1128
Anal. Chem. 1986, 58, 1128-1133
curve of ratio vs. known lifetime can be constructed. Equation 2 implies that this calibration curve can be linearized. A plot of In R vs. 1 / for ~ the synthetic decays is linear with a slope of -8.09 f 0.0033 for lifetimes 1 8 ns, but significant deviation from linearity occurs at short lifetimes (Figure 5b). The nonlinearity at short lifetimes is due to the large contribution made by the trailing edge of the instrument response to the observed decay. Equation 2 implies that the slope of the line should be -At. The actual At chosen for the convolutes was 9.2 ns, compared to the observed slope of -8.09 ns. The difference can be attributed to the fact that these ratios were obtained with tl positioned over the trailing edge of the instrument response. If tl is placed beyond the last vestiges of the excitation pulse, the calibration curve will be linear and of the correct slope. For comparison purposes, the ratios obtained from the quinine bisulfate experiments are also plotted in Figure 5b. The major advantages of the ratio method over more conventional methods of obtaining fluorescence lifetimes are as follows. The ratio method does not require an extensive amount of postrun data processing onCe an appropriate calibration curve has been obtained. The technique is fast, with applications to automated analyses or other areas where a high sample throughput is desired. The ability to measure lifetimes on the fly and the concentration independence of the method lend themselves well to liquid chromatography. The ratio
technique is less sensitive to drift, and it removes most of the contribution of source intensity fluctuations to the total noise. The major disadvantages are that the presence of multiple decays cannot be detected unless the component concentrations change and that complex instrumentation is required.
ACKNOWLEDGMENT We acknowledge Harry Pardue for the use of the autosampler and the peristaltic pump.
LITERATURE CITED Demas, J. N. Excited State Lifetime Measurements ; Academic Press: New York, 1983; Chapters 1 and 2. Hieftje, G. M.; Vogelsteln, E. E. "A Linear Response Theory Approach to Time-Resolved Fluorometry". I n Modern Nuorescence Spectroscopy; Wehry, E. H., Ed.; Plenum: New York, 1981; Vol. 4, Chapter 2. Rockley, M. G. Biophys. J . 1980, 193-198. Matthews, T. G.; Lytle, F. E . Anal. Chem. 1979, 51, 583-585. Chen, R. F. Anal. Blochem. 1974, 57(2), 593-604. Lytle, F . E. Photochem. Photobloi. 1973, 17, 75-78. Bevingtbn, P. R. Data Reductlon and Error Analysis for the Physical Sclences; McGraw-Hill: New York, 1969; Vol. 1, p 62. Almelda, M. C.; Seitz, R. Appl. Spectrosc. 1985, 39(1),84-90.
Received for review July 29, 1985. Resubmitted January 14, 1986. Accepted January 14,1986. This work was supported in part by the American Cancer Society, the Indiana Elks, and the National Science Foundation, Grant CHE-8320158.
Sensitized Fluorescence Spectrometry Using Solid Organic Substrate T. Vo-Dinh* and D. A. White'
Advanced Monitoring Development Group, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Thls study Investigated a slmple technlque based on sensltlred lumlnescence for detecting trace amounts of polynuclear aromatlc (PNA) compounds. Anthracene was used as the sensltizer deslgned to absorb excitation energy and funnel It to the guest analyte GOmpOundS spotted on anthracenetreated fllter paper. The data Indicate that anthracene can Improve the fluorescence slgnal of varlous PNA compounds such as perylene and benzo[a]pyrene by 2 orders of magnltude. For other compounds such as fluoranthene, no fluorescence sensltlratlon by anthracene was observed. The usefulness of the senslflred fluorescence detected by a fiberoptlcs lumlnoscope devlce for screening complex samples Is Illustrated In analyses of a coal llquld and an alr partlcular sample extract.
Sensitized luminescence by energy transfer in organic crystals has been a topic of extensive fundamental research (1-11). Although some earlier works have discussed the analytical features of sensitized luminescence (12-14) there has been, however, no current emphasis on practical applications of this photochemical process. A qualitative spot test procedure based on simple visual observation of sensitized Research Associate a t Oak Ridge National Laboratory and T h e University of Tennessee, Knoxville, TN.
fluorescence has been described for characterizing the content of polynuclear aromatic (PNA) compounds using naphthalene as the sensitizer (15, 16). Preliminary studies conducted in this laboratory further investigated the sensitized luminescence technique and led to the use of another efficient sensitizer, anthracene (17-19). Sensitized fluorescence based on naphthalene has also been recently reported (20). In this work we further evaluated the technique of sensitized fluorescence as a practical screening tool for detecting trace amounts of PNA compounds. Sensitized luminescence refers to the photophysical process by which the excitation energy absorbed by a donor molecule is transferred to an acceptor molecule, the luminescence of which is detected. This luminescence is not due to direct excitation of the acceptor molecules (usually present at low concentrations). Instead, acceptor molecules are intereacted with a large number of donor molecules (or sensitizers) acting as antennas collecting a higher amount of excitation energy and funneling this energy to the acceptor analytes. Under appropriate energy transfer conditions, this process could result in drastically increased luminescence of the acceptor analyte compounds. Emphasis is on anthracene used as the sensitizer. Since this technique has a' great potential as a simple tool for field use, we also evaluated the use of a portable fiber-optics luminoscope. This instrument was developed in this laboratory for monitoring skin contamination of workers and for remote sensing of pollutants in process streams at energyrelated technologies (21, 22). The results indicate that an-
0003-2700/86/0358-1128$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
thracene is a very efficient sensitizer for a variety of analyte compounds. Limitations as well as features of merit such as dynamic range, fluorescence enhancement, and detection limits of various sensitizer-analyte systems will be discussed. The use of the technique as a screening tool for field measurements will be illustrated by the characterization of several complex environmental samples such as a coal liquid and an air particular sample using the fiber-optics luminoscope. The sensitized luminescence technique described in this work and other previous studies (16-20) is best referred to as solid organic sensitized luminescence (SOSL) since the procedure involves incorporating the acceptor molecule uniformly throughout the donor or depositing the acceptor on a surface of the donor solid matrix after solvent evaporation. Upon solvent evaporation the sensitizer (anthracene in a saturated ethanolic solution) forms a thin layer of molecular crystalline aggregates that are easily detected by visual examination. Although these crystalline aggregates are far from perfectly grown pure crystals, many aspects of the sensitized luminescence mechanisms of the donor compounds should be related to spectrophysical properties of individual solid molecular aggregates. Energy transfer mechanisms in organic solids have been investigated by two basic theoretical approaches, viz., exciton diffusion (1,2) and long-range resonance interaction (3-5). The theoretical description of resonance interaction was originally developed by Forster ( 3 , 4 ) for dipole-dipole interactions and later extended by Dexter to include exchange and higher multipole interactions, which may become important at small intermolecular distances (5). The critical energy transfer distance, Ro,defined as the distance of which resonance energy transfer is equal to the rate of deexcitation by all other deactivation processes in the absence of energy transfers, is given by (3, 4 )
where K is a constant, is the quantum efficiency of the sensitizer, F,(v)is the spectral distribution of the sensitizer emission, EA(v)is the molar extinction coefficient of the acceptor, n(v)is the index of refraction in the range of spectral overlap, and (v) is the spectral frequency. The above equation indicates that the critical energy transfer distance Ro depends upon the spectral overlap between Faand EA. Typical values of Ro for aromatic crystals of interest here are in the range of 20-30 8, (9). These critical intermolecular distance values clearly make it necessary for the sensitizer molecule or exciton to come in physical contact with the acceptor molecule. Theories of energy transfer were therefore developed ( 1 0 , I I ) to combine the exciton diffusion mechanism and the long-range interaction mechanism. In these theoretical models, exciton diffusion is included as a perturbation to long-range resonance transfer. The singlet state of the sensitizer moves diffusively through the host lattice, with a finite probability of transferring its energy by long-range resonance transfer to a nearby acceptor after each step.
EXPERIMENTAL SECTION Instrumentation. All fluorescence spectra were measured with a commercial instrument, the Perkin-Elmer 43A fluorescence
spectrometer. A portable fiber-optics luminoscope developed at Oak Ridge National Laboratory (ORNL) for field measurements was used to evaluate the SOSL technique for practical applications. Figure 1shows the schematic optical arrangement of the fiber-optics luminoscope. The detailed description of an improved version of this instrument will be given elsewhere (23). (A field-portable version of this instrument is available from Environmental Systems Gorp., 200 Tech Center Dr., Knoxville, TN 37912.) Only the main features are discussed here. The purpose
MERCURY LIGHT SOURCE
-cfik\ I
11 . .
OPTICAL FILTER
1129
- -
MONOCHROMATOR c1-- DETECTION
PATH
FIBEROPTICS LIGHT PIPE STETHOSCOPIC
LENS
FLUORESCENCE
Figure 1. Schematic diagram of the portable fiber-optics lurninoscope for surface luminescence measurements.
of the bifurcated lightpipe (The Ealing Co.) is to transmit the excitation UV radiation onto the surface area being monitored and convey the fluorescence emission back onto the detector. Ultraviolet light from a small 125-W mercury lamp (PBL Electro-optics, Inc., Model HG-125) or a CW laser (helium cadmium type, Omnichrome) is focused onto the excitation entrace of the fiber optics. A set of optical filters (The Ealing Co.) were used for excitation. A miniature monochromator (PTR Optics) was used for selecting the emission wavelengths at which the fluorescence is monitored. A stethoscopic cap is mounted at the common leg of the bifurcated light guide. During the measurement, the open end of the stethoscopic cap is pressed against the targeted area of the filter paper. Safety of operation, simplicity of design and operation, portability, and low cost are the main features sought for the luminoscope. The use of a light guide to contain the UV light prevents inadvertent illumination of the person conducting the measurements. The detector is a miniature photon counting tube (Research Support Instruments, Inc., Model 2G-150). This device is a self-contained package with a photomultiplier tube, an adjustable high-voltage power supply, a charge-sensitive pulse amplifier, a level discriminator, and an internal power regulator. The digital counting circuits and the digital-to-analog circuits and a background nulling device are designed and constructed at ORNL. A trigger-operated shutter protects the photomultiplier tube from inadvertent exposure to light. The luminoscope is connected by a 5-m cable to a portable power supply (-5 kg) that uses conventional 120 V ac line power. The hand-held instrument is low cost, simple to operate, and weighs only a little over 5 kg without the power supply. Procedure. To perform the sensitized fluorescence measurement, a pencil was used to mark and label a number of circles (-1.5 cm in diameter) on filter paper to correspond to the series of dilutions made for the sample to be investigated. With a Gilson micropipet, a 2.5-pL portion of each dilution was applied to its respective marked spot and allowed to dry for approximately 1-2 min. The stethoscope of the luminoscope was then placed over each of the spots and, with the shutter opened, the number of photon counts recorded. Five readings were taken and averaged for each spot. A background-level photon count was obtained from an unspotted area of the filter paper and subtracted from the mean count for each concentration of the analyte solution. The results were plotted graphically as concentration vs. photon counts. Sensitized fluorescence measurements were performed by mixing equal volumes of analyte and sensitizer solutions and spotting the mixture on the filter paper. This procedure was adopted after having poor results applying first the analyte and then the sensitizer to the same spot. To record the background signal, a volume of sensitizer solution was mixed with an equal volume of ethanol (blank sample) and then spotted. Photon counts were measured and recorded with background level sub-
1130
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
tracted for each spot as above. To illustrate fluorescence enhancement due to the sensitizer, these results were plotted together. Corrections were made to compensate for dilution of the analyte solution caused by equal-volume mixing with the sensitizer solution. A saturated solution of anthracene (Aldrich Gold Label) in spectroscopic-grade ethanol was primarily used as the spot-test sensitizer. The use of naphthalene was eliminated because of practical limitations due to its volatility. Use of phenanthrene, carbazole, acenaphthene, acridine, chrysene, and fluorene as sensitizers was attempted but precluded by the reduced fiberoptics transmission of the higher energy UV necessary to excite these species. For the analyte mixtures, Signode oil, solventM anthracene was addirefined coal, and H-coal distillate, tionally employed as a sensitizing solution for comparison to the tests using saturated anthracene. Furthermore, a number of single compounds were utilized as analytes with saturated anthracene as the sensitizer. These were triphenylene, pyrene, perylene, fluoranthene, benz [a]anthracene, bem [a]pyrene, bem[e]pyrene, and chrysene. A series of ethanol dilutions was prepared for each of these compounds as described previously. When working with the analytes alone and with anthracene as the sensitizer, excitation light from the 125-W mercury lamp source was transmitted at 375 nm by a filter having a bandwidth of 50 nm. Emission radiations were monitored at 500 nm. Prior to each series of measurements, a filter paper blank standard was monitored at 415 nm to standardize instrumental response. Materials and Reagents. All polyaromatic compounds were purchased at their highest purity commerciallyavailable. Ethanol, spectroscopic grade, was used as the solvent. Samples supplied by various sources (US. EPA/ORNL Repository, Signode Co., PEDCo Environmental, Inc.) included an H-coal distillate product and an air sample extract.
FLUORESCENCE EMISSION OF ANTHRACENE
t
t v)
z W
c
z
W V
EXCITATION SPECTRUM OFFLUORANTHENE
]
\
2 W
0 v)
w
(L
0 3
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LL
RESULTS AND DISCUSSION Anthracene-Sensitized Fluorescence. After a variety of organic polyaromatic compounds were tested as sensitizers, anthracene was selected for several reasons. Anthracene fluorescence emission occurs between 370 nm and 480 nm, thus overlapping the absorption spectrum of many important PNA compounds having high ring numbers. A major advantage associated with the use of anthracene is the low volatility of this compound (compared to that of naphthalene). This feature results in substantial improvement in reproducibility of the measurements. Equation 1 indicates explicitly that no transfer is possible unless there is spectral overlap between the sensitizer (S) fluorescence and acceptor (A) absorption (or excitation) spectra. This important feature is illustrated in Figure 2 for the case of three A/S systems, benzo[a]pyrene/anthracene, perylene/anthracene, and fluoranthene/anthracene. The data in this figure show that the spectral overlap for the perylene/anthracene system is much larger than that for fluoranthene/anthracene. This feature is confirmed by the results of sensitized fluorescence experiments illustrated in Figure 3. Whereas fluorescence signals of fluoranthene showed no sensitization effect from anthracene, the emission of perylene increased about 2 orders of magnitude when this compound was spotted onto filter paper containing saturated anthracene sensitizer. The results of our measurements showed that benzo[a]pyrene fluorescence is enhanced 10-fold by anthracene sensitization. This degree of sensitization, which is more than with fluoranthene but less than with perylene, is in good agreement with the medium spectral overlap shown in Figure 2. The slight plateauing observed with perylene/anthracene (Figure 3a) and with some other systems could be caused by several effects. First, it could be due to filter effects caused by high concentrations of the combined donor-acceptor system; this deviation from linearity might also indicate a decrease in sensitization at high concentrations of the acceptor molecules (the donor concentration being saturated) and
EXCITATION PERYLENE
300
400
500
WAVELENGTH (nm)
Flgure 2. Spectral overlap between sensitizer (S) emission and acceptor (A) absorption: (a) anthracene: (b) anthracene (S) and benzo[a]pyrene (A): (c) anthracene (S) and perylene (A): (d) anthracene (S) and fluoranthene (A).
optimal acceptor-donor ratio has to be achieved to obtain maximum enhancement. Finally, it should be mentioned that the plateauing of analytical curves is predicted by theoretical calculations of luminescence intensities from solid surfaces (24, 25). Sensitized Fluorescence Spectra. The fluorescence of crystallite anthracene (dried spot of anthracene) differs from that of diluted anthracene in ethanol solution (Figure 4). The spectrum of crystallite anthracene shows some moderately resolved structure. The long-wavelength bands are significantly reduced, probably due to strong radiative reabsorption effects near the 0band of the absorption spectrum. Molecular interactions in the anthracene aggregates might also cause spectral changes. Addition of a trace amount of perylene (3 rL of 10-6 M) onto the filter paper impregnated with anthracene resulted in a
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1
1
1
1
1
L
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ON PAPER 2.5 I SATURATED E T O sOLUT~ON ~ SPOTTED ON W-42
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1
ANTHRACENE
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. V
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1131
--- IN SOLUTION
0 PERYLENE 0 PERYLENEt ANTHRACENE (SATURATED) A,,= 375 nm
A,,=soo
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nm
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1111I I l l I ~ 1 1 1 1 I II l l I ( 1 1 1 1 1 1 1 1 1 I 350
430
390
550
5tO
470
WAVELENGTH (nm)
Flgure 4. Fluorescence spectra of anthracene in solution (lo-' M In ethanol, dashed curve) and of anthracene crystallite (evaporated from solution, solid curve).
3
z
0
L
v)
z
c"
z
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-
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----- ANTHRACENE (SAT'D)
0 FLUORANTHENE 0 FLUORANTHENE +ANTHRACENE (SATURATED) is*= 375 nm A,,,= 500 nm
-
1'111I 1 1 1 I 11111II 1 1 1 I 111111I 1 1 1 I
.-
PERYLENE AND ANTHRACENE
> k v) z W
I-
z W
0 z W
0 v)
W
z
5J
390
430
470 510 550 WAVELENGTH (nm)
590
630
Flgure 5. (Curve A) Fluorescence spectrum of perylene (2pL of lo-' M solution spotted on paper) with anthracene as the sensitlzer. (Curve B) Fluorescence spectrum of anthracene (saturated)on paper. (Curve C) Fluorescence spectrum of perylene (2 pL of lo-' M solution) on
paper.
1132
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
*
/
k
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403
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0 H-COAL PRODUCT 0 H-COAL t ANTHRACENE (SATURATED ) i e , = 3 7 5 n m Xe,=500nm
I I I11111
to' 10-5
=I
I I1111111 10-3
to-4
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DILUTION 390
430
470 510 WAVELENGTH (nm)
550
--- PERYLENE (10-6M) DILUTED 1 W I T H ETOH -PERYLENE ( ~ o - ~ M+)ANTHRACENE ( 5 X ~ O - ~ M O .............. PERYLENE ( ~ o - ~ M+ )ANTHRACENE ( 1 x I O - ~ M ) ----- PERYLENE ( ~ o - ~ Mt) ANTHRACENE ( S A T ' D ) 1
,
Figure 8. Effect of the sensitizer (anthracene) concentration: (A) peryiene with no anthracene: (B) perylene on paper treated with 5 X lo-' M anthracene: (C) perylene on paper treated wlth M anthracene: (D) perylene on paper treated with saturated solution of anthracene.
The use of a saturated solution of anthracene ensures that the anthracene molecules will cover the filter paper surface in closely packed multilayers. The anal@ molecules that are subsequently added to the host matrix will be, therefore, in close contact with sensitizer molecules available for the energy funneling process. It is therefore important to employ a sufficiently concentrated solution of sensitizer to obtain maximum energy transfer. This feature is illustrated in Figure 6, demonstrating that perylene fluorescence is less sensitized when the paper is treated with a less concentrated solution of anthracene. These results are consistent with Forster's model predicting that the energy transfer rate between a sensitizer (S) and an acceptor (A) increases when the distance between S and A decreases (3, 4). Applications to Environmental Samples. The sensitized fluorescence technique can be used as a practical tool to improve the detection of PNA components in complex environmental samples. Great care should be taken in analyzing the results if only one sensitizer is used since not all compounds in the complex sample will be sensitized. The degree of sensitization is also different for different analytes and is determined by the spectral overlap expressed in eq 1. However, the sensitized fluorescence technique could find useful applications in many cases because of its simplicity and cost effectiveness. With the use of portable instrumentation such as the fiber-optics luminoscope described in this work, sensitized fluorescence has a great potential for routine field monitoring of organic pollutants collected at a variety of locations such as waste sites and process streams. Figures 7 and 8 illustrate some applications of the sensitized fluorescence technique for examining several complex environmental samples a t varying dilution factors. The results underscore the effectiveness of the SOSL technique and show that the
Flgure 7. Improved detectlon of H-coal product by sensitized fluorescence uslng anthracene.
- lo4 E
'
E
\
I
I I I I 1111
I I I IIIIII
0 AMBIENT AIR SAMPLE ( A A S )
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Flgure 8. Improved detection of an air sample extract by sensitized fluorescence using anthracene.
fluorescence intensities of the coal liquid samples and the air sample extracts are drastically increased, providing improved detection. Registry No. Anthracene, 120-12-7;benzo[a]pyrene,50-32-8; perylene, 198-55-0.
LITERATURE CITED (1) (2) (3) (4) (5) (6)
(7) (8) (9)
(IO) (11) (12) (13) (14)
Frenkel, J.. Phys. Z . Sow/etunlon 1936, 9 , 158. Franck, J.; Teller, E. J . Chem. Phys. 1938, 6 , 861. Forster, Th. Ann. Phys. (Le@&) 1948, 2 , 55. Forster. Th. Discuss. Faraday SOC.1959, 2 7 , 7. Dexter, D. L. J . Chem. Phys. 1953, 2 1 , 836. Soos, 2. G.; Powell, R. C. Phys. Rev. B : Solid State 1972, 6,4035. Chandrasekhar, S.Rev. Mod. Phys. 1943, 15, 1. Wolf, H. C. In "Advances in Atomlc and Molecular Physics"; Bates, D. R.. Estermann. R.. Eds.: Academic Press: New York. 1967; Vol. 3, D 119. Powell, R. C.; Soos, 2 . G. J . Lumin. 1975, 77, 1. Yokota, M.; Tarlmoto, 0. J . Phys. Jpn. 1967, 22, 779. Kurskii, Y. A.; Sellvanenko, A. S., O p t . Spektrosk. 1960, 8 , 340. Hornyak, I . J . Lumin. 1972, 5 , 132. Hornyak, I.J . Lumin. 1972, 5 , 430. Hornyak, I.J . Lumin. 1975/1976, 1 1 , 241.
1133
Anal. Chem. 1986, 5 8 , 1133-1137 (15) Smith, E. M.; Levins, P. L. I n “Polynuclear Aromatic Hydrocarbons: Chemistry and Biological Effects”; Bjorseth, A., Dennis, A. J., Eds.; Batelle Press: Columbus, OH, 1980; pp 973. (16) Johnson, L. D.; Luce, R. E.; Merriii, R. G. I n “Polynuclear Aromatic Hydrocarbons”; Bjorseth, A., Dennis, A., Ed.; Batteiie Press: Columbus, OH, 1980; p 119. (17) Vo-Dinh, T. I n ”Clean Surface Technology”; Mittai, K. L., Ed.; Plenum Press: New York, in press. (18) Vo-Dinh, T. Proceedings of 1982 Annual Meeting of Institute of Environmental Science, Atlanta, GA, April 20-23, 1982. (19) Vo-Dlnh, T. J. Environ. Sci. 1883, (January/February), 40. (20) Seyboid, P. G.; Hinckiey, D. A.; Heinrichs. T. A., Anal. Chem. 1983, 55, 1996. (21) Vo-Dinh, T.; Gammage, R. B., Am. Ind. Hyg. Assoc. J . 1881, 4 1 , 112. (22) Vo-Dinh, T.; Gammage, R. B. I n “Chemical Hazards In the Workplace:
Measurement and Control”; Choudhary, G., Ed.; American Chemical Soclety: Washington, DC, (1981; ACS Book Series, No. 149, p 269. (23) Vo-Dinh, T.; Blair, M. S.; Turner, J. C. “Luminoscope 11: Circuit Drawing”; Report ESCR-No-3X. Copyright 1985, Oak Ridge National Laboratory, Oak Ridge, TN. (24) Goidman, J. J. Chromafogr. 1873, 76. 7. (25) Zweidinger, R. A.; Windfordner, J. D. Anal. Chem. 1870, 42, 639.
RECEIVED for review July 23, 1985. Resubmitted December 16, 1985. Accepted December 16, 1985. This research was sponsored by the Office of Health and Environmental Research, U.S.Department of Energy, under Contract DEAC05-840R21400 with Martin Marietta Energy Systems, Inc.
Development of National Bureau of Standards Thin Glass Films for X-ray Fluorescence Spectrometry Peter A. Pella,* Dale E. Newbury, and E r i c B. Steel Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, Maryland 20899 Douglas
H.Blackburn
Center for Materials Science, National Bureau of Standards, Gaithersburg, Maryland 20899
Two thin glass film Standard Reference Materials, SRM 1832 and 1833, have been developed for the calibration of X-ray fluorescence spectrometers, especially for the elemental analysis of particulate matter collected on filter media. Each SRM consists of a silica-base film deposited by focused ionbeam coating onto a polycarbonate substrate mounted on an aluminum ring. The glass film (150-180 bg/cm2) is an amorphous layer 0.5-0.6 pm thick and contains known concentrations of selected elements (as oxides). SRM 1832 Is certified for Ai, Si, Ca, V, Mn, Co, and Cu and SRM 1833 for Si, K, Ti, Fe, Zn, and Pb. The fabrication and characterization of these films with respect to elemental homogeneity, composition, and sensitivity to moisture are described.
X-ray fluorescence spectrometry is a technique often applied to the analysis of materials in the form of a deposit on a filter, mesh, or membrane or as a thin pressed pellet. Collected airborne particulate matter, particulates in wastewater, and geochemical samples are just a few examples of these types of samples. In many respects, “thin samples” are ideal for X-ray spectrometric analysis because of the low detection limits attainable. Matric effects such as X-ray absorption/enhancement are often negligible. This provides a linear instrument response of X-ray intensity vs. element mass per unit area. Corrections are usually required, however, for X-ray self-absorption of low atomic number elements in particulates and for deposits in certain substrate materials. For a “thin” specimen, the count rate Iiof characteristic X-rays from an element is related to the element mass per unit area, mi (g/cm2) by the equation
Ii = Simi
(1) where Si is the sensitivity or calibration factor for element i .
Si is usually determined experimentally from thin standard samples. A thin specimen has been defined ( I ) as having a
total mass per unit area such that
m _I O.l/p
(2)
where p is the effective average mass absorption coefficient. Assuming X-ray photon excitation p = p1 cosec O1 k2 cosec O2 (3)
+
where p I is the mass absorption coefficient for the primary radiation, p2 is the mass absorption coefficient for the emitted characteristic X-ray radiation, and 81 and O2 are the corresponding angles of incidence and emergence, respectively. There are several types of thin samples presently available from commercial sources, such as Columbia Scientific Industries, Austin, TX, and Micromatter Co., East Seattle, WA. These generally consist of: (1)thin films of single metallic elements deposited by thermal evaporation on Mylar or thin aluminum substrates (The mass loading of the element is determined gravimetrically by the manufacturer.); (2) dried solution deposits on membrane filters (These are prepared by a multidrop technique. The elements are present in concentrations ranging from 1 to 50 mg/cm2 as stated by the manufacturer.); (3) particulate deposits consisting of ground minerals dispersed on membrane filters and fixed with a 30-40 mg/cm2 layer of paraffin wax. In addition to the above samples, a number of workers have reported on techniques for preparing thin samples for calibration (2-10). In the development of calibration standards, disagreements as high as i 1 5 % have been observed among standards from different commercial sources (6). Several important criteria must be met if thin samples are to be acceptable for calibration purposes. The thickness of the deposited layer and the elemental homogeneity across the layer should be within well-defined tolerance limits. If the sample contains particulate matter, both the particle size and the size distribution should be characterized. The mass loading of the thin deposit must be known within an acceptable tolerance, and finally, the thin sample should possess sufficient durability to permit distribution to users in other laboratories. In 1975, a joint NBS-
This article not subject to US. Copyright. Published 1986 by the American Chemical Society
