d. Remove disk tape, and trim tape to edge of disk

Transcription

1 METHOD OF XRF ANALYSIS FOR ENVIRONMENTAL Pb USING THIN FILM PRINCIPLES. PRELIMINARY TESTS Jacques Renault and Chrs McKee New Mexco Bureau of Mnes & Mneral Resources Dv. New Mexco Insttute of Mnng & Technology Campus Staton, Socorro, NM INTRODUCTION AND THEORY The XRF analyss of powders n the form of thn flms has been known for many years. Salmon (1962) summarzed the theory and appled the thnflm method to analyss of mneral samples. Salmon's approach was to encapsulate the powdered sample between sheets of mylar and present t upsde down to the X-ray beam n a spectrometer usng nverted geometry. He experenced dffcultes n preparaton of unform samples and n'attenuaton of the X-ray beam by the mylar flm. Durng the last 30 years the method has been used very lttle, but we have found that the use of modern closely coupled X-ray tube-sample geometres, uprght sample postonng and the elmnaton of the mylar flm between the X-ray tube and the sample make the thn flm method attractve for envronmental Pb analyss of mneral powders at acton levels greater than 100 ppm. The advantages of the method are that only small amounts of sample materal are requred, and that matrx effects are essentally elmnated. The dsadvantages are that the countrates requre long countng tmes for good precson and that weghng steps are requred. The absence of matrx effects allows a smplfed calculaton of results whch s based on a lnear proportonalty between unknown and a sngle standard. The countrate per concentraton per unt weght s the same for both unknown and standard when there are no nterferences. Usng the determnaton of Pb as an exampe, CX - CS... Where C s a countrate, w s a weght, subscrpt s s for standard, and subscrpt x s for unknown. Rearrangng Eq. (1) for determnaton of the unknown, [Pb],, cx cs ws wx. [Pbl, = [Pb],... (2) \ $ 1

2 The sample, as a fne powder, s to be mounted on the stcky surface provded by double faced adhesve flm prevously attached to a mount substrate approprate to the spectrometer. The mount substrate can be convenently made by compressng borc acd or mcrocrystallne cellulose n a de desgned for makn pressed powder brquets. The prepared mount s frst weghed (tare), then the sample powder s appled, then the mounted sample s reweghed and net weght of the sample determned by subtractng the tare. Detaled nstructons follow and are based on use of the Rgaku 3064 spectrometer. Prepare sample substrate. Double faced adhesve flm of most any type wll work well provded that t contans no nterferng elements. The flm cutter shown n Fgure 1 s desgned to cut a 1 nch crcle from flm attached to a 37 mm dsk that rdes n the spectrometer's sample changer. Refer to Fgure 1 for names of parts. 1. Apply tape a. Invert gude wth slots up, and place t on a table b. Insert labeled dsk, label toward table C. Apply tape to dsk through centerng slot d. Remove dsk tape, and trm tape to edge of dsk e. Smooth down tape wth plastc sheet or teflon rod 2. Cut dsk a. Place dsk on table wth tape' up b. Place gude over dsk wth slot toward the table c. Lower cutter nto gude untl t rests on tape d. Grp dsk through fnger recesses e. Rotate cutter on tape several tmes back and forth to cut t 3. Remove gude leavng cutter on dsk 4. Hold dsk plus cutter together and remove waste tape 5. Store dsk n a dust free place pror to weghng f the sample s not to be appled mmedately. Mount sample. The sample should be a powder wth a gran sze as fne as possble, for t s mportant that the grans fll the avalable space wthout overlap and consequent shadowng. 1. Wegh prepared pellet and tape (tare) 2. Apply sample to tape usng dsk cutter as a gude a. Completely cover tape wth sample powder by shakng t n dsk cutter 2

3 b. Use a soft artst's brush to dstrbute the sample evenly f necessary c.' Pour off excess sample powder and tap the mount upsde down to remove what remans. d. Carefully wpe out nsde of dsk cutter e. Press remanng powder nto tape wth end of clean teflon cylnder guded by the dsk cutter 4. Rewegh and subtract tare TEST OF THE METHOD The method was tested by usng several geochemcal standards havng a comprehensve range of Pb concentratons. The standards and nstrumental condtons used are lsted n Table 1. TABLE 1. Standards and Instrumental Condtons Samp No. Agency Sample type ppm FeR- 1 CCRMP Iron Formaton 5200 GXR- 2 So USGS SY- 2 Syente CCRMP 85 VS -N Glass ANRT 1000 Instrument: Rgaku 3064 Gegerflex WDXRF spectrometer; Kv/Ma: 60/45; Slts: coarse; Crystal: LF (200); peak: Pb Lp; Count: 200 sec Pb Samples were prepared n trplcate followng the procedures gven above. The data were reduced usng Eq. (2) and the standards treated as unknowns. The results were calculated once usng VS-N and agan usng FeR-I. Background was corrected after measurng peak ntensty on a blank. The results summarzed n Table 2 below should be consdered prelmnary. The results are graphcally dsplayed n Fgures 2 and 3. Inspecton of the last column of Table 2 shows that the precson of the analyss s on the order of 10 percent The average errors of FeR-1, VS-N, GXR-2, and Sy-2 are 11%, 5% lo%, and 8% respectvely. Excludng FeR-1,16, accuracy s good for the two hgher concentratons, moderate 690 ppm and poor at 85 ppm. The rogue value shown by sample FeR-1,16 s unexplaned at ths tme, but we suspect nhomogenety of the sample. Standard VS-N s an artfcal glass, and s expected to be more homogeneous n lead than the natural samples. It does, n fact show the oducblty. best I 3

4 Columns 3 and 4 of Table 2 show that results calculated by usng FeR-1,18 (5200 ppm) are systematcally lower than those calculated by usng VS-N,1 (1000 ppm). However, ths bas s substantally less than the analytcal varaton among replcat ons. Fgure 3 shows that calculated and reported Pb concentratons n the standards used for ths prelmnary study correspond closely. Fgure 2 shows that the varaton n net countrate per mllgram of sample versus reported Pb concentraton s substantally lnear over a range of three orders of magntude. Ths wll allow samples as dverse as sols, pant, and household dust to be analyzed wth a sngle well characterzed standard. As presently developed, the method s applcable to envronmental "acton level" evaluaton of total Pb n fnely dvded materals. The method s smple n that t demands no matrx correctons. In ths sense, t s exceptonally drect and free of the assumptons ordnarly found n XRF analyses. It does requre careful weghng of small amounts of sample and attenton to analytcal procedure. We thnk that ts most practcal use wll be n the analyss of dust collected from selected locatons n dwellngs. TABLE 2. Analyss of Pb usng FeR-1 and VS-N. Sampl # ppm rptd calc/vs calc/fer' cps unk mg unk cps/mg FeR-1, FeR- 1, FeR-1, VS-N, VS-N, VS-N, GXR GXR ,0.75 GXR sy-2, SY-2, SY-2, blank blank 1 13 blank 2 1

5 REFERENCES Salmon, Merlyn L. (1962) A smple multelement-calbraton system for analyss of mnor and major elements n mnerals by fluorescent X-ray spectrography. Advances n X-ray Analyss, Wm. M. Mueller, edtor, Plenum Press, vol 5., pp

6 Plan Vew b mm -4 Sde Elevaton Fgure 1. XRF/XRD Tape Cutter

7 Fgure 2. Cps/mg vs. Pb Concentraton n standards

8 Fgure 3. Calculated vs. reported Pb concentraton usng standard VS-N,l 6000 j $ E Q I"' n j j.- C Q h. +. :! 1 f Reported Pb Concentratons n ppm I I.