ASTM E516 Testing Thermal Conductivity Detectors Used in Gas Chromatography
8. Dynamic Range
8.1 Definition - The dynamic range of the detector is that range of concentrations of the test substance, over which a change in concentration produces a change in detector signal. The lower limit is given by the minimum detectability. The upper limit is the highest concentration at which a slight further increase in concentration will give an observable increase in detector signal, and the dynamic range is the ratio of the upper and lower limits. The dynamic range can be greater than the linear range, but obviously cannot be smaller.
8.1.1 The dynamic range may be expressed in three different ways:
8.1.1.1 As the ratio of the upper limit of dynamic range and the minimum detectability by simultaneously stating the minimum detectability (for example, 2 x 10(6) with a minimum detectability of 1 x 10(-6) mg/mL).
8.1.1.2 By giving the minimum detectability and the upper limit of dynamic range (for example, from 1 x 10(-6) mg/mL to 2 mg/mL).
8.1.1.3 By giving the dynamic plot itself with the minimum detectability indicated on the plot.
8.2 Methods of Measurement:
8.2.1 Using the exponential decay method (see 5.4), measure the detector output signal (E) at various concentrations (Cd ) of the test substance in the carrier gas. Plot E versus Cd on rectilinear graph paper, and draw a smooth curve through the data points as shown in Fig. 2. The upper limit of the dynamic range is the concentration at which the slope is zero.
8.2.2 Express the dynamic range according to 8.1.1.
8.2.3 In giving the dynamic range or the dynamic range plot, specify the test conditions in accordance with 5.2.5.
9. Noise and Drift
9.1 Definitions:
9.1.1 noise - the amplitude expressed in microvolts of the peak-to-peak envelope of the baseline which includes all observed random variations of the detector signal of a frequency on the order of one or more cycles per minute (see Fig. 3). This noise corresponds to the observed noise only. The actual noise of the system may be larger or smaller than the observed value depending upon the method of data collection or signal monitoring from the detector, since observed noise is a function of the frequency, speed of response, and the bandwidth of the electronic circuit measuring the detector signal.
9.1.2 drift - the average slope of the noise envelope expressed in microvolts per hour as measured over 1/2 h.
9.2 Methods of Measurement:
9.2.1 With the attenuator set at maximum sensitivity (minimum attenuation) adjust the detector output with the "zero" control to read near mid-scale on the recorder. Allow at least 1/2 h of baseline to be recorded.
9.2.2 Draw two parallel lines to form an envelope which encloses the random excursions of a frequency of approximately 0.0167 Hz (1 cycle per minute) or more. Measure the distance between the parallel lines at right angles to the edge of the chart paper. Express the value as microvolts of noise.
9.2.3 Measure the net change in microvolts of the lower line of the envelope over 1/2 h and multiply by two. Express as microvolts per hour drift.
9.2.4 In specifications giving the measured noise and drift of the TCD, specify the test conditions in accordance with 5.2.5.
