Portable spectrometers designed for users with little technical training must be more robust than spectrometers designed for professionals. Common measurement errors (and in fact any imaginable non-random error) must be accounted for, trapped, and the user gently directed to manipulate the system to yield a valid, useful measurement. Such measurements are likely cost-sensitive, as consumers typically want infinite performance at zero cost. These constraints require either simplicity, automation, or software-driven operation using inexpensive components whose limitations are compensated algorithmically. We describe progress towards an instrument of the latter description, providing absorption, diffuse reflectance, and luminescence measurements to be carried out in a hand-held grating spectrometer. The gratings are plastic films, stacked and mutually rotated to generate hundreds of orders, which in turn are detected by a consumergrade camera chosen to mimic those in typical low-end smartphones. Low camera dynamic range is compensated by observing orders with a wide range of throughputs, such that overall dynamic range in a single exposure may be 12 bits from an 8 bit image. This requires significant effort for wavelength calibration and inter-order intensity normalization. The paper discusses progress in automatic calibration algorithms, software modularization, and method development for quantifying nitrate and phosphate in agricultural runoff. While the overall approach has been established for some years, significant details requiring careful raytracing, numerical analysis, algorithmic modularization and error handling, materials choices, and mechanical engineering for manufacturability are of current importance.
Some uses of portable spectrometers require the same quality as laboratory instruments. Such quality is challenging because of temperature and humidity variation, dust, and vibration. Typically, one chooses materials and mechanical layout to minimize the influence of these noise and background sources. Mechanical stability is constrained by limits on instrument mass and ergonomics. An alternative approach is to make minimally adequate hardware, compensating for variability in software. We describe an instrument developed specifically to use software to compensate for marginal hardware. An initial instantiation of the instrument is limited to 430 – 700 nm. Simple changes will allow expansion to cover 315 – 1000 nm. Outside this range, costs are likely to increase significantly. Inherent wavelength calibration comes from knowing the peak emission wavelength of an LED light source, and fitting of instrument dispersion to a model of order placement with each measurement. Dynamic range is determined by the product of camera response and intentionally wide throughput variation among hundreds of diffraction orders. Resolution degrades gracefully at low light levels, but is limited to ~ 2 nm at high light levels as initially fabricated and ~ 1 nm in principle. Stray light may be measured in real-time. Diffuse stray light can be employed for turbidimetry fluorimetry, and to aid compensation of working curve nonlinearity. While unsuitable for, Raman spectroscopy, the instrument shows promise for absorption, fluorescence, reflectance, and surface plasmon resonance spectrometries. To aid non-expert users, real-time training, measurement sequencing, and outcome interpretation are programmed with QR codes or web-linked instructions.
Development of microfluidics has focused on carrying out chemical synthesis and analysis
in ever-smaller volumes of solution. In most cases, flow systems are made of either quartz,
glass, or an easily moldable polymer such as polydimethylsiloxane (Whitesides 2006). As the
system shrinks, the ratio of surface area to volume increases. For studies of either free radical
chemistry or protein chemistry, this is undesirable. Proteins stick to surfaces, biofilms grow on
surfaces, and radicals annihilate on walls (Lewis et al. 2006). Thus, under those circumstances
where small amounts of reactants must be employed, typical microfluidic systems are incompatible
with the chemistry one wishes to study. We have developed an alternative approach. We
use ultrasonically levitated microliter drops as well mixed microreactors. Depending on whether
capillaries (to form the drop) and electrochemical sensors are in contact with the drop or whether
there are no contacting solids, the ratio of solid surface area to volume is low or zero. The only
interface seen by reactants is a liquid/air interface (or, more generally, liquid/gas, as any gas may
be used to support the drop). While drop levitation has been reported since at least the 1940's,
we are the second group to carry out enzyme reactions in levitated drops, (Weis; Nardozzi 2005)
and have fabricated the lowest power levitator in the literature (Field; Scheeline 2007). The low
consumption aspects of ordinary microfluidics combine with a contact-free determination cell
(the levitated drop) that ensures against cross-contamination, minimizes the likelihood of biofilm
formation, and is robust to changes in temperature and humidity (Lide 1992). We report kinetics
measurements in levitated drops and explain how outgrowths of these accomplishments will lead
to portable chemistry/biology laboratories well suited to detection of a wide range of chemical
and biological agents in the asymmetric battlefield environment.
Developments in politics, communications, economics, and population have all had profound effects on the market for analytical chemical instrumentation. This essay examines the assumptions behind the current training of instrumentation scientists and marketing of instruments, and suggests changes in both. The market must be taken to be all of society, not just technically literate society. Cost tradeoffs between hardware and software are context- dependent. Chemometrics allows extraction of information from data that leaves the typical reductionist scientist queasy. And clever chemistry can sometimes obliterate entire markets. The implications of this evolution are explored.
Theoretically, a nonlinear system can be entrained to many selected behaviors by resonance excitation. A sufficiently accurate model of the nonlinear system is needed to compute the control sequence, but incomplete models may be used as the basis for experiments to probe the dynamics and extract additional model details. We report our efforts to control the chaotic oscillations of the Belousov- Zhabotinsky reaction and related chemical oscillators. Instrumentation and the interaction of laboratory constraints with the modeling algorithm are discussed.
The peroxidase-oxidase system is the only in vitro single-enzyme reaction which has been shown to oscillate chaotically. Difficulties in reproducing literature results have led to careful attempts to specify conditions for reproducing experimental parameters. Progress in specifying reaction conditions is reported. Parameters which require further elucidation before control can be adequately achieved are also specified.
The paper presents a spectrometer combining echelles with imaging detectors and optimizing the echelle design to match the behavior of the detector, to be used for simultaneous high-resolution multiwavelength observations of a variety of discharges. The instrument utilizes a dual Czerny-Turner mount, with compensation of astigmatism generated in the first spectrometer by arranging the second spectrometer in a plane orthogonal to the first. Software for displaying spectra in false color, extraction of orders from echellograms, and wavelength calibration is outlined. Application of the spectrometer to the study of three plasma systems is described.
Ultrasensitive Instrumentation for DNA Sequencing and Biochemical Diagnostics
8 February 1995 | San Jose, CA, United States
SC853: Tradeoffs in Spectrometer System Design
This course provides attendees with an overview of spectrochemical measurements and the instrumental parameters one must choose to obtain valid, precise results. Matching component specifications for an optimal overall design is emphasized. Time, space, and energy resolution, and the influence of noise, are considered. Components such as light sources, gratings, beam-splitters, lenses, mirrors, prisms, and detectors are discussed, as are their combination into absorbance, fluorescence, and scattering spectrometers. Optical aberrations and their influence on resolution and precision are emphasized. High resolution measurement problems (atomic spectroscopy, gas phase diagnostics) are contrasted with low-resolution problems (trace detection in solution).