TESTING & ANALYSIS _

Glow Discharge MassSpectrometry for Nuclear Materials

Keith Robinson and Edward F.H. Hall

INTRODUCTION

Glow discharge mass spectrometry(GDMS) has been shown to meet therequirements for bulk solids analysisin nuclear systems. It provides ele­mental analysis on all elements at ma­jor, minor and trace concentrationlevels, and has minimal matrix de­pendency.

Figure 1. The glow discharge source.

The technique of glow discharge mass spectrometry is rapidly becoming estab­lished in many areas of bulk solids analysis. It has great potential in character­izing materials used in the nuclear industry-uses ranging from generalmetallurgical analysis to spent reactor fuel analysis.

Glow discharge mass spectrometry offers the advantage of a small range ofsensitivity factors with a low matrix dependency for elemental analysis of solids.These features are derived from the mechanisms of the glow discharge source(Figure I)-neutral atoms are sputtered from the surface and subsequently ion­ized in the plasma by Penning charge exchange and/or electron impact. 1

The output from the glow discharge source consists mainly of singly chargedions with some background of doubly charged ions and singly charged molecularspecies. These range from a few ppb to around 100 ppm in concentration, relativeto the matrix.

Figure 2 shows relative ion yields for four different matrices. The relative ionyield is obtained by comparing the ion beam intensity of each element to thecertified concentration (by weight) of that element in the sample. The results arecompared by setting the response for Fe to be unity."

Two features are evident: First, the ion yields fall within one order of magni­tude (factor 3 about unity) for all elements measured from C to Pb. Second, theion yields vary by ± 30% at most for the four completely different matrices.Similar sensitivities for different matrices allow good "no standards" analysis onany matrix, and excellent quantitation even without the use of closely matchedstandards. In addition, the stability of the glow discharge source is superior tothe traditionally used spark source mass spectrometry (SSMS) enabling modernelectronic detection systems to be employed with a consequent improvement inprecision and analysis times.

ZIRCONIUM ALLOYS

Zirconium alloys (zircaloys) are used extensively in the nuclear power industryas a fuel element cladding material. In this application, it is necessary to deter­mine concentrations of a range of elements which influence the quality of thematerial in this special area. Three distinct and separate areas of concern are

Figure 2. Relative ion yields and matrix dependency in the VG9000 GDMS, showing (1) Uniformresponse element-to-element across periodic table. (2) Low matrix dependency. (3) Excellentcapability for no-standards analysis.

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ZIRCALOY STANDARDANALYSIS

Four zircaloy standards wereanalyzed. Determinations weremade on 36 elements, including al­loying elements, traces, gases andcarbon. Samples were prepared inthe form of a single pin electrode,typically 15 mm in length by 1.5mm in diameter. The pin electrodewas degreased in suitable organicsolvents followed by etching in amixture of nitric and hydrofluoricacids and a deionized water wash.Each sample was pre-sputtered for40 minutes to remove any remain­ing surface contamination prior toanalysis. A matrix beam current ofapproximately 6 X 10-JO A was re­corded at the working resolution of4300.

Determinations were made bymeasuring the ion intensity of theminor component isotopeon the Dalydetector versus the ion intensity ofthe zirconium major componentmeasured on the Faraday collector.The whole procedure is carried outautomatically by the menu drivendata system.

14 JOURNAL OF METALS. April 1987

influenced by the elemental composition of zircaloys. Neutron absorption is af­fected by the total composition of the alloy with respect to elements having ahigh neutron capture cross section. Elements of concern include boron, cadmium,rare earth elements, and others. Corrosion resistance is notably affected by theelements aluminum and phosphorus. Induced radioactivity is controlled by se­lected isotopes of certain elements. In a reactor they capture neutrons and trans­mute to radioactive isotopes. As an example, 59CO produces 60CO which has a halflife of 5.3 years and decays by emission of high energy gamma radiation.

The concentrations of uranium and its isotopic ratios are used as a means toascertain if the zircaloy is a "virgin material" or is recycled. To meet these re­quirements, a fast, accurate analysis of zircaloys is mandatory. In order to carryout a satisfactory analysis, several analytical procedures are normally required(viz., XRF, OES, combustion IR analyzers and SSMS). GDMS may provide anattractive "one method" alternative.

Table I shows the precision obtained on five repeat determinations in zircaloystandard 1. The mean concentration indicates a "no standards analysis" concen­tration that would be obtainable if no other standards were available for cali­bration. By comparing the mean value with the certificated value, a "relativesensitivity factor" (RSF) is obtained for each element.

This RSF is used to convert the "observed ion beam ratio concentration" to a"normalized concentration." Table II shows that excellent agreement is obtainedbetween the normalized VG9000 values and the certified values. Calibrationplots for several elements are shown in Figure 3.

For the certified elements, relative sensitivities fail to indicate any great var­iation from unity, with the exception of copper. Hence, semi-quantitative surveyanalyses may be carried out without standards within the "error factor" (RSF)shown.

Full quantitative analysis of gases and carbon is not possible since the stan­dards certificate does not provide a certified value. The relative sensitivity factorsfor these elements are not known in zirconium. However, using a typical RSFfrom Table I, these ion beam ratio concentrations could be converted to ''by weight"concentrations with an expected tolerance on accuracy approaching ± 30%. Fi­nally, detection limits of a few ppb are obtained.

URANIUM OXIDE

The analysis of uranium oxide reactor fuel is carried out as a product specifi­cation measure, both before irradiation and of irradiated material prior to re­cycling. It is necessary to check that the reprocessing plant is working efficientlyand that fission products, activation products and plant corrosion products areremoved. Again, particular attention is given to those elements with high neu­tron capture cross section (viz., Li, B, rare earth elements, Cd, etc.).

SSMS has traditionally been used for the determination of impurities to ppblevels. Glow discharge mass spectrometry offers a faster and more quantitativeapproach to this requirement without the need for lengthy and skilled interpre­tation of photographic plates.

Table I. Precision Data/Determination ofRelative Sensitivity Factors (RSF) for

Zirconium Alloys Using GDMS

5 RunMean RSD Cone.* RSF

%sn 0.44 10.0 1.83 4.2Fe 0.11 10.0 0.209 1.9Cr 0.010 5.7 0.041 4.1Ni 0.006 11.8 0.021 3.5

ppmHf 70 11.5 71 1.0B 0.37 9.6 1.1 3.0Cd 0.1U 1.4 10.9 0.8 0.6Cu 4.3 11.7 40 9.3Co 3.1 9.4 6 1.9Mn 1.8 9.8 5 2.8Ph 3.7 6.7 12 3.2Al 7.6 11.9 15 2.0Si 18 7.8 57 3.2Ti 45 11.3 28 0.6Mo 5.7 8.7 10 1.8W 4.9 10.1 13 2.7

C 9.2 28N 1.0 310 21 12Na .07Mg .26 19S 0.51 17 AdditionalV 1.0 10 ElementsGa 0.10 31 NotGe 0.006 CertifiedAs 0.6 10Nh 0.9 16Sh 0.31 30La .001Ce .001Eu .001Re .002Ir .002TI .002Bi .02 35Th .32 16*Certified

Table II. Comparison of Certified Analysis and GDMS Analysis ofZirconium Alloy Samples'

Standard 1 Cone. Standard 2 Cone. Standard 3 Cone.

VG 9000 VG 9000 VG 9000Normalized Certified Normalized Certified Normalized Certified

%sn 0.73 0.92 1.22 1.48 0.45 0.47Fe .122 .129 .126 .136 .086 .093Cr .012 .013 .087 .098 .155 .150Ni .097 .094 .057 .058 .0005 .001

ppmHf 121 128 69 72 219 220B 2.6 3.7 .03 0.2 0.14 0.2Cd N.D. 0.1 N.D. 0.1 N.D. 0.1U .92 1.1 .22 0.5 0.18 0.5Cu 89 98 7 8 11 11Co 22 20 0.8 3 55 49Mn 28 28 4 4 6 7Ph 12 11 4 5 3.5 3Al 111 86 63 53 139 130Si 82 95 13 21 113 124Ti 98 93 2 4 2 3Mo 44 46 0.3 2 0.2 2W 33 32 5 7 48 43All results expressed on a "by weight" basis.N.D.: Not Determined"The GDMS data are normalized using RSF values given in Table 1.

JOURNAL OF METALS. April 1987 15

Nonconducting powders such as UsOs may be successfully analyzed by glowdischarge mass spect rom etry, following a simple compaction technique with aconducting sub strate. The ion beam ratio principle employed is particularly at­tract ive since it removes the need for accurate weighing of the sample and sub­st rate. It al so removes errors due to incomplet e mixing/transfer of the sample orsubstrate.

Tabl e III shows the comparison of the VG9000 ion beam ratio concentrationswith the NBL provisional certified values. Figure 4 shows typical logarithmic­sca le plots of this data . One standard deviat ion error va lues are provided in eachcase where availa ble. Good correlation is obtained over a wide concentrationrange- sub ppm to several hundred ppm . Relat ive sensitivity facto rs within theexpected range ± 3 are confirmed for most elements. Sub ppm detection limitscan be obtained for most elements.

Table III. Comparison of VG 9000 Results with Certified Values for Analysis of NBL98Series Standard U30a

NBL98/7 (p p m) NBL98/5 (p p m) NBL98/3 (p pm) NBL98/1 (p p m)

Cert VG 1<1 Cert VG 10' Ce rt VG 10' Ce r t VG 10'-----

Li 0 *0.04 0.01 1.2 1.6 0.22 5.0 8.4 1.0 26.2 30.3 1.0Be 0 *0.04 0.01 0.8 1.0 0.16 5.3 6.1 0.8 25.7 23.8 4.0B 0.1 0.81 0.51 0.4 0.3 0.08 1.2 1.8 0.3 5.5 6.8 0.3Na 4.0 9.3 4.8 16.0 15.7 2.7 88.0 55.0 2.0 455.0 230 .0 12.6Mg 1.0 *0.4 4.0 2.5 0.45 17.0 13.5 2.2 91.0 42.0 3.9Al 5.0 0.8 0.05 25.0 20.0 4.9 115.0 120.0 7.7 522.0 429 .0 13.4Si 2.0 10.0 10.2 1.9 65.0 410.0 5.3 315.0P 3.7 0.7 0.04 23.0 7.2 0.82 99.0 39.5 2.1 505 .0 175.0 9.8K 2.3 4.8 2.1 28.0 22.2 2.6 138.0 102.0 7.3 725.0 582 .0 40.0Ca 4.5 9.6 5.0 19.0 40.0 5.1 100.0 167.0 19.0Ti 0.3 0.44 2.1 3.6 0.6 11.0 14.0 0.5 50.0 78.0 2.3V 0 *0.05 0.01 10.0 12.0 1.4 50.0 55.0 3.3 250.0 328.0 11.7Cr 2.0 *1.4 9.0 6.7 1.3 22.0 16.1 3.4 101.0 43.0 4.2Mn 0.8 2.9 1.3 0.11 10.6 7.5 0.9 49.0 28.0 0.5Fe 13.0 9.3 0.9 32.0 26.6 4 .0 110.0 87.3 6.4 515.0 394 .0 17.0Ni 2.0 1.0 0.2 5.6 3.5 0.66 22.0 12.8 1.7 103.0 61.0 2.1Co 0.06 *0.04 0.01 1.0 0.84 0.11 5.0 3.5 0.1 25.0 22.0 0.6Cu 0.4 *0.06 2.4 0.38 0.10 10.0 2.6 0.1 51.0 9.6 0.4Zn 1.5 *0.2 19.0 3.1 0.77 96.0 24.8 3.5 480.0 96.0 2.70Sr 0 2.6 1.9 0.21 10.0 9.0 0.9 55.0 52.0 2.0Mo *0.1 *0.28 0.07 2.0 2.3 0.62 10.0 9.5 2.7 51.0 45.0 0.8Ag 0.1 *0.08 0.3 0.25 0.1 0.8 0.43 0.1 6.0 2.7 0.5Cd 0.3 *0.33 0.6 1.9 *1.0 5.6 1.6 0.65In 0.2 *0.04 0.3 1.4 0.65 0.13 8.0 1.45 0.7Sn *1.0 *0.5 2.5 10.0 5.7 1.5 50 18.3 0.2Sb 0 0.14 1.0 1.0 5.0 1.8 0.3 25.0 6.4 0.5Ba 0 5.2 1.6 2.0 1.6 0.91 10.0 8.2 0.5 50.0 28.8 4.2W 0.1 *0.13 0.04 2.0 1.8 0.38 9.9 7.1 1.0 48.0 42.5 0.54Pb 0.8 *0.08 2.5 0.53 0.16 9.0 3.2 0.4 46.0 18.0 1.0Bi *0.2 *0.04 0.01 1.0 0.7 0.26 7.0 3.2 0.7 46.0 10.0 0.6

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Figure 4. Analysis of NBL98 series standardU30e powder by the VG9000 GDMS.

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Figure 3. Ca libration curves for analysis ofzirconium alloys by VG9000 GDMS.

URANIUM OXIDESTANDARD ANALYSIS

Analyses were carried out on fourNew Brunswick Laboratories U30.•powder standards (NBL 98 series98-1. 98-3, 98 -5 and 98-7) . Deter­minations were made on the com­plete 30 element suite and linearcorrelation plots determined .Around 150 mg ofeach ofthe U30S

powders were transferred to a 5 mlsample bottle in an "as received"condition . Approximately 50 mg ofhigh purity graphite (Carbon Lor­raine 300 mesh) were added to eachbottle and sealed with a polyethyl­ene stopper. Each bottle was ma­chine shaken for two minutes topromote efficient mixing.

Pin electrodes were prepared (15mm long x 2 mm diameter ) bycompacting the mixed powders at12-13 tons for 10-12 minutes in ahydraulic press. The samples weremounted into the glow dischargecell, and initially pre-sputtered un­der mild discharge conditions (0.5mA. 700 V) in order to allow sta­bilization. Analysis at 1.5 mA and1200 V produced a matrix beamcurrent in excess of10 11 A at 4500resolving power. Concentrations foreach of the 30 elements were deter­mined by measu ring ion beam ra­tios. impurity us. matrix, using anintegration time of 12 seconds perelement. Replicate analyses werecompleted on each sample. giving atotal analysis time inclusive of thepre-sputter period of 60 minutes/sample.

References

ABOUT THE AUTHORS

Keith Rob inson is currently with the ExportSales Depa rtment of the GDMS Divis ion ofVG Isotopes.

Edward F.H. Hall is currently senior sys­tems and development eng ineer for VG Iso­topes .

1. W.W. Harri son, K.R Hess, RK. Marcus and J.L. King,Anal. Chern.. 58 (2 ) . 341 (1986).2. Privat e communication , J . Huneke-Charles Evan sand Associates.

If you want more information on this subject.please circle reader service card number 50.

* = less th an.NBL Values: provisional certifica te concentrations by wt .VG9000 Values: Ion beam ratio concentrations.

16 JOURNAL OF METALS . April 1987