The spectrographic determination of impurities in small amounts of radioactive graphite by the cathode layer technique

ANALY’I-ICA CHIAMICA AC’I-A

548

THE

SPECTROGRAPHIC IN SMALL AMOUNTS BY

THE

DETERMINATION OF RADIOACTIVE

CATHODE

hl. S. \Y. W151313,J. /I lornic I:‘nc~gy

LAYER

TECHNIQUE

C. <‘.O’l”l-EI~II,I,

Itcscccvclr tlstublislr~mrll,

Woolwich

OF IMPURITIES GRAPHITE

AND

‘1’. W.

JONES

Or~fsl~itiort, Lorrclr~n (C;recit i3viloin)

(licccivctl OctoLcr I 7tl1,

~gC,r)

IN1’1ZOI3UC’rION In recent years thcrc has been considerable interest in the correlation of the chemical propertics of graphite with impurity content and the need arose in connection with this study for the determination of impurity clcments in small samples (about 50 mg) This report outlines the tccliniclue used for analysing such of raclioactivc graphite. samples.

Before this work was envisaged consiclcrablc espcriencc had alrcacly been gained in the analysis of grapliitc by the method of itshing the sample and applying conventional spectrographic methods i--3 to the analysis of the ash. This method was not considered suitable owing to the small sample size and the sensitivity required. A methocl was therefore sought which could bc appliecl directly to such samples without ashing. The catlioclc lnycr nicthod in which the enhanced emission of certain elements in the cathode layer of a d.c. arc is used, was clevclopcd by MANNICOWP’~for the analysis of a wide range of samples. MIWHIZLLG found that this method gave high sensitivity and good precision, and required only a few mg of sample. Furthermore since carbon is mised with sample to promote good burning characteristics in the arc this method secmccl particularly appropriate for the analysis cnvisagccl. ESI’EIZIMENTAI. Most worlccrs have followed MANNKOPFF AND PE’rEns4 in using a narrow cathode electrode macle of carbon with a clecp boring approximately I mm x 12 mm deep holding 5 to x0 mg of the sample admixed with spectrographic buffer and internal standard. mistures. Initially this procedure was adopted because it had been stated that carbon electrodes burnt more rapiclly and reproducibly than graphite. Most graclcs of carbon, however, wcrc found to contain significant quantities of calcium, magnesium ancl boron which contributed the equivalent of IO to 50 p.p.m. of these elements to the sample being analyscd. In spite of the greater thermal conductivity of graphite and, consequently, a potentially slower burning rate, it was necessary to make electrodes from spectroscopically pure graphite to overcome these blank difficulties. In point of fact it was Amd.

Chirta.

Acfa,

26

(1902)

548~55G. ‘I

IhIPURITIES

IN

SMALL

ANOUNTS

OF RADIOACTIVE

GRAPHITE

549

found that the burning time was only slightly increased, and no appreciable instability of the arc due to the greater CO(Jling of the cathode resulted. A variety of spectrographic buffers have been used by different workers in order to minimise the effect of variations in the composition of the samples, including carbon itself”+. In view of the fact that samples would consist of carbon in the form of graphite, it was considered that no additional buffer would bc required and in actual practice this assumption has been justified. Since the determinations of copper and cobalt were not required, the possibility of using a mixture of the osicles of these metals as internal standard was investigated by the “falling plate” technique, when it was found that the emission of the lines of these elements were homologous with the lines of most of the elements to be determined. Figures ra and b illustrate the curves obtained for two clcmcnts typical of the two main classes of elements. Further esperiments sl~owccl that the following misture gave satisfactory line densities : IO”/, copper osiclc, zQ% cobalt osidc and 874% sample (b)

(a) -Fe29732 -----co 2909s

,--..

I

1

50

100

lime

it-~?&

\

I

I

200

I

50 Time

Fig.

f b. Emission

I

100

ciwvcs

lx)

200

In see

for silicon

nnd copper.

Initial work was carried out using conventional cathode layer electrodes x:2 mm deep by I mm diameter and a wall thickness of about I: mm, but these required an extended exposure time to burn to completion. A series of esperiments was therefore carried out, varying the parameters of crater size, arc current and esposure time. and the optimum precision and sensitivity were obtained when an electrocle x mm internal diameter and 7 mm deep was used at a current of IO A with an csposure time of 3 min. The choice of photographic plate was govcrncd by the necessity or otherwise of were determining sodium. In actual practice it was found that blank difficulties experienced due to traces of sodium introduced with the internal standard. In these circumstances it was founcl more expedient to determine sodium by flame photometry on another portion of the sample and to USCan Ilford Ordinary plate in conjunction with a spectrograph of medium dispersion for the present investigation. The plate was evaluated by microphototnetry using the “blackening-separation” method of MITCHELL AND SCOTT~ and this necessitated exposing each sample through a seven-step rotating sector. In practice it was found convenient to measure Seidel Aural.

Cirim.

Act&

26

(x962)

5&3-&G

M. S. W.

550

WERB,

J. c.

COT-rEIIILL,

T. w.

JONES

densities either by the USCof a Scidel scale in the galvanometer mula Scidcl

density

(S)

=

log

& -

\

.G

--

or by use of the for-

\

1

1

whcrc GO = clear glass deflection and G = deflection on line measured. The sclcction of the cathode layer portion of the arc column was effected by means of a mask placed in front of the collimating lens of the spectrograph and alignment of the arc, which is critical, was maintained by observation of an enlarged image projected on to a screen placed at the further end of the instrument bar. The dimensions of the mask wcrc chosen to include z mm of the arc column immediately adjacent to the cathode. The form in which the impurities were present in the graphite was not known although it was thought likely that they would bc present in the form of oxides and carbides. HISGIZMANN AND rmSSlhNNU liave shown in the analysis Of graphite that there was no difference in the result produced by stanclarcls made from cithcr the oxides or the carbides of the impurity elements, presumably because the large excess of graphite prcscnt rcclucccl the aside to carbide in the first few WC of burning in the arc. Accordingly, synthetic standards wcrc made by succcssivc dilution of a misturc of all the osidcs of the metals and results sl~owecl this proccdurc to 1~ justified.

Imm

bore -----

hlE’THO1)

Note Some form of containment is necessary when handling radioactive samples. This should include at least a glove bos with a suitable filter at the cstraction outlet. An additional means of restricting the spread of radioactivity within the box is an inverted funnel suspended above the arc and connected through a filter to the eshaust. Sfiectrogvaphic

conditions

(a) FOY viwal

evaluation.

Spectrograph

: Hi&r

medium

quartz ; esternal

optics :

IXIPURITIES

IN SMALL

AXIOU!‘iTS

OF

RADIOACTIVE

GRAPHITE

551

arc 53 cm from slit, lens Fg57, 43 cm from slit, rotating step sector with lens Fro84 z cm from slit ; collimator mask : this should be of c&d or sheet metal with an aperture of 8 mm by 45 mm, painted matt black and placecl symmetrically across the front of the collimator mounting; slit length: x.S mm ; slit width: 0.010 mm; photographic plate : Ilford ordinary ; top electrode (+ve) : x/4 in. diameter N.C.C., freshly broken off, not shaped; bottom electrode (- ve): x/4 in. diameter N.C.C., specially machined (see Fig. 2) ; analytical gap: 10 mm ; sample charge: electroclcs filled to capacity using a small perspes funnel (see Fig. 3) and a rigid wire to ensure firm, even packing; current: 10 A t1.c. ; csposure: 3 min.

__,*___ --t4 T’

mm

lmm

bopc_____

-

8mm

____-

--

7’- f

8mm 1Omm _L__L

12

((,) ITor ,~)zicro~ltolol)rdvy. Conditions as above esccpt ttic mm; position of 7-step sector: :lt slit, sector ratio 2 : 1.

following:

slit lcn#h:

ant1 fis in acid hype until the l~cvclol~ in 1.11.2 for 4 tnin at 2o”. Rinse with wltcr plate is complctcly clear. M’asll for 30 min in an efficient washing tank and allow to tlry . Stundurds Prcpar~* by dry grinding: in an agate mortar a misturc of osicles of all elements rcquirctl to bc cletcrminetl and adjust the conccntrntion to x.oo/o by wei&t of each clement by the iKlditi011 of ‘Specpurc’ ammonium sulpliatc. Ry successive aclclitions of N.C.C. fqapliitc po~vclcr, prepare dilutions of this 1% niisturc containing x000, 500, 200, xoo, 50, 20, IO, 5, 2 and I p.p.m. of impurity elements in the ~raphitc. For use as an internal standard, prcparc by thy qincling a misture containing 400 nig of copper osiclc (CuO) ant1 100 rtig cobalt ositlc (ConO4).

Grind for IO min in an agate mortar 7 mg of the internal standard misture and 49 mg of sample. By means of a perspcs funnel (see Fig. 3) fill three clcctrodes with this misturc. Add a small portion of the misturc at a time repeatedly tamping with a rigid tungsten wire to ensure firm ant1 even filling of the clcctrodcs. ‘The presence of occludccl air in the cup will cause uneven volatilization of the impurities. Spectrogmphic

jhrocedurc

If required, first switch on the rotating step sector and allow it to attain its masimum r.p,m. Place a loadecl cathode and counter electrode in the arc stand and strike lTII

Anal.

Clritn. Acla, 26 (x962) 548-550

BI. S. W.

552

WEBB,

J. C. COTTERILL,

I-.

W.

JONES

the arc at 3 A by lowering the upper electrode. Rapidly (within 5 set) open the gap to IO mm and increase the current to IO A. Maintain the gap at IO mm by continuous adjustment of cathode and anode and allow to burn for 3 min. If any spluttering occurs during the but-n resulting in material being blown out of the electrode the exposure shoulcl be rejected. Kcpcat the above procedure to give duplicate exposures. Interfiretation

of spcctrrr

(a) Viwully. Using a suitable comparator and standard plates for lint identification read the spectra by comparing the densities of impurity and internal standard lines with those on the standard plate. The standard plates are prepared by mixing the standard dilutions above with the internal standard mixture and esposing an appro-

1\\1 Jj lb

I3c Iji Ch Cr

PC

3CJG I ._j 2497.7 493.). 3130.4 3131.

.ro-go0

S-500

I o-5OCJ

I

‘-SO

3067.7

l-50 20-- 500

,y)f,8.5

‘o-lo<>

‘fL10.7

10-1000

I

IO-500 IO--IO0

r843.3 425.1.3 2973.2

MO

Ni

co

3000.5 3 177.3

2801.1

co CO CO CO CO co

2803.X 2803.8 25H7.;! 25H7.r 2587.z 2803.8

313r.o

cu

3146.8

3’70.3

CO c:u co cu co co

3’77-3 2C)()7..) 3000.5 2gg7..t 3000.5 3307.1

co c.u cu co CU

2837.2 2882.() 2YY2.9 2837.r 2882.9

CO co c:o

1957.7 3307.1 3177.3

3002.5 3003. I 3393.0

Pb

2833.1

Si Sn

2881.6 28.40.0

Ti \’

2056.1 3361.2 3155.4

,,r.!b.70

0.5

0.G 0.3

IO-go0 c:O

i%Ig 2795.5 -80.2 -7 MI\ 2576.1 2593.7 ‘CJO5.7

(‘I’

l When calcium is dctcrmincd standard mixture.

s- 100 2- 100 5-200 5-200 5 -300 g-r00 so--500 50.-50” 5--200

I O-500

\Vl!ak ‘I’n 31 3L.b ‘I’a 3 170.3

0.3

Co liirc is bcwt for illicrol’hotoillctrv

0.3

20-500 LO-500 IO-500 :o-500

<‘II lint is best for micropliotomctry

0.5 0.5

Cu line is best. for nricropllotomctr~

0.5

50-500 20-so0

20-,500

0.3

_.it is csscntinl that chromiuni

bc nbscnt from the clcmcnts

And.

Chir~t. Ada,

26

(1962)

in the

548-556

IblPURITIES

IX SMALL

AMOUNTS

OF RADIOACTIVE

GRAPHITE

553

priate series of these by the method outlined for samples. The lines listed in Table I are pairs which have been found satisfactory. Other elements ma_v be determined if they are included in the standard mixture but suitable internal standard lines would need to be sought. (6) By nticro~lzolomel~y. (I) Using a non-recording microphotometer measure the Seidel densities of three appropriate steps of element and internal standard on the sectored pattern for samples and standards. The steps selected should bracket the recommended internal standard densit?; given in the last column of Table I. (2) Plot these densities against the log relative intensities as given by the steps of the rotating sector (see Fig. 4). (3) Measure the difference in mm on the log relative intensity scale at the chosen Seidel dcnsit_v lxtwecn the element and internal standard lines (set Fig. 4). (3) Plot these differences against the log concentration of the element for each standard (see Fig. 5). (5) Read off the log concentration of the clcmcnt present in the sample appropriate to the difference in “blnckcning separation” as given in mm in (3). iAl 3961.5 - 1.0

10-

45,.

*2

o-

! fj -1.5-

--l.5

-1 6

5

4

Steps on sector I:iK. .I. Ijkhckcning

1.0

sqxlration

1.5

I 2

.

.

_1L) 1

(3Omm Per steD)

curve’s

2.0

Log conc.of impurities(as

Fig. 5. Working

1 . 3

curves for nickel,

for Al/Co and Ni/C’o.

2.5 pp.m in graphite) iron

and

silicon.

Anal. Chim. Ada,

26

(xgG2)

:

_”

548-556

XI. S. W.

554

WISI313,

J. C.

CO’TTEHIJ.I,,

‘I’. \V. JONES

SENSITIVITY

A

series

&tection

of standards was exposed on a. number of plates wwc cletcrmincd. Thcsc arc shown in Table II.

__-.--

____ ___-.

---..

ant1 the visual

limits

of

_.-.__-_

ivily ia f~./~.vr. in grupkilc ..--.

Scmsif ISlC?m!#If ---

WClt~ClCIt~fh ._-----.---_------.

IrP

Alutniniuni

3w1 .s

Jjoron

2437.7

ljnrium Jjcryllium J3ismuth

4934.1 3 13”..# 3oG7.7

I 0

C;rlciurn

39fiX..5 “843.3 rrJ73:2 ;?Hor.7

.200

5 ‘0

Im1cl

2.503.7 3132.f’ 3oo~*s 2x33. I

Silicon

.2xX I .fJ

1
‘I’iII

2x.p.o

I0

‘I’itxiri\ini

33’11.2

10

\‘~llliltlilllll

3

Chroniiurii Iron

R1;tgwsiu111 Mtlll&LWX

hlolyldcnum Nickel

__-__. -____.. -_ n Scnsitivit>* lilnitctl

.5 I 10

1 C) 10

.!a

10 10

I X.j.‘1 IO .._-__-.__-- .___-.._.- ._-_ I)y

bl;rmk

‘llc precision of the tnctliocl was tlctcrminetl 11~7replic~~tion of a Ioo-p.lxm. standard on ;L number of plates and clctcrmining tlic coefficient of V:Uktioll of tllc results 0lHainccl by non-rccorcling micropl~otomctcr. l’hc VillUCS obtninccl arc shown in ‘I’xhlc I II.

l’hc

coefficients of vnrixtion for tin and lcncl Woulcl prOl>il\>l~ lx iniprowcl by the clement of similar volatility as intcr~~i~l standard. In orclcr to check tlic accuracy of tlie method for the clctcrminntion of calcium,

USC

ol

an

.‘I ~rtrl.Cltittr.

.4c/n,

a6

(1962)

548-556

I;\IPURITIES

IN S?tIALL

AMOUNTS

OF RADIOACTIVE

GRAPHITE

555

aluminium, vanadium and nickel, samples were esamined by the cathode layer and iron flus methods’ using the non-recording microphotometer in both cases and the comparative results are given in Table IV.

CO~ll’,\HISO.S

OF

RESULTS

OIIT,\ISI:.D

(I
I:‘lr?no:f --.-___-----.---.

Celcium r\luminium Vanadium Nickel ____._

IROS i1S

FLUS

ASD in

p.p.111.

C,\TIIODl~

LAYER

_-

srcmp1e B ..__.. .__ ___ ___.._ __.1.1:. C.L. /.I*‘. ~_.___.__

Src mjdr

.*I

___..___ C.L.

190 30

I.50 30

“00

I ., 0

‘20

sav,pl~ D su nlple c snmp1c E -.-__.. . ..- - .._.--.---_-_._ -_.-.__ C.L. I.F. C.L. 1.i:. C.L. I.F. --.

300 3”

380

50 6

2 10

.!dO

10

II


(IO 20 - .-.-__... --

__........_...

.I 10

350 50

NIJTIIODS

graphite)

______--_-__--_-

---_-__-_-_.-_

-_._-

UY

cxprcssctl

I

-_.....-

;?.)O

240

30

20

30

220

I go

30

__ _.

-..-...

..-

.._..._

2.5

SO0

I cm

50 ‘70

25

-

240

450

.+o

20

~.-_.---__-.---

samp1c 1: XL.

50
I.F. 280

50 H I

The accuracy of the method for the determination of silicon was assessed by comparison of the results obtained by the present method, the iron flus method and by chemical analysis using the silicomolybdate m&hoc!. The results arc given in Table V.

__.____.-_- .._-. _

_ __-.. ..-.__.-- ___-.._-..-. -.-.-.--

p.p.m. nJ silicutr in .wrphilc _-.._._.__-.~----

______

__.. __._.__ -_-.--_---

I\

100

c:

10

CJO

100

Ho

100

90

100

90

1)

.___ -.-

0.5

7”

1%

15

110

I .#”

I’

220

250

_______..-.___--__--_.-

-.-_-

90 <_I0

2.50 ----

.- .---

The accuracy for the determination of iron was assessed by comparison of results with those obtained by X-ray fluorescence analysis and chemical analysis (o-phenanthrolinc). The results arc given in Table VI.

___---

--

_- ___.-..-_---. p.p.m.

.sa,ap1e CaJhodc Joyrr ___---__-__~___

&\ 13 C 1) -

15 1:

X-ruy

irm

in gruphilc

/luorrarorcc

_-- -.-

Chemicnl (o-phrnatlthroliw)

. -_.__

00 6.5 40 60

5” 8.5 5.5 50

cm 8.5 50

I->0

I JO

50 70

150

‘30

150

_-__-..-_-_.-

---_.

ACKNOWI.EI)CEMEKTS

Grateful -.

acknowledgement

is made to the following:

Mr. J. AnaZ.

WALKDEN

Glrinr.Acfa,

of Chatham 26 (1962)

548-556

556

AI. S.

W. WEBIi,

J. C. COI’I’EI
‘I-. W. JONES

Outstation (A.E.R.E.) for arranging the chemical analyses; to Mr. H. ~.SHAI_COSKY, Woolwich Outstation for X-ray fluorcsccnce analyses and to Miss E. C. BUCKLOCK :UK~ Mr. M. L. WORDISGHAM who analysecl many of the samples connected with the work of this progrnmmc.

of the c~thdc layer tcclinicluc is ?iuitablc for the analysis of ratiioactivc yrnphitc using ccqqxr and cobalt us internal standards. ‘Cilc Si~lll~IlC! is ground with tllc internal StandiLrd mixture and introtiucccl into a specially clcsi(qxxl clcctrotlc whicl1 is burned in iL d.c. arc. The c;rthotlc IiLyCr portion of the wc plusrna is cx~~mincti using a mccliuin quartz spcctropaph with by compuison with stanti;rrtl spectra, plIC~t0gIXl~hiC recording. ‘The spectra arc L!V~~llli~t~ti visually, I\ iriotlific~rtion

or

by

non-rccortiing

saniplc elf gri~pllitc, I%i, I%, Si, Sn, ‘l’i ill: the

IOO-p.p.m.

using

microphotornctry

the

“blackening

separation”

mcthotl.

the cffcctivc conccntrirtion lXngc for AI, IS, IJil, 13c, I3i, (.1X, Cr. illlCl ttic cocfficiciit of wriation for ilntl 1' is from IO to 500 p.p.ni. lcvcl varies from z tm O’x, according to the clcmcnt tlctcrIninctl.

Using

a 5-mg

lie. %lg, IMn, MO. single cxposurcs

ixs nutcurs ant cffcctud unc dt11cic sur lc tlo~i1~u slxxtro~rnphiquc tl’iml~11rdds clans 1111Krnl)ilitc raclioactif, cn utilisant unc c!lcctrotlc spbcialc ct coii~mo &tilloIl intcrnc, Ic cuivrc ct Ic cobnlt. On ;L pu

:diisi

dbtcrinincr

tics

tcncurs

dc

io

Zr 500

p.l>.m.

tic:

:\I,

13, Isir,

I%c, I%, (::I,

(‘r,

i:c.

h1.g. >ln,

MO,

Ni, I%, Si. Sn, ‘I’i ct V. ZIJSAMMEK l3csclIrcilIiing

aktivon

cincr

Graphit

s~~cktro~rapiiiscllcrl

&~cthoth!

i:ASSIJN(; %ur

l~cStiIllnlllng

voll

\‘~~ll~~~!i~~~ll~l~:cll

in

riltlio-

untcr Vcrwcntlung cincr Spcxialclcktrotlc rnit Kupfcr untl Kc1lxrlt als intcrnc Sti~lltl~llYlSllI~Stilll%L’l1. Spurcn folgcntlcr 15lcmcntc itijnncn 1iachKcwiscn wcrtlcn: :\I, 13. IGl, IJc, IJi, Ca, (‘r, i;c, ivlg. AIn, No, Ni, L’b, Si, Sn, ‘I’i uncl V.