oratories are concerned with both types of analysis, the relative proportion of work performed in the twoareasmay varyconsiderably. Each area requires a completely different analytical approach. Wavelengthdispersive spectrometric analysis is usually employed for majorlminor element analysis while trace element XRF analysis is performed using either energy-dispersive or wavelength-dispersive spectrometers. S a m p l e Preparation X-ray fluorescence spectrometry is probably the most versatile of all instrumental analvtical methods with resneet t o the variety of specimen forms to which it s readily applmble nondestructively (Ibl. Samples may be presented as solids, small fabricatcd parts and forms, powders, hriquets, fusion products, liquids and solutions, and supparted specimens. Geological specimens are usually presented as powders, briquets, or fusion nroducts. At the United States Geolneicnl " ~ -Suwev. ---~ ~ -, an adaptation of the Clairse tetraburate fusion rntthud ia used for major element snnlysis. Molded glass disks were selected as the method of preparation because of their relstive stability and the necessity of eliminating possible contamination during t h e grinding and making of briquets (8). The loss-on-ignition (LOI) is determined first by heating an 800-mg sample t o 925 O C for 45 min in a tared 95% Pt-5% Au alloy crucible. After ignition and weighing, a small portion of the 8-g charge of lithium tetraborate flux is added to the crucible and pulverized with the specimen using an agate pestle. The bal~~~
~
~
.
~~~
~
~
ance of the flux is then added and the components thoroughly mixed before 250 @Lof a 50% solution of LiBr is added t o serve as a non-wetting agent to prevent the disk from sticking to the mold. An automatic fluxer, developed a t the Denver USGS laboratory (91,is loaded with seven crucibles with specimens and seven empty molds and fired in a muffle furnace a t 1120 OC for 40 min. After the specimen is melted and homogenized through the tilting action of the fluxer, the fused mixtures are poured into the molds and cooled. I t is important that the sample surface be flat and smooth. The two-piece mold can be taken apart and the bottom surface polished flat (after 50 or more uses) when needed. The two-piece mold virtually eliminates contamination of the analytical surface during grinding of the glass disks and saves significant labor compared t o grinding each sample disk. In the current study a t the USGS evaluating the performance of the ARL8420 for majorlminor element analysis, the results obtained on standard rock samples are given in Table 1 along with the accepted values reported in the literature and show excellent agreement as evidenced by the values of the standard error of estimate obtained.
Diflicultles In S a m p l e Preparation by Fusion Samples containing metals that alloy with platinum present difficulties. In their reduced form, those metals alloy with platinum more readily than in higher oxidation states. The oxidizing atmosphere of the muffle furnace minimizes the latter problem compared to heating in the reducing atmospheres of propane or natural gas flames. Samples containing up to 25 weight percent pyrite or 5 wt % chalcopyrite can be
prepared by fusions in Au-Pt alloy crueihles without a problem, but Pb, As, and T e are unacceptable above 0.2 w t % even in oxidized minerals such as cerussite or olivenite (8). Three elements found abundantly enough in geologic materials can cause the fused disks to stick t o platinumware-chromium, nickel, and copper. Samples with more than 10% copper, the worst of the three, should not be analyzed by XRF except by nonfusion preparation techniques. Organic-rich materials must be eonsidered in two categories. Samples containing between 5% and 25% organic matter such as oil shale, black shales, and soils sometimes support combustion but, more frequently, lead to small explosions that eject sample from the crucible. Those with more than 25% organic matter such as peat, coal, and vegetation support combustion, creating problems during the ignition stage. To avoid these limitations, the specimens are ashed alowly and the analysis performed on the residue (8). T r a c e Element Analysis In trace element analysis, sample preparation methods seek t o achieve minimum dilution because the X-ray count rate is low. Trace element analyses using E D X R F sometimes experience X-ray line interferences or inadequate spectral resolution that prevents determination of a few elements. Frequently, these elements can be determined by wavelength-dispersive XRF because of the greater resolution of the speetrometer or because a different X-ray source is used for excitation. At the USGS. vanadium and cobalt are determined bv WDXRF as were rubidium, strontium, and barium prior t o acquisition of EDXRF instruments.
~.
Table 1. Major and Mlnor Rock-Fmlng Elements In Slllcate Rocks Na.0 AGV-I Andesite BCR-1 Basalt BIR-1
Lit. Meas. Lit. Meas. Lit.
Basalt
Meas.
MRGl
Lit.
GBbbro
Meas. Lit. MeaS. Lit. Meas. Lit. Meas. Lit.
DTS-I Ounite PCC-1 Peridotita QLO QuartzLatite GSP Granodiorite RGM-1 Rhyolite
'StBndBld
MgO
A1203
SiOl
Meas.
Lit. Meas.
erra of estimate from calibration
SEOE = where & is me measured value,
F
Y.?
A202
P20s
E?
XI is me literature valw, and Nis the number of standards "red
Journal of Chemical Education
(46).
K20
CaO
Ti01
MnO
FetOa
Samples are prepared by mixing aportion of the sample with chromatographic cellulose paper in the proportion 85% sample to 15% cellulose powder. If a backing composed of 12% cellulose acetate and 28% wax is used as support, about 1.0 g of 85:15 sample:binder is required. The sample and backing are pressed into a briquet a t 25,000 osi for 1min usine., a soecial die (10). . . After removal from the die, the specimen briquet is stored in a deaiecatm until it is analyzed. Samples and standards are prepared similarly. Typical calibration data for vanadium include a coefficient of determination (COD) of 0.9991; a standard error of estimate (SEOE) of 4 ppm over the calibration range of 4 to 600 ppm. The vanadium intensity is corrected for three influencing elementa-titanium, calcium, and iron-using a multiple linear regression equation. Rubidium, strontium, and barium have been analyzed by a similar procedure using a 1:l sample-to-flux ratio to obtain a lower limit of detection of 2 ppm and a relative standard deviation of i 5 % at the 200-ppm level.
.
Optlmlzlng Geological Sample Analyclis Despite the relative simplicity of X-ray spectra and the versatility of X-ray fluorescence spectrometry, the rangeand complexity of geological materials challenge the spectroscopist in o~timizinetheir analvsis. in an earl& study;~nzelmi(11) sought t o ootimize some of the oarameters. The investigation included, for majorlminur element analysis: aensnivity and limits of detection far the light elements, sodium through phosphorus, in two sample types; analytical accuracy; and background measurement. For trace element analysis, the study evaluated the effects of X-ray tube voltage and current, detector performance, crystal and collimator selection, and matrix effects. Previously, Guwich (12) studied the effects of anode-to-sample distance, anode material, tube window design, and primary beam aperture. Other parameters to be considered include: pulse-beight-discriminator setting to extract background and higher order reflections or t o extract or include escape peaks and, finally, the state of the sample as presented to the beam.
-
SiOl A1&
Fe.08 CaO MgO MnO
Na20 K20
Ti01 P A
F S
(Continued on page A204)
A hybrid X-ray fluorescence spectrometer
Tab* 2. Elemem (oxide)
The gearless goniometer has a slew speed (the speed to go from one angular position to another) of up to 4WOo/min and permits rapid change of the flow proportional, sealed proportional, or scintillation detector available. (Conventional goniometers of sequential X-ray spectrometers employ a system of gears that connect movement of the crystal and detector and maintain the 8/28 angular ratio.) Thermal stabilization (i0.5') is maintained because the goniometers and manochromators are installed inside a vacuum tank. Standard USGS rocks were selected as samples and two methods of preparation used. Lithium tetraborate fusions of 51 flux-to-sample dilution ratio were made with one gram of NH4N03as an oxidant and two drops of hydrobromic acid as a release agent. Briquets were msdeby grinding5 g of sample and 1g of cellulose powder as binder with 10 mL of Freon for 5 min after which the mixture was pressed onto a boric acid backing.
The exoerimental work wes done on an Applied desearch i.aboratory (AHL) Model 8480 (see figure), a hybrid X-ray fluoreauence spectrometcr. Sequential analyses were performed on a gearless Moire fringe goniometer employing flat crystal optics with 0.15- and 0.45" coarse and fine primary collimators. The simultaneous or fixedchannel analvses were nerformed on monochromators &ine ~ohannsenfullv focussine opt~csw t h prmary and secondary slits set approxmately at the theoretical optimum ratio of 2 to 1. The hybrid structure permits the use of manochromators fixed a t certain wavelengths for elements analyzed routinely or requiring extended integration time (light elements, traces) and goniometers for the determination of backmound and consecutive analysis of other eiements concurrently with those analyzed by monochromators. Table 2 Lists the analysis times for 23 elements in a geological specimen and indicates the potential timesaving achievable by appropriate selection of goniometer or fixed-channel analysis (13).
%Ray Analysls of (ieolcglcal Samplera
Major wmponems Concentratim range, %
Analysis tlmd, P
35.0-75.0 10.0-40.0 1.0-10.0 0.2-15.0 0-10.0 0-0.3 0-5.0 0.1-8.0 0-2.0 0-0.5 0-0.5 S0.5
1 2 1 3 10 10 30C 4 4 20C looC 15
Element U Th Rb
MO
Zr Sr
As Pb Nb Y
Ba
Volume 64
Tram Concemration ran@, P D ~
Anaiysls time4,s
1-1000 1-2000 30-1000 1-1000 30-1000 50-1500 1-100 10-200 10-200 20-200 50-3000
Number 9
September 1987
100C 100r 5 100C 5 5 looC 40C 20C 15 10
A203
Literature Clted I. Bertin. ~ u r e n eP. himduction m X - m y s p ~ d r o m e t ri? Ar,oI.vsi?; Plenum: New Y w k , 1978; la) pp 281298; Ih) p 391. 2. Ro3e.H. J.,Jr.;Cuttitta,F.Adv. X-ray Anal. 1968.11,
21. 1. R0se.H.d...lr.:Cuttitta.Frsnk.Appl.Spadrmc.1968, 22,126430.
Table 3.
Silicate Rocks-Goniometer Sensitivity (S) and Limn of Detection (LOD) tor Fusions (51) and Powder Briqueta
4.
3.7,
5. Claisse, F. Noreleo Repailer 1957.1.3-7. R. Roae, Harry J., Jr.; Adler. Isidare: Flansgsn, Francis J. Use o/Lcz08a8aHeouy Abwrbsr in the X-ray Ruor m e n c e A a d y s i ~oisilirote Rocks; Us. Geological Survey, Pmfessional Pub. 450-8. 1962, Article 31. 880-882. 7. Rwe. Harry J.. Jr.; A d h , Isidore; Flanagan, Francis J. Appl. Spocfrose. 1963.17.81-85. 8. Tamart,J. E.. JI.: Lindmy, J. R.; Smtf,B. A.: vivit. D. A.; Barlel. A. J.: Skwart, K. C. Anolysk 01 Geologic Motwida by Wcluslan#th-Dis~ersius X-ray Fluorescence Spedromofry:Chapter 5 in U.S.G.S.Bulletin 1770,1986. 9. Tsggart,JsmesE., Jr.; Wshlherg, JsmesS.Adu.X-roy LO.
' F r m reference 13 with
Cuttitts,F.: RUM, H. J., Jr. A p p l Spectrore. 1968,22,
permission.
Anal. 1980.23.267-261. Fshbi. Brent P. A 'Die for Pelletizing Samples lor X my Fluorescence Analysis; in US. Gwl. Surv. Prof. Pub. 7W-B. Gwlogicsl Survey Research, 1970. 817n>na
11. Anzelmo. John A. "XRF Analvsis . of Gcnlmieal Sampier: optimization of parameters To Improve Speed and Accuraev": Dam. or~aentedst the Pittsb~reh ~
Table 5.
NapO Performance Data for Silicate Rock@ Standard
LOD (20)
Sample
Measurement
Error
(30S)
Concentration Range
Powder
Peak Peak-Background
0.07 0.08
0.008
2.0-9.0%
Fusion (5:l)
Peak Peak-Background
0.09 0.08
0.030
2.0-9.0%
'Fmm
reference 13 wllh
Paper No. 818.
A comparison of sensitivities for theliiht background regression analyses of rock elements, sodium through phosphorus in samples, soda and potassium feldspars, the two sample types, shows a 5:1 ratio plastic and flint clays, and various grades of (powder to fusion) for sodium and a 2.51 refractory ranging in silica concentration ratio for phosphorus (Table 3). Limits of from 20 to 70% in alumina from 14 to 70%, detection are4:l (fusion to powder) for sodiin iron from 0.05 to 0.14%.. and in calcium um through 2 3 for phosphorus. Sensitivoxide from 0.1 LO 710. While the general rule ities as well as limits of detection for the requires background measurement when ~~~. elements potavsium through manganese are the peak.10-background ratio is below 10:1, m the ratioof about 21. no improvement was obtained, in the case of Comparative sensitivity data were deterfusion beads, over the concentration range mined for analysea using fixed channels and investigated (Table 5). the goniometer. The elements sodium, aluIn the case of trace element analysis, the minum, calcium, and iron were selected to second maior area of concern to the eeorepresent the usually determined range of chemist, the study confirmed that careful lightest through heaviest elements in major/ attention to operating parameters can minor analyses. Tahle 4 shows the sensitivmarkedly improve results. ities to be approximately equal. ~
~~~~~~~~~~~
~~
~~~
~
Table 4. 8480 XRF Sensitivity (Kctslsl %) for Silicate Rocks-Powder Briqueta Element
Goniometer
Fixed Channel
Na
0.7 8.1 2.8 8.1
0.9 3.4 3.3 6.1
Al
Cs Fe
The important decision of whether or not to measure background in majorlminor analysis was resolved based on peak and
A204
Journal of Chemical Education
~
Conference Abstracts,
Gurvich,Y.M.Am. Lab. 1983, 15111),64.
a * .
permisoion
~
Pittsburgh
lWd
13.
-~~
Summary X-ray fluorescence spectrometry offers many advantages as an analytical technique for general research, materials research, and particularly for geological specimens and process industries such as cement and steel. The characteristic lines generated hy an element irradiated hy X-rays uniquely identify most elements, permitting qualitative, semiquantitative, and quantitative analysis with theuse of appropriate standards. Careful technique in sample preparation and handling permits the determination of elements in complexnatural and syntheticmaterials over a wide range of concentrations from parts per million to 1W%.
