Baseline restoration

The three above-mentioned types of kinetics also influence other aspects of sensor performance (Fig. 2.20). Thus, the signal-time profiles they provide are critically dependent on the kinetics of the processes involved for example, if the sensor regeneration is rather slow, baseline restoration is much too slow. As noted earlier, a slow chemical kinetics can be used to perform reaction rate measurements. 

Microbial sensors offer a number of assets, namely (a) they are less sensitive to inhibition by solutes and more tolerant to suboptimal pH and temperature values than are enzyme electrodes b) they have longer lifetimes than enzymes and (c) they are less expensive than enzyme electrodes as they require no active enzyme to be isolated. On the other hand, they lag behind enzyme electrodes in a few other respects thus, (a) some have longer response times than their enzyme counterparts b) baseline restoration after measurement typically takes longer and (c) cells contain many enzymes and due care must be exercised to ensure adequate selectivity e.g. by optimizing the storage conditions or using specific enzyme reactions) —some mutant microorganisms lack certain enzymes. 

Another possibility is to select a lower sampling rate, in order to permit quantitative baseline restoration between the monitoring of successive samples. This procedure cannot, however, always be recommended in view of its deleterious influence on sample throughput and hence on reagent consumption. 

Flushing time, the interval between achievement of the maximum analytical signal and baseline restoration, is typically 12-120 s. The next sample can therefore be introduced without a long delay time and this permits a high sample throughput, typically 30-300 h This is particularly advantageous in relation to, e.g., repetitive measurements, simultaneous determinations, titrations, standard additions, and quality control. 

Circuit enhancements for biopotential measurements, (d) Baseline restoration circuit the high-pass filter capacitor Cl is discharged by field-effect transistor F when activated manually or automatically by a baseline restoration pulse, (e) Electrical isolation transformer coupled using the transformer T (top) or optical using the diode D and the photodetector P (bottom). Note that the isolator separates circuit common on the amplifier side from the earth ground on the output side, (f) Electrical protection circuit resistance R limits the current, reverse-biased diodes D limit the input voltage, and the spark gap S protects against defibrillation pulse-related breakdown of the isolation transformer T. 

Correct PZ cancellation is essential for good resolution at any count rate other than a very low one (Figure 11.3 -this is part of Figure 4.20). It is also an essential preliminary to the baseline restoration (BLR) described in Section 11.3.6. The correction is not required with the transistor reset preamplifier. It is a dynamic online process. Chapter 4, Section 4.6 contains background information on pulse overshoot or undershoot. 

Switch the amplifier time constant to that reported in the detector specification. Ensure that the electronic system is set up properly. Pay particular attention to baseline restoration and pole-zero correction. 

Some microphonic effects may be filtered out at the amplifier. Moving to a shorter time constant could reduce the microphonic effect at the expense of increased system noise. If the amplifier has optional settings for the baseline restorer, try changing these to symmetrical or high or auto . 

BASELINE RESTORATION (BLR) A circuit at the amplifier output that maintains the baseline at its reference value. BECQUEREL (Bq) The SI unit of radioactivity, defined as one disintegration per second. 

Before proceeding to the repair, find in the manual the block diagram of the amplifier and try to understand the functions it implements. Although the spectroscopy amplifiers from the different manufacturers may differ considerably from each other, the differences are usually restricted to the more advanced parts, like baseline restorer, pile-up inspector, dead-time and live-time monitors. The basic functions still follow a well-established pattern, which is recognizable in the block diagram of Fig. 6.22. 

Output buffer stage for unipolar output it is reached by either direct feed or via delay line the output driver stage is controlled by the baseline-restorer-clrcultry. 

Switch off baseline restorer. Switch off pile-up rejector. 

Now switch on the baseline-restorer and compare the signal with the previous one. If its positive going lobe is left... 

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