Analytical Chem istry - DePauw University

Chapter 7 Collecting and Preparing Samples307riffle, dumping a gross sample into the riffle divides it in half. By repeatedlypassing half of the separated material back through the riffle, a sample ofany desired size may be collected.A sample thief (Figure 7.10) is used for sampling smaller particulatematerials, such as powders. A typical sample thief consists of two tubesthat are nestled together. Each tube has one or more slots aligned down thesample thief’s length. Before inserting the sample thief into the materialbeing sampled, the slots are closed by rotating the inner tube. When thesample thief is in place, rotating the inner tube opens the slots, which fillwith individual samples. The inner tube is then rotated to the closed positionand the sample thief withdrawn.slotsSa m p l e Pr e s e r v a t i o nWithout preservation, a solid sample may undergo a change in compositiondue to the loss of volatile material, biodegradation, and chemical reactivity(particularly redox reactions). Storing samples at lower temperatures makesthem less prone to biodegradation and to the loss of volatile material, butfracturing of solids and phase separations may present problems. To minimizethe loss of volatiles, the sample container is filled completely, eliminatinga headspace where gases collect. Samples that have not been exposedto O 2 are particularly susceptible to oxidation reactions. For example, thecontact of air with anaerobic sediments must be prevented.openclosedFigure 7.10 Sample thief showingits open and closed positions.Sa m p l e Pr e p a r a t i o nUnlike gases and liquids, which generally require little sample preparation,a solid sample usually needs some processing before analysis. There are tworeasons for this. First, as discussed in section 7B.3, the standard deviationfor sampling, s samp , is a function of the number of particles in the sample,not the combined mass of the particles. For a heterogeneous material consistingof large particulates, the gross sample may be too large to analyze.For example, a Ni-bearing ore with an average particle size of 5 mm mayrequire a sample weighing one ton to obtain a reasonable s samp . Reducingthe sample’s average particle size allows us to collect the same number ofparticles with a smaller, more manageable mass. Second, many analyticaltechniques require that the analyte be in solution.Re d u c i n g Pa r t i c l e Si z eA reduction in particle size is accomplished by a combination of crushingand grinding the gross sample. The resulting particulates are then thoroughlymixed and divided into subsamples of smaller mass. This processseldom occurs in a single step. Instead, subsamples are cycled through theprocess several times until a final laboratory sample is obtained.Crushing and grinding uses mechanical force to break larger particlesinto smaller particles. A variety of tools are used depending on the particle’s