ASTM E594 for Testing FIDs Used in Gas or Supercritical Fluid Chromatography
10. Dynamic Range
10.1 The dynamic range of the detector is that range of mass flow rates of the test substance, over which an incremental change in mass flow rate produces an incremental change in detector signal. The lower limit is given by the minimum detectability. The upper limit is the highest mass flow rate at which a slight further increase in mass flow rate will give an observable increase in detector signal, and the dynamic range is the ratio of these upper and lower limits. The dynamic range is larger than or equal to the linear range, but obviously cannot be smaller.
10.1.1 The dynamic range may be expressed in three different ways:
10.1.1.1 As the ratio of the upper limit of dynamic range to the minimum detectability. The minimum detectability must also be stated.
10.1.1.2 By giving the minimum detectability and the upper limit of dynamic range (for example, from 1 x 10(-12) g/s to 1 x 10(-3) g/s).
10.1.1.3 By giving the dynamic plot itself with the minimum detectability indicated on the plot.
10.2 Methods of Measurement:
10.2.1 Using the exponential decay method (see section 6.4), measure the detector output signal (E) at various mass flow rates ( •m ) of the test substance in the carrier gas. Plot E versus •m on rectilinear graph paper, and draw a smooth curve through the data points as shown in Fig. 3. The upper limit of the dynamic range is the concentration at which the slope is zero.
10.2.2 When giving the dynamic range or the dynamic range plot, specify the test conditions in accordance with 7.2.4.
11. Response Time
11.1 For an FID, response time is not an important parameter. The FID is a mass-sensitive detector and does not directly depend on flow rate or concentration. The time constant for ionization is negligible (sub-second). Because the detector is not diffusion based, the transit time of the sample through the detector has little influence. Provided that measurements are made within the linear range of operation, the response time should not have an appreciable effect. In actuality, in a typical FID system, the electrometer/amplifier is the limiting factor and imposes a time constant of a few hundred milliseconds.
12. Standard Values
12.1 Detector characteristics measured at optimum conditions recommended by the manufacturer may be expected to fall within the typical range of values listed in Table 1, which also indicates the way these values should be expressed. All data refer to n-butane as the test substance.
13. Keywords
13.1 flame ionization detector (FID); flame photometric detectors (FPD); gas chromatography (GC); packed columns; supercritical fluid chromatography (SFC)
