437
Anal. Chem. 1901, 53, 437-444 (16) Marshall, A. G.; Comisarow, M. E. Anal. Chem. 1975, 46, 491A. (17) Comisarow, M. B.; Marshall, A. G. J. Chem. Phys. 1978, 64. 110. (18) McIver, R. T., Jr.; Ledford, E. E., Jr.; Miller, J. S. Anal. Chem. 1975, 47, 692. (19) Hunter, R. L.; McIver, R. T., Jr. Anal. Chem. 1979, 51, 699. (20) Comisarow, M. E. In “Transform Techniques in Chemistry”; Grifftths, P. R., Ed.; Plenum Press: New York, 1978; p 282. (21) Feilgett, P. B. PhD. Thesis, University of Cambridge, Cambridge, England, 1951. (22) Ledford, E. E., Jr.; Ghaderl Sahba; Wikins, C. L.; Gross, M. L. Anal. Chem. 1880, 52, 463. (23) Horlick, G.; Yuen, W. K. Anal. Chem. 1978, 46, 1643. (24) Comisarow, M. E.; Meka, J. D. Anal. Chem. 1979, 51, 2198. (25) Sharp. T. E.; Eyler, J. R.; Ellen, Li Int. J . Mass. Spectrom. Ion Phys. 1972, 9 , 421.. Field, F. H.; Munson, M. S.E. J . Am. Chem. Soc.1967, 8 9 , 4272. Heller, S. R.; Mllne, G. W. A., “National Standard Reference Data System”, NSROS-NBS 63, 1976; Voi. 1. Hartman, K. N.; Lias, S.; Ausloos, P.; Rosenstock, H. M. “A Compendium of Gas Phase Basicity and Proton Affinity Measurements”, NBSIR, 79-1777. Huntress, W. T., Jr., Phinuotto, R. F., Jr.; Laudenslager, J. E. J . Am. Chem. Soc. 1973, 9 5 , 4107. Otvor, J. W.; Stevenson, D. P. J. Am. Chem. Soc. 1955, 78, 546. Tate. J. T.; Smith, P. T. Phys. Rev. 1932, 39. 270. Michnowicz, J.; Munson, 8. Org. Mass Spectrom. 1972, 6 , 765. Hunt. D. F.; McEwen, C. N.; Upham, R. A. Anal. Chem. 1972, 44, 1292. Martinsen. D. P.; Butbill, S. E., Jr. Org. Mass Spectrom. 1978, 1 1 , 762.
(35) Hunt, D. F.; Sethi, S. K.; Shabanowitz, J. 26th Annual Conference Mass Spectrometry-Allied Topics, M a y 28June 2, 1978, St. Louis, MO; p 146. (36) Freiser, B. S.;Woodin, R. L.;Beauchamp, J. L. J . Am. Chem. Soc. 1975, 9 7 , 6893. (37) Franchetti, V.; Freiser, E. S.; Cooks, R. E. Org. Mass Spectrom. 1978, 13, 106. (36) Comisarow, M. E.; Grassi, V.; Parisod, G. Chem. Phys. Lett. 1978, 57, 413. (39) Beauchamp, J. L.; Armstrong, J. T. Rev. Sci. Instrum. 198% 40, 123. (40) DeFrees, D. J.; McIver, R. T., Jr.. Rev. Sci. Insfrum. IB77, 48, 574. (41) Hourlet, R.; Gaumann, T. Int. J. Mass Spectrom. IonPhys. 1978, 28, 93. (42) Clow, R. P.; Futrell, J. H. J. Am. Chem. Soc. 1072, 9 4 , 3746. (43) Bwsey, M. M.; Elwood, T. A,; Hoffman, M. K.; Lehman, T. A.; Tesarek, J. M. Anal. Chem. 1970, 42, 1370. (44) Gross. M. L.; Lin, P. H.; Franklin, S.J. Anal. Chem. 1972, 44, 974. (45) FenerCorreia. A. J. V.; Jennings, K. R.; Sen Sharma, D. K. Adv. Mass Spectrom. 1978, 7A, 287.
RECEIVED for review August 12,1980. Accepted December 1, 1980. This work was supported by the National Science Foundation (Grants No. CHE-77-03964 and CHE-76-23549) and Gulf Oil Foundation. Conventional mass spectra were obtained at the Midwest Center for Mass Spectrometry, a NSF Instrumentation Facility (Grant No. CHE-78-18572).
Influence of Analyte-Heavy Atom Micelle Dynamics on Room-Temperature Phosphorescence Lifetimes and Spectra L. J. Cline Love,’ J. G. Habarta, and Marie Skrllec Department of Chemistty, Seton Hall Univeristy, South Orange, New Jersey 07079
The potential analytical utility of micelle-stabillzed room-temperature phosphorescence (MS-RTP) lifetimes is evaluated by extending a dynamic model of micelle-analyte interactions to several limiting cases of analytical significance. The experimental MS-RTP lifetimes of selected single-component systems and a two-component system in mixed heavy atom micelles are reported, and their use as qualitative indicators of the species is discussed. Additional information the lifetimes provide about molecular dynamics compared to conventional low-temperature phosphorknetry is considered. The average microenvironment for these analytes in heavy atom micelles is estbnated by studying their MSRTP, fluorescence, and absorbance characeristics in different polarity solvents and micelle systems. An instrument capable of measuring luminescence lifetimes from the microsecond to seconds range was constructed for these studles and its performance tested.
Micelle-stabilized room-temperature phosphorescence (MS-RTP) recently has been reported for the first time for several types of substituted arenes (I) and its analytical utility demonstrated for some simple aromatic hydrocarbons on the basis of their spectral characteristics (2). Limits of detection comparable to those observed by conventional phosphorescence at 77 K were obtained. Although the MS-RTP and 77 K triplet-state lifetimes were reported for all of the compounds discussed, their potential analytical utility or physicochemical significance were not thoroughly explored. A dynamic model for the micelle-analyte interaction has been developed by Almgren et al. (3) and Yekta et al. ( 4 ) for 0003-2700/8 1/0353-0437601 .OO/O
homogeneous micelles to aid in the study of the micellar assembly. The present study details the dynamics of the analyte-micelle interaction in mixed micelles containing substantial amounts of heavy atom counterions and discusses how the competing processes can affect the observed phosphorescence lifetime. The rate constants determining several limiting cases of analytical significance also are developed. The unique, dynamic nature of MS-RTP fluid systems provides additional parameters which can allow differentiation of similar molecules based on their triplet-state lifetbes where similar differentiation on solid substrates or at 77 K is impossible. The potential analytical utility of MS-RTP lifetimes is discussed and demonstrated for identification of a twocomponent mixture. The relative quenching of certain vibronic fluorescence bands of pyrene was determined in different polarity solvents and types of micellar systems to ascertain (i) the effect of heavy atoms on the micellar environment polarity and (ii) the approximate average location of the analyte in the micellar assembly. Both of these factors can have profound influences on the magnitude of the observed phosphorescence lifetime. Finally, an instrument capable of measuring lifetimes down to 10 ps was designed and constructed to evaluate the triplet-state decay rates of selected compounds. Overall instrument performance is discussed briefly, and the accuracy and precision of the lifetimes obtained for some simple arenes are evaluated.
EXPERIMENTAL SECTION Apparatus. Uncorrected room-temperature and 77 K phos-
phorescence spectra were obtained by using instrumentation described previously (2). Absorbance spectra were obtained with either a Cary 15 or Beckman Acta I11 spectrophotometer. 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
Table I. Components of Room-Temperature Phosphorescence Lifetime Instrument component model and manufacturer
SAMPLE HOUSING
RECORDER
1-
lamp power supply
POWER SUPPLY
I
I
Flgure 1. Block diagram of lifetime apparatus.
The components of the room-temperature phosphorescence lifetime instrument are listed in Table I and a schematic diagram is shown in Figure 1. The flashlamp was triggered by a +6 V, 100-300 Hz remote pulse source and is similar in design to that of Zynger and Crouch (5) but modified to be of simpler design. Thoriated tungsten electrodes (1-3 mm gap) were used. Reagents. The reagents and purification schemes were the same as described previously (2). Procedure. Samples were prepared in the same manner as for the standard curves described previously (2). A blank, omitting the analyte molecule, was also prepared and deoxygenated. For room-temperature phosphorescence lifetime measurements, the pulse generator pulse spacing and pulse width were set depending on (a) the pulse rate of the flashlamp desired, usually from 100 to 300 Hz (3-10 ms pulse spacing), and (b) the desired gate width on the photomultiplier tube (PMT), usually set from 250 to 300 ks. The +6 V amplitude of the square wave pulsed output triggered both the spark discharge lamp and the gated PMT. An auxillary output of +2 V amplitude triggered the external input of the TDH-9 waveform eductor. Gain settings on the current amplifier were adjusted to give maximum amplification without overload and minimum risetime to eliminate damping of the signal. The settings of the TDH-9 eductor were set on the basis of the anticipated lifetimes of the species under study. Depending on the sweep duration (from 2 to 10 ms), the characteristic time constant was set to maximize S/N ratios and minimize collection time. This was dependent on the “dead time” between the actual sweep of the signal averager and the pulses of the lamp. The usual settings were 10-20 s for the time constant of the system with the sweep duration varied to obtain optimum resolution. A delay of 300 p was placed at the beginning of each sweep, and the sweep duration was variable, dependent upon the particular species under study. An auxillary output allowed the monitoring of the buildup of the decay curve on a storage oscilloscope with appropriate time base. After the decay curve waveform was collected in the TDH-9 eductor, it was transferred to a servo recorder utilizing the readout and slow sweep settings. Data collection for each sample was complete in 15-45 s. A constant level dc component contributed to the dc offset of the output for each curve collected. For elimination of error due to this component in the calculations of the phosphorescence lifetimes, the Guggenheim and Mangelsdorf methods were used for graphical and computational determination, respectively, of the lifetimes and are described in ref 6. Each lifetime is the average of at least three separate determinations. The time scales for the lifetimes were calibrated by using (a) the time base of the TDH-9 averager, (b) the time base of the oscilloscope, (c) the compounds eosin and benzophenone of known phosphorescence lifetimes in ethanol at 77 K measured on the same apparatus, and (d) the lifetime of uranyl glass at ambient temperature. For measurement of lifetimes at 77 K, the apparatus described above was modified as follows. The +5 V trigger from the waveform eductor (internal mode) triggered both the PMT gating circuit and a shutter (Vincent Associates, Model 225XOROX5, Rochester, NY) via the shutter drive (Model 100-2). The negative edge of the pulse opened the shutter and, at the same time, was sent through two SN74121 monostable multivibrators and asso-
Model 437A nanopulser, Xenon Corp. (Medford, MA) lamp laboratory constructed spark discharge, Hacker Machine shop (Lansing, MI) sample compartment American Instrument Co. (Silver Springs, MD) phosphoroscope, modified for l-cma cells and excitationlemission filters filters Corning No, 7-54 (excitation), Corning No. 3-69 and 3-72 (emission), F. H. Gray, Inc. (Queens, NY) PMT power supply Model EU-42A operated at 1000 V, Heath, Co. (Eenton Harbor, MI) PMT Hamamatsu R928 (Middlesex, N J ) PMT housing Model E U-70 1-93, GCA/McPherson (Chicago, IL), modified for electronic gating circuitry as follows: addition of 1N5271, 100 V, Zener diode between photocathode and second dynode resistor; two O.Ol-pF, 3-kV capacitors from second dynode to ground; 1.2-kn load resistor from anode to ground PMT gating circuit Model GBlOOlA, EM1 Gencom, Inc. (Plainview, NY); + 5 V, 0 . 5 ~ s minimum variable width gate input used pulse generator Model 4001, Continental Specialties Corp. (New Haven, CT) amplifier Model 427, Keithley (Cleveland, OH) signal averager Model TDH-9 waveform educator, PARC (Princeton, NJ) servo recorder Model E 10015, series E, Esterline Angus (Indianapolis, IN) oscilloscope Type 564 storage scope with Types 3A74 and 3B3 plug-ins, Tektronix (Portland. OR) ciated pulse generator circuitry to the PMT gating circuit to turn off the PMT. On the next cycle, the +5 V pulse holds the normally open shutter closed during the duration of the waveform eductor sweep and turns the PMT on to measure the phosphorescence decay. The PMT output into the waveform eductor is treated in the same manner as described previously for room-temperature studies. A liquid nitrogen cooled Dewar was fitted to the sample compartment, and quartz sample tubes were used. A 150-W xenon continuum light source, Hanovia Model 901-0011 (Newark, NJ), in a lamp housing, Schoeffel Instruments Model LH150 (Westwood, NJ), was used for excitation.
THEORY In order that a unique and analytically useful lifetime, characteristic of each system, may be deduced, the dynamic equilibrium between the various competing processes deactivating the excited analyte as it migrates inside and outside of the stable micelle can be detailed and used to understand the factors affecting the observed lifetimes. Micelle structure and micelle-lumiphor interactions have been described elsewhere (7-11). A kinetic model for the partitioning of an excited triplet-state molecule between the aqueous phase and the micelle with general equations showing the effects of kinetic properties of the system on the observed lifetime for relatively long-lived species has been developed by Almgren e t al. (3). The implications of the model for the potential analytical usefulness of MS-RTP are discussed below. It will be shown that although this is a dynamic system, in contrast to conventional 77 K phosphorimetry or adsorption on solid
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
surfaces RTP, with proper control the observed lifetimes are reproducible and potentially useful for analysis. This dynamic property of the fluid system adds a useful parameter which allows, in some sytems, differentiation based on triplet-state lifetimes at room temperature where similar differentiation a t 77 K or on a solid substrate is not possible. In the absence of secondary processes, radiative deactivation of the triplet state follows a single exponential decay and the concentration of excited triplet-state species in the micelles, [MP*], at any point in time is described by the first-order kinetic model in eq 1. The phosphorescence lifetime is de[MP*It = [MP*Ioe-'/l
-E 9
k
%
a
.p:
+
t
B B
CL
z
a a
a
(1)
scribed by eq 2, whose derivation is given in Appendix I
h
8
g K a
(2) (supplementary material). Equation 2 indicates that the observed lifetimes depend on the following: (a) rate constants for exit from micelle (k-),deactivation within the micelle ( k w ) , and deactivation outside the micelle (k,); (b) products of concentration of internal quencher and its quenching rate (kqINIQ]IN)and external quencher and its quenching rate (kqEX[Q]m);and (c) reentry rate and concentration of micelles (k+[M]). It should be noted that radiationless deactivation rates of the analyte in each location, such as internal conversion, self-quenching, and effects due to the presence of heavy atoms, are included, along with the radiative rates, in km and k,. Certain assumptions and simplifications of eq 2 which result in analytically useful interpretations of experimental lifetimes will be discussed below. Development of three limiting cases of analytical interest are given in Appendix I (supplementary material), and some results are given in Table 11. One of the simplest cases, encounterd for many synthetic mixtures as well as real samples, is when the concentration of internal and/or external quenchers is negligible. Under those conditions, eq 2 reduces to eq 3. Since this is a general case,Tl+or any other
1
kk+[MI
7
k, + k+[MI
- = k- + kMp -
I
E
n
.
9
Et
8 a
c
.-I
+ I
8
B
a
(3)
heavy atom ions which induce MS-RTP are not considered explicity as quenchers of the excited state by not including a corresponding rate constant in eq 3. Examination of typical values for some of the terms in eq 3 and the dynamics of the system suggests that some rates may be significantly larger than others. For example, Almgren et al. (3) showed the k+ was much larger than k- for all cases studied. Also the assumption that k+ is much larger than k , is reasonable since the reentry rate constant, k+, has been estimated to be approximately lo9 M-' s-l (3). If k , were the same magnitude as k+, that would mean that the triplet-state lifetime in the external phase would be in the nanosecond range, which is unlikely on the basis of present knowledge of the triplet state. Another term of interest is the product k+[M], describing the reentry processes which follow second-order kinetics. Although k+ is on the order of lo9 M-' s- l, the product k+[M] depends on the concentration of micelles, [MI, and can be significantly smaller. The [MI is calculated from eq 4. The [MI = [(detergent) - CMC]/(aggregation number)
X
w 0 a
(4)
CMC is the critical micelle concnetration, that is, the concentration of surfactant at which the micelles start to form. The CMCs for the investigated TlLS/NaLS and AgLS/NaLS systems were determined in the authors laboratory by use of a tensiometric method and were found to be 3 X and 2
5
3
4
B
a
0 II
B E
n
II
1;: 9 4
n
-p:
+ !i a
II an E
n
E
E +
a
&
+
B
a
439
440
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
Table 111. Micellar Triplet-State Characteristics of Selected Arenes lifetime avo
77 K residence 20 O C C 295 K b solubility time, ~ r s ethanol ethanold EPAe compound (M)a in H,O (NaLS) Tl/NaLS (ms) Ag/NaLS (ms) (w) (SI (8) pyrene 6 x lo-' 244 0.93 (0.988/3.9%) 0.60 (0.989/4.9%) 10.2 0.47 0.58 biphenyl 4.1 x 10-5 10 0.43 (0.985/4.9%) 0.33 (0.979/5.7%) 3.6 4.15 3.7 naphthalene 2.2 x 4 0.45 (0.989/4.0%) 0.31 (0.978/5.7%) 2.35 2.3 a See ref 3. Average standard deviation = i0.04 ms; correlation coefficients and the average relative standard deviation for each regression coefficient used in calculating a lifetime are in parentheses; sample temperature ranged between 293 and 297 K;analyte concentration -4 X M. See ref 13. Average standard deviation = 20.05 s; analyte concentration -1.5 x M. e See ref 12. X M, respectively. From eq 4 a typical 0.1 M surfactant concentration, a CMC of 0.003 M, and an aggregation number of 100, the micelle concentration, [MI, is about 1 X M. Therefore, k+[M] is on the order of lo6 s-l or less. With these simplifying assumptions it can be seen that the observed lifetime depends primarily on k- and kMp when the concentration of the system is kept constant with respect to surfactant concentration. The contribution from k, is negligible under a number of conditions: when k- is greater than km and k+ is greater than k-, the excited analyte experiences a minimum amount of deactivation in the external phase due to rapid reentry into another micelle, (ii) if k- is approximately equal to km the majority of deactivation is experienced within a single micelle after excitation, before migration of the probe into the external phase. The second-order reentry rate constant, k+[M], may be adjusted only over an order of magnitude by varying the micellar concentration. Any adjustment outside of about 2 X to 2 X M gives either too viscous or too dilute (below the CMC) solutions. Since each of the rates is a physical property of a particular analyte in a specific micellar environment, the resulting lifetime is a constant and is a qualitative indicator of the analyte.
RESULTS AND DISCUSSION Single Component MS-RTP Lifetimes. The experimental MS-RTP lifetimes for the three arenes studied are given in Table 111. They all are less than 1 ms under the experimental conditions. Correlation coefficients of 0.98 or greater and average relative standard deviations of the linear regression coefficients (-5%) obtained from the logarithmic treatment of the data indicate that a single component was observed in each case over the time span measured. The lifetimes measured in a 5 5 2 mixture of ethyl ether, isopentane, and ethanol (EPA) at 77 K (121,in ethanol at 77 K, and in ethanol at room temperature (13)are also shown and will be discussed later in this paper. There is a great deal of information about the dynamic character of the system that is contained in the observed lifetimes. The data in both the thallium and silver micelle systems indicate that the lifetimes depend primarily on the kh8 and k-, case II(A) in Table 11. In addition to the inherent radiative decay for this system, km includes the specific spin-orbit coupling effects and internal conversion. These effects would be expected to influence the radiative decay of the different molecules studied in differing degrees. Although the lifetimes are generally proportional to the average residence time (inverse of exit rate k-) this relationship does not suffice to explain the data. Some factors that affect the residence time of an analyte in a micelle include internal polarity affinity, analyte size/mobility, and the analytes solubility in the external bulk solvent. For example, the inherent rates for homologous compounds in specific micellar systems are often different, whereas at 77 K no significant difference in lifetimes is noted. Examples of such compounds
are pyrene and pyrenebutyric acid and 2-naphthylacetic acid and 1-naphthylmethylcarbinolin which significant differences in lifetimes via MS-RTP were observed compared to lifetime differences a t 77 K (1). These differences can be attributed to many of the rate parameters discussed in addition to solvent or acid-base stabilizing effects of the microenvironment. The data in Table I11 show that the MS-RTP lifetimes for these simple arenes are inversely proportional to the analytes solubility in water. This then suggests a convenient method of predicting the approximate ordering of MS-RTP lifetimes based on their solubility characteristics alone when no other specific interactions are present (3). The dynamic nature of the micellar environment is underscored by comparing the lifetime data in solution at 25 OC with the 77 K frozen matrix data. The ordering of MS-RTP lifetimes is quite different from that a t 77 K, pyrene being the shortest at 77 K but the longest at room temperature. This illustrates the effect of the micelle fluid environment upon the analytes lifetime. Solvent reorientation, equilibration of the initial excited Franck-Condon state (Stokes shift), and many solvent/excited-solute reactions are negligible at 77 K but quite possible in fluid solution. Therefore, the values at 77 K represent base-line values, inherent to the molecules triplet-state oscillator strength, enhanced by lessened thermal internal conversion, and not appreciably influenced by microenvironmental effects. When one examines the lifetime values at 20 "C reproduced in Table III (13),a similar ordering to that observed in the MS-RTP systems is seen, further substantiating the effect of a fluid environment. This indicates that although k ~ ispgreatly influenced by micelle dynamics, it can reflect the relative ordering of phosphorescence lifetimes in a fluid environment. Comparisons of conventional lifetime data obtained at 77 K with MS-RTP lifetimes must be made with extreme caution because of the many other parameters coming into play in a dynamic system. Examination of MS-RTP lifetimes between the two different heavy atom micelle systems utilized yields distinct differences. In all cases observed, the thallium heavy atom micellar environment produced longer lifetimes and greater relative intensities compared to silver heavy atom micellar environment. This agrees with the expected increase in the spin-orbit coupling constant with increased atomic number, but direct comparisons should be cautioned against because of the unique interaction silver is known to have with aelectron systems (14).Since this silver a-electron interaction seems to affect both the ground and excited state of the analyte, as seen in our absorbance and luminescence spectra, and since the micellar dynamics in going from T1 to Ag is expected to be slightly altered due to the different counterion, the differences in observed lifetimes seem reasonable. The analyte may be closer in proximity to the silver counterions, and hence the micelle surface, than it is in the thallium system, which would cause an increase in k- and k M p It is also interesting to note that T1 appeared to produce little or no
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981 _ _ _ _ _ _ _ ~
441
~
Table IV. Triplet-State Lifetimes of a Two-Component Mixture by MS-RTP av re1 std dev mixturea of regression coefficient, % lifetimes 7,ms component
single-component lifetimes 7,ms
% error
10.6 pyrene 27.4 naphthalene Concentration ratio 1:2 molar pyrene to naphthalene in Tl/NaLS micelle; correlation coefficients in parentheses. 1.04 (0.984) 0.62 (0.998)
8.9 2.0
0.93 0.45
>
t z W
v)
t
z W
u z W
0 v)
W
5
z
2
300
42 5
550
675
WAVELENGTH
(nm)
800
925
Figure 2. Luminescence spectra of 0.5 X lo-' M pyrene and 1.0 X IO-' M naphthalene mixture in 0.15 M TILSINaLS micellar system with 30% TI'; X, = 284 nm. Peak at -570 nm is partially due to
second-order scatter.
enhancement of phosphorescence at 77 K (14) but did appear to give greater enhancement for several polyaromatic hydrocarbons vs. Ag at room temperature on a filter paper matrix (15). Thus, it appears that there may be a favorable factor, perhaps geometrical, entering in both the micellar and filter paper environments. Though heavy atoms have been shown to increase the rate of deactivation of the excited triplet state (16), it has been observed in the authors laboratory that as the relative amount of heavy atom is increased, in a micellar environment, the lifetimes tend to approach a limiting value. This value corresponds to that analyte's largest value of km in a specific micelle-heavy atom environment. This value should still be smaller than the k- value and an excited analyte would still be free to migrate to a second micelle to continue to undergo deactivation when no external quencher is present. Two-Component MS-RTP Lifetimes. The feasibility of measuring lifetimes and spectra for two-component probe mixtures was demonstrated. Figure 2 shows the luminescence spectra of 1:2 molar mixture of pyrene and naphthalene in a Tl heavy atom micelle. The two-component semilogarithmic decay curve is given in Figure 3. After data reduction of each segment by Guggenheim method described in ref 6, the best fitting straight lines were determined, as for the single components. These are shown in Table IV. The deviations from single-componentvalues (percent error) in the two-component lifetime determinations, particularly for naphthalene, can be explained by a combination of chemical and instrumental factors. First, to ensure that no more than one analyte occupies each micelle and thus prevent triplet-triplet anilhilation or analyte quenching and to minimize inner filter effects, we reduced the concentrations
0.4
0 .a
TIME(rnsl
Flgure 3. Multicomponent phosphorescence decay for 1.7 X M pyrene and 3.2 X 10" M naphthalene kr 0.15 M TLS/NaLS, with 30:70 molar ratio of counterions.
producing concomitant decreases in precision. Second, the fluorescence spectral distribution of naphthalene signifiFtly overlaps the longer wavelength absorbance bands of pyrene between and 310 and 360 nm, resulting in reabsorption of naphthalene emission by the pyrene. It can be seen in Figure 2 that there is negligible residual fluorescence emission occurring in the 310-330-nm region for naphthalene, while pyrene still contains significant residual fluorescence. Third, the 337-nm excitation wavelength used was very close to the optimum 342-nm line chosen for pyrene, but lacked appreciable intensity a t the optimum 280-nm line of naphthalene. In addition, the emission filters used had higher transmission characteristics near the pyrene emission compared to that of naphthalene. Thus, both excitation and emission intensities were diminished for naphthalene in the two-component measurement as compared to the optimized single-component measurement. Fourth, the precision was poorer, in part, because the number of channels available per component on the 100channel waveform eductor was reduced by half. ALSO,the lack of sensitivity of the output of this unit for the longer component of the exponential tail limited the useful range over which the curve could be observed. If a foreign component capable of quenching the excited triplet state and/or the excited singlet state of a molecule were present in a sample, three general cases of the quencher's preferred location in an anionic micellar system are possible:
442
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981
(i) in the aqueous phase (such as nitrite), (ii) in the micellar hydrocarbon interior (such as iodoheptane), and (iii) about equally distributed between the two phases (such as oxygen). Because MS-RTP is a unique, dynamic phenomena, the probability of an analyte-quencher interaction leading to the deactivation of the excited analyte is enhanced tremendously in solution, in contrast to the rigid 77 K glasses or roomtemperature solid surfaces phosphorimetry. It can be seen that the micelle-solubilized quenchers will be problematic in the applications of MS-RTP and that the aqueous-phase quenchers will interfere to a great extent when k- is greater than k ~ p which , is true for relatively long lived species ( 3 ) . Conversely,the knowledge of quencher locations and dynamics can be used advantageously for selective quenching of species located externally or internally, as has been demonstrated by many fluorescence studies in micellar systems (17,18). Other common causes of decreased sensitivity in fluorescent mixtures such as bimolecular quenching, self-quenching, or perturbations of the oscillator strength do not have any significant effect since each microscopic system is self-contained with a maximum of one molecule per micelle. This lack of chemical interaction of one luminescent molecule with another is further emphasized if one carefully examines the vibrational multiplets for both naphthalene and pyrene. Although the overall intensities are different, the spectral band intensity ratios within a multiplet are the same as those determined individually for each probe. In additon, even though this is a mixture, the individual lumiphors still should follow case II(A) dynamics as for the isolated individual species. Therefore, a molecule having a specific exit rate, k-, and micellar phosphorescence rate, km, relating to molecular structure, etc., can retain these characteristic properties in the presence of a second species. This protection from mutual interference, accorded by the micellar environment, thus allows for the individual characterization in a mixture without prior separation. Analyte Microenvironment in Heavy Atom Micelles. The location of the molecule in a heavy atom micelle should have a pronounced effect on both the formation rate of the excited triplet state species and on its radiative and radiationless rates of deactivation. The effect of different heavy atom counterions and the polarity of the microenvironment were related to the probable average location of the analyte in the micelle, which is, in turn, related to the average residence time, 1/k-. This was done by a series of solvent polarity studies using the absorbance, fluorescence, and MS-RTP spectra of pyrene. MS-RTP Spectral Characteristics. The MS-RTP spectra of pyrene in Ag/NaLS micelles and Tl/NaLS micelles are shown in Figure 4A,B, respectively. The silver counterion produces both broadening and blurring of the MS-RTP peak compared with thallium, which is consistent with earlier observations on other analyte species ( 2 , 1 4 ) . More differences are seen when the MS-RTP spectra are compared to that at low temperature in Figure 4C. First, the MS-RTP spectra are red shifted compared to low-temperature spectra. This is not unexpected since the heavy atom induces enhanced spin-orbit coupling which allows mixing of a higher energy S1singlet with a lower energy triplet, T1(19). Second, much more fine structure is seen at low temperature compared to room temperature. Also, drastic suppression of the high-energy vibronic bands of pyrene in the micelle stabilized spectra occurs compared to low-temperature spectra. Although recent reports have dealt with room-temperature phosphorescence of pyrene on solid substrates (15,20,21), no specific comments have been made concerning the high-energy bands observed in some pyrene spectra. The excitation spectra of the lowenergy bands and the faint high-energy bands (observable at
I:
C
380
480
WAVELENGTH
580
880
(nm)
Flgure 4. Luminescence spectra of 0.5 X lo4 M pyrene In (A) 0.08 M AgLSlNaLS micellar system with 50% Ag', (6)0.15 M TILS/NaLS micellar system with 30% TI', XEX = 336 nm in both cases, and (C) In ethanol solution at 77 K, X, via cutoff filter transmltting below 400 nm.
our highest sensitivity setting) were different in our studies in Tl/NaLS micellar media, indicating that the high-energy bands may be due to a trace impurity or to different excited states. However, HPLC analysis of the pyrene sample did not shown any extraneous peaks attributable to impurities. Fluorescence Spectral Characteristics. The effect of solvent environment on the vibronic band intensities in pyrene fluorescence are thought to be the results, predominately, of solute-solvent dipole-dipole coupling (22). The changes in intensity within the forbidden vibronic band progression have a qualitative relationship with variation in solvent dipole moment. The relative polarity of the analyte microenvironment is, in turn, considered an indication of the probable average location of the molecule within the micellar assembly (23,24). By examining the vibronic structure of the pyrene fluorescence in 0.1 M NaLS, water, and n-hexane, one can make a qualitative estimation concerning the probable location of the pyrene molecule in the micelle and the analytemicelle dynamics. Comparison of NaLS fluorescence with Tl/NaLS or Ag/NaLS residual fluorescence must be done with extreme caution because in the latter cases one may be observing fluorescence from molecules located in only certain parts of the heterogeneous micellar solution, thus giving misleading results. The residual fluorescence may not be representative of the total analyte distribution in the micellar solution. The three bands of interest in the pyrene fluorescence spectra are labeled I-III in Figure 5. It should be noted that the residual fluorescence observed in the Ag/NaLS micellar solution, curve E of Figure 5, also exhibits broadening of the bands relative to the other solvents. This is consistent with the broadening seen in the absorbance and phosphorescence spectral bands when silver is present as the heavy atom counterion. Bands I and I11 are due to the totally symmetric, forbidden vibration (includes the 0-0 band) and they increase in intensity as the solvent polarity increases (23,25). The nontotally symmetric, allowed b, vibration gives rise to band 11,which is relatively insensitive to solvent polarity (24).
ANALYTICAL CHEMISTRY, VOL. 53, NO. 3, MARCH 1981 443 time between excitation and emission (27,281. No significant wavelength shifts in the absorbance maxima at 334 and 272 nm of pyrene relative to those in hexane was seen, and no broadening was observed when the solvent system was changed to ethanol, ethanol-AgLS, or ethanol-AgN03. However, there were significant changes in the magnitudes of the absorbance band ratios a t the wavelength maxima relative to the ratios in hexane in going to the more polar ethanolic solutions. This was expected since the Ham effect was originally observed in absorbance spectra (26). The absorbance spectra of solutions of pyrene in aqueous NaLS and aqueous Tl/NaLS solvent systems shows red shifts at both wavelength maxima but no appreciable broadening of the bands. The aqueous Ag/NaLS solvent system also produced red shifta in the pyrene spectra relative to the hexane solvent system and showed pronounced broadening effects, analogous to those seen for both fluorescence and MS-RTP, indicating ground-state vibronic perturbations of the analyte by silver ions. Silver only broadened the spectrum when it was incorporated into the aqueous micellar assembly and not when introduced into an aqueous solution of the analyte. Similar results were observed for naphthalene. No meaningful interpretations could be made for biphenyl because it exhibits only one broad absorption band around 250 nm, which is close to the 240 nm optical cutoff point of heavy atom micellar solutions. Therefore, it appears on the basis of absorption spectra that the average environment of pyrene in the T1/ N U and Ag/NaLS heavy atom micelles is slightly less polar but similar to that of the NaLS micellar system.
t
v)
2
w
tW
0 2
YI
0 v) w
a
5 -1
IL
350
410
WA ’ELE 1Gl H
ACKNOWLEDGMENT
(nm)
Flgure 5. Fluorescence spectra of pyrene in (A) 0.15 M TiLS/NaLS micellar system with 30:70molar ratio of counterions, (B) water, (C) 0.1 M NaLS mlceilar system, (D) hexane, (E) 0.08 M AgLS/NaLS micellar system, with 50:50 ratio of counterlons. A and E are residual fluorescence. Pyrene concentration was -1 X lo-‘ M except in water where it was 1 X lo-’ M.
-
~~~
The authors thank Princeton Applied Research Corp. for the loan of the Model TDH-9 waveform eductor. We also thank Linda M. Upton for helpful discussions on the limiting rate equations and Andrew Mangini, Department of Chemistry Machine Shop, for his aid in the design and construction of sample compartments and the flashlamp.
~~
Table V. Pyrene Fluorescence Band Intensity Ratios in Different Polarity Solvents solvent system intensity ratioa II/I n-hexane 1.53 Ag/NaLS 1.05 TI/NaLS 1.04 NaLS 0.95 water 0.69 Band I1 taken at 384 nm; band I taken at 314 nm. This phenomenon is commonly referred to as the Ham effect (26). The intensity ratios of bands II/I are shown in Table V as a function of solvent. The data indicate that residual fluorescence from analytes in heavy atom micelles originates from a somewhat less polar environment than fluorescence from analytes in NaLS micellar solution. The presence of heavy atom counterions would have influences on micelle size (aggregation number) and penetrable volume into which both the analyte and bulk solvent water molecules can diffuse. We have found the critical micelle concentration (CMC) for these mixed counterion micelles is about 4 times lower than the CMC of NaLS. Absorbance Spectral Characteristics. The shapes and intensities of the absorbance bands of pyrene were studied as a function of solvent polarity (i) to determine if the changes in the luminescence spectral bands are a property solely of the excited state and (ii) to determine if the ground state is affected also by incorporation of heavy atoms into the micellar assembly. These data, together with the fluorescence data from NaLS experiments, could be helpful in deducing the probable average location of the analyte with respect to the
Supplementary Material Available: Appendix I, derivation of eq 2 and development of three limiting casea of analytical intrest (5 pages), will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper or microfiche (105 X 148 mm, 24X reduction, negatives) may be obtained from Business Operations, Books and Journals Division, American Chemical Society, 1155 16th SL, N.W., Washington, DC 20036. Full bibliographic citation (journal, title of article, author) and prepayment, check or money order for $5.00 for photocopy ($6.50foreign) or $4.00 for microfiche ($5.00 foreign), are required.
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(15) Yen Bower, E. L.; Winefordner, J. D. Anal. Chim. Acta 1978, 702,1. (16) Kalyanasundaram, K.; G-ieser, F.; Thomas, J. K. Chem. Phys. Len. 1977, 57, 501-505. (17) Habarta, J. G.; Cline Love, L. J. 30th Pittsburgh Conference on Analyilcal Chemistry and Applied Sectroscopy, Cleveland, OH, 1979; Abstract No. 601; ABC Press: Monroevllle, PA. (18) Khuanga, U.; McDonald, R.; Sellnger, B. K. 2. Phys. Chem. (Wesbaden) 1976, 101, 209-223; Chem. Abstr. 1976, 85, 1987582. (19) McGlynn, S. P.; Daigre, J.; Smith, F. J. J . Chem. Phys. 1963, 39, 675-679. (20) Vo-Dinh, T.; Yen, E. L.; Winefordner, J. D. Talenta 1977, 24, 146. (21) Vo-Dinh, T.; Hooyman, J. R. Anal. Chem. 1979, 57, 1915. (22) Kalyanasundaram, K.; Thomas, J. K. J . Am. Chem. Soc. 1977, 99, 2039-2044.
(23) Dorrance, R. C.; Hunter, T. F. J. Chem. Soc.,Faradiy Trans. 7 1977, 73, 1891-1899. (24) Thomas, J. K. Acc. Chem. Res. 1977, 70, 133-138. (25) Nakajima, A. Bull. Chem. Soc. Jpn. 1974, 4 4 , 3272. (26) Ham, J. S. J . Chem. Phys. 1953, 21, 756. (27) Cardinal. J. R.; Mukerjee, P. J . Phys. Chem. 1978, 82, 1614-1620. (28) Cardinal, J. R.; Mukerjee. P. J . Phys. Chem. 1978, 82, 1620-1627.
RECEIVED for review May 27, 1980. Accepted December
1,
1980. This work was presented in p& at the 31st Pithburgh Conference On Chemistry and Spectroscopy, Atlantic City, NJ, March 14, 1980; Abstract No. 820.
Matrix Interferences in Graphite Furnace Atomic Absorption Spectrometry by Capacitive Discharge Heating C. L. Chakrabartl," C. C. Wan, H. A. Hamed, and P. C. Bertels Department of Chemistry, Carleton University, Ottawa, Ontario, Canada, K1S 586
An analytical technique has been developed that employs an anisotropic pyrolytic graphite tube atomizer which is heated at very high heating rates (up to 100 K ms-') by capacitive discharge to produce high temperature (up to 3300 K) and an isothermal conditlon. Synthetic samples of saline water were analyzed by capacitive discharge technique and also by the conventional graphite furnace atomic absorption spectrometry (AAS), using for the latter the Perkin-Elmer HGA 76B. Recoveries by the capacitive discharge technique and the conventional graphite furnace AAS were typically about 100 % with the former technique and 12-75 % with the latter technique. There is also another very significant diffe r e n c e l h e background corrector was not required nor used with capacltlve discharge technique, whereas the background corrector was required and used with the conventional graphite furnace AAS. Since the sensitivities of capacitlve discharge technlques are independent of the matrix, calibration curves are neither necessary nor used for analysis. The sensitivity constant is evaluated for a given analysis line and for given experlmentai condltions using a single-point calibration with a standard of the anaiyte salt in ultrapure water. The mass of analyte M in unknown samples is evaluated from its AWakand the sensitivity constant.
Numerous workers (1-13) have reported matrix interferences in graphite furnace atomic absorption spectrometry (GFAAS). Slavin et al. (14-16) have reported various ways of reducing interferences especially with lead. Commercial electrothermal atomizers which are used in graphite furnace atomic absorption spectrometry have two serious limitations-their low heating rates limit them to much lower sensitivity than is achievable at much higher heating rates and they are nonisothermal (spatially and temporally). The result of the latter is that samples which are vaporized from hot central parts condense at the cold ends of the graphite tube resulting in much lower residence times, hence, lower sensitivities; also, the condensed analytes are revaporized in the next analysis producing memory effects and erroneous results (17). Also, their slow heating rates, nonisothermal conditions, and the much lower vapor temperatures into which the samples are vaporized cause severe matrix interferences 0003-2700/81/0353-0444%01.OO/O
of various kinds: spectral, chemical, and physical interferences. The effects of heating rates in graphite furnace atomic absorption spectrometry have been reported in earlier publications (18, 19).
CAPACITIVE DISCHARGE TECHNIQUE Cresser and Mullins (20) have suggested the use of a large electrolytic capacitor for rapid heating of metal filament atomizers. A practical device for capacitive discharge heating has been patented (21). L'vov (22) has reported the use of a capacitor bank as a source of electrothermal energy for heating graphite atomizers. Capacitive discharge technique has enhanced the sensitivity of GFAAS (23) and has made it relatively free from matrix interferences (24). This relative freedom from matrix interferences has been accomplished with an anisotropic pyrolytic graphite tube atomizer which is heated at very high heating rates (up to 100 K ms-') by capacitive discharge producing isothermal conditions and high temperatures (24). Conyentional instrumental analytical techniques require careful calibration of the instrument with chemically analyzed standards or synthetic standards of known composition. When analyses of miscellaneous materials are required, the task of providing the required range of standards becomes insurmountable and instrumental techniques then lose their accuracy, since accurate analyses generally necessitate the use of standards which are closely similar in composition to the sample for analysis. The reason for such close matching of standards and samples in their composition is that the sensitivity of conventional instrumental techniques depends on the sample composition, often in a complicted way. The necessity of preparing standards of the same composition as that of the unknown samples involves prior knowledge of the sample composition and chemical treatment or modification of the standards and/or samples-the latter exposes the samples to the risk of contamination and/or loss; the consequence is questionable results, especially in trace and ultratrace analysis. The new technique described in this paper employs electrothermal heating of an anisotropic pyrolytic tube atomizer in atomic absorption spectrometry producing very fast rates of heating (up to 100 K ms-') and an isothermal condition, both spatial and temporal (23). This technique dispenses with the calibration curve and yields analytical results directly from the absorbance of the unknown sample 0 1981 American Chemical Soclety
