Interface continuous sampling

This chapter demonstrated that microchip electrophoresis reached maturity and is appropriate for analysis of nitrated explosives. However, to create easy-to-operate field portable instruments for pre-blast explosive analysis would require incorporation of world-to-chip interface, which would be able to continuously sample from the environment. Significant progress towards this goal was made and integrated on-chip devices which allow microfluidic chips to sample from virtually any liquid reservoir were demonstrated [25,31]. 

Fang, Q., Xu, G.-M., Fang, Z.-L., High throughput continuous sample introduction interfacing for microfluidic chip-based capillary electrophoresis systems. Micro Total Analysis Systems, Proceedings 5th XTAS Symposium, Monterey, CA, Oct. 21-25, 2001, 373-374. 

These were attached to a fraction collector, whose timing mechanism was altered to allow continuous sampling of 28 separate diffusion cells with increased sample and timing capacity. Radiolabeled drugs were used and were measured in a liquid scintillation counter directly interfaced to a computer network. We describe the details of this system which enable us to conduct reproducible flow-through diffusion experiments, assay the samples, and analyze the data quickly and efficiently. 

Orthogonal injection provides a high-efficiency interface for sampling ions from continuous beam to a TOF analyser. The TOF analyser allows simultaneous transmission of all ions and therefore all the ions formed are analysed. However, the duty cycle is far from 100 % for the oa-TOF spectrometer and it is lower than for the TOF spectrometer. This is due to the orthogonal accelerator that samples to the analyser only a part of the ions produced in the source. The duty cycle, despite this fact, is between 5 and 50 %. This is a considerable improvement over the conventional techniques described for coupling a continuous source to a TOF spectrometer. 

Ultrasonic nebulizers have also been employed in continuous flow systems as interfaces between sample preparation steps in the analytical process and detection by virtue of their suitability for operating in a continuous mode. Thus, preconcentration devices have commonly been coupled to atomic spectrometers in order to increase the sensitivity of some analytical methods. An enhancement factor of 100 (10 due to USNn and 10 due to preconcentration) was obtained in the determination of platinum in water using a column packed with polyurethane foam loaded with thiocyanate to form a platinum-thiocyanate complex [51]. An enhancement factor of 216 (12 with USNn and 18 with preconcentration) was obtained in the determination of low cadmium concentrations in wine by sorption of metallic complexes with pyridylazo reagents on the inner walls of a PTFE knotted reactor [52]. One special example is the sequential determination of As(lll) and As(V) in water by coupling a preconcentration system to an ICP-AES instrument equipped with a USN. For this purpose, two columns packed with two different resins selective for each arsenic species were connected via a 16-port valve in order to concentrate them for their subsequent sequential elution to the spectrometer [53]. 

The increasing availabihty of mass spectrometers and on-Hne interfaces for sample preparation has made natural abundance measurements accessible to a broad community of biological oceanographers, and stable isotope measurements are becoming a routine tool in studies of marine ecosystems. In general, measurements are now carried out with continuous flow systems that integrate a... 

R.G. Christensen, H.S. Hertz, S. Meiselman, E. White, V, LC-MS interface with continuous sample preconcentration. Anal. Chem., 53 (1981) 171. 

Fig. 9.7.20 Continuous sampling interface designed by Z.-L Fang s group at Zhejiang University in Hangzhou. See text for details.

Unlike the field of microchip CE, there is almost a total lack of scientific literature covering continuous sampling interfaces for chip-based LC devices. However, the author has investigated sampling methods from continuous sample streams for chip-based LC intended for on-line analysis [52]. We have independently developed a sample flow-through strategy for our LC chips, similar to those described for... 

High throughput continuous sample introduction interfacing for microfluidic chip-based capillary electrophoresis systems. Micro Total Analysis Systems... 

A flow-gated interface allows sampling and injection from a continuous flow stream. 

Li, M. W., et al.. Design and characterization of poly(dimethylsiloxane)-based valves for interfacing continuous-flow sampling to microchip electrophoresis. Anal. Chem., 78, 1042, 2006. 

Samples are usually manually loaded into the sample reservoir using standard pipettors and their replacement requires extensive washing. Such manual or automated sample removal limits the sampling rate and is incompatible with the high performance of microchip devices. Efficient assays of real-world samples will require the incorporation of a continuous sampling capability (from the external environment) or rapid sampling of multiple discrete samples. Such ability to continuously introduce real samples into micrometer channels would make lab-on-a-chip devices compatible with real-life applications. Despite various approaches proposed recently for transferring samples to microchips, the world-to-chip interface remains a major barrier for real-life applications of microchip devices. 

Basile, F. Kassalainen, G.E. Williams, S.K.R. Interface for direct and continuous sample-matrix deposition onto a MALDI probe for polymer analysis by thermal field flow fractionation and off-line MALDI-MS. Anal. Chem. 2005, 77, 3008-3012. 

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