Detectors, HPLC 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... 

HPLC equipment has been designed and produced to assure correct volumetric delivery of the mobile phase, including the injected sample, and has low-noise detectors so that low concentrations of samples can be analyzed conveniently. Discussed below, briefly, are some of the important considerations for the HPLC equipment. More detailed discussion can be found in a recent text (see Chapter 3 of reference 3). 

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 baseline noise as offered by many UV-Vis detectors is in the range 1 to 2 x 10 5 AU and much lower than the limit of detection and quantitation required for most applications. This value is achieved under optimum conditions, such as with a reasonably new lamp, an ultraclean flow cell, stable ambient temperature, HPLC-grade solvents, and no microleaks in the entire HPLC system. These conditions are always valid at the manufacturer s final test and probably at the time of installation in the user s laboratory. However, after some time, optical and mechanical parts deteriorate (e.g., the lamp loses intensity and the flow cell may become contaminated). If we repeat the test after 3, 6, or 12 months, the noise of 1 x 10 5 AU may no longer be obtained. The recommendation is to select acceptance criteria according to the intended use of the system. 

Detectors. Fluorescence and UV detectors are used in the HPLC analysis. The high sensitivity and specificity of fluorescence detection in tocopherols and tocotrienols make the fluorescence detector the first choice. The fluorescence detector is ten times more sensitive and has less background noise than the UV detector. Electrochemical detectors are also used in the analysis of tocopherols and tocotrienols (Murphy and Kehrer, 1987 Sanchez-Perez et al., 2000). As a high-polarity mobile phase is needed for the electrolytes when using an elec-... 

An ideal on-line detector for HPLC or CE has versatility, high sensitivity, continuous monitoring of the column effluents, low noise level, wide linearity of response, stable baseline, insensitivity to flow rate and temperature changes, and response to all types of compounds. It should be rugged and not too expensive. The detector should be able to measure accurately a small peak volume without increasing its volume appreciably. Of these parameters, the terms noise, sensitivity, and linearity are typically used in describing detector performance. 

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