1990 - International Nuclear Information System
Transcript of 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
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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
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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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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
Counting time TMDLin seconds ppm.
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
Net peakcounts for0.01% standard
1816
688
2263
1158
1940
2106
Counting timein seconds
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
Counting time TMDLin seconds ppm.
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
Net peakcounts for0.01% standard
3060
1728
2067
2509
535
681
Counting timein seconds
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
Net peakcounts for0.01% standard
1365
778
746
792
1270
950
Counting timein seconds
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
Net peakcounts for0.01% standard
733
1237
1202
670
696
107
Counting timein seconds
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
Net peakcounts for0.01% standard
1731
1996
2179
3073
1894
1163*
Counting timein seconds
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
Net peakcounts for0.05% standard
17719
20810
14053
14891
10891
5540
12651
Counting timein seconds
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
Net peakcounts for0.05% standard
9550
16507
17940
7349
14324
29125
Counting timein seconds
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
Net peakcounts for0.05% standard
8741
12700
6288
7138
10515
Counting timein seconds
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
Net peakcounts for0.1% standard
25100
12700
35200
21200
20500
26900
4900
Counting timein seconds
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 -
Published by : M. R. Balakrishnan Head, Library & Information Services DivisionBhabha Atomic Research Centre Bombay 400 oe&