Page 206 - Photodetection and Measurement - Maximizing Performance in Optical Systems

Contamination and Industrial Systems

Contamination and Industrial Systems 199

2.5 Bromothymol Blue

2.0
Absorbance (10mm) 1.5 Detectors 1-5

1.0

0.5

0 1 2 3 4 5
200 300 400 500 600 700 800
Wavelength (nm)
Figure 9.8 More complex absorbance spectra allow combinations of on- and off-absorption
detectors to give improved suppression of background absorption.

tools for this generally known as linear multivariate calibration schemes, such
as multiple linear regression (MLR), principal component analysis (PCA), and
partial least squares regression (PLS). If these linear systems fail to give good
calibration because of nonlinearities of absorbance versus concentration, it may
be useful to try neural nets, which can provide similar calibration functions but
also deal with nonlinearities. Commercial spectral analysis software is available
with all these tools included, or you can develop your own using the available
mathematical descriptions and general programming software. Martens and
Næs (1993) give a very clear and practical description of the linear techniques.

9.4 Window Cleaning
Fouling should never be considered as an afterthought to a manufactured instru-
ment, as though we might get away without dealing with it! Fouling always
occurs, and on-line instruments have a tendency to be installed and forgotten.
Even laboratory instruments will suffer from similar types of fouling, but the
shorter periods actually in service, the proximity of staff, and the greater ease of
cleaning give fewer problems. Nevertheless, any instrument should be designed
from an early stage with fouling compensation, removal, or avoidance in mind.
Let’s now look at the second of these; removal means cleaning or replacement.

9.4.1 Disposables
The simplest approach for many measurements is to use disposable windows.
Usually this means a whole disposable sample cell, for example a plastic cuvette
with molded windows. Polystyrene and acrylic plastics are the most common

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