Instrumentation IRMS

GC-C-IRMS instrumentation enables the compound-specific isotope analysis of individual organic compounds, for example, n-alkanes, fatty acids, sterols and amino acids, extracted and purified from bulk organic materials. The principle caveat of compound-specific work is the requirement for chemical modification, or derivatisation, of compounds containing polar functional groups primarily to enhance their volatility prior to introduction to the GC-C-IRMS instrument. Figure 14.7 summarises the most commonly employed procedures for derivatisation of polar, nonvolatile compounds for compound-specific stable isotope analysis using GC-C-IRMS. 

GC-C-IRMS was first demonstrated by Matthews and Hayes (1978). However, it was somewhat later that Barrie and others (Barrie et al., 1984) coupled a GC, via a combustion interface, to a dual collector mass spectrometer to produce the forerunner of today s GC-C-IRMS instruments. Even so, true determinations of 815N values of individual compounds by GC-C-IRMS remained elusive until finally demonstrated by Hayes and co-workers (Merritt and Hayes, 1994). More recently the precision of GC-C-IRMS instruments has been improved further still with uncertainties in 813C values as small as 0.5 %o for samples containing 5 pmol C and 0.1 %o for 100 pmol samples having been demonstrated (Merritt and Hayes 1994). Instruments available commercially today, from several manufacturers, all conform to the same general principles of design. 

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. 

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... 

Isotope techniques provide some very powerful tools for scientists. However, IRMS instruments are relatively expensive and overcomplicated for laboratories with limited applications. 

Because little has been said concerning difficulties arising from derivati-zation of samples to render them suitable for GC analysis, replacement of GC by HPLC for non-volatile or thermally labile compounds is a possibility. However, the demands of reproducible solvent removal for a reliable LC-C-IRMS approach are formidable. Caimi and Brenna [685,686] have developed an instrument based on a moving wire transport system. The analytes are deposited on the wire as they elute from the HPLC column and, after solvent drying at 200 °C, are transported into an 800 °C combustion furnace loaded with CuO, where the resulting C02 is picked up by an He carrier stream and swept via a drying trap into the IRMS. 

The most widely used method for ionization is electron impact (El). In an El source the sample is placed in the path of an electron beam. Although many newer kinds of ion sources have been developed, El is the method commonly used in classical isotope-ratio mass spectrometers (IRMS), i.e. mass spectrometers designed for precise isotopic analysis. In this type of spectrometer the ions, once formed, are electrostatically accelerated, and then ejected through a slit into a magnetic field held perpendicular to the ion trajectory. In the magnetic sector part of the instrument the particles are deflected in an arc described by ... 

To avoid the kind of problems which trouble whole-molecule mass spectrometry it is better to use instrumentation especially designed for high precision measurements of isotope ratios isotope-ratio mass spectrometry (IRMS). 

Mass spectrometer (1) A scientific instrument used to identify organic compounds. (1) A scientific instrument used to measure the relative abundance of stable isotopes in a sample, e.g. isotope ratio mass spectrometer (IRMS). 

More recently, enantiomer ratios have been used as evidence of adulteration in natural foods and essential oils. If the enantiomer distribution of achiral component of a natural food does not agree with that of a questionable sample, then adulteration can be suspected. Chiral GC analysis alone may not provide adequate evidence of adulteration, so it is often used in conjunction with other instrumental methods to completely authenticate the source of a natural food. These methods include isotope ratio mass spectrometry (IRMS), which determines an overall 13C/12C ratio (Mosandl, 1995), and site-specific natural isotope fractionation measured by nuclear magnetic resonance spectroscopy (SNIF-NMR), which determines a 2H/ H ratio at different sites in a molecule (Martin et al 1993), which have largely replaced more traditional analytical methods using GC, GC-MS, and HPLC. 

The developments in instrumentation include production of dedicated machines. For example, with the introduction of the Attomole 2000 , HVEE (High Voltage Engineering Europe B. V., P.O. Box 99,3800 AB Amersfoort, The Netherlands, Phone +31 33 4619741 Fax +31 33 4615291) has made available a compact C Isotope Ratio Mass Spectrometer ( C IRMS) for biomedical applications. The system provides C/ C ratios down to 10 from sub-milligram samples in typically a few minutes. Both solid samples (carbon) as well as CO, can be analyzed. The Attomole 2000 combines a compact instruments package with the extreme sensitivity of large tandem accelerators that are normally found in big research centres. 

The main application of the instrument will be for tracer kinetic and pharmacological measurements in biomedical studies. Essentially, C IRMS is an alternative technique to C Isotope Ratio Mass Spectrometry ( C IRMS), which is widely accepted within the biomedical community. C IRMS is at least 1(X)-1000 times more sensitive than C IRMS. This allows, for example, that the kinetics of C-labelled enzymes, aminoacids or carcinogens can be studied in the human body at levels that are comparable to actual environmental exposure. 

However, a widespread acceptance of 0 IRMS in the biomedical community is presently handicapped by the size of the existing "C IRMS systems and their need for expensive support personnel. These shortcomings of the present instrumentation are overcome with the introduction of the Attomole 2000. The system is a compact, turnkey instrument that is user-friendly. Its characteristics are reflected in the following specifications ... 

The instrumentation is very expensive (two or three times as much in comparison to IRMS), the amounts of analyte (0.2 to 1 g pure compound)... 

In comparison to classical IRMS methods, the instrumental configuration of cGC-IRMS combines the precision of IRMS to the high purification effect of cGC-separa-tion, with large savings on laborious sample clean-up procedures. 

---