Determination of Light Elements (Carbon and Oxygen)
on IMPROVE Teflon Filters
Control Number: 1063
Omar F. Carvacho1, Carlos M. Castaneda1, Lowell L. Ashbaugh1,
Robert G. Flocchini1 and Jaspinder P. Singh1, Janice C.S. Lam1
1
University of California, Crocker Nuclear Laboratory, Air Quality Group
One Shields Avenue, Davis, California 95616
ABSTRACT
With the 1977 amendments to the Clean Air Act, Congress established mandatory Class I
areas where visibility must be protected from existing and future manmade air pollution.
These Class I sites include national parks and wildness areas larger than about 20 square
kilometers. Crocker Nuclear Laboratory (CNL) operates the particulate monitoring
network to track visibility in these areas. It has the responsibility to coordinate all field
operations and laboratory speciation work, and performs mass and elemental analysis on
IMPROVE samples using gravimetry, X-ray fluorescence (XRF), and Proton Elastic
Scattering Analysis (PESA).
To get a better reconstruction of the aerosol gravimetric mass, we are exploring the use
Rutherford Back Scattering to obtain the carbon (C) and oxygen (O) concentrations on
Teflon filters. The CNL Air Quality Group standard proton beam with energy 4.5 MeV
and beam intensity of 10nA was used. A surface barrier detector at 150 degrees from the
beam direction detected recoil protons.
To test the method, we analyzed filters with a known carbon deposit. Elemental carbon
was deposited on clean Teflon filters using the CNL resuspension chamber. We also used
PM2.5 aerosol samples collected in one three month period at Mesa Verde National Park,
Colorado to test the method. We compared the oxygen measurements by RBS to the
implied oxygen required to account for sulfate and soil in the aerosol samples.
INTRODUCTION
The Mesa Verde (MEVE1) site in the IMPROVE network is located in Mesa Verde
National Park at 37.1984N latitude and 108.4907W longitude. The site elevation is
2177 MSL. The site began operation in March 1988 and continues to operate at this time.
Samples are collected every third day following the national regulatory sampling
schedule. Prior to August 2000, samples were collected on the IMPROVE schedule of
every Wednesday and Saturday. The site is located near other sampling equipment in an
open area with a low density of trees that meets the IMPROVE siting criteria. Figure 1
shows a photo of the site.
Figure 1. Photograph of the Mesa Verde IMPROVE site from the Northeast
The sampler consists of four modules; three collect PM2.5 and the fourth collects PM10.
Module A collects PM2.5 on a Teflon filter for mass, elemental, and light absorption
measurements. Mass is measured gravimetrically, elements Na-U are measured using Xray fluorescence, H is measured with proton elastic scattering, and light absorption is
measured using a hybrid integrating plate and sphere. Module B collects PM2.5 on a nylon
filter for chloride, nitrate, and sulfate ion analysis by ion chromatography. Module C
collects PM2.5 on a quartz filter for carbon using thermal optical reflectance (TOR).
Module D collects PM10 for gravimetric mass measurements. Beginning August 13, 2003
a second Module A (designated Module X) was installed as part of a network-wide
quality assurance program. The samples from Module X were used in the analysis
reported here for the determination of carbon (C) and oxygen (O) using Rutherford Back
Scattering.
METHOD
The beam from the Crocker Nuclear Laboratory (CNL) 76-inch cyclotron, located at the
University of California at Davis, was used for this test.
All testing was done using the Air Quality Group standard proton beam with energy 4.5
MeV. A surface barrier detector (Si) for Rutherford Backscattering (RBS), was placed at
150 degrees to the beam axis and 170mm from the center of an ORTEC 2800 series
scattering chamber. Figure 2 shows the layout of the setup. The detector, 1000um thick
and 150mm2 in area, was collimated by a circular opening 4.8mm in diameter (18.1mm2).
The solid angle of the detector was measured to be 6.25x10-4steradians (sr). A similar
detector, to measure the forward proton elastic scattering, was placed at 30 degrees to the
beam axis. This detector measured the hydrogen in the filter. For completeness an X-ray
detector (AMPTEK XR_100CR) recorded characteristic X-rays from elements on the
filter.
Figure 2: ORTEC 2800 series scattering chamber with RBS, PESA, and XRF detectors
A Faraday Cup, FC, with electric and magnetic electron suppression, was located 105cm
from the center of the ORTEC chamber. To assure no protons were lost to the FC due to
dispersion by the sample filters, a secondary electron emission monitor (SEEM) was
placed before the samples. This SEEM was composed of three aluminized-Mylar foils of
thickness 0.29mg/cm2 each. The monitor was calibrated with the FC with no filter
present. Then for all the runs the monitor was used to count the protons impinging on the
samples. The size of the beam at the sample location was recorded using HD-810
Gafchromic film, and found to be 6.35mm in diameter. The background was measured
with beam on and no sample present, and no counts were observed from the detectors.
The spectra were collected with a multichannel analyzer using the Genie 2000 emulator
software by Canberra Inc. Peaks in the spectra were integrated using the PEAKFIT
software.
The mass calibration was done using the carbon and oxygen from carbon and Mylar foils
respectively. The thickness was 300µg/cm2 for the carbon foil and 280µg/cm2 for the
Mylar foil. Figure 3a and 3b shows the RBS spectra for both foils.
Carbon Foil 300 µg/cm²
4000
C
3500
Counts
3000
2500
2000
1500
1000
500
0
120
125
130
135
140
145
150
155
160
165
170
175
180
Channels
.
Figure 3a. RBS spectrum for carbon foil 300 µg/cm²
Mylar Foil 280 µg/cm²
2500
C
O
Counts
2000
1500
1000
500
0
120 125 130 135 140 145 150 155 160 165 170 175 180
Channels
Figure 3b. RBS spectrum for Mylar foil 280 µg/cm²
RESULTS
To test the method a Teflon filter was analyzed before and after loading with elemental
carbon (graphite) using the CNL resuspension chamber. The filter was weighed before
and after it was loaded. The difference in weight and area of the deposit gave an areal
density of 74.84 µg/cm2. Figure 4 shows the spectra for both the clean and loaded filter.
(Dec22 and Jan 5) Two analysis methods were used to determine the carbon
concentration: I) a simple subtraction of the unloaded spectrum from the loaded; II) a self
consistent method using a calculation with the proper fluorine and carbon ratio for Teflon
and fitted to reproduce the height of the fluorine peak. This calculation gives us the
height for the corresponding C peak. Comparing the height of the high-energy edge for
the simulated carbon peak with the height of the carbon distribution in the loaded filter
the fraction of the total carbon in the filter that corresponds to the carbon deposited can
be determined. Once the excess counts are known, we use the normalization factor
obtained from the thin carbon foil standard (see Figure 3) to obtain the areal density for
the extra carbon in the filter. The values extracted from the data using method I and
method II were 96±3 ug/cm2 and 69±6 ug/cm2, respectively. These values are 22% and
8% greater, respectively, than the value obtained by the difference in weight, 74.84
ug/cm2. The results are encouraging. The second method gives a result closer to the
known deposit, so we will use it for the analysis of the carbon on the MEVEX filters.
Method II also relies only on the height of the high-energy edge of the carbon peak. This
frees us from having to measure the thickness of the filters, thus avoiding analyzing the
blank filter before they are sent to the field.
Blank Teflon Filter
1000
Counts
800
C
F
600
400
200
0
120 125 130 135 140 145 150 155 160 165 170 175 180
Channels
Figure 4a. RBS spectrum of blank Teflon Filter
Teflon Filter loading with elemental carbon (graphite)
1000
Counts
800
600
400
200
0
120 125 130 135 140 145 150 155 160 165 170 175 180
Channels
Figure 4b.RBS spectrum of Teflon filter loading with elemental carbon (graphite)
The spectrum from a MEVEX samples is shown in Figure 5. The calculation using
SIMNRA (*) for a Teflon sample and normalized to the fluorine peak is also shown. In
the figure it can be seen that there is excess carbon and oxygen in the sample. The code
cannot simulate the width of the peaks in samples that are not homogenous, like this type
of filters. But the position and their heights can be calculated once the composition of the
material is known.
RBS- MEVE1 Teflon fi;ter
Counts
MEVE1 Teflon Filter
450
400
350
300
250
200
150
100
50
0
C
Simulated from ratio C/F
O
F
120 125 130 135 140 145 150 155 160 165 170 175 180
Channel
Figure 5 A MEVEX spectrum compared with the output from the code SIMNRA.
Only the height and energy position can be calculated.
To extract the oxygen is straightforward. Using the peak fitting routine, PeakFit ( ), we
can extract the number of counts under the oxygen peak. Fluorine has two excited states
that can be excited with 4.5 MeV. The position of those peaks and of the ground state is
known from kinematics. In the region of the fluorine and oxygen distribution, we fit four
peaks, one of them due to oxygen. The counts under the peak are normalized to the
counts under the oxygen peak from a 290ug/cm2 Mylar foil. See Figure 3 for the proton
spectrum from the Mylar.
The oxygen implied from the IMPROVE measurements is calculated from the sulfate and
soil elements. Sulfate (SO4=) is measured by ion chromatography. The oxygen present in
the sulfate amounts to 2/3 of the sulfate concentration. IMPROVE calculates a soil
parameter from the Al, Si, Ca, Ti, and Fe concentrations using the equation
SOIL = 2.2*[Al]+2.49*[Si]+1.63*[Ca] +2.42*[Fe]+1.94*[Ti]. The coefficients account
for the common oxide forms of these elements in crustal material plus a factor to account
for other elements not included in the equation. Using this equation, the implied oxygen
was calculated for comparison to the measured oxygen using RBS. Other chemical
species present in the aerosol deposit that include oxygen were not included in the
implied oxygen calculation.
Figure 6_ shows the comparison between the implied oxygen measured in IMPROVE
and the measured oxygen on the Teflon filter using RBS. The comparison show a good
linear relation for experimental data
IMPROVE Oxygen
(µg/cm²)
Comparison between: RBS-Oxygen from Teflon filters
and Oxygen measured in IMPROVE site MEVE1
18
16
14
12
10
8
6
4
2
0
y = 0.5942x + 0.6438
R2 = 0.7577
0
5
10
15
20
25
RBS-Oxygen (µg.cm²)
Figure 6. Comparison between the implied oxygen in IMPROVE with the measured
Oxygen on the Teflon filters usin RBS.
CONCLUSIONS
A Rutherford Back Scattering method has been developed and tested to measure the light
elements carbon and oxygen on Teflon filters from the IMPROVE network. The method
shows promise to extend the Crocker Nuclear Laboratory analytical techniques to more
completely determine the species that make up the measured mass on the filters. Further
development is needed and will be pursued in the coming months.
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Carvacho, O.F., L.L.Ashbaugh, M.S. Brown, R.G., Flocchini. 2001. Relationship
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Eldred, R.A., T.A. Cahill, P.J. Feeney. 1987. Particulate Monitoring at US National Park using PIXE.
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