Capillary action liquid chromatography

J. Sep. Sci. 2009, 32, 1831 –1837 B. Zhang et al. 1831

Bo Zhang1

Edmund T. Bergstr�m1

David M. Goodall1

Peter Myers1,2

1Department of Chemistry,University of York, York, UK

2Department of Chemistry,University of Liverpool,Liverpool, UK

Original Paper

Capillary action liquid chromatography

Capillary action LC (caLC) is introduced as a technique using capillary action as thedriving force to perform LC in capillary columns packed with HPLC type microparti-culate materials. A dry packing method with centrifugal force was developed to pre-pare capillary columns in parallel (10 columns per 3 min) to support their dispos-able use in caLC. Using a digital microscope for real-time imaging and recording sep-arations of components in a dye mixture, caLC was found to have flow characteris-tics similar to TLC. Based on the investigation of microparticulate HPLC silica gels ofdifferent size (1.5–10 lm) and a typical TLC grade irregular medium, Merck 60Gsilica, the van Deemter curves suggested molecular diffusion as the major contribu-tion to band broadening in caLC. With Waters Xbridge 2.6 lm silica, plate heightsdown to 8.8 lm were obtained, comparable to those achievable in HPLC. Assisted byan image-processing method, the visual caLC separation was converted to a classicalchromatogram for further data analysis and such a facility confirmed the observa-tion of highly efficient bands.

Keywords: Capillary action / Imaging / LC / Packed columns / Thin-layer chromatography (TLC) /

Received: December 11, 2008; revised: January 27, 2009; accepted: January 27, 2009

DOI 10.1002/jssc.200800723

1 Introduction

TLC is a simple and effective liquid phase separation tech-nique [1–3], with wide applications in pharmaceuticalindustry and synthetic laboratories [4–6] for liquid phaseseparations as an important complement to HPLC. Themost obvious difference between TLC and HPLC is thatTLC is operated on an open chromatographic bed with aplaner format and the solvent flow is driven by capillaryaction, whereas HPLC is performed on a closed columnsystem with a high-pressure pump delivering solvent.HPLC is a high efficiency separation technique and there-fore can provide high resolution for analysis of complexmixtures [7]. However, the sophisticated automaticinstrumentation, the hyphenated online detectors forquantitative analysis and structure elucidation, as wellas the cost of consumables make HPLC a relatively expen-sive separation technique. The solvent delivery system,required to maintain a precise and stable pumping flowat high pressure, is normally the most expensive compo-nent of the whole instrument: this is especially the casefor the recently introduced ultra high-pressure HPLC sys-tems [8, 9]. By contrast, TLC does not require such a

pumping system, since the dry chromatographic beditself can upload and transport mobile phase based oncapillary action. In this sense, TLC is an energy-free sep-aration tool. As well as low instrumentation costs andsimplicity, other attractive features of TLC include theability to run separations quickly and in parallel. TLC isroutinely used as an economical separation techniquefor many analytical tasks and provides realistic solutionsin cases where very high separation efficiency is notreally necessary [10].

Since the 1970s [11, 12], developments in porous par-ticulate stationary phases using high purity silica sup-ports with spherical regularity, small particle size andnarrow size distribution have allowed chromatographicdynamics to be optimised and HPLC to achieve impres-sive separation performance [13]. By contrast, irregularshaped adsorbents are still widely used to create TLCplates, although HPTLC has proved the usefulness ofsmaller particulate phases (down to 5 lm) with a narrowsize distribution [14].

The aim of this study was to see whether it was possibleto combine the low-cost instrumentation approach ofTLC with the use of the high-quality stationary phases ofHPLC. The method developed, capillary action LC (caLC),is similar to TLC in using capillary action as the drivingforce to draw solvent through the stationary phase. It dif-fers from TLC in using capillary columns packed withHPLC-grade microparticulate materials, rather than aplanar bed with TLC-grade irregular shaped adsorbentsand binder.

Correspondence: Professor David M. Goodall, Department ofChemistry, University of York, York, YO10 5DD, UKE-mail: [email protected]: +44-1904-432516

Abbreviations: caLC, capillary action LC; RGB, red, green andblue

i 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

1832 B. Zhang et al. J. Sep. Sci. 2009, 32, 1831 – 1837

Over the last thirty years, many different methodshave been introduced for the manufacture of capillarycolumns containing microparticulate stationary phases[15]. Slurry packing with a high pressure pump [16, 17] isprobably the most widely used strategy, although centri-fugal force [18, 19], gravity [20], vacuum [21] and electro-kinetic [22] packing methods have also been utilized.Packing methods described by Yan [22] and Perrett andcoworkers [23] have demonstrated high productivitiesthrough packing multiple columns in a single run. Thecentrifugal packing method introduced by Col�n andcoworkers [18, 19] also has potential for the productionof columns in parallel. Capillary columns have also beenpacked in a dry state [24, 25]; in this strategy, dry powersof chromatographic materials are directly packed intocapillary tubes without using carrier solvent as requiredin slurry packing methods. Microscale fittings were usedin most of the packing protocols reported, in order tosecure robust connections to the packing pipelines andto ensure minimal dead volume, and this normally onlyallowed columns to be packed one at a time.

Since the column bed must be dry prior to separationin caLC in order to enable capillary action driven flow,one of the objectives of this work was to develop a drypacking method using centrifugal force to produce mul-tiple capillary columns in a single run and allow theiruse as disposable items.

2 Experimental

2.1 Materials and apparatus

Glass micropipettes, normally for sample spotting in TLC,were purchased from Camlab (Cambridge, UK) and usedas capillary tubing in this study: dimensions were 10 lL(40 mm, 564 lm i.d./1060 lm o.d.), 5 lL (32 mm, 447 lmi.d./890 lm o.d.) and 2 lL (40 mm, 283 lm i.d./740 lmo.d.). Standard pipette tips of 1–200 lL to be used as capil-lary tubing holder and packing material reservoir to setup the micropackers were supplied by Thermo Fisher Sci-entific (Loughborough, UK). 1.5 mL standard centrifugetubes purchased from Sarstedt (N�mbrecht, Germany)were used as sheath tubes to fit micropackers into an MSEMistral 1000 benchtop centrifuge from MSB Midwest Sci-ence Biocenter (Albertville, MN, USA) for centrifugal pack-ing of glass capillaries. The nonspherical bare silica par-ticles Silica Gel 60G (TLC grade) with particle size of 90%a45 lm (as labelled on the pack) were supplied by Merck(Darmstadt, Germany). HPLC grade microparticulatesilica gels, 1–10 lm in particle size either porous or non-porous, hybrid and bare silica, were provided by Waters(Milford, MA, USA). A butane-powered microtorch waspurchased from RS Components (Corby, UK) and couldprovide a temperature up to 13008C for melting glasscapillary ends. The dye mixture IV (containing Fat Red 7B,

Solvent Green 3, Sudan II, Solvent Blue 35 and SudanOrange G) for normal phase separation was purchasedfrom AnalTech (Newark, DE, USA). Toluene and methanol(MeOH) of HPLC grade were purchased from ThermoFisher Scientific. Water was purified with an ElgastatUHQ II water purification system from Elga (Wycombe,UK). A Digital Blue QX5 digital optical microscope, pur-chased from dabs.com (Bolton, UK), was used to monitorand record the caLC separations of the dye samples andScion Image Beta 4.02 software (Scion Corporation, Mary-land, USA) for processing the image data.

2.2 Capillary column manufacture

The procedures for dry packing capillaries by centrifugalforce are shown in Fig. 1. In Fig. 1A, one end of the glasscapillary to be packed was heat shrunk into a taper shapewith a narrow orifice (c.f. Fig. 1H and I). To set up a micro-packer, the narrow end of a standard 1–200 lL pipettetip was first cut off to an appropriate length. As visual-ized in Fig. 1B, the capillary was then inserted taper endfirst into and through the pipette tip, leaving a very shortlength inside which was held tightly by the cut end ofthe pipette tip. The pipette tip was used as a reservoir forthe packing material, as shown in Fig. 1C. In Fig. 1D, thebottom of a 1.5 mL plastic centrifuge tube was choppedoff and then the setup in Fig. 1C was inserted into it: onemicropacker was made in this way. The centrifuge tubewas used here simply to provide a wide sheath tube to fitthe centrifuge cartridge. Figure 1E shows ten of themicropackers loaded into two centrifuge cartridges:each had a capacity of five micropackers. Before centrifu-gal packing, an appropriate amount of dry powder of thepacking material was loaded into the reservoir on thetop of each micropacker. The two cartridges were thenmounted symmetrically into the centrifuge and spun ata speed of 4000 rpm for 3 min. This method allowedpacking of ten glass capillary columns in one run andthey were ready for immediate use in caLC. Five columnspacked with Waters Xbridge 2.6 lm silica and ten col-umns of Waters Spherisorb 10 lm bare silica are shownin Fig. 1F and G, respectively, before separation fromtheir pipette tube reservoirs. A key step for capillary col-umn manufacture was making the taper ends to retainparticles through the keystone effect (Fig. 1A) [26–29]: col-umns packed with 10 and 2.6 lm material are shown inFig. 1H and I, respectively. The other end was left openand later inserted into a mobile phase reservoir as theinlet of the packed column in caLC separation.

It was experimentally proved that this dry packingmethod enabled a great variety of particulate packingmaterials with different particle sizes (1–40 lm) andchemistries (normal and RP, ion exchange, mixed-modeand chiral) to be packed into glass capillary tubes ofdifferent volumes (2, 5 and 10 lL) in parallel. By contrast


J. Sep. Sci. 2009, 32, 1831 –1837 Liquid Chromatography 1833

with the slurry packing strategy [30], there was no needto select and optimize solvent systems for different pack-ing materials.

2.3 caLC

A simple experimental platform was set up to allow caLCseparation and detection in an inexpensive manner,using a digital optical microscope. Fig. 2 shows the sep-aration device, involving a glass capillary column and amobile phase reservoir, placed on the object plate of themicroscope. The reservoir was made from a cap of a0.5 mL centrifuge tube and a hole of 1 mm diameter wasdrilled through its side to hold the glass capillary col-umn (10 lL column in this case). The open end of the col-umn was inserted into the reservoir through this hole tomake connection with mobile phase while the taper endwas cushioned by a glass rod, which kept the caLC col-umn horizontal during separation. A piece of white card-board was used as a white background for imaging thecaLC column during separation. Magnified by 106by themicroscope, the separation process that took place in thecapillary column could be monitored through snapshotsand recorded in real time by a computer connected via aUSB port to the digital microscope.

Normally, in TLC a sample is spotted near to one sideof the plate, usually 1.5–2 cm from the edge [31]. Thismethod was not applicable to the chromatographic bedused in caLC, due to the closed column format utilized.In this caLC study, the sample was directly loaded on thehead of the column through a dip-and-withdraw opera-tion in which the open end of the capillary column wasdipped into the vial containing the sample solution andwithdrawn quickly. A sample plug 0.5–1 mm long wasobtained each time in the experiment. After sample load-ing, the column was connected to the empty mobilephase reservoir via the injected end, as shown in Fig. 2,and caLC separation started right after about 100 lLmobile phase was loaded into the reservoir.

With dye mixture IV as sample and toluene as mobilephase, normal phase caLC was investigated on variousbare silica phases, a TLC grade Merck 60G silica mediumand three HPLC grade microparticulate materials:Waters Spherisorb 10 lm, Waters Xbridge 2.6 lm andWaters Spherisorb nonporous 1.5 lm bare silica. Allexperiments reported involve media packed into 10 lLglass tubes (40 mm, 564 lm i.d./1060 lm o.d.) and werecarried out at ambient temperature, l258C.

3 Results and discussion

3.1 Capillary action driven flow and caLC

Using the home-built experimental setup, caLC separa-tions were investigated in normal-phase mode on a vari-ety of bare silica phases. Since there is no solvent flowthrough capillary action beyond the end of the column,each column was only utilised once, as with disposableplates in TLC. Therefore, comparison of caLC separationswas at a column-to-column level. Snapshots of the separa-tion of dye mixture IV performed on five columns packedwith Waters Xbridge 2.6 lm silica particles are shown in


Figure 1. Packing glass capillaries with centrifuge force (seetext for details).

Figure 2. caLC separation device placed under a micro-scope.


Fig. 3. The excellent reproducibility of separations dem-onstrates that the centrifugal packing method for pro-duction of dry columns was fit for purpose. Fig. 3 alsoillustrates the change in appearance of the bed on wet-ting, from which it is easy to identify the position of themobile phase front: the dry section shows two apparentwall layers in this 2-D view, whereas in the wetted sectionthe wall appears to be far thinner.

In all caLC experiments, the rate of change of positionof the solvent front was observed to decrease with time.In TLC, the movement of the solvent front driven bycapillary action is related to time via a quadratic relation-ship [32]

Z2 ¼ �t ð1Þ

where Z is the migration distance of the solvent front tothe original spotted point, � the velocity constant and tthe separation time. Fig. 4 shows the dependence of Z2 ont for the range of stationary phases studied in the caLCexperiments. Linear regression fits to all lines had R2 val-ues greater than 0.994. This demonstrates that the TLCmodel is also applicable to caLC, as expected for permea-tion of dry-packed capillaries (Scurati, A., Manas-Zloc-zower, I., Feke, D. L., www.cwru.edu/cse/mcube/publicat/scurati2000.pdf).

Among the four silica phases investigated, WatersSpherisorb 10 lm bare silica presented the longestmigration distance of the solvent front for a given runtime, Waters Xbridge 2.6 lm had the shortest, whileMerck 60G and Waters Spherisorb 1.5 lm were inbetween. Comparing times to travel the same distance, ittook 168 s for the solvent front to travel 2.1 cm onXbridge 2.6 lm phase, but only 75 s on the Spherisorb10 lm phase.

Guiochon and Siouffi [33] carried out a comprehensivestudy of movement of solvent fronts in TLC, matchingexperimental observations and theoretical predictions.Their data and those of Ripphahn and Halpaap [34] usinga variety of solvents and LiChrosorb particles in therange of diameters 5–40 lm were shown to conform tothe relationship

� ¼ 2k0dp c=�ð Þ cos � ð2Þ

where k0 is the specific permeability (a dimensionlessconstant), dp the average particle size of the TLC station-ary phase, c the surface tension and � the viscosity of thesolvent, and � the wetting angle. For the combination ofmost nonpolar organic solvents and bare silica phases,wetting angles close to 08 are normally obtained on thesurface of bare silica phases, and the value of cos�may beassumed to be 1 [33]. Specific permeability values werefound to show little variation over the complete set ofsolvents tested, and the average values of the two groupsof workers were found to be in excellent agreement: k0

(7.9 € 1.2)610–3 [33] and (8.0 € 0.7)610–3 [34].Specific permeabilities were calculated from the caLC

results using the slopes in Fig. 4 for the velocity con-stants, the known values of dp, c/� for toluene from [33],and the assumption that cos� = 1 [33]. Results arereported in Table 1. No value of k0 was calculated forMerck 60G, since this material is polydisperse. Whilst ko

for Spherisorb 10 lm, 6.4610–3, is comparable to the val-ues obtained for the TLC phases, those for Xbridge2.6 lm and Spherisorb 1.5 lm are seen to be signficantlyhigher. Table 1 also presents estimates of the externalbed porosity ee (volume of interparticle space/volume ofbed) determined from the following equation [33]

k0 ¼ e3e=180ð1� eeÞ2 ð3Þ

A possible explanation for the increase in both ko and ee

observed as particle size decreases in caLC is the natureof the packing process, since the centrifugal packingprocess involves forces which scale with the mass of the


Figure 3. Visual caLC separation of dyes on five columnspacked with Waters Xbridge 2.6 lm silica, snapshot atZ = 1 cm. Experimental conditions: glass capillary,40 mm6564 lm i.d.; 21 mm column length (from inlet end)real-time imaged under digital microscope at 106 magnifica-tion; sample, AnalTech dye mixture IV; dip-and-withdrawinjection, sample plug length 0.5–1 mm; mobile phase, tol-uene.

Figure 4. Quadratic relation of solvent front versus separa-tion time on different silica phases in caLC. Experimentalconditions as in Fig. 3.


particle. As noted previously in TLC [33], external bedporosities are substantially greater than for HPLC col-umns, which typically show a randomly packed struc-ture with porosity ee l 0.4.

A brief study was conducted using columns packedwith alkyl bonded spherical silica gels in an attempt tocarry out RP caLC. The flow rates were found to be muchslower than in normal phase separations: for a 1 cmmigration length it took l300 s on a Spherisorb 5 lmODSB column, whilst for the slowest run on the normalphase Xbridge 2.6 lm column it took less than 50 s totravel this distance. This may be explained by referenceto Eq. (2) [33], since on the RP material, due to the hydro-phobic layer of alkyl groups chemically bonded onto thesurface of the silica gel, the water-rich mobile phase hasa wetting angle close to 908, resulting in a poorly wettedstationary phase surface [35]. The correspondingly lowvalues of cosh relative to those in normal phase caLC pro-vide the most likely reason why velocity constants andflow velocities were lower. Obviously, such a low capil-lary action flow and the limited migration distancemake RP caLC impractical.

3.2 Separation efficiency and plate heights

Both the van Deemter and Knox equations, the formulaeused to describe band broadening behaviour in columnLC [36], have also been applied to evaluate separation effi-ciencies in TLC [37, 38], where band broadening wastreated as a function of flow rate [39]. As in TLC [32], incaLC the analytes are separated in space and Eq. (4) and(5) can be used to calculate the efficiency, N, and plateheight, H, where Zx is the migration distance of the ana-lyte and w is the width at base of the analyte band.

N ¼ 16Zx

w

� �2

ð4Þ

H ¼ Zx

Nð5Þ

By contrast with HPLC, the solvent flow in caLC isdriven by capillary action, which changes dynamicallyaccording to Eq. (1) and is not manually adjustable.Therefore as the solvent front progresses with a deceas-

ing flow rate, the plate height changes accordingly, aspreviously well documented in TLC [32, 40, 41]. In HPLC,if the available pumping pressure is sufficient to main-tain a constant flow rate over a wide range, the plateheight is constant and the efficiency increases in propor-tion to the column length [42]. In caLC, however, due tothe variable plate height, such a proportional relation-ship between the effective column length (length trav-elled by the solvent front at a certain time point) and effi-ciency does not exist. Thus calculations based on Eq. (4)and (5) give an average plate height over the length thatthe mobile phase has travelled, rather than an instanta-neous plate height at a certain time point.

In the present study, w was measured manually via vis-ual estimation. With the least retained dye, Fat Red 7B asthe standard (pink band in Fig. 3), the average plateheight, H, as a function of instantaneous flow velocity, u,on the three silica phases is shown in Fig. 5. Data for astrongly retained dye, Solvent Blue 35 (blue band in Fig.3), are also presented on Xbridge 2.6 lm silica phase. Hwas calculated according to Eq. (4) and (5). u was calcu-lated from the velocity constant and migration distanceof the solvent front, according to Eq. (6) which follows bydifferentiation of Eq. (1).

u ¼ dZdt¼ �

2Zð6Þ

Comparing the three H-u plots for Fat Red 7B on differ-ent silica phases, the one for Xbridge 2.6 lm silica pre-sented the best performance. All curves show decreasingaverage plate height as flow velocity increased. Using thevan Deemter equation (Eq. 7) as the model to discuss theband broadening behaviour in caLC, as previouslyapplied for TLC [32, 40, 41], results are consistent withpeak dispersion in caLC being dominated by molecular


Table 1. Velocity constants, specific permeabilities andexternal bed porosities for centrifugally packed capillaries

Packing Particlediameter(lm)

Slope, �(cm2/s)

ko ee

Merck 60G – 0.0355Spherisorb 10 0.0618 6.4610–3 0.58Xbridge 2.6 0.0267 10.6610–3 0.63Spherisorbnonporous

1.5 0.0406 28610–3 0.72

Figure 5. Average plate height as a function of flow velocityon different silica phases: Fat Red 7B (weakly retained) asthe standard analyte on Merck 60G (g), Waters Spherisorb10 lm (f), Waters Xbridge 2.6 lm (h) silica phases and Sol-vent Blue 35 (strongly retained) as the other standard ana-lyte on Waters Xbridge 2.6 lm (6) silica phase. Experimen-tal conditions as in Fig. 3.


diffusion. At low velocity, plate height decreases as veloc-ity increases due to the B/u term.

H ¼ Aþ Buþ Cu ð7Þ

In HPLC, where flow rate is readily controllable, col-umns are normally operated at flow rates which give theminimum plate height (decrease in B/u term balanced byincrease in Cu term) [39]. Typically in HPLC with sphericalmicroparticulate (1–10 lm) packing material, the mini-mum in the van Deemter curve occurs at optimum linearvelocity higher than 0.5 mm/s [8]. This is higher than thevelocities in Fig. 5 (0–0.3 mm/s). In caLC, the limit to theavailable linear velocity of the capillary action flow didnot permit the technique to be operated with plateheights at the minimum value and as low as in HPLC.From Fig. 5, it may be seen that on the Xbridge 2.6 lm col-umn, the weakly retained Fat Red 7B presented the small-est average plate height, 8.8 lm, at the highest value of u(0.18 mm/s), measured at Z = 0.74 cm. For the stronglyretained Solvent Blue 35, the lowest plate height of 31 lmcorresponds to u = 0.13 mm/s (Z = 0.97 cm). On theXbridge column, bands for both analytes were welldefined and resolved from other bands under all condi-tions studied: the range of Z was 0.74–2.1 cm, and Zx 0.40–1.9 cm. This distance range was determined by the usableimaging area in the experimental setup (see Fig. 3). Thevery best HPLC columns achieve an optimum plate heightof Hl2dp [43] at the minimum of the van Deemter curve.In this regard, the lowest achievable plate height with2.6 lm particles should be Hl5.2 lm. The fact that it waspossible to approach close to this value, obtainingH = 8.8 lm with an extremely simple and low cost caLCsetup, was an excellent outcome of this study.

The difference in the H-u plots for Fat Red 7B on thethree phases can be attributed to the different particlesizes. Theoretically, the optimum values of Hl2dp are 5.2,20 and a90 lm for Waters Xbridge 2.6 lm, Waters Spher-isorb 10 lm and Merck 60G, respectively. The lowest val-ues obtained in this study were 8.8, 65 and 110 lm forthese three phases, and the results clearly illustrate thatthe benefits of using small particles hold in caLC as wellas in HPLC [11, 12].

3.3 Image analysis for caLC performanceevaluation

In the last section, caLC separation performance was dis-cussed based on experimental data measured manually.Since measuring bandwidths of coloured dye sample byvisual estimation is a subjective process, as previouslypointed out in TLC by Guiochon and his colleagues [44],computer-assisted image analysis was used for process-ing selected caLC chromatograms. Figure 6A shows asnapshot of a caLC separation on a Waters Xbridge

2.6 lm silica column. Scion Image software was used toseparate the colour bitmap of the snapshot into the red,green and blue (RGB) constituent images and to integratethe pixel values across the internal width (vertically asshown in Fig. 6A) of the column. Intensity profiles com-posed of 640 data points, one for each of the horizontalpixels along the axial column dimension, were gener-ated for each of the three colours. A spreadsheet wasused to calculate the ratio of each of the RGB values tothe average value for each point along the profile; theRGB ratio profiles are plotted in Fig. 6B. A single signalchromatogram was then generated by, at each point,subtracting the lowest of the RGB ratio values from theother two and then summing, and is shown in Fig. 6C.

Based on the chromatogram shown in Fig. 6C, an aver-age plate height of 35 lm was obtained for Solvent Blue35 (the blue band in Fig. 3 and 6A and the marked peakin Fig. 6C) at a flow front position Z = 2.1 cm. This is insatisfactory agreement with the corresponding averageplate height value of 41 lm calculated from Fig. 5 (whereu = 0.062 mm/s, Z = 2.1 cm), based on manual bandwidth(w) measurement. This shows that the manually meas-ured data is credible, at least in a semi-quantitative sense,and helps validate the use of visual estimates of band-widths for comparing band broadening on differentsilica phases. In future, as already exemplified in TLC[45], computer-assisted image analysis should become auseful tool for quantitative interpretation of caLC data.


Figure 6. Transforming a visual caLC separation (A) viacombination of filtered RGB signals (B) into a chromatogram(C). Each pixel represents 33 lm. caLC separation onWaters Xbridge 2.6 lm silica, snapshot at Z = 2.1 cm. Exper-imental conditions as in Fig. 3.


4 Concluding remarks

A column liquid chromatographic technique driven bycapillary force which can be thought of as a hybrid ofTLC and HPLC, caLC, has been investigated. The resultssuggest that normal phase caLC, using very simpleinstrumentation and HPLC-grade low-diameter particlesfor packing columns, can achieve efficiencies compara-ble to those in HPLC. The established fundamental theo-ries of TLC can be utilized to understand the transportprocess in caLC due to the same capillary action drivingnature. The centrifugal dry packing method developedin this study was a key enabling technology for prepara-tion of capillary columns in parallel for disposable use incaLC. In preliminary work towards caLC with UV detec-tion we have also packed fused silica capillary columns(75 and 100 lm i.d.) centrifugally but with a differentend-fritting technology [46]. There has been recent prog-ress in real-time monitoring of separations in TLC usinga CCD detector for area imaging [47], and an active pixelsensor area detector currently used for imaging CE sep-arations in parallel [48, 49]. These technologies could infuture be used for real-time on-column UV detection ofseparations in parallel caLC capillaries. Parallel caLCoffers advantages over running multiple TLC separationson a single plate, since confinement to the column for-mat would avoid smearing seen in TLC when bands runinto each other. Future developments of caLC will alsorequire improvements in the injection process.

We wish to thank Dr. Victor Chechik for providing access to thecentrifuge used to pack capillaries in this study.

The authors declared no conflict of interest.

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Capillary action liquid chromatography

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