Purge and trap preconcentration

Whole blood Purge-and-trap preconcentration thermal desorption GC/MS <0.5 pg/L No data Antoine et al. 1986... 

Drinking water, waste water Purge-and-trap preconcentration thermal desorption GC/MS (Standard Method 6210) < 0.1 pg/L (reagent water) using narrow bore capillary column 105 (3% RSD) narrow bore column Greenberg et al. 1992... 

Drinking water Purge-and-trap preconcentration onto Tenax/ silica/charcoal thermal desorption GC with PID and Hall in series 0.02 pg/L 98 (2.5% RSD) Ho 1989... 

Finished drinking/raw source water Purge-and-trap preconcentration onto Tenax/ silica/charcoal thermal desorption Cryofocusing (wide or narrow bore) HRGC/MS (EPA method 524.2) 0.03 pg/L (0.03 ppb, w/v) with wide bore column 0.02 pg/L (0.02 ppb, w/v) with narrow bore column 90 (6.1% RSD) at 0.5-10 pg/L (wide bore) 95 (3.2% RSD) at 0.1 pg/L with narrow bore column EPA 1992a... 

Groundwater, liquid, and solid matrices Direct injection of head-space gas (EPA method 5020) or purge-and-trap preconcentration and thermal desorption (EPA method 5030) GC/HSD (EPA method 8010) 0.5 pg/L (ppb, w/v) for groundwater, 0.5 pg/g (ppm, w/w) for low-level soil, 500 pg/L (ppb, w/v) for water-miscible liquid waste, 1,250 pg/g (ppm, w/w) for soil, sludge, and non-water-miscible waste Water 0.93C -0.39 where C = true concentration in pg/L. EPA1986a... 

Bulk oils Purge-and-trap preconcentration (with deuterated internal standards) onto Tenax thermal desorption GC/MS 1 ppb (w/v) 88 (6.7% RSD) at 93 ppb Thompson 1994... 

Larreta, J., Bilbao, U., Vallejo, A., Usobiaga, A., Arana, G., Zuloaga, O. Multisimplex optimization of the purge-and-trap preconcentration of volatile fatty acids, phenols and indoles in cow slurries. Chromatographia 67, 93-99 (2008)... 

Analysis of environmental samples is similar to that of biological samples. The most common methods of analyses are GC coupled to MS, ECD, a Hall s electrolytic conductivity detector (HECD), or a flame-ionization detector (FID). Preconcentration of samples is usually done by sorption on a solid sorbent for air and by the purge-and-trap method for liquid and solid matrices. Alternatively, headspace above liquid and... 

Gas purging and trapping is the most commonly used method for the preconcentration of 1,2-dibromoethane from water, waste water, soil, and various foods. This method also provides a preliminary separation of 1,2-dibromoethane from other less volatile and nonvolatile components in the samples, thereby alleviating the need for extensive separation of the components by a chromatographic column prior to quantification. 

While most preconcentration methods focus on the collection and concentration of analytes from vapor samples, the purge-and-trap method is used for collection and concentration of volatile and semivolatile analytes from liquid (usually aqueous) samples. 

Figure 3.3a shows a schematic of a purge-and-trap system for the collection of volatiles contained in an aqueous sample. Clean air is bubbled through the water sample to purge the water of the volatiles and then carry the volatiles into the IMS. If the concentrations of the analytes are too low for direct detection, a trap (preconcentrator) device is inserted between the purged water sample and the IMS. Figure 3.3b is a schematic of an exponential dilution system used to calibrate IMS instruments for... 

The sensitivity of a GC/FTIR system depends on the type of interface and the molar absorbance index of the analyte. Comparative studies of the three interfaces have shown that the highest sensitivity is achieved with the DD interface. In general, the limit of detection (LOD) of the hght-pipe GC/FTIR is about 10 ng on-column, while, the LODs of the DD/FTIR and MI/FTIR are 35 and 100 pg, respectively. In practice, the LODs of analytes in real samples are of the order of 0.5-25 ng on-column. To enhance the sensitivity of detection, additional clean-up and preconcentration methods are applied, such as head-space sampling, purge and trap, solid-phase extraction, and solid-phase micro-extraction. It would also be possible to achieve improved sensitivity by modifications of other parts of the analytical procedure, such as the use of large-volume sampling methods. 

EPA method 1631 uses oxidation followed by a purge-and-trap method and cold-vapor atomic fluorescence. Mercury is preconcentrated in the gold-coated sand trap by amalgamation. One commercial instrument uses an atomic fluorescence detection system and claims a working range from <0.05 ppt to 250 ppb. This method is quite useful for a variety of sample types ranging from seawater to sewage. 

The discussion of headspace methods for blood alcohol and solid-phase micro extraction (SPME) in Section 4.2 introduced the concept of creating an enriched head-space above a sample. Headspace methods may be passive or active and may involve heating the sample. Dynamic headspace (DHS) methods, used in arson analyses, exploit the equilibrium at the liquid-sample interface by sweeping tire headspace with a constant stream of gas, usually helium. DHS is also referred to as purge-and-trap (FT), allhough the latter can also mean a specific t) of sample preconcentrator used in environmental analysis. The trap material can be thermally desorbed or desorbed wifii a solvent. The thermal method is preferred, but is not always possible. The choice of trapping or sorbent materials depends on fire application arson typically requires charcoal or charcoal combinations. 

FIGURE 15.24 Three-stage flow path for the cryogenic preconcentration of VOCs from air and the removal of water vapor by microscale purge and trap, Entech Technologies, Inc. 

P/T (Pnrge-and-Trap) system An inert gas is bubbled through an aqueous sample causing insoluble volatile chemicals to be purged from the matrix. The volatiles are trapped on an absorbent column (known as a trap or concentrator) at ambient temperature. The trap is then heated and the volatiles are directed into the carrier gas stream. Samples requiring preconcentration or purification can be introduced via such a system, usually hooked up to the S/SL port. 

Thermal desorption, solvent elution, and solvent extraction are used in VOC preparation schemes for samples collected on solid sorbents. Thermal desorption methods require determination of the sensitivity of the target analytes to the desorption temperature. It is also critical to remove all traces of O2 from the gas used to purge the sorbent and transfer the analytes to the preconcentration trap. Quantitative recovery of monoterpenes from Tenax is accomplished at thermal desorption temperatures of 250°C. ° Multibed sorbent tubes consisting of graphitic carbon solids and molecular... 

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