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Measurement of Scattering and Absorption Cross Sections of Microspheres for Wavelengths between 240 nm and 800 nm.

Gaigalas AK, Wang L, Choquette S - J Res Natl Inst Stand Technol (2013)

Bottom Line: The value of ni at 266 nm was about 0.0060±0.0016 for microspheres with diameters of 1.5 μm, 2.0 μm, and 3.0 μm.The scattering cross section, absorption cross section, and the quantum yield at 266 nm of microsphere with a diameter of 2.0 μm was 5.65±0.01 μm(2), 1.54±0.03 μm(2), and 0.027±0.002 respectively.The styrene absorption reduces the scattering cross section by 20 % at 266 nm.

View Article: PubMed Central - PubMed

Affiliation: National Institute of Standards and Technology, Gaithersburg, MD 20899.

ABSTRACT
A commercial spectrometer with a 150 mm integrating sphere (IS) detector was used to estimate the scattering and absorption cross sections of monodisperse polystyrene microspheres suspended in water. Absorbance measurements were performed with the sample placed inside the IS detector. The styrene absorption was non zero for wavelengths less than 300 nm. Correction for fluorescence emission by styrene was carried out and the imaginary part of the index of refraction, ni, was obtained. Absorbance measurements with the sample placed outside the IS detector were sensitive to the loss of photons from the incident beam due to scattering. The absorbance data was fitted with Lorenz-Mie scattering cross section and a correction for the finite acceptance aperture of the spectrometer. The fit parameters were the diameter, the suspension concentration, and the real part of the index of refraction. The real part of the index was parameterized using an expansion in terms of powers of the inverse wavelength. The fits were excellent from 300 nm to 800 nm. By including the imaginary part obtained from the absorbance measurements below 300 nm, it was possible to obtain a good fit to the observed absorbance data over the region 240 nm to 800 nm. The value of ni at 266 nm was about 0.0060±0.0016 for microspheres with diameters of 1.5 μm, 2.0 μm, and 3.0 μm. The scattering cross section, absorption cross section, and the quantum yield at 266 nm of microsphere with a diameter of 2.0 μm was 5.65±0.01 μm(2), 1.54±0.03 μm(2), and 0.027±0.002 respectively. The styrene absorption reduces the scattering cross section by 20 % at 266 nm.

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(a) The emitted fluorescence spectrum from a suspension of 1.5 μm microspheres placed in a fluorimeter and excited with 266 nm light. The maximum emission is at 350 nm. (b) The measured absorbance from a suspension of polystyrene microspheres with diameter 1.5 μm. The trace was obtained by placing the sample in the holder inside the IS detector and subtracting the buffer contribution and a small background contribution (0.003).
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f4-jres.118.001: (a) The emitted fluorescence spectrum from a suspension of 1.5 μm microspheres placed in a fluorimeter and excited with 266 nm light. The maximum emission is at 350 nm. (b) The measured absorbance from a suspension of polystyrene microspheres with diameter 1.5 μm. The trace was obtained by placing the sample in the holder inside the IS detector and subtracting the buffer contribution and a small background contribution (0.003).

Mentions: The effect of fluorescence on absorbance measurements for samples inside the IS is demonstrated in Fig. 3a. The solid and dashed traces in Fig. 3a show the absorbance measurements carried out for 10 mol/L aqueous solution of tryptophan placed outside and inside the IS respectively (the solution did not have measurable scattering). The reduction of the tryptophan absorbance measured inside the IS was attributed to the fluorescence emitted by the excited tryptophan molecules. Figure 3b shows the observed fluorescence emission when the same tryptophan solution was placed in a fluorimeter and excited with 266 nm light. Clearly the emitted tryptophan fluorescence will reduce the observed tryptophan absorbance measured inside the IS because the IS detector will not distinguish between photons transmitted through the cuvette and photons emitted by the solution inside the IS. Figure 4a shows the emitted fluorescence spectrum from a suspension of 1.5 μm microspheres placed in a fluorimeter and excited with 266 nm light. A comparison of the spectra in Fig. 4a and Fig. 3b shows that the fluorescence emission from the microspheres is similar to the fluorescence emission from the tryptophan solution. Therefore it was assumed that the measure absorbance of a suspension of 1.5 μm microspheres shown in Fig. 4b would include a reduction by the emitted fluorescence and that the actual absorbance of the microsphere suspension is larger. In order to quantify the reduction in the measured absorbance due to fluorescence, it was assumed that the relative reduction in absorbance is proportionate to the quantum yield as indicated in Eq. (5).(5)A−A3A=mΦ


Measurement of Scattering and Absorption Cross Sections of Microspheres for Wavelengths between 240 nm and 800 nm.

Gaigalas AK, Wang L, Choquette S - J Res Natl Inst Stand Technol (2013)

(a) The emitted fluorescence spectrum from a suspension of 1.5 μm microspheres placed in a fluorimeter and excited with 266 nm light. The maximum emission is at 350 nm. (b) The measured absorbance from a suspension of polystyrene microspheres with diameter 1.5 μm. The trace was obtained by placing the sample in the holder inside the IS detector and subtracting the buffer contribution and a small background contribution (0.003).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4487312&req=5

f4-jres.118.001: (a) The emitted fluorescence spectrum from a suspension of 1.5 μm microspheres placed in a fluorimeter and excited with 266 nm light. The maximum emission is at 350 nm. (b) The measured absorbance from a suspension of polystyrene microspheres with diameter 1.5 μm. The trace was obtained by placing the sample in the holder inside the IS detector and subtracting the buffer contribution and a small background contribution (0.003).
Mentions: The effect of fluorescence on absorbance measurements for samples inside the IS is demonstrated in Fig. 3a. The solid and dashed traces in Fig. 3a show the absorbance measurements carried out for 10 mol/L aqueous solution of tryptophan placed outside and inside the IS respectively (the solution did not have measurable scattering). The reduction of the tryptophan absorbance measured inside the IS was attributed to the fluorescence emitted by the excited tryptophan molecules. Figure 3b shows the observed fluorescence emission when the same tryptophan solution was placed in a fluorimeter and excited with 266 nm light. Clearly the emitted tryptophan fluorescence will reduce the observed tryptophan absorbance measured inside the IS because the IS detector will not distinguish between photons transmitted through the cuvette and photons emitted by the solution inside the IS. Figure 4a shows the emitted fluorescence spectrum from a suspension of 1.5 μm microspheres placed in a fluorimeter and excited with 266 nm light. A comparison of the spectra in Fig. 4a and Fig. 3b shows that the fluorescence emission from the microspheres is similar to the fluorescence emission from the tryptophan solution. Therefore it was assumed that the measure absorbance of a suspension of 1.5 μm microspheres shown in Fig. 4b would include a reduction by the emitted fluorescence and that the actual absorbance of the microsphere suspension is larger. In order to quantify the reduction in the measured absorbance due to fluorescence, it was assumed that the relative reduction in absorbance is proportionate to the quantum yield as indicated in Eq. (5).(5)A−A3A=mΦ

Bottom Line: The value of ni at 266 nm was about 0.0060±0.0016 for microspheres with diameters of 1.5 μm, 2.0 μm, and 3.0 μm.The scattering cross section, absorption cross section, and the quantum yield at 266 nm of microsphere with a diameter of 2.0 μm was 5.65±0.01 μm(2), 1.54±0.03 μm(2), and 0.027±0.002 respectively.The styrene absorption reduces the scattering cross section by 20 % at 266 nm.

View Article: PubMed Central - PubMed

Affiliation: National Institute of Standards and Technology, Gaithersburg, MD 20899.

ABSTRACT
A commercial spectrometer with a 150 mm integrating sphere (IS) detector was used to estimate the scattering and absorption cross sections of monodisperse polystyrene microspheres suspended in water. Absorbance measurements were performed with the sample placed inside the IS detector. The styrene absorption was non zero for wavelengths less than 300 nm. Correction for fluorescence emission by styrene was carried out and the imaginary part of the index of refraction, ni, was obtained. Absorbance measurements with the sample placed outside the IS detector were sensitive to the loss of photons from the incident beam due to scattering. The absorbance data was fitted with Lorenz-Mie scattering cross section and a correction for the finite acceptance aperture of the spectrometer. The fit parameters were the diameter, the suspension concentration, and the real part of the index of refraction. The real part of the index was parameterized using an expansion in terms of powers of the inverse wavelength. The fits were excellent from 300 nm to 800 nm. By including the imaginary part obtained from the absorbance measurements below 300 nm, it was possible to obtain a good fit to the observed absorbance data over the region 240 nm to 800 nm. The value of ni at 266 nm was about 0.0060±0.0016 for microspheres with diameters of 1.5 μm, 2.0 μm, and 3.0 μm. The scattering cross section, absorption cross section, and the quantum yield at 266 nm of microsphere with a diameter of 2.0 μm was 5.65±0.01 μm(2), 1.54±0.03 μm(2), and 0.027±0.002 respectively. The styrene absorption reduces the scattering cross section by 20 % at 266 nm.

No MeSH data available.


Related in: MedlinePlus