1、Designation: E 1140 95 (Reapproved 2005)Standard Practice forTesting Nitrogen/Phosphorus Thermionic IonizationDetectors for Use In Gas Chromatography1This standard is issued under the fixed designation E 1140; the number immediately following the designation indicates the year oforiginal adoption or
2、, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers testing the performance of anitrogen/phosphorus thermioni
3、c ionization detector (NPD)used as the detection component of a gas chromatographicsystem.1.2 This practice applies to an NPD that employs a heatedalkali metal compound and emits an electrical charge from thatsolid surface.1.3 This practice addresses the operation and performanceof the NPD independe
4、ntly of the chromatographic column.However, the performance is specified in terms that the analystcan use to predict overall system performance when thedetector is coupled to the column and other chromatographiccomponents.1.4 For general chromatographic procedures, Practice E 260should be followed e
5、xcept where specific changes are recom-mended 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 thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish app
6、ro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use. For specific safetyinformation, see Section 5, Hazards.2. Referenced Documents2.1 ASTM Standards:2E 260 Practice for Packed Column Gas ChromatographyE 355 Practice for Gas Chromatography Te
7、rms and Rela-tionships2.2 CGA Standards:3CGA P-1 Safe Handling of Compressed Gases in Contain-ersCGA G-5.4 Standard for Hydrogen Piping Systems at Con-sumer LocationsCGA P-9 The Inert Gases: Argon, Nitrogen and HeliumCGA V-7 Standard Method of Determining Cylinder ValveOutlet Connections for Industr
8、ial Gas MixturesCGA P-12 Safe Handling of Cryogenic LiquidsHB-3 Handbook of Compressed Gases3. Terminology3.1 Definitions:3.1.1 For definitions of gas chromatography and its variousterms, see Practice E 355.3.2 Definitions of Terms Specific to This Standard:3.2.1 driftthe average slope of the noise
9、envelope ex-pressed in amps/h as measured over12 h.3.2.2 linear rangerange of mass flow rates of nitrogen orphosphorus in the carrier gas, over which the sensitivity of thedetector is constant to within 5 % as determined from thelinearity plot.3.2.3 minimum detectabilitythe mass flow rate of nitroge
10、nor phosphorus in the carrier gas that gives a detector signalequal to twice the noise level.3.2.4 noise (short term)the amplitude, expressed in am-peres, of the baseline envelope that includes all randomvariations of the detector signal of a frequency greater than onecycle per minute.3.2.5 selectiv
11、itythe ratio of the response per gram ofnitrogen or phosphorus in the test substance to the response pergram of carbon in octadecane.4. Significance and Use4.1 Although it is possible to observe and measure each ofthe several characteristics of a detector under different andunique conditions, it is
12、the intent of this practice that acomplete set of detector specifications be obtained at the sameoperating conditions, including geometry, flow rates, andtemperatures. To specify a detectors capability completely, itsperformance should be measured at several sets of conditionswithin the useful range
13、 of the detector. The terms and tests1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-tography.Current edition approved Feb. 1, 2005. Published March 2005. Originallyapproved in 1986. Last previo
14、us edition approved in 2000 as E 1140 95(2000).2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available fro
15、m Compressed Gas Association (CGA), 1725 Jefferson DavisHwy., Suite 1004, Arlington, VA 22202-4102.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.described in this practice are sufficiently general so that theymay be used under any
16、chosen conditions.4.2 Linearity and speed of response of the recorder shouldbe such that it does not distort or otherwise interfere with theperformance of the detector. Effective recorder response shouldbe sufficiently fast so that its effect on the sensitivity ofmeasurement is negligible. If additi
17、onal amplifiers are usedbetween the detector and the final readout device, theircharacteristics should first be established.5. Hazards5.1 Gas Handling SafetyThe safe handling of compressedgases and cryogenic liquids for use in chromatography is theresponsibility of every laboratory. The Compressed G
18、as Asso-ciation (CGA), a member group of specialty and bulk gassuppliers, publishes the following guidelines to assist thelaboratory chemist to establish a safe work environment.Applicable CGA publications include: CGA P-1, CGA G-5.4,CGA P-9, CGA V-7, CGA P-12, and HB-3.6. Application6.1 The N/P the
19、rmionic detector is an element-specificionization detector that is essentially a major modification ofthe flame ionization detector (FID). As in the normal FID, itmeasures increase in ionization current passing between twoelectrodes, one of which is polarized relative to the other.Usually these are
20、the inorganic salt source and the collector,with one often being at ground potential.6.2 The mechanism of the detector will only be discussedbriefly in this practice partly because full understanding of thedetector is not presently available and partly because thesubstantial differences in bead chem
21、istry, detector geometry,and bead heating mechanism prevent a singular view beinggiven.6.3 The addition of a heated alkali metal compound in thedetector area causes enhancement of the response for carbon-nitrogen and carbon-phosphorus bonds. In addition, the selec-tivity of response can be further e
22、nhanced when the bead iselectrically heated. Lower hydrogen and air flow rates thatdiminish the normal flame ionization response for hydrocarboncompounds can be used. This selective enhancement allows theNPD to be used for the detection of very small quantities ofnitrogen- and phosphorus-containing
23、compounds without in-terference from the signal of other molecular species.6.4 The selective response to C-N and C-Pbonds means thatthe detector is not suitable for permanent gas or elementalnitrogen or phosphorus analysis in the true definition of theterm. It should be noted, however, that some vol
24、atile inorganicphosphorous compounds do give a strong response with thisdetector, comparable to that of organophosphorus compounds.7. Detector Construction7.1 There is a wide variation in the method of constructionof this detector. It is not considered pertinent to review allaspects of the different
25、 detector designs available, but toconsider one generalized design as an example and recognizethat many significant variants may exist. Examples of signifi-cant differences may exist in bead chemistry and method ofheating, space jet and collector configuration, potential appliedacross the cell, its
26、polarity, and the flow rates and compositionof the three gases used.7.2 An essential part of the N/P thermionic detector is thepresence, in the active area of the detector, of an inorganicmaterial containing an alkali metal, often rubidium. Theinorganic material may be a salt or silicate. It is usua
27、lly, but notnecessarily, present in bead form and may be combined withother components for mechanical support, such as a ceramiccore.7.3 The inorganic salt mixture is usually connected to, orsupported by, a wire of platinum or other noncorrosive mate-rial. In some designs the bead is heated by passi
28、ng a currentthrough this wire; in others, the bead is heated by hydrogencombustion, for example, the burning flame itself.7.4 The carrier gas (usually helium or nitrogen) flowsthrough a jet as in normal FID practice and mixes, prior toleaving the jet, with a small volume of hydrogen. Combustiongas (
29、usually air) is fed around the jet in some manner and thenmoves over or around the bead before exiting from thedetector. It is worth noting that if this mixture is lean enough,due to low hydrogen flow, there will be insufficient fuel tomaintain a true flame.8. Equipment Preparation8.1 The detector s
30、hall be evaluated as part of a gas chro-matograph using injections of liquid samples that have a rangeof component concentrations.8.1.1 The detector shall be operated with carrier gas typeand hydrogen and oxidizer gas flow rates as recommended bythe manufacturer of the equipment. No attempt will be
31、made inthis practice to guide the selection of optimum conditions,except to state that because selectivity and sensitivity of theNPD are very dependent on the hydrogen flow rate, severalflow rates (in the range of 1 to 8 mL/min for the electricallyheated bead detector) should be tested for optimum d
32、etectorperformance.8.1.2 The complete set of performance specifications mustbe determined at the same operating conditions, since theabsolute sensitivity and noise vary independently over a widerange depending on the operating conditions. Once selected,the operating conditions should not be changed
33、during thedetermination of the detector characteristics.8.1.3 Detector stability over the course of the evaluation isessential for meaningful results. This may be monitored bychecking the bead temperature, the heating current, gas flows,and other parameters during the evaluation as dictated by thein
34、strument manufacturer. (Some electrically-heated beads tendto lose sensitivity continuously with operating time and requireincreasing the bead heating current to recover lost sensitivity.)8.2 ColumnAny column that fully separates the samplecomponents without causing overload or sample adsorptionmay
35、be used. One suitable column isa4ftby2mmglasscolumn packed with 100/120 mesh deactivated chromosorb Wcoated with 2 wt. % dimethyl silicone oil.8.3 GasesWith N/P thermionic detectors it is of criticalimportance that all gases are pure and that the gas lines are notcontaminated with oils, solder flux,
36、 etc. The use of wellconditioned molecular sieve traps in all lines helps to achieveE 1140 95 (2005)2this purity. If the chromatograph is fitted with in-line chemicalfilters after the gas regulators and flow controllers, they alsoshould be well conditioned to ensure that no contaminantsreach the col
37、umn from elastomeric diaphragms contained inthese parts.NOTE 1To condition a molecular sieve 5A column well, heat the trapwith a slow flow of carrier gas at 350C for a minimum of 2 h.8.4 Gas ConnectionsAll gas tubing and connectionsshould be made of cleaned copper or stainless steel, includingall fe
38、rrules and joints within the system. Vespel and graphiteferrules may be used for GC column connections provided thatthey are sufficiently conditioned after installation. These stepswill minimize contamination problems.9. Sample Preparation9.1 A solution containing three compounds dissolved inisoocta
39、ne should be used, with great emphasis placed on thepurity of all chemicals and particularly the solvent. Blank runsshould be made on the solvent to ensure that no interferingpeaks elute at the same time as the compounds of interest,which would invalidate the results. The three test compoundsare azo
40、benzene for nitrogen response (15.38 % nitrogen),malathion for phosphorus response (9.38 % phosphorus), andoctadecane for specificity (84.95 % carbon). Azobenzene andmalathion should be mixed in an appropriate ratio to allowcomparable peak heights under the isothermal conditions used.Typical ratios
41、are between 0.5 and 2.0, depending on detectorconstruction and operating conditions. Concentration limitsbetween 1 g/L and 1 mg/L are recommended initial values.The octadecane need be checked only at one concentrationlevel for specificity, and the recommended concentration forthis should be 1 g/L.9.
42、2 Because of the toxicity of malathion, it is recommendedthat a dilute solution be used as the starting material, and thatthis solution be purchased from one of the special supplyhouses that routinely make chemical standards. Precautions forhandling toxic materials must be followed throughout thedil
43、ution sequence as standard good laboratory practice.9.3 Sample InjectionThe recommended procedure foraccurate injection of liquid samples is the “solvent flush,” orBurke injection technique, in which a carefully washed 10-Lsyringe is loaded with 1 to 2 L solvent, 1 L air, 3 L sample,and 1 L air. Whi
44、le time consuming, this procedure allowsrepeatability of 62 % or better, and minimizes needle volumeeffects.10. Data Handling10.1 All manufacturers supply an integral electrometer toallow the small electrical current changes to be coupled torecorders/integrators/computers. The preferred system will
45、in-corporate one of the newer integrators or computers thatconverts an electrical signal into clearly defined peak areacounts in units such as microvolt-seconds. These data can thenbe readily used to calculate the linear range.10.1.1 Another method uses peak height measurements.This method yields da
46、ta that are very dependent on columnperformance and therefore not recommended.10.1.2 Regardless of which method is used to calculatelinear range, peak height is the only acceptable method fordetermining minimum detectability.10.2 CalibrationIt is essential to calibrate the measuringsystem to ensure
47、that the nominal specifications are acceptableand particularly to verify the range over which the output of thedevice, whether peak area or peak height, is linear with respectto input signal. Failure to perform this calibration may intro-duce substantial errors into the results. Methods for calibrat
48、ionwill vary for different manufacturers devices but may includeaccurate constant voltage supplies or pulse-generating equip-ment. The instruction manual should be studied and thoroughlyunderstood before attempting to use electronic integration forpeak area or peak height measurements.11. Test Subst
49、ances11.1 The test substance and the conditions under which thedetector sensitivity is measured must be stated. This willinclude, but not necessarily be limited to, the following: type ofdetector, detector geometry (for example, source of alkalimetal), carrier gas, carrier gas flow rate (corrected to detectortemperature), detector temperature, detector polarizing voltage,hydrogen flow rate, air flow rate, method of measurement, andelectrometer range setting.11.2 Azobenzene is the standard nitrogen-containing testsubstance. Malathion is the standard phosphorus-co
