- Email: [email protected]
Photon correlation spectroscopy for analysis of low concentration aerosols
J. Aerosol Sci., Vol. 26. Suppl 1, pp, S31-$32, 1995 Elsevier Science Ltd Printed in Great Britain 0021-8502/95 $9.50 + 0.00
Pergamon
PHOTON CORRELATION SPECTROSCOPY FOR ANALYSIS OF LOW CONCENTRATION AEROSOLS Alexander W. Willemse, Michiel A. van Drunen, llse L. Tuinman Jan C.M. Marijnissen, Henk G. Merkus and Brian Scarlett
Delft University of Technology Particle Technology Group Julianalaan 136 2628 BL Delft, The Netherlands Tel. + 31 15 78 43 72 Fax. + 31 15 78 44 52
KEYWORDS Light scattering; Aerosol sizing; Brownian motion; Diffusion; Photon Correlation Spectroscopy
INTRODUCTION Photon Correlation Spectroscopy (PCS), also referred to as Dynamic Light Scattering or Quasi-Elastic Light Scattering (QELS) has become a standard technique for measuring diffusion coefficients and particle sizes in liquid dispersions of macromolecules and submicrometer particles. In aerosols however, PCS is rarely used due to the particle number fluctuations and the large sampling times. As we have shown already (v. Drtmen et al., 1994), it can be used successfully to analyse in situ relatively high concentrated submierometer particles dispersed in gases. The technique is still limited however to hight concentration of aerosols, as shown by Itoh et al., 1991. In this paper we want to introduce a new system for the measurement of aerosols at ambient concentrations.
PHOTON CORRELATIONSPECTROSCOPY Particles suspended in a fluid exhibit Brownian motion This motion is due to collisions of the fluid molecules with the suspended particles. The smaller the particles are, the higher their frequency is. The (laser) light scattered by particles exhibiting Browuian motion will also fluctuate with time. The fluctuation frequency of the intensity of the scattered light will therefore depend on the size of the particles. Therefore the measured signal contains information about the particle movement. The main limitation for aerosol measurement however is that often the particle concentration is too low. Consequently, the fluctuations do not arise only from the Brownian motion, but also from the changing number of particles in the measuring volume (Weber et al., 1993). These number fluctuations occur when the number of particles in the measuring volume is less than about one hundred. As the measuring volume in PCS is usually 10-6cm3, the minimum particle concentration is about 108cm"3. $31
$32
A . W . WILLEMSEet aL
The fluctuations are quantified by auto-correlation. Mathematically, the autocorrelation function has the following form (Berne and Pecora, 1976):
(1)
G(z)=
where the brackets <... > indicate averaging over time and I is the scattered light intensity. In aerosols G(r) consists of three components (Chowdhurry et al., 1984): V2T 2
V2T 2
G(~) = 2 [i +exp (-2Dq2~) exp (-----7-) ] +7 exp (----7- )
(2)
0 q= ~on sin (-~)
(3)
with
where is the average number of particles in the scattering volume, D is the diffusion coefficient, q the length of the scattering vector, v the particle velocity, w the laser beam waist diameter, 7 is a geometric factor, n the refractive index of the fluid, ~'0 is the wavelength of the incoming laser beam in vacuum and 0 is the scattering angle. From equation 3 it can be seen that three components determine the autocorrelation function. The first is the Brownian motion, the second is due to the particle flow perpendicular to the scattering plane. The last term in equation 3 is caused by a fluctuating number of particles in the measurng volume. This term can be neglected when the total number of particles in the scattering volume is higher than 100, so 2 ~.
MEASURING SYSTEM In our research we have developed a measuring system for determination of particle concentrations of ambient aerosols. By electronic manipulation of the signal from the photo-multipliers, we are able to correct for the number fluctuations at low concentrations. Initial measurements have been done with latex particles in a liquid environment. At this moment we have developed a measurement system to go down to 107 particles/cm3. We are adapting the system now to the gas-phase and are trying to lower the concentration barrier below 107 particles/cm3. Both the electronic device and the first results of the PCS measurements at low aerosol concentration will be presented.
REFERENCES Berne, B. and Pecora, R. (1976) Dynamic Light Scattering. Wiley, New York. Chowdhurry, D.P., Sorensen, C.M. , Taylor, T.W., Merlin, J.F. and Lester, T.W. (1984) Application of PCS to flowing Brownian motion systems Appl. Opt.23, 4149 Drunen, M.A. van, T , inman, L., Marijnissen, J.C.M., Merkus, H.G. and Scarlett, B., (1994) Measurement of aerosols in a silicon nitride flame by optical fiber photon correlation spectroscopy, Y. Aerosol Sci., 25, 895 Itoh, M. and Takahashi, K. (1991) Measurement of aerosol particles by dynamic fight scatterlng-I Effects of non-gaussian concentration fluctuation in real-time photon correlation spectroscopy. J. Aerosol Sci., 22, 815 Weber, R., Rambeau, R., Schweiger, G. and Lucas, K. (1993) Analysis of a flowing aerosol by correlation spectroscopy: concentration, aperture, velocity and particle size effects. J. Aerosol Sci. 24, 485
Pergamon
PHOTON CORRELATION SPECTROSCOPY FOR ANALYSIS OF LOW CONCENTRATION AEROSOLS Alexander W. Willemse, Michiel A. van Drunen, llse L. Tuinman Jan C.M. Marijnissen, Henk G. Merkus and Brian Scarlett
Delft University of Technology Particle Technology Group Julianalaan 136 2628 BL Delft, The Netherlands Tel. + 31 15 78 43 72 Fax. + 31 15 78 44 52
KEYWORDS Light scattering; Aerosol sizing; Brownian motion; Diffusion; Photon Correlation Spectroscopy
INTRODUCTION Photon Correlation Spectroscopy (PCS), also referred to as Dynamic Light Scattering or Quasi-Elastic Light Scattering (QELS) has become a standard technique for measuring diffusion coefficients and particle sizes in liquid dispersions of macromolecules and submicrometer particles. In aerosols however, PCS is rarely used due to the particle number fluctuations and the large sampling times. As we have shown already (v. Drtmen et al., 1994), it can be used successfully to analyse in situ relatively high concentrated submierometer particles dispersed in gases. The technique is still limited however to hight concentration of aerosols, as shown by Itoh et al., 1991. In this paper we want to introduce a new system for the measurement of aerosols at ambient concentrations.
PHOTON CORRELATIONSPECTROSCOPY Particles suspended in a fluid exhibit Brownian motion This motion is due to collisions of the fluid molecules with the suspended particles. The smaller the particles are, the higher their frequency is. The (laser) light scattered by particles exhibiting Browuian motion will also fluctuate with time. The fluctuation frequency of the intensity of the scattered light will therefore depend on the size of the particles. Therefore the measured signal contains information about the particle movement. The main limitation for aerosol measurement however is that often the particle concentration is too low. Consequently, the fluctuations do not arise only from the Brownian motion, but also from the changing number of particles in the measuring volume (Weber et al., 1993). These number fluctuations occur when the number of particles in the measuring volume is less than about one hundred. As the measuring volume in PCS is usually 10-6cm3, the minimum particle concentration is about 108cm"3. $31
$32
A . W . WILLEMSEet aL
The fluctuations are quantified by auto-correlation. Mathematically, the autocorrelation function has the following form (Berne and Pecora, 1976):
(1)
G(z)=
where the brackets <... > indicate averaging over time and I is the scattered light intensity. In aerosols G(r) consists of three components (Chowdhurry et al., 1984): V2T 2
V2T 2
G(~) =
(2)
0 q= ~on sin (-~)
(3)
with
where
MEASURING SYSTEM In our research we have developed a measuring system for determination of particle concentrations of ambient aerosols. By electronic manipulation of the signal from the photo-multipliers, we are able to correct for the number fluctuations at low concentrations. Initial measurements have been done with latex particles in a liquid environment. At this moment we have developed a measurement system to go down to 107 particles/cm3. We are adapting the system now to the gas-phase and are trying to lower the concentration barrier below 107 particles/cm3. Both the electronic device and the first results of the PCS measurements at low aerosol concentration will be presented.
REFERENCES Berne, B. and Pecora, R. (1976) Dynamic Light Scattering. Wiley, New York. Chowdhurry, D.P., Sorensen, C.M. , Taylor, T.W., Merlin, J.F. and Lester, T.W. (1984) Application of PCS to flowing Brownian motion systems Appl. Opt.23, 4149 Drunen, M.A. van, T , inman, L., Marijnissen, J.C.M., Merkus, H.G. and Scarlett, B., (1994) Measurement of aerosols in a silicon nitride flame by optical fiber photon correlation spectroscopy, Y. Aerosol Sci., 25, 895 Itoh, M. and Takahashi, K. (1991) Measurement of aerosol particles by dynamic fight scatterlng-I Effects of non-gaussian concentration fluctuation in real-time photon correlation spectroscopy. J. Aerosol Sci., 22, 815 Weber, R., Rambeau, R., Schweiger, G. and Lucas, K. (1993) Analysis of a flowing aerosol by correlation spectroscopy: concentration, aperture, velocity and particle size effects. J. Aerosol Sci. 24, 485