Eliminating the Trade-off between Resolution and Throughput in Raman Spectroscopy

Introduction
Raman spectroscopy is gaining intense interest as an analytical tool due to the strong specificity of the Raman signature of many materials. However, the Raman signal is often very weak and traditional slit spectrometers typically have poor optical throughput — mostly due to the slit itself. In this application note, we introduce the Apex spectrometer, the first product in the Elite Series of high-performance spectrometers, light sources and sampling accessories. Apex is a modular, small-footprint spectrometer that rivals benchtop instruments with maximum resolution and exceptional sensitivity for superior performance. With a sophisticated design incorporating high end technology to overcome the trade-off between optical throughput and spectral resolution, the Apex is an ideal solution for Raman spectroscopy.

Apex HTVS Spectrometer
Conventional spectrometers use a narrow entrance slit to achieve higher resolution at the cost of throughput.  As shown in Figure 1, the slit rejects 75%-95% of the light entering the spectrometer in order to achieve high spectral resolution.

Figure 1: Comparison of Peak Shape with Different Spectrometer Slit Options Courtesy of Tornado Spectral Systems

Figure 1: Comparison of Peak Shape with Different Spectrometer Slit Options Courtesy of Tornado Spectral Systems

The Apex spectrometer incorporates sophisticated, high end design elements to achieve a dramatic throughput advantage that enables the acquisition of higher quality data more rapidly, enabling new applications and enhancing your existing ones.  With an integrated high-throughput virtual slit (HTVS), Apex spectrometers overcome the trade-off between light throughput and spectral resolution.

Apex incorporates a proprietary beam reformatting technology that eliminates the need for a physical slit in order to achieve high resolution. The virtual slit in the Apex enables the device to achieve the same high-resolution benefits of an actual slit aperture while maintaining high throughput, eliminating the classic trade-off between throughput and resolution.  When combined with the HTVS technology, advancements in optical thin film coatings, highly efficient volume phase holographic (VPH) gratings and solid state detector technology, spectrometer throughput as high as 95% is achievable.

Apex spectrometers use a comparatively large input aperture in combination with a series of specially configured mirrors, lenses and other elements. These components act to compress, reformat and then expand the light beam, with the result of narrowing the input aperture along the dispersion axis while preserving total flux, delivering dramatic performance improvements.

To maintain peak optical throughput, a 1200 line/mm volume phase holographic (VPH) grating blazed at 830 nm is used with an uncooled, NIR enhanced back-thinned CCD array detector. The Apex spectrometer provides wavelength range to 3800 cm-1 with a resolution of less than 10 cm-1 at 800 nm.

Results
Raman measurements of paracetamol (acetaminophen) and toluene were made using the Apex spectrometer (APEX-RAMAN-785), a 100 mW 785nm laser diode (LASER-785-LAB-ADJ), Raman sample holder (OOA-HOLDER-RFA) and 785 nm Raman fiber optic probe (RIP-RPB-785-FC).

As shown in Figures 2 and 3, the Apex measures spectral data over a wide Raman shift range beyond 3800 cm-1 with high spectral density delivering Raman data with more spectral features for better library matching and sample identification. Furthermore, the Apex spectrometer enables the acquisition of this high quality Raman spectral data at very short integration times.

Typical Raman measurements require seconds to achieve similar signal levels. This unprecedented measurement speed enables measurements in flowing or process control systems or for samples that change over time. The low integration times required for the Apex also allow for the measurement of multiple samples in the time it takes to get one spectrum from a less sensitive spectrometer, providing the opportunity for signal averaging to increase SNR improving measurement accuracy even further.

Figure 2:  Apex Raman Spectrum for Paracetamol (Acetaminophen) Measured at an Integration time of 750 msec

Figure 2: Apex Raman Spectrum for Paracetamol (Acetaminophen) Measured at an Integration time of 750 msec

Figure 3:  Apex Raman Spectrum for Toluene Measured at an Integration time of 195 msec

Figure 3: Apex Raman Spectrum for Toluene Measured at an Integration time of 195 msec

Conclusions
A new spectrometer design based on HTVS optical slicer technology has been shown to provide significant increase (a factor of 10-15x) in optical throughput without any detriment in optical resolution.

With the Apex, you can:

  • Achieve high sensitivity without the need for long integration times, detector cooling or high powered excitation sources
  • Use shorter measurement times for faster measurements
  • Acquire more accurate spectral “signatures” and Raman fingerprint data – better library matching
  • Use lower powered excitation sources for less photodestruction

The Apex spectrometer is ideal for Raman spectroscopy where there is always a demand for systems providing increased sensitivity or for high-speed measurements.

*HTVS technology is licensed from Tornado Spectral Systems.

References
Enhanced Chemical Identification Using High?Throughput Virtual?Slit Enabled Optical Spectroscopy and Hyperspectral Imaging – Tornado Spectral (http://tornado-spectral.com/wp-content/uploads/2012/09/20120829-Tornado-Spectral_white-paper.pdf)

On the web:

Apex 785 Raman Spectrometer

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