Portable Visible Absorption Spectrometry: Two Dimensions Instead of One

Think of a visible absorption spectrometer or a visible fluorimeter. What do you see? A scanned diffraction grating and photomultiplier? A fixed diffraction grating and a linear charge coupled device (CCD) or linear diode array? These have been mainstays for quantification for decades. Are there any other approaches one might take? One alternative is to use a color camera (a rectangular imager) as a detector. “Hey, I can use my digital camera to do spectrometry!” says the eager, but naïve, potential user. For qualitative use, of course this is correct. But for serious, quantitative work, there are problems.

The problems are associated with both the detector and the dispersion element. Cameras are optimized to give pictures that are appealing to people who prefer rich, saturated colors. Nonuniform illumination of the camera (due, for example, to lens vignetting) is usually compensated for in software or firmware. Some cameras are smart enough to brighten parts of images that are in shadow. The pictures look better than real life. But a spectrometer should not do such compensation. The imbalance in color is evidence of light absorption or fluorescence!

Spectrometers have to be robust with respect to all sorts of challenges: temperature changes, humidity, vibration, dirt, dust, and mechanical stress. Some of these are not much of a problem in the laboratory, but are severe for portable, field instruments. How can wavelength calibration be done in a 25 °C lab and still be accurate at the South Pole or in the Sahara Desert? Wouldn’t a smart approach calibrate the instrument at the time and point of use rather than in the factory?

As first reported at ScIX/FACSS 2013 and shown as a prototype (the AAH-200) at Pittcon® 2014, SpectroClick (Champaign, IL) is using novel grating, light source, and software technology to allow real-time calibration of absorption and fluorescence spectrometers. Instruments are handheld and targeted for on-site, at-sample use. The patented grating arrangement fills a camera with spectral information. Below we will look at an image, explain its unique properties, and contrast the image with that of conventional instruments.

Figure 1 – A SpectroBurst showing hundreds of diffraction orders of various dispersions and throughputs. While dispersion depends exclusively on radius, throughput is a function of both radius and the grating geometry giving rise to the order.

Figure 1 shows a SpectroBurst™ (SpectroClick), a centrosymmetric image with hundreds of diffraction orders all obtained from a single collimated beam. Near the center, dispersion is low, as much as 15-nm pixel–1. Since the undispersed 50-μm entrance aperture images to a spot 9 pixels in diameter, the resolution is roughly 150 nm, only as good as the resolution would be from the red/green/blue (RGB) encoding used in BMP and JPG files. Toward the edge of the image, the range from 420 nm to 700 nm covers hundreds of pixels, and demonstrated resolution is approximately 9 nm. If a 25-μm entrance aperture had been used, resolution would improve to 5 nm. It is obvious in the picture that some orders are cleanly imaged, while others are drastically overexposed. Only orders where intensity is transduced approximately linearly are useful for spectrometry.

What about the other orders? Suppose one is doing absorption spectrometry. The light will disappear from the weak orders at a modest absorbance. The orders previously in saturation will come out of saturation. Thus, for absorption spectrometry, as concentration increases, one uses less dispersed orders to keep the dynamic range of the measurement within the dynamic range of the detector. Within limits set by dispersion and stray light, the dynamic range of the grating/detector combination is the product of the dynamic range of the camera times the variation in throughput from order to order of the grating. Simply by spreading the light out more, third order would have one-third the illumination level (per pixel) of first order. Increasing the dynamic range from there is caused by some of the orders being diffracted multiple times by the grating stack and by the vignetting of regions far from the image center. An 8-bit per color camera can provide a dynamic range of 5000, or between 14 and 15 bits. This is not as great as the dynamic range of a good diode array or large pixel CCD, but the camera is significantly less expensive, and the gratings are also low-cost plastic transmission gratings.

The ability to use wide dynamic range is one thing; delivering useful chemical analysis is something else. There are numerous steps between obtaining a SpectroBurst and having a useful spectrum. A significant part of SpectroClick’s development work has been to devise wavelength and intensity calibration algorithms that are fast and adequate for spectroscopic needs. Simply plumbing the literature is inadequate. Let’s look at some of what is involved.

Dispersion

Figure 2 – How the stacked gratings generate hundreds of orders. The first double-dispersion grating generates 40+ orders (here we look side-on and show only two of these orders, pointed to by horizontal arrows). The second grating redisperses each order, some to greater distance from the optical axis, some to less, as identified by the vertical arrows. The result is the many families of orders seen in Figure 1.

In the grating stack, we employ surface relief transmission gratings (SRTGs). Each grating is in intimate contact with its predecessor, so the grating(s) closer to the light source set up diffraction patterns that are rediffracted by grating(s) closer to the camera. The position of each blue intensity peak corresponds to diffraction of the peak LED emission at 450 nm, so to the extent that dispersion is proportional to order number and λ(r) = 450 nm + kn (λ – 450 nm), determination of k for a single order calibrates every order, provided n can be identified. In fact, measuring r (450 nm) implies n for each order. However, dispersion varies by about 6% between 400 nm and 700 nm for the gratings we use (as with any grating, dλ/dx = d cos β/nf where f is the instrument focal length and β the diffraction angle for λ. Saying dispersion is constant can only be true in the absence of dispersion). With a camera field of view less than ±30°, cos β changes by less than 17%. For many applications, that is not negligible. Figure 2 illustrates that the first grating in the stack disperses several orders, the second redisperses those orders, and so on for as many gratings as one wishes. We find that three double-dispersion gratings are more than enough to generate the desired spectral richness.

Order overlap

One can readily see in a SpectroBurst that orders diffracted along a cardinal direction (parallel to one axis of the double dispersion gratings) overlap just as would multiple orders in an ordinary spectrometer. What’s different is that cross-dispersion generates numerous overlap-free orders. Part of the art being developed is automating the identification of clean, nonoverlapped orders. A second level of sophistication will be to add chemometrics to exploit overlapped orders and to determine where such use improves measurement precision.

Intensity calibration

What signal comes from a particular pixel? It is a sum of offset, dark current, and the sum, over relevant wavelengths, of photon flux times quantum efficiency. Dark current can be easily measured as a function of exposure time (at fixed temperature) and subtracted from raw signal. However, the slope response depends on both the detector and the optical throughput. To exploit the SpectroBurst, the relative response for each pixel must be normalized. Figure 3 shows four hypothetical pixels, each linear (solid line) or nonlinear (dashed line) in response. For high throughput, I = S (the true exposure I and the pixel readout signal S are the same) up to the point that the detector saturates. For weaker orders, I = 2S, I = 4S, or I = 8S. Clearly, such integer scaling is fanciful, but the cartoon of how varied throughput allows wide dynamic range from low range cameras is real.

Figure 3 – Possible responses of pixels to exposure. Normalization for linear differences in response is simple; normalization for nonlinear response requires linearization as well as rescaling.

Interorder information

The space between the orders contains useful information. Unlike almost all absorbance spectrometers, light is not refocused and recollimated after going through the cuvette. Fluorescence or light scattering will flood the field of view with uncollimated light. The bad news is that this will compromise dynamic range. The good news is that we can directly observe the phenomenon and, at low levels, subtract the stray light in real time. Unlike ordinary spectrometers, the instruments described here can detect when scattering or luminescence occurs.

Computation speed

With all the manipulations involved in turning a SpectroBurst into a spectrum, the algorithms must be fast or the improved data density of the image cannot be turned into chemically relevant information quickly enough to satisfy users. If initial work can be made general, an ordinary laptop should be able to turn an image into a spectrum in less than 10 sec.

Clearly, a great deal of computing is needed to connect the SpectroBurst to user needs. Since more than half the world’s population has adequate computing capacity, either on a computer, tablet, or cell phone, number-crunching is not a likely limitation. Making quality measurements and interpreting them on the spot is a concern. Here, we see yet another level of computing as an important part of the analytical chemistry ecosystem: using whatever camera is handy to read instructions from a QR code on a packet of reactants specific to whatever is being determined (for example, lead, nitrate, or coliform bacteria in drinking water). User instructions specific to the determination can also be displayed on the computing device, so that instruction occurs at point-of-use. In turn, as long as users follow the directions, procedures can be updated on the fly, either from hash codes embedded in the QR label or by downloading over the web.

Just as we now find it quaint to look back on the days of professional elevator operators, in a few years we will look back in wonderment on the days when only professionals were spectroscopists. The keys to such a transformation are those mentioned in this paper: using commercial off-the-shelf (COTS) components, simplified hardware, capable software, and strategic human-computer partnership to make adequate quality measurements cheap and plentiful.

Additional reading

  1. Martinez, A.W.; Phillips, S.T. et al. Simple telemedicine for developing regions: camera phones and paper-based microfluidic devices for real-time, off-site diagnosis. Anal. Chem.  2008, 80, 3699–3707.
  2. Scheeline, A.; Kelley, K. Cell phone spectrometer. J. Analyt. Sci. Digital Libr. Entry 10059, 11/30/09. Reprinted in m-Science: Sensing, Computing, and Dissemination, Cannesa, E.; Zennaro, M., Eds.; The Abdus Salam International Centre for Theoretical Physics, Nov 2010. Translation into Romanian, 9/2011.
  3. Scheeline, A. Focal point: teaching, learning, and using spectroscopy with commercial, off-the-shelf technology. Appl. Spectrosc.  2010, 64(9), 256A–68A.
  4. “Energy Dispersion Device,” T.A. Bui and A. Scheeline; Application 13/596,242, 20130093936 A1 filed 8/28/2012, published 4/18/2013.

Alexander Scheeline, Ph.D., is President, SpectroClickInc., 60 Hazelwood Dr., Rm. 213, Champaign, IL 61820, U.S.A.; tel.: 217-903- 3415; e-mail: [email protected]; www. spectroclick.com. Engineering by The Product Manufactory (Urbana, IL) and encouragement by High Purity Standards (Charleston, SC) and some scientists at the Construction Engineering Research Laboratory, U.S. Army Research and Development Command, are appreciated. Technology licensed from the University of Illinois at Urbana-Champaign.

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