FAQ: How do I monitor and maintain PicoRaman performance?

FAQ series: This series of publications explores some of the most often asked questions regarding Timegated^® spectrometers and measurements without having you read page after page of text.

Although Timegated^® PicoRaman spectrometers are designed to require minimal maintenance for everyday work, routine performance monitoring is advisable for ensured quality of measurement results. It is recommended that the spectrometer performance is checked before each measurement run e.g. in the morning before the beginning of sample measurements.

A basic performance monitoring measurement includes measuring a stable standard sample in a repeatable fashion. At Timegate Instruments this sample is often cyclohexane as it can withstand laser exposure, and it being a translucent liquid, the measurement results aren’t overly sensitive to variation in laser focusing. Cyclohexane is still somewhat volatile, but the sample can be replaced with a new batch of reagent grade cyclohexane without any significant changes in the sample structure of concentration.

Requirements for a performance monitoring sample:

• Must be able to withstand laser exposure
• Must be measurable in a repeatable fashion
• Must be stable or easily replaceable with an identical material
• Must have clearly defined Raman responses thorough the Raman shift range

Carrying out a basic performance monitoring measurement with cyclohexane:

• The laser focusing should be reproducible
  • This can be achieved by making sure that the probe head is always touching the cuvette window and that the laser focus point is within the liquid when doing this
• The performance monitoring measurements should always be carried out with the same measurement parameters
  • PicoRaman has the option for saving measurement parameters for later use
• The performance monitoring measurements should always be carried out using the same measurement setup e.g. the same measurement probe

Once the performance monitoring measurements have been carried out, it might be practical to save the data in a single location for easy comparison. The comparisons should include at least two variables (see figure below): 1. Are there any noticeable changes in the Raman signal intensities? 2. Are there any noticeable changes in the signal Raman shifts? Shifts of several peaks thorough the Raman shift range should be checked.

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Periodically measured spectra can be used to monitor if there are any significant changes in the signal intensities or Raman shifts.

A slight decline in laser power is normal over the laser lifetime but a significant decrease in observed Raman signal intensities might signal a need for a laser replacement. The laser power can also be monitored using a laser power meter. Similarly, significant changes in the signal Raman shifts might indicate a need for a re-calibration of the spectrometer. Laser replacements and re-calibrations are not routinely required but they might be necessary over the long lifespans of the spectrometers.

Other points for maintaining PicoRaman performance include:

• The laser should be switched off when not in use
• PicoRaman units should be shielded from mechanical impacts and vibrations
• PicoRaman units should be kept in spaces with stable temperatures
• The optical fiber tips and connectors should be checked (and cleaned when necessary) every time when connecting or re-connecting a measurement probe to a PicoRaman unit
• The probe windows/lenses should be kept clean
• Fiber-optic cables are quite sensitive and sharp bends in these cables should always be avoided (e.g. the probe cable)