PKULecture2analyticalmethods—穩(wěn)定同位素分餾資料文檔

1、Chapter II: Analytical approachesThe machines 1. Mass spectrometerGeneral coniguraticn forisoiopsHiding mdss spectiumeiermagnet io defiedIons according to rr)feFara 劃 cups fordelict Irg 13ns內(nèi)小哂、4,SLishasgas enters arc 世 by edltc用 tris are trenfTaccelerated using aaerirss of charged plates1) Gas sour

2、ce isotope ratio mass spectrometer (IRMS) What are we measuring- Gases! Charged gases!O2 as example for 18O measurementN2 for 15NCOSO2SF6CO2 as an example for 13C measurementMass 44, 45, and 46.Ion (isobaric) corrections:Relations between measured and desired isotope ratiosFor 15N measured using N2:

3、2915N14N14N1N15N28對N許下2%第1頁，共5頁Here the raw machine 29R is the ratio of the count, i.e. voltage or current, on cup 29 over that on cup 28. It is not the real isotope molar ratio of 15/14! Not even 2 time 15/14! This is because 29R is also affected by ionization efficiency of different isotopes, curr

4、ent, source voltage, resistance,However, when you 你彳門 pare thismachine raw R to that of the reference raw R, you get very accurate6 value.For O2, we have 34/32 as the 6 18O. (1618+18)/6+16. There is only a small and negligible isobaric interference from 1717.For CO2 你們 es the concept of "Ion Co

5、rrection"is important, but still it' s about 0.03 %ocorrection for 13C. 644: 16.12.1645: 16.13.16, 17.12.16, 16.12.1746: 17.13.16, 16.13.17, 17.12.17, 18.12.16, 16.12.18i) Dual-inlet mode: Dual-inlet,你們 pare reference and sample gas so that many factors cancel. Relative ratio difference is

6、much more accurately measured than for the absolute ratio.ii) Continuous flow mode: Good for small sample sizes.iii) ) PeripheralsTCEA (explained), EA, Gas bench, Kiel device,Standard, reference materials, and gas preparationRecall that the relative abundance ratio R/Rstd, or the difference of R bet

7、ween two substances can be measured to a much higher accuracy. This is the reason that why we have to have a reference material for individual element.Table 1.7 Worldwide standards in use for the i?so(opk vomposhlon of hydrogen, boron, carbon, niirogen. oxygen, silicium. sulfur, und chlorineEleinent

8、StandardStandardHStandard Menn Ocean WaterV-SMOWBBoric acid (NBS)SRM 9s ICBeknmitellu iiniericana Irojn ihc (.Pee deeV-PDBformEiti<)n. South CandinuNAir niimgenN? (atm.)OSiandard Meun Ocean aterV-SMOWSiQuariz sandNHSCKSTroiliic (FeS) from the Canyoii Diablo iron incieoriieV-CDTClSeagal er clilori

9、tkSMOC(From Hoefs 2021)第2頁，共5頁Issue with multiple standards or references:次-c = A-b + B-c + A-b X b拉Oxygen isotope 你們 position: two international standard VSMOW and VPDB, mention their relationship:18Ovsmow = 1.03091 18Ovpdb + 30.911) 1000 %。A C A C A B C BNote that if using the 6 ' notation, yo

10、u will haveSa-c =注時You have a tank of CO gas as your laboratory standard, a NBS127 (BaSOor your laboratory LSU-BaSO4 as your in-house reference, bw do you report your sample 18O' s 6 value in VSMOW?What are measured?8SM-co, NBs-co;What is known? nbS-vsmowNote that落m-vsmow = Sm-co + Co-vsmow + Sm

11、-co x cO-vsmow ；(1)and(Nbs-vsmow = Nbs-co + Co-vsmow + Nbs-co X cO-vsmow ; (2)Substract (2) frm (1), we get冷m-vsmow = Nbs-vsmow + (1+ 七o-vsmow)(冷m-co - Nbs-co);That means as long as theco-vsMoW is not too far away from the origin, the term(1+ (Co- vsmow) will be very close to 1, and adding or subtra

12、ctingthe ? (i. e "sM-co -(Nbs-co") will be a good approximation.第3頁，共5頁Llenwfitat Anarywr * open 郅"FUHO MSThree basic你們 ponents: ion source, analyzer (magnet), and collector (detector)2) . Solid-source IRMS: Thermal Ionization (TIMS) Good for elements that do not readily have a gas ph

13、ase. Cu, Fe, Se, Mo, Sr, Li, Ca 3) . Solid or liquid: Multi-collector Inductively coupled plasma MS (MC-ICP-MS)4) . Solid: Secondary Ion MS (SIMS) Cameca 1280SHRIMP (Sensitive high-resolution Ion micro-probe)“ Made the greatest contribution to stable isotope studies”2. Laser Absorption Spectroscopy-

14、 A totally new approach (Currently works for CO2 and H2O in gas phase)1) Cavity Ring-Down Spectroscopy (CRDS)“is a highly sensitive optical spectroscopic technique that enables measurement of absolute optical extinction by samples that scatter and absorb light. It has been widely used to study gaseo

15、us samples which absorb light at specific wavelengths, and in turn to determine concentrations down to the parts per trillion level. The technique is also known as cavity ring-down laser absorption spectroscopy (CRLAS).A typical CRDS setup consists of a laser that is used to 川uminate a high-finesse

16、optical cavity, which in its simplest form consists of two highly reflective mirrors. When the laser is in resonance with a cavity mode, intensity builds up in the cavity due to constructive interference. The laser is then turned off in order to allow the measurement of the exponentially decaying li

17、ght intensity leaking from the cavity. During this decay, light is reflected back and forth thousands of times between the mirrors giving an effective path length for the extinction on the order of a few kilometers.If something that absorbs light is placed in the cavity, the amount of light decrease

18、s faster it makes fewer bounces before it is all gone. A CRDS setup measures how long it第4頁，共5頁takes for the light to decay to 1/e of its initial intensity, and this "ringdown time" can be used to calculate the concentration of the absorbing substance in the gas mixture in the cavity." -from WikipediaMore on gas-source isotope ratio mass spectrometerI ship out a shell (CaCO), a week later, I get the measured 13C and 18O values for that shell. Please