Dual inlet system, IRMS

The purpose of this paper was to briefly describe fundamentals of isotope ratio mass spectrometry (IRMS), review the analytical systems currently available both for traditional dual-inlet (DI-IRMS) and the newer continuous-flow (CF-IRMS) and describe the specialized instruments that are in general use for isotopic measurements. 

As for all trace-level analyses, sample preparation and handling are of crucial importance. In addition to all the usual problems of GC-MS, measurements of isotope ratios must ensure that none of these steps introduce any isotope discrimination. Any chemical reactions, including conversion of the organic sample molecules to the simple gases which are those actually analyzed, must be quantitative (100% conversion) to avoid kinetic isotope effects [627]. Until relatively recently, all gas IRMS experiments employed a dual-inlet system to permit switching between sample and standard C02 contained in two bellows containers. The pressures in the two bellows are adjusted to be equal and,... 

Another important feature of IRMS is the provision of a dual inlet system which permits the operator to alternate the gas flowing into the ionization chamber between... 

Small variations in apparent isotope ratio are observed when inlet pressure changes. Therefore IRMS dual inlet systems are equipped with bellows mechanisms (Fig. 7.9) so that inlet pressures of both sample and standard can be adjusted to be the same. Furthermore, small systematic changes, if any, in the measured isotope ratio as the measurement progresses are compensated by appropriate interpolation. Both these corrections are handled automatically by the software which controls the modern IRMS. 

Fig. 7.9 Dual inlet system for IRMS (Ghosh, P. and Brand, W. A. bit. J. Mass Spectrom. 228, 1 (2003))...

Isotope ratios are given as deviations, in relation to a defined primary standard (zero point). The polyethylene foils CH 7 and NBS 22-oil are commercially available secondary standards, certificated and managed by the International Atomic Energy Agency. However, GC-lRMS systems cannot be calibrated without the aid of alternative peripheries like an elemental analyser (EA) or a dual inlet, owing to the lack of commonly accepted reference materials applicable in GC-IRMS techniques (Fig. 17.11). 

Isotope ratio mass spectrometers (IRMS) are specialized mass spectrometers that produce precise and accurate measurements of variations in the natural isotopic abundance of hght stable isotopes. The breakthrough in classical IRMS was the introduction of the dual inlet mass spectrometer by Urey in 1948 [26]. Since this time, the instrument has been considerably developed and automated, leading to the systems commercially available today. 

---