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INFRARED ANALYZER
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The above diagram provides a graphic representation of the basic design of an
infrared analyzer which is used to measure breath alcohol concentrations. The
design is based on the fact that specific wavelengths of infrared energy are
absorbed by ethyl alcohol molecules. In its simplest form, the instrument's
detector measures the change in the amount of a specific wavelength of infrared
energy that passes from the infrared source (lamp), through the sample chamber
and filter wheel to the detector. The change in response on the detector, as a
breath sample is submitted to the sample chamber , is monitored and analyzed by
a processor in the instrument. The change in the signal is used to calculate
an alcohol concentration
The difference between the amount of infrared energy that reaches the detector
when the sample chamber is free of compounds that absorb the infrared energy and
the amount of infrared energy that reaches the detector when a subject's breath
sample is within the sample chamber, provides an indication of the concentration
of the absorbing substances in the sample . If ethanol was the only molecule
found in a breath sample that would absorb energy at the wavelength being
recorded by the detector, the calculated difference in infrared energy reaching
the detector could be used by itself to establish the concentration of alcohol
in the breath sample. Unfortunately this is not always the case.
In order to deal with the lack of specificity, it is important that the primary
wavelength of infrared energy used to measure the concentration of alcohol is
selected based upon its limited cross sensitivity to other substances that are
commonly found in the human breath. The spinning filter wheel in the diagram
above is used to modulate the light through several different filters allowing
different wavelengths of infrared energy to be transmitted to the detector for
analysis. The secondary wavelengths of energy are selected based upon the
interfering compounds that could be found in a human breath sample. If these
compounds are identified and are in concentrations that would adversely effect
the calculated ethanol result, the analysis can be aborted.
One other important point is that the differing concentrations of alcohol or
other energy absorbing compounds found in the subject's breath sample are not
in a one to one relationship with the amount of energy that reaches the
detector. In other words, a .050 alcohol concentration may absorb X units of
infrared energy, but a .200 will not absorb 4X units of infrared energy. This
inherent non-linearity can be overcome by performing a multi-point calibration.
To ensure that the instrument is properly quantifying alcohol and the other
substances, it would be prudent to perform multi-point accuracy checks for each
substance of interest and ensure proper calibration over the entire range of
substances and concentrations.
Finally, infrared systems require power to light the IR source, heat the sample
chamber, power the light modulator and drive the detector and associated
circuits. Historically, the signal from the detector has been small relative
to the noise generated in the system. This has made it difficult to resolve
changes in the signal when low level concentrations of alcohol are presented to
the system. To solve this problem, several manufacturers have included cooled
detectors. These systems require additional power, but the cooled detector
decreases the noise on the detector and enhances the instrument's performance at
the low alcohol concentrations.
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Strengths of Infrared Based Analyzers |
Weaknesses of Infrared Based Analyzers |
Analyzes the breath sample continuously. Although it is commonly believed that the instrument can analyze a sample in real time, the dynamics of the sample inlet and sample chamber retard the detector's ability to truly generate real time data. Nonetheless, there is valuable information in the continuous stream of data that is generated. |
Cross reactivity in the ethanol absorption band to substances found on the breath other than alcohol
- Requires multiple wavelengths to identify these substances
- Requires Calibration Checking of interfering substances to ensure that these interfering substance systems are working properly.
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This technology is commonly used for generating evidential breath alcohol results. |
Noise in the system limits low level accuracy. |
Infrared testing systems can perform analyses in rapid succession. Once the sample chamber is purged from the last sample and a new "zero" baseline established, a subject can provide another sample for analysis. |
Since the detector output and concentration of alcohol are not a one to one relationship. Multiple point accuracy checks should be performed to ensure that the system is calculating alcohol concentrations and/or potential interfering substances properly throughout the full range of analysis. |
Single chamber systems have difficulty establishing a true .000 baseline. |
Limited life of the infrared source. |
Breath chambers tend to be large and limit the ability of the system to capture consistent deep lung samples or determine subtle changes in alcohol concentrations in the breath sample. |
Infrared systems tend to be relatively large and require a large amount of power to operate. |
The current systems are costly relative to other available technologies. |
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