Focussing Review

Chromatography, Vol.22 No.3 (2001)

Focussing Review

Recent Developments in Column Technology for
Fritless Packed Capillary Electrochromatography
Chuzo Fujimoto
Department of Chemistry, Hamamatsu University School of Medicine, 1-20-1, Handayama, Hamamatsu 431-3192, Japan
Received for review June 11, 2001.

Accepted September 4, 2001.

Abstract
During the past several years a large variety of fritless packed columns have been developed for CEC separations. These can be categorized
into two major types : in-situ polymerized columns and particle-immobilized columns ; organic polymer and silica-based columns are included in the former type. This review is concerned with the progress in fritless packed column preparation that has been made until April
2001. In addition to a short introduction and conclusion, this review contains four main sections : (i) synthetic polymer columns (SPCs) ; (ii)in-situ polymerized silica columns ; (iii) particle-immobilized silica columns ; and (iv) molecular imprinted SPCs. In each section, details ofdifferent approaches are summarized in tabular form.
Keywords: capillary electrochromatography, fritless columns, rod columns, continuous beds, capillary columns, packed columns

illary as well as fabrication of high permeable frits in a reproduc-

1. Introduction

ible manner continues to be practical problems.

With the goal of achieving a significant improvement in liquid
chromatographic (LC) efficiency, electroosmotic flow (EOF) has

The above-mentioned problems can be solved by using a so-been used to propel the mobile phase through silica-based particles

called fritless packed column. The formation of bubble would bepacked in a capillary [1-6]. This method is called capillaryminimized provided that a homogeneous stationary phase is pre-electroosmotically-driven chromatography, electro-driven chroma-pared by polymerizing monomers in a capillary. Obviously, trou-

tography, or capillary electrochromatography (CEC). EOF canblesome and less reproducible procedures such as frit-fabricationarise from the surface of LC packings and the inner wall of the cap-and particle-packing are eliminated. In our papers published inillary. When charged compounds are separated by CEC, both parti-

1995 [10, 11], we used the term fritless packed column to refer totioning and electromigration contributes to the separation. Thus far

columns whose stationary phases are prepared within the confines

a number of researchers have published their experimental works

of a fused silica capillary. In this approach, monomeric precursors

using packed capillaries. However, the method is not yet mature

are introduced into the capillary to be in situ polymerized, forming

enough to be used as a routine, practical tool. The formation ofa continuous bed. The concept of an in situ formed column is notbubbles within the packed section or the frits is the primary prob-

really new ; large i.d. columns consisting of a continuous bed werelem in using packed columns for CEC separations. The bubble for-prepared by Hjertén et al. [12] in 1989, and by Svec et al. [13] inmation was ascribed to the non-uniformity in EOF along the capil-

1992 for the purpose of use in LC. (Note that this concept further

lary [7-9], in addition to the self-heating effect. Pressurization ofdates from early studies on LC and gas chromatography columns inboth ends of the capillary column is useful for preventing the bub-the early 1970’s [14-17].) The principal difference in column

ble formation, but this requirement not only increases the overall

preparation between LC and CEC is that in order for the resultant

complexity of the equipment but also makes the coupling of CEC

column to generate strong EOF, functional monomers must be con-and mass spectrometry very difficult. Moreover, efficient packing

tained in the polymerization solution. We first utilized 2-of silica particles into a small inside diameter (i.d.) fused silica cap-

acrylamido-2-methylpropanesulfonic acid as the functional mono-

Tel, +81-53-435-2320 ; fax, +81-53-435-1626 ;e-mail, fujimoto@hama-med.ac.jp―１４５―

Chromatography, Vol.22 No.3 (2001)

mer to obtain enough EOF for CEC separation of neutral com-

tortuosity and attain enough EOF. The formed bed can be either

pounds.

soft or rigid. The surface chemistry of the columns can be modified

In this review we focus on the preparation of in situ polymer-

to yield selected separation mechanisms either by copolymerization

ized columns or fritless packed columns. This type of column iswith specific monomers or by chemical modification after polym-

often called a monolithic column, a rod column, or a continuous

erization. Depending on the chemistry or composition used for po-

bed column. There are a few cases in which the term a fritlesslymerization, it is possible to control the EOF to be generated inpacked column has different meanings : a column having fine ta-

the column. Also, polymer-based materials allow their use underpered ends to hold the packing in a capillary [18, 19] ; a column

extreme pH conditions. Using a fritless packed columns, proteins

packed with sol-gel bonded silica particles ; and a column packed

and peptides can be well separated at a low pH. From the stand-with thermally bonded silica particles. The latter two types of col-point of separation efficiency, this type of fritless packed columns

umns come within the scope of this review, but we are not con-possesses an advantage over particle-packed columns that the so-

cerned here with the first category.

called “wall effects” common to slurry-packed columns are minimized. To make good use of their experience acquired in the studies of fritless packed columns for use in HPLC, Hjertén et al. and

2. Synthetic polymer columns
Many groups have so far been involved in the preparation of

Fréchet et al. have reported various columns for CEC (see Table 1).

organic polymer-based columns. The polymerization mixture typi-

Extensive reviews of different approaches for the preparation ofcally includes a monovinyl monomer, a divinyl monomer (cross-synthetic organic polymers were recently published by Svec et al.

linker), and an ionogeneric monomer (an EOF functional mono-[21, 22]. A list of synthetic polymer columns (SPCs) which havemer). Porogenic solvents or polymeric surfactants have been usedbeen developed for achiral CEC separation is given in Table 1.

to form a porous structure. According to Luo and Andrade [20],Fritless column technology has been also used for enanti-

sub-µm channels should be formed in the bed in order to reduce theomeric separations. The derivatives of β-cyclodextrins, valine,chiral crown ether, and quinidine were incorporated into the sta-

Table 1. A summary of literature on SPCs for achiral separations.

tionary phase to be used as a chiral selector. In Table 2, a list of
Table 2. A summary of literature on SPCs for chiral separations.

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Chromatography, Vol.22 No.3 (2001)

Chuzo Fujimoto

Table 3. A summary of literature on in-situ polymerized silica columns.

chiral CEC separations carried out with synthetic polymer columns
is given. For enantioselective separations by molecular imprinted
polymers, see Section 5.
3. In-situ polymerized silica columns
Early studies on CEC have focused on the use of silica-based,
reversed-phase packing materials. The primary advantage of using
silica gels is that they have a long history in HPLC application.
With such stationary phases, our knowledge on retention and selectivity adjustment can be transferred directly for neutral compounds.
Moreover, silica particles can generate a reasonable EOF. In recent

Table 4. A summary of literature on particle-immobilized silica
columns.

years, the production of porous silica by a sol-gel process has received tremendous attention in HPLC. In a series of publications[51-53], Ishizuka et al. described a sol-gel process which consistedof acid-catalyzed hydrolysis and polycondensation of tetraethoxysilane in the presence of poly (ethylene glycol). This technique
has been modified for the fabrications of a silica-based column foruse in CEC [54-58]. A novel sol-gel chemistry was used to prepare
octadecylated silica columns in a single-step process [59]. Examples of the various in-situ prepared silica columns that have appeared in the literature are summarized in Table 3.
4. Particle-immobilized silica columns
Another alternative to prepare fritless packed columns is toimmobilize HPLC packing particles inside the capillary column.
The immobilization of the packing material has been accomplishedby the use of an entrapping solution or sintering the particles byheat. In the former approach, a capillary may be filled with a suspension of packing particles in a sol-gel solution, or a sol-gel solution may be introduced into a column that has been packed withparticles. Once the particles are fixed, the capillary no longer requires frits. Obviously, this renders a more stable packed bed without problems associated with the retaining frits. The immobiliza-

printed enantiomer being the most strongly retained. Althoughtion methods that have been employed for the fabrication of fritlessmolecularly imprinted columns are dominantly utilized for the

packed columns are listed in Table 4.

separation of a racemic mixture of the imprinted molecule, theyoften show good chiral recognition ability for other structural analogues. Table 5 summarizes molecular imprinted columns that have

5. Molecular imprinted SPCs

developed for fritless packed CEC.

Molecular imprinted SPCs would represent a complementaryalternative to chiral-ligand bonded SPCs. It is believed that the ap-

6. Conclusion

plication of molecular imprinting technology to CEC would greatly
improve the performance of imprinted polymer-based separations

A numerous number of methods for the preparation of fritlessin LC (see ref 78, for a review). For noncovalent imprinting, po-packed columns have been appeared over the last few years. Due tolymerization is performed in an environment as apolar as possible.

the advantages described in this article, fritless packed columns

In addition to the polymerization solvent, the choice of chiral func-

will hold an important place in CEC in time, although some re-tional and cross-linking monomers as well as the duration and tem-searchers believe that CEC will need more time to be used rou-

perature of the polymerization reaction are often important parame-tinely and find its unique applications. It is certain that the increas-ters for the polymerization system to form well-defined imprints.

ing research activity in this area accelerates the development of

Molecular imprinting provide a predictable elution order, the im-

CEC. This review only summarized fritless packed columns that

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Chromatography, Vol.22 No.3 (2001)

Table 5. A summary of literature on molecular imprinted synthetic
polymer columns.

PEO

poly (ethylene oxide)

PETEA

pentaerythritol tetraacrylate

PETRA

pentaerythritol triacrylate

TEOStetraethoxysilane

TMOS

tetramethoxysilane

TRIM

trimethylolpropane trimethacrylate

VBC

vinylbenzene chloride

2 Vpy

2-vinyl pyridine

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