ASTM E516 Testing Thermal Conductivity Detectors Used in Gas Chromatography
6. Minimum Detectability
6.1 Definition - Minimum detectability is the concentration of the test substance in the carrier gas which gives a detector signal equal to twice the noise level and is calculated from the measured sensitivity and noise level values as follows:
D = 2N/S
where:
D = minimum detectability, mg/mL,
N = noise level, mV, and
S = sensitivity of the detector, mV•mL/mg.
6.2 Test Conditions - Measure sensitivity in accordance with the specifications given in Section 5. Measure noise level in accordance with the specifications given in Section 9. Both measurements have to be carried out at the same conditions (for example, carrier gas identity and flow rate, detector temperature, and current) and preferably at the same time. When giving minimum detectability, state the noise level on which the calculation was based.
7. Linear Range
7.1 Definition - The linear range of a TCD is the range of concentrations of the test substance in the carrier gas, over which the sensitivity 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 different ways:
7.1.1.1 As the ratio of the upper limit of linearity obtained from the linearity plot, and the minimum detectability, both measured for the same test substance as follows:
L.R. = (Cd)max/D
where:
L.R. = linear range of the detector,
(Cd)max = upper limit of linearity obtained from the linearity plot, mg/mL, and
D = minimum detectability, mg/mL.
If the linear range is expressed by this ratio, the minimum detectability must also be stated.
7.1.1.2 By giving the minimum detectability and the upper limit of linearity (for example, from 1 x 10(-6) mg/mL to 2 x 10(-1) mg/mL).
7.1.1.3 By giving the linearity plot itself, with the minimum detectability indicated on the plot.
7.2 Method of Measurement:
7.2.1 For the determination of the linear range of a TCD, either the exponential decay or the dynamic methods described in 5.4 and 5.6 respectively may be used. The permeation tube method (5.5) will not be suitable except for detectors of extremely unusual characteristics because of the limited range of concentrations obtainable with that method.
7.2.2 Measure the sensitivity at various concentrations of the test substance in the carrier gas in accordance with the methods described above. Plot the sensitivity versus log concentration on a semilog paper as shown in Fig. 1. Draw a smooth line through the data points. The upper limit of linearity is given by the intersection of the line with a value 0.95 x Smax where Smax is the highest value of sensitivity on the fitted curve.
NOTE 9 - The dynamic method will give somewhat larger values for the upper limit of linearity as compared to the exponential decay method, because the integrated signal will average increments of signal obtained over the linear range of the detector with those obtained in the nonlinear range.
7.2.3 Express the linear range in accordance with 7.1.1.
7.2.4 In giving the linear range of the linearity plot, specify the test conditions in accordance with 5.2.5.
