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Total dissolved solids estimation with a fiber optic sensor of surface plasmon resonance
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Total dissolved solids estimation with a fiber optic sensor of surface plasmon resonance Lihang Feng a,∗ , Weigong Zhang a,b,∗∗ , Dakai Liang c , Jenkins Lee d a
School of Instrument Science and Engineering, Southeast University, Nanjing 210096, China Suzhou Research Institution, Southeast University, Suzhou 215000, China State Key Laboratory of MCMS, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China d Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, Los Angeles, CA 90095, USA b c
a r t i c l e
i n f o
Article history: Received 2 July 2013 Accepted 18 December 2013 Available online xxx Keywords: Surface plasmon resonance Fiber optic sensor Total dissolved solid, Ion concentration
a b s t r a c t In this paper, surface plasmon spectra of ion influence was investigated by using a fabricated fiber optic sensors, then spectrum subtraction of ion and non-ionic solutions were developed for field applications of total dissolved solids (TDS) estimation. We confirmed the SPR spectral difference between seven ionic and three non-ionic liquid samples, that for the same refractive index, resonance wavelength in SPR spectrum is much higher for ionic samples than that in the case of non-ionic ones due to the ions influence. The positive correlation of ion content and extra resonance wavelength shift has been established for TDS estimation in water. With three groups of water samples investigation and field testing, the proposed SPR technique showed a good performance comparable to the conductivity method. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction In recent years, surface plasmon resonance (SPR) sensor has been extensively studied that many applications are ranging from biomaterials analysis to environmental inspection [1]. Being a surface technique that is sensitive to changes of refractive index (RI), SPR technique (SPR-tech) is becoming popular for monitoring small changes that occur in the surface of sensors. Surface plasmons are charge density oscillations of conduction electrons coupled to an electromagnetic wave at a meta-dielectric interface which have been extensively described [2]. To excite surface plasmons, techniques named after Otto and Kretschmann are generally employed, respectively [3,4]. Both configurations are based on prism and the applications for SPR sensing mainly covered two methods. One is scanning the incident angle of light called angular interrogation [5] and the other is wavelength interrogation [6]. Moreover, the prism based SPR sensing device has a number of shortcomings which can be replaced by an optical fiber, that was first proposed by Jorgenson in 1993 [7]. Afterwards, many researchers take interests into fiber-optic (F-O) SPR sensors which are manufactured by removing a small portion of cladding
∗ Corresponding author at: School of Instrument Science and Engineering, Southeast University, Nanjing 210096, China. Tel.: +86 02583794157. ∗∗ Corresponding author at: Suzhou Research Institution, Southeast University, Suzhou 215000, China. Tel.: +86 138 0903 1886. E-mail addresses: [email protected] (L. Feng), [email protected] (W. Zhang).
from the fiber and coating with a thin layer of metal [8]. This dipprobe geometry of F-O SPR sensor expanded increasingly because it allows experiment to be designed around the sample instead of the sensor. Now the F-O SPR sensors are showing various advantages and widely used in many fields. As water status is linked with our health so closely, we concerned about the issue of water quality extremely. Domestic water and effluents from agricultures or industries such as irrigation water, oil field refinery, electrical and mining, etc. contain high amounts of dissolved solids. The major portions of dissolved solids in water are in ionic form such as sodium, potassium, calcium, chloridion, sulfate radical, carbonate and other particles that will pass through a filter with pores of around 2 microns in size [9]. Researchers have applied SPR technique into various kinds of ions detection selectively such as potassium, copper, and other metals by different methods [10,11]. Booksh group also fabricated robust F-O SPR sensors which can be used to detect salinity with a potential precision of 10 ppm [12]. But these do not precisely represent the total dissolved solids (TDS) well in water analysis. As studies of SPR spectrum show the effects of temperature [13], electric potential [14] on parameters such as sensitivity, robustness, accuracy, etc. which people made numerous efforts to improve, researchers also find the effect of ions on surface plasmons since SPR sensing species can be divided into two types, either ionic or non-ionic [15]. However, not too much attention on the importance of ion concentration has been paid, neither do the applications. As the data are still scarce, we provide more documents and make further studies that the SPR-tech based on the effect can be put into TDS measurement.
0030-4026/$ – see front matter © 2014 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.12.040
Please cite this article in press as: L. Feng, et al., Total dissolved solids estimation with a fiber optic sensor of surface plasmon resonance, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.12.040
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In the present study, surface plasmon spectra of ion influence were investigated further to estimate TDS levels in water. The positive correlation of ions content and resonance wavelength shift has been confirmed that response curves can be established for TDS estimation in water. Proposed SPR technique which is comparable to the conductivity method showed a good performance in TDS testing. The research will broaden the SPR application in water quality monitoring. 2. Materials and methods 2.1. Fiber optic SPR system An illustration of the F-O SPR probe which is 8 cm length with 1.5 cm of cladding from the end portion removed is depicted in Fig. 1, so are the scheme of SPR system and apparatus. To improve reproducibility of the probe, a stainless steel cylinder is required for affixing the probe into a standard FO connector (SMA.FC). The fabrication is similar to Pollet [16]. We use an ultrasonic cleaner and acetone to clean unclad portion of the fiber before coating with metal. After cleaning, the probes are sent into a sputtering machine (CFS-4ES, Shibaura, Japan). Typical 2 nm of Cr which acts as an adhesive is deposited and 50 nm of Au, too (see SEM in Fig. 1c). The average resolution of the sensors is approximately 5 × 10−5 refractive index units (RIU). By using a bifurcated optical fiber which connected with SPR probes, a tungsten-halogen light source (HL-2000, Ocean Optics, USA) is guided through and a UV–vis spectrometer is used to scan the reflected. Spectrometer is connected with a computer where data is recorded by software OOIBASE32 (Ocean Optics, USA) of spectral suite. As the spectrum in air is taken for reference signal, a spectral SPR-dip will show in reflection spectrum for a given solution and the resonance wavelength increases with the RI increasing. To improve the accuracy and minimize the noise, further data processing has been done in Matlab through several steps similar to Ref. [17]. 2.2. Samples preparation and data collection Sodium chloride (99.9%), potassium chloride (99.8%), sodium carbonate (99.9%), magnesium sulfate (99.8%), zinc sulfate (99.9%), calcium chloride (99%), magnesium chloride are Guarantee Reagent inorganics bases acids which purchased from Sinopharm Co. Led (China), so are glucose, glycerol and sucrose. Preparation of solution samples is processed at constant temperature (294 K) with Millipore water used. 7 ionic (NaCl, KCl, CaCl2 , MgCl2 , Na2 CO3 , MgSO4 , ZnSO4 ) and 3 non-ionic (glucose, glycerol, sucrose) solutions are selected transparent and colorless to avoid the influence of spectrum. Various ions in the samples including Na+ , K+ , Ca2+ , Mg2+ , Zn2+ , Cl− , CO3 2− , etc. which cover the most soluble salt ions about 95–99% of water impurities are considered.
Fig. 1. (a) Schematic diagram of F-O SPR/TDS system; (b) F-O SPR sensor; (c) SEM of the SPR probe after cleaning and Au coating.
Study site is chosen in JiangSu District of China (Fig. 2) and water samples are collected in clean polyethylene bottles from different sources. Hundreds of data observed and collected with different hydrological periods or drainage cycle is consumed including 3 simple specified groups of potable water, natural brackish, industrial waste and toxic brines for comparation (Table 2). High Millipore and desalinated samples are obtained in laboratory. Potable fresh water district spots of local area, natural fresh streams are Yangzi and branching rivers near, so are the natural brackish including eastern seawater samples. Industrial waste is simply collected with drainage from several companies and factories in Nanjing. Statistical research of all data collection is also assistance provided with different hydrology cycles by our fellows in Jiangsu Institute of Hydrology and Water Resources. Unmentioned matters which subject to the local environment are not strict discussed. 2.3. Methods and assay protocol RI measurements of all solutions were performed in lab by an Abbe refractometer with an accuracy of ±0.001. Each solution is independently prepared in different RI varying from 1.335 to 1.370 in steps of 0.005. For results calibration, we use standard gravimetric technique according to GB/T5750-2006 of National Standards (PR China) where method in lab for determining TDS after evaporation at 180 ◦ C is described. Measurements are accredited by the National Association of Testing Authorities of Jiangsu. Different ions in representative samples of tapwater, seawater, oil refinery saline are analyzed detailed (Sector 5.2) such as Ca, Mg, Cl, CO3 , Na, K by using ICP-AES and IC-HPLC spectrophotometer (Shimadzu, Japan) with standard techniques [18]. For example, Chloride determined routinely by an automated ferricyanide colorimetric method is using a segmented flow analyser with a detection limit of 1 mg/l. Before SPR and conductivity (conductivity meter, Mitutoyo, Japan) testing, organic components and suspended solids in less measurable samples are removed by filters. All samples obtained will be compared and analyzed with general estimation and specific measurement, respectively. Arithmetic mean values are also used because broad trends and verification tests are concerned primarily. 3. Spectral analysis for ionic and non-ionic solutions 3.1. Influence of ions on SPR spectrum To investigate the effect of ions on resonance wavelength, all 10 solutions including ionic and non-ionic ones are measured.
Fig. 2. Study site of Jiangsu in China around Yangtze River and its branches basin.
Please cite this article in press as: L. Feng, et al., Total dissolved solids estimation with a fiber optic sensor of surface plasmon resonance, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.12.040
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Fig. 3. Ions influence on SPR spectrum: (a) resonance wavelength (s ) as a function of refractive index (RI) in all solutions; (b) SPR spectrum of each liquid sample with RI 1.360. (Curves are the second order polynomial fits. Colors of spectra correspond to color of the text denote the solutions with experimental points, respectively. The zoomed inset shows the variation of s near RI = 1.360.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 3 shows the variation curves of resonance wavelength (s ) with refractive index (RI) of the samples. In all cases, due to wavelength dependence of the real part of the dielectric function of metal given by Drude model, the variation is nonlinear, s increases with the increase of solutions’ RI in accordance with previous studies [7,8]. However, the observed s is not same for all samples with RI ranging from 1.350 to 1.370. Taking out the SPR spectrum with RI about 1.360 of 10 solutions in Fig. 3 (inset figure) which is more clearly shown, for same RI of the solutions, s is much higher for 7 ionic samples in comparison to 3 non-ionic samples. No appreciable difference of s among non-ionic liquid samples is obtained and the mite of s may result from the presence of ions in solutions. The additional increase in s of ionic samples is due to the electrostatic interactions of the free ions with the surface electrons of the gold film, which has been proposed in Ref. [15]. The presence of ions causes an additional increase in s . The observation and results provide more documents, but our focus is on the importance of ion concentration and the applications which are based on this effect. 3.2. Relationship between ion concentration and SPR shift Further study of ion influence was conducted to obtain the extra resonance wavelength shift () from Fig. 3, non-ionic samples’ curve can be used as a reference (sucrose e.g.) and ion concentration as a function of RI for 7 ionic samples are shown in Fig. 4. Response curves are observed that ion concentration increases in a similar trend as with increase in RI for 7 ionic samples. Results indicate that, in such a solution, the increase of RI means an increase in ion content. A positive correlation can be confirmed that, the higher ions concentration in sample is, larger will be the increase in .
3
Fig. 4. Wavelength shift (a), ion concentration (b) as a function of RI for the different samples, respectively. Sucrose was used as a reference signal in (a) and more data were measured to enhance the reliability in (b).
Furthermore, if taking NaCl, KCl as example, for the same RI range, ions content in NaCl and KCl solutions are almost same and the curves overlap with each other approximately. Parallel results can be also found in other solutions. Hence, for the same RI, the additional increase , in the case of NaCl and KCl, is larger than that obtained for other solutions. We understand the consequence of the interaction between the ions and free electrons of metal film with a further qualitative explanation. For a given RI in ionic samples, the wave vector of surface plasmons with their momentum and energy decrease resulting in the extra increase in s [2]. In macroscopic view, when an electrolyte dissolves and ionizes in solution totally, the whole physicochemical system consisting of electrons, cations and neutral particles can be considered as a plasma which is electrically neutral. On microscopic level, all the particles are unstable and partly fluctuate because of thermal motion. The SPR effect is excited at a meta-dielectric interface, mass ions are influenced by evanescent field and the fluctuation enhanced. The electric interactions of free ions with the surface electrons of the gold film affect the resonance conditions of satisfaction. No matter how the ions influence worked, the surely positive correlation of and ions concentration in ionic samples can be introduced in SPR spectral analysis because of total dissolved ions estimation primarily concerned. 4. Principles of TDS estimation based SPR spectra Since TDS value is not easily measured, except under controlled conditions in reputable laboratories, many kinds of methods have been developed until now [19–25]. Distillation, the standard gravimetric method, is too time-consuming and processed in lab merely [20]. Deionization, reverse osmosis and electrochemical ion exchange methods are much inconvenient, complex and expensive due to the membranes and precision devices [21,22]. All above are
Please cite this article in press as: L. Feng, et al., Total dissolved solids estimation with a fiber optic sensor of surface plasmon resonance, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.12.040
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4 Table 1 Ion strength and valence typed of ionic solutions. Groups
1
Electrolyte solutions Valence type Ion strength I
NaCl 1–1 I=c
2 KCl 1–1 I=c
3
Na2 CO3 1–2 I = 3c
indirect ways that water is treated, purified or evaporated in the process. For on-line measurements and ease of implementation, practical methods are conducted on ionic parameter characteristics, such as ion concentration [23], activity coefficient [24], and ion conductivity [25,26]. Although electrical conductivity (EC) may be the most common way in practical use, one drawback is the correlation between EC and TDS varies greatly of nonlinearity, several meters of different measuring-ranges will be adopted [27,28], the other is the increasing electrophoresis effect which made it is not suitable in high concentration solutions [29,30]. Hence, for overall estimation of TDS concentration, this work introduced a relatively simple proportional scale factor of the calibration curve, which made SPR-tech can be put into use appropriately. As conductivity expresses how well a material conducts an electric current, when electrolyte solutions dissociate into positive and negatively charged ions, ions take over the charge transport in aqueous solutions. The more ions are present in liquid, the better it conducts the current. This relationship between ion concentration and the ability to conduct the electric current makes the conductivity an available process in TDS measurement [27–29]. As mentioned previously, the electrostatic forces which affecting the wave matching condition of SPR coupling play a role as localized enhancement modulation layer. The extra SPR shift reflected the interaction of ions and free electrons regularly, and it means the ions content relation can be established similar to conductivity. Generally, TDS is measured the sum concentration of the cations (positive charged) and anions (negative charged) ions in water and the particles are in a stable state of dynamic equilibrium. The charge-balance equation can be expressed as
z + mc =
z − ma
(1)
where ma and mc are molar concentration of cations and anions, respectively. For a certain water system especially, the electric neutrality equation is an approximate formula
− 2− Na+ + K+ + 2Ca2+ + 2Mg2+ = Cl− + 2SO2− 4 + NO3 + 2CO3 − − CO2− 3 + H2 O = HCO3 + OH
(2)
where HCO3− is micro-amount hydrolysis of carbonate. Since TDS accounts for the sum of ion concentration in water where Na+ , K+ , Mg2+ , Ca2+ , Cl− , SO4 2− , CO3 2− , HCO3− , etc. take up almost 95–99% of TDS content in natural water, the limiting error can be given by ıM = ±2 with a 95.44% mathematics probability of normal distribution for data collection. To determine the correlation between and TDS, the ion strength [30], which is a qualitative description of electric field intensity of ions, can be introduced and expressed as I=
1 2
ci zi2
CaCl2 2–1 I = 3c
MgCl2 2–1 I = 3c
MgSO4 2–2 I = 4c
ZnSO4 2–2 I = 4c
to the charge quantities of ionic atmosphere enlarge the electrostatic interaction, the larger the ion concentration is, the stronger the ion strength will be. Hence, the relationship between the and ions concentration can be processed. Linear fitting was made for 3 groups of electrolyte solutions based on ion strength of bond types (Fig. 5). The curve slopes with calculated values ki which are correlated with proportional scale factors in calibrations procedure, will play an important role in the following application of TDS measurement. Detailed theory analysis of SPR coupling considering the ionic activity and dielectric relaxation will be in a subsequent work. Here, we concern about the major portions of dissolved solids which are in ionic form of electrolyte solutions in natural water. For conductivity method, the well-known expression preferring a simple on-site rule of thumb always between EC and TDS is given by [27,28] CTDS = Ke EC
(4)
where linear factor K = 0.7 is general used. Some recommended appropriate K factors for distillates permeates, seawaters, etc. at 25 ◦ C were given in Ref. [27]. And EC in Eq. (4) can be also replaced with single ion concentration as Cl or Br for practical application [25]. For overall estimation of TDS, we rewrite the Eq. (4) by replace with total ion content Cm , and was substituted owning to the dependable linear approximation in Fig. 4, then the proposed equation is expressed as s =
1 10−3 · CTDS +b Cm + b = K KMr
(5)
where Cm is in mol/L, CTDS is mass concentration in mg/L, b is constant. Mr , the mean molecular mass of solution, is a stable constant in a given aqueous system. Total ions concentration of each component can be transferred to mg/L that Mr can be considered as weighting factor in Eq. (5) of each group. According to ion influence in 3 groups of ion strength, correction factors with ion parameters were introduced to replace K, linear fitting factor is expressed as
ki Mi K= Mi
(6)
(3)
where ci is the molar concentration of each ion, Zi is valence number of ions, c is the molar concentration. Then the electrolyte samples can be divided into 3 groups according to valence type (Table 1). Note that ion strength I showing the electrostatic field intensity of ions in each solution is consistent with the SPR shift (see Fig. 4). Moreover, in 1–1 type (NaCl, KCl) are much higher than other solutions for same RI as well. This is further evidence of the interaction between ions and free electrons exciting by SPR. Due
Fig. 5. Wavelength shift as a function of ion concentration for different valence type.
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Table 2 Measurement results of different categories of water sources. Category
Water, aqueous solution
TDS (g/L)
Conductivity (ms/cm)
Purity and potable water (group 0)
Puried millipore RO, desalinated water Rain water Urban tap-water Rural tap-water Lishui Rivera
0.005 0.02 0.42 0.78 1.18 1.58 ± 0.5
0.01 0.04 0.78 1.34 1.96 2.50 ± 0.30
SPR shift (nm) – – 0.20 0.60 0.70 0.75
K = TDS/EC 0.58
Natural brackish (group 1)
Surface brackish Underground minerala Saline lakesa Offshore seawater Eastern-Sea
5.1 ± 1.0 7.2 ± 1.5 11.5 ± 2.0 16.2 ± 3.0 34.0 ± 8.0
7.84 ± 0.60 10.04 ± 0.75 15.7 ± 1.0 23.1 ± 2.0 46.73 ± 5.0
1.05 1.50 1.65 2.40 4.21
0.73
Industrial waste and brinesa (group 2)
Domestic waste Pharmacy waste Textile effluents Heavy oil saline Chemical bittern High electrolyte solution
13.5 ± 4.0 18.5 ± 5.0 21.0 ± 5.0 50.5 ± 5.0 62.0 ± 10 80.0–100.0
18.57 ± 3.5 26.5 ± 4.0 29.0 ± 4.5 63.7 ± 5.0 81.27 ± 10 90–100
2.05 2.50 2.65 5.70 7.05–7.55 8.80–11.25
0.98
a Lishui River is a branch-stream of Yangzi river. Annotations in natural brackish is North Donnghai and Salt-City of JiangSu (Fig. 2), which lies between 34◦ 11 to 34◦ 44 N latitude and 118◦ 23 to 119◦ 10 E. Samples chosen with suspended solids, COD and colored substance preliminary filtration.
where ki is the curve slope in Fig. 5 and i subscripts the ionic solution type. To confirm the relationship, –TDS curve can be simply performed with all samples mixed above in lab. The progressive proportion with theoretical simulation can be Na:K:Mg:Ca:Zn:Cl: SO4 :CO3 = 1:3:2:1:1:6:2:1 in molar concentration, calculated linear correlation of and TDS by Eq. (5) and (6) was obtained in Fig. 6 with a K factor of 3.31. Note that is responsive to change with TDS and fit very well with measured samples. This confirms that the established relation will be appropriate in TDS estimation. Moreover, most of the sustainable water system, no matter natural water or industrial waste treatment, components tends to keep stable in a certain site and maintain a hydrological circle or drainage period, field testing of application can be implemented in statistical regression analysis without each component known. 5. Evaluation of TDS measurement with SPR-tech 5.1. Overall estimation and prediction For overall practical application, TDS level is an aggregative indicator preferring qualitative analysis in animals, plants growth and human health [31]. Not only compositions of samples not need precisely known with statistic analysis, the single ions component such as Cl/TDS relation in [25] was also used for prediction. According the
SPR spectral analysis previously, we used similar subtraction methods in SPR-tech for overall TDS measurement. The average results of TDS and EC obtained by standard gravimetric and conductivity meter are listed in Table 2 and Fig. 7. A wide scope evaluation from approximately 0 to 100 g/L was performed. Note that EC–TDS relationship shows a great nonlinear in a long range while segment detection and fitting is much linear, K factor varies from 0.50 to 1.01 due to the hindrance of ionic mobility by the crowding effect of ions at higher concentrations. This implies that EC–TDS relation in one group is not suitable in others, so we used more than 3 conductivity meters with different calibrations. However, SPR-tech shows high linearity and well generality in entire scope ranging from approximately 0 to 11.25 nm with an experimental detecting resolution of 0.05 g/L. The deviation in low concentration is much higher which can be result from resolution of SPR system and reference calibration of the impurities in water. However, the shortcomings will be weak as the TDS concentration increase. The one set of SPR instruments also implies that the preferable linearity will be more appropriate for unknown samples prediction. 5.2. Typical measurements and potential influences factors From the prospect of application, overall TDS estimation is general and field testing is more complex. Samples components proportion, seasonal variations, human activities, and other conditions, all influence the measurement accuracy and instantaneity. For example, natural brackish (group 1) and industrial waste (group 2) components are different, TDS level of natural water in saline lakes and seawater components will be stable while seasonal variations of total dissolved fluxes exist. Hence, to confirm the repeatability and feasibility, typical measurement in special monitor stations is implemented with segment correction. The practical prediction of statistic analysis by using auto regression time series model which can be expressed as t = Xt + st + Nt
Fig. 6. Wavelength shift as a function of TDS level in lab (mean reference signal of non-ionic solution is used in process).
(7)
Xt is time trend term due to the overall linear relation between and TDS, St is Seasonal items, Nt is noise item. We used a computing package “Statgraphics – Statistical Graphic System” to obtain the best model by using stepwise regression method. Since the focus is on SPR-tech, 3 typical monitoring sources as rural tapwater, eastern seawater, and oil refinery saline of each group are analyzed.
Please cite this article in press as: L. Feng, et al., Total dissolved solids estimation with a fiber optic sensor of surface plasmon resonance, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.12.040
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Fig. 7. Linear and polynomial fitting curves of EC and TDS (a and b); linear fitting of correlation (c). Groups of water samples and K factor are same in Table 2.
Parameters as EC, TDS, SPR shift and ion components of Ca, Mg, Cl, CO3 , Na, K, etc. are tested as well. Fig. 8 depicted repeatability, tendency, feasibility and cyclicity of the monitoring data. For rural tapwater shown in Fig. 8(a), SPR shift shows consistency with conductivity. The lower figure was processed with error correction and smoothing process. Although the measurement limitation exists, note that repeatability is still well. For seasonal variations which can be clearly observed in Fig. 8(b), EC increase sharply as seawater temperatures increasing in summer month (Jun-Aug). After temperature compensation, EC curve approaches SPR curve while wave trough is results from rainy seasons coming. For oil refinery saline samples analysis in Fig. 8(c), not only the drainage period (8:00 am–6:00 pm) is observed, measurement delay is also shown with plant shutdown at noon. Slow-moving results in EC curves is sharper than SPR curve, the obviously reason is that oil refinery saline has a large heavy metal component of 1–2 and 2–2 type ions such as SO4 , CO3 , Cl, etc. Further research of SPR-tech show accuracy and stability in a certain drainage period with a simple empirical weight correction of Eq. (5) as well.
Fig. 8. Monitoring results of rural tap-water (a), eastern seawater (b), and oil refinery saline (c) with different measurement cycle, respectively.
6. Conclusions In the present work, much more documents are provided that ions will have influence on SPR spectrum firstly. A simple and rapid method based on SPR spectral disparity analysis was developed for the determination of total dissolved ions in water. Then the proposed SPR-tech which is comparable to conductivity method was implemented into overall and field testing. The method could have potential applications in TDS measurement that it offers the advantages of anti-electromagnetic interference, rapidity and accuracy in effluents compared with conventional method based on conductivity. Acknowledgements The authors would like to thank the anonymous reviewers for their useful comments and suggestions. The work was supported by National Key Technology R&D Program of the Ministry of Science and Technology (Grants 2009BAG13A04) and Natural Science Foundation of China (Grants 51161120326).
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Total dissolved solids estimation with a fiber optic sensor of surface plasmon resonance Lihang Feng a,∗ , Weigong Zhang a,b,∗∗ , Dakai Liang c , Jenkins Lee d a
School of Instrument Science and Engineering, Southeast University, Nanjing 210096, China Suzhou Research Institution, Southeast University, Suzhou 215000, China State Key Laboratory of MCMS, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China d Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, Los Angeles, CA 90095, USA b c
a r t i c l e
i n f o
Article history: Received 2 July 2013 Accepted 18 December 2013 Available online xxx Keywords: Surface plasmon resonance Fiber optic sensor Total dissolved solid, Ion concentration
a b s t r a c t In this paper, surface plasmon spectra of ion influence was investigated by using a fabricated fiber optic sensors, then spectrum subtraction of ion and non-ionic solutions were developed for field applications of total dissolved solids (TDS) estimation. We confirmed the SPR spectral difference between seven ionic and three non-ionic liquid samples, that for the same refractive index, resonance wavelength in SPR spectrum is much higher for ionic samples than that in the case of non-ionic ones due to the ions influence. The positive correlation of ion content and extra resonance wavelength shift has been established for TDS estimation in water. With three groups of water samples investigation and field testing, the proposed SPR technique showed a good performance comparable to the conductivity method. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction In recent years, surface plasmon resonance (SPR) sensor has been extensively studied that many applications are ranging from biomaterials analysis to environmental inspection [1]. Being a surface technique that is sensitive to changes of refractive index (RI), SPR technique (SPR-tech) is becoming popular for monitoring small changes that occur in the surface of sensors. Surface plasmons are charge density oscillations of conduction electrons coupled to an electromagnetic wave at a meta-dielectric interface which have been extensively described [2]. To excite surface plasmons, techniques named after Otto and Kretschmann are generally employed, respectively [3,4]. Both configurations are based on prism and the applications for SPR sensing mainly covered two methods. One is scanning the incident angle of light called angular interrogation [5] and the other is wavelength interrogation [6]. Moreover, the prism based SPR sensing device has a number of shortcomings which can be replaced by an optical fiber, that was first proposed by Jorgenson in 1993 [7]. Afterwards, many researchers take interests into fiber-optic (F-O) SPR sensors which are manufactured by removing a small portion of cladding
∗ Corresponding author at: School of Instrument Science and Engineering, Southeast University, Nanjing 210096, China. Tel.: +86 02583794157. ∗∗ Corresponding author at: Suzhou Research Institution, Southeast University, Suzhou 215000, China. Tel.: +86 138 0903 1886. E-mail addresses: [email protected] (L. Feng), [email protected] (W. Zhang).
from the fiber and coating with a thin layer of metal [8]. This dipprobe geometry of F-O SPR sensor expanded increasingly because it allows experiment to be designed around the sample instead of the sensor. Now the F-O SPR sensors are showing various advantages and widely used in many fields. As water status is linked with our health so closely, we concerned about the issue of water quality extremely. Domestic water and effluents from agricultures or industries such as irrigation water, oil field refinery, electrical and mining, etc. contain high amounts of dissolved solids. The major portions of dissolved solids in water are in ionic form such as sodium, potassium, calcium, chloridion, sulfate radical, carbonate and other particles that will pass through a filter with pores of around 2 microns in size [9]. Researchers have applied SPR technique into various kinds of ions detection selectively such as potassium, copper, and other metals by different methods [10,11]. Booksh group also fabricated robust F-O SPR sensors which can be used to detect salinity with a potential precision of 10 ppm [12]. But these do not precisely represent the total dissolved solids (TDS) well in water analysis. As studies of SPR spectrum show the effects of temperature [13], electric potential [14] on parameters such as sensitivity, robustness, accuracy, etc. which people made numerous efforts to improve, researchers also find the effect of ions on surface plasmons since SPR sensing species can be divided into two types, either ionic or non-ionic [15]. However, not too much attention on the importance of ion concentration has been paid, neither do the applications. As the data are still scarce, we provide more documents and make further studies that the SPR-tech based on the effect can be put into TDS measurement.
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In the present study, surface plasmon spectra of ion influence were investigated further to estimate TDS levels in water. The positive correlation of ions content and resonance wavelength shift has been confirmed that response curves can be established for TDS estimation in water. Proposed SPR technique which is comparable to the conductivity method showed a good performance in TDS testing. The research will broaden the SPR application in water quality monitoring. 2. Materials and methods 2.1. Fiber optic SPR system An illustration of the F-O SPR probe which is 8 cm length with 1.5 cm of cladding from the end portion removed is depicted in Fig. 1, so are the scheme of SPR system and apparatus. To improve reproducibility of the probe, a stainless steel cylinder is required for affixing the probe into a standard FO connector (SMA.FC). The fabrication is similar to Pollet [16]. We use an ultrasonic cleaner and acetone to clean unclad portion of the fiber before coating with metal. After cleaning, the probes are sent into a sputtering machine (CFS-4ES, Shibaura, Japan). Typical 2 nm of Cr which acts as an adhesive is deposited and 50 nm of Au, too (see SEM in Fig. 1c). The average resolution of the sensors is approximately 5 × 10−5 refractive index units (RIU). By using a bifurcated optical fiber which connected with SPR probes, a tungsten-halogen light source (HL-2000, Ocean Optics, USA) is guided through and a UV–vis spectrometer is used to scan the reflected. Spectrometer is connected with a computer where data is recorded by software OOIBASE32 (Ocean Optics, USA) of spectral suite. As the spectrum in air is taken for reference signal, a spectral SPR-dip will show in reflection spectrum for a given solution and the resonance wavelength increases with the RI increasing. To improve the accuracy and minimize the noise, further data processing has been done in Matlab through several steps similar to Ref. [17]. 2.2. Samples preparation and data collection Sodium chloride (99.9%), potassium chloride (99.8%), sodium carbonate (99.9%), magnesium sulfate (99.8%), zinc sulfate (99.9%), calcium chloride (99%), magnesium chloride are Guarantee Reagent inorganics bases acids which purchased from Sinopharm Co. Led (China), so are glucose, glycerol and sucrose. Preparation of solution samples is processed at constant temperature (294 K) with Millipore water used. 7 ionic (NaCl, KCl, CaCl2 , MgCl2 , Na2 CO3 , MgSO4 , ZnSO4 ) and 3 non-ionic (glucose, glycerol, sucrose) solutions are selected transparent and colorless to avoid the influence of spectrum. Various ions in the samples including Na+ , K+ , Ca2+ , Mg2+ , Zn2+ , Cl− , CO3 2− , etc. which cover the most soluble salt ions about 95–99% of water impurities are considered.
Fig. 1. (a) Schematic diagram of F-O SPR/TDS system; (b) F-O SPR sensor; (c) SEM of the SPR probe after cleaning and Au coating.
Study site is chosen in JiangSu District of China (Fig. 2) and water samples are collected in clean polyethylene bottles from different sources. Hundreds of data observed and collected with different hydrological periods or drainage cycle is consumed including 3 simple specified groups of potable water, natural brackish, industrial waste and toxic brines for comparation (Table 2). High Millipore and desalinated samples are obtained in laboratory. Potable fresh water district spots of local area, natural fresh streams are Yangzi and branching rivers near, so are the natural brackish including eastern seawater samples. Industrial waste is simply collected with drainage from several companies and factories in Nanjing. Statistical research of all data collection is also assistance provided with different hydrology cycles by our fellows in Jiangsu Institute of Hydrology and Water Resources. Unmentioned matters which subject to the local environment are not strict discussed. 2.3. Methods and assay protocol RI measurements of all solutions were performed in lab by an Abbe refractometer with an accuracy of ±0.001. Each solution is independently prepared in different RI varying from 1.335 to 1.370 in steps of 0.005. For results calibration, we use standard gravimetric technique according to GB/T5750-2006 of National Standards (PR China) where method in lab for determining TDS after evaporation at 180 ◦ C is described. Measurements are accredited by the National Association of Testing Authorities of Jiangsu. Different ions in representative samples of tapwater, seawater, oil refinery saline are analyzed detailed (Sector 5.2) such as Ca, Mg, Cl, CO3 , Na, K by using ICP-AES and IC-HPLC spectrophotometer (Shimadzu, Japan) with standard techniques [18]. For example, Chloride determined routinely by an automated ferricyanide colorimetric method is using a segmented flow analyser with a detection limit of 1 mg/l. Before SPR and conductivity (conductivity meter, Mitutoyo, Japan) testing, organic components and suspended solids in less measurable samples are removed by filters. All samples obtained will be compared and analyzed with general estimation and specific measurement, respectively. Arithmetic mean values are also used because broad trends and verification tests are concerned primarily. 3. Spectral analysis for ionic and non-ionic solutions 3.1. Influence of ions on SPR spectrum To investigate the effect of ions on resonance wavelength, all 10 solutions including ionic and non-ionic ones are measured.
Fig. 2. Study site of Jiangsu in China around Yangtze River and its branches basin.
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Fig. 3. Ions influence on SPR spectrum: (a) resonance wavelength (s ) as a function of refractive index (RI) in all solutions; (b) SPR spectrum of each liquid sample with RI 1.360. (Curves are the second order polynomial fits. Colors of spectra correspond to color of the text denote the solutions with experimental points, respectively. The zoomed inset shows the variation of s near RI = 1.360.) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 3 shows the variation curves of resonance wavelength (s ) with refractive index (RI) of the samples. In all cases, due to wavelength dependence of the real part of the dielectric function of metal given by Drude model, the variation is nonlinear, s increases with the increase of solutions’ RI in accordance with previous studies [7,8]. However, the observed s is not same for all samples with RI ranging from 1.350 to 1.370. Taking out the SPR spectrum with RI about 1.360 of 10 solutions in Fig. 3 (inset figure) which is more clearly shown, for same RI of the solutions, s is much higher for 7 ionic samples in comparison to 3 non-ionic samples. No appreciable difference of s among non-ionic liquid samples is obtained and the mite of s may result from the presence of ions in solutions. The additional increase in s of ionic samples is due to the electrostatic interactions of the free ions with the surface electrons of the gold film, which has been proposed in Ref. [15]. The presence of ions causes an additional increase in s . The observation and results provide more documents, but our focus is on the importance of ion concentration and the applications which are based on this effect. 3.2. Relationship between ion concentration and SPR shift Further study of ion influence was conducted to obtain the extra resonance wavelength shift () from Fig. 3, non-ionic samples’ curve can be used as a reference (sucrose e.g.) and ion concentration as a function of RI for 7 ionic samples are shown in Fig. 4. Response curves are observed that ion concentration increases in a similar trend as with increase in RI for 7 ionic samples. Results indicate that, in such a solution, the increase of RI means an increase in ion content. A positive correlation can be confirmed that, the higher ions concentration in sample is, larger will be the increase in .
3
Fig. 4. Wavelength shift (a), ion concentration (b) as a function of RI for the different samples, respectively. Sucrose was used as a reference signal in (a) and more data were measured to enhance the reliability in (b).
Furthermore, if taking NaCl, KCl as example, for the same RI range, ions content in NaCl and KCl solutions are almost same and the curves overlap with each other approximately. Parallel results can be also found in other solutions. Hence, for the same RI, the additional increase , in the case of NaCl and KCl, is larger than that obtained for other solutions. We understand the consequence of the interaction between the ions and free electrons of metal film with a further qualitative explanation. For a given RI in ionic samples, the wave vector of surface plasmons with their momentum and energy decrease resulting in the extra increase in s [2]. In macroscopic view, when an electrolyte dissolves and ionizes in solution totally, the whole physicochemical system consisting of electrons, cations and neutral particles can be considered as a plasma which is electrically neutral. On microscopic level, all the particles are unstable and partly fluctuate because of thermal motion. The SPR effect is excited at a meta-dielectric interface, mass ions are influenced by evanescent field and the fluctuation enhanced. The electric interactions of free ions with the surface electrons of the gold film affect the resonance conditions of satisfaction. No matter how the ions influence worked, the surely positive correlation of and ions concentration in ionic samples can be introduced in SPR spectral analysis because of total dissolved ions estimation primarily concerned. 4. Principles of TDS estimation based SPR spectra Since TDS value is not easily measured, except under controlled conditions in reputable laboratories, many kinds of methods have been developed until now [19–25]. Distillation, the standard gravimetric method, is too time-consuming and processed in lab merely [20]. Deionization, reverse osmosis and electrochemical ion exchange methods are much inconvenient, complex and expensive due to the membranes and precision devices [21,22]. All above are
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4 Table 1 Ion strength and valence typed of ionic solutions. Groups
1
Electrolyte solutions Valence type Ion strength I
NaCl 1–1 I=c
2 KCl 1–1 I=c
3
Na2 CO3 1–2 I = 3c
indirect ways that water is treated, purified or evaporated in the process. For on-line measurements and ease of implementation, practical methods are conducted on ionic parameter characteristics, such as ion concentration [23], activity coefficient [24], and ion conductivity [25,26]. Although electrical conductivity (EC) may be the most common way in practical use, one drawback is the correlation between EC and TDS varies greatly of nonlinearity, several meters of different measuring-ranges will be adopted [27,28], the other is the increasing electrophoresis effect which made it is not suitable in high concentration solutions [29,30]. Hence, for overall estimation of TDS concentration, this work introduced a relatively simple proportional scale factor of the calibration curve, which made SPR-tech can be put into use appropriately. As conductivity expresses how well a material conducts an electric current, when electrolyte solutions dissociate into positive and negatively charged ions, ions take over the charge transport in aqueous solutions. The more ions are present in liquid, the better it conducts the current. This relationship between ion concentration and the ability to conduct the electric current makes the conductivity an available process in TDS measurement [27–29]. As mentioned previously, the electrostatic forces which affecting the wave matching condition of SPR coupling play a role as localized enhancement modulation layer. The extra SPR shift reflected the interaction of ions and free electrons regularly, and it means the ions content relation can be established similar to conductivity. Generally, TDS is measured the sum concentration of the cations (positive charged) and anions (negative charged) ions in water and the particles are in a stable state of dynamic equilibrium. The charge-balance equation can be expressed as
z + mc =
z − ma
(1)
where ma and mc are molar concentration of cations and anions, respectively. For a certain water system especially, the electric neutrality equation is an approximate formula
− 2− Na+ + K+ + 2Ca2+ + 2Mg2+ = Cl− + 2SO2− 4 + NO3 + 2CO3 − − CO2− 3 + H2 O = HCO3 + OH
(2)
where HCO3− is micro-amount hydrolysis of carbonate. Since TDS accounts for the sum of ion concentration in water where Na+ , K+ , Mg2+ , Ca2+ , Cl− , SO4 2− , CO3 2− , HCO3− , etc. take up almost 95–99% of TDS content in natural water, the limiting error can be given by ıM = ±2 with a 95.44% mathematics probability of normal distribution for data collection. To determine the correlation between and TDS, the ion strength [30], which is a qualitative description of electric field intensity of ions, can be introduced and expressed as I=
1 2
ci zi2
CaCl2 2–1 I = 3c
MgCl2 2–1 I = 3c
MgSO4 2–2 I = 4c
ZnSO4 2–2 I = 4c
to the charge quantities of ionic atmosphere enlarge the electrostatic interaction, the larger the ion concentration is, the stronger the ion strength will be. Hence, the relationship between the and ions concentration can be processed. Linear fitting was made for 3 groups of electrolyte solutions based on ion strength of bond types (Fig. 5). The curve slopes with calculated values ki which are correlated with proportional scale factors in calibrations procedure, will play an important role in the following application of TDS measurement. Detailed theory analysis of SPR coupling considering the ionic activity and dielectric relaxation will be in a subsequent work. Here, we concern about the major portions of dissolved solids which are in ionic form of electrolyte solutions in natural water. For conductivity method, the well-known expression preferring a simple on-site rule of thumb always between EC and TDS is given by [27,28] CTDS = Ke EC
(4)
where linear factor K = 0.7 is general used. Some recommended appropriate K factors for distillates permeates, seawaters, etc. at 25 ◦ C were given in Ref. [27]. And EC in Eq. (4) can be also replaced with single ion concentration as Cl or Br for practical application [25]. For overall estimation of TDS, we rewrite the Eq. (4) by replace with total ion content Cm , and was substituted owning to the dependable linear approximation in Fig. 4, then the proposed equation is expressed as s =
1 10−3 · CTDS +b Cm + b = K KMr
(5)
where Cm is in mol/L, CTDS is mass concentration in mg/L, b is constant. Mr , the mean molecular mass of solution, is a stable constant in a given aqueous system. Total ions concentration of each component can be transferred to mg/L that Mr can be considered as weighting factor in Eq. (5) of each group. According to ion influence in 3 groups of ion strength, correction factors with ion parameters were introduced to replace K, linear fitting factor is expressed as
ki Mi K= Mi
(6)
(3)
where ci is the molar concentration of each ion, Zi is valence number of ions, c is the molar concentration. Then the electrolyte samples can be divided into 3 groups according to valence type (Table 1). Note that ion strength I showing the electrostatic field intensity of ions in each solution is consistent with the SPR shift (see Fig. 4). Moreover, in 1–1 type (NaCl, KCl) are much higher than other solutions for same RI as well. This is further evidence of the interaction between ions and free electrons exciting by SPR. Due
Fig. 5. Wavelength shift as a function of ion concentration for different valence type.
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Table 2 Measurement results of different categories of water sources. Category
Water, aqueous solution
TDS (g/L)
Conductivity (ms/cm)
Purity and potable water (group 0)
Puried millipore RO, desalinated water Rain water Urban tap-water Rural tap-water Lishui Rivera
0.005 0.02 0.42 0.78 1.18 1.58 ± 0.5
0.01 0.04 0.78 1.34 1.96 2.50 ± 0.30
SPR shift (nm) – – 0.20 0.60 0.70 0.75
K = TDS/EC 0.58
Natural brackish (group 1)
Surface brackish Underground minerala Saline lakesa Offshore seawater Eastern-Sea
5.1 ± 1.0 7.2 ± 1.5 11.5 ± 2.0 16.2 ± 3.0 34.0 ± 8.0
7.84 ± 0.60 10.04 ± 0.75 15.7 ± 1.0 23.1 ± 2.0 46.73 ± 5.0
1.05 1.50 1.65 2.40 4.21
0.73
Industrial waste and brinesa (group 2)
Domestic waste Pharmacy waste Textile effluents Heavy oil saline Chemical bittern High electrolyte solution
13.5 ± 4.0 18.5 ± 5.0 21.0 ± 5.0 50.5 ± 5.0 62.0 ± 10 80.0–100.0
18.57 ± 3.5 26.5 ± 4.0 29.0 ± 4.5 63.7 ± 5.0 81.27 ± 10 90–100
2.05 2.50 2.65 5.70 7.05–7.55 8.80–11.25
0.98
a Lishui River is a branch-stream of Yangzi river. Annotations in natural brackish is North Donnghai and Salt-City of JiangSu (Fig. 2), which lies between 34◦ 11 to 34◦ 44 N latitude and 118◦ 23 to 119◦ 10 E. Samples chosen with suspended solids, COD and colored substance preliminary filtration.
where ki is the curve slope in Fig. 5 and i subscripts the ionic solution type. To confirm the relationship, –TDS curve can be simply performed with all samples mixed above in lab. The progressive proportion with theoretical simulation can be Na:K:Mg:Ca:Zn:Cl: SO4 :CO3 = 1:3:2:1:1:6:2:1 in molar concentration, calculated linear correlation of and TDS by Eq. (5) and (6) was obtained in Fig. 6 with a K factor of 3.31. Note that is responsive to change with TDS and fit very well with measured samples. This confirms that the established relation will be appropriate in TDS estimation. Moreover, most of the sustainable water system, no matter natural water or industrial waste treatment, components tends to keep stable in a certain site and maintain a hydrological circle or drainage period, field testing of application can be implemented in statistical regression analysis without each component known. 5. Evaluation of TDS measurement with SPR-tech 5.1. Overall estimation and prediction For overall practical application, TDS level is an aggregative indicator preferring qualitative analysis in animals, plants growth and human health [31]. Not only compositions of samples not need precisely known with statistic analysis, the single ions component such as Cl/TDS relation in [25] was also used for prediction. According the
SPR spectral analysis previously, we used similar subtraction methods in SPR-tech for overall TDS measurement. The average results of TDS and EC obtained by standard gravimetric and conductivity meter are listed in Table 2 and Fig. 7. A wide scope evaluation from approximately 0 to 100 g/L was performed. Note that EC–TDS relationship shows a great nonlinear in a long range while segment detection and fitting is much linear, K factor varies from 0.50 to 1.01 due to the hindrance of ionic mobility by the crowding effect of ions at higher concentrations. This implies that EC–TDS relation in one group is not suitable in others, so we used more than 3 conductivity meters with different calibrations. However, SPR-tech shows high linearity and well generality in entire scope ranging from approximately 0 to 11.25 nm with an experimental detecting resolution of 0.05 g/L. The deviation in low concentration is much higher which can be result from resolution of SPR system and reference calibration of the impurities in water. However, the shortcomings will be weak as the TDS concentration increase. The one set of SPR instruments also implies that the preferable linearity will be more appropriate for unknown samples prediction. 5.2. Typical measurements and potential influences factors From the prospect of application, overall TDS estimation is general and field testing is more complex. Samples components proportion, seasonal variations, human activities, and other conditions, all influence the measurement accuracy and instantaneity. For example, natural brackish (group 1) and industrial waste (group 2) components are different, TDS level of natural water in saline lakes and seawater components will be stable while seasonal variations of total dissolved fluxes exist. Hence, to confirm the repeatability and feasibility, typical measurement in special monitor stations is implemented with segment correction. The practical prediction of statistic analysis by using auto regression time series model which can be expressed as t = Xt + st + Nt
Fig. 6. Wavelength shift as a function of TDS level in lab (mean reference signal of non-ionic solution is used in process).
(7)
Xt is time trend term due to the overall linear relation between and TDS, St is Seasonal items, Nt is noise item. We used a computing package “Statgraphics – Statistical Graphic System” to obtain the best model by using stepwise regression method. Since the focus is on SPR-tech, 3 typical monitoring sources as rural tapwater, eastern seawater, and oil refinery saline of each group are analyzed.
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Fig. 7. Linear and polynomial fitting curves of EC and TDS (a and b); linear fitting of correlation (c). Groups of water samples and K factor are same in Table 2.
Parameters as EC, TDS, SPR shift and ion components of Ca, Mg, Cl, CO3 , Na, K, etc. are tested as well. Fig. 8 depicted repeatability, tendency, feasibility and cyclicity of the monitoring data. For rural tapwater shown in Fig. 8(a), SPR shift shows consistency with conductivity. The lower figure was processed with error correction and smoothing process. Although the measurement limitation exists, note that repeatability is still well. For seasonal variations which can be clearly observed in Fig. 8(b), EC increase sharply as seawater temperatures increasing in summer month (Jun-Aug). After temperature compensation, EC curve approaches SPR curve while wave trough is results from rainy seasons coming. For oil refinery saline samples analysis in Fig. 8(c), not only the drainage period (8:00 am–6:00 pm) is observed, measurement delay is also shown with plant shutdown at noon. Slow-moving results in EC curves is sharper than SPR curve, the obviously reason is that oil refinery saline has a large heavy metal component of 1–2 and 2–2 type ions such as SO4 , CO3 , Cl, etc. Further research of SPR-tech show accuracy and stability in a certain drainage period with a simple empirical weight correction of Eq. (5) as well.
Fig. 8. Monitoring results of rural tap-water (a), eastern seawater (b), and oil refinery saline (c) with different measurement cycle, respectively.
6. Conclusions In the present work, much more documents are provided that ions will have influence on SPR spectrum firstly. A simple and rapid method based on SPR spectral disparity analysis was developed for the determination of total dissolved ions in water. Then the proposed SPR-tech which is comparable to conductivity method was implemented into overall and field testing. The method could have potential applications in TDS measurement that it offers the advantages of anti-electromagnetic interference, rapidity and accuracy in effluents compared with conventional method based on conductivity. Acknowledgements The authors would like to thank the anonymous reviewers for their useful comments and suggestions. The work was supported by National Key Technology R&D Program of the Ministry of Science and Technology (Grants 2009BAG13A04) and Natural Science Foundation of China (Grants 51161120326).
Please cite this article in press as: L. Feng, et al., Total dissolved solids estimation with a fiber optic sensor of surface plasmon resonance, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.12.040
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Please cite this article in press as: L. Feng, et al., Total dissolved solids estimation with a fiber optic sensor of surface plasmon resonance, Optik - Int. J. Light Electron Opt. (2014), http://dx.doi.org/10.1016/j.ijleo.2013.12.040