Spurious drift noise

However in the multivariable case, such a normalization is not possible and it will be seen that a spurious or noise induced drift will arise as a result of the multiplicative noise term. This drift term will occur in the drift or first order coefficients of the Kramers-Moyal [31] expansion which are as follows ... 

Ito [51] obtains different expressions for the Kramers-Moyal coefficients in which the spurious drift term is absent. However use of Ito coefficients involves new rules for calculus and so Stratonovich s method will be used here since it is also in agreement with the original method of Brown [8] and is the correct definition to use in the case of a physical noise which always has a finite correlation time [58] (see B.2). 

The last term in Eq. (E.17) is as we have seen called the noise-induced or spurious drift [31]. 

Because of the relatively small number of experiments done on commercial-scale equipment before submission, and the often very narrow factor ranges (Hi/Lo might differ by only 5-10%), if conditions are not truly under control, high-level models (multi-variate regressions, principal components analysis, etc.) will pick up spurious signals due to noise and unrecognized drift. For example, Fig. 4.43 summarizes the yields achieved for... 

Spurious peaks could occur due to volatile compounds emitting from the flow controller diaphragm. This problem can be solved by use of metal diaphragms. Some manufacturers have used short molecular sieve traps between the flow controller and the inlet system, but, if a poor inlet system is used, the sample can flash-back upon injection and condense on the trap, later causing spurious peaks or noise and drift as the sample slowly comes back off the trap. 

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. 

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