Microfluidic devices limitations

Some reviews [5-7] have appeared on NCE-electrospray ionization-mass spectrometry (NCE-ESI-MS) discussing various factors responsible for detection. Recently, Zamfir [8] reviewed sheathless interfacing in NCE-ESI-MS in which the authors discussed several issues related to sheathless interfaces. Feustel et al. [9] attempted to couple mass spectrometry with microfluidic devices in 1994. Other developments in mass spectroscopy have been made by different workers. McGruer and Karger [10] successfully interfaced a microchip with an electrospray mass spectrometer and achieved detection limits lower than 6x 10-8 mole for myoglobin. Ramsey and Ramsey [11] developed electrospray from small channels etched on glass planar substrates and tested its successful application in an ion trap mass spectrometer for tetrabutylammonium iodide as model compound. Desai et al. [12] reported an electrospray microdevice with an integrated particle filter on silicon nitride. 

Mostly, mass spectrometers have been used widely for nanodetection in NLC and NCE due to its low detection limits and ease of hyphenation with microfluidic devices. However, attempts have been made to couple other detectors with NLC and NCE. The state of the art of hyphenation of detectors in NLC and NCE is still in its development stage. More advances are expected in the near future for detection at extremely low concentrations for a wide range of molecules. 

Simultaneous immunoassays for ovalbumin and anti-estradiol were performed on a six-channel microfluidic device within 60 s (see Figure 10.1). The limit of detection of anti-estradiol was determined to be 4.3-6.4 nM [1007], Later work reduced the LOD of the antibody to 310 pM, which equaled 2100 molecules. This compared with a theoretical detection limit of 125-525 pM [1008], In another report, chicken egg ovalbumin (600 nM) was determined by immunoassay on a Pyrex glass chip using Cy5-labeled anti-ovalbumin antibody (200 nM) [678]. 

Ueno, Y., Horiuchi, T., Niwa, O., Air-cooled cold trap channel integrated in a microfluidic device for monitoring airborne BTEX with an improved detection limit. Anal. Chem. 2002, 74(7), 1712-1717. 

Plastic microdevices for high-throughput screening with MS detection were also prepared for detection of aflatoxins and barbiturates. These devices incorporated concentration techniques interfaced with electrospray ionization MS (ESI-MS) through capillaries [2], The microfluidic device for aflatoxin detection employed an affinity dialysis technique, in which a poly (vinylidene fluoride) (PVDF) membrane was incorporated in the microchip between two channels. Small molecules were dialyzed from the aflatoxin/antibody complexes, which were then analyzed by MS. A similar device was used for concentrating barbiturate/antibody complexes using an affinity ultrafiltration technique. A barbiturate solution was mixed with antibodies and then flowed into the device, where uncomplexed barbiturates were removed by filtration. The antibody complex was then dissociated and electrokinetically mobilized for MS analysis. In each case, the affinity preconcentration improved the sensitivity by at least one to two orders of magnitude over previously reported detection limits. 

To overcome these limitations imposed on conventional and microfluidic methods for size separation, hydrophoretic methods have been developed. Here we provide a review of the methods for continuous size separation of microparticles, blood cells and cell-cycle synchrony, and for sheathless focusing of cells without external fields and sheath flows in microfluidic devices. We describe details of the separation mechanism and its application to particle and cell manipulation, comparing its advantages and disadvantages with other microfluidic methods. Finally, we present some challenges of the hydrophoretic technology. 

The coupling of microfabricated devices to mass spectrometers has been a great success through the efforts of many laboratories around the world. The depth of applications has been limited by our ability to perform protein chemistry and biochemistry on a scale compatible with microfluidic devices. To date, microfluidic devices have been generally used to handle small amounts of sample. The next frontier for this field of research will be the development and integration of in situ chemical and biochemical processing. 

The main limitation of the integrated microfluidics device is the poor fluidics control. By a proper design of the fluidic circuit and the integration of multiple sampling ports, direct, real-time monitoring of product formation during the reaction will become feasible, thus opening the way to kinetics studies. 

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