Comparing Spectrometers for Transmission Raman Measurements

Spatially offset and transmission Raman measurements are gaining traction in recent years in many different industries, including the forensic and pharmaceutical sectors. Transmission Raman is resistant to false returns as it yields a bulk measurement of the sample.

Transmission Raman Setup

Figure 1 shows a sketch of a typical transmission Raman setup, wherein the laser light is directed at the sample. The light then traverses the sample exciting Raman events. The Raman photons are observed from a large portion of the sample surface area, causing the sample source itself having a large étendue. Moreover, attenuation causes this signal to be weak for large samples with thickness ≥ 5m.

Typical Transmission Raman Setup

Figure 1. Typical Transmission Raman Setup.

Image Credit: IS-Instruments

The Requirement

A large étendue is essential for the spectrometer to collect a considerable portion of this light and to yield high resolution measurements. Nevertheless, a conventional fiber coupled Czerny Turner system requires an input slit of 50µm to obtain resolutions of < 10cm-1, limiting its light capturing potential. This problem can be addressed by using fiber bundles that collect the light from different points. This is an intricate and cost-intensive solution.

Solution from IS Instruments

IS Instruments’ HES2000 spectrometer is capable of capturing the light from a 1mm input fiber and maintaining a high level of spectral resolution (3 cm-1 per measurement bin) without slit. This article compares a HES2000 spectrometer with a fiber coupled Czerny Turner instrument to perform transmission Raman measurements of a 5-mm-thick paracetamol tablet.

Experimental Procedure

A fiber coupled 785nm CW laser was used to illuminate the paracetamol tablet. The light traversing was then collected through a 50mm focal length lens, where the light was collimated and then traversed through a 785nm long pass edge filter offered by Semrock. A 50mm focusing lens was used to focus the light into an optical fiber. The light was finally collected through a 50µm core and a 1mm core optical fiber, which was then utilized to direct the light into the Czerny Turner and HES2000 instruments.

The spectra for each analysis were acquired in 3s and 10s, respectively. The HES 2000 spectrometer is on the basis of a static Fourier transform spectrometer design, thus involving no moving components without compromising throughput. Since the HES 2000 spectrometer is a FT-based device, it is possible to post-process the data using methods such as zero filling and phase correction. Post processing of the data acquired was not performed to ascertain a fair comparison between the instruments.

Experimental Results

The spectrum acquired by the HES 2000 spectrometer using a 1mm coupling optical fiber and an integration time of 10s is illustrated in Figure 2. The image shows a clean Raman spectrum from the paracetamol tablet, with at least 13 visible peaks.

Paracetamol Raman spectrum acquired with HES200 instrument.

Figure 2. Paracetamol Raman spectrum acquired with HES200 instrument.

Image Credit: IS-Instruments

Figure 3 shows the spectrum collected by the Czerny Turner instrument using the same fiber and integration time of 10s. As can be seen, Raman photons are not collected by the Czerny Turner instrument using this fiber. This severely affects resolution and makes it difficult to determine the tablet due to the inability to observe clean peaks.

Czerny Turner spectrum of paracetamol tablet.

Figure 3. Czerny Turner spectrum of paracetamol tablet.

Image Credit: IS-Instruments

Figure 4 shows the direct comparison of the Czerny Turner and HES 2000 data after normalization. Red line is the HES 2000 measurement and the blue line is Czerny turner measurement.

Transmission Raman measurement through a paracetamol tablet 10 second integration.

Figure 4. Transmission Raman measurement through a paracetamol tablet 10 second integration.

Image Credit: IS-Instruments

Figures 5 and 6 show the measurements taken using an integration time of 3s. The pattern observed with the 10-second integration time is repeated in 3s. The HES 2000 spectrometer provides a clear paracetamol spectrum, while the Czerny Turner instrument is not able to resolve the spectrum albeit observing sufficient photons.

The considerable amount of laser light leakage in the spectra acquired does not affect the Czerny Turner instrument but does raise the noise in the HES 2000 spectrometer owing to its multiplex nature. Further filtering would be beneficial to the instrument.

HES2000 spectrum of paracetamol tablet 3 second integration time using 1mm core optical fiber.

Figure 5. HES2000 spectrum of paracetamol tablet 3 second integration time using 1mm core optical fiber.

Image Credit: IS-Instruments

Czerny Turner spectrum of paracetamol tablet 3 second integration time using 1mm core optical fiber

Figure 6. Czerny Turner spectrum of paracetamol tablet 3 second integration time using 1mm core optical fiber

Image Credit: IS-Instruments

A 50µm fiber was employed to collect the light for the Czerny Turner instrument to capture a spectrum with adequate resolution to resolve the Raman spectra above. Nevertheless, this causes a reduction in the signal by a factor of 400, as depicted in Figure 7 utilizing a 10-second integration time. Although the use of 50µm core optical fiber does not affect the resolution of the HES 2000, it drastically reduces the signal acquired by the instrument, yielding unclear spectrum within the 10 second integration time limit (Figure 8).

Czerny Turner spectrum of paracetamol tablet using a 10 second integration time and 50µm core optical fiber.

Figure 7. Czerny Turner spectrum of paracetamol tablet using a 10 second integration time and 50µm core optical fiber.

Image Credit: IS-Instruments

HES 2000 spectrum of paracetamol tablet using a 10 second integration time and 50µm core optical fiber.

Figure 8. HES 2000 spectrum of paracetamol tablet using a 10 second integration time and 50µm core optical fiber.

Image Credit: IS-Instruments

Conclusion

The results demonstrate that the spectrometer is limited by its étendue while making transmission Raman measurements. The HES spectrometer yielded high resolution spectra within a few seconds using a 1mm aperture optical fiber. Conversely, the Czerny Turner instrument needs to compromise resolution to collect the light, thus providing unclear spectrum. A 50µm core fiber is required for the Czerny Turner instrument to achieve adequate resolution.

In this experimental setup, the use of such a small fiber does not allow both spectrometers to observe the Raman spectrum. However, the addition of an edge filter into the HES 2000 could improve the performance of the spectrometer by lowering stray laser light. Furthermore, the contrast of the peaks in the data could be improved through post processing methods such as zero filling and phase correction.

This information has been sourced, reviewed and adapted from materials provided by IS-Instruments.

For more information on this source, please visit IS-Instruments.

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