Evaluation of Amount of Blood in Dry Blood Spots: Ring-Disk Electrode Conductometry.

A fixed area punch in dried blood spot (DBS) analysis is assumed to contain a fixed amount of blood, but the amount actually depends on a number of factors. The presently preferred approach is to normalize the measurement with respect to the sodium level, measured by atomic spectrometry. Instead of sodium levels, we propose electrical conductivity of the extract as an equivalent nondestructive measure. A dip-type small diameter ring-disk electrode (RDE) is ideal for very small volumes. However, the conductance (G) measured by an RDE depends on the depth (D) of the liquid below the probe. There is no established way of computing the specific conductance (σ) of the solution from G. Using a COMSOL Multiphysics model, we were able to obtain excellent agreement between the measured and the model predicted conductance as a function of D. Using simulations over a large range of dimensions, we provide a spreadsheet-based calculator where the RDE dimensions are the input parameters and the procedure determines the 99% of the infinite depth conductance (G99) and the depth D99 at which this is reached. For typical small diameter probes (outer electrode diameter ∼ <2 mm), D99 is small enough for dip-type measurements in extract volumes of ∼100 µL. We demonstrate the use of such probes with DBS extracts. In a small group of 12 volunteers (age 20-66), the specific conductance of 100 µL aqueous extracts of 2 µL of spotted blood showed a variance of 17.9%. For a given subject, methanol extracts of DBS spots nominally containing 8 and 4 µL of blood differed by a factor of 1.8-1.9 in the chromatographically determined values of sulfate and chloride (a minor and major constituent, respectively). The values normalized with respect to the conductance of the extracts differed by ∼1%. For serum associated analytes, normalization of the analyte value by the extract conductance can thus greatly reduce errors from variations in the spotted blood volume/unit area.

Miniature open channel scrubbers for gas collection.

An open channel scrubber is proposed as a miniature fieldable gas collector. The device is 100mm in length, 26 mm in width and 22 mm in thickness. The channel bottom is rendered hydrophilic and liquid flows as a thin layer on the bottom. Air sample flows atop the appropriately chosen flowing liquid film and analyte molecules are absorbed into the liquid. There is no membrane at the air-liquid interface: they contact directly each other. Analyte species collected over a 10 min interval are determined by fluorometric flow analysis or ion chromatography. A calculation algorithm was developed to estimate the collection efficiency a priori; experimental and simulated results agreed well. The characteristics of the open channel scrubber are discussed in this paper from both theoretical and experimental points of view. In addition to superior collection efficiencies at relatively high sample air flow rates, this geometry is particularly attractive that there is no change in collection performance due to membrane fouling. We demonstrate field use for analysis of ambient SO(2) near an active volcano. This is basic investigation of membraneless miniature scrubber and is expected to lead development of an excellent micro-gas analysis system integrated with a detector for continuous measurements.

Gas collection efficiency of annular denuders: a spreadsheet-based calculator.

Denuders are widely used for atmospheric analysis. Annular denuders are especially well-suited for preconcentration of trace gases compared to simpler single tube designs. While traditionally coated annular denuders have both bounding surfaces that behave as sinks, annular denuders/membrane-based scrubbers with the same basic geometric design and with only one of the annular surfaces functioning as sink (e.g., a membrane tube whose outer surface behaves as a sink disposed within an inert jacket tube) have become common. However, the gas collection efficiency of such devices cannot be expressed as a simple equation with fixed constants and there is no presently available tool to a priori determine the denuder performance or to design denuders with specific removal efficiencies at specific sampling rates. This paper presents a simple to use "spreadsheet calculator" for concentric annular denuders of any dimension based on known solutions to analogous heat transfer problems. The results from the present spreadsheet calculator are compared with results from a commercial computational fluid dynamics package (Fluent; this takes significant expertise and development effort to run)--the two approaches produce essentially the same results. The present spreadsheet calculator can be used easily and simply without training and will be a useful tool for denuder users and designers.

Cell detachment model for an antibody-based microfluidic cancer screening system.

We consider cells bound to the floor of a microfluidic channel and present a model of their flow-induced detachment. We approximate hydrodynamic force and cell elastic response using static finite-element simulation of a single cell. Detachment is assumed to occur when hydrodynamic and adhesive forces are roughly equal. The result is extended to multiple cells at the device level using a sigmoidal curve fit. The model is applied to a microfluidic cancer-screening device that discriminates between normal epithelial cells and cells infected with human papillomavirus (HPV), on the basis of increased expression of the transmembrane protein alpha6 integrin in the latter. Here, the cells to be tested are bound to a microchannel floor coated with anti alpha6 integrin antibodies. In an appropriate flow rate range, normal cells are washed away while HPV-infected cells remain bound. The model allows interpolation between data points to choose the optimal flow rate and provides insight into interaction of cell mechanical properties and the flow-induced detachment mechanism. Notably, the results suggest a significant influence of cell elastic response on detachment.

Durable microfabricated high-speed humidity sensors.

We describe a durable microfabricated humidity sensor made of interdigitated rhodium electrodes on a silicon substrate covered with a sensing film of Nafion perfluorosulfonate ionomer. Rhodium electrodes are much less prone to oxidative degradation compared to previously described gold electrode-based sensors. Even with dc excitation, Rh electrode sensors exhibit excellent long-term response stability. It has been found that low-amplitude (+/-1 V) square wave excitation can prolong the usability of gold electrode-based sensors to at least several months; however, this mode of interrogation cannot provide subsecond response times. Rhodium deposition on the microsensors is much more difficult than that of gold. We were able to attain crack-free Rh deposits by adaptation of pulsed electroplating techniques. At excitation voltages of >2 V dc, the Rh sensors respond to moisture with 10 <--> 90% rise and fall times of 30-50 ms. These are the fastest microfabricated water vapor sensors reported to date. We demonstrate applications as a breath monitor. Such sensors should also be of utility in atmospheric eddy measurements. Short-term repeatability is better than 0.6% RSD (n = 7).