|
 Technology:
A process analyzer is an analytical instrument used to automatically
measure the chemical content of samples extracted from a variable
process, for process monitoring or process control purposes.
When samples are continuously extracted from the process through
a direct connection to the analyzer, this is called On-line
analysis.
Process Analyzers are used to measure multiple chemicals at
municipal drinking water treatment plants and municipal wastewater
treatment plants or in an industrial process involving liquids.
Typical applications for process analyzers are characterized
by large flow volumes, a dynamic chemical matrix and a substantial
motive to obtain real-time chemical analysis information.
Motives for use of on-line analysis include economic issues,
quality control, and regulatory compliance.
A
process analyzer can be thought of as an on-line ultraviolet-visible
spectrometer, designed to collect the full absorbance signature
of a sample at 256 wavelengths. Spectroscopy is the study of
the absorbance and emission of electromagnetic radiation (light)
by matter. The collection of frequencies absorbed by a sample
is its absorption spectrum.
 When
light passes through or is reflected from a sample, the amount
of light absorbed is the difference between the incident radiation,
Io, and the transmitted radiation, I. the amount of light that
is absorbed is expressed either as transmittance or absorbance.
All spectrophotometers consist of four major sub-units:
· a source that generates electromagnetic radiation
· a dispersion device that selects a particular wavelength
from the broad band radiation
of the source
· a sample area
· a detector to measure the intensity of radiation
Diode
array:
Diode arrays are assemblies of individual photodiodes in a linear
array.
Self-scanned arrays have the read-out electronics included on
the chip with the array.
When displayed, all elements of the array must be read out in
series. The array has 1024 elements. Light of all wavelengths
falls on the diode-array and is measured simultaneously, that
is, data acquisition is done in parallel. Speed is the best
known advantage of diode-array spectroscopy. Data is acquired
in parallel, the detectors are read-out by "electronic
scanning", and microprocessors and computers are used to
process data. Measurements are made at different wavelengths
at the same time. Conventional spectrophotometers can make multi-wavelength
measurements but there is a time differential between each measurement.
This is accomplished by drawing a volume of sample into a flow
cell, transmitting known intensities of light through the sample
and detecting the intensity of light that remains at each wavelength
in the range.
The standard instrument detects light from 200 nm (nanometers)
through 450 nm and applies pattern recognition techniques to
the full absorbance signature of a sample in order to detect
multiple chemical parameters using a single analyzer. A manifold
system is used to monitor multiple sample points in a process.
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