ASTM E1698-95(2000)
(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
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 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 2000)
Standard Practice for
Testing Electrolytic Conductivity Detectors (ELCD) Used in
Gas Chromatography
This standard is issued under the fixed designation E 1698; 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 unique conditions, in particular its different modes of selectiv-
ity,itistheintentofthispracticethatacompletesetofdetector
1.1 This practice covers testing the performance of an
specifications should be obtained at the same operating condi-
electrolytic conductivity detector (ELCD) used as the detection
tions, including geometry, gas and solvent flow rates, and
component of a gas chromatographic system.
temperatures. It should be noted that to specify a detector’s
1.2 This practice is directly applicable to electrolytic con-
capability completely, its performance should be measured at
ductivity detectors that perform a chemical reaction on a given
several sets of conditions within the useful range of the
sample over a nickel catalyst surface under oxidizing or
detector. The terms and tests described in this practice are
reducing conditions and employ a scrubber, if needed, to
sufficiently general so that they may be used at whatever
remove interferences, deionized solvent to dissolve the reac-
conditions may be chosen for other reasons.
tion products, and a conductivity cell to measure the electro-
3.2 Linearity and speed of response of the recorder used
lytic conductivity of ionized reaction products.
should be such that it does not distort or otherwise interfere
1.3 This practice covers the performance of the detector
with the performance of the detector. Effective recorder re-
itself, independently of the chromatographic column, in terms
sponse should be sufficiently fast so that it can be neglected in
that the analyst can use to predict overall system performance
sensitivity of measurements. If additional amplifiers are used
when the detector is coupled to the column and other chro-
between the detector and the final readout device, their
matographic system components.
characteristics should also first be established.
1.4 For general gas chromatographic procedures, Practice
E 260 should be followed except where specific changes are
4. Principles of Electrolytic Conductivity Detectors
recommended herein for the use of an electrolytic conductivity
4.1 The principle components of the ELCD are represented
detector. For definitions of gas chromatography and its various
in Fig. 1 and include: a control module, a reactor assembly,
terms see Practice E 355.
and, a cell assembly.
1.5 This standard does not purport to address all of the
4.1.1 The control module typically will house the detector
safety concerns, if any, associated with its use. It is the
electronics that monitor or control, or both, the solvent flow,
responsibility of the user of this standard to establish appro-
reaction temperatures, and the conductivity detector cell. It can
priate safety and health practices and determine the applica-
be functionally independent of the gas chromatography or, in
bility of regulatory limitations prior to use.
some varieties, designed into the functional framework of the
2. Referenced Documents gas chromatograph. However, the reactor and cell assemblies
are designed for specific models of gas chromatographs so it is
2.1 ASTM Standards:
2 important the proper components be assembled on the appro-
E 260 Practice for Packed Column Gas Chromatography
priate chromatographic equipment.
E 355 Practice for Gas Chromatography Terms and Rela-
4.2 Fig. 2 is a block diagram representation of the GC/
tionships
ELCD system. The electrolytic conductivity detector detects
3. Significance and Use compounds by pyrolyzing those compounds in a heated nickel
catalyst (housed in the reactor), removing interfering reaction
3.1 Although it is possible to observe and measure each of
products with a scrubber (if needed), dissolving the reaction
the several characteristics of the ELCD under different and
products in a suitable solvent, and measuring the change in
electrical conductivity using a conductivity detector cell. Other
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
suitable non-catalystic reaction tubes can be used for more
Spectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-
selective response characteristics. Using the conditions set
tography.
forth in this practice, halogen (Cl, Br, I, F) compounds,
Current edition approved March 15, 1995. Published July 1995.
Annual Book of ASTM Standards, Vol 14.02.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1698–95 (2000)
5. Detector Construction
5.1 There is some variation in the method of construction of
this detector. In general, the geometry and construction of the
conductivity cell is the single distinguishing component be-
tween detector designs. It is not considered pertinent to review
all aspects of the different detector designs available but rather
to consider one generalized design as an example and recog-
nize that variants may exist.
5.2 Detector Base—The base extends into the gas chroma-
tography oven and permits an inert low dead volume interface
of the column to the reactor. The carrier gas, the reaction gas,
and the make-up gas (if needed) are introduced at the detector
base. The base is heated and controlled by the gas chromato-
graph or allowed to track the gas chromatograph oven tem-
perature.
5.3 Reaction Tube—The nickel pyrolysis tube interfaces to
the detector base and is heated by a heating element called the
reactor which surrounds the tube. The normal operating tem-
FIG. 1 ELCD—Principal Components
perature is 800 to 1000°C for most applications.
5.4 Scrubber—A coiled tube, used in either the nitrogen or
nitrogen compounds, and sulfur compounds can be measured
sulfur mode, containing a specific scrubbing material is placed
selectively, even in the presence of each other.
between the exit of the pyrolysis tube and the entrance of the
4.3 The electrolytic conductivity detector pyrolyzes com-
conductivity cell in order to remove certain reaction products
poundsastheyelutefromthechromatographiccolumnthrough
which may interfere in the specific mode of operation. Re-
a hot nickel reaction tube. Halogen and nitrogen compounds
placement of the scrubber is mandated by response to any
aredetectedunderreducingconditionswhilesulfurcompounds
halogen compound.
are detected under oxidizing conditions. The effluent from the
5.5 Conductivity Cell—The conductivity cell consists of a
gas chromatographic column is combined with either hydrogen
plastic block containing two metal electrodes that measure the
(reducing conditions) or air (oxidizing conditions) before
electrolytic conductivity of the solvent. It is connected to the
entering the heated (800 to 1000°C) nickel reaction tube. The
reactor exit by means of an inert (usually TFE-fluorocarbon)
compound is converted to small inorganic reaction products
transfertube.Itprovidestheconductivitysignalforthespecific
depending upon the reaction conditions as shown in Table 1.
compound. Gaseous products from the reaction tube enter into
4.4 Table 2 shows the chemistry and modes of selective
the front of the cell and contact the solvent which is introduced
response for the detector. Depending upon the mode of
through the side of the cell. Together, these entities pass
operation,variousinterferingreactionproductsareremovedby
through the electrode area and then out through the back of the
employing a selective gas scrubber before the product gases
cell.
reach the detector cell. In the nitrogen-specific mode, halogen
5.6 Solvent—The solvent is selected to provide the desired
and sulfur products are removed by reaction with a caustic
sensitivity and selectivity for each mode of operation. The
scrubber. In the sulfur-specific mode, halogen products are
solvent must be deionized, having a low conductivity, neutral
removed by a silver thread (or wire) scrubber. No scrubber is
pH, and must be able to dissolve the appropriate reaction
required for halogen mode operation.
products.The increase in conductivity of the solvent due to the
4.5 The reaction products pass to the conductivity cell
presence of the reaction products results in a peak response
where they are combined with the solvent. The following
corresponding to the original analyte. The solvent level in the
solvents are typically used for normal operation in each
reservoir should be maintained weekly and the solvent com-
indicated mode. Other solvents may be used to provide
pletely replaced every three months using highpurity solvents
changes in selectivity and sensitivity (see 6.7):
for best results.
Model Solvent
5.7 Solvent Delivery System—The system consists of a
pump and an ion exchange resin system which works to both
Halogen 1-Propanol
Sulfur 100 % Methanol
deionizeandneutralizethepHofthesolvent.Aby-passsystem
Nitrogen 10 %t-Butyl Alcohol/90 % Water
is used to allow the pump to run at a normal speed while still
4.6 The increase in electrical conductivity of the solvent as delivering the low solvent flow rates (30 to 100 µL/min)
a result of the introduction of the reaction products is measured required by the detector. For operation in the nitrogen mode
by the sensing electrodes in the conductivity cell. The solvent special solvent delivery systems may be required to ensure the
passes through the cell after being deionized through an ion pHofthewater-basedsolventremainsneutral.Refertospecific
exchange resin bed located between the conductivity cell and instructions provided by the manufacturer of the respective
solvent reservoir. In most instruments the solvent is recycled detector you are employing on your gas chromatograph. It is
by taking the solvent from the cell back into the solvent important to note that each mode will require specific resins
reservoir. which will require periodic replacement and attention given to
E1698–95 (2000)
FIG. 2 GC/ELCD System Overview
TABLE 1 Pyrolysis Reaction Products Formed Under Oxidizing
6. Equipment Preparation
or Reducing Conditions
6.1 The detector will be evaluated as part of a gas chro-
Oxidizing Element Reducing
matograph using injections of gases or liquid samples which
CO CCH
2 4
have a range of component concentrations.
HOH H
2 2
NO/N NNH 6.2 Gases—All gases passing through the reactor should be
2 3
HX, HOX X HX
ultra-high purity (99.999 %) grade. Helium or hydrogen can be
O OH O
2 2
used as the GC column carrier gas. Nitrogen is extremely
SO /SO SH S
2 3 2
detrimentaltotheperformanceofthedetectorinallmodes,and
therefore cannot be used as a carrier of makeup gas nor can it
be tolerated as a low level contaminant. No attempt will be
expiration dates for their useful life-time. Resins should be
madeheretoguidetheselectionofoptimumconditions,except
mixed thoroughly before adding or replacing as the anion/
to state that experience has shown that gases of the highest
cationmixtureusedbymostmanufacturerswillseparateunless
available purity result in far fewer detector problems and
a prepacked resin cartridge is used.
difficulties. Poor quality, hydrogen has been found to be the
5.8 Module—All operational functions, except for detector
cause of noise, low response, wandering baseline, and peak
base temperature, are controlled from the module. On some
tailing when operating in the halogen or nitrogen modes.
systems, vent time can be controlled from the gas chromato-
Similarly, the highest grade of air works best for the sulfur
graph as an external event.
mode.
5.9 Vent Valve—When opened, the vent valve provides a
6.3 Hardware—High-purity gases require ultra-clean regu-
way of preventing unwanted column effluents from entering
lators, valves, and tubing. Use of clean regualtors, employing
the reaction tube. These effluents may include substances such
stainless steel valves, diaphragms, and tubing have been found
as the sample injection solvent and column bleed which can
to result in far fewer detector problems and difficulties.
cause fouling or poisoning of the nickel reaction tube’s
catalytic surface. The valve is otherwise kept closed to allow 6.4 Columns—All columns, whether packed or capillary,
the compounds of interest to pass into the reaction tube so that should be fully conditioned according to supplier’s specifica-
they may be detected. The valve interfaces with the detector tions prior to connecting to the detector. Certain liquid phases
base by means of a vent tube connected at the column exit in that are not compatible with the mode of operation should be
thebase.Itisimportantthatthegasflowfromthevent(ifused) avoided. Use of silanes (such as those used in deactivation of
be measured daily to ensure reproducible results and retention glass liners and columns) should be avoided since they have
times. been shown to poison the reactor tube.
E1698–95 (2000)
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
6.5 Reactor Temperature—The target reactor temperature is 7.1.1 Halogen Mode—The headspace in a bottle of chloro-
800 to 900°C. However, other reactor temperatures may be form (CHCl ) or methylene chloride (CH Cl ).
3 2 2
found to provide better results with certain compound types.
7.1.2 Nitrogen Mode—The headspace in a bottle of ni-
Some typical reactor temperatures are given as follows:
tromethane, acetonitrile, one of the NOx gases, or some other
6.5.1 800 to 900°C for most halogen-mode applications,
low-boiling nitrogen compound.
6.5.2 850 to 925°C fo
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