Sampling detector noise

Precision The precision of a gas chromatographic analysis includes contributions from sampling, sample preparation, and the instrument. The relative standard deviation due to the gas chromatographic portion of the analysis is typically 1-5%, although it can be significantly higher. The principal limitations to precision are detector noise and the reproducibility of injection volumes. In quantitative work, the use of an internal standard compensates for any variability in injection volumes. 

Concentration assays are often the least demanding, since usually the component to be measured is abundant and minor components scarce. Even if resolution is poor or there is detector noise, accurate measurements of concentration can still be obtained. In concentration assays, the principal requirements are stringency in the precision of sample dilution and measurement of column losses of the major component. Detector calibration, another important issue in concentration assays, has been discussed above. 

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... 

From Figure 47-17 we note several ways in which the behavior of the transmittance noise for the Poisson-distributed detector noise case differs from the behavior of the constant-noise case. First we note as we did above that at T = 0 the noise is zero, rather than unity. This justifies our earlier replacement of E0 by E0 for both the sample and the reference readings. 

Previously, in the case of constant detector noise, we then set Var(A s) and Var(A r) equal to the same value. This is the point at which must we now depart from the previous derivation, since in the case of Poisson-distributed noise the sample and reference noise levels will rarely, if ever, be the same. However, we are fortunate in this case that Poisson-distributed noise has a unique and very useful property that we have indeed previously made use of the variance of Poisson-distributed noise is equal to the mean signal value. Hence we can substitute Es for Var(A s) and Er for Var(A r) ... 

Sampling Detector Changes rate Changes noise level in... 

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. 

Accuracy is a measure of the closeness of a measurement to the true value. Precision is a measure of how reproducible the measurements are. For many detectors, the accuracy of a measurement is maintained by user calibration. For some detectors, however, such as photodiode array detectors, accuracy relies on internal calibration. The linear dynamic range of the detector is the maximum linear response, divided by the detector noise. The detector response is said to be linear if the difference in response for two concentrations of a given compound is proportional to the difference in concentration between the two samples. Most detectors become nonlinear as the sample concentration increases. 

Figure 8. Close-up view of contrast-matched signals. The error hars are much larger for the large sample-detector distances d, because the signal intensity decreases as I let. The signal from the phenol is zero within the level of the noise. The phenol is therefore mainly distributed uniformly in the liquid. Symbols are the same as in Figure 7.

One of the most important decisions that is left to the analyst when operating a liquid chromatograph is the choice of detector sensitivity. In some instruments the output from the sensor is monitored continuously over its entire dynamic range and so sensitivity is not an optional experimental parameter. Nevertheless, in this case, the sample size determines the concentration range over which the eluted solutes are monitored and thus an optimum sample size must be chosen. The detector should never be operated at its maximum sensitivity unless such conditions are enjoined by limited sample size or column geometry. Provided that there is adequate sample available, and the sample concentration when eluted is within the linear dynamic range of the detector, the maximum sample size that the column can tolerate should be used. This ensures that the detector noise is always minimal... 

Although the dark signal contributes to the observed signal, it is distinguished from the background defined in Section 4.2.2 by the fact that it does not depend on laser intensity or sample variables. In fact, one test for detector noise is to run a spectrum with the laser completely off. The observed noise in this situation is due to detector and readout noise. Figure 4.6 shows a spectrum dominated by detector noise. The detector noise remains after the contributions of the cell and water are subtracted. 

SNRd is always smaller than that for the sample shot noise limit, all else (other than detector noise) being equal. As (p is decreased by improvements in the detector, SNRd approaches the value determined by sample and background shot noise. 

Fsnr and as defined in Eq. (4.24) and (4.25) assumed the case of the analyte shot noise limit, with negligible contribution from background scattering or detector noise. If the background noise is considered, as in Eq. (4.20) and (4.21), the Fsnr and expressions are more complex and probably less useful. However, Fsnr and F sj,ji still vary with (Fd6s) or (Fots), respectively, for a given sample. 

---