PAPER www.rsc.org/jaas | Journal of Analytical Atomic Spectrometry

Publ

ishe

d on

30

Sept

embe

r 20

10. D

ownl

oade

d by

St.

Pete

rsbu

rg S

tate

Uni

vers

ity o

n 09

/02/

2014

22:

44:0

5.

View Article Online / Journal Homepage / Table of Contents for this issue

Eu and Tb quantitation in luminescent g-ZrP-organometallics compounds byTXRF direct solid procedure

R. Fern�andez-Ruiz,*a Juan Carlos Rodr�ıguez-Ubis,b �Alvaro Salvador,b Ernesto Brunetb and Olga Juanesb

Received 4th June 2010, Accepted 24th August 2010

DOI: 10.1039/c0ja00043d

This work presents the analytical procedure developed to quantify Eu and Tb lanthanides by

Total-reflection X-Ray Fluorescence (TXRF) by a direct solid method for first time. These rare-earth

elements are doping the organic-inorganic hybrid compound g-ZrP-TerPy which is currently being

investigated as a highly efficient luminescent system. The developed procedure allows for the analysis of

very low sample quantities, ca. 5 mg, with an adequate expanded uncertainty, around 8%, and detection

limits lower than 0.002 wt% for Eu and Tb. The hereby described direct solid procedure has been tested

against the usual quantitative analysis by means of the acid digestion of the samples by TXRF and

ICP-MS. Discrepancies as large as one order of magnitude were found between the direct and the acid-

digestion measurements. The combination of several unanticipated chemical effects, produced in the

conventional digestion procedure of this material, explain its strong loss of Eu and Tb. The thorough

optimization of the digestion procedures finally confirmed the validity of the easy and simple direct

procedure carried out by TXRF in a solid way without sluggish sample digestion. This research shows

once again the versatility and confidence of the quantitative direct solid measurements performed by

TXRF.

Introduction

The preparation of phosphors or luminescent powders may find

immediate applications in many technological areas related to

the broad concept of photonics, the science and technology for

mastering the interaction of light with matter.1 These applica-

tions can be as diverse as telecommunications, solar energy,

artificial photosynthesis, lighting, displays, photo-signalled

molecular recognition, biotechnology, medical diagnostics, bio-

imaging, etc. To this effect, the sharp and intense luminescence of

lanthanides, due to their ff electronic transitions, has lots of basic

and applied research implications,2,3 yet direct lanthanide exci-

tation produces weak emission owing to the metal’s low molar

absorptivity. Significant emission enhancement can in turn result

when the lanthanide metals chelate with organic ligands that

efficiently take the light in and effectively transfer the absorbed

energy to the metal by the so-called antenna effect with

outstanding quantum yields achieved. In this way, the building of

organic–inorganic scaffolds rendering insoluble, thermally stable

solids for a variety of applications are nowadays being investi-

gated. The inorganic part is zirconium phosphate in its g form

(g-ZrP) which has been revealed as a very versatile carving board

where organic phosphorous functions can be covalently attached

by topotactic exchange.

In this work, g-ZrP laminar structure was funtionalized with

terpyrimidine (TerPy) and Eu and Tb were incorporated as

aServicio Interdepartamental de Investigaci�on, Facultad de Ciencias,Universidad Aut�onoma de Madrid, Cantoblanco, E-28049 Madrid, Spain.E-mail: ramon.fernandez@uam.es; Fax: + 34 91 4973529; Tel: + 34 914978581bDepartamento de Qu�ımica Org�anica, Facultad de Ciencias, UniversidadAut�onoma de Madrid, Cantoblanco, 28049 Madrid, Spain

1882 | J. Anal. At. Spectrom., 2010, 25, 1882–1887

luminescent lanthanides. So, the empirical formula of the studied

compounds can be written as

ZrPO4(H2PO3)x(C21H16N3O3P)1�x(H2O)y:Eu(Tb)z (1)

where the theoretical molar ratio between P and Zr is 2.

The correct quantitation of the lanthanide incorporated in the

structure of the material is one of the main goals due to its direct

relation with the quantum yield of the luminescent material.

Fig. 1 shows a simplified molecular model of the studied material

Fig. 1 Simplified model of the investigated material (upper). 3D struc-

ture model of the g-ZrP-Terpy compound (lower).

This journal is ª The Royal Society of Chemistry 2010

Publ

ishe

d on

30

Sept

embe

r 20

10. D

ownl

oade

d by

St.

Pete

rsbu

rg S

tate

Uni

vers

ity o

n 09

/02/

2014

22:

44:0

5.

View Article Online

where the inorganic part of laminar g-ZrP is represented as

bricks, and between them are intercalated TerPy molecules

together with Eu and Tb atoms.

TXRF is a technique well-known,4 and applied in very diverse

scientific fields. Many applications have been developed up to

now: catalytic process studies,5,6 biochemistry or cellular toxicity

studies,7,8,9 physics of materials,10,11,12 archaeometry13,14,15 and

many others. In this line, this work presents the first application

of the TXRF to the analytical study of g-ZrP-TerPy:Eu,Tb

compounds. In particular, the quantitation by the direct solid

procedure, by means of the previous optimization of the solid

suspension medium, has shown to be a powerful tool to solve

complex analytical questions in an easy, quick and accurate way.

The works of Fern�andez-Ruiz, related with matrixes of magne-

tite nanoparticles,16 carbon nanoparticles17 or quartz micro-

spheres,18 corroborates these facts. The basis to apply the direct

solid procedure by TXRF were given by Klockenk€amper and

Von Bohlen some time ago19 and more recently by Hellin et al.20

and Fern�andez-Ruiz.21

Lanthanide concentrations in this material are usually deter-

mined by spectroscopic techniques such as AAS or ICPS,

requiring digestion of the samples in acids of high concentration.

This fact requires a considerable amount of time as well as a high

cost of chemicals. TXRF can help to minimize the cost by means

of the simple, precise and accurate procedure developed in this

work. More important is the fact that Eu and Tb concentrations

can be seriously affected due to precipitation during the digestion

process of this material which could imply strong scientific and

industrial consequences. This work demonstrates that this fact

can produce distortions of Eu and Tb content as large as one

order of magnitude.

Keeping in mind the previous facts, a research program was

initiated with the aim of optimizing the application of the TXRF

in these materials.

Experimental

Instrumentation and reagents

The analysis by TXRF was performed using a TXRF 8030C

spectrometer (Cameca, France), equipped with a 3 kW X-ray

tube with a Mo/W alloy anode with a double-W/C multilayer

monochromator, adjusted to obtain an excitation energy of 17.4

keV (Mo-Ka), for P, Hf, Eu and Tb evaluation, and 35 keV

(W35), for Zr evaluation, and a Si(Li) detector with an active

area of 80 mm2 with a resolution of 150 eV at 5.9 KeV (Oxford

Instruments, England). The measurements were performed

working at 50 kV, and the intensity was adjusted automatically

so that a count rate of about 8500 cps was achieved. A fixed

acquisition time of 500 s was used.

The analysis by ICP-MS was performed by using an ELAN-

6000 spectrometer (Perkin Elmer Sciex, Ontario, Canada) under

the following instrumental conditions; RF power of 1200 W,

nebulizer gas flow of 0.8 L min�1, lens setting as auto lens, cross-

flow nebulizer, and nickel cones. The ICP-MS acquisition

settings were: dwell time of 50 msec, number of sweeps equal to

30, 5 replicates, peak hopping scan mode, 1 MCA channels per

peak, and dual detector mode.

This journal is ª The Royal Society of Chemistry 2010

The SEM images were acquired by means of a Philips XL-30

instrument, which it is equipped with a W source, detectors of

secondary and backscattering electrons and a vacuum sample

chamber with a vacuum lower than 4E-4 Pa.

The modification of the particle size distributions in suspen-

sions were performed with a Sonopuls HD-3200 ultrasonic

homogenizer (Bandelin, Germany) equipped with a titanium

probe and a maximum power transferring of 200 W. The particle

size distributions were performed with a MDS-2000 Diffraction

Light Scattering (DLS) system (Malvern, England).

The reagents used in this work were of Suprapure quality

(Merck, Germany) and the water was Milli-Q (18.2 MU).

Synthesis procedure of g-ZrP-TerPy:Eu,Tb compounds

Laminar g-ZrP contains two different kinds of phosphates, one

internal which sustains the integrity of the layers (by being

bonded to four different zirconium atoms through each of its

four oxygen atoms) and another in the lamellae surface pointing

to the interlayer region and using only two oxygen atoms to bond

two zirconium atoms. These surface phosphates are the ones

which can be exchanged by mild hydrothermal processes with

other phosphorous functions topotactically, i.e., maintaining

intact the layered structure. In this way, layered g-ZrP were first

exfoliated in order to make its interlayer region accessible to

TerPy molecules. This was achieved by colloidally suspending

g-ZrP in 1 : 1 water : acetone at 80 �C for several minutes. The

suspension was treated with the appropriate amount of TerPy

leading to material g-ZrP-TerPy. Later, the suspension of the

solid g-ZrP-TerPy for 48 h in a 0.1 M aqueous solution of LnCl3(Ln ¼ Tb3+ or Eu3+) followed by centrifugation, water washing

and drying at 100 �C for 3 h, generate the materials investigated

in this work.

TXRF direct solid preparation of the samples

The first approximation to the analytical problem was the use

of the TXRF for the analysis of a suspension of the solid samples

in a direct way. So, between 5 and 10 mg of the solid sample

was suspended in 10 mL of water and 500 mL of Se (Merck,

Germany) was added as internal standard to obtain a final

concentration of 50 ppm. The particle size distribution of the

suspension was modified in the following way, the suspension

was agitated during five minutes at 350 rpm with an orbital

agitator. By means of a high power ultrasound probe, an ultra-

sound program was applied to the suspension in a Falcon vial.

The program was as follows; 0.5 s to 50 W power and 0.5 s

without power in a cyclic mode during 5 min of exposition. To

minimize the excessive increment of temperature in the vials due

to the ultrasound energy transferred by mechanical vibrations to

the sample, vials were introduced in an ice/water bath during the

ultrasound program application. The suspension obtained was

stable for around 1 h which implies that the homogeneity during

the sampling process in the TXRF analysis was assured. In

addition, the suspension was always sonicated in an ultrasound

bath and agitated in a vortex previous to the TXRF analysis.

Five microlitres of the suspension were deposited on quartz

reflector and dried on a vitroceramic hot plate. Once cold, the

samples were analysed by TXRF.

J. Anal. At. Spectrom., 2010, 25, 1882–1887 | 1883

Fig. 2 Qualitative spectra of the g-ZrP-TerPy:Eu TXRF elemental

Publ

ishe

d on

30

Sept

embe

r 20

10. D

ownl

oade

d by

St.

Pete

rsbu

rg S

tate

Uni

vers

ity o

n 09

/02/

2014

22:

44:0

5.

View Article Online

Acid digestion procedures

The second approximation to the analytical problem was to try

the total digestion of the samples in several acid mediums. So,

three different protocols were applied. Due to the small quantity

of sample available only around 10 mg of sample were digested in

each protocol.

Protocol A. Applying the conventional procedure used in this

kind of material, 500 mL of HF and 500 mL of dimethyl sulfoxide

(DMSO) were added to the sample. The vial was introduced in an

ultrasound bath during 15 min up to obtain the complete

dissolution of the solid sample. The digestion was diluted to

25 mL with a 1% dissolution of HNO3.

Protocol B. With the goal of stabilizing Eu and Tb to obtain

the total digestion of the compounds, 1 mL of HF, 1 mL of

DMSO and 2 mL of HCl were added to the sample. The vial was

introduced in an open vessel digester at 100 �C during 1 h up to

obtain the complete dissolution of the solid sample. The diges-

tion was diluted to 25 mL with a 1% dissolution of HNO3.

Protocol C. With the goal of obtaining the complete leaching

of Eu and Tb contained in the interlayer terpyridine phase, 2 mL

of HCl, 2 mL of HNO3 and 2 mL of H2O2 were added to the

sample. The vial was introduced in an open vessel digester at

100 �C during 1 h. An insoluble solid precipitate, associated with

the g-ZrP phase, was obtained. The digestion was filtered and

diluted to 25 mL with a 1% dissolution of HNO3.

In these conditions, the samples were analysed by TXRF and

validated by ICP-MS.

profile for Mo-Ka (upper) and W-White (lower) X-ray excitation.

ICP-MS quantitation procedure

The ICP-MS quantification was carried out using external cali-

bration curves from dilutions of certified ICP-MS mono-

elemental standards (Merck, Darmstadt, Germany). The

quadratic correlation coefficients obtained in the regression lines

were up to 0.999 in all the cases. These coefficients assured the

linearity in the range of concentrations investigated. Rhodium

was used as the internal standard for correcting possible instru-

mental drifts. Each digestion was analysed under those condi-

tions exposed above.

Results and discussion

Qualitative TXRF elemental profile

Five microlitres of each one of the samples evaluated, without

internal standard, were deposited over a quartz reflector by

means of a vitroceramic hot plate inside an A-100 laminar flow

chamber. With the aim of obtaining the global qualitative profile

two excitation energies were applied; Mo-Ka for lighter elements

and W35 for heavy elements. Fig. 2 show the spectra associated

with the Mo-Ka (Up) and W35 (down) excitations for g-ZrP-

TerPy:Eu sample which display the characteristic TXRF

elemental profile present in this kind of sample.

The evaluation of elemental profile with MoK excitation

(Fig. 2 upper) shows the characteristic K and L lines of the

elements P, Zr, K, Ca, Eu(Tb), Hf, Zn and Br, associated to the

1884 | J. Anal. At. Spectrom., 2010, 25, 1882–1887

sample fingerprint, and two additional lines; one of Si, from the

quartz sample support, and the other, Ar, from the air present

between sample and detector respectively. These signals were

always present and from now on they are not going to be

considered. P, Zr and Eu(Tb) lines were expected but not K, Ca,

Hf, Zn and Br which were associated with induced contamina-

tion by reagents during the synthesis process. On the other hand,

P, K and Zr L lines strongly overlapped. This fact could produce

a bad quantitation of the P present in the samples. To check the

correct evaluation of P, the experimental molar P/Zr ratio was

evaluated, keeping in mind that theoretical molar P/Zr expected

should be 2. The evaluation of elemental profile with W35 exci-

tation (Fig. 2 down) shows a clean spectrum where only the

characteristic K and L lines of the elements Zr, Hf and Pd,

associated with the sample fingerprint, were observed. Hf is

a common contaminant in Zr compounds, such as this case, due

to its chemical similarities and to the difficulty to separate Hf

from Zr. Pd is present during the synthesis process of the g-ZrP,

therefore, this can be its origin. In conclusion of the qualitative

study, P, Eu(Tb) and Hf were quantified with MoK excitation

and Zr with W35 excitation. In both cases, Se was used as

internal standard.

Preliminary quantitative results

Eu and Tb compounds were initially analysed by two ways; first

the lixiviates obtained by the conventional A digestion procedure

This journal is ª The Royal Society of Chemistry 2010

Table 1 Comparative compositional results between conventional Adigestion protocol, measured by TXRF and validated by ICP-MS, anddirect solid TXRF procedure.

Eu (wt%) Tb (wt%) P (wt%) Zr (wt%)

A-Digestion Protocolg-ZrP-TerPy:Eu 0.028/0.022a 0.0/0.0 17.3/17.9 29.9/27.4g-ZrP-TerPy:Tb 0.0/0.0 0.072/0.077 18.1/18.6 25.1/24.6Direct Solid TXRF Procedureg-ZrP-TerPy:Eu 0.144 0.0 23.8 35.1g-ZrP-TerPy:Tb 0.0 0.267 21.3 30.3

a X.XX/X.XX indicate TXRF/ICP-MS results.

Publ

ishe

d on

30

Sept

embe

r 20

10. D

ownl

oade

d by

St.

Pete

rsbu

rg S

tate

Uni

vers

ity o

n 09

/02/

2014

22:

44:0

5.

View Article Online

were analysed by means of TXRF and ICP-MS for validation

reasons. Second, TXRF direct solid procedure was applied at

both samples. Table 1 shows the compositional results obtained

for evaluated compounds.

Results show strong differences for Eu and Tb concentrations

between both methods. Eu and Tb varies from 0.028 wt% to

0.144 wt% and from 0.072 wt% to 0.267 wt% respectively which

implies a loss of around 75% for Eu and Tb. In the case of P and

Zr these differences were not so pronounced but also poor

recoveries were found. A recovery loss around 19% for P and Zr

was found. The last results indicate that the loss of elements

during the A acid digestion protocol, could be associated to the

formation of insoluble phosphates or zirconates.

Validation of the digestion procedure

With the aim of investigating the origin of the leaching problems

and, additionally, validating TXRF direct solid results, two new

acid digestion protocols were applied. Protocol B was designed

with the goal of stabilizing Eu and Tb and simultaneously

obtaining the total digestion of the compounds. Protocol C was

designed with the goal of obtaining the complete leaching of Eu

and Tb contained in the organic phase of the compounds. At this

point of the research only the sample g-ZrP-TerPy:Tb was

investigated due to practical reasons. Additionally, stability

checking was carried out where lixiviations were analysed twice,

first after digestion and second after six days. Table 2 shows the

obtained compositional results.

Table 2 shows that results of B-protocol for Tb, P and Zr

lixiviate, just after digestions, were the same from an analytical

point of view that the obtained by direct solid TXRF method.

Table 2 Comparative compositional results between B and C digestionprotocol for g-ZrP-TerPy:Tb, measured by TXRF and validated byICP-MS, and direct solid TXRF procedurea

Tb (wt%) P (wt%) Zr (wt%)

B-Digestion ProtocolAfter digestion 0.246/0.256 19.4/20.6 30.5/30.1After 6 days 0.106/0.113 18.5/17.4 22.6/22.3C-Digestion ProtocolAfter digestion 0.266/0.263 6.1/5.8 0.94/1.2After 6 days 0.266/0.261 5.4/5.9 1.3/1.1Direct Solid TXRF Procedure

0.267 21.3 30.3

a X.XX/X.XX indicate TXRF/ICP-MS results.

This journal is ª The Royal Society of Chemistry 2010

Nevertheless, measures after six days indicates a poor stability

for all the elements. In this case, losses of around 57%, 10% and

23% for Tb, P and Zr respectively were found. This fact confirms

the existence of coprecipitation reactions in the acid medium

when g-ZrP is decomposed by HF. C-protocol results show that

Tb recovery was excellent compared with TXRF direct solid

measurements but P and Zr recoveries were very low, in agree-

ment with the weighted. In addition, measures after six days

indicate a complete stability of Tb in this lixiviate medium.

Analytical research of the TXRF direct solid procedure

The main analytical consequence of the results obtained in the

previous section is that the TXRF analysis in a direct solid way

can be an adequate analytical solution to investigate this kind of

materials in a quick, easy, precise and accurate way compared

with conventional acid lixiviation procedures which are neces-

sary for other techniques such as ICPS or AAS. In this way, an

additional research line was initiated with the aims of optimize

and evaluate the different analytical parameters associated to the

TXRF direct solid procedure. Three main objectives were

proposed. First, the study of the morphology of the solid parti-

cles in suspension phase, by means of DLS, and after deposition,

by SEM, were carried out. Second, the angular dependencies of

the TXRF intensities around critical angle (AD-TXRF) was

evaluated with the aim of checking the proportional behaviour of

the evaluated fluorescence signals. Third, detection limits and

expanded uncertainties for Eu, Tb, P and Zr in this matrix and by

this TXRF procedure were also evaluated.

Morphology of the solid particles

Once suspensions were prepared, such as previous sections

indicate, an aliquot was measured by means of DLS. Fig. 3 shows

the particle size distribution obtained for the g-ZrP-TerPy:Tb in

suspension phase.

Results indicate a maximum probability around 6 mm. From

the TXRF point of view, and in a first approximation, this

average particle size is very high, even more so considering the

presence of Zr as a structural element of the material. Never-

theless, analytical results indicate that an additional effect should

exist for this material. With this fact in mind, the samples

deposited over quartz reflectors were studied by SEM. Fig. 4

Fig. 3 Particle size distribution of the g-ZrP-TerPy:Tb sample in

suspension measured by means of DLS technique. Probability density

graph (,) and probability curve (-).

J. Anal. At. Spectrom., 2010, 25, 1882–1887 | 1885

Fig. 4 SEM images of a deposition of the g-ZrP-TerPy:Tb material in

different regions and magnifications.

Publ

ishe

d on

30

Sept

embe

r 20

10. D

ownl

oade

d by

St.

Pete

rsbu

rg S

tate

Uni

vers

ity o

n 09

/02/

2014

22:

44:0

5.

View Article Online

show a photograph composition of the deposition morphology

by SEM.

SEM images show the existence of a small border effect where

the mountain effect is not so pronounced. Furthermore, the

distribution of the particles over the reflector is very homoge-

neous. Both morphological aspects of the depositions minimizes

the presence of matrix effects. The more significant observation is

the laminar habit that the individual particles display. This

particle morphology, so planar, it is the macroscopic conse-

quence of the molecular lattice of the compound (see Fig. 1). By

means of the ImageTools v.3.0 software package (Texas

University, Healt Science Center, San Antonio, USA) the

particles sizes were evaluated. For statistical analysis a sampling

order around n ¼ 150 was used. The normalized probability

density graphs and its associated probability curve are shown in

Fig. 5.

Results give an indication of the 2D size dimensions of the

deposited particles with values around 6 mm. For z dimension, it

is possible to estimate a length lower than 1 mm which is

Fig. 5 Probability density graph (,) and probability curve (-)

associated to the 2D sizes of the deposited g-ZrP-TerPy:Tb particles.

1886 | J. Anal. At. Spectrom., 2010, 25, 1882–1887

sufficiently small so that the self-absorption effects can be avoi-

ded.19 Particle size evaluations by DLS and SEM were in

agreement, and around 6 microns. This implies that, in this case,

agglomeration effects do not exist in the suspension phase.

AD-TXRF intensities behaviour

Very recently Von Bohlen et al.22 published an interesting work

where they show that the analysis of solids in the nanometric

range, due to the consideration of the X-ray length coherence,

could imply quantitative distortions of the TXRF results.

In this sense, the evolution of the Tb, P, Zr, Se (Internal

Standard) and Si (reflector) AD-TXRF intensity profiles were

also performed with the aim of investigating the proportional

intensity relations around the critical angle. Fig. 6 shows the

achieved results. Angle scale was not corrected, and represents

the angular experimental measurements in our instrument.

Considering the angles down to the critical angle, the behaviour

of all the intensities are clearly proportional between them and

also the absence of unexpected intensity profiles distortions were

found. Both facts assure an adequate elemental TXRF quanti-

fication.14

Detection limits and uncertainties

Five samples of g-ZrP-TerPy:Tb and g-ZrP-TerPy:Eu materials

were prepared and analysed in parallel (n ¼ 5). Nominal

concentrations and detection limits were calculated after inte-

gration of each one of the elemental intensities for each one of the

elements evaluated for W35 (Zr) and Mo-Ka (P, Tb, Eu) exci-

tations. As is well-known,19 concentrations Cx and detection

limits DLx were calculated by means of equations

Cx ¼ CSe

Ix

ISe

SSe

Sx

and DLx ¼ 3Cx

Ix

ffiffiffiffiffiIB

t

r(2)

where I and S are net intensity, in counts per second, and relative

sensitivity for each one of the evaluated element x respectively.

Parameter t is the time of acquisition, 500 s in our case.

Subscripts B and Se indicate background and selenium used as

internal standard respectively. The formalism introduced by

Fern�andez-Ruiz to evaluate the uncertainties in the TXRF

measurements23 was applied in this part of the research. So,

Fig. 6 Angle-scan measurements for Tb, P, Zr, Se (Int. Std) and Si

(reflector) for one of the deposited g-ZrP-TerPy:Tb samples.

This journal is ª The Royal Society of Chemistry 2010

Table 3 Detections limits and uncertainties obtained for g-ZrP-Ter-Py:Tb and g-ZrP-TerPy:Eu materials by means of the direct solid TXRFprocedure (n ¼ 5)

Eu (wt%) P (wt%) Zr (wt%)P/Zr(molar rate)

g-ZrP-TerPy:Eu MaterialAverage C (wt%) 0.143 22.3 34.4 1.91s (wt%) 0.013 1.8 3.1 0.03U (%) 8.1 7.3 8.0 1.3DL (wt%) 0.0020 0.0663 0.0056 —g-ZrP-TerPy:Tb MaterialAverage C (wt%) 0.260 21.3 31.5 2.00s (wt%) 0.023 1.8 3.3 0.05U (%) 7.9 7.4 9.3 2.3DL (wt%) 0.0022 0.0660 0.0056 —

Publ

ishe

d on

30

Sept

embe

r 20

10. D

ownl

oade

d by

St.

Pete

rsbu

rg S

tate

Uni

vers

ity o

n 09

/02/

2014

22:

44:0

5.

View Article Online

TXRF expanded uncertainty Ux for each element x, can be

evaluated with 95% success probability (k ¼ 2), as

Ux ¼100�Cx

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4

n� 1

Xn

i¼1

�Ci

x � �Cx

�2

s(3)

where �Cxis the average concentration value for element x, n is the

sampling order, n ¼ 5 in our case, and Cix is the concentration of

each one of the concentration values obtained for element x.

Additionally, the P/Zr molar ratio was evaluated for both

materials, keeping in mind that the theoretical molar relations

should be of two atoms of P for each atom of Zr (see Ec 1). Table

3 shows the results obtained for both materials.

Expanded uncertainties around 8% were found for Eu, Tb, P

and Zr. Detections limits for Eu and Tb were found around

0.0021 wt% (21 ppm in weight) while for P and Zr were found

around 0.0661 wt% (661 ppm in weight) and 0.0056 wt% (56 ppm

in weight) respectively.

Conclusions

Two samples of g-ZrP-TerPy doped with Eu and Tb respectively

have been quantitatively evaluated by means of two ways, acid

leaching and direct solid quantification. Comparison of both

procedures has shown that Eu and Tb concentration can be

erroneous, as large as one order of magnitude, in function of the

acid digestion followed. This fact is of great importance due to

the scientific and industrial implications of the erroneous

lanthanide evaluation. In contrast, the direct solid analysis

developed by TXRF has proven to be an accurate, precise, quick

and not expensive procedure for the quantification of rare-earths

in this kind of compound. The validated composition of Eu/Tb,

P, Zr obtained for both compounds were as follows; g-ZrP-

TerPy:Eu: 0.143(13), 22.3(1.8) and 34.4(3.1) wt% respectively

and g-ZrP-TerPy:Tb: 0.260(23), 21.3(1.8) and 31.5(3.3) wt%

respectively. The achieved detection limits were in both cases

very similar, in particular, 0.002, 0.066 and 0.0056 wt% for Eu/

Tb, P and Zr respectively. The average uncertainty achieved for

all the evaluated elements was around 8% for k ¼ 2 (95%). The

results show that TXRF can compete with advantages with more

This journal is ª The Royal Society of Chemistry 2010

conventional techniques such as ICP-MS or AAS in this type of

material, and more importantly, avoiding the possible problems

associated to uncontrolled chemical alteration of the sample

during the acid digestion.

Acknowledgements

Authors are thankful for the financial support of the Universidad

Aut�onoma de Madrid and Comunidad de Madrid for EIADES

proyect S2009/AMB-1478. Authors also express their gratitude

to Esperanza Salvador and Inmaculada Rivas, from SEM and

ICP-MS laboratories of the SIdI-UAM for their availability and

expert advice.

References

1 P. Escribano, B. Julian-Lopez, J. Planelles-Arago, E. Cordoncillo,B. Viana and C. J. Sanchez, J. Mater. Chem., 2008, 18, 23–40.

2 M. H. V. Werts, Sci. Prog., 2005, 88, 101–131.3 E. Brunet, J. C. Rodriguez-Ubis and O. Juanes, Curr. Chem. Biol.,

2007, 1, 11–39.4 R. Klockenk€amper. Total reflection X Ray Fluorescence Analysis.

Wiley, New York, 1997.5 R. Fern�andez-Ruiz, M. Furi�o, F. Cabello Galisteo, C. Larese,

M. L�opez Granados, R. Mariscal and J. L. G. Fierro, Anal. Chem.,2002, 74, 5463–5469.

6 R. Fern�andez-Ruiz, F. Cabello Galisteo, C. Larese, M. L�opezGranados, R. Mariscal and J. L. G. Fierro, Analyst, 2006, 131,590–594.

7 T. Magalhaes, A. von Bohlen, M. L. Carvalho and M. Becker,Spectrochim. Acta, Part B, 2006, 61, 1185–1193.

8 R. Fern�andez-Ruiz, J. D. Tornero, V. M. Gonz�alez and C. Alonso,Analyst, 1999, 124, 583–585.

9 Mauricio Gonz�alez, Luc�ıa Tapia, Milton Alvarado, J. D. Torneroand R. Fern�andez-Ruiz, J. Anal. At. Spectrom., 1999, 14, 885–888.

10 R. Fern�andez-Ruiz and J. Capmany, J. Anal. At. Spectrom., 2001, 16,867–869.

11 M. Kr€amer, A. von Bohlen, C. Sternemann, M. Paulus andR. Hergenr€oder, Appl. Surf. Sci., 2007, 253, 3533–3542.

12 R. Fern�andez-Ruiz, J. P. Caba~nero, E. Hern�andez and M. Le�on,Analyst, 2001, 126, 1797–1799.

13 Franco Cariati, Paola Fermo, Stefania Gilardoni, Anna Galli andMario Milazzo, Spectrochim. Acta, Part B, 2003, 58, 177–184.

14 R. Fern�andez-Ruiz and M. Garcia-Heras, Spectrochim. Acta, Part B,2007, 62, 1123–1129.

15 R. Fern�andez-Ruiz and M. Garcia-Heras, Spectrochim. Acta, Part B,2008, 63, 975–979.

16 R. Fern�andez-Ruiz, R. Costo, M. P. Morales, O. Bomat�ı-Miguel andS. Veintemillas-Verdaguer, Spectrochim. Acta, Part B, 2008, 63, 1387–1394.

17 R. Fern�andez-Ruiz, P. Ocon and Manuel Montiel, J. Anal. At.Spectrom., 2009, 24, 785–791.

18 R. Fern�andez-Ruiz, Rom�an Andr�es, Ernesto de Jes�us andPilar Terreros, Optimization of the Quantitative direct solid TXRFanalysis of Glass Microspheres Functionalized with ZrOrganometallic Compounds, Spectrochimica Acta, Part B, 2010, InPress.

19 R. Klockenk€amper and A. Von Bohlen, Spectrochim. Acta, Part B,1989, 44, 461–469.

20 D. Hellin, W. Fyen, J. Rip, T. Delande, P. M. Mertens, S. De Gendtand C. Vinckier, J. Anal. At. Spectrom., 2004, 19, 1517–1523.

21 R. Fern�andez-Ruiz, Spectrochim. Acta, Part B, 2009, 64, 672–678.22 A. von Bohlen, M. Kr€amer, C. Sternemann and M. Paulus, J. Anal.

At. Spectrom., 2009, 24, 792–800.23 R. Fern�andez-Ruiz, Anal. Chem., 2008, 80(22), 8372–8381.

J. Anal. At. Spectrom., 2010, 25, 1882–1887 | 1887