1918
Anal. Chem. 1991, 63, 1918-1926
Though this technique of immobilization has so far been applied to the immobilization of enzymes over pH glass electrodes, it could be extended to other support surfaces of transducers commonly used for the construction of biosensors, such as treated metal, fibrous material, and artificial membranes. The technique is presently being studied for the immobilization of enzymes over optical fibers. Since the most striking feature of the technique lies in the ability to form thin deposits, it provides immense scope for the construction of reaction surfaces in flow-through bioreactors or typical batch fermentors.
LITERATURE CITED
ACKNOWLEDGMENT We thank Paul Jouffrey (Department de MatBriaux) for the scanning electron micrographs of the enzyme layers.
(1) Tor, R.; Freeman, A. Anal. Chem. 1988, 58. 1042-1046. (2) Tran-Mlnh, C.; Guyonnet, R.; Beaux, J. C . R . Aced. Sci. Parts. Ser. C 1978, 286, 115-118. (3) SuaudChagny, M. F.; Pujol, J. F. Analus& 1086, 73,25-29. (4) FatibelbFllho, 0.;Sulelman, Ahmad A.; Guilbault, George G.;Lubrano, Glenn J. AMI. Chem. 1088, 60, 2397-2399. (5) Bradley, C. R.; Rechnitz, 0. A. Anal. Chem. 1984, 56, 664-667. (6) Mullen, WIiHam H.; churchouse, Stephen J.; Vadgama. Pankaj M. AMlyst 1985, 7 70, 925-928. (7) Mlzutanl, F.; Yamanaka, T.; Tanabe, Y.; Tsuda, K. Anal. Chem. Acta 1985, 777, 153-166. (8) El Yamanl, H. Mesure de la toxicit6 de polluants par blocapteur. R6alisetion d'une 6lectrode 6 butyrykholinesterase. Automatlsation de la detection de pesticides. Thesis. Ecole Sup. des Mines de Park, France, 1987. (9) Cukn, L. F.; Rusiing. J. F.; Schleiier, A.; Paparielb,G. J. Anal. Chem. 1974, 46, 1955-1961. (IO) Matsumoto, K.; Seijo, H.; Karube; Suzukl, S. B/ote&. Bioeng. 1980, 22, 1071-1086.
Registry No. AChE, 9000-81-1;BuChE, 9001-08-5;penicillinase, 9001-74-5;urease, 9002-13-5.
RECEIVED for review October 31, 1990. Revised manuscript received April 29, 1991. Accepted June 6, 1991.
Topology and Vaporization Characteristics of Palladium, Cobalt, Manganese, Indium, and Aluminum on a Graphite Surface Using Electrothermal Atomic Absorption J. McNally' and J. A. Holcombe* Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712
Thw and spatiah resolved absorbance proRles, concentratbn dudles, and aerosol depositlon vs manual pipettlng studles have been obtalned for Pd, Co, Mn, In, and Ai in 0.5% nitric acld. The actlvatlon energies of release for Pd, Co, and Mn are 44 f 3,36 f 3, and 43 f 6 kcal/md, respectively. Two activatlon energk were obtained over the course of release for I n uslng manual pipetting: 57 f 7 and 33 f 5 kcai/moi. A strong Pd-graphite and Co-graphite interaction accounts for the characterlstically broader half-wldths and spatially nonunlfonn atom distribution on the rldng portbn of the peak. Data suggesl that Pd and Co desorb from the graphhe surface as individual atoms, which results in an apparent first-order release. The relatlvely weak metal-graphite Interactions observed for I n account for the narrower half-widths and a relatively uniform atom dlstrlbution withln the furnace during atomlzatlon. For In, the formation of aggregates with desorption occurring from the aggregate surface Is proposed as one of the generation mechanlsms. I n the case of Mn, vaporization occurs from aggregates of a Mn compound on the surface. However, an apparent firstorder release is observed. Finally, AI Is characterlzed by unlquely rapld generatlon from aggregates and undergoes gasphase reactions.
INTRODUCTION Recently we have presented two models concerning relative metal-graphite interactions in electrothermal atomizers (ETAS) (1,2).The f i t model details the absorbance profiles obtained for two elements, Au (1) and Ag (2),which have
* Author to whom correspondence should be addressed. Present address: Chevron Chemical Co., 1862 Kingwood Dr., Kingwood, TX 77339. 0003-2700/91/0363-1918$02.50/0
relatively weak metal-graphite but relatively strong metalmetal interactions. The occurrence of such interactions leads to the formation of metal microdroplets on the surface prior to vaporization. Subsequent metal atom collisions with the graphite surface are comparatively elastic, and the rate of atom removal from the furnace is diffusion limited. This explains the relatively uniform radial atom distribution for Ag and Au within the furnace during the entire atomization cycle. Diffusion-limited loss would also explain the relatively smaller peak half-widths observed in comparison to that for elements which are readsorbed onto the graphite surface. As the initial analyte mass pipetted onto the surface is increased, the average volume of the droplets increases; however, the number of droplets remain relatively constant. Absorbance profiles obtained for concentration studies with Ag and Au also show higher peak temperatures with an increase in the initial sample amount. Such a process is indicative of vaporization from metal aggregates. That the morphology of the metal at the time of vaporization can determine the signal shape has been discussed previously (1-3). Activation energies obtained for Au and Ag from Arrhenius profiles may represent either AHH,for the free metal, aasuming the droplets are sufficiently large to ignore the Kelvin effect (4-61,or the release of metal atoms at the edge of the metd-graphite interface, i.e., at the circumference of hemispheres or islands. This theory also predicts lower appearance temperatures for smaller aggregates for the same initial sample amount due to the increased surface area of the aggregates. A comparison of the results obtained from aerosol deposition vs manual pipetting for Au (I) and Ag (2) verified this prediction. Finally, Knudsen numbers (7,8)for typical analytical amounts of metal indicate that readsorption of the metal atoms back onto the metal droplets is unlikely. The second model concerns the vaporization of metals with moderately strong metal-graphite interactions. Cu was the 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
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Table I. Experimental Conditions (SIWand Concentration Study)'
element
wavelength, nm
mass, ng
dry
ash K)
atomize 1.5s
hold
(2970 K)
880
K)
1.5s
hold
(2970 K)
880
K)
1.5s
hold
(2570 K)
970
1.5s
hold
(2970
K)
850
1.5s
hold
(2770-2870
40 s (1170 K)
Al
309
0.7" 0.24-2.e
40-50 s (350
co
241
0.8"
50 s (350 K)
40 s (970
50-60 s (350 K)
40 s (770
50 s (350 K)
30-40 s (770
In
304
Mn
279
Pd
340
0.2-1.5b 0.20 0.4-5.06 0.04O 0.01-0.306
6.8"
30-50 s (350
K)
ramp rate, K/sb
40 s (1270
K)
K)
K)
880
1.4-40.0b
" SIW study conditions only. Concentration study conditions only.
A sheath gas flow rate of 2 L/min of Ar was used in all studies. Table 11. Aerosol-Manual Deposition Experimental Conditions
ZONE 9 ZONE 5 ZONE 1
Flglm 1. plctorlal repn#lentation ofthe bcatkn of three of the spatial viewing zones. The bottom, middle, and top (zones 1, 5, and 9, respectbety) wfthin a CRA-90 furnace are depicted. The sample solution is micropipetted at the bottom of the furnace near zone 1.
test element examined in this instance (1). Strong metalgraphite interactions may lead to the dispersal of metal atoms near the primary deposition site prior to the detection of an absorbance signal. After vaporization from the initial deposition site, metal atoms adsorb and desorb within the furnace. Thus, a quasi-equilibrium process exists between gas-phase metal atoms and adatoms on the graphite surface. The model predicts that during the early portion of the absorbance profile there exists an atom density gradient from the initial sample deposition area (i.e., at the bottom) to the furnace top. The use of random probability in the desorption of individual atoms has been used in the successfulsimulation of spatially resolved experimental profiles for Cu (9). Experimentally these predictions also have been verified (1). Typically, a uniform atom distribution occurs near the peak. The continuous adsorption-desorption process also accounts for the relative increase in signal half-widths for Cu in comparison to Au. Increasing the initial amount of analyte pipetted onto the graphite surface results in earlier appearance temperatures; however, the peak temperatures remain constant. Thus, two models have been proposed and experimentally verified concerning relative metal-graphite interactions for Cu, Au, and Ag. Five additional elements, Pd, Co, Mn, In, and Al, will be examined in light of the above theories. EXPERIMENTAL SECTION Instrumentation. A 3-mm4.d. X 9-mm-long CRA-90 mini Massmann furnace (Varian Techtron, Inc)was used in all studies. Details of the time and spatial optics and ancillary indrumentation can be found elsewhere (IO). Figure 1is a pictorial representation of three of the nine viewing zones within the furnace and indicates that each zone represents an integrated absorbance along the optical axis. Some degradation of the viewing zones occurs at the tube ends, which is not represented accurately in Figure 1. However, the signal from each viewing zone is independent of adjacent zones. The concentration study and the aerosol-manual depition experiments were performed on the same optical system but with a circular slit used in lieu of the spatial isolation wheel (SIW) attachment. For the aerosol deposition experiments, an IL254 FASTAC was used to deposit the sample at the bottom of the mini Massmann furnace described above. Temperature measurements were obtained from the output of a phototransistor that had been previously calibrated against
element
A1
Co
In
Mn
Pd
340 304 279 241 309 wavelength, nm 0.8b 0.11' 0.04" 5c initial mass, ng 0.7" (manual deposition) aerosol deposition 4 6 6 5 5 time, sf 2 L/min Ar at atomization sheath gas flow rate ODry, 50 s (350 K);ash, 40 s (1170 K);atomize, 1.5 s (2970 K). bDry, 50 s (350 K); ash, 40 s (970 K); atomize, 1.5 s (2970 K). 'Dry, 60 s (350 K); ash, 40 s (770 K); atomize, 1.5 s (2570 K). "Dry, 50 s (350 K); ash, 30 s (770 K); atomize, 1.5 s (2970 K). CDry,50 s (350 K); ash, 40 s (1270 K); atomize, 1.5 s (2770 K). /Same temperature, time conditions as in manual deposition with addition of 5 s of predeposition time. Nebulizer flow rate: 2.3-2.4 L/min.
a disappearing filament optical pyrometer (pyrometer Instrument Co., Inc., Model 95). The optical pyrometer was sighted at the bottom of the furnace through the sample deposition port. Solutions. Certified reagent grade Mn(NO& solution and solid Al(N03)3were used to prepare a stock solution that contained lo00 mg/L of Mn and Al, respectively. A minimal amount of concentrated nitric acid was used to dissolve Co and In metal and Pd powder to prepare stock solutionsthat contained lo00 mg/L of Co, In, and Pd, respectively. Working standards of the element being studied were prepared daily from the stock solution using doubledistilleddeionized water and were acidified in 0.5% HNO3 Procedures. The analytical conditions used in the spatially resolved studies and concentration studies are listed in Table I. For the spatially resolved studies, four to six absorbance profiles are averaged for each element. A 10-pL Hamilton syringe equipped with a Teflon needle was used to deposit a 2-pL aliquot of the standard solution unless indicated otherwise. A new tube was installed at the onset for each element in the concentration study. The study was performed four times for 10-14 concentrations. The spectrometer bandwidth was set at 0.8 nm for Pd and 0.2 nm for the other four elements. Table 11lists the analytical conditions used for the aerosol-manual deposition experimenta. For the aerosol sample introduction, the deposition time was adjusted so that peak areas equivalent to that from manual pipetting were obtained. Before the beginning of each experiment, the furnace was fired 15 times with a standard solution in order to condition the furnace.
RESULTS AND DISCUSSION Palladium. The interest in Pd stems, in part, from its involvement as a very useful matrix modifier (e.g., refs 11-15). At matrix modifier amounts of Pd (>1 pg), the formation of numerous Pd metal droplets has been observed by two groups using scanning electron microscopy after cooling down from a high-temperature ash cycle (14, 15). Voth-Beach and Shrader (14) postulated the "sticking" of analyte to the Pd droplets and have observed a more Gaussian analyte peak shape with a smaller, more uniform distribution of Pd droplets. These authors suggested that with a smaller droplet size
1920
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
-
1
9
ZONE
0
I
TIME
2
(SI
-
1
-
'
ZONE
1
1
ZONE
9
TIME
0
2
(SI
-
1
ZONE
LCI
--I H
x m I I V
A I I
W
n I
+
--ZONE
9
0
TIME
I
1
(5)
ZONE
-
'
9
1
1
ZONE
9
0
TIME
l (5)
-
i
ZONE
--I H
I
m I V
-
'
1
1
9
0
1
-
1
(SI ZONE Flgurr 2. (A) Absorbance vs time profiles at nine different heights within the furnace. Absorbance vs height contours at 40-ms time intervals for Pd and Co and 20-ms intervals for Mn, In, and AI (B) prior to and (C) after the peak absorbance. Plots are shown for Pd (6.8 ng), Co (0.08 ng), Mn (0.04 ng), In (0.2 ng), and Ai (0.7 ng). The time scale represents the onset of data collection. All samples are in 0.5% HNO,. Plots a-e are top to bottom, left to right. ZONE
TIME
distribution the total release of the analyte occurs over a shorter time period during the atomization cycle. Transmission electron microscopy of vapor-deposited Pd on thin layers of crystalline graphite also has revealed the presence of Pd droplets (16). However, the geometry of analytical amounts of Pd in GFAA during vaporization has not been elucidated previously. Figure 2a depicts time-resolved and spatially resolved absorbance profiles including that for 6.8 ng of Pd. Superimposed absorbance-time plots for all nine zones are shown in spectra A of Figure 2. Spectra B and C of N illustrate absorbance-zone contours at 40-ms time intervals during the rising portion and down the back edge of the curve, respectively. The increased noise encountered in all spatially resolved absorbance profiles results from the reduced light intensity associated with the SIW attachment and the 1-ms time resolution. Immediately apparent in Figure 2a for Pd is the atom
gradient from the bottom to the furnace top during the rising portion of the peak. The atom gradient is the result of a continuous adsorption-desorption process of Pd atoms with the graphite surface around the tube circumference until relatively uniform coverage is achieved. Thus, Pd, as in the case of Cu, is characterized by relatively strong metal-graphite interactions. The peak half-width is 3-4 times larger than the half-widths observed for elements exhibiting weak metal-graphite interactions (1, 2) and supports the continuous adsorption-desorption process described above. Figure 3e illustrates the spatially integrated absorbancetime profiles for various initial amounts of Pd deposited on the furnace bottom. As the initial amount of Pd is increased, there is a shift to an earlier appearance time (lower temperature); however, the peak times (temperatures) remain nearly constant. These features might indicate a first-order process as discussed previously (1). The average activation energy of release for 48 shots as determined by the method of Smets
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
TIME (SECJ
1921
TIME (SEC)
1.00
,800
.600 W
z
E p.
