- Email: [email protected]
Simultaneous PIXE-PIGE analysis of thin and thick samples
106
Nuclear
SIMULTANEOUS
PIXE-PIGE
C. BONI ‘), A. CARIDI ‘I CISE Tecnalogie ” Istiiuto di Fish
ANALYSIS
‘I*, E. CEREDA
Instruments
and Methods
in Physics
Research
B49 (1990) 106-110 North-Holland
OF THIN AND THICK SAMPLES
‘), G.M. BRAGA
MARCAZZAN
‘*2) and P. REDAELLI
‘)
P.O. Box 12081, 20134 Milan. 1taJy Generale Applicata dell’llniversitd, Milan, Italy Innoonriw,
In our laboratory PIXE-PIGE techniques have been set up for the analysis of thin samples (up to about 1 mg/cm2) of unknown weight and composition by using a proton beam whose energy is spread over a suitable interval in order to smooth the fluctuations in the prompt gamma reaction yields. The general features of the system as well as some typical applications to the analysis of urban particulate matter and coal fly ashes are discussed. PIGE has also been set up for the analysis of thick targets, to be applied to samples such as coals, for which the thin sample analysis does not have the required sensitivity.
1. Introduction The possibility of a rapid and nondestructive analysis. providing accurate concentration values for as many elements as possible, is one of the most attractive objectives of ion beam analytical techniques [l-3]. In principle, PIXE (particle induced X-ray emission) can detect all elements with Z > 11. but for light elements such as Na, Al and Si the accuracy is rather poor because of the self-absorption corrections, which strongly depend on the sample matrix and morphology. In particular, when dealing with particulate matter, these corrections are strongly affected by the particle sizes whose distributions are in general not unifo~. Moreover, very light elements in practice are not detectable at all. In this context, the simultaneous use of PIXE and PIGE (proton induced gamma ray emission) is a temarkable method for the detection of both light and heavy elements. So far, it has mainly been applied to the analysis of thick samples in many research fields such as geology, biology, art and archaeometry [4- 81. In our laboratory PIXE and PIGE techniques have recently been extended to the analysis of thin samples of unknown weight and composition and made up of particles whose size distribution is in general undetermined. The difficulties of applying PIGE to thin samples, associated with the strong energy dependence of the prompt gamma reaction yields [9], have been overcome by using an energy spread proton beam whose effect is just the smoothing of the fluctuations in the excitation functions. PIXE-PIGE applied to thin samples is a unique method for some applications such as the analysis of atmospheric aerosol and coal fly ashes.
* On leave from CNEA, 0168-583X/~/$03.50 (North-Holland)
Buenos Aires, Argentina.
0 Elsevier Science
Publishers
B.V.
On the other hand, in some applications such as the analysis of coals the sensitivity is very poor. In this case the analysis of thick pelletized samples can be more suitable. In addition, thick sample analysis improves the representativity when dealing with powders available in considerable amount. In our laboratory PIGE has been set up also for the analysis of thick samples in vacuum and an external beam line for both PIXE-PIGE is under way. In this paper the general features of the PIXE-PIGE setup for thin samples as well as some typical applications are presented and a comparison between PIGE detection limits for thick and thin samples is shown.
2. Experimental
2.1. PIXE- PIGE setup A 3.5 MV tandem Van de Graaff accelerator provides a proton beam which after passing through a bending magnet is switched into the PIXE-PIGE beam line whose layout is shown in fig. 1. A detailed description is given in ref. [lo]. For the analysis of thin samples, the peculiar feature is the simultaneous detection of X and gamma rays and the use of a proton beam with a suitable spread in energy obtained by making the beam cross a rotating disc made up of five Al sectors of different thicknesses (0.69, 1.38, 2.07, 2.76 and 3.45 mg/cm2) and of a sixth sector transparent to the beam placed at 40 cm from the irradiation chamber. In this way the proton beam gets a rectangular energy distribution about 270 keV wide resulting from the overlap of the six main components, partly superimposed by the straggling effect. This energy spread is suitable when both the coarse and fine
C. Boni et al. / PIXE- PIGE analysis of thin and thick samples
Fig. 1. Horizontal
fractions
of particulate
matter
cross-sectional
are present.
view of the PIXE-PIGE
When
dealing
only with the fine fraction, a narrow energy spread (about 140 keV), obtained using a rotating disc whose Al quadrants are 0.46, 0.92, 1.38 and 1.84 mg/cm2 thick, can be used. Fig. 2 shows the broadening of the narrow resonance at E, = 3.1 MeV for the 28Si(p, p, y)*‘Si obtained with the 140 and 270 keV energy spreads. The resulting trends can be assumed to be representative of the proton beam energy distribution because of the narrow width of the resonance itself. The energy scale reported on the x-axis for fig. 2B is the proton energy as measured by the analysis magnet (primary energy) and does not correspond to the energy on the sample because of the diffusers. The resulting gamma ray yield trend is reasonably constant over an energy interval larger than the proton energy loss in typical particulate matter samples, thus allowing straightforward quantitative analysis relative to standard reference samples. Similar trends are also obtained for the other reactions [ll]. As shown in fig. 1, before crossing the rotating Al diffuser, the proton beam passes through a 1.5 mg/cm2 Al foil in order to make the beam
beam line and irradiation
107
chamber.
get a uniform charge distribution over a large cross-sectional area on the rotating diffuser. The same effect can be obtained also by using a magnetic defocusing. Special care has been given also to the beam line inner wall shielding by means of a suitable collimation in order to suppress the gamma ray background which is now reduced to a few counts/tr,C for characteristic peaks of Na, Al and Si. As for thick samples, the same beam line is used. The 1.5 mg/cm2 thick Al foil is maintained while no rotating diffusers are used. 2.2. Sample preparation When particulate matter samples are directly collected on Nuclepore filters, no further handling is required. In general, when starting from powders (coals, fly ashes, etc.), a small quantity of previously ground and homogenized material is resuspended in cyclohexane and filtrated through Nuclepore filters which are finally coated with a thin layer of polyvinyl acetate in order to prevent material loss. For thick samples, the
w
(A) . . l
.
Fig. 2. Excitation function for the reaction 28Si(p , p, Y)~~S~. (A) No energy spread of the proton keV (triangles) proton beam energy distribution. The energy scale refers to the proton
beam; (B) 140 keV (circles) and 270 primary energy (see text).
II. EXPERIMENTAL
ARRANGEMENTS
C. Botu et al. / PIXE - PIGE anu!wis
108 Table
of rhm
and thick samples
1
Nuclear
reactions,
gamma
ray
Reaction
Li
‘Li(p.
p, y)‘Li
B
“B(p.
PI Y)“B
F
“F(p,
~1 Y )“F
478 2125 110 197 440 585 1014 1779 1266
‘jNa(p, p, y)“Na 25Mg(~. PI y)‘5Mg “Al(p. p2 v)*‘AI ‘BSi(p, p1 y)*‘Si 3’P(p, p, Y ?‘P
Mg Al Si P
of the PIGE
technique for a collected charge of 60 PC (see text)
g/cm* 1
E, [kevl
19F(p. pzy)19F Na
(DL)
energies and detection limits
DL [ cL
DL [ P g/g1
A
B
C
D
E
0.015 0.600 0.010
0.030 0.600 0.035 _ 0.080 2.000 0.300 0.400 0.300
0.015 0.600 0.025 _ 0.050 1.200 0.150 0.350 0.150
25 100 5 3 10 300 40 250 500
10 90 10 5 20 550 70 320 250
0.030 1.000 0.090 0.250 0.050
powder is ground and blended in preground graphite spiked with yttrium (as internal standard) and finally pressed in pellets. In general coals can be pelletized with no graphite diluition. while for coal fly ashes 1 : 10 dilution is applied in order to reduce the count rate and enhancement effects and to simplify matrix corrections.
3. Results and discussion
2.3. System
3.1. Thin samples
calibration
For the system calibration, Micromatter standard reference samples [12] have been used for thin samples, while for thick samples, home-made pellets containing known amounts of compounds in graphite have been taken as reference material. The linear trend between the number of emitted gamma rays per unit charge against the sample weight, has been checked by irradiating coal fly ash NBS 1633a and urban particulate
Table 2 Average values of concentrations between
( pg/m’
[tvs/m’l
Table 1 reports nuclear reactions and gamma ray energies used for PIGE applications. Columns A, B and C show the detection limits estimated for a blank Nuclepore filter and typical thin fly ash (NBS 1633a) and coal (NBS 1632) samples respectively. about 1 mg/cm2 thick. These values are defined as (3/S)( Nb/ where N, is the number of counts per unit QV2. charge in the background within an energy interval of
in winter (W) and summer (S) periods
and ratio (R)
elements EF
61 S
by filtration of Nuclepore filters
]111.
and %) and enrichment factors (EF)
winter and summer values for the detected
W
matter NBS 1648 samples prepared different amounts of material through
R
W
S
R
W
S
F
0.041
0.033
1.24
0.031
0.039
0.79
5
4
Na
2.18
1.02
2.14
2.15
1.06
2.03
6
2
Al
1.45
1.5
0.97
1.07
1.46
0.73
1
1
Si
5.06
4.54
1.11
3.45
4.33
0.8
1
1
S
7.73
5.6
1.38
5.26
4.57
1.15
K
0.73
0.33
2.21
0.5
0.3
1.67
1
Ca
1.74
1.8
0.97
1.36
1.79
0.76
3
3
Ti
0.065
0.062
1.03
0.044
0.065
0.68
1
1
V
0.069
0.017
4.06
0.046
0.01 I
2.71
21
8
Cr
0.066
0.018
3.67
0.048
0.025
1.92
26
9
1300
1184 1
Mn
0.067
0.054
1.24
0.043
0.057
0.75
4
3
Fe Ni
1.39 0.024
1.35 < 0.005
1.01 _
0.92 0.01 I
1.48 < 0.005
0.62
2 21
2 _
CU
0.048
0.059
0.81
0.034
0.064
0.53
Zn
0.61
0.76
0.8
0.44
0.73
0.6
0.53
1.55
0.56
0.61
0.92
Pb TSP
0.82 146.9
98.2
1.5
43
50
310
660
2510
2200
109
C. Boni et al. / PIXE - P/GE analysis of thin and thick sumples Table 3 Concentration
values of elements
Li [wml NBS1632a Measured Others a’ NBS1635 Measured Others ‘) NBS1633a Measured Others ‘)
detected
by PIGE in pelletized
F bml _
Na Ippml
170+ 10 162 b,
84O_t40 845 f. 50 850 i 40
_
35rtlO _
(2400) 2600 I 200 2600 + 100
210220 221
_ 1.50 + 60 87
17oo.t 100 1900+200 1769
40*10 47 h’ _
samples
Mg @I (0.1) 0.13kO.02 0.13*0.03
_ 0.13io.03 0.10 * 0.02 0.45 + 0.01 0.49 i 0.05 0.48
Si [%l
Al @I 3.07
_
3.41 + 0.25 2.94kO.13
6.21 kO.35 5.8OkO.10 _
(0.32) 0.37 + 0.03 0.30 rt 0.03
0.85 f 0.06 0.52 + 0.02 22.8+0.8 22.3 f 1.3 _
(14) 16.0*0.2 15.0
.I1 Ref. [14]. h’ Ref. [15].
two full widths at half maximum around the gamma peak, the sensitivity S is the yield calcutated as (number of counts per unit charge)/(element unit mass per unit area) and Q is the total irradiated charge that in the present case was 60 PC. Values given in column A should be taken as guide-line data while values reported in columns B and C are more representative since they correspond to actual samples where the Compton contribution due to the different gamma rays enhances the background yield. PIGE technique does not turn out to be particularly sensitive but fortunately the concentrations of light elements of interest are generally high enough in some applications such as coal fly ash and atmospheric aerosol samples. The proposed system has proved quite suitable for the analysis of atmospheric aerosol samples improving the previous routine analyses with F. Na. Al and Si without requiring longer irradiation times. It is now heing applied within an air quality monitoring programme in the city of Milan. Table 2 shows typical data for samples collected in the city centre of Milan at 20 m above ground level. The detection of Na was useful in sorting out some marine aerosol transport events [lo]; Al and Si resulted strongly correlated thus giving evidence of the soil contribution; finally, Al was used to calculate enrichment factors which resulted equivalent to values calculated with respect to Ti. The possibility of analyzing not only integral samples but also size fractionated atmospheric aerosol and fly ash samples collected by means of cascade impactors has been tested as well. Fig. 3 shows the average aerodynamic mass size distributions for some elements detected in a fractionated sample collected with a DeIron DCI-5 cascade impactor in a suburban area for 24 hours. The proposed method is particularly suitable since in these cases the deposit thickness and weight are in general difficult to be estimated thus making self-absorption corrections quite unreliable. Similar results for
both integral and fractionated ported in ref. [13].
fly ash samples
are re-
3.2. Thick samples Within the context of a research programme for the study of combustion processes leading to the partitioning of coal mineral constituents among products such as residual ashes, a complete elemental analysis of the starting coal is required. Typical concentrations are, on the other hand. often very low. To improve the sensitivity and the representativity of the analysis, pelletized samples seem to be more suitable. Test PIGE measurements have been performed on coal and fly ash
Fig. 3. Average aerodynamic mass size distributions for some elements detected by PIXE-PIGE analysis in a fractionated urban particulate matter sample. 11. EXP~Rt~ENTAI~
AR~~GEME~TS
110
C. Boni et al. / PIXE - PIGE ana&sis of thin and thrck samples
samples. Columns D and E of table 1 report the actual detection limits for typical ashes diluted in graphite and coals. The resulting improvement is within one order of magnitude, but is anyhow important for Mg whose concentrations are such that they cannot normally be detected in thin samples. To check the system accuracy standard reference samples NBS 1633a (coal fly ash). NBS 1632 (bituminous coal) and NBS 1635 (subbituminous coal) have been analyzed. The results are reported in table 3.
4. Conclusions The combined and simultaneous use of PIXE and PIGE techniques for the analysis of thin atmospheric aerosol and fly ash samples, thanks to the use of a proton beam with a large energy distribution, proved to be an exceptionally effective method for a rapid, nondestructive overall analysis. In particular, for multi-elemental air pollution studies with size fractionation which gives a small and not measurable mass loading, where PIXE seems to be the only realistic analytical tool, the possibility of quantifying also light elements is a noticeable improvement. When dealing with powders such as coals for which, in general, adequate amounts of material are available, thick sample analysis is more representative, sensitive and of simpler preparation. A. Caridi has carried out this work with the support of the “ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy”.
References [l] T.A. Cahill, Y. Matsuda. D. Shadoan, R.A. Eldred and B.H. Kusko, Nucl. Instr. and Meth. B3 (1984) 263. [2] M.O. Andrae and W.R. Barnard, Nucl. Instr. and Meth. 181 (1981) 383. Nucl. Instr. and Meth. B22 (1987) 356. [31 B.C. Martinsson. [41 L.E. Carlsson. Nucl. Instr. and Meth. B3 (1984) 206. G.R. Palmer, W.E. Hekman and K. [51 J.D. MacArthur. Budd. Nucl. Instr. and Meth. 218 (1983) 519. [61 J.D. Svalbe, M.A. Chandri. K. Traxel. C. Ender and A. Mandel. Nucl. Instr. and Meth. B3 (1984) 651. [71 G.S. Hall and E. Navon. Nucl. Instr. and Meth. B15 (1986) 629. PI P. Duerden, J.R. Bird, E. Clayton. D.D. Cohen and B.F. Leach, Nucl. Instr. and Meth. B3 (1984) 419. and V. De [91 C. Boni, E. Cereda. G.M. Braga Marcazzan Tomasi. Nucl. Instr. and Meth. B35 (1988) 80. [lOI C. Boni, A. Caridi, E. Cereda and G.M. Braga Marcazzan. Nucl. Instr. and Meth. B47 (1990) in press. [111 C. Boni, E. Caruso. E. Cereda. G.M. Braga Marcazzan and P. Redaelli, Nucl. Instr. and Meth. B40/41 (1989) 620. u21 L.M. Heageny and J.S. Heageny. Nucl. Instr. and Meth. 167 (1979) 137. 1131 C. Boni, A. Caridi, E. Cereda, and G.M. Braga Marcazzan. Nucl. Instr. and Meth. B45 (1990) 352. u41 M.S. Germani et al.. Anal. Chem. 52 (2) (1980) 240. u51 J.R. Bird and E. Clayton, Nucl. Instr. and Meth. 218 (1983) 525.
Nuclear
SIMULTANEOUS
PIXE-PIGE
C. BONI ‘), A. CARIDI ‘I CISE Tecnalogie ” Istiiuto di Fish
ANALYSIS
‘I*, E. CEREDA
Instruments
and Methods
in Physics
Research
B49 (1990) 106-110 North-Holland
OF THIN AND THICK SAMPLES
‘), G.M. BRAGA
MARCAZZAN
‘*2) and P. REDAELLI
‘)
P.O. Box 12081, 20134 Milan. 1taJy Generale Applicata dell’llniversitd, Milan, Italy Innoonriw,
In our laboratory PIXE-PIGE techniques have been set up for the analysis of thin samples (up to about 1 mg/cm2) of unknown weight and composition by using a proton beam whose energy is spread over a suitable interval in order to smooth the fluctuations in the prompt gamma reaction yields. The general features of the system as well as some typical applications to the analysis of urban particulate matter and coal fly ashes are discussed. PIGE has also been set up for the analysis of thick targets, to be applied to samples such as coals, for which the thin sample analysis does not have the required sensitivity.
1. Introduction The possibility of a rapid and nondestructive analysis. providing accurate concentration values for as many elements as possible, is one of the most attractive objectives of ion beam analytical techniques [l-3]. In principle, PIXE (particle induced X-ray emission) can detect all elements with Z > 11. but for light elements such as Na, Al and Si the accuracy is rather poor because of the self-absorption corrections, which strongly depend on the sample matrix and morphology. In particular, when dealing with particulate matter, these corrections are strongly affected by the particle sizes whose distributions are in general not unifo~. Moreover, very light elements in practice are not detectable at all. In this context, the simultaneous use of PIXE and PIGE (proton induced gamma ray emission) is a temarkable method for the detection of both light and heavy elements. So far, it has mainly been applied to the analysis of thick samples in many research fields such as geology, biology, art and archaeometry [4- 81. In our laboratory PIXE and PIGE techniques have recently been extended to the analysis of thin samples of unknown weight and composition and made up of particles whose size distribution is in general undetermined. The difficulties of applying PIGE to thin samples, associated with the strong energy dependence of the prompt gamma reaction yields [9], have been overcome by using an energy spread proton beam whose effect is just the smoothing of the fluctuations in the excitation functions. PIXE-PIGE applied to thin samples is a unique method for some applications such as the analysis of atmospheric aerosol and coal fly ashes.
* On leave from CNEA, 0168-583X/~/$03.50 (North-Holland)
Buenos Aires, Argentina.
0 Elsevier Science
Publishers
B.V.
On the other hand, in some applications such as the analysis of coals the sensitivity is very poor. In this case the analysis of thick pelletized samples can be more suitable. In addition, thick sample analysis improves the representativity when dealing with powders available in considerable amount. In our laboratory PIGE has been set up also for the analysis of thick samples in vacuum and an external beam line for both PIXE-PIGE is under way. In this paper the general features of the PIXE-PIGE setup for thin samples as well as some typical applications are presented and a comparison between PIGE detection limits for thick and thin samples is shown.
2. Experimental
2.1. PIXE- PIGE setup A 3.5 MV tandem Van de Graaff accelerator provides a proton beam which after passing through a bending magnet is switched into the PIXE-PIGE beam line whose layout is shown in fig. 1. A detailed description is given in ref. [lo]. For the analysis of thin samples, the peculiar feature is the simultaneous detection of X and gamma rays and the use of a proton beam with a suitable spread in energy obtained by making the beam cross a rotating disc made up of five Al sectors of different thicknesses (0.69, 1.38, 2.07, 2.76 and 3.45 mg/cm2) and of a sixth sector transparent to the beam placed at 40 cm from the irradiation chamber. In this way the proton beam gets a rectangular energy distribution about 270 keV wide resulting from the overlap of the six main components, partly superimposed by the straggling effect. This energy spread is suitable when both the coarse and fine
C. Boni et al. / PIXE- PIGE analysis of thin and thick samples
Fig. 1. Horizontal
fractions
of particulate
matter
cross-sectional
are present.
view of the PIXE-PIGE
When
dealing
only with the fine fraction, a narrow energy spread (about 140 keV), obtained using a rotating disc whose Al quadrants are 0.46, 0.92, 1.38 and 1.84 mg/cm2 thick, can be used. Fig. 2 shows the broadening of the narrow resonance at E, = 3.1 MeV for the 28Si(p, p, y)*‘Si obtained with the 140 and 270 keV energy spreads. The resulting trends can be assumed to be representative of the proton beam energy distribution because of the narrow width of the resonance itself. The energy scale reported on the x-axis for fig. 2B is the proton energy as measured by the analysis magnet (primary energy) and does not correspond to the energy on the sample because of the diffusers. The resulting gamma ray yield trend is reasonably constant over an energy interval larger than the proton energy loss in typical particulate matter samples, thus allowing straightforward quantitative analysis relative to standard reference samples. Similar trends are also obtained for the other reactions [ll]. As shown in fig. 1, before crossing the rotating Al diffuser, the proton beam passes through a 1.5 mg/cm2 Al foil in order to make the beam
beam line and irradiation
107
chamber.
get a uniform charge distribution over a large cross-sectional area on the rotating diffuser. The same effect can be obtained also by using a magnetic defocusing. Special care has been given also to the beam line inner wall shielding by means of a suitable collimation in order to suppress the gamma ray background which is now reduced to a few counts/tr,C for characteristic peaks of Na, Al and Si. As for thick samples, the same beam line is used. The 1.5 mg/cm2 thick Al foil is maintained while no rotating diffusers are used. 2.2. Sample preparation When particulate matter samples are directly collected on Nuclepore filters, no further handling is required. In general, when starting from powders (coals, fly ashes, etc.), a small quantity of previously ground and homogenized material is resuspended in cyclohexane and filtrated through Nuclepore filters which are finally coated with a thin layer of polyvinyl acetate in order to prevent material loss. For thick samples, the
w
(A) . . l
.
Fig. 2. Excitation function for the reaction 28Si(p , p, Y)~~S~. (A) No energy spread of the proton keV (triangles) proton beam energy distribution. The energy scale refers to the proton
beam; (B) 140 keV (circles) and 270 primary energy (see text).
II. EXPERIMENTAL
ARRANGEMENTS
C. Botu et al. / PIXE - PIGE anu!wis
108 Table
of rhm
and thick samples
1
Nuclear
reactions,
gamma
ray
Reaction
Li
‘Li(p.
p, y)‘Li
B
“B(p.
PI Y)“B
F
“F(p,
~1 Y )“F
478 2125 110 197 440 585 1014 1779 1266
‘jNa(p, p, y)“Na 25Mg(~. PI y)‘5Mg “Al(p. p2 v)*‘AI ‘BSi(p, p1 y)*‘Si 3’P(p, p, Y ?‘P
Mg Al Si P
of the PIGE
technique for a collected charge of 60 PC (see text)
g/cm* 1
E, [kevl
19F(p. pzy)19F Na
(DL)
energies and detection limits
DL [ cL
DL [ P g/g1
A
B
C
D
E
0.015 0.600 0.010
0.030 0.600 0.035 _ 0.080 2.000 0.300 0.400 0.300
0.015 0.600 0.025 _ 0.050 1.200 0.150 0.350 0.150
25 100 5 3 10 300 40 250 500
10 90 10 5 20 550 70 320 250
0.030 1.000 0.090 0.250 0.050
powder is ground and blended in preground graphite spiked with yttrium (as internal standard) and finally pressed in pellets. In general coals can be pelletized with no graphite diluition. while for coal fly ashes 1 : 10 dilution is applied in order to reduce the count rate and enhancement effects and to simplify matrix corrections.
3. Results and discussion
2.3. System
3.1. Thin samples
calibration
For the system calibration, Micromatter standard reference samples [12] have been used for thin samples, while for thick samples, home-made pellets containing known amounts of compounds in graphite have been taken as reference material. The linear trend between the number of emitted gamma rays per unit charge against the sample weight, has been checked by irradiating coal fly ash NBS 1633a and urban particulate
Table 2 Average values of concentrations between
( pg/m’
[tvs/m’l
Table 1 reports nuclear reactions and gamma ray energies used for PIGE applications. Columns A, B and C show the detection limits estimated for a blank Nuclepore filter and typical thin fly ash (NBS 1633a) and coal (NBS 1632) samples respectively. about 1 mg/cm2 thick. These values are defined as (3/S)( Nb/ where N, is the number of counts per unit QV2. charge in the background within an energy interval of
in winter (W) and summer (S) periods
and ratio (R)
elements EF
61 S
by filtration of Nuclepore filters
]111.
and %) and enrichment factors (EF)
winter and summer values for the detected
W
matter NBS 1648 samples prepared different amounts of material through
R
W
S
R
W
S
F
0.041
0.033
1.24
0.031
0.039
0.79
5
4
Na
2.18
1.02
2.14
2.15
1.06
2.03
6
2
Al
1.45
1.5
0.97
1.07
1.46
0.73
1
1
Si
5.06
4.54
1.11
3.45
4.33
0.8
1
1
S
7.73
5.6
1.38
5.26
4.57
1.15
K
0.73
0.33
2.21
0.5
0.3
1.67
1
Ca
1.74
1.8
0.97
1.36
1.79
0.76
3
3
Ti
0.065
0.062
1.03
0.044
0.065
0.68
1
1
V
0.069
0.017
4.06
0.046
0.01 I
2.71
21
8
Cr
0.066
0.018
3.67
0.048
0.025
1.92
26
9
1300
1184 1
Mn
0.067
0.054
1.24
0.043
0.057
0.75
4
3
Fe Ni
1.39 0.024
1.35 < 0.005
1.01 _
0.92 0.01 I
1.48 < 0.005
0.62
2 21
2 _
CU
0.048
0.059
0.81
0.034
0.064
0.53
Zn
0.61
0.76
0.8
0.44
0.73
0.6
0.53
1.55
0.56
0.61
0.92
Pb TSP
0.82 146.9
98.2
1.5
43
50
310
660
2510
2200
109
C. Boni et al. / PIXE - P/GE analysis of thin and thick sumples Table 3 Concentration
values of elements
Li [wml NBS1632a Measured Others a’ NBS1635 Measured Others ‘) NBS1633a Measured Others ‘)
detected
by PIGE in pelletized
F bml _
Na Ippml
170+ 10 162 b,
84O_t40 845 f. 50 850 i 40
_
35rtlO _
(2400) 2600 I 200 2600 + 100
210220 221
_ 1.50 + 60 87
17oo.t 100 1900+200 1769
40*10 47 h’ _
samples
Mg @I (0.1) 0.13kO.02 0.13*0.03
_ 0.13io.03 0.10 * 0.02 0.45 + 0.01 0.49 i 0.05 0.48
Si [%l
Al @I 3.07
_
3.41 + 0.25 2.94kO.13
6.21 kO.35 5.8OkO.10 _
(0.32) 0.37 + 0.03 0.30 rt 0.03
0.85 f 0.06 0.52 + 0.02 22.8+0.8 22.3 f 1.3 _
(14) 16.0*0.2 15.0
.I1 Ref. [14]. h’ Ref. [15].
two full widths at half maximum around the gamma peak, the sensitivity S is the yield calcutated as (number of counts per unit charge)/(element unit mass per unit area) and Q is the total irradiated charge that in the present case was 60 PC. Values given in column A should be taken as guide-line data while values reported in columns B and C are more representative since they correspond to actual samples where the Compton contribution due to the different gamma rays enhances the background yield. PIGE technique does not turn out to be particularly sensitive but fortunately the concentrations of light elements of interest are generally high enough in some applications such as coal fly ash and atmospheric aerosol samples. The proposed system has proved quite suitable for the analysis of atmospheric aerosol samples improving the previous routine analyses with F. Na. Al and Si without requiring longer irradiation times. It is now heing applied within an air quality monitoring programme in the city of Milan. Table 2 shows typical data for samples collected in the city centre of Milan at 20 m above ground level. The detection of Na was useful in sorting out some marine aerosol transport events [lo]; Al and Si resulted strongly correlated thus giving evidence of the soil contribution; finally, Al was used to calculate enrichment factors which resulted equivalent to values calculated with respect to Ti. The possibility of analyzing not only integral samples but also size fractionated atmospheric aerosol and fly ash samples collected by means of cascade impactors has been tested as well. Fig. 3 shows the average aerodynamic mass size distributions for some elements detected in a fractionated sample collected with a DeIron DCI-5 cascade impactor in a suburban area for 24 hours. The proposed method is particularly suitable since in these cases the deposit thickness and weight are in general difficult to be estimated thus making self-absorption corrections quite unreliable. Similar results for
both integral and fractionated ported in ref. [13].
fly ash samples
are re-
3.2. Thick samples Within the context of a research programme for the study of combustion processes leading to the partitioning of coal mineral constituents among products such as residual ashes, a complete elemental analysis of the starting coal is required. Typical concentrations are, on the other hand. often very low. To improve the sensitivity and the representativity of the analysis, pelletized samples seem to be more suitable. Test PIGE measurements have been performed on coal and fly ash
Fig. 3. Average aerodynamic mass size distributions for some elements detected by PIXE-PIGE analysis in a fractionated urban particulate matter sample. 11. EXP~Rt~ENTAI~
AR~~GEME~TS
110
C. Boni et al. / PIXE - PIGE ana&sis of thin and thrck samples
samples. Columns D and E of table 1 report the actual detection limits for typical ashes diluted in graphite and coals. The resulting improvement is within one order of magnitude, but is anyhow important for Mg whose concentrations are such that they cannot normally be detected in thin samples. To check the system accuracy standard reference samples NBS 1633a (coal fly ash). NBS 1632 (bituminous coal) and NBS 1635 (subbituminous coal) have been analyzed. The results are reported in table 3.
4. Conclusions The combined and simultaneous use of PIXE and PIGE techniques for the analysis of thin atmospheric aerosol and fly ash samples, thanks to the use of a proton beam with a large energy distribution, proved to be an exceptionally effective method for a rapid, nondestructive overall analysis. In particular, for multi-elemental air pollution studies with size fractionation which gives a small and not measurable mass loading, where PIXE seems to be the only realistic analytical tool, the possibility of quantifying also light elements is a noticeable improvement. When dealing with powders such as coals for which, in general, adequate amounts of material are available, thick sample analysis is more representative, sensitive and of simpler preparation. A. Caridi has carried out this work with the support of the “ICTP Programme for Training and Research in Italian Laboratories, Trieste, Italy”.
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