Sample Analysis

Fluorometric analysis is very sensitive (which was a necessity for the dilute

solutions encountered in this experiment). The method also lends itself to

simplicity and specificity. Since there is a linear relationship between

Total Flow 5 Ipm

1 1 1

Aerosol 1 Ipm 4 Ipm Filtered Sheath Air Laser Beams

Figure 4.6: Schematic of TSI Aerodynamic Particle Sizer

concentration and fluorescence, it is possible to construct a calibration curve

based on a standard. During the process of aerosolization from the VOMAG,

the alcohol volatilizes leaving fluorescently tagged oleic acid particles. These

particles can then be quantified in solution based on the resultant

fluorescence produced. The method is pH sensitive (optimal pH is

approximately 10). A Perkin-Elmer fluorescence spectrophotometer (Mode!

650-1 OS) was used to quantify the amount of aerosol that had been

collected by the cyclone and 47-mm filter cassette. This instrument is

specifically designed for the measurement fluorescence excitation and

emission spectra. The light source is a 150-watt xenon lamp. Both grating

20 nanometers (nm). The sample is irradiated by the light from the

excitation monochromator in the 220 to 830 nm wavelength range. The

emitted light from the sample passes through the emission monochromator

(selective measurement of intensity is 220 to 820 nm) to a photomultiplier

detector [Perkin-Elmer, 1978].

The relationship between fluorescence intensity and concentration has been

well described [Underfriend, 1962; Hercules, 1966; Guilbault, 1973]:

{S,), = mg{\)l^^fabc

^ abc abc abc"

21 3! (/7+1)!

(11)

where {Sf)j^ is the sample fluorescence intensity at a given wavelength, /(6)

is the geometry depending on the effective solid angle, g{X) is the response

characteristic of the detector (varies with wavelength), /q is the intensity of

the exciting radiation, 4>/ is the quantum efficiency of the molecule, a is the

molar absorptivity for the sample at the exciting wavelength, b is the sample

path length along the axis of irradiation, and c, is the concentration of the

fluorescing material in moles per liter. In situations, as were encountered in

this study, where the concentration of fluorescing material is very small (abc

< 0.05) the equation reduces to:

solution of uranine In 0.01 N Sodium Hydroxide (NaOH) to determine the

optimum excitation and emission wavelengths (shown graphically in Figure

4.7). Calibration curves, for the low and normal photomultiplier tube gain,

were then developed using a stock solution of 20 //g/ml uranine and 0.01 N

NaOH. This solution was added to 0.01 N NaOH to produce sequential

dilutions of 0.1, 0.05, 0.01, 0.005, 0.001, 0.0005, and 0.0001 /yg/ml. The

resultant calibration curve is shown in Figure 4.8.

For the analysis of the filter samples, the fluorescence spectrophotometer

slit widths were set at 5 and 10 nm for the excitation and emission

wavelengths, respectively. The narrower excitation slit width was selected

to guard against photo-chemical reactivity of the sample solutions. The

excitation wavelength was reduced to 484 nm, which is below the optimum

wavelength of 492 nm as determined by the spectral scan. The reduction of

the excitation wavelength and the narrower excitation band pass promoted

the minimization of interference of Rayleigh scatter peaks with emission

peaks due to the close proximity of the excitation and emission peaks

[Perkin-Elmer, 1978]. The optimum emission wavelength of 512 nm was

used for all analyses.

The 47-mm, 2 //m polytetrafluoroethylene (PTFE or Teflon®, Gelman

Sciences, Ann Arbor, Michigan) filter from each cyclone experimental run

was divided into three sections using a scalpel. The divisions between the

1.4- 512 nm A 1.2- Excitation 1 - i= 0.8- 0 > Emission

Response

o o *. at

11

492 nm J \

0.2-

_{/ l\}

0-

—.=.—JA V,

-0.2- 2C0 300 400 500 600

Wavelength (nm)

700 800

Figure 4.7: Excitation and Emission Spectrums for a 0.2 jt/g/ml Solution of

Uranine in 0.01 N NaOH

1200 0» 800

r^ = 0.99987

0.99976 4) 600

(S 400

ͤ Low PM Gain Normal PM Gain 0.02 0.04 0.06 0.08 0.1

Uranine/NaOH Concentration (ug/ml)

two outer sections and the inner section are represented by two tangentially

intersecting lines at the diameter of the cyclone exit tube. The outer and

inner sections were separately placed in two separate glass jars for later

analysis. The filters from the cyclone or the filter cassette were placed face

down in each jar. For the cyclone sample, 15 ml of 0.01 N NaOH was

added to each outer and inner section jar. For the filter cassette sample, 30

ml of 0.01 N NaOH was added to each jar. The samples were then placed

in an ultrasonic bath for 30 minutes. Analysis in the fluorescence

spectrophotometer was facilitated by placing approximately a 4 ml aliquot of

the sample solution into a cuvette. Response readings from the digital

display were converted to units of /jg/m^ of air based on the calibration

curve. The selection of the filter media, wash-off solution, and analytical

technique were based on the results of Tseng [Tseng, 1991]. Tseng

conducted experiments evaluating the effects of two filter medias (glass

fiber and PTFE filters) and various wash-off solutions (HjO with and without

a buffer, 0.001, 0.01, and 0.1 N NaOH solutions, and ethanol with and

without a buffer). Tseng found (1) no background reading from PTFE filters,

(2) 0.01 N NaOH is a suitable solvent for PTFE filters based on its high

extraction ability and non-existent background reading, and (3) 30 minutes

in an ultrasonic bath effectively removes all the uranine from the filter. All

sample aliquots were analyzed twice. Blanks, for the cyclone and the filter

cassette, were obtained at the conclusion of each days sampling run and

subtracted from each individual series result.

'Wmvww

In document 1206.pdf (Page 41-47)