Noise HPLC absorbance detectors

Such "non-optlcal" noise sources are significant in current HPLC optical absorbance detectors in that they limit the reduction of fixed wavelength detector noise below approximately 10 absorbance units (au) and limit the noise achievable in deuterium lamp-based photodiode array detectors to approximately 4x10 au, (The above noise values are given as peak to peak noise as seen on a recorder, which is approximately 6 times the rms noise for the case of a Gaussian noise distribution (1 ),... 

For HPLC optical absorbance detectors, the term (2qB/l ) 1.0 and thus an x-fold Increase In will reduce the absorbance noise contribution by >73ET... 

Optical absorbance detection In HPLC Is currently limited by several sources of "non-shot" or "non-optlcal" noise. As shown In Summary Table V, a state of art variable wavelength UV absorbance detector comes within a factor of two of Its 8x10 au shot noise limit. In this case the dominant noise sources are shot noise and Johnson thermal noise. 

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

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. 

The fundamental properties of fluorescence make this a particularly attractive basis for an HPLC detection system [27], for whereas photometers depend upon the measurement of fairly small differences between the intensity of a full and slightly attenuated beam the measurement of fluorescence starts in principle from zero intensity. At sufficiently low values of concentration (<0.05 absorbance) then the intensity of fluorescence is directly proportional to concentration with a linear range of three to four decades. Consequently, fluorescence detectors are more selective and sensitive than ultraviolet detectors in LC by a factor of 10 giving noise equivalent sensitivities of better 1 ngmP. ... 

A further important component of each HPLC system is the detection device. Often, Ultraviolet (UV)-detectors are used for this purpose because many analytes absorb light in the range of 200-350nm. UV detectors provide reasonable sensitivity and dynamic range as well as a low noise level. 

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