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Photoacoustic spectroscopy on ammonium sulphate and glucose powders and their aqueous solutions using a Co2 laser
Volume
22, number
OPTICS
2
PHOTOACOUSTIC
SPECTROSCOPY
AND THEIR AQUEOUS SOLUTIONS
COMMUNICATIONS
August
1911
ON AMMONIUM SULPHATE AND GLUCOSE POWDERS USING A CO2 LASER
Per-Erik NORDAL and Svein Otto KANSTAD Norwegian Defence Research Est., Division for Electronics, P.O. Box No. 25, N-2007 Kjeller, Norway Received
26 May 1977
Using a tunable CO? laser, we demonstrate the feasibility of photoacoustic spectroscopy on liquid and solid samples in the middle infrared. Spectra are recorded from glucose and ammonium sulphate powders and their aqueous solutions. A spectral feature, tentatively identified as the vr mode of SO4, is observed in ammonium sulphate for the first time. It is shown that a differential technique presently allows the detection of ammonium sulphate in water down to approximately 100 mg/!Z.
1. Introduction Following its laser-assisted revival as a tool for the detection and identification of gases at low relative concentrations [1,2], photoacoustic spectroscopy (PAS) has given new impetus to the spectroscopic characterization of solids [3,4] and liquids [5,6] too. This is particularly significant for specimens that are not easily subjected to conventional spectroscopic analyses, e.g., strong scatterers (powders, rough solid surfaces, smears, suspensions), media with extremely high [6] (or even extremely low [7] ) absorption coefficients, and solutions with strongly absorbing solutes and/or solvents, cfr. below. PAS of liquids and solids has mostly been performed in the UV-vis-NIR, in marked contrast to PAS of gases, which are mainly analysed in the middle infrared (MIR) due to their characteristic MIR absorption (“fingerprints”). With widely tuneable coherent MIR sources now becoming available [8], however, one would expect that related but broader spectral features (“footprints”?) shall soon promote PAS of condensed matter in the MIR to similar prominence. For instance, substances deposited or formed on surfaces (and in powders) in connection with pollution, corrosion and its inhibition, oxidation, adsorption, catalysis, filtration, chromatography, on biological membranes etc. may be quickly and nondestructively observed.
In the current investigation we demonstrate that (NH4)2S0, and D(t)-glucose can easily be studied by MIR-PAS in powder form as well as in aqueous solutions to high dilution. The choice of those samples was not entirely arbitrary, though motivated by their strong absorption spectra within the wavelength region of the radiation sources (a CO, laser): Abundant in atmospheric aerosols, (NH,),SO, is important as a long range carrier of sulphur pollutants [9,10] and plays a role in the global heat balance [ 1 l] ; sulphate ions present a major water pollution problem in many places; while glucose provides an example of relevance to biology. Water and aqueous solutions merit particular attention due to their pervasiveness in Nature.
2. Measurement techniques Spectra and concentration curves were recorded by illuminating the samples with radiation from a CO, laser that was chopped by means of cavity-length scanning using mirrors mounted on piezoelectric transducers [12]. The light was nearly collimated with an irradiated circular area apertured to 4 mm diameter. Between forty and fifty lines within the wavelength range 9.2-10.7 pm could be selected from the laser at pulse repetition frequencies up to 700 Hz without introducing pulse distortions; output powers varied from 185
Volume
22, number
2
OPTICS COMMUNICATIONS
10 to 100 mW across the spectrum. The normal scan frequency was f= 205 Hz, chosen so as to avoid harmonics of the AC mains frequency and to represent a compromise between high signal levels (decrease with f) and low background noise (largely l/f). Specimens were accomodated in a 7.7 mm diameter well drilled 10 mm into the O-ring sealed Perspex bottom plate of the photoacoustic cell, which was a 7 mm diameter by 24 mm height polished bore in a block of stainless steel, closed at the top by an Irtran-2 window. A short duct in the wall of the cell connected it to a General Radio type 1961 electret microphone, fitted with a GR type 1560-P42 preamplifier. Signals from the preamplifier were fed directly into a Princeton Applied Research model 186 lock-in amplifier that was operated in 2fmode. Since two pulses were emitted in each scan cycle of the laser mirror, this gave excellent immunity to coherent pick-up of acoustic noise from the chopping. At each wavelength two measurements had to be made: First the sample plate was clamped to the bottom of the photoacoustic cell (using a small lab-jack); the procedure was then repeated with another plate containing a suitable reference, to normalize the former reading. (Attempts to use the incident light for comparison, failed due to Fabry-Perot fringes from the window .) Measurements made with the same substance in different sample plates might scale the whole spectrum uniformly a few percent upwards or downwards due to unequal volumetric fillings. In the same run, however, the repeatability was impressing: Within one percent for the powders and usually within two percent for the aqueous solutions. The failure of the latter to conform with the former in this respect was due to evaporation of water, which served to increase the signal during the initial ten to fifteen second after closure of the chamber. Special care was therefore taken to avoid the absorption of radiation by water films adsorbed on surfaces inside the photoacoustic cell. Thus a water layer of 1 m thickness on the window would reduce the power reaching the specimen by ~7%~ sufficient to seriously impair several measurements. Satisfactory results were reached by heating the window to 6O”C, the cell itself remaining at 35°C due to the insulating rubber seal under the window. Measurements were made using readings taken immediately upon closure and after having allowed the signal to settle; they gave the same results when properly normalized. 186
August
1977
Distilled, deionized water was employed as the reference for the aqueous solutions. In the case of powders, an acetylene sooted piece of bakelite was used, that absorbed strongly and exhibited no discernible spectral structure, as evidenced in comparative measurements against pure water and from observations of laser power falling through the cell (bottom removed). Comparison of water signals with those from the sooted bakelite showed that the H,O absorption coefficient increased monotonously by a little more than 60% from 9.3 to 10.7 pm wavelength, in agreement with published data [ 131. The aqueous solution spectra presented below have not been corrected for the dispersion in H2 0.
3. Spectral measurements Powder samples of D(t)-glucose (monohydrate), Merck Art.8342, were finely ground, pressed firmly into the sample well and subjected to the procedures outlined above. Storage for several days in a desiccator caused no change in the spectrum. Aqueous solutions were made using distilled, deionized water, and spectra were recorded on solutions that were from l/2 hour to one week old. No ageing effects were observed. The glucose powder and solution spectra shown in fig. 1 are in essential agreement with those reported by Goulden [ 141. Powder and aqueous solution samples of ammonium sulphate (Merck Art. 12 17) were prepared and
* POWDER 0 SOLUTION
f
-4
WAVELENGTH Fig. 1. Spectra and in a 253.6
(pm)
of glucose as a fine-grained g/kg aqueous solution.
powder
(< 0.1 mm)
August 1911
OPTICS COMMUNICATIONS
Volume 22, number 2
analysed by routines analogous to those described above. Spectra are presented in fig.2. The aqueous solution spectrum agrees well with data obtained by Remsberg [15] and Downing et al. [16]. Our powder spectrum, however, differs in certain important respects from spectra previously published [ 11 ,17- 191 on single crystal and powdered (NH,),SO,. While earlier workers report an absorption maximum centered at or near 9 pm, we observed strong absorption extending to 9.6pm and beyond. This may originate from perturbations in the local symmetry of the sulphate ion in surface layers, contributions from which are emphasized in photoacoustic measurements. Indeed, both shape and position of the SO, v3 band near 9 pm have been found to depend strongly on the local symmetry [20,2 11. The v3 band position also differs somewhat for (NH4)2S04 powder in Nujol mull [17] and in KBr pellets [19]. The absorption peak at 10.26 fl has not previously been reported for (NH4)2S04, and is absent in our aqueous solution spectrum. We suggest that this feature is due to the v1 mode of SO,, which is not infrared active in the free ion but has been observed in cases where crystal fields lower the local symmetry [20]. Confirmation of this (tentative) identification would therefore support the view aired above, that surface effects contribute to our spectra. It would be interesting to investigate further the partial contributions from bulk and from the surface to the recorded spectrum. In order to rule out the possibility of geo-
metric
effects
being responsible
4. Concentration
(pm)
Fig. 2. Spectra of ammonium sulphate as a fine-grained powder (< 0.1 mm) and in a 58.5 g/kg aqueous solution, with the feature at 10.26 Mm shown for a coarse-grained (- 1 mm) powder sample too.
at
curves
For “thermally thick” samples 1221 where the incident radiation penetrates deeper into the specimen than the thermal diffusion length, the photoacoustic signal will be proportional to the optical absorption coefficient /3. Adding independent contributions from solute and solvent, the signal from the solution can be written Q(h) = K1~(V(c,P,(~)
+ cw P,(V),
(1)
where pw(h)>, P,(h) are the optical absorption coefficients and cw, cs the volumetric mixing ratios (c, t cs = 1) of pure water and pure solute, respectively. I(h) is the intensity of the incident radiation and K, is a constant. Similarly, the signal from the pure water reference is Q,(h)
= K2r(VPw(V>
(2)
where K2 is used rather than K, since acoustic efficiencies may differ when samples are changed. Assume that Q(h) and Q,(h) are recorded at h, and ho, chosen to make &(AI) large and P&ho) = 0. From eqs. (l)-(2) follows:
QOl>/Q,(Q Q(bYQ,(~o) -
WAVELENGTH
for tile observation
10.26pm, another spectral scan was performed on a coarse grained sample. The peak reproduced as shown (reduced signal amplitudes are expected for coarser grams).
’
where a, = p,c,/(p, +c,(p,-p,)) is the gravimetric mixing ratio of the solute, while ps and pw are the specific densities of solute and solvent, respectively. In fig. 3, data are presented on measurements that have been made on (NH,),SO, solutions of different 187
Volume
22, number
2
I
I
1 CONCENTRATION
10
(g . (NH,),SO,/kg
I 100
500 SOLUTION)
Fig. 3. Photoacoustic signals from dissolved ammonium shlphate versus strength of solution. The ordinate is calculated from the left hand side of eq. (3), where Q (hr)/Q,(ht) and Q(ho)/Qw(ko) are the ammonium sulphate signals relative to water at, respectively, the R 24 (9.25 pm) and P 20 (10.59 pm) lines.
strengths, referred to a pure water reference, as usual. X, = 9.25 pm and ho = 10.59 pm were chosen in accordance with fig. 2. Ordinate values, which correspond to the left hand side of eq. (3), are clearly proportional to the solute concentration over a wide range. At high optical densitites, the photoacoustic signal saturates in a manner analogous to that observed earlier [5] on aqueous solutions in the visible. (Care was taken to avoid saturation at all wavelengths in the spectra shown in figs. 1 and 2.) Near the lower detection limit, the problem of measuring the small contribution from (NH,),SO, in the presence of a large water background signal was compounded by the fact that this background differed from the solution to the pure water sample. The discrepancy was traced experimentally to slight dissimilarities in sample volumes (cfr. section 2) implying K, # K2 in eqs. (1) and (2). The background correction implicit in eq. (3) was essential at concentrations below -20 g(NH,),SO,/kg solution. The direct approach just described gave a detection threshold around 5 g(NH,),SO,/kg solution. Higher sensitivities can be reached through differential techniques, however. To illustrate this, we made our laser emit a train of pulses alternately at X, and ho, at a rateof395Hz.X1=9.25~mandXo=10.675~mwere 188
August
OPTICS COMMUNICATIONS
1911
chosen, corresponding to regions of strong and weak (NH4)2SO4 absorption, respectively. Any difference in the absorption at X, and ho will‘then appear as a signal at half the laser pulse frequency (197.5 Hz), With pure water in the cell, relative intensities at,h, and X0 were balanced for identical photoacoustic amplitudes at the two wavelengths, resulting in zero s!gnal on the lock-in amplifier at 197.5 Hz. Thus resetting the zero level using the water sample before inserting the various (NH,)2S04 solutions, the data in fig. 4 were recorded, indicating a sensitivity on the order of 100 mg(NH,),SO,/kg solution. Those measurements were limited by the long term drift in the relative pulse powers at h, and ho, resulting in an ill-defined zero level. Nevertheless, the results are already relevant for higher levels of water pollution, and further progress is expected soon using a more elaborate technique.
5. Discussion We have here been particularly concerned with the complex of questions relating to photoacoustic investigations of aqueous solutions. Transmitting well in the visible, water absorbs strongly at wavelengths beyond 2 F, a representative absorption coefficient being /3=700 cm-l near 10 pm [13]. As detailed above, attendant problems are: Attenuation of irradiating
0.1’ CONCENTRATION
0.1
1 (g - (NH4),S0,/kg
10 SOLUTION)
Fig. 4. Differential signal from dissolved ammonium sulphate versus strength of solution, as observed with the laser alternating between R 24 (9.25 pm) and P 28 (10.675 pm) lines at 395 Hz.
Volume
22, number
2
August
OPTICS COMMUNICATIONS
flux in a water film on the entrance window, strong background signals from the solvent and, possibly, from water films on the cell walls. Our results 5how that those difficulties can be circumvented, even in quite demanding measurements, by appropriate choice of apparatus design and experimental techniques. Conventional transmission spectroscopy of aqueous solutions in the MIR is an established analytical tool [14], which suffers, however, from limitations due to the strong water absorption. It was recently demonstrated, though, that high sensitivities can be achieved by employing CO, laser illumination, short (100 pm) cells and sophisticated differential techniques [23]. Apart from experimental complexities, such transmittance measurements are notoriously vulnerable to particles that scatter the radiation. The immunity of PAS to scattering-induced errors is therefore particularly valuable in a host of practical applications (e.g., turbid sewage, blood etc.). Our powder spectra support the much-published contention that PAS is well suited for investigations of solid samples that scatter strongly, like crystalline powders. The practical implications of this may be substantial: A three year effort has been reported [I l] to grow (NH,),SO, single crystals of satisfactory quality for spectroscopic analysis. Additional problems are encountered in polishing the crystals and in avoiding thermally induced damage [16]. Similarly mull and pellet techniques, while routinely being used for obtaining powder spectra, accept a range of samples more restrictive than that accessible to PAS. Photoacoustic techniques appear especially well suited for the investigation of surface properties. The depth to which the material is probed can be adjusted across a limited range by proper choice of the chopping frequency. Measurements of this type support our identification of the 10.26 pm peak in the (NH,),SO, spectrum as being related to surface effects and will be reported later.
6. Conclusion Whenever applicable, infrared spectroscopy offers a precise identification of substances. The feasibility of making photoacoustic spectroscopy in the MIR on
1977
aqueous and on strongly absorbing powders therefore opens up new vistas in the emerging powder technology and in pollution monitoring. It seems only a matter of technical developments to improve the present methods to yield the required sensitivities. Further studies will be needed to apply the methods to turbid solutions and to the chemistry and physics of surfaces, with the possibility of separating bulk and surface effects by simple means.
References 111E.L. Kerr and J.G. Atwood, Appl. Opt. 7 (1968) 915. 121 L.B. Kreuzer, J. Appl. Phys. 42 (1971) 2934. 131 A. Rosencwaig, Opt. Commun. 7 (1973) 305. [41 A. Rosencwaig, [51 J.F. McClelland (1976)
Anal. Chem. 47 (1975) 592A. and R.N. Kniseley, Appl. Phys. Lett. 28
467.
161 G.C. Wetsel Jr. and F.A. McDonald,
Appl. Phys. Lett. 30 (1977) 252. [71 A. Hordvik and H. Schlossberg, Appl. Opt. 16 (1977) 101. Proc. Loen Conf. [81 Tunable Lasers and Applications, Norway 1976, eds. A. Mooradian, T. Jaeger and P.A. Stokseth (Springer, Berlin, 1976), in particular Ch. II. [91 C. Brosset, Ambio 5 (1976) 157. [lOI R.E. Weiss, A.P. Waggoner, R.J. Charlson and N.C. Ahlquist, Science 195 (1977) 979. 1111 O.B. Toon, J.B. Pollack and B.N. Khare, J. Geophys. Res. 81 (1976) 5733. and T. Lund, J. Phys. E: [I21 S.O. Olsen, A. Bjerkestrand Sci. Instr., in press. (131 H.D. Downing and D. Williams, J. Geophys. Res. 80 (1975) 1656. Acta 15 (1959) 657. [I41 J.D. Goulden, Spectrochim. iI51 E.E. Remsberg, App. Opt. 12 (1973) 1389. [I61 H.D. Downing, L.W. Pinkley, P.P. Sethna and D. Williams, J. Opt. Sot. Am. 67 (1977) 186. [I71 F.A. Miller and C.H. Wilkins, Anal. Chem. 24 (1952) 1253. [I81 E.A. Chermack, The optical constants (n, k) of ammonium sulfate in the infrared, Ph. D. thesis (New York Univ., N.Y., 1970) results cited and reproduced in ref. [ll]. [I91 F.E. Volz, Appl. Opt. 12 (1973) 564. WI K. Nakamoto, Infrared spectra of inorganic and coordination compounds (J. Wiley & Sons, New York, 1963). 1211 G.D. Blyholder and G.W. Cagle, Environm. Sci. and Techn. 5 (1971) 159. [22] A. Rosencwaig and A. Gersho, J. Appl. Phys. 47 (1976) 64. (231 G. Kraus and M. Maier, Appl. Phys. 7 (1975) 287.
189
22, number
OPTICS
2
PHOTOACOUSTIC
SPECTROSCOPY
AND THEIR AQUEOUS SOLUTIONS
COMMUNICATIONS
August
1911
ON AMMONIUM SULPHATE AND GLUCOSE POWDERS USING A CO2 LASER
Per-Erik NORDAL and Svein Otto KANSTAD Norwegian Defence Research Est., Division for Electronics, P.O. Box No. 25, N-2007 Kjeller, Norway Received
26 May 1977
Using a tunable CO? laser, we demonstrate the feasibility of photoacoustic spectroscopy on liquid and solid samples in the middle infrared. Spectra are recorded from glucose and ammonium sulphate powders and their aqueous solutions. A spectral feature, tentatively identified as the vr mode of SO4, is observed in ammonium sulphate for the first time. It is shown that a differential technique presently allows the detection of ammonium sulphate in water down to approximately 100 mg/!Z.
1. Introduction Following its laser-assisted revival as a tool for the detection and identification of gases at low relative concentrations [1,2], photoacoustic spectroscopy (PAS) has given new impetus to the spectroscopic characterization of solids [3,4] and liquids [5,6] too. This is particularly significant for specimens that are not easily subjected to conventional spectroscopic analyses, e.g., strong scatterers (powders, rough solid surfaces, smears, suspensions), media with extremely high [6] (or even extremely low [7] ) absorption coefficients, and solutions with strongly absorbing solutes and/or solvents, cfr. below. PAS of liquids and solids has mostly been performed in the UV-vis-NIR, in marked contrast to PAS of gases, which are mainly analysed in the middle infrared (MIR) due to their characteristic MIR absorption (“fingerprints”). With widely tuneable coherent MIR sources now becoming available [8], however, one would expect that related but broader spectral features (“footprints”?) shall soon promote PAS of condensed matter in the MIR to similar prominence. For instance, substances deposited or formed on surfaces (and in powders) in connection with pollution, corrosion and its inhibition, oxidation, adsorption, catalysis, filtration, chromatography, on biological membranes etc. may be quickly and nondestructively observed.
In the current investigation we demonstrate that (NH4)2S0, and D(t)-glucose can easily be studied by MIR-PAS in powder form as well as in aqueous solutions to high dilution. The choice of those samples was not entirely arbitrary, though motivated by their strong absorption spectra within the wavelength region of the radiation sources (a CO, laser): Abundant in atmospheric aerosols, (NH,),SO, is important as a long range carrier of sulphur pollutants [9,10] and plays a role in the global heat balance [ 1 l] ; sulphate ions present a major water pollution problem in many places; while glucose provides an example of relevance to biology. Water and aqueous solutions merit particular attention due to their pervasiveness in Nature.
2. Measurement techniques Spectra and concentration curves were recorded by illuminating the samples with radiation from a CO, laser that was chopped by means of cavity-length scanning using mirrors mounted on piezoelectric transducers [12]. The light was nearly collimated with an irradiated circular area apertured to 4 mm diameter. Between forty and fifty lines within the wavelength range 9.2-10.7 pm could be selected from the laser at pulse repetition frequencies up to 700 Hz without introducing pulse distortions; output powers varied from 185
Volume
22, number
2
OPTICS COMMUNICATIONS
10 to 100 mW across the spectrum. The normal scan frequency was f= 205 Hz, chosen so as to avoid harmonics of the AC mains frequency and to represent a compromise between high signal levels (decrease with f) and low background noise (largely l/f). Specimens were accomodated in a 7.7 mm diameter well drilled 10 mm into the O-ring sealed Perspex bottom plate of the photoacoustic cell, which was a 7 mm diameter by 24 mm height polished bore in a block of stainless steel, closed at the top by an Irtran-2 window. A short duct in the wall of the cell connected it to a General Radio type 1961 electret microphone, fitted with a GR type 1560-P42 preamplifier. Signals from the preamplifier were fed directly into a Princeton Applied Research model 186 lock-in amplifier that was operated in 2fmode. Since two pulses were emitted in each scan cycle of the laser mirror, this gave excellent immunity to coherent pick-up of acoustic noise from the chopping. At each wavelength two measurements had to be made: First the sample plate was clamped to the bottom of the photoacoustic cell (using a small lab-jack); the procedure was then repeated with another plate containing a suitable reference, to normalize the former reading. (Attempts to use the incident light for comparison, failed due to Fabry-Perot fringes from the window .) Measurements made with the same substance in different sample plates might scale the whole spectrum uniformly a few percent upwards or downwards due to unequal volumetric fillings. In the same run, however, the repeatability was impressing: Within one percent for the powders and usually within two percent for the aqueous solutions. The failure of the latter to conform with the former in this respect was due to evaporation of water, which served to increase the signal during the initial ten to fifteen second after closure of the chamber. Special care was therefore taken to avoid the absorption of radiation by water films adsorbed on surfaces inside the photoacoustic cell. Thus a water layer of 1 m thickness on the window would reduce the power reaching the specimen by ~7%~ sufficient to seriously impair several measurements. Satisfactory results were reached by heating the window to 6O”C, the cell itself remaining at 35°C due to the insulating rubber seal under the window. Measurements were made using readings taken immediately upon closure and after having allowed the signal to settle; they gave the same results when properly normalized. 186
August
1977
Distilled, deionized water was employed as the reference for the aqueous solutions. In the case of powders, an acetylene sooted piece of bakelite was used, that absorbed strongly and exhibited no discernible spectral structure, as evidenced in comparative measurements against pure water and from observations of laser power falling through the cell (bottom removed). Comparison of water signals with those from the sooted bakelite showed that the H,O absorption coefficient increased monotonously by a little more than 60% from 9.3 to 10.7 pm wavelength, in agreement with published data [ 131. The aqueous solution spectra presented below have not been corrected for the dispersion in H2 0.
3. Spectral measurements Powder samples of D(t)-glucose (monohydrate), Merck Art.8342, were finely ground, pressed firmly into the sample well and subjected to the procedures outlined above. Storage for several days in a desiccator caused no change in the spectrum. Aqueous solutions were made using distilled, deionized water, and spectra were recorded on solutions that were from l/2 hour to one week old. No ageing effects were observed. The glucose powder and solution spectra shown in fig. 1 are in essential agreement with those reported by Goulden [ 141. Powder and aqueous solution samples of ammonium sulphate (Merck Art. 12 17) were prepared and
* POWDER 0 SOLUTION
f
-4
WAVELENGTH Fig. 1. Spectra and in a 253.6
(pm)
of glucose as a fine-grained g/kg aqueous solution.
powder
(< 0.1 mm)
August 1911
OPTICS COMMUNICATIONS
Volume 22, number 2
analysed by routines analogous to those described above. Spectra are presented in fig.2. The aqueous solution spectrum agrees well with data obtained by Remsberg [15] and Downing et al. [16]. Our powder spectrum, however, differs in certain important respects from spectra previously published [ 11 ,17- 191 on single crystal and powdered (NH,),SO,. While earlier workers report an absorption maximum centered at or near 9 pm, we observed strong absorption extending to 9.6pm and beyond. This may originate from perturbations in the local symmetry of the sulphate ion in surface layers, contributions from which are emphasized in photoacoustic measurements. Indeed, both shape and position of the SO, v3 band near 9 pm have been found to depend strongly on the local symmetry [20,2 11. The v3 band position also differs somewhat for (NH4)2S04 powder in Nujol mull [17] and in KBr pellets [19]. The absorption peak at 10.26 fl has not previously been reported for (NH4)2S04, and is absent in our aqueous solution spectrum. We suggest that this feature is due to the v1 mode of SO,, which is not infrared active in the free ion but has been observed in cases where crystal fields lower the local symmetry [20]. Confirmation of this (tentative) identification would therefore support the view aired above, that surface effects contribute to our spectra. It would be interesting to investigate further the partial contributions from bulk and from the surface to the recorded spectrum. In order to rule out the possibility of geo-
metric
effects
being responsible
4. Concentration
(pm)
Fig. 2. Spectra of ammonium sulphate as a fine-grained powder (< 0.1 mm) and in a 58.5 g/kg aqueous solution, with the feature at 10.26 Mm shown for a coarse-grained (- 1 mm) powder sample too.
at
curves
For “thermally thick” samples 1221 where the incident radiation penetrates deeper into the specimen than the thermal diffusion length, the photoacoustic signal will be proportional to the optical absorption coefficient /3. Adding independent contributions from solute and solvent, the signal from the solution can be written Q(h) = K1~(V(c,P,(~)
+ cw P,(V),
(1)
where pw(h)>, P,(h) are the optical absorption coefficients and cw, cs the volumetric mixing ratios (c, t cs = 1) of pure water and pure solute, respectively. I(h) is the intensity of the incident radiation and K, is a constant. Similarly, the signal from the pure water reference is Q,(h)
= K2r(VPw(V>
(2)
where K2 is used rather than K, since acoustic efficiencies may differ when samples are changed. Assume that Q(h) and Q,(h) are recorded at h, and ho, chosen to make &(AI) large and P&ho) = 0. From eqs. (l)-(2) follows:
QOl>/Q,(Q Q(bYQ,(~o) -
WAVELENGTH
for tile observation
10.26pm, another spectral scan was performed on a coarse grained sample. The peak reproduced as shown (reduced signal amplitudes are expected for coarser grams).
’
where a, = p,c,/(p, +c,(p,-p,)) is the gravimetric mixing ratio of the solute, while ps and pw are the specific densities of solute and solvent, respectively. In fig. 3, data are presented on measurements that have been made on (NH,),SO, solutions of different 187
Volume
22, number
2
I
I
1 CONCENTRATION
10
(g . (NH,),SO,/kg
I 100
500 SOLUTION)
Fig. 3. Photoacoustic signals from dissolved ammonium shlphate versus strength of solution. The ordinate is calculated from the left hand side of eq. (3), where Q (hr)/Q,(ht) and Q(ho)/Qw(ko) are the ammonium sulphate signals relative to water at, respectively, the R 24 (9.25 pm) and P 20 (10.59 pm) lines.
strengths, referred to a pure water reference, as usual. X, = 9.25 pm and ho = 10.59 pm were chosen in accordance with fig. 2. Ordinate values, which correspond to the left hand side of eq. (3), are clearly proportional to the solute concentration over a wide range. At high optical densitites, the photoacoustic signal saturates in a manner analogous to that observed earlier [5] on aqueous solutions in the visible. (Care was taken to avoid saturation at all wavelengths in the spectra shown in figs. 1 and 2.) Near the lower detection limit, the problem of measuring the small contribution from (NH,),SO, in the presence of a large water background signal was compounded by the fact that this background differed from the solution to the pure water sample. The discrepancy was traced experimentally to slight dissimilarities in sample volumes (cfr. section 2) implying K, # K2 in eqs. (1) and (2). The background correction implicit in eq. (3) was essential at concentrations below -20 g(NH,),SO,/kg solution. The direct approach just described gave a detection threshold around 5 g(NH,),SO,/kg solution. Higher sensitivities can be reached through differential techniques, however. To illustrate this, we made our laser emit a train of pulses alternately at X, and ho, at a rateof395Hz.X1=9.25~mandXo=10.675~mwere 188
August
OPTICS COMMUNICATIONS
1911
chosen, corresponding to regions of strong and weak (NH4)2SO4 absorption, respectively. Any difference in the absorption at X, and ho will‘then appear as a signal at half the laser pulse frequency (197.5 Hz), With pure water in the cell, relative intensities at,h, and X0 were balanced for identical photoacoustic amplitudes at the two wavelengths, resulting in zero s!gnal on the lock-in amplifier at 197.5 Hz. Thus resetting the zero level using the water sample before inserting the various (NH,)2S04 solutions, the data in fig. 4 were recorded, indicating a sensitivity on the order of 100 mg(NH,),SO,/kg solution. Those measurements were limited by the long term drift in the relative pulse powers at h, and ho, resulting in an ill-defined zero level. Nevertheless, the results are already relevant for higher levels of water pollution, and further progress is expected soon using a more elaborate technique.
5. Discussion We have here been particularly concerned with the complex of questions relating to photoacoustic investigations of aqueous solutions. Transmitting well in the visible, water absorbs strongly at wavelengths beyond 2 F, a representative absorption coefficient being /3=700 cm-l near 10 pm [13]. As detailed above, attendant problems are: Attenuation of irradiating
0.1’ CONCENTRATION
0.1
1 (g - (NH4),S0,/kg
10 SOLUTION)
Fig. 4. Differential signal from dissolved ammonium sulphate versus strength of solution, as observed with the laser alternating between R 24 (9.25 pm) and P 28 (10.675 pm) lines at 395 Hz.
Volume
22, number
2
August
OPTICS COMMUNICATIONS
flux in a water film on the entrance window, strong background signals from the solvent and, possibly, from water films on the cell walls. Our results 5how that those difficulties can be circumvented, even in quite demanding measurements, by appropriate choice of apparatus design and experimental techniques. Conventional transmission spectroscopy of aqueous solutions in the MIR is an established analytical tool [14], which suffers, however, from limitations due to the strong water absorption. It was recently demonstrated, though, that high sensitivities can be achieved by employing CO, laser illumination, short (100 pm) cells and sophisticated differential techniques [23]. Apart from experimental complexities, such transmittance measurements are notoriously vulnerable to particles that scatter the radiation. The immunity of PAS to scattering-induced errors is therefore particularly valuable in a host of practical applications (e.g., turbid sewage, blood etc.). Our powder spectra support the much-published contention that PAS is well suited for investigations of solid samples that scatter strongly, like crystalline powders. The practical implications of this may be substantial: A three year effort has been reported [I l] to grow (NH,),SO, single crystals of satisfactory quality for spectroscopic analysis. Additional problems are encountered in polishing the crystals and in avoiding thermally induced damage [16]. Similarly mull and pellet techniques, while routinely being used for obtaining powder spectra, accept a range of samples more restrictive than that accessible to PAS. Photoacoustic techniques appear especially well suited for the investigation of surface properties. The depth to which the material is probed can be adjusted across a limited range by proper choice of the chopping frequency. Measurements of this type support our identification of the 10.26 pm peak in the (NH,),SO, spectrum as being related to surface effects and will be reported later.
6. Conclusion Whenever applicable, infrared spectroscopy offers a precise identification of substances. The feasibility of making photoacoustic spectroscopy in the MIR on
1977
aqueous and on strongly absorbing powders therefore opens up new vistas in the emerging powder technology and in pollution monitoring. It seems only a matter of technical developments to improve the present methods to yield the required sensitivities. Further studies will be needed to apply the methods to turbid solutions and to the chemistry and physics of surfaces, with the possibility of separating bulk and surface effects by simple means.
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