L E NOTES

Spectroscopic Measurement of NaC1 and Seawater Salinity in the Near-IR Region of 680-1230 nm

J I E L I N and C H R I S W. B R O W N * Department of Chemistry, University o/Rhode Is- land, Kingston, Rhode Island 02881

Index Headings: Near-IR; Salinity; Principal component regression; Multi-layer regression; Electrolytes; NaCl; Sea water.

I N T R O D U C T I O N

Recently, nea r - IR spectroscopic measu remen t s of elec- t rolytes in aqueous solutions have been explored. The basis for this technique is the per tu rba t ions of electro- lytes on the nea r - IR water bands. Both measu remen t s of a single electrolyte and s imul taneous measu remen t s of several electrolytes have been investigated. 1-4 Deter- mina t ion of seawater salinity is also poss ib le ) Most of these measurements were made in the near - IR region f rom 1100 to 2500 nm. In contrast , the shor t -wavelength near- IR region f rom 700 to 1200 nm has received much less a t tent ion. 6

Due to very strong absorp t ion of water in the region of 1100 to 2500 nm, spectroscopic measu remen t s of aque- ous solutions mus t be pe r fo rmed with the use of a light pa th leng th of less t han 2 mm. I f the measu remen t s are to be made at different t empera tu res , t he rma l expansion or contract ion of the cell can cause a significant change in the pa th length , thus leading to a large error in the result. Compared to t ha t in the region of 1100 to 2500 nm, the absorp t ion of water is much lower in the region of 700 to 1200 nm. In the la t ter region, a pa th leng th up to 10 cm can be used. Therefore , measu remen t s in the la t ter region provide some advantages over those in the former region.

In this study, the concentra t ions of NaC1 (0 to 5 M) and seawater salinity (0 to 35%o) were measured in the region of 680 to 1230 nm. The spect ra were measured a t different t empera tu re s in order to invest igate the effect of t e m p e r a t u r e on the de te rmina t ion of NaC1 concen- t ra t ion and seawater salinity.

Received 25 September 1992. * Author to whom correspondence should be sent.

E X P E R I M E N T A L

Twen ty -one NaC1 solutions were p repa red with con- centra t ions f rom 0 to 5 M at an increment of 0.25 M. Eighteen seawater samples were p repa red with a salinity f rom 1 to 35%o at an inc rement of abou t 2%o. 6 The mea- su rements were made on an N I R S y s t e m Model 6250 spec t rophotometer (NIRSys tem, Inc., Silver Spring, MD). The spect ra were scanned f rom 680 to 1230 nm at a resolut ion of 0.79 nm; 20 scans were averaged for each spect rum. For the NaC1 solutions, a 2-cm cell was used, whereas for seawater samples, a 1-cm cell was used (due to the small volume of samples available). Five spec t ra were measured for each sample a t t empera tu re s f rom 23.0 to 28.5°C at app rox ima te ly the same increments . The (exact) t empe ra tu r e s do not need to be known for the calculat ion of NaC1 concentra t ion and seawater sa- linity. The NaC1 concentrat ion and seawater salinity were calculated by bo th l inear or mul t i - l inear regression and principal componen t regression. 7,s The procedure for da ta processing can be found elsewhere. 3,4

R E S U L T S A N D D I S C U S S I O N

N e a r - I R spec t ra of water and 5 M NaCI solution are shown in Fig. 1; the spec t ra are baseline zeroed a t 700 nm to remove the baseline shift. As seen, the dissolution of NaC1 causes a shif t of water bands to longer wave- length and a decrease in the absorp t ion intensity. Figure 2 shows difference spec t ra of NaC1 solutions minus water measured a t 28.5°C. The pe r tu rba t ions of NaC1 on water

1.0

0.8

¢~ 0.6

~2 0 0.4 (.t)

..Q <

0.2

0.0

7O0

FIG. 1. 28.5°C.

water ! ~ 5 M NaCI

i I i /

806 9~2 1o18 11=4 123o

Wavelength (nm)

Near-IR spectra of water and 5 M NaC1 solution measured at

Volume 47, Number 2, 1993 0003-7028/93/4702-023952.00/0 APPLIED SPECTROSCOPY 239 © 1993 Society for Applied Spectroscopy

0.00

0

..~ -0.05 w.-

0

<

<3 -0.10

NaCI

1M 2M 3M 4M 5M

-0.15 L . L~ L J

700 806 912 1018 1124 1230

Wavelength (nm) Fro. 2. Near-IR difference spectra of 5 M NaC1 solutions measured at 28.5°C with spectrum of water as reference.

0.002

0.000

-0.002 0

.0 -0.004

¢n _Q < <3 -0.006

-0.008

-0.010 t ' 11~24 700 806 912 1018 123q

Wavelength (nm)

FIG. 4. N e a r - I R d i f f e rence s p e c t r u m of 35%0 s e a w a t e r m e a s u r e d a t 28.5°C w i t h w a t e r as r e f e r ence .

bands can be seen clearly from these difference spectra. Several negative bands are observed in this wavelength region. The changes in the absorbances seem to be rea- sonably linear with increasing NaC1 concentrations. Therefore, linear regression (LR) using the absorbances at selected wavelengths can be performed to calculate NaC1 concentrations, and an optimum wavelength se- lected as the analytical wavelength.

Temperature has a significant effect on the near-IR water bands. Figure 3 shows the difference spectra of water and 5 M NaC1 solution measured at different tem- perature, with the spectra measured at 23.0°C as the reference. The magnitude of the temperature effect for 5M NaC1 solution is smaller than that of water, which may largely be due to the lower concentration of water

0.04

0 0

O

< <3

Water 0.03

0.02

0.01

0,00 ~ ' ~ ~ -

• 0.02

5 M NaCI

-~ 0.01

.JQ <

i

700 8(~6 912 1018 1124 1230

Wavelength (nm) Fro. 3. Near-IR difference spectra of water and 5 M NaC1 measured at different temperatures from 23.0 to 28.5°C with spectra measured at 23.0°C as reference.

in the NaC1 solution and hydration of the ions. Several isosbestic points are observed, at which the effect of tem- perature is zero. The perturbations at one of these wave- lengths, 1102.5 nm, are still large, and the absorbances at this wavelength can be used for the calculation of NaC1 concentration by linear regression (LRi~o).

The effect of temperature on the near-IR spectra of seawater can also be accounted for by performing the multi-linear regression (MLR) using the absorbances at several wavelengths or by principal component regres- sion (PCR) using entire spectra. In MLR and PCR, sev- eral variables are used to solve the changes in near-IR spectra caused by both NaC1 and temperature and, prob- ably, the nonlinear combinations of the changes.

Seawater is an aqueous solution containing a variety of electrolytes (so called sea-salts) at different concen- trations. With respect to the perturbations on the near- IR water bands, seawater can be treated as a "single electrolyte" system, due to the constant ratios among the major components (ions)? In this study, the salinity of seawaters was also determined by the use of their near- IR spectra. A seawater sample with a salinity of 35% is approximately equal to a 0.7 M NaC1 solution in ionic strength and in the intensity of the perturbations on the near-IR water bands. The perturbations by salts in 35%0 seawater are not obvious from the (original) spectra, but they can be seen clearly from a difference spectrum. As shown in Fig. 4, the pattern of the perturbations by sea- salts is similar to that by NaC1 because NaC1 is the major salt in seawater (86.5 % of the total sea-salts in terms of concentration in mol/kg). The difference spectrum is noisy, due to the shorter pathlength of the cell (1 cm) and the lower intensity of the perturbations caused by sea-salts (relative to those by 5 M NaC1). The salinity of seawaters is also calculated by MLR and PCR. Linear regressions (LR and LRi,o) were not applied to seawater, due to the relatively large noise encountered in the spec- tra.

All the results for the measurements of NaCl concen- trations and seawater salinity are listed in Table I. The sequence of errors is LR~,o > LR > MLR > PCR. Several points can be made regarding this sequence of errors: (1) LRi~o > LR, because LRiso is calculated for the spectra

240 Volume 47, Number 2, 1993

TABLE I. Results for NaCI concentration and seawater salinity mea- surements2

nA SEE SEP

NaC1 LR 1014.6 0.063 .. . LR~,o 1102.5 0.087 0.083 MLR 4 0.039 0.037 PCR 4 0.020 0.016

Salinity MLR 4 1.594 1.509 PCR 4 0.672 0.694

Analytical wavelengths selected in MLR: NaC1 896.4 1146.2 1150.9 1163.6 Salinity 680.0 681.6 976.5 1141.4

an = Number of wavelengths or eigen-vectors in MLR or PCR; h = wavelength at which LR or LR~so is performed. SEE = standard error of estimation; SEP = standard error of prediction.

measured at different temperatures. (2) MLR gives smaller errors than LR and LRiso, because four variables (wavelengths) are used in the MLR model for solving the perturbations caused by both salt(s) and tempera- ture. (3) MLR > PCR, because PCR employs successive average orthoganolization, which will increase the "ef- fective" signal-to-noise ratios (by averaging).

CONCLUSION

This study demonstrates that concentration of NaC1 and salinity of seawater can be determined by the near- IR spectrum in the region of 680 to 1230 nm. The mea- surements can be made at different temperatures, and the effect of temperature accounted for. Simple deter- minations can be performed by the measurement of ab- sorbances at selected wavelengths and the calculation using linear or multi-linear regression. Principal com- ponent regression using entire spectra gives the best re- sults; SEPs of 0.016 M for NaC1 and 0.694~/~ for salinity are obtainable. By means of optical fiber, the remote measurements are possible.

1. T. Hirschfeld, Appl. Spectrosc. 39, 740 (1985). 2. E. Watson, Jr., and E. H. Baughman, Spectroscopy 2, 44 (1987). 3. J. Lin and C. W. Brown, Appl. Spectrosc. 46, 1809 (1992). 4. J. Lin and C. W. Brown, Anal. Chem., paper in press (1993). 5. J. Lin and C. W. Brown, paper submitted to Environ. Sci. Technol.

(1992). 6. M. K. Phelan, C. H. Barlow, J. J. Kelly, T. M. Jinguji, and J. B.

Callis, Anal. Chem. 61, 1419 (1989). 7. D. E. Honigs, J. M. Freelin, G. M. Hieftje, and T. Hirschfeld, Appl.

Spectrosc. 37, 491 (1985). 8. S. M. Donahue and C. W. Brown, Anal. Chem. 63, 980 (1991).

Microwave-Induced Plasma Atomic Absorption Spectrometry with Solution Nebulization and Desolvation-Condensation

KIN C. NG* and T I M O T H Y J . G A R N E R t Department of Chemistry, California State Univer- sity-Fresno, Fresno, California 93740-0070

Index Headings: Atomic absorption spectroscopy; Instrumentation, atomic absorption; Microwave-induced plasma.

INTRODUCTION

Electrical discharge (plasma)-based analytical instru- ments are commercially available. These include the in- ductively coupled plasma atomic emission spectrometer (ICP-AES), the ICP-mass spectrometer (MS), the gas chromatograph/microwave-induced plasma (MIP)AES, the glow-discharge MS, the direct-current plasma AES, and the ICP atomic fluorescence spectrometer (ICP-AFS). High-temperature inert-gas plasmas are believed to be excellent sources for excitation spectrometry--namely, atomic emission, and ionization for mass spectrometry. The success of ICP-AFS, however, indicates that there also is an abundance of ground-state species for sensitive monitoring. The ICP-AFS technique offers parts-per- billion detection limits?

Recently, there has been renewed interest in the study of plasma atomic absorption spectrometry (AAS), a tech- nique that monitors the ground-state species. The main argument is that AAS offers a high degree of detection selectivity. Plasma-AAS will be even more attractive if it also provides good sensitivity and minimal matrix ef- fects. Thus Winefordner and co-workers 2,3 and Rayson and Shen 4,~ recently re-evaluated ICPAAS, Liang and Blades G studied capacitively coupled plasma AAS, and Ng e t a l . 7 and Jin and co-workers s,9 performed AAS with the microwave-induced plasma (MIP). A typical ICP op- erates at 1 kW power and 17 L/min argon flow, whereas a typical MIP operates at <0.2 kW and <1 L/rain argon. The lower MIP power might make it more suitable for providing ground-state species. Ng e t a l . 7 used the Bee- nakker cavity for producing the AA plasma, and Jin and co-workers s,9 employed the surfatron.

While the low-power MIP operation is attractive, it makes the plasma inefficient in accepting liquid aerosols. In atomic spectroscopy the most common sample type is liquid, and the sample introduction used is solution nebulization. In a procedure to improve a plasma's an- alytical capability, the solvent in an aerosol can be externally removed by employing a desolvation-conden- sation apparatus. 1° The performance of desolvation-con-

Received 14 September 1992. * Author to whom correspondence should be sent. t Present address: Department of Chemistry, UCLA, Los Angeles, CA

90024.

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