LASER ABLATION ICP-MS: DATA REDUCTION

Transcription

1 Lee, C-T A Lase Ablaton Data educton 2006 LASE ABLATON CP-MS: DATA EDUCTON Cn-Ty A. Lee 24 Septembe 2006 Analyss and calculaton of concentatons Lase ablaton analyses ae done n tme-esolved mode. A ~30 s gound o blank sgnal s taken whle the lase s off. Ths s followed mmedately by tunng on the lase and ablatng the sample, geneatng a tme-dependent sgnal (Fg. ). Measuements ae made fo anothe 3-4 mnutes unless the lase ablates entely though the sample o the sgnal dops sgnfcantly due to beam defocusng wth pogessve ablaton. efeence glass standads (BHVO2g, Bg, BC2g, NT60 and NT62) ae un befoe and afte a sequence of samples. AW DATA.E+09.E+08.E+07.E+06.E+05 cps.e+04.e+03.e+02.e+0.e Slce Numbe Fgue. Typcal tme-esolved sgnal fo an analyss of a glass efeence standad (BHVO2g). Ablaton of the sample begns at slce #4. Homogenety of the glass s evealed by the paallel natue of the tme-esolved sgnals. aw data fo each analyss ae each examned offlne n ode to pck out segments of tme coespondng to when the lase was ablatng though a homogeneous vetcal secton of a sample. The cteon used to dentfy a homogeneous secton s that the tme-esolved sgnals of all the elements ae paallel (Fgs., 2a). Ths s equvalent to sayng that the atos of all elements analyzed ae constant wth tme (Fg. 3). Cosscuttng tme-esolved elemental cuves can ndcate a tanston nto a new mateal (such as ablatng though the sample and nto the undelyng glass holde). f coss-cuttng occus at the onset of ablaton, ths may be due to suface contamnaton. n ths case, the analyst should avod the mmedate sgnal afte ablaton stats and focus on the sectons

2 Lee, C-T A Lase Ablaton Data educton 2006 havng hghe qualty data. Once appopate tme-esolved sectons ae chosen fo the gound and the sample (o efeence standad), the aveage gound sgnal ntensty s subtacted fom the sgnal ntenstes of the sample. AW DATA - 43-BC - CP.E+09.E+08 gound sample undelyng glass plate penetated lase tuned off.e+07.e+06.e+05 cps.e+04.e+03.e+02.e+0.e Slce Numbe Fgue 2. A tme-esolved sgnal fo a sample (clnopyoxene). A wndow s selected fo the gound and fo the sample. These wndows ae selected so as to coespond wth neapaallel tme-esolved sgnals. Note that at aound slce # 30, the sgnals stat to change n ntensty and coss each othe. Ths s due to a tanston nto the undelyng glass plate afte the lase ablated though the ente thn secton. BC (cpx) ato of element sgnal to sgnal of 24Mg ato slce numbe Fgue 3. me data as n Fgue 2. Plotted ae the atos of vaous elements elatve to a nomalzng sotope ( 24 Mg), whch s taken hee to be the ntenal standad. ntenstes of the sample sgnals have all been gound coected. The slces taken coespond to the slce wndow n Fgue 2. Note that the atos ae elatvely constant (even f the absolute sgnal ntenstes ae vayng). 2

3 Lee, C-T A Lase Ablaton Data educton 2006 Concentatons ae detemned as follows. Backgound-coected ntenstes fo a gven element ae all nomalzed to an ntenal efeence sotope, e.g., an ntenal standad. The ntenal standad s used to coect fo vaatons n absolute sgnal ntenstes due to matx effects and dffeent ablaton paametes (e.g., pt damete) but not to vaatons n actual concentaton n the sample. Ths means that the concentaton of the ntenal standad must be known n all efeence standads and sample unknowns. We typcally take Mg ( 24 Mg o 25 Mg) o Ca ( 43 Ca) as ntenal standads. The weght concentaton C of an element n an unknown sample s then gven by C C = C Eq. C whee C s the concentaton of the ntenal standad element n the sample, s the ato of gound-coected sgnal (cps) ntenstes of the element to the ntenal C standad element n the sample, s the ato of the concentatons of the element to C the concentaton of the ntenal standad n the efeence standad, and s the ato of the gound-coected sgnal (cps) ntenstes of the ntenal standad element to the element n the efeence standad. The fom of Eq. allows us to deal wth tme-vayng sgnal ntenstes. n many cases, the ablaton sgnal slowly decays wth tme (occasonally even nceasng) and ths tme-dependency dffes fom sample to sample. The measued quantty n Eq. s. Fo example, f 24 Mg was ou ntenal standad and we wee nteested n L concentatons, we would wok wth the ntensty ato of 7 L to 24 Mg athe than the absolute sgnal ntensty of 7 L. One daw of Eq. s that t only allows calbaton aganst one extenal efeence standad. Ths s the tadtonal appoach. Howeve, one daw of ths smplfed appoach would occu when the concentaton of an element n a sample unknown s much lage o smalle than that n the efeence standad. deally, one would lke to have a efeence standad whose concentatons ae smla to that n a sample unknown because measuement uncetantes can popagate nto vey lage uncetantes f extapolated to concentatons too hgh o too low. When one does not know the concentaton of an element n a sample, t s not always possble to match a sample to a efeence standad. One way to mpove on Eq. s to use multple extenal efeence standads n the hopes that the sample concentaton mght be eted by two o moe efeence standads. To do ths, seveal efeence standads ae analyzed. The C quantty s plotted vesus and a lne (foced though the ogn) s egessed C though the scatte plot. The qualty of the efeence calbaton can then be easly 3

4 Lee, C-T A Lase Ablaton Data educton 2006 assessed vsually fom the gaph o by examnng the egesson statstcs. The slope of ths lne s gven by C / C C m = = Eq. 2 / C and t follows that the concentaton of the element n the sample can be had fom substtuton of Eq. 2 nto Eq. : C = C m Eq. 3 Extenal calbaton (Mn) y = x 2 = (/s) (C/Cs) Fgue 4. An example of an extenal calbaton usng USGS glass standads, BHVO2g, Bg, and BC2g. Lmt of Detecton The lmt of detecton (LOD) epesents the mnmum sgnal that can be esolved fom the gound sgnal. The defnton taken hee fo the LOD s thee tmes the standad devaton of the gound sgnal 3 σ (cps). Whle the standad devaton of the gound sgnal s unlkely to change sgnfcantly thoughout most of the day, the actual LOD expessed n concentaton wll change f one changes the damete of the ablaton pt. To convet ths to concentaton, the quantty 3 σ nomalzed to the senstvty of the nstument, whch s expessed n sgnal pe concentaton unt (e.g, cps/ppm). The senstvty of the nstument s unque to each ablaton analyss. Thus, the followng pocedue must be adopted to estmate LOD n concentaton untes. Fst, the quantty 3 σ s dvded by the tme-aveaged sgnal ntensty of the ntenal standad n the sample that was analyzed wth the gound, that s, 3 σ /, whee the ba ove the s used to dstngush the tme-aveage sgnal fom each ndvdual sgnal. The LOD fo a gven element fo a gven sample analyss can then be expessed as 4

5 Lee, C-T A Lase Ablaton Data educton σ C LOD = C C Eq. 4 Concentaton detemnaton n the absence of a known ntenal standad Unde some ccumstances, an ntenal standad wth known concentaton may not be avalable. Howeve, the composton of an unknown can be stll detemned. Ths can be done by analyzng nealy all (>98%) of the majo (> wt. %) and mno (~0.2- wt. %) catons n an unknown. Fo most mneals, ths typcally eques measung S, T, Al, Fe, Mg, Mn, Ca, Na, K, P and C. Many of these sotopes can only be detemned n medum to hgh mass esolutons due to vaous sobac molecula ntefeences (fo example, 28 S and 30 S ae loaded wth ntefeences n low mass esoluton mode). Ths means that ths appoach can only be done wth secto feld CP-MS s wth hgh mass esoluton capabltes (e.g., the ThemoFnngan Element 2). Quadupole nstuments cannot esolve these sotopes fom ntefeences (although hexapole collson cell technology may help to elmnate some of the ntefeences). Denotng the caton weght facton n the sample as must hold: N =, the followng condton = Eq. 5 Dvdng Eq. 5 by a efeence caton (fo example by an sotope of Mg) and eaangng gves the followng expesson fo = Eq. 6 N + ( ) /, whee the summaton s ove all catons except that of the efeence caton. Once s known, the emanng caton factons ae detemned by multplyng / by. The quantty / s tself detemned fom the followng equaton C = Eq. 7 C whch has an dentcal fom to Eq. except that the ntenal standad subscpts ( ) have been eplaced by the efeence mass subscpt ( ). Once caton factons ae known, the caton factons can be conveted nto oxde factons and then e-nomalzed to 00%. n the fgue below, we show that fo volatlepoo mateals (such as a dy basalt), ths appoach s vey obust. Once the majo element composton of a mateal s detemned, any one of these elements can be chosen as an ntenal standad fo the subsequent detemnaton of tace elements n low mass esoluton mode. Uncetantes of couse ase f the valence state of Fe s not known vey well (e.g., FeO o Fe 2 O 3 ) and f thee ae othe volatles besdes oxygen that ae not accounted fo. We emphasze that ths appoach s only obust when all majo and mno catons ae detemned. 5

6 Lee, C-T A Lase Ablaton Data educton 2006 Concentatons (wt. %) fo B fom LA-CP-MS vesus Accepted Values (wthout use of ntenal standad) 00 0 LA-CPMS Acccepted Values Fgue 5. Oxde concentatons of majo and mno elements n Bg glass standad detemned by lase ablaton CP-MS (medum esoluton mode) wthout an ntenal nomalzaton standad vesus accepted values. 6