CF-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. 

The principles of CF-IRMS is that sample chemistry and gas purification take place in a He carrier to produce pulses of sample-derived N2, C02, N20, or S02 gases which flow directly into the ion source (Fig. 11). 

CF-IRMS provides reliable data on micromoles or even nanomoles of sample without the need for cryogenic concentration because more of the sample enters the ion source than in DI-IRMS. CF-IRMS instruments accept solid, liquid, or gaseous samples such as leaves, soil, algae, or soil gas, and process 100-125 samples per day. Automated sample preparation and analysis takes 3-10 min per sample. The performance of CF-IRMS systems is largely determined by the sample preparation technology. A variety of inlet and preparation systems is available, including GC combustion (GC/C), elemental analyzer, trace gas pre-concentrator and other. The novel... 

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. 

Final points to consider include the degree of replication and treatment of the initial, or time zero, concentrations and enrichment of both the product and source pools. The ability to replicate analyses has been greatly enhanced by the advent of automated CF-IRMS so that there is no longer a reason not to measure duplicate samples as a minimum. However, given the high analytical precision of modem mass spectrometers, the replication should be from the field by incubating replicate bottles... 

A recent improvement builds on the fluorometric analysis of ammonium concentration (Holmes et al., 1999) with OPA (ortho-phthalaldehyde). Johnston et al. (2003) extracted the OPA-NH4+ derivative (1 -sulfonato-iso-indole) by soHd phase extraction and found that derivatized amino acids were not retained along with the indole on the column so that the method was specific. Since they developed the method for natural abundance work, the requisite high precision obhged analysis by CF-IRMS and its attendant high mass requirement (7.14 pmol N for a standard deviation of 0.5%o) that is difficult to achieve in oligotrophic waters. Since tracer experiments do not require such high precision, a lower mass could likely be used with the inherently lower precision GC—MS. 

Arabian Sea. After extracting the gases by equihbration, they utilized cryofocusing of N2O with separation from CO2 on a capillary column before isotopic analysis by CF-IRMS of the N2O. They found the vast bulk of N2O was produced from N02 reduction, with htde contribution from nitrification at any depth within the suboxic waters. Their results also suggested an alternate source of nitrogen for dentrification, such as DON or DNRA via internal couphng to N2 production. 

The use of carbon isotopes to study DOC is becoming more prevalent due to technological advances in mass spectrometry. DOC generally occurs in natural waters in low concentrations, typically ranging between 0.5 ppm and 10 ppm carbon (Thurman, 1985 see Chapter 5.10). Thus, several liters to tens of liters of water were once necessary to extract enough DOC for conventional dual gas-inlet isotopic analysis. Today, automated total organic carbon analyzers (TOCs) are commercially available, and have been successfully interfaced with continuous flow isotope ratio mass spectrometers (CF-IRMS) for stable isotopic measurements of samples containing ppb concentrations of DOC (e.g., St-Jean, 2003). 

The CF-IRMS analytical method for sulphur is based on rapid oxidation of the samples by flash combustion at 1800°C. The released gases pass through a catalytic-oxidation-reduction column reactor from this they are chromatographically separated in a 0.8 m packed FIFE column, and then finally analysed in the mass spectrometer. The analytical cycle time is c. 430 s. The size of the samples analysed is optimized when possible to avoid potential linearity effects. One standard is analysed after every five samples, and a blank is run routinely after 10 analyses as a check. 

Grassineau, N. V., Mattey, D. P. Lowry, D. 2001a. Rapid sulphur isotopic analyses of sulphide and sulphate minerals by continuous flow-isotope ratio mass spectrometry (CF-IRMS) Analytical Chemistry, 1212), 220—225. 

In order to understand how H2O and CO2 are exchanged between leaf tissues and the air, measurement of the photosynthetic isotope fractionation by open flow gas analysis is conducted in a gas exchange chamber under controlled conditions of temperature, light intensity and humidity, and CF-IRMS and GC are employed. " When the same method is applied to ecosystem isotope fractionation in a field experiment, the variation in atmospheric CO2 concentrations and S values primarily reflect variations in net photosynthesis (daytime) and respiration. The S and CO2 data also reflect these processes, but with added complications because of oxygen isotope exchanges with soil and leaf water pools. 

Soil water samples taken from a forest are analyzed for their 5 0 values by GC-CF-IRMS to compare with those obtained from tree water on the same site. " The tree water 6 0 values differ from that of soil water near the surface, which imphes that tall trees (60 m) are likely to extract water from below the surface soil layers. In contrast, the 2 m tall shrubs have stem water 6 0 values that indicate the water source to be closer to the surface soil layers. 

Carbon isotope analysis on lake sediment cellulose may be performed by routine breakseal combustion (Boutton et al, 1983 Boutton, 1991a) or by continuous flow — isotope ratio mass spectrometry (CF-IRMS). Comparison of lake sediment samples analyzed by both methods at the UW-EIL show excellent agreement. 

CF-IRMS Continuous flow-isotope ratio mass spectrometry. A procedure that uses an instrument that is capable of repeatedly and rapidly measuring the masses of selected gases (e.g., carbon dioxide, hydrogen, nitrogen) delivered in a continuous gas stream from another instrument, such as an elemental analyzer or a gas chromatograph, to determine their isotopic compositions. 

Mass spectrometric approaches are also very useful for the measurement of stable isotopes in drug metabolism studies. The application of MS to the quantitative measurement of stable isotope has been limited due to the high cost and sophistication of the instruments necessary for stable isotope enrichment studies. Nonetheless, recent improvements in instrument design and performance, as well as computer software for instrument control, data acquisition, and analysis, have increased the sensitivity and reliability of stable isotopic enrichment studies. These new MS instruments, including continuous-flow isotope ratio mass spectrometry (CF-IRMS) and HPLC-chemical reaction interface mass spectrometry (HPLC-CRIMS) are increasingly less expensive, easier to operate, and accessible for mass balance/ metabolite identification studies with stable isotopes. 

Continuous-flow isotope ratio mass spectrometry (CF-IRMS) ... 

---