Focussing Review

Chromatography, Vol.22 No.3 (2001)

Focussing Review

The Use of Fibrous Polymer Materials in Capillary
Electrochromatography (Review)
Yoshihiro Saito and Kiyokatsu Jinno*
School of Materials Science, Toyohashi University of Technology, Toyohashi 441-8580, Japan
Received for review July 10, 200１.

Accepted September 5, 2001.

and also the extraction efficiency was significantly improved byIntroduction
Miniaturization of analytical systems is the most important is-

the multiple draw/eject procedure [14-16] or inserting a stainlesssue in recent analytical chemistry, since it realizes high perform-

wire inside of the capillary [18]. On the basis of the extensive in-ance and rapid analysis without a large solvent consumption andvestigations as described above, fiber-in-tube SPME [19] was de-with low cost operation. These miniaturized high performance/veloped recently. Fibrous synthetic polymer was packed into a cap-

throughput systems are, therefore, environmentally favorable notillary of fused-silica or polymeric tubing, such as polyetheretherke-

only economically preferable. There are two major processes in atone (PEEK) or polytetrafluoroethylene (PTFE), and used as the

typical analytical system, a sample preparation procedure and thatextraction medium. In terms of the miniaturization of the sample

for the subsequent analysis. In the former stage, the extraction and/preparation process, the fiber-in-tube SPME has a great advantage,or preconcentration of the analyte(s) of interest have been carriedbecause of the several hundreds of fine fibrous materials wereout, typically with liquid-liquid extraction (LLE) or solid phase ex-

packed inside and could have a function as the extraction medium.

traction (SPE). In the latter part, the separation of components inConsequently, the use of the fibrous polymer materials as the ex-

the analytical sample, and identification and quantification of eachtraction medium enables itself to have the extended applications tocomponent isolated should be performed by using chromatography

the sample preparation for microcolumn separation methods, suchand spectroscopy as the hyphenated system. For the miniaturization

as microcolumn LC (micro-LC) and capillary electrochromatogra-of the whole analytical systems, micro-scale sample preparationphy (CEC).
Since mobile phase is driven by electroosmotic flow (EOF) in

methods and the hyphenated system coupled to micro-scale separa-

CEC, peak broadening by the effect of parabolic flow is quitetion techniques should be developed.
Solid phase microextraction (SPME) is a solventless sample

smaller than that in LC. Therefore high separation performance be-preparation method developed by Pawliszyn et al., and originallyyond LC can be accomplished in CEC [20]. Although a large num-

employed as a miniaturized sample preparation technique for theber of LC packing materials can be utilized for the CEC stationary

analysis of volatile analytes in gas chromatography (GC) [1, 2].

phases, column preparation requires very skillful work to achieve a

The applications of SPME for the analysis of non-volatile or ther-good reproducible column, especially frit preparation that retains

mally liable compounds have been reported using a specially de-the particles in the column is quite difficult. Because the frits are

signed desorption device [3-13]. The extracted analytes aremain origin of bubble formation that prevents accurate analysis indesorbed from the polymer coatings on the fused-silica SPME rod

CEC [21, 22], pressurizing the buffer vials by using LC pump to-

into a small amount of solvent filled in the desorption device, andgether with high voltage power supply has been attempted [23, 24].

then transferred to the injection valve for the subsequent analysis

However, the required instrument is more complicated than simple

by liquid chromatography (LC) or capillary electrophoresis (CE).

CE instruments. As an alternative approach to improve the column

More effective coupling of the extraction method to LC was also

reproducibility and to avoid bubble formation, various column

made by the development of so-called “in-tube” SPME [14-17] andpreparation methods without the requirement of any frit preparation“wire-in-tube” SPME [18]. In these studies, a fused-silica capillary

have been reported [25-31]. Fiber-packed column is also one of theof coated inside was employed as the extraction medium. There-

promising format.
Jinno and Kiso et al. have studied the novel usage of fibrousfore, the effective on-line coupling to LC system was accomplished*Corresponding author : Tel, +81-532-44-6805 ; fax, +81-532-48-5833 ;e-mail, jinno@chrom.tutms.tut.ac.jp―１５１―

Chromatography, Vol.22 No.3 (2001)

material as the stationary phase in microcolumn separations [32-mechanical strength, these synthetic fibers were selected. The typi-33]. The results demonstrated the effectiveness of fibrous station-

cal outer diameter of each filament is about 11-12 µm and theseary phases for the separation of alcohols [32], polycyclic aromatic

filaments were packed as shown in Figure 2.
The overview of the fiber-in-tube SPME/micro-LC system ishydrocarbons (PAHs) [33] in micro-LC. The applications for CEC
were also reported with different type of synthetic fibers [34-36].
In these studies, no bubble formation was observed without thepressurization to the column. Taking advantage of the flow profile
of CEC, it has been also realized that an improvement over thefiber-packed LC columns, for example, a very fast separation by ashort fiber-packed capillary, could be obtained for the CEC separation [35, 36].
In this article, the recent use of fibrous polymer materials in
microcolumn separation science, especially CEC, and the related
techniques has been reviewed. Novel “fiber-in-tube” configuration
will be described as a specially designed sample preparation
method for the miniaturized separation systems. As the fibrous
polymeric material in this article, Zylon and Kevlar were mainly
introduced along with their variations : HM-type with heat treatment after the spinning process ; and AS-type with out the heat

Figure 2. SEM images of typical “fiber-in-tube”.

treatment. The applications of these fibrous materials as a stationary phase in microcolumn separations are also shown. The future
possibilities for the development of novel fibrous polymers forseparation science will be briefly described.
Miniaturized sample preparation process using fibrous materials
Fiber-in-tube SPME [19] is one of the miniaturized sample
preparation method, in which several hundreds of fine polymer
filaments play the role as an extraction medium. Figure 1 shows
the typical examples of the polymer structures : Zylon, poly(pphenylene-2,6-benzobisoxazole) (PBO) obtained from Toyobo Co.,
Ltd., Ohtsu, Japan and Kevlar, which is a typical aromatic amide
polymer, obtained from Du Pont-Toray Co., Ltd., Tokyo, Japan.
Taking into account the chemical structures, solvent resistance andFigure 3. Schematic diagram of fiber-in-tube SPME/micro-LC.
Syringe pumps for sample solution (a), desorption solvent (b) and mobile phase (c), and switching (d) and injection (e) valves.

Figure 1. Chemical structures of Zylon (A) and Kevlar (B).

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Chromatography, Vol.22 No.3 (2001)

Yoshihiro Saito and Kiyokatsu Jinno

depicted in Figure 3 and the resulting chromatogram for the analy-

wall-coated capillary (J & W Scientific, Folsom, CA, USA) and fi-

sis of n-butyl phthalate in water sample is shown in Figure 4. The

brous packing materials. For the sample preparation, the sample so-

extraction capillary was placed betwee the switching valve and thelution was pumped by a syringe and then the desorption solvent

injection valve. During the extraction process, an aqueous sample

was supplied to transfer the concentrated sample zone into thesolution was delivered to the extraction tube by a syringe pump,cross-section of the separation capillaries. Next, the voltage wasand the analyte was adsorbed onto the fibers. Next a certain amountapplied between the vials for the separation of preconcentrated ana-of the desoption solvent was also pumped after changing thelytes. Combining the inner-coated capillary and fibrous packings,

switching valve. At the same time, the injector was switched to fill

an improved extraction power was obtained for the mixture of anti-

the sample loop by the effluent from the extraction tube. The injec-

depressant drugs havingdifferent polarity. The volume of requiredtion was made after the sample loading. The preconcentration fac-solvent was less than about 2 µL for each run and, more than 200

tor of this method was calculated by injecting the standard solution

times preconcentration was demonstrated with the total sample vol-in a conventional way, and was estimated about 100 for the chro-ume of 1.0 mL and the extraction time of about 10 min.
No desorption solvent is needed for the “in-valve” configura-matogram in Figure 4.
For the on-line coupling of the fiber-in-tube SPME to elec-tion depicted in Figure 5 B. A small commercially available 2-way-trokinetic separation methods, two types of laboratory-made inter-4-ports valve (Model HV 4-1, Hamilton, Reno, NV, USA) was em-

faces, shown in Figure 5, have been developed. In Figure 5 A, aployed for this system. Fiber-packed PTFE capillary of 0.25 mm i.

short extraction capillary was incorporatein a modified cross con-d.×5.0 mm was inserted into the rotor. The number of packed fila-

nector. The extraction capillary was prepared with a DB-5 inner-

Figure 4. Typical chromatogram for the analysis of di-n-buthyl
phthalate in water samples with fiber-in-tube SPME/
micro-LC system. SPME conditions : SPME tube,
PEEK tube (0.25 mm i.d. × 24 mm) packed with Zylon
(HM) fibers ; extraction flow-rate and time, 16 µL/min ×
30 min ; desorption flow-rate and time, 2 µL/min × 4
min ; desorption solvent, methanol. Micro-LC conditions : column, fused-silica capillary (0.53 mm i.d. ×
200 mm) packed with Develosil ODS-5 (Nomura
Chemical, Seto, Japan) ; mobile phase, methanol/water= 90/10 (4 µL/min) ; injection volume, 1 µL ; detection,
UV at 254 nm.

Figure 5. Schematic diagram of fiber-in-tube SPME/CE (A) andfiber-in-tube SPME/CEC (B) system.

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Chromatography, Vol.22 No.3 (2001)

Figure 7. Typical chromatograms for the separation of cis-/transstilbene mixture (A) and di-n-alkyl phthalates (B) withfiber-packed column in LC. Conditions : mobile phase,methanol/water = 50/50 (A), 30/70 (B) at the flow-rateof 20 µL/min ; injection volume 1 µL ; detection, UV at
254 nm.

Figure 6. Typical CEC separation of di-n-alkyl phthalates in water
samples with fiber-in-tube SPME preconcentration.
SPME conditions : SPME tube, PTFE tube (0.25 mm i.d.
× 5 mm) packed with Zylon (HM) fibers ; extraction
flow-rate and time, 4 µL/min × 5 min. CEC conditions :column, fused-silica packed with Superiorex ODS(Shiseido, Yokohama, Japan, 0.15 mm i.d. × 50 mm ; total length 350 mm) ; mobile phase, methanol/water = 95/
5 (Tris 0.5 mM) ; applied voltage, 30 kV ; detection, UV
at 230 nm.
ments was about 380, and the packing density was about 80%. Figure 6 shows the typical chromatogram of a phthalates mixture. The
calculated preconcentration factors for these analytes were morethan 60 for the extraction of 20-µL sample volume (4 µL/min for 5min).
The use of fiber-packed capillaries in liquid-phase separations
For the use of fibrous cellulose acetate (CA) in chromatographic process, Jinno and Kiso et al. reported the possibility ofCA as a stationary phase for LC [37, 38], and demonstrated that theapplication for the separation of alcohols [32] and polycyclic aromatic hydrocarbons [33]. Other synthetic fibers, such as Zylon

Figure 8. Overview of the fiber-packed CEC system.

and Kevlar, were also introduced as the stationary phase in liquid-

Recently the applications of the fiber-packed capillary column

phase separation techniques. Figure 7 illustrates the typical chro-in CEC have been reported [35, 36]. With a specially designed

matograms for the separation of aromatic compounds in micro-LC,
in which about 1,300 filaments of Zylon (HM) are packed into asample injection device, shown in Figure 8, a short fiber-packed

PEEK capillary of 0.50 mm i.d. × 600 mm length. Although the ef-

ery during the separation [36]. A reproducible sample injection to

ficiency was not sufficient as a column for LC separation, it wasfiber-packed column could be made with the combination of elec-

shown that the fiber materials might have a possibility to be atrokinetic injection and micro-flow pumping. Taking an advantage

novel stationary phase in liquid-phase separations.

of the flat-flow profile of CEC, better separation performance overcapillary was connected to a blank capillary for mobile phase deliv-

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Chromatography, Vol.22 No.3 (2001)

Yoshihiro Saito and Kiyokatsu Jinno

Figure 10. Effect of methanol content in the mobile phase on theretention on alkyl benzoates. Conditions : column,
fused-silica (0.20 mm i.d. × 50 mm ; total length 500mm) packed with Zylon (HM) fibers, mobile phase,methanol/Tris-HCl buffer (2.5 mM, pH = 8.6). Other
conditions are the same as in Figure 9.

Figure 9. Chromatogram for the separation of p-hydroxy-n-alkyl
benzoates with fiber-packed column. CEC conditions :column, fused-silica (0.20 mm i.d. × 50 mm ; total
length 700 mm) packed with Kevlar (AS) fibers ; mobile phase, 2.5 mM sodium phosphate buffer (pH =7.0) ; injection 5 kV (5 sec) ; applied voltage, 20 kV ;detection, UV at 254 nm.

Table 1. Retention factor of n-alkyl benzoates with different
fibers.

LC was obtained with the column effective length of only 50 mm.
Another advantage of the fiber-packed CEC is its short analysis
time. As shown in Figure 9, the time required for this particularseparation is just 1 min. Even though more resolution should beneeded, the total analysis time will be less than a few minutes witha decreased applied voltage.
Furthermore, no frit will be required for the fibrous packings.
As described above, the frits are main origin of bubble formationthat prevents accurate analysis in CEC [21, 22], and it has been attempted to pressurize the buffer vials by using LC pump [23, 24].
Therefore, it can be considered that the fiber-packed capillary
should have a wide application as a separation media in CEC without the requirements for bubble elimination techniques.

with the retention factors for benzoates with different types of fiber-packed columns, as summarized in Table 1.

Characteristics of fiber-packed column in CEC
Figure 10 shows the logarithmic plots of retention factor to

As to the retentivity, HM-type always offers larger values thanthat of AS-type, and Kevlar fibers always mark longer retentions

column. As the sample probes, p-hydroxy-n-alkyl benzoate homo-needed for the differences among these fibers including the contri-

logues having different alkyl-chain length were employed. The re-bution of the heat treatment after the spinning process, it is clearlytention factors for these solutes decrease with increasing the metha-

shown that the fiber-packed column has a function similar to con-nol content, and the results are quite similar to that obtained withventional reversed-phase packing materials.

the organic solvent content in the mobile phase on Zylon (AS)than Zylon. Although the more extensive investigations should be

CA-packed columns [35]. The results also have a good agreement

Figure 11 shows the pH dependence of the retention factor for

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Chromatography, Vol.22 No.3 (2001)

Figure 11. Effect of pH on the retention of n-alkyl benzoates. Conditions : column, fused-silica (0.20 mm i.d. × 50 mm ;total length 700 mm) packed with different fibers. Mobile phase was prepared with various buffer solutions
having different pH values.
Figure 12. Typical GC separation of benzene (a) and toluene (b)with Zylon (HM) fiber-packed column. Conditions :column size, 0.32 mm i.d. × 2 m ; column head pressure, 100 kPa ; column temperature, 50℃ ; injector anddetector temperature, 250℃ ; split ratio, (50 : 1).

benzoates. Although a slight decrease of the retention factors wasobserved for all fibers with increasing the pH value of the mobile
phase, the retention factors were almost the same over the pH range
studied. The results indicate that the surface of each filament is notaffected by the pH value of the surrounding liquid. These plots also
demonstrate the excellent stability of these fibrous polymers from
acidic to basic environments.
Future possibilities of fibrous polymer materials in separation
science
The successful use of the fibrous stationary phases was shown
not only in liquid-phase separations but in gas-phase separations[40]. About 330 and 600 filaments of the polymer were packed
longitudinally into fused-silica capillaries of 0.32 and 0.53 mm i.d.,
respectively, and the separation of several test mixtures, such as nalkanes, was carried out with these fiber-packed capillary columns
in GC. In Figure 12, typical separation of benzene and toluene withfiber-packed capillary column is depicted. The separation was carried out isothermally at the column temperature of 50℃.
Temperature-programmed separations of n-alkane mixtures, as
shown in Figure 13, were also carried out with the fibrous packing
materials. The excellent separation performance was demonstrated
with only the column length of 2 m, and the retention on these col-Figure 13. Typical chromatograms for the separation of n-alkanes(C10-C18) mixture with Zylon (HM) packed column.
Conditions : column size, 0.32 mm i.d. × 2 m ; column
temperature, 80℃ to 200℃ at 5 ℃/min ; split ratio,(50 : 1) ; column head pressure, 200 kPa ; injector anddetector temperature, 300℃.

umns are quite reproducible because of the heat resistance of the fiber. Coated-fiber packings, in which a conventional polymer coating process was applied to the packed-filaments, were also evaluated to confirm the contribution of the fibers and the coatings to theseparation characteristics. Using the coated- and noncoated-fiber as
the stationary phase, isothermal GC separation of n-alkans was carried out. With the coating onto the fiber the retentivity was dramatically improved as shown in Figure 14.

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Chromatography, Vol.22 No.3 (2001)

Yoshihiro Saito and Kiyokatsu Jinno

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Figure 14. Comparison of fiber-packed column (A) and coatedfiber-packed column (B). For the fiber coating, HR1701 (Shinwa Chemical, Kyoto, Japan) was used.
Other conditions were the same as in Figure 12, except
for the column length of 1 m and the temperature at 80℃ (isothermal).

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