Detectors absorbance linearity

Intensity measurements are simplified when a detector always gives one electrical pulse for each x-ray quantum absorbed the detector remains linear so long as this is true. For low intensities, when the rates of incidence upon the detector are low, the Geiger counter fulfills this condition. As this rate increases above (about) 500 counts per second, the number of pulses per second decreases progressively below the number of quanta absorbed per second. This decrease occurs even with electronic circuits that can handle higher counting rates without appreciable losses. 

A second source of error may be in the detector. Detector linearity is an idealization useful over a certain concentration range. While UV detectors are usually linear from a few milliabsorbance units (MAU) to 1 or 2 absorbance units (AU), permitting quantitation in the parts per thousand level, many detectors are linear over only one or two decades of operation. One approach in extending the effective linear range of a detector is high-low injection.58 In this approach, an accurate dilution of a stock sample solution is prepared. The area of the major peak is estimated with the dilution, and the area of the minor peak is estimated with the concentrated stock. This method, of course, relies on linear recovery from the column. Another detector-related source of error that is a particular source of frustration in communicating... 

Fluorescence detectors can be made much more sensitive than uv absorbance detectors for favourable solutes (such as anthracene) the noise equivalent concentration can be as low as 10 12 g cm-3. Because both the excitation wavelength and the detected wavelength can be varied, the detector can be made highly selective, which can be very useful in trace analysis. The response of the detector is linear provided that no more than about 10% of the incident radiation is absorbed by the sample. This results in a linear range of 103-104. 

UV detector wavelength accuracy, absorbance linearity, and sensitivity ... 

Response factor When the response of a chromatographic detector is linear, the ratio of the component concentration to the detector signal (e.g., absorbance with a UV detector). 

The output of a thermal detector depends on the power absorbed by the detector. To make it absorb as much energy as possible, we blacken the surface of the detector. If the detector absorbs all of the incident energy, then the output of the bolometer is proportional to the arriving power, and the responsivity (in volts/watts) is independent of wavelength. This is very different from photon detectors, whose responsivity (in volts/watts) increases linearly with wavelength. 

The detector must be sensitive to the radiation falling on it, and the spectrum is very often displayed on a chart recorder. The spectrum may be a plot of absorbance or percentage transmittance (IOO///0 see Equation 2.16) as a function of frequency or wavenumber displayed linearly along the chart paper. Wavelength is not normally used because, unlike frequency and wavenumber, it is not proportional to energy. Wavelength relates to the optics rather than the spectroscopy of the experiment. 

Absorbance detectors are also commonly used in combination with postcolumn reactors. Here, most issues of detector linearity and detection limit have to do with optimization of the performance of the reactor. In a typical application, organophosphorus compounds with weak optical absorbances have been separated, photolyzed to orthophosphate, and reacted with molybdic acid, with measurement being performed by optical absorbance.58... 

The limit of detection for this instrument is about 10 pg/ ml for polystyrene in 2-butanone,163 which is close to two orders of magnitude higher than that of the deflection-type DRI. Moreover, the response of the ELSD is linear over only two decades in concentration.163 The ELSD is a useful backup detector when the DRI or UV detectors are not appropriate, e.g., when the UV absorbance or RI change is a function of copolymer composition as well as concentration or in gradient elution systems where changes in solvent composition cause drift in baselines of the UV and DRI detectors. Compounds about as volatile as the solvent are poorly detected by ELSD. 

Refractive index detectors are not as sensitive as uv absorbance detectors. The best noise levels obtainable are about 1CT7 riu (refractive index units), which corresponds to a noise equivalent concentration of about 10-6 g cmT3 for most solutes. The linear range of most ri detectors is about 104. If you want to operate them at their highest sensitivity you have to have very good control of the temperature of the instrument and of the composition of the mobile phase. Because of their sensitivity to mobile phase composition it is very difficult to do gradient elution work, and they are generally held to be unsuitable for this purpose. 

These detectors respond to UV/visible absorbing species in the range 190-800 nm and their response is linear with concentration, obeying the Beer-Lambert law (p. 357). They are not appreciably flow or temperature sensitive, have a wide linear range and good but variable sensitivity. 

In many instruments the meter read-out is calibrated in absorbance units using a logarithmic scale while other instruments retain the convenience of a linear scale but convert the signal from the detector to a logarithmic one by electronic or mechanical means. It is essential when using a photometric instrument to know if it is calibrated in absorbance or transmittance units. 

Linearity is the measurement of the linear range of detectability that obeys Beer s Law and is dependent on the compound analyzed and the detector used. In short there is a linear relationship between absorbance and concentration. To be within the linear range of the method you should be working within the absorbances and concentrations that form the linear part of the curve. 

The parameters that require qualification for a UV absorbance detector are wavelength accuracy, linearity of response, detector noise, and drift. These determine the accuracy of the results over a range of sample concentrations and the detection limits of the analysis. 

The wavelength accuracy and detector linearity and detector noise have the same effect on laser-induced fluorescence, as those of a UV absorbance detector. 

Detector linearity would normally be tested as part of an overall holistic test that examines the linearity of the complete instrument, the injector, as well as the detector. The test would normally be designed to cover the range up to 2 absorbance units (AU). 

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