Modular separation-based fiber-optic sensors for remote in situ monitoring

Abstract

A modular separation-based fiber-optic sensor (SBFOS) with an integrated electronically controlled injection device is described for potential use in remote environmental monitoring. An SBFOS is a chemical monitor that integrates the separation selectivity and versatility afforded by capillary electrophoresis with the remote and high sensitivity capabilities of fiber-optic-based laser-induced fluorescence sensing. The detection module of the SBFOS accommodates all essential sensing components for dual-optical fiber, on-capillary fluorescence detection. An injection module, similar to injection platforms on micro-analysis chips, is also integrated to the SBFOS. The injection module allows for electronically controlled injection of the sample onto the separation capillary. The design and operational characteristics of the modular SBFOS are discussed in this paper. A micellar electrokinetic capillary chromatography mode of separation is employed to evaluate the potential of the sensor for in situ monitoring of neutral toxins (aflatoxins). The analytical figures of merit for the modular SBFOS include analysis times of between 5 and 10 min, separation efficiencies of approximately 10⁴ theoretical plates, detection limits for aflatoxins in the mid-to-low nanomolar range, and controllable operation that results in sensor performance that is largely immune to sample matrix effects.

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Introduction

The separation-based fiber-optic sensor (SBFOS) is a chemical monitor that combines the high degree of selectivity and versatility afforded by capillary electrophoresis (CE) separation techniques with the remote and high sensitivity capabilities of fiber-optic-based, laser-induced fluorescence (LIF) sensing.^1–3 The basic principles of CE can be found in excellent monographs on the subject.^4–7 Basically, very efficient separations of charged solutes are achieved in very small diameter capillaries based on differential migration in an applied field. Neutral solutes can be separated by adding charged assemblies to the CE running buffer such as micelles or charged cyclodextrins. In this case, separations are based on differential association with the charged assemblies. Previously reported single-fiber (and dual-fiber) SBFOSs were based on mating an optical fiber(s) with a short CE capillary. This was accomplished within a commercially available micro-junction that was adapted as a post-capillary detection cell.^1–3 Excess dead volume within the adapted detection cells was problematic, as is the case with many post-column detection approaches, and an improved sensor design was needed.

The work described herein focuses on the fabrication and fundamental evaluation of a modular SBFOS that circumvents the aforementioned limitations of earlier designs. Rather than adapting existing components, the SBFOS is designed from the ground up to maximize performance and flexibility. This was accomplished with a modular design. The detection module was specifically designed to accommodate all essential sensing components for dual-fiber and on-capillary detection. Dual-fiber sensors have exhibited improved detection over single-fiber sensors.⁸ Moreover, the necessary critical alignments of the associated optical apparatus for dual-fiber sensors are much simpler than the critical optical alignments for single-fiber sensors. This former attribute is especially desirable in remote field monitoring applications, where it is necessary to align optical components reliably and routinely. The dual-fiber approach also allows for the implementation of different types of fibers, lens, filters, etc., to independently optimize the excitation and emission elements. An on-capillary detection approach was also implemented to minimize detector-related losses in separation efficiency.

Quantitative reproducibility and separation efficiency are often limited in CE by the nature of the sample introduction procedure. Sampling with the modular SBFOS described herein is accommodated by an integrated injection module. This module allows for electronically controlled sample injection onto the separation capillary of the SBFOS. This injection module is very similar to the injection platforms that are described in many CE microchip devices.^9–14 Although these types of injection platforms are more difficult to employ in remote applications, their integration to an SBFOS is more desirable for in situ sensing over alternative sampling approaches (i.e. frontal mode or a flow injection approach). Because the injection module is a “fixed-loop injector”, a constant volume of sample can be injected. This capability has implications when considering sample matrix effects that are often problematic in in situ monitoring. A micellar electrokinetic capillary chromatography (MEKC) separation mode, wherein separations are based on differential association with a charged micellar phase,¹⁵ was employed to evaluate the potential of the sensor for in situ groundwater monitoring of neutral toxins (aflatoxins).

Experimental

The modular SBFOS is composed of a detection module, an injection module, and a separation capillary. The detection and injection modules are affixed together with all critical and fragile components completely isolated. The resulting modular SBFOS is a compact device that should be easy to implement in remote environmental applications.

Dual-fiber detection module design and fabrication

Fig. 1 depicts the general design of the custom dual-fiber detection module. This dual-fiber modular design is an on-column approach utilizing an extended light path (ELP) separation capillary. The excitation fiber for all the sensors discussed herein was a 100 µm core/120 µm clad/150 µm jacket fiber-optic with a numerical aperture (NA) of 0.22 (Polymicro scientific, Phoenix, AZ, USA). Numerous sensors were constructed to evaluate the collection efficiency of a few emission-collection fibers: a 100 µm core/120 µm clad/130 µm jacket with an NA of 0.66; a 100 µm core/120 µm clad/150 µm jacket with an NA of 0.22; and a 400 µm core/450 µm clad/500 µm jacket with an NA of 0.22. Optical fibers that were 150 µm or less in diameter were housed within a 5 cm long section of 150 µm id/360 µm od capillary to ensure a snug fit within the aligning slits of the sensing module (refer to discussion below). The three dual-fiber SBFOSs varying in the type of emission-collection optical fiber were all mated to a ∼20 cm, 75 µm id/360 µm od separation capillary with a 3× (i.e. 225 µm path length) ELP detection window (“bubble”) (Hewlett-Packard, Bellefonte, PA, USA).

Fig. 1 Schematic of rugged detection module and associated components of the modular dual-fiber SBFOS (A) and photograph of a constructed dual-fiber SBFOS detection module (B).

The custom in-house fabricated detection module (1 in × 1 in × 1 in) consisted of two sections: a base plate (1 in × 1 in × 1/2 in) and a covering plate (1 in × 1 in × 1/2 in). In the base plate, numerous slits (refer to Fig. 1) were fabricated along the diagonal of the block that were 380 µm wide and 330 µm in depth. The slits served to house the fiber-optics and separation capillary and allowed for easy alignment of the fiber-optics with the detection window of the separation capillary. The depths of these slits were designed so that the separation capillary and the capillary-housed optical fibers could be secured in place with the covering plate. The slits for the fiber-optics were made along the diagonal of the square sensing module and thus a 45° angle relative to the separation capillary was created. This results in a 90° angle between the excitation and emission-collection optical fibers.

A 1/4 in diameter hole (1/8 in depth) was also made at the center of each section of the sensing module. This void space was created to minimize background noise contributions from module wall reflections. Additionally, this space facilitated the use of an index matching optical gel to minimize laser light scatter from the outer capillary walls. The final sensor component was the sensor-side reservoir/electrode assembly that was secured onto the detection module. This assembly consisted of a small 1 ml vial with an embedded platinum wire.

Electronically controlled injection device design and fabrication

Fig. 2 depicts the electronically controlled injection device. This device is an offset “X” junction provided by MicroQuartz Sciences (Phoenix, AZ, USA). The offset “X” is made with 75 µm id/360 µm od capillary and has four ports in which capillary sections (5 cm long; 75 µm id/360 µm od) are affixed. To eliminate dead volume between the offset junction and the adjoining capillary sections, the capillary sections, including the separation capillary from the constructed detection module, were polished with lapping paper prior to affixing them within the offset “X” junction. High viscosity epoxy (MicroQuartz Sciences) was used to affix the capillary sections within the offset “X” junction. The injection device is housed within an in-house fabricated module designed to protect the injection device and mount the reservoir/electrode assemblies necessary for operation.

Fig. 2 Electronically controlled injection device (A) and injection loading and separation processes (B).

Associated instrumentation

The associated optics and instrumentation for the dual-fiber SBFOSs used in this work require a laser excitation source, a laser line filter to reject laser plasma lines, a quartz lens to focus the laser radiation onto the excitation optical fiber, an appropriate optical filter to isolate emission, a photomultiplier tube (PMT) or other photodetector, and appropriate processing electronics. The lasers employed with the SBFOS were an Omnichrome He–Cd laser (325 nm, 20 mW; Chino, CA, USA), a Uniphase He–Ne laser (547 nm, 2 mW; Menteca, CA, USA) or a Cyonics Ar-ion laser (488 nm, 15 mW; San Jose, CA, USA). An appropriate laser line filter was employed as a primary filter and a biconvex, 25 mm diameter, f/2 quartz lens was used to focus the laser beam onto the excitation optical fiber. The emission optical fiber was positioned at the entrance of the PMT housing that also contained an appropriate filter to isolate the fluorescent signal. For the work involving Kiton Red as the analyte and the He–Ne laser, two types of filter were used: an interference filter/bandpass filter centered at 660 nm [full width at half maximum (FWHM) = 85 nm] and a 600 nm cut-on filter. For the limit of detection (LOD) work involving fluorescein and the Ar-ion laser, a 500 nm cut-on filter was employed. Finally, for the application involving aflatoxins with the He–Cd laser, a 400 nm bandpass filter (FWHM) = 40 nm) was used. An Edmund Scientific (Barrington, NJ, USA) Zoom Video microscope with 185× maximum magnification was used to visualize the critical components of the detection and injection modules. Three conventional CE high voltage power supplies were employed to drive the injection device as well as the electrokinetic separations. A single power supply with appropriate voltage dividers could be employed to simplify the instrumentation in actual remote applications.

Chemicals and materials

In most of the characterization work, the running buffer solutions were composed of 5 mM dibasic sodium phosphate (J. T. Baker, Phillipsburg, NJ, USA) and 3 mM sodium tetraborate (Fisher Scientific, Fair Lawn, NJ, USA). For the aflatoxin separations, the running buffer consisted of 50 mM sodium dodecyl sulfate (Sigma, St. Louis, MO, USA), 5∶3 phosphate–borate buffer, and 15% (v/v) HPLC grade methanol. HPLC grade water was used to prepare all samples and running buffer solutions. Kiton Red, fluorescein and the aflatoxins were all purchased from Sigma. Due to the toxicity of aflatoxins, extreme caution and care were taken when handling and preparing these samples. The aflatoxin samples were prepared by dissolving the solid aflatoxins in acetonitrile and then diluting the resulting solution in the SDS-based running buffer. Fused capillaries were obtained from Polymicro Technologies (Phoenix, AZ, USA) and rinsed with 1 M NaOH prior to use.

Results and discussion

Design and evaluation of the SBFOS detection module

Many sensors based on LIF employ a single-fiber for both excitation and emission. While these sensors are conceptually simple, they provide for very efficient overlap of excitation and signal-collection regions in the sample and are readily adapted to sensing applications, there are problems or complications. For example, a beam splitter and associated optics are needed to separate and isolate excitation and emission radiation. Optical background from fiber fluorescence and other sources can also be large. Conversely, dual-fiber designs do not require a beam splitter, they generally exhibit lower backgrounds, and it is possible to independently optimize the excitation and signal-collection fibers. With a properly designed dual-fiber design, a substantially better signal-to-noise (S/N) ratio relative to single-fiber designs is possible.⁸

Although bench-top CE instrumentation with fiber-optic-based LIF detection has been established and is commercially available,⁶ our goal was to develop a robust SBFOS for remote monitoring in potentially hostile environments. To develop a robust yet sensitive SBFOS, the following design factors were implemented: (i) on-capillary detection to minimize detector-related losses in separation efficiency; (ii) dual-fiber detection to improve detection as previously discussed; (iii) ELP separation capillaries to improve detection sensitivity; (iv) refractive index matching gel to minimize optical background; (v) evaluation of the overlap of the excitation cone of radiation with the viewing cone of the signal-collection fiber within the detection zone for optimized detection; (vi) evaluation of collection efficiency for a variety of signal-collection fibers; and (vii) a detection module to ensure easy assembly and alignment of critical components.

Optical background and associated measurement noise are due to inadequate spectral rejection of the laser scatter and LIF or Raman emission of the two fibers and the capillary. The use of on-capillary detection can exacerbate the background problem. Fiber and capillary fluorescence are particularly problematic when UV lasers are necessary for appropriate analyte excitation, as is the case in this work. Optical scatter of excitation radiation from the capillary wall also results in smaller fluorescence signals as less excitation radiation impinges onto the detection volume. While conventional focusing optics external to the excitation fiber-optic could be used to focus the excitation radiation onto the detection volume, accommodating these optics within the detection module would be difficult and probably compromise the ruggedness of the sensor. Moreover, such optics are not readily available for UV operation.

In this work, it was determined that refractive index matching gel was very effective in minimizing background levels resulting from laser scatter at the capillary and, if present, scatter of fluorescence that is generated in the excitation fiber. For example, when measurements of the test solute Kiton Red were performed using He–Ne laser excitation (see Experimental), optical background levels were typically over 250 nA without gel while less than 20 nA with gel. The filter used here may not have been ideal in terms of rejecting laser scatter. Nevertheless, the reduction in background is substantial. Although not studied, the reduction in background levels with gel could be greater when UV lasers are employed and excitation fiber fluorescence is involved.

Three sensors were constructed that varied in the type of signal-collection fiber-optic (see Experimental). Calibration plots were constructed for injections of Kiton Red over the concentration range of 10⁻⁵–10⁻⁸ M. Response factors and LODs (S/N = 2) obtained from these plots are shown in Table 1. For sensors with signal-collection optical fibers that differed only in numerical aperture (SBFOS A, NA 0.22; SBFOS B, NA 0.66), it is not surprising that the response factors are only slightly better for the larger NA fiber. Although the acceptance cone of SBFOS B is much greater than that of SBFOS A, visual inspection of Kiton Red flowing through the ELP bubble, with excitation radiation impinging alternately from the excitation or emission fibers, revealed excellent overlap of the excitation radiation with the acceptance cones of both the NA 0.22 and 0.66 collection fibers. Moreover, the positioning of both fibers relative to the excitation zone within the ELP bubble is such that the collection efficiency was limited mostly by the small solid angle of fluorescence emission impinging on the distal end of the collection fibers and not by the critical angle requirements for transmission within the fibers (the latter being NA dependent).¹⁶ The converse is true for the large diameter fiber (SBFOS C) with its large solid angle of collection (see Table 1).

Table 1 Performance comparison of SBFOS

Sensor (NA and core diameter of collection optical fiber)	SBFOS A (0.22; 100 µm)	SBFOS B (0.66; 100 µm)	SBFOS C (0.22; 400 µm)
Response factor (slope of calibration plot)	25.2	32.2	267
LOD/M	2.3 × 10^−7	1.1 × 10^−7	1.1 × 10^−8

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In most bench-top CE-LIF detection schemes, emission radiation is collected 90° from the excitation radiation. A 45° angle between the excitation or signal-collection optical fibers and the separation capillary was used in the current SBFOS. This configuration is easily and accurately fabricated by machining alignment slits along the diagonals of the square detection module block (see Fig. 1). Moreover, the 45° configuration results in a ∼35% increase in the optical path length across the detection window relative to collecting 90° to the separation capillary. ELP capillaries are often used to enhance detection sensitivity in CE. When collecting perpendicular to a 75 µm id capillary with a 3× ELP detection window, the optical path length across the center of the detection window is ∼225 µm. When collecting at 45° across the ELP detection window, the optical path length is ∼300 µm. Another advantage of the ELP capillary is that the wall thickness is reduced and hence there is less attenuation and/or fluorescence emission from the capillary itself.

Using SBFOS C, a calibration plot for fluorescein using the Ar-ion laser (see Experimental) was generated for injections over the range 1.0 × 10⁻⁷ to 6 × 10⁻¹⁰ M. The LOD (S/N = 2) was 5.1 × 10⁻¹⁰ M and the regression constant for the plot was 0.998. Based on an assumed injection volume of 10 nl, this corresponds to an absolute LOD of 5.1 attomol injected. This LOD compares reasonably well with most bench-top CE-LIF systems and is substantially better than previously reported single-fiber SBFOS prototypes.^1,2

Characteristics of the offset “X” junction

Fig. 2A depicts the offset “X” junction that is employed as an electronically controlled fixed-loop injection device. This offset “X” junction is integrated to the terminus of the CE separation capillary. The offset distance, and hence the injection length, is 3.4 mm. This injection length and the corresponding volume (∼15 nl) are slightly larger than normally used in CE and, in some instances, could noticeably degrade the separation efficiency. This offset “X” junction is made by high precision laser technology, in which adjoining capillaries are fused together. Efforts to visualize the operation of the injection device were not successful. This is unfortunate, as such visualization has proven to be a useful diagnostic when optimizing microchip CE injection systems.^9–11

Injection protocol

Numerous papers have reported fluid manipulation for sample injection via an electrokinetic phenomenon within multi-channel manifolds.^9–14,17 Most multi-channel CE systems are commonly constructed via etched channels on a microchip (i.e. “microchip capillary electrophoresis”). These electrokinetically driven systems are advantageous because they allow for valveless fluid control for sample injection and for nearly complete electronic automation of the sampling injection process (important in sensing applications). The advantages of an electronically controlled injection device were the motivation for its integration with the modular SBFOS developed in this work. Fig. 2B depicts the injection and separation processes of the injection device. The first step involves loading the injector by applying a primary voltage across ports C (sample) and B (sample waste; ground) and by applying an appropriate secondary voltage at ports A (running buffer) and D (separation channel). This injection approach results in a confined sample flow through the offset “loop” and no sample leakage occurs. Upon filling the injection loop, the system is switched to the separation mode by reconfiguring the voltage leads and readjusting the applied potentials. This mode of injection is designed to deliver a constant volume of sample onto the separation channel that is largely independent of sample matrix, injection timing, sample electrophoretic mobility, and electric field strength. This is potentially very important in in situ sensing. In conventional CE systems, sample matrix effects such as ionic strength can influence resulting analyte peak heights via sample stacking^18–20 due to the nature of sample injection (electrokinetic and hydrodynamic injection). When a “fixed-loop injector” is employed to deliver a constant volume sample onto a CE system, sample ionic strength effects are not problematic because peak area can then be used for calibration.

Characterization of the electronically controlled injection device

As described previously, the appropriate voltages need to be applied among the intersecting channels during loading and separation to prevent leakage and to provide appropriate fluid flow control within the multi-capillary network. Kirchhoff's rules for resistive networks were used to approximate the appropriate voltages for both the loading and separation processes. To simplify the equivalent circuit, the offset “X” junction was treated as a simple cross-junction. This is justified because the offset distance (3.4 mm) is small relative to the lengths of channels A through D, which were 5, 5, 5, and 30 cm, respectively. If the solutions in these channels have the same ionic strength, the resistances of the channels are proportional to their length. Summing the currents around the cross-junction yields where V is the applied voltage. For an injection voltage (V₁) of 3.0 kV, application of 1.5 kV at D (V₄) and A (V₂) yields a junction voltage (V_J) of 1.5 kV. Under these conditions, the leakage into the separation capillary (D) and running buffer (A) channels will be minimal. A similar Kirchhoff analysis is used to approximate the appropriate application voltages during the separation to prevent sample leakage and provide adequate fluid flow control.

Simple experiments were performed in order to validate the above treatment and to fine tune the system to account for the fact that the injection module includes an offset junction and not a true cross-junction. This was accomplished with a simple CE-LIF system consisting of 5∶3 mM phosphate–borate buffer, an He–Ne laser for excitation, and the laser dye Kiton Red as analyte. This simple system was used for initial characterization so that optimum conditions could be established prior to pursuing a more difficult separation mode (i.e. MEKC). Fig. 3 illustrates the peak response when the voltages across channels A and D are varied from 1.4 to 2.4 kV. As the potential increases, the volumetric flow rate into the offset injection loop increases (due to side channel flow) resulting in dilution of the injection volume and a decrease in the peak response. Based on these results, with a 3 kV primary voltage across the sample and sample waste channels, a secondary voltage of 1.4 kV across both the separation and running buffer channels was used for sample loading.

Fig. 3 Effect of loading conditions. Loading conditions: V_BC = 3 kV; voltage at channels A and D varied. Separation conditions: V_AD = 11 kV; V_B = 4 kV; V_C = 4 kV.

Examination of the separation process was also performed. Problems with sample leakage and inadequate flow are also solved with appropriate secondary potentials. When no secondary potentials are applied to side channels B and C (i.e. left floating) during the separation step, significant flow into these channels occurs and a lower effective flow rate results in the separation channel. Fig. 4A illustrates the resulting electropherogram that exhibits a long migration time. For proper fluid control, a secondary voltage across the side channels (channels B and C) must also be applied during the separation process. Although a side channel voltage that exactly matches V_J may produce an ideal flow rate in the separation channel, diffusive and hydrodynamic effects result in some sample leaking from the side channels into the separation channel during the separation. Under these conditions, peak shapes as seen in Fig. 4B often occur. Secondary voltages across the side channels that are slightly less than V_J will prevent this effect by causing a slight flow from the junction into the side channels while still maintaining adequate flow in the separation channel. Fig. 4C demonstrates the resulting electropherogram when the optimum side channel secondary voltages are applied. Under established optimum conditions for the loading and separation steps, the standard deviation for peak area reproducibility for seven consecutive injections of Kiton Red was 1.8%. Because the pinched injection experiments were conducted manually (i.e. without high voltage switching), improved reproducibility should be possible if the system is fully automated.

Fig. 4 Effect of floating channels B and C during separation (A), conditions where side channel leakage occurs during separation step (B), and conditions where applied side channel voltage is approximately optimum (C). Loading conditions: same as in Fig. 3 (V_AD = 1.4 kV). Separation conditions: V_AD = 11 kV; V_B = float; V_C = float (A); V_AD = 11 kV; V_B = 5 kV; V_C = 5 kV (B); and V_AD = 11 kV; V_B = 4 kV and V_C = 4 kV (C).

Applications

Remote and in situ monitoring of multiple analytes of interest offers analytical advantages that are significant in the biomedical, environmental, and industrial fields. Unfortunately, in situ measurements are typically very challenging and usually do not exhibit the selectivity, sensitivity, and reliability associated with conventional laboratory techniques. In previously reported work, a prototype single-fiber SBFOS was evaluated in frontal mode for in situ monitoring of multiple components. The MEKC separation mode was used to separate the target analytes, aflatoxins. Unfortunately, due to limitations of that approach and the prototype SBFOS, only two aflatoxins, G₁ and G₂, could be separated.² Aflatoxin B₁ is one of the most potent hepatocarcinogens known,²¹ and thus is a primary target analyte to monitor. Monitoring aflatoxins, such as B₁, B₂, G₁, and G₂, is potentially very important in many agricultural and environmental monitoring applications.

The established optimum loading and separation conditions were initially employed for the MEKC separation of aflatoxins B₁, B₂, G₁ and G₂. The aflatoxin samples were prepared in the SDS-based running buffer to maintain a uniform buffer system throughout the integrated SBFOS system. Upon establishing adequate separation of all components, these conditions were then used to construct a calibration plot from 3 × 10⁻⁶ to 1 × 10⁻⁸ M and to determine LODs for each of the aflatoxins. The LOD for each aflatoxin was based on a peak height that resulted in an S/N ratio that was twice the baseline noise (see Table 2). These LODs are well below the ppb level concentrations allowed in the feedstocks of most countries²¹ and thus should be adequate in trace environmental monitoring applications.

Table 2 Limits of detection of aflatoxins

Aflatoxin	Minimum detectable concentration	Minimum detectable amount^a	Correlation coefficient (r^2)
^a Minimum detectable amount based on 15 nl injected.
G2	8.6 × 10^−9 M (2.8 ppt)	43 fmol (14 pg)	0.997
G1	3.1 × 10^−8 M (10 ppt)	158 fmol (50 pg)	0.998
B2	7.2 × 10^−9 M (2.3 ppt)	33 fmol (10 pg)	0.997
B1	1.5 × 10^−8 M (4.7 ppt)	71 fmol (22 pg)	0.998

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In real world in situ monitoring, a sample such as groundwater will obviously not contain the SDS-based running buffer. In previously described electronically controlled injection studies, the running buffer and sample solvent were equivalent, and thus resistance per unit capillary length was constant throughout the SBFOS. A large difference between the sample's conductivity and the running buffer's conductivity, as is the case in in situ groundwater monitoring, will result in a significant change in the resistance along the sample and sample waste channel during the loading step. This will result in a change in the junction voltage during the loading step.

Applying the same circuit analysis as described above, the integrated SBFOS system can be modeled under the in situ conditions to determine appropriate secondary potentials. In establishing a realistic model, the ionic strength of common regional groundwater samples was determined to be roughly equivalent to a 1∶0.6 mM phosphate–borate running buffer.

Additionally, a typical 5∶3 mM phosphate–borate running buffer is assumed. When a 1.5 kV secondary voltage is applied during the loading step under in situ conditions (i.e. groundwater monitoring), V_J decreases from 1.5 kV to 0.8 kV as channels B–C fill. Under these conditions, significant leakage of running buffer from the separation and running buffer channels is expected as observed previously (refer to Fig. 3). If the secondary voltage is decreased to 1.0 kV and the equivalent circuit is modeled again, the junction voltage decreases from 1.32 kV to 0.75 kV. In this situation, upon initial application of the primary and secondary voltage, running buffer leakage from the injection loop initially occurs. However, by the time the sample reaches the offset injection loop, V_J decreases to a voltage approximately matching the secondary voltage and negligible leakage of sample should occur, i.e. a constant volume of sample should be injected.

Figs. 5A–5C illustrate the monitoring of aflatoxins in two regional groundwater samples and an approximately equivalent ionic strength running buffer (i.e. 1∶0.6 mM phosphate–borate buffer) spiked with 3 × 10⁻⁷ M of each aflatoxin. An essential factor in determining appropriate conditions for in situ groundwater monitoring is the approximation of the ionic strength of the groundwater. Because the groundwater sources investigated above were all collected within the same geographical region, their ionic strengths are relatively equivalent, and thus the same operating conditions were utilized for each sample. From source to source, the peak areas are approximately the same (within 3%). The main source of error from sample to sample is the variability in the resulting junction voltage. Because the ionic strength of each sample will not be perfectly matched to the approximate ionic strength of the sample used in the developed model for in situ monitoring, a mismatch between the secondary voltages and the actual junction voltages is likely. This would result in either leakage or dilution of the sample loop, depending upon the difference in the ionic strengths between the approximate equivalent running buffer and the sample. To determine appropriate in situ operating conditions for operation in a variety of environmental sites, a conductivity probe followed by the determination of the appropriate secondary voltage would be necessary to avoid the aforementioned effect. Additionally, generating calibration data based on standard solutions prepared at typical ionic strengths for groundwater would be necessary for adequate analyte quantification.

Fig. 5 Monitoring of aflatoxins in local river samples and equivalent buffer spiked with 3 × 10⁻⁷ M G₂, G₁, B₂, and B₁. Limestone Creek (Washington County, TN) (A), Tyson Creek (Knox County, TN) (B), and equivalent phosphate–borate buffer (C).

Conclusions

This work describes the development and fundamental evaluation of a sensor for in situ monitoring. Future work will involve further incremental improvements that will result in a field ready device. Specific improvements will involve developing a similar device on a microchip to further miniaturize the sensor. This is significant because it will reduce the voltages required for sensor operation. The primary challenge in developing a microchip SBFOS device will be the integration of the fiber-optics for adequate detection. Future work will also involve fine tuning the injection protocol. Specifically, a feedback control loop will be introduced that contains a conductivity probe for determining actual sample conductivity. This will eliminate the approximations in the model that were made in this work and thus will result in an optimized injection protocol for reliable constant volume injection regardless of the nature of the environmental sample. Finally, alternative detection approaches will also be pursued. The current sensor is based on fluorescent detection and thus limits the versatility of the sensor to certain analytes of interest. To expand the detection capability, UV–visible detection can be easily accommodated with the current sensor design. Although sensitivity is more limited with this detection approach, many viable applications could be pursued.

Acknowledgements

This work was sponsored by a gift from Proctor and Gamble Company and by the Division of Chemical Sciences, Office of Basic Energy Sciences, US Department of Energy under Grant DE-FG02-96ER14609 with The University of Tennessee, Knoxville. The authors wish to thank David Stokes and Stephen Jacobson (Oak Ridge National Laboratory) for their useful contributions to this work.

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