Sorbents porous polymers

Prepai ative isolation of nonvolatile and semivolatile organic compounds fractions (hydrophobic weak acids, hydrophobic weak bases, hydrophobic neutrals, humic and fulvic acids) from natural and drinking waters in optimal conditions was systematically investigated by solid-phase extraction method with porous polymer sorbents followed by isolation from general concentrate of antropogenic and/or toxic semivolatile compounds produced in chlorination and ozonation processes. 

Notes The most commonly used porous polymer sorbent is Tenax-GC, although the Porapakand Chromosorb Century series have also been used Tenax-GC has been used with thermal desorption methods, but can release toluene, benzene, and trichloroethylene residues at higher temperatures in addition to Tenax-GC, XAD 2-8, Porapak-N, and Chromosorbs 101, 102, 103, and 106 have found applications, sometimes in stacked sampling devices (for example, a sorbent column of Tenax-GC — Chromosorb 106 in tandem) Chromosorb 106, a very low polarity polymer, has the lowest retention of water with respect to organic materials and is well suited for use as a backup sorbent... 

Breakthrough Experiments. For the breakthrough experiments the sorbents were packed in 100-mg beds, 4 mm in diameter, and inserted into the six ports in the manifold. The effluent ends of the tubes were connected to a common line leading to the flame ionization detector. While the flame ionization detector, calibrated with the challenge atmosphere, monitored the concentration of vinyl acetate in the combined bed effluents, the pump in the detector drew the challenge atmosphere through each bed at approximately 0.2 L/min. The output from the detector, the breakthrough curve, was recorded with a strip-chart recorder. Most of the porous polymer sorbents tested were first washed with acetone and dried. 

A special case of porous polymer sorbent is polyurethane foam, which can be used to collect large air samples up to 100 m (Ligocki and Pankow, 1985). A major application of PU foams is the trapping of high boiling organopesticides such as lindane or per-methrine. 

Other adsorbents show a specific background as well Tirkkonen et al. (1995) present a compilation of compounds that were found under thermal desorption conditions for seven carbon-based and porous-polymer sorbents. 

The changes in system constants with composition of a binary mixture of carbon dioxide and 1,1,1,2-tetrafluoroethane on a column containing a porous polymer sorbent are illustrated by the results in Figure 7.7 [20]. Adding 1,1,1,2-tetrafluoroethane to... 

Hydrocarbons (mixed C3-C,q) Lab method using pumped porous polymer and carbon sorbent tubes, thermal desorption and gas chromatography 60... 

The principal adsorbents used in GSC are silica, alumina, graphltlzed carbon blacks, porous polymer beads, zeolites and cyclodextrlns [8,430,431,445]. The bonded phase sorbents discussed in section 2.2.3 could also be considered as modified adsorbents in many respects. 

Agricultural processing will still incorporate solvents. As an example, soybean flakes were extracted with supercritical carbon dioxide to produce a solvent-free, good-quality soybean oil. During the SFE process, volatile compounds were trapped on a porous polymer trap attached at the exhaust port of the SFE apparatus. The volatile profile obtained from the sorbent trap was found to be similar to the headspace profile from the SFE/soybean oil removed during the same extraction. In addition, crude soybean oil was heated in a stirred reactor and the volatiles, which were stripped by supercritical carbon dioxide in an attempt to improve oil properties, were collected on sorbent traps and analyzed by the above method for comparison. The described methodology permits the characterization of volatiles and semivolatUes in SEE soybean oil and can be used to monitor the extraction and quality of the resultant oil (Snyder and King, 1994). 

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. 

Table I shows some of the characteristics of the commonly used sorbents. The charcoals are by far the most frequently used solid sorbent for organic vapors. Over 130 methods have been validated in our five-year study using coconut, petroleum, and synthetic charcoal. The other sorbents include silica gel, used primarily for amines, and porous polymers, used for substances not amenable to collection on charcoal or silica gel. Other researchers have used other sorbents, including Florisil, alumina, and molecular sieves.

Chemical Properties. If a substance is highly polar, it may be readily collected by charcoal, but it may be difficult or even impossible to recover it. Also, some substances may be readily hydrolyzed and it may be best to collect these on hydrophobic sorbents like a porous polymer. 

It must be compatible with the sorbent. We have found that various solvents, notably CS2 and CH2CI2, dissolve some porous polymers and are unsuitable for use. Likewise, aqueous bases produce a gel-like siibstance with silica gel and should be avoided. 

The sorbents most commonly used in industrial hygiene sampling are charcoal, silica gel, and the porous polymers. In addition, a number of methods have been developed using coated sorbents. Each sorbent is discussed briefly in this section. 

The most serious limitation to the use of porous polymers is batch-to-batch variation. We have found tremendous variation in recovery and capacity tests with different manufacturer s lots of the same sorbent. Often even an extensive clean-up procedure did not result in reproducible data. For this reason, we recommend testing several manufacturer s lots. This often requires more extensive method development than required with charcoal and silica gel methods. 

An example of method development using porous polymers is the work done for n-butyl mercaptan. n-Butyl mercaptan collected on charcoal was found to oxidize readily to the dibutyldisulfide. It was not feasible to analyze for the mercaptan as the disulfide, because the disulfide could also be present in the workplace. Silica gel was an excellent collector for the mercaptan in a dry atmosphere however, at 80% relative humidity the sorbent collected moisture preferentially, and sorbent capacity was severely reduced. 

Coated Sorbents. When collection and recovery of a specific substance cannot be achieved using charcoal, silica gel, or porous polymers, chemical sorption with a coated sorbent may be necessary. Compounds requiring this method of collection are usually too reactive or unstable to be collected and stored by other means. In this case, a specific stable derivative or unique product characteristic of the compound of interest is desired. 

The report (5) on the study recommended that a less active charcoal be tested. A literature survey did not yield any conclusive evidence for successful collection by recovery from alternative sorbents. On the basis of the little information available, we selected petroleum charcoal and three porous polymers to test (petroleum charcoal is less active than coconut charcoal). All the porous polymers were based on aromatic monomers, and they should be good collectors for nonpolar aromatics. Aliquots of 113 pg diphenyl were spiked onto these sorbents, and the samples were desorbed with various solvents. The results are given below. 

The sorbents, Chromosorb 102 and XAD-2, which are styrene-divinyl benzene cross-linked porous polymers, proved to be most useful in our studies. Capacity of the sorbent sampling tubes was not a problem with the pesticides we studied since most were not extremely volatile. Sampling humid atmospheres of the pesticides also did not affect the sorbent capacity since these porous polymers are hydrophobic. 

---