Breakthrough volumes

A fundamental parameter characterizing the usefulness of a given precolumn for enrichment purposes is the breakthrough volume, Vg. This volume can be determined by monitoring continuously or discretely the detector signal at the outlet of the precolumn (35-37). The breakthrough volume can be defined by the following expression (37) ... 

Polar compounds present the most problems because of their low breakthrough volumes with common sorbents. In the last few years, highly crosslinked polymers have become commercially available which involve higher retention capacities for the more polar analytes (37, 38). Polymers have also been chemically modified with polar groups in order to increase the retention of the compounds previously mentioned (35, 37). 

Various forms of carbon are used to sample those analytes whose breakthrough volume is too low on Tenax for sufficient preconcentration [8,395-399]. Charcoal, graphitized carbon blacks, and ceurbosieves with wface areas from 5 to 900 w /g are commercially availablJ Bhe high surface area sorbents are used... 

BREAKTHROUGH VOLUMES (liters/g) OF SOME COMMON ATMOSPHERIC POLLUTANTS ON TENAX-GC... 

Modified silica with a C18 reversed-phase sorbent has historically been the most popular packing material, owing to its greater capacity compared to other bonded silicas, such as the C8 or CN types [22]. Applications of C18 sorbents include the isolation of hydrophobic species from aqueous solutions. The mechanism of interaction with such sorbents depends on van der Waals forces, and secondary interactions such as hydrogen bonding and dipole-dipole interactions. Nevertheless, the main drawbacks of such sorbents are their limited breakthrough volumes for polar analytes, and their narrow pH stability range. For these reasons, reversed-phase polymeric sorbents are also used frequently in environmental applications for the trace enrichment of soluble molecules that are not isolated by reversed-phase sorbents such as C18. 

The most significant figure of merit is the breakthrough volume and assuming a simple equilibrium model, this volume is used to determine the dissociation... 

A plot of [A]q + y versus reciprocal breakthrough volume supports the determination of Bt and by linear regression analysis. As with all FAC methods for ligand characterization, the cartridge does not require precalibration before a measurement is made, because is a product of the measurement. One disadvantage of this method is that any error in the earlier measurements is carried forward in subsequent ones however the speed of these measurements and the ability to accurately measure Vq usually makes this the method of choice. 

Fig. 6.13 Measurement of FAC data for a range of pyrimethamine concentrations applied to sol-gel-entrapped dehydrofolate reductase (A) overlay of breakthrough curves and (B) nonlinear regression analysis of the fit to the measured breakthrough volumes from A [28]. Adapted with permission from the American Chemical Society.

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