A small amount of background noise or scattered light is always created by the OPC’s laser interacting with the optical path.
Scatter from the capillary walls or optical windows can be measured. Normally, the amount of scatter from the capillary walls is minimal and has no effect on the data. As the capillary becomes contaminated or, in severe cases, damaged, the amount of scatter increases. In other designs, the optical windows from which the sample is viewed can become contaminated. Eventually, the amount of scatter coming from contaminated optics equals the amount of scatter from the smallest detectable particle size. In most cases, cleaning the capillary resolves this issue. See the blog series, LiQuilaz Particle Counter Maintenance, and the relevant paper.
Excessive background scatter can be recognized in the following ways:
- Accompanying software reports the amount of background scatter. Each OPC has a specification for this value. Once that specified value has been exceeded, the collected data should be considered suspect.
- Often the data will indicate excessive scatter. If the amount of scatter from contamination equals the amount of scatter from the smallest detectable particle size, there will be an excessive number of counts in the first channel. One will often see hundreds to thousands of counts in only the first channel on known clean DI water with zero to few counts in the other size channels, as shown in Figure 2.
Figure 2 Excess Background Scatter
Operators often ask if the first channel can be ignored. The answer is “No” for two reasons.
- First, the OPC electronics becomes busy processing the noise signal, which prevents it from accurately processing real particle signals.
- Second, a constant amount of light scatter hits the detector. The OPC is designed to measure total cumulative counts from the smallest detectable size and greater. The electronics and signal processing are not designed to handle such situations, resulting in compromised data across all channels.