Anal. Chem. 1986, 58,3126-3132
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Rapid Sulfur Determination in Bituminous and Subbituminous Coal by Slurry Atomization Inductively Coupled Plasma Emission Spectrometry David L. McCurdy and Robert C. Fry*
Department of Chemistry, Willard Hull, Kansas State Uniuersity, Manhattan, Kansas 66506
I n an effort to minimize sample preparation, coal slurries of unusually small particle size (median diameter -6 pm) were produced with 20-30 min of high intensity wet grinding of bituminous and subbltumlnous coal. Direct nebulization of coal slurries into an argon inductively coupled plasma emission spectrometer yielded accurate sulfur determinations under the following experimental conditions: (1) small coal particle size, (2) reduced nebulizer argon flow rate, and (3) removal of obstructions such as baffles, impactor beads, and tortuous bends from the spray chamber path. When these experimental conditions were met, the new slurry atomization inductively coupled plasma emission procedure for coal analysis gave quantitative sulfur recovery. Slurry atomization analysis was performed without the aid of wet or dry ashing, solvent extraction, matrix matching, standard addltlon, internal standards, or correction factors. Instead, simple aqueous standard (ammonium sulfate) callbration was sufficient to yield the same result as that certified by the NBS for sulfur in bituminous coal, SRM 1632b, and in subbituminous coal, SRM 1635.
The most time-consuming and unreliable aspect of performing plasma emission determinations of sulfur in coal is dissolving the sample. In an attempt to eliminate lengthy wet or dry ashing, fusion, and bomb digestion methods, O’Reilly et al. nebulized slurries of sieved coal powder directly into a nitrous oxide/acetylene flame for atomic absorption determinations of elements other than sulfur ( I , 2). (Sulfur cannot be determined with conventional atomic absorption apparatus and air-path optics.) Large suspended coal particle sizes from swing mill grinding led to aerosol mass transport losses and incomplete particle vaporization in O’Reilly’s system (1-3). The result was low signal recovery. All sample results were therefore multiplied by a 5-fold correction factor in order to provide a flame AA method for the rapid, semiquantitative determination of alkali, alkaline-earth, and transition metals in bituminous coal powder ( I , 2 ) . Several other authors have used more extensive grinding to reduce the particle size of soils, ores, and rocks prior to slurry injection into hot inductively coupled plasmas ( 4 , 5 ) . However, these authors still employed tortuous bends in the spray chamber and/or other transport obstructions such as aerosol impactor beads or baffles. As a result, the aerosol mass transport efficiency was low, and elemental signal recoveries for slurry samples were only about 24-79% of aqueous standards containing equivalent concentrations. The method of standard additions cannot correct for this type of interference. Instead, empirically determined correction factors had to be carefully evaluated and applied to all results. These correction factors varied widely with particle size, sample type, element, and plasma operating conditions ( 4 , 5 ) ,but they never reached the desired value of 1.00 (where they would have become unnecessary). McCurdy, Fry, and Wichman used a high efficiency McCrone Micronising Mill for rapid particle size reduction
in coal (6). When this approach was combined with a spray chamber modified to remove all aerosol impactor and baffle impediments together with elimination of all tortuous bends in the spray chamber path, the final result was elimination of the need for correction factors in rapid slurry atomization direct current plasma (DCP) emission determinations of Cr, Cu, Mn, Mg, Ni, and P b (6). However, air-path optics were employed, and sulfur was not included in the study. Furthermore, a slurry method for rapid, accurate, quantitative analysis of solid samples which exhibits 100% relative atomization efficiency ( E = 1.00) and is therefore free of the need for correction factors has not been published for the argon inductively coupled plasma. The present paper involves use of an inductively coupled plasma and argon purged optical system to facilitate sulfur determination a t 182.04 nm. A study was made of the interaction between ICP gas flow dynamics, spray chamber geometry, and coal particle size in an attempt to minimize the need for correction factors by ensuring efficient aerosol mass transport. The combination of small particle size, high temperature, and long residence time in the ICP was also studied as a means of achieving complete coal particle vaporization. The overall goal of the present study was to achieve quantitative ICP emission signal recovery and accurate sulfur determinations in coal with nothing more than aqueous standard calibration.
EXPERIMENTAL SECTION Apparatus. The apparatus is described in Table I. The internal monochromator path and the enclosed external optical path between the plasma and monochromator were both purged with argon gas at 6 L/min to eliminate atmospheric absorption below 185 nm. The Suprasil lens was used to focus a 1:l plasma image on the entrance slit of the monochromator. A McCrone Micronising Mill (McCrone Associates, Chicago, IL) was used to rapidly wet-grind the coal samples. The Micronising Mill was equipped with 48 interactive cylindrical agate grinding pellets stacked in a semiordered array (6 X 8) within a polypropylene canister. All particle and aerosol droplet sizing was performed with a Microtrac Analyzer (a laser Fraunhofer diffractometer available from Leeds and Northrup (L&N),Inc., Largo, FL). Standard “low gain” L&N amplifier boards were employed for coal particle sizing while factory supplied “high gain” boards were used along with optical rail modifications described in ref 7 for nonintrusive aerosol droplet size measurements by laser Fraunhofer diffraction. Reagents. Standard sulfur solutions in the range 1C-200 ppm (w/w) were prepared by dilution with distilled, deionized water from a 1000 mg/kg (as sulfur) standard solution of ammonium sulfate. A 0.5% (by weight) aqueous solution of Triton X-100 (Rohm and Haas Co.) was used as the slurry medium. Procedure. Subbituminous coal samples were prepared as follows. The clean, dry mill canister and grinding pelletes were tared to zero on a digital electronic balance. Approximately 0.5 g of National Bureau of Standards reference material SRM-1635 (subbituminous coal) was weighed t o the nearest milligram, directly onto the agate grinding pellets in the mill canister and diluted 20-fold by weight with 0.5% Triton X-100 (aqueous) in the same canister. The samples were then milled for 30 min producing a 5% coal slurry that was poured directly from the
C 1986 American Chemical Society 0003-2700/86/0358-3126$01.50/0
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
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Table I. Apparatus ICP rf generator nebulizer
Plasma-Therm ICP-2500 (Plasma-Therm, Inc., Kresson, NJ) HFP 2500D, 27 MHz, with APCS-3, automatching network and autopower control Applied Research Laboratories (ARL) MDSN Babington-type slurry nebulizer (Applied Research Laboratories, Sunland, CA) spray chamber an exact Pyrex glass copy of the standard ARL spray chamber, constructed at Kansas State University without the impactor bead
external optics lens optical path monochromator
Suprasil, 10 cm focal length, 2.5 cm diameter externally blackened Pyrex glass tubes, 32 mm diameter, purged with argon GCA-McPherson EU-700, Czerny-Turner 0.35-m focal length, f/6.8 (GCA-McPherson Instrument Co., Acton, MA) purged with argon 50 X 50 mm, 1200 grooves/mm, 250 nm blaze, first order grating PMT Hamamatsu RlO6UH (Hamamatsu Corp., Middlesex, NJ), housing purged with argon peristaltic pump Rainin Rabbit, 39-626 Tygon tubing (Rainin Instrument Co., Inc., Woburn, MA) Cole Parmer Instrument Co Model 355 (Cole Parmer Instrument Co., Chicago, IL) chart recorder
Table 11. Experimental Conditions SULFUR
ICP power incident 1.5 kw reflected 50 l m ) and will carry any internally suspended coal particles down the waste drain, even if the individual particles carried within that particular droplet were less than 19.7 pm in diameter. Only those coal particles that break free of water suspension upon nebulization or those coal particles suspended within water droplets that are nominally 19.7 pm or smaller are candidates for efficient aerodynamic transport through the unobstructed spray chamber. For this reason, the maximum aerosol transport efficiency of the most finely ground coal slurries may approach but not surpass that of an aqueous standard in this system. A specific gravity correction was next applied to the data for all particles equal to and larger than the median diameter of Figures 4 and 5 . The assumption was also made that coal
DIAMETER
(urn)
--j
Figure 6. Size distribution cutoff of coal particles that can be aero-
dynamically transported through each spray chamber at 0.9 L/min nebulizer argon flow rate: solid line (-), conventional spray chamber (dgO= 14.9 pm); dashed line (---), modified spray chamber (dg0= 17.3 pm). particles smaller than the corrected median of these figures
will transport at the maximum efficiency established by water droplets in which they are suspended. A new graph (Figure 6) was constructed to reflect the specific gravity correction and the maximum efficiency assumption for both spray chambers. Figure 6 gives us picture of the maximum coal particle size that can be aerodynamically transported through either spray chamber at 0.9 L/min of argon flow. In order for the sample to pass through the spray chambers with maximum efficiency, the grinder must produce a coal slurry of median particle diameter (dS0)less than or equal to 5.9 or 6.8 wm. The 90th percentile should be less than or equal to 14.9 or 17.3 ,um (see Figure 6). For accurate atomic emission analysis using normal argon flow rates, coal samples must be ground to particle size distributions that fit “under” the curves of Figure 6. The next step was to measure the actual particle size of ground coal and determine what fraction falls within the transportable ranges of Figure 6. Figure 7A shows the particle size distribution of NBS subbituminous coal powder before wet grinding with the McCrone Micronising Mill. The maximum transportable range from Figure 6 for the modified spray chamber is shown superimposed over the actual coal particle size distributions of Figure 7. Figure 7A indicates that approximately 57% of the particles are too large to transport without further treatment (see the “clear” region in the figure). Only those particles in the 43% “blackened” region of the figure are small enough to transport through the normal spray chamber without grinding. After 15 min of wet grinding in the McCrone Micronising Mill, the coal particle size has been greatly reduced (Figure 7B). A total of 94% of the coal particles now fall within the transportable “blackened” region of the distribution, with 6% remaining above the transport cutoff of the modified spray chamber. If aerodynamic transport losses in the spray chamber were the only factor affecting the atomic emission signal intensity in the new slurry atomization ICP method, we would predict a sulfur recovery of 94% based on aqueous standard calibration for the modified spray chamber. For the standard spray chamber and impactor bead, we would predict a sulfur recovery of 89%. For a 15-min sample milling time and a 0.9 L/min nebulizer argon flow rate, the actual sulfur recoveries from NBS subbituminous coal were only 65% and 59% for the modified and standard spray chamber geometries, respectively. The modified spray chamber has therefore given some improvement
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
0B
sy0vA;
I ON
\
A B Flgure 8. Plasma geometry and nebulizer argon flow rate effect: (A) 0.9 L/min argon and (B) 0.4 L/min argon; (C) argon purged external
optical path.
I PARTICLE
DIAMETER
(urn)
--f
Flgure 7. NBS subbftuminous coal (SRM-1635)particle size distribution (A) before wet grinding and (B) after 10 min of wet grinding with the McCrone Micronising Mill: clear region (O), size distribution of the MSS
fraction of coal rejected by the modified spray chamber: blackened region (D),size distribution of the mass fraction of coal passing through the modified spray chamber. in mass transport efficiency and sulfur recovery, but they are still considerably less than 100%. The particle size data indicate that only about 6% of the low sulfur recovery can be attributed to mass transport loss. The remainder of the problem is most likely a result of incomplete coal particle vaporization in the plasma. Flow Rate Effect. The next step was to reduce the nebulizer argon flow rate. This would presumably lengthen the residence time of coal particles in the plasma. It was also noted that the cooler central axial channel of the plasma constricted at lower nebulizer flow rates (e.g., 0.4 L/min), forcing the aerosol particles and droplets into a higher temperature region than normally experienced at the same viewing height and 0.9 L,/min. This is conceptually illustrated in Figure 8. Before the combined effect of increased residence time and more energetic collisions on the coal particle vaporization efficiency was assessed, the aerodynamic transport characteristics of the spray chamber were reevaluated a t the lower nebulizer gas flow rate. By use of the same type of laser measurements, density corrections, and maximum efficiency assumptions as those used earlier to generate Figure 6, new data establishing the maximum transportable coal particle size are shown in Figure 9 for 0.4 L/min nebulizer argon flow rates in this system. The coal slurry particle size distribution for a 15-min grind is also shown in the figure. A comparison of Figures 6, 7 , and 9 shows that the 90th percentile cutoff of the modified spray chamber has been increased from 17.3 to 23.2 pm by lowering the nebulizer argon flow rate from 0.9 to 0.4 L/min. The normal ARL spray chamber with impactor bead exhibits a similar flow rate dependence. Figure 9 shows that 100% of the wet ground coal slurry is in the aerodynamically transportable range of particle size when 0.4 L/min nebulizer argon is used in the modified spray chamber. This is an improvement over the 94% transport efficiency encountered with the normal 0.9 L/min nebulizer argon flow rate.
PARTICLE
DIAMETER
(urn)
j
Figure 9. Particle size distribution of the NBS analyzed subbituminous coal (SRM-1635)shown superimposed under the size distribution cutoff for particle transport through the modified spray chamber (0.4 L/min nebulizer argon flow rate): blackened region (H), aerodynamically transportable coal particles.
At 0.4 L/min, these laser measurements of particle size have shown that the aerodynamic transport of coal particles through this particular modified ARL spray chamber should be as efficient as that of aqueous standard aerosol droplets. The remaining question is simply whether the coal particles can be efficiently vaporized in the plasma when lowered nebulizer argon flow rates are employed. This question was answered by the measurement and calibration of sulfur emissions from NBS coal slurries. With the standard ARL spray chamber (with impactor bead) and the nebulizer argon flow rate reduced to 0.4 L/min, the sulfur recovery from NBS subbituminous coal slurry improved to the level of 85%. This is better than the 59% recovery encountered at normal nebulizer argon flow rates but is still considerably less than that needed for accurate quantitative analysis using only aqueous standard calibration. One of the final steps was removal of the spray chamber impactor bead. Figure 10 illustrates the effect of nebulizer argon flow rate on the percent sulfur recovery with the modified spray chamber. Normal ICP nebulizer flow rates (e.g., -1 L/min) will not allow quantitative sulfur determination with either spray chamber by the slurry atomization method. In contrast, a nebulizer flow rate of 50.4 L/min through the modified spray chamber yielded 100 5% sulfur recovery from finely ground NBS subbituminous coal slurry
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
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Table 111. Slurry Atomization ICP Emission Determination of Sulfur in NBS Coal sulfur results, wt %
NBS
bituminous coal (SRM 1632b) subbituminous coal (SRM 1635)
< "0.3
0.4
05
NEBULIZER
0 6
0.7
FLOW
0 8
09
(L/rnin 1
Flgure 10. Effect of nebulizer argon flow rate on sulfur recovery (subbituminouscoal in modified spray chamber only).
0
5
IO
MILLING
15
TIME
20
25
30
(min )
Flgure 11. Effect of milling time on coal particle size: (A)bituminolis coal (NBS SRM 1632b); (0) subbituminous coal (NBS SRM 1635).
with nothing more than aqueous standard calibration. Milling Time. The ICP excited sulfur results to this point were generated with subbituminous coal and a 15-min milling period. Our previous report on DCP emission determinations of trace minerals (other than sulfur) in bituminous coal involved only a 10-min milling period (6). In the present study, preliminary trials with a 10-min grind on the subbituminous sample (a softer type of coal), yielded only 60% sulfur recovery, even with the otherwise optimized nebulizer flow rate and modified spray chamber conditions reported in preceding paragraphs. A particle size measurement on this subbituminous sample next indicated that the 10-min grind did not produce as small a median diameter as that seen earlier with bituminous coal (6) or as that seen in Figure 9 for a 15-min grind. A study of the milling time was therefore undertaken to assess its effect on the particle size and sulfur recovery. This was done for both the subbituminous and bituminous coal samples. As we suspected, Figure 11indicates that the subbituminous coal is not as rapidly reduced in particle size as the bituminous material. At short grinding times (e.g., 10 min), the mill is apparently more effective with more brittle materials such as bituminous coal than with softer materials such as subbituminous coal. It should be noted that for short milling times (e.g., 10 min), some unaccountable variations in the particle size have occasionally been observed in this lab for some coal samples, even though the milling time was held constant. The variations have been on the order of an occasional 2- or 3 - ~ mshift
slurry method
certificate
1.9 0.33
1.89 0.06 0.33 f 0.03
*
in median diameter for a 10- or 15-min milling time. For samples experiencing such a shift toward larger particle diameters, the sulfur recovery may be low. The critical region appears to occur in the vicinity of 10- or 15-min milling time, with many samples and elements giving good results and a few samples and elements giving low results. This undesirable effect disappears at longer grinding times. Figure 11 indicates that an extra margin of safety may be added by increasing the milling time to a total of 20 or 30 min where no further adverse variation in particle size is likely to occur for either the bituminous or the subbituminous coal. If desired, and if the apparatus is available, a quality control check can be quickly made by rapid coal particle size analysis with a computerized laser diffractometer (after milling, but prior to ICP emission determination of sulfur in the coal slurry). Confidence may be placed in the sulfur determination if the modified spray chamber is employed a t 10.5 L/min nebulizer argon flow rate and preliminary laser determination of the coal particle size (after milling) yields a distribution similar to or smaller than the coal sample of Figure 9. This quality control test can be used as an aid in selecting the milling time. If, for some reason, the particle size distribution of the milled coal sample is found to be larger than that of Figure 9, then the milling time should be increased and another quality control check of the particle size should be made. It is generally not difficult to find a suitable milling time that allows a reasonable margin of safety in assuring an accurate sulfur determination for a given type of sample. With longer milling times (20-30 rnin.), Table I11 shows the final slurry atomization ICP emission results for the direct determination of sulfur in NBS soft coals. The slurry results are reported as an average of 19 determinations at 20-30 min milling times. The slurry results agree well with the certified NBS values. When the right experimental conditions of particle size, flow rate, and spray chamber geometry are employed along with the additional experimental conditions of Table 11, these data have shown that the new slurry method gives accurate results and can substantially reduce the time and effort expended in sample preparation for sulfur determinations in bituminous and subbituminous coal. Precision. The relative standard deviation for this lowresolution analog scanning instrument, nonintegrated strip chart recorder readout, and manual base line extrapolation method of background correction was found to be in the range of 4-6 70 relative standard deviation for low-sulfur, subbituminous coal. The percent relative standard deviation was found to be no worse for coal slurries than for the aqueous standard. It is anticipated that a modern instrument of higher resolution, more favorable grating blaze, improved line-tobackground ratio, vacuum path (instead of nitrogen purging), digital data processing (e.g., signal integration), and automated background correction would improve the measurement precision for both the coal slurry and the aqueous standard.
ACKNOWLEDGMENT The authors wish to thank Mitsugi Ohno of Kansas State University for fabricating the modified Pyrex spray chamber. Registry No. S, 7704-34-9.
Anal. Chem. 1986, 58, 3132-3135
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LITERATURE CITED (1) O'Reilly, J. E.;Hale, M. A. Anal. Lett. 1977, 10, 1095-1104. (2) O'Reilly, J. E.; Hicks, D. G. Anal. Chem.1979, 5 1 , 1905-1915. (3) Mohamd, N.; McCurdy, D. L.; Wichman, M. D.; Fry, R. C.; O'Reilly, J. E. Appl. Spectrosc. 1985, 3 9 , 979-983. (4) Fuller, C. W.; Hutton, R. C.; Preston, B. Analyst (London) 1981. 106, 9 13-920. (5) Dick, W. A,; Page, J. R.; Jewell, K. E. SollSci. 1985, 139, 211-218. (6) McCurdy, D. L.; Wichman, M. D.; Fry, R. C. Appl. Spectrosc. 1985, 3 9 , 984-988. (7) Mohamed, N.; Fry, R. C.; Wetzel, D. L. Anal. Chem. 1981, 5 3 , 639-645.
(8) Skogerboe, R. K.; Olson, K. W. Appl. Spectrosc. 1978, 32, 181-187. (9) Browner, R. F.; Boorn, A. W.; Smith, D. D. Anal. Chem. 1982, 5 4 , 1411-1419.
RECEIVED for review December 24, 1985. Resubmitted June 30, 1986. Accepted July 7, 1986. This project was supported in part by Applied Research Laboratories, Sunland, CA. The work was presented in part at the 1985 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, New Orleans, LA (Paper No. 400B).
Fluorometric Determination of Nitrite with 4-Hydroxycoumarin Takafumi Ohta,* Youichi Arai, and Shoji Takitani
Faculty of Pharmaceutical Sciences, Science University of Tokyo, 12, Ichigaya-Funagawara-Machi,Shinjuku-ku, Tokyo 162, Japan
A simple, sensitive, and reproducible fluorometric method for determination of nitrite has been developed. Thls method is based on the nitrosation of 4-hydroxycoumarin in acidic medium and subsequent reduction to 3-amino-4-hydroxycoumarin, which is fluorescent in alkaline medium. The fluorescence intensity is proportional to the nitrite concentration in the range of 3 ng/mL to 1 pg/mL in the sample solution, with a relative standard deviation of 0.5% (50 ng/ mL). The method has been applied to the determination of nitrite in saliva.
Nitrite ion is of importance not only as an intermediate in the nitrogen cycle, in which its concentration indicates the quality of water, but also as a precursor that forms carcinogenic and/or mutagenic nitrosamines by the reaction with various amines. Recent studies showed the presence of several N-nitroso amino acids in normal human urine, supporting the endogenous reaction of nitrite with amino acids ( I , 2). Thus, a simple, sensitive, and specific determination of nitrite present in foods and biological fluids such as saliva is highly desired. Various fluorometric methods have been developed for the determination of nitrite. However, these methods have several disadvantages. The method with 2,6-diaminopyridine ( 3 ) , 2,3-diaminonaphthalene (4),or resorcinol (5)is tedious because of solvent extraction or prolonged heating. The method with 5-aminofluorescein (6) is liable to variation probably due to the fact that the measurement is based on an increase in fluorescence intensity a t the emission maximum of its reagent: the sensitivity was largely affected depending on the difference in batches and brands of hydrochloric acid used as a reagent. The method with benzidine (7) has drawbacks of serious interferences and toxicity of the reagent used. The method with pyridoxal 5-phosphate 2-pyridylhydrazone (8) lacks sensitivity. Nitrite reacts rapidly with phenolic compounds in acidic solution t,o give their nitroso derivatives (9),which are generally nonfluorescent because of the electron-withdrawing effect of the nitroso group. These nitrosation reactions, therefore, have been used in spectrophotometric methods for the determination of nitrite ( 1 0 , l I ) . On the other hand, amino groups increase the fluorescence intensity of aromatic compounds (12). Therefore, nitrosation of a phenolic compound and subsequent reduction to an aromatic amine seems to
enable a fluorometric determination of nitrite. The present paper describes a simple, sensitive, and reproducible method for the determination of nitrite based on the nitrosation of 4-hydroxycoumarin and subsequent reduction to 3-amino-4hydroxycoumarin, which is fluorescent in alkaline medium.
EXPERIMENTAL SECTION Apparatus. Fluorescence measurements were made with a Shimadzu RF-510 spectrofluorometerequipped with a xenon lamp (Kyoto, Japan) using a 10 x 10 mm quartz cell at room temperature. The slit widths in terms of wavelength were 5 nm (excitation) and 10 nm (emission). All fluorescence excitation and emission spectra were uncorrected. Chemicals. All reagents used were of analytical grade except for sodium hydrosulfite (75% purity). A standard nitrite solution was prepared by drying sodium nitrite (Kanto Chemical Co.) at 110 "C for 4 h and dissolving it in deionized water (MilliporeRO-Q system) to give a 1 mg/mL solution. This standard solution was prepared weekly and kept in a refrigerator, and further dilution was made daily as required. 4-Hydroxycoumarin (Nakarai Chemical Co.) was dissoloved in acetonitrile2 M HCl (1:l) to give 0.04% solution. 3-Amino-4-hydroxycoumarinhydrocholoride was synthesized by the method of Heubner and Link ( 1 3 ) . Procedure. To 2.0 mL of sample solution in a 10-mL glassstoppered test tube was added 1.0 mL of 0.04% 4-hydroxycoumarin reagent. The tube was left in an ice bath for 5 min. To the reaction mixture was added 0.1 mL of 8% sodium thiosulfate, and this was left for 5 min at room temperature. The reduced mixture was made alkaline with 1.0 mL of 1.5 M NaOH and left for 10 min at room temperature. The fluorescence intensity was measured with excitation at 347 nm and emission at 453 nm. Each value in the figures and tables, except Table IV, represents the mean of duplicate runs. RESULTS AND DISCUSSION Fluorescence Spectrum. 4-Hydroxycoumarin, 7hydroxycoumarin, 7-hydroxy-4-methylcoumarin, 6,7-dihydroxycoumarin, 1-naphthol, and 2-naphthol were tested as fluorometric reagents for the determination of nitrite. Satisfactory results were obtained with 4-hydroxycoumarin; the other compounds were themselves highly fluorescent and therefore not usable. The fluorescence of the final reaction mixture resulting from nitrite and 4-hydroxycoumarin showed an excitation maximum a t 347 nm and emission maximum a t 453 nm (Figure 1). 3-Amino-4-hydroxycoumarin dissolved in a blank solution gave the same excitation and emission spectrum. In addition, this amino derivative was found to be present in the reaction mixture by means of thin-layer chromatography (data not
0003-2700/86/0358-3132$01.50/0@ 1986 American Chemical Society
