ASTM E1698-95(2010)
(Practice)Standard Practice for Testing Electrolytic Conductivity Detectors (ELCD) Used in Gas Chromatography
Standard Practice for Testing Electrolytic Conductivity Detectors (ELCD) Used in Gas Chromatography
SIGNIFICANCE AND USE
Although it is possible to observe and measure each of the several characteristics of the ELCD under different and unique conditions, in particular its different modes of selectivity, it is the intent of this practice that a complete set of detector specifications should be obtained at the same operating conditions, including geometry, gas and solvent flow rates, and temperatures. It should be noted that to specify a detector's capability completely, its performance should be measured at several sets of conditions within the useful range of the detector. The terms and tests described in this practice are sufficiently general so that they may be used at whatever conditions may be chosen for other reasons.
Linearity and speed of response of the recorder used should be such that it does not distort or otherwise interfere with the performance of the detector. Effective recorder response should be sufficiently fast so that it can be neglected in sensitivity of measurements. If additional amplifiers are used between the detector and the final readout device, their characteristics should also first be established.
SCOPE
1.1 This practice covers testing the performance of an electrolytic conductivity detector (ELCD) used as the detection component of a gas chromatographic system.
1.2 This practice is directly applicable to electrolytic conductivity detectors that perform a chemical reaction on a given sample over a nickel catalyst surface under oxidizing or reducing conditions and employ a scrubber, if needed, to remove interferences, deionized solvent to dissolve the reaction products, and a conductivity cell to measure the electrolytic conductivity of ionized reaction products.
1.3 This practice covers the performance of the detector itself, independently of the chromatographic column, in terms that the analyst can use to predict overall system performance when the detector is coupled to the column and other chromatographic system components.
1.4 For general gas chromatographic procedures, Practice E260 should be followed except where specific changes are recommended herein for the use of an electrolytic conductivity detector. For definitions of gas chromatography and its various terms see Practice E355.
1.5 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6 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.
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Designation: E1698 − 95 (Reapproved 2010)
Standard Practice for
Testing Electrolytic Conductivity Detectors (ELCD) Used in
Gas Chromatography
This standard is issued under the fixed designation E1698; 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. Referenced Documents
1.1 This practice covers testing the performance of an 2.1 ASTM Standards:
electrolyticconductivitydetector(ELCD)usedasthedetection E260Practice for Packed Column Gas Chromatography
component of a gas chromatographic system. E355PracticeforGasChromatographyTermsand Relation-
ships
1.2 This practice is directly applicable to electrolytic con-
ductivity detectors that perform a chemical reaction on a given
3. Significance and Use
sample over a nickel catalyst surface under oxidizing or
3.1 Although it is possible to observe and measure each of
reducing conditions and employ a scrubber, if needed, to
the several characteristics of the ELCD under different and
remove interferences, deionized solvent to dissolve the reac-
unique conditions, in particular its different modes of
tion products, and a conductivity cell to measure the electro-
selectivity, it is the intent of this practice that a complete set of
lytic conductivity of ionized reaction products.
detector specifications should be obtained at the same operat-
1.3 This practice covers the performance of the detector
ing conditions, including geometry, gas and solvent flow rates,
itself, independently of the chromatographic column, in terms
andtemperatures.Itshouldbenotedthattospecifyadetector’s
that the analyst can use to predict overall system performance
capability completely, its performance should be measured at
when the detector is coupled to the column and other chro-
several sets of conditions within the useful range of the
matographic system components.
detector. The terms and tests described in this practice are
sufficiently general so that they may be used at whatever
1.4 For general gas chromatographic procedures, Practice
conditions may be chosen for other reasons.
E260 should be followed except where specific changes are
recommendedhereinfortheuseofanelectrolyticconductivity
3.2 Linearity and speed of response of the recorder used
detector. For definitions of gas chromatography and its various
should be such that it does not distort or otherwise interfere
terms see Practice E355.
with the performance of the detector. Effective recorder re-
sponse should be sufficiently fast so that it can be neglected in
1.5 The values stated in SI units are to be regarded as
sensitivity of measurements. If additional amplifiers are used
standard. No other units of measurement are included in this
between the detector and the final readout device, their
standard.
characteristics should also first be established.
1.6 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the
4. Principles of Electrolytic Conductivity Detectors
responsibility of the user of this standard to establish appro-
4.1 The principle components of the ELCD are represented
priate safety and health practices and determine the applica-
in Fig. 1 and include: a control module, a reactor assembly,
bility of regulatory limitations prior to use.
and, a cell assembly.
4.1.1 The control module typically will house the detector
electronics that monitor or control, or both, the solvent flow,
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
Spectroscopy and Separation Science and is the direct responsibility of Subcom-
mittee E13.19 on Separation Science. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Nov. 1, 2010. Published November 2010. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1995. Last previous edition approved in 2005 as E1698–95(2005). Standards volume information, refer to the standard’s Document Summary page on
DOI: 10.1520/E1698-95R10. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E1698 − 95 (2010)
products with a scrubber (if needed), dissolving the reaction
products in a suitable solvent, and measuring the change in
electricalconductivityusingaconductivitydetectorcell.Other
suitable non-catalystic reaction tubes can be used for more
selective response characteristics. Using the conditions set
forth in this practice, halogen (Cl, Br, I, F) compounds,
nitrogen compounds, and sulfur compounds can be measured
selectively, even in the presence of each other.
4.3 The electrolytic conductivity detector pyrolyzes com-
poundsastheyelutefromthechromatographiccolumnthrough
a hot nickel reaction tube. Halogen and nitrogen compounds
aredetectedunderreducingconditionswhilesulfurcompounds
are detected under oxidizing conditions. The effluent from the
gaschromatographiccolumniscombinedwitheitherhydrogen
(reducing conditions) or air (oxidizing conditions) before
entering the heated (800 to 1000°C) nickel reaction tube. The
compound is converted to small inorganic reaction products
depending upon the reaction conditions as shown in Table 1.
FIG. 1 ELCD—Principal Components
4.4 Table 2 shows the chemistry and modes of selective
response for the detector. Depending upon the mode of
reactiontemperatures,andtheconductivitydetectorcell.Itcan
operation,variousinterferingreactionproductsareremovedby
be functionally independent of the gas chromatography or, in
employing a selective gas scrubber before the product gases
some varieties, designed into the functional framework of the
reach the detector cell. In the nitrogen-specific mode, halogen
gas chromatograph. However, the reactor and cell assemblies
and sulfur products are removed by reaction with a caustic
are designed for specific models of gas chromatographs so it is
scrubber. In the sulfur-specific mode, halogen products are
important the proper components be assembled on the appro-
removed by a silver thread (or wire) scrubber. No scrubber is
priate chromatographic equipment.
required for halogen mode operation.
4.2 Fig. 2 is a block diagram representation of the GC/
4.5 The reaction products pass to the conductivity cell
ELCD system. The electrolytic conductivity detector detects
compounds by pyrolyzing those compounds in a heated nickel where they are combined with the solvent. The following
catalyst (housed in the reactor), removing interfering reaction solvents are typically used for normal operation in each
FIG. 2 GC/ELCD System Overview
E1698 − 95 (2010)
TABLE 1 Pyrolysis Reaction Products Formed Under Oxidizing
thefrontofthecellandcontactthesolventwhichisintroduced
or Reducing Conditions
through the side of the cell. Together, these entities pass
Oxidizing Element Reducing
throughtheelectrodeareaandthenoutthroughthebackofthe
CO CCH
2 4
cell.
HOH H
2 2
NO/N NNH 5.6 Solvent—The solvent is selected to provide the desired
2 3
HX, HOX X HX
sensitivity and selectivity for each mode of operation. The
O OH O
2 2
solvent must be deionized, having a low conductivity, neutral
SO /SO SH S
2 3 2
pH, and must be able to dissolve the appropriate reaction
products.Theincreaseinconductivityofthesolventduetothe
presence of the reaction products results in a peak response
indicated mode. Other solvents may be used to provide
corresponding to the original analyte. The solvent level in the
changes in selectivity and sensitivity (see 6.7):
reservoir should be maintained weekly and the solvent com-
Model Solvent
pletely replaced every three months using high-purity solvents
for best results.
Halogen 1-Propanol
Sulfur 100 % Methanol
5.7 Solvent Delivery System—The system consists of a
Nitrogen 10 %t-Butyl Alcohol/90 % Water
pump and an ion exchange resin system which works to both
4.6 Theincreaseinelectricalconductivityofthesolventas
deionizeandneutralizethepHofthesolvent.Aby-passsystem
aresultoftheintroductionofthereactionproductsismeasured
is used to allow the pump to run at a normal speed while still
by the sensing electrodes in the conductivity cell. The solvent
delivering the low solvent flow rates (30 to 100 µL/min)
passes through the cell after being deionized through an ion
required by the detector. For operation in the nitrogen mode
exchange resin bed located between the conductivity cell and
special solvent delivery systems may be required to ensure the
solvent reservoir. In most instruments the solvent is recycled
pHofthewater-basedsolventremainsneutral.Refertospecific
by taking the solvent from the cell back into the solvent
instructions provided by the manufacturer of the respective
reservoir.
detector you are employing on your gas chromatograph. It is
important to note that each mode will require specific resins
5. Detector Construction
which will require periodic replacement and attention given to
5.1 Thereissomevariationinthemethodofconstructionof
expiration dates for their useful life-time. Resins should be
this detector. In general, the geometry and construction of the
mixed thoroughly before adding or replacing as the anion/
conductivity cell is the single distinguishing component be-
cationmixtureusedbymostmanufacturerswillseparateunless
tween detector designs. It is not considered pertinent to review
a prepacked resin cartridge is used.
all aspects of the different detector designs available but rather
5.8 Module—All operational functions, except for detector
to consider one generalized design as an example and recog-
base temperature, are controlled from the module. On some
nize that variants may exist.
systems, vent time can be controlled from the gas chromato-
5.2 Detector Base—The base extends into the gas chroma-
graph as an external event.
tography oven and permits an inert low dead volume interface
5.9 Vent Valve—When opened, the vent valve provides a
of the column to the reactor. The carrier gas, the reaction gas,
way of preventing unwanted column effluents from entering
and the make-up gas (if needed) are introduced at the detector
the reaction tube. These effluents may include substances such
base. The base is heated and controlled by the gas chromato-
as the sample injection solvent and column bleed which can
graph or allowed to track the gas chromatograph oven tem-
cause fouling or poisoning of the nickel reaction tube’s
perature.
catalytic surface. The valve is otherwise kept closed to allow
5.3 Reaction Tube—The nickel pyrolysis tube interfaces to
the compounds of interest to pass into the reaction tube so that
the detector base and is heated by a heating element called the
they may be detected. The valve interfaces with the detector
reactor which surrounds the tube. The normal operating tem-
base by means of a vent tube connected at the column exit in
perature is 800 to 1000°C for most applications.
thebase.Itisimportantthatthegasflowfromthevent(ifused)
be measured daily to ensure reproducible results and retention
5.4 Scrubber—A coiled tube, used in either the nitrogen or
times.
sulfur mode, containing a specific scrubbing material is placed
between the exit of the pyrolysis tube and the entrance of the
6. Equipment Preparation
conductivity cell in order to remove certain reaction products
which may interfere in the specific mode of operation. Re- 6.1 The detector will be evaluated as part of a gas chro-
placement of the scrubber is mandated by response to any matograph using injections of gases or liquid samples which
halogen compound. have a range of component concentrations.
5.5 Conductivity Cell—The conductivity cell consists of a 6.2 Gases—All gases passing through the reactor should be
plastic block containing two metal electrodes that measure the ultra-highpurity(99.999%)grade.Heliumorhydrogencanbe
electrolytic conductivity of the solvent. It is connected to the used as the GC column carrier gas. Nitrogen is extremely
reactor exit by means of an inert (usually TFE-fluorocarbon) detrimentaltotheperformanceofthedetectorinallmodes,and
transfertube.Itprovidestheconductivitysignalforthespecific therefore cannot be used as a carrier of makeup gas nor can it
compound. Gaseous products from the reaction tube enter into be tolerated as a low level contaminant. No attempt will be
E1698 − 95 (2010)
TABLE 2 Reaction Products Produced in the ELCD Using a Nickel Reaction Tube
Compound Main Reaction Products Comments
Reductive Conditions:
Halogen compounds HX HX can be removed by N-mode scrubber and is selectively detected in X-mode.
Sulfur compounds HSH S can be removed by N-mode scrubber and is poorly ionized in the X-mode.
2 2
Nitrogen compounds NH NH is poorly ionized in the X-mode and selectively detected in N-mode.
3 3
Alkanes CH , Lower Alkanes Products are not ionized in any mode.
Oxygen compounds HOH O gives little response in X-mode and N-mode.
2 2
Oxidative Conditions:
Halogen compounds HX, HOX HX can be removed by S-mode scrubber.
Sulfur compounds SO SO is selectively detected in S-mode.
2 2
Nitrogen compounds N and certain nitrogen oxides at No or little response.
elevated temperatures
Alkanes CO ,HOCO is poorly ionized in S-mode. H O gives little or no response.
2 2 2 2
madeheretoguidetheselectionofoptimumconditions,except
Sulfur Mode
Methanol Lower Higher
to state that experience has shown that gases of the highest
Methanol/20 % Water Normal Normal
available purity result in far fewer detector problems and
Ethanol Lowest Highest
difficulties. Poor quality, hydrogen has been found to be the
Nitrogen Mode
cause of noise, low response, wandering baseline, and peak
10 % t-Butyl Alcohol/Water Higher Higher
tailing when operating in the halogen or nitrogen modes.
50 % 1-Propanol/50 % Water Normal Normal
Similarly, the highest grade of air works best for the sulfur
6.7.1 Insolventsystemsrequiringwater,useonlydeionized
mode.
water with a resistivity of 18 MΩ or better. It should also be
6.3 Hardware—High-purity gases require ultra-clean
noted the binary solvent systems will change in their propor-
regulators,valves,andtubing.Useofcleanregulators,employ-
tions due to normal evaporation. It is suggested that those
ing stainless steel valves, diaphragms, and tubing have been
solventsbecheckedbiweeklyandthereservoirtoppedoffwith
found to result in far fewer detector problems and difficulties.
fresh solvent.
6.4 Columns—All columns, whether packed or capillary,
6.8 Solvent Flow—Electrolyte flow rates range from 25 to
should be
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