ASTM E697-96(2019)

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 higher specific activity and its
maintain effective electron capture without encountering other
ability to be used in a smaller volume detector. Because low
types of electron reactions and also small enough so as not to
3 63
levels of radioactive Hor Ni are released to the laboratory
lose any resolution that may have been achieved by high-
environment, it is a wise safety precaution to vent electron-
resolution chromatographic systems. Typical ECD cell vol-
capture detectors by means of hood exhaust systems.
umes range from approximately 2 to 0.3 cm . A detector cell
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 range of the particular β-rays.
current method (44-49). Within each mode of operation, there
lies the ability to optimize performance by selective adjust-
5.2 Radioactive Source:
ments of various ECD operational parameters. This may
5.2.1 Many β-ray-emitting isotopes can be used as the
include,amongotherthings,notonlythechoiceofreagentgas
primary ionization source. The two most suitable are H
to be used in the ECD (see 5.4) but also setting the detector’s
(tritium) (40, 41). and Ni (42).
pulsetimeconstantontheelectrometertocorrespondtothegas
5.2.1.1 Tritium—This isotope is usually coated on 302
used.
stainlesssteelorHastelloyC,whichisanickel-basealloy.The
tritium attached to the former foil material is in the form of 5.3.1.1 DC-Voltage Method—A negative d-c voltage is ap-
Ti H ; however, there is uncertainty concerning the exact plied to the cathode resulting in an increasing detector current
means of tritium attachment to the scandium (Sc) substrate of with increasing voltage until saturation is reached. The ECD
the Hastelloy C foil. The proposed methods of attachment response for the d-c mode is only linear over a narrow voltage
3 3
includeSc H and H astheoccludedgas.Thenominalsource range of approximately 10 to 15 V. Therefore, optimum
3 2
activity for tritium is 250 mCi in titanium sources and 1000 operation is obtained when the detector current is about 80%
mCi in scandium sources. Department of Energy regulations of the saturation level. At higher voltages, the response
permit a maximum operating temperature of 225°C for the becomes nonlinear and this nonlinearity becomes extreme on
3 3
Ti H source and 325°C for the Sc H source. Naturally, the saturation plateau. At d-c voltages below the optimum
2 3
E697 − 96 (2019)
range, the response-to-concentration slope is high at low mationtoarecordingdevice.Inactualoperation,thedifference
concentrations and decreases with increasing concentration. between the output current from the detector cell and a
reference current causes an integrating amplifier to change its
This effect will over-emphasize small chromatographic peaks
and tends to distort peak widths and heights. The d-c voltage output voltage, which in turn is applied to the input of
voltage-to-frequency (V/f) converter. The V/f’s output fre-
required for optimum operation can vary a great deal depend-
ing upon such factors as the type of radioactive source, quency therefore changes and is used to control the frequency
of the collection pulses. The setpoint of the reference current
effective source strength, interelectrode distance, detector
volume, detector cleanliness, detector temperature, flow rate affects both the detection limit and the linear range, so a
compromise is required on the chosen value of the reference
through the detector, liquid phase bleed from the column,
current to suit the particular analysis. As in the case of the
carrier gas, and its purity when it reaches the detector. Since
constant-frequency method, the amplitude of the collection
most of these parameters are difficult to change for a given
pulses is usually 50 to 60 V.
application, experimental variation of the voltage to achieve
5.3.1.4 Gas-Phase Coulometric Method—This unique tech-
maximum performance is recommended. Actual operational
nique is based on a 1:1 equivalency at 100%, or some known
voltages from+10 to+150 V may be required to obtain
constant, ionization between the solute molecules and the
optimumperformanceinthed-cmode.However,regardlessof
number of electrons absorbed by these molecules in the
the actual ECD operating voltage, the detector in the d-c mode
detector. Thus, the number of electrons consumed can be used
will still be limited to a narrow linear response range of 10 to
to calculate the molar quantity by means of Faraday’s law.
15V.Sincetheoptimumvoltagecanchangeduringcontinuous
With coulometric ECD, the peak area in ampere-seconds, or
operation, it is wise to check the current-versus-voltage re-
coulombs, is related to the mass in grams by the following
sponse frequently. This problem of variable response is suffi-
equation:
cientreasonforthefrequentuseofcalibrationstandardsduring
analyses. Because of the availability of electron-capture detec-
QM
g 5 (1)
tors that operate in the pulse sampling method and the analysis
F
problems inherent in the d-c mode, the dc-voltage method
where: g is the grams of analyte, Q is the number of
offers few advantages compared to its notoriety for yielding
coulombs, M is the molecular weight of the substance, and
anomalous results. Space charges, contact potentials, and
F=9.65×10 C/mol. This particular ECD method is appli-
unpredictable changes in electron energy are three significant
cable only to compounds with ionization efficiencies greater
factors which contribute to response problems in the d-c
than90%andtothosecompoundswhosereactionproductsdo
detector.
not capture electrons to a significant degree. Unlike the other
5.3.1.2 Constant-Frequency Method—The applied voltage
types of electron-capture detectors which function as
is pulsed at a constant frequency to the cathode in the form of
concentration-sensitive transducers, the coulometric ECD acts
a square wave. Thus, the pulse frequency is held constant and
asamass-sensitivedeviceprovidedthe1:1ratioismaintained.
the output variable presented on the recorder is the detector
Hence, the coulometric detector is to a considerable extent
current. The voltage pulses are of short duration, 1 µs or less,
unaffected by changes in temperature, pressure, or flow rate of
and should occur at infrequent intervals, for example, 1 to 10
thecarriergas.Althoughthecoulometricdetectorappearstobe
kHz.Ingeneral,theshorterthepulseandthelongertheinterval
an ideal analytical transducer, its use is presently limited to
that can be used to maintain reasonable current flow, the better
specific compounds that meet the coulometric criteria.
the performance of the detector. The sensitivity increases
5.3.2 There are certain advantages and disadvantages for all
directly with the time interval between collection pulses and
the basic ECD methods of operation. In general, the d-c mode
the response is normally linear with solute concentration up to
requires simpler electronics and can be initially adjusted for
absorption of 50% of the thermal electrons present in the
optimum response and concomitant sensitivity. However, at
detector. For this reason, optimization of the pulse cycle is
times the d-c mode is subject to anomalous responses which
recommended to achieve maximum response and to compen-
arerelatedtoanumberofinherentcharacteristics,forexample,
sate for the many other parameters that could affect detector
space charges, contact potentials, interference from non-
performance.The applied voltage (or pulse height) can also be
capturing compounds, etc. Furthermore, source contamination
varied, but as long as a minimum amount is used to promote
and subsequent decreases in the linear range and overall
current flow, it is not as critical a factor as the pulse cycle.The
sensitivity can often create difficulties during d-c operation.As
amplitude of the pulse is usually 50 to 60 V.
previously discussed, the use of Ni or tritiated scandium at
5.3.1.3 Variable-Frequency or Constant-Current Method—
hightemperaturescanalleviatetheproblemofsourcecontami-
The constant-current ECD has the advantage of an extended nation and significantly reduce the intervals between required
linear range, 10 . In this mode, the detector current is kept
cleanings. The higher detector temperatures also permit en-
constant by an electrical feedback loop which controls the hancementofsensitivitywithmanycompoundswhichundergo
pulse frequency. When an electron-absorbing substance enters
dissociative electron attachment. The techniques of ECD
the detector and removes electrons, the pulse frequency in- operation that involve pulse sampling methods are preferred to
creasestocollectmoreelectronsandtherebykeepsthedetector
thed-cmodeinrespecttoreproducibilityandtothediminution
current at its constant level. Thus, the change of pulse of anomalous responses. In many actual laboratory analyses,
frequency is proportional to the sample concentration, and a the ECD has been limited because of its relatively small range
frequency-to-voltage (f/V) converter is used to send the infor- oflinearity(refertoSection8foradescriptionoflinearrange).
E697 − 96 (2019)
5.3.2.1 For example, the linear range of the normal d-c and functional groups over other function groups that may be
constant-frequencyECDsisfrom50to100.Thislimitedlinear present in a sample matrix (19). The make-up gas must meet
range often means that a sample must be injected many times the requirements listed in 5.4.1.
to bring a peak into the linear range before accurate chromato-
NOTE 1—In an ECD where tritium is used as the ionization source,
graphicquantitationisfeasible.Priortothedevelopmentofthe
hydrogen may not be suitable for use in the carrier or make-up gas. Refer
constant-current mode of ECD operation, Fenimore (34) and to the detector’s manufacture for recommendations.
co-workers described an analog circuit that could be employed
5.4.3 Since the electron capture response can be affected
to increase ECD linearity. The constant-current ECD systems
markedlybycontaminantsinthecarriergas,theanalystshould
have been found to have comparatively large linear ranges of
use high purity gases. Additionally, gas scrubbers to remove
1000toapproximately10000.Besidesreducingthenumberof
residual oxygen and water from the carrier and make-up gases
reruns required for quantitation, the extended linear range of
should be installed on the gas lines. It is preferred that the
the constant-current detector permits the use of automated gas
scrubbersbemountedverticallyandlocatedasclosetotheGC
chromatographic systems in ECD analysis. In addition to the
systemaspossible.Thepotentiallydamagingroleofoxygenis
expanded linear response range, the pulsed mode is also more
due to its electron absorbing ability (50, 51). Several reports
sensitive than the d-c operation. Conceptually, the d-c mode is
haveshownthatlevelsofoxygenbelow10ppmcanreducethe
equivalent to a pulse-modulated ECD operating at such a high
standing current to less than half its maximum value. Besides
pulsefrequencythattheadjacentpulsesbegintooverlap.Since
absorbing the detector electrons, oxygen can form ions such as
− −
the average electron population within an ECD cell decreases
O and (H O) O , which can in turn undergo ion-molecule
2 2 n 2
with increasing pulse frequency, the pulsed modes result in
reactions with the chromatographic solutes. This situation
greater numbers of electrons within the cell than the d-c
complicates the response mechanism and is undesirable for
operationandhence,provideforincreasedsensitivity.Whereas
analytical purposes. Contamination of the carrier gas by
the coulometric detector has greater inherent detectability for
compounds desorbed from elastomeric parts of pressure and
those compounds with large rate constants for electron attach-
flow regulators, lubricants in metal tubing, compounds derived
ment (such as SF , CCl , etc.), the constant-current ECD has
6 4 from unconditioned injection port septa, etc. must also be
the larger linear-response range. At the present stage of
eliminated (52). Therefore, the use of metal diaphragm
development, the coulometric detector should only be consid-
diffusion-resistantpressureregulators,theuseofcleanedmetal
ered when the chromatographic analysis is dealing with
tubingforallgasconnections,theavoidanceofflowregulators
strongly electron-attaching compounds.
with plastic diaphragms, and the use of thoroughly baked
injection port septa are recommended for good performance.
5.4 Carrier Gas:
5.5 Detector Temperature and Flow Rate—The temperature
5.4.1 The carrier gas must fulfill the basic functions of
of the detector and flow rate through it are two variables that
reducing the electron energy to thermal levels and quenching
can affect detector response. Most of the time the choice of
unwanted side reactions, particularly metastable atom
these conditions is limited by the application at hand and the
formation, where possible. In pulsed-mode operation, electron
analytical conditions chosen for the gas-chromatographic col-
mobility should also be high. For these reasons, a mixture of
umn system. However, certain electron-capturing compounds
argon with 5 to 10% methane, or helium with 5% methane, is
show a marked dependence of response on detector tempera-
often recommended for use with pulse-operated detectors.
ture and this dependency can be used to increase significantly
Carbon dioxide can also be substituted for the methane in
the response for compounds with a dissociative mechanism
eithercase.Ford-coperation,nitrogenisrecommendedaslong
(53-55).Thedetectorflowratecanbeutilizedtoshifttheentire
as it is reasonably free of water and oxygen (prepurified or
linear range of a noncoulometric ECD by approximately an
oil-pumped grade). However, the gas mixture cited above for
order of magnitude since this type of ECD is a concentration-
pulsed operation can also be used for d-c operation. Similarly,
sensitive device. When a post column make-up gas is used, its
nitrogencarriergascanalsobeusedforpulsedECDoperation.
flowrate can be adjusted for optimum detector response with-
In fact, several of the constant-current ECDs can operate with
out changing the column efficiency. It should be recognized
either argon/methane or nitrogen. The use of nitrogen carrier
that changing the detector temperature and flowrate will affect
gas with certain designs of the constant-current ECD can
detector operation. When they are altered, steps to regain
actually increase the overall sensitivity, but the corresponding
optimum response, such as voltage or pulse-cycle adjustment,
linear range decreases by a factor of approximately three.
as cited in 5.3.1, should be taken.
However, at least one commercial ECD employs a displaced
coaxial-cylinder cell geometry to obtain both picogram detect-
5.6 Detector Contamination:
ability and equivalent 10 linearity with nitrogen carrier gas.
5.6.1 Contamination of the ECD occurs if various sub-
5.4.2 When a capillary column is being used, the low gas stances that elute from the chromatographic column are con-
flowrate through the ECD must be increased with a post- densed within the detector cell. These deposited films are
column make-up gas to ensure proper detector operation. It is usually derived from a combination of column bleed, septum
recommendedthatheliumorhydrogenbeusedasthecapillary bleed, and impurities in the carrier gas, solvent, and the actual
column carrier gas for optimum chromatographic performance sample.Theobservablesymptomsthatindicateacontaminated
andthatnitrogenorargon/methanebeusedasthemake-upgas detectorincludeareducedbaselinecurrentoranincreasedbase
for optimum detector response. Other types of make-up gases frequency (f ), a decreased dynamic range, a reduced sensitiv-
o
have been used to give enhanced sensitivities to specific ity and an increased baseline drift.
E697 − 96 (2019)
5.6.2 To minimize contamination of the ECD, the detector gas through the detector at high temperatures for 30 min or
should always be maintained at a temperature at least 10°C more. However, after cleaning, diminished response is ob-
above the injector, column, and interface temperatures. It is served toward oxygen and some chlorinated compounds for
also advisable to employ chromatographic columns prepared periods up to several hours. The procedure recommended in
the manufacturer’s manual should be consulted when detector
from high-temperature, low-bleed stationary phases which are
coated with low percentages (1.0 to 5%) of the liquid phase. contamination is suspected (56).
Allcolumnsshouldbethoroughlyconditionedatatemperature
5.7 Detector Maintenance:
of about 25°C above the maximum oven temperature to be
5.7.1 All ECD manufacturers sell their detector under a
employed in the chromatographic analyses.Always disconnect
general low level radioactive material license. In accordance
the column from the ECD during conditioning to prevent
with this license, the owner or operator of the detector is
contamination. Traces of water and oxygen impurities in the
requiredtoperformawipetestonthedetector’sbodytocheck
carrier gas can also affect the performance of the ECD.
for the event of a radioactivity leak. This test in most cases, is
Therefore, molecular sieve filters of the 5 Α or 13 X type
required once every six months. Wipe test kits are available
shouldbeusedincombinationwiththecommerciallyavailable
from the manufacturer of the detector and companies licensed
filters to remove water and oxygen, respectively, from the
to interpret the radioactive wipe test swabs. In the case of the
carriergas.Problemsduetoseptumbleedcanbeminimizedby
Ni ECD, the detector should not be disassembled to remove
several approaches including the use of TFE-fluorocarbon-
the radioactive foil.
coated septa, solvent-extracted septa which have been ther-
TERMS AND RELATIONSHIPS
mallyconditioned,andinjectionportswhichreducethecontact
betweenthecarriergasandtheseptum.Sincecertainanalytical
6. Sensitivity (Response)
samples may contain relatively large amounts of contaminants
in the natural sample matrix, it may be necessary to perform a
6.1 Description—The noncoulometric ECD generally acts
sample cleanup procedure before the actual GC/ECD analysis.
as a concentration-sensitive detector rather than a mass-
5.6.3 Recent data suggest that the contaminants deposited
sensitive detector. Therefore, the sensitivity (response) of the
ontheinsideofthedetectorinhibitchargecollectionbymeans
normalECDisthesignaloutputperunitconcentrationofatest
of polarization effects. The electrical polarization effects of an
substance in the carrier gas. In addition to the concentration of
insulating film can be diagnosed by operating the ECD a
the electron-capturing eluant, the signal of a noncoulometric
sufficient time to obtain a stable baseline. Then, reverse the
ECD also depends on the electron-capture characteristics of
anode and cathode connections on the ECD and continue the
each component. For quantitative analyses it is necessary to
reversed operation for several minutes. Finally, reconnect the
calibrate the ECD separately for every relevant compound. A
ECD leads to their normal positions and observe the recorder
simplified relationship for the sensitivity of an ECD is:
baseline as a function of time. If the above procedure lowers
A F
i c
the baseline to a stable position which persists for 2 to 4 min S 5 (2)
W
i
and then slowly returns to the pretest, high baseline, the test
where:
indicates a contamination film within the ECD. Another
experimental indication of a contaminated detector is the
S = sensitivity (response) in A·mL/pg or Hz·mL/pg (for
appearance of negative peaks subsequent to positive sample
constant-current mode),
peaks. A = peak area for substance, i, in A·min or Hz·min (for
i
constant-current mode),
5.6.4 Recommended Procedures for Cleaning a Contami-
F = carrier-gas flow rate in mL/min (corrected to detector
c
nated ECD:
temperature, refer to Appendix X1), and
5.6.4.1 The tritium radioactive foils and cell bodies can be
W = mass of test substance, i, in the sample, pg.
i
cleansed by immersion for 1 to2hin5%KOHin methanol,
Specificity of the detector for an analyte of interest is stated
followed by a thorough rinse with pure methanol.The detector
as the ratio of the sensitivity of the detector for the test
foil and cells are allowed to dry. Then the foil is inserted into
substance to the sensitivity of a potential interfering solute.An
the detector cell body and the ECD can be used for further
unsubstituted hydrocarbon that elutes close to test sample is
analyses after equilibration in the GC for1hat normal
generally used for this purpose. The ECD signal measured in
operatingtemperatures.AlwaysallowtheECDcelltowarmup
the absence of an electron capturing chromatographic species
in the GC before connecting the detector to the column. This
is called the detector background or baseline current. This
latter procedure will prevent condensation of column effluents
background signal is established by the sum of the signals for
on the cold ECD.
63 the carrier gas, make-up gas and other impurities. The sensi-
5.6.4.2 The Ni ECD contains a radioactive source which
tivity of the ECD for a sample is defined as the change in the
normally should not be opened for cleaning except by the
63 measured ECD signal resulting from a change in the concen-
manufacturer. However, the Ni detector can sometimes be
tration of the sample within the detector volume.
decontaminatedbyeitherpurgingtheECDat350to400°Cfor
12 to 24 h while maintaining carrier gas flow, or by injecting 6.2 Test Conditions:
several 100 µL aliquots of distilled water into a 300°C 6.2.1 Since individual substances have widely different
chromatographic system by means of an empty column. electron-capture rates, the test substance may be selected in
Another method of cleaning a Ni ECD is to pass hydrogen accordance with the expected application of the detector. The
E697 − 96 (2019)
testsubstanceshouldalwaysbewell-definedchemically.When 6.3.1.2 Regardless of which method is used to calculate
specifying the sensitivity of the ECD, the test substance used linear range, peak height is the only acceptable method for
must be stated. determining minimum detectability.
6.2.1.1 The recommended test substance is lindane (1,2,3, 6.3.2 Calibration—It is essential to calibrate the measuring
system to ensure that the nominal specifications are acceptable
4,5,6-hexachlorocyclohexane), with dieldrin (1,2,3,4,10,10-
hexachloro-6,7-epoxy-1,4,4a,5,6,7,8,8a-octahydro-endo-exo- andparticularlytoverifytherangeoverwhichtheoutputofthe
device,whetherpeakareaorpeakheight,islinearwithrespect
1,4:5,8-dimethanonaphthalene) as an alternative.
to input signal. Failure to perform this calibration may intro-
6.2.1.2 The ECD can also be calibrated for halogenated
duce substantial errors into the results. Methods for calibration
compounds using permeation tubes.
will vary for different manufacturers’devices but may include
6.2.2 The measurement must be made within the linear
accurate constant voltage supplies or pulsegenerating equip-
range of the detector, at a signal level between 10 and 100
ment.Theinstructionmanualshouldbestudiedandthoroughly
times greater than the minimum detectability, and 20 and 200
understood before attempting to use electronic integration for
times greater than the noise level at the same conditions.
peak area or peak height measurements.
6.2.3 The rate of drift of the base current for the detector at
the same conditions must be stated.
7. Minimum Detectability
6.2.4 The conditions under which the detector sensitivity is
measured must be stated. These should include but not neces-
7.1 Description—Minimum detectability (Note 3) is the
sarily be limited to the following:
concentration of test substance in the carrier gas that gives a
6.2.4.1 Geometryofdetector,radioactivesource,andsource
detector signal equal to twice the noise level and is calculated
activity,
from the measured sensitivity and noise:
6.2.4.2 Mode of operation,
2N
6.2.4.3 For the d-c mode: applied voltage; for the constant- D 5 (3)
S
frequency mode: duration and interval of pulses, and pulse
where:
height in volts; for the constant-current mode: the pulse
duration, pulse amplitude, and the reference detector current, D = minimum detectability, pg test substance/mL carrier
Iref,
gas,
6.2.4.4 Detector temperature, N = noise, A or Hz, and
S = sensitivity of the ECD, A·mL/pg or Hz·mL/pg.
6.2.4.5 Carrier gas, and if a capillary column is used, the
NOTE 3—Although the minimum detectable amount is frequently used
make-up gas must be specified,
to express the limits of detection for a specific analytical method, the
6.2.4.6 Carrier gas flow rate, and if a capillary column is
proper term for testing the detector is minimum detectability. It is the
installed, the total gas flow rate which includes the column
intention of Committee E-19 to delete reference to the term of minimum
flow and make-up gas flow (in either case the flow must be detectable amount in this practice on using detectors. By definition the
minimum detectability is independent of the peak width; the minimum
corrected to the detector temperature) (Note 2), and
detectable amount for a specific analytical method is not.
6.2.4.7 Specific test substance.
2Nt F
b c
NOTE 2—For the method of correction, see Annex A1.
D' 5 Dt F 5 (4)
b c
S
6.2.5 Linearity and response speed of the recording system
where:
orotherdataacquisitiondeviceusedshouldbesuchthatitdoes
notdistortorotherwiseinterferewiththeouputofthedetector. Dʹ = minimum detectable amount, pg,
D = minimum detectability, pg/mL,
Recorders should have a maximum 1-s response time corre-
N = noise, A or Hz
sponding to 90% of full scale deflection. If additional ampli-
S = sensitivity of the ECD, A·mL/pg or Hz·mL/pg,
fiersareusedbetweenthedetectorandthefinalreadoutdevice,
F = corrected carrier-gas flow rate in mL/min, and
their characteristics should be established, their time constants c
t = time corresponding to the width at base, min.
b
shouldbenoted,andtheirpossibleoveralleffectonpeakshape
of early eluted peaks determined. It should be noted that
7.2 Test Conditions—Measure sensitivity in accordance
manipulation of integrator and computer parameters to reduce
with Section 6. Measure noise in accordance with Section 11.
noise can distort the observed peaks (57).
Both measurements must be carried out at the same conditions
(see 6.2.4) and, preferably, at the same time. State the test
6.3 Data Handling:
substance and conditions in accordance with Section 6. Also
6.3.1 All manufacturers supply an integral electrometer to
state the noise level upon which the calculation was based.
allow the small electrical current changes to be coupled to
recorder/integrators/computers. The preferred system will in-
8. Linear Range
corporate one of the newer integrators or computers that
converts an electrical signal into clearly defined peak area
8.1 Description—ThelinearrangeofanECDistherangeof
counts in units such as microvolt-seconds.These data can then concentrations of test substances in the carrier gas passing
be readily used to calculate the linear range.
through the detector over which the sensitivity of the detector
6.3.1.1 Another method uses peak height measurements. is constant to within 6 5.0% as determined from the linearity
This method yields data that are very dependent on column plotspecifiedbelowin8.2.1.ThelinearrangeoftheECDmay
performance and therefore not recommended. be expressed in three different ways:
E697 − 96 (2019)
8.1.1 As the ratio of the upper limit of linearity obtained
from the linearity plot to the minimum detectability (or to the
lower limit, if it is greater), both measured for the same test
substance:
c c
max max
L.R. 5 or L.R. 5 (5)
D c
min
where:
L.R. = linear range of the detector,
c = concentration of the test substance corresponding to
max
theupperlimitoflinearityobtainedfromthelinearity
plot, pg/mL,
D = minimum detectability of the detector, pg/mL, and
c = concentration of the test substance in carrier gas
min
corresponding to the lower limit of linearity obtained
from the linearity plot, pg/mL.
If the linear range is expressed by this ratio, the minimum
detectability or lower limit must be stated.
8.1.2 By giving the minimum detectability or the lower
limit of linearity (whichever is greater) and the upper limit of
−2
linearity, for example, 1×10 pg/mL to 30 pg/mL.
8.1.3 By presenting the linearity plot itself, with the mini-
mum detectability indicated on the plot.
8.2 Method of Measurement:
8.2.1 Analyze various amounts of the test substance and
calculate the peak area sensitivity for each case in accordance
with Section 5. Plot the values of sensitivity as the ordinate
versusthelogofthesampleconcentration.Drawasmoothline
through the data points.The limits of linearity are given by the
intersectionofthelinewithvaluesof0.95·S and1.05·S
const const
where: S is the constant value of sensitivity on the graph,
const
FIG. 1 Example of a Linearity Plot for an Electron-Capture Detec-
and the lower limit of linearity cannot be less than the
tor
minimum detectability. The linearity plot for an ECD is
illustrated in Fig. 1.
8.2.2 Express the linear range according to 8.1.1. It should
lower limits. The dynamic range is larger than or equal to the
benotedthattheusablelinearrangeofanECDforqu
...