Kinetic samplers

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. 

Atmospheric kinetic samplers, sometimes referred to as diffusion samplers , have a long history of use in roles such as personal monitors or dosimeters to evaluate personal exposure or... 

Systematic attempts have been made to develop passive sampling systems that accumulate chemicals, and from which reliable exposure concentrations can be calculated. The passive samplers used in such systems are usually designed either as "kinetic samplers" or as "equilibrium samplers."... 

In the last decade the importance of polar organic pollutants has resulted in the development of kinetic samplers for polar organic pollutants. These types of samplers typically... 

Bartkow, M.E. Hawker, D.W. Kennedy, K.E. Muller, J.K. 2004, Characterizing uptake kinetics of PAHs form the air using polyethylene-based passive air samplers of multiple surface area-to-volume ratios. Environ. Sci. Technol. 38 2701-2706. 

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... 

Booij et al. (2003b) made an effort to model contaminant uptake by buried passive samplers. The major assumptions underlying this model are that the sampler can be regarded as an infinite sink for target contaminants, that the depletion of the bulk sediment phase is insignificant, and that the contaminant desorption kinetics are not rate-limiting. 

Few reports are available on the potential effect of chemical concentration on the BAF in an aquatic organism (e.g., Mayer, 1976). Yet, a key assumption of EP theory is the independency of BAF relative to exposure concentration. To our knowledge, there is only one report (Huckins et al., 2004) in the peer-reviewed literature, where the effect of chemical exposure level on concentration factors (CFs) or BAFs has been tested in side-by-side BMO and passive sampler exposures. Huckins et al. (2004) defined CF as the ratio of the concentration in a sample matrix (whole body [soft tissues in the case of bivalves] or whole SPMDs) relative to the concentration in the ambient exposure medium at any moment in time, whereas the A sw and BAF (includes biomagnilication) represent the maximal CF. Similar to ATs s and BAFs, CFs are expected to be independent of exposure concentrations, when residue exchange follows first-order kinetics. 

The apphcations described here illustrate the wide range of uses for robotic systems. This chapter is not intended to he exhaustive there are many other examples of successful applications, some of which are referenced below. For instance, Brodach et al. [34] have described the use of a single robot to automate the production of several positron-emitting radiopharmaceuticals and TTiompson et al. [3S] have reported on a robotic sampler in operation in a radiochemical laboratory. Both of these apphcations have safety imphcations. CHnical apphcations are also important, and Castellani et al. [36] have described the use of robotic sample preparation for the immunochemical determination of cardiac isoenzymes. Lochmuller et al. [37], on the other hand, have used a robotic system to study reaction kinetics of esterification. 

The exchange kinetics between a passive sampler and the water phase can be described by a first-order, one-compartment mathematical model ... 

With kinetic or TWA sampling, it is assumed that the rate of mass transfer to the sorption phase is linearly proportional to the difference between the chemical activity of the contaminant in the water phase and that in the sorption phase. During the initial phase of sampler exposure, the rate of desorption of analyte from the sorption phase to water is negligible and the sampler works in the linear uptake mode. The amount of analyte accumulated is therefore linearly proportional to its TWA concentration in water, even for situations where aqueous concentrations fluctuate over time (Figure 3.2). In this case Equation 3.1 reduces to... 

Analytes may accumulate in the sorption phase either by adsorption onto the surface of solid sorbent materials or by absorption in absorbent liquids or polymers that behave like subcooled liquids.The advantage of solid adsorbents is the potential to select materials with a high affinity and selectivity for target analytes. However, the sorption capacity of adsorbents is usually limited, and the description of adsorption/desorption kinetics of analytes to adsorbents is complex. Typically, the adsorbent materials used in passive samplers are similar to those used in solid-phase extraction techniques. 

A number of models has been developed to improve the understanding of the kinetics of analyte transfer to passive samplers.9,12,19,37 These models are essential for understanding how the amount of analyte accumulated in a device relates to its concentration in the sampled aquatic environment as well as for the design and evaluation of laboratory calibration experiments. Models differ in the number of phases and simplifying assumptions that are taken into account, for example, the... 

FIGURE 3.4 Biofouling reduces the exchange kinetics of PRCs (deuterated acenaphthene and fluorene) between the nonpolar Chemcatcher sampler (fitted with either fouled or unfouled LDPE membranes) and water. The experiment was performed in a laboratory flow-through calibration system at a water temperature of 11°C with simulated water turbulence of 40 rpm. MD(t)/MD(0) is the fraction of the PRC remaining in the sampler during exposure. 

Mazzella, N., J.-F. Dubemet, and F. Delmas. 2007. Determination of kinetic and equilibrium regimes in the operation of polar organic chemical integrative samplers application to the passive sampling of the polar herbicides in aquatic environments. J. Chromatogr. A 1154 42-51. 

A multipole cell at pressures around 1 to 15 mtorr, placed between the sampler-skimmer interface and the mass spectrometer, can serve two functions reduce the kinetic energy of the ions to nearly thermal energies (<0.5 eV) and carry out reactions with analyte or background ions. Of particular interest for ICP-MS are reactions that would dramatically reduce spectral overlaps due to elemental or polyatomic ions. Two potentially undesirable processes must be considered for successful use of a collision-reaction cell. Scattering losses can be severe if the mass of the collision or reaction gas is high compared to that of the analyte ion... 

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