Detectors noise and drift

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. 

Nowadays, most chromatographic software is capable of calculating the detector noise and drift. Typically, the detector should be allowed to warm up and stabilize prior to the test. Temperature fluctuations should be avoided during the test. The noise and drift tests can be performed under static and dynamic conditions. For a static testing condition, the flow cell is filled with methanol, and no... 

Make sure that the flow cell is clean and free of gas bubbles when performing the detector performance tests. Dirty flow cell and gas bubbles are the main reasons for poor results for detector noise and drift. 

Detector Noise and Drift. Transducers for modem spectrophotometric systems are usually vacuum phototubes or photomultiplier tubes. Noise and drift from the transducer can be quite troublesome in kinetic methods, and special care is normally taken to ensure low-noise operation. 

There are three different types of detector noise, short term noise, long term noise and drift. These sources of noise combine together to give the composite noise of the detector. The different types of noise are depicted in figure 3. 

Figure 1.18 Methods for calculating short- and long-ten noise and drift for chromatographic detectors.

All detectors need to be verified for meeting their noise and drift specifications. These tests are often performed with a dry cell that is sealed and has no actual solvent flow going through it. Some detectors have on-board diagnostics for these tests, with digital performance readouts. 

The first experiment of this set (row 1) is used to establish the noise and drift characteristics of the detector. 1 mL/min water (with 1% methanol) is pumped through the system. The system is allowed to equilibrate then, a 1 pL water blank is... 

Detector sensitivity is one of the most important properties of the detector. The problem is to distinguish between the actual component and artifact caused by the pressure fluctuation, bubble, compositional fluctuation, etc. If the peaks are fairly large, one has no problem in distinguishing them however, the smaller the peaks, the more important that the baseline be smooth, free of noise and drift. Baseline noise is the short time variation of the baseline from a straight line. Noise is normally measured "peak-to-peak" i.e., the distance from the top of one such small peak to the bottom of the next. Noise is the factor which limits detector sensitivity. In trace analysis, the operator must be able to distinguish between noise spikes and component peaks. For qualitative purposes, signal/noise ratio is limited by 3. For quantitative purposes, signal/noise ratio should be at least 10. This ensures correct quantification of the trace amounts with less than 2% variance. The baseline should deviate as little as possible from a horizontal line. It is usually measured for a specified time, e.g., 1/2 hour or one hour and called drift. Drift usually associated to the detector heat-up in the first hour after power-on. 

When characterizing copolymers, it is necessary to have two detectors in series, e.g., a refractometer with either a UV detector or an IR detector. An IR detector is preferred for the detection of polyalkenes at elevated temperatures because baseline noise and drift are much less than for the refractometer detector. 

Noise and Drift. Electronic, pump, and photometric noise poor lamp intensity, a dirty flow cell, and thermal instability contribute to the overall noise and drift in the detector. Excessive noise can reduce the sensitivity of the detector and hence affect the quantitation of low-level analytes [13,14]. The precision of the... 

It would be very convenient if the TCD had noise levels down in the nanovolt region. Amplifiers can be built without too much difficulty which contribute no more than 25 nV of their own noise. Unfortunately, the TC detector is subjected to many extraneous influences. It is the fluctuations in these which dominate the noise and drift seen on the recorder. 

Refractive Index Detectors These detectors respond to changes in refractive index (positive or negative) arising from the presence of a compound in the eluent. All the factors which can affect refractive index must be carefully controlled (e.g. temperature, eluent composition, pressure) otherwise noise and drift will limit the sensitivity. Thus the chromatograph is best placed in a thermostatically-controlled cabinet and good pumps are desirable to minimise pressure fluctuations. Changes in eluent composition will also cause spurious changes in refractive index. 

Detector noise is the term given to any perturbation on the detector output that is not related to an eluted solute. It is a fundamental property of the detecting system and determines the ultimate sensitivity or minimum detectable concentration that can be achieved. Detector noise has been arbitrarily divided into three types, short term noise, long term noise and drift all three of which are depicted in figure 4. 

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. 

Ideally, the transmittance scale on a linear detector is fixed by a 0% T measurement (dark current measurement on the detector) and a 100% T measurement (total illumination of the detector by Iq). A sample attenuates the Iq intensity signal, and the sample transmittance and hence the absorbance are obtained. All these individual measurements are subject to noise and drift errors and combine to give an overall measurement standard deviation, This standard deviation is related to the relative error of measurement, rearranging Equation (1.7) and obtaining the partial derivative. Note that the molarity, M, in Equation (1.5) has been replaced by C the concentration in g L-1. The relative error function [9] is given by Equation (1.8) ... 

According to another classification, the detectors can be divided into specific, universal and mixed. For the choice of the detection method, the properties of sample and mobile phase as well as experimental requirements are decisive however, sometimes several types of instruments based on the same detection principle are marketed. When judging the detectors one considers primarily (i) linearity, dynamic range and sensitivity, (ii) erroneous responses like noise and drift, caused in the former by the instability of operational variables such as temperature variations, pulsating eluent flow, etc. (iii) response distortion due to hydraulic broadening and skewing of the sample zone as well as response delay. 

The following features characterize the detection sensitivity, linearity, noise and drift, selectivity, suitability for gradient elution and computer compatibility. UV and RI detectors are the most common, but fluorescence, electrochemical and flame ionization detectors are also available. In Table III the main features of UV-detectors and in Table IV those of the Rl-detectors are summarized. Detectors are also offered by other manufacturers and should therefore be easily interchangeable (which requires standardized connection unions). 

Another group of detectors with sample transfer employs pyrolysis of macromolecules. Column effluent is deposited continuously on the appropriate moving transporter such as a wire, chain, or net. Next, the solvent is evaporated, sample is pyrolyzed and polymer concentration is assessed from the amount of caibon dioxide formed. The problems with cleaning of sample transporter to prevent base line noise and drift prevented broad use of detectors of this kind. The attempts to apply similar principle for monitoring polymer composition by engagement of complete gas chromatography of the fractions so far remained only on the level of laboratoiy experiments. Detection of such kind would provide valuable information on the sample composition, for example for statistical copolymers. There are, however, so far unsolved problems with the dependence of composition of pyrolytic products on presence of the neighboring units in copolymers. 

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