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A portable photometer based on LED for the determination of aromatic hydrocarbons in water
Microchemical Journal 103 (2012) 62–67
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Microchemical Journal journal homepage: www.elsevier.com/locate/microc
A portable photometer based on LED for the determination of aromatic hydrocarbons in water Kássio Michell Gomes de Lima ⁎ Federal University of Rio Grande do Norte, Institute of Chemistry, Grupo de Pesquisa em Quimiometria Aplicada, CEP 59072-970, Natal, RN, Brazil
a r t i c l e
i n f o
Article history: Received 26 December 2011 Accepted 11 January 2012 Available online 17 January 2012 Keywords: NIR Photometer-LED Optical sensor Aromatic hydrocarbons
a b s t r a c t The present work describes a portable photometer based on light-emitting-diodes (LED) for the determination of total aromatic hydrocarbons in water contaminated with gasoline. Two LED were employed as the light source with maximum emission at 1300 nm and 1689 nm. The radiation of each LED was collected with bundles of 24 silica optical fibers, which were assembled in a single bundle pointed toward the detection cell. An InGaAs photodiode was directly connected to the measuring cell. Software was written in VisualBasic.NET to control the photometer through a USB interface and for data acquisition. The determination of total aromatic hydrocarbons was performed using a silicone-sensing phase (length of 5 mm and diameter of 3.2 mm), which was employed to extract these compounds from the contaminated water. The extraction (60 min under constant stirring) was performed in a 100-mL flask filled with the water sample diluted in 2.0 mol L− 1 NaCl. A repeatability of 1.8% and 1.3% (expressed as the relative standard deviation of 10 measurements) was obtained at 1300 nm and 1689 nm, respectively. Absorbance values were calculated considering the signal at 1300 nm as reference because hydrocarbons do not absorb at this wavelength. Analytical curves up to 200 mg L− 1 and 400 mg L− 1 were constructed for benzene and toluene, providing limits of detection of 1.2 mg L− 1 and 1.7 mg L− 1, respectively. The sensitivity was similar to those obtained with a FT-NIR Bomem MB 160 spectrophotometer at the same wavelength. The photometer was applied to the determination of total hydrocarbons in water contaminated by gasoline. An analytical curve (50–300 mg L− 1) was constructed from reference solutions containing benzene and toluene in the ratio of 3:7, which is a ratio commonly found in Brazilian gasoline. The results provided by the photometer were compared with those obtained with the commercial spectrophotometer and did not show significant differences at a confidence level of 95%. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the interest of the miniaturization of instruments for field measurements, light-emission-diodes (LEDs) have become increasing popular in optical instruments in general and in photometers LED in particular. Among the main advantages of these instruments [1] are their simplicity of operation, low cost and power consumption, high stability and compatibility with optical fibers. In these LED photometers, the time delay between the sequential activation of the emitters and individual measurements on several wavelengths is very low (on the order of tenths of seconds), as in a diode array spectrophotometer. By employing LEDs that emit radiation at wavelengths previously selected (e.g., UV, Vis, NIR and MID), the information generated by a multi-LED photometer may be sufficient for use in analysis or discriminatory simultaneous determinations based on multivariate analysis. Several applications of LED photometers in different regions of the electromagnetic spectrum are present in the literature. In the UV
⁎ Tel.: + 55 84 3215 3828. E-mail address: [email protected]. 0026-265X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2012.01.003
region, instruments were developed for determining sulfate in water [2], acidity in fruit juices [3], and nitrite and nitrate in water [4], among others. For example, Zarate and colleagues [2] proposed an automatic procedure for the turbidimetric determination of sulfate in rain water based on a multicommutation flow analysis process using a photometer with an LED (λ = 420 nm), a flow cell with an optical path of 100 mm and a photodiode. In 2010, [3] Silva and colleagues described a low cost photometer based on an identical pair of LEDs (λ = 531 nm): one employed as a light source and the other as a photodetector. The equipment, including the photometer and the analysis module, was designed for an automatic titration procedure using multicommutation flow injection (MCFIA). Liu and colleagues [4] proposed an LED-photometer that operated in the UV region (λ = 255 nm) for the determination of total nitrite and nitrate in water. In this study, we found the detection limits for nitrite and nitrate to be 7 mol L − 1 and 12 mol L − 1, respectively. There are multiple examples of LED-photometers operating in the UV–vis region and combined with chemometric analysis. For example, Fonseca and colleagues [5] described the development of a portable multi-LED photometer operating in the visible region and applied for the simultaneous determination of Zn(II) and Cu(II) in pharmaceutical
K.M.G. de Lima / Microchemical Journal 103 (2012) 62–67
formulations. Multivariate calibration models based on multiple linear regression provided quadratic mean errors of prediction (RMSEP) of 0.06 and 0.12 mg L − 1 for Zn(II) and Cu(II), respectively. The same authors applied this multi-LED photometer in 2007 [6] for the discrimination of mineral water samples using chemometric tools. In addition, Hauser and colleagues [7] described a multi-LED photometer in the visible region to determine various species in solution, including ammonia, Cu(II), Al(III), Ca(II), Cr(III), phosphate and nitrite. The relative errors based on the measurements of concentration were between 0.2 and 5.0%. The NIR region offers additional advantages for analytical applications of LED photometers, such as a non-destructive nature, versatility and the possibility of in situ analysis. However, the large bandwidth of the LED (30 to 50 nm), pre-concentration steps and the need for chemometric tools limit work in this region. However, several applications of NIR LED-based photometers can be found in the literature for the determination of chlorophylls [8], adulteration of petrol [9], moisture tufa [10], lubricants [11] and others. For example, McClure and colleagues [8] developed a portable NIR photometer, coined “TWmeter” by the authors, using three LEDs with peak emissions at 700, 880 and 940 nm for the determination of chlorophyll in plants and moisture in paper. In this paper, multivariate calibration by multiple linear regression was used to determine the chlorophyll, resulting in a prediction error of 0.99 mg/cm2 of leaf surface, a dynamic range of 1.8 mg/cm2 and a humidity range of 30–65%. Additionally, Gaiao and colleagues [9] proposed a portable NIR photometer based on an LED for the low-cost detection of adulteration of Brazilian commercial gasoline with kerosene. The photometer contained an LED with a peak emission at 1550 nm and a detection system consisting of a PbSe photoresistor controlled by a microcontroller. The authors showed that the instrument could detect samples of gasoline contaminated with kerosene at 5% (v/v). Hyvarinen and colleagues [10] developed two NIR photometers for moisture determination in peat. This integration used a photometer detector and interference filters with narrow bands in a single semiconductor. Finally, Pignalosa and colleagues [11] presented an LED-based photometer for the analysis of lubricants with limits of detection and quantification of 0.07% and 0.16% (w/w), respectively. LED-based instruments have a simple assembly and low cost, are versatile and can be employed in the determination of many chemical parameters with promising results. Thus, the objective of this work was the development of a portable photometer-LED in the near infrared region (NIR) for the determination of BTEX compounds in contaminated water.
2. Experimental procedures 2.1. Reagents and solutions Benzene, toluene, and xylenes were purchased from Merck and used as received. Methanol (spectroscopic grade, Merck) was used to prepare stock solutions of BTX aromatic hydrocarbons. Sodium chloride (Vetec) was used to adjust the salinity of water samples. Silastic T2 (crosslinking agent) was provided by Dow Corning. Distilled and deionized water was used to prepare contaminated aqueous samples.
2.2. Reference solutions Initially, standard solutions of benzene and toluene at 50000 mg L − 1 were prepared, and ethylbenzene and xylene were prepared at 10 000 mg L − 1 in methanol. Then, 5.00 mL of these solutions was transferred to a 500 mL volumetric flask, and the solution was brought to its final volume with deionized water. The resulting stock solutions represented 500 mg L− 1 of benzene and toluene as well as 100 mg L − 1 of ethylbenzene and xylenes with 1% methanol.
63
2.3. Preparation of the sensing phase The polymeric sensing phase consisted of a silicone-based polydimethylsiloxane (PDMS). The monomer and crosslinking agent was mixed in a 10:1 ratio as recommended by the manufacturer. The homogenization was carried out in an ultrasound bath for approximately 3 min to remove the air bubbles formed during mixing. The mixture was subsequently transferred to Teflon containers molded into a disc shape (height of 15 cm and diameter of 4 cm). These containers were placed in a Teflon vacuum desiccator for 30 min to remove any bubbles that remained after homogenizing the mixture. The polymeric material was left for 36 h for polymerization/crosslinking in an airtight plastic container. After this period, the phase sensing polymer was placed in an oven for 6 h at 60 °C to complete the crosslinking. The phase sensors were then removed from the Teflon molds and cut into a cylinder with a diameter of 3.2 mm and a height of 5 mm. 2.4. Instrumentation An FT-NIR spectrophotometer (Bomen MB 160) was used for measurements in the region of 800 nm to 2500 nm. The data were obtained using Win-Bomem Easy 3.04 software. Each spectrum was recorded as an average of 100 scans with a resolution of 8 cm − 1. A transmission cell was made in our laboratory for the optical fibers. The optical path of the cell was determined by spacers of different length (1 mm, 2 mm, 5 mm and 10 mm), with the 5 mm spacer utilized in the following experiments. 2.5. NIR-LED photometer 2.5.1. Power supply The instrument used a commercial Mean Well power supply produced and distributed by LR Industrial Informatics. The source model was DP-2512 with a symmetrical output of ±12 V. 2.5.2. Array LED Six LEDs emitting radiation in different areas of the NIR spectrum were acquired from Roithner Lasertechnik. The wavelengths of the maximum emission indicated by the manufacturer were 1200 nm, 1300 nm, 1550 nm, 1650 nm, 2150 nm and 2350 nm. Each LED was fixed in a metal connector, and the arrangement of the LEDs was achieved on a crafted acrylic plate. 2.5.3. Collection of NIR radiation Twenty-four beams containing polymer optical fibers 200 μm in diameter were each (Ocean Optics) connected to each LED to guide the individual radiation emitters. These beams were grouped into a single output beam containing 144 fibers, which was connected directly to the transmission cell. 2.5.4. Circuit electronic photometer The circuit designed for activation of the LEDs allowed the use of a diode array with up to six LED. The circuit (Fig. 1a) consisted of current limiting resistors and drivers (commonly called transistor keys) for driving the LEDs with virtually infinite impedance between its input and its output. This ensured that overvoltage or short circuits occurring in the LED drive did not reach the acquisition interface, protecting the interfaced PC. The drivers used were Darlington (ULN2001 chip), produced by Texas Instruments. Sixty-eight ohm resistors and variable resistors with a maximum resistance of 1 kΩ were employed to determine the current through the LED. With the variable resistors, the emission intensities of the LEDs could be adjusted according to the sensitivity of the detector (photodiode G8373), which provides a stable signal to all the LEDs.
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K.M.G. de Lima / Microchemical Journal 103 (2012) 62–67
a
3 4
2
b
1
5 Fig. 2. Overview of NIR photometer: 1) LED and fiber optic assemblies, 2) power supply, 3) electronic circuit for powering the LEDs and signal amplification, 4) cell broadcast and the PDMS phase sensor, 5) USB interface for data acquisition.
radiation). The absorbance in each measurement was obtained from the difference described in Eq. (3). Fig. 1. (a) Electronic circuit of the NIR photometer powering the LEDs. (b) Electronic circuit built for signal conditioning of the InGaAs detector.
2.5.5. Detector The photometer employed here as a sensor was an InGaAs photodiode (model G8373) produced by Hamamatsu. The electronic circuitry used for conditioning the signal from the photodiode is detailed in Fig. 1b. Associated with the InGaAs photodiode, a voltage–current converter (Module I) that also operated as a first stage amplification was attached. The signal was amplified again in Module II. 2.5.6. Interface and data acquisition The interface used in this photometer facilitated communication between the dedicated software, controlling the instrument and data acquisition. Six of the twelve digital outputs were used to drive the LED, providing a logic low for those that must remain turned off and a high logic level for those that were fired. One of the twelve analog-digital (A/D converter) inputs was used for data acquisition from the photodiode. The interface used was a USB-6009 model from National Instruments. A program was written in the photometer VisualBasic.NET environment to control and acquire data from the photodiode through the electronic interface, which had an analog/ digital (A/D) of 14 bits. The written program drove the LEDs and collected the signals.
A ¼ A1689 −A1300
Fig. 2 shows a general outline of the developed NIR photometer. The InGaAs photodiode was connected directly to the optical fiber to collect direct radiation from the LEDs to prevent radiation loss. 2.6. Experimental procedure Measurements were performed by placing a PDMS monolith in an 85 mL bottle completely filled with the aqueous mixture of aromatic hydrocarbons and subjected to constant stirring (2700 rpm) for a time interval of 60 min at 22 ± 1 °C for the extraction of hydrocarbons. After this step, the monolith was removed from the mixture, quickly blotted dry and inserted into the measurement system. An adequate amount of NaCl was transferred to the bottle so that it was completely filled with the hydrocarbon solution, assisting in the transfer of compounds to the PDMS sensing phase. All extractions were performed in sealed vials. The spectrum of PDMS, previously immersed in an aqueous solution containing 1% methanol, was used as the reference for absorbance measurements. All measurements were employing new phase sensing material.
2.5.7. Calculation of absorbance and vision of the NIR photometer The absorbance for two LEDs operating at 1689 nm and 1300 nm can be calculated according to Eqs. (1) and (2), respectively. A1689 ¼ − log
I1689 −Esc R1689 −Esc I1300 −Esc R1300 −Esc
ð1Þ
A1300 ¼ − log
ð2Þ
where I is the average of the sample signals (phase sensor in contact with the BTEX compounds), R is the average of the signals relating to the reference measurements (only the phase sensing) and Esc is the average of the signals relating to measurements in the dark (no
ð3Þ
Fig. 3. Emission spectra of six standard LEDs in the NIR region.
K.M.G. de Lima / Microchemical Journal 103 (2012) 62–67
0,05
Table 1 Wavelengths of peak emission and bandwidths at half height for the LEDs used in the studied photometer. Measured wavelength (nm)
Relative error (%)
Effective bandwidth at half height (nm)
1200 1300 1550 1650 2150 2350
1197 1301 1504 1689 2186 2344
− 0.25 + 0.07 + 0.26 + 1.78 + 1.64 − 0.25
100 100 180 240 373 248
Day 1 Day 2 Day 3
0,04
Absorbance - a.u
Nominal wavelength (nm)
65
0,03
0,02
0,01
3. Results and discussion Using the spectrometer developed in our laboratory, the emission spectra of six LEDs were obtained. Fig. 3 shows the LED spectra after normalization. As observed, the LEDs had six NIR emissions that were the most informative for this application. Table 1 shows the wavelengths of maximum emission and the effective bandwidth at half height for each of the devices. The nominal wavelengths (indicated by the manufacturers) are also presented in this table. The results shown in Table 1 reveal small deviations (less than 2%) for the six wavelengths of the LEDs in relation to their nominal values. Variations in nominal wavelengths could be attributed to possible non-uniformities in the manufacturing process and/or calibration of the spectrophotometer. However, the bandwidths at half height for LEDs at longer wavelengths (1689 nm, 2150 nm and 2350 nm) were larger, indicating a loss of LED monochromaticity. The emission intensity profiles versus number of readings for all the LEDs were drawn from the signals found in the peak emission of 1000 readings obtained after the sequential activation of emitters. Fig. 4 shows the intensity profile for the LED with a peak emission at 1689 nm. For these devices, the intensity of the LED became steady after its minimum value was reached. The time signal “zero” was obtained immediately after the firing of the LED. The signal strength decreased by 4.1% after reaching its maximum value, achieving stability approximately 80 s after the drive. As noted in previous works, the more intense absorption peaks found for the BTEX compounds in NIR are 1675 nm, 2140 nm, 2165 nm and 2340 nm. As BTEX compounds do not absorb radiation at approximately 1300 nm and the variation in signal strength in this region reflects only the position of the sensing phase, it is necessary to use more than one LED. Therefore, the behavior of the LED with a peak emission at 1689 nm was initially assessed using a photometer cell
0,00 0
100
200
Toluene - mg
300
400
L-1
Fig. 5. Three analytical curves obtained for toluene on different days with an extraction time of 60 min in 2 mol L− 1 NaCl using a 5 mm sensing phase.
with a PDMS sensing phase having a 5 mm optical path. Those LED at 1500 nm, 2150 nm and 2350 nm showed extremely low intensities, indicating that virtually all the radiations were absorbed by the sensing phase. For this reason, LEDs with peak emissions of 1500 nm, 2150 nm and 2350 nm were not used in this application. The LED with a maximum intensity of 1689 nm gave a higher intensity and was instrumental in building the photometer, as 1689 nm was in the region related to the 1st C–H stretch of BTEX compounds. The performance of the LEDs with peak emissions at 1200 nm and 1300 nm were similar to those observed for the 1689 nm LED. As the standard curves of the spectrophotometer for the determination of BTEX were constructed using only the wavelengths 1300 nm and 1675 nm, the LEDs with maximum intensity at 1300 nm and 1689 nm were selected for this application. Therefore, the LED with peak emission at 1300 nm was employed in the construction of analytical curves of BTEX compounds in the photometer as a reference signal, and the LED with a maximum intensity at 1689 nm was used for the signal of the analytes. The accuracy of the proposed method using the NIR photometer was evaluated by constructing three analytical curves for toluene in the range from 0 to 400 mg L − 1. For this purpose, a transmission cell with optical fibers (Fig. 5) was used, resulting in an average slope with a standard deviation of 11.8% and a linear coefficient
Fig. 4. Profile of intensity vs. time for LED emission at 1689 nm.
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K.M.G. de Lima / Microchemical Journal 103 (2012) 62–67
a
Table 3 Determination of total benzene and toluene (mg L− 1) in water by different instruments at 1689 nm.
0,08 Spectrophotometer NIR (1675 nm) Spectrophotometer NIR (1689 nm) Photometer NIR
Absorbance - a.u
0,06
0,00 0
50
100
150
200
Benzene - mg L-1 0,10 Photometer NIR Spectrophotometer NIR (1675 nm) Spectrophotometer NIR (1689 nm)
0,08
Absorbance - a.u
Photometer NIR
Spectrophotometer NIR
1 2 3
156 ± 11 145 ± 16 157 ± 14
149 ± 14 140 ± 10 147 ± 11
0,04
0,02
b
Samples
0,06
0,04
0,02
0,00 0
100
200
300
400
Toluene - mg L-1
4. Conclusion
Fig. 6. Comparison of analytical curves obtained for benzene (a) and toluene (b) from the photometer and the spectrophotometer employing a PDMS phase containing a sensing optical path length of 5 mm in 2 mol L− 1 NaCl with an extraction time of 60 min.
with a standard deviation of 7.64%. Each of these curves was built on different days and with new standard solutions. These results indicated that the proposed method requires daily calibration. Analytical curves were obtained for benzene and toluene using the photometer operating under the following experimental conditions: 5 mm optical path, 2 mol L − 1 NaCl and 60 min of extraction. In this case, measurements were carried out on two different instruments, an NIR photometer and a commercial instrument (FT-NIR), to compare sensitivity. Once the signals from the LED emission maximums at 1689 nm and 1300 nm were registered by the photometer, the phase sensor was removed from the transmission cell photometer and immediately inserted into the other transmission cell coupled to the spectrophotometer for the registration of trade spectra. Fig. 6a shows the standard curves obtained for benzene compounds in the range from 0 to 200 mg L − 1, and Fig. 6b shows the standard curves for toluene in the range from 0 to 400 mg L − 1. The measurements performed at 1675 nm, which matched the wavelength of Table 2 Comparison of slopes obtained for the analytical curves of benzene and toluene using both the photometer and the spectrophotometer at 1689 nm. Compounds
Benzene Toluene
maximum absorption for the 1st C−H stretch of the compounds studied, are shown for comparison. As observed in the figures above, the sensitivity obtained for the spectrophotometer at 1675 nm was 1.7 to 2.6 times higher for benzene and toluene when compared to the results obtained by observing the maximum emission of the LED at 1689 nm. Comparing the sensitivities between the two instruments at 1689 nm (Table 2), it could be concluded that the sensitivities of the two instruments were quite similar. Ideally, the sensitivity would be greatly improved if the maximum emission of the LED was exactly 1675 nm. In addition, the detection limits for benzene and toluene in the NIR photometer were 1.2 and 1.7 mg L − 1, respectively, as calculated from the analytical curves shown in Fig. 6. To evaluate the developed NIR photometer against actual contamination, water samples were contaminated with type A gasoline (ratio of 2.5% (v/v)). Table 3 shows the results of the content of benzene and toluene for the three samples measured in triplicate using the total optical sensor developed for the photometer and the NIR spectrophotometer, both at 1689 nm. A t test of the results obtained from the two instruments (photometer and spectrophotometer) showed no statistically significant differences at a confidence level of 95%. Therefore, the NIR photometer-based LED could be considered a viable, low-cost alternative for the determination of benzene and toluene in water.
Instruments Spectrophotometer
Photometer
1.22 × 10− 4 1.13 × 10− 4
1.13 × 10− 4 8.25 × 10− 5
The potential of a multi-channel NIR photometer based on an array of LEDs was investigated in the determination of BTEX in gasolinecontaminated water using a PDMS sensing phase. The photometer based on LEDs was a simple, low-cost assembly, which was much cheaper than most NIR spectrophotometers currently available on the market. These results indicated that the instrument developed could be an economical and simple alternative for the application studied. Acknowledgments The author is grateful to professors Dr. Ivo M. Raimundo Jr. (UNICAMP-Brazil) and M.F Pimentel (UFPE-Brazil) for technical assistance and to CNPq and CAPES/PROCAD for financial support. References [1] P.K. Dasgupta, H.S. Bellamy, H. Liu, J.L. Lopez, E.L. Loree, K. Morris, K. Petersen, K.A. Mir, Light-emitting diode based flow-through optical-absorption detectors, Talanta 40 (1993) 53–74. [2] N. Zarate, R. Olmos-Pérez, B.F. Reis, Turbidimetric determination of sulfate in rainwater employing a LED Based photometer and multicommuted flow analysis system with in-line preconcentration, J. Braz. Chem. Soc. 22 (2011) 1009–1014. [3] M.B. Silva, C.C. Crispino, B.F. Reis, Automatic photometric titration procedure based on multicommutation and flow-batch approaches employing a photometer based on twin LEDs, J. Braz. Chem. Soc. 21 (2010) 1854–1860. [4] M. Zhang, Z. Zhang, D. Yuan, S. Feng, B. Liu, An automatic gas-phase molecular absorption spectrometric system using a UV-LED photodiode based detector for determination of nitrite and total nitrate, Talanta 84 (2011) 443–450. [5] A. Fonseca, I.M. Raimundo Jr., A multichannel photometer based on an array of light emitting diodes for use in multivariate calibration, Anal. Chim. Acta 522 (2004) 223–229. [6] A. Fonseca, I.M. Raimundo Jr., A simple method for water discrimination based on an light emitting diode (LED) photometer, Anal. Chim. Acta 596 (2007) 66–72.
K.M.G. de Lima / Microchemical Journal 103 (2012) 62–67 [7] P.C. Hauser, T.W.T. Rupasinghe, Simultaneous determination of metal ion concentrations in binary mixtures with a multi-LED photometer, Fresenius J. Anal. Chem. 357 (1997) 1056–1060. [8] W.F. McClure, D. Moody, D.L. Stanfield, O. Kinoshita, Hand-held NIR spectrometry. Part II: an economical no-moving parts spectrometer for measuring chlorophyll and moisture, Appl. Spectrosc. 56 (2002) 720–724. [9] E.N. Gaião, S.R.B. Dos Santos, V.B. Dos Santos, E.C.L. Nascimento, R.S. Lima, M.C.U. Araújo, An inexpensive, portable and microcontrolled near infrared LEDphotometer for screening analysis of gasoline, Talanta 75 (2008) 792–796.
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[10] T. Hyvarinen, P. Niemela, Rugged multiwavelenght NIR and IR analyzers for industrial-process measurements, SPIE (Process Optical Measurements and Industrial Methods) 1266 (1990) 99–104. [11] G. Pignalosa, A. Sixto, M. Knochen, Automatic determination of insolubles in lubricating oils by flow injection analysis employing an LED-photometer detector, Talanta 73 (2007) 959–961.
Contents lists available at SciVerse ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
A portable photometer based on LED for the determination of aromatic hydrocarbons in water Kássio Michell Gomes de Lima ⁎ Federal University of Rio Grande do Norte, Institute of Chemistry, Grupo de Pesquisa em Quimiometria Aplicada, CEP 59072-970, Natal, RN, Brazil
a r t i c l e
i n f o
Article history: Received 26 December 2011 Accepted 11 January 2012 Available online 17 January 2012 Keywords: NIR Photometer-LED Optical sensor Aromatic hydrocarbons
a b s t r a c t The present work describes a portable photometer based on light-emitting-diodes (LED) for the determination of total aromatic hydrocarbons in water contaminated with gasoline. Two LED were employed as the light source with maximum emission at 1300 nm and 1689 nm. The radiation of each LED was collected with bundles of 24 silica optical fibers, which were assembled in a single bundle pointed toward the detection cell. An InGaAs photodiode was directly connected to the measuring cell. Software was written in VisualBasic.NET to control the photometer through a USB interface and for data acquisition. The determination of total aromatic hydrocarbons was performed using a silicone-sensing phase (length of 5 mm and diameter of 3.2 mm), which was employed to extract these compounds from the contaminated water. The extraction (60 min under constant stirring) was performed in a 100-mL flask filled with the water sample diluted in 2.0 mol L− 1 NaCl. A repeatability of 1.8% and 1.3% (expressed as the relative standard deviation of 10 measurements) was obtained at 1300 nm and 1689 nm, respectively. Absorbance values were calculated considering the signal at 1300 nm as reference because hydrocarbons do not absorb at this wavelength. Analytical curves up to 200 mg L− 1 and 400 mg L− 1 were constructed for benzene and toluene, providing limits of detection of 1.2 mg L− 1 and 1.7 mg L− 1, respectively. The sensitivity was similar to those obtained with a FT-NIR Bomem MB 160 spectrophotometer at the same wavelength. The photometer was applied to the determination of total hydrocarbons in water contaminated by gasoline. An analytical curve (50–300 mg L− 1) was constructed from reference solutions containing benzene and toluene in the ratio of 3:7, which is a ratio commonly found in Brazilian gasoline. The results provided by the photometer were compared with those obtained with the commercial spectrophotometer and did not show significant differences at a confidence level of 95%. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the interest of the miniaturization of instruments for field measurements, light-emission-diodes (LEDs) have become increasing popular in optical instruments in general and in photometers LED in particular. Among the main advantages of these instruments [1] are their simplicity of operation, low cost and power consumption, high stability and compatibility with optical fibers. In these LED photometers, the time delay between the sequential activation of the emitters and individual measurements on several wavelengths is very low (on the order of tenths of seconds), as in a diode array spectrophotometer. By employing LEDs that emit radiation at wavelengths previously selected (e.g., UV, Vis, NIR and MID), the information generated by a multi-LED photometer may be sufficient for use in analysis or discriminatory simultaneous determinations based on multivariate analysis. Several applications of LED photometers in different regions of the electromagnetic spectrum are present in the literature. In the UV
⁎ Tel.: + 55 84 3215 3828. E-mail address: [email protected]. 0026-265X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.microc.2012.01.003
region, instruments were developed for determining sulfate in water [2], acidity in fruit juices [3], and nitrite and nitrate in water [4], among others. For example, Zarate and colleagues [2] proposed an automatic procedure for the turbidimetric determination of sulfate in rain water based on a multicommutation flow analysis process using a photometer with an LED (λ = 420 nm), a flow cell with an optical path of 100 mm and a photodiode. In 2010, [3] Silva and colleagues described a low cost photometer based on an identical pair of LEDs (λ = 531 nm): one employed as a light source and the other as a photodetector. The equipment, including the photometer and the analysis module, was designed for an automatic titration procedure using multicommutation flow injection (MCFIA). Liu and colleagues [4] proposed an LED-photometer that operated in the UV region (λ = 255 nm) for the determination of total nitrite and nitrate in water. In this study, we found the detection limits for nitrite and nitrate to be 7 mol L − 1 and 12 mol L − 1, respectively. There are multiple examples of LED-photometers operating in the UV–vis region and combined with chemometric analysis. For example, Fonseca and colleagues [5] described the development of a portable multi-LED photometer operating in the visible region and applied for the simultaneous determination of Zn(II) and Cu(II) in pharmaceutical
K.M.G. de Lima / Microchemical Journal 103 (2012) 62–67
formulations. Multivariate calibration models based on multiple linear regression provided quadratic mean errors of prediction (RMSEP) of 0.06 and 0.12 mg L − 1 for Zn(II) and Cu(II), respectively. The same authors applied this multi-LED photometer in 2007 [6] for the discrimination of mineral water samples using chemometric tools. In addition, Hauser and colleagues [7] described a multi-LED photometer in the visible region to determine various species in solution, including ammonia, Cu(II), Al(III), Ca(II), Cr(III), phosphate and nitrite. The relative errors based on the measurements of concentration were between 0.2 and 5.0%. The NIR region offers additional advantages for analytical applications of LED photometers, such as a non-destructive nature, versatility and the possibility of in situ analysis. However, the large bandwidth of the LED (30 to 50 nm), pre-concentration steps and the need for chemometric tools limit work in this region. However, several applications of NIR LED-based photometers can be found in the literature for the determination of chlorophylls [8], adulteration of petrol [9], moisture tufa [10], lubricants [11] and others. For example, McClure and colleagues [8] developed a portable NIR photometer, coined “TWmeter” by the authors, using three LEDs with peak emissions at 700, 880 and 940 nm for the determination of chlorophyll in plants and moisture in paper. In this paper, multivariate calibration by multiple linear regression was used to determine the chlorophyll, resulting in a prediction error of 0.99 mg/cm2 of leaf surface, a dynamic range of 1.8 mg/cm2 and a humidity range of 30–65%. Additionally, Gaiao and colleagues [9] proposed a portable NIR photometer based on an LED for the low-cost detection of adulteration of Brazilian commercial gasoline with kerosene. The photometer contained an LED with a peak emission at 1550 nm and a detection system consisting of a PbSe photoresistor controlled by a microcontroller. The authors showed that the instrument could detect samples of gasoline contaminated with kerosene at 5% (v/v). Hyvarinen and colleagues [10] developed two NIR photometers for moisture determination in peat. This integration used a photometer detector and interference filters with narrow bands in a single semiconductor. Finally, Pignalosa and colleagues [11] presented an LED-based photometer for the analysis of lubricants with limits of detection and quantification of 0.07% and 0.16% (w/w), respectively. LED-based instruments have a simple assembly and low cost, are versatile and can be employed in the determination of many chemical parameters with promising results. Thus, the objective of this work was the development of a portable photometer-LED in the near infrared region (NIR) for the determination of BTEX compounds in contaminated water.
2. Experimental procedures 2.1. Reagents and solutions Benzene, toluene, and xylenes were purchased from Merck and used as received. Methanol (spectroscopic grade, Merck) was used to prepare stock solutions of BTX aromatic hydrocarbons. Sodium chloride (Vetec) was used to adjust the salinity of water samples. Silastic T2 (crosslinking agent) was provided by Dow Corning. Distilled and deionized water was used to prepare contaminated aqueous samples.
2.2. Reference solutions Initially, standard solutions of benzene and toluene at 50000 mg L − 1 were prepared, and ethylbenzene and xylene were prepared at 10 000 mg L − 1 in methanol. Then, 5.00 mL of these solutions was transferred to a 500 mL volumetric flask, and the solution was brought to its final volume with deionized water. The resulting stock solutions represented 500 mg L− 1 of benzene and toluene as well as 100 mg L − 1 of ethylbenzene and xylenes with 1% methanol.
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2.3. Preparation of the sensing phase The polymeric sensing phase consisted of a silicone-based polydimethylsiloxane (PDMS). The monomer and crosslinking agent was mixed in a 10:1 ratio as recommended by the manufacturer. The homogenization was carried out in an ultrasound bath for approximately 3 min to remove the air bubbles formed during mixing. The mixture was subsequently transferred to Teflon containers molded into a disc shape (height of 15 cm and diameter of 4 cm). These containers were placed in a Teflon vacuum desiccator for 30 min to remove any bubbles that remained after homogenizing the mixture. The polymeric material was left for 36 h for polymerization/crosslinking in an airtight plastic container. After this period, the phase sensing polymer was placed in an oven for 6 h at 60 °C to complete the crosslinking. The phase sensors were then removed from the Teflon molds and cut into a cylinder with a diameter of 3.2 mm and a height of 5 mm. 2.4. Instrumentation An FT-NIR spectrophotometer (Bomen MB 160) was used for measurements in the region of 800 nm to 2500 nm. The data were obtained using Win-Bomem Easy 3.04 software. Each spectrum was recorded as an average of 100 scans with a resolution of 8 cm − 1. A transmission cell was made in our laboratory for the optical fibers. The optical path of the cell was determined by spacers of different length (1 mm, 2 mm, 5 mm and 10 mm), with the 5 mm spacer utilized in the following experiments. 2.5. NIR-LED photometer 2.5.1. Power supply The instrument used a commercial Mean Well power supply produced and distributed by LR Industrial Informatics. The source model was DP-2512 with a symmetrical output of ±12 V. 2.5.2. Array LED Six LEDs emitting radiation in different areas of the NIR spectrum were acquired from Roithner Lasertechnik. The wavelengths of the maximum emission indicated by the manufacturer were 1200 nm, 1300 nm, 1550 nm, 1650 nm, 2150 nm and 2350 nm. Each LED was fixed in a metal connector, and the arrangement of the LEDs was achieved on a crafted acrylic plate. 2.5.3. Collection of NIR radiation Twenty-four beams containing polymer optical fibers 200 μm in diameter were each (Ocean Optics) connected to each LED to guide the individual radiation emitters. These beams were grouped into a single output beam containing 144 fibers, which was connected directly to the transmission cell. 2.5.4. Circuit electronic photometer The circuit designed for activation of the LEDs allowed the use of a diode array with up to six LED. The circuit (Fig. 1a) consisted of current limiting resistors and drivers (commonly called transistor keys) for driving the LEDs with virtually infinite impedance between its input and its output. This ensured that overvoltage or short circuits occurring in the LED drive did not reach the acquisition interface, protecting the interfaced PC. The drivers used were Darlington (ULN2001 chip), produced by Texas Instruments. Sixty-eight ohm resistors and variable resistors with a maximum resistance of 1 kΩ were employed to determine the current through the LED. With the variable resistors, the emission intensities of the LEDs could be adjusted according to the sensitivity of the detector (photodiode G8373), which provides a stable signal to all the LEDs.
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a
3 4
2
b
1
5 Fig. 2. Overview of NIR photometer: 1) LED and fiber optic assemblies, 2) power supply, 3) electronic circuit for powering the LEDs and signal amplification, 4) cell broadcast and the PDMS phase sensor, 5) USB interface for data acquisition.
radiation). The absorbance in each measurement was obtained from the difference described in Eq. (3). Fig. 1. (a) Electronic circuit of the NIR photometer powering the LEDs. (b) Electronic circuit built for signal conditioning of the InGaAs detector.
2.5.5. Detector The photometer employed here as a sensor was an InGaAs photodiode (model G8373) produced by Hamamatsu. The electronic circuitry used for conditioning the signal from the photodiode is detailed in Fig. 1b. Associated with the InGaAs photodiode, a voltage–current converter (Module I) that also operated as a first stage amplification was attached. The signal was amplified again in Module II. 2.5.6. Interface and data acquisition The interface used in this photometer facilitated communication between the dedicated software, controlling the instrument and data acquisition. Six of the twelve digital outputs were used to drive the LED, providing a logic low for those that must remain turned off and a high logic level for those that were fired. One of the twelve analog-digital (A/D converter) inputs was used for data acquisition from the photodiode. The interface used was a USB-6009 model from National Instruments. A program was written in the photometer VisualBasic.NET environment to control and acquire data from the photodiode through the electronic interface, which had an analog/ digital (A/D) of 14 bits. The written program drove the LEDs and collected the signals.
A ¼ A1689 −A1300
Fig. 2 shows a general outline of the developed NIR photometer. The InGaAs photodiode was connected directly to the optical fiber to collect direct radiation from the LEDs to prevent radiation loss. 2.6. Experimental procedure Measurements were performed by placing a PDMS monolith in an 85 mL bottle completely filled with the aqueous mixture of aromatic hydrocarbons and subjected to constant stirring (2700 rpm) for a time interval of 60 min at 22 ± 1 °C for the extraction of hydrocarbons. After this step, the monolith was removed from the mixture, quickly blotted dry and inserted into the measurement system. An adequate amount of NaCl was transferred to the bottle so that it was completely filled with the hydrocarbon solution, assisting in the transfer of compounds to the PDMS sensing phase. All extractions were performed in sealed vials. The spectrum of PDMS, previously immersed in an aqueous solution containing 1% methanol, was used as the reference for absorbance measurements. All measurements were employing new phase sensing material.
2.5.7. Calculation of absorbance and vision of the NIR photometer The absorbance for two LEDs operating at 1689 nm and 1300 nm can be calculated according to Eqs. (1) and (2), respectively. A1689 ¼ − log
I1689 −Esc R1689 −Esc I1300 −Esc R1300 −Esc
ð1Þ
A1300 ¼ − log
ð2Þ
where I is the average of the sample signals (phase sensor in contact with the BTEX compounds), R is the average of the signals relating to the reference measurements (only the phase sensing) and Esc is the average of the signals relating to measurements in the dark (no
ð3Þ
Fig. 3. Emission spectra of six standard LEDs in the NIR region.
K.M.G. de Lima / Microchemical Journal 103 (2012) 62–67
0,05
Table 1 Wavelengths of peak emission and bandwidths at half height for the LEDs used in the studied photometer. Measured wavelength (nm)
Relative error (%)
Effective bandwidth at half height (nm)
1200 1300 1550 1650 2150 2350
1197 1301 1504 1689 2186 2344
− 0.25 + 0.07 + 0.26 + 1.78 + 1.64 − 0.25
100 100 180 240 373 248
Day 1 Day 2 Day 3
0,04
Absorbance - a.u
Nominal wavelength (nm)
65
0,03
0,02
0,01
3. Results and discussion Using the spectrometer developed in our laboratory, the emission spectra of six LEDs were obtained. Fig. 3 shows the LED spectra after normalization. As observed, the LEDs had six NIR emissions that were the most informative for this application. Table 1 shows the wavelengths of maximum emission and the effective bandwidth at half height for each of the devices. The nominal wavelengths (indicated by the manufacturers) are also presented in this table. The results shown in Table 1 reveal small deviations (less than 2%) for the six wavelengths of the LEDs in relation to their nominal values. Variations in nominal wavelengths could be attributed to possible non-uniformities in the manufacturing process and/or calibration of the spectrophotometer. However, the bandwidths at half height for LEDs at longer wavelengths (1689 nm, 2150 nm and 2350 nm) were larger, indicating a loss of LED monochromaticity. The emission intensity profiles versus number of readings for all the LEDs were drawn from the signals found in the peak emission of 1000 readings obtained after the sequential activation of emitters. Fig. 4 shows the intensity profile for the LED with a peak emission at 1689 nm. For these devices, the intensity of the LED became steady after its minimum value was reached. The time signal “zero” was obtained immediately after the firing of the LED. The signal strength decreased by 4.1% after reaching its maximum value, achieving stability approximately 80 s after the drive. As noted in previous works, the more intense absorption peaks found for the BTEX compounds in NIR are 1675 nm, 2140 nm, 2165 nm and 2340 nm. As BTEX compounds do not absorb radiation at approximately 1300 nm and the variation in signal strength in this region reflects only the position of the sensing phase, it is necessary to use more than one LED. Therefore, the behavior of the LED with a peak emission at 1689 nm was initially assessed using a photometer cell
0,00 0
100
200
Toluene - mg
300
400
L-1
Fig. 5. Three analytical curves obtained for toluene on different days with an extraction time of 60 min in 2 mol L− 1 NaCl using a 5 mm sensing phase.
with a PDMS sensing phase having a 5 mm optical path. Those LED at 1500 nm, 2150 nm and 2350 nm showed extremely low intensities, indicating that virtually all the radiations were absorbed by the sensing phase. For this reason, LEDs with peak emissions of 1500 nm, 2150 nm and 2350 nm were not used in this application. The LED with a maximum intensity of 1689 nm gave a higher intensity and was instrumental in building the photometer, as 1689 nm was in the region related to the 1st C–H stretch of BTEX compounds. The performance of the LEDs with peak emissions at 1200 nm and 1300 nm were similar to those observed for the 1689 nm LED. As the standard curves of the spectrophotometer for the determination of BTEX were constructed using only the wavelengths 1300 nm and 1675 nm, the LEDs with maximum intensity at 1300 nm and 1689 nm were selected for this application. Therefore, the LED with peak emission at 1300 nm was employed in the construction of analytical curves of BTEX compounds in the photometer as a reference signal, and the LED with a maximum intensity at 1689 nm was used for the signal of the analytes. The accuracy of the proposed method using the NIR photometer was evaluated by constructing three analytical curves for toluene in the range from 0 to 400 mg L − 1. For this purpose, a transmission cell with optical fibers (Fig. 5) was used, resulting in an average slope with a standard deviation of 11.8% and a linear coefficient
Fig. 4. Profile of intensity vs. time for LED emission at 1689 nm.
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a
Table 3 Determination of total benzene and toluene (mg L− 1) in water by different instruments at 1689 nm.
0,08 Spectrophotometer NIR (1675 nm) Spectrophotometer NIR (1689 nm) Photometer NIR
Absorbance - a.u
0,06
0,00 0
50
100
150
200
Benzene - mg L-1 0,10 Photometer NIR Spectrophotometer NIR (1675 nm) Spectrophotometer NIR (1689 nm)
0,08
Absorbance - a.u
Photometer NIR
Spectrophotometer NIR
1 2 3
156 ± 11 145 ± 16 157 ± 14
149 ± 14 140 ± 10 147 ± 11
0,04
0,02
b
Samples
0,06
0,04
0,02
0,00 0
100
200
300
400
Toluene - mg L-1
4. Conclusion
Fig. 6. Comparison of analytical curves obtained for benzene (a) and toluene (b) from the photometer and the spectrophotometer employing a PDMS phase containing a sensing optical path length of 5 mm in 2 mol L− 1 NaCl with an extraction time of 60 min.
with a standard deviation of 7.64%. Each of these curves was built on different days and with new standard solutions. These results indicated that the proposed method requires daily calibration. Analytical curves were obtained for benzene and toluene using the photometer operating under the following experimental conditions: 5 mm optical path, 2 mol L − 1 NaCl and 60 min of extraction. In this case, measurements were carried out on two different instruments, an NIR photometer and a commercial instrument (FT-NIR), to compare sensitivity. Once the signals from the LED emission maximums at 1689 nm and 1300 nm were registered by the photometer, the phase sensor was removed from the transmission cell photometer and immediately inserted into the other transmission cell coupled to the spectrophotometer for the registration of trade spectra. Fig. 6a shows the standard curves obtained for benzene compounds in the range from 0 to 200 mg L − 1, and Fig. 6b shows the standard curves for toluene in the range from 0 to 400 mg L − 1. The measurements performed at 1675 nm, which matched the wavelength of Table 2 Comparison of slopes obtained for the analytical curves of benzene and toluene using both the photometer and the spectrophotometer at 1689 nm. Compounds
Benzene Toluene
maximum absorption for the 1st C−H stretch of the compounds studied, are shown for comparison. As observed in the figures above, the sensitivity obtained for the spectrophotometer at 1675 nm was 1.7 to 2.6 times higher for benzene and toluene when compared to the results obtained by observing the maximum emission of the LED at 1689 nm. Comparing the sensitivities between the two instruments at 1689 nm (Table 2), it could be concluded that the sensitivities of the two instruments were quite similar. Ideally, the sensitivity would be greatly improved if the maximum emission of the LED was exactly 1675 nm. In addition, the detection limits for benzene and toluene in the NIR photometer were 1.2 and 1.7 mg L − 1, respectively, as calculated from the analytical curves shown in Fig. 6. To evaluate the developed NIR photometer against actual contamination, water samples were contaminated with type A gasoline (ratio of 2.5% (v/v)). Table 3 shows the results of the content of benzene and toluene for the three samples measured in triplicate using the total optical sensor developed for the photometer and the NIR spectrophotometer, both at 1689 nm. A t test of the results obtained from the two instruments (photometer and spectrophotometer) showed no statistically significant differences at a confidence level of 95%. Therefore, the NIR photometer-based LED could be considered a viable, low-cost alternative for the determination of benzene and toluene in water.
Instruments Spectrophotometer
Photometer
1.22 × 10− 4 1.13 × 10− 4
1.13 × 10− 4 8.25 × 10− 5
The potential of a multi-channel NIR photometer based on an array of LEDs was investigated in the determination of BTEX in gasolinecontaminated water using a PDMS sensing phase. The photometer based on LEDs was a simple, low-cost assembly, which was much cheaper than most NIR spectrophotometers currently available on the market. These results indicated that the instrument developed could be an economical and simple alternative for the application studied. Acknowledgments The author is grateful to professors Dr. Ivo M. Raimundo Jr. (UNICAMP-Brazil) and M.F Pimentel (UFPE-Brazil) for technical assistance and to CNPq and CAPES/PROCAD for financial support. References [1] P.K. Dasgupta, H.S. Bellamy, H. Liu, J.L. Lopez, E.L. Loree, K. Morris, K. Petersen, K.A. Mir, Light-emitting diode based flow-through optical-absorption detectors, Talanta 40 (1993) 53–74. [2] N. Zarate, R. Olmos-Pérez, B.F. Reis, Turbidimetric determination of sulfate in rainwater employing a LED Based photometer and multicommuted flow analysis system with in-line preconcentration, J. Braz. Chem. Soc. 22 (2011) 1009–1014. [3] M.B. Silva, C.C. Crispino, B.F. Reis, Automatic photometric titration procedure based on multicommutation and flow-batch approaches employing a photometer based on twin LEDs, J. Braz. Chem. Soc. 21 (2010) 1854–1860. [4] M. Zhang, Z. Zhang, D. Yuan, S. Feng, B. Liu, An automatic gas-phase molecular absorption spectrometric system using a UV-LED photodiode based detector for determination of nitrite and total nitrate, Talanta 84 (2011) 443–450. [5] A. Fonseca, I.M. Raimundo Jr., A multichannel photometer based on an array of light emitting diodes for use in multivariate calibration, Anal. Chim. Acta 522 (2004) 223–229. [6] A. Fonseca, I.M. Raimundo Jr., A simple method for water discrimination based on an light emitting diode (LED) photometer, Anal. Chim. Acta 596 (2007) 66–72.
K.M.G. de Lima / Microchemical Journal 103 (2012) 62–67 [7] P.C. Hauser, T.W.T. Rupasinghe, Simultaneous determination of metal ion concentrations in binary mixtures with a multi-LED photometer, Fresenius J. Anal. Chem. 357 (1997) 1056–1060. [8] W.F. McClure, D. Moody, D.L. Stanfield, O. Kinoshita, Hand-held NIR spectrometry. Part II: an economical no-moving parts spectrometer for measuring chlorophyll and moisture, Appl. Spectrosc. 56 (2002) 720–724. [9] E.N. Gaião, S.R.B. Dos Santos, V.B. Dos Santos, E.C.L. Nascimento, R.S. Lima, M.C.U. Araújo, An inexpensive, portable and microcontrolled near infrared LEDphotometer for screening analysis of gasoline, Talanta 75 (2008) 792–796.
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[10] T. Hyvarinen, P. Niemela, Rugged multiwavelenght NIR and IR analyzers for industrial-process measurements, SPIE (Process Optical Measurements and Industrial Methods) 1266 (1990) 99–104. [11] G. Pignalosa, A. Sixto, M. Knochen, Automatic determination of insolubles in lubricating oils by flow injection analysis employing an LED-photometer detector, Talanta 73 (2007) 959–961.