Injection techniques in SFC

Abstract

The objective of the injector is to transfer the whole sample or a predetermined fraction of the sample quantitatively to the column in the narrowest possible band. The first part of this objective, a quantitative transfer of all components, is not always obtained since sample discrimination occurs in some injection techniques in SFC as well as in GC. The second part of the objective, transfer in a narrow band, is a process which is affected by numerous factors, such as diffusion, linear velocity, temperature, solvents, loading and mixing efficiency, as well as phase transitions. Some of these factors also influence the first part of the objective, the yield of the transfer process. Injector temperatures exceeding room temperature are only used with solutes which need higher temperature to stay in solution, such as some polymers. Injection at high temperature should be performed with caution, since a solvent which starts to evaporate from a hot injector, before the valve is actuated, may result in sample loss and poor reproducibility.In most applications the sample is introduced as a relatively large (depending on the analyte concentration) liquid plug into the stream of mobile phase. Due to limited mixing, the major part of the sample often arrives at the column inlet dissolved in a liquid composed of the solvent partially mixed with the mobile phase. Thus, the transient elution strength of the sample may often be considerably higher than the elution strength of the mobile phase, and can adversely affect the peak shapes by flooding and dispersing the sample over the column inlet.It is well known that an exponential decay injection profile is obtained because of the laminar flow of the displacing mobile phase [1]. By moving the valve back to the load position from the inject position after a predetermined time, the solvent tail can be removed. The injection syringe should be removed first, since the pressure in the sample loop is released through the injection and waste ports when the valve is returned to the load position. With large sample loops the syringe might be pushed out of the port, or the plunger pushed out of the barrel, if the syringe is left in the injection port.By injecting very small volumes, using splitting or small sample loops, a more thorough mixing with the mobile phase can be achieved prior to the column. Currently the nominal volume of the smallest commercially available sample loop is 60 nl. However, small volume (< 100 nl) internal sample loops are difficult to manufacture with high accuracy, and the small volumes are difficult to maintain without changes, particularly with loops made from polymeric materials.Column overload is rarely caused by the mass of the solutes, but almost always by the amount of solvent. With a retention factor k = 1, the injected volume that produces 10 % peak broadening has been calculated to be 24 nl on a 50-µm-ID open-tubular column, and with a retention factor of 10, the volume was calculated to be 130 nl [2]. With an actual injection of 223 nl, 10 % loss of...

Supercritical Fluid and Unified Chromatography

“…There have been several improvements in injection techniques for SFC, but sample introduction is not yet as straightforward as in GC and HPLC. Greibrokk and Chester reviewed SFC injection techniques with emphasis on phase behavior and mass transfer (38). Molander et al described a high-temperature injector for packed-capillary SFC (39).…”

Determination of pressure–temperature coordinates of liquid–vapor critical loci by supercritical fluid flow injection analysis

“…These phase diagrams can be straightforwardly generated using high-pressure view cells or commercial phase analyzers (for example, [8]), but such equipment is not always available to chromatographers. Fortunately, knowledge of the temperature and pressure coordinates of the critical locus, rather than the complete phase diagram, is frequently all that is needed for setting parameters in SFC [9][10][11][12][13][14][15][16][17]. These coordinates can be estimated quickly using an open-tubular SFC instrument to perform a flow injection experiment [9,[14][15][16][17].…”