Video-taped sample evaporation in hot chambers simulating gas chromatography split/splitless injectors. I. Thermospray injection.
The processes in devices imitating vaporising injectors were visualised and video-taped using perylene as a fluorescent marker for non-evaporated samples. The observations in the context of thermospray injection are summarised. Partial evaporation inside the needle turns the solvent into a propellant which nebulises the sample liquid at the needle exit. Evaporation in the vaporising chamber occurs from fine droplets suspended in the gas phase. Empty injector liners are best suited; packings with glass wool or obstacles in the liner, like the cup, have no significant effect on the process observed. Non-evaporated (matrix) material forms aerosol particles which may enter the column together with the vapours, but most of them are transferred to the liner wall. Since solute material may be carried along, this is a possible source of matrix effects.
Injector-internal thermal desorption from edible oils. Part 1: visual experiments on sample deposition on the liner wall.
Injector-internal thermal desorption from edible oils or fats enables the analysis of a wide range of compounds in oils or extracts of fatty food without prior removal of the sample matrix. The oil or fat is deposited onto the wall of the injector liner. The solutes of interest are evaporated, leaving behind the sample matrix. The injector is kept at a temperature volatilizing the solutes of interest, but minimizing evaporation of the bulk material of the oil. This technique was optimized regarding sample deposition on the liner wall (Part 1) and desorption of high boiling compounds, such as migrants from food packaging materials into simulant D (olive oil) or fatty food (Part 2). The sample liquid should be transferred to the liner wall and spread to a thin film in order to facilitate the release of high boiling components. Visual experiments with perylene-containing solutions showed that the oil must be diluted to reduce the viscosity (separation from the needle tip). The oil concentration should not exceed 20% in order to rule out that squirting sample liquid drops to the bottom of the vaporizing chamber. Further dilution to about 10% oil improves spreading of the liquid to a thin film. A rather high boiling solvent should be used, such as n-butyl acetate, to prevent thermospray at the needle exit and violent evaporation from the liner wall with sputtering liquid. Using a 5-mm ID liner, 5-10-microL injections of 10-20% oil solutions were at the upper limit.
Conditions for accurate split injection of samples with wide boiling point range
The effect of injection temperature, carrier gas flow rate, geometry of the glass insert, and column temperature program on the precision and accuracy of split injections was measured. Three types of injection techniques were compared: injection into a hot isothermal injector, isothermal injection with the injector at the solvent boiling point temperature, and programmed injection temperature. The last of these techniques produced the best accuracy and precision of analysis. Conditions for complete sample trapping at the beginning of programmed temperature analysis are described.