Trends in Analytical Chemistry, Vol. 27, No.

4, 2008 Trends

Microfabricated chemical
preconcentrators for gas-phase
microanalytical detection systems
Ioana Voiculescu, Mona Zaghloul, Nachchinarkkinian Narasimhan

The gas or vapor preconcentrator is an analytical device that significantly improves the detection limit of a microanalytical
system by preconcentrating the analyte. The preconcentrator performs front-end sampling and preconcentration of analyte by
collecting and concentrating analyte over a period of time. After the analyte-collection phase is complete, a heat pulse releases
the analyte as a concentrated wave into the detector. Desirable features of the preconcentrator device include the capability of
operating at high flow rates, thermal heating with short-time constants, and selective collection of the analyte(s) of interest. The
preconcentrators presented in this review are used as a generic front-end modification to gas-phase microanalytical detection
systems, such as gas chromatographs, mass spectrometers, ion-mobility spectrometers, and microelectromechanical system
(MEMS)-based chemical sensors. The advantages of the detector in incorporating a preconcentrator device are enhanced sens-
itivity and improved selectivity. Target analytes concentrated by the preconcentrators described in this review include various
organic compounds in gas or vapor phase, such as explosives 2,4,6-trinitrotouluene (TNT) and 1,3,5 trinitro-1,3,5-triazine (RDX),
chemical agent dimethyl methylphosphonate (DMMP), a broad range of organic vapors, such as toluene, benzene, ethylene and
acetone, and mixtures of these gas-phase organic compounds. We discuss examples of the current trends in microfabricated
preconcentrator technology as well as several applications of microfabricated preconcentrators.
Published by Elsevier Ltd.

Keywords: Chemical preconcentrator; Gas chromatograph; Gas sensor; Ion-mobility spectrometer; Microanalytical detection; Microelectro-
mechanical systems (MEMS)

1. Introduction situations when analyte detection is per-

Ioana Voiculescu*,
formed over a period of time. Desirable
Mona Zaghloul,
Nachchinarkkinian Narasimhan Trace detection of analytes from minute features of the preconcentrator device in-
Mechanical Engineering quantities of vapors is challenging for any clude operation at high flow rates, thermal
Department, City College of City analytical system. A preconcentrator heating with short time constants, and
University of New York, incorporated at the front end of an ana- selective sampling of the analyte(s) of
NY 10031, USA
lytical system can enhance the overall interest.
performance of the detector. The gas or Conventional preconcentrators, so-
vapor preconcentrator can be used with a called microtraps, comprise a stainless-
variety of analytical systems, including steel tube or glass-capillary tube packed
gas chromatography (GC) [1–4], mass with one or more granular absorbent
spectrometry (MS) [5], ion-mobility spec- materials [8–15]. For desorption, current
trometry (IMS) [6], and microelectro- is passed through the stainless-steel tube
mechanical system (MEMS)-based or through a metal wire coiled around the
chemical sensors [7,8]. glass-capillary tube. These devices are
The preconcentrator device serves the characterized by large dead volumes and
important function of concentrating and limited heating efficiency due to their
purifying a chemical sample on a sorptive larger thermal mass, which subsequently
material at the inlet of an analytical contributes to the delivery of a broad time-
system. Concentrating analyte in a stage width pulse of vapor. Microfabricated
*
prior to the detection system can improve preconcentrator devices overcome these
Corresponding author.
Tel.: +1 (212) 650 5210;
detection limits for the analyte of interest. limitations through significant reduction
E-mail: The preconcentrator device requires time of dead volume and thermal mass by
voicules@ccny.cuny.edu for analyte collection and is used in delivering a concentrated sample that has

0165-9936/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.trac.2008.01.016 327

Trends Trends in Analytical Chemistry, Vol. 27, No. 4, 2008

a narrow time-width pulse for analysis (as illustrated in large collection area. To heat a large amount of sorbent
Fig. 1 [16]). material, the hotplate structure is designed in three
The preconcentrator device is an important subsystem dimensions and includes trenches that contain a large
of a trace detector that substantially improves detector amount of adsorbent granules. Some preconcentrators
sensitivity and selectivity. The microscale dimensions of have the heated surface packed with up to three different
the device allow integration with microfabricated elec- adsorbent materials to enhance the efficiency of collec-
trical, optical and mechanical components of a micro- tion and concentration of various organic compounds.
analytical system. Generally, adsorbents with large surface area are used to
trap compounds with high volatility. They are usually
placed downstream in the sampling gas-flow path, so
2. Operating principle of preconcentrators that the low-volatility compounds will not be trapped in
these adsorbents. The most significant figure of merit for
In essence, the preconcentrator is a microfabricated a preconcentrator is the concentration factor (CF), which
hotplate. A sorptive material layer is deposited on the is the ratio of the concentration of the analyte in the
active area adjacent to the heating element. The sorptive sample delivered to the detector to the concentration
layer selectively sorbs one or more chemical species of originally present in the inlet airflow.
interest over a time period, thereby concentrating the The preconcentrator devices presented in this review
chemical compound in the sorptive material. The sorp- are classified in two main groups: for target analyte
tive layer is subsequently heated and the thermally re- detection; and, for detection of complex organic mix-
leased analyte provides narrow desorption peaks with tures. In this article, we discuss design, fabrication and
relatively high concentration. This process allows the applications of several MEMS preconcentrators for gas-
analyte molecules present in a large air volume to be phase applications.
purified and concentrated, so increasing the efficiency of
detection.
Preconcentrator design and performance are influ- 3. Improving the detection of a specific analyte
enced by the specific application for which the device
was conceived. The preconcentrators used for detection Some microfabricated preconcentrator devices are used
of a specific analyte (e.g., an explosive or a chemical- for selective detection of a specific target analyte (e.g., an
warfare agent (CWA)) are designed with a suspended explosive or a CWA). These preconcentrators are incor-
hotplate membrane coated with a thin sorbent layer that porated into analytical systems used for explosive and
selectively sorbs the analyte of interest. When the ana- CWA detection. The range of analytes that could be
lyte of interest is combined with a background of inter- detected has been expanded over the years to include
fering compounds, the sorbent layer will collect only the pharmaceutical solvents, petrochemicals and toxic
analyte and will allow the interferents to pass. industrial chemicals. This type of preconcentrator com-
The preconcentrators used for the detection of com- prises a planar hotplate covered with a selective coating,
plex organic mixtures are conceived with a large sorbent generally a polymer. The target analyte is desorbed by
capacity using granular sorbent material that provides a applying a rapid heat pulse on the heater. The disad-
vantage of this type of preconcentrator is the limited area
Micromachined
of the hotplate that restricts the collection capability of
the device.
(MEMS)
preconcentrators 3.1. Planar micropreconcentrator fabricated from
silicon
Detector response

Sandia National Laboratories developed a planar

microfabricated preconcentrator that is incorporated in
Conventional the lChemLab detection system, which is a hand-held
preconcentrators device used for rapid trace detection of target analytes
[17,18]. The lChemLab is formed by three microfabri-
cated components: a deep-etched silicon (Si)-based GC
separation column; a chemically selective surface
acoustic wave (SAW) array sensor; and, the precon-
centrator [17–22].
Time (ms) The heated area of this preconcentrator is fabricated
from a free-standing silicon-nitride (Si2N3) membrane
Figure 1. Comparison of the analyte concentration and time
with dimensions 2.2 mm · 2.2 mm and thickness
constant in micromachined and conventional preconcentrators.
0.5 lm (see Figs. 2 and 3) [19–22]. The Si2N3

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Trends in Analytical Chemistry, Vol. 27, No. 4, 2008 Trends

membrane is suspended over a cavity fabricated in the Si [19]. The sample was collected by passing 5 ppm of
substrate using Bosch deep-reactive ion etching (DRIE) DMMP over the preconcentrator for 1 min. Desorption
technique or anisotropic potassium-hydroxide (KOH) was achieved by heating the micro-hotplate to 200C by
etching [19,21]. In this way, the Si2N3 membrane con- applying a square-wave voltage pulse for 10 ms. The
taining the embedded heater is thermally isolated from concentration factor for the flow of 3ml/min was 510.
the rest of the wafer and could serve as a micro-hotplate In this experimental test, background interferents were
for the preconcentrator. The heater is fabricated from a also used. Data for the concentration of analyte DMMP
thin layer of platinum (Pt) and achieves 200C in 4 ms and two interferents, xylene and methyl ethyl ketone
with an electrical power of 100 mW. The micro-hotplate (MEK), are given in Table 1 [19].
was spray coated with the microporous hydrophobic In summary, the rapid thermal desorption of the
surfactant-templated sol-gel layer [19]. This coating planar preconcentrator provides a narrow injection
provided selectivity in adsorption and allowed the pre- pulse in the GC channel. The small surface area of this
concentrator to collect and concentrate compounds of device limits the analyte collection and consequently the
interest even over a background with numerous inter- concentration factor.
fering compounds. The analyte flow was directed parallel
to the device. 3.2. Planar micropreconcentrator fabricated from
The planar micropreconcentrator was integrated in polyimide
the lChemLab and tested with CWA soman (also known Two types of preconcentrator devices based on the same
as GD) [22]. Soman was analyzed in a clean background principle, but fabricated in different microfabrication
and later in a background of diesel fumes. From Fig. 4, it technologies, were developed at the Naval Research
is explicit that the diesel fumes had no influence, pri- Laboratory (NRL), Washington, DC, USA, in collabora-
marily due to inefficient collection of the non-polar tion with the University of Louisville (U of L) and George
constituents of diesel on the sol-gel adsorbent. Likewise, Washington University (GWU). These devices were
humidity was rejected because of the adsorbentÕs affinity conceived to enhance the detection threshold of a nar-
for phosphonates over water. row class of analytes (namely organophosphates and
The planar preconcentrator was also tested with di- nitro-aromatic compounds). These preconcentrators are
methyl methylphosphonate (DMMP) using a flame-ion- based on a perforated planar hotplate coated with a
ization detector in a commercial, bench-top GC system polymer layer with the air flow directed perpendicular to
the device [23]. The preconcentrator fabricated at NRL
in collaboration with U of L is based on a perforated
Analyte Pt heater
polyimide hotplate, while that developed at NRL in col-
absorber layer laboration with GWU has the hotplate fabricated from
SiN Membrane
standard layers characteristic of complementary metal-
oxide semiconductor (CMOS) technology.
These two preconcentrators have the common char-
DRIE cavity
acteristic that the airflow is directed perpendicular to the
Si Substrate hotplate. The flow-through hotplate design facilitates
Figure 2. Cross-section of a planar preconcentrator. Reprinted with relatively large airflow through the device while pro-
permission from Ref. [20]. Copyright 1999 IEEE. viding intimate contact of the air with the sorbent layers,
so improving analyte collection. When the analyte vapor
flows parallel with the preconcentrator device, much of
the analyte vapor avoids contact with the sorbent layer
in its flow over the device.
The preconcentrator conceived at NRL in collabora-
tion with U of L is illustrated in Fig. 5 [24–27]. The
perforated polyimide hotplate membrane is 6 lm thick
and suspended over an Si frame. The array of rectan-
gular perforations (375 lm · 125 lm), in the polyimide
membrane (Fig. 5) allows the airflow to strike normal to
the sorbent layer [24–27]. A dual serpentine platinum
heater is embedded on the perforated membrane.
Desorption is achieved by applying an electrical power of
175 mW that will increase the heater temperature to
Figure 3. Potassium-hydroxide (KOH)-etched micro-hotplate pla- 120C. The microfabricated hotplate has an active
nar preconcentrator. Reprinted with permission from Ref. [22].
area of 6.65 mm · 6.65 mm that is significantly larger
Copyright 2006 IEEE.
than that of micropreconcentrator devices discussed in

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Trends Trends in Analytical Chemistry, Vol. 27, No. 4, 2008

Figure 4. Selectivity of the Sandia planar preconcentrator against chemical-warfare agent (CWA) soman (also known as GD). The desorption
peak of the preconcentrator, as viewed by a downstream surface acoustic wave (SAW), shows no collection of GD. Reprinted with permission
from Ref. [22]. Copyright 2006 IEEE.

previous sections. The large active area of the precon-

Table 1. Analysis of interferents by the Sandia planar preconcen-
centrator hotplate enhances the collection capabilities of
trator, with DMMP collection in 1 min
this device. The preconcentrator device was coated with
Compound Concentration factor polymer using an inkjet dispenser [28,29]. The sorbent
DMMP 510 polymer is a hyperbranched carbosilane with hexafluo-
Xylene 8 roisopropanol functional groups (designated HC), previ-
MEK 18 ously demonstrated as a useful sorbent material for
Reprinted with permission from Ref. [19].
hydrogen-bond basic analytes, such as organophos-
phates and nitro-aromatics [28].

8.5 mm Perforations
100μm x 375μm

Polyimide
6mm membrane

Platinum heater
9.5 mm

Figure 5. Micrograph of a 7 mm · 7 mm prototype preconcentrator with 35-lm wide heaters. The exploded area shows the device structure
more clearly. Reprinted with permission from Ref. [24]. Copyright 2007 Elsevier.

330 http://www.elsevier.com/locate/trac
Trends in Analytical Chemistry, Vol. 27, No. 4, 2008 Trends

This preconcentrator was developed to be used as the Tracer II, and was also tested with TNT [26]. Subsequent
front end of a portable IMS for the detection of trace- desorption produced at least an order of magnitude
explosive or CWA vapors [30]. Performance of the improvement in analyte concentration (as shown in
preconcentrator device was tested by interfacing the Fig. 8). The preconcentrator achieved desorption tem-
device to a lightweight chemical detector (LCD 3.1) perature of 120C at  30 V in less than 120 ms while
which is an IMS system (see Fig. 6) [25]. The precon- consuming a total power of 175 mW in DC-drive mode.
centrator was mounted on a printed circuit board and The large active area of this preconcentrator hotplate
integrated with the LCD 3.1 using a simple custom-made contributed to a large concentration factor. The device
Teflon chuck. The preconcentrator was tested with efficacy can be maximized by using multiple precon-
DMMP at various concentrations and explosives 2,4,6- centrator structures. The vapor-signal enhancement
trinitrotouluene (TNT) and 1,3,5-trinitro-1,3,5-triazine could be at least two orders of magnitude.
(RDX). Sensitivity enhancement for DMMP varied from
about one to two orders of magnitude for collection 3.3. Planar micropreconcentrator fabricated in CMOS
cycles of 10–120 s, using existing flow rates provided by technology
the LCD 3.1 pneumatics (see Fig. 7) [25]. The micropreconcentrator developed at NRL in collabo-
The preconcentrator device was connected to the front ration with GWU has the same operating principle as the
end of a commercial IMS detector for explosives, Vapor device fabricated from a polyimide hotplate. This precon-
centrator was fabricated in CMOS technology and has the
advantages characteristic of this technology: low cost;
and, low operating electric power. However, the reduced
dimensions of this preconcentrator led to a low precon-
centration factor. This device was also developed to en-
hance the detection threshold of a narrow class of analytes
and works in combination with IMS detectors [30].
This preconcentrator is formed by arrays of perforated
LCD3.1 micro-hotplate structures that allow the air to flow
through the device [16,31]. The preconcentrator was
fabricated in two-poly, two-metal standard CMOS AMIS
1.5 lm process through the metal-oxide-semiconductor
Teflon interface Preconcentrator device implementation service (MOSIS) [16,31]. Si was used for
chuck mounted to a printed circuit
the substrate, Si dioxide for the hotplate membrane,
board
polysilicon for the heater and aluminium for the thermal
Figure 6. A single preconcentrator device is integrated with LCD distribution plate. The heater was embedded in the sus-
3.1 using a Teflon interface chuck. Reprinted with permission from pended membrane. The preconcentrator chip was wet
Ref. [25]. etched using tetramethylammonium hydroxide (TMAH)
to release the hotplate membrane, followed by back
etching the substrate using the DRIE process.

Figure 7. LCD 3.1 reactive ion peak response after DMMP

desorption of the preconcentrator. The collection time is 100 s Figure 8. Preconcentrator testing using Vapor Tracer II IMS. RIP is
and the flow speed 0.05 mg/m3. The vapor-signal enhancement a Reactive Ion Peak used to transfer charge to the analyte before it
for the LCD 3.1 is two orders of magnitude. Reprinted with permis- is accelerated in the drift tube. Reprinted with permission from Ref.
sion from Ref. [25]. [26].

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Trends Trends in Analytical Chemistry, Vol. 27, No. 4, 2008

The hotplates were coated with a sorbent polymer, a

hyperbranched carbosilane synthesized at NRL with
hexafluoroisopropanol functional groups (designated
HC) [28,29]. A thermal desorption cycle was applied to
heat the preconcentrator to 180C in 40 ms to release a
concentrated wave of analyte. In this research, the
micro-hotplate was designed as a bridging structure in the
shape of a parallelogram, which was selected to ensure
that the anisotropic etchant undercut the entire micro- Preconcentrator
hotplate structure during the release process. The indi- Teflon
vidual micro-hotplates or microbridges, 180 lm wide interface chuck
and 300 lm long, were placed 40 lm apart. The chip
dimensions were 4 mm · 4 mm and the heated-area
dimensions were 3.4 mm · 3.4 mm. The preconcentra-
tor chip was mounted on an LDCC 100-pin package. A
circular hole was drilled in the middle of the package to LCD 3.1
allow the air to flow through the preconcentrator device
and the package. Fig. 9 shows a micrograph of the entire
chip bonded to the package.
This preconcentrator device was tested with CWA
DMMP and explosive TNT [31]. The preconcentrator
device was attached to the inlet of an LCD 3.1, a hand-
held IMS detector, during the tests with DMMP. The aim
of the experiments was to collect DMMP vapors on the
preconcentrator and to observe changes in the relative
intensities of the reactant ions and DMMP product ions
when the preconcentrator was desorbed. An interface
Figure 10. (a) Top view and (b) side view of the preconcentrator
chuck made of Teflon was used to connect the precon-
device integrated with LCD 3.1. Reprinted with permission from
centrator to the LCD 3.1, (see Fig. 10). The DMMP va- Ref. [31]. Copyright 2006 IEEE.
pors were collected for 60 s and desorption was achieved
by applying a voltage pulse of 12 V. After desorption, the
DMMP signal quickly returned to the initial value, which orbed after one heating cycle. The concentration factor
showed that the laden preconcentrator is largely des- was 6 (as shown in Fig. 11).
In order to test the preconcentrator with explosive TNT,
the device was integrated to the inlet of a Vapor Tracer II,
Preconcentrator chip a commercial hand-held trace-explosive and narcotics
IMS detector designed for particle- and vapor-sampling
and analysis. The preconcentrator was mounted on the
front end of the Vapor Tracer II and was evaluated by
exposing it to a TNT-vapor stream [16,31]. The experi-
ments involved varying the TNT-vapor collection time
and observing changes in the relative intensities of the
reactant ions and TNT product ions when the precon-
centrator was desorbed. In one of the experimental tests,
the preconcentrator collected TNT vapors at a constant,
but non-calibrated, concentration of 0.0002 mg/m3 for
20 min. After the collection phase, the preconcentrator
chip with the collected TNT was mounted on the front end
of the Vapor Tracer II detector and was then desorbed.
Desorption was induced by applying voltage of 12 V to the
preconcentrator chip. The concentration factor was 3 (see
LDCC 100 package Fig. 12). CMOS technology limits the device dimensions
and the preconcentration factor is low, compared to other
Figure 9. Preconcentrator chip mounted on the LDCC 100 devices discussed in this review.
package. Reprinted with permission from Ref. [31]. Copyright
In summary, the preconcentrators used for detection
2006 IEEE.
of a specific analyte are based on a planar hotplate

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Trends in Analytical Chemistry, Vol. 27, No. 4, 2008 Trends

1500

Preconcentrator
enhanced
DMMP signal
IMS signal counts

1000

Initial
DMMP
500
signal

Figure 13. 3D preconcentrator with flow perpendicular to the sub-

strate surface. Front and rear sides of the die show an adsorbent
0
600 650 700 support of Si cylinders, all suspended by an SiN membrane. The
central part of the SiN membrane was opened, using DRIE etching,
Time (ms) to allow flow through the device. Reprinted with permission from
Ref. [22]. Copyright 2006 IEEE.
Figure 11. DMMP-concentration enhancement, after collecting
DMMP for 60 sec and then desorbing at 12 V. Reprinted with per-
mission from Ref. [31]. Copyright 2006 IEEE.
on thick hotplates that contain deep trenches that can
hold a large quantity of absorbent material. The three-
dimensional (3D) hotplate could provide large adsorbent
Initial concen- Enhanced concen- capacity and a large heating surface capable of uniform
tration of TNT tration of TNT
heating of the adsorbent materials. The microheaters in
1200
this case are thick (> 500 lm) surrounded by air gaps
and provide large surface area and good thermal isola-
tion, which are important for a high-sensitivity, lower-
Vapor Tracer signal

800 power microGC system. The adsorbent layer comprises

granular material of different sizes that enable this type
of preconcentrator to trap complex organic compounds
with a broad range of volatility. In order to incorporate
400
two or three different adsorbent materials, a multi-stage
preconcentrator was developed. In this way, the vapors
that are less volatile are trapped on the absorbent
material with smaller surface area. These preconcen-
0 1 2 3 trators are characterized by higher thermal transfer
Time (s)
efficiency because of their larger contact area between
Figure 12. TNT spectra recorded after collecting TNT for 20 min the heaters and the sorbent materials, leading to very
and then desorbing at 12 V. Reprinted with permission from Ref. high preconcentration factors at low power. Their
[31]. Copyright 2006 IEEE. external dimensions are larger compared to the pre-
concentrators developed for collection of a specific ana-
lyte (Section 3). These types of preconcentrator device
covered with a thin layer of sorbent material. The sor- are used to enhance GC detection.
bent coating can be tailored to specific analytes, yielding
high values of concentration factor and selectivity over 4.1. 3D micropreconcentrator
interfering compounds. High selectivity permits low-level Based on the experience achieved with the planar hotplate
detection of analytes in real-world environments where preconcentrator (see Fig. 3), Sandia developed a 3D device
interferent concentrations could be much greater than that was conceived with an increased collection area,
that of the analytes. The devices with larger active areas while minimizing power consumption. This preconcen-
yield higher preconcentration factors. trator offers a 3D adsorbent support with a larger area in
comparison to the planar counterpart and it also saturates
after a longer time. This allows lower concentrations of
4. Improving the detection of complex organic analyte to be collected, and/or allows expansion of the
mixtures analyte set to compounds with very low volatility that are
not easily collected with the planar preconcentrator. The
The preconcentrator devices used for analytical systems analyte flow could be oriented parallel or perpendicular to
that detect complex organic mixtures are generally based the 3D preconcentrator-device surface.

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Trends Trends in Analytical Chemistry, Vol. 27, No. 4, 2008

The 3D preconcentrators are shown in Figs. 13 and Comparison of the planar preconcentrator (Fig. 3), 3D
14 [22]. The preconcentrator illustrated in Fig. 13 is preconcentrators (Figs. 13 and 14) and a customized,
designed for the airflow directed perpendicular to the mesoscale, desorption tube (0.3 cm diameter by 5 cm
device. For the device illustrated in Fig. 14, the airflow is long) for typical toxic industrial chemicals (TICs) is
oriented parallel to the device. Both 3D preconcentrators illustrated in Fig. 16. The parallel-flow 3D preconcen-
are fabricated from an Si wafer using the Bosch DRIE trator demonstrates a sharper, narrower desorption peak
etching process. After the etching process, the Si with desorption pulse widths of 2.3 s (full width at half
structures remain suspended on the central portion of maximum) compared to 3.1 s for the thermal desorption
the membrane. tube. Despite improved concentration capabilities, the
The planar (see Fig. 3) and 3D preconcentrators were parallel-flow 3D preconcentrator requires 0.6 W to
tested with DMMP [22]. Nanoporous carbon, sol gels, achieve a desorption temperature of 200C, while the 3D
and commercial packing materials were used as sorbent preconcentrator with perpendicular flow uses 150 mW
materials. Fig. 15 compares desorption peaks of a planar to achieve the same desorption temperature. The planar
preconcentrator and a perpendicular-flow 3D precon- device (Fig. 3) consumes only 100 mW under the same
centrator, both tested with DMMP. The planar precon- test conditions. The preconcentrator with air flow di-
centrator device loads more rapidly, but, due to the thin rected normal to the device has a smaller sorptive area,
sorbent layer, saturates after 2-min collection time, since the device is perforated by gaps designed for the air
while the 3D preconcentrator having a large adsorbent flow, so the preconcentration factor is lower. The planar
area saturates after about 20 min. preconcentrator (Fig. 3) also has a low preconcentration
factor due to the small sorbent capacity of the thin layer
of sorbent material. However, the parallel-flow 3D pre-
concentrator shows a better performance than the pla-
nar design, because the active area available for sorbent
materials has increased by a factor of 20 and the high
thermal conductivity of the 3D Si structures makes the
temperature of the active area more uniform during the
desorption phase.

4.2. Microfabricated preconcentrator based on a

hotplate
A microfabricated preconcentrator focuser, designed to
collect and concentrate complex organic vapor mixtures,
was developed and tested at The University of Michigan.
Figure 14. 3D preconcentrator with flow parallel to the substrate This preconcentrator was conceived for a microGC system.
surface. A fin-like Si adsorbent support is suspended by the SiN The preconcentrator is based on a micro-hotplate with
membrane. This structure is packed with granular sorbent material. high-aspect-ratio heating elements that provide a large
Reprinted with permission from Ref. [22]. Copyright 2006 IEEE.

Figure 16. Comparison of the desorption response of various pre-

concentrator devices with Tenax TA. Device response with toxic
industrial chemical (TIC) analytes after collecting for 300 s. The
parallel-flow micropreconcentrator had better power performance
Figure 15. Desorption peaks from planar and perpendicular flow
than the conventional desorption tube; the former took 0.6 W,
3D preconcentrators, as seen by a downstream flame ionization
and the latter 3 W. Reprinted with permission from Ref. [22].
detector (FID). Reprinted with permission from Ref. [22]. Copyright
Copyright 2006 IEEE.
2006 IEEE.

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Trends in Analytical Chemistry, Vol. 27, No. 4, 2008 Trends

adsorbent capacity and a large heating surface leading to enable a small quantity of spherical granules of the
a high preconcentration factor. The active collection adsorbent material to fit in this space. Repeated etch and
area is packed with granular adsorbent and the airflow is passivation cycles are used to produce thick, thermally-
directed parallel to the device. First, a preconcentrator isolated heating elements that are fabricated in Si. To
device with a single stage of preconcentration and using provide good thermal isolation, a 500-lm wide air gap is
one type of granular adsorbent material was designed. designed around the array of microheater elements that
To further enhance the performance obtained with the also helps to achieve low-power consumption. The
single-stage preconcentrator, a three-stage preconcen- granular adsorbent material is made of coarse spherical
trator using three different adsorbents was conceived. granules (200 lm) of Carbopack X, which is a graphi-
Each type of granular material used as analyte sorbent tized carbon. The microheater structure packed with
has its specific surface area, pore morphology, and pore- Carbopack X is sandwiched between two Pyrex glass
size distribution that enable the three-stage preconcen- plates, which contain inlet/outlet ports for sample flow.
trator to handle organic compounds spanning a wide The Carbopack X material is suitable for capturing (and
range of vapor pressures. The three-stage preconcen- releasing) compounds with vapor pressures in the range
trator was also developed for a microGC system that can 29–95 torr and shows no sign of degradation even after
perform the analysis of volatile organic compounds with 200 heating cycles [32,33]. The service life of the
concentrations of parts per billion (ppb) in indoor air. adsorbent is therefore not expected to be a limiting factor
in the performance of the preconcentrator. The signifi-
4.2.1. Single-stage preconcentrator. The single-stage cantly large microheater surface area enables high
preconcentrator (Fig. 17) comprises a micro-hotplate adsorption capacity and efficient uniform thermal
formed by an array of heating elements of 520-lm desorption of vapors captured in the adsorbent material
height, 50-lm width and 3000-lm length [32]. The within the structure.
distance between each heating element is 220 lm to For testing, the single-stage preconcentrator was
connected directly to the flame ionization detector (FID).
The device was individually tested with the solvents
benzene, toluene, m-xylene and a-pinene (which collec-
tively spanned a 19-fold range of vapor pressure and had
the concentration of 25 ppb/L) [32]. For a stop-flow time
of 25 s and at a power of 2.25 W, the preconcentration
factor was 5600 (as illustrated in Fig. 18). Benzene,
which has the highest vapor pressure, yielded the

Figure 17. (a) Preconcentrator focuser microheater (without top

cover) with heating elements [520 lm (h) · 50 lm (w) · 3 mm (l)
separated by 220-lm gaps] and a 500-lm peripheral thermal-
isolation air gap, anodically bonded to a 520-lm thick Pyrex base Figure 18. Dependence of the peak width at half height (PWHH)
plate; and, (b) enlarged view of the interconnection on dielectric and preconcentration factor on stop-flow period, showing a pre-
membrane (0.5-lm-thick poly-Si on 0.6/0.1/0.6-lm-thick concentration factor for benzene of 5600 and PWHH of 0.8 s for
oxide/nitride/oxide) spanning the 500-lm air gap. Reprinted with a 25-s stop-flow period. Reprinted with permission from Ref.
permission from Ref. [32]. Copyright 2003 IEEE. [32]. Copyright 2003 IEEE.

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sharpest peak, and a-pinene, which has the lowest vapor centrator extended the vapor pressure in the range
pressure, yielded the broadest peak (as illustrated in 0.01–231 torr [33,34].
Fig. 19). Less volatile components, such as a-pinene, The microheater elements for the three-stage precon-
desorb more slowly, yielding a much lower and broader centrator were divided into three stages, each loaded
peak profile. At the desorption temperature of 250C, the with three different granular materials (Carbopack B,
peak width at half height (PWHH) increased roughly Carbopack X, and Carboxen 1000). These adsorbents
four-fold over this 19-fold range of compound vapor have different surface areas (100 for Carbopack B, 250
pressures. Although further increases in the maximum for Carbopack X, and 1200 for Carboxen 1000) and are
temperature will tend to sharpen the peaks, the trend used to trap compounds with different volatilities [34].
shown in Fig. 19 will persist. Preconcentration factors Adsorbents with a large surface area are used to trap
for these vapors were in the range 5200–1260 under compounds with high volatility and they are usually
these test conditions. The experimental results also placed downstream in the sampling-gas flow path so that
reinforced that fact that the preconcentrator operating compounds with low volatility will not be trapped in
parameters are directly proportional to the strength of these adsorbents.
absorption of vapors. The adsorbent materials used for the multi-stage pre-
concentrator were tested with a wide range of vapors.
4.2.2. Multiple-stage preconcentrator. The multiple-stage Compounds with vapor pressures in the range 0.01–29
preconcentrator was conceived to further enhance the torr were trapped by Carbopack B (100 m2/g) in the first
performance obtained with the single-stage preconcen- stage, compounds with vapor pressures in the range 29–
trator [33–39]. The multiple-stage microfabricated pre- 95 torr were trapped by Carbopack X (250 m2/g) in the
concentrator was designed to provide larger adsorbent second stage, and higher volatility compounds with va-
capacity for complex organic mixtures and uniform por pressures in the range 95–231 torr were trapped by
heating of large amounts of different types of adsorbent Carboxen 1000 (1200 m2/g) in the third stage. A
granules. The major improvement in the three-stage micrograph of the preconcentrator with each stage filled
preconcentrator was its ability to collect and to con- with the corresponding granular adsorbent material is
centrate organic compounds spanning a wide range of shown in Fig. 20.
vapor pressures. The single-stage preconcentrator used a The three-stage preconcentrator featured a decreased
single type of adsorbent, Carbopack X, which can adsorb dead volume and one-third reduction in thermal mass to
and concentrate analytes with the vapor pressure in the increase its performance. The large adsorbent mass en-
range 29–95 torr [33]. The different types of graphitized abled this three-stage preconcentrator to trace sample
carbon adsorbents used for the multiple-stage precon- from a very large incoming airflow. These significant
improvements were due to a new design of the micro-
heater structure and cover-plate integration. Both the
microheaters for the single-stage prototype and
the three-stage device utilized Joule heating. However,
the new three-stage design decreased heater mass while
maintaining smaller dead volume than the single-stage
design by using tall heating elements to conduct heat
from a 50-lm thick Si membrane. The Si microheater
loaded with three types of adsorbent carbon granules
was covered with an Si cover plate with fluidic inter-
connection to the microGC system. The microheater and
the cover plate were fabricated using DRIE to obtain
steep, vertical sidewalls and a smooth morphology.
The heating-element dimensions in this case were
380 lm high and 50 lm wide, while the dimensions of
the channels containing the adsorbents were 220 lm
wide and 3000 lm long. As shown in Fig. 20, each stage
comprised thick Si heating elements, fluid-flow channels,
and containment channels that accommodated single
Figure 19. Flame-ionization detector (FID) response profiles for a rows of adsorbent granules. The size of each stage was
range of vapor pressures, pv, of test compounds (pv = 95, 60, 10, dictated by the mass of particular adsorbent necessary
and 5 torr for benzene, toluene, m-xylene, and a-pinene, respec- for the vapor pressure range of the compounds. The first
tively) using a desorption flow rate of 2.2 cm3/min (15-s stop-flow). stage comprised 16 adsorbent containment channels
PWHH values are given in parentheses. Reprinted with permission
that were packed with 1.6 mg of Carbopack B, while the
from Ref. [32]. Copyright 2003 IEEE.
second stage had nine channels packed with 1 mg of

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Trends in Analytical Chemistry, Vol. 27, No. 4, 2008 Trends

Figure 20. Three-stage microfabricated preconcentrator focuser using a microheater (upper center) packed with three carbon adsorbents to cover
a wide range of compound volatilities. Reprinted with permission from Ref. [34]. Copyright 2005 IEEE.

Carbopack X, and the third stage had six channels

packed with 0.6 mg of Carboxen 1000.
This preconcentrator provided a sharp injection plug
with a narrow desorption peak. The narrow desorption
peak increased the separation efficiency of the GC col-
umn and the high concentration peak increased the
sensitivity of the analysis. The geometry of the multi-
stage preconcentrator allowed air to be directed through
all the parallel, fluid-flow adsorbent channels. The pre-
concentration factor for the three-stage microprecon-
centrator has not yet been determined due to
breakthrough of high-volatility compounds resulting
from uneven packing of adsorbents in the channels [34–
36]. However, investigation of multi-stage micropre-
concentrator design is ongoing, and more uniform
adsorbent packing will avoid the breakthrough problem.
Figure 21. A chromatogram was obtained using single-stage
The preconcentrator factor is expected to be higher than
microfabricated preconcentrator-focuser, and 10 organic vapors
5600 [36]. were separated successfully. Reprinted with permission from Ref.
Both single-stage and three-stage preconcentrator [33]. Copyright 2003 IEEE.
were tested using the same testing configurations. The
preconcentrators were connected to a commercial GC
system with an FID. The micropreconcentrator adsorbed Extension of the range of volatility for the three-stage
the analytes followed by a stop-flow sequence for heating preconcentrator was demonstrated by the chromato-
the microheater and then desorption of the compounds gram showing the desorption of 30 organic vapors
into a GC instrument. The total heating time was 40 s spanning three order of magnitude in vapor pressure
with 20 s for stop-flow and 20 s for carrier-gas flow. The (see Fig. 22). The 10 compounds tested for the single-
power required to achieve 300C with carrier-gas flow stage prototype were also included in the 30 compound
was 0.8 W per stage [33]. samples. The most volatile compound desorbed was
The single-stage preconcentrator was tested with 10 acetone with a vapor pressure of 231 torr while the
different common organic vapors that were well lowest volatility compound was 3-octanone with a vapor
separated. The desorption peak widths was very small pressure of 1.3 torr. The PWHH for all of the compounds
(0.8–2 s) (as shown in Fig. 21). is less than 2.05 s at a flow rate of 2 ml/min and the

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Figure 22. A chromatogram showing successful separation of 30 organic vapors using three-stage microfabricated preconcentrator-focuser with
a conventional gas chromatography (GC) system. Reprinted with permission from Ref. [33]. Copyright 2003 IEEE.

shortest was 0.44 s for acetone. The total separation

time for 30 compounds was below 400 s (Fig. 22).
Comparing single-stage and multi-stage preconcen-
trators, it was evident that significant improvements
in heating efficiency and desorption performance were
obtained for the three-stage micropreconcentrator by
reducing the dead volume, thermal mass and pressure
drop, compared with the single–stage micropreconcen-
trator. The three-stage micropreconcentrator has effec-
tively extended the detection range of the volatile
compounds to cover up to four orders of magnitude in
volatility. This preconcentrator is incorporated in a GC
system [33–40]. Figure 23. Scanning electron microscope (SEM) image of the
anisotropic etched channel of the microconcentrator.
4.3. Micropreconcentrator based on a serpentine
microchannel
A preconcentrator used for concentrating a mixture of
organic vapors was developed at New Jersey Institute of
Technology [41]. This preconcentrator comprised a
serpentine microchannel, reactive ion etched (RIE) in an
Si wafer, and it was designed to work integrated with a
GC system. A thin layer of Al deposited on the serpentine
microchannel served as a heater. This heater could
reach 200C in less than 10 s and was protected with a
spin-on glass layer. A polymer layer that served as sor-
bent material was spin coated on the serpentine micro-
channel. The serpentine channel was covered with a
Figure 24. Cross-section of the etched channel of the microfabri-
quartz plate. The cross-sectional view of the micropre- cated microconcentrator. Reprinted with permission from Ref.
concentrator device is presented in Figs. 23 and 24. A [41]. Copyright 2006 Elsevier.
photograph of the micropreconcentrator is shown in
Fig. 25.
This micropreconcentrator was tested with a stream of pulse of AC electric current was applied on the precon-
organic vapors containing ppm levels of benzene, centrator heater at predetermined intervals to desorb the
toluene and ethyl benzene using a conventional FID. A trapped organic vapors. The signal enhancement of the

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Trends in Analytical Chemistry, Vol. 27, No. 4, 2008 Trends

Figure 27. Continuous monitoring of a stream containing ppm

Figure 25. Photograph of the microconcentrator, compared to a levels of benzene, toluene and xylene using the microconcentrator.
US penny. Reprinted with permission from Ref. [41]. Copyright Corresponding to each injection I1, I 2, I3 . . ., there is a chromato-
2006 Elsevier. gram C1 , C2 , C3 . . .. Reprinted with permission from Ref. [41].
Copyright 2006 Elsevier.

Figure 26. Continuous monitoring of a stream containing organics

using the microconcentrator. Corresponding to each injection I1, I2,
I3 . . ., there is a response C1 , C2, C3 . . .. Reprinted with permission
from Ref. [41]. Copyright 2006 Elsevier. Figure 28. PHASED chip lay-out. Integrated version shows sensors,
preconcentrator and separator. Reprinted with permission from
Ref. [45].

organic vapors by a factor of 14 is shown in Fig. 26. A security and medical diagnostics). As a result, the pre-
chromatogram was recorded corresponding to each concentrator included in this microanalyzer could collect
pulse, as illustrated in Fig. 27. and concentrate a broad range of gases.
This preconcentrator was coated with a thin film of The microanalyzer version was fabricated on an Si
polymer as sorbent material, so the preconcentration wafer and was formed by a string of 40 channels con-
factor is lower. This preconcentrator was not tested taining a large number of heaters (as illustrated in
individually and the preconcentration factor has not yet Fig. 28). Each channel was 100 lm wide and 50 cm
been determined. The preconcentrator was tested over a long [45]. Fig. 28 shows the chip layout, and Fig. 29
year of operation and the reproducibility of the retention shows a photograph of this miniaturized GC system
time and peak height was good. monolithically integrated with the micropreconcentra-
tor.
4.4. Micropreconcentrator based on a phased heater The preconcentrator channels contained typically
array 20–100 heaters that were heated in a controlled man-
Researchers at Honeywell developed a miniaturized GC ner, hence the device was named ‘‘PHASED’’ (Phased-
system containing a microfabricated preconcentrator Heater Array Structure for Enhanced Detection). The
monolithically integrated with the GC channel on the heaters were fabricated from a thin film of Pt on a thin
same Si substrate [42–45]. This microanalyzer was membrane of Si3N4. Fig. 30 shows the cross section of
conceived for gas-phase applications (e.g., industrial the preconcentrator channels. The channel was fabri-
chemical-process control, environmental monitoring, cated by DRIE of the Si substrate. An adsorptive polymer

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Trends Trends in Analytical Chemistry, Vol. 27, No. 4, 2008

Figure 31. Scanning electron microscope (SEM) micrograph of

etched PHASED channel before assembly. Reprinted with permis-
sion from Ref. [45].

Figure 29. Underside of PHASED sensor chip, with heater

elements in each of 2 · 20 etched grooves [46].
CONTROLLER

Gnd H1 H2 Hn-1 Hn TC

Preconcentrator Heaters
channel (H1-Hn)
Figure 32. Heater array (top view). The heaters are activated in a
phased manner. The TC sensor output increases with the activation
of each heater [47].

Figure 30. Cross-section of PHASED preconcentrator channel. second concentration pulse substantially overlapped the
Reprinted with permission from Ref. [45]. first concentration pulse to produce a higher concen-
tration pulse. The sample fluid carried the larger con-
centration pulse downstream toward the last heater
film was coated on the heated part of the microchannel. element of the heater array. After the nth heater was
Fig. 31 shows an SEM micrograph of the preconcentra- activated, the concentration of the analyte in the sample
tor channel. stream was 100 · n fold. The increase in the analyte
The heaters were connected to a controller that ap- concentration after the activation of each heater was
plied a voltage pulse on each of the heater elements in a monitored by thermal conductivity (TCD) sensors located
phased time sequence. Fig. 32 shows an array of heaters at the exit of the microanalyzer channel.
activated in a phased manner by a controller. The acti- The preconcentrator function is not tested separately,
vation of one heater can produce a concentration of since the preconcentrator is monolithically integrated
analyte 100-fold higher than the concentration of ana- with the GC separation channel. However, the PHASED
lyte in the gas sample. When the first heater was acti- chip could be operated as preconcentrator device only by
vated and the gas constituents were desorbed into the reversing the flow, in which a preconcentration pulse
sample stream, a first concentration pulse was produced. width of 3 ms was achieved [48]. Early on, the impor-
Then, in a phased manner, the second heater was acti- tance of synchronizing the gas flow with the rate of
vated and the gas constituents were desorbed into the advance of the heater wave was quantified [49]. The
sample stream and a second concentration pulse was preconcentrator capability to enhance the analyte con-
produced. The second heater was activated so that the centration is shown in Fig. 33. The gas sample was

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Trends in Analytical Chemistry, Vol. 27, No. 4, 2008 Trends

Figure 33. 20-stage sampling, desorption and pre-concentration of 720-ppm hexane in air with a PHASED chip, showing that the adsorption step
only takes 0.3 s or about two sample volumes to re-fill and saturate the stationary phase with hexane. Reprinted with permission from Ref. [50].

(SWNTs) were integrated into the microfabricated

channels. The carrier gas in this experiment was H2.
Hexane, octane, nonane and decane were separated in
less than 500 ms.
The PHASED multi-stage preconcentrator has a high
preconcentration factor due to the phased, synchronized
activation of a large number of heaters. The PHASED
chip and its preconcentrator are not yet available com-
mercially.

5. Conclusions

This article has presented current trends in microfabri-

cated preconcentrator technology as well as applications
of microfabricated preconcentrators for gas-phase ana-
lytical systems.
Figure 34. Experimental separation of C6–C10 alkanes on 50 cm/ The specific application for which a preconcentrator is
100 · 100-lm Si microchannels, coated on one side with single- conceived has an impact on the design and the perfor-
walled nanotubes (SWNTs) of 1lm in length, and temperature
mance of the device.
ramped from 90C to 150C. Achieved peak capacity: 16
between t0 and 4 s. Reprinted with permission from Ref. [51]. Preconcentrators used for the detection of a specific
Copyright 2006 American Chemical Society. analyte are designed with a suspended hotplate mem-
brane coated with a thin sorbent layer that selectively
sorbs the target analyte.
hexane in air deliberately set at high concentration of The preconcentrators used for the detection of com-
720 ppm. plex organic mixtures are conceived with a large sorbent
An important application of PHASED is detection and capacity using granular sorbent material that provides a
analyses of air pollutants in aircraft space (e.g., alde- large collection area. In order to heat a large amount of
hydes, butyric acid, toluene, and hexane). Fig. 34 illus- sorbent material, the hotplate structure is conceived in
trates the separation performance of PHASED-like 3D and includes trenches that can be filled with a large
microchannel with dimensions 100 lm · 100 lm amount of adsorbent granules. Some preconcentrators
square and 50 cm long. Single-wall carbon nanotubes have the heated surface packed with up to three different

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adsorbents materials to enhance the efficiency of col- tor concentrates trace amounts of target molecules and
lection and concentration of various organic com- can ignore background interferents.
pounds. Based on these considerations different There are many advantages in using preconcentrators
microfabrication techniques have been used for hotplate in microanalytical detection systems. The chemical pre-
fabrication. concentrator can be integrated with other elements of a
The micropreconcentrators used for the detection of a chemical-analysis system in a hybrid or monolithic
single analyte are based on a planar hotplate coated with fashion to provide a substantial improvement in selec-
a sorbent layer, typically a polymer. The planar mem- tivity of a particular chemical of interest and to increase
brane could be fabricated from a Si2N3layer, polyimide the sensitivity with which chemical-analysis measure-
or standard layers of CMOS technology. ments can be made. Furthermore, sorptive coating on the
The preconcentrators used for the detection of com- chemical preconcentrator can be tailored for chemical
plex organic mixtures have a 3D hotplate configuration selectivity to one or more chemical compounds of interest,
with the hotplate DRIE etched in an Si wafer. The which enables accumulation and concentration of the
adsorbent in this case is formed by granular materials desired compounds from an ambient or sample vapor over
that are able to trap complex organic compounds with a time while being relatively insensitive to other chemicals
broad range of volatility. Some micropreconcentrators that are of no interest. The accumulated chemical com-
used for the detection of complex organic mixtures and pounds of interest can be concentrated in a small area and
integrated with a GC system are based on a heated ser- subsequently released suddenly by thermal desorption to
pentine channel DRIE etched in a Si wafer. The sorbent form a sample pulse having a narrow time width and a
material in this case is a polymer deposited on the large relatively high concentration of the chemical compound,
heated area of the serpentine channel. thereby improving the detectability of each chemical
The most significant figure of merit for a preconcen- using the chemical-analysis system. The chemical pre-
trator is the concentration factor. The multiple-stage concentrator has a short time constant to allow rapid
preconcentrator (Fig. 20) has the highest concentration heating and release of the concentrated chemical com-
factor (5500 at a corresponding electric power of 0.8 W). pounds in a fraction of a second. One more advantage of
The disadvantages of this preconcentrator are the the chemical preconcentrator is that it is applicable to
microfabrication technique, which has several steps and different types of chemical analysis system, including
raises the cost of this device, and also the high operating those based on GC, MS and MEMS chemical sensors.
power. This preconcentrator has a heated area of
9 mm · 3 mm and uses an adsorbent granular material
that provides a large surface area for analyte collection,
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