Dual inlet

Assumes the pump features dual inlet connections and that an inlet manifold will be used. 

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. 

Figure 10. The changeover values used by dual-inlet IRMS...

There are now a number of options open to scientists who need to analyse stable isotopes as part of their research. The choice is dependent on many factors, such as the type of sample, the precision of measurements required, the amount of sample available, and the number of samples to be analysed. Dual inlet IRMS is still the technique of choice those scientists who need to measure a few samples to very great precision. More often, scientists want to determine a trend in conditions over time, or a flux integrated over an area. In these cases, CF-IRMS will be preferred. Continuous-flow IRMS is also preferred when a limited amount of sample material is available, as it is able to analyze far smaller samples (between 10 and several hundred times smaller) than DI-IRMS. 

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))...

Between the early 1950s, when the dual viscous-flow mass spectrometer was introduced by Nier and the mid 1980s only minor modifications have been made on the hardware of commercial mass spectrometers. Special efforts have been undertaken in the past years to reduce the sample size for isotope measurements. This has led to a modification of the classic dual inlet technique to the continuous-flow isotope ratio monitoring mass spectrometer in which the gas to be analyzed is a trace gas in a stream of carrier gas, which achieves viscous-flow conditions. Today, the majority of gas mass spectrometers are sold with the continuous flow system, instead of the dual inlet system. 

Absolute sensitivity is much more important than with the dual inlet system. Since sample sizes required for chromatography are significantly smaller, it is often important to use a significantly large set of samples in order to obtain a statistically sound data base. 

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). 

Figure 4 Schematic diagram of high-pressure liquid chromatography inductively coupled plasma mass spectrometry (HPLC-ICP-MS) system with dual-inlet system for isotope dilution analyses. (From Ref. 32.)...

For example, little is known about the isotopic composition of formaldehyde in the atmosphere. Formaldehyde is a chemical intermediate in hydrocarbon oxidation. The carbon (8 C) and hydrogen (8D) isotopic composition of atmospheric formaldehyde is analyzed using continuous flow gas chromatography isotope ratio mass spectrometry." Isotope ratios were measured using GC-IRMS (Finnigan MAT 253 stable isotope ratio mass spectrometer, single-sector field with electron impact ion source and multiple ion collection) with a precision of 1.1 and 50%(lo ) for 8 C and 8D, respectively. The accuracy of the online continuous flow isotope technique was verified by calibrating three aliquots of the gas phase standard via the offline dual inlet IRMS technique. The concentration of formaldehyde in ambient air was determined on IRMS major ion peak areas (i.e., mass 44 for 8 C and mass 2 for 8D)." ... 

AE measurements were performed on a gas isotope-ratio mass spectrometer (IRMS) (NucEde). This is a speciaEzed type of magnetic sector mass spectometer designed speciflcaEy to measure isotope raUos of pure gas samples. In the case of COp, different collectors allow the precise determination of the ratio of m/e 45 to m/e 44. A dual inlet allows the ratio of the sample gas to be expressed in relation to the ratio of m/e 45 to m/e 44 in a standard gas. 

Isotope ratios were measured on a Micromass 602E, a dual inlet, double collector mass spectrometer The mass 5 to mass ratio of CO2 from the sample material was compared with that of a standard gas of known isotopic composition. Results are expressed as ... 

A 13-mm holder for Nucleopore filters equipped with a dual inlet valve (Nucleopore Co., Cambridge, MA) nucleopore filters, 13 mm, 10 /xm pore size fraction collector syringes (1 and 50 ml) ... 

A dual-iidet ESI source, the LockSpray , was introduced by Waters, based on the rotating aperture of the MUX-source, for the co-introduction of a reference compoimd to act as a lock-mass for accurate-mass determination [65-66]. Dual-inlet devices to introduce a lock-mass compound have also been reported for sector [67] and Fourier-transform ion-cyclotron resonance MS (FT-ICR-MS) instruments [68]. 

Fig. 6.12 Sample pretreatment and conversion of organic compounds into gases for isotope ratio mass spectrometry (IRMS). The reductive pyrolysis is normally performed on glassy carbon (for a summary see e.g. [ 1291). In case of coupled HPLC-IRMS for determination wet oxidation is used [ 122f In the routine isotope ratio analysis of water often isotope equilibration with gases are used for 0-analysis CO2 [ 130], for S H-analysis H2 gas in the presence of Pt [131]. Recently an on-line method for S O and in water has been described showing dual-inlet like performance [ 132]. Low-temperature pyrolysis in connection with GC is used for compound fragmentation with the aim of partial isotope pattern analysis [133]. TC-EA = thermo conversion elemental analyser...

The reference principle for on-line methods has to be different as compared to classic dual-inlet isotope ratio analysis, as it is not possible to introduce sample and reference gas in exactly the same maimer into the mass spectrometer. Reference gas pulses have to be set at different points during one mn, but also laboratory standards with knovm 8-values have to be analysed periodically and exactly in the same manner and under the same conditions as the samples under investigation (identical treatment principle ]13]). A general problem of on-line HPLC and GC methods in isotope ratio analysis is the lack of international reference materials suitable to fulfil the above mentioned requirements. This implies the necessity to establish suitable laboratory standards by EA (elemental analysis) or classic measurements. Meanwhile, also efforts in supplying suitable international organic standards have been overtaken by the IAEA (e.g. caffeine, glutamic acid ]176, 177]). This programme (benzoic acid reference materials with different 8 0-values) will also support the on-line EA and GC 8 0-meas-urement by reductive pyrolysis (carbon reduction) methods (standardisation problems are compiled in ]178]). 

Bolanos, Hochgeschurtz, and Thies, 1991). Either a pure component or a solution mixture can be delivered to the system using dual inlet pumps shown in the diagram. 

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