Analytical Chemistry. LD Chemistry Leaflets. Determination of the chemical composition of a brass sample using x-ray fluorescence C3.6.5.

Transcription

1 SW alytcal Chemstry ppled aalyss Materal aalyss LD Chemstry Leaflets Determato of the chemcal composto of a brass sample usg x-ray fluorescece ms of the expermet To lear how to use a x-ray apparatus. To perform a x-ray fluorescece aalyss. To aalyse the composto of a alloy. To determe the quattatve cotet of the costtuets of a alloy. Prcples Whe atoms are rradated wth hgh-eergy x-rays, x-rays wth a characterstc eergy are emtted. Ths pheomeo s descrbed as x-ray fluorescece. The emtted eergy depeds o the atomc umber of the sample materal (Moseley's Law). The eergy suppled causes electros to be shot out of the er shells. The gaps the er electro shells are the flled by electros from the outer electro shells. These electros lose eergy the process, whch s the emtted the form of x-rays (x-ray fluorescece). The K spectrum (the K spectral seres) s caused by electro jumps out of the secod (Kα) or the thrd (Kβ) electro shell to the K shell. The L spectrum orgates aalogously whe electros fall back to the secod electro shell (L shell). Every elemet, each wth a dfferet atomc umber, emts x- rays wth dfferet eergy levels. Ths eergy s practcally depedet of the bodg form of the elemet, as the er electros have practcally o volvemet chemcal bdg. The x-ray fluorescece spectrum of a sample cotag several compoets s, as a frst approach, the addto of the dvdual spectra. The relatve proportos of the dvdual elemets a sample ca be stated based o the relatve testes of ther fluorescece les. Ufortuately, the physcal ad chemcal evromet of the elemets vestgated ca have a fluece o ths, referred to as the matrx effect. The surface qualty ad the geometrcal propertes of the sample must also be cosdered. d fally the measurg system also has a fluece o the measuremet. I order to obta a relable quattatve measuremet spte of ths, calbratos must be carred out the same strumet usg materals of a smlar matrx ad surface cosstecy. I ths expermet, the quattatve aalyss of the chemcal composto of a leaded brass sample wll be carred out. The compoets of ths alloy were detfed the LD Chemstry Leaflet C By comparso of the peak heghts of the sample wth those of the pure elemets, the composto s determed quattatvely. 1

2 LD Chemstry Leaflets Rsk assessmet The x-ray apparatus comples wth the regulatos regardg the desg of a school x-ray apparatus ad fully protected devce ad s type approved Germay (NW 897/97 Rö) as a school x-ray apparatus ad a fully protected devce. Owg to the bult- protecto ad screeg measures, the dose rate outsde of the x-ray apparatus s reduced to less tha 1 Sv/h, a value whch s the order of magtude of atural backgroud radato. Before puttg the x-ray apparatus to operato, spect t for damage. Protect the x-ray apparatus agast access by uauthorsed persos. Whe swtchg o the x-ray apparatus, check to make sure that the vetlator the tube chamber s turg. The goometer s moved exclusvely by electrc stepper motors. Do ot block the target arm ad sesor arm of the goometer ad do ot use force to move them. Equpmet ad chemcals 1 X-ray apparatus X-ray tube, Mo Goometer X-ray eergy detector Set of target alloys K-le fluorescece target set L-le fluorescece target set Pocket-CSSY 2 Bluetooth CSSY Lab MC box BNC cable, 1 m also requred: PC wth Wdows XP/7/8 Set-up ad preparato of the expermet The expermetal set-up s llustrated Fgure Feed the coecto cable of the power supply through the empty chael of the x-ray apparatus ad coect t to the m DIN socket o the x-ray eergy detector. 2. ttach the sesor holder assembled wth the x-ray eergy detector to the sesor arm of the goometer. 3. Coect the sgal output of the x-ray eergy detector to the BNC socket SIGNL IN of the x-ray apparatus usg the suppled th BNC cable. 4. Cotue to feed the cable so that t s suffcet to allow the sesor arm to rotate fully. 5. Press the SENSOR pushbutto ad adjust the sesor agle to 9 usg the DJUST rotato adjuster. 6. Mout the MC box oto the Pocket-CSSY ad coect t to the computer usg a USB cable. 7. Coect the output SIGNL OUT the coectos feld of the x-ray apparatus to the MC box usg a BNC cable. 8. Set the separato betwee the slt daphragm of the collmator ad the rotatoal axs ad betwee the rotatoal axs ad the etrace to the x-ray eergy detector to 5-6 cm each case. 9. Lay target 3 (leaded brass) oto the target table. Press the TRGET pushbutto ad adjust the target agle to 45 usg the DJUST rotato adjuster. Fg. 1: Expermetal set-up: a Collmator, b Target, c Target table, d Detector. Performg the expermet 1. Coect the power supply to the mas socket. The "gree" LED o the x-ray eergy detector wll lght up after about 2 mutes ad t s the ready for operato. 2. Load CSSY Lab settgs. 3. Set the tube hgh voltage to U = 35 kv ad the emsso curret to I = 1 m ad swtch o the hgh voltage supply. 4. Start recordg the spectrum by pressg or the butto o the Pocket-CSSY. 5. The record the spectra for the targets copper (Cu), zc (Z) ad lead (Pb) from the K-Le ad L-le fluorescece target sets. Results The recorded spectra are show Fgures 2a to 2d. Fg. 2a: Fluorescece spectrum of brass (target 3). 2

3 LD Chemstry Leaflets Fg. 2b: Fluorescece spectrum of copper (referece spectrum). Evaluato Eergy calbrato of the spectra The eergy calbrato wll be carred out based o the referece spectra of copper ad lead. 1. Ope "Eergy calbrato" the Eergy E settgs. Select "Global for all spectra o ths put" ad o the rght select the eergy values for the Cu-Kα le (8.4 kev) ad the Pb-Lα le (1.56 kev). 2. Mark the meda pot of the peak of the copper ad lead sgals. To do ths, select "Calculate peak meda pot" the cotext meu of the dagram ad mark the Cu-Kα le. The value s etered automatcally to the frst row. If ths does ot happe, eter the result maually to the eergy calbrato (e.g. by drag-ad-drop from the status bar). 3. Determe the meda pot of the Pb-Lα le a aalogous way ad also eter t o the left. 4. Dsplay the eergy o the x-axs. For ths, drag the butto E to the dagram usg drag-ad-drop. 5. Idetfy the les through the cotext meu of the dagram ad label them ( place mark X-ray eergy values Cu ad place mark X-ray eergy values Pb). Fg. 2c: Fluorescece spectrum of zc (referece spectrum). Fg. 3a: Fluorescece spectrum of the brass sample wth the les detfed of the alloy compoets. Idetfcato of les the spectra of brass The compoets of the brass sample ca ow be detfed (see Fg. 3a). 1. Select the spectrum ad establsh a sutable secto of the spectrum. To do ths, rght clck o the x-axs (Eergy E ) ad select a sutable secto. 2. I the cotext meu, load the perodc table to detfy the compoets of the alloy ( place mark x-ray eergy values). 3. over wth the mouse over the elemet symbols. Whe a elemet has bee "selected" ths way, ts relevat les appear the spectrum. Determe the approprate elemet by vsual comparso ad place the mark the dagram by clckg o ths elemet symbol. Determe the other compoets of the alloy the same way. 4. Close the perodc table wdow. Fg. 2d: Fluorescece spectrum of lead (referece spectrum). 3

4 LD Chemstry Leaflets It ca be see that the secod-hghest peak the spectrum cossts of two uresolved les. Z-Kα ad Cu-Kβ. The Cu- Kβ le partally overlaps the Z-Kα le. Determato of the mass proportos The umber of rradated atoms s proportoal to the area of a peak. s the peaks here are symmetrcal, the umber of rradated atoms s also proportoal to the heght of a peak. I the referece spectrum, ths umber s determed by the desty of the materal, ts atomc weght, the area rradated S ad the effectve thckess d of the rradated layer: S d. For the umber of atoms of each kd the alloy, the expresso V ca be used as a frst approach. Where ad are the heghts of the relevat peaks the spectrum to be aalysed or the referece spectrum, V = S d s the rradated volume. Thus the mass proporto C of elemet umber the alloy becomes C. (I) I order to calculate the mass proporto accordace wth equato I, the heghts of the strogest les the fluorescece spectrum of brass () ad the referece spectrum ( ) are compared wth oe aother. These les are: K of copper, K of zc ad L of lead. Determato of the peak heghts the fluorescece spectrum of the brass sample ad decovoluto of the overlappg les. To determe the heghts of the Cu-K ad the Z-K le, the fluorescece spectrum of brass s decovoluted the eergy rage 6 kev to 11 kev (see Fg. 3b). The spectrum s adjusted ths rage usg three Gauss curves of the same wdth at the kow eergy values of the Cu-K le (E = 8.4 kev), the Cu-K le (8.91 kev) ad the Z-K le (8.64 kev). 1. Delete the Pb LI le. Ths causes oly a small error, but allows a clearer evaluato. To do ths, double-clck o the le to take hold of t ad pull t out of the dagram. 2. I the cotext meu of the dagram, select the meu tem "Further evaluatos" "Gauss curves wth predefed eergy". 3. Mark measuremet pots the eergy rage of about 7.5 kev to 9.1 kev. The result s three Gauss curves (red) ad a adjusted cotour of the fluorescece spectrum (see Fg. 3b). 4. The peak heghts () of the three peaks are ow the status bar. Usg drag-ad-drop, drag the Kα peak heghts of copper ad zc from the status bar to the prepared table (o tab "Mass proporto") (see also colum Table 1). The Cu-Kβ peak s automatcally subtracted from the Z-Kα peak ths evaluato. To determe the heght of the L le of lead the fluorescece spectrum of the brass sample, t s adjusted usg a Gauss curve: Fg. 3b: Result of the adjustmet of the fluorescece spectrum of brass usg three Gauss curves wth predefed eergy (red les). The adjusted cotour s also show red. 5. I the cotext meu of the dagram, select the meu tem "Further evaluatos" "Gauss curves wth predefed eergy". 6. Mark the Pb-L le (eergy rage of about 1 kev to 11,5 kev). 7. Usg drag-ad-drop, drag the Lα peak heght of lead from the status bar to the prepared table (o tab "Mass proporto") (see also colum Table 1). Tab. 1: Determato of the mass proporto C of the alloy compoets of the brass sample. ρ g/cm 3 Le ρ / Copper 8.96 K Zc 7.1 K Lead L Determato of the heghts of the les the referece spectra 1. Select the referece spectra. 2. I the cotext meu of the dagram, select the meu tem "Further evaluatos" "djust Gauss curves (lt + G)". 3. Mark the Cu-K le the referece spectrum. 4. Usg drag-ad-drop, drag the Kα peak heght of copper from the status bar to the prepared table (o tab "Mass proporto") (see also colum Table 1). 5. The determe the heghts of the Z-K le ad the Pb- L le ther referece spectra ad drag the values to the prepared table (o tab "Mass proporto") (see also colum Table 1). Determato of the mass proportos The mass proportos of the alloy compoets are determed accordace wth equato (I) ad automatcally calculated CSSY Lab the prepared table (o tab "Mass proporto"). The values for the peak heghts the fluores- C % 4

5 LD Chemstry Leaflets cece spectrum of brass () ad the referece spectra ( ) are etered to Table 1. Results The mass proportos of the alloy compoets of the brass sample as determed agree well wth the kow chemcal composto (CuZ 39Pb 3). Tab. 2: Comparso of the expermetal values wth the stated values for the mass proportos of the alloy compoets. Elemet stated expermetal Copper 58% 62.4% Zc 39% 34.9% Lead 3% 2.7% ddtoal formato The example of the copper-zc alloys (brass) demostrates how secodary fluorescece ca alter the shape of the spectrum. Whe such a sample s rradated wth x-ray photos, exctato of the K les of both copper ad zc occurs. owever, as the K le of zc les over the edge of the K le of copper (E = 9.57 kev ad 8.99 kev, respectvely), t ca also cause exctato "secodarly" of the K les of copper. For ths reaso, the testy of the copper les the fluorescece emsso from the sample s hgher, at the expese of the Z-K le, ad the rato of the testes of the Z-K ad Z-K les s ot the same as the rato the sample made of pure zc. Ths s why the mass proporto determed from the K les of the alloy compoets shows a somewhat hgher proporto of copper. by LD DIDCTIC Gmb Leyboldstr. 1 D-5354 ürth Telefo: Fax: E-Mal: fo@ld-ddactc.de Techcal alteratos reserved