ASTM E1698-95(2005)
(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’ 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 E 260 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 E 355.
1.5 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
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(Reapproved2005)
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope E355 Practice for Gas Chromatography Terms and Rela-
tionships
1.1 This practice covers testing the performance of an
electrolyticconductivitydetector(ELCD)usedasthedetection
3. Significance and Use
component of a gas chromatographic system.
3.1 Although it is possible to observe and measure each of
1.2 This practice is directly applicable to electrolytic con-
the several characteristics of the ELCD under different and
ductivity detectors that perform a chemical reaction on a given
unique conditions, in particular its different modes of selectiv-
sample over a nickel catalyst surface under oxidizing or
ity,itistheintentofthispracticethatacompletesetofdetector
reducing conditions and employ a scrubber, if needed, to
specifications should be obtained at the same operating condi-
remove interferences, deionized solvent to dissolve the reac-
tions, including geometry, gas and solvent flow rates, and
tion products, and a conductivity cell to measure the electro-
temperatures. It should be noted that to specify a detector’s
lytic conductivity of ionized reaction products.
capability completely, its performance should be measured at
1.3 This practice covers the performance of the detector
several sets of conditions within the useful range of the
itself, independently of the chromatographic column, in terms
detector. The terms and tests described in this practice are
that the analyst can use to predict overall system performance
sufficiently general so that they may be used at whatever
when the detector is coupled to the column and other chro-
conditions may be chosen for other reasons.
matographic system components.
3.2 Linearity and speed of response of the recorder used
1.4 For general gas chromatographic procedures, Practice
should be such that it does not distort or otherwise interfere
E260 should be followed except where specific changes are
with the performance of the detector. Effective recorder re-
recommended herein for the use of an electrolytic conductivity
sponse should be sufficiently fast so that it can be neglected in
detector. For definitions of gas chromatography and its various
sensitivity of measurements. If additional amplifiers are used
terms see Practice E355.
between the detector and the final readout device, their
1.5 The values stated in SI units are to be regarded as
characteristics should also first be established.
standard. No other units of measurement are included in this
standard.
4. Principles of Electrolytic Conductivity Detectors
1.6 This standard does not purport to address all of the
4.1 The principle components of the ELCD are represented
safety concerns, if any, associated with its use. It is the
in Fig. 1 and include: a control module, a reactor assembly,
responsibility of the user of this standard to establish appro-
and, a cell assembly.
priate safety and health practices and determine the applica-
4.1.1 The control module typically will house the detector
bility of regulatory limitations prior to use.
electronics that monitor or control, or both, the solvent flow,
reaction temperatures, and the conductivity detector cell. It can
2. Referenced Documents
be functionally independent of the gas chromatography or, in
2.1 ASTM Standards:
some varieties, designed into the functional framework of the
E260 Practice for Packed Column Gas Chromatography
gas chromatograph. However, the reactor and cell assemblies
are designed for specific models of gas chromatographs so it is
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
important the proper components be assembled on the appro-
Spectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-
priate chromatographic equipment.
tography.
4.2 Fig. 2 is a block diagram representation of the GC/
Current edition approved Sept. 1, 2005. Published September 2005. Originally
approved in 1995. Last previous edition approved in 2000 as E1698 – 95 (2000). ELCD system. The electrolytic conductivity detector detects
DOI: 10.1520/E1698-95R05.
compounds by pyrolyzing those compounds in a heated nickel
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
catalyst (housed in the reactor), removing interfering reaction
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
products with a scrubber (if needed), dissolving the reaction
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1698–95 (2005)
by the sensing electrodes in the conductivity cell. The solvent
passes through the cell after being deionized through an ion
exchange resin bed located between the conductivity cell and
solvent reservoir. In most instruments the solvent is recycled
by taking the solvent from the cell back into the solvent
reservoir.
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
FIG. 1 ELCD—Principal Components
base. The base is heated and controlled by the gas chromato-
graph or allowed to track the gas chromatograph oven tem-
perature.
products in a suitable solvent, and measuring the change in
5.3 Reaction Tube—The nickel pyrolysis tube interfaces to
electrical conductivity using a conductivity detector cell. Other
the detector base and is heated by a heating element called the
suitable non-catalystic reaction tubes can be used for more
reactor which surrounds the tube. The normal operating tem-
selective response characteristics. Using the conditions set
perature is 800 to 1000°C for most applications.
forth in this practice, halogen (Cl, Br, I, F) compounds,
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
gaschromatographiccolumniscombinedwitheitherhydrogen
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 high-purity 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)
aresultoftheintroductionofthereactionproductsismeasured required by the detector. For operation in the nitrogen mode
E1698–95 (2005)
FIG. 2 GC/ELCD System Overview
TABLE 1 Pyrolysis Reaction Products Formed Under Oxidizing
thebase.Itisimportantthatthegasflowfromthevent(ifused)
or Reducing Conditions
be measured daily to ensure reproducible results and retention
Oxidizing Element Reducing
times.
CO CCH
2 4
HOH H
2 2
6. Equipment Preparation
NO/N NNH
2 3
HX, HOX X HX
6.1 The detector will be evaluated as part of a gas chro-
O OH O
2 2
matograph using injections of gases or liquid samples which
SO /SO SH S
2 3 2
have a range of component concentrations.
6.2 Gases—All gases passing through the reactor should be
special solvent delivery systems may be required to ensure the
ultra-high purity (99.999 %) grade. Helium or hydrogen can be
pHofthewater-basedsolventremainsneutral.Refertospecific
used as the GC column carrier gas. Nitrogen is extremely
instructions provided by the manufacturer of the respective
detrimentaltotheperformanceofthedetectorinallmodes,and
detector you are employing on your gas chromatograph. It is
therefore cannot be used as a carrier of makeup gas nor can it
important to note that each mode will require specific resins
be tolerated as a low level contaminant. No attempt will be
which will require periodic replacement and attention given to
madeheretoguidetheselectionofoptimumconditions,except
expiration dates for their useful life-time. Resins should be
to state that experience has shown that gases of the highest
mixed thoroughly before adding or replacing as the anion/
available purity result in far fewer detector problems and
cationmixtureusedbymostmanufacturerswillseparateunless
difficulties. Poor quality, hydrogen has been found to be the
a prepacked resin cartridge is used.
cause of noise, low response, wandering baseline, and peak
5.8 Module—All operational functions, except for detector
tailing when operating in the halogen or nitrogen modes.
base temperature, are controlled from the module. On some
Similarly, the highest grade of air works best for the sulfur
systems, vent time can be controlled from the gas chromato-
mode.
graph as an external event.
6.3 Hardware—High-purity gases require ultra-clean regu-
5.9 Vent Valve—When opened, the vent valve provides a
lators, valves, and tubing. Use of clean regulators, employing
way of preventing unwanted column effluents from entering
stainless steel valves, diaphragms, and tubing have been found
the reaction tube. These effluents may include substances such
to result in far fewer detector problems and difficulties.
as the sample injection solvent and column bleed which can
6.4 Columns—All columns, whether packed or capillary,
cause fouling or poisoning of the nickel reaction tube’s
should be fully conditioned according to supplier’s specifica-
catalytic surface. The valve is otherwise kept closed to allow
the compounds of interest to pass into the reaction tube so that tions prior to connecting to the detector. Certain liquid phases
that are not compatible with the mode of operation should be
they may be detected. The valve interfaces with the detector
base by means of a vent tube connected at the column exit in avoided. Use of silanes (such as those used in deactivation of
E1698–95 (2005)
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
glass liners and columns) should be avoided since they have 7. Performance Evaluation
been shown to poison the reactor tube.
7.1 Test for Response—The detector can be determined to
6.5 Reactor Temperature—The target reactor temperature is
be r
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