638
Anal. Chem. 1980, 52,
canceled by a negative CD due to a distinct transition of the amide chromophore. Johnson and Tinoco (16) have observed in the vacuum UV CD of LY helical polypeptides that a band expected near 180 nm on the basis of exciton theory is not observed. They have ascribed this effect to cancellation of the expected exciton band by CD due to a magnetically allowed amide n i * transition. A definitive explanation for this effect will require further experimental and theoretical study.
ACKNOWLEDGMENT T h e authors thank Sunil Rajadhyax for helping them construct a working analog signal splitting and CD voltage control device.
638-642 Johnson, W. C. Rev. Sci. Instrum. 1971, 42, 1283. Pysh, E. S. Ann. Rev. Biophys. Bioeng. 1976. 5 , 63. Brahms, S.;Brahms, J.; Spach, G.; Brack, A. Proc. Natl. Acad. Sci. (USA) 1977, 7 4 , 3208. Gedanken, A.; Levy, M. Rev. Sci. Instrum. 1977. 48, 104. Drake, A. F.; Mason, S. F., Tetrahedron 1977, 33, 937. Kemp, J. C. J . Opt. SOC.Am. 1969, 59, 950. Velluz. L.; Legrand, M.; Grosjean. M. "Optical Circular Dichroism"; Verlag Chemie-Academic Press: New York, 1965. Chen, G. C.; Yang, J. T. Anal. Lett 1977, 10, 1195. Bush, C. A. In "Excited States in Organic Chemistry and Biochemistry", Pullman, B., Goldblum, N., Eds.; D. Reidel: Dordrecht, 1977; p 209. Coduti, P . L.; Gordon, E. C.; Bush, C. A. Anal. Biochem. 1977, 78, 9. Buffington. L. A.; Pysh, E. S.; Chakrabarti, B.;Balazs, E. A. J . Am. Chem. Soc. 1977, 99, 1730. Bush, C. A.; Duben, A. J . Am. Chem. SOC. 1978, 100. 4987. Keilich, G.; Roppel, J.; Mayer, H. Carbohydrate Res. 1976, 57, 129. Johnson, W. C.; Tinoco, I., Jr. J . Am. Chem. Soc. 1972, 94, 4389.
LITERATURE CITED (1) Johnson, W. C. Ann. Rev. Phys. Chem. 1978, 29, 93. (2) Schnepp. 0.; Allen, S.; Pearson, E. F. Rev. Scr Instrum. 1970, 74, 1136
for review September 28, 1979. Accepted January 21, 1980. Research supported by NSF Grant CHE 76-16783.
Flame Geometry Effects on the Flame Photometric Measurement of Phosphorus in Aqueous/Organic Liquids T. L. Chester The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 39175 Cincinnati, Ohio 45247
Phosphorus-containing materials form HPO when burned in cool, hydrogen-based flames. HPO emits light which can be detected photometrically. However, severe interferences prevent the direct application of this phenomenon to phosphorus-selective detection in aqueous/organic samples or in typlcal liquid chromatography effluents. I n this study the usual geometric roles of hydrogen and air were reversed: air was flowed into a hydrogen atmosphere. Ignition resulted in formation of an Inverted air-hydrogen diffusion flame. Chemical and spectral Interferences caused both by organic and inorganic matrix components were substantially reduced. Both the primary flame and the secondary flame, formed when excess hydrogen burns in room air, can be used analytically.
Hydrogen-based diffusion flames have been known for many years to produce strong H P O emission when phosphorus-containing compounds are introduced into them (1-3). Brody and Chaney utilized this phenomenon in a phosphorus(and sulfur-) selective flame-photometric detector for gas chromatography (GC) ( 4 ) . These same principles were later used by Julin, Vandenborn, and Kirkland in a phosphorusselective detector for high performance liquid chromatography (HPLC) (5). In 1978, another version of this detector was built in our laboratory and used to detect diphosphonates separated from physiological fluids by ion exchange (6). With the flame configurations used in these detectors (4-7), severe interferences are routinely encountered. Co-eluting organic materials often lead to large losses of sensitivity. In the Brody and Chaney GC detector, the solvent peak may extinguish the flame. For HPLC applications, these organic interferences limit the possible solvent systems and separation modes. For example, Julin et al. ( 5 ) noted that adding only 1% methanol to aqueous solvents would reduce the HPO 0003-2700/80/0352-0638$01 .OO/O
signals by one half. Alkali metal and alkaline earth ions cause both negative chemical interferences and positive spectral interferences (5, 7). Fragments of organic compounds (such as CO and C,) may also cause spectral interference. The only solvents which give no interference are water and dilute hydrochloric acid (6). These factors essentially limit the possible HPLC modes of separation of phosphorus-containing compounds to aqueous ion exchange when using this type of detector. The goal of the present work was to investigate a flame configuration which is more tolerant of sample solvent systems and which will allow the direct introduction of common organic solvents with a minimum of quenching or interferences occurring. This configuration involves flowing air (in a laminar flow) into an atmosphere of hydrogen as shown in Figure 1. The resulting flame can be described as an inverted or "inside-out'' air-hydrogen diffusion flame. A secondary flame results when excess hydrogen, combined with combustion products, is ignited in air as it escapes the apparatus. -4similar configuration was reported by Draeger (8) and van der Smissen (9) for phosphorus and sulfur analysis of gases. Patterson et al. (10) used this basic geometry in a dual-flame photometric detector for GC. The inverted flame was compared directly to the conventional type of laminar diffusion flame used in References 5 and 6. The secondary flame was also studied.
EXPERIMENTAL Primary Flame. The equipment and settings used are listed in Table I. The capillary tubes specified in the burner head design ( 1 1 ) were omitted. Air was supplied to the nebulizer connection of the burner (7 L/min, gas flow rates are at 1 atm and room temperature). Hydrogen was introduced through the sheath attachment to the burner head (6 L/min). The fuel and oxidant fittings on the burner were capped. The flame was enclosed in a Pyrex tube (5.0-cm o.d., 4.6-cm i.d., 50 cm in length). The 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980 ~
639
~
Table 1. Apparatus model
component monochromator, 0.25 m photomultiplier housing photomultiplier tube photomultiplier power supply current amplifier
82-410
83-021 R106 141
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recorder
2-in. diameter 2-in. focal length 7127A
burner nebulizer
290-0107 303-0352
burner head
(see Ref. 11)
Pyrex tube flow controllers
(see text) 8944
lens (quartz)
Entrained ANr
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Entrained Air
H, Inerl Carner Gas and Sample Droplets
Figure 2.
Conventional hydrogen diffusion flame
secondary flame burned at the top of the tube in entrained room air. The configuration of the normal hydrogen-argon diffusion flame which was also studied is shown in Figure 2. Argon was supplied to the nebulizer inlet (7 L/min) and hydrogen to the (normal) fuel inlet (6 L/min). The Pyrex tube was removed and the hydrogen (with argon) was burned in entrained room air. With both the inverted and normal flame, an image was projected with the lens onto the entrance slit plane of the spectrometer. The inverted flame was viewed off-axis at the position of brightest HPO emission. The normal flame was viewed on-axis. Emission was measured a t 526 nm using conditions to give a spectral bandpass of 0.5 nm. Reagent grade (ACS) phosphoric acid (14.7 M) was used as the phosphorus standard. The HPO emission intensity was measured vs. phosphorus concentration for deionized water solutions of H3X'04 over the range of 0 to 2000 mg P / L using the inverted flame. Interferences from water miscible solvents were measured for both flames. The solvents tested were methanol, ethanol, iso-
manufacturer Jarrell- Ash, U'altham, Mass. Hamamatsu, Middlesex, N.J. Keithley, Cleveland, Ohio Esco Optics, Oak Ridge, N.J. Hewlett-Packard, San Diego, Calif. Perkin-Elmer, Norwalk, Conn.
conditions Wavelength = 526 nm. Slit width = 150 gm.
-740 V Gain = as required. Rise time = 300 mS. 1:1 imaging Span = 1 0 V Solution uptake rate = 6 mL/min. Nebulization efficiency = 11%(aqueous solution).
P&G, Miami Valley Laboratories Brooks, Hartfield, Pa.
(see text)
propanol, acetone, acetonitrile, acetic acid, formic acid, triethanolamine, sec-butylamine, dimethylformamide, nitric acid, and ammonium hydroxide (all ACS reagent grade). For each solvent a set of solutions was prepared to contain 136 mg P / L and varying amounts of the solvent under test. The balance of each solution was deionized water. Blanks containing no added phosphorus were also prepared for each solvent at every level tested. Each HPO emission measurement was corrected with the appropriate blank. Interferences due to two typical alkali metal and alkaline earth ions, sodium and calcium, were also checked in a similar manner. Standard solutions containing 1000 mg/L of sodium (as sodium chloride in deionized water) or calcium (as calcium carbonate in dilute nitric acid) were purchased commercially (Certified Atomic Absorption Standards, Fisher Scientific). Test solutions containing 136 mg P / L and varying amounts of sodium or calcium were prepared in deionized water and used immediately. A solvent containing a typical ion pairing reagent was evaluated for interferences: 100 mL of this solvent contained 1 g of tetrabutylammonium chloride, 10 mL of methanol, and 13.6 mg of phosphorus (as H,PO,). The balance was deionized water. A blank was also prepared in a similar manner but with no phosphorus added. The exhaust of the inverted flame (that is, the gas between the primary and the secondary flames) was sampled by gas-tight syringe through a 1-mm hole near the top of the Pyrex tube. Samples were collected while aspirating first water and then methanol into the flame. Samples were also taken while adding C 0 2 gas to the flame via the auxiliary gas port on the burner. These samples were analyzed by gas chromatography using a Hewlett-Packard 5700A chromatograph equipped with a thermal in. 0.d. conductivity detector. The column used was 9.5 ft X (290 X 0.3 cm) stainless steel packed with Spherocarb (spherical carbon molecular sieve) 100/120 mesh (Analabs, Inc.) and heated to 100 "C. Helium carrier gas was used a t 30 mL/min. Can Mix 10 (Scott Specialty Gases) was used as the standard for CO and C 0 2 (7.23% and 15.0%, respectively, by volume in nitrogen). Secondary Flame. The feasibility of using the secondary flame for phosphorus analysis was also investigated. In preliminary measurements it was observed that the intensity of the secondary flame signal for a given concentration of phosphorus standard would decrease over a period of several minutes immediately after lighting the flames. This led to an investigation of the dependence of HPO emission intensity on the temperature of the gases entering the secondary flame. The apparatus used is shown schematically in Figure 3. The Pyrex tube used previously was replaced with an aluminum tube of approximately the same dimensions. A hole was drilled in the wall of the tube near the top to admit a thermocouple probe (Type K, Omega Engineering). The tip of the probe was positioned on the axis of the tube about
640
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980 MonOchromator
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Figure 6. Chemical interference due to some organic nitrogen compounds (in water), inverted vs. normal flame
100
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Figure 4. Chemical interference due to alcohoVwater solvents, inverted flame vs. normal flame
1cm below the upper edge. The outside of the tube was wrapped with coils of 1/4-in.(0.6-cm) copper tubing over the upper three quarters of its length. Tap water was passed through the coil at various rates in order to select gas temperatures over the range of 588 "C (maximum cooling) to 752 "C (no water flow). The spectrometer was shifted to the secondary flame with the entrance slit placed approximately 1.5 cm from the edge of the flame. The lens was not used since, a t this distance, radiation sampled from the flame completely filled the grating with light. Using maximum cooling, HPO emission signals were measured for aqueous standards of phosphoric acid containing up to 900 mg P/L to check linearity. Then an aqueous standard containing 456 mg P/L (as phosphoric acid) was aspirated and its emission measured as a function of temperature. Interferences from several water miscible solvents were also measured for the secondary flame using maximum cooling. The solvents checked were methanol, ethanol, acetonitrile, acetone, formic acid, and acetic acid. The ion pairing solvent described above was also checked.
RESULTS Primary Flame-Inverted vs. Normal Configuration. T h e inverted flame configuration produced HPO emission signals which were directly proportional to the phosphorus concentration of aqueous phosphoric acid standards. The response was linear to approximately 200 mg P/L. Sensitivity was approximately equal to that of the normally configured flame. Although the system was not optimized for quantitative measurements, both the normal and inverted configurations produced detectable signals for 1 mg P/Laqueous standards.
For every solvent tested, interferences were greatly reduced with the inverted flame compared to the normally configured flame. The results for the alcohols and carboxylic acids are shown in Figures 4 and 5. These figures show the relative response vs. the concentration of the added solvent. For illustration, all responses were normalized to the pure aqueous standard for each flame. A 10% solution of acetone in water caused complete quenching of HPO with the normal flame. With the inverted flame, solutions of u p to 20% acetone caused essentially no quenching. At 5070 acetone about half of the signal was quenched. Solutions with more than 50% acetone usually extinguished the inverted primary flame. Of the nitrogen-containing compounds, triethanolamine, sec-butylamine, dimethylformamide, and ammonium hydroxide behaved similarly: 1 M solutions of these compounds in water caused greater than 90% of the HPO signal in the normal flame to be quenched but only about 25% quenching occurred in the inverted flame. Nitric acid (1 M) caused 75% quenching in the normal flame but no detectable quenching in the inverted flame. Acetonitrile (1 M in water) caused nearly complete quenching in the normal flame and approximately 76% quenching in the inverted flame. Figure 6 illustrates the quenching caused by the organic, nitrogen-containing solvents a t variolis concentrations in water for the inverted and normal configurations. For both flame configurations, spectral interferences were observed for every solvent tested and for sodium and calcium. For the organic solvents, the background increased roughly in proportion to the rate of carbon introduction to the flame. In every case, spectral interference was less than 20% of the signal recorded and was corrected with blank measurements.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980 I
I
0
550
600
650 700 Temperature (C)
750
800
Figure 7. Secondary flame HPO emission intensity for a 456 mg P/L standard as a function of gas temperature
The spectral interference was typically a factor of three smaller in the inverted flame than in the normal flame. Chemical interferences due t o sodium and calcium were observed in both the normal and the inverted flames. At metal concentrations of 250 mg/L, both sodium and calcium quenched more than 90% of the (blank corrected) HPO emission in the normal flame. However, for the inverted flame, the quenching due to calcium a t this concentration was only 50% of the HPO signal while sodium quenched only 26% of the signal. No chemical or spectral interference was observed in the inverted flame for the solvent containing 170 tetrabutylammonium chloride and 10% methanol. This solvent system quenched more than 90% of the HPO emission signal in the normal flame. However, most of this quenching was due to the methanol content of the solvent. All data were obtained a t a solvent uptake rate of 6 mL/min in order to show the gross effects of solvents on response. At flow rates more common to HPLC, for example, 2 mL/min and less, all of the nonnitrogen-containing solvents tested could be used undiluted with no more than a 20% loss in response using the inverted flame (assuming the nebulization efficiency is constant). GC analysis of the exhaust of the inverted flame showed t h a t oxidized carbon, whether originally introduced into the flame as aspirated methanol or as gaseous COa, exists solely as CO in the excess hydrogen exhaust. T h e overall reducing strength of the inverted flame was also demonstrated by extinguishing the secondary flame (caution,hydrogen released) and then briefly aspirating an aqueous solution of 10 000 mg P / L (as H3P0,). T h e odor of phosphine (PH3) was readily apparent. Secondary Flame. T h e HPO emission intensity a t the secondary flame varied linearly with the concentration of phosphorus in test solution from 0 to over 900 mg P / L under conditions of thermal equilibrium. HPO emission signals were found to be inversely proportional to the gas temperature between 588 and 752 OC. T h e slope was -0.54% 1°C (based on the sensitivity observed a t 588 O C ) as shown in Figure 7 . Both spectral interferences and quenching from the organic solvents tested were similar to what was observed in the inverted primary flame. Again, these data were obtained a t a 6 mL/min solvent uptake rate. With the use of maximum cooling, the sensitivity and limit of detection for the secondary flame were similar to what was observed with the primary flame.
DISCUSSION Chemical interferences in HPO formation or emission wcur in the single, normally configured flame partly due to the flame geometry. HPO formation has two main requirements: excess H2 must exist, and the temperature must be relatively cool; approximately 350 "C is optimum (7). In the normal flame
641
this region is situated on the interior side of the primary reaction zone (see Figure 3). When organic material is aspirated into the flame, it passes through the HPO formation region prior to contact with the hotter outer region where decomposition of organics chiefly occurs. Thus, organic materials pass through the HPO formation region prior to undergoing any significant decomposition and are available for quenching reactions. When the flame gases are reversed, the H P O formation region is on the exterior of the primary reaction zone (see Figure 1). In addition, the region of brightest HPO emission is spatially separated from the primary reaction zone by several millimeters. In this configuration, organic materials aspirated into the air stream are almost completely decomposed prior to reaching the HPO formation region. Thus, quenching of HPO formation reactions, or of I-IPO emission, by organic materials is greatly reduced compared to the normal flame case. A potential point of confusion may exist with respect to the work by Syty and Dean ( 2 ) . They did not observe quenching when methyl isobutyl ketone (MIKB) solutions containing phosphorus were aspirated into their flame. They used a total consumption burner to produce a turbulent air-hydrogen diffusion flame. I t was configured similar to Figure 2 (that is, hydrogen in the center) except that the sample was aspirated into the air stream. The fundamental reason that Syty and Dean did not observe quenching from MIKB is t h a t phosphorus in the sample must diffuse inward through the reaction zone and into the hydrogen for H P O formation to occur. In order t o cause quenching, MIBK (or hydrocarbon fragments) would also have to pass through the reaction zone but without undergoing oxidation. T h e probability of this occurring is so low that no significant quenching occurred. However, when Syty and Dean added a small amount of acetylene to the hydrogen, the H P O was totally quenched. In general, solvents which form only COz (or CO) and HzO when burned, produce much less quenching in the inverted flame than in the normal flame. Except for nitric acid, the compounds containing nitrogen are, in general, stronger quenchers than those composed of only C, H , and 0. Acetonitrile is the strongest quencher of the solvents tested. Laminar flow conditions in the inverted flame are also desirable for minimizing quenching. A thin, stable primary reaction zone allows the combination of relatively high temperatures and excess oxygen molecules near its inside surface. Thus, even though this flame may be considered strongly reducing owing to the large overall excess of hydrogen, organic constituents in the sample are exposed to oxidizing conditions prior to reaching the H P O formation region. Phosphorus compounds are most likely converted to P O in the oxidizing region, then subsequently reduced to HPO. Further reduction occurs above the HPO formation region, producing PH3. When the excess H2 is burned in air in the secondary flame, the PH, is oxidized first to HPO (resulting in light emission), and then probably to a mixture of phosphorus oxides and oxo acids. Chemical interferences by sodium and calcium are also decreased in the inverted flame. Thus, passage of the analyte through the primary reaction zone prior to H P O formation a t least partially releases phosphorus from compounds which do not form H P O readily. Spectral interferences may be easily reduced with the inverted flame. Atomic emission of interfering metals and molecular emission of organic fragments chiefly occur in the hottest part of the flame: the primary reaction zone. In the normal flame, the photometer must "look" through this zone to see HPO. However, in the inverted configuration, HPO is formed on the exterior of the primary reaction zone. Thus,
Anal. Chem. 1980, 52, 642-646
642
it is possible to view only one side of the flame, or the center may be blocked to prevent the interfering light from reaching the photometer. There are also several disadvantages in using the primary flame in the inverted configuration. The H P O formation region is large. Since it is experimentally difficult to measure the emission from the entire zone, optimum sensitivity cannot be easily achieved. Also, since any aspirated organic materials fuel the flame, both the shape of the primary reaction zone and the position of the brightest HPO emission (when viewed side-on) change as a function of the solvent composition. Concentrations of acetone and acetonitrile of 30% and 5070 by volume, respectively, when aspirated a t 6 mL/min, caused flame instability. Aspirating solutions containing higher than 50% of these solvents at this rate would extinguish the primary flame. However, at aspiration rates of 2 mL/min and less, there was no stability problem. These disadvantages do not exist when viewing the secondary flame. As long as the primary flame stays lit, the secondary flame geometry remains virtually unaffected by the solvent composition. All of the advantages of the inverted primary flame still apply to the upper flame except for spectral interferences: as with any normal flame, a photometer must “look” through a reaction zone to see H P O within. The use of the inverted ai-hydrogen flame or its secondary flame has several potential hazards. If the secondary flame should be accidentally extinguished, hydrogen will be released. Also, phosphorus, when present, will be released in the form of phosphine. Carbon will be released in the form of CO. Organic nitrogen may be released as hydrogen cyanide (especially if nitriles are aspirated in the sample). No special precautions other than routine venting are requiren33as long as the secondary flame is lit.
components of aqueous/organic liquid samples is greatly reduced by burning the ai-hydrogen flame “inside-out”. Either the primary or secondary flame may be used for the photometric measurement. Some quenching still occurs, especially when organic nitrogen-containing compounds are aspirated. Acetonitrile was the worst solvent encountered in this respect. However, if this flame configuration were used as the basis for an HPLC detector, methanol could be substituted for acetonitrile in most applications in order to allow phosphorus-selective detection. Ion pair chromatography of phosphonic acids may also be possible with phosphorus-selective detection using tetraalkylammonium pairing ions.
CONCLUSIONS
RECEIVED for review August 20,1979. Accepted January 11,
T h e quenching of H P O formation or emission by matrix
ACKNOWLEDGMENT The contributions of N. J. Holzschuh in gathering experimental data, of H. Ankenbauer and the MVL Machine Shop, and of R. J. Lloyd in assisting with the gas chromatography measurements are gratefully acknowledged.
LITERATURE CITED Salet, G. Ann. Phys. 1869. 737, 171. Syty, A.; Dean, J. A. Appl. Opt. 1968, 7 , 1331. Veiilon, C.; Park, J. Y. Anal. Chim. Acta, 1972, 60, 293-301. Brody, S. S.; Chaney, J. E. J . Gas Chromatogr. 1966, 4 , 42-46. Julin, B. G.; Vandenborn, H. W.; Kirkhnd, J. J J. Chromatogr. 1975, 172, 443-453. (6) Chester, T. L.; Lewis, E. C.; Benedict, J. J. Unpublished work, The Procter & Gamble Company, Miami Valley Laboratories, 1978. (7)Dagnall, R . M.; Thompson, K . C.: West, T. S. Analyst(London) 1968. 93, 72-78, (8) Draeger, 0. West German Patent 1 133918 (1962). (9) Van der Smissen, C. E. U S . Patent 3213747 (1965). (10) Patterson. P. L.: Howe. R. L.; Abu-Shumavs, A. Anal. Chem. 1978, 50. 339-344. (11) Haraguchi, H.; Winefordner, J. D. Appl. Spectrosc. 1977, 37, 195-199. (1) (2) (3) (4) (5)
1980.
Non-Dispersive Atomic Fluorescence Spectrometer for the Direct Determination of Metals D. S. Gough” and J. R. Meldrum CSIRO Division of Chemical Physics, P.O. Box 160, Clayton, Victoria 3 168, Australia
An atomic fluorescence spectrometer for the determination of elemental concentrations in metal samples is described. The atomic vapor produced from the sample by cathodic sputtering is irradiated by intense lamps and atomic fluorescence is detected at right angles to the incident light. The amplifier of the spectrometer incorporates a feedback loop so that compensation is made for fluctuations in the lamp intensity during a series of measurements. Detection limits and reproducibility of measurement have been determined for various elements in alloys of iron, aluminum, and copper. The precision of the measurements is typically f2% for minor and trace constituents in the samples. Detection limits are in the range 1-100 ppm.
T h e technique of cathodic sputtering is a convenient method for the production of atomic vapors from metal samples. T h e vapor produced is contained in an inert atmosphere, usually argon, which minimizes the possibility of chemical 0003-2700/80/0352-0642$01 . O O / O
reaction such as the formation of compounds. Vapors sputtered from solids have been analysed by emission (1-4), absorption (5-8) and fluorescence spectrometry (9). In an earlier paper (7),instrumentation was described for analysis of metals by atomic absorption in which a sputtering cell replaced a flame in a commercial atomic absorption spectrometer. The apparatus was convenient to use and the precision of the measurements was high. This paper describes a spectrometer in which the sample is atomized by cathodic sputtering but analysed by atomic fluorescence, The advantages of this spectrometer are that the detection limits achieved are approximately an order of magnitude lower than for absorption measurements, and the light path is easily purged with dry argon for the detection of elements whose resonance lines lie in the vacuum ultraviolet. The spectrometer is of the nondispersive type described by Larkins ( I O ) with approximately 1:l imaging of the fluoresced radiation on the detector, so that fluorescence is detected over a large solid angle. T h e light source intensity is measured continuously and a feedback loop within the amplifier provides compensation for variations in @ 1980 American Chemical Society
