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Photopyroelectric spectroscopy of H2O-D2O mixtures
I&red
Phys. Vol. 33, No. 6, pp. 575-519,1992
0020-0891/92$5.00 + 0.00 Fkrgamon Press Ltd
Printed in Great Britain
PHOTOPYROELECTRIC SPECTROSCOPY OF H20-D,O MIXTURES D. DADARLAT,A. FRANDAS and I. BRATU Institute of Isotopic and Molecular Technology, P.O. Box 700, Cluj-Napoca 5, R-3400 Romania (Received I June 1992) Abstract-The photopyroelectric technique, in a reflection mode contiguration, is proposed as an alternative method for quantitative analysis of H,O-D,O mixtures. The spectroscopic investigations were performed in the near infrared (2-5 cm), H,O and DzO being identified by their strong absorption bands from 3 pm and 4 pm, respectively. Due to the possibility of monitoring the sample thickness and the thermal diffusion length, the method can be applied to a large concentration range.
INTRODUCTION The photopyroelectric technique (PPE) ha8 been widely applied to the study of thermal and optical parameters of liquids and solids. Due to its intrinsic qualities the PPE method was often performed better than the classical techniques. For example, as spectroscopy, the PPE technique proved to be more suitable in investigating non-transparent solids,“) or highly absorbing liquids,‘2) as compared with classical transmission spectroscopy. Various configurations of PPE spectroscopy were proposed, depending on the experimental conditions, and with the purpose of increasing the sensitivity of the method and enriching the spectroscopic information.“) As is well known, in the PPE technique, the pyroelectric transducer is placed in thermal contact with the rear side of the sample slab or layer, thus measuring the rear temperature variation when the sample front side is being irradiated by modulated optical radiation. The overall PPE signal is produced by the vector sum of two periodic heat sources. The source Q,, generated upon absorption in the opaque pyroelectric, is proportional to the fraction of radiation transmitted through the sample. The detector acts as a radiation detector and produces a signal analogous to the conventional transmission spectrum. Another source, Q2, arises due to radiation absorption within the partially transparent sample and contains three types of information: optical transmission into the sample depth, optical absorption at various depths and the radiation-to-heat conversion quantum efficiency. For this signal component the pyroelectric sensor is a true thermal wave sensor, yielding a combined transmission-absorption spectrum. In the standard PPE configuration, the signal is often saturated due to dominance of the component Q,, especially for transparent samples. This is why it is sometimes useful to suppress it. The remaining PPE signal, due only to Q2, is expected to be richer in spectroscopic information on the sample. Experimentally one can achieve this by using a pyroelectric sensor provided with a highly reflecting front electrode, thus sending back out of the cell the transmitted radiation. This PPE configuration is called “reflection mode photopyroelectric” (RPPE) spectroscopy and was applied for the first time in investigating the optical properties of strongly absorbing liquids.” In this paper we propose the RPPE spectroscopy as an alternative method for quantitative analysis of H,O-D,O mixtures.
EXPERIMENTAL
AND RESULTS
In the near IR, Hz0 and D,O have three optical absorption bands at about 1.5, 1.9, and 3 pm (H,O) and 1.9, 2.6, and 4 pm (D,O). It should be expected that a PPE spectrum in the 2-5 urn
D. DADARLAT et al.
516
spectral range will be resolved with respect to the peaks from 3 and 4 pm, respectively. It follows that an RPPE spectroscopic investigation in the 2-5 pm spectral range is able to detect the presence of both H,O and D,O from a H20-D20 mixture. On the other hand the integral intensity of the bands and/or amplitude of the absorption peaks are proportional to the component concentration. In conclusion, one can determine the concentration of DzO in a H@-D20 mixture by measuring the peak amplitude of the band from 4 pm and the analogue at 3 pm for H20. In this type of investigation, the RPPE technique seems to be similar to classical one-beam transmission spectroscopy, but one important difference exists: the RPPE signal is proportional to the absorbed light, while in conventional transmission spectroscopy the signal is proportional to the transmitted radiation. For strongly absorbing liquids, as in the case of Hz0 and DzO in the near IR, when the optical absorption coefficient fl is sometimes larger than lo3 cm-‘, the transmitted intensity becomes very small. This fact recommends the use of the RPPE technique in some concentration ranges. The experimental cell and set-up were described elsewhere. (‘**)We will give here only some details. The pyroelectric sensor was a lead zirconate titanate ceramic disc (10 mm in diameter and 1 mm thick), provided with electrodes. An aluminium foil, 10 pm thick, serving as the reflecting surface is glued to the front side of the detector by means of silicone grease. Four mylar spacers of arbitrary thickness define the volume that accommodates the injected liquid sample that is retained in the cell by the capillarity. A highly transparent mica-sheet window (15 pm thick) is placed at the top of the spacers to complete the cell. Two thin metal springs hold the assembled structure together. The optical absorption measurements were performed in the 2-5 pm spectral range, the monochromator used being a SPM-2 (Carl Zeiss Jena) type, provided with a NaCl prism. The maximum equivalent slit width was 0.4,um. The radiation source was a Globar source of a Perk&Elmer 125 spectrophotometer. The modulation frequency of the radiation was 4 Hz. The signal from the detector was processed with an Unipan 232B (Poland) lock-in nanovoltmeter. During the experiment the signal-to-noise ratio ranged between 10 and 100. The measured signals were normalized to the signals obtained with a flat responsivity detector provided with the same window. The purpose of this normalization is to obtain a spectrum of constant incident energy in the whole spectral range.
0
2
I
I
2.4
2.8
I
3.2 X
I
I
I
I
3.6
4.0
4.4
4.0
(pm)
Fig. 1. The PPE-signal amplitude, normalized at a value of 4 pm for H&D20 content larger than 50%.
mixtures with a D,O
Photopyroelectric
spectroscopy of H,O-D,O
577
mixtures
0.6
U” 0.4
0.2
2
2.4
2.9
3.6
3.2 A
4.0
4.4
4.9
(pm)
Fig. 2. The PPE-signal amplitude, normalized at a vale of 3 pm for H&I-D,0 content smaller than 50%.
mixtures with a 40
Figures 1 and 2 contain the RPPE spectra of some H&D20 mixtures. The optical absorption bands of the mixture, in this spectral range, are shown. The layer thickness (L) was 6 pm and the chopping frequency (j) 4 Hz. For these experimental conditions the observed “peak inversion” phenomenon@) was eliminated. Each curve was normalized at the maximum value of the PPE signal, the maximum of which lies at 3 or 4 pm depending on the components’ concentration. For a quantitative analysis, a calibration of the method is necessary. There are many possibilities for calibration depending on the concentration range of interest. For high and intermediate 40 concentrations (50-98%) we proposed as calibration points the ratio of the PPE signal amplitudes from 3 ,um and 4 pm respectively. Such a calibration curve (L = 6 pm) is presented in Fig. 3. For smaller DzO concentrations, this calibration procedure becomes less sensitive. The two PPE signals from 3 and 4 pm are both almost saturated for a large concentration range (see Fig. 2) and the slope of the calibration curve is small due to the large (>40%) H,O contribution in the PPE signal at 4 pm. For calibration in this concentration range, we propose the use of only the signal from 4 pm, and the extraction of the 100% Hz0 contribution. Two such calibration curves are presented in Fig. 4 (L = 6 pm) and Fig. 5 (L = 20 pm). In Fig. 6 we plotted the D,O contribution in the PPE signal at 4pm for some mixtures with low D,O content (L = 20 pm).
Fig, 3. Calibration curve obtained by dividing the PPE signal amplitude from 3 pm to that from 4pm.
INF
33/6-I
/.-. D. DADARLATet al.
578
a-0
.”
0
I 20
I 40
I 60
0
1
3
5
7
CkO (%)
CD*O(%I Fig. 4. Calibration curve obtained by extracting the 100% H,O contribution from the PPE signal of the H,O-D,O mixture at I = 4 pm (co, = S-60%, L = 6 pm).
Fig. 5. As for Fig. 4. (c,=O.4-6.3%,
L =20pm).
Of course, the second calibration procedure proposed is more correct for mixtures with low D,O content. For intermediate concentrations the contribution of Hz0 is overestimated, but this overestimation, combined with the normalization procedure, leads to saturation of the calibration curve (Fig. 4) only at about 50% D,O concentration. As a consequence, the two calibration procedures proposed allow the use of the PPE method for quantitative analysis of H20-D20 mixtures in the whole concentration range. In Fig. 5 the saturation appears for lower D20 concentrations due to the sample quantity (thickness). It should be noted that each calibration of the PPE technique, independent of the procedure, is valid only for a given chopping frequency and a sample thickness. The second technique used in our experiment for calibration was classical IR double-beam transmission spectroscopy. CONCLUSION A new application of the PPE technique is proposed: namely the quantitative analysis of isotopic liquid mixtures. The well known H&D20 system have been chosen as test material. The RPPE spectroscopy proved to be capable of detecting in the near IR both components of this isotopic 0.4
x (pm) Fig. 6. Example of the remaining PPE signal for H&I-D,0 mixture, after extracting the 100% Hz0 contribution. The curves, including the extracted one, were normalized at an arbitrary value, in this case the signal from 3.2pm.
Photopyroelectric
spectroscopy of H,O-D,O
mixtures
579
mixture. For quantitative analysis an adequate calibration procedure has to be chosen, depending on the concentration range of interest. The sensitivity of the PPE technique is very high (one can detect temperature variations smaller than 1 PK), 07~)but its precision for this type of quantitative analysis could not be better than that of the technique used for calibration (e.g. classical transmission spectroscopy). The results are also influenced by the purity of the liquid samples. Three technical advantages of the method should be made clear: (i) due to the possibility of almost continuously monitoring the sample thickness and thermal diffusion length, the method can be used for a large concentration range; (ii) the necessary sample quantity is very small (about 1 mm3); and (iii) the RPPE experimental setup is simpler than those used in many classical techniques. REFERENCES 1. D. Dadarlat, M. Chirtoc, R. M. Candea and I. Bratu, Infrarred Phys. 24, 469 (1984). 2. M. Chirtoc, D. Dadarlat, I. Chirtoc and D. Bicanic, Specfrosc. Lett. 21, 413 (1988). 3. See for example: M. Chirtoc, D. Dadarlat and D. Bicanic, Proc. 7th Int. Topical Meeting on Photoacoustic and Photothermal Phenomenu, Doorverth, The Netherlands (1991) p. 54 (and references therein). 4. R. J. Miller and B. C. State (Editors), L&oratory Methods in Infrured Spectroscopy. Heyden, London (1972).
Phys. Vol. 33, No. 6, pp. 575-519,1992
0020-0891/92$5.00 + 0.00 Fkrgamon Press Ltd
Printed in Great Britain
PHOTOPYROELECTRIC SPECTROSCOPY OF H20-D,O MIXTURES D. DADARLAT,A. FRANDAS and I. BRATU Institute of Isotopic and Molecular Technology, P.O. Box 700, Cluj-Napoca 5, R-3400 Romania (Received I June 1992) Abstract-The photopyroelectric technique, in a reflection mode contiguration, is proposed as an alternative method for quantitative analysis of H,O-D,O mixtures. The spectroscopic investigations were performed in the near infrared (2-5 cm), H,O and DzO being identified by their strong absorption bands from 3 pm and 4 pm, respectively. Due to the possibility of monitoring the sample thickness and the thermal diffusion length, the method can be applied to a large concentration range.
INTRODUCTION The photopyroelectric technique (PPE) ha8 been widely applied to the study of thermal and optical parameters of liquids and solids. Due to its intrinsic qualities the PPE method was often performed better than the classical techniques. For example, as spectroscopy, the PPE technique proved to be more suitable in investigating non-transparent solids,“) or highly absorbing liquids,‘2) as compared with classical transmission spectroscopy. Various configurations of PPE spectroscopy were proposed, depending on the experimental conditions, and with the purpose of increasing the sensitivity of the method and enriching the spectroscopic information.“) As is well known, in the PPE technique, the pyroelectric transducer is placed in thermal contact with the rear side of the sample slab or layer, thus measuring the rear temperature variation when the sample front side is being irradiated by modulated optical radiation. The overall PPE signal is produced by the vector sum of two periodic heat sources. The source Q,, generated upon absorption in the opaque pyroelectric, is proportional to the fraction of radiation transmitted through the sample. The detector acts as a radiation detector and produces a signal analogous to the conventional transmission spectrum. Another source, Q2, arises due to radiation absorption within the partially transparent sample and contains three types of information: optical transmission into the sample depth, optical absorption at various depths and the radiation-to-heat conversion quantum efficiency. For this signal component the pyroelectric sensor is a true thermal wave sensor, yielding a combined transmission-absorption spectrum. In the standard PPE configuration, the signal is often saturated due to dominance of the component Q,, especially for transparent samples. This is why it is sometimes useful to suppress it. The remaining PPE signal, due only to Q2, is expected to be richer in spectroscopic information on the sample. Experimentally one can achieve this by using a pyroelectric sensor provided with a highly reflecting front electrode, thus sending back out of the cell the transmitted radiation. This PPE configuration is called “reflection mode photopyroelectric” (RPPE) spectroscopy and was applied for the first time in investigating the optical properties of strongly absorbing liquids.” In this paper we propose the RPPE spectroscopy as an alternative method for quantitative analysis of H,O-D,O mixtures.
EXPERIMENTAL
AND RESULTS
In the near IR, Hz0 and D,O have three optical absorption bands at about 1.5, 1.9, and 3 pm (H,O) and 1.9, 2.6, and 4 pm (D,O). It should be expected that a PPE spectrum in the 2-5 urn
D. DADARLAT et al.
516
spectral range will be resolved with respect to the peaks from 3 and 4 pm, respectively. It follows that an RPPE spectroscopic investigation in the 2-5 pm spectral range is able to detect the presence of both H,O and D,O from a H20-D20 mixture. On the other hand the integral intensity of the bands and/or amplitude of the absorption peaks are proportional to the component concentration. In conclusion, one can determine the concentration of DzO in a H@-D20 mixture by measuring the peak amplitude of the band from 4 pm and the analogue at 3 pm for H20. In this type of investigation, the RPPE technique seems to be similar to classical one-beam transmission spectroscopy, but one important difference exists: the RPPE signal is proportional to the absorbed light, while in conventional transmission spectroscopy the signal is proportional to the transmitted radiation. For strongly absorbing liquids, as in the case of Hz0 and DzO in the near IR, when the optical absorption coefficient fl is sometimes larger than lo3 cm-‘, the transmitted intensity becomes very small. This fact recommends the use of the RPPE technique in some concentration ranges. The experimental cell and set-up were described elsewhere. (‘**)We will give here only some details. The pyroelectric sensor was a lead zirconate titanate ceramic disc (10 mm in diameter and 1 mm thick), provided with electrodes. An aluminium foil, 10 pm thick, serving as the reflecting surface is glued to the front side of the detector by means of silicone grease. Four mylar spacers of arbitrary thickness define the volume that accommodates the injected liquid sample that is retained in the cell by the capillarity. A highly transparent mica-sheet window (15 pm thick) is placed at the top of the spacers to complete the cell. Two thin metal springs hold the assembled structure together. The optical absorption measurements were performed in the 2-5 pm spectral range, the monochromator used being a SPM-2 (Carl Zeiss Jena) type, provided with a NaCl prism. The maximum equivalent slit width was 0.4,um. The radiation source was a Globar source of a Perk&Elmer 125 spectrophotometer. The modulation frequency of the radiation was 4 Hz. The signal from the detector was processed with an Unipan 232B (Poland) lock-in nanovoltmeter. During the experiment the signal-to-noise ratio ranged between 10 and 100. The measured signals were normalized to the signals obtained with a flat responsivity detector provided with the same window. The purpose of this normalization is to obtain a spectrum of constant incident energy in the whole spectral range.
0
2
I
I
2.4
2.8
I
3.2 X
I
I
I
I
3.6
4.0
4.4
4.0
(pm)
Fig. 1. The PPE-signal amplitude, normalized at a value of 4 pm for H&D20 content larger than 50%.
mixtures with a D,O
Photopyroelectric
spectroscopy of H,O-D,O
577
mixtures
0.6
U” 0.4
0.2
2
2.4
2.9
3.6
3.2 A
4.0
4.4
4.9
(pm)
Fig. 2. The PPE-signal amplitude, normalized at a vale of 3 pm for H&I-D,0 content smaller than 50%.
mixtures with a 40
Figures 1 and 2 contain the RPPE spectra of some H&D20 mixtures. The optical absorption bands of the mixture, in this spectral range, are shown. The layer thickness (L) was 6 pm and the chopping frequency (j) 4 Hz. For these experimental conditions the observed “peak inversion” phenomenon@) was eliminated. Each curve was normalized at the maximum value of the PPE signal, the maximum of which lies at 3 or 4 pm depending on the components’ concentration. For a quantitative analysis, a calibration of the method is necessary. There are many possibilities for calibration depending on the concentration range of interest. For high and intermediate 40 concentrations (50-98%) we proposed as calibration points the ratio of the PPE signal amplitudes from 3 ,um and 4 pm respectively. Such a calibration curve (L = 6 pm) is presented in Fig. 3. For smaller DzO concentrations, this calibration procedure becomes less sensitive. The two PPE signals from 3 and 4 pm are both almost saturated for a large concentration range (see Fig. 2) and the slope of the calibration curve is small due to the large (>40%) H,O contribution in the PPE signal at 4 pm. For calibration in this concentration range, we propose the use of only the signal from 4 pm, and the extraction of the 100% Hz0 contribution. Two such calibration curves are presented in Fig. 4 (L = 6 pm) and Fig. 5 (L = 20 pm). In Fig. 6 we plotted the D,O contribution in the PPE signal at 4pm for some mixtures with low D,O content (L = 20 pm).
Fig, 3. Calibration curve obtained by dividing the PPE signal amplitude from 3 pm to that from 4pm.
INF
33/6-I
/.-. D. DADARLATet al.
578
a-0
.”
0
I 20
I 40
I 60
0
1
3
5
7
CkO (%)
CD*O(%I Fig. 4. Calibration curve obtained by extracting the 100% H,O contribution from the PPE signal of the H,O-D,O mixture at I = 4 pm (co, = S-60%, L = 6 pm).
Fig. 5. As for Fig. 4. (c,=O.4-6.3%,
L =20pm).
Of course, the second calibration procedure proposed is more correct for mixtures with low D,O content. For intermediate concentrations the contribution of Hz0 is overestimated, but this overestimation, combined with the normalization procedure, leads to saturation of the calibration curve (Fig. 4) only at about 50% D,O concentration. As a consequence, the two calibration procedures proposed allow the use of the PPE method for quantitative analysis of H20-D20 mixtures in the whole concentration range. In Fig. 5 the saturation appears for lower D20 concentrations due to the sample quantity (thickness). It should be noted that each calibration of the PPE technique, independent of the procedure, is valid only for a given chopping frequency and a sample thickness. The second technique used in our experiment for calibration was classical IR double-beam transmission spectroscopy. CONCLUSION A new application of the PPE technique is proposed: namely the quantitative analysis of isotopic liquid mixtures. The well known H&D20 system have been chosen as test material. The RPPE spectroscopy proved to be capable of detecting in the near IR both components of this isotopic 0.4
x (pm) Fig. 6. Example of the remaining PPE signal for H&I-D,0 mixture, after extracting the 100% Hz0 contribution. The curves, including the extracted one, were normalized at an arbitrary value, in this case the signal from 3.2pm.
Photopyroelectric
spectroscopy of H,O-D,O
mixtures
579
mixture. For quantitative analysis an adequate calibration procedure has to be chosen, depending on the concentration range of interest. The sensitivity of the PPE technique is very high (one can detect temperature variations smaller than 1 PK), 07~)but its precision for this type of quantitative analysis could not be better than that of the technique used for calibration (e.g. classical transmission spectroscopy). The results are also influenced by the purity of the liquid samples. Three technical advantages of the method should be made clear: (i) due to the possibility of almost continuously monitoring the sample thickness and thermal diffusion length, the method can be used for a large concentration range; (ii) the necessary sample quantity is very small (about 1 mm3); and (iii) the RPPE experimental setup is simpler than those used in many classical techniques. REFERENCES 1. D. Dadarlat, M. Chirtoc, R. M. Candea and I. Bratu, Infrarred Phys. 24, 469 (1984). 2. M. Chirtoc, D. Dadarlat, I. Chirtoc and D. Bicanic, Specfrosc. Lett. 21, 413 (1988). 3. See for example: M. Chirtoc, D. Dadarlat and D. Bicanic, Proc. 7th Int. Topical Meeting on Photoacoustic and Photothermal Phenomenu, Doorverth, The Netherlands (1991) p. 54 (and references therein). 4. R. J. Miller and B. C. State (Editors), L&oratory Methods in Infrured Spectroscopy. Heyden, London (1972).