SHORT COMMUNICATIONS 451

i G UKUAIN ANI> I-I LACOMBC. Cunrl>f Rwd . 133 (I 901) 874 2 H LAC0MBL. Cmpr Rcwl . I34 (1902) 772 3 C L PARIONS. J Amer. Chcnr Sot.. 26 (I 904) 72 1 4 M IVIIIJAU~I AND K MOKIJIMA, J C’hcw Sot J~~pafr. 72 (195 1 ) IO0 5 W BKAGC, ANI) G T MORGAN. PIOC Rev SOL. (Lo~rdotort). A 104 (1923) 437 6 J G VOGI L AND B G HOMKOCK, IS3rd Awwr Chm Sm. Me~trrry. MI~~II Bcc~clr. Fhrrrf~~, API II 1967

7 H D HAKDT. Z Army. AIlqetr~ Chetn , 3 14 ( 1962) 210. 8 R W M~IIIIZK AND R E SI~VI.KS, C~NJ C’lrro,llaroUrai)llI. of Merd Chclccte~ Pcrgamon Press, Oxford.

1965 9 A I GIU~XJH’LV. Rms J Jrroryj Ciwm . 8 (1963) 409 10 T MOEI,LIX. in L F AuI~~~TI~, Itwr~ctruc SJVII/I~WJS, Vol III. McGraw-Hill, New York. 1950. p 4 I I V AUCXH ANI) I RAISIN, Conr/>r Rod. I78 (I 924) 1546 12 GMIS IN. II~tidhmii rkr cl~iorl/Nitrtc/ierr C%rrtre, 32. Vcrlllg Chcmw, Wclnhclm. 1956. p 987 I3 A K Tb UI ANOV, A I GKI<,<x’I v ANII A V NOVOXLOVA. Kuss J frwrg C/ICW , 8 (1963) 127 14 A 1 GKIGOK’IX, A V. NUV~XEL~VA ANU K N SLMBNENKO, Z/I N~O~~JII WIIIII , 3 (1958) 1599 IS R D 1111 L. ANI) H GWSI.R. .I Gas Chormrtoyr . 1 (1963) I I

(Received 11 th June 1971)

Arwl Cilrly Acla, 57 (1971) 447-4s 1

Determination of phosphorus in silicate rocks by neutron activation and direct P-counting

Instrumental activation analyses for minor and trace elements in silicate rocks have almost exclusively been carried out by y-ray spectrometry. The only reported application of direct P-countmg to neutron-irradiated rocks seems to be for theYdeter- mmation of potassium’, with a!uminium absorbers to eliminate the contribution from 24Na. Another element which is often present m rclativcly high concentration m silicate rocks, and which might be determined similarly, is phosphorus. Previously published methods 2.3 for the determination of phosphorus in rocks by neutron activation have included decompositton of the sample and radiochemical separation steps before counting of the 32P lj--activity. The present work was done in order to study whether phosphorus can be determrned without the necessity of chemical separations. and, if so, to establish the interferences from other elements and proper counting conditions to reduce these interferences as far as possible.

Preliminary experiments The events recorded by a simple P-detector such as an end-window proportional

or Geiger-Mueller counter when exposed to an irradiated rock sample, represent a very complex mixture of nuchdes and radiations. Although p--particles are mainly responsible, conversion electrons, X-rays, y-rays and bremsstrahlung also contribute to the count-rate. Considering elemental abundances in common silicate rocks, and nuclear data of the radioisotopes induced by slow neutron bombardment, such as formation cross-section, half-life and characteristic radiation, it was estimated that isotopes of 26 elements might interfere with the determination of phosphorus by direct registration of the 32P P--activity, even after the decay of 24Na, 42K and other nuclides with similar or shorter half-lives The potential interfermg elements and their radioisotopes are listed m Table I.

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Anal. Chrm. Accu, 57 (1971) 451456

454 SHORT COMMUNICATIONS

If the activity of a nuclide like 32P, emitting high-energy P-rays, is to be shielded from interfering activities, a system of two absorbers may be advantageous. The thick- ness 1s so selected that one of the absorbers eliminates conversion electrons and low- energy P--particles, and the other is sufficiently thick to eliminate the P-rays from the high-energy /%emitter rn questlon, thus transmitting only y-rays and to some extent X-rays. The difference between the count-rates obtained with the two absorbers will mainly represent the contribution from high-energy p-emitters.

In order to study the feasibility of phosphorus determination by such a method, the following experiments were carried out.

Standard solutions of phosphorus and the 26 potential interfering elements were irradiated m sealed polyethylene tubes for 1 h in the JEEP II reactor (Kjeller, Norway) at a position with a thermal neutron flux of about 1.5 * 10’ 3 n cm’ 2 see- ’ and a cadmium ratio of gold of about 3. Aliquots (100 ~1) of the irradiated solution were adsorbed on circular plcces of filter paper placed in small circular alumimum cups (20 mm diameter) for P-actlvlty measurements In some cases, where the standard solution contained slgnlficant amounts of chloride or sulphate, yielding 32P by fast- neutron induced reactlons, or otherwise were expected to contain interfering radlo- active impurities, radiochcmical purification steps based on precipitation and filtering were employed. The samples were counted 10 d, 17 d and 25 d after the irradiation. A Geiger-Mueller counter with conventional shelf arrangement was used. The window thickness of the tube was 10 mg cmB2, and the effective diameter was 28 mm. The samples were counted with three different aluminium absorbers, namely 140 mgcmB2 (activity A,), 210 mg cm-’ (A,) and 650 mg cmm2 (A,), and the differences A1 --A3 and A2- A, were calculated and divided by the weight of element present. These specific activity values were divided by the corresponding value for phosphorus, and the “relative interference factors” thus obtained are listed in Table I.

Although several elements show serious interference when present in the same concentration range as pho$phorus, the possibility of obtaining useful data for phosphorus in rocks seemed to be good, since most of these elements are normally present in silicate rocks in concentrations far below that of phosphorus. It was decided to try this method on a series of U.S Geological Survey standard rocks with well known contents of phosphorus. From the results of Table I, counting after about 3 weeks seemed to be advantageous for this purpose, and the 210 mg cmm2 absorber was pre- ferred to the 140 mg cm -’ absorber because of less interference from several rare-earth nuchdes. An alummium absorber of 210 mg cm- 2 thickness stops all p--particles with energies less than 0.6 MeV.

Direct P-counting Rock samples and phosphorus standards (about 50 mg of potassium di-

hydrogenphosphate) were irradiated for 5 days at the same conditions as in the pre- lrminary experiments. After 20 days, 10”mg aliquots of the rock powder were weighed into the aluminium cups described before, spread evenly over an area of about 1 cm2 and covered with Scotch tape. The standards were dissolved in an appropriate volume of dilute nitric acid, and aliquots of 100 ~1 were applied to small circular pieces of filter paper placed in the aluminium cups, dried under an mfrared lamp and covered with Scotch tape. Counting was performed with the 210 mg cmW2 and 650 mg cmW2 absorbers. The differences in count-rate thus obtained were corrected for contribution

And Chm. Actu, 57 (1971) 451456

SHORT COMMUNICATIONS 455

from activities other than XP by applying the data avatlable for the US G.S. standard rock@’ and the empirical correction factors of Table I adjusted to 5 days’ trradtation. Values correspondmg to 21 days’ decay were estimated by interpolatton. It appeared that rubtdium and calctum, and antimony in the case of the ultrabasic rocks DTS-1 and PCC-I, were the only elements that caused interference exceeding loA,. The percentage of the recorded activity found to be due to 32P is given for each rock m Table II. The results for phosphorus corrected for interferences arc also gtven in Table II.

Radlochemrcul determtnutlon In order to assess the vahdtty of the dtrcct /&countmg procedure, accurately

known phosphorus data for the test samples were considered to be of vital importance. Previously a radiochemtcal neutron activatton method for phosphorus in rocks had been developed3 and data for the U.S.G.S. standard rocks had been obtained. Further experience indicated that the method used for chemtcal treatment of the standard did not bring about complete chemical exchange between acttve and carrier phosphorus, which would cause high results for the rock samples. Instead of hcatmg aliquots of the standard with hydrochloric acid/hydrogen peroxide, a new method of standard treatment was therefore introduced, mvolvmg fusion with sodium hydroxide pellets in the same manner as for the samples. Three series of standards with six parallels of each treated m this way showed rclatlve standard deviations of 2.1 ‘yO, 2.4”i0 and 2.2”A,, which was considerably better than the reproducibility observed with the previous standard treatment, thus mdicating more complete chemical exchange. The rocks AGV-1, BCR-1, G-2, GSP-1 and W-l were rc-analyzed using the same procedure as before, except for the standard treatment. Three parallels of each rock were run, and the results are given in Table II. The mean values arc 10(yO lower in average than the previous set of data, but are in good agreement with the neutron activation values of Greenland4, as well as the data obtained by conventtonal chemical analyses m the U.S G.S. laboratories’. Interference from the 32S( n,p)32 P reaction was neglected3.

Dlscussiott The values from the direct P-counting experiments are in good agreement with

the radiochemical values from thts work as well as the literature values (Table 11) The method is therefore considered to be well suited for use on a wide range of rocks with phosphorus contents exceeding 500 p.p.m Even for rocks contammg only 10 p p.m phosphorus, the method may yield the correct order of magnitude, as indicated by the results for DTS-1 and PCC-1.

Rubidium seems to be by far the most serious interference, and the accuracy of the results depends on a fairly good knowledge of the rubidium content. For calcium, an approximate knowledge of the content suffices m most cases. The rare-earth elements require special consideration. In the samples analyzed here, the total mter- ference from this group appears to be l”/, or less under the given condtttons, but for many rocks, espectally those which show a relative enrichment m the yttrium group, the interference would be appreciable. The nuchdes “Y and “‘Tm are the most important contributers m this group.

The direct P-counting method should be especially useful m connection with multi-element analyses by neutron activation and Ge(Li) y-spectrometry. In this case

Awl. Chm Actu, 57 (1971) 451456

456 SIIORT COMMUNICATIONS

Rb, Ca, Sb, as well as the rare earths, may bc determmed if present in concentrations high enough to Interfere with the o-countmg. Detcrminatlon of phosphorus by the present method should represent a useful addltlon to such a scheme.

It must be emphasized that the corrccilon factors given in Table I have been determined for a specific set ofexperimental conditions (energy distrlbutlon of neutron flux, irradiation time, decay time, performance of counting apparatus, absorber thlckncss, etc.) and cannot be appllcd dlrcctly to other experiments The number of clcmcnts for which a corrcctlon factor needs to bc determined, however, is small, as IS evldcnt from the preceding discusston.

Fmally, It should bc mcntloncd that the relatlvc rubidium mterferencc will be reduced if the activation is carried out in a more thcrmalized neutron flux, since “‘Rb has a hrgh resonance activation integral compared to its thermal neutron activation cross-sectlon, whtlc 31 l? follows the l/u law closely In the energy range to be con- sidercds. When changing from a position with R$i= 3.0 to another with R$s= 10, for example, the rclatlve rubldlum lnterfcrence will bc reduced by about 25”/0 Interference from antimony and thulium wtll also bc apprcclably reduced.

Instithtt fbr Atomenergi, Isotope Lahorotories, Kjeller (Norway)

E. Stclnncs

1 J. %' W1NCIII:SIT.I~. /Irtn/. Chnr , 33 (1961) 1007 2 P. I-ICNIXXSON, Ad Clrrm. Acta, 39 (1967) 512. 3 A 0 BIUJNI-1.1.1 AND E. SII INNLS, Arid Chrrti AC/N, 41 (1968) 154 4 L 1’ GKI:I.NI.ANI~. U S Gcol. Srr v PI oj I~upcr , 575-C (1967) I37 5 F. J FLANAGAN. Gcocltrrt~. Cmrr~ocl~rtn Acfcr, 31 (1967) 289 6 M. FIXISCH~I<. Gcocl,ml Ctwrrtoclttr,t. Acra, 33 (1969) 65 7 F. J FLANAGAN, Geoclrmr Co~~rrocl~rrn Acrrr, 33 (1969) 81. 8 E SICINNIS. In A 0. BRUNI 11~1 AND E SILINNLS, ~~crruc~tmi /~ticr/_vsls 111 Geocho?trclr_~~ atrdCostrloclre1~1.

~strv, Unlvcrsltctsforlagcl, Oslo, 197 I. p I 13.

(Received 5th July 1971)

Atrrrl Chrr,1 ncrcr, 57 (1971) 45 I-456 I.