PAPER www.rsc.org/loc | Lab on a Chip

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A fritless, EOF microchip pump for high pressure pumping of aqueousand organic solvents

Qin Lu and Greg E. Collins*

Received 18th September 2008, Accepted 11th December 2008

First published as an Advance Article on the web 13th January 2009

DOI: 10.1039/b816291c

A fritless, microchip electroosmotic flow (EOF) pump is microfabricated and demonstrated on a planar

soda lime glass substrate to be capable of supplying reasonable flow rates under high back pressures,

such as that required for micro-high pressure liquid chromatography (m-HPLC). The microchip EOF

pump is composed of a densely packed microchannel containing 800 nm silica particles and was capable

of generating a maximum pressure > 1000 psi (� 7 MPa) and a maximum flow rate of 282 nL/min

(aqueous cyclohexylamino alkyl sulfonate (CHES) buffer, 10 mM, pH 9.0, 200 V/cm). Other pumping

fluids, such as CHES buffer–acetonitrile mixture (50%, v/v), CHES buffer–methanol mixture (50%,

v/v), and pure acetonitrile were also used in a characterization of pump performance that included

determinations of the maximum flow rate, maximum pressure, and resulting flow rate against an

applied, downstream back pressure. The flow rate under a 200 psi (�1.4 MPa) back pressure at

an applied electric field strength of 250 V/cm ranged from 285 nL/min for aqueous CHES buffer to

44 nL/min for CHES buffer–acetonitrile mixture (50%, v/v), indicating that this EOF pump will meet

the future requirements of a m-HPLC system.

Introduction

In the last two decades, tremendous progress has been achieved

in the development of fully contained, micro total analysis

systems (mTAS).1,2 Ideally, a miniaturized analysis system should

perform all the analytical steps required for a given analysis,

including sample preparation, injection and transport, high

resolution separation of multiple components from a complex

sample matrix, and, finally, sensitive detection of each individual

component. An integral element to the continued expansion of

mTAS capabilities is in the exploration of novel, miniaturized

micropumps capable of operating within various types of

microfluidic networks. The most popular motive force for lab on

a chip devices has traditionally been electroosmotic flow (EOF).

EOF driven flow is easily integrated on-chip and can be utilized

by either applying an electric field across 1) an open channel

within a microfluidic network, i.e. capillary electrophoresis

(CE) microchip,3 2) a packed bed stationary phase, i.e. capillary

electrochromatography (CEC) microchip,4 or 3) an upstream

microchannel, open or packed, that microfluidically connects to

both a grounded, sealed reservoir and a downstream, field free

pumping region, i.e., EOF pump.5,6 There is growing interest in

expanding the separation capabilities of CE and CEC microchip

devices with a complementary approach, micro-high pressure

liquid chromatography (m-HPLC). The incorporation of high

back pressure, packed bed microcolumns onto lab on a chip

platforms presents a challenge for the integrated micropump.

EOF micropumps are a potential, on-chip solution to the

pumping requirements for m-HPLC because of their high pres-

sure compatibility, reasonable flow rate generation, ease of

integration and operation, and low fabrication cost.

Naval Research Laboratory, Chemistry Division, Code 6112, 4555Overlook Ave., S.W., Washington, DC, 20375-5342, USA

954 | Lab Chip, 2009, 9, 954–960

EOF pumps can be generally categorized into two types: open

channel EOF pumps and packed column EOF pumps. Dasgupta

and Liu, for example, reported an open capillary EOF pump for

flow injection analysis.7 Ramsey et al. used an open channel EOF

pump in a microchip format to generate electrospray.8 Efforts by

others have examined the fabrication of multiple, parallel,

shallow channels on a microchip device in the hopes of creating

an EOF pump with high flow rate generation at a reasonably

high pressure (> 50 psi (0.34MPa)).9–14Despite the fact that some

of these multiple, parallel channel microchip EOF pumps were

capable of transporting aqueous fluid with impressive flow rates

(> 10 mL/min), the inherent nature of these open channels and

their low hydraulic resistance, results in only low pressure

operation capability (# 5 psi (0.034 MPa)). Chen et al., for

example, presented a multiple, parallel open channel microchip

EOF pump that generated maximum flow rates as high 15 mL/

min, but with a maximum pressure of just 5 psi (� 0.034 MPa) at

an applied electric field strength of 10 kV/cm.11 In another report,

a microchip pump presented by Lazar et. al. generated flow rates

of 10–400 nL/min and a maximal pressure of 80 psi (�0.55 MPa)

at an applied electric field strength 1 kV/cm.12 In order to

enhance the output pressure of an EOF pump and, thereby,

extend its application into the area of fluid delivery under

significant back pressure, i.e., m-HPLC ($ 1000 psi (6.9 MPa)),

capillaries filled with monolith silica material or packed with

micron to sub-micron sized silica particles were used in the

construction of high pressure EOF pumps.5,6,15–18 In general,

EOF pumps utilizing packed capillaries had better performance

in terms of overall output pressure than the open channel EOF

pumps. Pressures in excess of 8000 psi (� 55 MPa) have been

generated utilizing packed capillary EOF pumps and these

pumps were attractive in the miniaturization of HPLC systems.5

Despite the success observed for generating high pressures

with packed capillary EOF pumps, there are only a few reports

This journal is ª The Royal Society of Chemistry 2009

Fig. 1 A) High pressure chip holder: a) bolts for compression sealing; b)

PEEK base substrate; c) microchip EOF pump; d) Nanoport fitting and

associated o-ring; e) PEEK top substrate, and B) chip design with inset

image of weir: PI—pump inlet; PO—pump outlet, ground; SR—sealed

reservoir, unused; CO—capillary outlet; f) 800 nm silica beads; g) 5 mm

silica particles.

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that detail the development of high pressure EOF pumps in

a microchip format.19–21 In our previous work, we reported

a packed microchannel, EOF pump using 3.3 um silica particles

that generated a maximum pressure of 368 psi (2.54 MPa) at an

applied electric field strength of 875 V/cm.21 A similar pump was

subsequently utilized in conjunction with an alkyl-modified sol-

gel monolith stationary phase, whereby a completely integrated

micro-liquid chromatography (m-LC) approach was demon-

strated for the on-chip separation of a mixture of explosives.22

The high porosity of the sol-gel monolith was such that this

pump design was able to manage the back pressures associated

with this stationary phase.

In this article, we present the characterization and perfor-

mance of a fritless, high pressure microchip EOF pump that is

composed of a densely packed microchannel containing 800 nm

silica particles, a design which results in considerable improve-

ments in high pressure generation for more effectively meeting

future m-HPLC requirements on a microchip platform. Two

advancements were necessary in order to realize this achieve-

ment: 1) the development of a simple approach for packing

nanoparticles into microchannels that avoids the difficult task of

fabricating extremely narrow weir structures in glass substrates

whose very nature severely limits fluid flow; and 2) the

construction of a home-made microchip holder capable of

providing high pressure sealing for both high quality column

packing, as well as high backpressure EOF micropump opera-

tion. At a modest applied electric field strength of 200 V/cm, the

EOF micropump presented here generated a maximum pressure

> 1000 psi (� 7 MPa) and a maximum flow rate of 282 nL/min

when aqueous cyclohexylamino alkyl sulfonate (CHES) buffer

(10 mM, pH 9.0) was used as the pumping fluid. To the best of

our knowledge, this is the highest pressure ever reported on

a microchip platform. The operation capabilities, such as

maximum flow rate, maximum pressure, and flow rate against

a back pressure, of this pump for m-HPLC-like applications, were

further assessed with pumping fluids from pure organic (aceto-

nitrile), to isocratic mixtures (water/methanol and water/aceto-

nitrile).

Materials and methods

Reagents

Ammonium hydroxide, acetonitrile (HPLC grade), hydrogen

peroxide and methanol were purchased from Fischer Scientific

(Fair Lawn, NJ). CHES and sodium silicate solution (�14%

NaOH, 27% SiO2) were purchased from Sigma-Aldrich (St.

Louis, MO). Chromium etchant and buffered oxide etchant (6:1)

were obtained from Transene Company (Danvers, MA). Porous

silica particles (5 mm in size, 50 A pore) and plain non-porous

silica particles (800 nm in size) were obtained from Macherey-

Nagel (Bethlehem, PA) and Bangs Laboratory (Fisher, IN),

respectively. Deionized water was obtained from a Milli-Q Plus

(Millipore, Billerica, MA) water system.

Microfluidic design and fabrication

The microchip EOF pump design utilized here is similar to

a previously published micropump, consisting of a microcolumn

packed with porous silica particles (3 cm in length) that is

This journal is ª The Royal Society of Chemistry 2009

connected to an open channel, downstream field-free region

(3 cm in length, Fig. 1B).21 The microchip device was fabricated

by standard photolithography and wet etching techniques, using

a soda lime substrate coated with chromium and photoresist

(Nanofilm, Westlake Village, CA). The design incorporates

a weir structure at the tail end of the micropump region to enable

packing of the silica particles; this feature requires two separate

exposures during the microfabrication process. The photomask

design of the microchip device was transferred to a soda lime

substrate using a 100 W, 365 nm ultraviolet flood lamp

(SB-100PC; Spectroline). The exposed photoresist and under-

lying chrome were removed by immersion in 0.1 N NaOH, fol-

lowed by chromium etchant. After the first exposure, isotropic

etching of the developed substrate using a mixture of hydro-

fluoric acid and nitric acid (HF:HNO3:H2O—17:12:71) formed

the microchannels � 65 mm deep and � 100 mm wide at half

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depth. The second exposure and subsequent etching with

buffered oxide etchant (6:1) provided the weir with a depth of

� 4.8 mm. After formation of the microchannels and weir

structure, but prior to complete removal of the photoresist and

chromium coatings, access holes were drilled at the end of each

channel using a Craftsman rotary power tool from Sears, Inc.

equipped with diamond coated drill bits from Crystalite

Lapidary and Glass Products (Lewis Center, Ohio).

Before bonding, the soda lime substrate and a soda lime cover

slide were sequentially cleaned for 30 min in 30% Branson

solution, basic piranha solution (H2O2: NH4OH: H2O—1:2:2),

and deionized water. The pre-bonding of the soda lime microchip

device was achieved using a hydraulic press (Carver Inc.,

Wabash, IN) under a pressure of 3 tons at a temperature of

150 �C for one hour. Final bonding was completed by placing

the pre-bonded microchip device in a box furnace (BF51800,

Lindberg Blue, Asheville, NC) at 550� C for one hour.

Fig. 2 Instrumental setup for flow rate measurement with/without back

pressure: a) EOF microchip pump microfabicated in soda lime substrate;

b) electric field application to pump inlet and pump outlet reservoirs; c)

microscope; d) close-up view of capillary indicating air/liquid meniscus;

e) pressure cell; f) capillary; g) argon gas tank.

EOF pump column packing

The EOF pump was formed by first packing a 0.2 cm length of

5 mm porous silica particles behind the weir structure, followed

by a 2.8 cm length of 800 nm silica particles (see Fig. 1B). This

approach enables the high pressure packing of small particles

(800 nm in size) using a weir 4.8 mm deep. Although some

mismatch does exist between the 5 mm porous silica and 800 nm

silica particles, no noticeable adverse impact was observed on the

flow, macroscopically. To obtain a densely packed column, it is

essential, however, nontrivial, to create a high pressure connec-

tion between the microchip access hole and the packing tool. In

this regard, the Nanoport� assemblies from UpChurch Scien-

tific (Oak Harbor, WA) are somewhat well suited for interfacing

the microchip’s inlet reservoir to a capillary (360 mm o.d.) that

can then be connected to a syringe whose needle tip has an o.d. of

360 mm. The high pressure utilized for packing was provided by

a handheld syringe pump from Unimicro Technologies (Pleas-

anton, CA). Problems, however, were encountered frequently

during the packing process due to leaking caused by the failure

of the epoxy (supplied by UpChurch Scientific) used to attach

the Nanoport onto the glass microchip. Other epoxies from

competing manufacturers resulted in similar failure rates. In

order to overcome this interface problem, an issue which was also

detrimental to subsequent EOF pump operation, especially when

high pressures were desired and organic solvents were used, we

fabricated a high pressure, microchip holder from 0.500 thick

PEEK substrate (Grainger, Inc., Washington D.C.) (Fig. 1A).

This microchip holder provided a high pressure seal between

each of the Nanoport fittings and the glass microchip substrate,

enabling us to pack a high quality column for EOF pumping and

to ultimately measure pressures as high as 1023 psi (7.05 MPa).

Note that the top portion of the holder was comprised of four

separate compression fittings, one for each Nanoport reservoir,

as opposed to a single fitting that compresses all four reservoirs,

simultaneously. Four separate compression fittings provided

optimal high pressure sealing for each individual Nanoport

reservoirs that, in our experience, was otherwise unachievable

with a single substrate.

Packing silica particles into the microchannel upstream of the

weir was achieved by using a syringe filled with a suspension of

956 | Lab Chip, 2009, 9, 954–960

2 wt. % silica particles in water and contained within the hand-

held syringe pump. Once the particles reached the end of the

microchannel, the Nanoport reservoir was connected to a pres-

sure cell (Next Advance, Rensselaer, NY) via a capillary.

Water was pushed through the column by applying 1000 psi

(� 6.9 MPa) of pressure from an argon tank overnight to afford

a densely packed column with high homogeneity.

Measurement techniques

Performance characterization of the microchip EOF pump

included measurements of the maximum flow rate and maximum

pressure in relation to the applied electric field strength, as well as

measurements of the flow rate at particular electric field strengths

against a series of known back pressures. The pumping fluids

utilized included an aqueous CHES buffer (10 mM, pH 9.0), pure

acetonitrile, a mixture of 10 mM CHES in 50% acetonitrile, and

a mixture of 10 mM CHES in 50% methanol. At least three

measurements were performed for each data point.

The technique used to measure the flow rate was based on

tracking the liquid/air meniscus within a 100 mm id fused silica

capillary. The measured meniscus velocity and the capillary

cross-sectional area were used to compute the flow rate. The flow

rate against a back pressure was determined similarly. The

microchip field-free outlet reservoir was connected to a pressure

cell via a 50 cm long capillary (100 mm id). The back pressure was

applied to the pressure cell via an argon gas tank (Fig. 2).

Although evaporation of the liquid at the liquid air meniscus is

unavoidable, we estimate this will give only a minor error, with

the reported flow rate values erring on the low side.

Maximum pressure was measured by connecting a 25 mm id

fused silica capillary to the field-free outlet reservoir. The capil-

lary was 30 cm in length, with the far end sealed. The volume of

air trapped within the capillary before (V0) and after (V1) the

application of an electric field across the pump region was

measured by tracking the motion of the liquid/air meniscus with

the aid of a Zeiss Stereomicroscope (Thornwood, NY), until it

reached an equilibrium position. The resulting pressure (P) was

estimated using Boyle’s Law:

P ¼ P0V0

V1

(1)

To monitor the packed column’s stability, the generation of

Joule heating, and the adverse impact of gas production due to

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electrolysis on the electrical connection, electric current through

the pump region was recorded and monitored continually during

the pressure and flow rate measurements by connecting a digital

electrometer (Keithley, Cleveland, OH) in series with the ground

electrode. The current was stable during the time frame of the

measurements (< 6.5 mA at an applied field strength of 200 V/

cm), and was linearly proportional to the applied electric field

strength (R2 > 0.99), indicating minimal problems associated

with Joule heating. The pumping fluid in the inlet and ground

reservoirs was replenished from time to time in order to minimize

variations in pH, buffer concentration, and composition of

aqueous–organic mixtures.

Table 1 Properties of pump working fluids

Solvent 324 h (cP)24 x (mV)25 meo � 10�4 (cm2/V s)25

Acetonitrile 37.5 0.34 207 19.90Methanol 32.7 0.54 108 5.82Water 80 0.89 99 7.8040% Acetontrile 56.026 0.9827 134 6.7560% Acetonitrile 46.526 0.7627 142 7.6440% Methanol 61.128 1.4227 117 4.4560% Methanol 51.828 1.4027 103 3.37

Results and discussion

The flow rate, Q, for a fluid electroosmotically pumped through

a packed capillary filled with porous media is defined by Zeng

et al. in Eq. [2]18

Q ¼ jDPAa2

8hs� j3zEA

hs

�1� 2lI1ða=lÞ

aI0ða=lÞ

�(2)

where I0 and I1 are the zero-order and first order modified Bessel

functions of the first kind; DP is the pressure difference along the

flow direction; A,s, and J denote the cross-sectional area,

tortuosity, and porosity of the porous medium; E is the applied

electric field; 3 and h are the permittivity and viscosity of the

medium; z is the zeta potential; a is the pore radius; and l is the

Debye length, indicating the double layer thickness. This rela-

tionship was derived from an earlier model by Rice and White-

head describing the velocity profile of fluid through a single, open

capillary with a pressure gradient.23 By setting Eq. [2] to zero, the

maximum pressure in a closed system can be described by Eq. [3].

Alternatively, the maximum flow rate, Eq. [4], can be obtained

when the backpressure is set to zero.

DPmax ¼83zEL

a2

�1� 2lI1ða=lÞ

aI0ða=lÞ

�(3)

Qmax ¼43zEA

hs

�1� 2lI1ða=lÞ

aI0ða=lÞ

�(4)

From Eqs. [3] and [4], a linear relationship between the flow

rate and the pressure of the EOF pump can be obtained.

Q ¼ Qmax

�DP

DPmax

� 1

�(5)

Despite the fact that Eqs. [2]–[4] were derived for a packed

capillary filled with porous media, they provide important

insights into the design of microchip EOF pumps with packed

trapezoidal channels, and help us to understand the behavior of

these pumps with different pumping fluids. Considering specifi-

cally the application of microchip EOF pumps for m-HPLC,

wherein the micropump must be compatible with large back

pressures evident from a downstream stationary phase, Eq. [3]

indicates that utilizing packing materials of smaller diameter will

greatly improve the pump performance. The reason for this is

that DPmax is inversely proportional to the square of the pore

radius of the packed bed. The smaller the effective radius of the

enclosed channels within the packed bed for EOF generation, the

This journal is ª The Royal Society of Chemistry 2009

higher the hydraulic resistance of the packed bed to counter the

pressure-driven flow opposite to EOF flow, and, therefore, the

higher the maximum pressure that can be generated by the EOF

pump. Furthermore, DPmax can also be increased as the length of

the packed bed microchannel is increased, provided that the

applied electric field strength remains the same. In addition to

generating high pressure, an EOF pump for m-HPLC must be

capable of supporting reasonably high flow rates. From these

considerations, the EOF pump examined here was fabricated by

packing 800 nm silica beads to a length of 2.8 cm in a micro-

channel with dimensions of � 65 mm deep and � 100 mm wide at

half depth. Once the pump dimensions, packing material type,

and particle size have been determined, EOF pump performance

will depend upon the nature of the pumping fluids used, such as

the permittivity, 3, the viscosity, h, and their affect on the zeta

potential, z, at the surface (see Eq. [2]). It is important to observe

the effect different pumping fluids have upon pump performance,

particularly when we consider the need for both aqueous and

organic solvents in typical m-HPLC applications.

The performance of the microchip EOF pump was evaluated

by focusing on four pumping fluids: aqueous CHES buffer (10

mM, pH 9.0), CHES buffer–acetonitrile mixture (50%, v/v),

CHES buffer–methanol mixture (50%, v/v), and acetonitrile.

CHES was chosen specifically due to its zwitterionic character

that leads to a lower ionic strength at a particular concentration,

as well as its low ion mobility that generates low current in an

applied electric field. In addition, its pKa of 9.3 (25 �C) makes it

an ideal buffer component for enhancing the zeta potential at the

surface of the silica particles. The organic pumping fluids,

acetonitrile and methanol, were selected due to their popularity

as mobile phases in HPLC, micellar electrokinetic chromatog-

raphy (MEKC) and CEC. The properties of water, acetonitrile,

methanol, water–acetonitrile mixture, and water–methanol

mixture relevant to the performance of the microchip EOF pump

are listed in Table 1.24–28

Maximum flow rate determination for microchip EOF pump

Certainly, one of the most important properties of the microchip

EOF pump is its capability for liquid delivery across a reasonable

dynamic range within a microfluidic network. For this reason, we

examined the maximal flow rates attainable by this EOF pump as

a function of the type of pumping fluid and the electric field

strength applied.

A plot summarizing the maximum flow rates determined for

four pumping fluids, CHES buffer (10 mM, pH 9), CHES buffer–

acetonitrile mixture (50%, v/v), CHES buffer–methanol mixture

(50%, v/v), and acetonitrile, as a function of the applied electric

Lab Chip, 2009, 9, 954–960 | 957

Fig. 3 Maximum flow rate, Qmax, versus the applied voltage: open

diamond ¼ aqueous CHES buffer (10 mM, pH 9.0); closed triangle ¼CHES buffer–acetonitrile mixture (50% v/v); open square ¼ CHES

buffer–methanol mixture (50% v/v); closed circle ¼ pure acetonitrile. The

standard deviation error bars are displayed as determined from three

measurements.

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field strength is shown in Fig. 3. Linear relationships between the

flow rate and applied field strength were observed in all cases.

Aqueous CHES buffer gave the highest flow rate at every applied

field strength (282 nL/min at 200 V/cm), followed by the CHES

buffer–acetonitrile mixture (236 nL/min at 200 V/cm), CHES

buffer–methanol mixture (96 nL/min at 200 V/cm), and pure

acetonitrile (19 nL/min at 200 V/cm). This order is in line with the

electroosmotic mobility (meo) values reported byWright et al. and

summarized in Table 1, with the notable exception of acetoni-

trile.25 Electroosmotic mobility is a function of the dielectric

constant, 3, and viscosity, h, of the pumping fluid, as well as the

zeta potential of the surface, z, with each of these parameters

represented in the flow rate Eqs. [2] and [4]. Based upon the low

viscosity and the high electroosmotic mobility calculated for

acetonitrile and the experimentally determined maximal flow rate

exhibited by pure acetonitrile in open capillaries,25 it might be

presumed that the EOF pump would generate the greatest flow

rates with acetonitrile as the pumping fluid. The primary reason

for the low flow rate observed when acetonitrile was used as the

pumping fluid is likely due to the double layer overlap. The

effective pore radius, a, in a packed bed with 800 nm silica

particles was estimated to be �200 nm (based upon a geometry

calculation using regular sphere packing and taking into consid-

eration the irregularity of the packed bed in a trapezoidal shaped

channel), whereas the estimated double layer thickness, l, in pure

acetonitrile is � 1 mm. In this case, the ratio of the effective pore

radius and the double layer thickness is a/l < 1, which means that

the term with Bessel functions of the first kind in Eq. [4] must be

taken into account, and a lower maximum flow rate results than

in the case of a/l [ 1. For comparison, the double layer thick-

ness is just 3 nm for symmetric, univalent electrolytes at 10 mM

concentration.29 To obtain higher flow rates for pure acetonitrile

958 | Lab Chip, 2009, 9, 954–960

in m-HPLC applications, low concentrations of electrolyte can be

added to achieve a more favorable a/l ratio.

Maximum pressure determination for microchip EOF pump

Maximal pressure attainment by an EOF pump is another very

important feature, both as it pertains to the development of

microvalves and microinjectors on lab on a chip devices,30 as well

as for the adaptability of these pumps to chromatographic

applications, such as m-HPLC. In comparison to an open

microchannel, the introduction of silica particles to pack the

microchannel of the micropump not only increases the surface

area and density of ionizable silanol groups for maximal gener-

ation of EOF, it also increases the hydraulic resistance in the

pump region such that any flow in a direction opposite to EOF is

significantly reduced.

When CHES buffer was used as the pumping fluid and the

applied electric field strength was increased from 50V/cm to

200 V/cm, the maximum pressure recorded increased linearly

from 546 psi (3.76 MPa) to 1023 psi (7.05 MPa). Repeated

attempts to increase the field strength beyond 200 V/cm resulted

in fracture or cracking of the glass chip in a region located

downstream from the weir structure. One potential explanation

for this result is that since the microchip EOF pump was fabri-

cated in soda lime glass, the thermal bonding process weakens

the structure due to soda lime glass’ low thermal shock resis-

tance. Improved pressure characteristics are expected from

borosilicate glass, which will be investigated in future designs. To

date, this microchip EOF pump has generated the highest pres-

sure (1023 psi) ever reported on a planar microchip device, and,

furthermore, this pressure was observed while applying only

a moderate electric field strength (200 V/cm). Obviously, higher

pressure is attainable with higher electric field strength, providing

that a more suitable substrate is used to fabricate the microchip

EOF pump. In comparison, a microchip EOF pump of similar

design packed with 3.38 um silica particles generated a maximum

pressure of 368 psi (2.54 MPa) while requiring an electric field

strength of 875 V/cm.21 The packed column for the EOF pump

presented here was found to have excellent stability, even under

pressures greater than 1000 psi and in the absence of a frit. We

ascribe this stability to 1) the small size of the silica beads, 2) the

relatively long length of the packed bed microcolumn, and 3) the

high pressure seal provide by our home-made microchip holder

which, when combined with a pressure cell, enabled the genera-

tion of a tightly packed column.

When pure acetonitrile was used as the pumping fluid, the

maximum pressure reached just 45 psi (0.31 MPa) at an applied

electric field strength of 200 V/cm. Increasing the electric field

strength to 875 V/cm gave a maximum pressure of 204 psi

(1.4 MPa). The low maximum pressure measured with acetoni-

trile as the pumping fluid may also be attributed to the effect of

the double layer overlap, since the term with Bessel functions of

the first kind in Eq. [3] is no longer negligible if the ratio of the

effective pore radius and the double layer thickness is a/l # 1.

Flow rate determination under a downstream backpressure

While determining the maximal flow rate under zero back pres-

sure conditions is important for pump characterization and for

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some fluidic applications, reasonable flow rate generation under

downstream back pressure conditions is a critical parameter

when these EOF pumps are integrated with packed bed

stationary phases such as found in m-HPLC, for example. A

packed bed stationary phase can generally be considered to act as

a back pressure or downstream load applied to the EOF pump.

In this study, we determined the net flow rates generated by the

microchip EOF pump when a back pressure was applied against

the EOF forward flow by an argon gas tank attached to the field-

free outlet reservoir via a pressure cell (see Fig. 2). As expected

from Eq. [5], linear relationships were observed in all cases

between flow rate and back pressure (Fig. 4), which indicates

minimal Joule heating generation due to both the low current in

the packed microchannel and efficient heat dispersion within the

glass microchip. The most remarkable result was obtained with

aqueous CHES buffer as the pumping fluid. At an applied elec-

tric field strength of 250 V/cm and a back pressure of 800 psi

(5.5MPa), a flow rate as high as 143 nL/min was still maintained.

When the fluids being pumped were aqueous buffer–organic

solvent mixtures, the flow rate under each applied back pressure

was much lower than that of the aqueous CHES buffer. These

lower flow rates can be attributed to the lower electroosmotic

mobilities of the mixtures, with CHES buffer–acetonitrile

mixture having higher flow rates than that of the CHES buffer–

methanol mixture (see Table 1). As the back pressure increased,

the flow rate of the CHES–acetonitrile mixture decreased more

drastically than the CHES buffer–methanol mixture. We can

attribute this result to the lower viscosity of the CHES

buffer–acetonitrile mixture (� 0.85) when compared to the

CHES–methanol mixture (� 1.4),27,28 suggesting a higher pres-

sure-driven flow rate in a direction opposite that of EOF (see

the term in Eq. [2] with jDPAa2=8hs). As the back pressure is

Fig. 4 Flow rate, Q, versus the applied backpressure, DP under an

applied electric field strength of 250 V/cm: open square¼ aqueous CHES

buffer (10 mM, pH 9.0); closed circle ¼ CHES buffer–acetonitrile

mixture (50% v/v); open triangle¼ CHES buffer–methanol mixture (50%

v/v). The standard deviation error bars are displayed as determined from

three measurements.

This journal is ª The Royal Society of Chemistry 2009

further increased, the flow rate for the CHES–acetonitrile

mixture eventually dropped below that of the CHES–methanol

mixture (at 200 psi (1.38 MPa)). Since all of the pumping fluids

used in this study had very low ionic strengths, higher electric

field strengths can be applied to meet higher flow rate needs, e.g.,

m-HPLC.

Efficiency of microchip EOF pump

The efficiency, hef, of a pump is defined as the ratio of the

hydraulic power generated, QDP, to the electrical power, IDV,

consumed:

hef ¼QDP

IDV(6)

In practice, thermodynamic efficiency11 can be calculated from

measured values as

hef ¼12Qmax

12DPmax

IDV¼ 1

4

QmaxDPmax

IDV(7)

The efficiency of our high pressure, microchip EOF pump

ranged from 0.1% to 0.5%, which is in the medium to low range

when compared to other reported EOF pumps. It can be further

increased by reducing the electrolyte concentration in the

pumping fluid. Previous research demonstrated that an electro-

lyte concentration as low as 1 mM is sufficient to maintain stable

EOF pump operation.31

Conclusions

We have successfully microfabricated a fritless microchip EOF

pump within a soda lime substrate whose high pressure

compatibility is expected to meet the demanding needs of future

micro-HPLC applications. The pump region was constructed by

tightly packing 800 nm silica particles against a weir structure

fabricated within the microchannel, taking advantage of a home-

made microchip holder which provides high pressure sealing to

pack the microcolumn with high density and homogeneity.

Electric current measurements and visual inspections made

continually during the pressure and flow rate measurements

suggested that the column remained intact even at a pressures >

1000 psi (� 7 MPa). Observation of a linear relationship between

the current through the pump’s packed bed and the electric field

strength applied indicate that there was minimal Joule heating

generated during the pump operation.

The flow rates generated by this microchip EOF pump in the

presence of high back pressure meet the anticipated requirements

of a HPLC pump on a chip. In future work, we will fabricate

a microchip device which integrates an EOF pump with a LC

column packed with C8–C18 beads as the stationary phase. It is

noteworthy, however, that the back pressure applied in this study

from a gas tank is not a perfect representation of the load that

results from an LC column connected to the outlet of the

microchip EOF pump, for example. In that case, the back

pressure will ultimately be determined by physical properties,

such as the column length, the particle size and porosity of the

stationary phase, as well as by the chemical properties of the

stationary phase, such as hydrophilicity and/or hydrophobicity.

If a pumping fluid is also used as the mobile phase for

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a downstream LC separation, careful planning is needed to

choose a suitable pumping fluid which not only delivers a high

flow rate at a high back pressure, but also is compatible with

the stationary phase and the analytes to be separated by the

stationary phase. The requirement that the mobile phase must

be able to sustain a reasonable EOF to initiate pumping, places

certain limitations on the mobile phase with regards to ionic

strength, pH, aqueous/organic composition and specific organic

solvent type.

Acknowledgements

The authors would like to thank the Office of Naval Research for

funding support of this effort through the Naval Research

Laboratory (NRL).

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This journal is ª The Royal Society of Chemistry 2009