VOCARB traps

VOCARB traps are special combinations of the adsorption materials Carbopack (GCB) and Carboxen (carbon molecular sieve). These combinations have been optimized in the form of VOCARB 3000 and VOCARB 4000 for volatile and lessvolatile compounds respectively, corresponding to the ERA methods 624/1624 and 542.2 (Supelco, 1992). VOCARB 4000 exhibits higher adsorptivity for less volatile components, such as naphthalenes and trichlorobenzenes. However, it shows catalytic activity towards 2-chloroethyl vinyl ether (complete degradation ), 2,2-dichloropropane, bromoform and methyl bromide. 

An example of a DHS application is the determination of aroma-active compounds in bambuu shoots. In this study, compoimds such as p-cresol, methional, 2-heptanoI, acetic acid, ( ,Z)-2,6-nonadienal, linalool, phenyl acetaldehyde, were extracted from the bambuu shoot samples and analysed by GC. The required sample amount was 10 g, and the extraction temperature was 60°C, using a 30 min extraction time. The stripped analytes were first trapped into a cooled adsorbent tube (VOCARB 3000, at 0 °C), and then thermally desorbed to GC. In DTD, the sample amount required for the analysis is typically smaller than in solid head-space (SHS). In the determination volatile components such as camphor, 1,8-cineoIe and 2,3,S,S-tetramethyl-4-methylene-2-cyclopenten-l-one, from Lavandula luisieri, only 10-20 mg of (dry) plant sample was required for the analysis. The volatiles were desorbed fi om the sample under a helium flow and then cryofocused on a Tenax TA trap at -30 °C. The trap was then quickly heated and the desorbed volatiles were transferred directly to the GC column through a heated fused-silica line (85). 

Extraction temperature —°C Purge gas helium Purge flow — ml/min Purge time 11 min Desorption temperature 180°C Desorption time 4 min Sample volume 5 ml Trap VOCARB 3000 trap Extraction temperature 20°C Purge gas helium Purge flow 40 ml/min Purge time 11 min Desorption temperature 250°C Desorption time 2 min... 

Trap VOCARB 4000 Extraction temperature 70°C Purge gas helium Purge flow 20 ml/min Purge time 30 min Desorption temperature 250°C Desorption time —... 

Sample volume 5 mL Trap both VOCARB 3000 (see next reference for P T conditions) and Tenax-silica gel-charcoal Purge gas He Purge flow 40 mL/min Purge time 11 min Purge temp. AT... 

Carboxen-1000 Adsrobent Vocarb 1200 Strong Strong Traps Freons compounds Permanent gases Light hydrocarbons Very volatile compounds Designed to be used as the innermost adsorbent bed Similar characteristics than Carbosieve-SIII Stable to temp, of 300°C Trap CO2. which can interfere in earlyeluting compounds in GC-MS Used in series after GCB and carbon molecular sieves Hydrophobic... 

The sorbent materials used to make the traps are mainly Tenax, silica gel, activated charcoal, graphitized carbon black (GCB or Carbopack), carbon molecular sieves (carbo-sieve such as Carbosieve-SIII), and Vocarb (Table 23.4). 

Desorption temperature As mentioned above, before the desorption step, the trap is preheated to desorb VOCs at a temperature approximately 5°C below the desorption temperature. The desorption temperature depends on the adsorbents with which the trap is made (from 180°C to 250°C). While traps containing Tenax are not recommended to heat above 200°C, a trap made of GCB, carbon molecular sieves, and Vocarb can desorb at 250°C (recommended desorption and bake temperatures are listed in Table 23.5). 

Table 2.12 Response factors and standard deviations for components with ERA method 624 using a VOCARB 3000 trap and 5 ml of sample (250 °C desorption temperature, reference bromochloromethane/difluorobenzene, concentrations in ppb, Supelco 1988). 

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