Micro and capillary HPLC

Most columns used for HPLC separations nowadays have an internal diameter of 4.6 mm, although they use double the amount of solvent and the performance is not better than with the 3.2 mm i.d. columns. There are many good reasons why the diameter should, in fact, be reduced even further  

Columns fall into three different categories  

Open capillaries of 5 pm i.d. and with 10 theoretical plates are rather easy to prepare. Theory shows that a capillary of i.d. less than 10 pm scores over a 

Practical High-Performance Liquid Chromatography, Fourth edition Veronika R. Meyer 2004 John Wiley Sons, Ltd ISBN 0-470-09377-3 (Hardback) 0-470-09378-1 (Paperback) 

Packed capillaries offer a larger capacity, i.e. the amount of sample may be greater than with open capillaries. Lower permeability is a disadvantage. 

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The ionspray (ISP, or pneumatically assisted electrospray) LC-MS interface offers all the benefits of electrospray ionisation with the additional advantages of accommodating a wide liquid flow range (up to 1 rnl.rnin ) and improved ion current stability [536]. In most LC-MS applications, one aims at introducing the highest possible flow-rate to the interface. While early ESI interfaces show best performance at 5-l() iLrnin, ion-spray interfaces are optimised for flow-rates between 50 and 200 xLmin 1. A gradient capillary HPLC system (320 xm i.d., 3-5 xLmin 1) is ideally suited for direct coupling to an electrospray mass spectrometer [537]. In sample-limited cases, nano-ISP interfaces are applied which can efficiently be operated at sub-p,Lmin 1 flow-rates [538,539]. These flow-rates are directly compatible with micro- and capillary HPLC systems, and with other separation techniques (CE, CEC). 

Dugo, R, Favoino, O., Presti, M. L., Luppino, R., Dugo, G., and Mondello, L., Determination of antho-cyanins and related components in red wines by micro- and capillary HPLC, Journal of Separation Science 27(17-18), 1458-1466, 2004. 

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