ASTM E1140-95
(Practice)Standard Practice for Testing Nitrogen/Phosphorus Thermionic Ionization Detectors for Use In Gas Chromatography
Standard Practice for Testing Nitrogen/Phosphorus Thermionic Ionization Detectors for Use In Gas Chromatography
SCOPE
1.1 This practice is intended to serve as a guide for testing the performance of a nitrogen/phosphorus thermionic ionization detector (NPD) used as the detection component of a gas chromatographic system.
1.2 This practice applies to an NPD that employs a heated alkali metal compound and emits an electrical charge from that solid surface.
1.3 This practice addresses the operation and performance of the NPD independently of the chromatographic column. However, the performance is specified in terms that the analyst can use to predict overall system performance when the detector is coupled to the column and other chromatographic components.
1.4 For general chromatographic procedures, Practice E260 should be followed except where specific changes are recommended herein for the use of a nitrogen/phosphorus (N/P) thermionic detector.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific safety information, see Section 5, Hazards.
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Designation: E 1140 – 95
Standard Practice for
Testing Nitrogen/Phosphorus Thermionic Ionization
Detectors for Use In Gas Chromatography
This standard is issued under the fixed designation E 1140; 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 CGAV-7 Standard Method of Determining Cylinder Valve
Outlet Connections for Industrial Gas Mixtures
1.1 This practice is intended to serve as a guide for testing
CGA P-12 Safe Handling of Cryogenic Liquids
the performance of a nitrogen/phosphorus thermionic ioniza-
HB-3 Handbook of Compressed Gases
tion detector (NPD) used as the detection component of a gas
chromatographic system.
3. Terminology
1.2 This practice applies to an NPD that employs a heated
3.1 Definitions:
alkali metal compound and emits an electrical charge from that
3.1.1 For definitions of gas chromatography and its various
solid surface.
terms, see Practice E 355.
1.3 This practice addresses the operation and performance
3.2 Definitions of Terms Specific to This Standard:
of the NPD independently of the chromatographic column.
3.2.1 drift—the average slope of the noise envelope ex-
However, the performance is specified in terms that the analyst
pressed in amps/h as measured over ⁄2h.
can use to predict overall system performance when the
3.2.2 linear range—range of mass flow rates of nitrogen or
detector is coupled to the column and other chromatographic
phosphorus in the carrier gas, over which the sensitivity of the
components.
detector is constant to within 5 % as determined from the
1.4 For general chromatographic procedures, Practice E 260
linearity plot.
should be followed except where specific changes are recom-
3.2.3 minimum detectability—the mass flow rate of nitrogen
mended herein for the use of a nitrogen/phosphorus (N/P)
or phosphorus in the carrier gas that gives a detector signal
thermionic detector.
equal to twice the noise level.
1.5 This standard does not purport to address all of the
3.2.4 noise (short term)—the amplitude, expressed in am-
safety concerns, if any, associated with its use. It is the
peres, of the baseline envelope that includes all random
responsibility of the user of this standard to establish appro-
variations of the detector signal of a frequency greater than one
priate safety and health practices and determine the applica-
cycle per minute.
bility of regulatory limitations prior to use. For specific safety
3.2.5 selectivity—the ratio of the response per gram of
information, see Section 5, Hazards.
nitrogen or phosphorus in the test substance to the response per
2. Referenced Documents gram of carbon in octadecane.
2.1 ASTM Standards:
4. Significance and Use
E 260 Practice for Packed Column Gas Chromatography
4.1 Although it is possible to observe and measure each of
E 355 Practice for Gas Chromatography Terms and Rela-
2 the several characteristics of a detector under different and
tionships
unique conditions, it is the intent of this practice that a
2.2 CGA Standards:
complete set of detector specifications be obtained at the same
CGA P-1 Safe Handling of Compressed Gases in Contain-
3 operating conditions, including geometry, flow rates, and
ers
temperatures. To specify a detector’s capability completely, its
CGA G-5.4 Standard for Hydrogen Piping Systems at
3 performance should be measured at several sets of conditions
Consumer Locations
3 within the useful range of the detector. The terms and tests
CGA P-9 The Inert Gases: Argon, Nitrogen and Helium
described in this practice are sufficiently general so that they
may be used under any chosen conditions.
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
4.2 Linearity and speed of response of the recorder should
Spectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-
be such that it does not distort or otherwise interfere with the
tography.
Current edition approved May 15, 1995. Published July 1995. Originally performance of the detector. Effective recorder response should
published as E 1140 – 86. Last previous edition E 1140 – 93.
be sufficiently fast so that its effect on the sensitivity of
Annual Book of ASTM Standards, Vol 14.02.
measurement is negligible. If additional amplifiers are used
Available from Compressed Gas Association, Inc., 1725 Jefferson Davis
between the detector and the final readout device, their
Highway, Arlington, VA 22202-4100.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 1140
characteristics should first be established. other components for mechanical support, such as a ceramic
core.
5. Hazards
7.3 The inorganic salt mixture is usually connected to, or
5.1 Gas Handling Safety—The safe handling of compressed
supported by, a wire of platinum or other noncorrosive mate-
gases and cryogenic liquids for use in chromatography is the
rial. In some designs the bead is heated by passing a current
responsibility of every laboratory. The Compressed Gas Asso-
through this wire; in others, the bead is heated by hydrogen
ciation, (CGA), a member group of specialty and bulk gas
combustion, for example, the burning flame itself.
suppliers, publishes the following guidelines to assist the
7.4 The carrier gas (usually helium or nitrogen) flows
laboratory chemist to establish a safe work environment.
through a jet as in normal FID practice and mixes, prior to
Applicable CGA publications include: CGA P-1, CGA G-5.4,
leaving the jet, with a small volume of hydrogen. Combustion
CGA P-9, CGA V-7, CGA P-12, and HB-3.
gas (usually air) is fed around the jet in some manner and then
moves over or around the bead before exiting from the
6. Application
detector. It is worth noting that if this mixture is lean enough,
6.1 The N/P thermionic detector is an element-specific
due to low hydrogen flow, there will be insufficient fuel to
ionization detector that is essentially a major modification of
maintain a true flame.
the flame ionization detector (FID). As in the normal FID, it
measures increase in ionization current passing between two
8. Equipment Preparation
electrodes, one of which is polarized relative to the other.
8.1 The detector shall be evaluated as part of a gas chro-
Usually these are the inorganic salt source and the collector,
matograph using injections of liquid samples that have a range
with one often being at ground potential.
of component concentrations.
6.2 The mechanism of the detector will only be discussed
8.1.1 The detector shall be operated with carrier gas type
briefly in this practice partly because full understanding of the
and hydrogen and oxidizer gas flow rates as recommended by
detector is not presently available and partly because the
the manufacturer of the equipment. No attempt will be made in
substantial differences in bead chemistry, detector geometry,
this practice to guide the selection of optimum conditions,
and bead heating mechanism prevent a singular view being
except to state that because selectivity and sensitivity of the
given.
NPD are very dependent on the hydrogen flow rate, several
6.3 The addition of a heated alkali metal compound in the
flow rates (in the range 1 to 8 mL/min for the electrically-
detector area causes enhancement of the response for carbon-
heated bead detector) should be tested for optimum detector
nitrogen and carbon-phosphorus bonds. In addition, the selec-
performance.
tivity of response can be further enhanced when the bead is
8.1.2 The complete set of performance specifications must
electrically heated. Lower hydrogen and air flow rates that
be determined at the same operating conditions, since the
diminish the normal flame ionization response for hydrocarbon
absolute sensitivity and noise vary independently over a wide
compounds can be used. This selective enhancement allows the
range depending on the operating conditions. Once selected,
NPD to be used for the detection of very small quantities of
the operating conditions should not be changed during the
nitrogen- and phosphorus-containing compounds without in-
determination of the detector characteristics.
terference from the signal of other molecular species.
8.1.3 Detector stability over the course of the evaluation is
6.4 The selective response to C-N and C-P bonds means that
essential for meaningful results. This may be monitored by
the detector is not suitable for permanent gas or elemental
checking the bead temperature, the heating current, gas flows,
nitrogen or phosphorus analysis in the true definition of the
and other parameters during the evaluation as dictated by the
term. It should be noted, however, that some volatile inorganic
instrument manufacturer. (Some electrically-heated beads tend
phosphorous compounds do give a strong response with this
to lose sensitivity continuously with operating time and require
detector, comparable to that of organophosphorous com-
increasing the bead heating current to recover lost sensitivity.)
pounds.
8.2 Column—Any column that fully separates the sample
components without causing overload or sample adsorption
7. Detector Construction
may be used. One suitable column isa4ftby2mm glass
7.1 There is a wide variation in the method of construction
column packed with 100/120 mesh deactivated chromosorb W
of this detector. It is not considered pertinent to review all
coated with 2 wt. % dimethyl silicone oil.
aspects of the different detector designs available, but to
8.3 Gases—With N/P thermionic detectors it is of critical
consider one generalized design as an example and recognize
importance that all gases are pure and that the gas lines are not
that many significant variants may exist. Examples of signifi-
contaminated with oils, solder flux, etc. The use of well
cant differences may exist in bead chemistry and method of
conditioned molecular sieve traps in all lines helps to achieve
heating, space jet and collector configuration, potential applied
this purity. If the chromatograph is fitted with in-line chemical
across the cell, its polarity, and the flow rates and composition
filters after the gas regulators and flow controllers, they also
of the three gases used.
should be well conditioned to ensure that no contaminants
7.2 An essential part of the N/P thermionic detector is the
reach the column from elastomeric diaphragms contained in
presence, in the active area of the detector, of an inorganic
these parts.
material containing an alkali metal, often rubidium. The
inorganic material may be a salt or silicate. It is usually, but not
NOTE 1—To condition a molecular sieve 5A column well, heat the trap
necessarily, present in bead form and may be combined with with a slow flow of carrier gas at 350°C for a minimum of 2 h.
E 1140
8.4 Gas Connections—All gas tubing and connections will vary for different manufacturers’ devices but may include
should be made of cleaned copper or stainless steel, including accurate constant voltage supplies or pulse-generating equip-
all ferrules and joints within the system. Vespel and graphite ment. The instruction manual should be studied and thoroughly
ferrules may be used for GC column connections provided that understood before attempting to use electronic integration for
they are sufficiently conditioned after installation. These steps peak area or peak height measurements.
will minimize contamination problems.
11. Test Substances
9. Sample Preparation
11.1 The test substance and the conditions under which the
9.1 A solution containing three compounds dissolved in
detector sensitivity is measured must be stated. This will
isooctane should be used, with great emphasis placed on the include, but not necessarily be limited to, the following: type of
purity of all chemicals and particularly the solvent. Blank runs
detector, detector geometry (for example, source of alkali
should be made on the solvent to ensure that no interfering metal), carrier gas, carrier gas flow rate (corrected to detector
peaks elute at the same time as the compounds of interest,
temperature), detector temperature, detector polarizing voltage,
which would invalidate the results. The three test compounds hydrogen flow rate, air flow rate, method of measurement, and
are azobenzene for nitrogen response (15.38 % nitrogen),
electrometer range setting.
malathion for phosphorus response (9.38 % phosphorus), and 11.2 Azobenzene is the standard nitrogen-containing test
octadecane for specificity (84.95 % carbon). Azobenzene and
substance. Malathion is the standard phosphorus-containing
malathion should be mixed in an appropriate ratio to allow test substance. Measurement of the test substance must be
comparable peak heights under the isothermal conditions used.
made within the linear range of the detector and at a signal
Typical ratios are between 0.5 and 2.0, depending on detector level at least 100 times greater than the minimum detectability
construction and operating conditions. Concentration limits (200 times greater than noise level).
between 1 μg/L and 1 mg/L are recommended initial values.
12. Test Conditions
The octadecane need be checked only at one concentration
12.1 Measure the noise level in accordance with the speci-
level for specificity, and the recommended concentration for
this should be 1 g/L. fications given in Section 13. Measure sensitivity in accor-
dance with the specifications given in 14.1. Both sensitivity
9.2 Because of the toxicity of malathion, it is recommended
that a dilute solution be used as the starting material, and that and noise level measurements must be carried out under the
this solution be purchased from one of the special supply same conditions (for example, carrier gas flow rate and
houses that routinely make chemical standards. Precautions for detector temperature) and preferably at the same time. When
handling toxic materials must be followed throughout the stating the minimum detectability, state the noise level on
dilution sequence as standard good laboratory practice. which the calculation was based.
9.3 Sample Injection—The recommended procedure for
13. Procedure for Noise and Drift Measurement
accurate injection of liquid samples is the “solvent flush,” or
13.1 Noise includes fluctuations of the baseline envelope of
Burke injection technique, in which a carefully washed 10 μL
a frequency less than one cycle per minute. The amplitude of
syringe is loaded with 1 to 2 μL solvent, 1 μL air, 3 μL sample,
these fluctuations may actually exceed the short-term noise.
and 1 μL air. While time consuming, this
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