Sampling onto Sorbent Tubes

The choice of the sorbent material employed for a specific samphng depends 

VOCs as well as the stability of the VOCs on the sorbent. Additionally, the amount of VOCs retained on a sorbent is determined to a large extent by the sorbent bed length and the sorbent mass. Thus, a standard sorbent tube has a length of 16 cm, an outer diameter of 6 mm and contains 0.1-1 g of the sorbent(s) [27,28]. Some parameters that should be considered when choosing the most appropriate sorbent method for a particular study include the hydrophobicity , the thermostability and the loadability of the sorbent [5, 29]. The less water is retained by the sorbent, the less interference is experienced during analysis the stabler the sorbent is, the more robust it is during thermal desorption of the analyte. Lastly, the more air that can be sampled onto a sorbent without sample breakthrough, the lower the detection limit that can be achieved. 

When a single sorbent is not sufficiently efficient in capturing a wide range of VOCs, combinations of sorbents are employed to increase the range of compounds that can be confidently sampled. Consequently, multibed sorbents made up of Anasorb GCBl, Carbotrap and CarbopackB have been employed in some validated methods [30]. Similarly, multibed sorbents consisting of CarbopackC, CarbopackB and Carbosieve Sill [27] and CarbopackB and Carbosieve Sill [10] have been used to trap a wide diversity of indoor VOCs. 

Active sampling onto sorbents entails storing known amounts of sorbent 

Most classes of VOCs found in indoor environments are sampled onto sorbents by adsorption but highly reactive VOCs like carbonyl compounds are sampled by chemical reactions with the sorbent. Thus aldehydes and ketones are sampled by their reactions with sorbent gels coated with 2,4-dinitro-phenylhydrazine to form stable hydrazones [38-40]. Similarly, formaldehyde has been sampled by its reaction with N-benzylethanolamine to give 3-benzyl-oxazolidine [41,42]. 

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US Environmental Protection Agency (1999) Compendium of methods for the determination of toxic organic compounds in ambient air. Compendium method TO-17 determination of volatile organic compounds (VOCs) in ambient air using active sampling onto sorbent tubes, US Environmental Protection Agency, Cincinnati, OH... 

Thermal Desorption Thermal desorption is an alternative GC inlet system particularly used for VOC analysis. However, the analytes subjected to thermal desorption must be thermally stable to achieve successful analysis. Otherwise, decomposition occurs. This technique is mainly used for determination of volatiles in the air. Such a methodology requires sample collection onto sohd sorbents, then desorption of analytes and GC analysis. Traditionally, activated charcoal was used as a sorbent followed by extraction with carbon disulfide. However, solvent desorption involves re-dilution of the VOCs, thus partially negating the enrichment effect. Therefore, the sampling method is to pump a sample of gas (air) through the sorbent tube containing certain sorbents in order to concentrate the VOC. Afterwards, the sample tube is placed in thermal desorber oven and the analytes are released from the sorbent by application of high temperature and a flow of carrier gas. Additionally, desorbed compounds are refocused in a cold trap and then released into the GC column. Such a two-step thermal desorption process provides a narrow chromatographic band at the head of the column. 

ACAMS alarms must be verified to ensure that they are not a false positive due to an interferent species or instrument malfunction. This verification is done using a depot area air monitoring system (DAAMS) deployed near an ACAMS. DAAMS is a passive system that draws an air stream through a sorbent tube. The tubes are collected and replaced periodically if there are no ACAMS alarms or shortly after an alarm occurs. They are transported to a laboratory and thermally desorbed onto a sample tube and analyzed on a laboratory scale GC/FPD system. Without confirmation by the more sensitive... 

GC plays a key role in the analysis of VOCs and SVOCs in industrial effluents. VOCs that have boiling points below 200°C can be extracted from water sample matrices using a P T system. Purged sample components are trapped in a tube containing suitable sorbent materials. When purging is complete, the sorbent tube is heated and backflushed with helium to desorb trapped sample components onto a GC... 

In the preferred procedure, samples of vinyl acetate on Chromosorb 107 were prepared. A glass U-tube was connected to the inlet end of a clean sampling tube, the outlet of which was connected to a small pump. As an aliquot of vinyl acetate in hexane was injected into the U-tube, the pump drew air through the system at 0.2 L/min such that the vapors were swept onto the sorbent bed. After 2-3 minutes, or approximately five volume changes, the pump was turned off and the apparatus was disassembled. Standard samples prepared in this manner were thermally desorbed and analyzed. A calibration curve thus... 

A schematic of an on-site system for collection of air samples is presented in Figure 7 [33]. In the on-hne mode, ambient air is drawn through a link tube and collected directly onto the cold trap of the thermal desorber and analysed immediately after the sampling. The cold trap contains two beds of sorbent (Tenax TA and Carbograph). The sampling flow was ca. 50mLmin , and the sampling time varied from 60 to 80 min. 

Voktile compounds were sparged from UHT milk samples in a Dynatherm Dynamic Thermal Stripper (DTS) 1000 and trapped onto multi-bed adsorbent traps. Sorbent traps were packed with a layered progression of Tenax TA/Ambersorb XE-340/Charcoal to effectively trap all purged volatiles. The oven, block, and tube heaters were set at 50 °C. Times and flow rates were 5 min oven pre-heat, 45 min. purge, 20 minute dry and 50 ml/min N2 purge, 30 ml/min N2 dry flow. 

The cheese samples were cut and placed in equal amounts into inert-coated heated extraction chambers for release and trapping of the volatile compounds. Nitrogen as an inert gas is passed through the chambers at a constant flow rate while sweeping the headspace vapours onto individual sorbent packed tubes. The used micro-chamber thermal extractor allows up to six samples to be extracted in parallel under identical conditions for comparable results. 

The sample chamber is set to a temperature of 40 C for pre-heating and extraction. This temperature was chosen for a good representation of the released VOCs of the cheeses as a mock-up of the typical mouth feeling. In this dynamic headspace sampling step the volatile headspace is flushed from the cheese samples by using the inert gas nitrogen for collection onto the sorbent packed tubes. After collection the tubes had been transferred to a thermal desorption unit. 

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