Isotope ratio mass spectrometry fractionation effects

Carbon isotope fractionation effects of individual compounds were observed in living organisms and also as a result of enzymatic isotope effects and reaction kinetics in biological systems. Such fractionation effects have to be examined by isotope ratio mass spectrometry in order to understand specific processes in life sciences or in environment.75... 

Mass spectrometric analysis of isotope ratios is in principle very accurate (isotope ratio mass spectrometers were developed for geochemical investigations), but the analytical problem comes from possible fractionation of isotopes in the workup procedure. Traditionally, isotope ratio mass spectrometry measurements of kinetic isotope effects have been confined to reactions which give a small molecule product (such as CO2), which can be directly introduced to the mass spectrometer. 

Ice samples are put into pre-cooled extraction vessels for evacuation. The ice is then melted and a gas extraction and purification similar to that used for water samples is performed. Because dynamic analysis requires far more sample than the static mode, ultrapure N2 is added to increase bulk pressure by a factor of 10. The mass spectrometric analysis follows conventional procedures of dynamic isotope ratio mass spectrometry, with modifications designed to avoid any fractionating effects, such as thermal diffusion... 

Jackson SE, Gunther D (2003) The nature and sources of laser induced isotopic fractionation in laser ablation-multicollector-inductively coupled plasma-mass spectrometry. J Anal At Spectrom 18 205-212 Jiang S-J, Houk RS, Stevens MA (1988) Alleviation of overlap interferences for determination of potassium isotope ratios by Inductively-Coupled Plasma Mass Spectrometry. Anal Chem 60 1217-1220 Lam JWH, Horlick G (1990) A comparison of argon and mixed gas plasmas for inductively coupled plasma-mass spectrometry. Spectrochim Acta Part B 45 1313-1325 Langmuir I, Kingdon KH(1925) Thermionic effects caused by vapours of alkali metals. Phil Trans R Soc A107 61-79... 

The ratio of a sample is measured in relation to a standard to improve the accuracy and precision of accelerator mass spectrometry measurements (Elmore and Phillips, 1987). Multiplying the ratio by 1000 results in the delta (del) values having units of parts per thousand, also know as per mil (%o). For standards, it is necessary to use wood from trees harvested before about 1850 pre-industrial, to avoid the Suess effects. The standard value for pre-industrialized atmospheric CO2 is 13.56 dpm g-1 or 14C/C equals 1.176 x 10-12 (Broecker and Peng, 1982). A correction term involving the effects of isotopic fractionation (IF) are also subtracted out of this equation. Isotopes are fractionated due to physical and chemical reactions (more details in the following section), thereby making the abundance of carbon isotopes (12C, 13C, and 14C) different in plants (Faure, 1986). The National Bureau of Standards currently provides an oxalic acid 14C standard that is used for this correction however, there have been many problems associated with development of this standard (Craig, 1954, 1961 Stuiver and Polach, 1977). 

Isotope ratios measured by mass spectrometry have to be corrected for mass discrimination and mass fractionation effects by the K-factors. In the case of equation (4.18) this correction is not necessary when the four isotope ratios are measured under the same conditions, because the correction K-factors are then cancelled out. This approach is only valid as long as there are no... 

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