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Selective versus Universal. This detector category refers to the number of analytes that can be detected. A universal detector theoretically detects all samples, while the selective type responds to particular types of com-
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Gas Chromatography Thermal conductivity (TCD) Flame ionization (FID)
Electron capture (ECD) Other ionization types
Flame photometric (FPD)
Refractive index (RI) Conductometric
UV/Vis Absorption Fluorometric Transport (FID) Amperometric
pounds or functional groups. Both types have advantages and disadvantages. For qualitative screening, a universal detector will provide information about the total number of analytes, while a selective one will pick out a select group that may aid in its identification. Often the selective type is more sensitive, which is also advantageous for trace analysis. A very crowded, complex chromatogram may be simplified by a selective detector. Both types can be used to advantage in quantitative analysis, but preferably after the sample has been characterized with a universal detector.
Destructive versus Nondestructive. Nondestructive-type detectors are necessary if the separated analytes are to be reclaimed for further analysis, as, for example, when identifications are to be performed using auxiliary instruments. One way to utilize destructive detectors in this situation is to split the effluent stream and send only part of it to the detector, collecting the rest for analysis.
Analog versus Digital. Most detectors produce analog (continuous) signals that must be digitized before they can be manipulated by a digital computer. The main exception is the radioactive detector.
The most important detector characteristic is the signal it produces, of course, and that topic is treated throughout this chapter. In this section we will define two other important characteristics, noise and time constant.
Noise. Noise is the signal produced by a detector in the absence of a sample. Usually it is given in the same units as the normal detector signal—volts, amps, absorbance units, and so on. It is caused by the electronic components from which it is made, from stray signals in the environment, and from contamination. Circuit design can minimize noise from the former source, shielding and grounding can help remove environmental noise, and sample pretreatment can often remove contaminants before the sample is chromatographed.
Figure 7.3 is taken from an ASTM recommendation for a GC detector2 and shows a typical noisy baseline. As indicated, the noise is defined as the detector signal range (in this case in mV) between the two parallel lines that enclose the random fluctuations. In some other specifications, noise is further categorized as short term (0.5 to 1 min) and long term (10 min). In addition, the figure shows a long term drift over a period of 30 min. Sometimes drift is referred to as long term noise.
The level of noise restricts the minimum signal that can be detected and attributed to an analyte, so it is important to keep it to a minimum. A detector characteristic that is often more meaningful than the noise is the ratio of the signal-to-noise, SIN. In most chromatographic work it is
Figure 7.3. Example for the measurement of noise level and drift for a TCD. Copyright ASTM. Reprinted with permission.
agreed that the smallest signal that can be attributed to an analyte is one whose S/N is 2.
Time Constant. The time constant ò is a measure of the speed of response of a detector. Specifically, it is the time (usually in seconds or milliseconds) a detector takes to respond to 63.2% of a sudden change of signal, as shown in Figure 7.4. The full response (actually 98%) takes four time constants and is referred to as the response time. Unfortunately, some workers define response time as 2.2 time constants (not 4.0), corresponding to 90% of full scale deflection (not 98%); others define a rise time as the time for the signal to rise from 10 to 90%. To further confuse the situation, some use the terms time constant and response time interchangeably. This lack of consistency can be found in the ASTM specifications.
Nevertheless, Figure 7.5 shows the effect of increasingly longer time constants in distorting the shape of a chromatographic peak. The deleterious effects on chromatographic peaks are the changes in retention time and peak width, both of which get larger as the time constant gets larger. The area is unaffected, so quantitative measurements based on area will still be accurate, but those based on peak height will be in error.
Figure 7.4. Example for the measurement of response time of a TCD. Copyright ASTM. Reprinted with permission.