1、Designation: E516 95a (Reapproved 2013)Standard Practice forTesting Thermal Conductivity Detectors Used in GasChromatography1This standard is issued under the fixed designation E516; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision,
2、the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice is intended to serve as a guide for thetesting of the performance of a thermal conductivity
3、detector(TCD) used as the detection component of a gas chromato-graphic system.1.2 This practice is directly applicable to thermal conduc-tivity detectors which employ filament (hot wire) or thermistorsensing elements.1.3 This practice is intended to describe the performance ofthe detector itself in
4、dependently of the chromatographiccolumn, in terms which the analyst can use to predict overallsystem performance when the detector is coupled to thecolumn and other chromatography system components.1.4 For general gas chromatographic procedures, PracticeE260 should be followed except where specific
5、 changes arerecommended herein for the use of a TCD. For definitions ofgas chromatography and its various terms see Practice E355.1.5 For general information concerning the principles,construction, and operation of TCD see Refs. (1-4).21.6 The values stated in SI units are to be regarded asstandard.
6、 No other units of measurement are included in thisstandard.1.7 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 appro-priate safety and health practices and determine the applica-b
7、ility of regulatory limitations prior to use. For specific safetyinformation, see Section 4.32. Referenced Documents2.1 ASTM Standards:4E260 Practice for Packed Column Gas ChromatographyE355 Practice for Gas Chromatography Terms and Relation-ships2.2 CGA Standards:5CGA P-1 Safe Handling of Compresse
8、d Gases in ContainersCGA G-5.4 Standard for Hydrogen Piping Systems atConsumer LocationsCGA P-9 The Inert Gases: Argon, Nitrogen and HeliumCGA V-7 Standard Method of Determining Cylinder ValveOutlet Connections for Industrial Gas MixturesCGA P-12 Safe Handling of Cryogenic LiquidsHB-3 Handbook of Co
9、mpressed Gases3. Significance and Use3.1 Although it is possible to observe and measure each ofthe several characteristics of a detector under different andunique conditions, it is the intent of this practice that acomplete set of detector specifications should be obtained atthe same operating condi
10、tions. It should be noted also that tospecify a detectors capability completely, its performanceshould be measured at several sets of conditions within theuseful range of the detector. The terms and tests described inthis practice are sufficiently general so that they may be used atwhatever conditio
11、ns may be chosen for other reasons.3.2 Linearity and speed of response of the recorder usedshould be such that it does not distort or otherwise interferewith the performance of the detector. Effective recorderresponse, Refs. (5, 6) in particular, should be sufficiently fastthat it can be neglected i
12、n sensitivity of measurements. If1This practice is under the jurisdiction of ASTM Committee E13 on MolecularSpectroscopy and Separation Science and is the direct responsibility of Subcom-mittee E13.19 on Separation Science.Current edition approved Jan. 1, 2013. Published January 2013. Originallyappr
13、oved in 1974. Last previous edition approved in 2005 as E516 95a (2005).DOI: 10.1520/E0516-95AR13.2The boldface numbers in parentheses refer to the list of references at the end ofthis practice.3See Appendix X1.4For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Cust
14、omer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.5Available from Compressed Gas Association (CGA), 4221 Walney Rd., 5thFloor, Chantilly, VA 20151-2923, http:/.Copyright ASTM International, 100 Barr H
15、arbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1additional amplifiers are used between the detector and thefinal readout device, their characteristics should also first beestablished.4. Hazards4.1 Gas Handling SafetyThe safe handling of compressedgases and cryogenic liquids
16、 for use in chromatography is theresponsibility of every laboratory. The Compressed GasAssociation, (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: C
17、GA P-1, CGA G-5.4,CGA P-9, CGA V-7, CGA P-12, and HB-3.5. Sensitivity (Response)5.1 Definition:5.1.1 Sensitivity (response) of the TCD is the signal outputper unit concentration of a test substance in the carrier gas, inaccordance with the following relationship (7):S 5 AFc/W (1)where:S = sensitivit
18、y (response), mVmL/mg,A = integrated peak area, mVmin,Fc= carrier gas flow rate (corrected to detectortemperature5), mL/min, andW = mass of the test substance in the carrier gas, mg.5.1.2 If the concentration of the test substance in the carriergas, corresponding to a detector signal is known, the s
19、ensitivityis given by the following relationship:S 5 E/Cd(2)where:E = peak height, mV, andCd= concentration of the test substance in the carrier gas atthe detector, mg/mL.5.2 Test Conditions:5.2.1 Normal butane is the preferred standard test substance.5.2.2 The measurement must be made within the li
20、nearrange of the detector.5.2.3 The measurement must be made at a signal level atleast 100 times greater than the minimum detectability (200times greater than the noise level) at the same conditions.5.2.4 The rate of drift of the detector at the same conditionsmust be stated.5.2.5 The test substance
21、 and the conditions under which thedetector sensitivity is measured must be stated. This willinclude but not necessarily be limited to the following:5.2.5.1 Type of detector (for example, platinum-tungstenfilament type),5.2.5.2 Detector geometry (for example, flow-type,diffusion-type),5.2.5.3 Intern
22、al volume of the detector,5.2.5.4 Carrier gas,5.2.5.5 Carrier gas flow rate (corrected to detectortemperature),5.2.5.6 Detector temperature,5.2.5.7 Detector current,5.2.5.8 Method of measurement, and5.2.5.9 Type of power supply (for example, constantvoltage, constant current).5.2.5.10 For capillary
23、detectors, the make-up gas, carrier,and reference flows should be stated.5.3 Methods of Measurement:5.3.1 Sensitivity may be measured by any of three methods:5.3.1.1 Experimental decay with exponential dilution flask(8, 9) (see 5.4),5.3.1.2 Utilizing the permeation tube (10), under steady-state cond
24、itions (see 5.5),5.3.1.3 Utilizing Youngs apparatus (11), under dynamicconditions (see 5.6).5.3.2 Calculation of TCD sensitivity by utilizing actualchromatograms is not recommended because in such a case theamount of test substance corresponding to the peak cannot beestablished with sufficient accur
25、acy.5.4 Exponential Decay Method:5.4.1 A mixing vessel of known volume fitted with amagnetically driven stirrer is purged with the carrier gas at aknown rate. The effluent from the flask is delivered directly tothe detector. A measured quantity of the test substance isintroduced into the flask, to g
26、ive an initial concentration, Co,ofthe test substance in the carrier gas, and a timer is startedsimultaneously.5.4.2 The concentration of the test substance in the carriergas at the outlet of the flask, at any time is given as follows:Ct5 Coexp2Fct/Vf# (3)where:Ct= concentration of the test substanc
27、e at time t afterintroduction into the flask, mg/mL,Co= initial concentration of test compound introduced in theflask, mg/mL,Fc= carrier gas flow rate, corrected to flask temperature4mL/min,t = time, min, andVf= volume of flask, mL.5.4.3 To determine the concentration of the test substance atthe det
28、ector, Cd, it is necessary to apply the followingtemperature correction:Cd5 Ct Tf/Td !(4)where:dCdd= concentration of the test substance at the detector,mg/mL,Tf= flask temperature, K, andTd= detector temperature, K.5.4.4 The sensitivity of the detector at any concentration canbe calculated by:S 5 E
29、/Cd(5)where:S = sensitivity, mVmL/mg,E = detector, signal, mV, andE516 95a (2013)2Cd= concentration of the test substance at the detector,mg/mL.NOTE 1This method is subject to errors due to inaccuracies inmeasuring the flow rate and flask volume. An error of 1 % in themeasurement of either variable
30、will propagate to 2 % over two decades inconcentration and to 6 % over six decades. Therefore, this method shouldnot be used for concentration ranges of more than two decades over asingle run.NOTE 2A temperature difference of 1C between flask and flowmeasuring apparatus will, if uncompensated, intro
31、duce an error of13 %into the flow rate.NOTE 3Extreme care should be taken to avoid unswept volumesbetween the flask and the detector, as these will introduce additional errorsinto the calculations.NOTE 4Flask volumes between 100 and 500 mL have been found themost convenient. Larger volumes should be
32、 avoided due to difficulties inobtaining efficient mixing and likelihood of temperature gradients.5.5 Method Utilizing Permeation Tubes:5.5.1 Permeation tubes consist of a volatile liquid enclosedin a section of plastic tubing. They provide low concentrationsof vapor by diffusion of the vapor throug
33、h the walls of thetubing. The rate of diffusion for a given permeation tube isdependent only on the temperature. As the weight loss over aperiod of time can be easily and accurately measuredgravimetrically, the rate of diffusion can be accurately deter-mined. Hence, these devices have been proposed
34、as primarystandards.5.5.2 Accurately known concentrations can be prepared bypassing a gas over the previously calibrated permeation tube atconstant temperature. The concentration of the test substancein the gas can then be easily calculated according to thefollowing relationship:C 5 RT/Fc(6)where:C
35、= concentration of the test substance in the gas, mg/mL,RT= permeation rate of the test substance at the temperatureof the permeation tube, mg/min, andFc= flow rate of the gas over the tube at the temperature ofthe tube, mL/min.NOTE 5If the flow rate of the gas is measured at a temperaturedifferent
36、from the tube temperature, correction must be made, as describedin Appendix X1.5.5.3 When using a permeation tube for the testing of aTCD, the carrier gas is passing over a previously calibratedpermeation tube containing the test substance at constanttemperature and introduced immediately into the d
37、etector, keptat the desired temperature. Knowing the concentration of thetest substance in the carrier gas leaving the permeation tube atthe temperature of the tube, the concentration at detectortemperature can be calculated directly, by applying the correc-tion specified in 5.4.2. Knowing this valu
38、e and the detectorsignal, the sensitivity of the detector can be obtained accordingto the equation given in 5.4.4.NOTE 6Permeation tubes are suitable only for preparing relativelylow concentrations in the part-per-million range. Hence for detectors ofrelatively low sensitivity or of higher noise lev
39、els, this method may notsatisfy the criteria given in 5.2.3, which requires that the signal be at least100 times greater than the noise level.5.6 Dynamic Method:5.6.1 In this method a known quantity of test substance isinjected into the flowing carrier gas stream. A length of emptytubing between the
40、 sample injection point and the detectorpermits the band to spread and be detected as a Gaussian band.The detector signal is then integrated by any suitable method.This method has the advantage that no special equipment ordevices are required other than conventional chromatographichardware. For dete
41、ctors optimized for capillary column flowrates, uncoated, deactivated, fused silica tubing should be used.5.6.2 The sensitivity of the detector is calculated from thepeak area according to 5.1.1.NOTE 7Care should be taken that the peak obtained is sufficientlywide so the accuracy of the integration
42、is not limited by the response timeof the detector or of the recording device.NOTE 8Peak areas obtained by integration (Ai) or by multiplying peakheight by peak width at half height (Ac) differ by 6 % for a Gaussian peak:Ac5 0.94 Ai(7)6. Minimum Detectability6.1 DefinitionMinimum detectability is th
43、e concentrationof the test substance in the carrier gas which gives a detectorsignal equal to twice the noise level and is calculated from themeasured sensitivity and noise level values as follows:D 5 2N/S (8)where:D = minimum detectability, mg/mL,N = noise level, mV, andS = sensitivity of the detec
44、tor, mVmL/mg.6.2 Test ConditionsMeasure sensitivity in accordancewith the specifications given in Section 5. Measure noise levelin accordance with the specifications given in Section 9. Bothmeasurements have to be carried out at the same conditions(for example, carrier gas identity and flow rate, de
45、tectortemperature, and current) and preferably at the same time.When giving minimum detectability, state the noise level onwhich the calculation was based.7. Linear Range7.1 DefinitionThe linear range of a TCD is the range ofconcentrations of the test substance in the carrier gas, overwhich the sens
46、itivity of the detector is constant to within 5 %as determined from the linearity plot specified in 7.2.2.7.1.1 The linear range may be expressed in three differentways:7.1.1.1 As the ratio of the upper limit of linearity obtainedfrom the linearity plot, and the minimum detectability, bothmeasured f
47、or the same test substance as follows:L.R. 5 Cd!max/D (9)where:L.R. = linear range of the detector,(Cd)max= upper limit of linearity obtained from the linearityplot, mg/mL, andD = minimum detectability, mg/mL.If the linear range is expressed by this ratio, the minimumdetectability must also be state
48、d.E516 95a (2013)37.1.1.2 By giving the minimum detectability and the upperlimit of linearity (for example, from 1 106mg/mL to2101mg/mL).7.1.1.3 By giving the linearity plot itself, with the minimumdetectability indicated on the plot.7.2 Method of Measurement:7.2.1 For the determination of the linea
49、r range of a TCD,either the exponential decay or the dynamic methods describedin 5.4 and 5.6 respectively may be used. The permeation tubemethod (5.5) will not be suitable except for detectors ofextremely unusual characteristics because of the limited rangeof concentrations obtainable with that method.7.2.2 Measure the sensitivity at various concentrations ofthe test substance in the carrier gas in accordance with themethods described above. Plot the sensitivity versus logconcentration on a semilog paper as shown in Fig. 1. Draw asmooth l
