How to Design a Spectrometer Scheeline, Alexander
Applied Spectroscopy,
10/2017, Volume:
71, Issue:
10
Book Review, Journal Article
Peer reviewed
Open access
Designing a spectrometer requires knowledge of the problem to be solved, the molecules whose properties will contribute to a solution of that problem and skill in many subfields of science and ...engineering. A seemingly simple problem, design of an ultraviolet, visible, and near-infrared spectrometer, is used to show the reasoning behind the trade-offs in instrument design. Rather than reporting a fully optimized instrument, the Yin and Yang of design choices, leading to decisions about financial cost, materials choice, resolution, throughput, aperture, and layout are described. To limit scope, aspects such as grating blaze, electronics design, and light sources are not presented. The review illustrates the mixture of mathematical rigor, rule of thumb, esthetics, and availability of components that contribute to the art of spectrometer design.
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Portable instruments for use by non-experts require engineering focused on several, often mutually exclusive, features. Cost must be low. User skill must be presumed negligible, although familiarity ...with common computer, tablet, and cell phone interfaces is likely. Consequently, expertise and critical evaluation of data must be automatic, with instructions for sample and chemical manipulation carefully synchronized to the details of each determination. In contrast to professional acquiescence to the benefits of replicate measurements, the number of replicates by non-experts must be minimized, ideally working with n = 1. Instrument self-calibration, self-diagnosis, and mechanical robustness are all essential. Means to track user behavior, sequence of operations, and common missteps should be employed to improve measurement reliability. Time and position stamping, data encryption, and tracking of access to the data is necessary for reliable data provenance. Giving results as action recommendations rather than numbers (but with the numbers accessible for quality assurance purposes) may be desirable. So long as the most significant contribution to measurement variance is correlated with a variable or parameter that can solve the user's problem, other variances may be poorer than in common laboratory instruments. With the right instrument, the highest supplier cost is method development, suggesting that measurements likely to be made in the millions to billions annually are preferable for commercialization. With an emphasis on low resolution visible absorption quantitative minor component determinations, difficulties encountered in using detectors already in consumer's hands are outlined.
•Cell phone spectrometers are rarely consumer products.•Instrumental limitations include inadequate photometry and poor calibration.•Method limitations include lack of operator training or sensitivity to method detail.•Tight capital restricts solution to the instrumental and method limitations.•Problems are only solvable with economies of scale.
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Microreactors are desirable for exploring chemical and biological processes, as reactant consumption is minimal and safety issues are easily managed. Levitated drops are a class of microreactors for ...which mixing is continuous and solid/liquid interfaces are absent or of lesser importance than in channeled microfabricated flow reactors. Thus, reactant adsorption or wall catalysis possibly of importance in ordinary microfluidic systems is absent in levitated drops. Transport of gaseous reactants or products is facile. Levitated drop microreactors are amenable to batch or continuous flow study of biochemical reactions. The possibility of studying oscillatory enzyme-catalyzed reactions in drops is apparent. This review explains the physics and chemistry of levitated drop microreactors and describes practical aspects of their design, fabrication, implementation, and optimization. Such considerations as drop evaporation, thermal control, protein behavior at the gas/liquid interface, and observation with spectroscopic and electrochemical probes are discussed.
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► Ultrasonically-levitated drops show promise as reactors for studying biochemical kinetics. ► Such drops are most interesting for systems in which free radicals are reactants or products. ► Fluid handling and diagnostics are sufficiently developed for near-term application. ► Rate constants ≤1s−1 (first order) and ≤106M−1s−1 (second order) should be measurable. ► Complex dynamics such as chaotic oscillations should be well-suited for study in levitated drops.
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•Explains why cell phone spectrometry is as yet rare.•Suggests ways that personal spectrometry may evolve.•Discusses trade offs among convenience, performance, portability, and cost.
While a number ...of applications for chemical analysis using cell phones as the readout module have come into common use, the characteristics of cell phone cameras and demands of precision spectrometry have prevented the development of a general use cell phone spectrometry platform. The paper reviews the basis for cell phone limitations and explores several approaches to developing such a general platform. Aspects of light sources, sample holders, dispersion, focusing, and camera behavior are discussed.
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The astonishing propagation of sophisticated electronics across the globe has attracted the attention of many for developing disease diagnoses, water purity measurements, and security applications ...using cellular telephones and the complementary metal oxide semiconductor (CMOS) cameras with which many are equipped. This Focal Point article builds on the theme of using technology already in the hands of students/consumers to teach spectrophotometry and, by extension, to suggest how one might inexpensively develop an instrumental analysis curriculum at extraordinarily low cost. A grating spectrophotometer using a white light-emitting diode (LED) light source, plastic sample cuvette, holographic transmission grating, and any camera that produces JPG files as output is described. Optical characteristics are explained and a sample working curve provided. The raw data for that curve are available as Supplementary Material on-line. The behavior of the “Cell Phone Spectrometer” is inadequate for routine analysis but outstanding for teaching the workings of optical instrumentation components and systems.
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A two-channel sensor capable of almost instantaneous simultaneous detection of superoxide radical and hydrogen peroxide in the concentration range 10-7−10-4 M is very important for understanding of a ...number of rapid kinetics processes. A glassy carbon working microelectrode covered by an electrodeposited polypyrrole/horseradish peroxidase (PPy/HRP) membrane was employed as a H2O2 sensor. Another glassy carbon microelectrode covered by a composite membrane of an inside layer of PPy/HRP and an outside layer of superoxide dismutase was employed as a working electrode for superoxide detection. These two working electrodes with Pt counter and tungsten oxide (WO3) reference electrodes were contained in one 6 mm diameter Teflon cylinder. Simultaneous measurements were performed at a potential of −60 mV (vs WO3 reference, pH 5.1). Additional sensor characterization was performed for pH 5.1−9.0. Superoxide sensor behavior as a function of membrane deposition conditions and coating time is reported. Sensors' mutual influence, selectivity, response times, linearity, stability, and sensitivity for hydrogen peroxide and superoxide are presented and discussed. A mathematical model of sensors' responses is proposed, with model calculation corresponding to experiment within 10%.
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Many modern spectrometric instruments use diode arrays, charge-coupled arrays, or CMOS cameras for detection and measurement. As portable or point-of-use instruments are desirable, one would expect ...that instruments using the cameras in cellular telephones and tablet computers would be the basis of numerous instruments. However, no mass market for such devices has yet developed. The difficulties in using megapixel CMOS cameras for scientific measurements are discussed, and promising avenues for instrument development reviewed. Inexpensive alternatives to use of the built-in camera are also mentioned, as the long-term question is whether it is better to overcome the constraints of CMOS cameras or to bypass them.
In this review, we have attempted to retain the overall organizational structure of the previous review (Crouch, S.R.; Cullen, T F.; Scheeline, A.; Kirkor, E.S. Anal. Chem. 1998, 70, 53R-106R). The ...majority of the papers selected from Chemical Abstracts appeared there during the October 1997-October 1999 period.
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