Routine Determinations of Traces of Cobalt in Soil ...

Routine Determinationsof Traces of Cobalt in Soil and Plant Tissue by Instrumental Neutron Activation Analysis O. JOHANSEN and E. STEINNES Institutt for Atomenergi, Isotope Laboratories, Kjeller, Norway

l n tro d u c ti o n In field studies related to cobalt deficiency in domesticanimals,the determinationof low concentrationsof this elementin vegetabletissueas well as in soilsmay be desirable.Neutron activation analysis,exhibiting a very high sensitivity for cobalt determination, should be a suitable technique for this purpose. Neverthelessthe applicationsof activationanalysisin this field have been few. Cobalt determinationsin soils by neutron activation analysishave been reported by Yamada (1964), using a radiochemicalseparationmethod for the isolation of the induced 80Coactivity. More recently Kline and co-workers(1965,1969) havereportedthe determinationof Co in soilsby purely instrumentalactivation analysisusing a 5 in. x 4 in. NaI(Tl) detector. The experience reported by theseauthors did not appearespecially favourablefor the useof neutronactivation analysisfor Co determinationin soil samples. Applications on vegetable tissue have been reported by Fourcy (1966),using a radiochemical neutron activation method for the simultaneous determination of Co, Fe and Zn in fodder, and by Nadkarni & Ehmann (1969), who demonstrated the determination of a number of elements, including Co, in a biological standard using Ge(Li) y-spectrometryfollowing the activation. Previous experienceon the determination of cobalt in rocks (Brunfelt & Steinnes,1966),employing a well-type NaI(Tl) detector for the registration of the 2.50 MeV sum coincidence peak of 60Co,indicated that this techniquemight be applicableto soils as well. Moreover,although the determination of Co in vegetabletissue by instrumental activation analysisusing a NaI(Tl) detector has been considered to be "difficult

using 60Co" (Bowen, 1967a), the technique describedin this work has beenappliedto cobalt determinationin plant ashesas well as directly on dry plant material and found to yield satisfactory results. E xperi mental Soil samples were prepared for irradiation by wrapping about 200 mg in a piece of aluminium foil, without further grindingof the samples.Airdried samples of homogenized plant material were prepared by weighing an amount of about 500 mg and wrapping in Al foil. Plant ashes, produced from vegetationsamplesby dry ashing at 550"C for 4 hours, were dried for I hour at 105"C before weighing 50-100 mg into the aluminium foil packing. As cobalt standardwas useda solution of Co (20 sglml) in 0.1 M nitric acid. About 500 pl of this solution was sealedin a quartz ampoulefor irradiation. Samplesand standardswere irradiated in the JEEP I reactor (Kjeller, Norway) at a thermal neutronflux of about 2 x 10il n cm-2s-r. For the soil samples, 20 hours' irradiation was found appropriate; for the plant samples and ashes, one week's irradiation period was used. The sampleswere stored for 4 weeks before starting the radioactivity measurements,for short-lived activities to decay. Before counting, the samples were transferredto glasscounting vials. Quantitative transfer was used for plant samplesand ashes; for the soils, an appropriate amount (50-200 mg), dependingon the activity level, was weighed into the counting vials. For cobalt standard, 100 pl of the solution was withdrawn from the quartz ampo-ulewith a microiripette. The quantitative analysis was basedon meas. urements of 7-radiation from 60Co (halfJife Acta Agriculture Scandinavica 22 (1972)

104 O. Johansenand E. Steinnes rates. Possibleinterferencefrom pile-up effects $'ereestimatedby counting a weak 00Cosource separately and together with s'9P sources of graduallyincreasing activity,at thesamecorrnting geometryas used for the samples.For the soil samples,the activity was always kept below a Ievelcorrespondingto 20o/odeadtime, and pile-up effectswereestimatedto be negligible.If operating at higher count-rates,possiblepile-upinterferencernight be studiedin a correspondingway as for the ash samplesby meansof mixturesof {6Sc and 60Co.Appreciable gain-shift was not observedin the activityrangeusedwith the equipment employedin this work. For the ash samples,possibleloss of cobalt during the ashingprocessmight be a sourceof error. In order to investigatethis point, four samplesof timothy grasswhich had been anal1,zedby the direct method were ashedand reanalyzed.The resultsof this experimentare given in Table l, u'herethe resultsfor ashedsamples havebeencorrectedfor loss of weight on ignition. Loss of cobalt evidentlydoesnot occur to a great extentduring the ashingstep.

g

s

3 g o 6

0.5

1,0

1,5 2.0 En e r g{yM e Vi+

2.5

Frg. .1. Gamma-spectra of irradiated samples, recorded 4 weeks after the irradiation. (A) Plant ash sample. (B) Soil sample.

R esul ts and D i scussi on Resultsfor cobalt obtained by duplicate analyses of a numberof soil samplesas well as a seriesof plant ashesare given in Table 2. The valuesfor the latter refer to the ash weight; the cobalt content of the originaldry tissuewould in most cases be a factor of 15-20lower.

5.27years),usingr/-spectrometry basedon a welltype 3 in. x 3 in. NaI(Tl) scintillation detector connected to an "Intertechnique" 4o0-channel pulse-heightanalyzer. The counting time used Table 1. Cobalt content of timothy grass before was 10 min for soils,30 min for ashesand 60 min and after the dry-ashingprocess for vegetationsamplesanalyzeddirectly.PhotoDirect analysis Results for ashed peak areas were calculated according to the samples, corrected of air-dried method of Covell (1959). for ignition loss Sample samples Typical y-energyspectraof a soil and a plant no. (ppm Co) (ppm Co) ash sample respectivelyare shown in Fig. l. In the soil sample,{oScwas the major componentin 0.046 most spectra;in the plant and ash samples,more 0.040 than 90% of the counts arouse from bremso.o42 strahlung due to the high energyp-particlesfrom 0.037 32P.In the caseof ash samplesthe count rate was 0.037 0.039 in severalcaseshigh enoughto causea deadtime of 50-60% in the analyzer. In order to avoid a 0.017 0.023 0.019 correspondingprolonging of the counting time, 0.016 pulseecorrespondingto /-energiesbelow 1.5MeV 0.040 0.033 were discriminatedbefore enteringthe analogue0.039 to-digital converter. This, however, does not 0.043 eliminateerrors due to pulsepile-up at high count Ac t a Agricul t ure Scandinavica 22 (1972)

Determinationof cobalt in soil and plant tissue 105 Table 2. Results of duplicate determinationsof cobalt in soils and plant ashes by instrumental neutron act ioat ion analy sis Soil samples No.

ppm Co

t 2 3 4

3.17 4. 18 1. 74 ll.2 2.08 2.8r 5.86 7.91 6. 14 16. 6 4. r 4 3.98 0.91 ll. 3

6 8 9 t0 lt l2 l3 l4

Plant ashes No.

3.03 4. r 2 1. 81 10.2 1.81 2.67 6.40 8.70 5. 75 15. 6 5. 13 4.23 1.00 il. I

I 2 J

4 5 6 7 8 9 IO 1l l2 l3 l4

ppm Co

No.

ppm Co

l5

0.294 0.172 0.912 0.494 0.405 0.277 0.689 0.462

0.307 0.174 0.906 0.517 0.357 0.297 0.671 0.450

r.69 0.328 1.24 0.143 0.659

I .68 0.290 t.22 0.172 0.70e

o.27t 0.153 2.04 0.452 0.813 0.679 0.655 0.368 1.62

0.215 0.151 l.8r 0.377 0.829 0.656 0.699 0.400 l;tl

t6 t7 l8 l9 20 2l 22 23

) 'tl

) 'r 9

.A

0.465 t.58 0.214 t.04

0.455 I .40 0.215 0.97

25 26 27 28

r.sz

t.64

Relative standard deviation: 7.2% for soils, 6.0% for plant ashes. a

A relative standard deviation of 6% is observedfor the cobaltdeterminationin plant ashes, which is consideredto be quite satisfactoryat this concentrationlevel.The spreadobservedfor the soil samples,holvever, is somewhatlarger than that observedwhen applying this technique for rock samples with similar concentrations,in which casea precisionoI + 4Yohas beentypically observed in the authors' laboratory. The less satisfactory precision observed for the soil samples may be due to inhomogeneitiesin the samplesanalyzedin this work. The accuracyof the presentmethod for plant materialwas investigated by anafyzing"Standard Kale" (Bowen, 1967)which has'previouslybeen analyzedby other researchgroups using spectrophotometryaswell as neutronactivationmethods involving radiochemicalseparations. The results are given in Table 3, and show reasonableagreement with previousdata. The method described in this paper involves little work per sample,and eventhough the time interval betweenirradiation and measurementis considerable, the methodshouldstill be attractive for cobalt determinationin large seriesof soil samples,providedthat the immediateacquisition of data is not necessary.The method also appears promising for analysis of plant material. Vegetable tissue may be analyzeddirectly, but dryashedsamplesare more convenientto irradiate as well as easierto handle after the irradiation. The latter thus seemsto be preferable,provided that no cobalt contamination takes place during

the ashingstep.For the plant tissueand the ash samples,more preciseresultsmay be obtainedby increasingthe irradiation time and postponing the activitymeasurements until 2-3 monthsafter the end of the irradiation,at which time the 3tP activity will have decreasedto a less disturbing level. Direct determinationof cobalt by neutron activationanalysiscan also be performedusinga Ge(Li) detector system for the 7-spectrometric measurements, as demonstrated recently by Nadkarni & Ehmann (1969)for vegetabletissue.

Table 3. Determination of cobalt in "standard kale" Literature data Presentwork (ppm)"

0.046 0.045 0.059 0.059 0.052 Mean0.052*0.07

ppm

Technique employed

0.052;0.052; 0.060;0.081

Neutron activationD

0.05;0.065; 0.005;0.058;0.06

Spectrophotometr y o

0.054

Neutron activationc

o Samples were corrected for drying loss as describedby B orvcn(19670). D values cited in Bowen, 1969. c Nadkarni & Ehmann (1969), instrumental activation analysis using a Ge(Li) detector. Acta Agriculturte Scandinavica 22 (1972)

106 O. Johansenand E. Steinnes As the counting efficiencyfor the Gc,(LDsystems - 1967b, Comparativeelementalanalysisof a standard plant matcrial. Analyst 92, 124-131. availableto-day is low comparedto the one used 1969. Standard materials and intcrcomparisons.In in the present work, and the price is far higher, J. M. A. Lcnihan & S. J. Thompson(cds,),Adoances in the use of the well-type NaI(Tl) detectorfor this actioatlon analysis, vol. I, pp. l0l-113. Acadcmic Prcss,London. purpose s€emsto be a reasonablechoice.

ncutronBrunfelt,A. O. & Steinnes, E. 1966.Instrumental activation analysis ol "standard rocks". Geochim. Cosmochim.Acta 30, 921-928. Summary Covcll, D. F. 1959.Detcrmioation of y-ray abundancc directlyfrom thc total absorptionpak. Anal. Chem.3I, 1785-1790. The determinationof cobalt in soilsand vegetable tissue using instrumental neutron activation Fourcy, A. 1956.Quelquesapplicationsde I'analysepar radioactivationneutroniquc cn biologic vegetabloct analysis with a well-type NaI(Tl) detector has cn agtronomie.Report CEA-R 2967.C*ntre d'Etudcs been studied. The amount of work per sample is Nuclcaircsde Grcnoblc. low, and tho sensitivity is adequate for direct Kline, J. R., Brar, S, S., Gustafson,P. F. & Rust, R. H. 1965.Use of ncutron activation analysisto detcrminc determination in plant samples.The precisionof biological availability of coppcr in soils and for nonthemethod in routine application is of the order destructivcanalysisof soils. Proc. Int. Conf. Modern of 5%. Trendsin Actioation Analysis, pp. 319-323. Collego Station, Tcxas. Kline, J. R. & Brar, S. S. 1969.Instrumentalanalysisof Acknowledgement ncutrotrirradiatcdsoils.Soil Scl.Amer. Proc.33,234238. The authors wish to thank Dr G. Semb,Depart- Nadkarni, R. A. & Ehmann,W. D. 1969.Determination ment of Soil Science,Agricultural College of of tra@elementsin biologicalstandardkale by neutron activationanalysis.t. Radioanal.Chem.3,175-185. Norway, for co-operation during this work. Yamada, Y. 1964.Thc dctcrmination of micronutricnt clementsin soil by radioactivation analysis. Radrolsotopes(Tokyol I 3, 32-38.

References

Bowen, H. J. M. 1967a.Activation analysisin botany and agricultur€: Survcy papcr. Proc. IAEA Synp. 5, Nuclcar Actioation Techniquesin the Lifu Sciences, MS roceivcdNovernber l97l Vienna,p. 287, Printed May 30, 1972

Acta Agriculture Scandinavica?2 (1972)