Anal. Chem. 1999, 71, 3265-3272
Size Determination of Diesel Soot Particles Using Flow and Sedimentation Field-Flow Fractionation Won-Suk Kim, Young Hun Park, Ji Young Shin, and Dai Woon Lee*
Department of Chemistry, Yonsei University, Seoul 120-749, Korea Seungho Lee
Department of Chemistry, Hannam University, Taejon 306-791, Korea
The applicability of field-flow fractionation (FFF) was investigated for determination of size and size distribution of diesel soot particles. A sample preparation procedure was developed for FFF analysis where soot particles are recovered from filters in an ethanol bath sonicator, and then they are dispersed in water containing 0.05% Triton X-100 and 0.02% NaN3. Mean diameters obtained from sedimentation FFF (SdFFF) and flow FFF (FlFFF) agree well with each other and are in good agreement with diameters obtained from photon correlation spectroscopy (PCS) and scanning electron microscopy. The relative error was less than 11%. Data show diesel soot particles have broad size distributions ranging from 0.05 up to ∼0.5 µm with the mean diameters between 0.1 and 0.2 µm. The use of FlFFF is more convenient as FlFFF fractograms can be converted directly to size distributions, while the conversion of the SdFFF fractogram needs the particle density information. The density needed for SdFFF analysis was obtained by combining the SdFFF retention data with the PCS size data. For samples whose density is known, SdFFF may be more useful as SdFFF provides a wider dynamic range than FlFFF under constant field strength. The automobile has been regarded as one of the main sources of environmental pollution. There are several classes of environmental pollutants emitted from automobiles. The first class is the exhaust gases including carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbons (HC). The second class is inorganic compounds containing heavy metals. The third class is organic compounds such as polycyclic aromatic hydrocarbons (PAHs), which are known as potent mutagens and carcinogens. The automobiles emit particulate matter (“soot”) normally having diameters of less than ∼2.5 µm.1 These combustion-generated particulates consist of solid carbonaceous particles (carbon black) that are often associated with various compounds including the inorganic or organic compounds mentioned above. Diesel engines are typically used in buses, trucks, construction equipment, stationary power sources, etc. Diesel engines have (1) Cho, K. R.; Eom, M. D.; Kim, C. C. A study on smoke control technology of diesel vehicle. Final report of the National Institute of Environmental Research, Korea, 1988; pp 3-20. 10.1021/ac990048z CCC: $18.00 Published on Web 06/24/1999
© 1999 American Chemical Society
advantages over gasoline engines because they have a higher thermal efficiency, producing less CO and HC.2 Diesel engines are generally more durable than gasoline engines.3 Heavy-duty diesel engines are considered to be one of the major sources of particulate matter, mainly fine carbonaceous particles. Automobile soot particles cause atmospheric visibility impairment and may also result in permanent deposition of particles (especially those with diameters smaller than ∼0.3 µm) in the human respiratory tract once inhaled.1,4 Its final destiny and probability of deposition in the human body depend on the particle size and size distribution.4,5 Accurate determination of soot particle size is thus important. As the role and function of submicrometer particles in human health and the environment are of increasing interest, a number of methods6-8 have been studied to determine the size and size distribution of automobile soot particles. Unfortunately, size analysis of engine soot particles remains troublesome. First, there is no proper standard for size measurement of soot particles. Second, the physical properties of soot particles are not accurately known. Finally, the sample preparation procedures are usually time-consuming and multistep, resulting in poor reproducibility in size measurement. Automobile soot particles are in irregular shapes (usually in chain forms) formed by aggregation of several tens to hundreds of primary spherical particles.9 Primary particles are formed by coagulation of roughly spherical fine particles (∼3-4 nm in diameter). Existing techniques for determining the size and the size distribution of environmental particulate matter include scanning electron microscopy (SEM), photon correlation spectroscopy (PCS), and electrical aerosol analysis (EAA). SEM has a unique advantage over other techniques in that it provides images of (2) Cuddihy, R. G.; Griffith, W. C.; McClellan, R. O. Environ. Sci. Technol. 1984, 18, 14A-21A. (3) John, H. J.; Thomas, M. B. Diesel Particulate Emissions: Measurement Techniques, Fuel Effects and Control Technology; John Wiley & Sons: New York, 1992; pp 163-171. (4) Vincent, J. H. Analyst 1994, 119, 19-25. (5) Vincent, J. H. Analyst 1994, 119, 13-18. (6) Pitts, J. N.; Finlayson-Pitts, B. J. Atmospheric Chemisty: Fundamentals and Experimental Techniques; John Wiley & Sons: New York, 1986; Chapter 12. (7) Jambers, W.; De Bock, L.; Van Grieken, R. Analyst 1995, 120, 681-692. (8) Kirkland, J. J.; Liebald, W.; Unger, K. K. J. Chromatogr. Sci. 1990, 28, 374378. (9) Ishiguro, T.; Takatori, Y.; Akihama, K. Combust. Flame 1997, 108, 231234.
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particles from which the size of individual particles can be measured. However, the size data obtained from SEM may not be accurate because the sample suspension is dried and put into a high-vacuum chamber during measurement, which may cause shrinkage and agglomeration of particles. SEM is also timeconsuming and requires experience and skill. PCS is a widely used technique for the determination of particle size in the range from a few nanometers up to a few micrometers.10 PCS is fast and easy to operate. It usually provides accurate sizing for particulate samples having relatively narrow size distributions. However, it is difficult to obtain accurate size distributions for samples having broad size distributions or multimodal distributions.11,12 EAA (or mobility analysis) is a technique that is particularly useful for size determination of aerosol particulates.6 The size measured by EAA is the electrical mobility-equivalent diameter, that is, the diameter of a spherical particle having the same electrical mobility as the sample particle. EAA has a unique advantage over other techniques in that it can directly take in aerosol particles. Therefore, EAA can be directly connected to the automobile exhaust, allowing on-line measurements. Thus, the sample preparation step can be eliminated. Neverzertheless, it has been recognized that the reproducibility of EAA may be doubtful.3,6 Field-flow fractionation (FFF) is an elution technique that is useful for high-resolution separation and characterization of a broad range of colloidal particles,13,14 polymers,15,16 and biological macromolecules.17,18 FFF uses a thin (usually 50-500 µm thick) ribbonlike flow channel that provides a well-defined parabolic (laminar) flow profile. An external field is applied perpendicularly to the flow axis and forces the sample particles (or molecules) to migrate toward the accumulation wall of the channel. An equilibrium layer is established as a result of a counteraction between the field-driven migration and the diffusion of sample components away from the accumulation wall. Parabolic channel flow carries sample components down the channel. The downstream migration velocity of a sample component depends on the thickness of the component’s layer. FFF is divided into a few subtechniques according to the type of the external field employed. Sedimentation FFF (SdFFF) uses a centrifugal acceleration, and it is particularly useful for the analysis of particulate samples of colloidal size.19 Flow FFF (FlFFF) uses a field formed by flowing the carrier liquid across the channel thickness through the semipermeable membrane.20 Because of this cross-flow force field, larger components move closer to the
accumulation wall and are intercepted by slower flow streams. Thus, elution in flow FFF is increasing in order of component size (or hydrodynamic volume). FlFFF has been used for characterization of various water-soluble polymers as well as particulates.21 One of the merits of FFF over other particle-sizing techniques is that it actually separates particles according to their size. FFF is particularly useful for characterizing macromolecules or particles as the openness of the channel minimizes shear degradation and sample adsorption. Each of the FFF subtechniques has unique advantages and disadvantages for particular application. The potential of SdFFF for analysis of diesel soot particles has already been shown.8 The goal of this study was to develop an efficient sample preparation method for FFF analysis of soot particles and to investigate and compare the capabilities of SdFFF and FlFFF for accurate determination of particle size distribution of diesel soot particles. THEORY In FFF, retention time, tr, is given by13
tr ) (t0w/6kT)F
where t0 is the channel void time, w the channel thickness, k the Boltzmann constant, T the temperature, and F the force exerted on sample components by the external field. Equation 1 shows the retention time, tr, is directly proportional to the force, F. In the normal mode of SdFFF, the force, F, is given by22
F ) πd3∆FG/6
3266 Analytical Chemistry, Vol. 71, No. 15, August 1, 1999
(2)
for spherical particles, where d is the particle diameter, ∆F the density difference between the particle and carrier, and G the centrifugal acceleration. Combining eqs 1 and 2 gives
d ) [(36kT/π Gwt0∆F)tr]1/3
(3)
Thus using eq 3, d can be determined by measuring retention time, tr, if the density of particle (thus ∆F) is known. Or the particle density can be determined if d is known for each retention time, tr. Rearranging eq 3 gives
∆F ) 36kTtr/πGwt0d3 (10) Phillies, G. D. J. Anal. Chem. 1990, 62, 1049A-1057A. (11) Barth, H. G.; Sun, S.-T. Anal. Chem. 1995, 67, 257R-272R. (12) Lee, S.; Rao, S. P.; Moon, M. H.; Giddings, J. C. Anal. Chem. 1996, 68, 1545-1549. (13) Giddings, J. C. Science 1993, 260, 1456-1465. (14) Park, Y. H.; Kim, W. S.; Lee, D. W. J. Liq. Chromatogr. Relat. Technol. 1997, 20, 2599-2614. (15) Benincasa, M. A.; Giddings, J. C. Anal. Chem. 1992, 64, 790-798. (16) Ratanathanawongs, S. K.; Shiundu, P. M.; Giddings, J. C. Colloids Surf. A: Physicochem. Eng. Aspects 1995, 105, 243-250. (17) Barman, B. N.; Ashwood, E. R.; Giddings, J. C. Anal. Biochem. 1993, 212, 35-42. (18) Zhang, J.; Williams, P. S.; Myers, M. N.; Giddings, J. C. Sep. Sci. Technol. 1994, 29, 2493-2522. (19) Giddings, J. C.; Ratanathanawongs, S. K.; Moon, M. H. KONA: Powder Part. 1991, 9, 200-217. (20) Giddings, J. C.; Yang, F. J. F.; Myers, M. N. Science 1976, 193, 12441245.
(1)
(4)
Equation 4 can be used to determine the density of a sample using SdFFF provided the diameter d is known. In case of FlFFF, F is given by22
F ) 3πηUd
(5)
where η is the viscosity of carrier and U the field-induced velocity (or cross-flow velocity). Combining eqs 1 and 5 gives (21) Giddings, J. C.; Benincasa, M. A.; Liu, M. K.; Li, P. J. Liq. Chromatogr. 1992, 15, 1729-1747. (22) Giddings, J. C. Anal. Chem. 1995, 67, 592A-598A.
d ) (2kTV0/t0w2πηVc)tr
(6)
where t0 is the channel void time, Vc the cross-flow rate, and V0 the channel volume. Although both SdFFF and FlFFF can be used for particle size determination, there are some fundamental differences between them. The size measured by SdFFF is the diameter of a spherical particle having the same effective mass with the sample particle, while the size measured by FlFFF is the hydrodynamic diameter. Theoretically, in spherical particles, both SdFFF and FlFFF yield the same results ideally. However, for nonspherical particles (such as soot particles), the results from SdFFF and FlFFF could be different. SdFFF provides higher size-selectivity than FlFFF, as the force F is proportional to d3 in SdFFF instead of d as in FlFFF. As seen from eq 3, the use of SdFFF requires the density of the sample to be known, while that of FlFFF does not need the density information. Thus, FlFFF could be more convenient for size determination of unknown samples. However, the use of FlFFF may not be always advantageous, particularly for highly adsorptive materials such as carbon black, due to possible adsorption of sample onto the channel membrane. For size determinations in this work, eq 3 was used for SdFFF and eq 6 for FlFFF, respectively. The density needed for SdFFF calculation was obtained by combining SdFFF and PCS, where the diameter is first determined by PCS and then used to calculate the density using eq 3. EXPERIMENTAL SECTION Standard Materials. All polystyrene latex standards used in this work were obtained from Duke Scientific (Palo Alto, CA). For test separations in the FFF system, the standards were diluted with the carrier ∼100-200 times and then mixed together to prepare a standard mixture. CAB-O-JET (Cabot Corp., Billerica, MA) is an aqueous dispersion of black pigment intended to be used in ink-jet printers. CAB-O-JET was filtered through a 1-µm disposable syringe filter and was used as a reference material for size measurement of diesel soot particles. The manufacturer provided the mean diameter of CAB-O-JET as 0.125 µm. Soot Collection and Sample Preparation. A sample preparation procedure was developed for fast recovery of soot particles with minimum loss and degradation. First, diesel soot particles were collected by using three filters placed in the diesel engine exhaust line as shown in Figure 1. The steel net filter is placed first, followed by the Horiba filter, and then by the thimble-type filter. The soot samples are then prepared in the following three steps: (1) recovery of soot particles from filters (recovery step), (2) removal of soluble components (extraction step) from the soot particles, and (3) dispersion of soot particles in an appropriate solvent system (dispersion step). The first step (recovery step) involves a selection of a solvent system for complete recovery of the soot particles from the filters. Among various solvents tested (water, ethanol, hexane, dichloromethane, etc.), ethanol was chosen for it seems to provide the most efficiency in recovery (recovered amount per time). The second step (extraction step) is to remove the compounds associated with the soot particles. This step is necessary as our earlier study indicated that the soluble compounds might promote the aggregation of the soot particles.23 The final step (dispersion
step) is to obtain a homogeneous dispersion of soot particles for size determination using FFF or other techniques. Optimum dispersion of soot particles requires formation of a continuous monolayer of dispersing agent on the surface of the particles.24 Among various cationic, nonionic, and anionic surfactants tested, Triton X-100 (polyoxyethylene p-tert-octylphenol), a nonionic surfactant, was chosen as a dispersing agent for soot particles in this study. In the first step (recovery step), each filter is bath-sonicated for 20 min in 50 mL of ethanol for recovery of soot particles from the filter. This step is repeated several times to achieve complete recovery of soot particles. For the next two steps (extraction and dispersion steps), two different procedures (named the “focusing method” and “hexane-decant method”, respectively) were used. In the focusing method, the mixture of ethanol and soot particles obtained in the first step (recovery step) is mixed with n-hexane and water containing 0.05% Triton X-100 in a separatory funnel in 1:1:0.5 (v/v) ratio. Shaking the funnel for 20 min resulted the soot particles populated (“focused”) at the boundary between two liquid layers of n-hexane and the mixture of ethanol and water) as seen in Figure 2. This extraction step is repeated three or four times. After the removal of ethanol and n-hexane, the particle-containing aqueous phase was heated at 60 °C for ∼1 h on a stirring hot plate for complete removal of the remaining n-hexane and ethanol. Finally 2-3 mL of water containing 0.05% Triton X-100 and 0.02% NaN3 is added to the aqueous phase, and then the mixture is sonicated for 10 min in a probe-type sonic dismembrator for the dispersion of the particles. In the hexane-decant method, the mixture of 50 mL of ethanol and soot particles obtained in the first step is heated in a water bath (70 °C) for 8 min until 4-5 mL of ethanol remain. Then 20 mL of n-hexane is added, and the mixture is heated for 30 min on a stirring hot plate (60 °C) for extraction of organic compounds. After decanting n-hexane, the extraction step is repeated three times. Then 2-3 mL of water containing 0.05% Triton X-100 and 0.02% NaN3 is added, and the mixture is sonicated for 10 min in a probe-type sonic dismembrator for the dispersion of the particles. Finally, the mixture is heated on a stirring hot plate for complete removal of n-hexane. Five diesel soot samples were prepared for this study, and are listed in Table 1. Soots 1-4 are samples prepared from various filters that were placed in the exhaust line of a Hyundai Motor Co. (HMC) diesel engine running at a constant rpm between 3000 and 5000 rpm. Soots 1, 3, and 4 were prepared by the focusing method from a Horiba, steel net and thimble-type filter, respectively. Soot 2 was prepared by the hexane-decant method from a Horiba filter. Soot 5 was prepared by the focusing method from a glass fiber membrane filter obtained from the Motor Vehicle Emission Research Laboratory (MVERL) of Korea, where the glass fiber membrane filter was placed in the exhaust line of a diesel engine running in a D-13 mode. The D-13 mode is an exhaust gas restriction mode for heavy-duty diesel engines that has been used in Korea since 1996.25 This mode consists of 13 different driving patterns according to the engine speed, load rate, and driving time. Soot particles from the diesel engine are collected using the engine dynamometer and minidilution tunnel (23) Park, Y. H. Ph.D. Thesis, Yonsei University, Korea, 1997. (24) Foster, J. K.; Sims, E. S. Polym. Paint Color J. 1994, 184, 314-316. (25) Lim, C. S. Master Thesis, Konkuk University, Korea, 1996.
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Figure 1. Schematic diagram of a diesel engine emission measuring instrument.
Figure 2. A picture of diesel soot particles focused at the phase boundary. Table 1. Carbon Black Standards and Diesel Soot Samples Used in This Study soot sample
filter type
datea
sampling method
source
1 2 3 4 5
Horiba Horiba steel net thimble type glass fiber membrane
1/2/98 1/2/98 1/2/98 1/2/98 2/28/98
focusing decant focusing focusing focusing
HMCb HMC HMC HMC MVERLc
a Sample preparation date. b Hyundai Motor Co. c Motor Vehicle Emission Research Laboratory in Korea.
(MDT). The exhaust particles enter the primary dilution tunnel where the particles are mixed with clean air. Particles continue to travel down the filter holder equipped with the Teflon-coated glass fiber membrane filter. The filter’s face temperature was maintained below 50 °C and the total sampling time was ∼25 min. Field-Flow Fractionation. Two subtechniques of FFF (SdFFF and FlFFF) were employed in this study. The SdFFF system used in this study is the model S101 colloid/particle fractionator purchased from FFFractionation, LLC (Salt Lake City, UT). The channel length, breadth, and thickness are 89.1, 2.0, and 0.0254 3268 Analytical Chemistry, Vol. 71, No. 15, August 1, 1999
cm, respectively. The rotor radius is 15.1 cm. The channel volume, measured as the elution volume of acetone, is 4.49 mL. The elution of particles was monitored using a M720 UV/visible detector (Young-In Scientific Co., Seoul, Korea) with the wavelength fixed at 254 nm. The detector signal was processed using the FFF software obtained from FFFractionation, LLC. Samples were injected using a Rheodyne model 7125 loop injector (Rheodyne, Inc., Cotati, CA). The injection volume was between 10 and 20 µL, depending on the sample concentration. Carrier liquid was doubly distilled and deionized water containing 0.05% (w/v) Triton X-100 and 0.02% (w/v) NaN3. Triton X-100 was added as a dispersing agent, and NaN3 as a source of salt as well as a bactericide. The pH and the ionic strength of the carrier liquid were 8.0 and 0.0041 M, respectively. The pH of the carrier was adjusted by adding the appropriate amount of 0.1 N NaOH. All SdFFF experiments were performed by using a power-programming mode26 to reduce the time for analysis. The FlFFF system used in this study is a homemade system that is similar to the Universal Fractionator model F-1000 (FFFractionation, LLC). The length and breadth of the channel are 28.1 and 2.0 cm, respectively. The effective channel thickness was determined from the channel volume measured by the rapidbreakthrough method.27 The channel volume was measured to be 1.21 mL, from which the effective channel thickness was calculated to be 0.0230 cm. The channel membrane is YM-10 (Amicon, Beverly, MA), a regenerated cellulose having the cutoff pore size of MW 10 000. The injection and detection of the sample along with the data analysis were accomplished in the same manner as in SdFFF. Carrier liquid for FlFFF is the same as that used in SdFFF except that the concentration of Triton X-100 is lowered to 0.01% to prevent membrane distortion.23 Photon Correlation Spectroscopy and Electron Microscopy. The PCS system used in this study is a model 4700C (26) Williams, P. S.; Giddings, J. C. Anal. Chem. 1987, 59, 2038-2044. (27) Giddings, J. C.; Williams, P. S.; Benincasa, M. A. J. Chromatogr. 1992, 627, 23-35.
Table 2. Diameters of Polystyrene Standards Measured by SdFFF and FlFFF diameter (µm) nominal (µm)
Figure 3. FIFFF (a) and SdFFF (b) separations of polystyrene latex standards. FIFFF conditions: channel flow rate, 6.00 mL/min; crossflow rate, 0.89 mL/min; stop-flow time, 2 min. SdFFF conditions: initial field strength, 1450 rpm; final field strength, 75 rpm; predecay time, t1, 7 min; stop-flow time, 10 min; flow rate, 4.05 mL/min.
purchased from Malvern Instruments Ltd. (Worcestershire, U.K.), which uses He-Ne laser (632.8 nm) as the light source. Experimental parameters were as follows: temperature 25 °C, viscosity 0.009 cp, refractive index 1.33, and collection angle 90°. A JEOL model JSEM-5410LV (Tokyo, Japan) high-resolution scanning electron microscope (SEM) and an Hitachi model H-600 (Tokyo, Japan) a transmission electron microscope (TEM) were used for electron microscopy of soot particles. RESULTS AND DISCUSSION SdFFF and FlFFF systems were tested with mixtures of narrow polystyrene (PS) latex standards. Figure 3a shows FlFFF separation of four PS standards having diameters of 0.064, 0.155, 0.222, and 0.343 µm. A baseline separation was achieved within ∼20 min. For FlFFF, the channel flow rate was 6.0 mL/min, cross-flow rate 0.89 mL/min, and relaxation time 2 min. Figure 3b shows SdFFF separation of four PS standards having diameters of 0.222, 0.343, 0.502, and 0.705 µm. The separation time was ∼50 min. A power programming was used for SdFFF, with an initial field strength of 1450 rpm, a final field strength of 75 rpm, a time lag duration of 7 min, a relaxation time of 10 min, and a flow rate of 4.05 mL/ min. Both FlFFF and SdFFF showed good separations.
measured (µm)
relative error (%)
0.222 0.343 0.502 0.705
SdFFF 0.232 0.351 0.513 0.732
4.5 2.3 2.2 3.8
0.064 0.155 0.222 0.343
FlFFF 0.070 0.161 0.232 0.333
9.4 3.9 4.5 2.9
Table 2 shows diameters determined by FlFFF and SdFFF for each of the standards shown in Figure 3. Diameters were calculated by using eq 3 (for SdFFF data) or eq 6 (for FlFFF data) with the retention time (tr) determined from Figure 3. All SdFFF and FlFFF data show reasonable agreement with nominal values provided by the manufacturer. The relative error was of lower than 10%. Figure 4(a) shows fractograms of four diesel soot samples (soots 1-4) and the CAB-O-JET obtained by FlFFF. Experimental conditions were the same for all samples with a channel flow rate of 6.25 mL/min and cross-flow rate of 0.48 mL/min. As explained with eq 6, a FlFFF fractogram can be directly converted to a size distribution. It is noted that the fractograms of soots 1 and 2 show a slight rise followed by a rather rapid return to baseline at the end of the fractograms, which is a symptom of the steric inversion phenomenon.28 This steric inversion phenomenon indicates soots 1 and 2 contain some large particles. Figure 4b shows size distribution curves obtained from the fractograms shown in Figure 4a. Although the detector is intended for monitoring the absorption of light at 254 nm, the light scattering of particles also contributes to the attenuation of light. To accurately convert a fractogram to a size distribution, the detector response must be corrected by taking the size dependence of the light scattering into account. No light-scattering correction was performed in this study, as all the necessary optical parameters for the light-scattering correction were not available for the soot particles. As shown in Figure 4b, soot particles recovered from the Horiba filter (soots 1 and 2) are larger than those from the steel net (soot 3) or those from the thimble-type filter (soot 4). The quantity of the soot particles recovered from the Horiba filter was also higher than that from either the steel net or the thimbletype filter. Soots 1 and 2 are particles recovered from the same type of filter (Horiba) but with different preparation methods. Soot 1, prepared by the focusing method, has a smaller size distribution than soot 2, prepared by the hexane-decant method. The heating after the recovery step for the removal of ethanol in the hexanedecant method may have promoted aggregation of soot particles. The mean diameters were determined for each sample as the first moments of the size distribution curves. For all FlFFF and SdFFF experiments, each sample was injected at least three times and the relative errors in the measured mean diameters were lower than (5%. (28) Lee, S.; Giddings, J. C. Anal. Chem. 1988, 60, 2328-2333.
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Figure 4. FIFFF elution curves (a) and size distributions (b) of diesel soot samples. Experimental conditions: channel flow rate, 6.25 mL/ min; cross-flow rate, 0.48 mL/min; stop-flow time, 90 s.
Figure 5. SdFFF elution curves (a) and size distributions (b) of diesel soot samples. Experimental conditions: initial field strength, 1680 rpm; final field strength, 75 rpm, predecay time, t1, 4.5 min; stopflow time, 10 min; flow rate, 1.04 mL/min.
Figure 5a shows the fractograms of five diesel soot samples (soots 1-5) and the CAB-O-JET obtained by SdFFF. For SdFFF, a power programming was used with an initial field strength of 1680 rpm, a final field strength of 75 rpm, a time lag duration t1 of 4.5 min, a stop-flow time of 10 min, and a flow rate of 1.04 mL/ min. Unlike in FlFFF (Figure 4), SdFFF fractograms of soots 1 and 2 do not show any symptoms of the steric inversion phenomenon, which indicates the dynamic range of SdFFF extending to a larger size than FlFFF under the conditions used in this study. As explained earlier with eq 3, density is needed to obtain a size distribution from a SdFFF fractogram. Density can be calculated using eq 4 if the diameter d is known. Table 3 shows densities of soots 1-5 calculated from eq 4 using the diameters determined by PCS for each sample. The same samples as those used to obtain the fractograms shown in Figure 5a were used in PCS measurements. Diameters obtained from FlFFF (Figure 4) were not used for density calculations, as they may not be accurate due to the steric inversion phenomenon at the higher end of the distributions. As shown in Table 3, the density values range from 1.21 to 1.36 with the average value of 1.28. However, the differences in density values among soot samples are small, and a density value of 1.3 is used for all samples in this study. It is also noted that the density is assumed to be constant over the
Table 3. Density of Diesel Soot Determined Using PCS and SdFFF
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soot sample
density (g/mL)
1 2 3 4 5
1.36 1.36 1.21 1.25 1.24
av
1.28
whole range of the size distribution of the soot samples. Figure 5b shows size distributions obtained from the fractograms shown in Figure 5a. Overall, SdFFF data look similar to those obtained from FlFFF (see Figure 4b). As expected, diesel soot particles have broad size distributions ranging from 0.05 to ∼0.5 µm, which covers the size range that could result in permanent deposition in the human respiratory tract once inhaled. Like SdFFF and FlFFF, PCS is a particle-sizing technique that does not require system calibration. When tested with a narrow polystyrene standard having a nominal diameter of 64 nm, the mean diameter determined by a unimodal analysis of PCS data showed a good agreement with the nominal value. The unimodal analysis assumes the size distribution is unimodal and usually yields a reliable measure of the mean diameter when the sample
Table 4. Mean Diameters of Diesel Soot Samples Measured by FFF and PCS diameter (µm) measured from peak max sample
FlFFF
SdFFF
CAB-O-JET soot 1 2 3 4 5
0.081
0.069
0.171 0.193 0.110 0.106 0.112
0.203 0.215 0.118 0.110 0.140
first moment PCSa
0.174 0.206 0.116 0.117
FlFFF
SdFFFb
PCSc
0.091
0.074
0.113
0.196 0.219 0.130 0.122 0.159
0.217 0.233 0.129 0.126 0.161
0.203 0.217 0.145 0.132
a PCS data obtained by collecting a fraction from FlFFF runs at the peak maximum. b SdFFF data obtained using a density of 1.86 g/mL for the CAB-O-JET and 1.30 g/mL for the soot samples. c PCS data for whole sample.
Figure 7. SEM pictures of fractions collected at the peak maximum of FIFFF fractogram for soot 2 (a) and soot 5 (b).
Figure 6. SEM pictures of original sample of soot 1 (a) and the fraction collected at the peak maximum of FIFFF fractogram for the same sample (b).
has a relatively narrow size distribution. For all PCS determinations of soot samples, unimodal analysis was employed, except for soots 1 and 2 in which a bimodal analysis was used as they contained some large particles. As in FlFFF, PCS measurement is based on the Brownian motion (or diffusion) of particles, yielding the hydrodynamic size, while SdFFF yields the size based
on the effective mass of the particles. It is thus expected that FlFFF data will agree better with PCS data than SdFFF data. Mean diameters determined by SdFFF, FlFFF, and PCS for all soot samples are summarized in Table 4. FlFFF data show better agreement with the PCS data than with the SdFFF data as expected, although the data from the three techniques (SdFFF, FlFFF, PCS) were generally in reasonable agreement. Figure 6a shows a SEM picture of soot 1 taken prior to step 2 (extraction step) of the sample preparation procedure, and Figure 6b shows a SEM picture of the fraction collected for 1-2 min at the peak maximum of FlFFF fractogram for the same sample. Particles in Figure 6a are much larger than those in Figure 6b. Panels a and b of Figure 7show SEM pictures of fractions collected for 1-2 min at the peak maximum of FlFFF fractogram for soots 2 and 5, respectively. As in Figure 6b, both (a) and (b) of Figure 7 show all particles are well-dispersed. Image analysis of pictures shown in Figures 6b and 7a and b yielded mean diameters of 0.178, 0.194, and 0.132 µm, respectively, which are in reasonable agreement with those obtained from either FFF or PCS. CONCLUSIONS FlFFF and SdFFF provide some advantages for size determination of automobile soot particles over other particle-sizing Analytical Chemistry, Vol. 71, No. 15, August 1, 1999
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techniques. FlFFF yields the size distribution as well as the mean diameter in a relatively short period of time. A FlFFF elution profile can be converted to a size distribution without the need for the density information. However, the dynamic range of FlFFF is relatively narrow compared to that of SdFFF, and under the experimental conditions used in this study, the use of FlFFF is limited to samples having particles smaller than ∼0.4-0.5 µm due to the steric inversion phenomena in larger particles. A field programming may extend the dynamic range of FlFFF. Also care must be taken in selecting a dispersing agent, as there is a possibility of sample adsorption onto the membrane or of membrane distortion. SdFFF requires longer time for analysis than FlFFF, even with field-programming operations. And the size determination using SdFFF requires the density of particles to be known. Despite these problems, SdFFF could be more useful than FlFFF once the density of the particles is known. As in FlFFF, a SdFFF elution profile can be easily converted to a size distribution. There is no restriction in selecting a dispersing agent, and the applicable size range is wider than FlFFF. PCS is a convenient tool for size determination of various particulate samples. However, PCS may not be as useful as FFF for accurate determination of sizes of samples having broad distributions such as soot particles. Size determination using SEM
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requires each particle to be measured individually. Thus, the use of SEM is not convenient for size analysis of particles having broad distributions or irregular shapes such as soot particles. Of the two sample preparation procedures tested, the focusing method seems more effective than the hexane-decant method for the following reasons. First, the focusing method does not involve heating for the removal of ethanol. Data showed the soot sample prepared by the hexane-decant method to have a larger mean size than the sample prepared by the focusing method, which indicates that the heating of the hexane-decant method may promote the sample to aggregate. Second, the focusing method allows extraction of hydrophilic as well as hydrophobic compounds from the soot particles. Third, the focusing method is quicker and simpler than the hexane-decant method. ACKNOWLEDGMENT This research was supported by the Ministry of Education of Korea through the Basic Science Research Institute program (BSRI-97-3425). This work was also partially supported by Center for Molecular Catalysis (KOSEF). Received for review January 19, 1999. Accepted April 15, 1999. AC990048Z
