECD (20, 21). For applications requiring less sensitivity, other
4.2 A radioactive source inside the detector provides a
detectors are recommended.
sourceofβ-rays,whichinturnionizethecarriergastoproduce
4.6 A compound with a high electron-capture rate often
a source of electrons (18). A constant or intermittent negative
contains an electrophoric group, that is, a highly polar moiety
potential,usuallylessthan100V,isappliedacrossthereaction
that provides an electron-deficient center in the molecule.This
chamber to collect these electrons at the anode. This flow of
group promotes the ability of the molecule to attach free
“secondary” electrons produces a background or “standing”
electrons and also may stabilize the resultant negative
current and is measured by a suitable electrometer-amplifier
molecule-ion. Examples of a few electrophores are the
and recording system.
halogens, sulfur, phosphorus, and nitro- and α-dicarbonyl
4.3 As sample components pass through the detector, they
groups (22-26).
combine with electrons.This causes a decrease in the standing
4.7 A compound could also have a high electron-capture
current or an increase in frequency of potential pulses depend-
ratewithoutcontaininganobviouselectrophoreinitsstructure,
ingonthemodeofECDoperation(see5.3).Themagnitudeof
or its electron-capture rate could be much greater than that due
current reduction or frequency increase is a measure of the
totheknownelectrophorethatmightbepresent.Inthesecases
concentration and electron capture rate of the compound. The
certain structural features, which by themselves are only
ECD is unique among ionizing detectors because it is this loss
weakly electrophoric, are combined so as to give the molecule
in electron concentration that is measured rather than an
its electrophoric character. A few examples of these are the
increase in signal.
quinones, cyclooctatetracene, 3,17-diketosteroids, o-phthalates
4.4 The two major classifications of electron-capture reac-
and conjugated diketones (27-33).
tions in the ECD are the dissociative and nondissociative
4.8 Enhanced response toward certain compounds has been
mechanisms.
reported after the addition of either oxygen or nitrous oxide to
4.4.1 In the dissociative-capture mechanism, the sample
the carrier gas. Oxygen doping can increase the response
moleculeABreactswiththeelectronanddissociatesintoafree
toward CO , certain halogenated hydrocarbons, and polycyclic
−
radicalandanegativeion:AB+e→A+B .Thisdissociative
aromatic compounds (34). Small amounts of nitrous oxide can
electron-capture reaction is favored at high detector tempera-
increase the response toward methane, carbon dioxide, and
tures. Thus, an increase in noncoulometric ECD response with
hydrogen.
increasing detector temperature is evidence of the dissociative
4.9 While it is true that the ECD is an extremely sensitive
electron-capture reaction for a compound. Naturally, detect-
ability is increased at higher detector temperatures for those detector capable of picogram and even femtogram levels of
detection, its response characteristics vary tremendously from
compounds which undergo dissociative mechanisms.
one chemical class to another. Furthermore, the response
4.4.2 In the nondissociative reaction, the sample molecule
characteristic for a specific solute of interest can also be
AB reacts with the electron and forms a molecular negative
−
enhanced or diminished depending on the detector’s operating
ion:AB+e→AB . The cross section for electron absorption
temperature (35) (see 4.4 and 5.5). The detector’s response
decreases with an increase in detector temperature in the case
characteristic to a solute is also dependent on the choice of
of the nondissociative mechanism. Consequently, the nondis-
reagent gas and since the ECD is a concentration dependent
sociativereactionisfavoredatlowerdetectortemperaturesand
detector, it is also dependent on the total gas flow rate through
the noncoulometric ECD response will decrease if the detector
the detector (see 5.5). These two parameters affect both the
temperature is increased.
absolute sensitivity and the linear range an ECD has to a given
4.4.3 Beside the two main types of electron capture
solute. It is prudent of the operator of the ECD to understand
reactions, resonance electron absorption processes are also
−
the influence that each of the aforementioned parameters has
possible in the ECD (for example, AB+e=AB ). These
on the detection of a solute of interest and, to optimize the
resonancereactionsarecharacterizedwhenanelectronabsorb-
parameters prior to final testing.
ing compound exhibits a large increase in absorption cross
section over a narrow range of electron energies. This is an
5. Detector Construction
extremely temperature sensitive reaction due to the reverse
reaction which is a thermal electron deactivation reaction. For
5.1 Geometry of the Detector Cell:
solutes in this category a maximum detector temperature is
5.1.1 Three basic types of β-ray ionization-detector geom-
reached at which higher temperatures diminish the response to
etriescanbeconsideredapplicableaselectron-capturedetector
the analyte (19).
cells: the parallel-plate design, the concentric-tube or coaxial-
tube design, and recessed electrode or asymmetric type (36-
39). The latter could be considered a variation of the
concentric-tube design. Both the plane-plate geometry and
The boldface numbers in parentheses refer to a list of references at the end of
this practice. concentric geometry are used almost exclusively for pulsed
E697 − 96 (2011)
operation.Although the asymmetric configuration is primarily sources is their availability at much higher specific activities
employed in the d-c operation of electron-capture detectors, a than nickel-63 sources; therefore, Sc H sources are smaller
unique version of the asymmetric design (referred to as a and permit the construction of detector cells with smaller
displaced-coaxial-cylinder geometry) has been developed for internal volumes. The maximum energy of the β-rays emitted
pulse-modulated operation.The optimum mode of operation is by tritium is 0.018 MeV.
usually different for each detector geometry and this must be
5.2.1.2 Nickel-63 ( Ni)—This radioactive isotope is usu-
considered, where necessary, in choosing certain operating
ally either electroplated directly on a gold foil in the detector
parameters.
cell or is plated directly onto the interior of the cell block.
5.1.2 In general, more efficient operation is achieved if the Sincethemaximumenergyoftheβ-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 (43), constant-frequency method, and the constant-
the
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