Anal. Chem. 1988. 6 0 , 2487-2493
locities have been shown to yield faster mixing. However, the driving pressures necessary for smaller capillary tips and their frequent clogging tends to offset any immediate advantages gained by using small (99.5%. Pyrene was dissolved at a concentration of about 1.5 mM in methanol (p.a., Merck) and added t o the suspensions; the concentration of methanol in the suspensions was always less than 1% (w/w). In all measurements the particle concentration was 2.0 (=tO.l) mg of particles/g of HzO. The particles were suspended in water with the aid of 1.2-1.8 mM tetradecyl sulfate, the concentration used in a particular experiment is given in the figure legend. It has been shown (18, 19) that ground-state dimers of pyrene can be formed on solid surfaces. However, it is possible to differentiate monomers from the dimers by an analysis of the excitation spectra with observation in the monomer (383 nm) and excimer (470 nm) regions. We have shown in a previous paper (15)that no ground-state dimers were detected on RP-18 and RP-2. In the present work the concentration of pyrene on the surfaces was much higher in some experiments. The same type of analysis performed at room temperature on the systems with the highest pyrene concentration gave no evidence of ground-state dimers. Steady-State Fluorescence Spectrometry. All fluorescence spectra were obtained with a Perkin-Elmer Model MPF-3 scanning fluorescence spectrophotometer,using an excitation slit width of 2 nm and an emission slit width of 2 nm for the monomer emission and usually 6 nm for the excimer emission. The check for ground-state dimers was performed with a slit width of 2 nm in both monochromators. All measurements were performed in the right angle mode, usually with the excitation wavelength set at 337 nm. A correction
ANALYTICAL CHEMISTRY, VOL. 60, NO. 22, NOVEMBER 15, 1988
Table I. Influence of the Concentration of Surfactant on Emission Intensity total concn of surfactant, mmol/L of type of particle"
H20
RP-18 RP-18 RP-18 RP-18 RP-18 RP-8 RP-8 RP-8 RP-2 RP-2 RP-2 RP-2
0.869 1.70 2.22
2.73 3.24 0.24 0.92 1.58 0.073 0.51 0.96 1.77
2489
1~11~
em intens (470 nm)/em intens (383 nm) 1.36 1.46 1.43 1.43 1.53 6.96 7.61 8.85 1.98 2.61 3.02 3.34
a Particle concentration: 2 mg/g of HzO. bExcitation wavelength: 337 nm.
RP-la
for the light scattered by the particles was made by measuring a suspension with the same concentration of particles and surfactant. Fluorescence Decay Measurements. The argon deaerated sample in a 1-cm cuvette was excited with a N2 laser at 337 nm with a pulse length
