identical. Other studies have found FID responses to be essentially identical for aromatics and alkyl aromatics (8). For example, naphthalene, methylnapthalene, and dimethylnaphthalene, dissolved in toluene to a known concentration, showed GC peak areas per gram to be identical within experimental error of 1-270. Therefore, quantitative GC was chosen to be the most accurate available technique to determine concentrations in LVMS analytical solutions. Previously published GC sensitivities using a flame ionization detector (9) reported a difference between PNAs, their methyl derivatives, and their hydro derivatives. Data in this study showed this not to be representative of compounds which are completely soluble in the solvent used for GC injection. The reported change in sensitivity with methylation included differences in behavior in the separation scheme used. Methylated PNAs were seemingly less sensitive than the parent PNA, but this decrease in sensitivity was due to differences in solubility rather than to a change in true detector response characteristics.
ACKNOWLEDGMENT The author thanks David J. Miller who performed some of the experimental work reported.
LITERATURE CITED H. R. Appell and I. Wender, Prepr., Div. Fuel Chem., Am. Chem. SOC., 12, (3),220 (1968). B. H. Johnson, and T. Aczel, Anal. Chem., 39,682 (1967). A. W. Peters, and J. G. Bendaraitis, Anal. Chem., 48, 968 (1976). T. Aczel, J. Q. Foster, and J. H. Karchmer, Prepr., Div. Fuel Chem., Am. Chem. Soc., 13, (l),8 (1969). J. L. Schuttz, R. A. Fiedel, and A. G. Sharkey, Jr.. U.S.Bur. Mines Rept. R I 7000 (1967). H. E. Lumpkin and T. Aczel, Anal. Chem., 36, 181 (1964). J. E. Schiller and D. H. Neal, “Preparation of Hydroaromatic Compounds for Mass Spectrometry Standards in Coal Conversion Analysis”, ERDA Rept. Invest., GFERC/RI-75/2 (1975). J. C. Sternber, W. S. Galloway, and D. L. T. Jones, “Gas Chromatography”, Academic Press, Inc., New York, 1962, Chap. 18. R . C. Lao, R. S. Thomas, H. Oja, and L. Dubois, Anal. Chem., 45,908
(1973).
JoseDh E. Schiller Grand Forks Energy Research Center Grand Forks, North Dakota 58202
RECEIVED for review February 10,1977. Accepted May 2,1977. Reference to specific manufacturers, their brands, or models is for identification only and does not represent endorsement by ERDA.
Analytical Possibilities of Phase Resolved Phosphorimetry Sir: The phase characteristics of several organic phosphors have been investigated as the basis of an analytical technique ( I ) . The sample is excited by a continuum source, the intensity of which is modulated, and the phase of the phosphorescence emission is compared with that of the exciting light. The phase shift (0) of the phosphorescence emission signal is a function of the frequency of modulation and the lifetime of the phosphorescence. By measuring the phase shift, phosphorescence lifetimes can be calculated from the basic equation, 0 = tan-’ ( w ~ T )where T is the phosphorescence lifetime and wJ is the angular frequency, usually the fundamental frequency, wl. The same theory applies to fluorescence, and a review by Birks and Munro (2) covers phase resolution via fluorimetry. Because the instrumentation and its operation are fairly simple in phase resolved phosphorimetry, efforts were made in this laboratory to extend the analytical capabilities of this technique with the hope of doing routine analysis of drugs. In the present work, minor modifications were carried out on the system used in the earlier work cited (1). EXPERIMENTAL Apparatus. The experimental setup used in this study was similar to that of Mousa and Winefordner ( I ) . The source of excitation was an Eimac 150-W lamp whose intensity was modulated mechanically rather than electronically as was previously done. The intensity of this lamp was far greater than the one used in prior work because of the use of an integral parabolic reflector. The lamp was powered by a Varian Illuminator Power Supply kept at 12 A. A block diagram of the instrumental system is shown in Figure 1 (Comments are described on the caption). Reagents. Benzophenone (Fisher Scientific Co., Fair Lawn, N.J.), 4-bromobiphenyl (Pfaltz and Bauer, Flushing, N.Y.), morphine, and codeine (Applied Science Labs., Inc., State College, Pa.), were all used as received. The solvent used in all cases was ethanol (U.S. Industrial Chemicals, Co., New York, N.Y.) which was made anhydrous using the method of Lund and Bjerrum ( 3 ) . Procedure. The operational procedure used was identical to the one described by Mousa and Winefordner (1). 1262
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8 , JULY 1977
Table I. Phosphorescent Lifetimes from Phase Data Lifetimes, ms Phase Phasea (Ref. Molecule (this work) 1) Time resolvedb 5.9 I Benzophenone 6‘ ( 2 5 Hz) 5.3d ( 2 5 Hz) 4-Bromobiphenyl 17‘ (10 Hz) 14 17 32‘ (10 Hz) --39 Codeine a Values in parentheses are the optimum modulation frequencies in each case. The capillary cell for all lifetime Data were taken from measurements was not spun. Snowy matrix. Ref. 4-6; Glassy matrix.
RESULTS AND DISCUSSION Quantitative analysis and measurement of lifetimes of benzophenone and 4-bromobiphenyl in a clear-glassy matrix of pure ethanol were readily carried out with the experimental system described. The results obtained indicate good agreement with other published works as shown in Table I. It appears that phase resolved phosphorimetry would be useful for the analysis of these two compounds where the slope of the log-log analytical curve is close to unity and the range of linearity is fairly large ( lo3). The phosphorinietric limits of detection obtained were: 0.1 ppb (5 X lo-’’ M) for benzophenone; and 2 ppb (7 X lo-’ M) for 4-bromobiphenyl. The detection limit is defined as that concentration of analyte resulting in a signal-to-noise ratio of 3. The same procedure was carried out on benzophenone in a 10% ethanolfwater mixture which produced a snowy matrix to avoid the tedious handling of a clear-glassy matrix because formation of such a matrix sometimes proved to be a nuisance. The measured lifetime of benzophenone in this snowy-matrix (capillary cell not spinned) agreed quite well with that found in a clear glassy matrix (see Table I), but no quantitative analysis could be carried out. The inhomogeneity and opaqueness of the snowy N
I
CONCLUSIONS
I
G I
P
li
Figure 1. Block diagram of Instrumental system. A = 10-15 V, 7-21.5 A, d.c. Power Supply (Varian Illuminator Power Supply PS 300-1, Varian,
San Carlos, Calif.). B = Eimac 150-W Xenon Arc Lamp (Varian-Eimac Division, San Carlos, Calif.). C = Quartz lens. D = Mechanical Chopper (Model 382B, Ithaco, Ithaca, N.Y.). E = Optional Monochromator (Model H10, J. Y. Optical Systems, Metuchen, N.J.). F, G = Excitation and emission monochromators (Aminco-BowmanSPF, American Instruments Co., Silver Spring, Md.). H = Photomultiplier tube and housing (1P21, Hamamatsu Corp. Middlesex, N.J.). I = High Voltage Power Supply (240 Regulated High Vottage Power Supply, Keithley Instruments, Cleveland, Ohio). J = Preamplifier (Model 164 Preamplifier, Ithaco, Ithaca, N.Y.). K = Lock-in amplifier (Dynatrac 391A, Ithaco, Ithaca, N.Y.). L = X-Y Recorder (Plotamatic 715, MFE Corp., Salem, N.H.). S = Sample compartment (same as reference 1) matrix made necessary spinning of the capillary cell which introduced additional noise and random phase shifts and prevented correct phase measurement which limited the accuracy and precision of lifetime and quantitative measurements. Similar measurements (in clear glasses and snowy matrices) were attempted for morphine and codeine, but the results were not nearly as encouraging. A solution of morphine (400 ppm) gave no appreciable phosphorescence signal, while a solution of codeine (750 ppm) gave a weak signal. In these cases, where emission was very weak, increased amplification led to an enhanced phosphorescence signal as well as noise level. In addition, spectral interferences from stray and scatter light, which could not be phased out, contributed relatively much more to the noise than in the benzophenone and 4-bromobiphenyl cases above. A second monochromator in series was placed between the light source and sample to reduce stray light in the system. In this case, benzophenone and 4-bromobiphenyl were still easily detectable with the limit of detection of benzophenone being raised to =1 ppb. No significant increase in the signal-to-noise ratio of morphine or codeine was seen so that the reduction in stray light was insufficient to allow suitable detection of these drugs.
These observations do not dispute those of Mousa and Winefordner ( I ) because the phase resolution technique apparently works in the case of strong phosphors or concentrated solutions of poorer phosphors if no fluorescence or stray light interference is present. I t also appears to be successful in resolving binary mixtures ( I ) of highly phosphorescent species if the difference between the phase shifts Ba and OB of molecules A and B is of a certain magnitude or greater. In the theoretical treatments ( I ) , stray light and fluorescence were assumed to be negligible. In practice, however, these two factors are usually present unless only strongly phosphorescent species are involved or unless one uses more sophisticated instrumentation to minimize these interferences which would limit the analytical use of this technique to rather restricted conditions. Also, in phase resolved phosphorimetry, molecules with short phosphorescent lifetimes have enhanced response over those with long phosphorescence lifetimes; this situation is desirable when the background phosphorescence from the solvent is of a long-lived nature. However, an inherent disadvantage results because only phosphorescent molecules with lifetimes between 1-50 ms can be easily measured using phase resolved phosphorimetry (7). If the lifetime is too long (>50 ms), measurements must be made at a low frequency where bubbling noise of the liquid nitrogen and other low frequency noises make it difficult, if not impossible, to take accurate and precise data. If the lifetime is too short (
