order to use the above manifold in the range of 0-25 mg/l. chloride, using oil bath temperature of 75 "C, it was necessary to use 0.19% color reagent in 86% v/v sulfuric acid and 370 mg/l. nitrate through the reagent and nitrate pump tubes, respectively, in the manifold. Absorption spectra of the resultant solutions indicated that the maximum absorbance of the solutions changed with the concentration of nitrate in the manifold. For example, the absorbance maxima for chromotropic reagentnitrate vs. chromotropic reagent-nitrate-chloride solution was found to shift from 450-505 nm when the final concentration of nitrate was changed from 25-100 mg/l. in the manifold. The apparent increase in response of the reaction was found to be due to the fact that the maxima lay closer to the 505-nm narrow pass filter as the concentration of nitrate was changed from 25-100 mg/l. In our original communication ( I ) , a 505-nm narrow pass filter was selected, instead of the filter corresponding to maximum absorption, to cover the concentration range of chloride between 0.25100 mg/l.
LITERATURE CITED (1) B. K. Afghan, R. Leung. A. V. Kulkarni, and J. F. Ryan, Anal. Chem., 47, 556 (1975). (2) M. Bartusek and L. Havelkova, Collect. Czech. Chem. Commun., 32, 3853 (1967). (3) J. W. Robinson and C. J. Hsu. Anal. Cbim. Acta., 44, 5 1 (1969). (4) D. D. Perrin, W. L. F. Armarego, and Dawn R. Perrin, "Purification of Laboratory Chemicals", Pergammon Press, Oxford, 1966, p 114. (5)B. K. Afghan and J. F. Ryan, Anal. Chem., 47, 2347 (1975). (6) G. Pastuska and H. Trinks, Chem. Ztg., 86, 135 (1962). (7) D. Waldi, in "Thin-Layer Chromatography, a Laboratory Handbook", E. Stahl, Ed., Academic Press, New York, 1965, pp 485-502. (8) J. F. Lechner and I. Sekerka, J. Elecfroanal. Chem. lnterfacial Electrocbem., 57, 317 (1974). (9) I. Sekerka. J. F. Lechner, and R. Wales, WaterRes., 9, 663 (1975).
Badar K. Afghan* James F. Ryan Analytical Methods Research Section Applied Research Division Canada Centre for Inland Waters Burlington, Ontario, Canada L7R 4A6 RECEIVEDfor review June 23, 1975. Accepted January 15, 1976.
X-ray Excited Optical Luminescence of Polynuclear Aromatic Hydrocarbons Sir: The carcinogenic properties of polynuclear aromatic hydrocarbons (PAH's) are now well known ( I ) . Because virtually all phases of coal conversion into coke and liquid or gaseous fuels may involve potential exposure to PAH compounds at some time in the process ( 2 ) , there is increasing interest in new analytical concepts for the detection and quantitative determination of these compounds a t ultratrace levels ( 3 ) . When PAH's are dissolved in n-alkanes (e.g., n-heptane) and the resulting liquid is frozen a t 77 K, the solid solution formed emits sharp-line luminescence under ultraviolet (uv) illumination. These sharp line spectra are a manifestation of the Shpol'skii effect ( 4 ) . The application of the Shpol'skii effect to the determination of trace amounts of 3,4-benzopyrene, the well known carcinogen, as well as other PAH under uv excitation is well demonstrated (5-8). In earlier publications, we have demonstrated that x-rayexcited optical luminescence (XEOL) is a sensitive analytical technique capable of detecting impurities a t fractional parts per billion levels in appropriate solid or gaseous environments (9, I O ) . In this communication, we report on the extension of the scope of application of the XEOL technique to organic systems, in particular to PAH compounds. We report here the first observation of x-ray excited sharpline fluorescence and phosphorescence emitted by a selected group of PAH's present as trace constituents in n-heptane, a Shpol'skii solvent. The primary x-radiation was obtained from a tungsten target x-ray tube (OEG-50-T, Machlett Laboratories, Springdale, Conn.) which was operated a t 50 kV dc and 40 mA. Under these conditions x-rays in the range of 1to 10 8, are obtained. The frozen solutions were obtained by injecting the liquid samples into 1-cm diameter by 2-mm deep A1 planchets which were inserted into a recess in a cold finger cooled by liquid N2. The frozen samples, which attained a temperature of 90 K, were irradiated inside a shielded chamber modified to facilitate the introduction of the cold finger assembly. A flow of cold dry Nz over the sample prevented moisture condensation on the frozen samples. The simple spectrometric system previously described (11) was
used to record the XEOL spectra at a spectral bandwidth of 0.7 nm and a scan rate of 20 nm/min. Because the spectrometric system has not been calibrated to provide corrected luminescence spectra, the spectral radiances of the most intense lines in the spectra have not been determined. The spectra shown in Figures 1 and 2 were obtained from 0.2-ml solutions containing 10 wg of PAH/ml in n-heptane. The relative intensity of the spectra emitted by various compounds can be gauged by the attenuation factors indicated in parentheses under the names of the compounds in the figures. Thus perylene, benzo(ghi) perylene, coronene, and 3,4-benzopyrene, the highly toxic carcinogen exhibited the most intense spectra. A few of the spectral lines have been tentatively designated as F (fluorescence) or P (phosphorescence) emission based on their wavelength similarity with uv excited luminescence spectra. The relative simplicity of the spectra should facilitate the quantitation of a number of these compounds in a mixture using simple spectrometric instrumentation. The frozen n-heptane solvent blank spectra, shown in Figure 3 at attenuation factors of 1, 6, and 10, reveal that the solvent contributes a weak unidentified broad band in the 400-460 nm region. The sharp spectral lines in the 350-420 nm region are those of the second positive and first negative system of molecular N2 observed during x-ray irradiation of air or N2 ( I O ) . A significant advantage of using the Shpol'skii effect for the detection of PAH's is illustrated in Figure 4. In this figure, the relative intensity of the luminescence spectra obtained a t 90 K of perylene in the non-Shpol'skii solvent cyclohexane and the Shpol'skii solvent n-heptane are compared. The relative attenuation factors indicated in parentheses in the figure emphasize that the luminescence of perylene in n-heptane is enhanced by over two orders of magnitude in comparison to that observed in cyclohexane. This observation portends significant advantages for the use of the Shpol'skii effect in the observation of XEOL of organic molecules. The limits of detection for perylene, coronene, and the highly toxic carcinogen 3,4-benzopyrene were estimated to be a t nanogram levels from spectral recordings typical of ANALYTICAL CHEMISTRY, VOL. 48,
NO. 6, MAY 1976
915
I
I
I
IF
400
600
500
Wovelength
400
500
600
(nm)
Figure 1. X-ray excited optical fluorescence (F) and phosphorescence (P) spectra of 3,4-benzopyrene, perylene, 1,a-benzanthracene and benzo[ghi]perylene in n-heptane at 90 K
400
SO0
600
Wavelength (nm:
Figure 4. Comparison of x-ray excited optical luminescence of perylene in the non-Shpol'skii solvent cyclohexane and in the Shpol'skii solvent n-heptane obtained at 90 K
I
PERYLE NE 3,4 -BENZOPYRENE
I
CORONENE
jF I u 410
400
500
400
600 Wovelength
500
600
n
I
I
500
400 i,zv-Ierq*t
600
17
Figure 3. X-ray excited optical luminescence spectra of n-heptane at 90 K recorded at three attenuation factors
916
u
430 470 540 WAVELENGTH lnmi
580
Figure 5. X-ray excited optical luminescence intensities of the most sensitive lines of 3,4-benzopyrene. perylene, and coronene obtained with 20 ng of the three compounds in n-heptane at 90 K
lnm)
Figure 2. X-ray excited optical fluorescence (F) and phosphorescence (P) spectra of coronene, phenanthrene, anthracene, and 1,2,5,6-dibenzanthracene in n-heptane at 90 K
I
450
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
those shown in Figure 5 of the most sensitive lines of the three compounds. The spectra observed a t 90 K of the nheptane solvent in the wavelengths regions indicated are also shown. The spectra of PAH's shown in Figure 5 were obtained with samples containing 20 ng of one of the three compounds. I t must be emphasized that the instrumentation, Le., the combination of grating blaze and detector spectral sensitivity, were not optimum for recording the spectra in the 400-600 nm region. It is premature a t this time to elaborate on the relative merits of x-ray or uv excitation. One obvious advantage for x-ray excitation is the freedom from optical cross-talk between the exciting and luminescence radiation commonly encountered when uv excitation is used. A few general comments based on investigations of luminescence in other organic systems (12), suggest other potential advantages. X-ray irradiation may result in populating electronic levels in molecules not accessible to ultraviolet excitation. Hence, XEOL spectra may reveal spectral components not observed under uv excitation, specially in the near and vacuum uv spectral regions. These spectra may lead to identification of high-lying energy levels as well as aid in characterizing energy transfer processes. I t has been observed that x-ray excitation also leads to substantial phosphorescence even when negligible phosphorescence is observed
under uv excitation (12).This observation portends significant advantages to the application of time-resolved phosphorimetry (13) as a refinement of the XEOL technique. X-ray irradation of organic compounds is known to cause thermoluminescence ( 1 4 ) . In multicomponent systems, it may then be possible to differentiate components with similar luminescence characteristics by their dissimilar thermoluminescence behavior. Thus, the predicted advantages of the XEOL technique enumerated above may lead to the development of a versatile analytical system for the determination of trace organic compounds.
(8)G. F. Kirkbright and C. D . DeLima, Analyst, (London), 99,338(1974). (9)V. A. Fassel, E. L. DeKalb, and A. P. D'Silva. in "Analysis and Application of Rare Earth Materials", 0.B. Michelson, Ed, NATO Advanced Study Institute. Kjeller, Norway, Universitetsforlaget, 1973, pp 109-
122. (10) S. A. Goldstein, A. P. D'Silva, and V. A. Fassel, Radiat. Res., 59, 422
(1974). (11)A. P. D'SilvaandV. A. Fassel, Anal. Chem., 44, 2115(1972). (12) H. B. Steen. 0.i. S@orensen, and J. Aa. Holting, lnt. J. Radiat. Phys. Chem., 4, 75 (1972). (13)J. D. Winefordner, Acc. Chem. Res., 2, 361 (1969). (14)C. A. Parker, in "The Triplet State", A. B. Zahlan, Ed., Cambridge University Press, 1967,pp 353-390.
A. P. D'Silva G . J. Oestreich V. A. Fassel*
LITERATURE CITED (1)A. Haddow. in "The Physiopathology of Cancer" (Homburger, Ed.), 2nd ed.. Harper, New York, 1959,Chap. 14. (2)R. I. Frendenthal, G. A. Lutz, and R. I. Mitchell, "Carcinogenic Potential of Coal and Coal Conversion Products", Battelle Columbus Laboratories, Ohio, 1975. (3)C. E. White and R. J. Argauer, "Fluorescence Analysis", Marcel Dekker, New York, 1970,chap. 12. (4)E. V. Shpol'skii, A. A. il'ina, and L. A. Klimova, Dokl. Akad. Nauk SSSR,
87,935 (1952). (5)B. Muel and G. Lacroix, Bull. SOC.Chim. Fr., 2139 (1960). (6)R . I. Personov and T. A. Teplitskaya, Zh. Anal. Khim., 20, 1125 (1965). (7)P. P. Dikun, Zh. Prik. Spektrosk., 6 , 202 (1967).
Ames Laboratory-ERDA Iowa State University Ames, Iowa 50011 RECEIVEDfor review July 17, 1975. Accepted January 14, 1976. Work performed for the U.S. Energy Research and Development Administration under Contract No. W-7405eng-82.
Existence of Gas-Liquid Interfacial Adsorption in Solutions of Alcohols and Ketones in Saturated Hydrocarbons Sir: There has been much debate in recent years as to whether low molecular weight alcohols, ketones, and amines are adsorbed a t the gas-liquid interface of saturated hydrocarbon solvents. According to some authors (1-3), adsorption is negligible while, according to others (4-6), the adsorption contribution to retention is very significant in comparison with that of partition between the gas and bulk liquid phases. The question is of great importance in gas-liquid chromatography (GLC), both in current efforts to classify systems exhibiting liquid surface adsorption (7), and in making accurate measurement of thermodynamic solution parameters by GLC. When determining activity coefficients and surface tensions by static, nonchromatographic methods for methanol, ethanol, acetone, and diethylamine in squalane, Pecsok and Gump ( 4 ) were the first to conclude that a considerable retention contribution from gas-liquid adsorption would exist in the corresponding GLC systems a t infinite dilution. In contrast, Urone and Parcher's (2) GLC study of methanol and acetone in squalane supported on silanized firebrick appeared to show only substantial adsorption by the solid support. Conder, Locke, and Purnell (6) subsequently sought to resolve the argument by calculation based on both the static and GLC results. They showed that the retention contributions-partition, and both types of adsorption-were of comparable orders of magnitude, so that support adsorption masked liquid surface adsorption in Urone and Parcher's analysis. In addition, the latter analysis was questioned as being based on simultaneous variation of too many parameters. This resulted from keeping the solute sample size constant as the liquid loading was varied, so that solute concentration became an adventitious variable. Some further gas chromatographic support for Pecsok and Gump's hypothesis has been provided by Liao and Martire ( 5 ) for the systems rz-propanol and sec-butanol in n-octadecane on silanized Chromosorb W. However, their experi-
mental method, which relied on extrapolation from large sample data, has been criticized ( 3 ) . Recently, Parcher and Hussey ( 3 )have overcome the difficulty in interpretation arising from Urone and Parcher's use of constant sample size. They effectively kept the solute concentration constant as the liquid loading was varied, as recommended by Conder et al. (6). Data were obtained for the system 2-propanol in n-heptadecane on silanized Chromosorb P using the FACP finite concentration technique (8). The results were plotted as total uptake of solute Q vs. volume of liquid phase a t constant solute concentration c in the gas phase, for which condition the retention equation (at low finite concentration) is
where VL is the volume of liquid phase, A I the area of gasliquid interface, A s the active area of support surface and K L , K I , and K s the corresponding condensed phase/gas phase distribution coefficients at concentration c. The plots were linear over a wide range of VL, were of positive slope, and extrapolated to only a very small intercept which agreed with the measured adsorption on the uncoated support. In this way, Parcher and Hussey demonstrated unambiguously that adsorption on the liquid surface was negligible under their experimental conditions. There thus exists, a t the present time, an apparent conflict of evidence as to whether liquid surface adsorption exists in these types of system. Our purpose is to show that the problem can be resolved if due account is taken of the different solute concentrations considered by Pecsok and other authors (4-6) on the one hand, and Parcher and Hussey ( 3 ) on the other. In their static tensiometer measurements, Pecsok and Gump ( 4 ) investigated the effect of varying the solute mole fraction in solution on surface tension. Methanol, ethanol, aceANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
*
917
