Quantitative Chemical Analysis

Seventh Edition

Chapter 28

Sample Preparation

Copyright © 2007 by W. H. Freeman and Company

Daniel C. Harris

Depth profile of nitrate in sediment from

freshwater Lake Sobygard in Denmark.

Measeruments were made with a biosensor

containing live bacteria that convert NO3-

into N2O, which was then measured

amperometrically by reduction at a silver

cathode.

Data are for Mn in powdered algae, showing that the sampling constant, Ks, is

approximately proportional to sample mass, m, over six order

Triturador Wig-L-Bug

H3PO4

Vtotal= 23 mL

Pmax= 80 bar

Vacid= 15 mL

1 g de muestra inorgánica

0.1 g de muestra orgánica

Tc (CO2)= 304.2 K

Pc(CO2) = 73.83 bar

MeOH/H2O 20/80 % mol a 24.0

MPa (238 bar)

Extracción líquido-líquido

Extracción sólido-líquido

The analyte may interact with a solid-phase sorbent in at least four ways:

1. Through absorption, the analyte may interact with the sorbent by penetrating its three-

dimensional structure, similar to water being absorbed by a sponge. Three-dimensional

penetration into the sorbent is a particularly dominating process for solid-supported liquid phases.

In the absorption process, analytes do not compete for sites; therefore, absorbents can have a high

capacity for the analyte.

2. The analyte may interact two-dimensionally with the sorbent surface through adsorption due to

intermolecular forces such as van der Waals or dipole–dipole interactions. Surface interactions

may result in displacement of water or other solvent molecules by the analyte. In the adsorption

process, analytes may compete for sites; therefore, adsorbents have limited capacity. Three steps

occur during the adsorption process on porous sorbents: film diffusion (when the analyte passes

through a surface film to the solid-phase surface), pore diffusion (when the analyte passes

through the pores of the solid-phase), and adsorptive reaction (when the analyte binds, associates,

or interacts with the sorbent surface).

3. If the compound is ionogenic (or ionizable) in aqueous solution (as discussed earlier), there

may be an electrostatic attraction between the

Preconcentración

Derivatización

Five questions should be considered when designing a sampling

plan:

1. From where within the target population should samples be

collected?

2. What type of samples should be collected?

3. What is the minimum amount of sample needed for each

analysis?

4. How many samples should be analyzed?

5. How can the overall variance be minimized?

1. From where within the target population should samples

be collected?

Random sample. A sample collected at random

from the target population.