Ion Chromatography - The Thermospray Interface 3

 

 

This pump eliminates about 99% of the vaporized mobile phase, whereas the heavier molecules pass through an ion aperture in the sampling cone, and into the mass spectrometer. The ions are formed immediately after the nebulization, process and pass alongside a repeller plate, held at a high potential, that impels the ions through a hole into the ion source. Once in the ion source, the ion optical system directs them into the analyzer section of the spectrometer. The reagent gas is methane and its flow is controlled by separate needle valves.

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Figure 41. The Vestec Model 210 Thermospray Interface

 

This pump eliminates about 99% of the vaporized mobile phase, whereas the heavier molecules, having greater momentum, continue on their path and pass through an ion aperture in the sampling cone, and into the mass spectrometer. The ions are formed immediately after the nebulization, process and pass alongside a repeller plate, held at a high potential, that impels the ions through the aperture into the ion source. Once in the ion source, the ion optical system directs them into the analyzer section of the spectrometer. The reagent gas is methane and its flow is controlled by separate needle valves.

 

McCracken et al (4) used a thermospray ionization source to measure furazolidone in some pharmacokinetic studies. Furazolidone (N-(5-nitro-3-furfurylidene-3-amino)-2-oxazolodinone is one of the 5-nitrofuran antibiotics that are added to animal feeds to help control such infections as Escherichia coli and Salmonella in cattle, pigs and poultry. In Europe the maximum level that is tolerated is 5mg/kg of animal foodstuff.

 

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Figure 42. Single Ion Chromatograms (m/z=243) Furazolidone Demonstrating the Sensitivity Levels Obtainable

 

Furazolidone is light sensitive and so its extraction must be carried out under artificial yellow light. As an example, samples of liver and muscle tissue were analysed. 2g samples were homogenized with 40ml of a mixture of McLlvaine buffer and methanol (7+3) and centrifuged for 15minutes. The supernatant liquid was concentrated by evaporation to 15ml at 40°C and 25ml of dichloromethane was then added and the mixture shaken for about 1minute. The upper aqueous layer was discarded and the solvent extract evaporated to dryness and taken up in 2ml of dichloromethane and 6ml of hexane. The sample was passed through a prepared Bond-Elut NH2 extraction cartridge, washed with 5ml hexane/ dichloromethane mixture (1+1) and 2ml of hexane/chloroform mixture (1+1). The cartridge was extracted with 5ml of chloroform/methanol mixture (7+3), the extract evaporated to dryness and re-dissolved in 100ml of mobile phase. The sample was injected onto a reversed phase column, 12.5cm long, 4mm I.D. containing RP18 stationary phase bonded to 5mm particles. The instrument was a Vestec thermospray LC/MS Model 201A. Chromatograms showing the elution of the Furazolidone by single ion monitoring are shown in figure 42.