Development of a Computer-Controlled Versatile Luminescence Spectrometer for Room-Temperature Phosphorescence Spectrometry

speed of 18520 samples/s was obtained when a timer output with a 27-tLs pulse interval was used as the trigger source (Fig. 5B, dotted line). But these data have no accurate timing relationship with the trigger source, because if each pulse is used for the ADC trigger then a throughput of 37037 samples/s must be obtained. An optimum speed, with true timing relationship with the trigger source, was obtained with a 62-tts pulse interval, which gave a throughput of 16130 samples/s (Fig. 5B line).

One of the simple applications of the IBM-DACA is to acquire data from many instruments which normally use a strip chart recorder. Many scientific instruments generate output signals in the range of mV to V for connection to a chart recorder. Simple application of

those output signals to approximately 10 V is sufficient for connection the IBM-DACA. Figure 6 is the fluorescence spectrum of an Ovalene standard captured from Perkin-Elmer LS-5B, when both the data acquisition system and spectrometer were running at the same speed.

1. P. M. Wiegand, K. K. Trischan, and S. R. Crouch, Anal. Instrumen. 14(2), 127 (1985).

2. AM9513 System Timer Controller Technical Manual (Advanced Micro Device, Sunnyvale, California, 1983).

3. IBM Personal Computer Data Acquisition and Control Adaptor Technical Manual (IBM Corp., New York, 1984).

4. Data Acquisition Databook (Analog Device, Norwood, Massachu- setts, 1984).

5. Microsystem Component Handbook Microprocessors and Periph- erals (Intel Corp., Santa Clara, California, 1985).

Development of a Computer-Controlled Versatile Luminescence Spectrometer for Room-Temperature Phosphorescence Spectrometry

H A I D O N G K I M , M A T T H E W J . Z A B I K , * and S T A N L E Y R. C R O U C H * Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (H.K., S.R.C.); and Pesticide Research Center, Michigan State University, East Lansing, Michigan 48824 (M.J.Z.)

An inexpensive, but versatile computer-controlled luminescence spectrometer has been developed for fluorescence, room-temperature phosphorescence (RTP), and RTP lifetime measurements. A continuum xenon-arc lamp was modulated with a rotating single-disk type chopper, and the chopping phase was accurately detected by a simple and flexible circuit. An integrated, menu-driven software package was developed for data acquisition, instrument control, and data manipulation with IBM- XT compatible personal computer. The system developed has a number of advantages, compared with the conventional rotating-can type phosphoriscope, including increased sensitivity, high selectivity, and versatile applicability for short-lived RTP. Index Headings: Room-temperature phosphorimeter; Computer inter- facing; Data acquisition.

INTRODUCTION

Room-temperature phosphorescence (RTP) spectrometry is a very convenient method in comparison to the conventional low-temperature phosphorimetry, which is typically performed in glass matrices at liquid-nitrogen temperature. 1 Since the first introduction of RTP as an analytical tool by Roth in 1967, various RTP methods have been developed to observe phosphorescence from many organic compounds. 2 These methods include solid- surface RTP, 3 micelle-stabilized RTP, 4 sensitized R T P ) and cyclodextrin-induced RTP. 6

Although RTP has grown as an important analytical method over the last two decades, there are few commercial instruments available for RTP. Modifications to

Received 9 December 1988. *Authors to whom correspondence should be sent.

commercial instruments for RTP study have been made to aid individual needs, depending on the RTP technique employed. Cline Love et al. 7 constructed a phosphorimeter from commercially available components and used it in micelle-stabilized RTP studies. Vo-Dinh et al. s designed an automatic phosphorimeter for solid-surface RTP with a continuous filter paper device. Nithipatikom et al. 9 constructed a spectrometer for solid-surface RTP lifetime measurement.

Several commercial spectrophosphorimeters, including one by Aminco-Bowman (SPF-500), use a rotating can to modulate the exciting source. The major disad- vantage of the rotating-can type phosphoriscope is that the modulation frequency of the excitation source and the luminescence detection time window are limited. But the RTP lifetimes of most organic phosphors are very short, especially in solution. As a result, rotating-can type phosphoriscopes give very low or negligible luminescence signals for short-lived phosphors. The short delay time and the right gate time will increase the RTP sensitivity of the short-lived phosphors. A pulsed source or a single- disk type chopper gives better performance in terms of flexibility of selecting appropriate delay time and gate time for the experimental requirements.

This paper describes the construction of an inexpensive but versatile computer-controlled luminescence instrument using a single-disk type chopper and a counter/ timer for measuring fluorescence, RTP, and RTP lifetimes with a comprehensive data acquisition and analysis software. This system allows semiautomatic luminescence data acquisition and analysis on a personal computer without sacrifice of a tight budget.

Volume 43, Number 4, 1989 0003-7028/89/4304-061152.00/0 APPLIED SPECTROSCOPY 811 © 1989 Society for Applied Spectroscopy

I - 0n0 I. ....... I,en0.-o01

Fro. 1. Block diagram of the luminescence spectrometer.

EXPERIMENTAL

Material and Sample Preparation. Naphthalene (East- mann) and pyrene (J. T. Baker) were recrystalized from ethanol. Sodium sulfite, anhydrous (98%, J. T. Baker) was recrystalized from warm water according to the method reported by Diaz Garcia e t al . lo Sodium dodecyl sulfate (99%, Sigma) and thallium (I) nitrate (99.99%, Aldrich) were used as received. All the stock solutions used in the experiment were prepared with distilled, deionized water prior to the experiments. A Lab Master board (33 MHz version, Scientific Solution, Solon, OH) was used for data acquisition and monochromator control. An IBM-XT compatible clone computer [Turbo- XT (8 MHz), Liuski International, Long Island City, NY] was used for the Lab Master board control and data manipulation.

Principle of Operation of the Instrument. In this work, we used the optical unit of the Aminco-Bowman spec- trofluorometer (SPF-500) including the PMT, both excitation and emission monochromators, the xenon-arc lamp, and the sample compartment unit. A block diagram of the instrument is shown in Fig. 1. A continuous Xe-arc lamp (250 W) was used as an excitation source. To discriminate phosphorescence from fluorescence and delayed fluorescence, we modulated the exciting radia- tion by a rotating single-disk type chopper.

For data acquisition and monochromator control, a Lab Master board was used with an IBM-XT compatible computer. The Lab Master board 11 contains a 12-bit analog-to-digital converter (ADC) with a built-in 16-channel multiplexer (MP 6812), a 24-bit parallel port (Intel 8255), two 12-bit digital-to-analog converters, and a five- channel 16-bit counter/timer (AM9513). The AM9513 LSI chip is one of the most powerful counter/timers available. 12 For both excitation and emission monochromator control, a parallel output was used, along with a latch (74LS75). The direction of rotation and the number of turns of the monochromator, to scan wavelength, were controlled by the software, by sending the appropriate digital output to the latch inputs--whose outputs were connected to the control lines of both monochromators. This approach allows total computer control of the scanning modes (emission, excitation, synchronous, excitation-emission) and automatic initial wavelength

Photo Diode

Opto- Interrupter

MulU- ~brator

FIG. 2. Timing diagram of the chopper phase detector. The opto- interrupter output is connected to the multivibrator, and the pulse width of the multivibrator is adjustable with a variable register.

selection of both monochromators. The current signal from the P M T (Hamamatsu R928) was converted into a voltage signal, and a variable gain amplifier (AD 507, Analog Device) was used to get the desired input voltage signal (0-10 V) for the ADC converter. To reduce high frequency noise, we used a low-pass filter for fluorescence and RTP measurements, which was turned off by a manual switch when RTP decays were being measured.

Source Modulation and Detection. In this design, the Xe-arc lamp was modulated by a replaceable rotating single-disk type chopper. The chopper disk was constructed with a stiff, thin plastic blade (10 cm diameter) and coated with non-glare black paint. The chopper was driven by a synchronous motor, and its speed was controlled by a variable, precision dc power supply. Two types of disks, which have a different number of open slots, were used, depending on the type of experiment (RTP or RTP lifetime). When fluorescence was measured, the disk was stationary, with its open slot facing the incoming excitation beam so that all the exciting light could pass through the chopper disk.

The phase detection of the rotating chopper was ac- complished through the use of a photodiode and an opto- interrupter. A photodiode (EG&G UV-100BQ) with a Schmitt trigger (74LS14) was placed on the opposite side of the chopper across the sample cell, and an opto-interrupter (GEH21B1) with monostable multivibrator (74LS121N) was placed under the chopper blade. The

Enable Counter

Counter ~ Chopper 0ate _ _ Phase

i Counter I Delay

Terminal ,['1 Count 1 RTP

i Counter 2 I ~ I I ~ Gate

Terminal H H ~ RTP Count 2 ~ ~ Lifetime

FIG. 3. Timing diagram of the counters for RTP and RTP-lifetime measurement.

612 Volume 43, Number 4, 1989

phase detection signal was adjusted in the visible range, as described below.

At the beginning of the experiment, signals from photodiode circuit and opto-interrupter circuit were dis- played concurrently on a dual-trace oscilloscope. The output of the photodiode circuit is the true chopped signal of the excitation source, whereas the output from the opto-interrupter circuit is not the true source chopping signal, since its signal depends on the mounting position of the opto-interrupter. The low-going pulse output of the opto-interrupter was adjusted manually by a variable register of the multivibrator until the falling edges of both signals from the photodiode and the opto- interrupter were exactly matched (Fig. 2).

RTP and R T P Lifet ime Measurement. For RTP and RTP lifetime measurements, an AM9513 timer/counter on the Lab Master board was used. A single-disk type chopper with four opening slots was rotated in order to obtain a 200-500 Hz modulated source for RTP measurements. The chopper signal from the one-shot of the phase detector circuit was connected to gate 1, and counter 1 was programmed to generate a pulse at the terminal count. The on-board 1.000 MHz clock was used as the clock source. The output of counter 1 was connected to the ADC external-start conversion pin. By varying the count number, one can acquire at different time delays after excitation for time-resolved phosphorimetry (Fig. 3). For the RTP lifetime measurement, two counters were used simultaneously. A chopper with one or two open slots was rotated in order to obtain a 5-10 Hz modulated source. Gate 1 was connected to the chopper signal, and the output of the counter 1 was connected to gate 2 of counter 2. The analog-to-digital conversion was triggered by the output of counter 2. At the falling edge of the chopper signal, counter I will start the count-down for a given delay time and will generate a pulse at the terminal count. The rising edge of the terminal count signal will then initiate counter 2 to count down for a given gate time repetitively, to generate pulses at each gate time interval. At each terminal count pulse of counter 2, an RTP signal was read in order to obtain a decay signal. The delay time is the time delay from the falling edge of the chopper signal to the starting point of the data reading. Gate time (33 #s minimum) is the time interval between the successive data read points.

Software. A menu-driven program, EMISPEC, has been developed with the use of TURBO PASCAL (Version 4.0, Borland International) and MACRO ASSEMBLER (Version 4.0, MicroSoft) for instrument control, luminescence data acquisition, and manipulation on the IBM- XT compatible computer. A block diagram of the program, EMISPEC, is shown in Fig. 4. There are six items in the main menu: File, Data, Set-up, Plot, Tools, and Lifetime. For noisy data, smoothing can be employed with the use of a Fourier transformation technique according to the unique algorithm developed by Eric E. Aubanel. 1~ Set-up parameters include Mode (Fluores- cence, RTP, RTP Lifetime), initial setting and scan ranges for both monochromators, number of data per wavelength or number of decay cycles for signal averaging, scan mode (emission, excitation, synchronous, excitation-emission), delay time, and gate time. Fluorescence and RTP spectrum data are plotted on the screen in real

~ISPEC-II I

File I

i I _ LOad

_ Save

_ List

_ Dos Shell

Display

Print

~aoothing

1 Start Read

FIG. 4.

Set-up

Mode

- Screen Plot

- Printer Plot

- Plotter Plot

- Plotter File

_ MulPlot

Lifetime I

Vale LTM Help

Simulate LTM ~_ Change Parm

Semilog Plot ~_ Test Data Read

Block diagram of the EMISPEC program.

time as the monochromator scans. The plot routine plots data in five ways: screen plot, printer plot, plotter plot, plotter file. Spectra drawn on the screen can be printed with either an EPSON printer or an HP plotter. Data plot can also be routed into a file in HPGL format.

RESULTS AND DISCUSSION

The primary output of any experiment in which chemical information is to be extracted is the signal which measures the phenomenon under observation. In most cases the signal measured has superimposed noise which arises from various sources. Noise can be distinguished from the signal by its frequency characteristics and by the time of occurrence or phase coherence of the frequency component. 14 The removal of noise can be performed by hardware and software.

In this system we employed both methods. A simple, passive low-pass filter (cut-off frequency 10 Hz) was placed between the current-to-voltage converter and the variable gain amplifier '5 for RTP and fluorescence measurement. Because the low-pass filter has a long time constant which can affect the phosphorescence decay signal it was turned off by a manual switch when RTP lifetime was measured. Two other methods, signal averaging and Fourier smoothing, are used to further en- hance the signal-to-noise ratio (S/N) of the measure~ ment. If the number of measurements is N, then noise is proportional to square root of N, while signal is proportional to N. As a result S/N will be improved by the square root of N with signal averaging. 16 Also, by per- forming Fourier smoothing, one can further reduce noise components from the measured data. The method of Fourier smoothing used is a modified Discrete Fourier Transformation (DFT) which runs much faster than reg- ular DFT. In this method, the user can control the degree of smoothing, and the data size is not limited to a mul- tiple of 2, which is the limiting factor of the Fast Fourier Transformation (FFT).

Figure 5 is a typical luminescence data set obtained with an aqueous solution of naphthalene in a sodium dodecyl sulfate (SDS) micellar solution. This spectrum was obtained in the fluorescence mode. Peaks below 400 nm are fluorescence, and the peaks above 400 nm are phosphorescence at room temperature. If the data are read in the phosphorescence mode, the fluorescence peaks can be eliminated by reading data after a certain delay

APPLIED SPECTROSCOPY 613

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FIG. 5. Luminescence spectrum of naphthalene at room temperature. Peaks below 400 nm are fluorescence and peaks above 400 nm are RTP. Fluorescence peaks will be removed if a de lay t ime is used in the R T P mode. Dot ted l ine = real data; solid line = Four ier smoothed data . Exc i t a t i on = 276 nm; [Naphtha lene] = 1.5 x 10 -5 M; [SDS] = 0.02 M; [T1N031 = 1.75 x 10 -2 M; [NaS03] = 0.01 M.

time. The dots in Fig. 5 are the real data without signal averaging, and the solid line is the smoothed data after Fourier smoothing.

The general principle and application of the time-resolved phosphorimetry was well demonstrated by Wine- fordner. 17 Fisher and Winefordner is showed a detailed comparison of the pulsed source and continuous mechanical phosphoriscope system in phosphorimetric instrumentation. The major advantage of the pulsed source phosphorimetry over the rotating-can type mechanical phosphoriscope is the possibility of selectz'ng experimental conditions, such as delay time and gate time, so that the species of interest is measured at the optimum conditions. For the measurement of short-lived RTP, the rotating-can phosphoriscope was found to be inadequate, since it takes a relatively long time (1.5 ms minimum) to open the emission window by rotating 90 ° after excitation. As a result, the intensity of a short-lived RTP signal has decreased significantly before the detection of actual RTP signal is started. On the other hand, if the exciting light source is modulated by a disk-type blade or pulse, the detector is always open to the phosphorescence signal. The measurement of the phosphorescence signal can be initiated at any time by choosing the proper delay time after excitation for time-resolved spectrometry. This system gives more versatility in the measurement of a short-lived RTP than does the rotating- can phosphoriscope.

Accurate detection of the end of the modulated excitation source pulse is very important in the RTP and RTP lifetime measurements. Typically, an opto-interrupter or a photodiode is used to detect chopper phase. However, manual adjustment of the position of the opto- interrupter so that its output matches exactly with the true source chopping is very difficult. Also, most pho- todiodes (even those that are UV sensitive) are not very sensitive to the light below 280 nm, but many organic compounds require excitation under 300 nm. In addition,

typical RTP micellar sample solutions are turbid, due to the high concentration of heavy atoms. These conditions make it difficult to obtain a reliable and accurate phase detection signal from the rotating chopper. To overcome these difficulties, we used a combined system using a photodiode and an opto-interrupter for chopper phase detection. This system gives a very reliable and accurate chopper signal that is independent of any variation of the motor speed of the chopper, excitation source wavelength, and condition of the sample solution.

A single component decay signal at time t from an RTP sample can be described by

I ( t ) = A e x p ( - t / r ) + B (1)

where A is the initial decay intensity, r is the RTP lifetime, and B is the background signal. The background signal is a convolute of the detector dark current, in- strumental offset, ground loops, light scatter signal, etc., with solvent or substrate and sample impurity signals. Many methods have been developed for the determination of a phosphorescence lifetime from a single ex- ponential decay. These are the linear least-square method, 19 the Guggenheim method, 2° the phase plane method, 21 and the rapid lifetime determination (RLD) methodY 2 Each method has advantages and disadvantages, depending on the data used. Among these methods, phase plane and RLD methods give the most accurate lifetime from the raw decay data which have finite noisy background. The RLD method calculates the RTP lifetime, background, and initial decay intensity of the decay data but does not give a correlation coefficient. The RLD method--considering that it determines the base line and initial decay amplitude and that it offers ease of programming and faster execution time, in comparison to other methods--was our choice. EMISPEC calculates the RTP lifetime by the RLD method and can simulate decay data according to Eq. 1 if A, r, and B are provided.

The phosphorescence lifetime of naphthalene (1.5 x

-614 Volume 43, Number 4, 1989

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TIME (MICROSECONDS) FIG. 6. RTP decay of naphthalene in micellar solution. Dotted line is the real data and solid line is the simulated decay obtained with calculated RTP decay information of naphthalene. [Naphthalene] = 1.5 x 10 ~ M; [SDS] = 2.0 x 10 -2 M; [T1N03] = 4.25 x 10 .2 M; [Na~S03] = 1.0 x 10 -2 M; excitation = 276 nm; emission = 512 nm.

10 -5 M) was measured in an aqueous 0.02 M SDS micellar solution with tha l l ium ni t ra te (4.25 x 10 -2 M ) as the external heavy a tom source. This sample solution was deoxygenated with 0.01 M sodium sulfite and s tored under nitrogen. Figure 6 shows the decay da ta for naphthalene in the 0.02 M SDS micellar solution. A R T P l ifet ime of 486 gs was ob ta ined with the use of the R L D method. The dot ted line in Fig. 6 is the real decay da ta af ter signal averaging, and the solid line is the s imula ted da ta with calculated decay information. The R L D meth - od works well, as can be judged visually in the over lapped da ta plot in Fig. 6. The R T P lifetime of naph tha lene ob ta ined with this sys tem is comparab le with the repor ted value of 450 ~s by Cline Love e t a l . 4 under similar conditions, except for the me thod of deoxygenat ion of the sample solution. The R T P lifetime of m a n y poly- nuclear a romat ic hydrocarbon molecules is ex t remely sensit ive to the condit ion of the sample solution used.

The concentra t ion of dissolved oxygen, heavy a tom, and other impuri t ies in the sample solution direct ly affects the R T P lifetime. With a decreased concentra t ion of heavy a toms, a much longer R T P lifetime of naph tha lene was observed.

Figure 7 shows the exci ta t ion-emission spec t rum of naph tha lene in SDS micellar solution. The excitat ion- emission spec t rum is useful in order to examine the spec- t ra l response of the sample molecules over certain exci tat ion and emission wavelength ranges. Also, any two similar spec t rums of the samples at a par t icular excita- t ion will show a different exci ta t ion-emission spect rum.

C O N C L U S I O N S

The i n s t rumen t designed in our labora tory for luminescence m e a s u r e m e n t shares mos t of the advantages of the pulsed source phosphor ime te r bu t does not share the d isadvantages of the convent ional ro ta t ing-can type me-

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A P P L I E D S P E C T R O S C O P Y 615

chanical phosphoriscope. The major advantages of this i n s t rumen t over the convent ional phosphor ime te r s are:

(1) Fluorescence, R T P , and R T P l ifet ime can be measured on the same in s t rumen t wi thout addi t ional ins t rumenta l modification.

(2) Unlike the ro ta t ing-can type mechanica l phosphor - iscope, the i n s t rumen t puts no l imita t ion on the t ime discr iminat ion procedure. Delay t ime and gate t ime can be easily changed by software, wi thout adjust- m e n t of the ro ta t ion speed of the chopper.

(3) This i n s t rumen t is less dependen t on the precision of the modula t ion f requency of the exci tat ion source. T h e chopper phase de tec tor detects every closing edge of the exciting source accurately.

(4) T h e i n s t rumen t gives equally good pe r fo rmance in R T P and R T P l ifet ime m e a s u r e m e n t for short- l ived phosphors and long-lived phosphors . By changing the delay t ime, gate t ime, or ro ta t ion speed of the chopper , one can obta in an o p t i m u m detect ion t ime window.

(5) This i n s t r u m e n t gives a more reliable reproducibi l i ty and less r f in terference than does a pulsed source, due to a chopped cont inuous source.

(6) Finally, this i n s t rumen t is control led by a low-cost I B M - X T compat ib le personal computer . Lumines- cence da ta are acquired and man ipu la t ed easily on site.

1. T. Vo-Dinh, Room Temperature Phosphorimetry for Chemical Analysis (John Wiley and Sons, New York, 1984).

2. R. J. Hurtubise, Anal. Chem. 55, 669A (1983). 3. R. A. Paynter, S. L. Wellons, and J. D. Winefordner, Anal. Chem.

46, 736 (1974). 4. M. Skrilec and L. J. Cline Love, Anal. Chem. 52, 1559 (1980). 5. J. J. Donkerbroek, C. Gooijer, and N. H. Velthorst, Anal. Chem.

54, 891 (1982). 6. S. Scypinski and L. J. Cline Love, Anal. Chem. 56, 322 (1984). 7. L. J. Cline Love, M. Skrilec, and J. G. Habarta, Anal. Chem. 52,

754 (1980). 8. T. Vo-Dinh, G. L. Walden, and J. D. Winefordner, Anal. Chem.

49, 1126 (1977). 9. K. Nithipatikom and B. D. Pollard, Appl. Spectrosc. 39,109 (1985).

10. M.E. Diaz Garcia and A. Sanz-Medel, Anal. Chem. 58, 1436 (1986). 11. Lab Master User's Guide (Scientific Solutions, Solon, Ohio, 1984). 12. P. M. Wiegand, K. K. Trischan, and S. R. Crouch, Anal. Instrum.

14(2), 127 (1985). 13. E. E. Aubanel and K. B. Oldham, Byte Feb., 207 (1985). 14. G. M. Hieftje, Am. Lab. Oct., 110 (1988). 15. H. V. Malmstadt, C. G. Enke, and S. R. Crouch, Electronics and

Instrumentation for the Scientist (Benjamin/Cummings Publish- ing, Menlo Park, California 1981).

16. J. D. Ingle and S. R. Crouch, Spectrochemical Analysis (Prentice Hall, Englewood Cliffs, New Jersey, 1988).

17. J. D. Winefordner, Acct. Chem. Res. 2, 361 (1969). 18. R. P. Fisher and J. D. Winefordner, Anal. Chem. 44, 948 (1972). 19. G. D. Boutilier and J. D. Winefordner, Anal. Chem. 51, 1384 (1979). 20. E. A. Guggenheim, Philos. Magazine 2, 538 (1926). 21. J. R. Bacon and J. N. Demas, Anal. Chem. 55, 653 (1983). 22. R. J. Woods, S. Scypinski, L. J. Cline Love, and H. A. Ashworth,

Anal. Chem. 56, 1395 (1984).

The Effects of Temperature on the Solid-Surface Luminescence Properties of Benzo(f)quinoline Adsorbed on Filter Paper

S. M . R A M A S A M Y and R. J . H U R T U B I S E * Chemistry Department, University of Wyoming, Laramie, Wyoming 82071

Experimental values of fluorescence quantum yield, phosphorescence quantum yield, and phosphorescence lifetime were acquired at temper- atures from 23 ° to -180°C for the protonated form of benzo(f)quinoline adsorbed on filter paper. From the experimental values, triplet formation efficiencies, rate constants for phosphorescence, and rate constants for the radiationless transition from the triplet state were calculated. The results showed the various interrelationships of the solid-surface radia- tive and nonradiative processes. In addition, the results indicated that the modulus of the solid matrix is an important factor in enhancing phosphorescence quantum yield.

Index Headings: Luminescence spectroscopy; Temperature variation; Solid-surface.

I N T R O D U C T I O N

Solid-surface r o o m - t e m p e r a t u r e fluorescence (RTF) and solid-surface r o o m - t e m p e r a t u r e phosphorescence (RTP) have been used ra ther extensively in organic t race analysis. 1,2 Al though some advances have been made in clarifying the physiochemical interact ions for R T F and R T P , more work is needed to develop an analyt ical model

Received 9 December 1988. * Author to whom correspondence should be sent.

for solid-surface luminescence analysis. Several spect ra l techniques were used to show tha t the anion of p - a m i - nobenzoic acid was held by two sodium ace ta te molecules which pe rmi t t ed s t rong R T P signals to be obtained. ~ Other repor ts have considered the physicochemical interact ions of p -aminobenzo ic acid adsorbed on sodium acetate 4~ and p-aminobenzoic acid, 4-phenylphenol , benzo(f)quinoline, and phenan th rene adsorbed on a-cyclo- dex t r in - sod ium chloride mixtures . 7,8

Fil ter pape r is the mos t widely used solid surface in R T P analysis. To date, little is unders tood abou t the interact ions required for R T P f rom compounds adsorbed on filter paper . Schu lman and P a r k e r 9 pos tu la ted t ha t hydrogen bonding of organic molecules to hydroxyl groups on filter pape r was i m p o r t a n t in yielding R T P . Andino et al. 1° employed x-ray photoelec t ron spect roscopy to s tudy surface processes in R T P with heavy a toms pres- ent. T h e y concluded tha t the ex ten t of pene t ra t ion of heavy a toms or ions exceeded t h a t of the adsorbed organic compound . In addit ion, Su and Winefordner 11 used x-ray photoe lec t ron spect roscopy to invest igate heavy a tom/ana ly te / subs t ra te interactions in RTP . Suter et al. 12

618 Volume 43, Number 4, 1989 0003-7028/89/4304-061652.oo/0 APPLIED SPECTROSCOPY © 1989 Society for Applied Spectroscopy

Documents

Development of a Computer-Controlled Versatile Luminescence Spectrometer for Room-Temperature Phosphorescence Spectrometry