Breakthrough sorbents

Breakthrough A condition that exists when the backup section of a sorbent tube is found to contain 20-25 percent of the total amount of contaminant captured in the front section. 

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). 

An alternative way of eliminating water in the RPLC eluent is to introduce an SPE trapping column after the LC column (88, 99). After a post-column addition of water (to prevent breakthrough of the less retained compounds), the fraction that elutes from the RPLC column is trapped on to a short-column which is usually packed with polymeric sorbent. This system can use mobile phases containing salts, buffers or ion-pair reagents which can not be introduced directly into the GC unit. This system has been successfully applied, for example, to the analysis of polycyclic aromatic hydrocarbons (PAHs) in water samples (99). 

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... 

Adsorption/desorption kinetics the time of the adsorption-regeneration cycle greatly depends on the kinetics of the C02 adsorption-desorption profile, which is measured in breakthrough experiments. Sorbents that adsorb and desorb C02 in a shorter time are preferred as these reduce the cycle time as well as the amount of sorbent required, and ultimately the cost of C02 separation. 

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. 

Selection of Collection Media. The methods developed for HCCP, HCBD, and 1,2-DCP involve the collection of the analytes from air on solid sorbent materials in small Pyrex tubes. Each tube is 7 cm long by 6 mm o.d. and 4 mm i.d. The recommended sampling tubes contain two beds of sorbent material—one layer for sorption and a second, smaller, backup layer to monitor breakthrough if the capacity of the sorbing layer is exceeded. 

To determine the capacity of a sorbent material, generator effluent was sampled into sorbent tubes at a known rate and breakthrough from the tubes was monitored. 

In designing and developing methods based on solid sorbents it is desirable to have some guidelines to predict the collection (breakthrough) of various compounds. 

Other solid sorbents have been found more suitable than charcoal for a number of compounds. Silica gel and alumina have been used as a complement to charcoal when sampling polar compounds, but water vapor is strongly adsorbed on these sorbents which leads to deactivation of the sorbent and breakthrough of the compounds by frontal elution. Difficulties also arise with compounds that hydrolyze easily. Alternative sorbents for the collection of polar organic compounds which are sensitive to hydrolysis are porous polymers such as the Chromosorb porous polymer series, Porapak porous polymer series, Tenax-GC and Amberlite XAD sorbent series. 

The collection efficiency of a filter sampler was demonstrated by sampling test atmospheres with a backup collector at the proposed sampling rate and time, and analyzing the collected samples. For sorbents or filter/sorbent sampling trains, the breakthrough volume was determined (to demonstrate capacity) at 80% relative humidity. 6... 

Figures 2 through 4 are graphical representations of the tests performed on the filter/sorbent sampling trains to test for sorbent capacity or breakthrough, and to best determine aerosol/vapor partitioning.

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