Detector/system linearity

Experiments 10-27 are designed to check the autosampler injection precision, pump repeatability and detector/system linearity. One programs the system to automatically inject multiple replicate volumes of a certified test standard. One typically injects 6-10 replicates per volume. The standard component s peak areas are used for calculated injection precision (reproducibility) and system linearity whereas, the retention times are used to calculate pump repeatability. 

The diffraction equipment used for the study of conducting polymers in no way differs fi-om that used for the study of conventional polymers. This short section does not cover the experimental methods in any technical detail, however, but merely presents some considerations about their applicability. Details can be found in the standard books on this topic [3-5]. Admittedly, these books are somewhat dated they do not, for instance, reflect the impact of computers on both automation of equipment and data evaluation. Another result of the ever-accelerating progress in microelectronics (still based on metals and inorganic semiconductors instead of polymers), is to be found in the field of x-ray detector systems linear photodiode array detectors, Charge-Coupled-Device area detectors and Image Plate detectors have all become available recently. 

Taking a value for (oe) of 2.5 pi (which would be typical for a well-designed column detector system) and using equation (8), values for (ropt) are shown plotted against separation ratio in Figure 7. It is seen that the optimum column radius increases linearly with the separation ratio of the critical pair (ranging from 0.1 mm... 

A] could represent the spectral information and [C] the chromatographic (time dependent) information. Assumed is that the mutual Muence of the components is neglectable, i.e. that the system including the detector is linear. In principle only [A] has to be determined if a model exists for the columns of [C] ... 

This means that the detector systems must still be responding linearly to the absolute amount of each component even if one represents 99% of the sample and is not the component of interest. Certainly smaller sizes can be used but here again practicality enters in. 

Flow-injection analysis is a versatile technique to evaluate the performance of a detector system. CHEMFETs may have an advantage over ISEs because of their small size and fast response times. We have tested our K+-sensitive CHEMFETs in a wall-jet cell with a platinum (pseudo-)reference electrode. One CHEMFET was contineously exposed to 0.1 M NaCl and the other to a carrier stream of 0.1 M NaCl in which various KC1 concentrations in 0.1 M NaCl were injected. The linear response of 56 mV per decade was observed for concentrations of KC1 above 5 x 10"5 M (Figure 9). When we used this FIA cell (Figure 10) for determination of K+ activities in human serum and urine samples, excellent correlations between our results and activities determined by flame photometry were obtained (Figure 11). 

During the development of our OMA-based detector system, we performed a number of experiments checking the performance by measuring the absorbances of uniform solutions spun in the ultracentrifuge. It was observed that the flat absorbance profiles for the hundreds of points for each cell varied only by about 0.002 absorbance units and a plot of the average absorbance for each cell vs. the concentration was linear from 0 to 1 absorbance units and even higher ( 1,2). However, such a demonstration with uniform solutions does not prove that the correct absorbance profile is obtained from a cell with a concentration gradient. 

The basic principles of operation which have been explained so far, are valid for one dimensional or linear detectors as well as for two-dimensional gas filled detector systems. 

Some commercially available linear detector systems use this rise-time technique. The resolutions that are specified are 50 pm FWHM, at a signal charge of 0.5 pico-Coulomb, for an overall detector length of 50 mm (bl/l = 10 ). 

Sweedler J. V., Jalkian R. F. and Denton M. B. (1989) A linear charge coupled device detector system for spectroscopy, Appl Spectrosc 43 953-961. 

Recently, an indirect method to measure the free radical scavenging activity of extracts and, thus, their antioxidant activity has been developed using HPLC coupled to a coulometric array detector system [61]. This was developed to characterise the overall antioxidant activity status of fruit and vegetables. A significant positive linear correlation was demonstrated between the total antioxidant activity determined by using the oxygen radical absorbance capacity assay and that measured using the electrochemical data obtained from the coulometric array detectors. 

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