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Photon correlation spectroscopy at low concentrations
J A e r o s o l S c i Vol. 30, Suppl. 1, pp. S 143-S144, 1999 1999 Published by Elsevier Science Ltd. All tights reserved Printed in Great Britain 0021-8502/99/$ - s e e front matter
Pergamon
Photon Correlation Spectroscopy at low concentrations E J Nijman, J C M Marijnissen, B Scarlett Delft University of Technology, Particle Technology Group, Julianalaan 136, 2628 BL, Delft, The Netherlands, Tel +31 15 278 4372, Fax +31 15 278 4452, e j [email protected] nl
Keywords Photon Correlation Spectroscopy, Low Concentration Aerosols Photon correlation spectroscopy (PCS) is a light-scattering technique for measuring the size of sub-micrometer particles It measures the fluctuations of the intensity of scattered laser light from an ensemble of particles exhibiting Brownian motion PCS is a standard technique for liquid dispersions, both in research and in industry Some applications in aerosols have been described (King et al., 1982, Tuinman et a l , 1997), but until now, the use of PCS in aerosols has often been limited by concentration. The upper limit is determined by multiple scattering of the incident light, and the lower limit is determined by the number of particles in the measuring volume, which is the intersection between the incident laser beam and the detection optics. Lowering of this detection limit would create new fields of application for PCS, such as measuring particle size in ambient air With our PCS setup, we have been able to measure the particle size down to about 500 particlesJcm3. In order to explain these results, we look at two concentration regions separately. The first concentration region is that in which the measuring volume contains up to about 100 particles At such concentrations, the number of particles in the measuring volume is no longer a constant but changes because of diffusion in and out of the measuring volume The average scattered intensity therefore, will also fluctuate in time. An example of measured intensity fluctuations is given in Figure 1
Intensity
10-3 % v/v 10-4 % v/v
10 -3 % v / v
Figure I Measured scattered intensity o f 501 nm latex particles at different concentrations. Note the slow intensity fluctuations at 10"~ and 10 .6 % v/v.
10"6 % v/v
10.7 % v/v
t=60s
S!43
S144
Abstracts of the 1999EuropeanAerosolConference
From the high frequency intensity fluctuations, the particle size can be determined. The timescale of the low frequency intensity fluctuations is in agreement with the timescale of particles diffusing through the measuring volume. In combination with Poisson statistics to describe the number of particles in the measuring volume, we can use these signals to determine the concentration. New methods of signal processing will be used for this. The second concentration region is that in which the measuring volume contains, on average, less than one particle. The only light that will reach the detector is then light that is scattered by two particles, whereby the first particle is in the laser beam, and the second particle is in the detected "oeam'. An example is given in Figure 2.
laser beam Figure 2. The laser light is first scattered by a particle in the laser beam. The light scattered under an angle 01 is then scattered by a second particle under an angle 02.
~tor
At a scattering angle of 90 °, PCS measurements at concentrations down to 0.001 particle in the measuring volume yielded correct particle sizes. In order to explain this and to predict the results at other scattering angles, we are working on a model to describe the intensity fluctuations for such double scattered light.
References King, G.B., Sorensen, C.M., Lester, T.W. and Merklin, J.F., Appl. Optics (1982) 21 976-978 Tuinman, I.L., Van Drunen, M.A., Marijnissen, J.C.M., ScarieR, B. and Schoonman, £, Aerosol Sci. Techn. (1997) 26 55-73
Pergamon
Photon Correlation Spectroscopy at low concentrations E J Nijman, J C M Marijnissen, B Scarlett Delft University of Technology, Particle Technology Group, Julianalaan 136, 2628 BL, Delft, The Netherlands, Tel +31 15 278 4372, Fax +31 15 278 4452, e j [email protected] nl
Keywords Photon Correlation Spectroscopy, Low Concentration Aerosols Photon correlation spectroscopy (PCS) is a light-scattering technique for measuring the size of sub-micrometer particles It measures the fluctuations of the intensity of scattered laser light from an ensemble of particles exhibiting Brownian motion PCS is a standard technique for liquid dispersions, both in research and in industry Some applications in aerosols have been described (King et al., 1982, Tuinman et a l , 1997), but until now, the use of PCS in aerosols has often been limited by concentration. The upper limit is determined by multiple scattering of the incident light, and the lower limit is determined by the number of particles in the measuring volume, which is the intersection between the incident laser beam and the detection optics. Lowering of this detection limit would create new fields of application for PCS, such as measuring particle size in ambient air With our PCS setup, we have been able to measure the particle size down to about 500 particlesJcm3. In order to explain these results, we look at two concentration regions separately. The first concentration region is that in which the measuring volume contains up to about 100 particles At such concentrations, the number of particles in the measuring volume is no longer a constant but changes because of diffusion in and out of the measuring volume The average scattered intensity therefore, will also fluctuate in time. An example of measured intensity fluctuations is given in Figure 1
Intensity
10-3 % v/v 10-4 % v/v
10 -3 % v / v
Figure I Measured scattered intensity o f 501 nm latex particles at different concentrations. Note the slow intensity fluctuations at 10"~ and 10 .6 % v/v.
10"6 % v/v
10.7 % v/v
t=60s
S!43
S144
Abstracts of the 1999EuropeanAerosolConference
From the high frequency intensity fluctuations, the particle size can be determined. The timescale of the low frequency intensity fluctuations is in agreement with the timescale of particles diffusing through the measuring volume. In combination with Poisson statistics to describe the number of particles in the measuring volume, we can use these signals to determine the concentration. New methods of signal processing will be used for this. The second concentration region is that in which the measuring volume contains, on average, less than one particle. The only light that will reach the detector is then light that is scattered by two particles, whereby the first particle is in the laser beam, and the second particle is in the detected "oeam'. An example is given in Figure 2.
laser beam Figure 2. The laser light is first scattered by a particle in the laser beam. The light scattered under an angle 01 is then scattered by a second particle under an angle 02.
~tor
At a scattering angle of 90 °, PCS measurements at concentrations down to 0.001 particle in the measuring volume yielded correct particle sizes. In order to explain this and to predict the results at other scattering angles, we are working on a model to describe the intensity fluctuations for such double scattered light.
References King, G.B., Sorensen, C.M., Lester, T.W. and Merklin, J.F., Appl. Optics (1982) 21 976-978 Tuinman, I.L., Van Drunen, M.A., Marijnissen, J.C.M., ScarieR, B. and Schoonman, £, Aerosol Sci. Techn. (1997) 26 55-73