- Email: [email protected]
PIXE analysis of carbon black for elemental impurities
414
Nuclear
Instruments
and Methods
in Physics
Research
B49 (1990) 414-417 North-Holland
PIXE ANALYSIS R.S. SOKHI
OF CARBON
‘), C . GRAY
BLACK FOR ELEMENTAL
3), K. GARDINER
IMPURITIES
2, and L.G. EARWAKER”
” School oj Physics and Space Research and ” Institute of Occuputtonal Health, lJnirlerst!v of Birmingham. -” Deakin
Birmingham
B15 -7TT. UK
Unioersity, Geelong, Victoriu 3217, Australia
Carbon black has been analysed for elemental contamination using the particle induced X-ray emission (PIXE) technique. A 3 MV Dynamitron accelerator was employed to produce the 2.5 MeV proton beam used for the PIXE measurements. The carbon black particulates were pelletised to form thick targets and also collected on 25 mm diameter cellulose-ester filters for thin target measurements. The targets were placed normally to the incident proton beam in a multisample chamber for analysis. A conventional Si(Li) X-ray detector, placed at 135 o to the proton beam was employed to detect the emitted X-rays. The results obtained from the
thick and thin target analyses are compared and discussed. Calculations of the maximum Stoke’s diameter of the carbon black particles are included
1. Introduction Carbon blacks are powdered forms of elemental carbon manufactured by the controlled vapour phase pyrolysis of natural gas or oil or a mixture of both. They have small particle sizes (17-500 nm), high surface areas per unit mass, low content of ash and extractable materials and varying degrees of particle aggregation [ 11.There are several methods for manufacturing carbon black and details of these methods can be found in ref. [I]. Depending on the process of manufacture and the type and source of feedstock the chemical composition of carbon black will vary greatly (see table 1 in ref. [I]). Epidemiological investigations of the hazards associated with occupational exposure to carbon black were initiated in the early 195Os, when the incidence of cancer in industries associated with its use or manufacture was studied. As with a number of substances, the carcinogenic properties of carbon black are due to the presence of certain impurities. In the case of carbon black, these impurities are either elemental. which result from contamination of the feedstock, and/or from the generation of polynuclear aromatic hydrocarbons (PNAs) [2] which are formed during combustion of organic material and high temperature processing of crude oil. The elemental contamination, which includes sulphur, calcium and vanadium is integral to the structure of the carbon black whereas the PNAs are adsorbed onto the surface. This paper is concerned with the measurement of elemental contamination of carbon black using the particle induced X-ray emission (PIXE) technique. Occupational hygiene sampling in epidemiological studies relies on sampling a number of homogeneous groups within any given industrial plant and between a number of 0168-583X/90/$03.50 (North-Holland)
‘B Elsevier Science Publishers
B.V.
plants, and then to derive meaningful exposure groups from this combination against which health effects can be correlated. When conducting an investigation, however, it is difficult to decide whether the particulates consist of particles from, for example. welding fumes or household dust exclusively or as a mixture with carbon black. By performing elemental analysis of the airborne contaminants and detecting the elements specific to carbon black it would be possible to determine what proportion of the particulates constitute carbon black. Furthermore, when assessing the toxicity of airborne particulates it is essential to identify and, if possible, quantify all elemental contamination. PIXE will. thus, allow the detection of not only metals such as nickel, chromium and iron, which are known to cause medical disorders. but also other elements whose roles may not be understood fully and may be erroneously excluded from the toxicological assessment. The initial results, which form part of a major study being carried out at Birmingham University of the toxic effects of carbon black, are reported in this paper. Concentrations of the element impurities in carbon black. in the form of thick and thin targets are presented. Calculations of the maximum Stoke’s diameter of the carbon black particles are also included in this paper.
2. Procedure 2. I. Sumpling
The only carbon black available for this study was that in its pelletised form. This is the form in which the “fluffy” carbon black, as manufactured. is formed into
415
R.S. Sokhi et al. / Ana!vsis of impurities in carbon black
were used to collect the carbon black particulates on cellulose-ester filters. These IOM heads were used because of the even distribution and density of deposition achieved on the filters. These filters loaded with carbon black then acted as thin targets for subsequent PIXE analysis. The particulate loading was typically 300 p g/cm’.
--
0.05
n+
2.2. PIXE measurements
Fig. 1. Schematic diagram of the vertical elutriator used collect carbon black particles on filters (not to scale).
to
dense pellets with a diameter of about 1 mm. Measurements were made on thick and thin targets of carbon black. Typically, about 200 mg of carbon black, after agating to a fine powder, were compressed into 1 cm diameter pellets. In order to prepare thin targets cellulose-ester filters (0.8 pm diameter pore size and 25 mm diameter) were employed. It was necessary to fractionate the sample to ensure that the pellets, which are totally unrepresentative of occupational exposure due to airborne carbon black, did not impact on to the filter. This was achieved by designing and building a vertical elutriator. see fig. 1. A small amount of carbon black was poured into a polyethylene container and a cylindrical cardboard tube of cross sectional area 19.6 cm* and length 83.5 cm was securely attached and supported above the container. Finally an airtight seal was formed between the container and chimney. To aerolise the sample and fractionate the particulates, air was forced into the container, through a tube at its base, at a constant flow rate of 10.2 l/min. This nebulised the carbon black forcing the smaller particles, as a function of their aerodynamic diameter. up the chimney to the sampling head. Those particles with a large aerodynamic diameter also rise up the chimney but do not reach the top. IOM (Institute of Occupational Medicine. Edinburgh, Scotland) inspirable heads, with a backing disk,
A 2.5 MeV proton beam from the 3 MV Dynamitron accelerator at the School of Physics and Space Research Radiation Centre was employed for the PIXE measurements. The targets were placed normally to the beam in a vacuum scattering chamber. Twelve samples could be mounted on an Al disk at one time. The incident proton beam was diffused by passing it through a 6 km Al foil and then collimated down to 5 X 5 mm’ size before allowing it to impinge on the targets. For thin targets the target current, which was measured with a Keithley electrometer, was maintained at 10 nA. Typically the samples were irradiated for a charge of 10 PC. For thick targets currents of 100 nA were employed and the samples bombarded for a total charge of 100 PC. An Ortec Si(Li) X-ray detector was placed at 135” to the incident beam direction to measure the proton induced X-rays. The signals from the preamplifier of the detector were processed with a 572 Ortec amplifier and then fed into a Canberra 8077 fast ADC. A typical PIXE spectrum for a pelletised carbon black target is shown in fig. 2. Data manipulation and storage was carried out with a Canberra SlOO counting system based on an IBM PS/2 model 50 computer. The accumulated X-ray spectra were analysed for elemental impurities with a PIXE analysis software package written in Fortran 77. The analysis programme fits Gaussian functions to the X-ray peaks and models the background with a polynomial. The peak areas derived from the fitted functions were then used to determine the concentrations of the elements. For further details the reader is referred to ref. [3] which deals with the analysis of thin and thick targets comprehensively.
3. Results and discussion Table 1 lists the mean values of the levels of the measured elemental impurities. The values for thin targets are quoted in kg/cm’ and for thick targets concentrations are given in mg/kg. The aerodynamic diameter of the particles reaching the sampling head were calculated from the following equation in cgs units
VI. AEROSOL
STUDIES
R.S. Sokhi et al. / Analysis of rmpurities in carbon black
Zn
.: *.. . ... . . .- ‘: 10’ =
:.
“.
._
.
*._ .:
.
: .
.:
.
. .
_. .
.
. .
.._
I
I
0
I
I
60
I
PIXE spectrum
where v is the settling velocity (cm/s), g is the acceleration due to gravity, a is the particle radius (cm), p, and pa are the densities of the particle and air respectively and n is the viscosity of air (dynes/cm*). For the system described earlier v = 8.67 cm/s and assuming the values g = 981 cm/s*, p, = 0.0012 g/cm3 at a temperature of 18” C [4], p, = 1.8 g/cm3 [l] and n = Table 1 Average concentrations
S K Ca Ti V Cr Mn Fe Ni CU Zn
of elemental
impurities
in carbon
Thin target
Thick target
[ P g/cm2 1
[mg/kgl
3.11 0.060 0.100
kO.08 + 0.006 f 0.005
1.110*0.004% 169 &l 259 _+I 18 *2 3.0 +0.4
0.004 0.006 0.051
+ 0.002 +0.005 f 0.005
0.0020 f 0.0005 0.0015 & 0.0006
3 8.7 72 1.0 2.1 1.9
*1 +_0.9 fl f0.2 kO.5 50.5
black
I
180 CHANNEL
Fig. 2. A typical
I
120
I
240
. .
.
. .
.
.
10°
.
.
.
. .
.
.
.
,
.
.
.
.
_.
1
300
NUNBER
of a thick pelletised
carbon
black target.
182.7 x lop6 dyne s/cm*, the aerodynamic diameter (maximum) is calculated to be 40 urn. If laminar flow is assumed then the largest particle to be carried out of the elutriator would have a Stoke’s diameter of 40 urn. Turbulence and other causes of non-laminar flow, such as, wall drag would tend to carry out some larger particles. In general, particles which have physical diameters of less than 40 pm would be able to contaminate the environment and would pose an occupational hazard as the sizes of these particles lie within the definition of inspriability/inhalability [5]. The filters used in this study were the cellulose-ester membrane type. Although care was taken to keep loadings of the filters below 300 kg/cm* the nature of the carbon black particles makes it very difficult to avoid loss of particulate matter during sample handling. Thus, smaller loading than that used here is recommended. Furthermore, an adhesive agent on the filter prior to loading will also reduce this problem. An additional complication when analysing substances like carbon black on filters is that environmental dust particulates can also be trapped on the filters and will lead to erroneous results. However, it is hoped that the analysis
R.S. Sokhi et al. / Ana!vm
of thick pellets of ‘pure’ carbon black will provide an initial set of elemental concentrations characteristic of this substance. It should be noted that because the cellulose-ester filters are not conductors beam currents were kept to less than 10 nA to avoid damaging the targets, whereas, for the carbon black pellets 100 nA beam current was used. This enabled higher sensitivities to be reached as indicated by the concentrations listed in table 1. Several elements such as Ti, V, Cr, Mn. Ni and Cu. which are known to be toxic, particularly carcinogenic [6]. have been identified. Comparison of the thin and thick target results (table 1) reveal some inconsistencies in the ratios of the elemental concentrations. Larger amounts of the heavier elements are observed in the filter targets relative to. say, S. Impurities of Mn, Fe, Cu and Zn elements were also observed in the filters and may partially account for this discrepancy. Although considerable care was taken the possibility of contamination during the loading and transport of the filters also cannot be excluded.
4. Conclusions Thick and thin target analyses of carbon black are presented. Eleven elements ranging from S to Zn have been quantified and it is hoped that this will serve as an initial elemental data set characteristic of carbon black. Simultaneous analysis of these elements gives PIXE
411
of impurities in carbon black
considerable advantages over other techniques such as atomic absorption spectrometry (AAS) which, although requiring complicated sample preparation procedures using acid digestion techniques, is often employed by occupational hygienists for elemental analysis. This work forms an integral part of a major study being carried out at Birmingham University of the toxicological assessment of carbon black in occupational areas. Further work is being planned to improve detection sensitivities by using different filters and sampling heads.
The expertise of the Dynamitron ing staff is appreciated.
accelerator
operat-
References PI I.A.R.C.,
Polynuclear Aromatic Hydrocarbons, part 2. Carbon blacks. Mineral Oils (Lubricant Base Oils - Derived Products) and some Nitroarenes, vol. 33. PI A. Gold. Anal. Chem. 47 (1975) 1469-1472. [31 M.R. Khan and D. Crumpton, Crit. Rev. Anal. Chem. II (1981) 103-155 and 12 (1981) 161-193. 4th ed. [41 R.C. West (ed.). Chemical Engineers’ Handbook. 1963. On the quantitative defi[51 J.H. Vincent and L. Armbruster. nition of the inhalability of airborne dust, Am. Occup. Hyg. 24 (1981) 245-248. [61 H.A. Waldron (ed.). Metals in the Environment (Academic Press, 1980).
VI. AEROSOL
STUDIES
Nuclear
Instruments
and Methods
in Physics
Research
B49 (1990) 414-417 North-Holland
PIXE ANALYSIS R.S. SOKHI
OF CARBON
‘), C . GRAY
BLACK FOR ELEMENTAL
3), K. GARDINER
IMPURITIES
2, and L.G. EARWAKER”
” School oj Physics and Space Research and ” Institute of Occuputtonal Health, lJnirlerst!v of Birmingham. -” Deakin
Birmingham
B15 -7TT. UK
Unioersity, Geelong, Victoriu 3217, Australia
Carbon black has been analysed for elemental contamination using the particle induced X-ray emission (PIXE) technique. A 3 MV Dynamitron accelerator was employed to produce the 2.5 MeV proton beam used for the PIXE measurements. The carbon black particulates were pelletised to form thick targets and also collected on 25 mm diameter cellulose-ester filters for thin target measurements. The targets were placed normally to the incident proton beam in a multisample chamber for analysis. A conventional Si(Li) X-ray detector, placed at 135 o to the proton beam was employed to detect the emitted X-rays. The results obtained from the
thick and thin target analyses are compared and discussed. Calculations of the maximum Stoke’s diameter of the carbon black particles are included
1. Introduction Carbon blacks are powdered forms of elemental carbon manufactured by the controlled vapour phase pyrolysis of natural gas or oil or a mixture of both. They have small particle sizes (17-500 nm), high surface areas per unit mass, low content of ash and extractable materials and varying degrees of particle aggregation [ 11.There are several methods for manufacturing carbon black and details of these methods can be found in ref. [I]. Depending on the process of manufacture and the type and source of feedstock the chemical composition of carbon black will vary greatly (see table 1 in ref. [I]). Epidemiological investigations of the hazards associated with occupational exposure to carbon black were initiated in the early 195Os, when the incidence of cancer in industries associated with its use or manufacture was studied. As with a number of substances, the carcinogenic properties of carbon black are due to the presence of certain impurities. In the case of carbon black, these impurities are either elemental. which result from contamination of the feedstock, and/or from the generation of polynuclear aromatic hydrocarbons (PNAs) [2] which are formed during combustion of organic material and high temperature processing of crude oil. The elemental contamination, which includes sulphur, calcium and vanadium is integral to the structure of the carbon black whereas the PNAs are adsorbed onto the surface. This paper is concerned with the measurement of elemental contamination of carbon black using the particle induced X-ray emission (PIXE) technique. Occupational hygiene sampling in epidemiological studies relies on sampling a number of homogeneous groups within any given industrial plant and between a number of 0168-583X/90/$03.50 (North-Holland)
‘B Elsevier Science Publishers
B.V.
plants, and then to derive meaningful exposure groups from this combination against which health effects can be correlated. When conducting an investigation, however, it is difficult to decide whether the particulates consist of particles from, for example. welding fumes or household dust exclusively or as a mixture with carbon black. By performing elemental analysis of the airborne contaminants and detecting the elements specific to carbon black it would be possible to determine what proportion of the particulates constitute carbon black. Furthermore, when assessing the toxicity of airborne particulates it is essential to identify and, if possible, quantify all elemental contamination. PIXE will. thus, allow the detection of not only metals such as nickel, chromium and iron, which are known to cause medical disorders. but also other elements whose roles may not be understood fully and may be erroneously excluded from the toxicological assessment. The initial results, which form part of a major study being carried out at Birmingham University of the toxic effects of carbon black, are reported in this paper. Concentrations of the element impurities in carbon black. in the form of thick and thin targets are presented. Calculations of the maximum Stoke’s diameter of the carbon black particles are also included in this paper.
2. Procedure 2. I. Sumpling
The only carbon black available for this study was that in its pelletised form. This is the form in which the “fluffy” carbon black, as manufactured. is formed into
415
R.S. Sokhi et al. / Ana!vsis of impurities in carbon black
were used to collect the carbon black particulates on cellulose-ester filters. These IOM heads were used because of the even distribution and density of deposition achieved on the filters. These filters loaded with carbon black then acted as thin targets for subsequent PIXE analysis. The particulate loading was typically 300 p g/cm’.
--
0.05
n+
2.2. PIXE measurements
Fig. 1. Schematic diagram of the vertical elutriator used collect carbon black particles on filters (not to scale).
to
dense pellets with a diameter of about 1 mm. Measurements were made on thick and thin targets of carbon black. Typically, about 200 mg of carbon black, after agating to a fine powder, were compressed into 1 cm diameter pellets. In order to prepare thin targets cellulose-ester filters (0.8 pm diameter pore size and 25 mm diameter) were employed. It was necessary to fractionate the sample to ensure that the pellets, which are totally unrepresentative of occupational exposure due to airborne carbon black, did not impact on to the filter. This was achieved by designing and building a vertical elutriator. see fig. 1. A small amount of carbon black was poured into a polyethylene container and a cylindrical cardboard tube of cross sectional area 19.6 cm* and length 83.5 cm was securely attached and supported above the container. Finally an airtight seal was formed between the container and chimney. To aerolise the sample and fractionate the particulates, air was forced into the container, through a tube at its base, at a constant flow rate of 10.2 l/min. This nebulised the carbon black forcing the smaller particles, as a function of their aerodynamic diameter. up the chimney to the sampling head. Those particles with a large aerodynamic diameter also rise up the chimney but do not reach the top. IOM (Institute of Occupational Medicine. Edinburgh, Scotland) inspirable heads, with a backing disk,
A 2.5 MeV proton beam from the 3 MV Dynamitron accelerator at the School of Physics and Space Research Radiation Centre was employed for the PIXE measurements. The targets were placed normally to the beam in a vacuum scattering chamber. Twelve samples could be mounted on an Al disk at one time. The incident proton beam was diffused by passing it through a 6 km Al foil and then collimated down to 5 X 5 mm’ size before allowing it to impinge on the targets. For thin targets the target current, which was measured with a Keithley electrometer, was maintained at 10 nA. Typically the samples were irradiated for a charge of 10 PC. For thick targets currents of 100 nA were employed and the samples bombarded for a total charge of 100 PC. An Ortec Si(Li) X-ray detector was placed at 135” to the incident beam direction to measure the proton induced X-rays. The signals from the preamplifier of the detector were processed with a 572 Ortec amplifier and then fed into a Canberra 8077 fast ADC. A typical PIXE spectrum for a pelletised carbon black target is shown in fig. 2. Data manipulation and storage was carried out with a Canberra SlOO counting system based on an IBM PS/2 model 50 computer. The accumulated X-ray spectra were analysed for elemental impurities with a PIXE analysis software package written in Fortran 77. The analysis programme fits Gaussian functions to the X-ray peaks and models the background with a polynomial. The peak areas derived from the fitted functions were then used to determine the concentrations of the elements. For further details the reader is referred to ref. [3] which deals with the analysis of thin and thick targets comprehensively.
3. Results and discussion Table 1 lists the mean values of the levels of the measured elemental impurities. The values for thin targets are quoted in kg/cm’ and for thick targets concentrations are given in mg/kg. The aerodynamic diameter of the particles reaching the sampling head were calculated from the following equation in cgs units
VI. AEROSOL
STUDIES
R.S. Sokhi et al. / Analysis of rmpurities in carbon black
Zn
.: *.. . ... . . .- ‘: 10’ =
:.
“.
._
.
*._ .:
.
: .
.:
.
. .
_. .
.
. .
.._
I
I
0
I
I
60
I
PIXE spectrum
where v is the settling velocity (cm/s), g is the acceleration due to gravity, a is the particle radius (cm), p, and pa are the densities of the particle and air respectively and n is the viscosity of air (dynes/cm*). For the system described earlier v = 8.67 cm/s and assuming the values g = 981 cm/s*, p, = 0.0012 g/cm3 at a temperature of 18” C [4], p, = 1.8 g/cm3 [l] and n = Table 1 Average concentrations
S K Ca Ti V Cr Mn Fe Ni CU Zn
of elemental
impurities
in carbon
Thin target
Thick target
[ P g/cm2 1
[mg/kgl
3.11 0.060 0.100
kO.08 + 0.006 f 0.005
1.110*0.004% 169 &l 259 _+I 18 *2 3.0 +0.4
0.004 0.006 0.051
+ 0.002 +0.005 f 0.005
0.0020 f 0.0005 0.0015 & 0.0006
3 8.7 72 1.0 2.1 1.9
*1 +_0.9 fl f0.2 kO.5 50.5
black
I
180 CHANNEL
Fig. 2. A typical
I
120
I
240
. .
.
. .
.
.
10°
.
.
.
. .
.
.
.
,
.
.
.
.
_.
1
300
NUNBER
of a thick pelletised
carbon
black target.
182.7 x lop6 dyne s/cm*, the aerodynamic diameter (maximum) is calculated to be 40 urn. If laminar flow is assumed then the largest particle to be carried out of the elutriator would have a Stoke’s diameter of 40 urn. Turbulence and other causes of non-laminar flow, such as, wall drag would tend to carry out some larger particles. In general, particles which have physical diameters of less than 40 pm would be able to contaminate the environment and would pose an occupational hazard as the sizes of these particles lie within the definition of inspriability/inhalability [5]. The filters used in this study were the cellulose-ester membrane type. Although care was taken to keep loadings of the filters below 300 kg/cm* the nature of the carbon black particles makes it very difficult to avoid loss of particulate matter during sample handling. Thus, smaller loading than that used here is recommended. Furthermore, an adhesive agent on the filter prior to loading will also reduce this problem. An additional complication when analysing substances like carbon black on filters is that environmental dust particulates can also be trapped on the filters and will lead to erroneous results. However, it is hoped that the analysis
R.S. Sokhi et al. / Ana!vm
of thick pellets of ‘pure’ carbon black will provide an initial set of elemental concentrations characteristic of this substance. It should be noted that because the cellulose-ester filters are not conductors beam currents were kept to less than 10 nA to avoid damaging the targets, whereas, for the carbon black pellets 100 nA beam current was used. This enabled higher sensitivities to be reached as indicated by the concentrations listed in table 1. Several elements such as Ti, V, Cr, Mn. Ni and Cu. which are known to be toxic, particularly carcinogenic [6]. have been identified. Comparison of the thin and thick target results (table 1) reveal some inconsistencies in the ratios of the elemental concentrations. Larger amounts of the heavier elements are observed in the filter targets relative to. say, S. Impurities of Mn, Fe, Cu and Zn elements were also observed in the filters and may partially account for this discrepancy. Although considerable care was taken the possibility of contamination during the loading and transport of the filters also cannot be excluded.
4. Conclusions Thick and thin target analyses of carbon black are presented. Eleven elements ranging from S to Zn have been quantified and it is hoped that this will serve as an initial elemental data set characteristic of carbon black. Simultaneous analysis of these elements gives PIXE
411
of impurities in carbon black
considerable advantages over other techniques such as atomic absorption spectrometry (AAS) which, although requiring complicated sample preparation procedures using acid digestion techniques, is often employed by occupational hygienists for elemental analysis. This work forms an integral part of a major study being carried out at Birmingham University of the toxicological assessment of carbon black in occupational areas. Further work is being planned to improve detection sensitivities by using different filters and sampling heads.
The expertise of the Dynamitron ing staff is appreciated.
accelerator
operat-
References PI I.A.R.C.,
Polynuclear Aromatic Hydrocarbons, part 2. Carbon blacks. Mineral Oils (Lubricant Base Oils - Derived Products) and some Nitroarenes, vol. 33. PI A. Gold. Anal. Chem. 47 (1975) 1469-1472. [31 M.R. Khan and D. Crumpton, Crit. Rev. Anal. Chem. II (1981) 103-155 and 12 (1981) 161-193. 4th ed. [41 R.C. West (ed.). Chemical Engineers’ Handbook. 1963. On the quantitative defi[51 J.H. Vincent and L. Armbruster. nition of the inhalability of airborne dust, Am. Occup. Hyg. 24 (1981) 245-248. [61 H.A. Waldron (ed.). Metals in the Environment (Academic Press, 1980).
VI. AEROSOL
STUDIES