Isotope liquid nitrogen trap

Mass Spectrometric Analysis Combusted sample tubes were attached to a purification vacuum line connected to the inlet system of the mass spectrometer. Sample tubes were opened under vacuum using a tube-cracker (35), and the gases were passed through a dry ice trap to remove water vapor and a liquid nitrogen trap to collect CO 2. Noncondensible gases were pumped away. The purified CO2 was thawed and admitted into the inlet system of the mass spectrometer for determination of isotopic composition ... 

The contents of the liquid-nitrogen traps (SiD and traces of ether) are combined into one trap, and the material in the —95° trap (mainly diethyl ether) is discarded. The product is freed of last traces of ether by passing five times through a —130° trap into a —196° trap. About 0.013 mole of SiD4 (87% yield based on LiAlD4), having an isotopic purity of >96%, is obtained. 

Demeny and Haszpra have demonstrated a procedure to elfectively measure CO2 concentration plus its stable isotope compositions. Air samples are pumped through silica gel and Mg(C104)2 traps and 3 1 glass bottles equipped with two Teflon stopcocks. Moisture can thus be removed from the collected air. In spite of its small amounts in the air samples, N2O must be separated from CO2 because they have very similar nth values. This effect is important especially when an IRMS is employed as the detector. A Chromosorb 101 column is used and shows acceptable separation characteristics. Another alternative to separate CO2 from N2O is to pump the sample by helium flow through a liquid nitrogen trap. Under the experimental conditions, CO2 can be detected at about 4.5 min and N2O at 6 min. The acmal volume of CO2 in the 3 1 air sample at 1 atmospheric pressure is around 2 to 3 ml. 

Carbon-13. The increasing availability of C-labelled substrates has provided a marked impetus in the use of these tracers in clinical studies. Most recent interest centres round the use of C02 breath tests for the in vivo estimation of the rate of oxidation of specific C-labelled substrates. Some clinical implications of these tests will be discussed later. Mention should be made of the available methods for respiratory COj collection. They include the collection of whole breath in evacuated glass tubes prior to CO2 separation [92], the direct cryogenic trapping of exhaled CO2 in liquid nitrogen [93] and the precipitation of CO2 as carbonate in sodium hydroxide [94]. The latter method is suspect in that it results in isotopic fractionation which may not be reproducible. 

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