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Techniques & Tools Mass Spectrometry, Technology

Closing the Gap

The development of diode array spectrometers in the 1990s allowed spectroscopy to escape the lab by providing instant spectra in a compact, low-cost footprint. It meant that many questions requiring basic qualitative or quantitative answers could finally be measured at the point of sampling, using UV-VIS, fluorescence, Raman, or near-infrared spectroscopy. Since then, a proliferation of companies has grown up around this technology, each adding their own unique features, wavelengths, accessories, interfaces, and detectors.

As the market has matured, many spectroscopy-based applications have been commercialized in process monitoring, materials analysis, environmental monitoring, and particularly health and safety. Handheld Raman systems are now used routinely by first responders for explosives and narcotics detection, while advanced blood oxygenation systems provide precise results to guide clinical decisions in real time.

However, although many incremental gains have been made in the performance of traditional diode array spectrometers through scientific-grade detectors, cooling, and choice of optics, their performance still falls just short of their analytical lab counterparts – and applications requiring greater measurement speed, sensitivity, signal to noise ratio (SNR), and limit of detection (LOD) remain just out of reach. This gap forces many critical industrial measurements to be made offline or in remotely located labs, resulting in lost time, materials, and product. When it comes to safety and clinical outcomes, the hours, days, or weeks required for offline analytical testing have an incalculable impact.

The traditional diode array spectrometer employs a crossed Czerny-Turner design with reflective grating and mirrors, which though compact, is prone to image aberrations that limit its resolution unless designed with f/4 or higher input. This, in turn, directly limits the amount of light that can be collected at the sample and therefore sensitivity and measurement speed. Use of larger, aspheric optics can compensate to a certain point, but increases size and cost of the spectrometer.

What we need is a new breed of spectrometers – and it appears that transmissive, low f-number spectrometers are stepping up to close this performance gap. Transmissive spectrometers are already favored for low-light applications like Raman, in this case they provide a considerable increase in sensitivity and LOD over their predecessors.

Transmissive spectrometers sidestep the limitations on f-number, correcting for aberrations with lenses, and without an increase in footprint. Commercial transmission spectrometers designed as f/2 or lower collect 4–9× more light than an f/4 spectrometer. In addition, transmission gratings (whether fused silica or volume phase holographic) can easily offer 40 percent more efficiency than reflection gratings, with more smoothly varying efficiency profiles. The benefit of this is seen in increased sensitivity, and also reduced stray light. These advantages can be optimized with gratings designed to deliver broader bandwidths and less polarization sensitivity for better performance across wavelength and in volume.

The question still remains – how do these advances in technology translate into measurable performance? As an example: our team has developed a fluorescence system capable of measuring fluorescein down to 5 picomolar concentrations, approaching the performance of benchtop fluorimeters. In addition, in the UV-VIS, the systems enable linear absorbance calibration up to 3.7 AU at 300 nm, a range typically accessible only to much larger, more expensive benchtop systems. Either system could fit on a piece of paper.

With this step change in sensitivity and SNR, the next generation of diode array spectrometers could enable analytical lab-grade measurements to be performed in entirely new environments and applications. Ultimately, we believe they will enable faster, better decisions in the plant, the field, and the clinic, closing the gap with benchtop systems and making the analytical lab a tool for confirmation rather than decision-making.

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About the Author
Cicely Rathmell and David Creasey

Cicely Rathmell and David Creasey are based at Wasatch Photonics, Logan, Utah, USA.

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