- Email: [email protected]
Novel applications of near-infrared spectroscopy of water and aqueous solutions from physical chemistry to analytical chemistry
320
trends in analytical chemistry
vol. 13, no. 8, 1994
Novel applications of near-infrared spectroscopy of water and aqueous solutions from physical chemistry to analytical chemistry Jie Lin ‘, Chris W. Brown* Kingston, RI, USA The results of many early investigations in physical chemistry have useful implications in present day analytical chemistry. Optical spectroscopy was one of the major tools used to elucidate structures and physical/ chemical properties of water and aqueous solutions by physical chemists. Recently, similar spectroscopic techniques have been used to solve analytical problems. For example, near-IR spectra of water and aqueous solutions have been used for the determinations of temperature, concentrations of electrolytes, and physical and chemical properties. Interdisciplinary investigations can be very productive and can benefit from previous knowledge in other disciplines.
1. Introduction Spectroscopic techniques have been widely used in the investigations of physical chemistry. Spectra, produced by the interactions between light and matter can give useful information about the changes in electronic, vibrational and rotational energies, the bonding in a molecule, the interactions between atoms in a molecule or between molecules in a sample. Before 1970, many investigations were conducted to study the structures of water using spectroscopic techniques in the mid- and near-IR regions as well as Raman spectroscopy [ l-141. Many of those investigations were made in the near-IR region where weak overtones and combinations of vibrations of hydroxyl groups allowed * Corresponding author. ’ Present address: Department of Chemistry, versity, Medford, MA 02155, USA.
0165.99X6/94/$07.00
Tufts Uni-
the easy use of relatively long path lengths. The structures of water, e.g., the number of hydrogen bonds that each water molecule forms and the strength of the hydrogen bonding between hydroxyl groups, were studied using near-IR spectra of water as a function of temperature and of aqueous solutions as a function of types and concentrations of electrolytes [ 5-121. From these investigations, it was found that temperature had a strong effect on the near-IR spectrum of water and that many electrolytes changed the spectrum of water in different ways to different degrees. However, the changes in the spectrum of water by electrolytes were not used for the determination of electrolytes in aqueous solutions until 1985 when Hirschfeld first reported the determination of NaCl by near-IR analysis [ 151. The spectral changes in near-IR water bands induced by changes in temperature were not used as a spectroscopic temperature scale until a near-IR fiber-optic temperature sensor based on this phenomenon was developed in 1993 [ 161. In the 1960’s and 1970’s, many physical chemists tried to develop models for the structures of water, the physical and chemical properties of water, and their functions of temperature [ 3-6,1 O12,17-201. Some of the models were based solely on theoretical considerations, whereas others were based on spectroscopic measurements together with some assumptions. These models were restricted by incorporating inappropriate assumptions or by using a limited amount of spectral information (e.g., the use of the absorbance at a single wavelength for the calculation of one water species such as the water ‘monomer’). The properties of water, which are determined by the intermolecular interactions, are reflected in the spectrum or changes in spectrum as characteristic ‘patterns’, instead of as the absorbances or changes in absorbantes at specific wavelengths. Therefore, the properties should be related to the entire spectrum of water or the absorbances at several selected wave0
1994 Elsevier Science B.V. All rights reserved
321
trends in analytical chemistry, vol. 13, no. 8, 1994
(6) ReflectIon
(A) Open Type of Sensor
Type of Sensor fiber
fiber
craw plexiglass plate aluminum sheet first surface reflecting mirror
(C) Capillary
Type of Sensor (D) Sandwich
Type of Sensor
epoxy resin
fiber
a layer of water entrapped fiber
~
aluminum sheets
Fig. 1. Diagrams of fiber-optic sensors: (A) open type of sensor; (B) reflection type of sensor; (C) capillary type of sensor; and (D) sandwich type of sensor. (Reproduced with permission from Ref. [16].)
Spectra of water measured with a fiber-optic sensor (capillary type, pathlength = 0.4 mm) at temperatures from 5 to 65°C are shown in Fig. 2A and the difference spectra of water using the spectrum at 5°C as reference are shown in Fig. 2B. As seen, the increase in temperature causes an increase in the absorption intensity of the water band and a shift of the band to a higher frequency. The spectral changes appear to be linear with temperature in some wavelength regions; thus, temperature can be calculated by linear regression (LR) using the absorbance at selected wavenumbers. In reality, the changes in temperature will not only cause changes in spectra, but also cause changes in refractive index and thermal expansion of the sensor materials. Therefore, temperature was also calculated by multilinear regression (MLR) using the absorbantes at several selected wavenumbers, and by principal component regression (PCR) using entire spectra in the 9000-5350 cm- ’ region. With the use of the different types of sensors shown in
0.7 I
lengths. By correlating spectra with properties, near-IR spectroscopy can be used for the simultaneous determinations of different properties of water as a function of temperature and of aqueous solutions of NaCl or methanol at different concentrations, The results of many early spectroscopic investigations in physical chemistry have very useful implications in analytical chemistry. Interdisciplinary investigations involving the applications of physical chemistry to analytical chemistry can be very productive. In this paper, we will discuss the results of our investigations using this strategy.
0.2
8
s 0.1 -!? z:
2. Recent investigations 2.1. A near-/R fiber-optic temperature
sensor
Simple fiber-optic sensors were fabricated inhouse for the temperature measurements based on near-IR spectra of water. The designs of four sensors are shown in Fig. 1 [ 161. The sensors were fabricated by leaving a small gap between two fibers for water to pass (open and reflection types) or by entrapping a small amount of water between two fibers (capillary and sandwich types). The fiber-optic sensors were interfaced to a spectrometer for spectral measurements.
9
Q
0.0
I
-0.1 I 9000
\W/
v
8085 Wavenumber
7170
, 6255
I 5340
(cm-l)
Fig. 2. (A) Spectra of water measured at temperatures from 5 to 65°C; and (B) difference spectra of water with that measured at 5°C as reference. Measurements were made using a capillary type of fiber-optic sensor with a pathlength of 0.4 mm. (Reproduced with permission from Ref. [16].)
322
trends in analytical chemistry, vol. 13, no. 8, 1994
300,
0.15 (
m
A
,’ I'
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HCI /’ ,/
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1’
1. NaCl 2. NaHC03 3. Na2G03
9085
7170
Wavenumber I
1300
1500
1900
1700
Wavelength (nm)
Fig. 3. (A) Spectra of water and 5 M NaCl solution measured at 285°C; (B) difference spectra of NaCl solutions measured at 285°C with water as reference; and (C) difference spectra of 2.5 M NaCl solution measured at temperatures from 23.0 to 285°C with that measured at 23.O“C as reference. Measurements were made with a 2 mm cuvette.
6255
I 5340
(cm-l)
Fig. 4. Difference spectra of 1 Msolutions of electrolytes minus water measured using a cell type of fiberoptic probe with a pathlength of 1 mm at 23.O”C. (A) NaCl and HCI; (B) NaCI, NaHCO,, and Na,CO,. (Reproduced with permission from Ref. [22].)
stant temperature (e.g. room temperature). Several groups have reported the quantitative analyses of
Fig. I, the standard errors of prediction (SEPs) ranged from 0.53 to 1.64”C for the LR model, 0.22 to 0.85”C for MLR model, and 0.16 to 0.32”C for the PCR model over a temperature range of 5 to 85°C. These fiber-optic temperature sensors are reagentless, very simple, readily fabricated, and most importantly, immune to electrical interference. Therefore, they can be used for remote senselectrical strong temperature in ing of environments such as in the caustic industries that involve electrolysis of aqueous solutions. 1300
2.2. Near-/R spectroscopic NaCl in aqueous solutions
determinations
of
Traditionally, near-IR spectral measurements of aqueous solutions are made using cuvettes at a con-
1500
1700
1900
Wavelength (nm)
Fig. 5. Near-IR difference spectra of 35%0 seawater and 1 MNaCl solution measured at 28.5”C with water as reference. (Reproduced with permission from Ref. 1231.)
trends in analytical chemistry, vol. 13, no. 8, 1994
electrolytes with near-IR spectroscopy prior to our investigations [ 15,25-271. Further investigations were conducted to study the effects of temperature on the determination of NaCl using near-IR spectra between 1100 and 1900 nm [ 211. Fig. 3A shows the spectra of water and 5 M NaCl solution, Fig. 3B shows the difference spectra of NaCl solutions minus water, and Fig. 3C shows the difference spectra of 2.5 A4 NaCl solutions measured at different temperatures (from 23.0 to 28.5”C with the intermediate temperatures unknown). Models expressing the concentration of NaCl were developed with linear regression of the absorbances at a selected wavelength and with principal component regression (PCR) using entire spectra. The effect of temperature on the determination of NaCl can be removed by linear and non-linear regressions using the absorbances at the wavelengths where the temperature effects are zero (so called isosbestic points), or be accounted for by the PCR model. Therefore, concentrations of NaCl can be measured at different temperatures and temperature control is not necessary. An SEP of 0.005 M was obtainable for NaCl; this corresponds to a detection limit of 0.015 M (three times the SEP) . 2.3. Fiber-optic probes for electrolytes in aqueous solutions
Fiber-optic probes for the remote determination of electrolytes with near-IR spectroscopy were also developed [22]. Three types of probes were used including the open and reflection types of the temperature sensors (see Fig. 1) , which were dipped into solutions for the measurement, and a cell type probe [ 221, for which only about 0.3 ml solution was needed for the measurement. Since each electrolyte has its characteristic perturbations on the water bands (as seen in Fig. 4), near-IR analysis can be used to determine single and multiple electrolytes in aqueous solutions. Fiber-optic probes were also used to determine single electrolytes including NaCl, NaHCO, and HCl [ 221. We also extended the near-IR spectroscopic technique for the simultaneous determination of several electrolytes in mixtures including NaCl-HCl and NaCl-NaHCO,-Na,CO, solutions. Both NaCl and temperature, which have different patterns of perturbations on the water band (as seen in Fig. 3B and C), were also determined simultaneously in a solution. Standard errors less than 0.010 M were attainable for concentrations in the
323
range of 0 to 1 A4 for single electrolytes. It was found that smaller standard errors were obtained for the electrolytes having larger perturbations on the water bands, e.g., the sequence of perturbations is Na,CO, > NaCl > NaHCO, whereas the sequence of SEPs is Na,CO, (0.005 M) < NaCl (0.008 M) < NaHCO, (0.012 M) . The near-IR spectroscopic technique was also used for the determination of seawater salinity [ 23,241. Seawater is an aqueous solution containing various electrolytes (which bear constant ratios to each other). NaCl is the predominant electrolyte and the total perturbations of sea-salts on the nearIR water bands are similar to those of NaCl solutions (Fig. 5). Therefore, seawater salinity can be determined in a way similar to NaCl concentration. An SEP of 0.22%0 was obtained for salinity in the range of 0 to 35700 [23]. 2.4. A universal approach for determination of physical and chemical properties of water by near-/R spectroscopy Modelling and determination of physical and chemical properties of water and their functions of temperature have been traditional tasks in physical chemistry. Generally, each property is measured by a specific method. Spectroscopic methods have not been explored for this purpose. Theoretically, all physical and chemical properties of water are determined by its structures (i.e., hydrogen bonding of the hydroxyl groups) and will be reflected in the near-IR spectrum of water. Therefore, it is expected that these properties can be determined from near-IR spectra of water and a universal spectroscopic approach can be developed. Principal component regression (PCR) and multilinear regression (MLR) were used to correlate the spectra of water measured at temperatures from 5 to 65°C (as seen in Fig. 2) with the physical and chemical properties of water. Fifteen properties were studied including density, refractive index, dielectric constant, viscosity, surface tension, vapor pressure, sound velocity, isothermal compressibility, thermal expansivity, thermal capacity, thermal conductivity, enthalpy, free energy, entropy, and ionization constant. The literature values for these properties as functions of temperature are shown in Fig. 6. Very good correlations were found between the PCR predicted values from a direct validation and the literature values for all these properties as shown in Fig. 7. This study demonstrated that fifteen properties of water can
324
trends in analytical
chemistry,
vol. 13, no. 8, 7994
1 005
,.ooo
relative
dielectric
0995
constant
0990 0965 0.660 0.975
1
60’
vapor pressure 150
100
50
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49
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46 45
4.16
44 43
15.0 14.5
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6.6
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5.6
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entholpy
250
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3.6
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3.4
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10
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150~
20
30
40
Temperature
50
(C)
60
70
(2.5
0
(JMW
10
M
30
40
60
6u
70
Temperature (C)
0
lO203a
4a
6060
Temperature
(C)
70
Fig. 6. Literature values of the properties of water as functions of temperature. (Reproduced with permission from Ref. [28].)
all be determined simply by measuring a set of near-IR spectra of water. This method was also used for the determinations of physical and chemical properties of NaCl solutions and water-methanol mixtures [ 29,301. We demonstrated that near-IR spectroscopy can be used as a universal method for the determinations of physical and chemical properties of water and aqueous solutions.
3. Applications
and future investigations
We have demonstrated that the results of many early investigations in physical chemistry can be
applied in analytical chemistry. The investigations using this strategy have led to several novel applications of near-IR spectroscopy of water and aquesolutions for the determinations of ous temperature, electrolytes, and physical and chemical properties. Interdisciplinary investigations can be very productive if one can benefit from the previous knowledge in other disciplines. The near-IR spectroscopic methods developed in these investigations are simple and fast. The fiber-optic sensors and probes developed can potentially be used for remote sensing in industrial process control and environmental monitoring. They can be used in caustic industries to determine both temperature and concentrations of electrolytes
325
trends in analyfical chemistry, vol. 13, no. 8, 1994
I 975
I 9eo
865
990
995
loot
,005
ACtlJal 1.335
I
0.2
1
1.o
0.6
60
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00
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6.4
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100
150
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65
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200
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3.6
3.6
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-550
600
450
400
450
a00
-250
Actual
Fig. 7. Near-IR predicted (by PCR models) vs. actual properties of water at temperatures (Reproduced with permission from Ref. [28].)
in an electrolysis broth, where electrically based temperature transducers (e.g., thermal couples and thermistors) can not be used and the environment is hostile for humans. They can also be applied in semiconductor industries to monitor the chemical compounds such as fluorides in an etching broth. In addition, they can be used in pharmaceutical industries where the active ingredients are dissolved in aqueous solutions of electrolytes or alcohols. For environmental monitoring, the fiber-optic sensors can be used to determine the temperature and salinity of seawater in coastal regions, or to monitor specific pollutants. Furthermore, they can be used in waste water treatment plants.
from 5 to 65°C.
References L11 H.R. Wyss and M. Falk, Can. J. Chem., 48 ( 1970) 607.
[El G. Brink and M. Falk, Can. J. Chem., 48 ( 1970) 3019.
[31 M.A. Thiel, E.D. Becker and G.C. Pimentel, J. Chem. Phys., 27 ( 1957) 486. [41M. Falk and T.A. Ford, Can. J. Chem., 44 ( 1966) 1699. [51 K. Buijs and G.R. Choppin, .I. Chem. Phys., 39 (1963) 2035. (61 G.R. Choppin and K. Buijs, J. Chem. Phys., 39 ( 1963) 2042. [71J.D. Worley and I.M. Klotz, J. Chem. Phys., 45 ( 1966) 2868.
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trends in analytical chemistry, vol. 13, no. 8, 1994
[8] K.W. Bunzel,J. Phys. Chem., 71 (1967) 1358. [ 91 W.A.P. Luck and W. Ditter, J. Phys. Chem., 74 ( 1970)
3687.
[ lo] O.D. Bonner and G.B. Woolsey,
J. Phys. Chem., 72 (1968) 899. [ 111 W.C. McCabe, S. Subramanian and H.F. Fisher, J. Phys. Chem., 74 (1970) 4360. [ 121 W.C. McCabe and H.F. Fisher, 1. Phys. Chem., 74 (1970) 2990. 1131 G.E. Walrafen, J. Chem. Phys., 35 ( 1962) 1035. [ 141 G.E. Walrafen, J. Chem. Phys., 44 ( 1966) 1546. 1151T. Hirschfeld, Appl. Spectrosc., 39 ( 1985) 740. 1161J. Lin and C.W. Brown, Appf. Spectrosc., 47 (1993) 62. [I71 W.A.P. Luck, Truns. Faraday Sot., 64 (1968) 115. 1181 G.E. Walrafen, in A.K. Covington and P. Jones (Editor), Hydrogen-Bonded Solvent Systems, Tayler and Francis Ltd, London, 1968, p. 9-29. L191 G. Nemethy and H.A. Scheraga, J. Chem. Phys., 36 ( 1962) 3382. [201 G. Nemethy and H.A. Scheraga, J. Chem. Phys., 36 (1962) 3401.
[21] J. Lin and C.W. Brown, Appl. Spectrosc., 46 ( 1992) 1809. [ 221 J. Lin and C.W. Brown, Anal. Chem., 65 ( 1993) 287. ~231 J. Lin and C.W. Brown, Environ. Sci. Technol., 27 (1993) 1611. v41 J. Lin and C.W. Brown, Appf. Spectrosc., 47 (1993) 239. [251 E. Watson Jr. and E.H. Baughman, Spectroscopy, 2 (1987) 44. [261 M.K. Phelan, C.H. Barlow, J.J. Kelly, T.M. Jinguji and J.B. Callis, Anal. Chem., 61 ( 1989) 1419. ~271 A. Grant, A.M.C. Davis and T. Bilverstone, Analyst, 114 (1989) 819. [281 J. Lin and C.W. Brown, Appl. Spectrosc., 47 (1993) 1720. ~291 J. Lin and C.W. Brown, Vib. Spectrosc., ( 1994) in press. [301 J. Lin, C.W. Brown, J. Near Infrared Spectrosc., ( 1993) in press. Jie Lin and Chris W. Brown are at the Department of Chemistry, University of Rhode Island, Kingston, RI 0288 1, USA.
International Conference - H-TAP Hyphenated Techniques in Analytical SeparationsSM January 22-25, 1995, Orlando, Florida, USA lnternational
Conference and Exhibition Techniques in Analytical Separations is one of the most exciting and significant instrumentation areas to meet the new challenges for real-time and highly sensitive measurements. Now, in one place, speakers from throughout the world, meet to share original presentations, tutorials and keynote addresses to the instrumentation and specific applications of hyphenated techniques. You’ll have the opportunity to meet with your colleagues...the people who are establishing the foundation of Hyphenated Strategies. Come share in discussions, give input and get ideas for the future well being of the industry. CALL FOR PAPERS Prospective speakers are requested to submit an abstract of 150-200 words to H-TAS Committee c/o InfoScience, Box 153, Northbrook IL 60065 USA. The three day conference will focus on emerging techniques, the applications and the instrument development integrating/hyphenating the techniques GC, MS, PTV, Purge & Trap, FTIR, SED, Air Traps, Extractors, LC, NMR, FIA, DAD, SFC, SFE, CZE, ITP. A special mini-conference on Sample Preparation will also be held. The speakers include distinguished industry, government, and university professionals. A comprehensive exhibition of equipment and services will be on display during the conference.
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trends in analytical chemistry
vol. 13, no. 8, 1994
Novel applications of near-infrared spectroscopy of water and aqueous solutions from physical chemistry to analytical chemistry Jie Lin ‘, Chris W. Brown* Kingston, RI, USA The results of many early investigations in physical chemistry have useful implications in present day analytical chemistry. Optical spectroscopy was one of the major tools used to elucidate structures and physical/ chemical properties of water and aqueous solutions by physical chemists. Recently, similar spectroscopic techniques have been used to solve analytical problems. For example, near-IR spectra of water and aqueous solutions have been used for the determinations of temperature, concentrations of electrolytes, and physical and chemical properties. Interdisciplinary investigations can be very productive and can benefit from previous knowledge in other disciplines.
1. Introduction Spectroscopic techniques have been widely used in the investigations of physical chemistry. Spectra, produced by the interactions between light and matter can give useful information about the changes in electronic, vibrational and rotational energies, the bonding in a molecule, the interactions between atoms in a molecule or between molecules in a sample. Before 1970, many investigations were conducted to study the structures of water using spectroscopic techniques in the mid- and near-IR regions as well as Raman spectroscopy [ l-141. Many of those investigations were made in the near-IR region where weak overtones and combinations of vibrations of hydroxyl groups allowed * Corresponding author. ’ Present address: Department of Chemistry, versity, Medford, MA 02155, USA.
0165.99X6/94/$07.00
Tufts Uni-
the easy use of relatively long path lengths. The structures of water, e.g., the number of hydrogen bonds that each water molecule forms and the strength of the hydrogen bonding between hydroxyl groups, were studied using near-IR spectra of water as a function of temperature and of aqueous solutions as a function of types and concentrations of electrolytes [ 5-121. From these investigations, it was found that temperature had a strong effect on the near-IR spectrum of water and that many electrolytes changed the spectrum of water in different ways to different degrees. However, the changes in the spectrum of water by electrolytes were not used for the determination of electrolytes in aqueous solutions until 1985 when Hirschfeld first reported the determination of NaCl by near-IR analysis [ 151. The spectral changes in near-IR water bands induced by changes in temperature were not used as a spectroscopic temperature scale until a near-IR fiber-optic temperature sensor based on this phenomenon was developed in 1993 [ 161. In the 1960’s and 1970’s, many physical chemists tried to develop models for the structures of water, the physical and chemical properties of water, and their functions of temperature [ 3-6,1 O12,17-201. Some of the models were based solely on theoretical considerations, whereas others were based on spectroscopic measurements together with some assumptions. These models were restricted by incorporating inappropriate assumptions or by using a limited amount of spectral information (e.g., the use of the absorbance at a single wavelength for the calculation of one water species such as the water ‘monomer’). The properties of water, which are determined by the intermolecular interactions, are reflected in the spectrum or changes in spectrum as characteristic ‘patterns’, instead of as the absorbances or changes in absorbantes at specific wavelengths. Therefore, the properties should be related to the entire spectrum of water or the absorbances at several selected wave0
1994 Elsevier Science B.V. All rights reserved
321
trends in analytical chemistry, vol. 13, no. 8, 1994
(6) ReflectIon
(A) Open Type of Sensor
Type of Sensor fiber
fiber
craw plexiglass plate aluminum sheet first surface reflecting mirror
(C) Capillary
Type of Sensor (D) Sandwich
Type of Sensor
epoxy resin
fiber
a layer of water entrapped fiber
~
aluminum sheets
Fig. 1. Diagrams of fiber-optic sensors: (A) open type of sensor; (B) reflection type of sensor; (C) capillary type of sensor; and (D) sandwich type of sensor. (Reproduced with permission from Ref. [16].)
Spectra of water measured with a fiber-optic sensor (capillary type, pathlength = 0.4 mm) at temperatures from 5 to 65°C are shown in Fig. 2A and the difference spectra of water using the spectrum at 5°C as reference are shown in Fig. 2B. As seen, the increase in temperature causes an increase in the absorption intensity of the water band and a shift of the band to a higher frequency. The spectral changes appear to be linear with temperature in some wavelength regions; thus, temperature can be calculated by linear regression (LR) using the absorbance at selected wavenumbers. In reality, the changes in temperature will not only cause changes in spectra, but also cause changes in refractive index and thermal expansion of the sensor materials. Therefore, temperature was also calculated by multilinear regression (MLR) using the absorbantes at several selected wavenumbers, and by principal component regression (PCR) using entire spectra in the 9000-5350 cm- ’ region. With the use of the different types of sensors shown in
0.7 I
lengths. By correlating spectra with properties, near-IR spectroscopy can be used for the simultaneous determinations of different properties of water as a function of temperature and of aqueous solutions of NaCl or methanol at different concentrations, The results of many early spectroscopic investigations in physical chemistry have very useful implications in analytical chemistry. Interdisciplinary investigations involving the applications of physical chemistry to analytical chemistry can be very productive. In this paper, we will discuss the results of our investigations using this strategy.
0.2
8
s 0.1 -!? z:
2. Recent investigations 2.1. A near-/R fiber-optic temperature
sensor
Simple fiber-optic sensors were fabricated inhouse for the temperature measurements based on near-IR spectra of water. The designs of four sensors are shown in Fig. 1 [ 161. The sensors were fabricated by leaving a small gap between two fibers for water to pass (open and reflection types) or by entrapping a small amount of water between two fibers (capillary and sandwich types). The fiber-optic sensors were interfaced to a spectrometer for spectral measurements.
9
Q
0.0
I
-0.1 I 9000
\W/
v
8085 Wavenumber
7170
, 6255
I 5340
(cm-l)
Fig. 2. (A) Spectra of water measured at temperatures from 5 to 65°C; and (B) difference spectra of water with that measured at 5°C as reference. Measurements were made using a capillary type of fiber-optic sensor with a pathlength of 0.4 mm. (Reproduced with permission from Ref. [16].)
322
trends in analytical chemistry, vol. 13, no. 8, 1994
300,
0.15 (
m
A
,’ I'
0.10 -
HCI /’ ,/
8 5 fl
0.05 -
B
0.00
s
/’
I I’ /’
a
$
a
0.02
-0.10 -
5 a 6 0.00 In 9
a
-0.02
9000
I t,‘I,,4 I : ( 1 ‘r:
1’
1. NaCl 2. NaHC03 3. Na2G03
9085
7170
Wavenumber I
1300
1500
1900
1700
Wavelength (nm)
Fig. 3. (A) Spectra of water and 5 M NaCl solution measured at 285°C; (B) difference spectra of NaCl solutions measured at 285°C with water as reference; and (C) difference spectra of 2.5 M NaCl solution measured at temperatures from 23.0 to 285°C with that measured at 23.O“C as reference. Measurements were made with a 2 mm cuvette.
6255
I 5340
(cm-l)
Fig. 4. Difference spectra of 1 Msolutions of electrolytes minus water measured using a cell type of fiberoptic probe with a pathlength of 1 mm at 23.O”C. (A) NaCl and HCI; (B) NaCI, NaHCO,, and Na,CO,. (Reproduced with permission from Ref. [22].)
stant temperature (e.g. room temperature). Several groups have reported the quantitative analyses of
Fig. I, the standard errors of prediction (SEPs) ranged from 0.53 to 1.64”C for the LR model, 0.22 to 0.85”C for MLR model, and 0.16 to 0.32”C for the PCR model over a temperature range of 5 to 85°C. These fiber-optic temperature sensors are reagentless, very simple, readily fabricated, and most importantly, immune to electrical interference. Therefore, they can be used for remote senselectrical strong temperature in ing of environments such as in the caustic industries that involve electrolysis of aqueous solutions. 1300
2.2. Near-/R spectroscopic NaCl in aqueous solutions
determinations
of
Traditionally, near-IR spectral measurements of aqueous solutions are made using cuvettes at a con-
1500
1700
1900
Wavelength (nm)
Fig. 5. Near-IR difference spectra of 35%0 seawater and 1 MNaCl solution measured at 28.5”C with water as reference. (Reproduced with permission from Ref. 1231.)
trends in analytical chemistry, vol. 13, no. 8, 1994
electrolytes with near-IR spectroscopy prior to our investigations [ 15,25-271. Further investigations were conducted to study the effects of temperature on the determination of NaCl using near-IR spectra between 1100 and 1900 nm [ 211. Fig. 3A shows the spectra of water and 5 M NaCl solution, Fig. 3B shows the difference spectra of NaCl solutions minus water, and Fig. 3C shows the difference spectra of 2.5 A4 NaCl solutions measured at different temperatures (from 23.0 to 28.5”C with the intermediate temperatures unknown). Models expressing the concentration of NaCl were developed with linear regression of the absorbances at a selected wavelength and with principal component regression (PCR) using entire spectra. The effect of temperature on the determination of NaCl can be removed by linear and non-linear regressions using the absorbances at the wavelengths where the temperature effects are zero (so called isosbestic points), or be accounted for by the PCR model. Therefore, concentrations of NaCl can be measured at different temperatures and temperature control is not necessary. An SEP of 0.005 M was obtainable for NaCl; this corresponds to a detection limit of 0.015 M (three times the SEP) . 2.3. Fiber-optic probes for electrolytes in aqueous solutions
Fiber-optic probes for the remote determination of electrolytes with near-IR spectroscopy were also developed [22]. Three types of probes were used including the open and reflection types of the temperature sensors (see Fig. 1) , which were dipped into solutions for the measurement, and a cell type probe [ 221, for which only about 0.3 ml solution was needed for the measurement. Since each electrolyte has its characteristic perturbations on the water bands (as seen in Fig. 4), near-IR analysis can be used to determine single and multiple electrolytes in aqueous solutions. Fiber-optic probes were also used to determine single electrolytes including NaCl, NaHCO, and HCl [ 221. We also extended the near-IR spectroscopic technique for the simultaneous determination of several electrolytes in mixtures including NaCl-HCl and NaCl-NaHCO,-Na,CO, solutions. Both NaCl and temperature, which have different patterns of perturbations on the water band (as seen in Fig. 3B and C), were also determined simultaneously in a solution. Standard errors less than 0.010 M were attainable for concentrations in the
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range of 0 to 1 A4 for single electrolytes. It was found that smaller standard errors were obtained for the electrolytes having larger perturbations on the water bands, e.g., the sequence of perturbations is Na,CO, > NaCl > NaHCO, whereas the sequence of SEPs is Na,CO, (0.005 M) < NaCl (0.008 M) < NaHCO, (0.012 M) . The near-IR spectroscopic technique was also used for the determination of seawater salinity [ 23,241. Seawater is an aqueous solution containing various electrolytes (which bear constant ratios to each other). NaCl is the predominant electrolyte and the total perturbations of sea-salts on the nearIR water bands are similar to those of NaCl solutions (Fig. 5). Therefore, seawater salinity can be determined in a way similar to NaCl concentration. An SEP of 0.22%0 was obtained for salinity in the range of 0 to 35700 [23]. 2.4. A universal approach for determination of physical and chemical properties of water by near-/R spectroscopy Modelling and determination of physical and chemical properties of water and their functions of temperature have been traditional tasks in physical chemistry. Generally, each property is measured by a specific method. Spectroscopic methods have not been explored for this purpose. Theoretically, all physical and chemical properties of water are determined by its structures (i.e., hydrogen bonding of the hydroxyl groups) and will be reflected in the near-IR spectrum of water. Therefore, it is expected that these properties can be determined from near-IR spectra of water and a universal spectroscopic approach can be developed. Principal component regression (PCR) and multilinear regression (MLR) were used to correlate the spectra of water measured at temperatures from 5 to 65°C (as seen in Fig. 2) with the physical and chemical properties of water. Fifteen properties were studied including density, refractive index, dielectric constant, viscosity, surface tension, vapor pressure, sound velocity, isothermal compressibility, thermal expansivity, thermal capacity, thermal conductivity, enthalpy, free energy, entropy, and ionization constant. The literature values for these properties as functions of temperature are shown in Fig. 6. Very good correlations were found between the PCR predicted values from a direct validation and the literature values for all these properties as shown in Fig. 7. This study demonstrated that fifteen properties of water can
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chemistry,
vol. 13, no. 8, 7994
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all be determined simply by measuring a set of near-IR spectra of water. This method was also used for the determinations of physical and chemical properties of NaCl solutions and water-methanol mixtures [ 29,301. We demonstrated that near-IR spectroscopy can be used as a universal method for the determinations of physical and chemical properties of water and aqueous solutions.
3. Applications
and future investigations
We have demonstrated that the results of many early investigations in physical chemistry can be
applied in analytical chemistry. The investigations using this strategy have led to several novel applications of near-IR spectroscopy of water and aquesolutions for the determinations of ous temperature, electrolytes, and physical and chemical properties. Interdisciplinary investigations can be very productive if one can benefit from the previous knowledge in other disciplines. The near-IR spectroscopic methods developed in these investigations are simple and fast. The fiber-optic sensors and probes developed can potentially be used for remote sensing in industrial process control and environmental monitoring. They can be used in caustic industries to determine both temperature and concentrations of electrolytes
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trends in analyfical chemistry, vol. 13, no. 8, 1994
I 975
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in an electrolysis broth, where electrically based temperature transducers (e.g., thermal couples and thermistors) can not be used and the environment is hostile for humans. They can also be applied in semiconductor industries to monitor the chemical compounds such as fluorides in an etching broth. In addition, they can be used in pharmaceutical industries where the active ingredients are dissolved in aqueous solutions of electrolytes or alcohols. For environmental monitoring, the fiber-optic sensors can be used to determine the temperature and salinity of seawater in coastal regions, or to monitor specific pollutants. Furthermore, they can be used in waste water treatment plants.
from 5 to 65°C.
References L11 H.R. Wyss and M. Falk, Can. J. Chem., 48 ( 1970) 607.
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[8] K.W. Bunzel,J. Phys. Chem., 71 (1967) 1358. [ 91 W.A.P. Luck and W. Ditter, J. Phys. Chem., 74 ( 1970)
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[21] J. Lin and C.W. Brown, Appl. Spectrosc., 46 ( 1992) 1809. [ 221 J. Lin and C.W. Brown, Anal. Chem., 65 ( 1993) 287. ~231 J. Lin and C.W. Brown, Environ. Sci. Technol., 27 (1993) 1611. v41 J. Lin and C.W. Brown, Appf. Spectrosc., 47 (1993) 239. [251 E. Watson Jr. and E.H. Baughman, Spectroscopy, 2 (1987) 44. [261 M.K. Phelan, C.H. Barlow, J.J. Kelly, T.M. Jinguji and J.B. Callis, Anal. Chem., 61 ( 1989) 1419. ~271 A. Grant, A.M.C. Davis and T. Bilverstone, Analyst, 114 (1989) 819. [281 J. Lin and C.W. Brown, Appl. Spectrosc., 47 (1993) 1720. ~291 J. Lin and C.W. Brown, Vib. Spectrosc., ( 1994) in press. [301 J. Lin, C.W. Brown, J. Near Infrared Spectrosc., ( 1993) in press. Jie Lin and Chris W. Brown are at the Department of Chemistry, University of Rhode Island, Kingston, RI 0288 1, USA.
International Conference - H-TAP Hyphenated Techniques in Analytical SeparationsSM January 22-25, 1995, Orlando, Florida, USA lnternational
Conference and Exhibition Techniques in Analytical Separations is one of the most exciting and significant instrumentation areas to meet the new challenges for real-time and highly sensitive measurements. Now, in one place, speakers from throughout the world, meet to share original presentations, tutorials and keynote addresses to the instrumentation and specific applications of hyphenated techniques. You’ll have the opportunity to meet with your colleagues...the people who are establishing the foundation of Hyphenated Strategies. Come share in discussions, give input and get ideas for the future well being of the industry. CALL FOR PAPERS Prospective speakers are requested to submit an abstract of 150-200 words to H-TAS Committee c/o InfoScience, Box 153, Northbrook IL 60065 USA. The three day conference will focus on emerging techniques, the applications and the instrument development integrating/hyphenating the techniques GC, MS, PTV, Purge & Trap, FTIR, SED, Air Traps, Extractors, LC, NMR, FIA, DAD, SFC, SFE, CZE, ITP. A special mini-conference on Sample Preparation will also be held. The speakers include distinguished industry, government, and university professionals. A comprehensive exhibition of equipment and services will be on display during the conference.
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