Gas Chromatography - Data Acquisition and Processing > The Scaling Ampifier > Page 47

Data Acquisition and Processing

Originally, analytical results were calculated from measurements made directly on the chromatogram provided by the chart recorder. This is still true for many chromatographs in use today, but analyses obtained from contemporary instruments commonly process the results using a computer. The output from the detector (which is only rarely the direct output from the detector sensor) is usually in millivolts and is suitable for direct connection to a potentiometric recorder. This output represents a voltage that is linearly related to solute concentration being measured by the detector sensor and as the sensor response is often nonlinear, the signal usually requires nonlinear processing to provide the equired output. This is carried out by the detector electronics. The FID is an exception to this, as the ion current from the flame itself happens to be linearly related to the mass of carbon passing through it per unit time. A block diagram showing the essential elements of a data acquisition and processing system is given in figure 31.

Figure 31. Data Acquisition and Processing System

The Scaling Ampifier

The output from the detector usually passes directly to a scaling amplifier that modifies the signal to a range that is appropriate for the analog-to-digital (A/D) converter. The output can alternatively pass to a potentiometric recorder and produce the chromatogram in real time. The computer system can also produce a real time chromatogram but, to do so, the data must be processed and the chromatogram presented on the printer. The output from most detectors ranges from 0 to 10 mV? whereas the input required by most A/D converters is considerably greater e.g. 0 to 1.0 V. For example, if the FSD of the signal is 10 mv, the instantaneous measurement of 2 mV (assumed from the detector) must be scaled up to 0.2 volt, which is carried out by a simple linear scaling amplifier having a gain of 100.