560
Anal. Chem. 1988, 60,
(16) Evans, J. F.; Kuwana. T. Anal. Chem. 1977, 49, 1632-1635. (19) Dole. M. I n The Radiation Chemistry of Macromoiecuies; Dole, M.. Ed.; Academic: New York, 1973; Vol. 2, Chapter 6. (20) Clem, R. G.;Sciamanna, A. F. Anal. Chem. 1975, 47, 276-260. (21) &Vaev, V. M.: Ryabchikova, G. G.: Chalykh, A. E.; Plavnik, G. M. R O C . Tlhany Symp. Radiat. Chem. 1977, 4 , 465-470. Chem. Abstr. 1978, 88, 752299. (22) Hiemenz, P. C. Polymer Chemistry: The Basic Concepts; Dekker: New York, 1964; 394-396. (23) Chapiro, A. Radiation Chemistry of Polymer Systems ; Interscience: New York, 1962; Chapter 12. (24) Hawley, M. D.: Tatawawadi. S.V.; Piekarski, S.; Adams, R. N. J. Am. Chem. SOC. 1967, 8 9 , 447-450. (25) Ishimitsu, T.; Hirose, S.; Sakurai, H. Taianta 1977, 2 4 , 555-560. (26) Price, W. P., Jr.; Edens, R.; Hendrix, D. L.; Deming, S. N. Anal. Biochem. 1979, 9 3 , 233-237. (27) Wang, A; Tuzhi, P. Anal. Chem. 1086, 58, 3257-3261.
560-564
(28) Nagy, G.; Gerhardt, G. A.; Oke, A. F.; Rice, M. E.; Adams, R. N.; Moore, R. B., 111; Szentirmay, M. N.; Martln, C. R. J. Eiecfromal. Chem. Interfacial Electrochem. 1985, f88, 85-94. (29) Mueller. K. fharmacol. Biochem. Behav. 1986, 25, 325-328. (30) Justice, J. B., Jr. Microchem. J. 1986, 3 4 , 11-14. (3 1) CRC Handbook of Chemistry and Physics ; CRC Press: Boca Raton, FL, 1981: p D-142.
RECEIVED for review April 21, 1987. Accepted November 13, 1987. This work was supported by Army Research Office Grant DAAG29-82-K-0161, National Science Foundation Grant CHE-8217045, and the Edison Sensor Technology Center. L.A.C. acknowledges support of the University of Cincinnati Research Council.
Alternating Current Amperometry at a Carbon Paste Electrode with Charging Current Nulling M a r k B. Gelbert' and D. J. C u r r a n *
Department of Chemistry, University of Massachusetts, Amherst, Massachusetts 01003
Phase sensitive detectlon and charging current nuiilng have been used with carbon paste electrodes to explore alternating current (ac) amperometry. The chemical system used was o-dlanlsldlne which Is klnetkally fast and revereible In the ac sense at the frequencles used. A circuit Is presented that allows phase and amplitude control of the nuilng current. I t 1s demonstrated that the charging current can be reduced by about 3 orders of magnitude before 11 enters the current-tovoltage converter. This Instrumental capability enabled ac amperometric measurements of solutions as dilute as 0.5 nM. Peaks In the ac background current at carbon paste eiectrodes were found that are attributed to redox active surface functionaiities.
In the following work, phase sensitive alternating current amperometry is used with the carbon paste electrode to produce an electrochemical detector capable of working at extreme dilution. The ac instrument used in the study was developed in these laboratories by Kingsley (6) and was modified to include an alternating current nulling circuit connected to the summing point of the current-to-voltage (I/E) converter. By use of the nulling current, a large fraction of the ac charging current could be removed from the signal entering the I/E converter and ac signals produced by nanomolar to subnanomolar concentration changes of o-didanisidine (0-DIA) were measured. Some observations on background currents at carbon paste electrodes are made which indicate that surface currents can be studied in the ac mode.
Sinusoidal alternating current (ac) polarography has been known for about 5 decades but very little work has appeared involving electrode materials other than mercury, despite the fact that solid electrodes have great utility in the anodic region for the study of organic oxidations. Among the several types of carbon electrodes, the carbon paste electrode is known for its very low background currents (1-3). However, the publication by Walker, Adams, and Juliard appears to be the only one where the carbon paste electrode has been used with ac voltammetry ( 4 ) . The oxidation of a number of organic compounds was studied, and the response was reported to be unaffected by stirring the solution. More recently, Hanekamp, Bos, and Vittori used the phase sensitive alternating current technique with flow stream detectors (5). Glassy carbon and a horizontal dropping mercury electrodes were used. The interesting observation was made that it was analytically useful to operate at amplitudes for the applied ac voltage as large as 10-fold above the values commonly used. Some of their data was obtained by running complete current-voltage curves and some flow injection work was done at a fixed direct current (dc) potential.
Apparatus. The ac voltammetric potentiostat with digital phase sensitive detection was constructed in-house (6). The analog
EXPERIMENTAL SECTION
'Present address: The Procter & Gamble Co., Sharon Woods Technical Center, 11511 Reed Hartman Highway, Cincinnati, OH 45241.
signal from the I/Econverter is filtered by a tuned amplifier and then converted to a digital signal with a voltage-to-frequency (V/F) converter. The output frequency is proportional to the amplitude of the input sine wave signal. The pulses are counted by a set of up/down counters which are controlled by timing circuitry triggered by a reference square wave derived from the internal oscillator which drives the electrochemical cell. The net count is converted to a dc voltage by a digital-to-analog converter (DAC) which is connected to either a strip chart or X-Y recorder. A more detailed description of the instrument is available (6). The carbon paste electrode was constructed in a Teflon holder which was attached to a piece of glass tubing containing a copper rod for electrical contact. The diameter of the carbon paste well was 0.25 in. The paste consisted of Ultra Carbon USP graphite powder and Nujol in the ratio of 1.65/1 by weight. The paste was prepared by wetting the mixture with 10 mL of spectroscopic grade toluene and stirring well. The toluene was blown off with a stream of purified nitrogen gas and the paste was placed under vacuum in a desiccataor for 12 h. The solution of electrolyte was contained in an ordinary electrochemical cell with a standard calomel reference electrode (SCE) and a platinum button counter electrode. For sensitive work, the cell was shielded with aluminum foil connected to instrument ground. A magnetic stirring system was used when mixing of the solution was necessary.
0003-2700/86/0360-0560$01.50/0 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988 Resistor Network
561
1nu11
I
T 0.64 u A
Oscillator
1
Phase Shifter 0
Voltage Divider
360‘
Figure 2. Block diagram of the ac nulling current circuit.
-
b
00
R;5K
0 - 4
90 K
E (mv) vs SCE
Figure 1. ac cyclic voltammogram for the oxidation of odianisidine in 0.1 M H,SO, at a carbon paste electrode (Cb= lo4 M, f = 10 Hz, A€ = 2.5 mV, v = 0.5 mV/s).
9K lK
I
JRt.
lout
0
Reagents. The 3,3’-dimethoxybenzidine (0-dianisidine,o-DIA) was obtained from Aldrich Chemical Co. with a purity of 88% (the remainder being water) and was used without further purification. A 0.1 M H2S04electrolyte solution was prepared from reagent grade H2SO4 and distilled deionized water obtained from a cartridge-type Barnstead water purification system. Experimental Procedures. ac cyclic voltammetry was performed typically with applied ac peak potentials (AE) of 2.5 or 5.0 mV, a frequency of 10 Hz, and a dc linear sweep rate of 5 mV/s unless otherwise noted. The dc initial and switching potentials were 400 and 700 mV, respectively. Phase angle adjustment of the reference signal to the lock-in amplifier was made according to the procedure described by Kingsley (6). With the dc potential adjusted to a value where no ac faradaic current flows, the ac charging current is monitored at the input to the VFC. Coarse adjustment of the phase angle control was accomplished by aligning the peak of the ac charging current with the downward slope of the reference square wave, as observed on an oscilloscope. This achieves approximately a 90° phase shift of the charging current with respect to the reference square wave. Fine tuning was accomplished by adjusting the phase angle until the output of the lock-in amplifier was zero. ac amperometric experiments were performed by setting the instrument controls to the desired frequency and peak amplitude of the applied ac signal and adjusting the dc potential to a value corresponding to the ac peak current for the system under study. The cell was filled with 50 mL of the supporting electrolyte, and usually 100-pLaliquots of o-DIA in the same supporting electrolyte was pipetted into the cell with a Gilson Pipetman. The DAC output of the lock-in amplifier was connected to a strip chart recorder and, as the aliquots were added, the increase in current was recorded as a function of time. RESULTS AND DISCUSSION ac Voltammetry. At the carbon paste electrode, o-DIA undergoes a reversible two-electron oxidation in acidic electrolytes (I). The alternating current cyclic voltammogram for a solution of o-DIA at the carbon paste electrode is shown in Figure 1. The experimental peak current is 3.47 PA. The digital lock-in amplifier measures the in-phase component of the peak current which is given by IJmeasd) = I, cos 45O = 0.7071, (1) where
Ip = n2FA(wD)‘J2CbAE/4RT A theoretical peak current of 3.61 pA was calculated by using the geometric area of the electrode and a diffusion coefficient
Figure 3. Schematic diagram of the ac nulling current circuit: A, outputs from the oscillator circuit: B, analog phase shifter; C, voltage foilowerlcurrent buffer; D, voltage divider; E, inverter amplifier; F, resistor network for voltage-to-current conversion.
of 0.47 X 10” cm2/s ( I ) . The system is reversible in the ac sense as can be seen by the complete overlap of the anodic and cathodic scans. The agreement between predicted and measured currents is very good. The width of the anodic wave at half the peak height was 47.5 mV, in good agreement with the theoretical value of 45 mV. Repeated experiments, each on a freshly renewed carbon paste surface, yielded an average current of 3.47 FA with a relative standard deviation of h5.570, which is typical of carbon paste electrodes ( 2 ) . ac Amperometry-Charging C u r r e n t Nulling. In contrast to dc amperometry where the charging current is transient and rapidly disappears, ac amperometry produces a sinusoidal charging current. Although the digital lock-in amplifier can discriminate against much of this quadrature component, problems arise with the I / E converter. For very dilute concentrations, the ac charging current can be 3 or more orders of magnitude larger than the faradaic current. The largest current component (the charging current in this case) determines the maximum sensitivity that can be set on the I/E converter. If the magnitude of the ac charging current can be decreased before entering the I / E converter, the sensitivity setting can be increased and a larger signal corresponding to the faradaic component can be sent to the band-pass filter and lock-in amplifier. The ac charging current is given by
i, = ACdlAE cos w t
(3) where Cdl is the double layer capacitance which is sensibly constant a t constant dc potential and small ac amplitudes. Thus, all terms on the right-hand-side of eq 3 are constants for a given experiment and the charging current can be “nulled” at the summing point of the I/E converter by adding an alternating current which is 180’ out of phase with the charging current. A block diagram of a circuit to accomplish this is shown in Figure 2. The oscillator signal is sent to a 0-180° analog phase shifter which has been described by Kingsley (6). The signal is then attenuated by a voltage divider (10-turn potentiometer). The signal can be shifted another 180’ by an inverter, thus providing 0-360’ phase shift capabilities. The operational amplifier used for the inverter is arranged with a three-pole, two-position rotary switch and can function as an inverter or a follower so that the signal is
562
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
+ill" r"
Table I. ac Charging Current as a Function of Frequency and the Efficiency of the Nulling Current Experimental Conditions: 0.1 M H2S04,C&! f, Hz
i,, A (p-p)
10.66 21.25 31.94 42.53 53.2fi 63.86 74.81 96.03 99.17
4.10 X 8.20 X 1.22 x 1.60 x 2.00 x 2.40 X 2.80 X 3.55 X 3.70 X
lo-' 104 104 104 lo* lo*
lo* lo4
i, (after null) 1.5 X 1.6 X 2.6 x 2.9 x 5.9 x 5.6 X 5.8 X 6.0 X 6.8 X
lo-' 10-9 10-9 10-9 lo-$
lo-$
10-$ IO-$
= 5 mV % null"
99.63 99.80 99.79 99.82 99.71 99.77 99.79 99.83 99.82
T
26 n A
1
"Average Nulling Efficiency = 99.77 h 0.06%. Table 11. ac Charging Current as a Function of Peak Amplitude and the Efficiency of the Nulling Current
AE,mV
i,, A (p-p)
i, (after null)
%
null"
f = 10 Hz
2.5 0
10 25 50
2.45 X lo-' 5.00 X 1.02 x 1 0 4 2.50 X lo* 4.95 x io*
7.5 X 6.0 X 7.5 x 10-10 7.6 X 2.0 x 10-9
99.69 99.88 99.93 99.97 99.96
2.35 X 4.80 X lo4 9.80 x 104 2.45 X 10-5 4.70 x 10-5
5.0 X lo-$ 5.0 X lo-' 7.5 x 10-9 3.0 X 5.0 x 10-8
Figure 4. Alternating current vs time for incremental additions of 100 WLof 4 X loT4M odianisidine to 50 mL of 0.1 M H,SO,. Each step represents a change in bulk concentration of 0.8 pM. A€ = 5 mV, f = 10 Hz, E,, = 540 mV.
A
T
f = 100 Hz 2.5 5 10 25 50
-I
O0nA
99.79 99.90 99.92 99.88 99.89
1
"Average nulling efficiency: 99.89 f 0.10% (10 Hz); 99.88 f 0.047~(100 Hz). buffered in either case by a high input impedance (Figure 3). The switchable resistor network provides nulling currents of 0-4 pA, 0-40 wA,0-400, FA, and 0-4 mA peak to peak. The output of the current nulling circuit is connected to the summing point of the I / E converter. All the leads in the circuit were shielded with the shields connected to instrument ground. Both operational amplifiers used were Analog Devices AD 510 FET input types. Resistors were all low-temperature coefficient, 1% components except the resistor network from Ohmtek, which was laser trimmed to a precision of 0.1%. The switches used in the circuit were rotary ceramic types to reduce leakage currents. The ability to efficiently null the charging current at the input of the I / E converter was studied by use of the carbon paste electrode by varying the frequency and amplitude of the applied ac signal in separate experiments. The charging current component was monitored at the V / F converter input. As seen in eq 2 and 3, both the charging and faradaic currents are directly proportional to the amplitude of the applied ac The frequency dependence is direct for the voltage (a). former and a square root relationship for the latter. Table I shows the increase in the charging current as the frequency of the applied signal is changed from 10 to 100 Hz. The amplitude of the applied voltage was 2.5 mV peak. The measured amplitude of the charging current after nulling is also listed in Table I. The average nulling efficiency is 99.8%. A plot of the data for the experiments without nulling was linear with a slope of 3.7 X A/Hz. By use of eq 3 and this slope, a double layer capacitance of 7.5 pF/cm2 was calculated. Adams reported a value of 6.55 pF/cm2 for a carbon paste electrode in a HCI/NaCl supporting electrolyte using dc techniques (8).
B
T 13nA
k 1 mind
Figure 5. A comparison of the ac amperometric experiments for 20 nM concentration steps of odianiskline with I/E converter sensitivities of lo5 (A) and lo6 V/A (B).
Table I1 lists the charging current, with and without nulling, as a function of the amplitude of the applied ac voltage for frequencies of 10 and 100 Hz. The average nulling efficiency was 99.89% at 10 Hz and 99.88% a t 100 Hz. For all of the experimental conditions investigated, the nulling circuit is capable of reducing the charging current component by 3 orders of magnitude. As a consequence, it is possible to increase the sensitivity of the I / E converter for very dilute solutions and it is possible to increase the sensitivity of the measurement by increasing the amplitude of the applied ac voltage. Amperometric Solution Measurements. Figure 4 shows the change in output with time of the DAC output of the digital lock-in amplifier after successive additions of 100 pL of a 4 X M solution of o-DIA to the electrochemical cell containing 50 mL of 0.1 M H2S04. The solution was stirred after each addition. The dc potential was held constant at 540 mV, the potential of the ac peak current. The sensitivity of the I/E converter was 105 V/A, and charging current nulling was not used. Each step corresponds to a change in bulk
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
563
Table 111. Alternating Current Amperometry of o-Dianisidine Near the Detection Limit"
T 0.65 nn
1
i
rs r' t-
2
Cb,nM
sample vol, p L
100
2
1
50
min4
Figure 6. Alternating current amperometric step experiment for the addition of 100 pL of 10" M odianisldine to 50 mL of 0.1 M H,SO, wlth A€ = 25 mV, f = 10 Hz, and E, = 540 mV. Each step corresponds to a change in concentration of 2 nM.
concentration of o-DIA of 0.8 pM. The relative standard deviation of the current for the five additions is *1.2%. Figure 5A shows the recorded signal for successive additions of 1 X 10" M solutions of o-DIA. Each step in the current correM. The sponds to a bulk concentration change of 2 X relative standard deviation for the current measurements is &4.4%for six injections and the detection limit, calculated for S I N = 2, is about 13 nM. With the nulling circuit used to lower the amount of charging current entering the I/E converter, the sensitivity of the I/E converter was increased by an order of magnitude to lo6 VIA. Figure 5B shows the result of these experiments. For the data in Figure 5A, the gain of the band-pass filter amplifier was set to 10 with a Q (quality factor) of 100, and for Figure 5B, the gain was 1 and the Q was 100. The signal to noise ratio (SIN) for the data in Figure 5B is more than 5 times greater than that for the data in Figure 5A. Working curves were linear with correlation coefficients of at least 0.999 over the concentration range 20 nM to 1.6 pM. For example, a working curve for concentration changes of 0.2-1.6 pM had a slope of 0.43 AIM and a correlation coefficient of 0.9993. Using the nulling current lowered the ratio of the charging current to faradaic current entering the I/E converter from 350 with no nulling to about 0.71 for 20 nM solutions of o-DIA. Following the work of Hanekamp et al., the amplitude of the applied alternating voltage was increased to increase the ac peak current (5). Figure 6 shows the results of some experiments with AE = 25 mV. Current nulling was used and each step represents a change in bulk concentration of o-DIA of 2 nM. The relative standard deviation of the measurements was &8.6%. The S I N is 9.5, which yields a detection limit of about 421 pM at SIN = 2. This is about an oder of magnitude better than the detection limit cited by Kissinger for LCEC detectors using dc amperometric techniques (9). The detection limits given by Wang for the sinusoidally modulated rotated disk electrode and the stopped-flow porous plug electrode are 2000 and 1000 pM, respectively (10). Measurements were made at concentrationsnear the detection limit by adding smaller volumes of solution of o-DIA to the 50 mL of 0.1 M H2S04. These data are shown in Table 111. These results are very satisfactory. Since the only experimental difference between these data and the data obtained for the working curve quoted above is the difference of a factor of 5 in AE,a direct comparison of the measured current changes and those predicted from the slope of the working curve is possible. Using the value of the slope and the concentrations shown in Table 111 and multiplying by 5, predicted current changes of 430, 215, and 108 pA are found corresponding to the entries in the last column of Table 111. It is clear that the combination of large AE and charging current
25
0.5
i , PA
av i, pA
450 394 466 402 482 386
430 *5%
241 209 217 209 209 217
217 f6%
96.4 120 96.4 104 98.3 112
104.5 &9%
" Conditions: f = 10 Hz, AE = 25 mV, Edc = 540 mV, I/E set at lo6 V/A, 50 mL of 0.1 M H,SO,.
T 32 n A
1
I
200
I
I
400
I
I
m
1
E (mV) vs SCE Figure 7. Cyclic ac voltammograms of the background current at a carbon paste electrode in 0.1 M H,SO,: A, first scan; E,second scan; C, scan after electrochemical pretreatment.
nulling provides a successful approach to the determination of concentrationsbelow the 1 nM level. It is important to note that o-DIA is a model compound for a reversible oxidation and that compounds with slower rates of electron transfer will yield poorer detection limits. Totally irreversible systems yield ac peak currents that are only a few percent of those for reversible systems and it is expected that the detection limits
564
Anal. Chem. 1988. 60,564-568
for such systems would reflect this loss in current. Observations on the Background Current at Carbon Paste Electrodes. Kakutani and Senda ( I I ) have shown theoretically that currents produced in the ac experiment due to strongly or irreversibly adsorbed species and species chemically bonded to the surface by other means are 90° out of phase with the applied ac potential for systems that are reversible in the ac sense, but kinetically slower systems have phase angles which are not in quadrature. Currents for the latter will pass through the lock-in amplifier. Figure 7A shows the first ac cyclic voltammogram at a freshly prepared carbon paste electrode cycled between 200 and 700 mV. On the anodic scan, a shoulder appears a t about 560 mV. On the return cathodic scan, a definite alternating current peak appears a t 480 mV. Figure 7B shows the second ac cyclic scan on the same carbon paste surface. A more defined set of peaks appears in this voltammogram with the peak potential for the anodic scan at 511 mV and that for the cathodic scan at 480 mV again. Holding the dc potential constant at 700 mV with a superimposed ac peak potential of 25 mV at 10 Hz for 2 h produced the cyclic voltammogram shown in Figure 7C. The change in the magnitude of the current at 540 mV after 2 h is approximately 5 nA. Other peaks have become evident in the background shown in Figure 7C. Graphite has been found to contain many types of carbon-oxygen surface groups such as quinones, quinhydrones, phenols, and carbonyls (12-14). It is reasonable that these currents are due to surface functional groups and the evidence in Figure 7 suggests that while anodization at a fixed dc potential may reduce the background current at that potential, surface currents may be increasing at another potential due to the change in the nature of the
surface functional group. The detail shown in the ac background scans in Figure 7 is much greater than can be observed in dc scanning experiments because the latter are obscured by the charging current, but the digital lock-in amplifier discriminates against a major portion of the charging current without current nulling. Registry No. C, 7440-44-0; o-dianisidine, 119-90-4.
LITERATURE CITED (1) Adams, R. N. ,Electrochemistry at Solid ,Electrodes; Marcel Dekker:
New York. 1969. (2) McAiiister. D. L.: Drvhurst. G. I n Laboratow Techniuues in Electroanalytical Chemistry; Kjssinger, P. T., Heineman, W. R.: Eds.; Marcel Dekker: New York, 1984; pp 289-319. Stulik, K.; Pacakova, V.; Starkova, B. J . Chromatogr. 1981, 213, 47. Walker, D. E.; Adams, R. N.; Juliard, A. L. Anal. Chern. 1960, 32, 1528. Hanekamp, H. B.; Bos, P.; Vittori, 0. Anal. Chlm. Acta 1981, 131, 149. Kingsley, E. D. Ph.D. Dissertation, University of Massachusetts, Amherst, MA, 1982. Bard, A. J.. Faulkner. L. R. ,Electrochemical Methods in Analytical Chemistry; Marcel Dekker: New York, 1980; pp 288-389. Adams, R. N. Rev. Polarogr. 1963, 11, 71. Kissinger, P. T. I n Laboratory Technlques in ,ElectroanalyticalChemis try; Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1984; Chapter 22, pp. 611-612. Wang, J. Talanta 1981, 28, 369. Kakutani, T.; Senda, M. Bull. Chem. Soc. Japn. 1979, 52, 3236. van der Linden, W. E.; Deiker, W. E. Anal. Chim. Acta 1980, 179, 25. Engstrom, R. C.; Strasser, V. A. Anal. Chem. 1984, 56, 136. Panzer. R. E.; Elving, P. J. J . Electrochem. Soc. 1972, 40, 99.
RECEIVED for review June 26,1987. Accepted November 19, 1987. M.B.G. wishes to express his thanks for support in the form of a Grant-in-Aid of Research from Sigma Xi.
Solid-Surface Luminescence Interactions of Nitrogen Heterocycles Adsorbed on Silica Gel Chromatoplates Submerged in Chloroformln -Hexane Solvents G . J. Burrell and R. J. Hurtubise* Chemistry Department, University of Wyoming, Laramie, Wyoming 82071 The room-temperature fluorescence (RTF) and room-temperature phosphorescence (RTP) of benzo[f)quinoiine and benro[h]quinoiine, obtalned from the samples adsorbed on silica gel chromatoplates submerged in chioroform/n-hexane solvents, revealed several of the Interactions of the nitrogen heterocycles with the sdid matrix. The RTP resuits showed the emitting phosphor was protected from coillsionai deactivation by the matrlx and that the adsorbed chloroform mlnimally disrupted the phosphor adsorption Interactions. I n addition, at least two popuiatlons of phosphors were indicated. The RTF data and RTP data showed that different interactions were occurring in the singlet state compared to the triplet state. A comparison of RTF intensity and RTP intensity as a function of chromatographic solvent strength indicated that the protonated forms of the nitrogen heterocycles in their triplet states were interacting with the matrix more strongly than the protonated forms of the nitrogen heterocycles in their singlet states.
The observation of room-temperature phosphorescence (RTP) from molecules adsorbed on solid surfaces has been
shown to depend on several factors such as how rigidly the phosphor is held, the extent of hydrogen bonding with the surface, and other factors ( I , 2). Schulman and Walling ( 3 ) observed strong R T P from ionic organic molecules adsorbed on filter paper. I t was concluded that hydrogen bonding interactions between the molecules and hydroxyl groups on the surface were important in the restriction of collisional and other nonradiative deactivation processes ( 4 ) . An enhancement in the RTP from benzolflquinoline (BMQ) was observed when the carboxylate functionality of a polyacrylate binder in a commercial thin-layer chromatoplate was converted to the acid form (5). Strong hydrogen bonding between B m Q and the carboxyl groups in the binder resulted in the strong R T P signal. Recent work has demonstrated the importance of a rigidly held mechanism to PABA adsorbed on sodium acetate (6). R T P has also been observed from a system in which no hydrogen bonding is possible. Smith and Hurtubise ( 7 ) observed strong R T P from the dianion of terephthalic acid adsorbed on the sodium salt of polyacrylic acid. With this system it would not be possible to form hydrogen bonds. Other mechanisms for enhancing RTP have been suggested. Matrix packing (8),where added sugars or salts effectively
0003-2700/S8/0360-0564$01.50/0@ 1988 American Chemical Society
