1710
Anal. Chem. 1986, 58, 1710-1714
waist. The thermal diffusivity of fused silica is 0.0037 cm2/s compared to that of water being 0.001 46 cm2/s. Thus, surface-adsorbed species could result in an observed characteristic time constant that is as little as 0.4 that of the aqueous solution. The apparatus reported here was not optimized for signal magnitude. An achromat microscope objective was used to focus the output of the laser fiber illuminator. This resulted in a coincidence of both pump and probe laser focus positions. The optimum TLS signal is obtained when the offset between the pump and probe laser focus positions is approximately 1/31/2of the probe laser Rayleigh range (4).On the basis of 4 of ref 4,it is estimated that the signal magnitude of the current apparatus is only 2.5% of the theoretical maximum. This could be corrected by using chromatic focusing optics. The enhancement factor, defined as the ratio of the sensitivity of TLS to that of conventional spectrophotometry, is only 1 for water and 10 for ethanol solutions (2). These poor enhancement factors are due to the solvents used and the low pulse energy. The matched focus positions also decrease the enhancement by an additional factor of 0.025. However, with the SNR enhancement inherent in the correlation estimation procedure utilized here, and the decreased pointing noise, the present apparatus is very likely to be better than that previously reported in terms of SNR (8). Registry No. Fused silica, 60676-86-0.
LITERATURE CITED (1) Fang, H. L.; Swofford, R. L. I n Ultresensltive Laser Spectroscopy; Kllger, D. S., Ed.; Academic Press: New York. 1983;Chapter 3. (2) Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54, 2034-2038. (3) Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55, 1075-1079. (4) Long, G. R.; Bialkowski, S. E. Anal. Chem. 1984, 56, 2806-2811. (5) Long, G. R.; Bialkowskl. S. E. Anal. Chem. 1985, 57, 1079-1083. (6) Long, G. R.; Bialkowski, S. E. Anal. Chem. 1088,58,80-86. (7) Nickohisen, S.L.; Bialkowski, S. E. Anal. Chem. 1085, 57,758-762. (8) Nickolaisen, S. L.; Bialkowski, S. E. Anal. Chem. 1986, 58, 215-220. (9) Dovichi, N. J.; Harris. J. M. Anal. Chem. 1081, 53,689-692. (10)Welmer. W. A.; Dovlchi, N. J. Anal. Chem. 1085, 57, 2436-2441. (11) Weimer. W. A.; Dovichi, N. J. Appl. Opt. 1085, 2 4 , 2981-2986. (12) Carter, C. A.; Harris, J. M. Anal. Chem. 1084,56, 922-925. (13) Kateman, G.;Pljpers, F. W. Qualify Control in Analytical Chemistry; Wiiey: New York, 1981. (14) Yariv, A. Opffcal Electronics, 3rd ed.; Holt, Rinehart, and Winston: New York, 1985. (15) Nickohisen, S. L.; Bialkowski, S. E., submitted for publication in J. Chem. Inf. Compuf. Sci.
(16) Papouiis, A. PrObablHty. Random Variables, and Stochastic Processes, 2nd ed.; McGraw-Hill: New York, 1984. (17) Golub. G. H.; Van Loan, C. F. Matrix Computaflons; Johns Hopklns Press: Baltimore, MD. 1983. (18) Carter, C. A.; Harris, J. M. Appl. Spectrosc. 1083, 37, 166-172. (19) Jackson, W. B.; Amer, N. M.; Boccara, A. C.; Fournier, D. Appl. Oot. 1081, 20,1333-1243. (20) Bialkowskl, S. E. Appl. Opt. 1985, 2 4 , 2792-2796.
RECEIVED for review December 31,1985. Accepted March 3, 1986. This research was funded by a grant from the Vice President for Research Office a t Utah State University.
Matrix Effects in Thermal Lensing Spectrometry: Determination of Phosphate in Saline Solutions C. M. Phillips,' S. R. Crouch,* and G. E. Leroi* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824
The thermal Ienrlng dgna(8 from heteropdy blue 8oiuilons
A lenslike element is produced that causes the beam to di-
c o n t a l n l n g p h o o p h a t e u n k r ~ e d ~ ~ h a v everge, and the magnitude of the beam divergence (measured as a loss of intensity a t beam center in the far field) depends been measuted. The d@lkanl flndkrgr were: (1) the satkre sensitively on the sample absorbance. The first analytical mairlx has a real and hrporlant eifed on the thermal lendng study involving thermal lensing was performed by Dovichi and properties of the sokrtlor#r-an average signal enhancement Harris in 1979 (2).The technique has rapidly become a widely of 28 f 5 % was calculated between conconused analytical method with applications in trace analysis tratbns of aqueou~ and salhe s o M m (2) when the anatyte (%14), chromatographic detection (15-23),and flow injection Is extracted into kabueyil slcohd an e n h m m m t In mponse analysis (24). over that of the parent aqueous solution Is Observed, due to Inherent in the analytical power of thermal lensing is the the Merence In the thermoop#cal ptop.rtkb of the solverhie ability to detect ultratrace quantities of analyte. However, and (3) moreover, solvent extractkn doerr not overcome the as was pointed out by Harris and Williams (W), the significant effects of the sallne matrix. Large changes In the thermal quantity in any spectrometric technique is not the minimum lenslng parameter are obrerred for corresponding concendetectable absorbance, but the minimum detectable change tratkns between exiracts from aqueoue rdutknr and extracts in absorbance. As the sensitivity of any spectrometric techfrom sallne sduilons, and the slopes of the worklng curves nique is improved, limits in the precision due to instrumental are also affected. factors and sources of absorbance (both controllable and
The thermal lensing effect, first reported by Gordon et al. in 1964 (I), is due to the formation of a refractive index gradient within a sample upon irradiation with a laser beam.
'
Present address: D e p a r t m e n t of Chemistry, Massachusetts Ins t i t u t e of Technology, Cambridge, MA 02139.
uncontrollable) unrelated to the analyte concentration must be carefully considered. In transmission methods, reduced precision can arise from absorption and scattering of light from several sources: (1)excesa reagent, (2) solvent, (3) the matrix (such as dissolved salts, organics), and (4)impurities. In calorimetric methods such as thermal lensing spectrometry (where absorbed power is converted into heat), these interferents may have an effect on the thermal properties of the system, thus modifying the resultant signal even in the absence
0003-2700/86/035&17lO$Ol.50/0 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
of any change in analyte absorbance. Thermal lensing has been proposed as a technique for determining phosphorus (as the “heteropoly blue”) in seawater (13);however, any effect of the saline background on the thermal lensing signal was neglected in that study. The sample matrix cannot be ignored in thermal lensing measurements. In this study we report on the effect of the matrix on the thermal lensing signal arising in the colorimetric determination of phosphorus in saline solutions, along with results of efforts to reduce this effect. We conclude that the background contribution must be carefully considered in assessing the precision of analytical measurements made by thermal lensing spectrometry. EXPERIMENTAL S E C T I O N Apparatus. For these experiments a conventional thermal lensing spectrometer was employed (2). A Spectra-Physics (Model 165) Kr+ laser is used as the pump laser (except where noted in the text). The laser output beam (X = 647.1 nm) is folded with two flat mirrors (Melles Griot 02 MFG 001) to facilitate spatial and optical requirements and chopped with a Uniblitz (23X) shutter. The output beam is focused with a simple plano-convex lens (Oriel; focal length = 350 mm, f/13.8) so as to define the position of the beam waist. The cells, mounted on an optical rail (Newport Research Corp., URL 36), can be variably positioned with respect to the beam focus and may be tilted to reduce optical interference from multiple reflections. High-quality, fused silica cells (Hellma 110-QS)were employed due to their low absorptivity in the region of interest. The lensed light is folded with a four-mirror configuration (Melles Griot), which provides approximately 5 m between the cell and the detector. The detector assembly consists of a 100-bm pinhole (Melles Griot 04 PIP 015) tht is used as a mask for a photodiode detector (Silicon Detector Corp. Model SD 041-12-12-211). The photodiode current is converted to a voltage with an LF-351 operational amplifier-based current-to-voltage converter (Rfof 680 kQ). The thermal lensing experiment is controlled by an Intel 8088-based microprocessor (programmed in the FORTH language); a detailed description of the microprocessor and ita components is given by Newcome and Enke (26). The converted voltage is then input to one channel of an eight-channel differential multiplexer (Date1 Model MVD 807); the multiplexer output is connected to a programmable gain amplifier (Burr Brown Model 3606 BG). The amplified analog voltage signal is digitized by a 12-bit analog-to-digital converter (Analog Devices Model AD 574). The laser beam shutter is controlled by setting one bit of a parallel input/output chip (Intel Model 8255). Microsecond timing in the experiment is handled with a 1.0-MHz crystal and a counter/timer chip (Intel Model 8253). Normally, 100 thermal lensing transients are acquired in one experiment. The time-dependent thermal lensing data ( Z ( t ) ) stored in memory are written to a FORTH formatted floppy disk. This floppy disk is then transferred to an LSI 11/23 minicomputer where the average thermal lensing signal, with appropriate statistics, is calculated. A procedure similar to that first described by Dovichi and Harris (27) is used. The data are fit (with u2 weighting) to an equation of the form (28) Z ( t ) = Z(O)[l.O - %tan-’ (0.577 t / ( t + tJ)l
(1)
with the fitting routine DATFIT. In this formalism, Z ( t ) is the detected intensity at any time t , Z(0) is the detected intensity at time t = 0, t , is the characteristic time constant of the lens ( t , = u 2 c p / 4 k , where w is the beam radius at the sample, c is the solvent heat capacity (calg-’ K-’), p is the solution density (g an4), k is the solution thermal conductivity (cal cm-’ s-’ K-l), 8 is the analytically significant thermal parameter (8 = 0.24 Pal(dn/ dT)/Xk], where P is the incident laser beam power (W), a is the absorbance of the sample per unit path length (cm-‘), 1 is the sample path length (cm), X is the wavelength of the laser radiation (cm), and dn/dT is the thermooptical constant for the solvent (K-’)). The core routines used here are contained in program MINPACK (29),which uses an algorithm derived by More et al.
1711
Table I. Results of the Relative Signal Change in a 400 pg/mL Phosphorus Solution with a Change in the Background NaCl Concentration sample
relative &‘
0.0 M NaCl 0.01 M NaCl 0.1 M NaCl 0.5 M NaCl
100.0 f 3.5 100.8 f 5.3 111.9 f 4.9
120.0 f 5.0
DRelativeto 0.0 M NaC1. Table 11. Test of Interference of NaCl on the Chromogenic Reaction of Phosphorus NaCl sample concn
0
0.1 M added before color development 0.1 M added after color development 0.5 M added before color development 0.5 M added after color development
0.105 f 0.004 0.109 f 0.003 0.114 f 0.004 0.119 f 0.004
(30)to compute a gradient search for the solution with minimal storage. The resulting fit gives a best estimate for Z(O), 8, and t,. Reagents. The colorimetric procedure followed here is similar to that described by Murphy and Riley (31);it relies on the reduction of the antimonyl complex of 12-molybdophosphoricacid (12-MSbPA)to form a colored product (‘mixed heteropoly blue”). A standard sample solution made up of 100 pg/mL phosphorus is prepared by dissolving 0.4395 g of potassium dihydrogen phosphate (KHzP04)in 1.0 L of distilled, deionized water. Subsequent dilutions are performed to give a stock solution of 10 ng/mL phosphorus. Aliquots up to 10 mL are introduced into 100-mL volumetric flasks. To this is added 5 mL of a stock reagent solution. This reagent solution is made by dissolving 8.5 g of sodium molybdate dihydrate (NazMo042H20)and 0.1371 g of potassium antimonyl tartrate hemihydrate (K(SbO)C4H4062H20)) in distilled, deionized water followed by careful addition of 83.2 mL of concentrated sulfuric acid, with subsequent dilution to 1.0 L. The 12-MSbPA is reduced with 5 mL of a 1.75% (w/w) ascorbic acid solution. Aliquots of a 1.0 M NaCl solution are added to the solution mixture to give a final NaCl concentration selected to lie in the range between 0.0 and 0.5 M. The solution is then diluted to volume and transferred into the fused silica cell for thermal lensing measurements.
RESULTS AND DISCUSSION The effect of saline solutions on the thermal lensing measurements was investigated. Several identical solutions, each containing 400 pg/mL phosphorus, were prepared, the color was developed, and varying amounts of 1 M NaCl were then added prior to dilution to create final solutions of 0.0, 0.01, 0.1, and 0.5 M NaCl. Initial experiments were performed with an Ar’ laser-pumped dye laser system (A = 620 nm, power = 70 mW). After the experimental data were least-squares fit to eq 1,B values at each NaCl concentration were referenced to the 0.0 M NaCl value. The results, given in Table I, show a definite enhancement in the thermal lens upon the addition of NaC1. The salinity of the world’s oceans is approximately equivalent to that of a 0.5 M NaCl solution. Thus the analysis of Fujiwara et al. (13) is suspect, since apparently no account was made for the ionic strength change between standard solutions prepared in the laboratory and the analyzed seawater samples. As a check for possible Na+ or C1- interference in the chromogenic reaction, solutions of 400 pg/mL phosphorus were again prepared, and NaCl was added either before or after color development; results are listed in Table 11. The 8 value for a given NaCl concentration is statistically invariant, regardless of the order of salt addition. Thus there is no discernible interference from Na’ or C1- on the chromogenic reaction within the time frame of the measurement.
1712
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
Table 111. Calculated Signal Enhancement for Various NaCl Solutions (Referenced to 0.0 M NaCl) [NaCl], M
k , cal s-' cm-' K-'*
0.0 0.01 0.1 0.5
14.60 X 14.60 X 14.53 X 14.48 X
dn/dT, K-'*
70 signal
[NaCl], M
co
Pb
kC
t,, ms
1.135 X 1.137 X 1.160 X lo-' 1.260 X
100.0 100.2 102.7 112.0
0.0 0.01 0.5 0.5
1.0000 1.0000 0.9910 0.9607
1.0000 1.0007 1.0043 1.0207
14.60 X IO4 14.60 X lo4 14.53 X lo4 14.48 X
49.5 49.5 49.5 48.9
nExtrapolated from data in ref 36. bExtrapolated from data in ref 35.
1
'30.0c
m 'O'%O
0.1
0.2
Table IV. Calculated Values of t , for Selected NaCl Concentrations
0.3
0.4
0.5
[NaCi] (M)
Figure 1. Comparison of calculated ( e )and observed (standard deviation bars) thermal lensing enhancements as a function of NaCl concentration for a 400 ng/mL phosphate solution. For clarity, each set of points has been connected.
Another possible explanation for the thermal lens enhancement upon addition of salt is a change in the bulk refractive index, which might alter the absorption coefficient of the reduced 12-MSbPA species. A change in solute molar absorptivity with the refractive index of the solvent was proposed many years ago by Kortum and Seiler (32),who suggested that n / ( d + 2)2be applied as a correction factor to the molar absorptivity. From this factor and the accepted values for the refractive index of water (1.333) and a 0.5 M NaCl aqueous solution (1.338) (33),a relative change in the molar absorptivity of 0.33% was calculated. Since this is 2 orders of magnitude smaller than the observed enhancements, the bulk refractive index change contributes insignificantly to the saline matrix effect. In the expression for 8 (8 = 0.24 Pal(dn/dT)/Xk) ( B )the , only other terms that can vary are d n / d T and k . Values of d n / d T for pure solvents are abundant (34);however, for electrolyte solutions there is a paucity of reliable data. Recently, d n / d T values for KBr solutions were determined by interference refractometry (35). Due to the similarity of the electrolyte solutions, extrapolated values of d n / d T should reasonably approximate NaCl solution data. Since the major constituent of seawater is NaC1, tabulated thermal conductivity values ( k ) for seawater (36) should also reasonably approximate NaCl solutions. The extrapolated values for k and d n / d T (at 25 "C) and the calculated enhancement in 8 are listed in Table 111. The calculated relative 8 values are graphically compared to the experimentally observed values in Figure 1. The latter are consistently higher than the predicted values; in light of the approximations made, this is not surprising. I t is clear that the magnitudes of the calculated and observed enhancements are approximately equal, and thus one can conclude that the variation in the thermooptical parameters is the primary reason for the change in 8 between aqueous and NaCl solutions. It was thought that the saline matrix might also have a measurable effect on the characteristic time constant of the system, t,. Calculations were performed for solutions of 0.0, 0.01, 0.1, and 0.5 M NaCl concentrations, using tabulated values of heat capacity (c, cal g-' K-l) for seawater (37) to approximate the NaCl solutions. The density ( p ) values (g
Extrapolated from data in ref 37, in cal g-' K-'. *From ref 33, in g ~ m - ~ 'Extrapolated . from data in ref 36, in cal cm-' s-l K-'. ~ m -were ~ ) obtained from the CRC Handbook (33), and the values for k (cal s-l cm-' K-l) were as before (36). These parameters, along with a measured beam radius of 170 mm, were used to calculate the values oft, (ms) given in Table IV. Within the experimental uncertainties inherent in the measurement of t, (a nominal-fit t, value of 49 f 3 ms was observed in these experiments) these differences are insignificant, and therefore no further investigation of this parameter was made. In an effort to minimize the effects of the matrix, solvent extraction was employed to separate the analyte from the saline solution matrix. Much literature exists on the subject of extracting reduced 12-molybdophosphoric acid or its antimonyl analogue from aqueous solution with oxygenated organic solvents (38). Isobutyl acetate or propylene carbonate can be used; however, the most common solvent is 2methyl-1-propanol, also known as isobutyl alcohol (IBA) (39-41). For this reason, IBA was chosen for these extraction studies. Phosphate solutions ranging from 0 to 1000 pg/mL phosphorus were formed in both distilled, deionized water and 0.5 M NaCl solution (the latter being chosen because it closely resembles the ionic strength of seawater and gives the most noticeable thermal lensing enhancement). Both series of solutions (50-mL aliquots) were extracted with IBA (25 mL) and shaken for 2 min. The extract was then diluted to 50 mL. The aqueous solutions were analyzed first, followed by the extracted solutions to allow (1) adjustment to the lower laser power needed for the IBA solvent, since its thermal lensing enhancement factor is 1order of magnitude larger than that of water, and (2) adjustment of the mirror assemblies to allow the beam center to fall on the pinhole (there is a bulk refractive index change). The results are enumerated in Table
v.
With the exception of a few anomalies, each series of experiments gave a signal response that varied linearly with concentration. The results (after blank subtraction) for each series are plotted in Figure 2. Differences in the relative thermal lensing signal enhancements are reflected by a change in the slope of the constrained least-squares line. (For clarity the error bars have been removed, and data that vary by more than 20 from the least-squares line have been omitted.) The previously mentioned effect of the saline matrix on the relative signal enhancement is again demonstrated by the change in slope between the two lower curves. The two upper curves reflect the change in the partition coefficient for 12-MSbPA between water and IBA due to a change in the salinity of the aqueous phase. This implies that less of the analyte is extracted from the saline solution, resulting in a smaller increase in the relative thermal lens signal with increasing analyte concentration. In order to investigate this differential extraction, three separate 500 pg/mL phosphorus solutions were prepared in 0.1, 0.5, and 0.8 M NaC1. Each solution was extracted with IBA as before; the extracts were then diluted and analyzed. The results of this experiment are given in Table VI. The ionic strength of the solution does affect the efficiency of the extraction.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
1713
Table V. Results of IBA Solvent Extraction species
blank aqueous solution 100 pg/mL P aqueous solution 200 pg/mL P aqueous solution 500 pg/mL P aqueous solution 700 pg/mL P aqueous solution 1000 pg/mL P aqueous solution extract of blank aq solution extract of 100 pg/mL P, aq solution extract of 200 pg/mL P, aq solution extract of 500 pg/mL P, aq solution extract of 700 pg/mL p, aq solution extract of 1000 pg/mL p, aq solution
0
species
0
0.136 f 0.003 blank 0.5 M NaCl solution 0.136 f 0.006 100 pg/mL P in NaCl solution 200 pg/mL P in 0.138 f 0.006 NaCl solution 500 pg/mL P in NaCl solution 0.142 f 0.005 700 pg/mL P in NaCl solution 1000 pg/mL P in 0.156 f 0.008 NaCl solution extract of blank 0.5 M NaCl 0.156 f 0.010 extract of 100 pg/mL P, 0.5 M NaCl 0.582 f 0.008 extract of 200 pg/mL P, 0.5 0.591 f 0.012 M NaCl extract of 500 pg/mL P, 0.5 0.550 f 0.009 M NaCl extract of 700 pg/mL P, 0.5 0.654 f 0.019 M NaCl extract of 1000 pg/mL P, 0.5 0.686 0.013 M NaCl
0.174 f 0.007 0.166 f 0.006 0.177
* 0.010
0.215 f 0.005 0.208 f 0.002 0.208 f 0.009 0.300 f 0.020 0.313 f 0.003 0.319 f 0.013 0.339 f 0.008 0.309 f 0.014 Concentration P (ng/mL) 0.364 f 0.007
*
Figure 2. Corrected (blank-subtracted) thermal lensing signals as a functlon of analyte In aqueous solution, 0.5 M NaCl solution, IBA extract of 0.5 M NaCl solution, and IBA extract of aqueous solution. Observable changes in slope are evident through comparison of the lower two curves and the upper two curves.
0.735 f 0.019 matrix interferences are not present or can be overcome. Registry No. NaC1, 7647-14-5; phosphate, 14265-44-2.
Table VI. Results of IBA Extraction from Solutions of Varying NaCl Concentration sample
0
500 pg/mL P in 0.1 M NaCl 500 pg/mL P in 0.5 M NaCl 500 pdmL P in 0.8 M NaCl
0.505 f 0.004 0.344 f 0.006 0.312 f 0.005
As a further investigation, a 0.5 M NaCl solution was extracted with IBA; the extract was diluted to 50 mL; and 10 mL of this solution was placed into a previously weighed drying bottle. The solvent was volatilized, and visual inspection of the bottle revealed a white coating, obviously NaC1. The bottle was reweighed, and the solubilized amount was determined by subtraction; a solubility of 0.009 M NaCl in IBA was calculated. This value is 270 000 times larger than the maximum possible concentration of the 12-MSbPAspecies in IBA. From these experiments it is clear that the activity of the highly charged 12-MSbPA species is altered by the change in ionic strength. The higher ionic strength of the aqueous phase stabilizes these species, as predicted by the Debye-Huckel theory. In addition, the salt that is extracted partially excludes the 12-MSbPA species, resulting in a lowered activity.
CONCLUSIONS In those situations where the interfering matrix cannot be precisely compensated for or effectively separated from the analyte of interest, thermal lensing analyses require preparation of a working curve in a matrix approximating the environmental sample. Alternatively, the method of standard additions could be performed. On the other hand, the extraordinary sensitivity of thermal lensing makes it attractive for trace analysis providing that
LITERATURE CITED (1) Gordon, J. P.; Leite, R. C. C.; Moore, R. S.;Potto, S.P. S.;Whinngry, J. R. J . Appl. Phys. 1985, 36,3. (2) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1979, 51, 728. (3) Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55, 1075. (4) Higashi, T.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1983, 55, 1907. (5) Mori, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54, 2034. (6) Aifheim, J. A.; Langford,C. H. Anal. Chem. 1985, 57,881. (7) Long, G. R.; Biaikowski. S. E. Anal. Chem. 1984. 56, 2806. (8) Haushalter, J. P.; Morris, M. D. Appl. Spectrosc. 1980, 34, 445. (9) Miyaishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chim. Acta 1981, 124, 381. IO) Imasaka. T.; Miyaishi, K.; Ishibashi, N. Anal. Chim. Acta 1880, 115, 407. 11) Miyaishi, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1982, 54, 2039. 12) Beitz, J. V.; Hessler, J. P. “Oxklation State Specific Detection of Transuranic Ions in Solution”. Presented at The Argonne National Labora-
tory Specialists’ Workshop on Basic Research for Nuclear Waste
13) (14)
Management, Sept 5-6, 1979; Argonne National Laboratories, Argonne. IL. Fujiwara, K.; Lei, W.; Uchiki, H.; Shimokoshi, F.; Fuwa, K.; Kobayashi, T. Anal. Chem. 1982, 54, 2026. Nakanishi, K.; Imasaka. T.; Ishibashi, N. Anal. Chem. 1985, 57, 1219.
(15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (26) (29) (30) (31) (32) (33)
Leach, R. A.; Harris, J. M. J . Chromatogr. 1981, 218, 15. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1981, 53, 689. Buffett, C. E.; Morris, M. D. Anal. Chem. 1982, 54, 1824. Nickoiaisen. S.L.; Biaikowski, S. E. Anal. Chem. 1985, 57,758. Sepaniak, M. J.; Vargo, J. D.; Kettler, C. N.; Maskarinec, M. P. Anal. Chem. 1984, 56, 1252. Buffett, C. E.; Morris, M. D. Anal. Chem. 1983, 55, 376. Pang, T.-K. J.; Morris, M. D. Anal. Chem. 1984, 56, 1467. Leach, R. A.; Harris, J. M. Anal. Chem. 1984, 56, 2801. Carter, C. A.; Harris, J. M. Anal. Chem. 1984, 56, 922. Yang, Y.; Hairrell, R. E. Anal. Chem. 1984, 56, 3002. Mrris. T. D.; Williams, A. M. Appl. Spectrosc. lQ85, 39, 28. Newcome, B.; Enke, C. G. Rev. Scl. Instrum. 1984, 55, 2017. Dovichi, N. J.; Harris, J. M. Anal. Chem. 1081, 53, 106. Sheldon. S.J.; Knight, L. V.; Thorne, J. M. Appl. Opt. 1982, 21, 1663. More, J. J. NTIS Publication ANL 80-74, Argonne National Laboratories, Argonne, IL, 1980. More, J. J. In Lecture Notes in Mathematics 630: Numerical Ana&sis; Watson, G. A., Ed.; Springer-Veriag: New York, 1977; p 105. Murphy, J.; Riley, J. P. Anal. Chim. Acta 1962, 27,31. Kortum, G.; Seller, M. Angew. Chem. 193% 52,687. CRC Handbook of Chemlstry and Physics; Weast, R. C., Ed.; CRC: Cleveland, OH.
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Anal. Chem. 1986, 58, 1714-1716
(34) Solimini, D. J. Appl. Phys. 1966. 37, 3314. (35) Appleby, R.; James, D. W.; Bowie, C. A. Spectrochlm. Acta, Part A 1904, 40A. 785. (36) Jam!eson, D. T.; Tudhope, J. S. Desalination 1970, 8 , 393. (37) Jamleson, D. T.; Tudhope, J. S.; Morris, R.; Cartwright, G. Desalination 1970, 7. 23. (38) Boltz, D. F.; Howell, T. A. I n Chemical Analysis; Wiley: New York. 1978; Vol. 8,p 340. (39) Proctor, C. M.; Hood, D. W. J . Mar. Res. 1954, 13, 122. (40) Stephens, K. Limnol. Oceanogr. 1963, 8 , 361.
(41)
Henriksen, A.
Analyst (London) 1965, 9 0 , 29.
RECEIVED for review January 21,1986. Accepted March 10, 1986. This work was supported in part by National Science Foundation Grants CHE-8320620 (SRC) and CHE-7921319 (GEL). A summary of the results was presented a t the 12th Annual Meeting of FACSS, Philadelphia, PA, Sept 1985.
Helium-Neon Laser Intracavity Photothermal Beam Deflection Spectrometry Chieu D. T r a n '
Department of Physics and Atmospheric Sciences, Jackson State University, Jackson, Mississippi 3921 7
paratus be developed. One possibility is to replace the inA novel, compact, inexpensive He-Ne laser lntracavlty p h s t ~ a l b e a m d e t l e c t k n ~ w h s s b e e n d . v ~ .terferometer with a laser cavity resonator, Le., measuring photothermicity of a sample placed inside a cavity of a laser. I n this Instrument, a sample Is placed M d e the cavity of the Morever, the advantages of this method are not only limited He-Ne laser, which serves as a probe. An argon ion exclto its compact size but also include its inherent higher sentatlon laser is traversed in the horizontal plane collinear with sitivity due to the intracavity enhancement effect. the He-Ne laser beam Inside the sample cell. The apparatus It is well-known that when an absorbing sample is placed is 28 thnes more sensltlve than the conerpawlhg extracavity inside the laser resonator, the so-called intracavity enhanceinstrument. The detection lbnit for cyanine dye aqueous sois generated from such effects as multiple passes, ment that lution ls 1.0 X M. A mechanism for the lntracavity mode competition, and threshold can become as large as lo7 enhancement Is discussed.
Applications of laser to chemical analysis have increased significantly in recent years (1,2). Laser absorption technique.3 are the most widely used among those applications. Measurement of small absorption such as those in trace analysis requires an ultrasensitive laser spectrometer. The two most widely used techniques to enhance the sensitivity of laser absorption spectrometry are based upon photothermal and intracavity effects. When a laser beam passes through a material of finite optical absorption, the heat generated by the absorption of light increases the temperature within the sample, which changes the index of refraction, which in turn affects the optical beam. The results include a defocusing or focusing of the beam (photothermal lensing), a change in phase delay as the refractive index changes, and a deflection of the beam (photothermal deflection). Certain of the effects are observalbe for beams in the power range of only milliwatts in samples normally thought to be transparent, and are thus useful for measurement of low absorptivity. In fact, absorhave been measured by using photothbances as low as ermal techniques (3-7). Recent attempts to improve the sensitivity of photothermal measurement have utilized Mach-Zehnder interferometers to detect the change in the phase delay as refractive index changes (8-10). These methods cm-') were able to detect part-per-trillion levels (i.e., a = of SF, in nitrogen. Unfortunately, inherent disadvantages of the interferometric photothermal apparatus such as its bulk and the strict requirements for its accurate alignment make this apparatus impractical for general analytical use. Thus, it is important that a compact system having the same or better sensitivity than the interferometric photothermal apPresent address: Department of Chemistry, Marquette University, Milwaukee, WI 53233. 0003-2700/86/0358-1714$01.50/0
(12).In spite of its great potential, intracavity photothermal deflection spectrometry has not been exploited. All but two studies were reported (13,14).Although these systems hold great promise, they are still inferior compared to the interferometric photothermal techniques because: (1) The sample that was a gas in the former study and developed chromatogram in the latter was placed inside a cavity of a dye laser or an argon ion laser, which is complicated and bulky compared to interferometer. (2) The necessity for a sample to absorb dye laser radiation restricts applications of the apparatus to only a few gases (13). (3) The perpendicular overlap between the excitation and probe beams in the latter decreases the sensitivity of the system (14). Such considerations prompted inititation of the present study, which aims to develop for $he first time a compact, simple, inexpensive, ultrasensitive He-Ne laser intracavity photothermal deflection spectrometer. It will be demonstrated in this communication that such an intracavity photothermal beam deflection spectrometer (IPD) can be constructed by placing a sample inside the cavity of a simple He-Ne laser, which serves as a probe laser. The argon ion excitation laser beam will be collinear with the probe beam inside the sample cell. By use of the indicated apparatus, cyanine dye a t a M was detected. concentration of 1.7 X
EXPERIMENTAL SECTION Typically, a commercial H e N e laser has its high reflector and output coupler sealed into its plasma tube. The laser used in this work, which was specially constructed according to our requirements, was purchased from the Particle Measurement Systems, Inc., Boulder, CO. It consists of a plasma tube with one end sealed with a concave high reflector ( H E R = 100 cm) while the other end is a Brewster window. Ita output couple (OC) was a concave mirror with R = 100 cm and dielectrically coated for 1% transmission at 632.8 nm. The output coupler was mounted on a translational stage to provide tilt, vertical and horizontal adjustments for the coupler, and a horizontal adjustment for the cavity length. The distance from the Brewster window to the output coupler was fixed at 20 cm during this work. A schematic 0 1986 American Chemical Society
