Ilidian Juurnal ul' ChclI1i stry Vo l. ..J5A. Oc tober 2006, pp. 22:n-2237

Ex ternal proton-induced X-ray emissio n: A simple method for multi-elemental analysis

of waler

P K Nayak"' ;"*, T R Rautr;l)'" & V Vij aY;lll h

"Depal1mCnl ur Basic Sc icnccs. i\.K. Colk ge or Ellgi lleerillg, Kri shllallkll il ()26 1<)0. Illdi ;1

Emai l: pranab;l@hutma il .com hilistitutc or Phys ics . Sachi valaya 1vlal·g.

Bhuballeswar 75 1 005. Indi a

/? ecc: i\ 'c:d 25 No\,clI/!;('/' 200-1: /'('/'Cl'iscd 25 .III/\' 2006

All acce ler;\tor hased r;ljlid method 1'0 1' ll1ull i-ciL lilell lal allal ys is or wa leI' samples has bccn prcsc lll ed usillg pre­concentralioll (mctal ion ;\s car!J;ullates) techlliquc rollu",ed by thc usc 01' cxternal I'I XE set-up. Thc problems rcg; \I'dilig time requ ired ill crcating vacuum alld the irrad i;ll ioll or 11l1111 ber or spccimells (usuall y rour to six numbers ill a ,i llglc rU II ) ill cOllvelltiolla l vacuum-PI XE techll ique has bce ll successru lly overcomc. The method is cX Jlcc ted to bc usei'ul ror l11ulti­clemental anal ysis or vari ous liquid samples on rout inc basis.

I PC Code: Int. CI S GO I N23122 I

Development of rapid ana ly tical techniCjues 1'0 1'

address ing env ironmental po lluti on problem s is o r

high importance. Water is the primary source th l'Ough

which populati ons are ex posed to t o~ ic substances o r

anthropogeni c o r geo log ica l origin t,) . Hi gh levels o r

heavy metal s in water rro lll natural sources are ve ry cOlllmon,I,X . U ltramafi c rock s and th eit' all er;ltion

products have been ident iri ed as the main source o r

chrolll ium contaminati on. Th e arsen ic cO ll ce ntrati on s

in groundwater reported in West [3 engal ( India) and

[3angladesh (more than 50 di stri cts) are co nsidered as

one of the hiohest co ncentrati ons in the world . '='

Concent ration of trace elemenl s li ke /\ s, Cd , Ilg, T h.

TI , U and Zr in groundwater is allec ted hy the

g round w ater host litho logy w ith incre;l se in 1)11 . whereas th e concentrali ons of some 11C;lvy nlel;" s,

such as cadmium , lead , ni ckel and zi nc In

oroundwater are round 10 dec rease. Conce ntr;llion () r

~th cr Illclal ions, most nl1l ;ll1 ly ;l r ~ l' l lil' ; 11 1( 1 ;: I ~( , mlllybdenulll and lungs tcn i s ;" S(1 cOl l tr(1!kd I) \' 1111, but ex hibit the oppos it e Ircnd ( illnl';l.sc lIilll

increas ing 1111 ). rlll'lherill Ore , it is lI'orthy 1(1 l11l'llti on

"l'rcscl1 l "ddl'cs,: Il igh I:ncrgy Cl' '; lllic Ibv (j rOll 1'. ' 1';,1 ;, 11l , lillliL' or Fundamen tal I{csca rch. I I(lI11 i I3h;lbha I ~ lla d . 1\ 11IIl l i>; li ,100005. India

here th at due to the relati ve ly hi gh mobility of trace

elel11en ts in surface ,md ncar surface env ironments,

the l11 easurel11 ent o r l11et,li s in natural water may be

used as a basis 1'0 1' Qeochem ica l exp loration. U nder

these c ircumstances, a rapid and multi-elemental

melal delec ti on method and/or dev ices are · o f hi gh

importance. Severa l X- ray techniCju es like particl e-induced

X -ray emi ss ion (PIX E) anc! energy di spersi ve X-ray

rIu o rescence (EDX RF) have been used by vari ous

wo rk ers 1'0 1' anal ys ing env ironmental material s . ~ , 1 1 TI I incl uding wa ter anc! secl lments' . lOug 1

co nventi onal PIXE (al so call ed as vacuum PIXE) has

th e ad Va nlaQe o r anal ys ing any ty pe of so l id sampl es . ~ 1-

there are seve ral technica l prob lems ) . The foremost

dirricu lt y is \"hil e analysing objects of large size w ith

co mpl ex shape. Th is can be ana lysed 'as such' by

EPIX E, w hich is particularl y use fu l for study ing

archaeo log ica l (artefacts, manuscripts, etc. ),

bi o loQ ical ( ti ssues , b lood etc. ) and geo logical

mat e l~a l s ( like b ig rocks), w here sample handling and

pos iti o ni ng is Cjuite convenient. The second important

aspec t co mes rro m the sample preparation procedure.

In con ve nti onal PIX E, pell et-making procedure is

usuall y a prereCjui sit e, w hi ch makes thi s techniquc o f

l imit ed usc since th e sampl e or a part o f it needs to be

ciL:sl l'Oyeci al Ihe sal11ple prepar;lt ion stage itse l f . Thi s

ca n be signiri cantl y avo ided by ex ternal PIXE

techniCjuc. Anot her impOl' tant advantage is in the

studi es in vo l v ing vo latil e substances o r metal

cO l11pouncl s, w hi ch arc difli cult to handle in vacuum.

Thi s (; In furth er be avo ided using erfici ent coo ling by

;Ii r or hel iu nl rI ow that ca n signiricantly reclu ce the

I'i sk o r d:II11:I QC due to heatin !l .

. /\ s pan o!- our prog ram !-o r developing a suit ab lc

Ill elhod 1'0 1' characteri sing wa ter and sediment sampl es

using X- ra y tech niques, wc herein repo rt a sim ple,

lil11 c- savi ng ,]Cce lerat or basecl method 1'0 1'

ch:l r;l c tCI'i 7i ng 111 1111 i -(' k 111 C III; Ii II'\' 1111 i <t Il l' .

I·: \ P l' I' i 111l' ll ( ; II

w:l ter S:lI11plc <; Ii sing si mlllt;meOli s

Il o ll-des lrll c ti\'c ex tel'l1:1i PIXr:

SalllJlk ('olk('lioll alld pr('parali o ll

/\11 W;l tl'l' S;II11p IeS were collcc leci in pol ye thelene

g /;I ss I)()tt\cs. Th e bu llies wue w ;\shed rirst by

2234 I DI AN J CHEM . SEC A . OCTOB ER 200()

inl 1l1ersIIlg in a Ini xturc or Hel and HNO.1 follovved by several rin sing with pure water (from a Millipore deio ni ser plant). The rain water was co ll ec ted from Bhubaneswar, a densely populated and busy city, surrounded by a wide and crowded industrial belt. Ri vcr water was coll ected from ri vc r Daya, si tuated near Bhubaneswa r and cont ain s substantial amount of effluents com ing from th e city. Seawater was collected from th e Bay of Bengal ncar Puri (India), and groundwater from a we ll , situated In Bhubanes war.

Palladium, as PdCI2 (200 ~ll , ! OO-ppm), was added to 50 1111 of the water samples, wh ich served as an . ! !! ! I .. 11 - 1\ Intern a stanc arc, anc a so, as a co-prec ipitant . . The pH of the so lution was maintail ~ ed at 9.0 by treating "vith ammonia so lution. Abou t 2.5 ml of saturated sodium di ethyldithiocarbamate (NaDDTC, CSHI ONaS2.H20 ) was added to prec ipitat e the metal as th ei I' res pect i ve carbamates I, . The preci pi tate as obtained was coll ected on 25 mm dia membranes (Nuc leopore) o r pore size 0.4 mi crons by vacuum filtrati on on a Millipore vacuum riltrati on unit. A thi n uniform layer (about 23 mm in diameter) was formed on the membrane, which was used as the target for the PIXE meas urements. Blank targets compri sing the chemi ca ls used were also prepared in the same way. A ta rget of NIST water standard (S RM-1643d) was also prepared by same procedure for verificati on of the obtained result.

Sample irradiation

The proton beam energy of 3 MeV obtained from a 3 MV tandem type pell etron accelerat or was co llimated by a graphite collimator to a beam of 3 mm diameter. The beam was ex tracted usin e a Kapton™ foil at exit point of th e chamber (8 mi cI~o n s thi ckness) 15. The beam was allowed to trave l 3 cm in air. About 2.4 MeV proton beam was used to irradi ate the thin targets. Charge measurement s were carried

. . h 1(, TI ' k out uSing a rotat ing copper . lin targets were ' ept in air over a sample stand making an angle 45 degree with the beam direction. The targets were irradiated wi th maximum beam current of I nA. A Si(Li) detector with a full- width at half maximum (FW HM ) of ] 80 eV at 5.9 keV (act ive area 30 mm", beryllium window thi ck ness 8 ~lm , placed at 90 degree with respect to the bea m di rect ion) was lL cd to detect I .. X . I ,. I 17'11 A c wracten sLI c -rays elllillec Tom t le targets '-.

25 ,lim thic k alumini sed Mylar absorber was kept in front of the detector to attenuate the bremsstrahlung

background and dominant low energy X-ray peaks. Spectra were recorded by using a Canberra S- IOO multi channe l analyser, which was calibrated with 2.J IAm X-ray sourcel.J.I S-I') . The ex ternal PI XE system was ca li brated with single element standards (macro mater) and NIST (USA) water standard.

The PIXE spec tral analyses wcre perform ed using GUPIX-2000 so ft ware"l. Thi s prov ides a non-linear least sC]uares fittin g of th e spectru m, together with subscC]uent conversion of the fitted X-ray peak intensiti es into elemental co ncentration~" utili zing the fundam ell tal parameter method (FPM) fo r C]uanti tati ve analysis. It reC]uires parameters of ex perimen ta l geometry and Si (Li ) detector (I-I-parameter), chamber window thickness, energy o r the incid ent parti cle and the nct charge co ll ec ted I X-2 1. The concentrati ons of the middle Z clements ( 19<Z<39) were es timated rrom the intensities of K X-ray lines whil e rhat of lead was from the inte;l siti es of L X-ray lines .

Results and discussion The analys is of the external PIX E spectra was

carri ed out by GUPIX soft ware. In Fig. I, a spectrum of blank , prepared by mi xing va ri olls reagents used in the preparati on and cleaning process al ong with internal standard PdCI", shows that th ere arc no contami nants present as addi ti onal trace elements. In Fig. 2, a spectrum obtained by extern al PIX E method of NIST water standard has been presented. A co mpari so n table between certi heel and obtai ned elemental co ncen tration for NIST water standard has been prov ided in Table I, which clearly indicates tha t

V)

c OJ o U

10'

10'

100 200 300 400 500 60C 700 800

Channel number

Fi g. I- Ex ternal PI XE spectruill o f the blank (by mi xing lI sed n~age nt s and relel ,) .

NOTES 2235

thi s method can suitably be used for analysis of water samp les. A total of 12 water sa mples of various types were analysed by external PIXE. The average elemental composi tions of di fferent water samp les are provided in Table 2. In Fig. 3, the external PIXE spectrum of a sample of ri ver water shows that the elements such as calcium(II), iron(II ) and lead(II) are mostl y dominant. The typical spectra obtained from seawater, rain water and groundwater are given in Figs 4-6, respectively. Titanium( II ) was present in very low quantity in seawater and groundwater, and was absent in other samples. Chromium(II) was absent in ri ver water. Manganese( II ) had relatively hi gher concentrati on in ground water whil e iron(l l) was found in lower concentration in rainwater as compared to all other water sa mples. Concentration of

10'

Vl c 5 10' ()

Ca

Fe Ka.

100 200 300 400 500 600 700 800

Channel number

Fig. 2-External PIXE spec trum o f the NIST water standard (w ith PdC IJ.

zinc( II ) was less in river water while it maintained an almost constant level in al l other water samples . Selenium(ll) was found to be absent in river water whi le lead(n) , another toxi c element, was found in all the water samples. Lead(ll) showed very hi gh concentration in ri ver water whi le it was found to be low in all other water samples with almost no variation irrespective of whether it is seawater, rainwater or groundwater.

We wou ld like to menti on here that the pre­concentration method em ploying paliadium(II ) as internal standard gives a bett er estimate about error in each measurement. Another important observation is that during vacuum PIXE on ly 4-6 samples can be loaded, whereas in the present case thi s problem is mostly avoided as the beam is outside the PIXE chamber and samples can directly be irradi ated wi th proton beam. Also, additi onal time to create vacuum inside the PIXE chamber after sample loading is not req uired. Furthermore, the target preparation and the external PIXE measurement are not time-consuming. Less than 10 minutes is required per target and peak

Table I-External PIXE results o f NIST water standard (NIST-I643d)

Meta l Trace c leme nts in wa ter standard (ppb)

Found Cert. valuc

Ca 30S42 ± 775 3 1040 ± 500 C r 19.2± 1.7 IS.53 ± 0.20

Mn 39.2 ± 2.7 37 .66 ± 0.83 Fe 93.4 ± 1.6 91.2±3.9 Ni 56.2 ± 9.7 58 . 1 ±2.7 C u 19 .9 ± 3.2 20.5 ± 3.8 Zn 69 .5 ± 2.6 72.48 ± 0.65

Pb 19.3 ± 4.9 18. 15 ± 0.64

Table 2--Tracc metal conce ntrati on in diffe rent types of water samples as dete rmined by ex te rnal -PIXE tec hnique (co nce ntratio ns are in ppb unless o the rwise menti oned)

Metal Ri ver wate r (n=4) Seawater Rain water (n=2) Grou ndwater LaD (n=3) (n=3)

Ca" 20.5 ± 0.5 10.4 ± 0.2 16.9 ± 0.4 36.3 ± 0.3 7.2 Ti BDL. n.9 ± 1.2 I3DL 7. 1 ± 0 .5 6.9 C r BDL 4 .3 ± 0.4 3.6 ± 0.3 4 .2 ± 0 .3 1.5 Mn 29.5 ± 2 .1 11.5 ± D. 8 20.3 ± 1.4 53.0 ± 3.7 ? ~ _.J

Fe 349.6 ± 5.3 304.4 ± 5.5 159.2 ± 2.4 206.8 ± 3. 1 2.6 Ni 9.0 ± 1.6 7.8 ± 1.0 12. 1 ± 1.9 6.6 ± 09 1.8 C u 4.8 ± 0.7 13.6 ± 1.6 14.6 ± 1.9 9.2 ± 0 .9 2.S Z n 136.S ± 4.S 307 .9 ±7. 1 3 10.9 ± 8.5 363.5 ± 9.6 3. 1 Sc: BOL 3 .3 ± 0.2 5 .2 ± 1.1 BDL 2.4 I'b 35.4 ± 6.4 12.3 ±4.4 12. 1 ± 3.2 13.9 ± 4. 1 5.5

n = Numbe r o f samples ana lysed : LaD = Lim it o f de tectio n; BOL = Be low de tec tio n limit : " = in ppm

2236 INDI AN J CHEM. SEC A. OCTOB ER 2006

1 !'l 10 § o U

10'

Ca

Pd-K "u

Pd-K~

100 200 300 400 500 600 700 800 900

Channel number

Fig. 3--External PIXE spee trul11 of river wate r (from Daya ri ver. Bhu ba neswa r).

V>

C ::J o U

10' .,.---------------------,

Fe Ku

10' Pd Ku

Pd KP

10' Pb LP

100 200 300 400 500 600 700 800 900

Channet number

Fi g. 5--Ex tern al PIXE speetrul1l of rainwater (fro lll 13 h ubaneswar).

areas with good stati stica l prec Ision can be obtained within IS minutes of irradi ati on through the present ex ternal PIXE set-up at Institute of Physics, Bhubanes war. Hence, it can be reasonably argued that the use of ex ternal PIXE as a analyti cal technique has advantages over other quantitative techniques for the analys is of various water samples. In our opinion , if Ihi s technique can be combined with other low-Z elemental data (either by particle-induced gamma-ray emi ss ion or other analytical method) along with other related parameters of water quality index, it will solve many environmental and analytical problems quite successfull y_ Attempts are under progress towards the

, 10

J!l c ::J o U

10

P Ku

Pd KP

P La

100 200 300 400 500 600 700 BOO 900

Channet number

Fig. 4--Externa l PIXE spec trum of seawater (frol1l Bay of Bengal , Puri ).

10'

Ca Ka

Ca KP

Fe Ka Zn Ka

MnK Pd Ka

100 200 300 400 500 600 700 800 900

Channel number

Fig_ 6-External PI XE spec trulll of groundwater (from a we ll in I3hubaneswar).

es timation of uranium 111 seawater with mintmum sample pre- treatment by uSll1g ex ternal PIXE technique presented here.

A rapid method based on the use of accelerator has been presented for the analys is of vari ous types of water samples. Various middle Z and heavy metal s can quantify simultaneously from the ex ternal PIXE spectrum. The pre-concentration method employ ing palladium as internal standard gives <l \ eLter es timate on error in each measurement. This method can suitably be employed for analys is of water sa mples on rout ine basi s with satisfactory results, espec ially

NOTES 2237

where a large number of samples are to be analysed and data on severa l heavy metals are of interest.

Acknowledgement We thank Dr R K Choud hury and Dr B R Sekhar of

Institute or Phys ics, Bhubaneswar for their encouragement duri ng the various stages of the present in vestiga tion. Thanks are also due to the scien tifi c and technical sta ff members of Ion Beam Lab, lnstitut e of Phys ics for accelerator faci lity and various other necessary supports.

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