Anal. Chem. 2000, 72, 4282-4288
Multichannel Mode-Filtered Light Detection Based on an Optical Fiber for Small-Volume Chemical Analysis Kemin Wang,* Dan Xiao, Zheng Zhou, Leiji Zhou, Xiaohai Yang, and Du Li
College of Chemistry and Chemical Engineering, Hunan University, Changsha, 410082, P.R. China
A novel mode-filtered light detection method is described in which an unjacketed optical fiber is inserted into a transparent capillary tube and three or more detection channels are set on the capillary side for different distances from the port of the fiber. This new method is the basis of synchronization of separation and analysis, with which a modern multidimensional analysis apparatus will be constructed. For samples of different concentration, the more close to the laser incidence port of the fiber the detector has been set up, the greater the change of the intensity profile of mode-filtered light (∆IF) that is obtained. Vice versa, if a parameter r ) I0/In is established and, instead of a mode-filtered light signal, the reversed alteration trend of ∆r is found in comparison with the alteration trend of ∆IF, the reason is that the background rapidly lowers with the increasing distance to the laser incidence port of the fiber; moreover, the mode-filtered light signal decreases slowly with it. With the present method and apparatus, glucose and glycerol have been determined, with good reproducibility and stability and a small sample volume. Furthermore, a real sample of glucose injection is measured for a detection volume of 5 µL, and an acceptable result is observed. In the field of optical waveguide-based chemical sensors, there is an ongoing interest in an optical fiber based chemical analyzer for a variety applications such as process analysis and biologic monitoring.1-9 The optical fiber sensor plays an active role in current research, which always has been divided into two groups, extrinsic and intrinsic.6,10 The intrinsic optical fiber sensor has its advantages, such as swift response and high performance. Commonly, the detector is put in the end of the fiber to measure the intensity of transmitted light.1,3,5,6 But, the mode-filtered light detection transfers the detector to the side of the fiber in order (1) Wolfbeis, O., S. Trends Anal. Chem. 1996, 15, 225-231. (2) Synovec, R. E.; Sulya, A. W.; Burgess, L. W.; Foster, M. D.; Bruckner, C. A. Anal. Chem. 1995, 67, 473-481. (3) Wang, W.; Liao, Y.; He, Q.; Wang, J.; Feng, M.; Wu, G. Anal. Chim. Acta 1998, 375, 261-267. (4) Bruckner, C. A.; Synovec, R. E. Talanta 1996, 43, 901-907. (5) Ross, I. N.; Mbanu, A. Opt. Laser Technol. 1985, February, 31-35. (6) Smela, E.; Santiago-Aviles, J. J. Sens. Actuators 1988, 13, 117-129. (7) Foster, M. D.; Synovec, R. E. Anal. Chem. 1996, 68, 1456-1463. (8) Brandenburg, A. Sens. Actuators, B 1997, 38-39, 266-271. (9) David, D. J.; Shaw, D.; Tucker, H. Rev. Sci. Instrum. 1976, 47, 989-997. (10) Prahl, B. W.; Tracey, P. M. Sensors 1986, August, 48-52.
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to acquire a low background and possess a signal-to-noise advantage over the conventional approach.2,4,7,11 Developing a cladding material enhances the optical fiber sensor’s intrinsic chemical selectivity, so it shows a potential capacity of acting as liquid chromatography, flow injection analysis, and gas chromatography.2 A multichannel detector is equipped, along the capillary side with different distances to the laser incident port of the fiber, which monitors the procedure of compound separation, that makes the ideal of synchronization of separation and analysis become a reality. Our aim, to build upon the fundamental of this previous work the possibility of multichannel detection incorporating a mode-filtered light instrument and the potential benefit of acquiring chemical information, is considered in this work. Furthermore, the intrinsic optical-fiber-based sensor’s future application, such as monitoring the concentration of some compounds in aqueous solution, is studied in this paper. The optical-fiber-based sensor by mode-filtered light detection devised by Robert E. Synovec et al. contains a fiber and a transparent capillary,2 which forms a small-volume annular column. When the samples are injected into this annular column surrounding the fiber, the environment’s refraction index (RI), the critical angle, and the intensity of mode-filtered light (IF) are changed. Whether the fiber surface has cladding or not, the mode-filtered light emanates in the whole fiber by a change in the critical angle that is measured directly along the side of the capillary. So, it is very easy to image that during the sample moving in the annular column; the mode-filtered light along the capillary side represents the sample’s chemical information. In other words, if the sample is separating in the annular column, using the multichannel detection method to measure the intensity of mode-filtered light at several points along the capillary side shows the alteration in concentration of every compound contained in the sample. With a thin cladding fiber or a thin cladding capillary, the sample separates by the differential distribution in the mobile phase and the cladding. Meanwhile, the concentration of compounds contained in the sample is measured by mode-filtered light detection because the critical angle changed by the refraction index (RI) in the cladding and the mobile phase varied. The whole procedure described above is the approach of the synchronization of separation and analysis. This novel method provides a convenient approach to obtaining the separating capacity of a cladding fiber (11) Wang, K.; Zhou, L.; Mao, D.; Yang, X. Chem. J. Chin. Univ. 1999, 20 (7), 1047-1048. 10.1021/ac0000527 CCC: $19.00
© 2000 American Chemical Society Published on Web 08/17/2000
Figure 1. Schematic drawing of the multichannel annular sensor using mode-filtered light detection.
Figure 2. Schematic drawing of the structure of the multichannel annular sensor using mode-filtered light and the propagated condition of the incident laser beam.
or capillary and the length of a multichannel sensor when all the compounds in a sample are completely separated. The synchronization of separation and analysis requires that at every point along the capillary side the signal is measurable and all signals are obtained by the same detector. A design of a chemical sensor fulfilling the above idea is illustrated in Figure 1. As an application of the above sensor in a simple system, it is used as an apparatus with which to monitor a compound’s concentration in aqueous solution. I. N. Ross devises a way, consisting of a fiber being put into sugar solution and photo detection measuring the transmission beams at the end of the fiber, of monitoring the glucose concentration. But, this way is incapable of acquiring high sensitivity and a detection range at the same time.5 The annular sensor by multichannel mode-filtered light detection solves this problem easily. From a different detection channel, the S/N ratio and detection range is optimized. This sensor can respond to glucose solution within a detection range from 0 to 15%. THEORY For the light resource in this experiment, a Helium-neon laser is used, the beams discussed following all are polarized laser light. Figure 2 represents a section of a multichannel annular sensor and shows schematically the principle of refraction of an incident beam. The incidence laser beam transmits from the atmosphere into the fiber, the total intensity profile of transmission beam labeled by I0 in Figure 2. By Snell’s law (the parameters in the following equation are labeled in Figure 2.)
n0 sin θ0 ) n1 sinR
where n0 and n1 are the RI of incident medium and the refraction
Figure 3. (A) The curve of IR/I0 versus the times of reflection. (B) The curve of F versus n2.
medium, respectively; θ0 represents the incidence angle; R is the refraction angle. In the multichannel annular sensor, disregarding losses other than by transmission at the interface I0 ) IR + IT (IR and IT represent the intensity profile of reflected and transmitted light, respectively); when divided by I0, the IR and IT are given by 1) F + τ where F represents the reflection ratio and τ represents tthe ransmission ratio, F ) [tan (θ - φ)/tan (θ + φ)]2, and where θ is the angle of incidence and φ is the angle of refraction. At the interface of the fiber and the liquid phase, the fiber’s RI is n1 and the liquid’s RI is n2; then by Snell’s law n1 sin θ1 ) n2 sin θ2 (θ1 and θ2 represent the angle of incidence and refraction, respectively) Since the refractive index of the fiber is the same as quiz (n1 ) 1.47), the total IF varies with different refractive indexes of the analyzed sample. If there is no cladding at the fiber and capillary Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
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(as in Figure 2), the following reasoning is valid. First, a simple process of a laser beam propagating into the detection channel has been estimated. At the interface between the fiber and the liquid phase, before the beam arrives in the detection channel, it has already reflected n times, then
IR ) I0 Fn
(1)
where F ) [tan (θ1 - θ2)/tan (θ1 + θ2)], θ1 ) π/2 - R, θ2 ) sin-1 (1.47(sinθ1/n2)). On the assumption that the liquid phase is pure water and that the incidence beam that arrives at the detection channel already has reflected n times, on the basis of eq 1, some results are calculated and are shown in Figure 3. From the figure it is easy to find an angle a little smaller than the crisis angle F decreasing rapidly with the reflection times and F almost reaching zero when the incidence beam is only reflected 3 times in the fiber at an incident angle 0.03 rad smaller than the crisis angle. When the transmission beam reaches the interface between the liquid phase and the capillary wall, the intensity of transmitted light (ITN) is given by
ITN ) I0Fn-1(1 - F)
(2)
When the transmission beam reaches the interface between the capillary and the detection channel
IF1 ) I0Fn-1(1 - F)2(1 - F′)
K ) 1 - F′ ) constant
(n,m))(n1,m1)
IF )
∑
2
KI0F (1 - F) (1 - F)
(5)
(n,m))(n1,m1)
It expresses that the total intensity profile of mode-filtered light (IF) in a channel equals the sum of the intensities of beams reaching the detection channel after n times of reflection and m 4284
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(6)
In the experiment, the relationship between RI and the concentration of glucose solution is found, that is, RI equals a constant multiple of the concentration in the experimental concentration range (0-15 wt %). So, eq 7 is valid. Where k is a constant and C represents the concentration of glucose solution (wt %)
n2 ) kC + k0
(7)
When θ1 and n1 are fixed in the experiment, K0 ) n1 sin θ1 ) constant. On the basis of Figure 3, F decreases rapidly with n2 increasing, so F ∝ 1/n2.
(4)
IF1 only represents a part of the mode-filtered beam that is detected in a detection channel. An overview of the incident beam propagation in the whole multichannel annular sensor shows that when the incident beam reaches an interface it will divide into two beams, the reflection beam and the transmission beam. Each of the two beams divides into two parts, respectively, at another interface. This process occurs at the interface of the whole multichannel annular sensor, after 10 or more times of reflection, there are hundreds of beams propagating in the sensor. But, the mode-filtered light is detected only at the laser beam if it refracts at least three times through three interfaces and transmits into the detection range without consideration of its individual route. So, eq 5 is a derivation of the above m
IF ) clFl + cl+1Fl+1 + cl+2Fl+2 + cl+3Fl+3 + ... + cmFm
(3)
The incident angle of the interface between capillary and detection channel equals θ1.When θ1 is fixed, F′ is a constant if the RI of the quartz capillary and the RI of the detection channel are constant.
n
times of refraction, except that three times of refraction are necessary, where (n,m) represents all probable modes. If there are i possible modes, from 1 to i represents those modes. For example, if the mobile phase is H2O (n2 ) 1.333), the incidence angle θ1 ) 1.135, and then F ) 0.7752. The core of the fiber radius R ) 0.15 mm and the inner radius of the capillary R ) 0.265 mm, at every 0.64425 mm in the fiber or every 0.064425 mm in the capillary wall or every 4.72424 mm in the annular column the beams will reflect one time. The detection channel is at a length of 157-158 mm away from the incidence port, and the front is 90 mm out of the annular column. Since the crisis angle for the fiber and air is smaller than θ1, the incident beam reflects hundreds of times at, first, 90 mm, and no refraction happens. It does 10 or more refractions and reflections when the beams happens to transmit into the detection channel, so IF is the total intensity of these beams. It is clear that the possible modes are related with the distance of the channels. On the basis of eq 5 and the definition of cx as the coefficient of Fx, (where x represents an arbitrary number) IF is deduced as follows:
F)
tan2 [θ1 - arcsin (K0/n2)] tan2 [θ1 + arcsin (K0/n2)]
(8)
From all the above, F ∝ 1/n2, and substituting eq 7 into eq 8
F)
tan2 {θ1 - arcsin [K0/(kC + k0)]} tan2 {θ1 + arcsin [K0/(kC + k0)]}
(9)
If we define eq 9 as equal to f(C), then
IF ) cl[f(c)]l + cl+1[f(C)]l+1 + Cl+2[f(C)]l+2 + ... + cm[f(C)]m (10) In the experiment, the IF value reduces with the glucose concentration increasing (Figure 4, (plot xz)), and it provides evidence that the signal obtained from the multichannel sensor continuously responds to the analyte concentration. Furthermore, the experimental results suggest that the variable is not present in the first degree but is raised to powers in the equation. This result conforms to eq 10. If n2 changes very little, when the dIF/ d(1/C) > 0, the theory is consistent with the experimental
Figure 4. The 3D curve of glucose solutions, which is their concentration versus RI versus signal (Channel 3), in which water and 2, 4, 6, 8, 10, 15% (wt %) glucose solution are detected by abbe refractometer at 299 K, and the data of the signal is the same as the data of channel 1 in Figure 9A.
Figure 6. The IF values of glucose solution with different concentrations measured by three channels.
Figure 5. The curve of CCD element number and its corresponding IF value. These data are obtained by three channels. Figure 7. The R value versus the concentration of glucose solution measured by three channels.
phenomenon. But it is seems that accurate calculation of the IF is a very difficult attempt, at present, for after n times of reflection and refraction, one beam light becomes 2n beams of light. In the experimental conditions, the n equals 50 or more. The theory provides an academic interpretation for the experimental results. EXPERIMENTAL SECTION Chemical and Reagent. Glucose and glycerol of analytical grades were obtained from Reagent and Glass Apparatus Corp. (Changsha, China) and were used without further purification. The glucose which was injected (5%) was purchased from Medicine Corp. of Hunan Province (Changsha, China). Aqueous solutions were prepared with redistilled water. Construction of the Annular Sensor. A fused-silica capillary (YL Fiber Factory, China) and a naked optical fiber (Glass
Institution of Beijing, China) make the annular column. The capillary tube which surrounds the fiber had an inner diameter of 530 µm, in which the diameter of a quartz fiber without cladding is 320 µm. Between the inner capillary wall and the fiber outside diameter, the average 105-µm gap provides a flow channel for the mobile phase and analytes. From the first channel, every 40 mm a channel is set up in the dark box, so there are six channels total distributing at the capillary side. The three selected detection channels are located at 157, 197, and 237 mm, respectively, away from the flat end of the fiber coupling of the incidence beam. To reduce friction and damage to the surface of the fiber, the capillary tube filled with mobile phase before the insertion process. The exposed end of the fiber is fixed by an optical microadjustment and is aligned with the He-Ne laser light source. An end of the Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
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Figure 8. The IF value of glycerol solution detected by channel 2.
capillary is supported by an equipment base without any seals to let the position of the fiber be adjusted freely. Another end of the fiber inserts into a plastic tube (purchased by RS Components Ltd., Co. HK), and its upward curving part just holds the fiber. The plastic tube also fastens the capillary tightly for its inner diameter is the same as the capillary’s out diameter. Before the capillary is inserted into the plastic tube, the plastic tube is put into boiling water, for at high temperature, it becomes flexible and expandable. The plastic tube is fixed in the equipment base except for the upward curving part. To reduce shaking, a thick sponge makes the foundation of this device. A cross-sectional view of the multichannel annular sensor is shown in Figure 2; this sensor is incorporated into an analyzer as shown schematically in Figure 1. Apparatus. This sensor is particular for its special multichannel detection device, which is constructed with several plastic optical fibers (provided by RS Components Ltd., Co. HK.) and a CCD (Toshiba Corporation, Japan.) with its data-gathering system (LHL Corp., Tianjing, China). The flat end of the optical fiber aligned with the He-Ne laser light source (Optical Instrument Corporation of Nanjing, China) operates with continuous wave output at 632.8 nm. An optical microadjustment (LY Equipment of Fiber and Laser Corp., China) supports the fiber and adjusts its position. The CCD gathering system communicates with a compatible personal computer. The software programmed by our lab transfers the digital data into a graph shown on the screen, in order that the whole process of sample injected in and flowing out can be monitored. The environment temperature is controlled by air conditioning, keeping room temperature during the experiment time. An Abbe refractometer (Shanghai Specimen and Model factory, Shanghai, China) is used to determine the RI of analytes, while concentration (wt %) is detected by a polarimeter (Shanghai Optical Instrument Corp., China). Procedure. After the best incident angle is found, the incidence angle is fixed during the experimental period. The water has been injected into the annular column for 48 h before the experiment is started, with the effect that a high reproducibility 4286 Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
Figure 9. The calibration curves for glucose solution: The calibration curve of IF and glucose concentration is shown in part A. Part B shows the calibration curve of R and glucose concentration.
is obtained. Helium-neon laser light is focused with a 5× objective into a 450-mm length of the optical fiber, the last 400 mm of which were inserted into the capillary tube. The incidence angle of the laser beam is fixed at about 1.13 rad in the experiment. The multichannel annular sensor detects the analyte from low concentration to high concentration in sequence. Then, an Abbe refractometer and a polarimeter at the same temperature measure samples’ RI and concentration (wt %), respectively. The maximum intensity of light that the CCD can detect is defined as 4096 units; moreover, the mix is defined as 0 unit. The dark signal at room temperature is about 351 units. RESULTS AND DISCUSSION The Possibility of Synchronization of Separation and Analysis. The possibility of realizing the idea of synchronization of separation and analysis is most important. It is must be
Table 1. Results of Detection by a Polarimeter and a Multichannel Annular Sensora method multichannel annular sensor Csample(wt %)
5.03
4.94
5.07
polarimeter 5.109 5.004 5.057 avg: 5.057
a The data of the multichannel annular sensor is obtained from the calibration curve of Figure 10A and represents the detection results from channel 1 to channel 3 in order. The polarimetry determination results are observed from three samplings of the glucose injection.
Figure 10. The reproducibility of the multichannel annular sensor. (These data are the IF values of the glucose solution corresponding to the concentration labeled in this figure).
investigated whether the sensitivity of different detection channels is enough to meet the requirement of chromatographic analysis. To solve the problem of how to deal with the original signal so that the signal obtained from different detection channels is comparable, the original signal needed be transformed. The parameter R is defined as R ) I0/In. There are 2048 elements in a CCD; furthermore, each of them can detect the intensity of mode-filtered light. What is the best sensitive element in a channel must be found out, for one hundred or more elements are arranged in a channel. Through analyzing data, which include the element number and its corresponding signal value, it is found that the element at the center of the channel reveals the channel’s high sensitivity. This conclusion is supposed by Figure 5, which shows the plot of element number versus its corresponding signal value. Those data record the CCD element number and IF value detected by the number’s corresponding element, and this form of data is named Form 1. Those data of Figure 5, recorded in data Form 1, include the detection information for the analyte which is the glycerol solution of 0, 1, 5, 10% (V/V). As shown in Figure 5, it is clear that collected IF descends from channel 1 to channel 3, in sequence, and so does the sensitivity if the result is calculated from the original data. So, the most sensitive CCD element represents the detection of one multichannel sensor’s channel. Then, a series of glucose solutions are measured by the multichannel annular sensor. Those analytes are 0, 2, 4, 6, 8, 10, 15% (wt %) glucose solution and a real glucose injection sample. Figure 6 illustrates the IF value detected by the most sensitive element of each channel, respectively. Figure 6 opens out the trend between IF and glucose concentration, that is, the IF value reduces as the glucose concentration increases. In all the detection range, the trend is the same in the three channels. Consequently, every one of the three channels is available when the sample’s concentration is measured. But, when a separation process is monitored, the original signals from different channels cannot be compared directly. For example,
when the glucose concentration rises from 0 to 15%, the IF value decreases 1719.6 units in channel 1, while the IF value decreases 1350.7 units in channel 2 and 1245.9 units in channel 3. The function R ) I0/In is utilized to deal with the original data; the result is shown in Figure 7. The most sensitive CCD element of R is in channel 3, whether the concentration is high or low. Although channels 2 and 1 are not as good as channel 3 in sensitivity of R, they also offer a distinct alteration of R when the concentration changes. If more than three channels are equipped, the observed signal of different analytes is feeble in some channels where IF is too faint. But, if R takes the place of the IF value, this problem will be solved, for R is not concerned with the absolute IF value. On the basis of the above, the synchronization of separation and analysis will be achieved as a consequence in the coming experiment. Response of the Multichannel Annular Sensor. Furthermore, this device also acts as an intrinsic sensor that can monitor the changing concentration in some solutions which only contain a solute. This sensor has been used to measure a series of glucose solutions and a series of glycerol solutions, such as shown in Figures 6 and 7, where the signals of glucose solution are detected by channels 1, 2, and 3, respectively. Figure 8 is the curve of glycerol solution detected by channel 2. (Where the fiber and the capillary structure and the two multichannel annular sensors, which detected glucose solution and glycerol solution, respectively, are of the same kind.) On the basis of Figures 6 and 7, the average IF value of each concentration is counted and is shown at Figure 9. The IF value is not lined with glucose concentration, which is reasonable according to eqs 6 and 10. The order of F and C in the two equations is not present in the first degree, but raised to at least l power. According to eq 5, the possible modes of mode-filtered light which transmits into detection channels are not the same. In other words that the coefficients of the different channels in eq 6 and eq10 are different. As shown in the Figure 9 the three curves of three channels respectively are not the same. This experimental result is good agreement with the theory. Reproducibility and Reversibility. It is well known that the intrinsic sensor has good reproducibility and reversibility and so does this multichannel annular sensor. The redistillated water, 2, 4, and 6% glucose solution are injected into it in turn. The relative standard deviations are obtained, which are 2.49, 2.26, 2.47, 0.76%, corresponding with sample order. The IF values measured in this experiment are 3153.50 ( 3.09, 3049.61 ( 2.81, 2973.69 ( 3.01, 2873.15 ( 0.94. Those results are shown in Figure 10. Analytical Chemistry, Vol. 72, No. 18, September 15, 2000
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Application. A real injection sample, which contained 5 wt % glucose, is measured by the multichannel annular sensor. Compared with the results observed by the polarimeter, the multichannel annular is accurate enough to provide a new instrument for quick determination of glucose. (Those data are shown in Table 1.) From the above table, an interesting thing is that the signal of channel 2 is lower according to the signal of the others, but there is not a satisfactory theory to explain this now. CONCLUSION From the theory, two deductions are obtained. One is that the IF value is not in a linear relationship with the concentration of glucose solution (C). The other is that the different channels possess different coefficients in the equation. The experimental results are consisted with the two deductions. It has been demonstrated that the feature of this multichannel annular column optical-fiber sensor makes the foundation for the synchronization of separation and analysis. Meanwhile, this
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additional information endows multichannel mode-filtered light detection with the capacity of gathering more chemical information. Furthermore, the application of the multichannel annular column optical-fiber sensor is also attempted. It displays good reproducibility and swift response as expected, which has potential application in monitoring the concentration of solutions such as injections and liquid drugs. The real glucose injection is measured using it and an acceptable result is observed. ACKNOWLEDGMENT This work was supported by the National Nature Science Foundation (29675005) and National Outstanding Youth Foundation of P.R.China (29825110).
Received for review January 13, 2000. Accepted June 8, 2000. AC0000527
