Nondestructive and Nonintrusive Determination of Chemical and Isotopic Purity of Solvents by Near-Infrared Thermal Lens Spectrometry
C H I E U D. TRAN,* VICTOR I. GRISHKO, and MAURICIO S. BAPTISTA Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233
A novel instrument which is based on the use of the thermal lens effect to facilitate the sensitive measurements of the absorption in the near- infrared region has been developed. In this instrument, the near-IR excitation light was provided by a solid-state, spectra-tunable (from 860 to 1060 nm) titanium:sapphire laser. The heat generated as a conse- quence of the sample absorption of the excitation beam was monitored in the visible region by a He-Ne laser. The data obtained were analyzed by multivariate calibration methods for the nondestructive, noninvasive determinations of chemical and isotopic impurities in solvents. Water in D20 and in tetrahydrofuran can be detected at levels as low as 0.006 and 0.3% (v/v). The method can also be used for the simultaneous de- termination of water and DMSO-h6 in DMSO-d6 and CD3OH in CD3OH, CD2HOH, and CDH2OH at levels as low as 10 3% (w/w).
Index Headings: Near-infrared; Laser; Thermal lens; Isotope.
INTRODUCTION
Absorption measurements in the near-infrared (NIR) region have increased significantly in recent years? -5 The popularity stems from factors such as the wide applica- bility of the technique (i.e., NIR absorption is due to the overtones and combination transitions of the -OH, -NH, and -CH groups); its easy-to-use characteristic (no need for sample preparation); and the availability of multi- variate methods which facilitate the analysis of multi- component samples. 1-5 However, it is precisely due to these advantages that the NIR absorption measurements suffer from low sensitivity. A variety of reasons are re- sponsible for this consideration, including the low ab- sorption coefficients in the NIR region and the low sen- sitivity of the NIR detectors. The former limitation stems from the fact that NIR absorption bands, as described above, are due to the overtones and combination tran- sitions, and these transitions are known to have very low absorption coefficients. Compounding this difficulty is the fact that, compared to the photomultiplier tubes used in the UV and visible regions, detectors in the NIR suffer from not only low sensitivity but also high dark current. Various attempts have been made to alleviate these draw- backs, the most notable probably being the NIR fluores- cence technique. 4,6 While this technique has proven to be effective, it is not widely used because few molecules are fluorescent in the NIR. Additionally, it is not usually possible to label all types of compounds with NIR fluo-
Received 8 December 1993; accepted 5 April 1994. * Author to whom correspondence should be sent.
rescent reagents. It is, therefore, particularly important that a novel technique which has high sensitivity and wide applicability and is capable of measuring absorption in the NIR be developed.
The thermal lens technique is based on the measure- ment of the temperature rise that is produced in an il- luminated sample by nonradiative relaxation of the en- ergy absorbed from a laser. 7-16 Because the absorbed energy is directly measured in this case, the sensitivity of the technique is similar to that of the fluorescence technique and is relatively higher than conventional absorption measurements. In fact, it has been calculated and exper- imentally verified that the sensitivity of the thermal lens technique is 237 times higher than that by conventional absorption techniques when a laser of only 50 mW is used for excitation. 8-1° Absorbances as low as 10 -7 have been measured with the use of this ultrasensitive technique. 7-16 Furthermore, the background noise (i.e., the noise of the NIR detector and the instability of the light source) is very low in the thermal lens measurements. This is be- cause the thermal lens measurement can be performed with the use of the dual-beam optical configuration (i.e., NIR pump and visible probe), and in this system, the signal is detected more sensitively and with less noise in the visible region by a phase detection technique (at the chopping frequency of the pump beam) with the probe laser. Potentially, the technique should serve as an ex- cellent NIR absorption method because it has high sen- sitivity and wide applicability (compared to the fluores- cent technique that is applicable to only fluorescent samples). It should, therefore, be particularly suited for the trace characterization of samples which are available only at very low concentrations and small volumes (e.g., biological, pharmaceutical, and isotope-enriched sam- ples). Thermal lens measurements in the near-infrared have, in fact, been performed previously. 17,18 Unfortu- nately, these studies were not able to demonstrate the full potentials of the NIR/thermal lens technique. The limi- tations are due to the fact that, for any type of spectral chemical analysis in the NIR region, rather than using calibration curves only at a few discrete wavelengths, it is important that one use the spectrum of the whole region to construct the calibration curves in order to accurately and precisely determine the concentration of the analyte. This requirement stems from the fact that virtually all substances absorb in the NIR region, and because the NIR bands are broad and extensively overlapped, it is very inaccurate and imprecise to use only absorbances at a few discrete wavelengths to construct calibration curves to determine the concentration of a certain species in a
mixture. The accuracy and precision can be mathemat- ically enhanced by use of the multivariate calibration techniques (e.g., principal component analysis, partial least-squares) to analyze the whole NIR absorption spec- trum. Unfortunately, the multivariate calibrations could not be used in these previous NIR thermal lens studies because, in these studies, the signals were measured either at a single wavelength (3.39 ~m) or with a highly fluc- tuated and difficult-to-reproduce stimulated Raman light. In fact, to our knowledge, to date, the ultrasensifive ther- mal lens technique has not been used to measure the absorption spectra for the entire NIR region. As a con- sequence, its synergistic use with the multivariate tech- niques to facilitate sensitive and accurate trace chemical characterization has not been realized.
The information presented is indeed provocative and clearly indicates that it is possible to synergistically use the NIR, the thermal lens technique, and the multivariate calibration technique to develop a novel method for chemical analysis of samples that could not be accom- plished otherwise. Such considerations prompted us to initiate this study, which aims to exploit the use of the thermal lens effect for the absorption measurements in the entire NIR region. In this paper we will report the development of the first NIR thermal lens spectropho- tometer, as well as preliminary results on the synergistic use of this spectrometer with the multivariate method for making unique measurements--measurements which we have strategically designed not only to demonstrate the full potentials of the technique but also to illustrate that it is not possible to perform these measurements with existing techniques. Specifically, the use of the technique for the determination of the isotopic purity of D20, DMSO-d6, and deuterium-substituted methanols, as well as the dryness of organic solvents such as tetrahydrofuran, will be reported.
EXPERIMENTAL
A thermal lens apparatus based on a pump/probe con- figuration was used in this work. The schematic diagram of the apparatus is shown in Fig. 1. As illustrated, a Ti : sapphire laser (Coherent Corp., Model 890) which was pumped by an argon-ion laser (Coherent Corp., Model Innova 70-5) was used as an excitation source. With a set of long-wavelength high reflectors and output cou- plers, this Ti: sapphire laser provided output radiation
FIe. 2. Output power of the Ti : sapphire laser near-IR laser used in this experiment.
in the near-infrared from 860 to 1060 nm. The laser output was spectrally tuned by means of an intracavity bireffingent filter that was driven by a computer-con- trolled stepping motor. After being modulated at 25 Hz by a mechanical chopper (Stanford Research System Model SR450), the excitation beam was focused onto the sample by an achromatic lens having a focal length of 120 mm. The probe beam, provided by a green (543.5 nm) He-Ne laser (Melles Griot Model 05-LGR-173), was aligned to colinearly overlap with the plump beam at the sample by means of a dichroic filter which reflects the 543.5-nm probe beam but transmits all other wave- lengths. The pump and probe beams were then separated by an equilateral prism (P). The heat generated by the sample absorption of the pump beam changed the beam center intensity of the probe beam. The fluctuation in the center intensity of the probe beam was measured with a PIN photodiode (PD1, United Detector Technology PIN 10-DP) placed 2.0 m from the sample and behind a pin- hole (PH). A lens with a focal length of 50 mm was used to focus the probe beam, and its relative distance from the sample was adjusted to give maximum thermal lens signals. Demodulation and amplification of the thermal lens signal were accomplished by means of a lock-in am- plifier (Princeton Applied Research Model 5207). It was necessary to normalize the thermal lens signal with the intensity of the pump beam. This was because of the variation in the output intensity of the Ti : sapphire laser at different wavelengths (Fig. 2). The normalization was accomplished by initially splitting a small portion of the pump into a reference photodiode (PD2, United Detector Technology PIN 10-DP) by a beamsplitter. A ratio value between the lock-in ~/mplifier output signal and the ref- erence photodiode signal was then obtained by means of a ratio meter. The output of the meter was then connected to a microcomputer (IBM AT compatible) with a 386 microprocessor (Northgate Computer Systems) through the A/D of the 12-bit DAS-16 board (Metra-Byte). By use of the LAB-CALC or GRAMS 386 program (Galactic Industries Corp.), the thermal lens spectra obtained were analyzed for linear regression at selected wavelength and for partial least-squares (PLS) and principal component regression (PCR) analysis using entire spectra.
834 Volume 48, Number 7, 1994
6
5
J
® 3
~- 2 h
0 ~ I i I i
881 920 960 1000
Wavelength, nm
FIGG. 3. Thermal lens spectra o f 99.77% D20 (a), o f pure water (b), and of sample a with 0.099 (c); 0.66 (d); 0.99 (e); 1.32 (f) ; 1.64 (g); 2.28 (h); 3.22 (i); and 4.76% (j) added water.
Deuterium oxide (99.95 atom % D, Fluka or 99.9 atom % D, Aldrich) and methylsulfoxide-d6 (99.95 atom % D, Fluka) were used as received. Deuterium-substituted methanols (namely, CD3OH , CHD2OH, and CH2DOH) were obtained from Cambridge Isotope Laboratories and were specified by the manufacturer to contain 99, 98, and 97 deuterium atom %, respectively. Tetrahydrofuran was freshly distilled from lithium aluminum hydride prior to use. A quartz cell with a 2-ram pathlength was used for all measurements.
RESULTS AND DISCUSSION
This study was designed to demonstrate the full poten- tials of the NIR/thermal lens technique and to illustrate that this technique can be used for measurements which are impossible otherwise. Specifically, the high spatial resolution of the laser and the sensitivity of the thermal lens effect will be exploited for the nondestructive deter- minations of isotopic purity of samples. These measure- ments are currently not possible because there is a lack of nondestructive techniques which have the required sensitivity and also because the isotopic samples are available only in a small quantity and have very small isotopic impurity. Since the near-IR region used in this work (i.e., from 865 to 1050 nm, Fig. 2) covers the com- bination transitions and overtone absorption of the O-H and C-H groups, 3,19 the present study was focused on the determination of (1) the isotopic purity of D20 (i.e., the amount of water impurity in D20); (2) the dryness of tetrahydrofuran; (3) the isotopic purity and dryness of dimethyl sulfoxide-d6 (i.e., the amount of the water and DMSO-h6 impurities in DMSO-d6); and (4) the isotopic purity of CD3OH, CHD2OH, and CH2DOH.
Water in Deuter ium Oxide. S h o w n in Fig. 3a is the thermal lens spectrum of deuterium oxide which, ac- cording to the manufacturer, contains 99.95% deuterium atoms. For comparison, the thermal lens spectrum of pure water is also shown in Fig. 3b. Since water has much stronger absorption in this region than D20, the actual
spectrum of water was reduced by a factor of 10 with respect to the one shown in Fig. 3b. Spectra of D20 sam- ples having different amounts of added H20 are also shown in the figure. Specifically, Fig. 3c, 3d, 3e, 3f, 3g, 3h, 3i, and 3j show the thermal lens spectra of DzO sample con- mining 0.099, 0.66, 0.99, 1.32, 1.64, 2.28, 3.22, and 4.76% added H20, respectively. Two features are clear from these spectra: (1) there is a large background under the spectrum of the supposedly pure D20 sample; and (2) when the concentration of the hydrogen isotopes in the D20 sample increases, the spectrum exhibits a blue shift concomitant with a generation of a new peak centered at about 975 nm.
The background absorption under the spectrum of pure D20 shown in Fig. 3a can be due to the water impurity in the sample and/or to the residue absorption of the D20 in this region. Therefore, it was necessary to determine the absolute concentration of water in this sample in order to elucidate the origin of the background absorption. This task was accomplished by use of the N M R technique. Specifically, a known amount of purified, dried DMSO was added to the D20, and the intensity of the proton signal of the water impurity was compared with the pro- ton signal of the DMSO. Results obtained show that this D20 contains up to 0.23% water. Because the Fig. 3a spectrum is much higher than the difference between 3c (which corresponds to 0.23% added H20) and 3a (which corresponds to 0% added H20), it is not unreasonable to assume that the 3a spectrum is not entirely due to the water impurity in the sample. Another possible source for this background is, as previously mentioned, the res- idue of the absorption band of the D20 itself. In fact, it has been reported that the 2v, + v3 combination transition for D20 is centered at about 1339 nm? ° However, it should be realized that this spectrum for the D20 sample is more complicated because of the existence of the HOD species. The HOD species is present in the D20 sample because this sample, as mentioned earlier, contains some H20, and D20 is known to undergo rapid exchange with water to form HOD. z°,21 The absorption band for the same 2v~ + v3 transition by HOD is reported to be at 1240 nm. 2° Additionally, HOD also has a band at 1000 nm, and this band has been tentatively assigned to the com- bination transitions 2,~ + ,2 + //3 and v2 + 3v3 .2° As a consequence, the bands due to the absorption of the 0 2 0 and HOD species may have some residues under the absorption region of the H20. Therefore, the broad back- ground under the Fig. 3a spectrum may be due to a com- bination of the water impurity (in the D20 ) and the res- idues of the combination transition bands by D20 and HOD species.
It is evident from Fig. 3 that water, when added to the solution of D20, produced a clear and distinct change in the thermal lens spectrum of the latter. The spectrum of D20 underwent a blue shift concomitant with a new ab- sorption band centered at 975 nm. The intensity of this band increased as more water was added to the 0 2 0 solution. Obviously, this 975-nm band must be due to the absorption of water. In fact, it has been previously assigned to the combination transition 2v~ + v3, where Vl is the symmetric O-H stretch and v3 is the antisymmetric stretch)
It seems, from Fig. 3, that there is a correlation between the amount of added water and the thermal lens signal
APPLIED SPECTROSCOPY 835
intensity. A multilinear statistical analysis using the prin- cipal component and the partial least-squares methods was performed to determine the wavelength which would give the least error and the best correlation for analysis. A plot of the correlation coefficient vs. wavelength for the data set shown in Fig. 3 is illustrated in Fig. 4A. Very good correlation coefficients were found for added amounts of water and the thermal lens signal intensity for the region from 930 to 1000 nm. It is therefore evident that any of the wavelengths in this range can be used to determine the amount of water in the D20 sample. In fact, when the partial least-squares method was used to predict the amount of water with the use of the set of spectra shown in Fig. 3, a very good correlation was ob- tained between the actual (i.e., added) and the predicted concentrations of added water (Fig. 4B). As illustrated, a linear relationship was obtained with an intercept of -0.0149, a slope of 1.01, and the R 2 value of 0.99653 [the standard error of prediction (SEP) and the prediction residual error sum of squares (PRESS) values were 0.05534 and 0.04287, respectively]. The limit of detection (LOD), defined as the amount of added water that yielded a sig- nal-to-noise ratio of two, was determined with the use of a calibration curve constructed with data at 975 nm. It was found to be 0.0066%. This LOD value is, in fact, much lower than LOD values of 0.8 and 1.0% which have been previously achieved with the use of currently avail- able techniques including NMR 22-24 and luminescence lifetime. 25 However, the more desirable feature of this NIR/thermal lens method is its nondestructive and non- invasive characteristic (existing techniques are invasive because they all require the addition of some type of compound into the D20 sample22-25).
Water in Tetrahydrofuran. The thermal lens spectrum of dried tetrahydrofuran (THF) is shown in Fig. 5A. The spectrum exhibits a strong band at 906 nm and a shoulder at 938 nm. These bands are in agreement with those previously observed 4,26 and have been assigned to the third overtone of the C-H in the methylene groups. 17,27 These C-H bands are well separated from the O-H band, and since THF does not absorb in the region where water absorbs (namely, the region around 975 nm), it is possible to use this technique to determine the amount of water in THF. In fact, as can be seen in Fig. 5A, which also shows the spectra of THF solutions with different amounts of water, adding water to the THF solution leads to a decrease in the intensity of the C-H band at 906 nm and an increase in the intensity of the O-H band of water at 975 nm. It is thus hardly surprising that good correlation coefficients between the signal intensity and the amount of added water were found for both regions: from 900 to 925 nm, which corresponds to the absorption band of the C-H group, and from 960 to 1000 nm, where the O-H group absorbs (Fig. 5B). Partial least-squares analysis was then used, based on the set of spectra shown in Fig. 5A, to predict the amount of water. Figure 5C shows the plot of the actual (added) and predicted concentration of add- ed water in THF. As illustrated, a good linear relationship was obtained (slope = 0.959, intercept = 0.035, corre- lation coefficient = 0.999965, SEP = 0.0448, and PRESS = 0.0100). A calibration curve (not shown) was then con- structed with the use of data at 975 nm. On the basis of this calibration curve, it was found that this thermal lens
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(A) Plot of the correlation coetficient vs. wavelength for the set of spectra shown in Fig. 3. (B) Plot of actual water concentration vs. concentration predicted by the PLS method based on the set of spectra shown in Fig. 3.
technique can be used to detect water in THF at concen- trations as low as 0.03%. This value is relatively higher than the LOD value obtained for water in D20. A variety of reasons might account for this deterioration, but the most likely cause is probably the overlapping between the O-H band at 975 nm and the C-H band at about 1010 nm (this 1010-nm band, which is due to the combination of the stretching and bending vibrations of the C-H, can
836 Volume 48, Number 7, 1994
"~ 6 E
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F[G. 5. (A) Thermal lens spectra of tetrahydrofuran without and with different amounts of water. (B) Plot of the correlation coefficient vs. wavelength for the set of spectra shown in A. (C) Plot of actual water concentration vs. concentration predicted by the PLS method based on the set of spectra shown in A. Concentrations of water (in v/v %) are listed in A.
be clearly observed in the case of DMSO-h6 in DMSO- d6 in the following section). While the present LOD value is much lower than those obtained by the Karl Fischer titration technique, 28 it is either comparable to or higher than those obtained by NMR, 29 gas chromatography, 3°,31 HPLC, 32-34 or flow injection analysis techniques. 35 How- ever, the most important and desirable feature of this NIR/thermal lens technique is (as described above in the section that discusses water in D20) its nondestructive and noninvasive characteristic.
Water and DMSO-h6 in DMSO-dr. The results pre- sented have clearly demonstrated that minute amounts of water in solvents such as D20 and THF can be sen- sitively determined by use of the thermal lens technique (through the measurement of the absorption of the O-H group). The technique can also be used for the sensitive measurement of the C-H group, and because the O-H absorption band is well separated from that of the C-H, it is possible to use this technique for the simultaneous determination of the two impurities in a sample (assum- ing that there is no C-H or O-H group in the sample and that each of the impurities has either a C-H or O-H group). This possibility was investigated in the following
study, which aims to develop a method for the simulta- neous determination of water and DMSO-h6 in DMSO- d6.
Water in D M S O - d 6. The thermal lens spectrum of a sample of supposedly pure DMSO-d6, which is shown in Fig. 6A, exhibits a small peak at about 975 nm. This sample, which was purchased from Fluka, was specified by the manufacturer to contain 99.95 atom % of deute- rium. Since neither the C-D nor the S=O group should have any absorption in this near-IR region, the 975-nm band must be due to the water impurity presence in the sample. This possibility is confirmed upon examination of other spectra (also shown in Fig. 6A) of the DMSO-d6 samples containing different amounts of added water. It is evident from these spectra that the 975-nm band is due to water, because the intensity of this band increases as the concentration of water increases. In fact, when the partial least-squares method was used to analyze the set of spectra shown in Fig. 6A, it was found that the thermal lens signal intensities were indeed well correlated with the concentrations of added water in solution for the wavelength region ranging from 920 to 1050 nm (Fig. 6B). Also shown in Fig. 6C is the plot of the actual and
FiG. 6. (A) Thermal lens spectra o f DMSO-d6 without and with different amounts of water. (B) Plot of the correlation coefficient vs. wavelength. (C) Plot o f actual water concentration vs. concentration predicted by the PLS method.
APPLIED SPECTROSCOPY 837
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FIG. 7. (A) Thermal lens spectra of DMSO-d6 without and with different amounts of DMSO-h 6. (B) Plot of the correlation coefficient vs. wavelength. (C) Plot of the actual concentration of DMSO-h6 vs. the concentration predicted by the PLS method.
predicted concentration of water. As illustrated, very good agreement between the actual and predicted values (the straight line shown in Fig. 6C has a correlation coefficient of 0.995588 and SEP and PRESS values of 0.0761546 and 0.0347971, respectively). The LOD value, based on the calibration curve constructed with the use of data at 988 nm (not shown), was found to be 2.5 x 10-3%. This LOD value is comparable with the above LOD of 6.6 x 10-3% found for water in D20. It is much lower than other LOD values of 0.01 and 0.05% determined by N M R 29 and luminescence lifetime. 36 It is interesting to note that, by the use of this method, the concentration of water in the sample of pure DMSO-d6 used in this work was determined to be 0.158%. This result is hardly sur- prising considering the hygroscopic nature of the solvent and the difficulty in removing the water.
DMSO-h6 in DMSO-d6. Figure 7A shows the thermal lens spectra of samples of pure DMSO-d6 and of DMSO- d6 with different amounts of added DMSO-h6. As illus- trated, the spectrum of the DMSO-d6 has a band at 975 nm. This band, as explained in the above section, is due to the water impurity in the sample. The spectrum un- derwent significant change when DMSO-h6 was added to the sample. Two new bands were generated: a strong one
at 902 and a weaker one at 1010 nm. These bands were assigned, respectively, to the third overtone and the com- bination of the stretching and bending vibrations of the C-H groups. ~9 As expected, the intensities of these two bands increase concomitantly with the increase in the concentration of DMSO-h6 in the sample. Partial least- squares analysis was performed, and the results obtained (Fig. 7B) show that good correlation coefficients were found between the signal intensity and the concentration of the DMSO-h6, for the wavelength ranges which correspond to the overtone and combination bands of the C-H (i.e., from 890 to 910 nm and from 990 to 1040 nm). Good agreement was also found for the actual and predicted concentration of the added DMSO-h6 (Fig. 7C shows a straight-line relationship with a correlation coefficient of 0.994666 and SEP and PRESS values of 0.10437 and 0.0653586, respectively). Limits of detection, determined at the peak of the three bands (902, 975, and 1010 nm) were found to be 6.4 × 10 -3, 0.125, and 6.5 x 10-3%, respectively. The LOD value at 975 nm is relatively high- er than those at other wavelengths because the 975-nm band is due to the absorption of water impurity present in the sample, and this background absorption increased the noise level in the signals of DMSO-d6 at this wave-
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FIG. 8. (A) Thermal lens spectra of DMSO-h6 without and with different amounts of water. (B) Plot of the correlation coefficient vs. wavelength. (C) Plot of the actual concentration of water vs. the concentration predicted by the PLS method.
838 Volume 48, Number 7, 1994
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F[6. 9. Thermal lens spectra of seven three-component samples with different concentrations of water and DMSO-h6 in DMSO-d6.
length (and as a consequence, increased the limit of de- tection). By the use of this technique, the concentration of DMSO-h6 in the sample of pure DMSO-d6 was deter- mined to be 0,0386%. This value is in good agreement with the value of 0.05% specified by the manufacturer.
Water in DMSO-h6. The thermal lens spectra of DMSO- h6 without and with different concentrations of added water are shown in Fig. 8A. As expected, the strong band at 902 and the weaker one at 1010 nm in the spectrum of the pure DMSO-h6 are due to the third overtone and combination transition of the methyl group. Adding wa- ter into the solution increased the signal intensity in the region around 975 nm. Because the 975-nm absorption band of water and the 1010-nm band of the C-H group are very broad, they overlapped; as a consequence, the spectra are not resolved. However, a good correlation was obtained (Fig. 8B) between the signal intensity and the concentration of H20. The predicted concentrations of H20, calculated according to the PLS method (Fig. 8C), agree well with the actual concentration of added water (the straight line shown in Fig. 8C has a correlation co- efficient of 0.995588 and SEP and PRESS values of 0.0761546 and 0.0347971, respectively). Limits of de- tection, determined at 975 and 990 nm, were found to be 2.1 x 10 -2 and 5.4 x 1 0 - 2 % , respectively. These LOD values are relatively higher than those found for H20 in D20 and in DMSO-d6. This result is hardly surprising considering the extensive overlap between the 975-nm band of water and the 1010-nm band of the C-H group.
Simultaneous Determination of Water and DMSO-h6 in DMSO-d6. It is evidently clear from the results pre- sented that this thermal lens technique can be used for the sensitive determination of water and DMSO-h6 in DMSO-d6. The determinations need not be done sepa- rately, as in the sections above, but can be done simul- taneously (i.e., the concentrations of water and DMSO- h6 in DMSO-d6 can be simultaneously calculated from a
FIG. 10. Plots of predicted against actual concentrations of water (A) and DMSO-h6 (B) in DMSO-d6.
measured single thermal lens spectrum). We investigated this possibility by measuring spectra of 60 different sam- ples. These were three-component samples, prepared by mixing water, DMSO-h6 and DMSO-d6 in different com- binations of concentrations. The spectra of seven different samples are shown in Fig. 9. In this figure, the % (v/v) water in each sample used can be read directly from the linear scale on the annotated axis, while the % (v/v) DMSO-h6 concentrations for the respective spectra are denoted at the top of the graph. As expected, increasing the concentrations of water and DMSO-h6 led to an in- crease in the intensities of the O-H band at 975 nm and of the C-H bands at 902 and 1010 nm. On the basis of this set of 60 spectra, the partial least-squares method was used to calculate the concentrations of water and DMSO-h6 in the samples. The calculated concentrations were plotted against the actual concentrations for added water and DMSO-h6 (Fig. 10A and 10B). Very good linear relationships were obtained for both cases: the correlation coefficients and the SEP and PRESS values were found to be 0.998177, 0.0622557, and 0.209291 for water and 0.995369, 0.131499, and 0.933777 for DMSO-h6, re- spectively. On the basis of these relationships, the con- centrations of water and DMSO-h6 in four different "un- known" samples were simultaneously calculated; the results obtained are listed in Table I. Again, good agree-
APPLIED SPECTROSCOPY 839
TABLE I. Simultaneous determination of water and DMSO-h6 in DMSO-d6.
Water DMSO-h6
Added Calculated Actual Calculated concentration concentration concentration concentration
ment was found between the calculated and actual con- centrations for all four cases.
Isotopic Purity of Deuterium-Substituted Methanols. The absorption of either the O-H or the C-H group was used in the previous sections to determine the concen- tration of H/O and/or DMSO-h6 in different solvents. It is, therefore, possible to increase the accuracy and pre- cision of the technique by simultaneously measuring the absorption of both groups. Absorption spectra of meth- anol and its deuterium-substituted compounds (namely, CD3OH, CHDzOH, and CHzDOH) were measured with the use of this NIR thermal lens technique. The spectra obtained, shown in Fig. 11A-1 to 11A-4, confirm this possibility, i.e., that there are two different bands in the spectrum of methanol (Fig. 11A-1; a well-defined band at 925 nm and a broad band with a maximum at 1040 and a shoulder at about 1025). As illustrated in Fig. 11B, this broad band can be resolved into two bands (11B-a and 11B-b) with maxima at 1025 and 1040 nm. On the basis of the literature 4,a9 and of information from previous sections, it is tempting to assign the two bands at 925 and 1040 nm (of Fig 11A-l) to the third overtone and the combination of the stretching and bending vibrations of the C-H group/9 respectively, and the 1025-nm band (of Fig. 11A-l) to the combination transition (2Vl + v3) of the O-H group. This assignment is further confirmed when the spectrum of methanol is compared with those of the isotopic species (shown in Fig. 11A-2, -3 and -4). As illustrated in Fig. 11A-2 and -3, replacing each of the hydrogen atoms in the CH3 group with a deuterium atom (to form C H D / O H and CH/DOH) leads to a substantial decrease in the intensity of the two C-H bands at 925 and 1040 nm. The intensity of the shoulder at 1025 nm that is due to the O - H group remains the same. When all the hydrogen atoms in the CH3 group are replaced with deuterium atoms (CD3OH) the C-H band at 925 nm disappears completely (Fig. 11A-4). These results in- dicate that it is possible to use the present technique for the determinations of the isotopic purity of deuterium- substituted methanols. Results on the use of this tech- nique for the determination of CH3OH in CD3OH, CHDzOH, and CHzDOH will be described in the follow- ing section.
The thermal lens spectra of CH3OH and of CD3OH without and with different concentrations of added CH3OH are shown in Fig. 12A. As expected, adding CH3OH into CD3OH sample produces a new band at 925 nm concomitantly with an increase in the intensity of the band at 1040 nm. Because these bands are due to the absorption of the C-H group, their intensities are ex- pected to be correlated with the concentration of CH3OH
= 2
I-
A
0 . . . . P . . . . I . . . . i . . . . i . . . . p . . . . q . . . .
875 900 925 950 975 1000 1025 1050
Wavelength, nm
2 ._m
E O
F--
a
. / , . '
0 i p i i I i i I i i , , . ~ . . . ~ J " l I i
960 975 990 1005 1020 1035 1050
Wavelength, nm
FIG. 1 I. (A) Thermal lens spectra of (1) CH3OH , (2) CH2DOH, (3) CHD2OH, and (4) CD3OH. (B) Deconvoluted components of spectrum of CH3OH; component a with its maximum at 1025 nm is due to the combination transition 2v~ + v 3 of the O-H group, and component b with its maximum at 1040 nm is due to the combination of the stretching and bending vibrations of the C-H group.
in the sample. A plot of the correlation coefficient as a function of wavelength, calculated according to the PLS method, is shown in Fig. 12B. As illustrated in Fig. 12B, a very good correlation (close to 1) was obtained for the region between 910 and 942 nm, which is due to the third overtone band of the CH group for the entire 0-100% concentration range of CH3OH in the CH3OH:CD3OH mixture. A correlation was also obtained for the region between about 1020 and 1040 nm but with relatively smaller values (less than 0.996). This result is expected because the third overtone band at 925 nm is well isolated from other bands, whereas the band at 1040 nm is almost entirely overlapped with the OH band. The predicted concentrations of C H a O H , calculated according to the PLS method for the region from 910 to 940 nm (not shown) agree well with the added concentrations (corre- lation coefficient, SEP, and PRESS values are 0.99, 0.130, and 0.136, respectively). The limit of detection, deter- mined at 920 nm, was found to be 0.05% (w/w).
840 Volume 48, Number 7, 1994
I -
0 875
A CH OH (% v/v) loo.oo 3 ' 2a.6r / \
16.~7 / \ | 13.79 / i
L ;0; | 4 1 7 6 /
/ 3.85 / ~ , / 2.91 J ~ , ~ ^ _ \
llsa
I \
. . . . t . . . . i . . . . i . . . . i . . . . i . . . . i , , , I
900 925 950 975 1000 1025 1050
Wavelength, nm
1.000
~ec 0.998
i-_o 0.996
o
O
0.994
O
o 0.992
0.990 . . . . i . . . . i . . . . . . . i . . . . i 87! 900 925 950 975 1000 1025 1050
Wavelength, nm
FIG. 12. (A) Thermal lens spectra of CD3OH without and with different concentrations of CH3OH. (B) Plot of correlation coefficient vs. wave- length for the set of spectra shown in A.
Similar measurements were also performed to deter- mine the concentrations of CH3OH in CHDEOH and in CHEDOH. Again, as expected, in both cases it was found that the concentration of added CH3OH is well correlated with the intensity of the C-H band in the region between about 910 and 940 nm (figure not shown). The calibration curves constructed at 925 nm exhibited a very good linear relationship; the correlation coefficients were found to be 0.999 and 0.999 for CHDEOH and CHEDOH, respec- tively. The slope of the calibration curve for CHDEOH is 1.342 x 10 -2 (w/w %)-1, which is much higher than that for CHEDOH [6.752 × l0 -3 (w/w %)-1]. This result is expected because overtones and combination bands are proportional to the mass of the compound, and the mass difference between CHD2OH and C H a O H is 6%, which is two times higher than that between CH2DOH and CH3OH. As a consequence, the limit of detection for CH3OH in CHDEOH was found to be 0.075%, which is about two times lower than the LOD of0.15% for CH3OH in CH2DOH (0.1%).
It has been successfully demonstrated that near-infra-
red absorption can be sensitively measured by use of the thermal lens technique. The sensitivity of the technique stems from the inherent sensitivity of the thermal lens effect--namely, in this technique the signal intensity is directly proportional to the excitation laser power. As a consequence, its intensity is higher than transmission measurements by an enhancement factor E, which is giv- en by 9'16
P( dn/ d T) E (1)
1.91 Xk
where P is the excitation laser power, k and (dn/dT) are the thermal conductivity and temperature coefficient of the refractive index of the solvent, and X is the wavelength of the probe beam. For methanol solution (dn/dT = 3.85 × 10 -4 K-I; k = 1.95 mW cm -1K -116), the enhancement factor E was calculated to be 26 for measurements made with 13.5-mW excitation laser power and monitoring at 632.8 nm with a He-Ne laser. It is therefore clear that, due to this sensitivity enhancement, the technique was able to provide a unique means for the nondestructive, noninvasive, and sensitive determination of the isotopic purity of samples such as water and DMSO-h6 in DMSO- d6, and C H 3 O H in C D 3 O H , CD2HOH, and CDH2OH, at limits of detection as low as 10-3% (v/v). It is important to point out that these low limits of detection were achieved by the use of a cell with only a 2-mm pathlength and a laser power as low as 13.5 mW. Even lower LOD values may be obtained with higher laser power. A longer-path- length cell can also be used to improve the detection limit. However, this is probably not possible in practice because of the limited availability and the cost of isotopic com- pounds. This reason also explains why conventional ab- sorption techniques cannot be used for such isotopic de- terminations. Specifically, a cell with a pathlength of about 10 cm is needed in the absorption measurement in order to achieve detection limits that are similar to those of the thermal lens measurement. Even with the use of a mi- crocell (e.g., NSG Precision Cells, Inc. Model T-521), this approach will require at least 4.8 mL of isotopic samples, which is about 48 times more than the 100 ~L required in the present thermal lens measurement.
Another interesting feature which is illustrated by Eq. 1 is the fact that the sensitivity enhancement is inversely proportional to the wavelength of the monitoring laser. As a consequence, the sensitivity of the present thermal lens technique was further enhanced by about 1.8 times because the thermal lens effect is monitored not in the NIR (e.g., 1000 nm) but in the visible region at 543.5 r im .
In terms of applicability, it is important to add that water, DMSO, and methanol are not the only compounds which this technique can determine but rather are only examples which we used to demonstrate the applications of the technique. The technique is based on the currently available Ti : sapphire near-IR laser which has its output radiation from 680 to 1050 nm. As a consequence, the technique can be used for the nonivasive, sensitive, and simultaneous determination of any trace chemical and isotopic impurities which have O-H or C-H groups. Ex- periments are now in progress to use the technique for the determination of isotopic purity Of H E t 8 0 , H2170 , and carbon compounds with '3C. The results obtained, corn-
APPLIED SPECTROSCOPY 841
bined with our knowledge of the use of the thermal lens for the kinetic determination of fast chemical reac- tions, 37,3s will allow the use of this technique for following pathways of biochemical and biological reactions initi- ated by substrates with 1so and/or 13C isotopes.
ACKNOWLEDGMENTS
M. S. Baptista acknowledges CAPES for his fellowship. The National Institutes of Health, National Center for Research Resources, Biomed- ical Research Technology Program is gratefully acknowledged for fi- nancial support of this work.
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