Full PaperNew Fully Portable Instrument for the Versatile Determination of Cations and Anions by Capillary Electrophoresis with Contactless Conductivity Detection Pavel Kuba´nˇ,a, b Huon
Trang 1Full Paper
New Fully Portable Instrument for the Versatile Determination of Cations and Anions by Capillary Electrophoresis with Contactless Conductivity Detection
Pavel Kuba´nˇ,a, b
Huong Thi Anh Nguyen,a, c
Mirek Macka,d, e
Paul R Haddad,d
Peter C Hausera
*
a
Department of Chemistry, University of Basel, Spitalstrasse 51, 4004 Basel, Switzerland
*e-mail: peter.hauser@unibas.ch
b
Institute of Analytical Chemistry, Academy of Sciences of the Czech Republic, Veverˇ* 97, 61142 Brno, Czech Republic
c
Centre for Environmental Technology and Sustainable Development (CETASD), Hanoi University of Science, Nguyen Trai Street
334, Hanoi, Viet Nam
d
Australian Centre for Research on Separation Science, School of Chemistry, Faculty of Science, Engineering and Technology, University of Tasmania, 7001 Hobart, Tasmania, Australia
e
Department of Chemical Sciences, Dublin City University, Glasnevin, Dublin 9, Ireland
Received: April 3, 2007
Accepted: May 21, 2007
Abstract
A new portable capillary electrophoresis instrument with capacitively coupled contactless conductivity detection was
developed and optimized for the sensitive field measurements of ionic compounds in environmental samples It is
powered by batteries and the high voltage modules are capable of delivering up to 15 kV at either polarity for more
than one working day Inorganic cations and anions, including ions of heavy metals and arsenate, could be determined
with detection limits in the range from about 0.2 to 1 mM The instrument was field tested in a remote region of
Tasmania and nitrite and ammonium could be determined on-site at concentrations as low as 10 ppb in presence of
other common inorganic ions at concentrations which were 2 to 3 orders of magnitude higher.
Keywords: Capillary electrophoresis, Contactless conductivity detection, Portable instrument, Environmental
analyses, Arsenic, Heavy metals, Inorganic ions
DOI: 10.1002/elan.200703908
1 Introduction
The development of portable analytical instrumentation is
an important trend in analytical research Field analysis is
attractive as it minimizes complications with sample storage
and transport, enables fast decisions at the sampling site and
therefore reduces the overall analysis time Portable
instru-ments are often used in environmental applications for
monitoring of water quality, soil contamination and air
pollution, in process analysis, but are desirable also for
clinical applications, in forensics and in the detection of
chemical warfare agents In comparison to conventional
bench-top instruments compromises usually have to be
made in performance, mainly with regard to sensitivity and
precision
The most widely used portable instruments are based on
electrochemical sensors, for example portable pH-meters,
conductometers, ion-selective electrodes and devices for
measuring dissolved or atmospheric gases such as oxygen
However, field instruments based on more complex
ana-lytical techniques have also been described These include
photometry [1], voltammetry [2], flow-injection analysis [3]
and X-ray fluorescence [4, 5] Moreover, portable
instru-ments based on the separation methods of gas
chromatog-raphy (GC) [6], high-performance liquid-chromatogchromatog-raphy (HPLC) [7] and ion chromatography (IC) [8, 9] have also been reported
Capillary electrophoresis (CE) is well suited for portabil-ity as only a separation capillary, a high voltage (HV) power supply and small volumes of buffer solutions are needed to perform the separation High-pressure pumps as required for liquid chromatography are not necessary The most serious challenge in CE is detection and this has always been
a critical issue since its introduction in the early 1980s For a portable instrument, the most difficult task is therefore to develop a suitable detection system that can be powered from batteries, can achieve good sensitivity, and is rather universal for the range of analytes with quite different physicochemical properties that can be separated in CE The most commonly used detection modes in bench-top CE are UV/vis absorbance, laser induced fluorescence and mass spectrometry None of these is well suited for small portable instruments because of the power requirements and com-plexity The simplest detection approach in CE are the electrochemical methods, which mainly consist of electron-ics and therefore can also be easily translated into the portable format Indeed, electrochemical detection is the only detection scheme that has been implemented in
Trang 2accessible Potential gradient detection (PGD), which can
be seen as an indirect conductometric detection method, has
been used on portable instruments employing conventional
or microchip CE for the determination of DNA fragments
[13] and for alkaloids and inorganic cations [14], however,
the LODs achieved (from about 104to 105M) also do not
meet the requirements for the analysis of many samples On
the other hand, conductometry can be considered an
attractive universal detection method for CE; most
inor-ganic as well as orinor-ganic cations and anions can be
determined The capacitively coupled contactless
conduc-tivity detector (C4
D) is a highly sensitive tool in CE (see, for example, the following review articles [15 – 17]) A C4
D-cell
is constructed from two axially positioned tubular
electro-des through which the separation capillary is fitted No
alignment of the electrodes is necessary and electrode
fouling is not possible as these are not in direct contact with
the solution [18, 19]
Preliminary results obtained with an early, non-optimized
and mains powered prototype of a C4
D tested on a previous version of a portable CE instrument (which had been
designed for potentiometric and amperometric detection)
were indeed promising [12] In the current contribution, a
new and improved design of a fully portable CE instrument
with integrated contactless conductivity detection is
pre-sented and characterized, which includes a portable and
battery powered data acquisition system connected to a
laptop computer for immediate data processing and
quan-tification The C4
D was designed for low background noise operation, which ensures low LOD The complete system
was tested by field measurements in the remote Tasmanian
wilderness
2 Experimental
2.1 Chemicals, Reagents, and Methods
All chemicals were of analytical grade Stock solutions of
anions (1 mM) were prepared from their corresponding
potassium or sodium salts Stock solutions of cations (1 mM)
were prepared from their nitrate salts and stock solutions of
heavy metal cations (1 mM) were prepared from their
chloride or sulfate salts (all chemicals were purchased from
Fluka, Buchs, Switzerland or Sigma-Aldrich, Steinheim,
Germany) All multi-ion anionic and cationic standard
360 mm o.d.) (Polymicro Technologies, Phoenix, AZ, USA) was used for the separations The capillary was precondi-tioned with 1 M NaOH for 5 min, deionized water for 5 min,
1 M HCl for 5 min, deionized water for 5 min and finally with electrolyte solution for 15 min Environmental samples for field measurements were taken and analyzed in the South West Conservation Area, UNESCO World Heritage, Tasmania (Sample 1: water from Lake Pedder; Sample 2: water from Condominium Creek in the Mount Anne Range) Other samples for laboratory measurements were collected in the Ha Nam provinces and in Ha Noi City (Thanh Tri district), Vietnam (ground water) The samples were used without any pretreatment except for filtration with 0.2 mm syringe filters (Millipore); the samples from Vietnam were diluted 1 : 4 with deionized water
2.2 Apparatus 2.2.1 Portable Capillary Electrophoresis Instrument
A sketch of the instrument is given in Figure 1 The main part consists of a box with the dimensions of 310 220
260 mm which is made from poly(methylmethacrylate) (PMMA) plates Two handles are fixed to the side walls so that the instrument can easily be carried The case is divided into two parts One compartment is positioned on the rear of the PMMA box and is used to accommodate the high voltage supply and a 12 V rechargeable battery to power the instrument The battery is of the lead-acid type (NP 3.2-12, Yuasa Battery UK Ltd., Swindon, UK) and has a capacity of 3.2 Ah If the instrument is operated at a site where mains power is available then this may be utilized with an adapter
as connectors are provided on the rear These connectors are also used for recharging the battery with a suitable charger connected to the mains A socket for tying the electrical ground of the portable system to an external earth (in a mains socket when available or as a metal rod pushed into the ground for on-site measurements) is also provided An additional aluminum case (70 205 160 mm) mounted on the left side of the PMMA box contains the controls for operating the unit
The second compartment is in the front and contains on the left side a sample tray with six positions where electro-lyte and sample solutions are placed, a holder for the high voltage (HV) electrode, connectors for switching between
Trang 3positive and negative HV and a holder for the separation
capillary The sample tray is moved by lowering and turning
it manually and its height is fixed; samples are injected
manually in the electrokinetic or hydrodynamic mode The
detection part is positioned on the right and contains an
adjustable vial holder, which can be moved up and down in
order to match the height of the electrolyte solution to the
level of the solution in the sample tray The C4
D-cell is mounted on top The cell is connected to the detector
electronics via cables fed through a hole in the right wall of
the PMMA box The front lid of the box can be opened over
its entire width to facilitate manipulations inside
Standard high voltage power supplies specified for CE
have typical output voltages of 30 kV at 300 mA, a weight of
several kilogram and sizeable dimensions This component
would be the most expensive and also heaviest part of a
purpose made CE-instrument, which is also the main drain
for its battery Experience also showed us that for
measure-ments in the outdoors a voltage of 30 kV is not suitable
because under conditions of high humidity such high
voltages can lead to electrical arcing Furthermore when
using conductivity detection, low conductivity buffers are
generally employed and capillaries with diameters larger
than 50 mm are avoided to minimize Joule heating
There-fore the currents drawn from the high voltage power
supplies seldom exceed 50 mA when using this mode of
detection For this reason it was possible to make a
compromise in the specification of the high voltage supply
and the two modules (DX150 and the DX150N, EMCO,
Sutter Creek, CA, USA) chosen to provide both polarities
have maximum output voltages of 15 kV at 100 mA The
dimensions of each module are 95 76 25 mm and total
weight of both modules is 400 g so that these contribute very
little to the size and weight of the whole portable instrument
The polarity of the applied separation voltage is set by a
switch but also requires manual replugging of the high
voltage lead into the appropriate socket A circuit diagram
which details the wiring of the high voltage supplies of the portable instrument is shown in Figure 2 Note that for operator safety a microswitch to interrupt the power to the high voltage modules on opening the instrument is included
2.2.2 Battery Powered Contactless Conductivity Detector and Data Acquisition
The excitation sine wave in the new circuitry was generated with a MAX038 integrated circuit oscillator (Maxim Integrated Products Inc., Sunnyvale, CA, USA), which delivers a maximum amplitude of 2 Vpp(peak-to-peak) in a frequency range between about 100 and 1000 kHz The sine wave from this internal oscillator is multiplied from 2 Vppto
20 Vppby an operational amplifier (OPA627, Texas Instru-ments, Dallas, TX, USA) and is applied directly to the excitation electrode of the C4
D-cell The resulting current, flowing through the detection cell, is subsequently picked up
by another electrode and converted to voltage using an OPA655 (Texas Instruments, 1 MW feedback resistor) operational amplifier located in the cell itself Details on the construction of the detector cell can be found in reference [20] The signal is then rectified, amplified, low-pass filtered and offset using the electronic circuitry described in an earlier publication [21] The detection circuitry, except for the cell itself, is located in a separate shielded aluminum case It is powered by two additional
12 V lead-acid batteries (NP 1.2-12, Yuasa, capacity 1.2 Ah)
to provide a split 12 V supply
Data acquisition was carried out using an e-corder 201 data acquisition system (Product No ED201 with option ED9000 from eDAQ Pty Ltd., Denistone East, NSW, Australia, note that this is a 12 V DC-power variant of a high resolution 16 bit, low noise laboratory grade unit) A separate 12 V lead-acid battery (NP 3.2-12, Yuasa) with a capacity of 3.2 Ah was used as power supply The system was connected through a USB cable to a portable laptop computer for data acquisition with the Chart software (eDAQ) and the acquired data was further processed for quantitative analyses with the Peaks software package (eDAQ)
Fig 1 Schematic drawing of the portable capillary
electropho-resis system 1) control electronics, 2) sample tray, 3) capillary
holder, 4) vial holder, 5) detector cell.
Fig 2 Circuit diagram for the instrument.
Trang 4been reported previously [23] It was found that a
perfor-mance in terms of precision and detection limit very similar
to the other two detectors could be achieved when
evaluat-ing these on a commercial CE-instrument When carryevaluat-ing
out preliminary tests with the battery powered detector on
the new portable instrument, by elevating the injection end
of the capillary to effect hydrodynamic injection rather then
pressure driven injection as used on the commercial
instru-ment, similar results were obtained A performance
com-parable to the combination of a standard contactless
conductivity detector with a commercial benchtop
instru-ment is therefore possible with the portable battery
powered instrument Note however, that the portable
instrument is not thermostatted As several parameters
related to injection and separation as well as detection are
dependent on temperature it has to be expected that the
reproducibility for measurements performed outdoors is
not as good as for those carried out in the laboratory where
temperature fluctuations are not as pronounced On the
other hand, active cooling of capillaries is not needed when
using conductivity detection In contrast to optical
detec-tion, Joule heating can be minimized by using narrow
capillaries, as the sensitivity of conductivity detection shows
only little dependence on cell size
The separate batteries used for powering the detector and
the data acquisition system were chosen to allow an
operating time of approximately 9 h for both parts Longer
operating times could be achieved by using batteries with
higher capacity, however when using a standard notebook
computer for the measurements this is the limiting factor; 3
or more sets of notebook batteries are needed to achieve the
same operating time as for the other parts of the portable
CE-C4
D system
3.2 Application Examples
3.2.1 Cations of Heavy Metals
A CE-C4
D method for determination of several inorganic
cations was optimized An electrolyte solution based on
histidine and acetic acid was employed and 18-crown-6 was
added for baseline separation of ammonium and potassium,
while different concentrations of weak complexing agents
were examined in order to achieve baseline separation of
heavy metal cations a-Hydroxyisobutyric acid and lactic
range from 1 to 200 mM The LOD values were determined
as 3 S/N ratio for the most diluted standard solution used for linearity measurements and are also summarized in Table 1 The system was tested by analyzing a sample of potable water obtained from a domestic well in Vietnam The resulting electropherograms of the separation of inorganic and heavy metal cations in a sample from Vietnam (a), spiked sample from Vietnam (b) and standard solution (c) are depicted in Figure 3A An extracted portion of the electropherograms giving detailed information on the determination of heavy metal cations is subsequently shown
in Figure 3B Fe2þwas spiked to the sample at a concen-tration of 5 mM, while other heavy metal cation standards were added at a concentration of 2.5 mM The concentration
of manganese found in the sample using the standard addition method was 8.0 mM Fe2þ was not found in the sample although iron is expected to be present All iron in the sample may have been in the form of Fe3þas it had been exposed to ambient air in transport and storage However, the determination of the oxidized species was beyond the scope of the present investigation For correct speciation of iron, on-site measurements would be mandatory
3.2.2 Arsenic As(V) in the form of the anionic arsenate (H2AsO4 ) was determined in an electrolyte solution consisting of dilute acetic acid This was chosen as it enabled separation of arsenate from phosphate, which can be expected to be
Table 1 Performance parameters for the determination of heavy metal cations using the portable CE-C 4
D The relative standard deviation ( RSD ) values of the peak areas were calculated for 6 consecutive injections of standard solutions containing 5 mM of the heavy metal cations The correlation coefficients (r 2
) were obtained for calibration curves in the range from 1 to 200 mM (5-point calibration), and the limits of detection ( LOD ) are the concentrations giving peak heights corresponding to 3 times the baseline noise.
RSD peak area (%) r 2
LOD (mM )
Trang 5present in natural water samples A calibration curve was
acquired for the range from 1 to 100 mM and a correlation
coefficient (r2
) of 0.9997 was achieved (based on 5 point
calibration) The lower limit of detection, the concentration
giving peak heights equivalent to 3 times the baseline noise
was determined as 0.2 mM RSD values for peak areas in the
concentration range from 1 to 100 mM were found to be
between 1.8 and 6.9 % (n¼ 3)
Three Vietnamese ground water samples, known to be
contaminated with arsenic, were subsequently analyzed
with the portable instrument As(V) was detected in
presence of major other anions as is illustrated in Figure 4
The concentrations were determined based on the
calibra-tion curve established in the previous procedure and
0.17 0.02 mM, 1.82 0.09 mM, and 0.18 0.03 mM of
As(V) were found in the 3 samples a, b and c respectively
(n¼ 3)
3.3 On-Site Measurements For the on-site measurements of inorganic anions and cations, it is desirable that a buffer is employed which is suitable for the determination of ions of either charge as otherwise a lengthy set-up procedure is required between measurements Electrolyte solutions with a relatively low pH-value, between 3.0 and 5.0, were deemed suitable for this purpose The electroosmotic flow is significantly reduced in this pH-range and fast anions can be analyzed in the negative electrophoretic mode while cations can be ana-lyzed in the positive electrophoretic mode using one common electrolyte solution [24, 25] The following anions (Cl, NO3 , SO4 , ClO4 , and NO2 ) and cations (NH4 þ, Kþ,
Ca2þ, Naþand Mg2þ) were considered as target analytes and
an electrolyte solution based on histidine and acetic acid was optimized in order to achieve baseline separation of all ions Best separation and good sensitivity was achieved for 7.5 mM histidine and 40 mM acetic acid at its natural pH-value of 4.1 Baseline separation of ammonium and potassium was achieved by addition of 2 mM 18-crown-6, which has only negligible effect on the separation efficien-cies for other cations and anions [24] Cations and anions could be separated in this electrolyte solution by simply switching the high voltage module to positive or negative polarity
The analytical parameters of the method for on-site determination of inorganic ions were validated and are summarized in Table 2 The detection limits for the inor-ganic anions and cations are very similar and between about
107 and 106M These values are one to three orders of magnitude lower than LOD previously reported for port-able CE instruments, employing potentiometric or potential gradient detection [10 – 14] and in the same concentration
Fig 3 Determination of metal cations using the portable
CE-C 4
D system Electrolyte solution: 11 mM His, 50 mM acetic acid,
1.5 mM 18-crown-6, 0.1 mM citric acid, pH 4.1 Capillary:
fused-silica, 50 mm i.d., L total ¼ 62 cm (L eff ¼ 55 cm) Hydrodynamic
injection by elevating the capillary end by 20 cm for 40 s.
Separation voltage: þ 15 kV A) a) sample of well water, b)
sample spiked with 2.5 mM Mn2þ, Cd2þ, Co2þ, and Zn2þand 5 mM
Fe 2þ , c) standard solution containing 50 mM NH 4 þ , K þ , Ca 2þ , Na þ ,
Mg 2þ and 5 mM Mn 2þ , Cd 2þ , Co 2þ , and Zn 2þ B) detailed
electro-pherograms for the time range from 7.8 to 9.8 min.
Fig 4 Determination of As(V) in the form of arsenate in 3 different ground water samples (a – c) from different places in Vietnam Electrolyte solution: 45 mM acetic acid, pH 3.2 Capil-lary: fused-silica, 50 mm i.d., L total ¼ 75 cm (L eff ¼ 68 cm) Hydro-dynamic injection by elevating the capillary end by 20 cm for 60 s Separation voltage: 15 kV.
Trang 6range as LOD for amperometric detection [11, 12] Note,
however, that amperometry is not suitable for the
compre-hensive determination of inorganic species as only a few
inorganic ions can be detected by this means
The analysis of real samples was carried out in the South
West Conservation Area of the remote Tasmanian
wilder-ness, which belongs to the UNESCO World Heritage list
Only trace concentrations of inorganic pollutants are expected to be found in this area and therefore only highly sensitive instruments can be used for their determination
No sample storage and transport issues arise for portable CE-C4
D system, which enables precise determination of persistent (perchlorate) as well as non-persistent (nitrite) inorganic pollutants The results for the determination of inorganic ions in two real samples (#1: Lake Pedder and #2: side branch of Condominium Creek that joins Lake Pedder
in the valley) are summarized in Table 3 Nitrite was determined in one of the samples, while perchlorate was not found in either of the samples investigated The determination of the inorganic cations at lake Pedder is illustrated by the electropherogram of Figure 5 and an electropherogram for the anions is shown in Figure 6 Note, that the sample had been spiked with 0.5 mM of perchlorate and 1 mM of nitrite to illustrate the possibility of the determination of these potential pollutants at this level
4 Conclusions
The entire system, including capillary electrophoresis instrument, detection and data acquisition is powered by batteries and can be carried into the field by one person as the dimensions of the equipment do not exceed the size of a regular outdoor rucksack Its performance in terms of
NO 2
Fig 5 Determination of inorganic cations in the field (at Lake
Pedder, Tasmania) Electrolyte solution: 7.5 mM His, 40 mM
acetic acid, 2 mM 18-crown-6, pH 4.05 Capillary: fused-silica,
50 mm i.d., L total ¼ 62 cm (L eff ¼ 55 cm) Hydrodynamic injection
by elevating the capillary end by 20 cm for 20 s Separation
voltage: þ 15 kV.
Fig 6 Determination of inorganic anions in the field (at Lake Pedder, Tasmania) The sample had been spiked with 0.5 mM ClO 4 and 1 mM NO 2 Electrolyte solution: 7.5 mM His, 40 mM acetic acid, 2 mM 18-crown-6, pH 4.05 Capillary: fused-silica,
50 mm i.d., L total ¼ 62 cm (L eff ¼ 55 cm) Hydrodynamic injection
by elevating the capillary end by 20 cm for 60 s Separation voltage: 15 kV.
Table 3 Concentrations (in mM ) of selected inorganic ions in natural water samples determined on-site ND: not detected
Cl NO 3 SO 4 ClO 4 NO 2 NH 4 þ K þ Ca 2þ Na þ Mg 2þ
Trang 7sensitivity was found to be comparable to conventional
bench-top instrumentation In field work direct sunlight
onto the portable system should be avoided during
oper-ation as this could lead to an increased temperature in the
injection and separation compartment as no thermostatting
is available Conductivity detection is versatile, and while
generally good detection limits can be achieved, please note
that these are not sufficient for all trace level environmental
analysis However, it is possible for instance to determine
perchlorate at a level below the current EPA guideline for
drinking water (24.5 ppb) [26]
5 Acknowledgements
This work was supported by the Swiss National Science
Foundation (Grant Nos 105176/1 and
200020-113335/1) and a grant from the Science, Engineering and
Technology Unit, Department of Prime Minister and
Cabinet, Federal Government of Australia The authors
would also like to thank Boris Schlensky (eDAQ) for his
help and donation of a prototype of the portable data
acquisition system and John Davis (University of Tasmania)
for helpful discussions on the noise reduction of the portable
C4
D PK thanks ACROSS for funding his stay in Tasmania
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