(second harmonic mode) waveform provides increased amplifier output when compared with sinusoidal modulation. The improvement is greater for the second than the first harmonic mode and increases if larger plate amplitudes are required. With the stepped wave, the signal obtained is nearly independent of modulation amplitude and the size of the steps can be independently adjusted to provide a correction at any selected wavelength within the modulation range. This later feature is particularly important when the background contains fine structure in the vicinity of the analytical line necessitating careful selection of exact wavelengths for correction. Sinusoidal modulation does not allow for such precise selection. A disadvantage of the present system is the limited frequency a t which this torque motor responds. At 100 Hz, for example, the plate would be in transit about 40% of the time as opposed to 10% a t the frequencies used in this study. The frequency limitation is not likely to be important unless fast transient signals must be measured. We are investigating electronic and mechanical means to overcome this problem.
LITERATURE CITED E. E. Pickett and S. R. Koirtyohann, Anal. Chem., 41(14), 28A (1969). A. AnticJovanovlc, V. Bojuvic, and M. Maunkovic, Spectrochim. Acta. Part 8 , 25, 405 (1970). D. W. Goiightly, R. N. Kniseley, and V. A. Fassel, Spectrochim. Acta, Pat? 8 , 25, 451 (1970). V. G. Mossottl, F. N. Abercromble, and J. A. Eakin, Appl. Specfrosc., 25, 331 (1971). R. J. Sydor, J. T. Sinnamon, and G. M. Heiftje, Anal. Chem., 48, 2030 (1976). W. Snelleman, T. C. Rains, K. W. Yee, H. D. Cook, and 0. Menis, Anal. Chem., 42, 394 (1970). F. Brech, FLsher Scientific Cap.,Waltham, Mass., Personal Communication, 1978
I. S.Maines, D. G. Mitchell, J. M. Rankin, and B. W. Bailey, Spectrosc. Lett., 5, 251 (1972).
M. S. Epstein and T. C. O'Haver, Specfrochim. Acta., Part 8 , 80, 135 (1975). T. C. O'bver, M. S.Epstein, and A. Zander, Anal. Chem., 49, 458 (1977). E. E. Pickett and S. R. Koirtyohann, Specfrochim. Acta., Part 8 , 24. 325 (1969). E. Allen and W. Rieman, Anal. Chem., 25, 1325 (1953).
RECEIVED for review December 20, 1976. Accepted April 6, 1977.
Instrument for the Facilitation of Room Temperature Phosphorimetry with a Continuous Filter Paper Device T. Vo-Dinh,' G. L. Walden, and J. D. Wlnefordner" Department of Chemistty, University of Florida,
Gainesville, Florida 326 1 1
The design of a slmple and unlque automated phosphorimetric instrument is described. The apparatus, which was developed for a new method of analysis based on room temperature phosphorescence of adsorbed organic molecules on fllter paper, uses a simple rotating mirror device to perform phosphorimetric time-resolution. The results show that the RTP technique could be easily automated for large scale and fast routine analysis and therefore has a great potential In certain areas of appllcatlon, such as cllnlcal and environmental chemlstry.
Room Temperature Phosphorescence (RTP) of ionic and non-ionic adsorbed molecules on filter paper has been previously investigated for analytical purposes (1-3). In this work, the construction and performance of a simple and unique automated phosphorimetric instrument are discussed. The device was modified from a commercial instrument using a rotating mirror device to achieve phosphorimetric timeresolution. The influence of several experimental factors upon phosphorescence signals has also been investigated in detail. Analytical figures of merit and limitations of the novel instrument are discussed. The results show that the R T P technique could be applied on a routine basis in laboratories which are called upon to perform daily analyses on a very large number of samples. Our investigation indicates potential use of this technique in automated equipment to meet the demands of large scale routine analysis. Present address, Oak Ridge National Laboratory,Health Physics Division, P.O. Box X, Oak Ridge, Tenn. 37830. 1126
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
EXPERIMENTAL Apparatus. A schematic block diagram of the instrumental system is shown in Figure 1. The detection unit was an Aminco-Bowman Spectrophotofluorometer (SPF, Aminco Instrument Co., Silver Spring, Md.) which has a modified sample compartment equipped with a laboratory-constructed rotating mirror assembly for phosphorimetric detection. The operating principle of the rotating mirror is illustrated schematically in Figure 2. The mirror consists of a reflecting surface made from a diagonally cut section of an aluminum cylindrical rod. The surface is well-polishedin order to offer good reflection for visible and ultraviolet light. The plane of the mirror presented a 45' angle with respect to the horizontal plane (determined by the optical axes of the monochromators). As shown in Figure 2A, the excitation light beam from the excitation monochromator is reflected by the reflecting surface onto the surface of the filter paper which moves horizontally across the slit located at the top of the sample compartment (the excitation cycle). As the cylindrical reflecting surface is rotated, the reflection plane is moved into the emission path, and the phosphorescence signal emitted from the sample on the filter paper is reflected back into the detection system-emission monochromatorphotodetector system-(detection cycle). During the detection cycle, the excitation light is not "seen" by the detector and scattered light is greatly reduced by inserting the reflecting cylindrical surface into the Aminco-Keirs phosphoroscope attachment (shown in Figure 2B). The exciting light from the exit slit of the excitation monochromator is focused onto the sample by a (1-in. diameter, 11/4-in.focal length) quartz lens. A cylindrical tubing allowed easy and practical adjustment and focusing procedures with any lens size. An aperture located at the upper part of the sample compartment allowed a continuous warm air flush over the filter paper during the experiment. Samples are spotted on the filter paper roll of the Technicon Continuous Filter (a module of the Technicon AutoAnalyzer, Technicon Instrument Co., Tarrytown, N.Y.) which is used to
I
Figure 1. Schematic diagram of an AutoAnalyzer continuous filter with the room temperature phosphorescence detection system. (1) Light source, (2) excitation monochromator, (3) rotation motor-phosphoroscope, (4) reflecting surface, (5) optics, (6) filter paper, (7) emission monochromator, (8) detection unit, (9) recorder, (IO)filter paper roll, (11) spotting syringe, (12) drying 1R lamp, (13) dry air supply, (14) AutoAnalyzer continuous filter
, % Excitation
o i
\\
c
Recorder
Phosphore
SCW
v
Pholomultipller
Figure 2. (A) Principle of phosphorimetric excitation and detection with the rotating mirror phosphorimeter. (6)Design of the rotating mirror assembly
separate liquids from solids in fluid streams containing precipitates, flocculants,and agglutinates. The drying unit is a black metal chamber equipped with an infrared lamp. A 150-W Varian Eimac xenon arc lamp powered by a Varian dc power supply (Model 250 S-2, Varian, Eimac Div., San Carlos, Calif.), was used as the excitation light source. The photomultiplier, a Hamamatsu 1P21 tube (Hamamatsu T V Co. Ltd., Hamamatsu, Japan), is connected to a laboratory-constructed nanoammeter ( 4 ) . The signal current output is recorded on a strip-chart recorder (Model SRG, Sargent Welch Scientific, Skokie, Ill.). In preliminary optimization procedures (variation of drying time, different types of paper support, etc.), measurements were performed with a slightly-modifed version of the finger type sample holder described previously (1). Removable tips were used such that the paper could be easily positioned in the holder prior to the spotting process. This design improved the reproducibility of the measurements and decreased the analysis time without affecting the detection limits. It allowed the study of numerous types of filter paper and other supports. Procedure. The Eimac xenon arc lamp, the SPF, and the Technicon Continuous Filter were operated according to the manufacturers' instructions. In the Technicon filter unit, a roll
of filter paper was loaded and drawn into the drying chamber and the cell compartment. Samples were delivered drop by drop onto the moving filter paper by manually spotting 3 pL of sample solution using a hypodermic syringe. This step could, however, be automated in further studies. The filter paper was then fed into the drying chamber such that each sample remained in the chamber for approximately 2 min. The drying time could be controlled by selecting the paper roll speed. The paper was then passed over the sample compartment of the SPF. For each new type of sample investigated, the system was optimized (choice of excitation and emission wavelength) by stopping the filter paper when the sample was directly over the upper slit of the sample compartment and observing the signal. The optical adjustments were made on the first sample used and remained unchanged throughout the given experiment. For the preliminary optimization procedures, 3 pL samples of 145 pg/mL pyrene in a solution of ethanollwater (l/l;v/v) containing 0.1 M AgNOs were used with a flow of dry, warm (-60 "C) air through the sample compartment. Reagents. All compounds investigated were purchased and used without further purification: p-aminobenzoic acid (Fisher ScientificCo., Fair Lawn, N.J.), phenanthrene (Eastman Organic Chemicals, Rochester, N.Y.) and pyrene and chrysene (Research Organic/Inorganic Chemical Corp., Sun Valley, Calif.). More than 20 different types of paper support were used and the optimum sample support for analytical investigations was Schleicher and Schuell 604 (12l/~-cmcircles) and 591-C (1-in. width paper roll). Sodium hydroxide (Matheson, Coleman and Bell, Cincinnati, Ohio), sodium iodide (J. T. Baker Chemical Co., Phillipsburg, N.J.), silver nitrate (Mallinckrodt Chemical Works, St. Louis, Mo.), and ethanol (US. Industrial Chemicals Co., New York, N.Y.) and distilled water were used to prepare the solutions.
RESULTS AND DISCUSSION Influence of Various Experimental Parameters. In order to study the influence of various factors on the phosphorescence signals, preliminary measurements were performed with pyrene using the finger-type sample holder and the SS 604 filter paper (see next section). Two important experimental parameters which seemed to have an important influence on the phosphorescence efficiency were the predrying process of the sample (before the actual measurement) and the continuous flushing of dry gas through the sample compartment (during the measurement). Previously, we reported the effect of the second process (Z ), namely the presence of normal laboratory atmosphere could reduce significantly the emission signal. This may have been due to the quenching effect of moisture present in the air and the humidity of the atmosphere which might have altered the rigidity of the emitting species adsorbed on the paper support. In order to gain a better understanding, we undertook a more detailed study of both parameters by investigating the time behavior of the phosphorescence signal of a series of identical samples which had various drying times and had been measured under different gas flows. The results shown in Figure 3 correspond to measurements of 3-pL samples of pyrene performed under a dry, warm air flow (60 "C). Curve 1 of Figure 3 (dashed line) shows the growth of the phosphorescence signal of a sample which had not been pre-dryed and was directly put into the sample compartment after the solution has been spotted on the filter paper. The phosphorescence signal, which was not detectable in the beginning, increased sharply after about 1 min and remained nearly unchanged. Because an additional 0.5 min was required to mount each sample on the finger holder, 11/2 min was an indication of the time required for the sample to be dry enough for phosphorescence emission to be detected. The increase, which was not a gradual growth but a sudden change in the signal level, reflected the rapid evolution of the physical state of the substances adsorbed on the paper support. Curve 2 in Figure 3 shows the evolution of phosphorescence from a ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1127
Table I. Relative Room Temperature Phosphorescence Signals of Pyrene and the Background with Various Supporta Approximate Analyte signal/ analyte signal background signal (X l o 9A) Company Paper type E.D. 613 Eaton-Dikeman Co. (Mount Holly Springs, Calif.) 1.4 8 E.D. 615 LOb 5 b Gelman Instrument Co. (Ann-Arbor, Mich.) ITLC/SG b _ - _b ITLC/SAF b - - -b ITLC/SA C - - -c Metricel (GM-4) 0.8 pm RA 202 H. Reeve Angel & Co. (Clifton, N.J.) 1.0 6 RA 201 1.0 10 Scientific Products (Evanston, Ill.) S.P. (F2402-9) Eli Lily Co. (Indianapolis, Ind.) 1.5b 'b Powder paper Will 13061 Will Corp. (Atlanta, Ga.) 0.8 6 Will 13071 0.9 4 SS 2049 Schleicher & Schuell (Keene, N.H.) 2.1 7 SS 604 2.2 20 SS 591-C 1.5 10 Whatman Chromatographic Fisher Scientific (Fair Lawn, N.J.) 1.8 6 Whatman 1 1.9 8 Whatman 30 2.1 10 Whatman 40 1.8 10 2.0 10 Whatman 4 1 1.8 10 Whatman 42 EM Laboratories (Elmsford, N.Y.) EM 60 F-254 Silicagel precoated - - -b _ - _b Aluminum support - - -b - _ _b Plastic support N o t deteca A,, = 350 nm, A,, = 600 nm, 3 pL pyrene (145 pg/mL) in ethano1:water (l/l;v/v) with 0.1 M AgNO,. Support became brownsignal not detectable. table under experimental conditions.
___ ___ ___ ___
___
___
___
4
c
I -
LL_, , ,
3
2
,
,
'
4 EVC-JTlOhI
0 TIME
m n)
Figure 3. Time evolution of the phosphorescence signal of pyrene (145 pg/mL in ethanokwater with 0.1 M AgNO3). Curves: (1) No predrying procedure, (2)Two-min pre-drying time, (3) Five-min pre-drying time, (4) Ten-min pre-drying time, (5) Fifteen-min pre-drying time, (6) Twenty-min pre-drying time, (7) Thirty-min pre-drying time
sample which had had a 2-min pre-drying time prior to measurement. The initial phosphorescence signal started from a certain level and remained approximately constant within the usual 8-min observation time. Curves 3, 4,5,6, and 7 in Figure 3 correspond to samples which had been pre-dryed for 5 , 10, 15, 20, and 30 min, respectively. In all cases, the phosphorescence signal remained relatively constant during the measurements. This shows that, with adequate pre-drying treatment in order not to alter chemically the species involved, the drying process had an effect only upon the initial conditions of the samples (e.g., state of adsorption, incorporation into the matrix, interaction with heavy atoms, etc.), and these properties should and did not change greatly with time. The second noteworthy feature provided by these results is the existence of an optimal pre-drying time (between 5 and 10 min) when the sample provided the greatest phosphorescence emission. It has been reported that oxygen does not seem to quench effectively room temperature phosphorescence of adsorbed molecules on paper supports (1-3). Our present study 1128
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
2
4
6
'
TIME i m n i
Figure 4. Influence of oxygen and nitrogen as on the phosphorescence signal conclusively shows that measurements performed under a nitrogen atmosphere provided more intense emissions (about twice) than those performed under an oxygen atmosphere. Figure 4 shows the temporal behavior of phosphorescence emission from samples which had not been pre-dryed and directly placed into the sample compartment flushed by nitrogen (solid lines) and oxygen gas flow (dashed lines). Selection of Paper Support. For the choice of sample support, over twenty different kinds of commercial filter papers and several TLC supports were tested (pyrene was used as the standard). Our criteria for selection was based upon the relative phosphorescence emission signal of the sample with respect to the phosphorescence background of the paper. In addition to various types of filter and chromatographic papers, TLC plastic and aluminum sheets precoated with silica gel (EM 60F-254), and glass fiber paper (Gelman, ITLC) were also tested. About half of the supports used in this study provided pyrene phosphorescence signals larger than the phosphorescence background. The S&S filter papers yielded the highest signal-to-background ratio and so were used in all further measurements with the rotating-mirror phosphoroscope. The results are summarized in Table I.
P e r f o r m a n c e of the R o t a t i n g M i r r o r Device. Separation of the fast decaying fluorescence from the much slower phosphorescence is usually achieved mechanically by two basic types of phosphoroscopes (5, 6): (i) the Becquerel phosphoroscope; and (ii) the rotating cylinder phosphoroscope. The rotating mirror used in this work performed time resolution by achieving a similar effect as with the rotating can phosphoroscope. As described in the Experimental section, the rotation of the mirror allowed the excitation beam to strike the sample surface periodically and permitted the phosphorescence emission from the sample to be reflected back and reach the detector (out of phase) between two consecutive excitation cycles. The rotating mirror produced a series of periodic exciting light pulses on the sample. The pulse frequency can be varied by changing the rotation speed. Similarly, the detector can also observe the emitting signal only for a short time when the rotating reflecting surface plane is in the appropriate position. The detector is therefore “optically gated” for a short period a t a specific delay after the exciting pulse. In our experiments, the photocurrent signal from the detector is measured with a dc nanoammeter, the response of which is proportional to the average area under the current pulses. Expressions, which related the measured and instantaneous phosphorescence signal to the decay time 7 of the phosphorescence species and to the characteristics of the mechanical shutter mechanism, were given by O’Haver and Winefordner (7). Those equations were extended to apply to a measurement system using pulsed excitation and a gated photomultiplier for observation of sample luminescence (8). For a long-lived phosphor, the present system results in an a-factor (the fractional measured phosphorescence signal as compared to a dc system with continuous excitation/measurement) which was =36 times smaller than with the conventional phosphoroscopic can devices used in the Aminco SPF system for phosphorimetry; therefore considerable improvements in signal levels are possible in our system. Because the rotating reflecting surface, like any stroboscopic device, could also allow enhancement of the signal of one luminescent species with respect to other species of longer or shorter lifetimes (time resolution), time resolved operation could be easily achieved by selecting the appropriate rotating speed for any specific component (a certain 7) in order to obtain a selective maximum enhancement. Time Required for Analysis. One of the most important criteria to be considered in analysis is the requirement of analytical speed. Because control analyses involve routine and high-volume measurements, they must necessarily be based on simple procedures where the instrumentation operates continuously with only a minimum of operator handling and preparation. In this regard, the present system with the rotating reflecting surface for room temperature phosphorimetric analysis seems to be quite suitable and presents several advantages over conventional methods. In contrast with most analytical methods, the detection is performed directly on the paper without the need for time-consuming separations and removal of the chemical substance from the support resulting in a simpler and more rapid analytical procedure. The idea of using phosphorimetry as a detection method for paper or thin-layer chromatography is not recent, but the approaches used in a present work are quite different. Previous workers have utilized the phosphorescence of polyaromatic hydrocarbon compounds separated by paper chromatography, but in order to detect the phosphorescence spectra, the paper had to be at low temperature, thus requiring cryogenic equipment (9). Recently, p-aminobenzoic acid has been determined by room temperature phosphorescence using sodium acetate powder (IO). Although this method did not require any cryogenic equipment, the sample preparation
TIME
+
Figure 5. Shape of the phosphorimetric peaks. (A) p-Aminobenzoic acid (390 ng, A,, = 280 nm, he, = 430 nm) in a solution of Na0H:NaI (0.5 M:0.25 M). (B) Chrysene (42 ng, hex = 305 nm, A,, = 515 nm) in an ethanokwater (l/l;v/v) solution containing 0.1 M of AgN03
requirements were not sufficiently simple for high volume and fast routine analysis. In our system, the time required for analysis depended only upon the optimal drying time chosen for the measurement. Because the drying chamber had a length of 15 cm, a paper advance speed of 8 cm min-’ was used to obtain a 2-min drying time (within the sample chamber) which was adequate to obtain excellent signal-to-noise ratios. Because a 15-cm paper strip could contain a maximum of 15 sample spots, under our specific conditions, 7-8 measurements could be performed per minute. Here again, it should be kept in mind that if higher temperatures were used for the drying process, with the attendant risk of degradation of the chemicals involved, a much shorter drying time would be required, and the analysis time could consequently be decreased. S h a p e of the S a m p l e Spot. When the filter paper containing the circular sample spots was moved across the sample compartment aperture a t a constant speed, the recorded emission peaks were observed (the peak height of each curve is related to the concentration of the emitting species). The optical gating effect of the rotating reflecting surface was negligible at high rotation speed and typical filter paper speeds of 8 cm min-l. Figure 5 shows the phosphorescence peaks recorded using 390 ng of p-aminobenzoic acid (Aern = 430 nm) in a solution of NaOH:NaI, (1M0.25 M) and 42 ng of chrysene (he,,, = 15 nm) in a solution of silver nitrate (0.1 M) in ethano1:water ( l / l ; v/v). Although in both cases, the shapes of the peaks were symmetric, the chrysene peak showed a maximum in the center region whereas the p-aminobenzoic acid showed two maxima a t the border of the curve; this indicated that a major part of p-aminobenzoic acid diffused towards the outside border of the spot, whereas chrysene stayed mostly in the middle part. The difference in the shape of the peaks of various compounds was typical for other PAH compounds (in silver nitrate solution) that we have studied. The differences in the peak shape of various types of compounds could provide an additional factor of selectivity in the analysis of mixtures and deserves further investigation. However, for quantitative analysis, care must be taken because it is necessary to integrate the area under each specific curve to obtain an accurate measure of the parameter related to the concentration of various compounds. Reproducibility of the Measurement. Several series of 10 to 15 identical samples of various substances were measured. The peak heights of the recorder response were taken as the phosphorescence signal intensity proportional to the amount of the prosphorescing compounds. The relative standard deviations of most series of measurements were found to be less than 5% which is superior to the 15% found ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1129
when the finger-type holder was used (1-3). The experimental procedures with the finger-type holder involved more operator steps resulting in poorer reproducibilities, i.e., tedious spotting of samples on each paper circle, careful timing of the predrying period for each sample, mounting separately each paper circle with sample on the finger holder, etc. With the use of the continuous filter paper assembly and the direct introduction of the samples into the detection path of the rotating reflecting surface device, only the sample spotting operations were still manually performed in our case (although this step could be further performed automatically when a complete AutoAnalyzer is used). The pre-drying and drying times for each sample were strictly determined by the speed of the continuous filter and therefore were exactly the same for every sample. Improvement in Signal-to-Noise Ratio. A straightforward application of the RTP method is its extension as a detection technique to chromatographic separation methods, such as TLC and paper chromatography. The results from our device indicated also that solution samples from liquid chromatographs could be further spotted on a paper support in order to be detected by the R T P technique. Because of the spectral selectivity and sensitivity, inherent to a phosphorimetric detection method, the R T P technique should complement the conventional absorption and fluorescence detectors used in chromatography. Although most of the basic components used for fluorimetric detection in TLC, paper, or liquid chromatography are similar to those used in this present work (in fact, many commercial fluorimeters are available with TLC scanning attachments) without a phosphoroscope device (a rotating reflective surface phosphoroscope in our case), scattered exciting light would lead to an excessively large background. Because wide monochromator slit widths (>1mm) are commonly used in analytical measurements to increase the sensitivity, scattered light from the excitation source is very often a serious problem. Such a problem cannot always be overcome by the use of additional filters but in our case, because of the rotating reflecting surface phosphoroscope, there is considerable reduction in stray light, by time-resolution. It has been observed that without the rotating reflecting surface device, the recorded signals of scattered light with 2-5 nm spectral slit widths were in the range of lo4 A, which was 2 to 3 orders of magnitude higher than the signal levels occuring in most of our actual analytical measurements (lo-' to A). When the rotating device was used without the phosphoroscope can attachment (see Experimental section), the level of scattered background was
reduced to lo-' to lo-' A. However, when the reflecting surface was inserted into the cylindric phosphoroscope can and the sample compartment was carefully fitted with appropriate wall baffles, the observed scattered light was very weak (of the order of lo-'' to lo-'' A) and allowed a substantial enhancement of the signal-to-noise ratio for the analyte. Sensitivity of t h e I n s t r u m e n t a l Method. Room temperature phosphorimetry has been previously shown to be a sensitive analytical spectroscopic detection technique. In previous reports, this method has been applied to a large variety of biologically-important compounds (1-3). It was shown that R T P could be used, not only for ionic molecules (I, 21, but also for other types of organic molecules, such as the important PAHs (3). The purpose of this report was not to provide a more extensive list of substances which could be detected, but to present a unique, simple, and sensitive instrumental device which was especially developed with regard to rapid sample processing and potentially with application to automated analysis and which therefore has potential use in clinical and environmental laboratories. As an illustration of the detectability achieved with this instrument, our measurements provided a limit-of-detection (Signal/Noise,= 3) at the nanogram and sub-nanogram range for the following compounds: 60 pg of phenanthrene (Aex = 310 nm, A,, = 500 nm) in a solution of ethano1:water (l/l;v/v) containing 0.1 M of silver nitrate, 1.3 ng of chrysene (Aex = 305 nm, A,, = 515 nm) in the same solvent and 1.6 ng of p-aminobenzoic and (Aex = 280 nm, A,, = 430 nm) acid in an aqueous solution of Na0H:NaI (0.5 M:0.25 M). ACKNOWLEDGMENT The authors thank Technicon Corporation, Tarrytown, N.Y. for the loan of an AutoAnalyzer Continuous Filter. LITERATURE C I T E D (1) '
S.L. Wellons, R. A. Paynter, and J. D. Winefordner, Saecfrochim. Acta.
Part A , 30, 2133 (1974). (2) T. Vo-Dinh, E. Lue Yen, and J. D. Winefordner, Anal. Chem., 48 1186 (1976). \
- I
(3) T. Vo-Dinh, E. Lue Yen, and J. D. Winefordner, Talanfa, in press. (4) T. C. O'Haver and J. D. Winefordner, J. Chem. Educ., 46, 241 (1969). (5) J. D.Winefordner, Acc. Chem. Res., 2, 361 (1969). (6) J. J. Aaron and J. D.Winefordner, Talanfa, 22, 707 (1975). (7) T. C. O'Haver and J. D. Winefordner, Anal. Chem., 38, 602 (1966). (8) T. C. O'Haver and J. D.Winefordner, Anal. Cbem., 38, 1258 (1966). (9) M. Zander and U. Schimpf, Angew. Chem., 70, 503 (1958). (IO) R. M. A. von Wandruszka and R . J. Hurtubise, Anal. Cbem., 48, 1784 (1976).
RECEIVED for review February 25,1977. Accepted April 25, 1977. This research was supported by NIH-GM-11373-14.
Modulation Photolysis as an Alternative to Flash Photolysis J. A. Burt Physics Department, York University, 4700 Keele Street, Downsview, Ontario, Canada M3J lP3
The mathematical principles are developed whereby a low intensity continuously modulated light source is employed to secure kinetic data for a photochromic solution. The detector used is a spectrophone. The method Is not restrlcted to photochromism and spectrophones but may be used In any experiment whlch also permits study by flash photolysls.
T o determine kinetic parameters such as the dark, thermal time constant for a photochromic reaction flash photolysis 1130
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8,JULY 1977
offers distinct advantages since the system under study is pumped instantaneously to the excited state. The flash illumination ceases and the system natural decay time may be simply observed. Two preconditions are implied here: (a) that the flash is indeed of sufficient intensity to pump a large fraction of the system to its excited state and (b) that the system response is much slower than the duration of the flash. If the flash duration is comparable to the system time response,the true response may be unfolded from the observed response by using the method of Demas ( I ) . Alternatively,
