Continua tabla…

Permeability: An SPE device must be

permeable in order to process a sample.

The rate at which the sample is processed, for

example in mL/min., depends upon several

properties of the system. These include the

viscosity and particulate content of the sample;

and the length, pressure difference (or

centripetal g force) and specific permeability,

B,, of the sorbent bed. The specific

permeability parameter is defined for a packed

bed, where the fluid is particle-free, in Equation

below:

where q = flow rate, = fluid viscosity, L = bedlength, ΔP = pressure difference across bed,and A = cross-sectional area of bed.

Performance Characteristics

Using SI units throughout, the units of B,are m2 and the larger its value, the morepermeable is the bed. Table 6 lists variousSPE devices and a 5 m HPLC column forcomparison, showing typical values of B0.

2. Processing Systems

This section describes basic manual systems and accessories with which anSPE device is processed. Manual processing systems can be classified intotwo types. They are:

1) Single sample, single device, serial processing2) Multiple sample, multiple devices, parallel processing

Continúa..

Surface Coverage: This property is often used to characterize a silica based

sorbent used in SPE and is commonly defined as the ratio of the number of moles

of ligands per gram of sorbent, to the surface area of the sorbent in square meters

per gram. Another definition uses the ratio of the moles of carbon (in a

hydrocarbon-based bonded phase) per gram to the surface area in meters per gram.

Table 17 is a set of data from sorbents of various manufacturers, giving values ofcarbon coverage for specific C18 sorbents.

One of the important performance criteria of SPE sorbents and products that is

frequently measured is that of recovery. As it was explained above, the analyst will

seek 100% recoveries and perhaps more importantly, reproducibility of this property.

Most of the larger manufacturers of SPE devices have published data in support of

reproducibility of recovery.