Tech Note: Extending the Reach of Portable Spectroscopy

Spectroscopy has come into its own over the last decade, making the leap from lab to industry and the field with handheld and field-portable analysis systems. As with many technologies, however, the reduction in size has often meant a compromise in performance as compared to traditional benchtop Raman systems.

At Wasatch Photonics, we see three opportunities to extend the reach of portable spectroscopy to new applications through good product design:

  • Sensitivity – optical design with >10x more throughput than conventional f/4 spectrometers
  • Miniaturization – robust, intelligent mechanical design to reduce footprint, not performance
  • Wavelength – optimization of analysis wavelength(s) to get the best SNR for each sample

In this tech note, we’ll talk about our unique approach to each, and how we strive to stretch the limits of applied spectroscopy by providing a step change in performance and available products.


Sensitivity – The Wasatch Advantage

We talk about this a lot, because it’s key to some of the most interesting emerging industrial applications: trace level detection, through-package analysis, high-throughput inspection, and tagging. The conventional compact crossed Czerny-Turner (CCT) spectrometers that have been on the market for 20 years are able to serve many applications, but still fall short of benchtop spectroscopy systems by 1-2 orders of magnitude due to the limitations in design.f/1.3 spectrometer schematic

Our design takes a different approach – by working in transmission instead of reflection, we’re able to reduce optical aberrations to below the diffraction limit, reducing stray light and achieving consistently good focus across the detector image plane. Use of our own patented and proprietary transmissive VPH gratings also gives us a 20-40% increase in efficiency over the reflective gratings used in CCT spectrometers for a 1.2-1.4x increase in signal. The efficiency of VPH gratings vary much more smoothly than reflective gratings, and are ultra-low scatter for less stray light. We amplify that further by increasing the collection angle of our spectrometers to 42º (f/1.3 vs the typical f/4 of a CCT spectrometer), for a >9x improvement in signal. Together, these factors give us >10x more signal by design!

This improvement in optical design translates into several concrete advantages in terms of competitive performance:

830 nm Raman measurements of cyclohexane for WP 830 vs f/4 crossed Czerny-Turner spectrometer, same signal

Higher sensitivity:

  • >10x more signal in the same integration time
  • Capture brief phenomena, or low light levels
  • Minimizes radiant exposure for delicate samples
  • Perfect for portable Raman or fluorescence
1064 nm Raman measurements of coconut oil for WP-1064-C vs f/4 crossed Czerny-Turner spectrometer, same signal

Shorter integration times:

  • Capture the same signal in 1/10th the time
  • Allows increased averaging to maximize SNR
  • Enables better spatial resolution in 2D scanning
  • Perfect for high-throughput scanning
Fluorescein Limit of Detection, WP VIS with quick-fit cuvette holder, 50 ms integration time

Lower limit of detection:

  • >20x lower LOD in Raman & absorbance
  • Detect trace levels of analytes in solution or on surfaces
  • Analyze new phenomena or complex mixtures with ease
  • Allows increased averaging to maximize SNR

Miniaturization – Reducing Size, not Performance

The smaller a spectroscopy system can be made, the more places it can go. Modular systems are ideal for proving out a new concept and developing algorithms and chemometric models for analysis, but seldom stand up to the rigors of industrial use. In adapting modular components for use in a portable or OEM application, the number of components used is typically reduced, eliminating fibers in favor of free-space coupling for durability and increased signal, and replacing active components with more robust OEM models. The challenge then is to achieve the same performance with the new components.

The impact of sample coupling

Raman sample coupling - WP probe vs other probe vs integrated laser

Using sampling accessories and optical fibers which are matched to our f/1.3 input aperture allows you to collect the most light possible from your sample (read the tech note). That’s why we recommend our WP probes for Raman and use of 0.36 NA fibers otherwise. Free space coupling gives you another boost in signal – choose our integrated laser systems for Raman, or our quick-fit cuvette holder for fluorescence.

At Wasatch Photonics, we’ve built our standard spectrometers around a highly compact, robust OEM optical bench, allowing us to deliver the same performance in a fraction of the footprint, at any volume. The example below shows how the WP 785 Raman spectrometer (red) with a laser module (blue) and probe (black) can be combined into a integrated Raman system (center), and further miniaturized to create an integrated OEM module with onboard laser.

Size reduction - Modular to OEM for the WP 785

 

Standard product vs OEM module performance

Since our core optical bench remains the same at every stage, so does the performance. This can be seen in a comparison of Raman measurements taken of cyclohexane at 50 ms integration time using a WP 785-A-L Raman system (785 nm integrated laser), as compared to the equivalent OEM module (image shown above, right).

Standard WP 785-A-L vs the OEM module equivalent

Ensuring Thermal Stability

Thermal stability is extremely important in both Raman and NIR spectroscopy. In Raman, it determines the need for compensation or frequency of recalibration. In NIR, it affects the spectral reproducibility, which is extremely important in chemometric modeling to ensure that accurate models are built up. To validate the optomechanical design, we monitored Xe emission spectra with our WP 785 ambient (uncooled) Raman spectrometer during temperature cycling, finding thermal shift to be <2 pixel over 0-40°C, compared to ~5 pixels for a similarly configured f/4 CCT spectrometer.

Thermal stability of WP 785 Raman spectrometer, uncooled detector Thermal stability of competitive Raman spectrometer, uncooled detector

We repeated the same study with our WP NIR I spectrometer, finding thermal shift to be just 0.5 pixel over 0-40°C, compared to 2 pixels for a typical f/4 CCT NIR spectrometer. It is also interesting to note that the f/4 CCT spectrometer showed considerable astigmatism as a function of temperature, significantly degrading resolution.  The peaks of the WP NIR I remain much more symmetric with temperature, giving you the thermal stability and resolution you need to achieve high accuracy, in chemometric analysis for NIR spectroscopy or for library matching in Raman.

Thermal stability of WP NIR I spectrometer, f/1.3 transmissive design Thermal stability of NIR f/4 crossed Czerny-Turner spectrometer

Unit-to-Unit Reproducibility

Reproducibility is one of the most important aspects to spectrometer performance for our OEM customers. Peak shape consistency between units is important for library building in Raman, and crucial for chemometric analysis of other spectra. Our VPH grating and transmissive optical design has inherently less instrument response function variance as compared to a typical f/4 CCT spectrometer. We also offer sensitivity matching as part of in-house QC. The units shown here were intensity matched to <10% across the full spectrum –  every wavelength, not just a few discrete wavelengths!Unit-to-unit reproducibility, 830 nm Raman OEM module


Wavelength – Extend the Choice, Extend the Science

Knowing what wavelengths to look at is a prerequisite to getting useful answers from spectroscopy. The instrumentation can then be designed around those needs, taking into account bandwidth, resolution, and the amount of signal available.

Example HD grating, Wasatch Photonics

As a manufacturer of volume phase holographic (VPH) gratings, Wasatch Photonics is uniquely positioned to design and offer customized gratings perfectly tuned to OEM applications, balancing needs for bandwidth and dispersion against efficiency using our multiple patented and proprietary grating technologies. Our gratings maximize efficiency and are ultra-low scatter, optimizing SNR.

In Raman spectroscopy, excitation wavelength is also important, determining signal level and background fluorescence as discussed in our application note, Wavelength Matters. Shorter wavelengths offer more signal, while longer wavelengths or near-UV excitation reduce background fluorescence for organic and biological samples. Cost comes into play as well, with many OEMs choosing 830 nm excitation over 1064 nm due to the relative cost of a silicon detector spectrometer vs one with an InGaAs array and TEC cooling. The excitation source is a factor, both in relative cost and size, and availability and lifetime (particularly for blue or UV lasers).

Raman wavelength choices for standard or OEM

 

By offering 6 standard  wavelengths for Raman – 405, 532, 633, 785, 830, and 1064 nm – and the option to customize wavelength further for OEMs, we give you the flexibility to design the system best suited to your specific sample or application.


Conclusion

Wasatch Photonics’ compact, f/1.3 transmissive optical design offers an order of magnitude greater sensitivity and lower limit of detection than conventional f/4 crossed Czerny-Turner spectrometers. Designed for both research and OEM use, they transition easily from modular to reduced size footprints with no loss of performance. Additionally, they offer the thermal stability and unit-to-unit reproducibility that is key to advanced analysis methods used in Raman, NIR spectroscopy, and other techniques. If you’re looking to stretch the limits of spectroscopy, this is certainly a good place to start.