Introduction to Analytical Chemistry

Introduction to Analytical Chemistry

• Analytical chemistry measures matter and consists of powerful ideas and methods.
• It is a measurement science useful in science, engineering, and medicine.
• It characterizes matter both qualitatively and quantitatively.
• Qualitative analysis reveals the identity of elements and compounds of a sample.
• Quantitative analysis indicates the amount of each substance in a sample.
• An analyte is a component of a sample that is determined.

Role of Analytical Chemistry

• Analytical chemistry is used throughout industry, medicine, and all sciences.
• In medicine, it is the basis for clinical laboratory tests that help physicians diagnose disease and chart progress in recovery.
• In industry, it tests raw materials and assures the quality of finished products whose chemical composition is critical, such as fuels, paints, and pharmaceuticals.
• For environmental quality, the air in cities is analyzed for suspected contaminants like carbon monoxide.
• For nutritional value of food, chemical analysis determines major components like protein and carbohydrates, as well as trace components such as vitamins and minerals.
• Analytical chemistry relates to biology, geology, environmental science, agriculture, social science, material science, medicine, physics, chemistry and engineering

Common Analytical Techniques

• Classical techniques include gravimetry and titrimetry.
• Instrumental techniques include spectroscopy and chromatography.

Classical Methods

• Gravimetric methods determine the mass of the analyte or some compound chemically related to it.
• Volumetric titration measures the volume of a solution that contains sufficient reagent to react completely with the analyte.

Instrumental Analysis

• Spectroscopic methods analyze the interaction between electromagnetic radiation and analytes, like IR and UV-VIS spectroscopy.
• Chromatographic methods employ separation techniques based on differences in the rates at which substances are carried by a gaseous or liquid mobile phase through a stationary phase.

Typical Quantitative Analysis

• Step 1: Choose a method that requires experience and intuition. The method should also consider required accuracy, economic factors, and complexity of the sample
• Step 2: Acquire a sample that has the same composition as the source material, especially for large and heterogeneous samples; this is the most difficult step, and a source of errors.
• Step 3: Process the sample where sometimes it is not necessary based on circumstances, where sample preparation is dependent on the different states:
  • Solids must be ground, homogenized, dried, and stored.
  • Liquids must be kept in a sealed container, as solvents may evaporate and change the analyte concentration.
• Step 3: Replicate samples are where portions of a material of approximately the same size are carried through an analytical procedure at the same time and in the same way, which improves the quality of the results and reliability.
• Step 3: Preparing solutions requires a suitable solvent that dissolves the entire sample rapidly and completely by chemical dissolution with heating acids/bases and oxidizing/reducing agents where a sample has a property that is proportional to analyte concentration
• Step 4: Eliminate interferences, or interferents, which are species that cause an error in analysis by enhancing or attenuating the quantity being measured.
• Step 5: Calibrate and measure concentration by determining the proportionality between analyte concentration and a measured quantity.
• Step 6: Calculating results using calculators and computers are based on raw experimental data collected in measurement, the characteristics of instruments, and the stoichiometry of the analytical reaction.
• Step 7: Results are evaluated by estimating reliability by measuring the measure of uncertainty, and performing statistical analysis.

Feedback Control Systems

• A feedback system is a continuous measurement and control process.
• A feedback loop is a cycle of measurement, comparison, and control.

Deer Kill Case Study

• Analytical chemistry is a powerful tool in environmental investigations.
• A park ranger found dead white-tailed deer near a pond, starting an investigation.
• Cause of death needed to be determined so deer kills could be prevented.
• Dead grass around power lines suggested herbicide use, and arsenic is an ingredient in herbicides.
• Samples of dead grass and organs from the deer were taken to test for arsenic and its concentration.
• The Association of Official Analytical Chemists (AOAC) methods determined arsenic quantitatively in biological samples.
• Deer kidneys were removed and analyzed since arsenic eliminates rapidly through the urinary tract.
• Each kidney was cut, homogenized, and blended to reduce size.
• Three 10-g samples of the homogenized tissue were placed in porcelain crucibles as replicates.
• Samples underwent dry ashing to convert organic material to carbon dioxide and water to convert organic matrix.
• Samples were heated cautiously over an open flame followed by heating in a furnace at 555°C for two hours.
• Dry ashing freed the analyte from organic material and converted it to arsenic pentoxide.
• The dry solid was dissolved in dilute HCl for a soluble solution tested for amount of arsenic.
• Arsenic was separated from the solutions with arsine, a toxic gas.
• The solutions from deer and samples were combined with Sn2+ and iodide ions to catalyze reduction.
• The reaction was carried out in flasks to collect the arsine in the absorber solution.
• Interferences remained in the reaction flask to ensure the arsine was collected in the transparent containers called cuvettes.
• Arsine then reacted with silver diethyldithiocarbamate in the cuvette to form a colored complex compound.
• Amount of arsenic in each sample was determined by measuring the intensity of a red color formed in the cuvettes with a spectrophotometer.
• The spectrophotometer provided a number called absorbance that is directly proportional to the color intensity and color concentration of the color.
• Calibration generates a curve by measuring the absorbance of solutions with known concentrations of the analyte.
• Absorbance values are plotted against the concentrations to read the unknown solutions.
• Arsenic is toxic at levels of >10 ppm in kidney tissue.
• Grass samples contained 600 ppm arsenic, which suggested that the deer died from poisoned grass.
• Statistical methods were used to analyze data.
• The average absorbance for replicates is a reliable measure of arsenic concentration.
• Investigators recommended that use of arsenical herbicides be suspended to protect deer and animals in the area.