Sample preparation in analytical chemistry refers to the ways in which materials are treated prior to their analysis. Proper preparation is critical, as the analytical results are worthless if not representative of the full process.
The sample preparation path includes collecting samples from a running process or out of inventory, size reduction to a size suitable for the analytical tools, and a final subdivision to the minute amount needed for the analysis—all without corrupting the materials with contaminates. Steps such as extraction, chemical reactions, dissolution, chelating, dilution, filtering, and so on are part of sample preparation and these would be handled by the analytical laboratory.
Acquisition of the sample can range from very large amounts to minute quantities. The critical point is that this initial sample collection must represent the entire mass. In production facilities, the incoming raw materials can consist of solid minerals or chemicals, as well as liquids.
The sample can come from a stationary location, such as pulling a sample from a storage pile with a thief, auger or lance. These rod-type devices are basically tubes that are pounded into the mass; with a twist to secure the sample at the end, the sample is trapped and pulled back out of the pile. Due to the way solids fall and land into a pile (angle of repose), this method requires extractions from various locations throughout the mass to be somewhat representative. Digging deep into a large pile is problematic.
When the sample can be recovered off of running conveyor belts, the collection can be at one localized area in the path of the materials. Samples can be swept off the top of the belt or grabbed when spilling off the belt’s end when being dropped down to a lower level. A bucket moving over time back and forth through the flowing stream can collect a representative sample.
Another capture method involves a bent rotation tube within a housing. As the tube sweeps past an outlet port, a subsection of the mass is removed for later analysis. These are designed with the option of pulling one or more samples. The size of the outlet port determines the fractional collection ratio. A gate valve on the outlet allows for adjustments in the field.
Later, during the processing steps, the samples can be drawn from powder mixes prior to casting or press forming, from pressed parts still in the green state, or final fully sintered parts. Using green or sintered parts will most likely mandate the destruction of each of these.
Sample lots should be placed in sealed containers with proper labeling and matching documentation, which ensures that the completed analytical work can be traced back to a particular processing activity. If the batch passes quality control standards, the goods can be released for sale. If a failure arises, then process adjustments are mandated.
Each analytic instrument has a multitude of physical requirements for the intended samples. Generally speaking, the samples must be small and obviously representative of the mass from which they originated. Methods for grinding samples to the required size can broadly be grouped into four classes: impact (hammering), compression (squeezing), abrasion (filing), cutting (slicing), and combinations of these.
Impact equipment such as hammer mills feature rotating arms that strike the inflowing samples. The arms can be fixed to the hub to be rigid or hinged to be flexible. Once small enough, the ground powder exits through an outlet screen that controls particle size. The screen traps oversized particles for additional passes until they are small enough. Usually, the output is about half the screen opening. This is because the sample hits at an obtuse angle, not perpendicular to the screen’s exit hole.
Another class of impact mills has loose impact elements; these include ball mills and dish-and-puck mills. Here the sample is loaded into a container with a grinding media (e.g., balls, rods, puck, rings, and other shapes). The entire container is then shaken, rolled or vibrated (or another violent motion) to force the media against the subject samples.
The compression method might be viewed as a slower moving impact. There is a pinch point where the samples need to crumble in order to escape. Jaw crushers have two plates fitted at a “Vee” angle, with the samples entering the larger top opening and exiting through a gap at the bottom. This gap separation sets the size of the largest particle allowed to exit. Another configuration would comprise two or more parallel rollers with just a small gap between them. This gap is the pinch point that breaks the samples.
Abrasion (galling) is where a hard, irregular surface such as a file or stone discs shave off sections from the sample. The separation gap between faces sets the outlet size. This equipment, which does not have outlet sizing screens, is often used for wet, soggy samples that would blind a screen if present. In addition, this method could be used when a sample is needed from a finished product for quality testing, but the entire workpiece is not to be sacrificed. A scraping can be removed from a non-critical surface of the workpiece with a file.
The cutting action on the samples can be from a free moving knife edge (similar to a sword) or a pinching action of rotating knives moving very close to stationary knives (similar to a scissors). The gap distance in this second design sets the particle size in one direction. If a long, skinny particle is developed, it will be trapped by an outlet screen and returned for additional comminution. Even with the rework and random tumbling of the sample, there is a slight chance that long skinny particles can still be found in the outlet. The dulling of the knife edge can occur if the samples are too hard.
For any piece of equipment, there can be combinations of these four actions occurring simultaneously. For example, in a rolling ball mill, the samples are broken by the direct impact of the grinding media, but some size reduction happens by abrasion as particle rub against their neighbors in the bed of solids.
It is likely that only a minute amount of the prepared sample is needed for feeding the analytical equipment. A subdividing step is once more needed and can be accomplished through a number of different methods. Quartering is an older, manual method whereby the entire sample is placed onto a flat surface or fabric material (e.g., cloth, paper, canvas). The pile is then mixed by a spatula or by lifting the fabric’s corners. Next, the pile is cut into four sections, with two opposite sections removed. The remaining pile is again mixed, cut into fours, and two opposite sections removed. This is done until the final needed small sample quantity remains.
With a simple, low-cost riffle sample splitter, the sample is placed into a hopper that sits above several chutes or slots. Then the powder falls through the bottom, being cut into two subsamples (one to be discarded). This process can be repeated until a final sample quantity is attained
Two mechanical methods include the spinning tube described earlier for larger sample quantities, and a spinning bottle riffler for smaller amounts (see Figure 1). In the latter, the sample is fed from a hopper by a vibrating feeder into a spinning manifold with multiple outlet ports. Beneath these ports, bottles or collection bags are fixed that catch the samples. Each collection vessel has similar composition and quantity.
A final method to produce a representative subsection of the milled sample is simply by blending. All of the ground sample is placed into a high-precision powder blender. When the entire batch is homogenous, any small amount scooped out would represent a valid subsection.
Mlling Equipment Selection
Points to be addressed when selecting milling equipment include:
- Size in and target size out. It is hard to get more than 1:10 or 1:15 size reduction in one step. Multiple runs with different outlet screen sizes may be needed. Also, a very large starting chunk will require a very large mill opening, even if only a small amount of material is to be processed.
- Contamination of the sample as parts of the grinding equipment are worn. The wear of equipment surfaces, or dulling of knife blades, will be accelerated when processing very hard materials. Use of materials of construction that are not in the samples prevents mill wear from corrupting the analysis.
- Be sure the particles are brittle and will break in impact or compression equipment. Some materials will just smear, such as malleable gold. Softer or filamentous materials can be cut in a knife mill.
- Safety issues need to be addressed, including employee dust exposure or explosion of fines.
- Clean out between samples so that cross-contamination between samples is reduced.