Measurement of the Detector Response

The application of dual detection [UV and refractive index (RI)] to the SEC analysis of polystyrene-poly(methyl methacrylate) (PS-PMMA) has already been studied in this laboratory (2). Both MWD and CCD were determined using a methodology outlined by Runyon et al. (3). This approach relies on SEC column calibration with narrow polydis-persity standards for each of the homopolymers as well as a measure of the detector response factors for each homopolymer to produce a copolymer MWD. In the case of PS and PMMA this is feasible, but in other block copolymer systems the availability of suitable molecular weight standards may be more limited. In addition, this procedure does rely on true SEC and is not valid for block copolymers for which the universal calibration does not hold true for both blocks in a given solvent system. 

An intense work of Research Design is being carried out in the community for the measurement of the detector response to gamma rays in the large energy range (0.1 - 20... 

To enable qnantitative measurements to be made, the analyst requires the ability to determine the areas or heights of the detector responses of analyte(s) and any internal standard that may be present and then, from these figures, to derive the amount(s) of analyte(s) present in the unknown sample. The software provided with the mass spectrometer allows this to be done with a high degree of automation if the analyst so desires. 

In robustness tests, peak measurementfanalysis parameters can also be considered. Such parameters are related to the measurement of the detector signal and they affect responses, such as peak areas, peak heights, retention time, and resolution. They allow improving the quality of these responses. These factors can be found in the data treatment software of an instrument, where often only the default settings are used by the analyst. 

Chromatogram. A plot of the detector response (which uses effluent concentration or other quantity used to measure the sample component) versus effluent volume or time. 

A plot of the detector response present at the column outlet as a function of time is called a chromatogram (Fig. 31.6). The time from injection of the sample until the peak elutes from the column is called the retention time, t. The amount of compound present for a given peak can be quantified by measuring the peak height or area (most useful) and comparing it with the response for a known amount of the same compound. 

Mp. This is illustrated in Figure 1 in which the response calculated for a LALLS detector (by taking the product of concentration and molecular weight of each slice) is superimposed on the measured MWD (DRI, for the standard labeled Mp == 17,000 by the supplier) from which it was generated. The magnitude of this shift depends both on the MWD of the calibration standard and on the molecular weight dependency of the detector response. 

There are several important characteristics of a good detector sensitivity, dynamic range, stability, and for specific ones selectivity. Sensitivity should be in fact characterized by two parameters the ratio of the detector response to the amount of sample (sensitivity slope) and the minimum detectable level of a given compound (commonly measured for a signal to noise ratio of 3). The dynamic range is the range... 

It is seen that a truly linear detector will have a response index (a) equal to unity and the numerical value of (a) will provide an accurate measure of the proximity of the detector response to strict linearity. The real merit of (a), is that if its numerical value is known, it can also be used to correct for any non-linearity that might exist and thus improve the accuracy of the analysis. 

Problems are also introduced into this combined technique by the concentration detector. As the LALLS detector gives considerably more response for the higher molecular masses, there is often difficulty regarding the calculation of molecular mass at the high end of the distribution when there may be a strong LALLS response, but the concentration detector does not respond to the small amount of material present (an underestimate of the amount of material present produces a large overestimate of molecular mass in the limit, a LALLS response with no measurable concentration detector response corresponds to an infinite molecular mass ). Usually the concentration detector will be a differential refractometer. These are difficult to calibrate reliably for response, and this will lead to errors in the calculated molecular masses. Alternatively, a known mass of sample is injected into the SEC system and it is assumed that this corresponds to the total area of the detector response. This alternative approach works as long as no sample is lost... 

The linearity error represents a predetermined acceptable deviation from a constant sensitivity value, with a constant linearity coefficient. The linearity error is selected, and it is evident that selection of a larger error causes widening of the specified linear dynamic range but that it does not lead to improved linearization of the detector response. The linearity error should not be chosen greater than +5% to permit correct interpretation of the measured response. 

The linear dynamic range of a measuring system can be obtained from the linearized form of the detector response, i.e., from the logarithmic form of eqn [1] (Figure 9) or from eqn [6], as demonstrated in Figure 11. The latter procedure does not permit determination of the linearity coefficient and can only indicate its deviations from unity. 

All instruments measure some chemical or physical characteristic of the sample, such as how much light is absorbed by the sample at a given wavelength, the mass-to-charge ratio of an ion produced from the sample, or the change in conductivity of a wire as the sample passes over it. A detector of some type makes the measurement and the detector response is converted to an electrical... 

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