SOLID-SURFACE LUMINESCENCE SPECTROMETRY
Robert J. Hurtubise Chemistry Department University of Wyoming Laramie, WY 82071 In its broadest meaning, solid-surface luminescence spectrometry includes the theory and methodology involved in luminescence emitted from components adsorbed on or chemically honded to solid matrices as well as luminescence emitted from solid compounds. The speed, simplicity, sensitivity, selectivity, and moderate cost of solidsurface luminescence make it applica0003-2700/89/0361-889A/$01.50/0
@ 1989 American Chemical Society
hle to many areas of chemical interest from column chromatography to the analysis of pharmaceutical tablets. Because both fluorescence and phosphorescence signals can he obtained at room temperature in many situations, their combined use can yield a suhstantial amount of luminescence information about a sample, particularly mixtures. Principles of solid-surface luminescence Several types of solid-surface luminescence techniques are suitable for a wide variety of applications (Figure 1).The
term luminescence refers to both fluorescence and phosphorescence. Fluorescence results when a radiative transition occurs from the lowest vibrational level of the lowest excited singlet state to a singlet ground state of a molecule, whereas phosphorescence occurs when the transition is from the lowest vibrational level of the lowest triplet state to the singlet ground state (I). The transition from an excited singlet state to the singlet ground state (fluorescence) is spin-allowed and will thus occur with a high degree of probability. Fluorescence decay times range from approximately to s. The radiative transition from an excited triplet state to the singlet ground state (phosphorescence) is spin-forbidden, bowever, which results in long lifetimes (from approximately to 10 s ) for a triplet electronic state and greatly increases the probability of collisional transfer of energy with other molecules. Because collisional transfer of energy is efficient in solution at room temperature, micelle, sensitized, and solid-surface room-temperature phosphorescence are widely used for chemical analysis (2). Theory, instrumentation, and applications of solid-surface luminescence are discussed in recent monographs (3, 4 ) , and will he covered only briefly here. Several solid materials, including filter paper, silica gel, aluminum oxide, silicone rubber, sodium acetate, potassium bromide, and cellulose, are used in solid-surface luminescence. In general, one of two methods is used to deposit luminescent components on solid surfaces. Syringes or micropipets have been employed that can deliver microliter amounts of solution to a solid surface such as filter paper. In addition, it has been possible to adsorb luminescent components on powders by mixing the powder with a solution of luminescent compounds and then evaporating the solution in which the components are dissolved. Various chemical reac-
ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989 * 8 8 9 A
REPORT tions are carried out to chemically bond a luminescent species to a surface. One obvious difference between solid-surface and solution luminescencei s that in solid-surface luminescence the luminescent molecules are usually attached to small particles or a solid surface such as filter paper or silica gel, whereas in solution luminescence, the luminescent molecules are dissolved in a solvent. In solid-surface luminescence, both the source and luminescence radiation are scattered and diffusely reflected from the surface of the solid matrix. Radiation can also he transmitted through the solid material. Ordinarily the diffusely reflected luminescence is measured. For adsorbed species, a fraction of the sample of interest penetrates into the solid matrix, and the sample luminescence is excited a t the surface and within the solid matrix a t a given depth, depending on the properties of the solid matrix. The most important breakthrough in solid-surface luminescence analysis in the past 15years is the development of solid-surface roomtemperature phosphorescence (RTP), which will he discussed later. Why use solid-surface
luminescence? Solid-surface luminescence analysis is very sensitive and selective for organic trace analysis (3, 4). Nanogram an(’ subnanogram detection limits are easi Iy obtained, and in many cases, picogram levels can he detected. In addition, solid-surface luminescence methodology is simple, inexpensive, and rapid, and it can he used selectively for mixture analysis. Although solutions are most often used, a solid surface may be the only vehicle that can be employed. Small samples are easily handled hy solidsurface luminescence techniques, a capability that can be important when toxic materials or biological samples are used. In solution phosphorimetry, it is usually necessary to cool the sample solution to liquid nitrogen temperature. However, with solid-surface RTP, the analysis can be performed a t room temperature, which eliminates the need for cryogenic equipment. In addition, both fluorescence and phosphorescence data (rather than just solution fluorescence data) often can be obtained a t room temperature from a sample adsorbed on a solid surface. In some cases, solid-surface luminescence is also more rapid than solution luminescence. It can he automated, usee with HPLC, TLC, and paper chroma tography, and readily adapted to fiek and process control work. BSOA
Any analytical approach has disadvantages, however, and solid-surface luminescence analysis is no exception. Although depositing a material on a surface is not as easy as dissolving a sample in solution, with a little practice excellent reproducibility can he achieved. Fluorescence and phosphorescence background signals emitted from the solid surface can raise the limits of detection and also adversely affect reproducibility. Moisture and oxygen quenching can he a problem in RTP analysis, hut under the appropriate experimental conditions these prohlems can be minimized. Although the use of powders requires more sample preparation time than does a flat surface such as filter paper, the sensitivity and selectivity obtained with powders often outweigh the additional preparation time.
Solid-surface fhomcence Some new developments in solid-surface fluorescence are considered in a recent renew on solid-surface luminescence analysis (5).Many of the current applications center around the analysis of environmental and biological samples. Numerous applications of this type have appeared in the past and should appear in the future because of the simplicity of adsorbing a tiny ali-
quot (-1 p L or less) of a solution on a solid surface and because of the sensitivity that can he obtained by measuring the fluorescence of the adsorbed compounds. A relatively simple and very sensitive laser fluorometric system has been described (6)with which a detection limit of --8ooo molecules of Rhodamine 6G was obtained. The Rhodamine 6G was adsorbed on silica spheres (10-wn diameter) that were viewed individuallywith a fluorescence microscope (Figure 2). A CW Ar-ion laser was used as an excitation source, and an optical fiber was used to direct the output of the laser to a quartz slide on which the silica spheres were spread. To obtain a calibration curve, the fluorescence emission was measured for each of 10 individual spheres and an average value and a percent relative standard deviation were calculated. The main advantages of this system are that there is no solvent fluorescence or Raman scattering, and particles are viewed from a stationary position. This fluorometer could he useful for the ultratrace measurements of polycyclic aromatic hydrocarbons on air particulates. Several types of filter papers have been evaluated for use as substrates for solid-surface room-temperature fluorescence and photochemical fluores-
TECHNIQUES
APPLICATIONS
Synchronous
Chromatography
Constant-energysynchronous
Polymers
Derivative
Pollutants
Time-resohred
DNgS
Sensitized
Clinic4 compounds
Quenching
Biologica! compounds
Room-temperature immunoassay
Forensic samples Fmd pmducts
Fossil fuels
Figure 1. Salic!-surface luminescence techniques and applications
ANALYTICAL CHEMISTRY, VOL. 61. NO. 15. AUGUST 1. 1989
,
cence (7). Analyte fluorescence signals were two to four times higher on filter paper than on silica gel TLC plates, and absolute detection limits of between 0.6 and 40 ng were obtained by using Whatman filter papers. Based on these results, filter paper was determined to be a convenient, inexpensive, and easy-to-handle substrate for roomtemperature fluorescence and photochemical fluorescence measurements. An in situ fluorescence detection approach has been developed for polycyclic aromatic hydrocarbons preconcentrated on C18 chromatographic silica (8). Pyrene was employed as a model compound, and the analyte was preconcentrated from a methanol-water solution onto the CISsilica after appropriate adjustment of the solution composition. With an arc-lamp fluorometer the detection limit for pyrene was 15 pg/mL, whereas with a laser fluorometer, the detection limit dropped to 0.17 pg/mL. The concentration of pyrene sorbed on the surface can be related to the solution concentration by chromatographic retention values obtained from a column packed with the same CISpacking. Bacon and Demm (9)used the oxygen quenching of the luminescence c tris(4,7-diphenyl-l,lO-phenanthro line)ruthenium(II) perchlorate immc bilized in silicone rubber to develop a method for measuring oxygen concentrations in both solutions and the gas phase. The quenching can be quantitated by either lifetime or intensity quenching measurements, and the degree of quenching of the excited complex is related to the partial pressure of oxygen in contact with the polymer film.Although essentially all small gaseous molecules penetrate the sensor film, only sulfur dioxide and chlorine interfere, and they do not degrade the sensor. This system can be used to analyze oxygen concentrations below l torr partial pressure of oxygen, and more sensitive sensors could be fabricated by using less permeable polymers or sensors with shorter excited-state lifetimes. Several researchers recently employed fluorescence spectrometry to study fluorescent species that are chemically bonded or physically adsorbed to chromatographic stationary phases. Lochmfiller et al. (IO)studied the fluorescence of pyrene silane molecules chemically bonded to microparticulate silica gel a t several surface concentrations. They were interested in determining the proximity and distribution of chemically bound molecules on the native silica gel. The fluorescence of the monomer and the excimer of the chemically bonded pyrene silane
I I
-
Figure 2. Schematic of the laser-excit-
ed microscopic fluorometer. (Adapted from Reference 6.)
was measured as a function of surface concentration (Table I). As the percent carbon increases, the pyrene excimerto-monomer ratio increases, which shows that pyrene molecules chemically bonded to microparticulate silica were not evenly distributed hut were clustered into regions of high density. A reversed-phase stationary phase
#mer
=Q
pyrene
silica Percent (max. at 26.600 sample carbon cm-’) A B
C D E F
1.1 1.6 2.6 4.4 5.4 8.2
483 564 316 115 69 14
would thus have “clusters” of bonded alkyl ligands rather than a uniform distribution, as previously reported. The influence of the mobile phase on the environment and structure of the C18 stationary phase was explored by using pyrene fluorescence as a probe (11). The intensity ratios of the vibronic emission bands were used to study changes in the polarity around the pyrene sorbed to the CISsurface. In other experiments, the quenching of pyrene fluorescence by potassium iodide was used to correlate changes in polarity to exposure of pyrene to the mobile phase. With high concentrations of methanol, pyrene was partitioned or fully surrounded by alkyl chains, which were most likely well solvated by methanol. At lower methanol concentrations, the onset of quenching by solution-phase ions showed that some fraction of sorbed pyrene was ex. posed to the surrounding solvent as the stationary-phase volume collapsed. Solid-surface fluorescencespectrometry has also been used to investigate interactions in normal-phase liquid chromatography (12)by studying the chromatographic interactions of benzo[h]quinoline with water-deactivated silica gel. The experimental conditions significantly affected the extent to which benzo[h]quinoline would undergo excited-state protonation with silica. Fluorescence data were obtained for benzo[h]quinoline as it migrated on water-deactivated silica gel packed into a quartz column. The results showed that some of the water was adsorbed on sites in silica that were incapable of proton donation to benzo[h]quinoline in its excited singlet state. By combining the fluorescence data with the chromatographic data, the mass balance was calculated for three benzo[h]quinoline species that were responsible for the chromato-
Excimer Intensity (max. al21,lOO cm-’) 78 121 459 505 536 653
LOe
Excimer-to-
mon ri 0.16 0.21 1.45 4.39 7.77 48.64
(ercimer. ionomi
ratio) -0.80 -0.68 0.16 0.64 0.89 1.67
Note. Adapted horn Reference
ANALYTICAL CHEMISTRY. VOL. 61, NO. 15, AUGUST 1. 1989
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REPORT graphic bands (Figure 3). These results provided the fmt fluorescence data in support of the Snyder displacement model for liquid-solid chromatography (13).
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S o l l d Q U l a c e ~ In the six years since ANALYTICAL CHEMISTRYpublished an INSTRUMENTATION article on phosphorimetry (21,considerable progress has been made in developing an understanding of interactions in aolid-surface RTP, in obtaining analytical RTP data, and in applying RTP to important analytical areas (4, 5). Fewer developments have occurred recently in instrumentation, however, because much of the instrumental development in solid-surface phosphorescence happened during the late 1970s and early
19808. Why does a phosphor give RTP when adsorbed on a particular surface? The answer is not an easy one; factors such as solution chemistry, evaporation of the solvent, drying of the solid surface, and phosphor-solid matrix interactions must he considered. Schulman and Parker (14) were the fnst to study the effects of moisture, oxygen, and nature of the supportphosphor interactions. Their results showed that hydrogen bonding of the phosphor to the support was an important factor in preventing collisional deactivation of the excited phosphor and that the sample matrix resisted the penetration and quenching efforts of
oxygen. Niday and Seybold (15) showed that packing fiter paper with substances such as salts or sugars inhibited the internal motion of the adsorbed phosphor, and they suggested that the added subtances “plugged up” the channels and interstices of the solid matrix and therefore decreased the oxygen and moisture permeability. McAleese and Dunlap (16) proposed a matrix isolation mechanism for solidsurface RTP with filter paper based on the swelling property of cellulose in the presence of strongly polar solvents, such as when ethanol is used to deposit the phosphor on the solid surface. Swelling of fiter paper would favor entry of phosphor molecules into the suhmicroscopic pores in the paper. After the solid matrix is dried, the moledes could become trapped between cellulose chains, providing the necessary rigidity for the adsorbed phosphor. To pursue the interactions of phosphors with solid surfaces in more depth. a method has heen developed for determining fluorescence and phosphorescence quantum yields for compounds adsorbed on a variety of solid surfaces (17). By using this method, one can calculate some fundamental luminescence parameters that yield insights into solid-surface RTP interactions. For example, solid-surface fluorescence quantum yield (4,) and phosphorescence quantum yield (4,) values along with phosphorescence lifetime (TJ values for the anion of p-aminobenzoic acid adsorbed on d i u m ace-
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Fraction of neubal beruo[hlqulrmllne In !ha mobile phaae (- -), hactlon of pmtDnated benw[h]qulrmllne On i b StationaF, PhMe (-), and fractlm of neutral bsnzo[h]qulnolineon me stnlonary pheae (. . .). (A@W hwn Reference 12.)
CHEMISTRY. VOL. 61. NO. 15, AVGUST 1, 1989
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REPORT tate have been obtained from 23 OC t -180 "C (18).The phosphorescencquantum yield increased as the temperature was lowered and the rate constant for the radiationless transition from the triplet state increased with increasing temperature, whereas the triplet formation efficiency (bt) remained constant, leading to the conclusion that a "rigidly held" mechanism was operative. The stronger the interaction between the phosphor and the solid matrix, the higher the phosphorescence quantum yield. In other work, the anion of p-aminobenzoic acid was studied on several sodium acetate-sodium chloride mixtures (19).Again, 6,, &, and T , were obtained for the anion of p-aminobenzoic acid. To obtain strong RTP the dried matrix must be packed efficiently and a saturated sodium acetate solution must be used in the sample preparation step. Mixtures of both or-cyclodextrinand j3-cyclodextrin with sodium chloride have also been shown to induce strong RTP from a variety of adsorbed compounds (20, 21). By obtaining &, $p, and +c values, it was possible to construct energy-level diagrams that showed how the absorbed energy was distributed radiatively and nonradiatively (22). It was assumed that quenching was minimal; however, even if it did occur, the extent of such quenching would show up in the "nonradiative" part of the diagram. Figure 4 shows typical diagrams for 4-phenylphenol adsorbed on 80% or-cyclodextrin-NaC1 at 23 "C and -180 "C. The percentage of fluorescence, of intersystem crossing from singlet to triplet state, of intersystem crossing from triplet to ground state, and of phosphorescence all increase with lower temperature, and internal conversion decreases to 0% at -180 "C. A sizable amount of the absorbed energy is lost through internal conversion from the singlet state a t room temperature. By minimizing internal conversion from the singlet state, it is possible to make more energy available to increase q+ or h.If & were increased, this could favor an increase in .&,. Such energy diagrams can be useful for adjusting experimental conditions to maximize + p and for studying interactions in solidsurface RTP. Filter paper is used extensively in solid-surface RTP. In recent work, temperature variation experiments were performed with the protonated form of benzov]quinoline adsorbed on fdter paper (23). Several luminescence parameters were calculated, and a relationship was found between 4pand the modulus (softness or stiffness) of cellu894A
(a)
S, T, F 33%
IC 33%
P 8%
ISC 26%
nLm ISC 66%
34%
G%
ISC
29%
37%
L Flgure 4. Energy diagrams for 4-phenylphenol adsorbed on a-cyclodextrin-NaCI mixtures. (a) 80% a-cyckdealn mixtam at 23 OC and (b) 80% a-cyckdaalnmldure at -180 OC. %, gourd state; st.singlet state; T,, mpim mate.; F. nyore-; IC. Interne1 ccnmmion; ISC. Intersystem crlng; P. phosphorescenca. (Adapted wwh permlsslmw a n Referenca22.)
lose. The modulus of the fdter paper, which is related to the hydrogen bonding network in the paper, is an important factor in obtaining a high &for a phosphor adsorbed on filter paper. X-ray photoelectron spectroscopy (XPS) has been used to better understand the surface proceases in RTP in the presence of a heavy-atom enhancer (24). XPS data were obtained before and after the spotting of a luminescent compound or a heavy-atom solution onto the surface of the filter paper. There was no evidence that a strong chemical interaction occurred between the heavy atom and the phosphor, and penetration into the bulk of the paper of both the heavy a t o m and luminescent molecules was apparent. A considerable amount of RTP research involves acquiring analytical data for model compounds in the hope of developing analytical applications. The effects of surfactants on RTP of several polyaromatic hydrocarbons and carbazole have been studied by using filter paper and thallium nitrate as a heavy atom (25). When an anionic surfactant was added or when phosphors were spotted from micellar solutions, sensitivity enhancements of 2-9 orders of magnitude were obtained. The RTP signal on filter paper was totally quenched, however, in the presence of a cationic surfactant. Analysis of multicomponent mixtures using RTP without the need for separation has also been achieved by using mixtures prepared from four or
ANALYTICAL CHEMISTRY, VOL. 81, NO. 15, AUGUST 1. 1989
five toxic s u b t a m e standards (26).In other work, Long et al. (27) considered the feasibility of surface analysis of homogeneous tablets by RTP using propranolol, p-aminobenzoic acid, and acetylsalicylic acid as model compounds. For commercially prepared tablets, RTP can be employed in quality control work but correlation of RTP intensities with a reference method such as high-performance liquid chromatography is necessary. A personal dosimeter badge based on molecular diffusion and direct detection by RTP of polynuclear aromatic pollutants has been developed (28). The dosimeter is a pen-sized device that does not require sample extraction prior to analysis. The paper substrate is simply removed from the badge and the RTP of the adsorbed components measured. The dosimeter badge has been used to detect various polynuclear aromatics (e.g., pyrene, phenanthrene, and quinoline) a t the parts-per-billion level after 1h of exposure. White and Vo-Dinh (29) used RTP to determine potentially carcinogenic compounds that permeate protective clothing. RTP was used to analyze the surface of a filter paper substrate exposed to petroleum products that permeated protective glove samples. The petroleum product passes from a vial through a fine mesh wire screen to the protective clothing, permeates the clothing, and eventually contacts the filter paper disk. RTP spectra were obtained from the material on the Titer
paper without the need for any sample extraction, illustrating the simplicity and cost-effectiveness of the R T P technique.
Hunnicutt, M. L.; Harris, J. M. Anal. Chem. 1983,55,1344. (11) Carr, J. W.; Harris, J. M. Anal. Chem.
__ ~~~~~~~. __
-l'lR7.59 .,- -,%dfi (12) . Chem. ~Burrell. ~1988.60,2178. , G J . 1 Hurtuhise, R.J. Anal. (13) Snyder, L. R. HIgh Performance Li ~
Future develqwnents The state of analytical theory in solidsurface luminescence requires further development. More complete theoretical equations are needed that describe luminescence reflected or transmitted from solid surfaces as a function of the amount of adsorbed luminescent components. Also, a more complete analytical model is required to describe the interactions between the luminescent compounds and the solid matrix that yield high luminescence. Generally, commercial and research instrumenta are effective for obtaining solid-surface luminescence data. However, some instrumental improvements could be made. In some cases, incorrect positioning of the adsorbed sample can cause measurement errors. More extensive use of computers and digital electronics would minimize this source of error. Although laser sources have not yet been used much in solid-surface luminescence work, they have been used in fluorescence line-narrowing experimenta with pyrene adsorbed on a TLC plate (30) and in obtaining fluorescence line-narrowing information on a polycyclic aromatic hydrocarbon-nucleoside adduct adsorbed on a TLC plate (31). Such use of laser sources should continue to increase. Given the speed, simplicity, sensitivity, selectivity, and moderate cost of solid-surface luminescence, many applications should continue to appear. As advancements are made in theory and instrumentation, solid-surface luminescence should be exploited even more extensively to solve future analytical problems.
uid Chromatography; Horvath, C., E& Academic: New Y ork, 1983; Vol. 3. (141 Schulman. E . M.; Parker, R. T. J . Phys. Chem. i977,81,1932. (15) Niday, IG. J.; Seybold, P. G. Anal. Chem. 1918.,50,1577. (16) McAIeesi?._D.-L.;Dunlap, R.B. Anal. Chem. 1984,56,2244. (17) Ramasamy, S.M.; Senthilnathan, V. P.; Hurtuhise, R. J. Anal. Chem. 1986,
58.612 (18)'Ramasainv. .. S.M.; Hurtuhise, R. J. Anal. Chem. 1987,59,432. (19) Ramasamy, S;. M.; Hurtubise, R.J. Anal. Chem. 1987,59,2144. (20) Bello. J. M.: IHurtubise. R.J. Anal. Lett. 1986,19,115. (21) Richmond, M.R.;Hurtubise, R. J.
Appl. Spectrose., in press.
(22) Bello, J. M.; Hurtubise, R. J. Appl. Speetrose. 1988,42,619. (23) Ramasamy, S.M.; Hurtuhise, R. J. Appl. Spectrosc. I989,43,616. (24) Andino, M. M.; Kosinski, M. k,Winefordner,J. D. Anal. Chem. 1986,58,1730. (25) Ramos, G.R.; Alvarez-Cque, M.C.G.; O'Reilly, A. M.; Khasawneh, I. M.; Winefordner,J. D. Anal. Chem. 1988,60,416. (26) Asafu-Adjaye, E. B.; Yun, J. I.; Su, S.Y. Anal. Chem. 1985,57,904. (27) Long, ,W. J.; Su, S.Y.; Kames, H. T. Anal. Chrm. Acta 1388,205,279. (28) Vo-Dinh,
T. Enuiron. Sei. Teehnol.
1985.19,991. (29) White, D.A,; Vo-Dinh, T. Appl. Spectrosc. 1988,42,285. (30) Hofstraat, J. W.; Engelsma, M.; Cofino, w. p.; Hoornweg,G. Ph.; Gooijer. C.; Velthorst, N. H. Anal. Chim. Acta 1984, 1.59 A 1 9 -II,""". (31) Cooper, R.S.;Jankowiak, R.; Hayes, J. M.; Pel-qi, L.;Small, G. J. Anal. Chem. 1988,60,2692.
Hamamatsu sngle and multielement do1 ow Cathode Lamps offer super or stab lily spectral writyand output intensity.even for s x n elements as arsenic and selen Jm They are compatible with most commercial spectrophotometers inckding Beckman. Zeiss ano Perkin-Elmer And oest of all. they re available from your loca lab supplier
RetWWK%!3 (1)
Birka. .I. B. Phoroph>sicrof Aromotic
Molecules; Wiley New York. 1970. (2) Hunubise, H.J. Anal. Chem. 1983.55, fifi9 n """ . (3) Hurtubise, R.J. Solid-Surface Luminescence Analysis; Dekker: New York, 1981. (4) Vo-Dinh, T.Room TemperaturePhos-
.
phorimetry for Chemical Analysis; Wiley: New York, 1984. (51 Hurtuhise. R. J. Molecular Luminescence Sperlroscopy:MelhodJ and Applicalu,,ns-Port I I ; Schulman. S. J.. Ed.; Wiley: New Sork, 1988; Chapter 1. (6, Kirsch,B.; Vaigtman. E.; Winefordner, .I. I). Anal. Chm. 1985.57. 2007. (7)Fidanza, J.; Aaron, J. J.'Talanta 1986, 33,215. (8) Carr, J. W.;
Harris. J. M. Anal. Chem.
1988,60,698. (9) Bacon, J. R.;Demas,J. N. Anal. Chem. 1987,59,2780. (10) Lochmilller, C. H.; Colborn, A. S.;
Hamamatsu Hollow Cathode Lamps are now available from major lab suppliers.
Robert J. Hurtubise received his Ph.D. i n analytical chemistry from Ohio Uniuersity. After working at Rockhurst College and Pfizer Inc., he accepted a faculty position a t the Uniuersity of Wyoming, where he is professor of chemistry. His research interests include physical and chemical interactions in solid-surface luminescence analysis, the application of solid-surface luminescence to organic trace analysis, solutelmobile-phase and solutelstationary-phase interactions in liquid chromatography, and luminescence properties of polycyclic aromatic hydrocarbon-DNA adducts.
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 15. AUGUST 1, 1989
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