Determination of trace pollutants in urban snow using PIXE techniques

Nuclear Instruments and Methods 193 (1982) 323-329 North-Holland Publishing Company

323

P a n VII. P I X E

D E T E R M I N A T I O N OF T R A C E P O L L U T A N T S IN URBAN SNOW USING P I X E T E C H N I Q U E S * R.E. J E R V I S , S. L A N D S B E R G E R Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada, M5S IA4

R. L E C O M T E * * , P. P A R A D I S and S. M O N A R O Department de Physique Nucl£aire, Universit£ de Montrdal, Case Postale 6128, Montreal, Quebec, Canada, H3C 3J7

The absolute concentrations of 25 elements in urban snow samples have been determined by PIXIEtechniques. Detection limits between 0.2 and 147 ppb for the following elements: Na, Mg, AI, P, C1, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Br, Rb, Sr, Mo, Cd and Pb were achieved by a simple pre-concentration method. Proton beams of 1.6 MeV and 3.0 MeV were employed to bombard the targets. Comparisons of unconcentrated and pre-concentrated snow samples, blank determinations, sensitivity measurements vs bombarding time and reproducibility tests were also carried out.

1. Introduction It is generally perceived today that one of the most prominent and vexing challenges in the field of public health is the proper investigation and control of air pollution in the highly industralized cities. O v e r the last years it has become increasingly clear that substances released into the air by various industrial plants and automobile exhausts are known to produce adverse physiological changes in man. These airborne pollutants include oxides of nitrogen and sulphur, photochemical oxidants, synthetic organic and inorganic compounds, fine particulates and trace metals. In particular, the detection and m e a s u r e m e n t of trace elements has received a lot of attention [1]. Their primary paths back down to earth are by rain, snow or dry deposition. Invariably these trace pollutants cause epidemiological damage by two main pathways, i.e., absorption through the respiratory system and from drinking water and food chains. The knowledge of these types of toxins in the environment is extremely important in correlating their concentrations with morbidity and mortality rates. As well, a knowledge of their amounts is essential in setting toxicological

standards for the public at large. It is therefore clear that a reliable and precise m e t h o d is needed to detect and analyse qualitatively and quantitatively these pollutants. Precipitation, wet and dry, is one of the prime indicators of atmospheric pollution. Moreover, it can prove to be very useful in giving a general profile of trace elemental pollution of particular areas. With the advent of highly sensitive nuclear analytical microprobes, such as proton-induced X-ray emission (PIXE), it is possible to detect the presence of trace elements in the range of parts per billion. In fact its sensitivity and reliability in multi-elemental analysis has been well documented [2-4]. Although trace elemental analysis of aqueous environmental samples has received little attention [5-7], our group has successfully used this technique in studying fiver water pollution [8, 9] and trace elemental concentrations of other aqueous solutions [10]. It is the purpose of this work to report on the use of P I X E techniques in the quantitative determination of elemental concentrations in snow from an industrialized city; Montr6al, Qu6bec, Canada.

2. Experimental procedures * This work is supported by the Natural Sciences and Engineering Research Council of Canada. ** D6partement de M&ieeine Nucl6aire et Radiobiologie, Centre Hospitalier Universitaire de Sherbrooke, Sherbrooke, Qu6bec, Canada, J1H 5N4.

2.1.. Sample collection

Four samples, representing the north (N), north-west (NW), south-west (SW) and south-

0029-554X/82/0000-0000/$02.75 ~) 1982 North-Holland

vii. PIXE

324

R.E. Jervis et al. / Trace pollutants in urban snow

east (SE) parts of the island of Montr6al, were gathered over a 24 h period. To obtain the maximum amount of undisturbed snow in a single season, before any meltdown, the last week of February 1979 was chosen. T h e snow, about ten liter per site, was collected using a 95 cm x 11 cm pre-cleaned plastic cylinder which was inserted into the snow at the same approximate depth at each of the locations. The sampling sites were chosen away from the roadsides to avoid large quantities of salt sprinkled on them. The snow samples were then placed in polyethylene bags and kept in cold storage to minimize any adsorption on the sides of the bags and to prevent algae growth.

inserted into a Millipore Swinex water filtration apparatus. An acid-washed pre-weighted Pyrex flask collected the filtered snow-water. The Swinex apparatus was modified in two ways: (a) the standard viton O-ring was replaced with a teflon O-ring to prevent any external contamination [11] (b) a teflon fitting tube from the bottom of the Swinex to the Pyrex flask was constructed, so that a low pressure vacuum could be applied to the flask to speed up the filtration process. After filtration, the flask was pre-frozen in a mixture of dry-ice and then quickly placed in a freeze-drier under a pressure of 200/zm of Hg for a period of about 36 h. At the end of the freeze-drying time the flask was subjected to an ultrasonic water-bath for fifteen minutes to prevent any adsorption onto the walls. A quantity of 2 c m 3 of high purity acid (Ultrex HNO3) was added and swirled about to re-dissolve the residue. The ultra-sound and swirling operation was

2.2. S a m p l e preparation ]:or P I X E analysis

A b o u t 100g of frozen snow was filtered through a 0.45/xm H A type Millipore filter,

5000

--

NO

X I/IQ U N C O N C E N T R A T E D SNOW ( SITE 15 ) PROTON ENERGY: I 6 MeV T I M E 7 2 0 0 SECONDS

Si

S i ;~,.

3000

-

i

!

:

w

' I

: :

_J

I000

--

Z Z

~

i

!

.

i :.

"~ . < L

"

. : / ~

Ne

IOOm~ F R E E Z E - D R I E D S N O W ( S I T E 15) CONCENTRATION FACTOR= 5 0 PROTON E N E R G Y : 1 6 M e V T I M E ; 5 6 0 0 SECONDS

Si 7000

t-

JLMg

Z

-.

5000

Co

Ae

~.

K

i i

i¢': -

-

'

:(

1 ,. "-':

I00

:

!11

5000

I000

.

X 1/10

tlr" hi ¢~

o

K

'

T 0

(,0

~.

' CI

: /

Co (internal slondord)

CO

.,, ' ~

,,-:

200

-~,.

:

300 CHANNEL

F i g . 1. C h a r a c t e r i s t i c X - r a y s p e c t r a f r o m u n c o n c e n t r a t e d w i t h 1.6 M e V p r o t o n s .

400

500

600

NUMBER

(top-half) and pre-concentrated

(bottom-half)

snow sample bombarded

R.E. Jervis et al.

UNCONCENTRATED SNOW SITE(15) PROTON ENERGY~ 3 0 MeV TIME:7200 SECONDS

Co Ti VCr Fe Ni Cu i Zn iMn

104

K~'i

io 3

•

7

"

j

102 ,:•-r? / i01 w z z 7-

325

Trace pollutants in urban snow

Pb Se Br

,..:,, , .~

sr

.4

L" ~

_

1

•'..:7.:.%" ...

": !.' ,.

CQ

F, ,,co z.

!. i /~ivcr L h'4, I [Mn

O9 F-z

I04

0 t~ I 0 3

IOOme FREEZE- DRIED SNOW (SITEI5) CONCENTRATION FACTOR, 50 PROTON ENERGY, 3 0 M e V TIME, 7 2 0 0 SECONDS

i

o. 105 _

I

, \~7

] ~7 Pb SeBr

~-I

_ :. .,

i02 _

t

"

'

'

Rb

:'..~':

vij:...7

,; :;.

~

:-'

~ .-

.t.g

. .

St

i?

4

,' !

Cd

?,

.... ,:.. :,~..~ .i

i01

..':'~:'.

_

...

I

I

I00

200

t 3OO

I 400

CHANNEL

'

,

!!

='-."

: .:

.. .....

,:

.

I ............ i .......... 500 600

i .......... 700

NUMBER

Fig. 2. Characteristic X-ray spectra from unconcentrated (top-half) and pre-concentrated (bottom-half) snow sample b o m b a r d e d with 3.0 M e V protons.

subsequently repeated. The entire procedure was repeated twice with samples of distilled and deionized water used as blanks. The targets were prepared according to a procedure used in previous work on fiver water pollution [8,9]. A quantity of 0.2cm 3 of the preconcentrated snow sample was doped with 0.2 cm 3 of a cobalt internal standard (300 ppm for the samples and 30 ppm for the blanks). Cobalt was chosen as the internal standard since this element was not present in any of the samples. Then 0.02 cm 3 of the well-stirred solution was deposited on a 0.1 # m Nuclepore filter and was then quickly placed in a vacuum chamber and freeze-dried for about two hours. Targets from unconcentrated snow water samples were also prepared for the purpose of comparison.

2.3. Experimental set-up The completely automated set-up incorporat-

ing an Ortec Si(Li) detector having a 160 eV (fwhm) resolution is fully described elsewhere [12]. The X-ray yields were measured with the targets at 45 ° with respect to both the beam axis and the detector. The spectra were recorded on a 1024-channel A D C interfaced to a CDC 3100 computer. Typical X-ray spectra are shown in figs. 1 and 2. The analysis was done using a completely automated computer program purposely developed at the Laboratoire de Physique Nucl6aire de l'Universit6 de Montr6al [12, 13].

3. R e s u l t s a n d d i s c u s s i o n s

3.1. Sensitivity and blank determinations Preliminary tests on unconcentrated filtered snow samples (top halves of figs. 1 and 2) gave spectra poor in statistics with few characteristic X-rays. Even after two hours of bombardment VII. PIXE

R.E. Jervis et al. / Trace pollutants in urban snow

326

the detection limits (defined as three times the standard deviation of the background under the peak in an interval of two times the fwhm) [4] were unacceptable. The use of the pre-concentration techniques as outlined in section 2.2 gave dramatic increases in sensitivity. This can be observed visually in the bottom halves of figs. 1 and 2 and from the graph showing elemental sensitivity vs atomic n u m b e r Z presented in fig. 3. As can be seen these curves, which follow the expected trend [14], give an approximate gain in sensitivity of 50 for all the elements studied. Optimization of bombarding time was also carried out. Fig. 4 demonstrates the sensitivity increase between 15 min and 120min. It was felt that a one hour bombarding time was sufficiently long to achieve good statistics at 1.6MeV. An increase in time beyond one hour did not result in any higher precision for the elements Mg and AI. This was due to the high bremsstrahlung background at the low end of the energy spectrum. The other element of potential interest, notably sulphur, had sufficient statistics. The runs at 3 . 0 M e V were all conducted with bombarding times of 120 min to obtain m a x i m u m precision. Two blank determinations, with distilled and de-ionized water, were done. Fig. 5 shows typical spectra for 1.6 and 3.0 proton beams, their concentrations are presented in the ninth column of table 1. These elements represent the con-

No 5000

SENSITIVITY OF Ka X'RAYS VS Z OF ELEMENT OF UNCONCENTRATED AND

Mg

"At

lO00 500

PRE-CONCENTRATED

i ~SC'KcQ Ti

oo i ! m

O. 0,.

>-

l0

~-

5

Mo

co

z~ .

T • Nicu

Ag ,, "

L a X-RAYS ....

I ! i,i

,I, ,

i iI li I;

0.5 I

I

14

I

I

I

I

18 22 26

'

t t~B, ,!

~ i

I0

PROTONS

3 . 0 MeV

PROTONS

500

I

. . '~ " ~

O0 50

....

.

,~

- ,~__ _

A

Ill I0

. ..

5

,,~

--

No

:::8 ~, 8:

O. O.

)-

.

*,Mg ~p . . . .

~,s

Co

- . ::~ _~.~

-

I/)

~Ti

°V

~

zn',Aa.sa0.1

I I 900 1800

~

Fe

::-%

'1~. . . . . . . . ! Rb 3600 "/~00

T I M E (SECONDS)

Fig. 4. E l e m e n t a l sensitivity vs p r o t o n b o m b a r d i n g t i m e .

taminations coming from the nitric acid and the Nuclepore filter. Since the same amount of acid was added to all the snow samples, the contaminants could be measured and subsequantly subtracted from the results, without loss of precision. It is well-worth noting that elements such as sodium and chlorine, often prevalent in snow, did not interfere significantly in the 1.6MeV spectra. This was due to the relatively low efficiency of detection for sodium and to the evaporation of most of the chlorine due to the preparation procedure and to the heat dissipated by the proton beam.

I

t

50 34. 58 42

3.2. Experimental results and reproducibility

Hg Pb

y'~

L I ! "Z"A~SeRS~

"'

0,1

1.6 MeV

•

Cd

[ Mn '~Fe

i

''

~~"

?

~

-

o

TV r

Ii

50 • ~

SNOW (SITE 15)

" UNCONCENTRATED o FREEZE-DRIED

S E N S I T I V I T Y VS BOMBARDING T I M E

I

I .-''1

4 6 50

I

8 0 84

Z OF ELEMENT

Fig. 3. E l e m e n t a l s e n s i t i v i t y o f u n c o n c e n t r a t e d a n d p r e - c o n c e n t r a t e d s n o w s a m p l e vs a t o m i c n u m b e r Z .

Twenty-five trace elemental concentrations from four sample sites are presented in the first five columns of table 1. All these results have been corrected for the blank determinations which are shown in column nine, while the average sensitivities are tabulated in the eighth column. In dealing with experiments such as these, which encompass pre-concentration, volumetric pipetting, doping, freeze-drying techniques, etc., it is of the utmost importance

R.E. Jervis et al. / Trace pollutants in urban snow

327

Xl/lO BLANK:lOOm{ DE- IONIZED WATER CONCENTRATION FACTOR : 5 0 PROTON ENERGY: 16 MeV TIME; 7 2 0 0 SECONDS

1200 Si

! J

800

•.

.:

Co (internal standard )

•~/."~" • *

: _I w z z <

:

:

400

S !

\\

'.2

f'!

I

:<~.~"~v:..--_~-.'.~'~,.',; " oc I,I O-

Co

104

. . . .

BLANK:IOOm~ DE-IONIZED WATER

Cr Fe NiCuZn

CONCENTRATION FACTOR: 5 0 PROTON ENERGY~ 5,OMeV T I M E : 7 2 0 0 SECONDS

oo

I-z 0 o

" ";

i0 3 /

Br

-.~ ~ -" " "J: i J i l a; ..! '*

:

102

Pb

i' ::i,! ,

'i

i

1 i.

!: ,

i01

--

)

".i_:~..... , .

:. ;"

i

I

I

I

i

I

I

I00

200

300

400

500

600

700

CHANNEL NUMBER F i g . 5. C h a r a c t e r i s t i c X - r a y s p e c t r a f r o m d i s t i l l e d a n d d e - i o n i z e d and with 3.0 MeV protons (bottom half).

that reproducibility tests be undertaken. To that end, three additional separate targets, from an arbitrary pre-concentrated stock solution (NW site), were prepared. The standard deviation of the reproducibility results are shown in the sixth column. It should be pointed out that these reproducibility tests did not begin with three separate 100cm 3 samples of frozen snow, but after the additions of nitric acid. This is obvious in that there was no way of knowing if the entire original ten liter sample was totally homogeneous in trace elemental distribution. As well, a comparison of the elements analysed without pre-concentration to those with, agreed Well within the assigned error bars. This consequently gave us confidence that any of the trace elements which may have adhered to the walls of the flasks after the freeze-drying procedure, were completely re-dissolved in the acid solution. The overall error, presented in the seventh

water sample blank bombarded

w i t h 1.6 M e V p r o t o n s ( t o p - h a l f )

column is the sum of the standard deviation of the reproducibility, pipetting procedure (2%), relative detection efficiency (3%) and the standard deviation of the internal doping standard (3.7% for 1 . r M e V and 3.3% for 3.0 MeV). A total of .10 elements have an overall error of 17% or less. The ones which are directly situated on the high bremsstralung background, notably Mg and AI for the 1.6MeV proton bombardment and Ti, V and Cr for the 3.0 MeV proton bombardment, have as expected higher overall errors. The remaining elements have overall errors between 22--60%. This is caused by their poorer sensitivities or by their extremely low concentrations (1 ppb or less). Some of the important heavy elements such as Mn, Fe, Ni, Cu, Zn, Se and Pb can be easily detected with good precision using PIXE. Other elements of interest such as Mg, AI, Ti and V may be detected but with less precision. Sulphur VII. PIXE

328

R.E. Jervis et al. / Trace pollutants in urban snow

Table 1 Final results" Element

Na Mg AI P S CI K Ca Ti V Cr Mn Fe Co Ni Cu Zn As Se Br Rb Sr Mo Cd Pb

N (ppb)

SE (ppb)

SW (ppb)

NW (ppb)

Reproducibility %b Standard Deviation

6406 324 93 101 1310 242 879 1978 13 5 <7 9 27 <0.5 63 24 97 <0.2 0.4 7 0.7 5 <0.8 15 12

13845 429 100 151 2665 455 1930 6486 8 7 <7 8 2 <0.5 48 35 106 <0.2 <0.2 9 0.8 19 <0.8 3 3

9084 403 <37 27 1650 240 846 3170 13 4 <7 25 34 0.7 20 14 84 <0.2 0.8 9 0.4 9 <0.8 <2 7

13788 192 77 88 871 253 746 1459 19 3 <7 6 39 1 48 22 72 <0.2 0.5 4 1 4 <0.8 6 9

8.1 30.4 33.2 13.1 1.5 8.4 3.8 5.2 16.8 40.7 ND f 5.5 10.0 8.5 3.0 1.5 1.5 ND 21.5 18.0 24.6 15.8 ND 51 12.5

% ErrorC

-+17 -+39 ---42 -+22 -10 -+17 -+13 -+14 -+25 -+50 ND -+14 -+19 -+17 -+12 -+10 _+10 ND -+30 -+26 -+33 -+24 ND -+60 -+21

Sensitivityd (ppb)

Blank" (ppb)

127 45 37 27 19 18 13 13 4 3 2 1 0.9 0.5 0.3 0.2 0.2 0.2 0.3 0.2 0.2 (1.2 0.8 2 0.5

<516 <72 <52 <41 74 -<37 -<50 <20 <4 <3 7 0.8 15 <0.6 4 5 12 <0.2 <0.2 7 <0.3 <0.2 <0.5 <1 2

a Final results with blank contaminations subtracted. b Reproducibility of 4 prepared samples, NW site. c Error analysis is the sum of standard deviation of reproducibility, pipetting (2%), relative detection efficiency (3%), and standard deviation reproducibility of the internal standard: for 1.6 MeV, (3.7%), for 3.0 MeV, (3.3%). d Sensitivity is the average of the four sample sites. e Blank contaminations are the average two samples. f ND Not detected.

w h i c h h a s l o n g b e e n i m p o r t a n t in s t u d y i n g u r b a n city air p o l l u t i o n a n d m o r e r e c e n t l y in a c i d p r e c i p i t a t i o n h a s b e e n m e a s u r e d in its e l e m e n t a l f o r m w i t h n o t e w o r t h y success. A l l t h e e x i s t i n g m e t h o d s m e a s u r e t h e specific c o m p o u n d s of s u l p h u r (e.g. s u l p h i d e s , s u l p h a t e s a n d s u l p h i t e s ) . E v e n t h e s e n s i t i v e t e c h n i q u e s of a t o m i c abs o r p t i o n a n d n e u t r o n a c t i v a t i o n a n a l y s i s a r e unsuited for this particular element. PIKE offers a v e r y p r e c i s e , r e l i a b l e a n d q u i c k m e t h o d in d e t e r m i n i n g t h e t o t a l s u l p h u r c o n c e n t r a t i o n , as w e l l as o t h e r n u m e r o u s e l e m e n t s in p r e c i p i t a t i o n . I m p r o v e m e n t s in s e n s i t i v i t y f o r all t h e e l e m e n t s c a n b e a c h i e v e d by p r e c o n c e n t r a t i n g higher amounts of snow water. Furthermore, the u s e of h i g h e r p r o t o n e n e r g i e s , w h i c h w o u l d l e a d

to higher X-ray excitation cross-sections for the heavier elements, could yield better detection limits and precision for those elements.

4. Conclusions It h a s b e e n s h o w n t h a t P I K E t e c h n i q u e s , in conjunction with preconcentration freeze-drying methods, offer a suitable and convenient way to i n v e s t i g a t e t r a c e e l e m e n t a l p o l l u t i o n in s n o w , as w e l l as in rain, r i v e r o r l a k e w a t e r . A t o t a l of 25 elements can be measured, many with a prec i s i o n of 1 7 % o r b e t t e r . In s u c h a w a y a g e n e r a l profile of trace elemental atmospheric pollution can be determined.

R.E. Jervis et al. / Trace pollutants in urban snow

References [1] P.B. Hammond and R.P. Beliles, Casarett and Doull's Toxicology: The Basic Science of Poisons, eds., J. Doull, C.D. Klaassen and M.O. Amdur, (Macmillan, London, 1980) ch. 17 and references therein. [2] S.A.E. Johansson and T.B. Johansson, Nucl. Instr. and Meth. 137 (1976) 473. [3] T.A. Cahill, Ann. Rev. Nucl. Part. Sci. 30 (1980) 211. [4] S. Monaro and R. Lecomte, Int. J. Nucl. Meal. and Biol., 8 (1981) 1. [5] P. Sioshanisi, A.S. Lodhi and H. Payrovan, Nucl. Instr. and Meth. 142 (1977) 285. [6] P. Simms and F.A. Rickey, Rep. (1978) EPA/600/178/058; order PB-287832. [7] F.A. Rickey, P.C. Simms and K.A. Mueller, IEEE Trans. Nucl. Sci. NS-26 (1979) 1347. [8] G. Desaulniers, A. P'an, R. Lecomte, P. Paradis, S.

[9]

[10] [11] [12]

[13] [14]

329

Landsberger and S. Monaro, Int. J. App. Rad. Isot. 30 (1979) 261. S. Monaro, R. Lecomte, P. Paradis, S. Landsberger and G. Desaulniers, Proc. Intern. Conf. on Particle Induced X-rays Emission and its Application, Lurid, Sweden, (1980) Nucl. Instr. and Meth. 181 (1981) 231. R. Lecomte, P. Paradis, S. Lanclsberger and S. Monaro, to be published. A. Wyttenbach, S. Bajo and K. Furrenkothen, Radiochem. Radioanal. Letters 42 (1980) 307. R. Lecomte, P. Paradis, S. Monaro, M. Barrette, G. Lamourex and H.A. Menard, Nucl. Inst. and Meth. 150 (1978) 289. R. Lecomte, P. Paradis, S. Landsberger, G. Desaulniers and S. Monaro, X-ray Spect. 8 (1981) 113. M. Barrette, G. Lamoureux, E. Lebel, R. Lecomte, P. Paradis and S. Monaro, Nucl. Instr. and Meth. 134 (1976) 189.

VII. PIXIE