Microchip-mass spectrometer interfaces

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. 

The first mode to interface the sample effluent from a microchip to a mass spectrometer (MS) was based on electrospray ionization (ESI). For electrospray generation, a sharp tip is usually used as an emitter. For instance, a sheath flow micro-ion sprayer was used to interface a microchip to a mass spectrometer (see Figure 7.30). CE separation was first carried out, and then the separated components were transferred to the mass spectrometer for MS analysis [812]. 

Henry, C. Micro Meets Macro interfacing microchips and mass spectrometers. Analytical Chemistry News and Features, 69, 359A-361A. 

Ramsey and Ramseyalso described microchip interfacing to an ion trap mass spectrometer. Microfluidic delivery was realized by electroosmotically induced pressures and electrostatic spray at the channel terminus was achieved by applying a potential between the microchip and a conductor spaced 3-5 mm from the channel terminus. Tetrabutylammonium iodide was tested as a model compound with this device. Later, Ramsey et reported use of a microchip nanoelectrospray tip coupled to a time-of-flight mass spectrometer for subattomole sensitivity detection of peptides and proteins. A fluid delivery rate of 20-30 nL/min was readily obtained by applying an electrospray voltage to the microchip and the nanospray capillary attached at the end of the microfabricated channel without any pressure assistance. 

FIGURE 53.3 A microchip-MS interface with a nebulizer that includes an aerodynamic focusing chamber for directing the ESI plume into the mass spectrometer sampling orifice. (Reprinted from Grym, J., et ah. Lab Chip, 6, 1306-1314, 2006. Copyright 2006. With permission from The Royal Society of Chemistry.)... 

Adaptation of CIEF to the microchip format can provide higher performance compared to conventional capillaries. Whole channel imaging can be performed, thereby eliminating the need for mobilization. The chip can be interfaced to the mass spectrometer while maintaining the short length of the separation channel which is a distinct advantage compared to the conventional capillary. 

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