Chip chromatography

Iversen, W., Single-chip chromatography, and more www.automationworld.com, Dec. 11, 2003. 

As far as applications of on-chip chromatography go, we will most likely see developments geared toward two areas miniature portable analysis systems and highly multiplexed systems. The benefits of miniaturization will enable the creation of detection and diagnostic instruments that are small and easy to use for point of care and military applications. Micro-chromatography devices will also enable development of systems of unprecedented complexity that will allow analysis of highly complex samples with compounds of interest in trace amounts. With such systems, presymptomatic diagnosis of various diseases may become a reality. 

To further integrate on-chip LC, it is necessary to miniaturize LC components. Terry Lee s group has worked to utilize the gas produced from hydrolysis to act as the pumping mechanism for on-chip chromatography [7]. In order to effectively develop this pumping mechanism, high back pressure needed to be considered when delivering the solvent for on-chip LC. Back pressure causes the flow rate to be inadequate to practically perform a separation. Their electrochemical pump was able to deliver 70 nl/min at a back pressure of 80 psi. 

Cowen, S. and Craston, D. H., An on-chip miniature liquid chromatography system design, construction and characterization, in Micro Total Analysis Systems, van den Berg and Bergveld, Eds., Kluwer Academic, The Netherlands, 1995, 295. 

Ericson, C., Holm, J., Ericson, T., and Hjerten, S., Electroosmosis- and pressure-driven chromatography in chips using continuous beds, Anal. Chem. 72, 81,... 

If total analysis systems are not exactly the best option for polymer/additive analysis, then neither are chip-based hyphenations. The reasons here are completely different minute sample size (homogeneity problems for more traditional samples), and (theoretical) limits to chromatography and concentration-based sensitivity, etc. 

When elution chromatography is used in both dimensions, the valve configurations are similar for the different column combinations. However, when CE is utilized as the second dimension, other types of interfaces not based on valves have been implemented with unique advantages. These and the microfluidic implementation of sampling systems for chip-based two-dimensional separations will be discussed below. 

With the introduction of modern electronics, inexpensive computers, and new materials there is a resurgence of voltammetric techniques in various branches of science as evident in hundreds of new publications. Now, voltammetry can be performed with a nano-electrode for the detection of single molecular events [1], similar electrodes can be used to monitor the activity of neurotransmitter in a single living cell in subnanoliter volume electrochemical cell [2], measurement of fast electron transfer kinetics, trace metal analysis, etc. Voltammetric sensors are now commonplace in gas sensors (home CO sensor), biomedical sensors (blood glucose meter), and detectors for liquid chromatography. Voltammetric sensors appear to be an ideal candidate for miniaturization and mass production. This is evident in the development of lab-on-chip... 

Protein toxins such as botulism, staphylococcal enterotoxin B, or ricin can be separated with gas or liquid chromatography, electrophoresis, or a combination. The pChemLab (Sandia National Laboratories Albuquerque, NM) series of instruments includes a hand-held Bio Detector. Proteins in the sample are labeled with fluorescent tags, and nanoliter volumes of samples are separated by microchannels etched into a glass chip. The separation occurs as the sample moves through the channels and identification is based on retention times. The analyses can be completed within 10 min. 

Abbott et al. [163] described a pyrolysis unit for the determination of Picloram and other herbicides in soil. The determination is effected by electron capture-gas chromatography following thermal decarboxylation of the herbicide. Hall et al. [164] reported further on this method. The decarboxylation products are analysed on a column (5mm i.d.) the first 15cm of which is packed with Vycor chips (2-4mm), the next 1.05m with 3% of SE-30 on Chromosorb W (60-80 mesh) and then 0.6m with 10% of DC-200 on Gas Chrom Q (60-80 mesh). The pyrolysis tube, which is packed with Vycor chips, is maintained at 385°C. The column is operated at 165°C with nitrogen as carrier gas (110ml min-1). The method when applied to ethyl ether extracts of soil gives recoveries of 90 5%. Dennis et al. [165] have reported on the accumulation and persistence of Picloram in bottom deposits. 

Very low flow electrospray is called nanoelectrospray [26] where the samples are infused into the mass spectrometer at the nanoliter flow rate range. The infusion of a few microliters will result in a stable signal for more then 30 min, using pulled capillaries or chip-based emitters [27]. With infusion, signal averaging allows to improve the limit of detection in tandem mass spectrometry. Nanoelectrospray is particularly important in combination with nanoflow liquid chromatography or chip-based infusion for the analysis of peptides and proteins. 

Hopfgartner, G. The combination of liquid chromatography/tandem mass spectrometry and chip-based infusion for improved screening and characterization of drug metabolites. Rapid Commun. Mass Spectrom. 2005, 19, 618-626. 

H. Irth A microfluidic-based enzymatic assay for bioactivity screening combined with capillary liquid chromatography and mass spectrometry. Lab Chip 2005, 5,... 

Lab-on-a-chip/high-performance liquid chromatography 2005 LOC/HPLC 215 Collins, 13.4... 

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