Equilibrium sampling devices

S. A. S. Werdnski, ed.. Solid Phase Microextraction A Practical Guide (New York Marcel Dekker, 1999) Z. Zhang, M. J. Yang, and J. Pawliszyn, Solid-Phase Microextraction, Anal. Chem. 1994, 66, 844A P. Mayer, J. Tolls, J. C. M. Hermens, and D. MacKay, Equilibrium Sampling Devices. Environ. Sci. Technol. 2003,37, 185A. 

Mayer, P., J. Tolls, L. Hermens, and D. Mackay. 2003. Equilibrium sampling devices. Environ. Sci. Technol. 37 184A-191A. 

ESD Equilibrium sampling device Use of PDMS membrane that equilibrates with the surrounding water sample Extraction of halogenated compounds from sea and river water samples... 

If the aim of an investigator is to determine equilibrium concentrations in samplers, then the residence time (tm) is a logical parameter to compare among samplers. The tm is the mean length of time that a molecule spends in a passive sampling device, where solute exchange follows first-order kinetics. Residence time is given by... 

Connect gas lines and begin flow at 20 ml/sec. Allow gas to run for 10 sec through the system (but not through the sampling device) to purge any equilibrium or psuedo-equilibrium gas phase from the reservoir. 

Passive sampling can be defined as any sampling technique based on the movement (by diffusion) of analyte molecules from the sampled medium to a receiving phase contained in a sampling device. This mass transfer process is driven by a difference in chemical potentials of the analyte in the two media. This process continues until equilibrium is reached in the system, or until the sampling process is stopped.14 Analytes are retained in a suitable medium within the device, known as a receiving or sorption phase. This can be a solvent, chemical reagent, absorbent, or... 

The theory of equilibrium passive sampling devices has been published by Mayer et al.15 The basic requirements of the equilibrium sampling approach are that equilibrium concentrations are... 

FIGURE 3.2 Passive sampling devices operate in two main regimes kinetic and equilibrium. 

The substance-specific kinetic constants, kx and k2, and partition coefficient Ksw (see Equations 3.1 and 3.2) can be determined in two ways. In theory, kinetic parameters characterizing the uptake of analytes can be estimated using semiempirical correlations employing mass transfer coefficients, physicochemical properties (mainly diffusivities and permeabilities in various media), and hydro-dynamic parameters.38 39 However, because of the complexity of the flow of water around passive sampling devices (usually nonstreamlined objects) during field exposures, it is difficult to estimate uptake parameters from first principles. In most cases, laboratory experiments are needed for the calibration of both equilibrium and kinetic samplers. 

The conventional polymeric coating, PDMS, employed in SPME has a film thickness of 100 pm, which corresponds to a volume of about 0.5 pL for the whole fiber. In SPME, the thin PDMS film provides the highest enrichment when equilibrium between the film and sample is realized, the attainment of which depends largely on analyte hydrophobicity and distribution to the coating. With thin PDMS films, SPME does not generally afford quantitative exhaustive extraction, which renders SPME a less sensitive technique even for nonpolar compounds and an unsatisfactory sampling device that fails to extract polar analytes. Implementing SPME for total exhaustive extraction is conceivably plausible if the PDMS film thickness is increased dramatically. 

Other observations of the reaction of hydrazine and nitrogen tetroxide substantiate the production of non-equilibrium combustion products. Non-equilibrium product concentrations were found in combustion gases extracted from a small rocket combustion chamber through a molecular beam sampling device with direct mass spec-trometric analysis (31) (39). Under oxidizer rich conditions excessive amounts of nitric oxide were found under fuel rich conditions excessive amounts of ammonia were found. A correlation between the experimentally observed characteristic velocity and nitric oxide concentration exists (40). Related kinetic effects are postulated to account for the two stage flame observed in the burning of hydrazine droplets in nitrogen dioxide atmospheres (41) (42). 

To determine these values in a fairly simple apparatus about 250 to 500 ml of the mixtures are placed in flask a (Fig. 47) and the liquid is heated to boiling. The vapour liquefies in condenser b, and the condensate returns to flask a as long as tap c is kept closed. The temperature established is read off on thermometer d. After equilibrium has become established a sample of about 0.1 to 0.2 ml is taken at tap c.. At the. same time a sample of the flask contents is taken through tap e. The determinations are continued until three successive concentrations are the same. When measiireiuents are carried out in vacuum a sampling device (which is of course also suitable for... 

The rate constants, the adsorption equilibrium constants, and the deactivation parameters a, and oto were determined from the measurement of Pa and pj, as a function of time and position (i.e., W/F o) in the bed, through a special sampling device. In addition, the coke profile was measured at the end of the run. The parameters were found to be statistically significant, and the rate coefficients obeyed the Arrhenius temperature dependence. 

In another attempt to develop devices for selective, passive preconcentration of explosives, vapor-deactivated quartz fiber filters impregnated with metal P-diketonate polymers were used. The Lewis acidic polymers selectively interact with Lewis base analytes, such as explosives. The uptake kinetics of TNT and RDX vapor from a saturated atmosphere were characterized through passive equilibrium sampling. Approximately 5 ng of RDX were collected on the filter at the end of 1 month, and this amount was readily detected by the IMS. 

FIGURE 13.3 Hollow-fiber devices for membrane extraction (a) hollow-fiber loops for equilibrium extraction. (Reprinted from Liu, J.-F., Jonsson, J.A., and Mayer, R, Equilibrium sampling through membranes of freely dissolved chlorophenols in water samples with hollow fiber supported liquid membrane, Anal. Chem., 77, 4800. Copyright 2005 American Chemical Society.) (b) Liquid-phase microextraction device... 

In atomic spectrometry, the sample is introduced, by means of a sampling device, into a high-temperature source or atom cell (plasma, flame, etc.). Here, the sample is vaporized, e.g., by thermal evaporation or sputtering. It is important to supply as much energy as possible, so that the volatilization processes, which involve a physical or chemical equilibrium, result in complete atomization, irrespective of the state of aggregation, solid state structure, or chemical composition of the sample. This is very important, both to ensure maximum sensitivity and to minimize matrix interference in the analysis. The effectiveness of the volatilization processes involved, the plasma temperature, and the number densities of the various plasma components will all influence sample atomization. 

---