Investigation of Detection Limits for Solutes in Water Measured by Laser Raman Spectrometry Kirkwood M. Cunningham,' Marvin C. Goldberg,* and Eugene R. Weiner2 U.S. Geological Survey, Water Resources Division, Mail Stop 424,P.O.Box 25046,Denver Federal Center, Denver, Colo. 80225
The influence of experimental parameters on detection sensitivity was determined for laser Raman analysis of dissolved solutes in water. Individual solutions of nitrate, sulfate, carbonate, bicarbonate, monohydrogen phosphate, dihydrogen phosphate, acetate ion, and acetlc acld were measured. An equation is derived which expressesthe signal-to-noise ratlo in terms of solute concentration, measurement time, spectral slit width, laser power fluctuations, and solvent background intensity. Laser beam Intensity fluctuations at the sample and solvent background intensity are the most important limlting factors.
R a m a n spectrometry has been used for water analysis by many investigators (1-8). Although our purpose was to evaluate the utility of conventional R a m a n spectrometry as an analytical technique for aqueous solutions, p a r t of the general interest in the method is due to its special advantages i n remote sensing applications. Because the instrument geometry can be arranged to measure Raman radiation backscattered at 180", a single-ended instrument can be built, having both the light source and the detector on the same side of the sample, a requirement for a true remote sensor (1,2).Devices of this type a r e now being used for altitude profiling of atmospheric composition a n d measurements of air pollution (9). I n a n earlier s t u d y of sensitivities, Brown a n d co-workers (7, 8) reported detection limits between 25-75 mg/l. for strongly scattering anions such as N03-, S042-, CO&, and PO4:+. W h e n t h e y changed the scattering geometry of their laboratory laser R a m a n spectrometer from the normal 90" to 180", simulating a remote sensing instrument, their sensitivity for NO:j- changed from 25 to 150 mgh. They did not, however, quantitatively evaluate the limiting experimental parame-
ters. The purpose of this report is to evaluate, under laboratory conditions, the effect of solute concentration, measurement time, spectral slit width, solvent background intensity, and fluctuations in laser beam intensity on the signal-to-noise ratio (S/N). For ideal conditions (no luminescent background or laser beam fluctuations), a spectra slit width of 13.0 cm-l, S/N of 2, and a measurement time of 5 min, our treatment predicts a detection limit for N03- of 4.4 mg/l. with our laboratory equipment a n d also permits an estimate of sensitivity for a remote sensing configuration under field conditions.
EXPERIMENTAL Our equipment was comparable to that of Baldwin and Brown ( 7 ) , consisting of a Spectra Physics model 165argon ion laser and a Spex 1401 double monochromator with a dispersion of 54 WrnIcm and a slit height adjusted to 1cm. Samples were illuminated with the 488.0-nm line at approximately 0.8 watt. Unwanted radiation from the laser was removed with a Claassen filter. A mirror served to double pass the 1 Present address, Smithsonian Institution, Room AB 070, MHT Bujlding, Washington, D.C. - University of Denver, 2199 So. University Blvd., Denver, Colo. 80210.
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
laser beam through a cylindrical sample cell. The sample volume was focused sharply on the entrance slit. Samples were made from stock solutions of reagent grade salts in singly distilled water and filtered through 0.2-mp Millipore filters to remove any suspended matter. Solutions of NO?-, Sod2-, Cos2-, HCO3-, HzP04-, HP0d2-, acetate ion, and acetic acid were studied. Bicarbonate concentrations were in error by about 10% because some of the bicarbonate forms CO2, which is partially lost to the atmosphere (10). For each solute, the most intense Raman lines were measured, all of which were in the region between 800-1200 cm-l. Brown et al. ( 7 , 8 ) ,working with natural water samples, reported an improvement in sensitivity by shifting the excitation wavelength from 488.0 to 514.5 nm because the photoluminescent background intensity was decreased at the longer wavelength. Our filter stock solutions had no significant luminescence and little difference in S/N was observed with excitation a t either 488.0 or 514.5 nm. Natural water usually will contain photoluminescent materials and it should be advantageous to operate a laser Raman sensor using an excitation wavelength as far towards the red region as is practical. Tobin ( 1 1 ) drew similar conclusions about the advantages of shifting the laser excitation wavelength for Raman analysis of powders having fluoresecent backgrounds. Photon counting and signal processing were done with an SSR 1151B photomultiplier tube, 1105 ratemeter, 1110 digital synchronous computer, and Digital Equipment Corp. PDP-11 computer. A t low signal levels S / N measurements are more reproducible when determined from solute peak heights rather than peak areas because the band wings, which have relatively high noise levels, are a large fraction of the total area. When making quantitative comparisons at low concentrations ( 4 ) ,band peak heights are satisfactory. To determine the true signal level a t a peak maximum, the background signal a t the wavenumber of the peak maximum must be subtracted from the peak signal. Because the water solvent background is nonlinear, a simple straight-line interpolation of the background curve between the extremes of the band wings will introduce an error. As shown below, it is necessary to use a pure water spectrum in order to determine the shape of the background curve at a solute peak. A matter of concern, however, is the fact that the presence of ionic solutes affects interactions between water molecules and can modify the entire water spectrum (11).At solute concentrations of 0.1 to 0.001 M, there was no significant change in the shape of the water background curve in the wavenumber region of our measurements. The water background at a solute peak was determined by using the water bending band at 1650 cm-1 as a internal reference.The ratio of the water signal a t 1650 cm-l and a t the wavenumber of a solute peak was taken to be the same in pure water and in the solution. The 1650 cm-' reference peak also allows correctionsfor variations in laser power and optical alignment between measurements. The peak heights of the water bending and solute bands were determined as follows: (based on Figure 2). For each peak, measurements were made a t 3 wavenumbers to obtain the signalslVL,Np, and N H ,where the subscripts refer to the low wavenumber boundary, peak maximum, and high wavenumber boundary respectively; as determined visually. A straight-line interpolation between N L and N H gives
+
NB' = U N H bNL
(1)
where a = ( i p - VL)/(~JH - 5 ~ and ) b = 1- a. NH' serves as a matching point to which the peak height and the pure water background a t Up are referred. To measure an anion concentration, four measurements of N L ,Np, and N Hare needed; they are: at the solute bend, a t the 1650 cm-' water reference band for solution, at the 1650 cm-I water reference band for pure water, and a water background for pure water taken a t the solute band position using the same VL, Yp and 5~as for the solute band. Then the quantity Ai = Np NB' is calculated, where the index references are: i = 1for
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the solute band, i = 2 for the 1650 cm-' reference band for the solution, i = 3 for the 1650 cm-' reference bapd for pure water, and i = 4 for the pure water background at the solute band.
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Because the water background is slightly concave upward in the 800-1200 cm-I region, where all of the solute peaks fall, A4 is a small negative number. Thus 12 measured points in all are used for each anion. At each of the 12 points, 30 separate counts of 4 s each were averaged. From Figure 2, the solute peak height will be
$
e
'"1
3500
1 ,5001
Ns is the solute signal and NBis the water background signal at the wavenumber of Np. The ratio A*/A3 corrects the pure water background for the presence of the solute. This care in dealing with the
background is necessary at low solute concentrationsbecause A1 may be of the same order of magnitude as A4. The 1650 cm-l reference peak is much larger than the solute peaks so that NB' may be taken as the background count for the reference peaks. The quantities used in the calculation of S/Nare:
The quantity directly proportional to solute concentration is the ratio (4h
R = NsIN2
(3)
This proportionality was found to hold for all solute species measured and supports the assumption that changes in both the water solvent band shape and solute band shapes may be neglected over the small solute concentration ranges used.
THEORY In Figure 1, the large, nonlinear background signal ih the vicinity of the nitrate peak illustrates a fundamental problem inherent to the use of Raman spectrometry for the analysis of aqueous solutions. This background is due almost entirely to the Raman scattering signal from the water solvent. The dark noise count is only 30 per second, extraneous laser emissions have been removed by a Claassen filter and scattered Rayleigh light has been reduced to negligible levels in this region by the use of a double monochromator (stray light at 65 cm-1 from exciting line is less than 1O-l1, according to the manufacturer). Walrafen (12) has published the Raman spectrum of water and it is evident that many solutes of interest to the remote sensing of water quality inevitably will have their Raman signals superimposed on a relatively large, nonlinear background signal from the water solvent (1, 2). This means that small signals from solutes must generally be measured as a small difference between two large numbers. This problem has been emphasized by others (1-5). In order to quantitatively express S/N, an expression for the noise level is needed. Let D equal the root mean square (rms) deviation of the signal from its true value. Assume that D is caused by statistical variations in photon counting and by fluctuations in the laser beam intensity in the sample. The portion of the rrns variations which are due to photon counting are described by a Poisson distribution (13, 14) and may be given by CT = N1/2, where N is the total number of counts collected in a given time period. Laser beam fluctuations in the sample may arise from several causes, such as statistically random noise in the laser output, laser power supply ripple, refractive index fluctuations in the laser beam path, beam scattering by particulates, optical misalignments due to vibrations and temperature effects, and the like. The rrns laser beam intensity fluctuations may be expressed by: d = x f = xam
where f = average value of the beam intensity, here treated as a constant equal to the true intensity value in the aksezce of any fluctuations. x = rrns fractional deviation from I. N = average number of counts received during the counting period; treated as a constant in the_sam_e manner as I. a = proportionality factor converting N to I .
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ACM-I
Flgure 1. Raman spectrum,of 100 mg/l. NO3- in water. Spectral slit width = 5.6 cm-'. Argon laser excitation: 488.0 nm at 0.8 watt. The background is the Raman signal from the water
Because the incident beamenergy is much larger than the scattered Raman energy, I >> N and a will be a large number, the quantity x can be treated as the sum of two rms fractional deviations; xa, due to statistically random fluctuations and x b , due td nonrandom fluctuations as would arise from causes such as power supply ripple, refractive index gradients, and particle scattering. Then x = x , f Xh. Because x , follows Poisson statistics, it becomes very small for large counts (x, = N-lI2), whereas X b is independent of the number of counts. Because of thd water background, the count rates are always large, around lo4 counts per 4-s period, even in the absence of a solbte signal. If laser beam intensity fluctuations are significant a t all, x,
