1990 - International Nuclear Information System

BARC-1526

OB

oon

X-RAY FLUORESCENCE ANALYSIS OF HIGH PURITY RARE EARTH OXIDES FORCOMMON TRACE RARE EARTH IMPURITIES

by

L. C. Chandola, R. M. Dixit, P. P. Khanna, S. S. Deshpande, I. J. Machado and S. K. KapoorSpectroscopy Division

1990

R.A.R.C-1526

GOVERNMENT Ol" INDIAATOMIC ENERGY COMMISSION

O

CC

X-RAY FLUORESCENCE ANALYSIS 01 HIGH PURITY RARE EARTH

OXIDES FOR COMMON TRACE RARE EARTH IMPURITIES

by

L.C. Chandola, R.M. Dixit, P.P. Khanna, S.S. Deshpande,1.3. Machado and S.K. Kapoor

Speccroscopy Division

RHARHA ATOMIC RESEARCH CFNTPEBOMBAY, INDIA

1990

B.A.R.C.-1526

BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT

(as per IS : 9100 - 1980)

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Technical Report

B.A.R.C.-1526

10 Title and subtitle : X-ray fluorescence analysis of niqhpurity ra.r<s earth oxides for commontrace rare earth impurities

II Collation ;

13 Project No. :

20 Personal author(s) :

S4 p., 5O tabs., 2 figs.

L.C. Chandola; R.M. Dix.it;P.P. Khanna; S.S. Deshpande;I.J. Machado; S.K. Kapoor

21 Affiliation of author(s) : Spectrascopy Division, Bhabn-3 At' TU.CResearch Centre, Bombay

22 Corporate author(s) Bhabha Atomic Research Centre,Bombay—400 085

Spectroscopy Division, B.A.R.C.,Bombay

24 Sponsor(3) Name :

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31 Publication/Issue date

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Sovernment

October 1990

November 199O

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40 Publisher/Distributor : Head, Library and InformationDivision, Bhabha Atomic ResearchCentre, Bombay-400 085

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English

38 refs.

60 Abstract : Methods for the determination of individual tracecommon rsre earth (RE) elements have been developed far fifteenRE oxide matrices viz. L a p H to Lu.,0a and Y Q,,. In general, foreach matrix two or three neighbouring elements on both sides ofthe matrix element are determined. The minimum determinationlimit (MDL) achieved is 0.0022 for most of the elements. Specialefforts were made to use a small amount of sample (as low as 400mg) for the analysis by the use of double layer pellet techniqueand critical thickness studies. Pratical experiences with 15 REmatrices, most of which ara investigated for the first time, arediscussed. Details of selection of instrumental parameters andanalysis lines, precision and accuracy and preparation ofsamples and synthetic standards are given. Theoretical minimumdetection limit (TMDL) for each snalyte element is calculated inall the 15 matrices.

70 Keywords/Descriptors : X-RAY FLUORESCENCE ANALYSIS; RARE F-AFMHS;IMPURITIES; INTERFERING ELEMENTS; Vin-JUJM OXIDES; LANTHANUMOXIDES; CERIUM OXIDES; PRASEODYMIUM GXIDE3; .NLODYMiUM OXiL-.S;SAMARIUM OXIDES; EUROPIUM OXIDES; GADOLINIUM OXIDES; TERBIUMOXIDES; DYSPROSIUM OXIDES; HOLMIUM OXIDES; ERBIUM CIXIDES;THULIUM OXIDES; YTTERBIUM OXIDES; LWTFTIUM OXIDES: MONAZITES;OXALATES; OXALIC ACID; NITRIC: ACID; CALIEtRATION STANDARDS;SA.'-iilh PRfiPARATiUN; BORIC ACID; BINDERS

71 Class No. : JNIS Subject Category : Bl'.iO

99 Supplementary elements ;

X-RAY FLUORESCENCE ANALYSIS OF HIGH PURITY RARE EARTH OXIDES FOR

COMMON TRACE RARE EARTH IMPURITIES

by

L.C. Chandola, R.M. Dixit, P.P. Khanna, S.S. Deshpande,

I.J. Machado and S.K. Kapoor

1. INTRODUCTION

Rare earths (REs) are increasingly being used in modern

industries and technologies. REs are important in Atomic Energy

programme due to the fact that they are mostly found associated

with the deposits of thorium and uranium and are also produced

during nuclear fission. The chemical properties of REs resemble

closely those of uranium, thorium and Plutonium which are members

of actinide series. Some REs are neutron poisons and therefore

these are used to have effective control over neutron flux in

nuclear reactors.

Large scale separations of individual REs is being carried

out at Indian Rare Earths, Alwaye, Kerala State. The separated

REs are around 99.99% pure. It is thus necessary to develop

rapid, sensitive and accurate methods for their analysis. The

conventional chemical methods of analysis are not suitable for

determining trace RE impurities in individual REs because of their

similar chemical properties. Instrumental methods of- analysis

such as spectrophotoraetry, neutron activation analysis (NAA),

X-ray excited optical luminescence (XEOL) etc. can be used to

determine the REs only in a limited way. Optical emission

spectroscopic (OES) methods though-applicable for the analysis of

all REs are sometimes restricted in their applications due to

complex spectra of REs and accompanying interference problems.

X-ray fluorescence (XRF) method is, however, applicable to

all REs and the* interference problem in it are minimum due to

simple X-ray spectra of rare earths. This report describes the

wavelength dispersive XRF methods developed in Spectroscopy

Division of Bhabha Atomic Research Centre,Trombay,Bombay (India)

for the analysis of individual REs in high purity REs.

Rare earths are generally taken -as a group of elements

starting from lanthanum <Z^5Y). to lutetium (Z=Y1) and yttrium

(Z=39) and sometimes scandium (Z-21). • It is well known that

proraethiara (Z=61) does not occur in nature and that the chemical

properties of Sc are much different from rest of the REs.

Therefore, for .the purpose of this report only- 15 elements via.

yttrium (Y)., lanthanum (La), c«rium (Ce), praseodymium (Pr),

neodymium (Nd), samarium (Sm), ear opium (Eu), gadolinium (Gd),

terbium (Tb), dysprosium (.By), holmium (Ho), erbium (Er) . thulium

(Tm), ytterbium (Yb) and lutetium (Lu) are supposed to constitute

RE group of elements. The series of elements starting from

lanthanum to lutetium (Z=57 to 71) is known as lanthanide series

and these elements are known as lanthanons.

The electron configuration of a normal lanthanide atom is. , 2 _ Z , , ri _ Z , , < S _ . 1 O « 2 . <S . ji'> . .^i Z r rt

written as : Is , 2s 2p , 3s Jp 3d , 4s 4p 4d .4f , 5s 5p ,

5d16s where n=0, 1, 2,....,14 for different Lanthanide atoms.

There are slight departures for some lantbanides from the regular

pattern. Filling up of inner 4f shell for different REs while

outer valence electrons bd 6s remained undisturbed, explains the

similarity of chemical properties of REs. For an exhaustive

discussion of electron configuration of REs, see Atomic Spectra

and Atomic Structure by G. Herzberg [I].

- 2 -

1.1. Importance and Uses of Rare Earths

1.1.1. Importance of Rare Earths

The importance of REs in Atomic Energy programme is due to

the following factors :

1. Rare earths are mostly found associated with the deposits of

thorium and uranium in ores, therefore these are obtained as

a by-product in large quantities in processing of 0 and Th

from their ores.

2. Some REs are produced in the process of fission when uranium

is used as a nuclear fuel.

3. Many REs have very high thermal neutron absorption cross

section and act as neutron poisons. As such nuclear grade

U, Th and reactor materials have to be free from REs.

4. Some REs like gadolinium with high thermal neutron

absorption cross section are used as burnable poisons to

have effective control in neutron flux in nuclear reactors.

1.1.2. Uses of Rare Earths

Rare earths can be used as a group or as individual elements

for certain applications. The separation process of individual

REs is costly and therefore if in any application, the individual

RE has a marginal advantage over mixed group REs, the latter is

obviously preferred. Cerium dioxide is used for polishing glass

but the natural RE oxide mixture obtained from nonazite is rich in

cerium oxide and as such can be used equally effectively for the

purpose. Rare earths mixture is used in petroleum cracking

catalysts, misch metal-iron lighter flints and RE silicide is an

additive to ductile iron and steel.

Rare earth flourides added to carbon arc cores increase the

arc intensity by a factor of ten and the resultant light quality

is nearly identical to sunlight.

— 3 —

The other important uses of individual REs are as hosts and

activators in phosphors, magnetic and electronic materials, fibre

optics, lasers, alloy additives etc.

Recently rare earths have found use in high temperature

superconductor materials of the type RE-Ba-Cu-0 where RE

represents an individual RE element oxide. Though almost all the

RE elements are being used for the research, the oxides of yttrium

and lanthanum are found to be especially useful.

AB., type permanent magnets where A is a RE metal (mi.sch

metal, Sm or Pr) and B is cobalt are produced by powder metallurgy

techniques. These magnets have high coercivity.

Lanthanum, cerium and yttrium are added at the level of 0.1

to 2% to cobalt, nickel and chromium base alloys to make super

alloys having high corrosion and oxidation resistance in various

environments like high temperature and salt, water. Cerium dioxide

is used to decolourise flint glass.

Europium is the most important RE activator which is used in

colour television as the red phosphor. Europium is also used in

high pressure mercury vapour lamps as a colour correcting

phosphor. Cerium, samarium and terbium are some other important

activators.

Many of the '4f!containing RE atoms exhibit laser action in

suitable media. Only neodymiurn Lasers have any commercial

importance. v2°., >

G d2 ° 3 >

K2 ° z

S (R-La,Y,Gd) and YVO^ are useful as

host material for RE phosphors. Yttrium aluminium garnet (YAG),

yttrium iron garnet (Y1G) and glasn also serve as good hosts for

Nd lasers.

Several ME elements viz. samarium, europium, gadolinium,

dysprosium and erbium have high thermal neutron absorption cross

section. They are used in control rods and as burnable poisons to

keep effective control over thermal neutron flux in a reactor.

- 4 -

1.2. Rare Earths-Occurrence, Separation and Analysis

1.2.1 Occurrence

The most important sources of REs are monazite, bastnasite

and xenotime ores. Monazite and xenotime are orthophosphates of

light (Ce group) and heavy (Y group) REs respectively.

Bastnasite is a fluorocarbonate of Ce group of REs. Table 1

shows average RE content of these ore minerals.

Table 1

Average Composition of Rare Earth Elements (X)

Element Monazite Bastnasite Xenotime

LaCePrNd

SmEuGd

TbDyHoErTmYbLu

2046!>19

30.011.7

0.160.50.090.130.010.060.006

93

4.7

322b413

0.0.0.

5115

98.7

0.75

2.0 I

0.9

2.0

0.0

I

1.21.013.6

1.07.52.06.21.2Y6.00.63

60.0

10.6

4.8

24.6

60.0

At present India, Brazil and Australia are producing REs

from monazite while U.S.A.. is producing the same from bastnasite

ore. Malaysia has xenotime with their placer tin deposits and it

is the major producer of REs from this ore.

- 5 -

1.2.2 Separation

A RE mineral or ore is brought into solution or "cracked" by

different methods. RE group of elements are then separated as

hydroxides, oxalates, fluorides et' The methods of decomposition

of ore are :

(i) Treatment with sulphuric acid, nitric acid or hydrofluoric

acid,

(ii) Bisulphate or alkali fusion

In India, France and Brazil, for commercial production,

caustic soda digestion process is followed and the REs are

recovered as chlorides.

The monazite ore (which is available in plenty in India) is

finely ground to 300 mesh size and digested with caustic soda

flakes. The reaction is completed in 8 hours by passing steam at

140°C. The reacted mass is dumped in water where trisodium

phosphate formed during the reaction is dissolved and the mixed

-thorium and RE hydroxides settle down as insolubles. The

precipitate is filtered and washed free of phosphate and NaOH and

enough HC1 is added to attain a pH of about 3.2. If the reaction

is done at 60-745 C, the RE hydroxides are dissolved and crude

thorium hydroxide is left behind. Clear RE chloride solution is

deactivated by co-precipitation with barium suJphate and further

treated to remove lead impurity. The solution is evaporated and

concentrated in stages to get RE chloride product.

Cerium oxide of 99.95% purity in obtained from crude cerium

hydrate of 92-96% purity. The didymium (Nd+i'r) chloride solution

J.eft behind after precipitation of cerium hydrate from RE chloride

is enriched in La, Wd, Pr etc. This forms starting matericil lor

the production of lanthanum by a separation method making use of

basicity differences. Rare earths other than La are precipitated

with ammonia. La remaining in solution i is converted to carbonate

and dissolved either in EJC1 or in HNOg to form respective salts.

- 6 -

A counter current crystaiiisation technique is used to produce

99.99% purity L a ^ .

The remaining REs are prepared by the mixed technique of

solvent extraction and ion-exchange.

1.2.3 Methods of Analysis of High Purity REs

The following are a few instrumental methods which can be

used for the analysis of high purity rare earths:

1. Neutron activation analysis (NAA)

2. X-ray excited optical luminescence (XEOL)

3. Spectrophotometry

4. Optical emission spectroscopy (OES)

5. X-ray fluorescence (XRF) spectrometry

The above methods are described in brief highlighting their

main limitations to show how X-ray fluorescence spectrometric

methods are most suitable for this problem.

1.2.3.1 Neutron Activation Analysis (NAA)

In neutron activation analysis two discrete steps are

involved:

(i) Production of a radioactive nuclide from the element

generally by thermal neutron irradiation and

(ii) Measurement of the amount of induced radioactivity of

r;jdionuclide.

The main factors determining the sensitivity of NAA are; the

neutron flux available, the activation cross section and the

irradiation time. A number of RE elements have high thermal

neutron absorption cross section; shielding effects are therefore

likely to occur in many cases. In some cases the matrix may

become extremely radioactive because of high activation cross

section. In such cases there is a neeci of post irradiation

cooling under proper shieLdiritf. In the light RE group, the

- 7 -

nuolides which are used for NAA determination may also be formed

by nuclear fission of uranium. This interference is specially

serious for Ce and Nd when RE impurities are to be determined in

metallic uranium. The radiochemicai separation which becomes

necessary in most cases, in order to separate matrix element, are

relatively complicated. Thus, although many RE elements can be

determined with high sensitivity, NAA is tedious, time consuming

and requires presence of powerful neutron source (reactors) in the

vicinity.

1.2.3.2 X-ray Excited Optical Luminescence (XEOL)

X-ray excited optical luminescence has emerged as an

extremely sensitive technique for the detection and determination

of RE impurities in certain RE hosts. La, Ce, Pr, Gd, Tb, Lu and Y

are reported to be useful hosts to support the fluorescence of

other RE elements. It is often necessary to convert the sample to

chemically convenient form to obtain optimum XEOL yield. RE

phosphates and vanadates are found to be good hosts for many RE

impurities in high purity RE oxides. Detection limits of RE

impurities in parts per giga (ppg; 1 in lif) level in yttrium

oxide have been achieved.

Though, the exact mechanism of XEOL emission is not fully

understood it is suggested that the secondary electrons/excitants

generated by absorption of X-rays by host atoms transfer their

energy to RE impurities fco excite luminescence.

XEOL is largely influenced by the concentration quenching

due to presence of the other RE impurities in the host. XEOL is

strictly dependent on concentration of particular impurity only if

impurity levels are vary low.

1.2.3.3 Spectrophotometry

Analysis of REs by speetrophotinetry is carried out by

- 8 -

measuring the intensities of characteristic absorption bands for a

RE ion in solution. Though interference between the lighter REs

as a group is relatively minor and easily adjustable, these

problems are manyfold in the case of heavier REs. Thus the method

cannot be made universally applicable to analyse entire range of

REs.

1.2.3.4 Optical Emission Spectroscopy (OES)

Optical emission spectroscopy is one of the powerful

instrumental methods for the analysis of REs. Host of the REs

have rich and complex spectra and therefore present line

interference problems. This fact imposes certain limitations for

the analysis of a RE impurity in a RE matrix.

For most of the OES methods, graphite electrodes are used.

A direct current arc with graphite electrodes emits intense CN

bands where most of the sensitive lines of REs fall. This,

therefore, introduces another limitation for the analysis of REs

by OES. These limitations are overcome to some extent by using

high resolution spectrographs and an inert atmosphere for a D.C.

?rc. However, the arrangement becomes cumbersome.

lnspite of the above limitations, the OES has emerged to be

one of the powerful analysis methods for REs. The pioneering work

in the OES analysis was carried out by Fassel and co-workers

[2-5].

In Spectroscopy Division of Bhabha Atomic Research Centre,

Trombay, Bombay, OES methods of analysis for most of RE oxides

have been developed [6-18} with comparable detection limits with

those obtained by Fassei and co-workers.

One advantage of OES is that individual elements can be

determined simultaneously with a single exposure using only a few

milligram of the sample. In many cases XRF and OES methods are

used to complement each other.

- 9 -

1.2.3.5 X-ray Fluorescence (XRF) Spectrometry

X-ray fluorescence (XRF) spectrometric methods are broadly

divided into two categories'-

(i) Wavelength dispersive (WD)

(ii) Energy dispersive (ED)

The EDXRF methods are not suited for the analysis of

individual REs in high purity RE oxides because of the matrix

radiation itself saturates the counter making trace analysis

difficult. For this reason only WDXRF methods are discussed.

Wavelength dispersive XRF methods are ideally suited for the

analysis of individual REs in hiah purity RE oxides due to

following main factors;

(a) The K or L X-ray spectra of RE elements are simple.

Hence interelement line interferences are minimum. In cases where

interferences are encountered the extent of interference can be

quantitatively evaluated.

(b) X-ray spectra are independent of the chemical nature of

the sample. Gome matrix effects like absorption-enhancement

effects can be quantitatively assessed/estimated.

(c) The precision obtained in WDXRF methods is vex-y good and

approaches those of wet chemical methods at higher concentrations.

The disadvantage of WDXRF method, specially for the analysis

of costly RE elements like Eu, Ho, Tm and Lu is that it needs

comparatively higher amount of (about 500 mg) sample whereas the

sample needed by D.C. arc-OES methods is only about 20 mg. ii

some cases, lower detection limits can be achieved by D.C. arc-OES

methods as compared by XRF method.-,.

- 10 -

2. EXPERIMENTAL

2.1. Outline of the Methods

The individual RE oxide samples are converted to oxalate by

dissolution in nitric acid and precipitation with oxalic acid.

The dry oxalate sample is then mixed with boric acirt binder in the

ratio 1:1, 2:1 or 3:1 and the mixture is pressed into a double

layer pellet over a boric acid backing pellet. These sample

pel lets are irradiated by high intensity X-rays obtained from a

tungsten anode tube and the fluorescent X-rays are dispersed by a

LiF(200) crystal in a Philips PW 1220 X-ray fluorescence

spectrometer. The intensities of the selected characteristic

X~ray lines are measured by a flow proportional counter (by a

scintillation counter for Y only) at selected 2© angles.

Standards are prepared synthetically by adding known amounts of

RE oxide solutions to the individual matrix RE solution and

precipitating them as oxalates. The intensity of the background

at various 2P angles of the analyte RE element is also measured

from a RE matrix pellet containing no impurities, which is then

subtracted from the total counts (measured separately) to arrive

at net counts. The net counts for the standards are plotted

against impurity element concentration to obtain the working

curve. The concentration of the impurity in sample is obtained by

reading the net counts for the particular element from these

working curves.

2.2. Preparation of Standards and Samples

2.2.1 Introduction

Each RE matrix needs a separate set of standards for

quantitative analysis. Standards have therefore been prepared on

each of the fifteen RE matrices viz. oxides of La, Ce, Pr, Nd, Sm,

Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y. The standards are

- 11 -

initially prepared in solution form, on oxide to oxide basis, by

adding the solutions of analyte elements to the matrix element

solution. The procedure followed is described below :

The oxides of the above fifteen K£ elements, obtained from

Johnson Matthey & Co. or Rare Earths Products, Cheshire, England,

are dissolved in nitric acid (Merck, G.R.). From these solutions

further dilutions 1, II and III are prepared such that each

solution contains the individual oxide at a concentration of 500

r/ml, 50 r/ml and 5 /snl. The required weight of matrix RE oxide

is weighed out separately and dissolved in nitric acid. To the

matrix RE oxide solution, the individual RE oxide solution 1, II

or III is added according to the requirement. The standards thus

prepared, contain a blank (no added impurities) and 10, 20, 50,

100, 200, 506, 1000, 5000 and 10000 ppm by weight of each RE oxide

analyte in individual matrix oxide. As an example the preparation

of terbium oxide standards is given in Table 2. Similar tables

are prepared for each RE matrix prior to preparation of standards.

Table 2

Preparation of Terbium Oxide Standards for XRF Analysis

Sr.No Std.No. Solution addedAmountadded

r

Tb O taken4 V

ran

Standardobtained

ppm

1 .

2.

3.

4.

5.

6.

A 10 ml of -5/Vml 50

B 20 ml of 5}-/ml 100

C 4 ml of 50,r/ml 200

D 10 ml of 50?/ml 500

E 20 ml of 502/ml 1000

F 10 ml of 500r/ml 5000

Gd 02 3

I)y 02 3

and

999.70

999.40

993. ;;0

997.00

994.00

970.00

each.

50

100

200

500

1000

5000

- 12 -

These standard solutions are precipitated as oxalates by

adding saturated oxalic aci'i solution to them. The precipitates

are allowed to settle down overnight and filtered on a filter

paper kept in a glass funnel. The funnel is kept for drying in an

oven at 8QTC. These dry oxalate standards are mixed with proper

amount of binding material which is either 1:1 or 2:1 or 3:1 as

given in Table 3. The total amount of mixture used is also given

in Table 3.

Table 3

Rare Earth Oxalate to Boric Acid Weight Ratios and Their Weights

for Preparation of Double Layer Pellets.

1.

2.

3.

4.

RE Matrix

CeO , Pr 0 &2 d 1 1

Nd2O , Eu203, Tb

La203, Sm203. Gd

Er2Oa, Tm 20 3 & Yb

Y2°a

i40, & I

? 0 3 & i

)y203

RE Oxalate /

Weights, mg

400/400

400/200

600/200

450/150

n ou

Ratio

1:1

2:1

3: 1

3:1

X-ray fluorescence analysis is based on the principle that

the surface irradiated by the primary X--ray i is representative of

whole sample and the sample preparation should ensure that such a

situation exist;;. The pressed pellet technique provides a smooth

r.urface for irradiation and ensures a reproducible geometry. The

particle size affects the intensity of fluorescent X-rays

immensely. The uniformity of particle size in the samples and the

standards is ensured by dissolving the samples and the

- 13 -

constituents of the standards in nitric acid and precipitating

them as oxalates.

A series of experiments [19] given below were done before it

was decided to take the samples as oxalates. The experiments were

directed towards finding a RE salt which could be mixed well with

the binding material boric acid for double layer pellet

preparation. The experiments were :

(i) Precipitating the RE solutions as hydroxides and using

the hydroxide as such for pelletisation.

(ii) Converting t.he hydroxides of (i) to RE oxide by

ignition and using the oxide for pelletisation.

(iii) Precipitating the RE solutions to oxalate and using

the dried oxalates for pelletisation.

(iV) Igniting oxalates in (iii) to oxide and using the

oxides for pelletisation.

Procedures (i), (ii) and (iv) were found to be unsuitable

for mixing and grinding as they tended to form cakes and this

presented sample handling problems.

The sample in oxide form is, therefore, dissolved in nitric

acid and precipitated as oxalate which is then treated similar to

the standards for the preparation of pellets.

Samples of cerium dioxide sometimes give problem in

dissolution with nitric acid. The same is achieved by adding a

few drops of hydrogen peroxide to nitric acid while the sample if.

being dissolved.

2.2.2 Sample to Binding Material Ratio

To get a seLi-sustaining peilet of 31 mm diameter, about .'» tf

weight of RE oxalate sample is needed. for high purity materials

and especially the costly REs like Eu, llo, Tm and Lu, it is not

possible to spare this large amount of sample for analysis.

Therefore, the scimple amount for analysis should be kept to a

- 14 -

minimum. This is achieved by resorting to the technique of double

layer pellet. In this technique the mechanical strength to the

pellet is provided by the backing pellet of boric acid (4 g) and

sample (400-600 m g) is uniformly spread to get 'infinite

thickness' and briquetted over the backing pellet. To ensure that

the sample layer does not peel off from the backing pellet the

sample is also mixed with certain amount of boric acid. The boric

acid contained in the sample binds itself properly with boric acid

of backing pellet and holds the sample along with it. To obtain

as low a detection limit as possible i.e. to get high count rate

for the analyte, the amount of binding material mixed with the

sample should be as small as possible and on the other hand more

the binding material mixed with the sample the less chance it has

of getting peeled off, which is quite common after repeated

irradiation by X-rays. Therefore an optimum ratio of sample:boric

acid was found for each case which has been given in Table 3.

2.2.3 Saturation Thickness Experiments

Saturation thickness, infinite thickness and critical

thickness refer to the same minimum quantity of sample beyond

which with an increase in the amount of sample there is no further

increase in fluorescent X-ray intensity. Ascertaining the weight

of sample to obtain saturation thickness is necessary to keep the

sample weight to a minimum without sacrificing the detection

limit. As a typical example of these experiments the work done on

erbium oxide matrix has been described here.

An erbium oxide standard containing 750 ppm of Tb, Dy, Ho,

Tm, Yb, Iiu and Y oxides in erbium oxide was converted to oxalato.

It was mixed with boric acid binder in the weight ratio 3:1. From

the sample-binder mixture, 50, 100, 200, 400, 600, 1000 and 1600

ing portions were weighed out and pressed to form • double layer

pellets over 400 mg boric acrid backing pellet. Intensities of Lrt

- 15 -

lines of the eiements Tb, Uy, Ho, "I'm, Yb and Lu were measured for

100 seconds each. These intensities are plotted against the

sample weights which are shown in Fig.l. It was seen from these

curves that there is no contribution to the analyte line intensity

after a weight of 400 mg.

y. Milk/

rC 12H

% 1011

**" E'"»"a : "BW VIII - :i:A l l .

I 1 I;..) COO 4110 KOO tdi l l f

d.UII'I.K I)IM>!.I< MIVIIIIIK. IU<

HC I TOTM. INTINSin tl>' IMI'HUIl 1 , , , I l l lk i III MillH'U OHl*TS FHOU

1 I I m m UUUKIEI: I ' U U I 14 lii'tli ' 1' !l.

[8(1(1

Similarly, the intensity ol Kr line of Y was measured for

seconds. The sample, weight versus intensity curve for this

element is of a different shape which is .shown in Fig.2.

i . U)

i : r , d , , : | l [ i { , | ( \ ( III

V'M'.IPM , . , ! ) | , | ,m

.IUL \

U

*-- hi u

airL..,. it) i n o i 'od i on

NAMI'll IIINIM.li MlM'Hlir., in:

1 " '

TOTAL ANH NKT INTKNSITIKS UK YTTIilMM K,,L I N K K I . O M IVI1- I - 'KI;[ • : . . 1 l U ' l i . l i l S U K S A M I ' I . K -I I I M H ' l i M I V I I ' l : ! , r i i l S . S K I I O V I - . K A : i l m ml ' K I . ] , K T i : F i l i l l i l , A ' i l l .

- 16 -

The low atomic number elements in H BO scatter the high

energy (14.b keV) K^ line and there is a high background from the

boric acid supporting pellet as there was no sample on the top of

it. Addition of the sample to the supporting pellet attenuated

the background count till at 400 rag mixture a saturation was

reached. At this point the intensity is purely due to K line of

Y and the contribution of the background from the supporting

pellet was nil. Fig.?., also shows the net intensity versus the

sample mixture weight curve. This curve is similar to the curve

for other analyte elements shown in Fig.l.

From these studies it is found that 400 mg sample-binder

mixture gives a saturation thickness when converted to a pellet of

31 mm diameter . To be on safe side a 600 mg sample mixture

is taken for the analysis in this case.

2.3 Instrument and Operating Conditions

The semiautomatic Philips X-ray spectrometer PW 1220 used in

these analyses has four portholes which can hold pellet of a

maximum of 31 mm diameter. The background count intensity N, is

measured at each element position from the blank pellet at

position 1 which is subtracted from total peak counts N ofp

standards and/or samples loaded in other 3 positions to get thenet intensity N - N, of the characteristic line. The same

P b

counting time is used for both N and Nfc. The net intensities

obtained from the set of synthetic standards is use.d for plotting

of working curve. The net intensity of a particular element i.s

referred to its working curve to obtain its concentration. The

operating conditions for the instrument are given in Table 4. The

lowest point on the concentration axis gives the minimum

determination limit (MDL) for the particular element. The MDL:;

obtained for different, elements in 1b RF. matrices are given in

- 17 -

Table-5.

Table 4

Instruments and their Parameters used in RE Analysis

Spectrometer

Generator

X-ray Tube

Collimator

X-ray Path

Detector

Pulse Height

Counting Time

Analysing Crystal

Philips Semiautomatic X-raySpectrometer PW 1220.

Philips Ultrastable GeneratorPW 1140 voltage and currentregulated to 0.01%.

Philips 3kW tungsten targetoperated at 60 kV and 35 mA.

Fine 160 Min for all.Coarse 480 t-im for Y and Lu II

Vacuum 0.f> torr for all.

Flow proportional operatingwith P-10 gas in general.Scintillation Nal(Tl) type forY only.

Discriminator-250, Window-500.

40, 60 and 100 sec.

LiF (200); I order for all; IIorder for Lu in Er only.I.IF(220) I order for Yb in G.I;Pr in Sm; II order for Ho & Krin Dy. To;iax for Nd in La.

- 18 -

Table b

Minimum Determination Limits (%) in RE Matrices by XRF

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Y

La 0.005 0.1 0.00b 0.005

0.01 Ce 0.02 0.02 0.01

0.01 0.01 Pr 0.01 0.01

0.01 0.01 Nd 0.01 0.005 0.01

0.005 0.01 0.005 Sm 0.01 0.005 0.01

0.01 0.02 0.01 Eu 0.01 0.01 0.01 0.01

0.005 0.005 Gd 0.005 0.005 0.01 0.005

0.01 0.01 0.01 T_b 0.01 0.01 0.01

Eu Gd Tb Dy Ho Er Tin Yb Lu Y

0.005 0.005 0.005 Dy 0.01 0.005 0.01

0.005 0.005 0.00b Ho 0.01 0.01 0.02

0.005 0.005 0.005 Er 0.005 0.01 0.005 0. \Mii

0.005 0.002 0.005 Tin 0.01 0.002 0.002

0.00b 0.005 0.005 Yb 0.01 0.005

0.002 0.005 0.002 _Lu B.C-502

0.01 0.005 0.005 0.005 0.01 0.01 - 0.01 - Y

3. RESULTS AND DISCUSSION

The problems of line interferences, the lines selected for

analysis, the precision obtained for each element, the theoretical

detection limit etc. will be discussed separately for each RE

matrix.

3.1 The Line Interferences and Selection of Analysis Lines.

3.1.1 Lanthanum Oxide (Ref.19)

Cerium l*» (X = 2.463 A) does not have any interference

from other analyte elements or matrix lines.

Praseodymium l&t (X = 2.561 A) has a strong interference

from La W (X - 2.458 A) line. The next best line Pr W ('*•

2.258 A) is strongly absorbed by La (L absorption edge at v r

2.259 A) but has no line interference. Praseodymium 1/3 (A -

2.1 IS A) is also absorbed by La (L absorption edge at. X = 1.973

A) and in addition has an interference from La Lj' . Pr Isy is too

weak to be chosen as analysis line. Therefore Pr Wf is chosen as

analysis line as a compromise even though due to matrix absorption

"the minimum determination limit is poorer.

Neodymium L« has interference from Co I/if and a 1 ino

overlap from La \fin matrix lint:. Since Ce is present only in

trace amounts in high purity La 0. samples d: j liter tV.renci. U; :hi

is negligible. Nd L><t can be conveniently :,e)>cirated Y;y usintj

topaz crystal in the spectrometer. Therefore Nd L> line is used

for analysis with topaz as a dispersing crystal.

Samarium U-* also has an interference from Ct: U\, .««i L'r L/*

whereas Sm Mi is free from interferences which Is chosen as

analysis line. The chosen analysis lines and the determination

range covered by them ar& giver* in Table (5.

- 20 -

Table 6Analysis Lines for Lanthanum Oxide

Element

Ce

Pr

Nd*

Sm* Topaz

Analysis Lines

Line WavelengthA

Lat 2.561

Wt 2.258

Loi4 2.370

W± 1 - 998

crystal

DeterminationRange

0.005 -

0.1

0.005 -

0.005 -

- 1.0

- 1.0

- 1.0

- 1.0

CountingTime

Sec.

100

100

100

100

3.1.2. Cerium Oxide (Ref.20)

Lanthanum Lot line does not have any interference from other

analyte lines or matrix element lines and therefore has been used

as analysis line. Praseodymium Lo< (A. = 2.666 A) line has

interference from La l/i± line (X = 2.459 A ) . Therefore L/5 line

(X. = 2.259 A) has been used for analysis. The Lo^ lires of Nd and

Sm are masked by the matrix lines of Ce. Therefore the next best

lines W± are used for Nd and Sm analysis. Table 7 gives the

analysis lines used in cerium oxide matrix and the determination

range covered by them.

Table 7

Analysis Lines for Cerium Oxide

AnalyteElement

Analysis Lines

Line WavelengthA

DeterminationRange

%

0.01 - 0.5

0.02 - 0.5

0.02 - 0.5

0.01 - 0.5

Count i ngTime

Sec.

100

100

100

100

La

Pr

Nd

Sm

2.666

2.259

2.167

1.998

- 21 -

3.1.3 Praseodymium Oxide (Ref. 21)

It was possible to use sensitive Lot lines for La, Ce and

Nd. For Sm, the U>± line falls inside the matrix line Pr IV? (*• -

2.217 A) which appears strongly in the spectrum. Therefore for Sm

the IJ?i line (A = 1.998 A) was used. The analysis lines used and

the determination range obtained are given in Table 8.

Table 8.

Analysis Lines for Praseodymium Oxide

Analyte

Element

La

Ce

Nd

Sm

Analysis Lines

Line Wavelength

A..

L» 2.666

L=< 2.562

L« 2.370

U\ 1.998

Determination

Range

%

0.01 - 1.0

0.01 - 1.0

0.01 - 1.0

0.01 - 1.0

Counting

Time

Sec.

100

100

100

100

3.1.4. Neodymium Oxide (Ref. 22)

The sensitive kx line could be used only for Ce. For all

other elements W lines are used. The Pr La (X = 2.463 A) line

is interfered by La L/? line (K = 2.459 A) whereas in the case of

Sm, Eu and Gd, the interference to La lines is from Nd matrix

lines. The Pr La line can be used for samples in which La is

absent. The analysis lines used and the determination range

obtained with them are given in Table 9.

- 22 -

Table 9.

Analysis Lines for Neodymium Oxide

Analyte

Element

Ce

Pr

Sm

Eu

Gd

Analysis Lines

Line Wavelength

A

La 2.562

W 2.259

Wi 1.998U\ 1 • 920

L/?i 1.847

Determi nation

Range

%

0.01 - 1.0

0.01 - 1.0

0.01 - 1.0

0.005- 1.0

0.01 - 1.0

Counting

Time

Sec.

60

40

40

40

40

3.1.5. Samarium Oxide (Ref. 23)

Koi line of Y and Lo< line of Ce are free from interferences

and therefore used for the analysis. Praseodymium Lo<t (X - 2.463

A) has an interference from La = 2.459 A) but Pr is

free from RE element interferences. Former interference can be

resolved by the use of LiF (220) crystal; alternatively Pr L/?f line

can be used. The intensities of analysis lines are corrected

whenever the samples contain an interfering element in appreciable

quantity. For example Gd Is* intensity is corrected for Ce lv

interference by determining the intensity correction factor (ICK)

of Ce Lrt/Ce l&t in Gd free Sm.,03 samples. Ce U>- intensity i:v

estimated by measuring the Ce \J* intensity. This Ce Uv

intensity is subtracted from measured intensity at Gd Lv

position. The analysis lines and determination range are given

in Table 10.

Table 10

Analysis Lines for Samarium Oxide

Analysis Lines „ . . . „. , . Determination CountingAnalyte naT>«« T i _ oElement Line Wavelength Kange lime

A % Sec.

Ce Lo«i 2.561 0.01-1.0 100

Pr* Lci 2.463 0.01 - 1.0 100

Nd La, 2.370 0.01 -1.0 100

Eu Wt 1.920 0.01-1.0 40

Gd L=«i 2.046 0.01 - 1.0 40

Y Kot 0.328 0.01 - 1.0 40

* LiF (220)

3.1.6. Europium Oxide (Ref. 24)

Praseodymium Lo< (X = 2.463 A) has a strong interference

from La L/?t (X = 2.459 A). As La is usually present in Eu O , Pr

Lot is chosen as analysis line. In case sample contain La an

alternate line Pr L/5 (X = 2.258 A) has to be used. Neodymium Ivs

(X = 2.370 A) is overlapped by Eu L (X _ 2.394 A). Neodymium I/*

(X = 2.166 A) has slight overlap by strong matrix line Eu La (X -

2.121 A ) while Nd L/?2 is too weak to be used for analysis.

Therefore Nd [/? though with some Eu I,:* background, i.s used for

analysis. Samarium L/* and Tb l/:» are comparatively interference:

free and as such are chosen as analysis lines.

Dysprosium l^*i (X = 1.908 A) has an interference from

intense matrix line Eu U'it (k "- 1.920 A). Dysprosium Lft (X

- 24 -

1.711 A) has an interference from a comparatively weaker matrix

line Eu I^5 which causes high background at Dy Lfi position.

Dysprosium I/? which is already weak line is further reduced in

intensity by absorption in Eu LXJ absorption edge. Therefore, Dy

Lftt is selected as- analysis line even though there is a high

background at its position due to a weak matrix line.

Gadolinium te*t (A - 2.046 A) is interfered by one analyte

element line Nd Wz (>>• = 2.036 A). Line overlap corrections to Gd

1/3 intensities may be necessary in case samples contain Nd in

large concentrations. For Gd the 1/ (X = 1.806 A) and L/?2 (X =

1.745 A) can not be used as alternate lines as these are

interfered by matrix lines Eu ty and Eu l& respectively.

The analysis lines selected and the concentration range

covered by these lines are given in Table 11.

Table 11

Analysis Lines for Europium Oxide

AnalyteElement

Pr

Pr

Nd

Sm

Gd

Tb

Dy

Y

Analysis Lines

Line WavelengthA

Lci 2.463

Wt 2.258

IV? 2.167

Lvt 2.199

U*t 2.046

I/*t 1.976

Wt 1.710

Ra 0.829

DeterminationRange

0.01

0.01

0.02

0.01

0.005

0.01

0.02

0.01

- 0.2

- 0.2

- 0.2

- 0.2

- 0.2

- 0.2

- 0.2

- 0.2

CountingTime

Sec.

1 0 0

100

100

4 0

1 0 0

4 0

4 0

1 0 0

- 25 -

3.1.7. Gadolinium Oxide (Ref. 2b)

La lines of Sm, Eu, Tb and Dy do not present interference

Holmium Lo<i (Xproblems and have been chosefi as analysis lines.

1.845 A) is interfered by Gd hfit (X - 1.849 A) therefore Ho U\ (X

= 1.647 A) is chosen as analysis line. However, this line has an

interference from Y Ka <II order) line which can be eliminated by

pulse height selection.

Ytterbium Lo^ (X = 1.672 A) has close lines Tb W2 (X =

1.683A) and Dy U) (X = 1.662 A) near it but these three lines are3

well resolved by LiF (220) crystal. Therefore, Yb Lo<i is

selected as analysis line with LiF (220) crystal. Yb 1/3 is also

equally suitable for analysis.The selected analysis lines and

concentration range covered by them is given in Table 12.

Table 12

Analysis Lines for Gadolinium Oxide

AnalyteElement

Analysis Lines

Line WavelongthA

DeterminationRange

0.005 -

0.01 -

0.01 -

0.005 -

0.01 -

0.01 -

0.01 -

0.01 -

0.

0.

0.

0.

0.

0.

0.

0.

1

1

1

1

1

1

1

1

CountingTime

Sec.

100

40

40

40

40

100

100

100

Sm Lo^

Eu If*

Tb La t

Dy La t

Ho Wt

Yb* Lci

Yb U\

Y Ko.

* LiF (220)

2.199

2.121

1.976

1.908

1.647

1.672

1.476

0.828

- 26 -

3.1.8. Terbium Oxide (Ref. 26)

The sensitive Lc«i lines of Sin, Eu, Gd, Dy and Ho pose no

problem of interference in Tb 0 matrix. An interference of Eu

l/it line (X - 1.920 A) to Dy l&t line (X = 1.909 A) was suspected

because of their proximity but experiments showed that these two

lines were clearly separated by LiF (200) crystal. Dysprosium Let

line was therefore used for the analysis.

The selected analysis lines and the concentration range

covered is given in Table 13.

Table 13

Analysis Lines for Terbium Oxide

AnalyteElement

Sm

Eu

Gd

Dy

Ho

Y

Analysis Lines

Line WavelengthA

Lc«i 2.200

Lc<i 2.121

L»± 2.047

Lc<i 1.909

Laj 1.845

K« 0.828

DeterminationRange

0.01

0.01

0.01

0.01

0.01

0.005

- 0.5

- 0.5

- 0.5

- 0.5

- 0.5

- 0.5

CountingTime

Sec.

1 0 0

1 0 0

100

• 1 0 0

100

1 0 0

3.1.9. Dysprosium Oxide. (Ref. 27)

Europium I/» (X - 2.121 A) has interference from Pr Ifi (X =

2.119 A ) and Nd L/*3 (X - 2.127 A) but Pr and Nd are usually not,

found in Dy O . Therefore Eu L^ has been selected as analysis

line.

Gadolinium La (X = 2.047 A) has interference from Nd Lfi (X

- 27 -

= 2.036 A) and a line overlap from Ce Lr4 (X = 2.049 A) but Ce and

Nd usually are not found in Dy 0 - Therefore,

selected as analysis line.

Gd La has been

Terbium U*x (X = 1.977 A) has interference from Pr hy% (x

(X = 1.962 A). Since Pr and Sm are not

the Tb Lot line is selected for analysis.

1.961 A) and Sm I

expected in Dy O

Holmium Lc*t (X = 1.845 A) has interference from Gd hft± (X =

1.849 A). With LiF (200) in second order, Ho La± and Gd Lft± are

sufficiently resolved. Similarly, Er La (X =. 1.784 A) has

interference from Tb W3 = 1.747 A) and Gd L/?2 (X =1.746 A) which

are satisfactorily resolved in LiF (200) second order. Ho La and

Er La are therefore measured in second order of LiF (I J0).

The selected lines and the concentration range covered by

them are given in Table 14.

Table 14

Analysis Lines for Dysprosium Oxide

AnaxyL6Element

Eu

Gd

Tb

Ho*

Er*

Y

* LiF (200)

Analysis Lines

Line WavelengthA

Lai 2.120

Lr*t 2.046

Lai 1.976

Lat 1.845

l&t 1.784

Koi 0.828

second order

DeterminationRange

0.005

0.005 -

0.005

0.01

0.005 -

0.01 -

- 0.5

- 0.5

- 0.5

- 0.5

- 0.5

-0.5

CountingTime

Sec.

40

40

40

100

100

100

- 28 -

3.1.10. Holmium Oxide (Ref. 28)

Gadolinium tof,..Tb La^, Dy U*t and Tm Lo^ have interferences

mostly from Ce, Pr, Nd and Sm. These elements are usually not

present in Ho203 samples the interference is not serious and l/*±lines are selected as analysis lines. Erbium Lo< (X = 1.784A) has

interference from Tb Wa (X = 1.747 A) and Er W± (X = 1.587A) has

much more serious interference from Ho I/? (X - 1.567 A).

Therefore, Er La^ with intensity correction for Tb IV? is used as

analysis line. Ytterbium Lo^ (X = 1.672 A) has interferences from

Tb Wz (X = 1.683 A ) , Dy hfti (X = 1.711 A) and Ho hftt (X = 1.648

A). Lithium fluoride (200) crystal in its second order is used to

resolve these lines.

The selected lines and the concentration range covered by

them is given in Table 15.

Table 15

Analysis Lines for Holmium Oxide

AnalyteElement

Gd

Tb

Dy

Er

Tm

Yb

Analysis Lines

Line WavelengthA

Lc*t 2.046

ls.*t 1.976

Lo«i 1. 908

I/*t 1.784

U*i 1.726

Lc»i 1.672

DeterminationRange

0.005

0.005

0.005 -

0.01 -

0.01 -

0.02 -

- 0.5

- 0.5

0.5

- 0 5

- 0.5

- 0.5

CountingTime

Sec.

40

40

40

40

40

100

- 29 -

3.1.11. Erbium Oxide (Kef. 29)

It was possible to use sensitive L»a lines for all elements

except for Y for which Kot line was used. Lutetium La (X = 1.620

A) lies close to Dy I/*., (X = 1.624 A). However, these two lines

are properly separated when LiF (200) erystal is used in second

order.

The selected Iine3 and the concentration range covered by

-them are given in Table 16.

Table 16

Analysis Lines for Erbium Oxide

AnalyteElement

Tb

Dy

Ho

Tin

Yb

Lu*

Y

Analysis Lines

Line

1

1

1

L«1

Lat

K«

WavelengthA

1.977

1.909

1.845

1.727

1.672

1.624

0.828

DeterminationRange

0.005 -

0.005 -

0.005 -

0.005 -

0.005 -

0.00b -

0.005 -

- 1. 0

- 1.0

- 1.0

- 1.0

- 1.0

- 1.0

- 1.0

CountingTime

Sec.

100

100

100

100

100

100

100

LiF (200) second order

3.1.12 Thulium Oxide (Ref. 30)

It was possible to use sensitive La lines for elements Dy,

Ho, Er, Yb and Lu. For Y, Koc line is used.The selected lines and

concentration range covered are given in Table 17.

Table 17

Analysis Lines for Thulium Oxide

AnalyteElement

Vy

Ho

Er

Yb

Lu

Y

Analysis Lines

Line WavelengthA

Let 1.909

La 1.845

La 1.784

Lat 1.672

La 1.620

K« 0.828

DeterminationRange

0.005 -

0.002 -

0.005 -

0.01 -

0.002 -

0.002 -

- 1.0

- 3.0

- 1.0

- 1.0

- 1.0

- 1.0

CountingTime

Sec.

40

40

40

40

40

100

3.1.13 Ytterbium Oxide (Ref. 31, 32)

It was possible to use Lot lines which give high sensitivity

for all elements except for Y. For Y, the K-M line was used.

The selected lines and concentration range covered in

ytterbium oxide matrix are given in Table 18.

Table 18

Analysis Lines for Ytterbium Oxide

AnalyteElement

Ho

Er

TmLu

Y

Analysis Lines

Line WavelengthA

h* 1.845t

La 1.784

U* 1.726

La 1.620

Ka 0.828

DetermiRan^

0.005 -

0.005 -

0.00b -

0.01

0.005 -

Lnationje

- 1.0

- 1.0

- 1.0

- 1.0- 1.0

CountingTime

Sec.

40

40

40

40

100

3.1.14 Lutetium Oxide (Ref. 33, 34)

It was possible to use l&t lines which give high sensitivity

for all elements except for Y. For Y, K« line was used.

The selected lines and concentration range covered in

lutetium oxide matrix are given in Table 19.

Table 19

Analysis Lines for Lutetium Oxide

AnalyteElement

Er

Tm

Yb

Y

Analysis Lines

Line WavelengthA

Lo^ 1.784

L«t 1.726

U*t 1.672

Kot 0.828

DeterminationRange

0.002 -

0.005 -

0.002 -

0.002 -

- 1.0

- 1.0

- 1.0

- 1.0

CountingTime

Sec.

40

40

40

100

3.1.15 Yttrium Oxide (Ref. 35, 36)

Holmium Lftt (X = 1.648 A) line is interfered by Y K<=< (X -.-

0.828 A) second order line. Pulse height selection .eliminate;-,

this interference. Yb L«l(X = 1.476 A) is interfered by Ho U\ (•>•

= 1.648 A ) , Eu L?^ (X =:1.6f>7 A) and Tb Wz (X = .. 1.683 A) lines.

These lines are resolved properly in second order of LiF

(200)crystal and therefore Yb l/*j line is selected for analysis.

The lines selected and concentration range covered by them

are given in Table 20.

Table 20

Analysis Lines for Yttrium Oxide

AnalyteElement

Analysis Lines

Line WavelengthA

Determination CountingRange Time

% Sec.

Eu

Gd

Tb

Dy

Ho

Er

Yb" Lot.

2.121

2.046

1.975

1.710

1.647

1.587

1.476

0.005 -

0.005 -

0.005 -

0.01 -

0.005 -

0.005 -

0.01 -

1.0

1.0

1.0

1.0

1.0

1.0

1.0

100

100

100

100

100

100

100

* LiF (200) second order

3.2 Precision

The precision is calculated in terms of relative standard

deviation percent (RSD %) from a minimum of eleven values each of

total peak counts and background counts using the formula :

RSD % -

2 n Z

( peak " background ) X 100

Where c2 = Z d2 /(n-1) and o* . = £ df /(n-1)pe-CLk p background >

Here, 5T d is the sum of the squares of differences ofp

count N .p

individual peak counts from their average peak

Similarly, £ d^ is sum of the squares of differences of

individual background counts from their average background count

The RSD % values are calculated for each analyte element in

all matrices at each concentration of various standards. The

precision values are given in Tables 21 to 35.

it is clearly observed in all rare earth matrices that the

precision is very good at higher concentrations and worsens as the

minimum determination limit is approached. XRF methods are known

to be the most precise methods, for intermediate concentrations.

Table 21.

Precision for Lanthanum Oxide (Ref. 37)

Concent ra t i on%

0.005

0.025

0.1

0.5

1.0

Ce

IP.

7

6

4.5

0.5

RSD %

Pr

-

13

6

1.6

0.8b

Nd

-

14

2.5

2.5

1.6

Sm

35

16

5

1.2

1.0

Table 22

Precision for Cerium Oxide (Ref. 20)

ConcentrationX

0.01

0.02

0.05

0.1

0.5

La

23.2

12.7

6.4

2.6

0.8

RSD %

Pr

-

38.8

5.8

1.2

1.1

Nd

-

40.8

8.3

4.4

1.1

Sm

23.2

9.4

7.6

3.3

0.7

Table 23

Precision for Praseodymium Oxide (Ref. 21)

Concentration%

0.01

0.025

0.1

0.5

1.0

La

42.0

17.1

3.4

1.03

0.9B

RSD %

Ce

37.4

7.0

7.7

0.69

0.35

Nd

21.1

9.1

2.5

0.74

0.51

Sm

25.7

7.7

7.8

1.7

0.16

Table 24

Precision for Neodymium Oxide (Ref. 22)

Concentration%

0.005

0.01

0.025

0.1

0.5

1.0

Ce

-

18.3

8.1

2.7

0.4

0.3

Pr

-

40.0

20.3

5.5

1.0

0.7

RSD %

Sm

' -

35.8

18.2

3.0

0.9

0.5

Eu

28.4

12.5

B.b

2.3

0.6

0.5

Gd

-

13.8

8.9

4.0

1.0

0.7

Table 25

Precision for Samarium Oxide (Ref. 3V)

Concentration

0.005

0.01

0.05

0.1

0.5

1.0

Y

22

11

3

1.

0.

0.

8

3

3

Ce

22

10

2.2

1.8

0.5

0.4

RSD

Pr

-

36

10

4

1

0

%

.7

.0

.77

Nd

27

16

3.6

1.6

0.58

0.44

Eu

-

30

5

4

1

0

.0

.6

Gd

32

19

4.4

2.5

0.6

0.3

Table 26

Precision for Europium Oxide (Ref. 24)

Concentration

0.005

0.01

0.02

0.05

0.1

0.2 .

Y

-

20

8

2

1

.0.82

Pr

20

10

3.8

2.6

2.1

0 - 6b.

RSD %

Nd

-

-

26

7

6

. 3

Sm

-

11

6.

4

2.

-.0 -

3

5

85

Gd

28

11.

9.

4

1.

... 1.

b

4

7

1

Tb

-

7

4

5

2

1

Dy

-

-

30

11

8

4

- 36 -

Table 27

Precision for Gadolinium Oxide (Ref. 37)

Concentrat ion%

0.005

0.01

0.02

0.05

0.1

Y

-

28

10

4

3

Sm

11

10

5

2

1.7

RSD

Eu

-

12

7

3

1.

X

5

Tb

-

19

6

2.2

2

Dy

10

9

6

3.

1.

6

7

Yb

-

24

15

10

5

Table 28

Precision for Terbium Oxide (Ref. 26)

Concent rat i onX

0.01

0.02

0.05

0.1

0.5

Sm

-

9.2

5.3

1.9

1.1

Eu

-

16.6

5.9

3.5

1.3

RSD *

Gd

-

51.4

19.6

9.8

3.5

Dy

-

21.

9.

3.

1.

2

7

4

2

Ho

28.

13.

4.

3.

0.

6

5

7

1

7

Y

21.

16.

3.

2.

1.

2

2

4

3

6

- 37 -

Table 29

Precision for Dysprosium Oxide (Kef. 27)

Concentration%

0.005

0.01

0.05

0.1

0.5

1.0

Y

-

10.7

3.6

2.3

0.66

0.32

Ku

23.5

11.6

4.5

1.2

0.46

0.50

RSD %

Gd

15

9

2.3

1.3

0.51

0.20

To

30

18

4

1.

0.

0.

8

66

55

Ho

-

19

4.

1.

0.

0.

3

8

63

42

Er

28

15

3.

2.

1.

0.

0

5

1

46

Table 30

Precision for Holmium Oxide (Ref. 28)

Concentration%

0.005

0.01

0.02

0.05

0.1

0.5

Gd

40

16.7.

8.5

3.5

2.0

0.6

Tb

42

14.3

8.9

3.4

1.8

0.5

RSD %

Dy

31.7

18

11.7

6.3

3.5

0.8

Er

-

23.

10.

3.

1.

0.

4

8

1

9

4

Tm

-

45

20.

9.

3.

1.

0

6

6

0

Yb

-

-

24.

11.

6.

1.

3

3

5

5

Table 31

Precision for Erbium Oxide (Kef. 29)

ConcentrationX

0.005

0.01

0.02

0.6

1.0

Tb

27.9

10.4

5.2

2.1

1.2

Dy

15.0

7.0

3.7

l.b

0.5

RSD %

Ho

28.1

16.7

7.1

3.7

1.4

Tm

16.5

11.6

6.2

3.5

1.7

Yb

65.0

29.0

6.5

7.9

3.1

Lu

24.7

18.3

15.0

5.4

3.8

Y

44.1

21.1

12.1

7.3

3.6

Table 32

Precision for Thulium Oxide (Ref. 30)

Concentration%

0.002

0.005

0.01

0.02

0.05

0.1

0.5

1.0

Dy

-

18.2

7.5

4.0

1.8

1.5

0.5

0.3

Ho

20.

12.

4.

3.

2.

1.

0.

0.

9

5

4

3

7

0

6

5

RSD

Er

-

28.

13.

6.

3.

2.

0.

0.

%

1

3

4

4

0

7

4

Yb

-

-

26.

13.

8.

3.

1.

1.

8

3

7

9

6

5

Lu

31.

17.

9.

4.

2.

1.

1.

1.

1

2

4

7

4

3

3

3

Y

34.6

16.1

8.8

4.5

2.5

1.8

0.6

0.4

- 39 -

Table-33

Precision for Ytterbium Oxide (Ref. 32)

Concentrat ion%

0.005

0.01

0.02

0.05

0.1

0.6

1.0

Ho

23.7

12.6

8.0

2.7

1.2

0.4

0.2

Er

17.8

2.9

5.0

1.4

1.0

0.3

0.3

RSD %

Tin

39.1

22.9

15.3

6.2

3.0

0.9

0.3

Lu

_

36.6

16.4

5.2

2.2

0.8

0.3

Y i.

'is.'f

10.7

6.8

4.3

1.9

1.0

0.4

Table 34.Precision for Lutetium Oxide (Ref. 34)

Concentration%

0.002

0.005

0.01

0.02

0.05

0.1

0.5

1.0

Er

17.7

11.8

5.8

2.2

1.6

0.5

0.3

0.2

RSO %

Tm

-

16.8

8.9

4.8

2.2

1.9

0.6

0.4

Yb

38.2

14.5

11.8

•6.8

3.6

2.2

0.9

0.4

Y

19.8 •

14.8

5.8

3.4

2.7

2.1

1.2

1.1

Table 35

Precision for Yttrium Oxide (Ref. 36)

ConcentrationX

0.005

0.01

0.05

0.1

0.5

1.0

Eu

21.0

8.5

2.0

0.7

0.4

0.1

Gd

16.3

6.1

2.0

1.6

0.8

0.2

SSD X

Tb

9.7

4.9

1.3

0.9

0.2

0.1

Dy

-

10.7

2.2

1.1

0.7

0.4

Ho

26.4

13.8

2.8

1.9

0.6

0.4

Er

21

10.3

2.8

1.3

0.3

0.1

Yb

-

29

5.8

2.6

0.7

0.4

3.3 Theoretical Minimum Detection Limit (TMDL)

The theoretical minimum detection limit (TMDL) is defined as

a concentration for which signal above background is equal to

three times standard deviation of background counts (N, ) for ab

given counting time. This is also known as three sigma limit

whare sigma denotes the standard deviation of background counts NK

by using the formula

TMDL = 3 T T I ~ X concentration/(N - Nfe)

In practice it is difficult to attain the theoretical

detection limit because of many experimental constraints. This is

the ultimate detection limit which could be attained when all the

limiting factors are taken care of. The TMDL's have been

calculated for each analyte element in all the 15 matrices

- 41 -

investigated. These have been given in Tables 36 to 50.

Table 36

Theoretical Minimum Detection Limits in Lanthanum Oxide (Ref. 19)

Element BackgroundCounts Nb

0 - 0 1 % s t a n d a r d

Counting time TMDLin seconds ppm.

Ce

Pr

Nd

Sm

23600

12280

14210

7080

1080

184

1400

768

100

4 0

4 0

4 0

4 0

1 1 0

2 0

6 0

Table 37

Theoretical Minimum Detection Limits in Cerium Dioxide (Ref. 20)

Element BackgroundCounts N.

Net peakcounts for0.02% standard


La

Pr

Nd

Sm

15830

45863

42102

16187

160

1638

1211

1984

100

100

100

100

47

80

102

47

- 42 -

Table 38

Theoretical Minimum Detection Limits in Praseodymium Oxide

(Ref. 21)

Element

La

Ce

Nd

Sm

BackgroundCounts N.

9430

19419

32052

25652

0.02% standard

2239

3575

6148

3376

Counting timein seconds

100

100

100

100

TMDLppm

33

29

22

36

Table 39

Theoretical Minimum Detection Limits in Meodymium Oxide (Ref. 22)

„, ' Background N e t £***£ Counting time TWDL»—* Cunts N, S ° ^ 5

Ce

Pr

Sm

Eu

Gd

10228

1773b

36731

12636

19822

6671

4333

7610

6723

5987

60

40

40

40

40

45

92

76

50

70

Table 40

Theoretical Minimum Detection Limits in Samarium Oxide (Ref. 23)

Element

Ce

Pr

Nd

Eu

Gd

Y

BackgroundCounts N,

b

18213

15689

21098

47588

38497

26711


1816

688

2263

1158

1940

2106


100

100

100

40

40

40

TMDLppm.

20

90

20

60

30

20

Table 41

Theoretical Minimum Detection Limits in Europium Oxide (Ref. 24)

ElementBackgroundCounts R

Net peakcounts for0. 1% standard


Pr

Nd

Sm

Gd

Tb

Dy

Y

14400

50562

20840

25482

32920

44280

37456

17427

41bl

21308

13b97

16535

4009

38700

100

40

40

40

40

40

100

20

160

30

35

33

160

20

- 44 -

Table 42

Theoretical Minimum Detection Limits in Gadolinivim Oxide (Ref.25)

Element

Sm

Eu

Tb

Dy

Yb

Y

BackgroundCounts N

20998

19381

21340

25640

5210

11034


3060

1728

2067

2509

535

681


100

40

40

40

100

100

TMDLppm.

10

20

20

20

80

40

Table 43

Theoretical Minimum Detection Limits in Terbium Oxide (Ref. 26)

Element

Sm

Eu

Gd

Dy

Ho

Y

BackgroundCounts N.

b

11115

25358

21606

22353

23420

13927


1365

778

746

792

1270

950


100

100

40

40

40

100

TMDLppm.

73

61

59

56

36

37

- 45 -

Table 44

Theoretical Minimum Detection Limits in Dysprosium Oxide (Ref. 27)

Element

Eu

Gd

Tb

Ho

Er

Y

BackgroundCounts N,

13994

12315

28090

13428

9934

13727


733

1237

1202

670

696

107


40

40

40

100

100

100

TMDL

50

30

40

50

43

30

Table 45

Theoretical Minimum Detection Limits in Holmium Oxide (Ref. 28)

Element

Gd

Tb

Dy

Er

Tm

Yb

BackgroundCounts N.

194640

182840

40040

42880

35080

25500


1731

1996

2179

3073

1894

1163*


40

40

40

40

40

100

TMDLppm.

25

20

27

20

30

80

* Value for 0.02%

- 46 -

Table 46

Theoretical Minimum Detection Limits in Erbium Oxide (Ref. 29)

Element

Tb

Dy

Ho

Tm

Yb

Lu

y

BackgroundCounts N

45790

48876

103466

94321

138666

83182

22983


17719

20810

14053

14891

10891

5540

12651


100

100

100

100

100

100

100

TMDLPPm-

18

16

34

31

37

78

10

Table 47

Theoretical Minimum Detection Limits in Thulium Oxide (Ref. 30)

Element

Dy

tlo

Er

Yb

h,i

Y

BackgroundCounts Nt

b

17647

18826

3392b

65456

27785

23961


9550

16507

17940

7349

14324

29125


40

'1.5

40

40

40

40

TMDLPPm.

21

13

lb

52

9

8

- 47 -

Table 48

Theore'tical Minimum Detection Limits in Ytterbium Oxide (Ref. 31)

Element

Ho

Er

Tin

Lu

Y

BackgroundCounts R

19694

20063

37336

46327

25504


8741

12700

6288

7138

10515


40

40

40

40

40

TMDLppm.

24

17

46

45

23

Table 49

Theoretical Minimum Detection Limits in Lutetium Oxide (Ref. 33)

Element Background ^ J ^ J o r Counting time TMDLCounts Nb 0 _ 0 b % s t a n d a r d m seconds PPm.

Er 18957 16H15

Tm 20306 9586

Yb 54209 107bb

Y 24049 17017

40

40

40

40

12

13

33

14

- 48 -

Table 50

Theoretical Minimum Detection Limits in Yttrium Oxide (Ref. 36)

Element

Eu

Gd

Tb

Dy

Ho

Er

Yb

BackgroundCounts N,

b

11410

5752

16664

34030

40010

36000

8210


25100

12700

35200

21200

20500

26900

4900


100

100

100

100

100

100

100

TMDLppm.

13

18

11

26

30

. 21

55

3.4 Conclusions

For the first time, the work, spread over several years, on

the development of XR1<" analytical methods for all the icembers of

the rare earth group has been reported. Here, wave]en^th

dispersive X-ray fluorescence methods for the determination of

trace raru earth imparities in fifteen matrix <J!, viz La 0. , Ce;>, ,

Tci. 0. , Yb. 0

and Hn-it.y

Lu 0 ' and Y^O have been reported. Earlier Lytle

130J have developed XliF methods for rare earth

• n- uriti^s in five matrices viz;. CeOz

Pr 0oil Nd 0

Z 3

Sm 0V 1

anil

Y 0 . We have investig<"ited 10 more matrices.

Our XJtF methods are superior to those of Lytie and Head>

- 49 -

[381 in several respects. We have used the pressed pellet

technique for presenting the sample for analysis which gives more

reproducible results than obtained by packed powder technique used

by them. Due to double layer pellet technique the amount of

sample required has been reduced to as low as 400 ing compared to

several grammes used by them. The counting time is small in our

methods (100 s maximum and 40 s minimum) compared to 1 to 10

minutes used by Lytle and Heady. Apart from these, we have used a

vacuum path in the XRF spectrometer thus avoiding the costly He

gas in spectrometer.

Acknowledgements

The authors express their sincere thanks to Dr. V.B. Kartha,

Head, Spectroscopy Division for his interest in this work. The

thanks are due to Drs. N.A. Narasimham and S.L.N.G.

Krishnamachari, the former Heads of Spectroscopy Division, for

initiating and sustaining this programme. Our thanks are due to

Shri R.M. Agrawal for practical help and theoretical discussions

from time to time. Thanks are due to Mrs. A.N. Mohile and Mrs.

Annamraa Thomas, our previous colleagues, who participated in some

of these programmes. Finally, we thank Ms Sangeeta for laboriously

typing the report and to Shri S.S.Bhattacharya for drawing the

figures and help with the computer programming.

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- 52 -

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