ASTM E1140-95(2005)
(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
SIGNIFICANCE AND USE
Although it is possible to observe and measure each of the several characteristics of a detector under different and unique conditions, it is the intent of this practice that a complete set of detector specifications be obtained at the same operating conditions, including geometry, flow rates, and temperatures. 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 under any chosen conditions.
Linearity and speed of response of the recorder 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 its effect on the sensitivity of measurement is negligible. If additional amplifiers are used between the detector and the final readout device, their characteristics should first be established.
SCOPE
1.1 This practice covers 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 E 260 should be followed except where specific changes are recommended in this practice 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 , Hazards.
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NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
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Designation:E1140–95 (Reapproved 2005)
Standard Practice for
Testing Nitrogen/Phosphorus Thermionic Ionization
Detectors for Use In Gas Chromatography
This standard is issued under the fixed designation E1140; 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 CGAP-1 Safe Handling of Compressed Gases in Contain-
ers
1.1 This practice covers testing the performance of a
CGAG-5.4 Standard for Hydrogen Piping Systems at Con-
nitrogen/phosphorus thermionic ionization detector (NPD)
sumer Locations
used as the detection component of a gas chromatographic
CGAP-9 The Inert Gases: Argon, Nitrogen and Helium
system.
CGAV-7 Standard Method of Determining Cylinder Valve
1.2 This practice applies to an NPD that employs a heated
Outlet Connections for Industrial Gas Mixtures
alkalimetalcompoundandemitsanelectricalchargefromthat
CGAP-12 Safe Handling of Cryogenic Liquids
solid surface.
HB-3 Handbook of Compressed Gases
1.3 This practice addresses the operation and performance
of the NPD independently of the chromatographic column.
3. Terminology
However,theperformanceisspecifiedintermsthattheanalyst
3.1 Definitions:
can use to predict overall system performance when the
3.1.1 For definitions of gas chromatography and its various
detector is coupled to the column and other chromatographic
terms, see Practice E355.
components.
3.2 Definitions of Terms Specific to This Standard:
1.4 For general chromatographic procedures, Practice E260
3.2.1 drift—the average slope of the noise envelope ex-
should be followed except where specific changes are recom-
pressed in amps/h as measured over ⁄2 h.
mended in this practice for the use of a nitrogen/phosphorus
3.2.2 linear range—range of mass flow rates of nitrogen or
(N/P) thermionic detector.
phosphorus in the carrier gas, over which the sensitivity of the
1.5 This standard does not purport to address all of the
detector is constant to within 5% as determined from the
safety concerns, if any, associated with its use. It is the
linearity plot.
responsibility of the user of this standard to establish appro-
3.2.3 minimum detectability—themassflowrateofnitrogen
priate safety and health practices and determine the applica-
or phosphorus in the carrier gas that gives a detector signal
bility of regulatory limitations prior to use. For specific safety
equal to twice the noise level.
information, see Section 5, Hazards.
3.2.4 noise (short term)—the amplitude, expressed in am-
2. Referenced Documents peres, of the baseline envelope that includes all random
variationsofthedetectorsignalofafrequencygreaterthanone
2.1 ASTM Standards:
cycle per minute.
E260 Practice for Packed Column Gas Chromatography
3.2.5 selectivity—the ratio of the response per gram of
E355 Practice for Gas Chromatography Terms and Rela-
nitrogenorphosphorusinthetestsubstancetotheresponseper
tionships
gram of carbon in octadecane.
2.2 CGA Standards:
4. Significance and Use
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
4.1 Although it is possible to observe and measure each of
Spectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-
the several characteristics of a detector under different and
tography.
unique conditions, it is the intent of this practice that a
Current edition approved Feb. 1, 2005. Published March 2005. Originally
approved in 1986. Last previous edition approved in 2000 as E1140–95(2000). complete set of detector specifications be obtained at the same
DOI: 10.1520/E1140-95R05.
operating conditions, including geometry, flow rates, and
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
temperatures. To specify a detector’s capability completely, its
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Standards volume information, refer to the standard’s Document Summary page on performance should be measured at several sets of conditions
the ASTM website.
within the useful range of the detector. The terms and tests
Available from Compressed Gas Association (CGA), 1725 Jefferson Davis
Hwy., Suite 1004, Arlington, VA 22202-4102.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E1140–95 (2005)
described in this practice are sufficiently general so that they heating, space jet and collector configuration, potential applied
may be used under any chosen conditions. across the cell, its polarity, and the flow rates and composition
4.2 Linearity and speed of response of the recorder should of the three gases used.
be such that it does not distort or otherwise interfere with the 7.2 An essential part of the N/P thermionic detector is the
performanceofthedetector.Effectiverecorderresponseshould presence, in the active area of the detector, of an inorganic
be sufficiently fast so that its effect on the sensitivity of material containing an alkali metal, often rubidium. The
measurement is negligible. If additional amplifiers are used inorganicmaterialmaybeasaltorsilicate.Itisusually,butnot
between the detector and the final readout device, their necessarily, present in bead form and may be combined with
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
supported by, a wire of platinum or other noncorrosive mate-
5.1 Gas Handling Safety—Thesafehandlingofcompressed
rial. In some designs the bead is heated by passing a current
gases and cryogenic liquids for use in chromatography is the
through this wire; in others, the bead is heated by hydrogen
responsibility of every laboratory. The Compressed GasAsso-
combustion, for example, the burning flame itself.
ciation (CGA), a member group of specialty and bulk gas
7.4 The carrier gas (usually helium or nitrogen) flows
suppliers, publishes the following guidelines to assist the
through a jet as in normal FID practice and mixes, prior to
laboratory chemist to establish a safe work environment.
leaving the jet, with a small volume of hydrogen. Combustion
Applicable CGA publications include: CGAP-1, CGAG-5.4,
gas (usually air) is fed around the jet in some manner and then
CGAP-9, CGAV-7, CGAP-12, and HB-3.
moves over or around the bead before exiting from the
detector. It is worth noting that if this mixture is lean enough,
6. Application
due to low hydrogen flow, there will be insufficient fuel to
6.1 The N/P thermionic detector is an element-specific
maintain a true flame.
ionization detector that is essentially a major modification of
the flame ionization detector (FID). As in the normal FID, it 8. Equipment Preparation
measures increase in ionization current passing between two
8.1 The detector shall be evaluated as part of a gas chro-
electrodes, one of which is polarized relative to the other.
matograph using injections of liquid samples that have a range
Usually these are the inorganic salt source and the collector,
of component concentrations.
with one often being at ground potential.
8.1.1 The detector shall be operated with carrier gas type
6.2 The mechanism of the detector will only be discussed
and hydrogen and oxidizer gas flow rates as recommended by
briefly in this practice partly because full understanding of the
themanufactureroftheequipment.Noattemptwillbemadein
detector is not presently available and partly because the
this practice to guide the selection of optimum conditions,
substantial differences in bead chemistry, detector geometry,
except to state that because selectivity and sensitivity of the
and bead heating mechanism prevent a singular view being
NPD are very dependent on the hydrogen flow rate, several
given.
flow rates (in the range of 1 to 8 mL/min for the electrically
6.3 The addition of a heated alkali metal compound in the
heated bead detector) should be tested for optimum detector
detector area causes enhancement of the response for carbon-
performance.
nitrogen and carbon-phosphorus bonds. In addition, the selec-
8.1.2 The complete set of performance specifications must
tivity of response can be further enhanced when the bead is
be determined at the same operating conditions, since the
electrically heated. Lower hydrogen and air flow rates that
absolute sensitivity and noise vary independently over a wide
diminishthenormalflameionizationresponseforhydrocarbon
range depending on the operating conditions. Once selected,
compoundscanbeused.Thisselectiveenhancementallowsthe
the operating conditions should not be changed during the
NPD to be used for the detection of very small quantities of
determination of the detector characteristics.
nitrogen- and phosphorus-containing compounds without in-
8.1.3 Detector stability over the course of the evaluation is
terference from the signal of other molecular species.
essential for meaningful results. This may be monitored by
6.4 TheselectiveresponsetoC-NandC-Pbondsmeansthat
checking the bead temperature, the heating current, gas flows,
the detector is not suitable for permanent gas or elemental
and other parameters during the evaluation as dictated by the
nitrogen or phosphorus analysis in the true definition of the
instrument manufacturer. (Some electrically-heated beads tend
term. It should be noted, however, that some volatile inorganic
tolosesensitivitycontinuouslywithoperatingtimeandrequire
phosphorous compounds do give a strong response with this
increasing the bead heating current to recover lost sensitivity.)
detector, comparable to that of organophosphorus compounds.
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
E1140–95 (2005)
this purity. If the chromatograph is fitted with in-line chemical 10.1.2 Regardless of which method is used to calculate
filters after the gas regulators and flow controllers, they also linear range, peak height is the only acceptable method for
should be well conditioned to ensure that no contaminants determining minimum detectability.
reach the column from elastomeric diaphragms contained in 10.2 Calibration—It is essential to calibrate the measuring
these parts. system to ensure that the nominal specifications are acceptable
andparticularlytoverifytherangeoverwhichtheoutputofthe
NOTE 1—To condition a molecular sieve 5Acolumn well, heat the trap
device,whetherpeakareaorpeakheight,islinearwithrespect
with a slow flow of carrier gas at 350°C for a minimum of 2 h.
to input signal. Failure to perform this calibration may intro-
8.4 Gas Connections—All gas tubing and connections
duce substantial errors into the results. Methods for calibration
should be made of cleaned copper or stainless steel, including
will vary for different manufacturers’ devices but may include
all ferrules and joints within the system. Vespel and graphite
accurate constant voltage supplies or pulse-generating equip-
ferrulesmaybeusedforGCcolumnconnectionsprovidedthat
ment.Theinstructionmanualshouldbestudiedandthoroughly
they are sufficiently conditioned after installation. These steps
understood before attempting to use electronic integration for
will minimize contamination problems.
peak area or peak height measurements.
9. Sample Preparation
11. Test Substances
9.1 A solution containing three compounds dissolved in 11.1 The test substance and the conditions under which the
isooctane should be used, with great emphasis placed on the
detector sensitivity is measured must be stated. This will
purity of all chemicals and particularly the solvent. Blank runs include,butnotnecessarilybelimitedto,thefollowing:typeof
should be made on the solvent to ensure that no interfering
detector, detector geometry (for example, source of alkali
peaks elute at the same time as the compounds of interest,
metal), carrier gas, carrier gas flow rate (corrected to detector
which would invalidate the results. The three test compounds temperature),detectortemperature,detectorpolarizingvoltage,
are azobenzene for nitrogen response (15.38% nitrogen),
hydrogen flow rate, air flow rate, method of measurement, and
malathion for phosphorus response (9.38% phosphorus), and electrometer range setting.
octadecane for specificity (84.95% carbon). Azobenzene and
11.2 Azobenzene is the standard nitrogen-containing test
malathion should be mixed in an appropriate ratio to allow substance. Malathion is the standard phosphorus-containing
comparable peak heights under the isothermal conditions used.
test substance. Measurement of the test substance must be
Typical ratios are between 0.5 and 2.0, depending on detector made within the linear range of the detector and at a signal
construction and operating conditions. Concentration limits
level at least 100 times greater than the minimum detectability
between 1 µg/L and 1 mg/L are recommended initial values. (200 times greater than noise level).
The octadecane need be checked only at one concentration
level for specificity, and the recommended concentration for
12. Test Conditions
this should be 1 g/L.
12.1 Measure the noise level in accordance with the speci-
9.2 Because of the toxicity of malathion, it is recommended
fications given in Section 13. Measure sensitivity in accor-
that a dilute solution be used as the starting material, and that
dance with the specifications given in 14.1. Both sensitivity
this solution be purchased from one of the special supply
and noise level measurements must be carried out under the
housesthatroutinelymakechemicalstandards.Precautionsfor
same conditions (for example, carrier gas flow rate and
handling toxic materials must be followed throughout the
detector temperature) and preferably at the same time. When
dilution sequence as standard good labo
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