Analytical Chemistry

Sample preparation affects how accurate a chemical measurement will be. In analytical chemistry, a sample must be treated carefully before analysis so it can serve as a standard or an unknown sample.

Errors in analytical methods are usually grouped as random or systematic. Random errors come from change and are often linked to noise in the instrument. Systematic errors come from investigator bias or instrumental bias, which creates an offset in the measured value.

Errors made during sample...

Accurate chemical analysis depends on careful sample handling and transfer. When a sample is moved or prepared, some of it can be lost. That loss can make it harder to measure the true concentration of an unknown sample.

One way to improve accuracy is to reduce the number of handling steps. For example, a solid sample can be weighed directly into the flask where the solution will be made. If a sample must be moved from one flask to another during a dilution, triple rinsing the glassware helps...

The method of standard additions is a quantitative analysis method used to correct matrix effects in a sample. Matrix effects happen when other components in the sample reduce or enhance the analyte absorbance signal, which can lead to major errors in the final result.

This approach is often used when a sample contains multiple components that may interfere with the measurement. It is also useful for correcting chemical phase separations that occur during extraction. The key idea is that the...

Calibration curves help measure how an instrument responds to an analyte and how to estimate the concentration of an unknown sample. To build one, scientists prepare standard samples at several concentrations that include the expected range of the unknown. They then record the instrument response for each standard.

To improve accuracy, the response at each concentration can be measured more than once. Repeated measurements help show the error, often as error bars. The collected data are then...

UV-Vis spectroscopy is a common analysis method used to study how samples absorb ultraviolet and visible light. It works by sending UV-Vis light through a sample and measuring how much light passes through. From that transmittance, or light that gets through, the absorbance can be calculated as A = -log(T).

The result is an absorbance spectrum. This graph shows how much a compound absorbs at different wavelengths. The absorbance pattern depends on the chemical structure of the molecule.

Raman spectroscopy identifies molecules and materials by their Raman fingerprint. It measures vibrational and other low-frequency modes in a sample. In chemistry, this makes it useful for identifying molecules. In solid-state physics, it helps characterize materials and examine crystal structure or crystallinity.

Raman micro-spectroscopy has several practical advantages for material testing. It is non-destructive, usually needs no sample preparation, and can be used with very small sample...

X-ray fluorescence (XRF) is a type of induced radiation that gives spectroscopic information. It is used to detect emitted signals from metals after they are excited by X-rays.

X-ray fluorescence microscopy is a non-destructive imaging method. It uses the fluorescence emission of metals to identify where they are found in a sample and to measure their spatial distribution.

Gas chromatography (GC) separates and detects small, volatile molecules in the gas phase. The sample can start as a gas or as a liquid that is vaporized in the injection port. GC is usually used for compounds under 1,000 Da because larger molecules are harder to vaporize. It is often chosen for non-aqueous samples and for organic compounds in environmental and industrial work.

GC is especially useful when the target molecules are small and nonpolar. Liquid chromatography (LC) is more common...

High-performance liquid chromatography (HPLC) is used to separate and measure components in liquid samples. It works by pumping a solution through a column packed with small porous particles. A second phase is bound to the particle surface inside the column. Because sample components dissolve differently in these two phases, they move through the column at different average speeds and separate from one another.

In this method, the pumped solution is the mobile phase, and the material inside...

Ion-exchange chromatography is a separation method that cleans up samples by using charge. A column is packed with an ion-exchange resin, which is a charged stationary phase on a solid support. Strong cation-exchange chromatography uses a negatively charged resin to separate cations. Strong anion-exchange chromatography uses a positively charged resin to separate anions.

This method is often used to prepare samples before further tests. It can help clean up protein samples or nucleic acid...

Capillary electrophoresis is a separation method that uses an electric field to sort molecules by size and charge. It is carried out in a small glass tube called a capillary. The capillary is filled with an electrolyte solution, which helps the analytes move during the separation.

Molecules separate because they have different electrophoretic mobility, or different speeds in the electric field. That mobility depends on charge, solvent viscosity, and size. Because the capillary has a large...

ICP-MS is a mass spectrometry method used to detect elements in a sample, with iron shown here as the example. Mass spectrometry helps identify unknown compounds, measure known materials, and study the structure and chemical properties of molecules. It does this by turning chemical compounds into ions so they can be analyzed.

A mass spectrometer has three main parts: an ionization source, an analyzer, and a detector. In inductively coupled plasma, or ICP, samples with elements of interest...

Scanning electron microscopy, or SEM, uses electrons to create detailed images of conductive samples. It is a powerful tool for seeing surface features at magnifications that are much higher than a traditional light microscope can reach. Modern light microscopes can reach about 1,000X, while a typical SEM can reach more than 30,000X.

Because SEM does not use light to form an image, the pictures appear in black and white. The sample is placed on the SEM stage and the chamber is brought to a...

This page explains how supported catalyst electrodes are prepared for electrochemical testing with a potentiostat/galvanostat. A potentiostat/galvanostat measures current at an applied potential, or potential at an applied current. It is a standard tool for studying anode and cathode materials used in fuel cells, electrolyzers, batteries, and supercapacitors.

The setup uses a three-electrode electrochemical cell. The potentiostat leads connect to the reference electrode, counter electrode, and...

Cyclic voltammetry is an electrochemical method that scans a range of potential voltages while current is measured. It uses a stationary electrode that is immersed in the sample. The potential is swept from a starting value to a switching potential and then swept back again.

This forward scan and reverse scan create a cyclic sweep of potentials. The current versus potential plot from the data is called a cyclic voltammogram. The minimum and maximum potentials in the test define the scan window.