Detector wavelength accuracy

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

Esquivel, J. B., Wavelength accuracy testing of UV-visible detectors in liquid chromatography, Chromatogmphia, 26, 321, 1988. 

On some detectors, a holmium oxide filter or special holmium oxide cell is integrated into the detector these can then be moved into the optical path during calibration mode. The detector itself or associated control software may then be able to check for wavelength accuracy. Note If the filter or cell is made of glass, and not quartz, 241 nm may not be used. 

Experiment 4 is intended to check the detector s wavelength accuracy at 205nm and 273nm. Using the same caffeine solution as for the OQ test, this test is performed and calculated as in the procedure described earlier under subsection (i) of Detector in section (j) of the OQ guidelines. 

It is recommended that OQ test the following on an HPLC system flow accuracy, pump compositional accuracy, pressure pulsations, column oven temperature accuracy/stability, detector noise/drift and wavelength accuracy, autosampler injection precision and carryover. 

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. 

Linearity problem Broken optical fiber Wavelength accuracy Detector linearity Noise... 

The detection method can also be a source of potential variations from instrument to instrument. For example, UV detectors usually specify the wavelength accuracy to within +/-1-3 nm, and a change of this value could prove significant. Another, often ignored, factor is the time constant of the detector. Too low a setting of this factor can show significant noise levels and too high a value can distort the peak shape. 

UV-Vis detector The detectors should operate with a wavelength range from 200 to 800 nm. The detector should have a wavelength accuracy of +2nm. The detector response should be linear with a correlation coefficient r2 not less than 0.999 over the full dynamic range. The linear range should be up to 2.0 Absorbance Unit Full Scale (AUFS). 

Pump flow rate accuracy and gradient accuracy Detector linearity of response, noise, drift, and wavelength accuracy Injector precision, linearity, and carryover Column heater temperature accuracy... 

Detector linearity of response, noise, drift, and wavelength accuracy... 

There are many ways to check the wavelength accuracy of a UV-Vis detector. For detectors with built-in wavelength verification, the deuterium line at 656 nm or the absorption bands at 360, 418, 453, and 536 nm in a holmium oxide filter... 

The wavelength accuracy of the diode array detector is commonly determined by comparing a measured absorbance with the absorbance maxima of a reference... 

If a system comprises several modules, it is recommended that system tests be performed for parameters that are affected by multiple modules (holistic testing) rather than performing tests module by module (modular testing). Individual module tests should be performed if the parameter is affected by that module only (e.g., the wavelength accuracy of an HPLC variable-wavelength detector or the temperature accuracy of a column compartment). 

Wavelength accuracy. In order to evaluate the ability of each system to locate spectral lines, a preliminary wavelength calibration was carred out with the emission spectrum of a mercury pen lamp and then the peak maxima of several atomic lines from an iron hollow cathode lamp were located. The root mean square (RMS) prediction error, which is the difference between the predicted and the observed location of a line, for the vidicon detector system was 1.4 DAC steps. Because it is known from system calibration data that one DAC increment corresponds to 0.0125 mm, the absolute error in position prediction is 0.018 mm. For the image dissector, the RMS prediction error was 7.6 DAC steps, and because one DAC step for this system corresponds to 0.0055 mm, the absolute error in the predicted coordinate is 0.042 mm. The data in Table II represent a comparison of the wavelength position prediction errors for the two detectors. 

Operating devices may become miscalibrated after a while (e.g., the temperature accuracy of a gas chromatography [GC] column oven or the wavelength accuracy of a UV/visible detector s optical unit). This can have an impact on the performance of an instrument. Therefore, a calibration program should be in place to recalibrate critical items of an instrument. 

The detector. The parameters that have to be checked are wavelength accuracy detection, refractive index sensitivity, linearity, spectral quality (PDAD) short-term or long-term noise, drift, and flow sensitivity. 

Noise and drift are measured in static (dry detector cell) and in dynamic mode at different wavelengths, e.g., 200, 254, and 390 nm. The change in the absorbance as a function of flow rate at the same wavelengths reflects flow sensitivity. Noise is expressed in AU/cm, drift in AU/hr, and flow sensitivity in ALf min/mL. Some equipment units can automatically perform calibration for accuracy. For example, some HPLC-UV/Visible detectors include holmium oxide filters for measurement and calibration of the wavelength accuracy. 

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