Energy-Dispersive X-rayFluorescence Methods forEnvironmental Characterization ofSoilsS T E V E N J . G O L D S T E I N , *A L I C E K . S L E M M O N S , A N DH E A T H E R E . C A N A V A N

Chemical Science and Technology Division, MS K484, LosAlamos National Laboratory, Los Alamos, New Mexico 87545

With recent requirements for rapid, field-based methodsfor environmental characterization, we have evaluatedenergy-dispersive X-ray fluorescence (EDXRF)techniques for elemental analyses of soils at LosAlamos using laboratory, transportable, and portableinstruments. Fundamental parameters providereasonably accurate standardization, and spectralinterferences are generally absent. Detection limitsare below screening action levels or backgroundsoil abundances for all elements of concern exceptAs and Be. Results for certified materials indicatethat accuracy is typically better than (10%, althoughsome elements have few or no suitable referencematerials to evaluate accuracy. Portable and fixed-base instruments typically give consistent results.However, large positive biases (2-78×) are generallyfound between EDXRF and standard EPA nitric aciddigestion methods. This reflects the fact that EDXRFmeasures total amounts of the analyte, whereas EPAmethods measure only the components labile in nitricacid and not the matrix. Consequently, EDXRF andEPA methods are not directly comparable forpristine soils, whereas contaminated soils shouldgive more comparable results for the two techniques.Our data indicate that EDXRF can vastly exceedanalytical requirements for field screening, and thatthis simple and fast technique can yield fullyquantitative elemental analyses for soils in environmentalstudies.

IntroductionRecently, there has been a growing need for rapid, field-based methods for elemental analyses of soils and otherdifficult matrices for environmental characterization andrestoration purposes (1). Energy-dispersive X-ray fluores-cence spectroscopy (EDXRF) has several potential advan-tages for this type of analysis (2-8). It is nondestructive

with rapid throughput and simple sample preparationamenable to a field setting, a broad range of elements fromNa to U are characterized simultaneously, and the sensitivityof ∼10 ppm is appropriate for field screening for mostmetals. Possible disadvantages are related to spectralinterferences for certain elements such as Pb and As andpoorer sensitivity for lighter elements, with hazardousconstituents of atomic number lower than Na such as Benot detected. For soils and other complex matrices,empirical methods for calibration can be difficult orcumbersome, and theoretical calibration methods such asfundamental parameters models are not always viewed asreliable (e.g., ref 9). It can also be potentially difficult todirectly compare total analyte measurements by EDXRFwith the more standard acid leach methods designed toanalyze only acid-labile components, although most studieshave shown good agreement for contaminated soils (2, 3,5, 8, 10).

However, recent developments in field-based EDXRFhave potentially overcome many of these problems (e.g.,ref 4). High resolution Si(Li) detectors have improvedenergy resolution dramatically, thereby reducing spectralinterferences. The development of personal computerswith high speed and memory has also allowed fundamentalparameter algorithms to be quickly performed usingmultiple standards, resulting in rapid and more accuratestandardization and analyses for multicomponent, complexmatrices over standard empirical methods (9, 11, 12).

Here, we evaluate the EDXRF technique for elementalanalyses of soils at Los Alamos with two fixed-base andfield-transportable instruments and compare results witha portable instrument and standard nitric acid/ICP methodsmandated by the Environmental Protection Agency (EPA).We show that fixed-base and field-transportable EDXRFtechniques provide data of sufficient sensitivity, accuracy,and precision in almost all cases to meet the data qualityobjectives for environmental characterization purposes atLos Alamos.

Experimental SectionSample preparation and analysis follow techniques de-scribed by Watson et al. (6). Briefly, 20-30-g samples aredried and physically homogenized, first by drying in amicrowave oven for 5 min or in a convection oven overnight.Samples with large objects are sieved through a 10-meshpolypropylene screen to remove the coarse fraction.Samples are then milled and mixed for 5 min in a zirconiaball mill assembly and are sieved through a 100-mesh nylonscreen, from which a ∼10-g fraction passing through thescreen (<100 µm) is analyzed. Cleaning of all samplepreparation equipment with a dry towel leads to a cross-contamination factor of 1 part in 1000-10 000, which isadequate for field characterization. The sample preparationprocedure is quite rapid (∼20 min/sample) and environ-mentally safe, but is quite dusty and should be carried outwithin a standard chemical hood or outdoors to provideadequate protection and ventilation for the individualpreparing samples.

Instrument analysis procedures are also quite rapid,requiring ∼20 min/sample. We routinely analyze 18elements ranging from K to U. Samples are analyzed in

* Author to whom all correspondence should be addressed;telephone: 505-665-4793; fax: 505-665-5982; e-mail address:sgoldstein@lanl.gov.

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standard sample cups with polyethylene film used toprovide maximum transparency for X-rays. Similar pro-cedures are used for both the field-transportable Spectrace6000 and the fixed-base Spectrace 5000. Three excitationconditions are used: a low atomic number condition forK-Mn K lines, with a 0.13-mm aluminum filter, a tubevoltage∼13-15 kV, and a live time of 200 s; a middle atomicnumber condition for Fe-Se K lines and Hg-U L lines, witha 0.13-mm Rh filter, a tube voltage ∼35 kV, and a live timeof 200 s; and a high atomic number condition for Cd-BaK lines, using a 0.63-mm Cu filter, a tube voltage ∼50 kV,and a live time of 200 s. Instrument software stores eachspectrum for each condition, performs background sub-traction and overlap correction, creates peak intensity files,and performs quantitative analysis using fundamentalparameters techniques.

A fundamental parameters algorithm is used for stan-dardization, from which pure element count rates aredetermined for each element. The algorithm calculatesX-ray tube emission, absorption by sample, sample excita-tion, and self-absorption from theory, but empiricallycalibrates iteratively using standards (9, 11-14). For mostaccurate results, this standardization method requiresmultiple standards with similar matrix composition. Weused seven soil/rock standards to represent a broad rangein composition and location: NIST 2704 (Buffalo Riversediment), NIST 1648 (urban air particulate matter), NIST2709 (San Joaquin soil), NIST 2710 and 2711 (contaminatedMontana soils), CCRM SY-2 (Syenite rock standard), andCCRM BL-4 (uranium ore). These are all essentially silicate-based materials with standards 1648, 2710, and 2711 havingelevated transition metal concentrations and standards SY-2and BL-4 having high actinide and lanthanide concentra-tions.

Results and DiscussionStandardization results are presented in Figure 1 as thetypical relative standard deviation in pure element countrate for each analyzed element. This parameter providesan indication of the agreement of pure element count ratesfor a particular element among the standards used and istherefore a measure of the inherent uncertainty in analysisdue to calibration. Typical agreement of pure element

count rates is (12% (1σ), with best standardization (<10%)for U, Mn, Ca, K, Fe, Cr, Cd, and Ba and less precise (∼20%)calibration for Ti, Ni, and As.

Detection limits, screening action levels, and soilbackground are shown in Table 1. Detection limits for thefixed-base, transportable, and field-portable instrumentsare all calculated at the level of three times the standarddeviation of the background for a 200-s live time. Thedetection limits decrease with increasing atomic numberas expected and are typically 5-15 ppm for the fixed-baseand transportable instruments. Detection limits are some-what higher for the portable instrument, with a typical rangeof 10-100 ppm. Screening action levels (SAL) for eachelement are derived from dose assessment calculations orare set equal to the upper tolerance level of the background,defined as the 99th percentile of the background soildistribution at Los Alamos (15). From these data, it isevident that the detection limits for the fixed-base andtransportable instruments are lower than screening actionlevels for all elements with the exception of arsenic.

Results for 10 certified soil/rock materials analyzed asopen quality control check standards utilizing the fixed-base instrument are summarized in Tables 2 and 3. Thesestandards include the CCRM reference soil materials SO-1,SO-2, SO-4; the CCRM reference stream and lake sedimentstandards STSD-1, STSD-2, STSD-3, STSD-4, and LKSD-3;the CCRM syenite rock standard SY-3; and the NISTestuarine sediment reference material 1646. Typically,average accuracy is in the range of 90-110%. Deviationsfrom this are primarily related to the lower concentrationrange for certain elements (i.e., Cr, Ni, Cu, Zn, As, Sb, U,Th) in these check standards. For results of standards withelements present at high concentration (>2×DL; Table 3),only Ti appears to show a significant bias, but this reflectsthe fact that one of the check standards (SY-3) has a 0%accuracy due to misidentification of the Ti K-R line as alanthanide L line. For certain elements including Hg, Se,

FIGURE 1. Standardization results based on fundamental parameterscalibration of the fixed-base EDXRF spectrometer. The typical relativestandard deviation in pure element count rate for each analyzedelement for up to seven soil/rock standards is presented. Thisparameter provides a measure of the inherent uncertainty in analysisdue to calibration. Typical agreement of pure element count ratesis 12% (1σ), with best calibration for U, Mn, Ca, Fe, and K and poorestcalibration for Ni, As, and Ti. Results for the transportablespectrometer are similar.

TABLE 1

Detection Limits,a Screening Action Levels, andNatural Soil Background Abundancesb for LosAlamos Soils (ppm)

elementlaboratoryEDXRF DL

transportableEDXRF DL

portableEDXRF

DL

screeningactionlevel

Los Alamosbackground

K 120 100 180Ca 120 100 80Ti 195 30 63Cr 11 12 345 400 34Mn 16 16 318 11 000 1030Fe 110 10 150Ni 28 13 96 1 600 27Cu 13 8 57 3 000 16Zn 12 5 57 24 000 101As 33 4 36 12 12Se 5 4 23 400 2Cd 2 3 89 80 3Sb 3 4 33 32 3Ba 11 10 10 5 600 1140Hg 10 5 49 24 1Pb 12 7 18 400 39Th 8 8 9 46 25U 8 8 11 177 6

a Detection limits are defined at three times the standard deviationof the background for each X-ray line. b Soil background abundances(15) are those at the upper tolerance limit, which is defined as the 99thpercentile of the natural concentration distribution at Los Alamos.Average soil abundances are generally a factor of 5-10 lower than theupper tolerance limit.

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and Cd, no check standards were available with high enoughconcentrations to permit quantification of these elementsand an estimate of accuracy. Accuracy results for thetransportable EDXRF were similar to those for the laboratoryinstrument.

Results for the portable instrument (Spectrace 9000) andfixed-base instrument are compared in Table 4 for a set of25 soil samples from Los Alamos. These results are obtainedon the same sample cup for both instruments, hencedifferences reflect biases due to instrumental analysis only.Consistent results are obtained for the two instruments,with agreement to within 20% for most elements. However,some elements show large positive biases (factor of 5-40)for the portable instrument including As, Th, and U. Thismay be due to the somewhat poorer energy resolution andgreater spectral overlap for these elements with the portableinstrument, which has a HgI detector. Nominal energyresolution for the fixed-base and portable instruments are160 and 240 eV for the Mn K-R, respectively.

In contrast, large biases are found between EDXRF andstandard nitric acid digestion/ICP methods prescribed by

EPA, as shown in Table 5. Average XRF/ICP results are 78for K; 2-6 for Mn, Fe, Pb, Cr, Cu, Ca, and Ba; and 0.8-1.1for As, Ti, and Zn. Although the two methods utilizeddistinct sample preparation methods, the XRF/ICP differ-ences are too large and consistent to be due to samplingvariability between the procedures. The divergent resultsmost likely reflect the fact that the XRF technique ismeasuring total amounts of the analyte, whereas standardEPA methods are measuring only the nitric acid-digestiblepart of the soil and not the matrix. These results aresomewhat consistent with prior studies comparing wetchemical analysis after hydrofluoric acid digestions withnitric acid digestions. The hydrofluoric acid digestionprovides total dissolution of the sample, whereas the nitricacid digestion does not dissolve the silicate matrix con-stituents. Results of this comparison show that totallydissolved samples can have up to factors of 2-10 greaterconcentrations for certain elements than nitric acid-digested samples (15-17). From these data, it is evidentthat XRF and EPA methods are not directly comparable for

TABLE 2

Quality Control Check Standard Results for theLaboratory EDXRFelement concn range accuracy range av accuracy biasa

K 1.00-3.49% 92-141% 105% noCa 0.83-5.90% 91-129% 104% noTi 900-8600 ppm 0-95% 83% ?Cr 12-160 ppm 97-525% 164% ?Mn 370-3950 ppm 81-133% 102% noFe 2.37-6.00% 91-114% 100% noNi 30-94 ppm 61-260% 128% noCu 16-65 ppm 20-124% 85% noZn 94-246 ppm 84-128% 97% noAs 7-42 ppm 25-114% 82% noSe ?Cd ?Sb 3-7 ppm 58-189% 112% noBa 400-2000 ppm 97-117% 104% noHg ?Pb 16-130 ppm 78-146% 105% noTh 9-990 ppm 63-117% 88% noU 8-650 ppm 103-137% 120% noa Elements marked with ? may show a small bias based on the

accuracy results or have no suitable standards for which to evaluateaccuracy.

TABLE 3

Quality Control Check Standard Results (>2 × DL)for the Laboratory EDXRFelement concn range accuracy range av accuracy biasa

K 1.00-3.49% 92-141% 105% noCa 0.83-5.90% 91-129% 104% noTi 900-8600 ppm 0-95% 83% ?Cr 61-160 ppm 97-164% 118% noMn 370-3950 ppm 81-133% 102% noFe 2.37-6.00% 91-114% 100% noNi 47-94 ppm 69-170% 106% noCu 22-65 ppm 83-124% 96% noZn 94-246 ppm 84-128% 97% noBa 400-2000 ppm 97-117% 104% noPb 28-130 ppm 95-122% 109% noTh 17-990 ppm 95-117% 109% noU 19-650 ppm 103-137% 120% noa Elements marked with ? may show a small bias based on the

accuracy results.

TABLE 4

Comparison of Portable and Fixed-Base EDXRFelement no. of samples av portable/fixed-base biasa

K 25 125% noCa 25 120% noT 25 86% noCr 1 66% ?Mn 13 102% noFe 25 98% noNi 0 ?Cu 2 71% ?Zn 24 144% noAs 7 1660% yesSe 1 4090% ?Cd 0 ?Sb 0 ?Ba 25 77% noHg 0 ?Pb 6 79% noTh 22 460% yesU 20 3380% yes

a Elements marked with yes show a large bias between portable andlaboratory EDXRF based on a large population of samples, while thosewith ? may show a small bias or are based on too small a samplepopulation to evaluate the two methods. Elements with no data hadresults below detection for either or both of the EDXRF instruments.

TABLE 5

Comparison of EDXRF and Nitric Acid Dissolution/ICP Dataelement no. of samples av XRF/ICP standard deviation commentsa

K 20 77.5 49.9 >Ba 20 6.2 3.4 >Ca 20 3.9 1.7 >Cu 7 3.3 1.4 >Cr 6 3.1 1.9 >Pb 17 2.6 1.5 >Fe 20 2.3 0.9 >Mn 20 2.1 1.0 >Zn 20 1.1 0.4 )As 1 1.0 0.4 ?Ti 1 0.8 ?a Elements marked with ) show no difference between methods,

whereas elements with > indicate a significant difference. Elementswith ? have too few quantifiable data to make a valid comparison. Nocomparison was possible for Cd, Hg, Sb, Se, Th, and U since resultsfor these analytes were below detection.

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relatively pristine soils. For these samples, the EDXRFtechnique will provide a much more conservative estimateof elemental contamination than standard EPA procedures.However, in contaminated soils where most of the analyteis nitric acid-leachable, EDXRF and standard EPA methodsshould give more similar results.

These data indicate that the EDXRF technique, whencombined with steps to create a physically uniform andreproducible sample, vastly exceeds capabilities requiredfor field screening of environmentally contaminated sites.In most types of environmental studies and others whereaccuracy of ∼(10% is required, this simple and fasttechnique can be used as a fully quantitative method forelemental analysis of soil samples.

AcknowledgmentsRon Conrad and Albert Dye kindly provided the portableXRF data. Cathy Smith provided the comparison data forthe EDXRF and standard EPA methods and also helpedwith constructive criticism. We also thank the anonymousreviewers for helpful suggestions. Financial support wasprovided by the Environmental Restoration Program at LosAlamos National Laboratory.

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Duffy, C.; Ryti, R. Natural Background Geochemistry, Geomor-phology, and Pedogenesis of Selected Soil Profiles and BandelierTuff, Los Alamos, New Mexico; Report LA-12913-MS; Los AlamosNational Laboratory: Los Alamos, NM, 1995.

(16) Kane, J. S.; Wilson, S. A.; Lipinski, J.; Butler, L. Am. Environ. Lab.Appl. Note 1993.

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Received for review October 5, 1995. Revised manuscriptreceived February 22, 1996. Accepted March 15, 1996.X

ES950744Q

X Abstract published in Advance ACS Abstracts, May 1, 1996.

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