Analyte trapping

A 1-g sieved, air-dry soil sample is placed in an extraction cell with methanol as a modifier. The sample is extracted at a C02 flow rate of 1.5 mT/min with supercritical carbon dioxide for 15 minutes and analytes trapped in an octadecyl siloxane microextraction disk for subsequent analysis (adapted and condensed from Reference 9). 

Polycyclic aromatic hydrocarbons Polycyclic aromatic hydrocarbons co2 Pretreatment of sample with (a) 15% water, (b) 5% (ethylenedinitrilo) tetraacetic acid tetrasodium salt or (c) 50% methanol then extraction with C02 Use of liquid-solid traps compared to analyte trapping 60 - 98% recovery using Na4 EDTA-C02. Only 7 - 63% recovery with C02 alone [133]... 

For accurate analysis, we must know the total volume of air passed through the adsorbent tube, the mass of the analyte trapped, and the desorption efficiency of the solvent. Before sampling, the pump must be calibrated using a bubblemeter, a rotameter, or a gasometer to determine the flow rate. Using the flow rate and the time sampled, the total volume of air sampled can be determined. 

Aqueous samples heated in a purging vessel under helium purge. Analyte trapped over an adsorbent (i.e., silica gel or Tenax) and then thermally desorbed out from the absorbent trap and transported onto a polar GC column for FID or GC/MS determination. 

Aqueous samples purged by an inert gas analyte trapped on a sorbent trap transferred onto a GC column by heating the trap and backflushing with helium determined by GC/MS. 

Storage of solvent extracts or sorption tubes with analytes trapped, in freezer in glass vials closed with PTFE stoppers 1 month 115,116... 

The traps should be dried at about 40°C in a mercury-free N2 flow for 5 min prior to analysis, after which they should be connected to the AFS detector on-line with the helium gas flow. The mercury is then thermally desorbed either directly into the detector or onto an analytical trap. If an analytical trap is used, a second heating step should be performed before the detection. The advantages of dual amalgamation are that the influence of any interfering substances adsorbed on the first trap may be reduced and that the mercury adsorbed on the second analytical trap will be more easily desorbed, thus yielding a sharper peak. 

The elevated pressure and temperature used in ASE affects the solvent, the sample, and their interactions. The solvent boiling point is increased under high pressure, so the extraction can be conducted at higher temperatures. The high pressure also allows the solvent to penetrate deeper into the sample matrix, thus facilitating the extraction of analytes trapped in matrix pores. At elevated temperatures, analyte solubility increases and the mass transfer is faster. The high temperature also weakens the solute matrix bond due to... 

In aqueous solutions the micellar assembly structure allows sparingly soluble or water-insoluble chemical species to be solubilized, because they can associate and bind to the micelles. The interaction between surfactant and analyte can be electrostatic, hydrophobic, or a combination of both [76]. The solubilization site varies with the nature of the solubilized species and surfactant [77]. Micelles of nonionic surfactants demonstrate the greatest ability for solubilization of a wide group of various compounds for example, it is possible to solubilize hydrocarbons or metal complexes in aqueous solutions or polar compounds in nonpolar organic solutions. As the temperature of an aqueous nonionic surfactant solution is increased, the solution turns cloudy and phase separation occurs to give a surfactant-rich phase (SRP) of small volume containing the analyte trapped in micelle structures and a bulk diluted aqueous phase. The temperature at which phase separation occurs is known as the cloud point. Both CMC and cloud point depend on the structure of the surfactant and the presence of additives. Table 6.10 gives the values of CMC and cloud point for the surfactants most frequently applied in the CPE process. 

In aqueous matrices, most compounds have quite small values (< 0.25), so the headspace has a low analyte trapping capacity. As a result, the sensitivity of headspace SPME is almost the same as that of direct SPME. The sensitivity loss is only significant when the target analytes partition well into the headspace (i.e. when they possess large... 

A second method of sample acquisition, based on use of a solid-phase adsorbent as an analyte trap, is widely used for low volatility species that are less suitable for sampling using canister methods because of the increased capacity of analyte condensation on the container walls. The adsorbent used in the trap is chosen in such a way to introduce an element of selectivity to the trapping mechanism, although in practice a trap-all approach is commonly used. 

In off-line collection, the effluent is depressurized, and the analyte is collected in a solvent, an open container, or an analyte trap packed with a solid support. Off-line collection is simpler, and the collected sample can be analyzed by several methods. Thus, the SFE instrument is decoupled from the analytical instruments. Other advantages of the off-line approach are accommodation of larger sample sizes, feasibility of multiple analyses from a single extraction, and accommodation of a wider range of analyte concentrations. In the analyte trap approach, the use of different solvents for elution can serve as an additional experimental parameter for increased selectivity and cleanup. In the open-container collection approach, aerosol may form because depressurization of the supercritical fluid produces a high flow rate of gaseous carbon dioxide. 

The extracted analytes are collected after the depressurized step. The analytes travel through the restrictor, where the SF decompresses, and analyte deposits in some type of trapping device. Analyte trapping after the extraction step can be carried out with either a small amount of collection solvent (an appropriate solvent placed in a cooled vial) or in an adsorbent trap (solid surface cryogenically cooled by means of liquid nitrogen). The trapping system is selected depending on the nature of extracted analyte. 

The simplest principle for analyte trapping was described in Section 13.2.1.1. In order to perform enrichment of an acidic compound, the pH of the acceptor is held enough alkaline, so the main fraction of the acidic analyte becomes charged and thus nonextractable in the acceptor. This is analogous with the principle of back extraction in LLE, where an organic extract of an acidified sample is extracted with a second aqueous phase, in order to isolate acidic compounds. 

Semivolatile organic analytes trapped on air filters or on solid phase adsorbents or in water, soil, and other solid samples are extracted with an organic solvent, or a solvent mixture, and the extracts are concentrated by evaporation of the solvent before the determination of the analytes. Solid samples are often extracted with the classical Soxh-let apparatus or with a variety of other techniques including several that use organic solvents at elevated temperatures and pressures. If a sample is highly concentrated,... 

Reliable hardware and software tools for coupling techniques and system control have enabled the establishment of more complex two- or multidimensional HPLC systems, which allow consecutive chromatographic steps performed online on different stationary phases also including steps of analyte trapping or sample preparation. 

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