55 1
Anal. Chem. 1992, 64, 551-557
nonlinearities in the first few of these. The following questions/objections are often put forward when attempting to use neural networks for quantitative analy~is:'~J~ 1. Neural networks are difficult to design and will not generalize satisfactorily. 2. Can neural networks extrapolate? Will neural networks predict better on data outside the calibration set range than PLS and PCR? 3. Neural networks are prohibitively slow to train. The first of the above points has certainly proven wrong in these two data sets (and in other cases we have tried). Measured by standard error of prediction the neural network performs a factor 4 better than PLS and PCA in a real-world application on both interpolation and extrapolation data sets. We have demonstrated that neural networks are able to predict well on new objects that fall outside the calibration set range. In the "real-life experiment" OMNIS3 out-performed both PLS and PCR completely. As for the third objection, calibrations OMNISl, OMNIS2, and OMNIS3 were produced on a 20-MHz PC using Turbo Pascal software. Training time varied from 10 to 30 min corresponding to 300-1000 traversals of the calibration set. This included the initial calculation of the PCA eigenvectors on the training set. The main reason for these very short training times is that we have used direct connections from input to output. Most nonlinear effects in "Et spectroscopy can be thought of as relatively small perturbations on a linear solution. Therefore it is only the direct connections that need adjusting to get the largest part of the calibration in place. Ale0 the networks we have used are very small. This has been possible because we have used PCA to reduce the number of inputs.
Earlier attempts to use artificial neural networks for calib r a t i o n ~have ~ ~ not been quite so optimistic about the future of this technique. However, the use of direct connections in neural networks and principal component scores as input ensures that the performance (generalization) is never worse than that obtained with PCR, because PCR is part of the model.
REFERENCES (1) Martens, Harald; Naes, Tonnod. kkJbharfete Celbration; John Wlby b. SOWS: b W YO&. 1989: DD 97-111. (2) Stom, M.; Brooks, R. J. J . R . Shtbt. SOC. B 1990, 52 (No. 2),
237-269. (3) Thomas, Edward V.; Haaland, Davld M. Anal. Chem. 1990. 62. 1091- 1099. (4) Rumelhart. D. E.; Hinton, 0. E.; Wlwiams, R. J. In pere/h9l#slMsMbuted processing; Rumelhart, D. E., McCblland, J. L., Eds.; MIT Press: Cambrldge, MA, 1988; Vol. 1, pp 318-362. (5) Uppmann, Rlchard P. IEEE ASSP Meg. 1987, April, 4-22. (6) Jansson. Peter A. Anal. Chem. 1991, 63,357A-362A. (7) Gemperllne, Paul J.; Long, James R.; Gregoflou, Vadlis G. Anal. chsm.1991, 69, 2313-2323. (8) Lang, Kevin J.; witbrock, Mlchael J. Connecbbnlst kkdeEs, Proceedings of the I988 Summer School; Morgan Kauhnenn Publisbrs, Inc.: San Mateo, CA, 1960 pp 52-59. (9) bum, E. D.; Hausskr, D. Neural Computath 1 ; MIT Press: Cambridge, MA, 1989 151-160. (10) CottreN. Q a d n W. connecdknhrr Wd&, Proceedings of the 1990 Summer Schod; Morgan Kaufmann PubHshers, Inc.: Sen Mateo, CA, 1QQI. .. DD 328-337. --(11) webnd, A.; Hubennen, B.; ~u-rt, D. ~ n tJ. . Neuelsyst. 1990, 1 (No. 31. 103-209. (12) Thbdbecg, Hans H. rnt. J . ~scuelsyst. 1991, 1 (NO. 4), 317-326. (13) Long, J. R.; Vasills, G. G.; Gemperline, P. J. Anal. Chem. 1990. 62, 1791-1 797. (14) Zupan, J.; Ciestelger, J. Anal. CMm. Acta 1991, 248, 1-30. F.
RECEIVED for review August 23,1991. Accepted November 27, 1991.
W ater-i n- OiI MicroemuIsions as Solvents for Laser- Excited Multiphoton Photoionization Mitchell E. Johnson and Edward Voigtman* Department of Chemistry, University of Massachusetts at Amherst, Amherst, Massachusetts 01003-0035 Lasor-oxcltod muttlphoton photolonlzatlon spectroscopy In polar re# #krtknrk by rdrrtlvdy high nobe bvols. Dotoctbn Ilmtts aro 2-3 orders of magnltudo wolw than In apolar solvents. Wator-lnoll microomulrlons of wator/AOT [rodkm bk(2-othylhoxyl) rutforucclnate~nheptanewore ovaulatod as altornatlvo rdvonts for polar analytes. Dkpordon of watu/wrtactanl aggmgater In hoptaw crdded excess kw-froqwncy n o k relatlvo to pur. heptano, but lt was pordbb to flttor out tho O X C ~ S Sn o k . Root moan square noko Improvomonts of as much a0 30 the8 wore poulble. Sgnal amplltudo was a h reducod In m l c r m for tho tert compounds 1-naphthrlenosuMonlc acM, 4-amlno-1naphthabneWonk add, and p+honylenodlrrmh, howevor, wch that 110 Improvomenl In tho d.toctbn lhdt was reallzed. Photolonlzatlon offkkncy Is vow ~nsltlvoto tho mlcroonvironrmnt. S.nkrlt.rlng of tho wator pool k propoud as a pordMs mochanbm for rlgnal supprelon. Howovor, procbkn and Ilnoartty of rospon8o WHO bnprovod, partkularly In d.hnnhakn of tho addsatts. llw potontlal oxlstsfor uso
hfmdamuWaudsrofpkWo&Wnandforanalysoswlth M w o d or hbh lonlc strongth rolutlonr.
* Author to whom correspondenceis to be addressed.
INTRODUCTION Laser-excited multiphoton photoionization spectroscopy (MPI) has been shown to be a useful and sensitive method for the determination of molecular species in the condensed phase.'" Briefly, photoionization is the process whereby a molecule absorbs two photons whose combined energy is enough to promote an electron into the ionization continuum. The geminate ion-electron pair thus produced lasts only a few tens of nanoseconds in liquid solvents; however, if the ionelectron pair ie produced in an electric field gradient, a current proportional to the number of charge carriers will be induced in the electrodes and can be detected and amplified by conventional methods. One barrier to the general usefulness of MPI stems from the detection of the ionization event in polar solvents. When the bias voltage is applied across polar analyts solutions, residual ionic impurities and auto-ionized solvents carry a certain amount of current, the leakage current.2 The dc component of the leakage current is easily removed, but the noise is responsible noise associated with it is not. This exfor detection limits that are at least 2-3 orders of magnitude worse for analytss in polar solvents than in nonpolar solvents.'."' The amount of noise and its spectral character depend strongly on the polarity and grade of the solvent? but
0003-2700/92/0384-0551$03.00/0 0 1992 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1. 1992
been chosen for this study is water/AOT [Aerosol OT, sodium polar solvents are always worse than nonpolar solvents. It bis(2-ethylhexyl) sulfosuccinat.e]/n-heptane.This system has clearly follows that ionic solutions (e.g., buffered aqueous been studied extensively and can be considered to be fairly solutions) are out of the question for use in conjunction with well characterized (it should be noted that there is not yet MPI. a general agreement concerning the exact state of the miUnfortunately, polar solvents and ionic solutions comprise croemulsion, particularly in the interphase). a large proportion of the matrices found in real analytical situations. For example, the analysis of mixtures by MPI EXPERIMENTAL SECTION suffers due to a somewhat limited selectivity (in common with Photoionization Apparatus. The experimental arrangement most molecular spectroscopic techniques). Chemical sepafor photoionization is shown in Figure 1. The XeCl excimer laser ration is therefore often necessary, which in turn often implies delivers 80-100 mJ per pulse at 308 nm (4.05 eV); approximately polar solvents. While reversed-phase liquid chromatography 1% of the pulse energy reaches the flow cell. For some experiis possible with MPI there is still the problem ments, “ghosts” from the mirror and aperture were removed by of excess noise with polar solvents. replacing the second aperture with a pinhole spatial filter. The It has been known for some time that the addition of ionic flow system consisted of a gravity feed pump based on the surfactants to nonpolar solvents allows the solubilization of Marriotte flask design;= a pair of Rheodyne Type 50 Teflon rotary valves, one for injecting analyte and one for filling the injector significant amounts of water. Depending on the amount of loop; and a windowless flow cell constructed in-house. The flow water and surfactant added to the nonpolar solvent, various cell was similar to the one designed by Voigtman et al.’ Stainless aggregation states are observed. At very low water concensteel HPLC tubing formed the liquid inlet and the bias electrode trations and above a characteristic concentration of surfactant, and was positioned above a solid titanium collector electrode of reverse micelles form. The ionic surfactant head groups cluster the same dimensions. The electrodeswere spaced 1.7 mm apart, together, stabilized by counterions and hydrogen bonding.12 the flowing liquid was suspended between the electrodes by surface At higher concentrations of water, the micelles swell. Above tension. A voltage of -900 V was applied to the bias electrode. a certain water concentration, a new phase is formed in which The collector electrode was connected directly to the inverting a core of “free” water molecules are surrounded by a surfacinput (virtual ground) of the preamplifier. Experiments were tant-water interphase, again stabilized by counterions and performed either in the flow-injectionmode or by f i g the pump with analyte solution. “bound” water molecules. This phase is known variously as Signal Processing. Signal processing options are shown in a water-in-oil (W/O) microemulsion, L2 phase, or, under more Figure 1. The preamplifier was either a simple current-to-voltage restricted conditions, aqueous nanophase.13 This is the phase converter (transimpedance amplifier) with a 1.1-MQfeedback of interest; for convenience, it will simply be designated a (40 kHz, 3 dB bandwidth) or a base-line restorer (BLR)? resistor microemulsion, and a single surfactant-water aggregate will a single-pole band-pass filter with a gain of 120 dB and cutoff be called an aggregate. frequencies (-3 dB) of 460 Hz and 4.4 kHz. The output of the These microemulsions are monodisperse, thermodynamipreamplifier was quantified with a gated integrator (SR250, cally stable, and exist, in some cases,over a wide range of water Stanford Research Systems, Palo Alto,CA) and optionally boxcar averaged. The output of the gated integrator was digitized and surfactant volume fraction^.'^-'^ The location of a given (SR245) and collected with the IBM PC/XT, using software system on a ternary phase diagram can be specified by the provided with the boxcar (SR265). Leakage currents were obweight percentages of the two components. It is generally tained with an electrometer based on an AD515L FET op amp more useful, however, to give the molar water-to-surfactant (AnalogDevices, Norwood, MA) with a 100-MQfeedback resistor, ratio (w,) and weight percentage or concentration of surfactant followed by a low-pass filter. (% w/w or m~l-dm-~). The size of the water pool increases Noise Spectrum Estimation. For each case, 16 noise records with w0,l5-l8with a concomitant drop in pool core v i s c o ~ i t y ~ ~ J ~of 1024 points each were collected with a digital storage oscillofor a given concentration of surfactant. At constant wo, the scope (HM 2052, Hameg, Port Washington, NY)and transferred number of aggregates increases with increasing surfactant to the Macintosh SE via a GPIB interface (Hameg H079) and board (GPIB-SE, National Instruments, Austin, TX). Data concentration, but the aggregation number (number of surcollection and processing were performed with programs written factant molecules per aggregate) does not. Therefore, water in QuickBASIC (Microsoft, Redmond, WA) using GPIB routines pool characteristics, which generally correlate with w,, are supplied with the National Instruments board (NI-488). The independent, to a first-order approximation, of surfactant resulting noise records were analyzed by standard Fourier concentration. transform techniques, either on the IBM PC/XT using spreadThere are several regions of distinctly different environsheets and Fourier Perspective I1 (Alligator Transforms, Costa ments in a single surfactant/water aggregate. Above w, = 6, Mesa, CA) or on the Macintosh using a QuickBASIC program. the core of the water pool consists of yfree” water molecules, All QuickBASIC program details are availablefrom the authors. The 16 periodograms were averaged, without windowing or other with some physical properties approaching those of bulk smoothing, to form the power spectrum estimate. Total root mean water.l9N There is a semidiffuse electrical double-layer region square (RMS)noise was found by integratingthe power spectrum at the interphase where the sulfate head groups of the AOT and taking the square root. As no anti-aliasingfilters were used, reside, in which sodium ions and water molecules are more the bandwidth of the RMS calculations were the same as that rigidly bound and viscosity is high. There is a less polar region of the preamplifier (see above). where the carboxylic acid groups reside (though these may Reagents and Chemicals. The test species l-naphthaleneah0 be complexed with the counterions), and there is the tail sulfonic acid (NSA) and 4-amino-1-naphthalenesulfonicacid region, which is similar to the bulk heptane phase. The an(ANS) were obtained from Eastman Organic Chemicals (Rochalyte may reside in any of these regions, depending on its ester, NY); p-phenylenediamine (PPD) and HPLC grade nchemical properties. heptane were obtained from Fisher Scientific (Fair Lawn, NJ); and AOT [bis(2-ethylhexyl) sulfosuccinate, sodium salt] was The premise of this work is that it is possible to use an obtained from Fluka (Ronkonkoma, NY). Water (>16 MQecm aqueous nanophase dispersed in a nonpolar solvent to soluresistivity) was obtained from a Barnstead Nanopure I1 system. bilize polar or ionic analyte molecules in order to perform MPI All chemicals were used as received. without the inherent disadvantages of aqueous solution. %an Microemulsion Solutions. The microemulsionsolutions were has reported an analogous technique for signal enhancement prepared by first dissolving AOT in heptane to form a stock in thermal Aqueous analyte solutions were dispersed solution. The appropriate amount of water or aqueous analyte in nonpolar solvents, which have much better thermooptical solution was added, and the solution was shaken or sonicated to clarity. Alternatively, the analyte was dissolved directly in the properties than water. The microemulsion system that has
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1,
II;
1992 553
GIIBA
F PD I
I
MAC
optional spatial filter
Flgure 1. Experimental apparatus for MPI and slgnal/data processing. Photolonlzatlon: XeCl exclmer laser (UVL), apertures (A), front-surface mirror (M), lens (L), flow cell (FC), beam dump (BD), and bias voltage source (HV). The optional spatial filter consists of lenses (L) and a plnhole (P). Trlggerlng: flber optlc (FO) and photodiode (PD). Flow system: gravlty feed pump (GFP), valves (V), sample Injection polnt (S), and waste 0.slgrel fxocesW: preampliRer (PA), gated lntegrator/boxcar averager (GVBA), and IBM PC/XT (PC). Noise data co#ectkn: optbnal Inverting ampllfler (-lox),dlgltal storage oscllloscope (DSO), and Apple Macintosh SE (MAC). methanol (e0pA) water, acetonitrile (60pA) ethanol (50 pA)
Table I. Composition of Water/AOT/Heptane Microemulsions w0
50 0.4' 10 20 30 40 50 60 70 80 50 50 50
%
w/wAOT 0.10 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.50 0.74 0.98
6W-
[AOTI, mM
% w/wH20
HzOa
r,Ab
1.53 3.83 3.83 3.83 3.83 3.83 3.83 3.83 3.83 3.83 7.66 11.5 15.3
0.20 0.004 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 1.0 1.5 2.0
0.14 0.003 0.069 0.14 0.21 0.28 0.34 0.41 0.48 0.55 0.69 1.0 1.4
74
% v/v
s a
15 30 45 59 74 89 100 120 74 74 74
Not corrected for AV of mixing. *Waterpool radius, estimated from eq 1 in ref 18. CBasedon the water content of heptane as specified by the manufacturer. microemulsion. Solutions were allowed to equilibrate overnight. Table I summarizes the composition of the microemulsions used in this work. Various units are shown to facilitate comparison with other work.
RESULTS AND DISCUSSION Noise Behavior of Microemulsion Solutions. The leakage current of a solvent is an indication of the fitness of that solvent for MPI high leakage currents entail poor performance. Figure 2 shows the leakage current of typical microemulsions as a function of the amount of surfactant. Also shown are leakage currents of common solvents. The latter were taken from Voigtman and Winefordner.2 As their values were obtained at a bias voltage of -100 V, the values shown here were scaled for an estimate a t the -900-Vbias voltage. In terms of leakage current, the microemulsions
SW-
-3w-
1w
0 0
0.k
o.b
-chloroform (47 nA) -io:% ethanokhexane (36nA) 0.i~
1.0
Flgure 2. Leakage currents of typical microemulsions (ao= 50) as a fundon of surfactant concentration. Leakage currents of common solvents are shown at right as the values In parentheses.
behaved as moderately polar solvents. The solutions with a lesser amount of surfactant had leakage currents nearly 3 orders of magnitude lower than the strongly polar solvents such as water, acetonitrile, and short-chain alcohols. The leakage current is only an average value of the background current. The noise power and its spectral density are far more important in determining solvent suitability and in designing a signal-processingscheme. Note that throughout the following discussion, noise spectra are shown with the proper units of V2/Hz.In referring to the spectra as power density spectra, the 1-Qapproximation has been made. Figure 3 shows noise power spectral density (NPSD) of some microemulsion solutions as a function of the amount of surfactant. Clearly the dispersion of microemulsion aggregates into pure heptane added excess low-frequency (ELF) noise, relative to pure heptane. The amount of ELF noise added depended on the amount of surfactant, as predicted from the leakage current data. These results were very useful. To begin with,
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
Table 11. Comparison of RMS Noise of Various Microemulsion Solutions with RMS Noise of Transimpedance Preamplifier, Base-Line Restorer, Pure Heptane, and Water Transimpedance Preamplifier w,,
= 50, % w/w AOT =
0.25% AOT, w,,
00 0.27 0.4 0.33
=
Ob
0.10
0.28 10 1.6
0.75 20 2.3
0.98 2.7 40 2.1
0.25 1.4 30 2.3
50
70 0.98
60 1.3
1.4
80 0.56
Base-Line Restorer (BLR) Preamplifier unconnected BLR
0.25%
0.71
AOT, w, = 0-80
water
0.71
21
Unconnected transimpedance preamplifier. "Pure"heptane. For details on preamplifier construction, see Experimental Section; for details on microemulsion solution composition, see Table I.
m
3 a
C
b a 1 "
0
XXI
4ca
Boa
BQ)
1000
frequency, Hz
frequency, Hz Flguro 3. Noise power spectral density plots for some typlcal microemulsion solutlons as a functlon of surfactant concentration. The concentration axis 1s not linear. For ail solutlons, w, = 50. Preamplifier: translmpedance preampllfler. Collectlon frequency: 2 kHz.
a low amount of surfactant was indicated for further studies. Recall from the Introduction that the amount of surfactant used has no fundamental consequences, in terms of the physical properties of the aggregatea, as long as stable solutions are formed (at least within the decade of concentration used in this work). The RMS noise and NPSD of the pure heptane solution were nearly the same as those of the unconnected preamplifier, see Table 11. Also, the base-band noise power of the microemulsions (i.e., the noise power above the ELF noise comer frequency) was the same, within experimental error, as that of pure heptane. The base-band noise power of the 0.25% w/w AOT case was slightly higher, but these data were obtained 5 months after the other and can, therefore, be considered relatively close. No attempt was made to control temperature, which might easily account for the discrepancy. Thus,the noise added by dispersing water in heptane had only ELF components. The situation for microemulsions of different w, was similar. The solutions differed slightly in the amount and character of ELF noise, but the base-band noise was the same as that of heptane for all cases. The cutoff frequency was roughly the same (ca.130 Hz) for w, = 10-60,but dropped to 110 and 80 Hz for w, = 70 and 80, respectively. The frequency dependence of NPSD of the portion below the cutoff frequency also depended on w,, though not strongly. Table 11shows the dependence of total RMS noise on w,. Low-frequency noise is always troublesome, but in all cases reported here, the noise corner frequency was very low, less than 150 Hz. The signal bandwidth was much higher, being
F I g m 4 . CQmparlsonofndsepowerspectrawtthvarkuspreempliRer
and filter comblnatlons: (a) pure heptane with transimpedance preamplifier; (b and c) 0.25% w/w AOT, w, = 50 mlcroemulskn wlth (b) wnslmpedance preampltlier and (c)base-line restorer preampliffer; (d) pure water wRh base-line restorer preamplifier. limited by the bandwidth of the preamplifier. Thus, it was possible to fiiter out the ELF noise without decreasing the signal bandwidth significantly. Attempts to simply add a high-pasa fiter were unsuccessful, however, a baseline restorer (see Experimental Section) worked well and allowed direct comparison to aqueous solutions. Figure 4 shows the noise power spectra of various solution and preamplifer combinations. This f i i e clearly shows that the base-line restorer was successful in filtering out the ELF noise. Unfortunately, the noise level was not reduced to the base-band level (i.e., that of heptane). Noise power spectral density and total noise power did not vary with w, when using the baseline reatorer. The cause of the latter two observations is simple: the noise added by the base-line restorer circuit itself dominated the total noise power. The NPSD of the base-line restorer alone was almost identical to those for all of the microemulsion solutions. Thus,the noise power could not be reduced further without different preamplifier construction. The reasons for the observed noise behavior are not immediately obvious. The linear dependence of leakage current on surfactant concentration (or, equivalently, aggregate concentration, assuming monodispersity) suggested that either free surfactant molecules or the aggregates themselves carried the current. Leakage current, which is directly proportional to steady-state conductivity, was also linearly dependent on bias voltage (not shown). However, with no water added (0, = 0.4), the leakage current was very low (a. 400 PA), the noise was white, and the RMS noise was nearly the same as that of pure heptane (Table 11). These findings suggested that
ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
there was little free surfactant, at least at very low water content. The most plausible explanation is that the aggregates, at w, 1 10, had a substantial dipole moment and carried the current themselves. An early study by Eicke and Shephard showed that the addition of water to Aerosol AY (sodium di-Zpentyl sulfosuccinate)reversed micelles in benzene causes a drastic change in the dielectric properties of the solution at w, = 3.* The observed behavior was attributed to a change in dipole moment with added water. Langugs and Sauterey reported that, in a low concentration regime, the mean electrical charge on a microemulsion droplet is proportional to the aggregation number." Therefore, assuming their results for sodium dodecyl sulfate/cyclohexane/ 1-pentanol/waterare applicable, each AOT/water aggregate had an appreciable charge that was independent of surfactant concentration, and the linear leakagecurrent response to surfactant concentration and bias voltage can be interpreted in terms of simple ionic conduction. The fact that the response was linear also suggests that particle interaction was negligible and that viscosity (and diffusion coefficienta) did not decrease significantly with increasing concentration of aggregates. Ideal behavior of monodisperse spherical particles dispersed in solution (Stokes-Einstein relation) predicts only a 3% increase in viscosity for a 10-fold increase in concentration at the volume fractions used here. Finally, the observed behavior indicates that the concentration of aggregates was well below the percolation threshold for all cases.24 The behavior observed as a function of w, is more difficult to explain. Conductivity was not observed directly, but RMS noise was found to be roughly proportional to leakage current (see Figure 2 and Table II). Ideally, the mobility of Spherical charged particles is inversely proportional to the particle radius (if charge is independent of radius). However, an increase in noise power was observed for w, = 0.4-30, followed by a decrease. One possible explanation is that the dipole moment increased with aggregate radius, and the balance between increasing dipole moment and decreasing mobility shifted between w, = 20 and 30. To our knowledge, the dipole moment or other electrical properties of microemulsion solutions in the concentration ranges employed in this work have never been studied. The important pointa are that the noise was whitened with filtering, which allows boxcar averaging of the signal for improved signal-to-noise ratios (SNR),and the RMS noise level was reduced to within a fador of 3 of that of heptane. Also, the RMS noise of the microemulsion solutions with the b l i n e restorer was much lesa than that of water (see Table 11),and the NPSD of water was non-white-the spectrum shown in Figure 4 has a distinct l / f character out to 1kHz. Thus, the first goal of this work was met: noise power and leakage current were greatly reduced relative to aqueous solution while still maintaining a polar environment for the anal*. Photoionization in Microemulsion Solutions. The reduction in noise power made by using microemulsions was offset by a large reduction in signal power. Figure 5 shows calibartion curves for pphenylenediamine (PPD) in water and microemulsion. The signal was as much as 2 orders of magnitude greater in water than in the microemulsion. This phenomenon was observed with other compounds, 1naphthalenesulfonic acid (NSA) and sodium l-amino-4naphthaleneeulfonic acid (ANS), see Figure 6,as well. In the caae of PPD, the signal reduction more than offset the noise difference. The detection limit (defined to be 3 times the standard deviation of the blank divided by the slope of the calibration curve) was -2-3 times worse in the microemulsion (due to nonlinearity of the calibration curve in aqueous so-
555
'1
/ 0
1
1
,
3
2
log (conc, pM)
-
Figure 5. Callbratlon cwves for p-phenylenedlamlne In (m) water (pH 5) and (0)microemulsion (0.25% w/w AOT, w, = 50), obtalned from Rowinjecbkn analysis results. For water, both quadatic and linear least-squares fits are shown; the linear flt excludes the lowest concentration.
a)
";"I
>
0
0
1
2
log (conc, IJM)
-' -
time, s
3
time, s
Ftgw 8. CaUbratbn cuves (a and c) and representative RowinJeCtion analysls results (b and d) for the sodium salt of 1-amlno-4naphthelenegulfonate. The results In (a) and (b) are In a microemolsion (0.25% w/w AOT, w, = 50); (c) and (d) are In water (pH 5). The FIA results are for the sokrtlons Indicated by a &de on the caltbratkm curves. The curve fits are quadratic least-squares fits.
-
lution, the detection limit was only a rough estimate). The poorer detection limit was due entirely to the reduction in signal power, since the blank standard deviation (i.e., RMS noise) measured at the output of the boxcar was 25 times less than with aqueous solution, as predicted by the resulta given above. However, the determination in the microemulsion solution showed a much higher degree of linearity than in water. The results for ANS, shown in Figure 6, are an example of one of the positive aspects of MPI in microemulsion solutions-the ability to reliably determine salts by MPI. Neither calibration curve is linear, but the higher precision available with the microemulsion allowed a more accurate calibration. Qumtitation of aqueous ANS solutions using peak mea was not even possible, due to splitting of the peak (Figure 6d),especially at higher concentrations. Calibration by peak height was clearly not reliable (Figure 6c). These latter results were not unexpected, because of the excess ions added by a salt, as discussed in the Intlbduction. However, with ANS and its counterion restricted to the microemulsion interior, the excess ions caused little problem (though their presence may have been the cause of nonlinearity, due to small changes
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 5, MARCH 1, 1992
interpretation of this relatively small effect. Absorption spectra of the three test compounds showed differences between water and microemulsion solutions in the near-UV and visible regions. In the case of ANS and NSA, the spectral differences were slight, though for ANS a small (2-fold)decrease in molar absorptivity at 308 nm was observed. For PPD, the differences were much greater, with wavelength shifts of as much as 70 nm. The spectra were also independent of w,. Certainly the absorptivity differences observed were much too small to account for the 1-2 order of magnitude drop in signal. Figure 7 shows the absorption spectra for PPD in the microemulsion solution and in the aqueous solution at pH 3 and pH 5.2. The UV peak shifts (at ca. 300 nm) could not be attributable to pH effects alone, though the visible region shows a better correspondence with the lower pH. The pH effect implied by the absorption spectra could be attributed to natural buffering of the water pool by acid impurities generated in the manufacturing process of AOT.28 Interaction with the diffuse double-layer,sodium cations, and the sulfate headgroups of the AOT micelle also act to buffer the water pool. El Seoud has reported pK, shifts as great as 2 pK units.29 Such a pH shift is enough to cause substantial protonation of both amino groups of PPD, for example. Protonation of amino groups affects the lone pair on nitrogen, impairing photoionization efficiency. Such an explanation does not apply to NSA, though it is not unreasonable to suppose that pH may have an effect on the photoionization efficiency of all acidic or basic compounds.
308
wavelength, nm Comparison of UV-vis absorption spectra of p phenylenediamine in (a)aqueous solutbn, pH = 5 . 2 (b) microemulsion solutlon, 0.25% w/w AOT, w, = 30; (c)aqueous solution, pH = 3.0. The absorptlon axis of (c) was expanded by 70 % for comparison purposes. Flgurr 7.
N
in the microenvironment of the ANS molecule). There are several possible explanations for the decreased signal: decreased electron or ion mobility, either due to location of the analyte in the "bound" water layer or to increased viscosity (thus making the signal-reduction mechanism similar to the noise-reduction mechanism); decreased lifetime of the ion-electron pair; decreased effective electric field strength from screening by the double-layer associated with the polar head groups; lowered absorption cross section due to solvatochromic shifts; or chemical effects, such as effective pH, which affect photoionization efficiency. The possible contributions of these various effects are difficult to predict. Pileni et al. and Visser and Fendler both reported drastically-reduced (relative to aqueous solution) lifetimes of electrons produced by pulse radiolysis in AOT water pools.25926However, Abdel-Kader and Krebs and CalvePerez et al. reported longer lifetimes (on the order of microseconds) for photolytically-produced electrons in AOT water p 0 0 l . ~ ~There J ~ appears to be general agreement that the viscosity of the water pool, especially in the bound layer, is greater than that of water;13 therefore, the mobility of photoproduced species is expected to be lower. It also appears that the lifetime of photoproduced species is very dependent on the method of production and the location of the analyte in the water pool.n These effects are also generally dependent on w,. The photoionization signal of PPD in a 0.25% w/w AOT microemulsion depended slightly on w,. The signal at w, = 10 was a factor of 2 greater than the signal at w, = 70, and the values for w, = 20-60 lay in between. The complexity of the possible mechanism does not allow for unambiguous
CONCLUSIONS Though noise reduction was achieved, the reduction was offset by signal reduction, such that detection limits for the test compounds were 2-3 times worse in microemulsion solutions that in aqueous solution. The signal reduction was due to an unknown mechanism, though pH was shown to be a possible culprit, at least in one case. However, the test compounds, particularly ANS, a salt, showed much better linearity and precision in microemulsion solution. The microemulsion system studied in this work not only provides an overall better environment for performing MPI on aqueoussoluble analytes but also provides a previously unavailable means of studying the effect of pH on MPI efficiency, provided the pH of the interior can be reliably calibrated. Buffering of the water pool is possible, but the system must be carefully characterized because of its complexity.29At a given w, and % w/w AOT, several of the potential variables can be expected to remain constant, regardless of buffering, so exact knowledge of all variables would not be necessary. Microemulsions can often accommodate rather large salt concentrations,30so the effect of ionic strength on photoionization efficiency could also be studied. Finally, other microemulsion systems 'might be more appropriate. For example, microemulsionsbased on the cationic surfactant didodecyldimethylammoniumbromide are known to be stable and "non-c~nducting".~~ ACKNOWLEDGMENT We thank Merck, Sharpe & Dohme Research Laboratories for financial support of this work. We also thank the Analytical Division of the American Chemical Society and Eli LiUy and Company for support in the form of an Analytical Division Fellowship, and the Graduate School of the University of Massachusetts for a Graduate Fellowship (both for M.E.I.). This work was presented, in part, at PittCon '92 in New Orleans, LA. REFERENCES (1) Voigtman. E.; Winefordner, J. D. J . Liq. Chromatogr. 1982, 5, 2 113-2 122. (2) Voigtman, E.; Wlnefordner, J. D. Anal. Chem. 1982. 52, 1834-1839. (3) Yamada, S.; Kano, K.;Ogawa, T. Bunseki K8gaku 1982, 3 1 , E247E250.
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Yamada, S.; Ogawa, T. Rog. A M I . Spectrosc. 1988, 9 , 429-453. Hlno, A.; Yarnada, S.; Nagamura, T.; Ogawa, T. Bunsekl Kagaku 1984, 33, E3934396. Yamada, S.; Hlno, A.; Kano, K.; Ogawa, T. Anal. Chem. 1983, 5 5 , 1914-1917. Vdghnan, E.; Jwgensen, A.; Wlnefordner, J. D. Anal. Chem. 1981, 53, 1921-1923. Voigtman, E.; Snyder, P. A. Anal. Instrum. 1990, 79, 1-13. Voigtman, E.; Winefordner, J. D. J . Llq. Chromatcgr. 1983, 6 , 1275-1289. Yamada, S.; Hlno, A.; Ogawa, T. Anal. Chlm. Acta 1984, 756, 273-277. Yamada, S.; Hno, A,; Ogawa, T. BunseklKagaku 1984, 33, E37-E40. Ekke, H.-F. In Micelles; Sprlnger-Verlag: New York, 1980 pp 85- 145. €&e. H.-F. In Intwtaclal FWnomena h ApoJar Msdia; Ekke, H.-F., Parfltt, 0.D., Eds.; Marcell Dekker: New York, 1987; pp 41-92. Berthod, A. J . Chlm. pnys. pnys.-ChIm. Bid. 1983, 80, 407-421. Berthod, A.; Nlcolas, 0.;Porthault, M. Anal. Chem. 1990, 62, 1402-1407. AbddKadw, M. H.; Krebs, P. J. Chem. Soc. Faraday Trans. 11988, 8 4 , 2241-2245. Sahyun, M. R. V. J . h y s . Chem. 1988, 92. 6028-6032. Wonft. M.: Thomas, J. K.; Gratzel. M. J. Am. Chem. Soc. 1978. 98. 2397-2397. Cako-Perez, V.; Beddard, G. S.;Fendler, J. H. J . ~ Y S Chem. . 1981, 85, 2316-2319.
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RECEIVED for review September 9,1991. Accepted November 29, 1991.
Quantitative Analysis of Short-Chain Phosphates by Phosphorus-3 1 Nuclear Magnetic Resonance and Interlaboratory Comparison with Infrared and Chromatographic Methods David R. Gard,* John C. Burquin, and Janice K. Gard* Momanto Company, 800 North Lindbergh Boulevard, St. Louis, Missouri 63167
A mothod for the analysis of commercial inorganlc oiigophoqhates has been developed based on the use of hlgb r m -31 Fourler transform nuclear magnetic resonance (NMR). Experknentai parameters were selected to optlmize accuracy, precision, and analysts time. The accuracy and pNdrkn of the NMR method have been demonstrated to be comparable to chromatographic methods and ru(mkr to IR and XRD In controlled Intedaboratory analyses wlng commercial sodium tripolyphosphate. Thls study r e p resent8 the only thorough and dlrect comparlson of methods for phoqhato apsclos analysts. Tho maw source of error In the NMR mothod Is In obtaining reproducible integrationsfor algnak wlth proximate ch.mlcai shifts, but the preddon may be greatly improved wlng a Ilne-shape analysis and curvefitting routine. Detection limns of 1-10 mg P/L may be attained with phosphorus-31 NMR by scanning overnight.
INTRODUCTION Phosphates are materials of great importance and are an ubiquitous part of modern life. Their role in foods, environmental issues, human and animal health, plant and microbial vitality, herbicide chemistry, and industrial and consumer products demands exacting analytical methods for their characterization. Phosphorus-31 nuclear magnetic resonance (NMR) has been demonstrated as a useful method for the quantitative analysis of small inorganic phosphate^.'-^ NMR
involves minimal sample preparation and reasonably short analysis times and is applicable to moderately low concentrations. Limitations of phosphorus-31NMR include lower sensitivity and the complexity of spectra for polyphosphates higher than tripolyphosphate. Slow relaxation times can generally be overcome with the use of a paramagnetic relaxation agent. As the basis for quantitative determination of phosphate species, phosphorus-31 NMR offers several potential advantages compared to chromatography, including (1)simultaneous observation of all phosphorus-containing species and only the phosphorus-containing species, (2) structural information which may complement or aid quantitation, (3) application to systems for which other methods do not exist or are not appropriate (e.g., degradation on column packing), and (4) quantitation with or without standards, and quantitation with an elemental as opposed to a molecular standard. In regard to the use of standards, Rabenstein and Keire point out that NMR differs from chromatographicand other spectroscopic methods in that a predetermined response factor for each compound being determined is not r e q ~ i r e d . ~In NMR, concentrations can be obtained directly from relative resonance integrals and pure samples of the compounds of interest are not required for calibration. N M R is therefore particularly suited to an analysis of condensed phosphates because highpurity standards are not obtainable for many condensed phosphates (in contrast to orthophosphates) because of the considerably more complicated phase chemistry of condensed phosphates and their mixtures. For example, at least 0.5%
0003-2700/92/0364-0557$03.00/00 1992 American Chemical Society
