Anal. Chem. 1991, 63,343-348
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Direct Analysis of Solid Powder Biological Samples Using a Magnetron Rotating Direct-Current Arc Plasma and Graphite Furnace Sample Introduction David S l i n k m a n ' and Richard Sacks* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109
Solid powder samples deposited in a graphite tube furnace are vaporized and introduced into a magnetron rotating direct-current arc plasma. The arc plasma has the form of a radial current sheet that completely covers the end of the graphite anode. The sample vapor from the graphite furnace is forced to pass through the current sheet. This ensures adequate sample-plasma interaction and resuits In detection limits in the mlcrogram/gram range. Anaiytlcai data will be presented for pure solid powder samples and for a number of NIST Mologlcal reference materials. Percent error values for the determlnatlon of various metallic elements using the method of standard additions ranged from 1.2% to 19.6%. Absolute detection limits at the subnanogram level are reported.
While most direct-current plasma (DCP) research has focused on solution sample analysis, recently there has been increased interest in the analysis of direct solid samples. The advantages of solid sampling include the saving of time and effort in the preparation of samples as well as the reduced risk of sample contamination and loss due to the sample preparation methods. A number of authors (1-3) have used various forms of sample digestion and dissolution to determine various metals by DCP in complex matrices including animal tissue (4),plant tissue (5), aircraft lubricating oils (6),and environmental samples (7). While these methods produce adequate results, the complicated and time-consuming chemical pretreatments can be very cumbersome. Fry and co-workers have investigated a variety of solid samples by nebulizing a slurry of suspended solids into the DCP (8-10). In this method, the solids are milled and suspended in a solution that is introduced into aclog-free Babington-type nebulizer. Laser ablation DCP spectrometry has been used by Sneddon and co-workers to sample metallic species in solid and pelletized powdered samples ( I 1,12). In this work, a low-energy Nd:YAG laser ablates the sample, which is then carried to the DCP by a flow of Ar. Other forms of direct solid sampling include the introduction of a solid powder in a gas stream to a two-jet plasmatron (13)and the use of a ceramic nebulizer with a plastic spray chamber as a means of introducing dilute aqueous suspensions into a DCP (14).
Slinkman and Sacks (15,16) recently described a magnetron rotating DCP device that combines a hollow cylindrical graphite anode, a coaxial W / T h (4%) cathode wire, and a magnetic field parallel to the electrode axis to produce a rotation of the current channel in a motorlike fashion between the wire cathode and cylindrical anode. A t sufficiently high rotational frequencies, the current channel assumes the form of a relatively uniform current sheet, which covers the end of the graphite tube anode. A tail flame extends for several
* Corresponding author.
Present address: Nalco Chemical Co., Naperville, IL. 0003-2700/9 1/0363-0343$02.50/0
millimeters above the current sheet. Sample is introduced by passing the aerosol or vapor through one end of the anode tube and forcing it to pass through the plasma tail flame a t the other end of the tube. The relatively short sample residence time in the hightemperature current sheet makes the device more useful for solution samples introduced from a graphite furnace (17) than for solutions nebulized into the plasma (16). The interface of the graphite furnace to the magnetron rotating DCP also makes possible the introduction of solids. The analysis of solids has been carried out for years in graphite furnace atomic absorption spectrometry (18-20) with samples ranging from soil (21) to bovine liver (22). Electrothermal atomization has also been used to introduce solids into the ICP (23-25). An electrothermal atomizer has also been interfaced to a DCP to determine gold in solid algal cells (26). This paper describes the application of the magnetron rotating DCP with graphite furnace sample introduction to the analysis of solid samples. Both pure metal powder samples and NIST biological reference materials were studied. Percent recovery data for both types of solid samples and detection limits for the determination of various metals in the NIST reference materials are presented.
EXPERIMENT DESIGN AND APPARATUS Rotating Arc Design and Furnace Interface. The principle of operation of the magnetron rotating direct-current arc plasma has been presented in detail (15). Figure 1shows a diagram of the magnetron rotating DCP, the graphite furnace, and the optical system used for this study. A radial electric field is generated in the arc plasma by the use of a cylindrical electrode geometry consisting of a W/Th (4%) wire cathode (W) and a coaxial graphite tube anode (A). The electrode axis is vertical, and an argon gas flow through the anode tube (Arl) results in the arc forming between the cathode tip and the top inner edge of the anode tube. When this is combined with a coaxial magnetic field generated by a ceramic ring magnet (M), the E X B drift motion of the plasma electrons is in the azimuthal direction, and one end of the arc current channel rotates around the inner edge of the anode tube. The resulting plasma (P) is a diffuse, stable plume that covers the end of the anode cylinder. A ceramic insulator tube (T) prevents internal arcing. This system has been described in detail (15, 16). A graphite tube furnace (F) (InstrumentationLaboratory Model 555) was interfaced to the rotating DCP (17). A 7.5-cm-long, 3.0-mm-i.d.ceramic tube was placed in one end of the graphite furnace. Two pieces of Tygon tubing (9 cm long, 3-mm id.; and 6.5 cm long, 9-mm i.d.) connected the other end of the ceramic tube t o the arc assembly. The sample was deposited into the furnace through the sample door (S). The argon sample vapor transport gas (Ar2) was introduced into the furnace chamber and flowed into the open end of the furnace to carry the sample vapor through the ceramic interface (I) and into the arc. The graphite furnace and DCP are powered by power supplies PS2 and PSI, respectively. Construction details and experimental conditions are found in Table I. Optical and Electrical Monitoring. The monochromator conditions and the data collection procedure have been described previously (17). The electrode assembly was oriented with the 0 1991 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 63,NO. 4, FEBRUARY 15, 1991
Schematic representation of instrumental setup. P, plasma; M, magnet; A, cylindrical hollow anode; T, ceramic insulator tube; W, W/Th cathode; Arl, Ar cooling gas; PS1, arc power supply; I, interface tube; F, graphite furnace; Ar2, Ar sample vapor transport gas; S. sample inlet; PS2. graphite furnace power supply; M 1 and M2, over-and-under mirror configuration; V, spectrometer entrance slit; H, horizontal mask; G, monochromator; C, computer; R, analtye ring. inset on left (a) shows the view from above the arc and the horizontal viewing axis (E). Inset on right (b) shows t h e side view of the arc and the vertical observation window (H). Figure 1.
Table I. Construction Details and Experimental Conditions
rotating arc anode
graphite cylinder (Ultra Carbon Type IIF/4S), S-mm i.d., 8-mm ad., 18 mm long cathode W/Th (4'30) rod, 1-mm diam cathode insulator A1203 tube, 3.0-mm i.d., 5.0-mm o.d., 25 mm long current/voltage 12 A/74 V gas flows 4.8 I,/min Ar cooling gas, 1.0 L/min Ar sample vapor transport gas magnet ceramic alloy, 22-mm i.d., 60-mm o.d., 1%" thick; peak field strength (vertical component) 0.90 kG furnace tube pyrolytic graphite, 5.0-mm i.d., 6.4-mm o.d., 88 mm long gas flow
interface
1.0 L/min Ar A1203 tube, 3.0-mm i.d., 5.0-mm ad., ra-mm-long Tygon tube, 3-mm i.d., 9 cm long plus 9-mm i.d., 6.5 cm long -?
electrode axis in the vertical direction, parallel to the spectrometer slits. The image-transfer system used two 50-mm-diameter, 500-mm focal length spherical front-surface mirrors, M1 and M2, in an over-and-under configuration. The system had a lateral magnification of 2.0. The arc device was mounted on a translational stage, permitting the selection of both vertical and horizontal (normal to the optical axis) observation coordinates. The plasma was viewed through a segment of the magnet that was cut out. The plasma was focused onto the vertical entrance slit (V) and the horizontal mask (H). The observation zones in the plasma are shown by the two insets. The side view of the arc (inset b) shows the vertical observation window (H), extending from the anode surface to 5 mm above the anode surface. The view from above the arc (inset a) shows the horizontal observation zone (E; optical axis). Previous studies (16) have shown that the sample appears to penetrate the plasma near the anode surface.
Table TI. Furnace Temperature Program
element
ashing temp, "C
atomization temp, "C
Fe
1400 1400
2400 2500 1800 2300 1700 2200
Ni
Zn CU Mg
Mn
700
1000 900 1400
The result is that the greatest analyte line intensities are found in a ring just above the inside anode wall. This is shown more clearly in inset 8, where R represents the ring of greatest analyte emission. Materials and Procedures. Both solid particle and solution residue samples were evaluated. Particle sizes of 5-10 and 20-30 pm of pure Mn and Ni were used for these studies. These size fractions are operationally defined as the portion of sieved powder retained by a specific size sieve and passed by another size sieve. The preparation of the NIST biological reference materials has been described in detail (27). The average particle size for these samples was reduced to less than 30 pm. Atomic absorption standards (1000 pg/mL, Aldrich, Milwaukee, WI) were used to prepare solution samples. Lower concentration solutions were prepared daily by serial dilution with distilled, deionized water. The powder standards and NIST materials were weighed on a microbalance and suspended in 0.2% HNOP These suspensions were then mixed with an ultrasonic bath and a vortex mixer. Typically, 30-pL aliquots of suspensions were drawn and deposited into the graphite furnace while the suspension was still thoroughly mixed. The furnace temperature program consisted of a 1.0-min desolvation step at 100 "C followed by a 5.0-s ashing step and a 5.0-s atomization step. The temperature value that resulted in the highest signal-to-background ratio for each element is listed in Table IT. No visible condensation of analyte was observed along the transport lines. The initiation of the furnace heating cycle did not result in a major change in the plasma structure. The
ANALYTICAL CHEMISTRY, VOL. 63, NO. 4, FEBRUARY 15, 1991
160 140
345
1
0.2
0.4
0.6
0.8
Cancentration
1
1.2
1.4
1.6
(us)
Figure 3. Analytical curves for Mn and Ni powder samples. I n both cases, 5-10-pm powder samples were used. Each point represents the average peak area from four experiments.
0
1
2 3 Time (seconds)
4
5
Flgure 2. Emission wave forms for the Mn(I1) 259.4-nm line (a) and the Ni(1) 305.0-nm line (b). All samples were present at the 1.2-pg level. I n plots a, A is the signal from 20-30-pm Mn powder samples and B is from Mn(NO,), solution residue samples. In plots b, A is the signal from 20-30-pm Ni powder samples and B is from Ni(NO,), solution residue samples.
procedure for data manipulation has been described previously (17).
RESULTS AND DISCUSSION Study of Pure Solids. Emission intensity vs time wave forms from pure solid powders were evaluated to test the ability of the furnace-DCP system to vaporize and excite the solids efficiently. Metal powders with particle sizes of 20-30 pm were vaporized in the furnace, and the emission wave forms were compared to wave forms from a corresponding amount of solution sample for the same metal. The emission wave forms are shown in Figure 2 where the relative intensity is plotted as a function of time for the Mn samples (a) and the Ni samples (b). The plots labeled A are for the solid powder samples, and plots labeled B are the signal from the solution samples. For both metals, the solution samples were prepared from the nitrate salts. The solid powder suspension and solution samples were prepared so that a 20-pL aliquot contained 800 ng of either Mn or Ni. The Mn plots in Figure 2a show that the powder wave form and the solution wave form are very similar. The peak areas from 3 to 5 s show that the powder sample signal is only 1.3% larger than the solution signal. The Ni wave forms in Figure 2b show that the solid powder waveform A is symmetric, while the solution waveform B has a shoulder on the rising edge of the peak. The slight difference in peak structure could be an indication of different rates of atomization for the two sample forms. The peak area integrals from 3 to 5 s show that the signals are within 1% of each other. These results also indicate that aqueous solution standards could be used in establishing calibration curves for the direct
elemental analysis of these solid powder samples. This would eliminate the need for using extremely small masses of powder samples and the errors due to powder-sampling statistics. Analytical curves for both the Mn and Ni solid powder samples are shown in Figure 3. Particle sizes of 5-10 pm were used to reduce sampling errors. Each point is the average of a t least four determinations. Correlation coefficients for both curves are greater than 0.999. The relative standard deviation (RSD) for most points was below &8%; however, slightly larger values were obtained for the lower concentration samples. Biological Sample Study. The rotating DCP-furnace technique was used for the analysis of six NIST solid powder biological reference materials, including pine needles (SRM 1575), oyster tissue (SRM 1566), bovine liver (SRM 1577), citrus leaves (SRM 1572),tomato leaves (SRM 1573), and rice flour (SRM 1568). These materials were chosen to represent a wide range of compositions. Figure 4 shows the rotating DCP emission wave forms for an Fe(I1) line in tomato leaves (a), pine needles (b),and bovine liver (c). Emission wave forms labeled A are average intensities from NIST reference materials, and wave forms labeled B are average intensities from equal amounts of Fe (with respect to the corresponding NIST reference material) present in aqueous solutions of Fe(NO& In all cases, peak area values are larger for the solid powder sample than for the solution sample. The enhancement factors, defined as the ratio of the Fe signal in the solid powder sample to the signal in the solution sample, are 1.7, 1.5, and 2.0 for plots a, b, and c, respectively. The solid powder peaks not only are higher but are also wider than their corresponding solution peaks. All of the peaks are fairly symmetric except for the bovine liver sample in c where there is a significant decrease in the Fe(I1) intensity from 3 to 3.5 s. This is probably the result of concomitant effects. Previous work with the magnetron rotating DCP has shown that concomitant species can have a large effect on analyte signals (28,29). I t has been shown that the addition of easily ionized elements (EIEs) to the rotating DCP can increase the temperature of the plasma up to 1000 K and can double the electron density (28). This results in an increase in the analyte signal for most elements (29). A decrease in the background intensity was also observed with the addition of an EIE. The NIST reference materials used here all contain significant amounts of EIEs, and these elements are probably the cause of the results in Figure 4. The most prevalent concomitant species are K, Ca, and Na with weight percents nearing 5% for some of the samples. For the samples used in Figure 4, the tomato leaves contained 4.46% K and 3.00% Ca, the pine needles 0.41% Ca and 0.37% K, and the bovine liver 0.996% K and 0.243% Na. On the basis of previous work
346
ANALYTICAL CHEMISTRY, VOL. 63,NO. 4, FEBRUARY 15, 1991 0.35 r
1.8
--
1
1.2 1.6
0.3
1.4
0.25
0.2
A
1 -
-
0.15
0.8
"r
0.6 0.4 -
0.2 0 -0.9 -
0.1
0.05
0
0.8
-
0.6
-
0.7 -
0.04
0.4 0.3 0.5
0.02 0 r
0.2 0.1
-
01.2 1.1 1 0.9 0.8
0.7 0.6
0.5 0.4
0.3 0.2
Time (seconds) Figure 4.
Emission wave forms from NIST reference materials for the
Fe(I1) 259.9-nm line. (a)Tomato leaves, (b) pine needles, (c)bovine liver. In all three sets, curve A is the emission wave form from Fe in the N E T reference material (solid powder),and curve B is the wave form from an equal amount of Fe as Fe(N0J2. (29),the concentrations of concomitant species in these samples could easily account for the enhancement effects as well as the decrease in background intensity observed. Figure 5 shows Mg(1) wave forms for 1.0 pg/mL amounts of Mg in oyster tissue (a), citrus leaves (b), and bovine liver (c). Once again, the wave forms labeled A are average intensities from the NIST reference materials, and wave forms labeled B are average intensities from equal amounts of Mg present in aqueous solutions of Mg(NO,),. The Mg(1) plots for the NIST samples also show large concomitant enhancement factors with values of 3.0, 1.5, and 3.6 for plots a, b, and c, respectively. The concomitant concentrations for oyster tissue are 0.15% Ca, 0.969% K, and 0.51% Na, and for citrus leaves, they are 3.15% Ca and 1.82% K. Also note in Figure 5 that there is a temporal shift in the Mg(1) wave forms between the solid and solution samples. In all cases, the NIST reference material peak comes before the solution peak. This is seen dramatically in the oyster tissue (a) and the citrus leave (b) plots with the solid sample peak intensity value coming over 0.5 s before the corresponding solution sample peak intensity. A much smaller change occurs in the bovine liver (c) wave form. This suggests that the sample matrix is influencing the temporal release of the Mg during atomization in the furnace and acting as a matrix modifier in increasing the volatility of the Mg. The matrix-effect data from Figures 4 and 5 as well as data from other elements in the NIST samples are presented in Table 111. The lowest enhancement factor of 1.1 was obtained for Zn(1) in rice flour, while the highest enhancement factor of 3.6 was obtained for Mg(1) in bovine liver. Note that the concomitant concentrations in the rice flour are very low with only 0.112% K and 0.014% Ca. These enhancement factors suggest that it would be difficult to use aqueous solutions as standards for the NIST reference
0.1 0
Emission wave forms from N E T reference materials for the Mg(I1)285.2-nm line. (a) Oyster tissue, (b) citrus leaves, (c) bovine liver. In all three sets, curve A is the emission wave form from Mg in the NIST reference material (solid powder),and curve 6 is the wave form from an equal amount of Mg a s Mg(NO&. Figure 5.
Table 111. Concomitant Effects Obtained with Biological Samples
concn, rglmL
concomitant enhancement factor
sample
A, nm
oyster tissue SRM 1566
213.8
2
2.4
285.2 279.5 259.9 285.2
1 1 1 1
3.0 2.6 2.1 3.6
259.9 259.4
1
2
2.0 2.4
259.9 285.2
0.6
1.5 1.5
279.5 259.9
2
1.9 1.7
213.8
0.4
1.1
bovine liver
SRM 1577 pine ne ed 1es
SRM 1575 citrus leaves SRM 1572 tomato
1 1
leaves
SRM 1573 rice flour
SRM 1568 materials. Two attempts were made to reduce the matrix enhancement effects so t h a t it would be possible to use solution standards. The first attempt involved using the ashing step t o volatilize selectively the concomitant species. The second involved matrix matching. It was found that most K solutions were volatilized with the ashing temperatures used; however, the Ca and Na solutions were not. T h e ashing
ANALYTICAL CHEMISTRY, VOL. 63, NO. 4, FEBRUARY 15, 1991
Table IV. Determination of Elements in NIST Reference Materials sample
element line, nm
bovine liver
Zn(1) 213.8 Mn(I1) 259.4 Zn(I) 213.8
SRM 1577
Cu(1)
tomato leaves
SRM 1573
324.7 MdI) 285.2 Fe(I1) 259.9 Mn(I) 259.4 I
pine
needles
SRM 1575 oyster tissue
SRM 1566 citrus leaves SRM 1572
.
.
ZnU)
213.8 Mg(1) 285.2
certified value, r g / g
sample 70 error
*6
73
* 7.3
+17.7
238 f 7
218
* 15.8
-8.4
62
Table V. Detection Limits Obtained for NIST Reference Materials
found value,
rglg
citrus leaves
SRM 1572 tomato
A, nm
SRM 1573
123 f 8
125 i 3.4
+1.6
158 f 7
189 i 1.9
+19.6
560 f 23.5
-6.7
SRM 1577
200 f 10
192 f 3.1
-4.0
pine
675 f 15
651 i 41.7
-3.5
852 f 14
865 i 100.3
+4.7
5800 f 174
5869 f 255.2
* 15
+1.2
temperatures needed to clean the system of Ca and Na often lead to a major loss in the analyte as well. Matrix matching was attempted with the NIST materials that had the lower concentrations of concomitant species. A bovine liver sample and a corresponding solution sample were made 0.04 M in Na and 0.001 M in Ca. These concentrations are over an order of magnitude higher than the concentrations in the solid sample so that any enhancement seen should be due almost entirely to the added matrix modifiers. The enhancement factors did decrease with the use of the matrix modifiers, but the values were not acceptable. Without the matrix modification, a Mg(I1) line in bovine liver showed an enhancement factor of 2.8, and with the modified matrix, the enhancement factor decreased to 1.6. Similar decreases in the enhancement factor were observed for Cu(1) and Zn(1) lines; however, the values were still significantly greater than 1.0, suggesting that matrix effects are associated with the atomization processes in the furnace. These severe matrix effects also have been seen in the analysis of similar samples with graphite furnace ICP techniques (24). Due to the observed matrix effects, determination of metallic elements in the NIST reference materials was carried out by the standard addition method. Table IV shows the results for standard additions of various elements in five NIST reference materials. Seven of the nine determinations have errors of less than l o % , with the largest error being about 20%. These percent error values compare favorably to similar determinations in biological samples using graphite furnace techniques ( 1 4 2 1 )and graphite furnace ICP techniques (23, 24). These encouraging results demonstrate that, even for difficult sample matrices, standard addition methods can be used with the rotating DCP to determine metallic elements in solid powder samples. The limits of detection achieved for the NIST solid samples for various elements are listed in Table V. Detection limits are defined as the concentration producing a net analyte emission intensity equivalent to 3 times the standard deviation of background emission intensity. The detection limits are in the microgram/gram range with absolute detection limits in the nanogram and subnanogram range. It is interesting to note that for both Mg(1) and Mn(I1) there is an order of magnitude difference in detection limits in the different NIST reference materials, while the Zn(1) detection limits show very little dependence on the sample. The detection limits for Mn(I1) in pine needles is 6.4 pg/g, and in
bovine liver
needles SRM 1575 oyster tissue SRM 1566
concn, pg/g abs amt, ng
285.2
7.8
0.122
259.4
0.5
0.122
213.8 259.9 285.2
0.4 17.2 4.3
0.180 3.000 0.090
324.7 213.8 259.4
0.7 0.8 6.4
0.300 0.300 1.000
259.9 285.2
5.3 0.2
1.320 0.010
213.8
2.8
0.210
leaves
600
347
tomato leaves, the value drops to 0.5 pg/g. The concomitant species concentrations in the tomato leaves are nearly an order of magnitude higher than in pine needles, and increased signal enhancement caused by the greater concentrations of EIE in the matrix could account for the improved detection limits. The invariance of the detection limits for Zn(1) could result from Zn not being as susceptible to matrix effects as the other elements. Previous work (29)with solution samples showed that Zn signals were relatively independent of the presence of an EIE while Mg and Mn signals showed relatively large EIE enhancements. The detection limits achieved with the rotating DCP-furnace system compare very well with those obtained by other forms of solid sample introduction into the DCP. The 20 and 10 pg/g detection limits for Cu in laser ablation DCP (12) and by sample dissolution techniques ( I ) , respectively, are significantly greater than the 0.7 pg/g limit achieved in this work. The 3 pg/g detection limit for Mn in the two-jet plasmatron (13) is close to the 6.4 pg/g limit obtained in this study. Detection limits as low as 10 ng/g have been obtained for Fe in a solid powder sample with an electrothermal ICP system (25);however, the significantly better detection limits achieved with the ICP may be offset in part by the lower initial and operating costs of the magnetron rotating DCP system. The preliminary study presented here demonstrates that solid powder biological samples can be vaporized, atomized, and excited in the magnetron rotating DCP with graphite furnace sample introduction. With a few minutes of grinding as the only sample preparation needed, this method provides a rapid and direct method for trace and minor components analysis in a variety of biological materials. The ability to use solution standards with the standard addition method for the direct analysis of solids also eliminates the need for preparing powder standards, which can be difficult, timeconsuming, and lead to significant sampling errors with lowconcentration standards.
LITERATURE CITED (1) Natansohn, S.; Czupryna, G. Specfrochim. Acta 1983, 388, 317. (2) Bankston, D.C.; Fisher, N. S . Anal. Chem. 1977, 4 9 , 1017. (3) Potter, N. M.; Vergosen, H. E., 111. Talanta 1985. 32, 545. (4) Frank, A,; Peterson, L. R. Spectrochim. Acta 1983, 386, 207. (5) Lajunen, L. H. J.; Kubin, A. Talanfa 1986, 33, 265. 161 Brown. J. R.: Saba. C. S.:Rhine. W. E.: Eisentraut. K. J. Anal. Chem. 1980, 52, 2365. (7) Grogen, W. C. Specfrochim. Acta 1983, 386, 357. (8) McCurdy, D. L.; Wichman, M. D.; Fry, R. C. App/. Specfrosc. 1985, 39,984. (9) ig88, ~. Wichman. M. D.:FN. R. C.: Hoffman. M. K. ADD/. . . S~echOSc. . 40, 351. (10) Vim, S.H.; Fry, R. C. Appl. Specfrosc. 1988, 42. 381. (11) Mitchell, P. G.; Sneddon, J.; Radziemski, L. J. Appl. Specfrosc. 1986, 4 0 , 274. I
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(12) Mitchell, P. G.; Sneddon, J.; Radziemski, L. J. Appl. Spectrosc. 1987, 4 1 , 141. (13) Yudelevich, I. G.; Cherevko, A. S.; Engelsht. V. S.;Pikaiov, V. V.; Tagiltsev. A. P.; Zheenbajev, Zh. Spectrochlm. Acta 1984, 3 9 8 , 777. (14) Derie, R. Anal. Chim. Acta 1984, 166, 61. (15) Slinkman. D.; Sacks, R. Appl. Spectrosc. 1990, 4 4 , 76. (16) Slinkman, D.; Sacks, R. Appl. Spectrosc. 1990, 4 4 , 83. (17) Slinkman, D.; Sacks, R. Anal. Chem. 1990, 6 2 , 1656. (18) Rettberg, T. M.; Holcombe, J. A. Anal. Chem. 1986, 58, 1462. (19) Langmyhr. F. J. Anayst 1979, 104, 993. (20) L'vov, B. V. Talanta 1976, 2 3 , 109. (21) Vollkopf, U.; Grobenski, 2 . ; Tamm, R.; Welz, 6. Anahst 1985, 710, 573. (22) Chakrabarti, C. L.; Karwowska, R.; Hollobone. B. R.; Johnson, P. M. Spectrochim. Acta 1987, 428, 1217.
(23) Bhkemore, W. M.; Casey, P. H.; Collie, W. R. Anal. Chem. 1984, 56, 1376. (24) Aziz, A.; Broekaert. J. A. C.: Leis, F. Spectrochim. Acta 1982, 3 7 8 , 369. (25) Reisch, M.; Nickel, H.; Mazurkiewicz, M. Spectrochim. Acta 1989, 4 4 8 , 307. (26) Greene, B.; Mitchell, P. G.;Sneddon, J. Spectrosc. Lett. 1986, 19 (2), 101. (27) Brewer, S. W., Jr.; Sacks, R. D. Anal. Chem. 1988, 6 0 , 1769 (28) Slinkman, D.; Sacks, R. Appl. Spectrosc., in press. (29) Slinkman, D.; Sacks, R. Appl. Spectrosc., in press.
RECEIVED for review August 9, 1990. Accepted November 5, 1990.
Quantitation of Acidic Sites in Faujasitic Zeolites by Resonance Raman Spectroscopy Robert D. Place' and Prabir K. Dutta* Department of Chemistry, The Ohio State University, 120 W. 18th Avenue, Columbus, Ohio 43210
This paper examines the selective excitation of the Raman spectra of dye molecules adsorbed on acidic zeolite surfaces. By taking advantage of the strongly allowed transitions In these dye molecules (large extinction coefficients) and the different absorption maxima of the conjugate acid and base forms of the dye, selective enhancements of the Raman bands specific to each form can be obtained. The focus has been on the dye molecule, 4 4 phenyiazo)diphenyiamine (PDA), adsorbed onto the faujasitic zeolite, Nay. A calibration curve of Raman Intensity (peak area) versus number of protons In supercages was obtained. Because of the inner finer effect, at loadings significantly greater than 1 proton per supercage, the Raman intensity was found to decrease. The sensitivity of the Raman method at low proton loadings appears to be considerably better than the typical infrared methods used to estimate acidity on catalyst surfaces.
INTRODUCTION The acidic properties of zeolites play a central role in their catalytic behavior in a wide variety of chemical- and petroleum-related processes ( I , 2). Considerable research has been done in developing methods for measurement of distribution of acidic functionalities in these and other solid materials. Amongst the classical methods for measuring acidic properties of solid acid surfaces are color changes of indicators adsorbed on surfaces and butylamine titration of the surface in the presence of Hammett indicators (3,4). Infrared spectroscopy of basic molecules such as ammonia and pyridine adsorbed onto the acidic sites also provides for quantitative estimation of Bronsted and Lewis acid sites (5). The attractive feature of the Hammett indicator method is that the distribution of acid strengths in zeolites can be studied by choosing indicators of various pKa's. The cautionary aspect of this method is to choose dyes that can penetrate into the zeolite supercages through the 12-membered-ringopenings (-7-8 A). However, there are several shortcomings of the Hammett indicator approach. First, in order for the dyes to be successfully used, I Permanent address: Department of Chemistry, Otterbein College, Westerville, OH 43081.
there must be a perceptible change in color, implying that the spectral shift between the acid and the base forms be significant. This is especially a problem with lower pKa (el) indicators. In addition, there is often a change in color due to adsorption effects alone (6-8). These problems can be circumvented to some degree by obtaining the electronic spectra of the solid (9,lO). However, because of significant band broadening on the solid surfaces, often there is overlap in the electronic spectra between the acid and basic forms. New electronic bands may also appear in the spectrum, e.g., 4-nitrotoluene exhibits only one band at 380 nm in acidic solutions, but three bands were reported a t 300,340,and 500 nm on an acidic aluminosilicate surface (11). We reasoned that it may be possible to use the resonance Raman effect in order to distinguish between the acid and base forms, as well as to quantitate the amount of acidic form (and hence the acidity of the solid surface) from the Raman signal. In this paper, we illustrate this principle by using the dye molecule 4-(phenylazo)diphenylamine(PDA, see below) and various loadings of protons for the acidic form of zeolite Y.
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Resonance Raman spectroscopy involves excitation into an electronic band of the chromophore, which can result in enhancement of the Raman signal by orders of magnitude. Since the Raman spectrum is a vibrational signature of the chromophore and the Raman bandwidths are typically 10 cm-I, this technique provides the potential for high selectivity and sensitivity (12). The latter can be particularly high in the case of dye molecules, since the extinction coefficient, or absorptivity, of these molecules is large and the resonance Raman intensity scales as the square of this parameter (transition moment terms in the numerator for the dipole-allowed transition in the A term) (13). In principle, this makes it possible to examine zeolite materials with low levels of acidity. In addition, since laser excitation is readily available in a continuously tunable fashion ranging from the near ultraviolet to the visible, the choice of dye molecules is also clearly no longer restricted (14). We illustrate below some of these features of resonance Raman spectroscopy using the dye
0003-2700/91/0363-0348$02.50/0 0 1991 American Chemical Society
