Anal. Chem. 1980, 52,2079-2083
ACKNOWLEDGMENT We thank Ron Pause for generating spectra of polluted well water samples a n d Terry Fuller for technical assistance. LITERATURE CITED (1) Bush, B.: Narang, R . S.: Syrotynski, S. Bull. Environ. Contam. Toxicol. 1977, 18, 436. (2) Kaiser. K. L. E.; Oliver, 8 . G. Anal. Chem. 1976, 48, 2207.
2079
(3) Nicholson, A. A.; Meresz, 0. Bull. Environ. Contam. Toxicol. 1975, 14, 453. (4) Bellar, T. A.; Lichtenberg. J.; Kroner, R . C. J . Am. Wafer Works Assoc. 1974, 703. (5) &ob, K.: Gob, J. J . Chromatogr., 1974, 9 0 , 303. (6) Fujii. T. Anal. Chem. 1977, 49, 1985. (7) Driscol, J. N. J . Chromatogr. 1977, 49, 134.
RECEIVED for review March 12,1980. Accepted July 25,1980.
Comparison of Three Commercially Available Gas Chromatographic-Flame Photometric Detectors in the Sulfur Mode J. F. McGaughey' and S. K. Gangwal' Energy, Engineering and Environmental Sciences Group, Research Triangle Institute, Research Triangle Park, North Carolina 27709
Three commercially available flame photometrlc detectors (FPD) in the sulfur mode have been evaluated by Comparing the parameters of dynamic range, llnearky, minimum detection limn, sensitivity, peak shape, selecthrity, and ease of operation. Sulfur compounds used for this study included hydrogen sulflde, carbonyl sulflde, and sulfur dioxide. The experimental parameters were kept as nearly Identical as possible for the three systems wnh only minor equipment modlfications belng necessary. The selection of an FPD should be based on the intended use because none of the systems proved to be superior In all categories.
Accurate determination of low levels of sulfur gases has received considerable attention in recent years (1-15). Usually, the measurement device of choice has been the flame photometric detector (FPD) since its introduction by Brody and Chaney (1) in 1966. In combination with a gas chromatograph (GC), the F P D has become a significant tool for determining individual sulfur gases because of its sensitivity and specificity (2-8). T h e Research Triangle Institute (RTI) has been involved for several years in the determination of sulfur gases [specifically hydrogen sulfide (H,S), carbonyl sulfide (COS), and sulfur dioxide (SO,)] in concentrations ranging from parts per billion to low percent levels. One of the authors recently described the applicability of a novel F P D to the analysis of sulfur emissions from emerging energy technologies (9). RTI also conducts outdoor smog chamber experiments that require the determination of parts per million to parts per billion levels of these sulfur gases. Additionally, RTI, under contract to government and private organizations, carries out both fugitive a n d stack emission measurements from many industries. These analyses constitute a wide range of sulfur concentrations a n d prompted the present instrument study. Three commercial FPDs are currently in use at RTI. Experiments comparing the three systems were conducted to determine the most suitable system to be used for each specific application. I t is believed t h a t the results of this study will aid other investigators in avoiding some of the pitfalls (10-14) Present address: TRW, Inc., Energy Systems Group, Environmental Engineering Division, Research Triangle Park, NC 27709. 0003-2700/80/0352-2079$01 .OO/O
Table I. Cylinder Concentrations label concn (ppm in nitrogen) low H,S high H,S
low cos high COS low so, high SO,
6.45
100.00
7.03 100.00
measd concn (ppm in
nitrogen) 4.09 84.60 6.41 95.20
1.12
1.12
50.80
50.40
of F P D (e.g., nonlinearity and compound dependence) and will allow them to advantageously use these instruments for measurement of sulfur gases.
EXPERIMENTAL SECTION FPD. The three systems used in this study were a Varian-3700 FPD (Varian Associates, Walnut Creek, CA), a Perkin-Elmer-3920 FPD (Perkin-Elmer Corp., Norwalk, CT), and a Tracor-560 FPD (Tracor Instruments, Austin, TX). The Perkin-Elmer system had been in use the longest (14 months) at the time of this study. The Varian FPD can be used as both a single- and a dual-flame detector as described by Patterson et al. (15,161. The PerkinElmer and Tracor systems are single-flame detectors. In addition, the electrometer for the Varian instrument is equipped with two filter time constants: 0.2 and 1.2 s. The electrometer was preset at the factory at the lower value and was used throughout this study. The larger value is intended to be used only rarely and in unusually high noise situations. The 0.2-9 mode is necessary in order to detect early peaks with rapidly changing slopes, which was the situation in this study. The Perkin-Elmer or Tracor electrometers do not have this option and the time constant is not specified in the manuals, but presumably they have a time constant comparable to that of the Varian electrometer. GC Column, Gases and Standards. A 6 f t (1.8 m) X 1/8 in. (0.32 cm) 0.d. Teflon (FEP) column packed with Carbopack B/1.5% XE-60/1% H3P04obtained from Supelco, Inc. (Bellefonte, PA), was used for all measurements. Grade 0.5 helium and grade 0.1 hydrogen obtained from Airco (Research Triangle Park, NC) were used as the carrier and fuel gas, respectively. A Bendix (Louisberg, WV) clean air system was used to provide dry hydrocarbon-free air to the detectors. The standard gases used for this evaluation were H2S, COS, and SOz. These were contained in compressed gas cylinders with nitrogen as the balance gas. The concentration of each cylinder was certified by Scott Environmental Technology (Plumsteadville, PA) to within *2%. These concentrations were verified with a 0 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
2080
VALCO HASTALLOY-C HELIUM
VALVE
!\,
'
TEFLON VALVE
NUPRO STAINLESS
loG C
,--STEELTOGGLE VALVES
Table 111. Detector Conditions PerkinElmer Tracor
VACUUM PUMP HElSE VACUUMPRESSURE G A U G E
SAMPLE GAS BOTTLE
detector temp, 'C air flow, mLimin
150
H flow, m L min
6s 0.36
02.H, electrometer range output current, nA ( f u l l scale)
Figure 1. Sample injection system.
Table 11. Column Conditions
124
150 109 82
Varian Varian S.F. D.F. 150 205 0.0 144
0.30
x 100
0.23 x 100
0.16
0.5
0.1
150
166 76 144 0.31
1O"O
0.1
Varian Varian PerkinElmer Tracor S.F. D.F. column temp, " c retention times, min
so, cos H 2s
He flow, mL/min
50
50
50
50
1.28 0.82
1.30 0.80 1.32 30
1.27 0.80 1.26 30
1.28 0.80 1.27 30
1.27 30
sulfur permeation tube system available in the standards and quality assurance laboratory at RTI. The values obtained from the permeation tube system and those stated on the cylinder labels are given in Table I. The former were used for quantitative purposes in this study. GC Injection System. For a comparison of the three systems to be meaningful, it was absolutely essential that these systems be identical in all respects from the point of sample injection to the detector. The injection system used was essentially the same as the one employed by Gangwal and Wagoner (9) in an earlier study (Figure 1). This remained identical for the three systems. A Teflon sample loop was used with an internal volume of 1.0 mL. The basic components of the system are a zero dead volume Hastalloy-C valve (Valco Instruments Co., Houston, TX) for sample injection, a Heise vacuum-pressure gauge (Dresser Industries, Starford, CT) with a range of -760 to -1500 mmHg in 2-mm graduations for accurate measurement of the sample loop pressure, and a Direc-Torr vacuum pump (Sargent Welch Scientific Co., Skokie, 11) capable of evacuating the sample loop to in. (0.32 cm) o.d., less than 1 mmHg, absolute. All gas lines '/I6 in. (0.16 cm) i.d.1 and appropriate fittings extending from the gas cylinder containing the standard to beyond the sample loop were constructed of Teflon. All fittings and shutoff valves were capable of leak-proof operation at pressures from 1 mmHg, absolute, to several atmospheres. Column-Detector Interface. The Perkin-Elmer system required a 13.5-in. (34.3 cm) transfer line between the column and the detector. A ' / g in. (0.32 cm) o.d., 0.5 mm i.d. glass-lined stainless steel tubing (GLT) and a ' / I 6 in. (0.16 cm) o.d., 0.4 mm i.d. GLT were combined through a zero dead volume I/*- to l/le-in. (0.32- to 0.16-cm) reducing union (Scientific Glass Engineering, Austin, TX) to give an interface 13.5 in. (34.3 cm) long. I t was necessary to have two 90" bends in the GLT transfer line a t appropriate positions between the column and the detector. So that the column and the detector interface for the Varian and the Tracor systems would be essentialy the same (considering dead volume and pressure drop) as that for the Perkin-Elmer system, an 8.64 in. (21.9 cm) long ['/g-in. (0.32 cm) o.d., 0.5 mm i.d.1 GLT bent a t SOo in the center was used. The Varian and Tracor systems have the added advantage of using the column itself as the interface. Procedure. A uniform evaluation procedure was followed for each FPD. The Varian system was evaluated in both the singleand dual-flame modes. Identical column oven temperatures were used for each system. These temperatures were monitored by an external thermocouple to eliminate the possibility of any offset from system to system. (The Tracor system had to be set at 58 "C with the oven door slightly ajar in order to maintain a temperature of 50 "C). Conditions and flow rates maintained during the procedure are given in Tables I1 and 111. First, the column and detector temperatures were set, and then a carrier gas flow of 30 mL/min was obtained by using a pressure regulator and needle valve. The flows were measured directly
Perkln-Elmer
1 I
!
I
1
1
2 TIME l l i l 0 MINI
Flgure 2. Peak shapes.
at the detector exits by using a calibrated soap bubble flow meter. This procedure was followed for the Varian and Tracor systems. For the Perkin-Elmer system, the FPD cap was temporarily replaced with a Perkin-Elmer FID chimney (photomultiplier tube off) and the flows were measured at its exit. The air and hydrogen flows were then set so that the maximum response for a 1.12 ppm SOzsample was obtained. (For the Perkin-Elmer system the 6.41 ppm COS sample was used to maximize the response). The optimized air and hydrogen flows were measured after all necessary chromatographic measurements were completed. The air and hydrogen flows and the oxygen-to-hydrogen ratios at optimum conditions are given in Table 111. The responses of the FPD for each of the six cylinders on the four systems were measured. The pressure range used was from 50 to 2260 mmHg, absolute. The peaks were kept on scale on a 1 mV Linear Instruments (Irvine, CA) recorder using the electrometer output range and attenuator. The peak areas were measured by using a Hewlett-Packard HP3352 laboratory A/D converter-computer (Avondale, PA) system a t the 0-1 V output. Prior to sample injection, the gas cylinder regulator was first flushed for 5 min at 20 psig. Then the sample loop was flushed for 30 s prior to each sample injection. The ideal gas law was assumed for calculation of the weight of sulfur in the injected sample. Peak heights were measured manually and converted to nanoampere (na) units and corresponding peak areas were converted to an equivalent electrometer range for further analysis. These range settings are shown in Table 111. The atmospheric pressure was monitored frequently during each experiment.
RESULTS A N D D I S C U S S I O N Peak Shape. A qualitative comparison of the effects of the detectors on peak shape was made as follows. Peaks for each system (Figure 2) were obtained by operating the strip chart recorder at 10 cm/min and injecting a low concentration
ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
2081
Table IV. Linear Range and Selectivity Perkin-Elmer exptl manufr
exptl
Tracor manufr
Varian S.F. exptl manufr
Varian D.F.
exptl
manufr
linear range
so
cos
selectivity, S : H
2
io5
low high a
2.4
2.1 1.6 1.6
2
H2S
104
2.3 2.1
2.5
104
5 x 104
2.8 2.7 2.6
a
io4
a
2.7 2.8 2.8
3
io4
103 1 0 6
N o data available.
10M-
1M-
w SlWK3
8
za
- Varian S F
/
I
0. 1
01
1
10K-
+
I
Varian D F
I
1 10 100 NANOGRAMS SULFUR
l&
1
001
1
01
I
I
1 10 NANOGRAMS SULFUR
I
100
I
loo0
--
Figure 3. Calibration curve with SO2.
Figure 4. Calibration curve with H,S.
sample of carbonyl sulfide. In each case, the peak width a t one-fourth height was approximately 2.5 s. The peaks were nearly Gaussian for all but the Tracor system where a noticeable tailing of the peak was observed. Because the system as a whole remained constant for each instrument, this tailing can be attributed to dead volume or adsorption in the detector itself. As higher and higher concentrations of samples were injected, the peaks became closer to Gaussian for all systems. Saturation points were reached for the Varian and the Perkin-Elmer systems as indicated by the flat-top peaks that developed. The Tracor F P D did not show flattening of the peak tops but was limited, in fact, by the electrometer range. Linear Range. Calibration curves were developed for each system utilizing the three compounds a t concentrations ranging from below the detection limit to well into the saturation region. The curves were plotted on log-log paper as area counts vs. nanograms of sulfur (Figures 3-5) The linear ranges are given in Table IV as orders of magnitude, along with the manufacturer’s specifications. The upper and lower limits of the linear range were determined by first calculating the slope of the calibration curve using points well within the linear region and then recalculating the slope using additional points individually a t each extreme of the curve. When the slope differed by more than 5% (arbitrary value) using any given point, that point was considered out of the linear range.
The correlation coefficients and slopes were then recalculated with all permissible points that met the above criteria. The results of the linear range experiments were generally in agreement with those specified by the manufacturer. Manufacturer’s specifications for the Varian in the single-flame mode were not available because the instrument was intended to be used only in the dual-flame mode. The Tracor was still linear as the electrometer limit was reached. The Varian displayed the greatest linear range in both the single-flame and the dual-flame modes, followed closely by Tracor and then Perkin-Elmer. Of the four systems, the Perkin-Elmer deviated the greatest and the most frequently from the straight-line calibration curve. Selectivity. The selectivity, or sulfur-to-hydrocarbon ratio, is defined simply as that mass of hydrocarbon compound that gives a response in the FPD that would be equivalent to some given mass of sulfur species. Due to the quadratic dependence of the response for sulfur species and the linear response of hydrocarbons, the selectivity ratio must be qualified as being determined a t either low or high sulfur concentrations. Table IV lists the experimental selectivity ratios determined a t low sulfur concentrations along with the manufacturer’s values. Varian was the only manufacturer that made the distinction between the selectivity ratios determined at either low or high sulfur concentrations. Both ratios were available for the dual-flame mode but again no data were available for the
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 13, NOVEMBER 1980
Table V. Lower Detection Limit Perkin-Elmer exptl manufr Pg S b ppb SO, Pg S I S Pg s ppb H,S Pg SIS Pg s ppb C O S Pg S t s a
N o data available.
Table VI.
500 350 100 300 350 100 500 350 100
1500 1146
300 1700 1299 340 1000 764 200
Tracor
Varian S.F.
exptl
manufr
exptl
20 17 4 20 15 4 10 8 2
25 20 5 26 20 5 25 20 5
190 145 38 80 61 16 90 69 18
manufr a
a
a
Varian D.F. exptl manufr 200
500
153
350
40 100 76 20 110 84 22
100 500 350 100 500 350 100
Width at one-fourth height, 5 s.
Sensitivity (Slope) Perkin-Elmer
so2 H2S
cos
Tracor
Varian S.F.
Varian D.F.
slope
CC“
slope
cca
slope
cc a
slope
CCQ
1.82 1.92 2.07
0.9989 0.9795 0.9974
2.10 2.00 1.95
0.9843 0.9987 0.9955
1.92 1.85 1.84
0.9977 0.9988 0.9977
1.94 1.81 1.83
0.9981 0.9879 0.9984
linearizer available
Yes
yes
Yes
Yes
C C = correlation coefficient.
1WM-
IOM-
1M-
E 100K2
a
8
a
10K-
1ooO-
Perkin-Elmer
100-
aTracor
.Vanan S.F. +Varian D.F
with those determined experimentally. Limited information was available from Perkin-Elmer as to the detection limits for the compounds studied, but the product literature did give the LDL for hexanethiol as 1 x g of sulfur/s, which is the value listed. The experimentally determined LDLs, given as nonograms of sulfur per second, are based on a peak width at one-fourth height (17) of 2.5 s. For easy comparison to the manufacturer’s literature, other equivalent units for the LDL are based on that value and extrapolated t o a peak width of 5 s and a 1-mL sample size. For convenience, the LDLs are given for each of the three compounds as picograms of sulfur, parts per billion (ppb) of the individual compounds as well as picograms of sulfur per second. The Varian (dual flame) and Tracor instruments exceeded the manufacturer’s specifications, whereas the values determined for the Perkin-Elmer were two to three times higher. The Varian single-flame had a slightly lower LDL than the dual flame. Graphical comparisons of the LDLs of the four FPDs are shown in the calibration curves, Figures 3-5. Sensitivity. The sensitivity of an instrument or detector as used here is defined as the ability t o distinguish between small changes in concentrations. A large positive slope on the calibration curve indicates a greater degree of sensitivity. For FPD detectors in the sulfur mode, a log-log plot of response vs. mass of sulfur should presumably yield a theoretical slope of 2 (18). Table VI lists the experimentally determined slopes and the correlation coefficients for each detector with the three sulfur compounds. Each detector exhibited a slope equal to or near the value of 2 for a t least one compound. Electronic signal linearizers, which eliminate the quadratic dependence, are available for each instrument. T h e major disadvantage of these devices is that they assume a slope of 2 for the calibration curve, which can often lead t o potential errors in quantitation. This problem and its remedies have been discussed in detail by Burnett e t al. (19). Ease of Operation. Although subtle differences in features such as ease of operation and portability of the gas chromatographic unit may not significantly affect test procedures, they are important. Flow rates were more easily determined for the Tracor and Varian instruments than for the PerkinElmer. The Tracor was very sensitive to adjustments in flame conditions with occasional “flame-outs” during the optimization procedure.
l a
0 01
NANOGRAMS SULFUR
Figure 5. Calibration curve with COS.
single-flame mode. The experimental value for the Varian was found to exceed the manufacturer’s specifications. On the assumption that the values given by Perkin-Elmer and Tracor were determined a t low sulfur concentrations, the experimentally determined values either equaled or exceeded the manufacturer’s specifications. Lower Detection Limit. The lower detection limit (LDL) is defined here as the mass of sulfur which gives a response equal to twice the noise level. The noise level was determined manually from the chromatograms by measuring, in millimeters, the average noise or background oscillations that are typical of any F P D with the electrometer sensitivity set a t a high value. Table V lists the manufacture’s values along
Anal. Chem. 1980, 52, 2083-2087
Consideration must be given to the instrument's portability if it is to be used for field studies utilizing a mobile laboratory. In this laboratory, Perkin-Elmer has proven reasonably portable. The Tracor is physically the smallest of the three but exhibits some operating difficulties in the best of laboratory conditions. The Varian is designed primarily as a laboratory research instrument but presumably is rugged enough to be used for field studies. Conclusions a n d Recommendations. I t is evident from the data presented that actual experimental parameters may vary from the manufacturer's specifications as a result of laboratory conditions or differences in individual instruments. Therefore, it should be standard practice to characterize each F P D before placing any confidence in resulting data. The choice of an FPD system should be based upon careful consideration of individual laboratory needs and budgetary constraints. To select the instrument with the lowest detection limit would be unwise if the linear range does not extend to the concentration levels pertinent to a particular application. Additionally, instrumental options, such as other detectors, injection systems, and capillary accessories should be considered before purchasing.
2083
(3) Pearson, C. D.; Hines, W. J. Anal. Chem. 1977, 4 9 . 123-126. (4) Desouza, T. L. C.; Lane, D. C.; Bhatia, S. P. Anal. Chem. 1975, 47, 543-545. (5) Baumgardner, R. E.; Clark, T. A.; Stevens, R. K. Anal. Chem. 1975, 47, 563-566. ... .... (6) Bruner, F.; Liberti, A.; Possanzini, M.; Allegrini, I. Anal. Chern. 1972, 4 4 . 2070-2074. (7) Stevens, R . K.; Mullk, J. K.; O'Keefe, Krost, K. J. Anal. Chem. 1971, 4 3 , 827-831. (8) Stevens, R. K.: O'Keefe, A. E.; Ortman. G. C. Envlron. Scl. rechnol. 1969, 3, 652-655. (9) Gangwal. S . K.; Wagoner, D. E. J. Chromatogr. Scl. 1979, 77, 196-201. (10) Farwell, S . 0.;Rasmussen, R. A. J. Chromatogr. Scl. 1976, 74, 224-234. (11) Mizany, A. E. J . Chromatogr. Scl. 1970, 8 , 151-154. (12) Burnett. C. H.; Adams, D. F.; Farwell, S. 0. J . Chromatogr. Sci. 1978, 76, 68-73. (13) Sugiyama, T.; Suzuki, Y.; Takeuchi. T. J. Chromatogr. Scl. 1973, 7 1 , 639-641. (14) Shiftman, S.; Frank, C. W. Anal. Chem. 1974, 4 6 , 1804-1887. (15) Patterson, P. L.; Howe, R. L.; Abu-Shumays, A. Anal. Chem. 1978, 50, 339-344. (16) Patterson, P. L. Anal. Chem. 1978, 50, 345-348. (17) Patterson, P. L., private communication, Varian Associates, Walnut Creek, CA, Oct 1978. (18) Sugiyama, T.; Suzuki, Y.; Takeuchi, T. J . Chromatogr. 1973, 77, 309-316. (19) Burnett, C.H.; Adams, D. F.; Farwell, S. 0. J . Chromatogr. Sci. 1977, 75, 230-232.
ACKNOWLEDGMENT T h e authors wish t o thank Mr. Raymond Michie for his technical assistance.
RECEIVED for review April 22, 1980. Accepted July 28, 1980. This work was presented a t the 178th National Meeting of the American Chemical Society, Washington, DC, Sept 1979, and was supported in part by the Fuel Process Branch, U.S. Environmental Protection Agency, Research, Triangle Park, NC, under Grant No. R8040979.
LITERATURE CITED (1) Brody, S. S.; Chaney, J. E . J . Gas. Chromatogr. 1986, 4 , 42-46. (2) Pescar, R. E.; Hartman, C. H. J. Chromatogr. Sci. 1973, 11, 492-502.
High Repetition Rate Atmospheric Pressure Nitrogen Laser for Lifetime Measurements Totaro Imasaka and Nobuhlko Ishibashi" Faculty of Engineering, Kyushu University, Hakozaki, Fukuoka 8 12, Japan
organic molecules in the condensed phase (7 N 0.1-10 ns). Though a sophisticated design of a dye laser cavity enables the generation of short dye laser pulses (5-8),the nitrogen laser with subnanosecond pulses might be more useful for the generation of the shorter dye laser pulses and for the use of the nitrogen laser itself as an exciting source. The direct generation of the subnanosecond pulses (0.05-1 ns) from the nitrogen laser has been reported by several authors on the basis of transversely excited atmospheric (TEA) discharge excitation (9-18). However, very few applications of this TEA nitrogen laser to fluorometry have been reported. A sensitive photodetection system such as a photon counter combined with a repetitive pulse laser is useful for ultratrace analysis (19). However, low repetition rate operation of the nitrogen laser (-20 Hz) requires a long time for recording a spectrum or a fluorescence decay curve, and it limits the convenient use of the apparatus. A higher repetition rate nitrogen laser is commercially available (- 100 Hz); however, it requires a large amount of nitrogen gas and a high-flow-rate vacuum pump to supply fresh ion-free gas to the laser for each pulse. Through the use of a closed-cycle transverse gas flow system, operation of a nitrogen laser has been demonstrated a t the repetition rate up to 1200 Hz (20). If the system could be simplified, this type of the laser might be useful for practical use. A transverse gas flow, TEA nitrogen laser might be very
An output of 24 mW of average power at 337.1 nm is obtained from an atmospheric pressure nitrogen laser Operating at a repetltlon rate of 600 Hz through the use of a transverse gas flow system to supply fresh ion-free nitrogen gas. A laser fluorometrlc system for the lifetime measurement of nanosecond fluorescence decay is constructed by using this nitrogen laser as an exctting source and comblning wlth a fast response photomultipller. The increase of the fluorescence quantum yleld of the zinc 5-sulfo-8-quinolinolate complex by the surfactant of Zephiramine is concluded to originate partlally from lengthening the fluorescence llfetime (3.6 5.0 ns). The use of the organic solvent of 1,2-dichloroethane for the measurement of ANS-Zephiramlne Increases the fluorescence intenslty and lengthens the fluorescence lifetime.
-
Nitrogen lasers and nitrogen laser pumped dye lasers have found wide-spread appications as short intense light sources for an ultratrace analysis of organic molecules (1-4). This short pulse is essential for the application of lasers for time-resolved fluorometry. The usual nitrogen laser, which is operated a t the reduced nitrogen pressure (-50 torr), has a pulse width of several nanoseconds (3-10 ns), and it is not too short for the study of time-resolved fluorometry of the 0003-2700/80/0352-2083$01 .OO/O
C
1980 American Chemical Society
