Formation of droplets of alternating composition in microfluidic channels and applications to indexing of concentrations in droplet-based assays

For screening the conditions for a reaction by using droplets (or plugs) as microreactors, the composition of the droplets must be indexed. Indexing here refers to measuring the concentration of a solute by addition of a marker, either internal or external. Indexing may be performed by forming droplet pairs, where in each pair the first droplet is used to conduct the reaction, and the second droplet is used to index the composition of the first droplet. This paper characterizes a method for creating droplet pairs by generating alternating droplets, of two sets of aqueous solutions in a flow of immiscible carrier fluid within PDMS and glass microfluidic channels. The paper also demonstrates that the technique can be used to index the composition of the droplets, and this application is illustrated by screening conditions of protein crystallization. The fluid properties required to form the steady flow of the alternating droplets in a microchannel were characterized as a function of the capillary number Ca and water fraction. Four regimes were observed. At the lowest values of Ca, the droplets of the two streams coalesced; at intermediate values of Ca the alternating droplets formed reliably. At even higher values of Ca, shear forces dominated and caused formation of droplets that were smaller than the cross-sectional dimension of the channel; at the highest values of Ca, coflowing laminar streams of the two immiscible fluids formed. In addition to screening of protein crystallization conditions, understanding of the fluid flow in this system may extend this indexing approach to other chemical and biological assays performed on a microfluidic chip.     

Monitoring of ethylene by a pulsed quantum cascade laser

We report on the development and performance of a gas sensor based on a quantum cascade laser operating at a wavelength of approximately 10 microns to measure ethylene (C2H4) concentrations by use of a rotational component of the fundamental nu 7 band. The laser is thermoelectrically cooled and operates in a pulsed mode. The influence of pulse-to-pulse fluctuations is minimized by use of a reference beam and a single detector with time discriminating electronics. Gas absorption is recorded in a 100-m optical path-length astigmatic Herriott cell. With a 10-kHz pulse repetition rate and an 80-s total acquisition time, a noise equivalent sensitivity of 30 parts per billion has been demonstrated. The sensor has been applied to monitor C2H4 in vehicle exhaust as well as in air collected in a high-traffic urban tunnel.     

High-power deuterium Raman laser at 632 nm

We demonstrate a continuous-wave deuterium Raman laser that generates more than 160 mW of Stokes output power despite severe thermal effects. This output power represents nearly an order-of-magnitude increase over any previously reported continuous-wave Raman laser and is the first such system to our knowledge that uses deuterium gas as the Raman medium. The high output power is achieved through careful consideration of the electronic feedback design, frequency actuators, and pump-laser intensity noise.     

Atmospheric formaldehyde monitoring in the Greater Houston area in 2002

A laser spectrometer based on difference frequency generation (DFG) was deployed for real-time long-term monitoring of HCHO concentrations at an environmental monitoring site located at Deer Park, Texas, in the Greater Houston area. Three HCHO concentration measurements were made during the periods of July 20-31 (period I), August 2-14 (period II), and August 24-September 25 (period III), 2002. In periods I and II, differences in HCHO concentrations are apparent between day and night measurements, with elevated concentrations during daylight hours. Most of the HCHO peak values are less than 20 ppbV except for two intense peaks on August 02 (approximately 25 ppbV) and August 04 (approximately 30 ppbV). The formaldehyde concentration levels in ambient air at the measurement site are produced mainly by the photochemical oxidation of volatile organic compounds (VOCs) caused by intense sunlight during periods I and II. This observation was made based on a comparison with the ozone concentration, solar radiation, temperature, relative humidity, and wind speed data obtained from the Texas Commission on Environmental Quality (TCEQ). During period III, data collected by a time-integrating wet-chemical technique are compared to the data collected by the spectroscopic instrument.     

Accuracy and precision of manual baseline determination

Vibrational spectra often require baseline removal before further data analysis can be performed. Manual (i.e., user) baseline determination and removal is a common technique used to perform this operation. Currently, little data exists that details the accuracy and precision that can be expected with manual baseline removal techniques. This study addresses this current lack of data. One hundred spectra of varying signal-to-noise ratio (SNR), signal-to-baseline ratio (SBR), baseline slope, and spectral congestion were constructed and baselines were subtracted by 16 volunteers who were categorized as being either experienced or inexperienced in baseline determination. In total, 285 baseline determinations were performed. The general level of accuracy and precision that can be expected for manually determined baselines from spectra of varying SNR, SBR, baseline slope, and spectral congestion is established. Furthermore, the effects of user experience on the accuracy and precision of baseline determination is estimated. The interactions between the above factors in affecting the accuracy and precision of baseline determination is highlighted. Where possible, the functional relationships between accuracy, precision, and the given spectral characteristic are detailed. The results provide users of manual baseline determination useful guidelines in establishing limits of accuracy and precision when performing manual baseline determination, as well as highlighting conditions that confound the accuracy and precision of manual baseline determination.     

Quantification of low dose signal in EPR tooth dosimetry--a novel approach

For radiation exposures below 100 mGy, the dosimetric signal in tooth enamel is too small to be measured by using the traditional dose reconstruction procedure. This is because low amplitude zero-added-dose signal can not be identified in an EPR spectrometer. A technique is presented wherein, zero-added-dose signal. when amplified by a proper known dose, can be measured in the EPR spectrometer. Mathematically, the accidental dose x is modified by a known amount of exposure, y (large enough so that the signal is now visible), and total exposure becomes x' = x + y, which is the modified-zero-added dose. The exposure x' is then quantified using the conventional backward extrapolation method and the accidental dose can be measured. In a laboratory controlled experiment, the feasibility of dose reconstruction in the 100 mGy range has been demonstrated. This may enable measurements of dose even due to suspected low exposure in tooth enamel.     

Measuring coping at a university using a Rasch model

Coping with academic difficulties at a university was based on six aspects: Motivation and Planning, Friends and Planning, Studying and Planning, Emotions, Spiritual Help and Coping by Doing Nothing. Stem-items for each aspect were conceptually ordered by difficulty. Each of the stem-items was answered from three perspectives, Good Coping Strategies, Actual Coping Strategies, and Stress Reduction Strategies. The three response categories were No, not on any occasion this semester; Yes, on 1 to 3 occasions this semester, and Yes, on 4 or more occasions this semester. The convenience sample was 337 students studying education at an Australian university and data were analysed with a Rasch measurement model. A scale was created in which the difficulties of the items were ordered from easy to hard and the student measures of Coping were ordered from low to high. Coping by Doing Nothing and Using Spiritual Help stem-items didn't fit the measurement model and were deleted. This left an effective item sample of 21 (7 stem-items times 3). The proportion of observed student variance considered true was 0.88. The results supported the theory behind the construct of Coping as using Motivation and Planning, Friends and Planning, Studying and Planning, and Emotions, in which Expected Good Coping Strategies are easier than Actual Coping Strategies which, in turn, are easier than Stress Reducing Coping Strategies.     

Spill behaviour using REACTPOOL. Part II. Results for accidental releases of silicon tetrachloride (SiCl(4))

Silicon tetrachloride is a toxic, corrosive water reactive substance that is used widely in the process industries. On spillage from containment it creates liquid pools that can either boil or evaporate. The main feature of the pool behaviour is the exothermic reaction with water. There are three sources of water available for reaction: free ground water, substrate water and atmospheric moisture. Hydrogen chloride gas and ortho-silicic acid solid (or silica gel) are produced by the hydrolysis reaction. The purpose of this paper is to describe the dangers involved in cases of accidental releases of silicon tetrachloride, to report its properties, referring to toxicity data, major accidents and mitigation tests. It also describes pool behaviour using REACTPOOL [1]. Model results indicate that the pool behaviour is governed mainly by the amount of water available for reaction. Surface roughness and wind speed also have a significant effect on the results. Results are compared with those for other water reactive chemicals in Part III of this series of papers [3]. The generated cloud will initially contain silicon tetrachloride and hydrogen chloride with numerous processes taking place.Although silicon tetrachloride has been involved in many major hazard incidents, there are no experimental data relevant to the modelling requirements.     

Biological perchlorate reduction in high-salinity solutions

Perchlorate (ClO4-) has been detected in numerous ground and surface waters, and has recently been added to the drinking water Candidate Contaminant List in the United States. Perchlorate can be removed from drinking water using ion exchange, but this results in the production of highly saline (7-12%) perchlorate-contaminated brines. Perchlorate-degrading microbial enrichments capable of growth in highly saline water were obtained by screening six salt water environments including marine and lake surface waters, salt marshes, subtidal sediments, and a biofilm/sludge from a seawater filter. Perchlorate reduction was obtained in three of these samples (seawater, saline lake water, and biofilm/sludge) at a salinity of 3%. The salinity range of two of these cultures was extended through serial transfers into media having higher salt concentrations (3-7%). Growth rates were measured over a salinity range of 1-15%. The maximum growth rate measured for the saline lake-water enrichment was 0.060+/-0.003 d(-1) (doubling time of 11.6+/-0.8 d) at a salinity of 5%. Growth rates decreased to 0.037+/-0.002 d(-1) at a salinity of 11%, and no growth was observed at salinities of 13 or 15%. These results demonstrate for the first time that biological perchlorate reduction is possible in solutions having a salinity typical of ion exchange brines.