630
Anal. Chem.
1982,5 4 , 630-634
Alteration of Metal Release Mechanisms in Graphite Furnace Atomizers by Chemisorbed Oxygen Stephen G. Salmon and James A. Holcombe" Depatfment of Chemistty, The University of Texas, Austin, Texas 78712
The appearance temperatures of several metals In graphlte furnace atomic absorptlon spectrometry are affected by the presence of 0, In the sheath gas. Chemisorption of 0, on the actlve sites of the graphlte surface is responslbie for the shlft in appearance temperature due to a change In the rate of reductlon of the analyte when a stable surface oxlde Is formed on the graphite. Metal vaporization Is preceded elther by metal oxlde reduction at two major types of graphite actlve sites or by thermal dlssociatlon of the metal oxide. Blockage of the actlve sites by Chemisorbed 0, affects the rate of the reduction reaction whlch determines the appearance temperature of the metal. The appearance temperature may be raked to a polnt where elther the thermal dlssoclation of the metal oxide becomes favorable or the actlve reducing sites are made available due to desorptlon of oxygen as CO or CO, from the surface. The metals that exhlbit a shift In appearance temperature vaporlre wlthln the temperature range of optlmum 0, chemlsorptlon at approximately 850 K and total desorption at approxlmateiy 1200 K. The effect of chemisorbed O2 on the appearance temperatures of Cd, Zn, Pb, Ag, In, Ca, and Cu Is presented.
The widespread employment of graphite furnace atomizers and the need to develop a better understanding of the basic atomization processes have led to a number of studies employing both mathematical modeling and theoretical mechanisms for atom formation and loss (1-20). A likely process preceding atomization would involve the reduction of a metal oxide by the graphite atomizer. Both thermodynamic and kinetic approaches have been used to evaluate the graphite reduction step. Aggett and Sprott (5)considered the graphite reduction of the metal oxide to form the metal vapor above a graphite filament. The involvement of graphite in the reduction mechanism was justified by comparing the appearance temperatures of several metals atomized from a graphite filament with the temperatures obtained using a T a ribbon. Sturgeon et al. (12)have noted that care must be taken in this comparison since Ta reduction of metal oxides is also thermodynamically favorable at the atomization temperatures used. They also noted that the T a reduction can be ruled out if the atomizer surface is covered with a coherent layer of Ta205,which is probably a valid assumption. Campbell and Ottaway (6) also have used the thermodynamic approach in considering the reductive role of graphite in a furnace atomizer. In this study they suggested a direct correlation between the appearance temperature and the temperature where the thermodynamic value of G o for the reduction reaction becomes negative. The correspondence between these two temperatures was quite good for a large number of the elements studied. They noted that their approach does not take reaction rates into account but assumes that graphite reduction rates are reasonably fast if the free energy for the reaction is negative. Fuller (4,9,14) proposed a kinetic approach to the study of atomization, after noting that the thermodynamic approach
used by Campbell and Ottaway would predict the formation of stable carbides before metal oxide reduction for several metals. The rates of atom formation and loss were calculated for Cu and it was determined that the reduction of Cu20 by graphite is the rate-controlling step. This, however, is inconsistent with DTA studies by Eklund and Holcombe (21), which indicate that CuO is reduced to Cu in the presence of graphite before a temperature is reached where Cu20forms. Sturgeon et al. (11)also have proposed a model for the formation of Cu atoms which proceeds through the reduction of CuO by graphite. Smets (20) also noted the drawbacks of the thermodynamic approach and used a kinetic approach to determine the mechanisms for atom formation and dissipation for several metals using a graphite minitube furnace and a graphite-strip heater. Sturgeon et al. (11) have commented on the problems with the thermodynamic approach but also point out that since most atomizers are not isothermal during the formation of analyte atoms and that the temperatures measured are those of the atomizer wall rather than the gas phase, the kinetic approach to the study of atomization also has limitations. Their approach was to combine thermodynamic and kinetic considerations to describe the mechanisms of atom formation in a graphite furnace. From this study four mechanisms were proposed for different metals. Their mechanisms were based on the assumption that activation energies, obtained by plotting the log of the absorbance against the reciprocal of the absolute temperature, were good approximations of rate-limiting enthalpies; i.e., they assumed no activation barrier existed for the processes and E, i= AH'. As has been shown previously (22), the rate of release of Pb, reflected by the appearance temperature, is not fixed and can be changed by altering the graphite surface. In this case the alteration of the surface was accomplished by the chemisorption of O2on the active edge sites of graphite. From the effect of chemisorbed O2on the appearance temperature of Pb, it can be assumed that previous reports of P b formation occurring by thermal dissociation (5)may be invalid for certain atomizer surface conditions. It was proposed that alterations in P b appearance temperatures were observed because the range of release temperatures for this element coincides with the range over which oxygen binds to the surface and desorbs from the graphite as CO or C02. The present study provides data on the appearance temperature changes, i.e., changes in release mechanism, for several other volatile metals to show that the effect reported for P b is a function of the graphite surface and not unique to P b only. EXPERIMENTAL SECTION Apparatus. A Varian CRA-90 furnace atomizer was enclosed in an airtight box and was used in the study. Details of this atomizer and the furnace enclosure have been given previously (22). The optical system, gas flow system, and data collection system were the same as that described in previous studies for Pb (22-24). Reagents. The metals chosen for this study were Cd, Zn, Pb, Ag, In, Ga, and Cu. Excluding Hg, these elements were chosen from a list of average literature values of appearance temperatures
0003-2700/82/0354-0630$01.25/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
Table I. Average Values for Appearance Temperaturesa A A appear AA appear element temp, K element temp, K Cd 730 In 1190 Zn 1050 Ga 1350 Pb 1060 cu 1460 Ag 1120 a
From ref 19.
I6O0
-
p ; x ]
631
1400
!!
g 1200
c
$
F
(19) to cover a range from the lowest appearance temperature available up t o an appearance temperature that should place the . atomization beyond the point of desorption of 0, from graphite. A list of the elements examined and their average appearance temperatureti are given in Table I. All working solutions were prepared each day from stock solutions that contained lo00 mg/L of the metal. Zn, In, Ga, and Cu stock solutions were prepared by dissolving the pure metals in a minimal amount of "OB. Cd, Pb, and Ag stock solutions were prepared by dissolving reagent grade nitrate salts of these metals in deionized and doubly distilled water. Procedure. A Hamilton syringe, fitted with a short length of Teflon tubing, was used to deposit a 2-pL drop of solution on the atomizer In the CRA-90 furnace, the sample was dried for 30 s at 100 OC!, ashed for 30 s, and atomized at a final temperature of 2000 "C, which was held for 1 s. Ash temperatures of approximately 500 "C for Pb, Ag, In, Ga, and Cu, and 350 " C for Cd and Zn were used. The actual ash temperatures used in each case were measured with a W5% Re vs. W:26% Re microthermocouple (Omega Elngineering, Inc., Stamford, CT). The ramp rate of the furnace atomizer was 800 OC/s. The atomizer was imaged onto a phototransistor whose output was calibrated against an optical pyrometer and produced a voltage reading whose logarithm was inversely proportional to the absolute temperature. A temperature vs. time plot was obtained by extrapolating the atomizer temperatures back tto the measured ash tempwature, and the appearance temperatures of the metals were then calculated based on their appearance times. Rotameters were used to mix Ar and Ar containing 1.0'% 0, to obtain various 0, concentrations while maintaining a conetant flow of sheath gas at 2.5 L/min. The samples were atomized in a sheath gas differing O2concentrations, and1 absorbance- time data were collected. Sufficient time was allowed for the atomizer to be flushed completely when sheath gas compositions were changed to eliminate carry-over. A Savitzky-Golay procedure (25) was used to smooth the data, and diagnostic information on the absorbance signal, such as peak height, area, appearance time, etc. was calculated for each shot on a laboratory microcomputer. The appearance time of the atomic signal, was defined as the time required from the start of the atomize step for the absorbance signal to reach two stanidard deviations above the average base line noise.
RESULTS AND DISCUSSION Alteration of the Graphite Surface. In a previous gaper (22)it was shown that the appearance of P b in both a graphite filament and a graphite furnace atomizer was shifted to a higher temperature when 0, was present in the Ar sheatlh gas or when the atomizer was fired in an 0, environment ]prior to the atomization of a P b sample. It was proposed that the chemisorpticin of 0, on the active edge carbons of the graphite crystallites deactivated these sites and either forced the reduction of P bO to occur at a site requiring a higher activation energy or removed the reducing capability of the suirface altogether which leads to the thermal dissociation of PbO. The temperature range over which this surface alteration should occur extends from an optimum chemisorption temperature of 850 K to a desorption temperature of 1200 K. Since the appearance temperature of P b falls within this range, the mechanism of P b vaporization is altered by chemisorbed 0,. If this change in the ability of the graphite surface to reduce a metal oxide has an effect on metal release mechanisms, and, hence, the appearance temperature, other metals should 13how
P 1000 a P n
600
-2 -I log (% O*'
-3
0
Flgure 1. Appearance temperature shift with increasing O2concentratlon in the sheath gas for Cd, Pb, Zn, Ag, In, Cu, and Ga.
I
0
1.0
0.5
L
0
Relative
05 time
AI O (5)
Figure 2. Absorbance vs. time profiles for Zn and Cd obtained in (a) and (c)and Ar sheath an Ar sheath, (b) a sheath containing 1.O% 02, immedlately followlng atomization in 1.O% 02.
a similar effect to that reported for Pb. According to the appearance temperatures listed in Table I, Cd, Zn, Ag, and In would fall within a range where chemisorbed 0, could affect the metal release mechanism. In contrast, the appearance temperatures for Ga and Cu fall beyond the desorption temperature of chemisorbed O,,and these metals should not be affected by the presence of O2in the atomizer. The appearance ternperatures were determined for the metals listed in Table I for an Ar sheath gas, containing O2 ranging in concentrations from 0.001% to 1.0%. Atomization with tit Graphite Furnace. Figure 1 shows the appearance temperature vs. log (% 0,) curves for several metals for the CRA-90 graphite furnace. The general dependence of appearance temperatures for Pb, Zn, Cd, and Ag on increasing amounts of 0, in the sheath gas is very similar. The general shape of the absorbance-time curves for Zn and P b both exhibited a shoulder which shifted with increasing O2and was similar to that shown previously for P b (22). In contrast, Cd and Ag showed no detectable shoulder in their absorbance profiles. Figure 2 shows the general shape of Zn and Cd profiles under various conditions including the first shot back in Ar after an atomization in 1.0% 02.This retention of a high-temperature shift for the first shot back in Ar demonstrates that the cause of the increased shift in ap-
632
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
pearance temperature is due to a surface process rather than a gas-phase involvement of O2 during atomization. For the last shot in 02,the O2is present in the sheath gas during the cooling down period and probably adsorbs onto the surface to form a stable surface oxide at this time and thus alters the chemical nature of the surface for the next shot where O2 is absent in the sheath gas (22). The appearance temperature for In lies in the temperature range where the rate of desorption of oxygen from the surface becomes rapid. While the appearance temperature vs. time curve for In in Figure 1 increases, the linear dependence is significantly different than that observed for the other volatile metals studied. This linearity was also observed when atomizing from a graphite filament-type atomizer. The In signal was enhanced as the O2concentration increased in the graphite furnace with the largest signal obtained when switching from a 1.0% O2sheath to an Ar sheath. This general enhancement seen with oxygen pretreatment of the surface was also observed by Beaty et al. (26). These results again demonstrate that the observed enhancement is due to a surface reaction rather than a gas-phase process. Although the net effect on the In absorbance with O2present was an enhanced signal, it is likely that the less substantial enhancement in comparison with the “first shot back in Ar” is caused by a gas-phase reaction with 02.The lack of any memory effect also dismisses the idea that the increase signal is a result of a build up of In on the surface with repeated analyses. The oxygenated graphite surface must allow In to be vaporized as the free metal, whereas the nontreated surface must foster vaporization of a molecular species containing In. It is possible that the presence of the more reducing nature of the unoxygenated surface produces a reduced form of the In at a significantly lower temperature. This species may then react on the surface to form a more volatile compound which does not dissociate completely in the gas phase and subsequently is lost by diffusion. The lack of any sample matrix and the expected chemical differences in the two surface characters would make the loss of In as Inz a prime candidate since elemental In is the most stable reduced form of the element. Unfortunately, the relatively weak dissociation energy of 22.4 kcal/mol for In2 (27) is not strongly supportive of this conclusion. Figure 1 also shows the appearance temperature vs. log (% 0,) curves for the less volatile metals, Cu and Ga. These metals are vaporized at temperatures above those cited for the persistence of chemisorbed oxygen on graphite. Significant desorption of CO or C02 should have renewed the active sites and the appearance temperatures of these metals should not be affected significantly by the presence of O2in the furnace. Consequently, the appearance temperatures do not shift with increasing amounts of 02. Mechanism of Atom Formation. If it is assumed that the vaporization of the free metal from the atomizer surface is rapid compared to the graphite reduction of the metal oxide MO, the production of gas-phase metal atoms can be expressed by MO(s,l) + C M(g) f CO(s)
-
where the oxygen may attach to carbon as a surface bound species, e.g., “CO(s)” and later desorb as CO(g). The rapid conversion of condensed phase metal to metal vapor has been omitted from this equation for simplification. The appearance temperature of the metal can then be defined as the temperature at which the reduction reaction is sufficiently fast to produce a measurable amount of metal vapor. The existence of active sites on graphite, which have different activation energies for the chemisorption of 02,and the effect that blockage of these sites by oxygen has on the appearance temperature of P b have been discussed previously (22). The similar behavior of the other volatile metals, when
vaporized from a graphite surface that has been treated with 02,suggests that the graphite reduction of MO cannot be expressed by a single rate equation, and the two major types of active sites provide two different reaction pathways, with different activation energies for the metal oxide reduction. Thus, the reduction of MO at a particular active site on the graphite surface can be more specifically designated as
MO(s,l) + CI
ki +
M(g)
+ C,O(S)
where the subscript I references a particular type of site. If the rate of formation of M(g) is limited by the rate of surface reduction of MO, then the free metal formation can thus be related to the rate of change in surface coverage of MO, aMo, which is given by
where t is the time and u represents the surface coverage in cm-2 which is analogous to a surface concentration of a particular species. This equation is not a rigorous representation of the analytical signal but represents the supply function only, Le., diffusional loss and readsorption of M onto the surface (20,28)have been omitted for simplification in this discussion. A similar set of equations can be writtm for a second surface site I1 whose chemical reactivity, e.g., kII, is less than that of the type I sites and whose surface coverage ucIImay be different than that of the type I sites.
The existence of sites with varying activity on a graphite surface has been documented (29, 30). Additionally, it has been estimated that these sites may constitute only a small fraction of the total surface area (30-32), although the exact number is dependent on the type of graphite surface being discussed. If site I has a lower activation energy for the metal oxide reduction reaction, eq 1 will determine the rate of reaction and, consequently, the appearance temperature of the metal. Assuming that site I is also the most active site with respect to chemisorption of 02,then the number of available type I sites, ucI, decreases as O2reacts with the surface. The first measurable absorbance can be loosely viewed as that temperature where -daMo/dt reaches a maximum, critical value. Thus, with a decrease of ucI due to occupancy of these sites by adsorbed oxygen, the temperature must be raised to increase the magnitude of the rate constant kI until the critical value of -daMo/dt is again realized. As the number of type I sites continues to decrease with increasing amounts of O2 added to the sheath gas, the atomizer temperature can reach a value where metal oxide reduction at type I1 sites can contribute significantly to the reduction of MO and consequently the production of M(g). This second release mechanism can also explain the shoulder seen on the absorbance-time profiles for some metals. As the amount of O2 is increased further, the dominant release of M(g) is given by eq 2. Eventually, the availability of type I sites is sufficiently low that eq 2 accounts for the majority of the metal vapor produced and what was once a shoulder on the trailing edge of the absorbance signal becomes the predominant peak. Similarly, the shoulder on the rising edge, which had been the main peak, decreases then disappears. The appearance temperature of the atomic signal should then continue to increase as act, decreases in eq 2. At elevated O2 concentrations, the change in appearance temperature for Pb, Cd, Zn, and Ag in Figure 1 tends to approach a limiting value. With a high O2 surface coverage, i.e., 21%O2in the gas phase, the appearance temperatures of these four metals lie within k75 K and near the temperature that is often sited as that temperature where
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
the oxygen desorption proceeds at a rapid rate. This strongly suggests that a general, rate-limiting step of surface redulction of the oxide may exist for all of these metals, and the appearance temperatures are strongly influenced by the a,vailability of reducing sites. Once the surface has become nlearly saturated, this availability is governed by the net rate at which they are “uncovered” due to the desorption processes ocurring at the elevated temperatures. Thus, these metals should1 and do exhibit appearance temperatures which show the relatively narrow temperature spread of f75 K. Thus, on a hiighly oxygenated graphite surface, the appearance temperature of the volatile metals is not strongly dependent on the partilcular metal but rather is governed by the desorption rate of oxygen.
where C,-O(s) represents the oxygenated surface, C+&) represents the unoxygenated surface after desorption, and k’ is the rate constant for the process. A 300 K spread in appearance temperatures for these 13ame metals exist13for the unoxygenated surface. The appearance temperature on this “active” surface is dependent on the rate of reduction of this particular metal oxide by the graphite, which should be elemental dependent and slower due to the reduced value of the rate constant at the lower temperature. The relative sensitivities (e.g., oscillator wtrengths) of the various met& will also govern the amount of metal that need be released before detection is possible. This latter factor may also contribute to a lesser extent toward the slight spread in appearance temperatures at the higher 0, concentrations. Another mechanism of metal vapor formation must be considered as the appearance temperature reaches higher values. If the temperature a t which significant metal vapor is produced by the metal oxide reduction reaction becomes sufficiently high, the rate of free metal formation by thermal dissociation of the metal oxide may become competitive with the reduction reaction. As the appearance temperature reaches a value a t which the thermal dissociation of the rnetal oxide is the imain source of metal vapor production, a further increase in the amount of chemisorbed O2would have little effect on the appearance temperature of the metal. This also would account for a decrease in the slope of the appearance temperature vs. log (% 0,) curve a t higher O2concentration. Cu and Ga behave as would be expected since their normal appearance temperatures on an unoxygenated surface lie above the point where the oxygen should be desorbed. Similar results were found for several other moderate to high boiling metals. However, fior certain metals researchers have reported a depressed analytical signal if the surface is pretreated with O2and atomization is conducted in an inert atmosphere (e.g., ref 26). For example, preliminary studies in our laboratory with Cr show that the first shot back after O2treatment is severely attenuated. However, the signal returns to its original value after the atomizer has been heated to a high temperature several times. T h i s strongly suggests that the surface has been degraded by the 0, but, with time and elevated temperatures, anneals itself. This annealing may be viewed as simple graphitization of the surface which occurs at elevated temperatures. Further stulrfies are needed to better understand this phenomenon, which may be only distantly related to the studies reported in this paper. While the kinetic approach was taken iin the above discussion, similar arguements can be presented for the case of a “pseudoequilibrium” (17) existing at the graphite/vapor interface. This is possible since the equilibrium constant for these procecises should show similar dependencies on the activity (“concentration”) of CI,which is relatable to ncI. The equilibrium constant K also exhibits a temperature dependence similar to that shown by the rate constant k , Le., K =
exp(-AG”/RT) and k
633
exp(-E,/RT).
CONCLUSION As noted in the introduction, the involvement of the graphite as a reducing agent preceding vaporization has been suggested previously. These studies support that hypothesis and show that the graphite activity can be moderated by reducing the effective number of active sites through the chemical binding of ox:ygen to the surface by the chemisorption of Oz(g) onto the graphite. Similarly, the thermal decomposition of oxyanion salts present in a real sample can produce oxidant gases which may behave in a similar fashion to produce appearance time shifts. For example, such shifts were observed for Pb in1 the presence of a NaN03 matrix (28). Additionally, these results suggest that similar temperature shifts may be realized by other means of forming surface oxides of carbon. For example, solution oxidation by HN03 to form surface bound oxygen is known to occur (33, 34). Other extensions of these studies include a mechanism to partially explain the analytical performance of metal carbide coated tubes. If the metal carbide forms at the edge carbons, i.e., active sites, one would expect (a) a higher temperature release of the volitile metals which normally require carbon assisted reduction prior to atomization and (b) an increased resistance of the tube toward oxidation (i.e., increased lifetime) since oxidative attack on the graphite will occur preferentially at the edge carbons and not at the relatively inert carbons located in the basal plane. Blockage of the active sites by some means other than the formation of surface oxides should also increase the maximum appearance temperature for some metals. As was discussed in the text, with an oxygenated surface the metal release may occur coincident with oxygen desorption due to the renewed availability of active sites for reduction. With a more refractory metal bound to these surface sites, the availability of these reducing centers may not exist until much later in the atomization cycle. Thus, the only paths for the release of the metal may be via thermal decomposition or volitilization of the metal compound. Altering the methods used to generate the pyrolytic coating in order to maximize the size of the graphitic crystallites and thereby decreasing the ratio of edge carbons to basal plane carbons may be another means of reducing the number of active sites. In contrast, where low temperature graphite reduction is desirable, the use of unlpyrolyzed furnaces may provide better performance by supplying an increased number of active sites. The potential benefit of this approach may be offset by the problems associated with the more porous nature of uncoated furnaces, e.g., diffusion of the sample into these pores and a decrease in analytical sensitivity due to a reduction in the release rate of the analyte from these pores (e.g., ref 35). On pyrolytic surfaces, it may be possible to protect the active sites from oxidation by the siample matrix or entrained oxygen by providing a more reducing environment. The often used method of adding organic acids (e.g., ref 36-39) for the analysis of the volatile metals may provide this needed reducing environment either in the solution or in the gas phase where the pyrolysis of these compounds generates radicals which may be capable of maintaining a larger percentage of active sites. It has been suggested that the carbon residue left by these organic acids may assist in the metal oxide reduction (36). This again is consistent with the mechanisms proposed in the paper. The obvious benefits of oxygen ashing are confirmed in this study. Not only should the O2present in the sheath gas assist in the combustion of complex marices, higher ash temperatures should be availablle without analyte loss. As long as the atomizer temperature is kept below about 1200 K while in O,, there should be minimal reduction in the furnace lifetime.
Anal. Chem. 1902, 54, 634-637
LITERATURE CITED L'vov, E. V. "Atomlc Absorption Spectrochemical Analysls"; Adam Hllger: London, 1970. Torsl, G.; Tessari, G. Anal. Chem. 1973, 45, 1812-1816. Paverl-Fontana. S. L.; Torsi, G.; Tessarl, G. Anal. Chem. 1974, 4 6 , 1032- 1038. Fuller, C. W. Analyst (London) 1974, 99, 739-744. Aggett, J.; Sprott, A. J. Anal. Chim. Acta 1974, 72, 49-58. Campbell, W. C.; Ottaway, J. M. Talanta 1974, 21, 837-844. Tessari, G.; Torsl, G. Anal. Chem. 1975, 47, 842-849. Johnson, D. J.; Sharp, E. L.; West, T. S.;Dagnall, R. M. Anal. Chem. 1975, 47, 1234-1240. Fuller, C. W. Analyst (London) 1975, 100, 229-233. Paverl-Fontana, S.L.; Torsl, G.; Tessarl, 0. Ann. Chlm. (Rome) 1976, 66,691. Sturgeon, R. E.; Chakrabartl, C. L.; Langford, C. H. Anal. Chem. 1978, 48, 1792-1807. L'vov, E. V.; Katskov, D. A.; Krugllkova, L. P.; Polzlk, L. K. Spectrochlm. Aqta, Part 8 1976, 3 1 8 , 49-80. Katskov, D. A. Zh. Prikl. Spectrosk. 1976, 2 6 , 598. Fuller, C. W. Analyst (London) 1978, 101, 798-802. Torsl, G.; Tessari, G. Anal. Chem. 1978, 4 8 , 1318-1324. Van de Broek, W.; de Galan, L. Anal. Chem. 1977, 49, 2176-2186. Tessarl, G.; Torsi, G. Ann. Chlm. (Rome) 1978, 68. 967-989. ZsakB, J. Anal. Chem. 1978, 50, 1105-1107. L'vov, E. V. Spectrochim. Acta, Part 8 1978, 3 3 8 , 153-193. Smets, E. Spectrochlm. Acta, Part 8 1980, 3 5 8 , 33-42. Eklund, R. H.; Holcombe, J. A. Talanta 1979, 2 6 , 1055-1057. Salmon, S. G.; Davis, R. H., Jr.; Holcombe, J. A. Anal. Chem. 1981, 5 3 , 324-330.
(23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (38) (37) (38) (39)
Salmon, S. 0.; Holcombe, J. A. Anal. Chem. 1978, 50, 1714-1716. Holcombe, J. A. Anal. Chem. 1979, 5 1 , 648-650. Salmon, S. 0.; Savitzky, A.; W a y , M. J. E. Anal. Chem. 1964, 3 6 , 1627-1638. Eeaty, M.; Earnett, N.; Grobenski, At. Spectrosc. 1980, 1 , 72. Gaydon, A. G. "Dlssoclation Energies and Spectra of Diatomic Molecules", 3rd ed.; Chapman and Hall: London, 1968. Holcombe, J. A.; Rayson, G. D.; Akerllnd, N., Jr. Spectrochim. Acta, In press. Vastoia, F. J.; Walker, P. L., Jr. Carbon 1967, 5.591. Lussow, R. 0.; Hart, P.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1987, 5 , 363. Vastola, F. J.; Hart, P.; Walker, P. L., Jr. Carbon 1964, 2 , 65. Lalne, N. R.; Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1983, 67, 2030. Puri. E. R.; Slngh, S.;Mahajan, 0. P. J. Indian Chem. SOC.1980, 3 7 , 171. Cookson, J. T., Jr. I n "Carbon Adsorption Handbook"; Cheremisinoff, P. N., Ellerbusch, F., Eds.; Ann Arbor Sclence: Ann Arbor, MI, 1978; p 249. L'vov, B. V.; Eayunov, P. A.; Ryabchuk, G. N. Spectrochim. Acta, Part 8 1981, 3 6 8 , 397. Regan, J.; Warren, J. Analyst (London) 1976, 101, 220. Fuller, C. W. At. Absorpt. Newsl. 1977, 16, 106. McLaren, J. W.; Wheeler, R. C. Analyst (London) 1977, 102, 542. Regan, J.; Warren, J. At. Absorpt. Newsl. 1978, 17, 89.
RECEIVED for review August 6, 1981. Accepted January 11, 1982. This work was supported in part by National Science Foundation Grant No. CHE78-15438 and the Robert A. Welch Foundation.
Time-Resolved Rejection of Fluorescence from Raman Spectra via High Repetition Rate Gated Photon Counting 1.L. Gustafson" The Standard Oil Company (Ohio), 4440 Warrensviiie Center Road, Warrensviiie Heights, Ohio 4 4 128
F. E. Lytle Department of Chemistty, Purdue U n i v e r s i ~ ,West La fayette, Indiana 47907
High repetition rate gated photon countlng was used In conJunction with a synchronously pumped cavlty dumped dye laser to reject undesirable fluorescence background from Raman spectra. The system performance was evaluated by using benzene doped with rubrene (7,= 16.5 ns). The degree of background rejection was determined by comparing the level of fluorescence In a tlme-resolved experiment to that for contlnuous excitation using the 992-cm-' solvent peak as an Internal standard. The resulting Raman to fluorescence enhancement was 35. The same technlque was used to reject lmpurlty fluorescence from ultravloiet exclted preresonance Raman spectra.
The problem of fluorescence interference in Raman spectrometry has led to many attempts to reduce this residual background (1-8). In general, two experimental approaches have been taken to reduce the background interferences. The first of these involves the use of nonlinear techniques where the Raman signal is carried on a collimated beam. The second approach which has been used to reduce the fluorescence contribution to Raman spectra is based on the temporal difference between scattered and emitted photons. In the former category, such methods as coherent antistokes Raman spectrometry (CARS) (9-13), inverse Raman spectrometry (14-20),and Raman gain spectrometry (21-26) are capable of producing Raman spectra of fluorescing ma0003-2700/82/0354-0634$01.25/0
terials. For the normal Raman spectroscopist, however, these techniques present some formidable obstacles. The signal intensity depends on the electric field strengths of more than one laser. In addition, the CARS signal depends on the square of the normal Raman cross section and on the square of the number of scattering molecules (27). This nonlinearity with respect to concentration makes it difficult to use differencing techniques for studying perturbations due to molecular interactions (28,29). Also, these techniques require a considerable change in the physical layout of a spontaneous Raman experiment. Often the spectroscopist does not have the leisure or the facilities to conveniently switch from a conventional spontaneous Raman experiment to a coherent Raman technique. The technique described here used pulsed excitation combined with synchronous optical detection. The major advantage of this method is that it is relatively transparent to an existing spectrometer. Several authors have reported schemes using this approach. Yaney describes a system based on a 100-ns Q-switched Nd:YAG laser and fast photon counting for the duration of the pulse (2). The system operates at 450 Hz and works best for samples with long fluorescence lifetimes. Van Duyne et al. employ a system which uses a mode locked argon ion laser as the pulsed source and a time-to-amplitude converter with a level discriminator for gated detection of the Raman signal ( 4 ) . Their system is limited primarily to the measurement of Raman spectra of samples with short fluorescence lifetimes due to the fiied high 0 1982 Amerlcan Chemlcal Soclety
