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Figure 2. Sheathless flow CE-MS interface as presented by Mehdi Moini2
Currently, the most used interface technique is a sheath liquid approach where a make-up flow of 1-5 µL/min is added via a pump (LC or syringe device). In an adjustable triple tube needle (see Figure 1), the CE capillary is combined with the sheath liquid and a nebulizing gas flow. This needle unit then enters into the MS ion source. Performance of this setup is very robust as there is a constant flow rate for the MS granted by the sheath liquid. Also, the chemistries for separation and ionization can be decoupled efficiently by adding acidic or basic sheath liquids with organic contents to an aqueous buffer solution. Ideally, the capillary end is set on ground potential to avoid difficulties connecting to the ion source and its high voltage requirements.
in 2007.
Another option is to use sheathless interfaces (see Figure 2) which are currently in development2
. This approach can avoid the
dilution effect by the sheath liquid and result in improved sensitivities. In this process, CE nanoliter flowrates are provided directly to the MS. As there is only one background electrolyte for separation and ionization this must be optimized accordingly to fulfill both needs.
There is a broad field of interest for CE-MS with accurate mass detection applications ranging from food, forensics, and biotechnologies to basic research in biology, such as in metabolomics. Metabolomics is a good example to show the breadth of CE-MS capabilities. The combined performance of CE and MS allows the detection and analysis
of organic acids and very polar small molecules in a complex biological and sometimes sample limited situation. An example for such a metabolomics analysis using CE-TOF MS is shown in Figure 3. CE provides a fast and robust separation of this complex mixture requiring minimal or no sample preparation. TOF-MS offers very fast and full range spectra with an extraordinary mass accuracy. There are currently many groups in metabolomics research using this approach3
, and commercial offerings for industrial requirements also exist.
In summary, CE-MS has evolved into a reliable system suitable for industrial applications. It has become a standard analytical tool for applications not ideally suited for LC-MS (e.g. ionic, very polar or very large compounds). Via ESI, a portfolio of MS options are now available, ranging from simple Quadrupole and Ion trap instruments to Triple- Quadrupole, TOF or Q-TOF systems. For small and less polar compounds or suppressing buffer conditions, there are other promising ionization methods available, such as Atmospheric Pressure Chemical Ionization (APCI) or Atmospheric Pressure Photo Ionization (APPI).
References 1) J.A. Olivares, N.T. Nguyen, C.R.Yonker, R.D. Smith Anal. Chem. 59 (1987) 1230
2) M. Moini Anal. Chem. 79 (2007) 4241 3) R. Maria, N. Monton, S. Tomoyoshi J. Chromatogr. A 1168 (2007) 236
Figure 3. CE-TOF MS ion electropherograms for a standard mixture of anionic metabolites in the components of glycolysis, pentose phosphate and the TCA pathways (Soga et al.4
).
4) T. Soga, K. Igarashi, C. Ito, K. Mizobuchi, H-P. Zimmermann, M. Tomita Anal. Chem. 81 (2009) 6165
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