Elution Chromatography Coupling Techniques

The separation methods furnish a given flow of eluate (liquid or gaseous) generally under atmospheric pressure. One way or another, the eluted substances must find their way into the mass spectrometer source, where a high vacuum is necessary. We saw in the Introduction that the mean free path could be evaluated according to the following equation, where the pressure p is expressed in Pa and the distance L is expressed in centimeters  

A pressure of about 10 4 Torr (0.01 Pa or 100 nbar or 10 7 atm) ensures a mean free path of about 50 cm, and constitutes the upper limit of tolerable pressure for the source. Under this pressure, 1 cm3 of gas under atmospheric pressure becomes 107 cm3. A flow of 1 cm3 min 1 thus requires a pumping flow of 107 cm3 min 1 under this pressure, i.e. 1661 s 1 we thus obtain an approximation of the pumping capacity required by this chromatographic flow. 

If the eluent is liquid, 1 mol will yield about 241 of gas under atmospheric pressure. To gain an idea, suppose the eluent is water and its flow rate is 0.1 cm3 min-1. A gas flow of (0.1/18) x 25000= 139 cm3 min-1 under atmospheric pressure will be observed, which would require a pump flow at the source of 139 x 166 = 23 0001s1. This flow is exceedingly high the eluate from a liquid chromatograph cannot be entirely evaporated in the source. We will examine the existing solutions to this problem. 

In fact the actual capacities of pumps in mass spectrometry range from 50 to 1000 Is1, so that the maximum gas flow hovers around 5 cm3 min 1 under 1 atm pressure. 

Graphical representation of the probability that there be more than one compound per peak. The peak capacity is along the n axis. The m axis is the number of acceptable constituents that have the probability indicated next to each curve of having only one compound per peak. The statistical analysis used to produce this graph is slightly different from that leading to the equation in the text. Redrawn from Martin M., Herman D.P. and Guiochon G., Anal. Chem., 58, 2200, 1986, with permission. 

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An on-line supercritical fluid chromatography-capillary gas chromatography (SFC-GC) technique has been demonstrated for the direct transfer of SFC fractions from a packed column SFC system to a GC system. This technique has been applied in the analysis of industrial samples such as aviation fuel (24). This type of coupled technique is sometimes more advantageous than the traditional LC-GC coupled technique since SFC is compatible with GC, because most supercritical fluids decompress into gases at GC conditions and are not detected by flame-ionization detection. The use of solvent evaporation techniques are not necessary. SFC, in the same way as LC, can be used to preseparate a sample into classes of compounds where the individual components can then be analyzed and quantified by GC. The supercritical fluid sample effluent is decompressed through a restrictor directly into a capillary GC injection port. In addition, this technique allows selective or multi-step heart-cutting of various sample peaks as they elute from the supercritical fluid... 

Keywords Crystallization analysis fractionation Field Flow Fractionation High performance liquid chromatography Hyphenated techniques Liquid chromatography Polyolefin analysis SEC-NMR coupling Size exclusion chromatography Temperature rising elution fractionation Two-dimensional liquid chromatography... 

The modulator is the heart of the GCxGC system, and is positioned at the confluence of the coupled chromatography columns. The role of the modulator is to trap or isolate compounds present in a given time fraction eluting from the first-dimension column and reinject these components rapidly into the second column. This essentially yields a time-sampled chromatogram, from the first dimension ( D) to the second dimension ( D). It is critical that the modulator is capable of representatively and faithfully sampling peaks eluting from onto D. This can be achieved by either complete or partial transfer of the first-column eluent, however, both techniques are considered comprehensive. 

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