Coated capillary

Fig. 13. Enantiomeric separations of monohalohydrocarbons on a 2,6-0-dipentyl-3-0-trifluoroacetyl-y-cyclodexttin coated capillary column (10 m, 0.25...

Bentone-34 has commonly been used in packed columns (138—139). The retention indices of many benzene homologues on squalane have been determined (140). Gas chromatography of C —aromatic compounds using a Ucon B550X-coated capillary column is discussed in Reference 141. A variety of other separation media have also been used, including phthaUc acids (142), Hquid crystals (143), and Werner complexes (144). Gel permeation chromatography of alkylbenzenes and the separation of the Cg aromatics treated with zeofltes ate described in References 145—148. 

There are several methods of buffer and capillary surface modification used to prevent electrostatic interactions. Two modes have been examined mn buffer with pH >10 in the uncoated capillary and anionic polymer coating capillary, developed in lAI. 

Poole, S. K., Patel, S., Dehring, K., Workman, H., Dong, J. Estimation of octanol-water partition coefficients for neutral and weakly acidic compounds by microemulsion electrokinetic chromatography using dynamically coated capillary columns. J. Chromatogr. 

Solid phase extraction (SPE) is a very simple, rapid and reproducible cleanup technique that is now widely accepted as an alternative to the time-consuming liquid-liquid extractions. Additionally, SPE uses relatively small volumes of solvents, and is easy to automate. It is available in a number of different formats, including cartridges, disks, loose material, well plates or SPME using film-coated capillaries. SPE can be considered as an extraction technique when used for isolation and concentration or a cleanup technique when used to remove co-extractives from solvent extracts. The use of SPE for cleanup is discussed later. 

The products were identified by comparing the retention times of the reaction products with commercial compounds, and by GC-MS analysis in a Hewlett-Packard 5973/6890 GC equipped with an electron impact ionization at 70 eV detector and a cross-linked 5% PH ME siloxane (0.25 mm coating) capillary column. The reaction products were separated from the catalyst with filter syringes and analyzed in an Agilent 4890D and a Varian 3400 GC equipped with a flame ionization detector, and CP-Sil 8CB (30 m x 0.53 mm x 1.5 pm) and DB-1 (50 m x 0.52 mm x 1.2 pm) columns, respectively. Decane was used as an internal standard. The catalyst was thoroughly washed after reaction with acetonitrile, acetone and water, and dried overnight under vacuum at 40°C. 

Physical surface modifications Adsorption is probably the simplest way to change the EOF on purpose by using appropriate additives. EOF modification by adsorption can be used on both uncoated and coated capillaries. The surface of uncoated fused silica... 

Three bacterial species (E. coli, P. putida, and S. rubidae) were separated on isoelectric focusing in methylcellulose coated capillaries, and three bacterial species (P. fluorescens, E. aerogenes, and M. luteus) and the yeast S. cerevisae, were separated by capillary electrophoresis in the presence of polyethylene oxide.101 The polymers served to minimize adsorption to the walls without causing cellular lysis. 

Implementation of SFC has initially been hampered by instrumental problems, such as back-pressure regulation, need for syringe pumps, consistent flow-rates, pressure and density gradient control, modifier gradient elution, small volume injection (nL), poor reproducibility of injection, and miniaturised detection. These difficulties, which limited sensitivity, precision or reproducibility in industrial applications, were eventually overcome. Because instrumentation for SFC is quite complex and expensive, the technique is still not widely accepted. At the present time few SFC instrument manufacturers are active. Berger and Wilson [239] have described packed SFC instrumentation equipped with FID, UV/VIS and NPD, which can also be employed for open-tubular SFC in a pressure-control mode. Column technology has been largely borrowed from GC (for the open-tubular format) or from HPLC (for the packed format). Open-tubular coated capillaries (50-100 irn i.d.), packed capillaries (100-500 p,m i.d.), and packed columns (1 -4.6 mm i.d.) have been used for SFC (Table 4.27). 

Karim, M.R., Shinagawa, S., Takagi, T. (1994). Electrophoretic mobilities of the complexes between sodium dodecyl sulfate and various peptides or proteins determined by free solution electrophoresis using coated capillaries. Electrophoresis 15, 1141-1146. 

Although protein behavior in SDS-containing buffers is qualitatively different from small molecules, applications of MEKC-type conditions have been applied to many protein separations. In applications using uncoated capillaries, protein-wall interactions are eliminated because of the anionic character of SDS-protein complexes. In applications using coated capillaries with no EOF, the high electrophoretic mobility of SDS-protein complexes can decrease analysis time. 

Selection of capillary length is dictated by the type of capillary used and the required resolution. When using coated capillaries with insignificant EOF, separations can be achieved with relatively short capillaries of 20 to 30 cm effective length... 

The low level of EOF in coated capillaries permits separations to be carried out with very short effective capillary lengths. Earlier work using chemical mobilization was performed using capillaries as short as 11 cm with internal... 

FIGURE 3.3 Screening and optimization scheme developed in Perrin et al. [28]. Other conditions Injection 0.5 psi, 3.5 s Fnsed-sihca capillary or polyacrylamide-coated capillary of 30 cm X 50 xm. (Reprinted from Perrin et al. Electrophoresis 2001, 22, 3203-3215. With permission from Wiley VCH.)... 

Capillary wall Generally, the most straightforward approach is to use an uncoated fused silica capillary. But sometimes this is not possible because of adsorption problems to the capillary wall, or other wall properties are needed to control the electroosmotic flow. In literature, there are multiple examples. Besides permanently coated capillaries, there are several descriptions of dynamic coatings available, e.g., triethanolamine, Triton X-100, Polybrene, and quaternary ammonium salts. The advantage of these dynamic coatings is that the coating can be renewed between injections, which could improve repeatability and reproducibility of the separation. 

In the pH range 4—7, small pH changes have large effects on the charge of the fused silica capillary wall, and thus the EOF. If a BGE pH in this range is needed, coated capillaries (permanently or dynamically) should be considered. Also, more robust systems are obtained if the BGE consists of both a buffering co-ion and a buffering counterion. ... 

Kohr, J., and Engelhardt, H. (1991). Capillary electrophoresis with surface coated capillaries. J. Microcol. Sep. 3, 491—495. 

Catai, J. R., Somson, G. W., and de Jong, G. J. (2004). Efficient and reproducible analysis of peptides by capillary electrophoresis using noncovalently bilayer-coated capillaries. Electrophoresis 25, 817-824. 

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