In-Situ Measurements in Microscale Gas Flows—Conventional Sensors or Something Else?

Abstract

Within the last few decades miniaturization has a driving force in almost all areas of technology, leading to a tremendous intensification of systems and processes. Information technology provides now data density several orders of magnitude higher than a few years ago, and the smartphone technology includes, as well the simple ability to communicate with others, features like internet, video and music streaming, but also implementation of the global positioning system, environment sensors or measurement systems for individual health. So-called wearables are everywhere, from the physio-parameter sensing wrist smart watch up to the measurement of heart rates by underwear. This trend holds also for gas flow applications, where complex flow arrangements and measurement systems formerly designed for a macro scale have been transferred into miniaturized versions. Thus, those systems took advantage of the increased surface to volume ratio as well as of the improved heat and mass transfer behavior of miniaturized equipment. In accordance, disadvantages like gas flow mal-distribution on parallelized mini- or micro tubes or channels as well as increased pressure losses due to the minimized hydraulic diameters and an increased roughness-to-dimension ratio have to be taken into account. Furthermore, major problems are arising for measurement and control to be implemented for in-situ and/or in-operando measurements. Currently, correlated measurements are widely discussed to obtain a more comprehensive view to a process by using a broad variety of measurement techniques complementing each other. Techniques for correlated measurements may include commonly used techniques like thermocouples or pressure sensors as well as more complex systems like gas chromatography, mass spectrometry, infrared or ultraviolet spectroscopy and many others. Some of these techniques can be miniaturized, some of them cannot yet. Those should, nevertheless, be able to conduct measurements at the same location and the same time, preferably in-situ and in-operando. Therefore, combinations of measurement instruments might be necessary, which will provide complementary techniques for accessing local process information. A recently more intensively discussed additional possibility is the application of nuclear magnetic resonance (NMR) systems, which might be useful in combination with other, more conventional measurement techniques. NMR is currently undergoing a tremendous change from large-scale to benchtop measurement systems, and it will most likely be further miniaturized. NMR allows a multitude of different measurements, which are normally covered by several instruments. Additionally, NMR can be combined very well with other measurement equipment to perform correlative in-situ and in-operando measurements. Such combinations of several instruments would allow us to retrieve an “information cloud” of a process. This paper will present a view of some common measurement techniques and the difficulties of applying them on one hand in a miniaturized scale, and on the other hand in a correlative mode. Basic suggestions to achieve the above-mentioned objective by a combination of different methods including NMR will be given.

The Fabrication of Amino Acid Incorporated Nanoflowers with Intrinsic Peroxidase-like Activity and Its Application for Efficiently Determining Glutathione with TMB Radical Cation as Indicator

“…Although these methods present the ideal sensitivity and selectivity for assaying the level of GSH in human cells or tissues, the application of a fluorometric assay suffers from the difficulties caused by the necessity for the derivatization procedures of the samples before measurement, the fact that the utilization of an electrochemical detector needs an operating electrochemical cell with a higher applied oxidation potential, resulting in its shortened life span, and high-performance liquid chromatography, capillary electrophoresis, mass spectrometry, magnetic resonance spectroscopy and surface-enhanced Raman spectroscopy require the use of the expensive, sophisticated and specialized apparatus as well as the requirement for experienced technicians, which severely confines the applications of these techniques in clinical chemistry [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 ]. Conversely, the colorimetric assay has received continuous attention as an attractive alternative strategy because it is a simple, convenient, sensitive and accurate route to accomplish the assessment.…”

Editorial for the Special Issue on Gas Flows in Microsystems

“…Unfortunately, the amount of experimental data on gas microflows is very limited, compared to the high number of numerical studies. The main difficulty, as explained in the review by Brandner [6], is due to the fact that conventional measurement techniques (for temperature, pressure, etc.) cannot be adapted to gas microflows, due to their intrusiveness and/or low signal delivery, especially when timely and spatially correlated measurements are required.…”