In e labo a o y
No e
De e mina ion o U iso opic a ios in en i onmen al samples by
ICP-MS
Shigeo Uchida,*
a
Ra ael Ga cõÂa-Teno io,
b
Keiko Tagami
a
and Manuel Ga cõÂa-LeoÂn
c
a
En i onmen al and Toxicological Sciences Resea ch G oup, Na ional Ins i u e o
Radiological Sciences, Anagawa 4-9-1, Inage-ku, Chiba-shi 263-8555 Japan.
E-mail: s_uchida@ni s.go.jp
b
Dep . FõÂsica Aplicada, Uni e sidad de Se illa, A da Reina Me cedes, 2, 41012-Se illa, Spain
c
Facul ad de FõÂsica, Uni e sidad de Se illa, Apdo 1065, 41080-Se illa, Spain
Recei ed 17 h Feb ua y 2000, Accep ed 17 h Ap il 2000
Published on he Web 14 h June 2000
Induc i ely coupled plasma mass spec ome y (ICP-MS) has been used o he de e mina ion o ace elemen s
in a a ie y o ma e ials. Fo u anium, he concen a ion o
238
U can be measu ed wi h a de ec ion limi o less
han 0.1 pp in a ew minu es. Howe e , because o he ex emely low
234
U concen a ions in en i onmen al
ma e ials, i is necessa y o sepa a e U om he ma ices and o emo e in e e ing elemen s om he sample
solu ion o measu emen o he
238
U/
234
U a io by ICP-MS. In his s udy, a simple and apid sepa a ion
me hod o U wi h an ex ac ion ch oma og aphic esin (TEVA esin) was de eloped o ICP-MS and he U
iso opic a ios in phosphogypsum samples, collec ed in Spain, we e measu ed. The esin e ®cien ly e ained U
in 6 M HCl medium and mo e han 98% o he U was easily elu ed wi h he ® s 30 mL o 0.1 M HNO
3
. The
sepa a ed solu ions we e ee om mos o he ma ix elemen s and he concen a ions o U in he solu ions
anged om 35 o 70 ppb. The
238
U/
234
U iso opic a ios in he solu ions we e measu ed by ICP-MS. Mo eo e ,
due o he p esence o a highe numbe o
235
U a oms in compa ison wi h
234
U, he
238
U/
235
U iso opic a ios
we e also de e mined wi h high p ecision by ICP-MS. The esul s ob ained ag eed well wi h he a ios measu ed
by alpha-spec ome y. The me hod o his de e mina ion is mo e apid han alpha-spec ome y, and is
conside ed o be mo e sui able o en i onmen al moni o ing.
In oduc ion
Induc i ely coupled plasma mass spec ome y (ICP-MS) has
been used wi h g ea success o he accu a e and p ecise
de e mina ion o ace elemen s in en i onmen al ma e ials.
Due o i s low de ec ion limi s, analy ical speed, lack o
chemical in e e ence, and mul ielemen al capabili y, he
me hod has been success ully applied o he de e mina ion o
mo e han 30 elemen s in en i onmen al samples wi hou any
chemical sepa a ions.
1±4
This echnique has p o ed o be
powe ul enough o he de e mina ion o long-li ed adio-
nuclides, such as
238
U,
237
Np,
232
Th,
99
Tc and
239
Pu, in
en i onmen al samples.
5±12
P ecise de e mina ion o U and Th
wi hou adiochemical sepa a ion can be achie ed o wa e
samples,
13±15
o soil samples in which ea ed amoun s a e
abou 0.1 g.
4
In ela ion o iso opic a io s udies, he mal ioniza ion mass
spec ome y (TIMS) has been he con en ional me hod o
high p ecision analysis, bu his echnique is ime consuming
and expensi e. Recen ly, iso opic a io measu emen s using
ICP-MS ha e been epo ed.
16±20
Howe e , he e is no much
in o ma ion in he cu en li e a u e on he capabili y o ICP-
MS o measu ing
238
U/
234
U a ios in en i onmen al sam-
ples.
11,12,21,22
This is mos ly due o he ex emely low
234
U mass
concen a ion in hese ma ices. In ac , by assuming adio-
ac i i y equilib ium in en i onmen al samples, he
238
U/
234
U
iso opic a io can be expec ed o be a ound 1.8610
4
. Ne e -
heless, he knowledge o such a ios can gi e ele an
in o ma ion on na u al sys ems.
23
I e eals, o ins ance, he
exis ence o an h opogenic inpu s in aqua ic sys ems
24
o can
help o da e geological samples h ough he U/Th da ing
me hod. Also, he de e mina ion o he
238
U/
235
U a io is
use ul, o ins ance, o en i onmen al s udies a ound nuclea
plan s. I is clea ly wo hwhile, he e o e, o in es iga e
whe he ICP-MS could be applied o he de e mina ion o U
iso ope a ios.
T adi ional U de e mina ion by ICP-MS equi es almos no
chemis y wo k. Howe e , he ex emely low
234
U mass
concen a ion a en i onmen al le els makes i necessa y o
ca y ou some p econcen a ion p ocesses, i.e. some chemical
manipula ions, so as o su ®cien ly dec ease he limi s o
de ec ion and be able o gi e ue U iso ope a ios.
In ecen yea s, selec i e ex ac ion ch oma og aphic esins,
such as TEVA, TRU and UTEVA esins, ha e been de eloped
by EICh oM Indus ies Inc. (Da ien, Illinois, USA) o he
sepa a ion o ac inides om solu ions. A no el and simple
me hod o sepa a ion and concen a ion o U-iso opes om
en i onmen al samples, based on he use o hese esins, has
been de eloped and alida ed.
25±29
In pa icula , we a e
in e es ed in TEVA esins om he iewpoin o U de e mina-
ion by ICP-MS. The easons a e as ollows.
(1) Adso p ion o U by TEVA esin unde sui able acid
solu ion condi ions is highe han ha by UTEVA esin.
(2) Deso p ion o U om TEVA esin by a weak ni ic acid
solu ion is achie ed mo e easily han o he o he wo esins.
This means ha TEVA esin can concen a e U in a small
amoun o weak ni ic acid solu ion. I is also p e e able ha
he ®nal solu ion is less han 0.5 M HNO
3
o easy
de e mina ion by ICP-MS.
(3) TRU esin shows he highes adso p ion cha ac e is ics
o U, bu deso p ion by a small amoun o a weak ni ic acid
solu ion seems o be di ®cul , because o he ela i ely high
adso p ion ac o unde low pH acid solu ion condi ions.
In his pape , we sepa a e and concen a e U in en i on-
men al samples by using TEVA esins. La e ICP-MS will be
used o de e mine he U iso opic a ios in some phospho-
gypsum samples.
DOI: 10.1039/b001332n J. Anal. A . Spec om., 2000, 15, 889±892 889
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Expe imen al
Samples
Phosphogypsum samples (mos ly CaSO
4
?2H
2
O), he by-
p oduc o med in he p oduc ion o phospho ic acid om
phospha e ocks, we e collec ed in Spain. They we e chosen o
es he pe o mance o he chemical sepa a ion o U and he U
a io de e mina ion because hey showed ela i ely high U
concen a ions. The
238
U,
235
U and
234
U concen a ions had
been p e iously de e mined by alpha-spec ome y,
30
which
se ed o alida e he me hod.
Reagen s
P epacked columns o TEVA esin, including ca. 2 mL o he
esin, we e used in he expe imen s. S anda d, washing and
s ipping solu ions we e p epa ed wi h ni ic and hyd ochlo ic
acids o ul a-analy ical g ade (Kan o Chemical Co. Inc.,
Tokyo, Japan). Deionized wa e (w17 MO) was used h ough-
ou he wo k.
Sample p epa a ion
The samples we e p epa ed acco ding o a newly de eloped
p ocedu e, which is shown schema ically in Fig. 1. A 0.1 g
sample was placed in a poly e a¯uoe hylene (PTFE) beake
co e ed wi h a PTFE co e and diges ed unde e¯ux wi h
conc. HNO
3
o 2 h a 120±140 ³C on a ho -pla e. A e his
s ep, he co e was emo ed and he sample e apo a ed o
d yness. The esidue was hen dissol ed in o 10 mL o conc.
HCl. The solu ion was ®l e ed o emo e insoluble ma e ials.
The ®l e was insed wi h 10 mL o wa e which was added o
he ®l a e. A e all hese p ocesses he ®nal solu ion o be
passed h ough he TEVA esin was ca. 20 mL o ca. 6 M HCl.
We conside ed ha his p ocedu e could no ha e comple ely
diges ed he phosphogypsum. Howe e , we decided o use his
we -diges ion me hod, because i was he one used o he
U de e mina ion by alpha-spec ome y.
30
In o de o com-
pa e he esul s by ICP-MS wi h hose ob ained by alpha-
spec ome y, i was p e e able i he diges ion p ocedu es we e
he same.
Sepa a ion and concen a ion
Isola ion o U and pu i®ca ion o he solu ion was done wi h a
TEVA esin. The isola ion s ep is based on a s ong endency
o he oxidised o m o U o be e ained on he esin column
a high HCl mola i y. This a ®ni y disappea s a low acid
condi ion, e.g., 0.1 M HNO
3
o 0.1 M HCl.
25
Thus, a e
condi ioning he esins wi h 6 M HCl, he solu ions ob ained
by we -diges ion o he samples, in which he acid s eng h was
also 6 M HCl, we e passed h ough he columns, p oducing he
elua e A.
Nex , he esin column was washed wi h an addi ional 20 mL
o 6 M HCl (elua e B) o elimina e o he elemen s ha ha e low
®xa ion on he esin. The U e ained was ®nally elu ed om he
esin by passing h ee olumes o 30 mL o 0.1 M HNO
3
(elua es 1, 2 and 3). The ¯ow- a e was kep a abou
1 mL min
21
du ing he expe imen s. 0.1 M HNO
3
was selec ed
as a s ipping solu ion, because Cl o igina ing om HCl o ms
complexes wi h se e al elemen s, such as Au and Hg. When U
iso opes we e measu ed by a quad upole ype ICP-MS, hese
complexes, especially,
197
Au
37
Cl,
198
Hg
37
Cl and
200
Hg
35
Cl,
ha e o be ejec ed in o de o a oid in e e ences. In his s udy,
we ied o de e mine di ec ly he concen a ion o U iso opes
in he ®nal solu ion by ICP-MS. Fo di ec measu emen o an
elemen wi h an ICP-MS, a high acid solu ion is ha m ul o he
de ec o . The 0.1 M HNO
3
solu ions can be di ec ly in oduced
in o he ICP-MS, i o he ma ix elemen s in he ®nal solu-
ions ha e been emo ed du ing sepa a ion and pu i®ca ion
p ocedu es. Mo eo e , due o he low concen a ion o
234
Uin
he samples, i is necessa y o make he coun ing ime as long as
possible. High concen a ions o ma ix elemen s in he ®nal
solu ions damage he in oduc o y pa h and de ec o o he
ICP-MS. Bo h sepa a ion o U om o he elemen s and good
eco e y o U a e necessa y o he de e mina ion o U iso opic
a ios by ICP-MS.
Measu emen s
A Yokogawa (Tokyo, Japan) PMS-2000, quad upole ype
ICP-MS, was used o he analysis o U iso opes in he samples.
The ins umen al pa ame e s a e summa ised in Table 1.
Unde hese analy ical condi ions, he oxide o ma ion o Ce
was ound o be less han 2%. As an in e nal s anda d, Bi was
used o compensa e o changes in he analy ical esul s du ing
ope a ion. Also, a s anda d solu ion was p epa ed om Spex
mul ielemen al plasma s anda ds (XSTC 13, Spex Indus ies,
Edison, NJ, USA), and used o calib a ion pu poses. Th ee
eplica e measu emen s we e done o e e y sample, looking
o 2, 50 and 150 s coun ing imes o
238
U,
235
U and
234
U,
espec i ely, in each un.
Resul s and discussion
E ec i eness o he sepa a ion me hod
A ce ain amoun o each elua e (elua es A, B, 1, 2 and 3)
ob ained om wo phosphogypsum samples (iden i®ed as A
and B), was measu ed by ICP-MS. The amoun s o U (
238
U) in
each elua e a e lis ed in Table 2.
I was clea ha only a negligible amoun o U was de ec ed
in he d ain o he load solu ion and he washing solu ion o
6 M HCl (20 mL62). Almos all U in he samples was
concen a ed in he ® s s ip solu ion o 0.1 M HNO
3
. Mos o
Fig. 1 Expe imen al scheme o he sepa a ion o U in phosphogypsum
sample.
Table 1 Ope a ing condi ions o ICP-MS
R powe 1240 W
Coolan gas ¯ow a e 13 L min
21
Auxilia y gas ¯ow a e 1.2 L min
21
Ca ie gas ¯ow a e 0.83 L min
21
Dis ance om load coil
o ape u e
5.4 mm
Dwell ime 300 ms o
234
U, 100 ms o
235
U and 4 ms o
238
U
Numbe o sweeps 500
Replica ion 3
890 J. Anal. A . Spec om., 2000, 15, 889±892
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he ma ix elemen s passed h ough he column wi h he 6 M
HCl solu ion, wi h he excep ion o Fe. Some Fe was elu ed
wi h he 6 M HCl solu ion (elua e A), bu he es beha ed in a
simila manne o U. The concen a ion o Fe in elua e 1 was
mo e han 100 ppm. This migh be due o he o ma ion o a
s ongly adso bed nega i ely cha ged i on complex in he 6 M
HCl condi ions. The samples used in his s udy con ained a
ela i ely high concen a ion o Fe. We hough ha some Fe±
Cl complex ions in he in oduc o y solu ion we e adso bed on
he esin, bu he complex ions exceeded he capaci y o he
esin o adso p ion and we e elu ed wi h he solu ion (elua e
A). The ions adso bed on he esin hen we e deso bed wi h
0.1 M HNO
3
solu ion (elua e 1).
Ou esul s show ha he TEVA esin e ®cien ly e ained U
in 6 M HCl medium, while i was easily elu ed wi h he ® s
30 mL o 0.1 M HNO
3
. The concen a ion o U iso opes in he
solu ion can be de e mined di ec ly by ICP-MS, because majo
elemen s we e emo ed om he solu ion by he esin. In ou
expe imen , we ®nally collec ed U in 30 mL o 0.1 M HNO
3
,
bu we hough ha U could be concen a ed in he s ip
solu ion, o which he olume is less han 30 mL.
The esul s ob ained p e iously in o he aliquo s o he same
samples by alpha-spec ome y a e lis ed in Table 3.
30
The
esul s ag ee well o bo h samples A and B. I can be deduced
ha no app eciable ac ion o U emained in he column.
Consequen ly, he solu ions o 0.1 M HNO
3
, co esponding o
elua e 1, con ain mo e han 98% o he U o iginally p esen in
he sample, and a e ee om mos o he ma ix elemen s
excep Fe. These solu ions con ain a ound 50 ppb o
U. The e o e, his sepa a ion me hod can be used o he
de e mina ion o he
238
U/
234
U and
238
U/
235
U iso opic a ios
by ICP-MS.
Iso opic a ios o U in phosphogypsum samples
The U iso ope amoun s we e measu ed by ICP-MS in he
pu i®ed solu ions (elua es 1) ob ained om h ee mo e
phosphogypsum samples (samples C, D and E). These
expe imen al esul s a e compiled in Table 4. The esul s
ob ained o a blank sample a e also included. The blank
sample unde wen exac ly he same chemical sepa a ion
p ocedu e as he phosphogypsum samples.
F om hese esul s, we d ew wo conclusions: (1) e y good
accu acy was ob ained in he measu emen o di e en U
iso opes (in all he cases, he s anda d de ia ions we e lowe
han 4%); and (2)
234
U in hese samples was clea ly de ec ed,
wi h coun ing alues a leas 10 imes highe han ha in he
blank sample.
These da a made us con®den in calcula ing he iso opic and
ac i i y a ios o U iso opes. The esul s a e summa ised in
Table 5. A e age alues de e mined in a se o aliquo s o he
same phosphogypsum samples by alpha-spec ome y a e also
gi en.
30
F om he esul s ob ained p e iously, a secula
equilib ium was ound be ween
234
U and
238
U, and he
235
U/
238
U ac i i y a io was 0.047, which is expec ed in na u al
samples.
We concluded ha , a he U concen a ions exis ing in he
®nal measu ed solu ions (in he ange 35±70 ppb), he
238
U/
234
U iso opic a ios can be measu ed wi h good accu acy.
These a ios ag ee wi h hose p e iously de e mined by alpha-
spec ome y. Due o he p esence o a highe numbe o
235
U
a oms in compa ison wi h
234
U, he
238
U/
235
U iso opic a ios
can also be de e mined wi h high p ecision by ICP-MS.
The esul s show he capabili y o he ICP-MS me hod
de eloped in his wo k o he de e mina ion o he
238
U/
234
U
a io a he concen a ion le els desc ibed. Conce ning he
measu emen ime, he ICP-MS me hod can be conside ed
supe io o o he adi ional adiome ic echniques, e.g.,
Table 2 U anium esul s (
238
U in ng) in he ® e elua es ob ained om wo phosphogypsum samples by he applica ion o he adiochemical me hod
in Fig. 1
Sample A U/ng U in elua e (%) Sample B U/ng U in elua e (%)
Elua e A 7.56¡6.95 0.47 Elua e A 9.8¡6.9 0.68
Elua e B 8.72¡5.50 0.50 Elua e B 5.3¡6.4 0.37
Elua e 1 1603¡8 98.86 Elua e 1 1415¡9 98.77
Elua e 2 0.94¡0.08 0.06 Elua e 2 2.0¡1.0 0.15
Elua e 3 1.30¡0.7 0.08 Elua e 3 0.38¡0.10 0.03
To al 1622¡12 100.01 To al 1433¡13 100.0
Table 3
238
U speci®c ac i i ies (mBq g
21
) de e mined in wo phospho-
gypsum samples by ICP-MS and alpha-spec ome y
a
Sample
238
U/mBq g
21
ICP-MS Alpha-spec ome y
Sample A 179¡2 170¡7
Sample B 161¡2 170¡8
a
¡:1ss a is ical e o by ICP-MS and 1scoun ing e o by
alpha-spec ome y.
Table 5 Mass and ac i i y u anium iso opic a ios ob ained in he measu emen s o h ee phosphogypsum samples. The a e age alues o hese
iso opic a ios, de e mined by alpha-spec ome y, a e also lis ed
Sample
238
U/
234
U mass a io
238
U/
235
U mass a io
234
U/
238
U ac i i y a io
235
U/
238
U ac i i y a io
C 18601¡681 144¡4 1.00¡0.04 0.044¡0.001
D 18884¡847 144¡3 0.98¡0.04 0.044¡0.001
E 18967¡588 147¡4 0.98¡0.03 0.043¡0.001
Alpha-spec ome y
a
18550¡370 139¡3 1.00¡0.02 0.047¡0.002
a
The a e age alues de e mined om a se o o he aliquo s o he same phosphogypsum samples by alpha-spec ome y.
Table 4 Analy ical esul s ob ained in he de e mina ion o he U-
iso opes in h ee phosphogypsum samples and a blank sample
a
Sample
238
U coun s
235
U coun s
234
U coun s
2 s 50 s 150 s
C 797778¡14592 138372¡3220 3573¡102
D 799421¡11815 138903¡2285 3538¡134
E 1424194¡27592 241231¡5345 5978¡135
Blank 403¡15 204¡13 358¡9
a
¡:1ss a is ical e o by ICP-MS.
J. Anal. A . Spec om., 2000, 15, 889±892 891
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alpha-spec ome y. In ac , o ob ain he same accu acy by
alpha-spec ome y a coun ime o a leas 72 h is necessa y,
assuming 100% eco e y yield and 0.1 g o phosphogypsum.
This ®gu e mus be compa ed wi h 10 min, which is he ypical
coun ing used along his wo k. Besides, i he de e mina ion o
he
238
U/
235
U a io is also o in e es , ICP-MS can ob ain i
accu a ely because he adioac i i y o
235
U is clea ly lowe
han ha o
234
U in he samples.
Acknowledgemen s
One o he au ho s (RGT) g a e ully acknowledges he
®nancial suppo o he Science Technology Agency o Japan
o a scien i®c isi o he Ins i u e o Radiological Sciences in
Nakamina o. RGT is also e y g a e ul o all he s a o his
ins i u e o hei help and wa m hospi ali y.
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