Liquid Chromatography Isotope Ratio Mass Spectrometry (LC/IRMS)
The IsoLink interface enables the coupling of an HPLC to an IRMS. This direct coupling allows δ13C values for underivatized compounds in aqueous solution to be determined directly without the need for derivatization. The LC IsoLink interface can be used in one of two injection modes. Either the injected compounds are separated by aqueous HPLC and then pass into the IRMS or, alternatively, the HPLC column can be removed and direct injection of aqueous solutions can be performed, resulting in a bulk δ13C value representing all the carbon in the sample.
Once the samples have passed into the LC IsoLink, either directly or from the eluent of an HPLC column, the compounds pass through a T-piece where the oxidation reagents, sodium peroxodisulfate (Na2S2O8) and phosphoric acid (H2PO4), are added. Silver nitrate may also be added to the peroxodisulfate as a catalyst for compounds that require particularly strong oxidizing conditions. This mixture of reagents and compounds in the mobile phase pass through an oxidation reactor maintained at 99.9 °C where they are oxidised (quantitatively) to CO2 The CO2 in solution is cooled and then the gas is separated from the mobile phase by being transferred through three thin membranes into a counter flow of helium where the difference in the CO2 concentration between the inside and outside of the membranes and the acidity of the solution result in a close to 100% degassing of the liquid phase for flow rates <400 mL/min. The CO2 separated is then dried by passing through a Nafion membrane and then introduced to the isotope ratio mass spectrometer via an open split and subsequently analysed in an identical way to the CO2 obtained in GC/C/IRMS.
Figure 1 A schematic of a typical LC/IRMS instrument