ASTM E516 Testing Thermal Conductivity Detectors Used in Gas Chromatography
10. Response Time
10.1 Definition - The response time (speed of response) of the detector at a given flow rate is the time required for the output signal to reach 63 % of the new equilibrium value when the composition of the gas entering the detector is changed in a stepwise manner, within the linear range of the detector. Internal volume and geometry of the detector must also be stated as part of this specification. Response time may be determined and specified at various flow rates.
10.1.1 The response time of the detector is determined by a combination of effective volume and time constant of the sensing elements. Effective volume determines the time required to introduce a compositional change into the measuring region. Time constant is the time required for the elements to respond to concentration changes. Effective volume and time constant may also be separately specified in addition to the response time.
NOTE 10 - For diffusion geometry detectors, the time required for the composition to come to equilibrium in the measuring region depends primarily on the volume, length, and diameter of the measuring region and diffusion channels, and of the diffusion constant of the carrier gas-test substance pair. For flow-geometry detectors, the response time is dependent primarily on the volume of the measuring region and the carrier gas flow rate. Hence a statement of the internal volume and geometry of the detector and of the flow rate is necessary for the specification to be complete.
10.2 Method of Measurement:
10.2.1 The composition of the gas entering the detector is changed in a stepwise manner and the output signal recorded. The time required for the signal to reach 63.2 % of the new equilibrium value is determined from the recorder chart (see Fig. 4). The test may be repeated at various flow rates.
10.2.2 Flow rates for the measurement should be chosen with due regard for the types (diameter) of columns for which the detector is designed. Table 1 lists typical flow rates used with various column types. The flow rates should be corrected to detector temperatures.
10.2.3 A stepwise change in composition may be obtained by means of high-speed valves which rapidly substitute carrier gas with added test substance for pure carrier gas or for gas containing a different amount. The test substance may be added by means of a saturator which saturates the carrier gas with the vapor of a volatile liquid in the manner of Schmauch (12).
NOTE 11 - Design of the apparatus should be such as to prevent backmixing of the two gases ahead of the detector, which would result in a diffuse composition change.
10.2.4 Alternatively, a mixture of the carrier gas containing a known amount of the test substance may be used.
10.2.5 The concentration of the test substance must be within the linear range of the detector.
NOTE 12 - Extreme care must be exercised to adjust the flow rates so that a change in flow rate does not occur when compositions are switched; otherwise errors due to flow sensitivity of the detector are introduced.
10.3 Calculation:
10.3.1 The response time of TCD may be expressed in two ways:
10.3.1.1 As the time, in seconds, obtained from a plot similar to Fig. 4 and the flow rate in millilitres per minute at which the test was made.
10.3.1.2 As a rectilinear plot of response time, in seconds, versus carrier gas flow rate, in millilitres per minute (see Fig. 5).
NOTE 13 - Use of the plot of response time versus carrier gas flow rate is strongly recommended because it provides a better picture of the contributions of effective volume and sensing element time constant.
10.3.1.3 In either case, the internal geometry (for example, diffusion- or flow-through type) and the internal volume (in microlitres) of the detector must be stated.
10.3.2 The effective volume of the detector and the time constant of the sensing element may be established from the same data in the following way:
10.3.2.1 Plot the response time in seconds versus the reciprocal of the flow rate (that is, seconds per millilitre), draw a straight line through the data points, and extrapolate back to zero reciprocal flow rate (for example, infinite flow rate), as shown in Fig. 6.
10.3.2.2 The effective volume of the TCD is given by the slope of the straight line:
Ve = [Δt/Δ(1/Fc)60]10(3)
where:
Ve = effective volume of the detector, µL,
t = response time of the detector, s, and
Fc = carrier gas flow rate corrected to detector temperature, mL/min.
10.3.2.3 The time constant of the sensing element is equal to the value of the ordinate at the intersection of the straight line at zero reciprocal flow rate.
10.3.3 The effective volume of the detector and the time constant of the sensing element may also be determined by the method of Kieselbach (13), in the following way:
10.3.3.1 Use a high-speed sampling valve to inject a plug sample of air into the detector. Determine the response time at various flow rates from the shape of the recorded peak, by drawing tangents through the inflection points. The response time is equal to the time interval from the intersection of the tangents to the point at which the signal has returned to 37 % of the value of the height of the intersection of the tangents (see Fig. 7).
10.3.3.2 Plot the data as described in 10.3.2.1 and calculate effective volume and time constant as described in 10.3.2.2 and 10.3.2.3.
