ASTM E516-95A

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
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Designation: E 516 – 95a
Standard Practice for
Testing Thermal Conductivity Detectors Used in Gas
Chromatography
This standard is issued under the fixed designation E 516; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope CGA P-1 Safe Handling of Compressed Gases in Contain-
ers
1.1 This practice is intended to serve as a guide for the
CGA G-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-
CGA P-9 The Inert Gases: Argon, Nitrogen and Helium
graphic system.
CGA V-7 Standard Method of Determining Cylinder Valve
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
CGA P-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
E 260 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 E 355.
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).
this practice are sufficiently general so that they may be used at
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) in particular, should be sufficiently fast that
information, see Section 4.
it can be neglected in sensitivity of measurements. If additional
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:
E 260 Practice for Packed Column Gas Chromatography
4. Hazards
E 355 Practice for Gas Chromatography Terms and Rela-
4 4.1 Gas Handling Safety—The safe handling of compressed
tionships
gases and cryogenic liquids for use in chromatography is the
2.2 CGA Standards:
responsibility of every laboratory. The Compressed Gas Asso-
ciation, (CGA), a member group of specialty and bulk gas
suppliers, publishes the following guidelines to assist the
laboratory chemist to establish a safe work environment.
Applicable CGA publications include: CGA P-1, CGA G-5.4,
This practice is under the jurisdiction of ASTM Committee E13 on Molecular
CGA P-9, CGA V-7, CGA P-12, and HB-3.
Spectroscopy and is the direct responsibility of Subcommittee E13.19 on Chroma-
tography.
5. Sensitivity (Response)
Current edition approved Sept. 10, 1995. Published November 1995. Originally
published as E 516 – 74. Last previous edition E 516 – 95.
5.1 Definition:
The boldface numbers in parentheses refer to the list of references at the end of
this practice.
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, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 516
5.1.1 Sensitivity (response) of the TCD is the signal output 5.4 Exponential Decay Method:
per unit concentration of a test substance in the carrier gas, in 5.4.1 A mixing vessel of known volume fitted with a
accordance with the following relationship (7):
magnetically driven stirrer is purged with the carrier gas at a
known rate. The effluent from the flask is delivered directly to
S 5 AF /W (1)
c
the detector. A measured quantity of the test substance is
where:
introduced into the flask, to give an initial concentration, C ,of
o
S = sensitivity (response), mV·mL/mg,
the test substance in the carrier gas, and a timer is started
A = integrated peak area, mV·min,
simultaneously.
F = carrier gas flow rate (corrected to detector tempera-
c
5.4.2 The concentration of the test substance in the carrier
ture ), mL/min, and
gas at the outlet of the flask, at any time is given as follows:
W = mass of the test substance in the carrier gas, mg.
C 5 C exp @2 F t/V # (3)
t o c f
5.1.2 If the concentration of the test substance in the carrier
gas, corresponding to a detector signal is known, the sensitivity
where:
is given by the following relationship:
C = concentration of the test substance at time t after
t
S 5 E/C (2)
introduction into the flask, mg/mL,
d
C = initial concentration of test compound introduced in
o
where:
the flask, mg/mL,
E = peak height, mV, and
F = carrier gas flow rate, corrected to flask temperature
c
C = concentration of the test substance in the carrier gas at
d
4 mL/min,
the detector, mg/mL.
t = time, min, and
5.2 Test Conditions:
V = volume of flask, mL.
f
5.2.1 Normal butane is the preferred standard test substance.
5.4.3 To determine the concentration of the test substance at
5.2.2 The measurement must be made within the linear
the detector, C , it is necessary to apply the following
d
range of the detector.
temperature correction:
5.2.3 The measurement must be made at a signal level at
C 5 C ~T /T ! (4)
d t f d
least 100 times greater than the minimum detectability (200
times greater than the noise level) at the same conditions.
where:
5.2.4 The rate of drift of the detector at the same conditions
C = concentration of the test substance at the detector,
d
must be stated.
mg/mL,
d
5.2.5 The test substance and the conditions under which the T = flask temperature, K, and
f
T = detector temperature, K.
detector sensitivity is measured must be stated. This will
d
include but not necessarily be limited to the following: 5.4.4 The sensitivity of the detector at any concentration can
5.2.5.1 Type of detector (for example, platinum-tungsten be calculated by:
filament type),
S 5 E/C (5)
d
5.2.5.2 Detector geometry (for example, flow-type,
where:
diffusion-type),
S = sensitivity, mV·mL/mg,
5.2.5.3 Internal volume of the detector,
E = detector, signal, mV, and
5.2.5.4 Carrier gas,
C = concentration of the test substance at the detector,
d
5.2.5.5 Carrier gas flow rate (corrected to detector tempera-
mg/mL.
ture),
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
measurement of either variable will propagate to 2 % over two decades in
5.2.5.8 Method of measurement, and
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 Sensitivity may be measured by any of three methods:
NOTE 3—Extreme care should be taken to avoid unswept volumes
5.3.1.1 Experimental decay with exponential dilution flask between the flask and the detector, as these will introduce additional errors
into the calculations.
(8, 9) (see 5.4),
NOTE 4—Flask volumes between 100 and 500 mL have 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:
5.3.2 Calculation of TCD sensitivity by utilizing actual 5.5.1 Permeation tubes consist of a volatile liquid enclosed
chromatograms is not recommended because in such a case the in a section of plastic tubing. They provide low concentrations
amount of test substance corresponding to the peak cannot be of vapor by diffusion of the vapor through the walls of the
established with sufficient accuracy. tubing. The rate of diffusion for a given permeation tube is
E 516
dependent only on the temperature. As the weight loss over a signal equal to twice the noise level and is calculated from the
period of time can be easily and accurately measured gravi- measured sensitivity and noise level values as follows:
metrically, the rate of diffusion can be accurately determined.
D 5 2N/S (8)
Hence, these devices have been proposed as primary standards.
where:
5.5.2 Accurately known concentrations can be prepared by
D = minimum detectability, mg/mL,
passing a gas over the previously calibrated permeation tube at
N = noise level, mV, and
constant temperature. The concentration of the test substance
S = sensitivity of the detector, mV·mL/mg.
in the gas can then be easily calculated according to the
6.2 Test Conditions—Measure sensitivity in accordance
following relationship:
with the specifications given in Section 5. Measure noise level
C 5 R /F (6)
T c
in accordance with the specifications given in Section 9. Both
measurements have to be carried out at the same conditions
where:
(for example, carrier gas identity and flow rate, detector
C = concentration of the test substance in the gas, mg/
temperature, and current) and preferably at the same time.
mL,
When giving minimum detectability, state the noise level on
R = permeation rate of the test substance at the tempera-
T
ture of the permeation tube, mg/min, and which the calculation was based.
F = flow rate of the gas over the tube at the temperature
c
7. Linear Range
of the tube, mL/min.
7.1 Definition—The linear range of a TCD is the range of
NOTE 5—If the flow rate of the gas is measured at a temperature
concentrations of the test substance in the carrier gas, over
different from the tube temperature, correction must be made, as described
in Appendix X1. which the sensitivity of the detector is constant to within 5 %
as determined from the linearity plot specified in 7.2.2.
5.5.3 When using a permeation tube for the testing of a
7.1.1 The linear range may be expressed in three different
TCD, the carrier gas is passing over a previously calibrated
ways:
permeation tube containing the test substance at constant
7.1.1.1 As the ratio of the upper limit of linearity obtained
temperature and introduced immediately into the detector, kept
from the linearity plot, and the minimum detectability, both
at the desired temperature. Knowing the concentration of the
measured for the same test substance as follows:
test substance in the carrier gas leaving the permeation tube at
the temperature of the tube, the concentration at detector
L.R. 5 C /D (9)
~ !
d max
temperature can be calculated directly, by applying the correc-
where:
tion specified in 5.4.2. Knowing this value and the detector
L.R. = linear range of the detector,
signal, the sensitivity of the detector can be obtained according
(C ) = upper limit of linearity obtained from the
d max
to the equation given in 5.4.4.
linearity plot, mg/mL, and
NOTE 6—Permeation tubes are suitable only for preparing relatively
D = minimum detectability, mg/mL.
low concentrations in the part-per-million range. Hence for detectors of
If the linear range is expressed by this ratio, the minimum
relatively low sensitivity or of higher noise levels, this method may not
detectability must also be stated.
satisfy the criteria given in 5.2.3, which requires that the signal be at least
7.1.1.2 By giving the minimum detectability and the upper
100 times greater than the noise level.
−6
limit of linearity (for example, from 1 3 10 mg/mL to
5.6 Dynamic Method:
−1
2 3 10 mg/mL).
5.6.1 In this method a known quantity of test substance is
7.1.1.3 By giving the linearity plot itself, with the minimum
injected into the flowing carrier gas stream. A length of empty
detectability indicated on the plot.
tubing between the sample injection point and the detector
7.2 Method of Measurement:
permits the band to spread and be detected as a Gaussian band.
7.2.1 For the determination of the linear range of a TCD,
The detector signal is then integrated by any suitable method.
either the exponential decay or the dynamic methods described
This method has the advantage that no special equipment or
in 5.4 and 5.6 respectively may be used. The permeation tube
devices are required other than conventional chromatographic
method (5.5) will not be suitable except for detectors of
hardware. For detectors optimized for capillary column flow
extremely unusual characteristics because of the limited range
rates, uncoated, deactivated, fused silica tubing should be used.
of concentrations obtainable with that method.
5.6.2 The sensitivity of the detector is calculated from the
7.2.2 Measure the sensitivity at various concentrations of
peak area according to 5.1.1.
the test substance in the carrier gas in accordance with the
NOTE 7—Care should be taken that the peak obtained is sufficiently methods described above. Plot the sensitivity versus log
wide so the accuracy of the integration is not limited by the response time
concentration on a semilog paper as shown in Fig. 1. Draw a
of the detector or of the recording device.
smooth line through the data points. The upper limit of
NOTE 8—Peak areas obtained by integration (A ) or by multiplying peak
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