Mastering Analytical Chemistry: Error Sources and Sample Preparation

Errors of Measurement

Large errors are due more often, even when incorrect sampling is applied, or an application of the method is not suitable. The most likely causes of errors are:

1. Material is strategically in the form $k$ is done, but does not take into account the disposition of strata; i.e., $k$ is not proportionately represented.
2. Analytical property $k$ is measured, which varies non-uniformly from the surface to the center.
3. $k$ in suspension during transport; there is a separation of particles, making it impossible to shake the entire volume for a homogeneous sample.

Preparation of Materials

Reagent Concentrations:

• Aqueous reagents: Physical concentration (% P, % V).
• Aqueous reagents: Chemical concentration (M, N, m).

Treatment of the Sample

Before conducting the analysis, it is necessary to homogenize the gross sample.

• If it is a fine powder, liquid, or a homogeneous suspension, it may be enough to obtain a laboratory sample.
• If it is a solid, it needs to be sprayed in a mortar. Additionally, this should be dried at 110 °C in a muffle furnace. Subsequently, dissolve it to perform the analysis. If the sample does not dissolve under moderate conditions, or if the reagents dilute it under moderate conditions, acid digestion or fusion can be used. Acids such as HCl, HNO3, H2SO4 may be used. In some cases, the organic matter should be diluted to free the inorganic elements (e.g., in the determination of certain metals in plants).

Elimination of Interference

The analysis often shows species that may interfere with the determination of the analytes.

It is important to distinguish whether they produce a signal or by the attention being paid; thus, eliminating them is necessary. Interference elimination can be accomplished in two ways:

• Chemically: Using masking agents that react with the interfering substance to form an inert species. This is often achieved with pH adjustment.
• Physically: Separating or determining them previously by precipitation, distillation, or extraction chromatography.

Separation Mechanisms

The following separation mechanisms exist:

• Separation by precipitation: Based on the existence of large differences in solubility between the analyte and the interference.

Some of the most commonly used precipitating agents are:

1. Separation based on the control of acidity.
2. Separation of sulfides, with the exception of alkaline earth metals and alkali metals.
3. Other inorganic precipitates, such as chlorides and sulfates.
4. Organic precipitation.
5. Ionic separations of constituents in trace quantities.
6. Separation by electrolytic precipitation.

Other aspects to be addressed, complementary to the elimination of interferences, are:

• Separation by extraction.
• Ion-exchange separation.
• Inorganic species separation by distillation.
• Chromatographic separation.