1、Designation: E 516 95a (Reapproved 2005)Standard Practice forTesting Thermal Conductivity Detectors Used in GasChromatography1This standard is issued under the fixed designation E 516; 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 (e) 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 conductivi
3、ty 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
4、 independently of the chromatographic col-umn, 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, PracticeE 260 should be followed except where sp
5、ecific changes arerecommended herein for the use of a TCD. For definitions ofgas chromatography and its various terms see Practice E 355.1.5 For general information concerning the principles, con-struction, and operation of TCD see Refs. (1-4).21.6 The values stated in SI units are to be regarded as
6、standard. 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
7、applica-bility of regulatory limitations prior to use. For specific safetyinformation, see Section 4.32. Referenced Documents2.1 ASTM Standards:4E 260 Practice for Packed Column Gas ChromatographyE 355 Practice for Gas Chromatography Terms and Rela-tionships2.2 CGA Standards:CGA P-1 Safe Handling of
8、 Compressed Gases in Contain-ers5CGA G-5.4 Standard for Hydrogen Piping Systems atConsumer Locations5CGA P-9 The Inert Gases: Argon, Nitrogen and Helium5CGA V-7 Standard Method of Determining Cylinder ValveOutlet Connections for Industrial Gas Mixtures5CGA P-12 Safe Handling of Cryogenic Liquids5HB-
9、3 Handbook of Compressed Gases53. 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 sam
10、e operating conditions. 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 at
11、whatever conditions 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 recorder re-sponse, Refs. (5, 6) in particular, should be sufficiently fast thatit
12、 can be neglected in sensitivity of measurements. If additionalamplifiers are used between the detector and the final readoutdevice, their characteristics should also first be established.4. Hazards4.1 Gas Handling SafetyThe safe handling of compressedgases and cryogenic liquids for use in chromatog
13、raphy is theresponsibility of every laboratory. The Compressed Gas 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.1This practice is under the jurisdiction of ASTM Committe
14、e E13 on MolecularSpectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-tography.Current edition approved Sept. 1, 2005. Published September 2005. Originallyapproved in 1974. Last previous edition approved in 2000 as E 516 95a (2000).2The boldface numbers in parentheses refe
15、r 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 Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe AS
16、TM website.5Available from Compressed Gas Association, Inc., 1725 Jefferson DavisHighway, Arlington, VA 22202-4100.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.Applicable CGA publications include: CGA P-1, CGA G-5.4,CGA P-9, CGA V
17、-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 = sensitivity (response), mVmL/mg,A = integr
18、ated peak area, mVmin,Fc= carrier gas flow rate (corrected to detector tempera-ture5), 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 sensitivityis given by the foll
19、owing 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 linearrange of the detector.5.2.
20、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 and the conditions under whic
21、h 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 Internal volume of the detector,5.2.
22、5.4 Carrier gas,5.2.5.5 Carrier gas flow rate (corrected to detector tempera-ture),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, constant volt-age, constant current).5.2.5.10 For capillary detectors, the make-up gas
23、, 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 conditions (see 5.5),5.3.1.3 U
24、tilizing 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 accuracy.5.4 Exponential Decay
25、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 give an initial concentrati
26、on, 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 Coexp 2 Fct/Vf (3)where:Ct= concentration of the test substance at time t afterintroduc
27、tion into the flask, mg/mL,Co= initial concentration of test compound introduced inthe flask, 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 detector, Cd, it is necessar
28、y to apply the followingtemperature correction:Cd5 CtTf/Td! (4)where:Cdd= 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/Cd(5)where:S = sensitivity
29、, mVmL/mg,E = detector, signal, mV, andCd= 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 will propagate to 2 % over two decades inco
30、ncentration 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, introduce an error of13 %into the flow rate.NOTE
31、 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 avoided due to difficulties inobtaining ef
32、ficient mixing and likelihood of temperature gradients.E 516 95a (2005)25.5 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 through the walls of thetubing.
33、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 measured gravi-metrically, the rate of diffusion can be accurately determined.Hence, these devices have been proposed as primary standards.5.5.2
34、 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 = concentration of the te
35、st substance in the gas, mg/mL,RT= permeation rate of the test substance at the tempera-ture of the permeation tube, mg/min, andFc= flow rate of the gas over the tube at the temperatureof the tube, mL/min.NOTE 5If the flow rate of the gas is measured at a temperaturedifferent from the tube temperatu
36、re, 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 detector, keptat the des
37、ired 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 value and the detectorsigna
38、l, 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 levels, this method may no
39、tsatisfy 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 sample injection point
40、 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 detectors optimized for cap
41、illary 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 is not limited by the r
42、esponse 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 the concentrationof the t
43、est 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 detector, mVmL/mg.6.2 Test C
44、onditionsMeasure 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, detectortemperature, and
45、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 sensitivity of the detector
46、 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 for the same test substa
47、nce 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 thelinearity plot, mg/mL, andD = minimum detectability, mg/mL.If the linear range is expressed by this ratio, the minimumdetectability must also be stated.7.1.1.2 By giving the
48、 minimum detectability and the upperlimit of linearity (for example, from 1 3 106mg/mL to2 3 101mg/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 linear range of a TCD,either the expone
49、ntial 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 line through the data points. The upper limit ofE 516
