Anal. Chem. 1990, 62, 1943-1947
1943
Effect of Particle Size and Modulation Frequency on the Photoacoustic Spectra of Silica Powders Raghoottama S. Pandurangi and Mohindar S. Seehra* Physics Department, West Virginia University, Morgantown, West Virginia 26506 Investlgatlons of the Infrared spectra of crystalline and amorphous silica particles of different sizes (0.05, 5, 10, 15, 30, 45, and 260 pm) in the range of 400-4000 cm-' using Fourier transform Infrared/photoacoustIc spectroscopy are reported. The change In the intenslty I of the signal with poroslty e of powders follows the e/( 1 - e) dependence for strong bands and e dependence for weaker bands as predicted by the theory of McGovern et ai. For strong bands, I also follows the empirical relation I D-", where D is the particle diameter In cm and n = 0.34-0.42. The anomalous posltive frequency dependence of I on the modulation frequency I observed for the stronger bands in the 45-pm and 260-pm particles is beileved to result from saturatlon. Some observatlons on the effect of changlng the background from (carbon black) powder to pellet are also made.
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INTRODUCTION Since the publication of the Rosencwaig-Gersho (RG) theory on the photoacoustic effect in solids (1, 21, Fourier transform infrared/ photoacoustic spectroscopy (FTIR/PAS) has become an important technique for obtaining the IR spectra of surfaces and materials in their natural state (1-12). In the one-dimensional RG theory, the incident radiation, modulated at frequency f , is absorbed by the material to a depth p6 = 1/@ ( p = absorption coefficient), thus setting up temperature variations in the material. These variations heat the adjacent gas/solid interface leading to the pressure fluctuations of the gas above the sample, which in turn is detected by microphone as a PA signal. The thermal wave originating from no deeper than the thermal diffusion length pa= ( 2 k / ~ p c f )in ~ /the ~ material contributes to the PA signal (here k , p , and c are respectively the thermal conductivity, density, and specific heat of the material). Since ps can be varied by varying the experimental parameter f , depth profiling by PAS is possible and a number of papers on this subject have appeared in recent years (2-10). Another important phenomenon is saturation which occurs when the material thickness 1 > p, > ps (1-12). The original promise of PAS for use with powder samples, without the need to "prepare" samples as in FTIR spectroscopy, has not yet been fully realized because of several complications. Theoretical works by Monchalin et al. (13) and McGovern et al. (14) showed that the interstial gas in the voids of the powders can act as amplifier and that, for the same mass, a finer powder is expected to yield higher PA signal than a coarser powder. Although in a number of earlier studies (15-21) an increase of PA signal with decreasing particle size has been observed, a quantitative check on the equations developed by McGovern et al. (14) has not yet been made. It is necessary to understand the particle size effects in PAS if the promise of PAS for IR spectroscopy of powders is to be realized. In this paper, we report results of a detailed study on the PAS of silica powders for the particle size range of 0.05-260 pm and compare the results with theory (14). We have also investigated the frequency dependence of the various
Table I. Tabulation of the Porosity c and c / ( l - c) for Different Particle Sizes particle diameter, pm
porosity c
0.05 (powder) 0.05 (pellet) 5 (powder) 5 (pellet) 10 (powder) 30 (powder) 45 (powder) 260 (powder)
0.97 0.66 0.79 0.61 0.78 0.64 0.48 0.46
c/(l
-4
32.33 1.94 3.76 1.56 3.55 1.78 0.92 0.85
IR modes in these powders and show that the saturation phenomenon affects a t least some of the observations. Some observations on the effects of pellet vs powder background are also reported (22-24). Details of these findings are presented and discussed in this paper.
EXPERIMENTAL SECTION Here, measurements are reported on Min-U-Sil, a crystalline form of SiOz, and Cab-0-Sil, an amorphous form of SOz. In a recent paper (24),we have given the details of our studies on the electron microscopy, FTIR spectroscopy, and cytotoxicity of these two forms of silica. For the work reported here we used Min-U-Sil particles of mean size 5, 10, 15, 30, 45, and 260 pm, and these samples were obtained from several sources (U.S. Silica Inc., Berkely Springs, WV, and Mill Creek, OK; Alfa Products; 88316 and 88777). Cab-0-Sil, of mean particle size 0.05 pm (25),was obtained from Cabot Corp. of Tuscola, IL. All the spectra reported here were taken with a Mattson Instruments Cygnus-100 FTIR spectrometer, equipped with an MTEC photoacoustic cell. The spectrometer and the sample zone are flushed continuously with dry air using the Balston air filter system, in order to avoid interference from carbon dioxide and water bands. Typically 32 scans at resolution of 4 cm-I were used to collect the spectra from 400 to 4000 cm-' range. The spectra are modulated with different mirror speeds v ranging from 0.08 to 0.18 cm/s, leading to modulation frequency f = 209. This gives, e.g., f = 1.44 kHz for u = 4000 cm-l at u = 0.18 cm/s. In the theory by McGovern et al. (14),porosity t of the powden defined by c = ( p , - p ) / p , is important where p is the apparent density of the powder and p, is the density of silica in solid state (p, = 2.65 g/cm3). To measure p and hence e, we measured masses of different.powders in a standard 1-mL flask, and these values oft for different powders are given in Table I. For Cab-0-Sil, our measured value of t = 0.97 agrees very well with the value given by McGovern et al. (14). For pellets, volumes were easily measured by measuring physical dimensions with a micrometer.
THEORY OF PAS OF POWDERS The main result of the calculations by McGovern et al. (141, which in turn were based on the theory by Monchalin et al. (13), is that for powders with porosity t, there are two contributions to the intensity of the PA signal: the thermal signal from the solid which is well described by the RG theory and a pressure signal due to the interstitial gas which acts as an amplifier. The pressure signal depends on the porosity E and the total signal Z can be written in a simple form (14) as
* Author to whom correspondence should be addressed. 0003-2700/90/0362-1943$02.50/00 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
t
Table 11. Values of p,, j3, and fig for Different Bands and for Different Modulation Frequencies band, cm-' frequency, Hz w8, wm 1080
85 70 56 100 85 68 107 92 13 67 54 44 46 38 31
173 260 389 126 190 284 111 166 249 300 450 675 600 900 1350
797 693 1875 3750
0,''cm-'
w p = 1/P
(wm)
960
10.4
294
34
53
189
25
400
5
,o
800
,
,
400
797 cm '(XIO)
/ loEo
0
7
200
100
300
D ( I o-'cm)
b
n C"
Figure 2. Intensity I of the 797- and 1080-cm-' bands plotted as a function of mean particle diameter D . Inset: In I vs In D for the same data. The soli lines are fits to the empirical eq 2, with n = 0.42 and 0.34 for the 1080- and 797-cm-' bands, respectively.
A n ril
0 50
4000
;. .1
1200
1
'Spitzer, W. G.; Kleinman, D. A. Phys. Reu. 1961, 121, 1324. *Not known.
0ooi
.t
I
I
I
I
3500
3000
2500
2000
I
1500
1000
I
500
Wavenumber (cm ' ) -2
Flgwe 1. PA spectra of 5-pm particles of 3.5 mg of silica, both in the powder and the pellet form.
where the second term in eq 1is the pressure term and A and B are constants depending upon the properties of the solid and the gas. If (1 - e)@ >1 and the exponential term in eq 1 0. This leads to € / ( I - e) dependence of I. Note that in the limit e 0, I = A, the thermal
-
-
signal. We will check the validity of eq 1 with the observations reported here on silica particles and with the data on other materials reported in literature (21). In Table 11, we list the calculated values of the thermal diffusion length ps and the optical absorption length ps = 1 / P for different bands of silica at different modulation frequencies (see Figure 1). These quantities are important for discussing the saturation phenomenon for various bands in particles of different sizes.
RESULTS The PA spectra of the 5-pm particles of Min-U-Sil, both in the powder and pellet forms, with carbon black powder as background, are shown in Figure 1. In this case, 3.5 mg of silica is used and the pellet was made under a pressure of 1500 1b/ine2(1.03 X lo7 N/mZ). It is clear that the intensity of all the bands is higher for the powder sample as compared to the pellet, increase being of the order of 75%. Also some of the weaker bands, viz. the 1875- and 1609-cm-' bands, are welldefined only for the powder sample. By use of the porosity values of Table I, it is clear that the above difference may simply be related to the pressure signal due to the interstial gas which is lowered in the pellet.
14
-12
-10
-8
-6
-4
In D Figure 3. Plot of In I vs In D (similar to inset of Figure 2) for the 1875-cm-' band. The soli line is a least-squares fit to the data for the six larger particles with n = 1.06, whereas the dashed curve is drawn by connecting the data points.
T o compare the signal I for different particle sizes and porosity values, we used 1 mg of each sample to take the spectra. The intensity I of each band was computed in inverse centimeters by computing the area under each band with a computer option available on the Cygnus-100. In Figure 2, we have plotted I for two intense bands, viz. 797 and 1080 cm-', against the mean diameter D of the particles. (The data for the 693-cm-' band is not plotted because this band is absent in amorphous silica (24)and its intensity is too weak for the larger particles of 45 and 260 pm size). The increase in I with decreasing D is qualitatively similar to some earlier reports (16-18). However a quantitative understanding of this kind of data has been lacking so far. In the inset of Figure 2, In I vs In D is plotted. The excellent fit obtained suggests that I varies with D as
-
I D-" (2) The solid lines are the least-squares fit to the data with n = 0.34 f 0.04 for the 797-cm-' band and n = 0.42 f 0.03 for the lOSO-crn-' band. These values of n are intriguingly close to 1/3. However for the weaker 1875-cm-' band (Figure 3), the data fit eq 2 only if the 0.05-pm Cab-0-Si1 particles are excluded and magnitude of n = 1.06 0.11 is much larger. The results presented above were all taken a t the lowest mirror velocity u = 0.08 cm/s and the spectra are ratioed against carbon black spectra taken at the same velocity. The
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ANALYTICAL CHEMISTRY, VOL. 02, NO. 18, SEPTEMBER 15, 1990
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9 1.21 W C
0.6
m
n
pellet background
ln 0.8
n
a 0.4
1.4
45pm
U
a
+
1080 cm ' 797 c m '
693cm A 1875 cm
4000
' '
3500
3000
2500
2000
1500
500
1000
Wavenumber (cm ')
Flgve 5. Effect of changing the background (from powder to pelletized carbon-black) on the PA spectra of 5-pm particles.
1.0
m Y
1 .o
0.8 0
200
400 f
600
I
(Hz)
Figure 4. Effect of modulation frequency f on the intensity of the PA signal of various bands for four particle sizes. The PA signals are normalized to unity at the lowest f . The solid lines are drawn connecting the data points. normalized PA intensities of various bands at other velocities (i.e. at different modulation frequencies) are shown in Figure 4 for particles of 0.05-, 5-, 45-, and 260-pm sizes. Again, the spectra are ratioed against carbon black at each new velocity. A systematic trend is evident from these observations. For the smallest particles (0.05 and 5 pm), the frequency dependence is similar to the predictions of the RG model in that the intensities of all the bands decrease as the modulation frequency f increases. However, for the larger 45- and 260-pm particles, the intense bands at 797 and 1080 cm-' have now a positive frequency dependence (Z increases as f increases) whereas the weaker bands at 693 cm-' and 1875 cm-' retain the negative frequency dependence as expected from the RG theory (I, 2 ) . In the discussion section below, we argue that this phenomenon arises from the saturation of the intense bands for the larger particles. The question of a proper background reference material in PAS has been raised in several recent papers (22-24). In Figure 5, we show the effect of changing the background from powder carbon black to pelletized carbon black. Note that the intensities of all the bands are enhanced with the pellet as the background compared to the powder background. Measurements of Z show that all bands are enhanced by exactly the same factor. In Figure 6, we have plotted the frequency dependence of the 1080-cm-' band for the 0.05-pm particles with carbon powder and carbon pellet as backgrounds. The frequency dependence is different in the two cases, although in both cases the signal decreases as the frequency increases. A discussion of these and other results is presented below. DISCUSSION In order to check whether eq 1describes our data of Figure 2, we have plotted the intensity I of the 797-, 1080-, and 1875-cm-' bands as a function of porosity e in Figure 7 and
100
200
400
300
(Hz) Figure 0. Effect of changing the background (from pellet to powder of carbon black) on the frequency dependence of the 1080Gm-' band of 0.05-pm particles of silica. f
1200
600
-5 U
400
0
E
Figure 7. PA intensity I plotted against porosity c for three bands. The solid curves are drawn connectlng the data points for visual aid. However note the near linear dependence on t for the weaker 1875-cm-' band. as a function of e / ( l - t) in Figure 8. It is clear that for the stronger bands at 797 and 1080 cm-', Z varies very nearly as c / ( l - e), and for the weaker 1875-cm-' band, Z varies as c. This is what is predicted by eq 1. This verification of the calculations by McGovern et al. (14) is a major result of this work and provides a quantitative estimate of the amplification factor of the PA signal in powders by the pressure term. For powders for which pCla > D, considerable amplification of the PA signal results. It is noted that in the recent work of Belton et al. (21) on the P A spectroscopy of sucrose and carbon black powders of particle size ranging between 28 and 212 pm, a near linear dependence of the intensity of several relatively weaker bands
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 18, SEPTEMBER 15, 1990
120 I
$
i
~
1
1080 crn ' band of SiO,
4
260pm
4 1200
45rm
1
-
800
-,400
I 0
.
z
0.8 100
300
200
400
500
i
I
10
,
,
,
,
I
,
20
,
1
4
1
30
8
,
f
'0
(Hz)
Effects of modulation frequency f o n the PA intensity of the 1080-cm-' bands for several particle sizes.
Figure 9.
[ G I ( 1-E)J Figure 8. Data of Figure 6 replotted against e / ( 1 - e ) . Now for the 797- and 1080-cm-' bands, the solid lines are least-squares linear fits, whereas for the 1875tm-' band, the solid curve is drawn connecting the data points for visual aid.
on porosity c is evident from their plots. Although they did not interpret their results in terms of the calculations of McGovern et al. (14), we believe that their observations are also supportive of this theory. It would, however, be of interest to extend their measurements to smaller particles. The empirical relation, eq 2, equally well fits our data for the stronger bands (Figure 2) and it allows a very convenient description of the intensity of the signal on the particle size D. However why the exponent n is close to 1/3 for the stronger bands and much different for the weaker band at 1875 cm-' (Figure 3) is not clear at present. An interpretation of the empirical eq 2 is highly desirable. The dependence of the PA intensity on the modulation frequency f (normalized to that at the lowest f , for different bands and different particle sizes (Figure 4) is considered next. For smaller particles (0.05and 5 pm), pus>> D and the volumes occupied by the interstial gas are much larger than those occupied by the particles. Hence the pressure term in eq 1 dominates. Decrease of wS by increasing f decreases the signal but mainly because of the decrease in the pressure term. As discussed by McGovern e t d . ( I 4 ) ,the frequency dependence is then similar to that of homogeneous samples, following the prediction of the RG theory ( I , 2). For the larger particles of 45 and 260 pm, the frequency dependence of the intensity for the strong bands at 797 and 1080 cm-' does not follow the prediction of the RG theory. Instead the signal increases as f increases (Figures 4 and 9). However the weaker band a t 1875 cm-' follows the normal behavior of the decrease in I with an increase in f (Figure 4). Although we do not have a quantitative interpretation of this anomalous result, the following argument is advanced as an explanation. Consider the relative magnitude of k s ,p f l ,and D (Tables I and 11). Note that for the larger particles D ps but F~ < ps for the stronger bands and pug> ps for the weaker bands. The condition for saturation is that p f l < D. The crux of the argument is that the 797- and 1080-cm-' bands are severely saturated and the contribution of the pressure term to the signal for larger particles is also negligible. As f increases, pa decreases leading to a less severe condition for the inequality pug< ps. The increase of I with increasing f then results as we move from a more severe condition of saturation to one of less severity. For the nonsaturated bands such as that a t 1875 and 693 cm-', the normal frequency dependence is observed. The effect of background on the PA signal is considered next in light of several recent publications (22-24) and the
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data presented in Figures 5 and 6. As noted earlier, our measurements show that with carbon black pellet as a background (vis-a-vis carbon black powder), the intensities of all the bands are enhanced by exactly the same factor. This enhancement results from the reduction in the carbon background signal in the pellet compared to that in the powder because of reduction in porosity upon pelletizing. Note that the PA signal is ratioed against the background signal so that the above observation is completely understandable. We have also observed that pelletizing the carbon black for reference material reduces the errors due to the pressure signal in the finely divided powder when the finely divided powder is redistributed in repeated measurements. Carter et al. (22,24) have suggested other solid materials (such as painted substrates, carbon-filled rubber, and solid graphite) as reference materials for PAS. These findings suggest that reference material in PAS may not be limited just to carbon black powder. The frequency dependence of the PA signal for a particular band, viz. that a t 1080 cm-' for the 0.05 pm particles, with carbon black powder and pellet as background, was presented in Figure 6. Although in both cases the signal decreases as the frequency increases, the magnitude of the decrease is less with the pellet as the background, probably because the pressure term is reduced in the pellet a t all frequencies. In conclusion, the intensity of PA signal for powders is strongly dependent on the particle size (or equivalently the porosity of the sample), and we have demonstrated that this dependence is satisfactorily explained by the theory McGovern et al. Also the effect of modulation frequency on the PA signal of various particle sizes has been explained qualitatively. We have also shown that pelletized carbon black gives higher PA intensity and it may be a better reference material for a number of cases. From this work it is evident that quantitative analysis of powders by PAS is possible only by properly taking into account the particle size dependence of the PA signal or by using powders of exactly the same particle size. The latter situation poses a major experimental difficulty so that recourse to the analysis by using the theory of McGovern et al., as e.g. done here, appears unavoidable.
ACKNOWLEDGMENT Several useful discussions with Dr. W. E. Wallace are gratefully acknowledged. Registry No. Si02, 7631-86-9; vitreous silica, 60676-86-0. LITERATURE CITED (1) Rosencwaig, A.; Gersho. A. J . Appl. Phys. 1976, 4 7 , 64-69. (2) Rosencwaig, A. Photoacoustics and Photoacoustic Spectroscopy; John Wiley and Sons: New York, 1960. (3) Low. M. J. D.; Parodi. G. A. Appl. Spectrosc. 1980, 3 4 , 76-80.
Anal. Chem. 1990, 62, 1947-1953 (4) Yang. C. Q.; Ellis, T. J.; Bresee, R. R.; Fateley. W. G. Po/ym. Meter. SCi. €ng. 1985, 53, 169-175. (5) Teramae, N.; Tanaka. S. Appl. Spectrosc. 1985. 39, 797-799. (6) Urban, M. W.; Koenig, J. L. Appl. Spectrosc. 1988, 4 0 , 994-997. (7) Yang, C. Q.; Bresee, R. R.; Fateley, W. G. Appl. Spectrosc. 1987, 4 1 ,. 889-896. ... ...
(8) Zerlia, T. Appl. Spectrosc. 1988, 4 0 , 214-217. (9) Carter 111, R . 0.: Paputa Peck, M. C. Appl. Spectrosc. 1989, 43, 46%-473. (10) Michaelian, K. H. Appl. Spectrosc. 1989. 43, 185-190. (11) McClelland, J. F.; Kniseiey, R. N. Appl. Phys. Lett. 1978, 26, 467-469. (12) Krishnan, K. Appl. Spectrosc. 1881, 35, 549-556. (13) Monahalin, J.-P.; Bertrand, L.; Rousset, G.; Lepoutre, F. J . Appl. Phys. 1984, 56, 190-210. (14) McGovern, S. J.; Royce, E. S. H.; Benziger, J. E. J. Appl. Phys. 1985, 5 7 , 1710. (15) King, D.; Davidson, R. S.; Phillips, M. Anal. Chem. 1982, 5 4 , 2191-2194. (16) DavMson, R. S.; King, D. Anal. Chem. 1984, 56, 1409-1411. (17) Rockley, N. L.; Woodard, M. K.; Rockley, M. 0.Appi. Spectrosc. 1984, 36, 329-334.
I947
(18) Yang, C. Q.; Fateley, W. G. J. Mol. Struct. 1988, 141, 279-284. (19) McGovern, S. J.; Royce, E. S. H.; Benziger, J. E. Appl. Surf. Sci. 1984, 18, 401-413. (20) Seehra, M. S.; Pandurangi, R. S. J . Phys.: Condens. Metter 1989, 1. 5301-5304. (21) Beiton, P. S.; Wilson, R. H.; Saffa. A. M. Anal. Chem. 1987, 59, 2370. (22) Carter 111. R. 0.; Paputa Peck, M. C.; Samus, M. A,; Klllgoar, P. C., Jr. Appl. SpeCtrOsC. 1989, 43, 1350-1354. (23) Vidrine. D. W. Appl. Spectrosc. 1980, 3 4 , 314-319. (24) Carter 111, R. 0.:Paputa Peck, M. C. Appl. Spectrosc. 1989. 43, 468-473. (25) Pandurangi, R. S.; Seehra, M. S.; Razzaboni, E. L.; Bolsaitis, P. € m i ron. Health Perspect. in press.
RECEIVED for review February 1,1990. Accepted May 31,1990. This work was supported in part by grants from the US. Bureau of Mines under the Generic Technology Center for Respirable Dust (Grant No. G1135142).
Chemical Control of Reaction Time in an Enzyme Assay and Feasibility of Enzyme Spot Tests J a n e N.Valenta’ a n d Stephen
G.Weber*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Robert C.Elser
York Hospital, 1001 South George Street, York, Pennsylvania 17405
There are many circumstances in which the understanding of a patlent’s status would be Improved by knowing one or mare enzyme actlvltles. Such data are routinely produced in cllnIcal iaboratorles, but simple, nonlnstrumental tests for enzymes are a rarlty, so thelr extralaboratory determination Is also rare. The essentlal problem Is that effectlve cilnlcal determlnatlons of enzyme actlvltles are typlcally carried out by measwing readon rates, 80 the reaction time needs to be controlled. The reactlon tlme of a sample can be controlled by using a passlve, ion exchange-based tltratlon. I n this work, OH-, H+, and qulnidlne have been used to stop the enzymes LDH (EC 1.1.1.27) (wlth H+ and OH-) and chollnesterase (EC 3.1.1.8) (with quinldlne). The Ion exchange materlal containing the enzyme-stopplng ion is separated from the sample by a fllter. The sample contains ions that can exchange wlth the enzyme-stopping Ion In the ion exchange materlai, and lt may contain species that buffer the enzymestopplng ion. The reactlon time is governed by the exchanging Ion’s concentration in the sample, the quantity of buffer In the sample, the thickness of the filter between the Ion e x c h a w material and the sample, and the temperature. A test for LDH requiring 50 pL of serum and no Instrumentation can be made so that resulls from sera wlth elevated levels appear different than those wlth normal levels.
INTRODUCTION The philosophy behind the development of sensors and spot tests is that analyses can be performed by nonprofessionals. Current address: PPG Industries, Inc., Research Center, P.O. Box 9, Allison Park, PA 15101. 0003-2700/90/0362-1947$02.50/0
Functions ordinarily carried out instrumentally should be carried out chemically in an extralaboratory analytical device (EXLAD). For example, the sequence of mixing reagents, ordinarily carried out by other means in clinical laboratory apparatus, can be achieved by creating a series of reagent layers through which sample seeps (1-3). In order to perform quantitative determinations of catalysts, and in particular enzymes, with an EXLAD, a means for the control of timing is required. This is in distinct contrast to the situation for most analytes; EXLAD’s for noncatalytic solutes yield a steady signal (e.g., color) after a brief incubation period and during some long time determined by the stability of the signal. What would be ideal for catalyst EXLAD’s is some method that irreversibly destroyed the catalyst after a certain time. Physical methods, such as heating or cooling, have the advantage of being conceptually simple, electronically controllable, and fast. On the other hand they would draw considerable power and would influence other reactions, such as color-forming reactions. Such means would also be reasonably expensive to the user. Enzymes typically have pH optima, and there are generally ranges of pH over which an enzyme activity is much less than the maximum. This leads to a method for turning off enzyme activity after a certain time-the addition of acid or base. The slow addition of H+ or OH- to a buffered solution containing an enzyme will gradually alter the pH of the solution, thus causing the activity of the enzyme to decrease. The ultimate goal of an EXLAD requires that the addition be simple, safe, and operable under a variety of environmental conditions with low power. A noninstrumental, passive acid-base titration has been developed for such devices. The source of H+or OHfor the passive titration is an ion exchange material. There are cases in which an alteration in pH would be inappropriate, for example in a case where the substrate, 0 1990 American Chemical Society
