ASTM E516-95a(2000)
(Practice)Standard Practice for Testing Thermal Conductivity Detectors Used in Gas Chromatography
Standard Practice for Testing Thermal Conductivity Detectors Used in Gas Chromatography
SCOPE
1.1 This practice is intended to serve as a guide for the testing of the performance of a thermal conductivity detector (TCD) used as the detection component of a gas chromatographic system.
1.2 This practice is directly applicable to thermal conductivity detectors which employ filament (hot wire) or thermistor sensing elements.
1.3 This practice is intended to describe the performance of the detector itself independently of the chromatographic column, in terms which the analyst can use to predict overall system performance when the detector is coupled to the column and other chromatography system components.
1.4 For general gas chromatographic procedures, Practice E260 should be followed except where specific changes are recommended herein for the use of a TCD. For definitions of gas 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).
1.6 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 4.
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Designation:E516–95a (Reapproved 2000)
Standard Practice for
Testing Thermal Conductivity Detectors Used in Gas
Chromatography
This standard is issued under the fixed designation E516; 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 (e) 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 is intended to serve as a guide for the
CGAG-5.4 Standard for Hydrogen Piping Systems at
testing of the performance of a thermal conductivity detector
Consumer Locations
(TCD) used as the detection component of a gas chromato-
CGAP-9 The Inert Gases: Argon, Nitrogen and Helium
graphic system.
CGAV-7 Standard Method of Determining CylinderValve
1.2 This practice is directly applicable to thermal conduc-
Outlet Connections for Industrial Gas Mixtures
tivity detectors which employ filament (hot wire) or thermistor
CGAP-12 Safe Handling of Cryogenic Liquids
sensing elements.
HB-3 Handbook of Compressed Gases
1.3 This practice is intended to describe the performance of
the detector itself independently of the chromatographic col-
3. Significance and Use
umn, in terms which the analyst can use to predict overall
3.1 Although it is possible to observe and measure each of
system performance when the detector is coupled to the
the several characteristics of a detector under different and
column and other chromatography system components.
unique conditions, it is the intent of this practice that a
1.4 For general gas chromatographic procedures, Practice
complete set of detector specifications should be obtained at
E260 should be followed except where specific changes are
the same operating conditions. It should be noted also that to
recommended herein for the use of a TCD. For definitions of
specify a detector’s capability completely, its performance
gas chromatography and its various terms see Practice E355.
should be measured at several sets of conditions within the
1.5 For general information concerning the principles, con-
2 useful range of the detector. The terms and tests described in
struction, and operation of TCD see Refs. (1-4).
thispracticearesufficientlygeneralsothattheymaybeusedat
1.6 This standard does not purport to address all of the
whatever conditions may be chosen for other reasons.
safety concerns, if any, associated with its use. It is the
3.2 Linearity and speed of response of the recorder used
responsibility of the user of this standard to establish appro-
should be such that it does not distort or otherwise interfere
priate safety and health practices and determine the applica-
with the performance of the detector. Effective recorder re-
bility of regulatory limitations prior to use. For specific safety
3 sponse,Refs.(5,6)inparticular,shouldbesufficientlyfastthat
information, see Section 4.
itcanbeneglectedinsensitivityofmeasurements.Ifadditional
2. Referenced Documents amplifiers are used between the detector and the final readout
device, their characteristics should also first be established.
2.1 ASTM Standards:
E260 Practice for Packed Column Gas Chromatography
4. Hazards
E355 Practice for Gas Chromatography Terms and Rela-
4 4.1 Gas Handling Safety—Thesafehandlingofcompressed
tionships
gases and cryogenic liquids for use in chromatography is the
2.2 CGA Standards:
responsibility of every laboratory. The Compressed GasAsso-
ciation, (CGA), a member group of specialty and bulk gas
suppliers, publishes the following guidelines to assist the
This practice is under the jurisdiction ofASTM Committee E13 on Molecular
Spectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-
laboratory chemist to establish a safe work environment.
tography.
Applicable CGA publications include: CGAP-1, CGAG-5.4,
Current edition approved Sept. 10, 1995. Published November 1995. Originally
CGAP-9, CGAV-7, CGAP-12, and HB-3.
published as E516–74. Last previous edition E516–95.
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this practice.
3 5
See Appendix X1. Available from Compressed Gas Association, Inc., 1725 Jefferson Davis
Annual Book of ASTM Standards, Vol 14.01 Highway, Arlington, VA 22202-4100.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E516–95a (2000)
5. Sensitivity (Response) 5.3.2 Calculation of TCD sensitivity by utilizing actual
chromatogramsisnotrecommendedbecauseinsuchacasethe
5.1 Definition:
amount of test substance corresponding to the peak cannot be
5.1.1 Sensitivity (response) of the TCD is the signal output
established with sufficient accuracy.
per unit concentration of a test substance in the carrier gas, in
5.4 Exponential Decay Method:
accordance with the following relationship (7):
5.4.1 A mixing vessel of known volume fitted with a
S 5AF /W (1)
c
magnetically driven stirrer is purged with the carrier gas at a
known rate. The effluent from the flask is delivered directly to
where:
the detector. A measured quantity of the test substance is
S = sensitivity (response), mV·mL/mg,
introducedintotheflask,togiveaninitialconcentration, C,of
A = integrated peak area, mV·min, o
the test substance in the carrier gas, and a timer is started
F = carrier gas flow rate (corrected to detector tempera-
c
simultaneously.
ture ), mL/min, and
5.4.2 The concentration of the test substance in the carrier
W = mass of the test substance in the carrier gas, mg.
gas at the outlet of the flask, at any time is given as follows:
5.1.2 If the concentration of the test substance in the carrier
gas,correspondingtoadetectorsignalisknown,thesensitivity
C 5 C exp @2 F t/V # (3)
t o c f
is given by the following relationship:
where:
S 5 E/C (2)
d
C = concentration of the test substance at time t after
t
introduction into the flask, mg/mL,
where:
C = initial concentration of test compound introduced in
E = peak height, mV, and o
the flask, mg/mL,
C = concentrationofthetestsubstanceinthecarriergasat
d
F = carrier gas flow rate, corrected to flask temperature
the detector, mg/mL. c
4 mL/min,
5.2 Test Conditions:
t = time, min, and
5.2.1 Normalbutaneisthepreferredstandardtestsubstance.
V = volume of flask, mL.
f
5.2.2 The measurement must be made within the linear
5.4.3 Todeterminetheconcentrationofthetestsubstanceat
range of the detector.
the detector, C , it is necessary to apply the following
d
5.2.3 The measurement must be made at a signal level at
temperature correction:
least 100 times greater than the minimum detectability (200
C 5 C T /T ! (4)
~
d t f d
times greater than the noise level) at the same conditions.
5.2.4 The rate of drift of the detector at the same conditions
where:
must be stated.
C = concentration of the test substance at the detector,
d
5.2.5 The test substance and the conditions under which the
mg/mL,
d
detector sensitivity is measured must be stated. This will T = flask temperature, K, and
f
T = detector temperature, K.
include but not necessarily be limited to the following:
d
5.4.4 Thesensitivityofthedetectoratanyconcentrationcan
5.2.5.1 Type of detector (for example, platinum-tungsten
be calculated by:
filament type),
5.2.5.2 Detector geometry (for example, flow-type,
S 5 E/C (5)
d
diffusion-type),
where:
5.2.5.3 Internal volume of the detector,
S = sensitivity, mV·mL/mg,
5.2.5.4 Carrier gas,
E = detector, signal, mV, and
5.2.5.5 Carrier gas flow rate (corrected to detector tempera-
C = concentration of the test substance at the detector,
d
ture),
mg/mL.
5.2.5.6 Detector temperature,
NOTE 1—This method is subject to errors due to inaccuracies in
5.2.5.7 Detector current,
measuring the flow rate and flask volume. An error of 1% in the
5.2.5.8 Method of measurement, and
measurement of either variable will propagate to 2% over two decades in
concentration and to 6% over six decades.Therefore, this method should
5.2.5.9 Type of power supply (for example, constant volt-
not be used for concentration ranges of more than two decades over a
age, constant current).
single run.
5.2.5.10 For capillary detectors, the make-up gas, carrier,
NOTE 2—A temperature difference of 1°C between flask and flow
and reference flows should be stated.
measuring apparatus will, if uncompensated, introduce an error of ⁄3 %
5.3 Methods of Measurement:
into the flow rate.
5.3.1 Sensitivitymaybemeasuredbyanyofthreemethods:
NOTE 3—Extreme care should be taken to avoid unswept volumes
5.3.1.1 Experimental decay with exponential dilution flask
betweentheflaskandthedetector,asthesewillintroduceadditionalerrors
into the calculations.
(8, 9) (see 5.4),
NOTE 4—Flask volumes between 100 and 500 mLhave been found the
5.3.1.2 Utilizing the permeation tube (10), under steady-
most convenient. Larger volumes should be avoided due to difficulties in
state conditions (see 5.5),
obtaining efficient mixing and likelihood of temperature gradients.
5.3.1.3 Utilizing Young’s apparatus (11), under dynamic
conditions (see 5.6). 5.5 Method Utilizing Permeation Tubes:
E516–95a (2000)
A 50.94 A (7)
5.5.1 Permeation tubes consist of a volatile liquid enclosed
c i
in a section of plastic tubing.They provide low concentrations
6. Minimum Detectability
of vapor by diffusion of the vapor through the walls of the
tubing. The rate of diffusion for a given permeation tube is
6.1 Definition—Minimum detectability is the concentration
dependent only on the temperature. As the weight loss over a
of the test substance in the carrier gas which gives a detector
period of time can be easily and accurately measured gravi-
signal equal to twice the noise level and is calculated from the
metrically, the rate of diffusion can be accurately determined.
measured sensitivity and noise level values as follows:
Hence,thesedeviceshavebeenproposedasprimarystandards.
D 52N/S (8)
5.5.2 Accurately known concentrations can be prepared by
passingagasoverthepreviouslycalibratedpermeationtubeat
where:
constant temperature. The concentration of the test substance
D = minimum detectability, mg/mL,
in the gas can then be easily calculated according to the
N = noise level, mV, and
following relationship:
S = sensitivity of the detector, mV·mL/mg.
6.2 Test Conditions—Measure sensitivity in accordance
C 5 R /F (6)
T c
with the specifications given in Section 5. Measure noise level
where:
in accordance with the specifications given in Section 9. Both
C = concentration of the test substance in the gas, mg/
measurements have to be carried out at the same conditions
mL,
(for example, carrier gas identity and flow rate, detector
R = permeation rate of the test substance at the tempera-
T
temperature, and current) and preferably at the same time.
ture of the permeation tube, mg/min, and
When giving minimum detectability, state the noise level on
F = flow rate of the gas over the tube at the temperature
c
which the calculation was based.
of the tube, mL/min.
NOTE 5—If the flow rate of the gas is measured at a temperature 7. Linear Range
differentfromthetubetemperature,correctionmustbemade,asdescribed
7.1 Definition—The linear range of a TCD is the range of
in Appendix X1.
concentrations of the test substance in the carrier gas, over
5.5.3 When using a permeation tube for the testing of a
which the sensitivity of the detector is constant to within 5 %
TCD, the carrier gas is passing over a previously calibrated
as determined from the linearity plot specified in 7.2.2.
permeation tube containing the test substance at constant
7.1.1 The linear range may be expressed in three different
temperatureandintroducedimmediatelyintothedetector,kept
ways:
at the desired temperature. Knowing the concentration of the
7.1.1.1 As the ratio of the upper limit of linearity obtained
test substance in the carrier gas leaving the permeation tube at
from the linearity plot, and the minimum detectability, both
the temperature of the tube, the concentration at detector
measured for the same test substance as follows:
temperature can be calculated directly, by applying the correc-
L.R. 5 ~C ! /D (9)
d max
tion specified in 5.4.2. Knowing this value and the detector
signal,thesensitivityofthedetectorcanbeobtainedaccording
where:
to the equation given in 5.4.4.
L.R. = linear range of the detector,
(C ) = upper limit of linearity obtained from the
d max
NOTE 6—Permeation tubes are suitable only for preparing relatively
linearity plot, mg/mL, and
low concentrations in the part-per-million range. Hence for detectors of
D = minimum detectability, mg/mL.
relatively low sensitivity or of higher noise levels, this method may not
satisfy the criteria given in 5.2.3, which requires that the signal be at least If the linear range is expressed by this ratio, the minimum
100 times greater than the noise level.
detectability must also be stated.
7.1.1.2 By giving the minimum detectability and the upper
5.6 Dynamic Method:
−6
limit of linearity (for example, from 1 310 mg/mL to
5.6.1 In this method a known quantity of test substance is
−1
2 310 mg/mL).
injected into the flowing carrier gas stream.Alength of empty
7.1.1.3 Bygivingthelinearityplotitself,withtheminimum
tubing between the sample injection point and the detector
detectability indicated on the plot.
permitsthebandtospreadandbedetectedasaGaussianband.
7.2 Method of Measurement:
The detector signal is then integrated by any suitable method.
This method has the advantage that no special equipment or 7.2.1 For the determination of the linear range of a TCD,
devices are required other than conventional chromatographic eithertheexponentialdecayorthedynamicmethodsdescribed
hardware. For detectors optimized for capillary column flow in 5.4 and 5.6 respectively may be used. The permeation tube
rates,uncoated,deactivated,fusedsilicatubingshouldbeused. method (5.5) will not be suitable except for detectors of
5.6.2 The sensitivity of the detector is calculated from the extremely unusual characteristics because of the limited range
peak area according to 5.1.1. of concentrations obtainable with that method.
7.2.2 Measure the sensitivity at various concentrations of
NOTE 7—Care should be taken that the peak obtained is sufficiently
the test substance in the carrier gas in accordance with the
wide so the accuracy of the integration is not limited by the response time
methods described above. Plot the sensitivity versus log
of the detector or of the recording device.
concentration on a semilog paper as shown in Fig. 1. Draw a
NOTE 8—Peakareasobtainedbyintegration(A)orbymultiplyingpeak
i
heightbypeakwidthathalfheight(A )differby6%foraGaussianpeak: smooth line through the d
...
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