Atomic spectroscopy is the technique for determining the presence and concentration of an elemental analyte by its electromagnetic or mass spectrum. It is increasingly applied as part of impurity analysis of products and package systems as referenced in USP chapters and their EP counterparts such as <232 / 233>, <661.1 / 661.2>, <381>, and much more. Several analytical techniques, or methodologies, exist, and an understanding of their relative performance and limitations is critical in achieving accurate and reliable real-world results.
There are three widely accepted analytical technologies for atomic spectroscopy – atomic absorption, atomic emission, and mass spectrometry. The most common types of instruments incorporating these technologies include:
- Flame Atomic Absorption Spectroscopy (Flame AA)
- Graphite Furnace Atomic Absorption Spectroscopy (Graphite AA)
- Inductively Coupled Plasma Optical Emission Spectroscopy. (ICP-OES)
- Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
The operating principle, potential benefits, and disadvantages of each major technology will be outlined below.
An Overview of Atomic Absorption (AA) Spectroscopy:
Atomic Absorption (AA) refers to the phenomenon in which electrons in a ground state atom absorb light energy and reach an excited state, moving to a higher energy level. Generally, different atoms (elements) are excited by different wavelengths of light. By passing light at the appropriate wavelength through an atomized sample, the amount of energy absorbed at the wavelength corresponding to the element of interest (analyte) can be measured. As concentration of the analyte increases, the amount of light energy absorbed at this wavelength will increase. This relationship between energy absorption and the concentration of analyte can be used to determine unknown sample concentrations by interpolation based on measurements of known standards.
Performing atomic absorption spectroscopy requires the following:
- A primary light source
- An atomizer / atom source, which breaks the sample down into its individual elements
- A monochromator to isolate the specific wavelength of light to be measured
- A detector to measure the light accurately
- Electronics to process the data signal
- A display or software to interpret and report the results
The general sequence of steps for Atomic Absorption are as follows, with additional considerations depending on the type of AA technology used.
A lamp is installed in the AA unit. Generally, the light source is a hollow cathode lamp (HCL) or an electrodeless discharge lamp (EDL), with different lamps being used for different analytes (elements of interest). Since different elements are excited by different wavelengths of light, lamps able to emit light at the desired wavelength must be used. In some cases, a few elements may be combined in a multi-element lamp.
The liquid sample is nebulized and sprayed into an atomizer, or atom source. In AA, the source of energy for free-atom production is heat, though as discussed below, different approaches to this are available. The heat source drys the sample and atomizes it, breaking it down into individual elements. The light at a certain wavelength is passed through the atomized sample, where it is absorbed by elements of interest, and that energy absorption is measured.
Flame Atomic Absorption Spectroscopy:
In flame AA, the atomizer, or atom source, is most commonly in the form of an air/acetylene or nitrous oxide/acetylene flame. The sample is introduced as an aerosol into the flame, which is aligned so that the light beam passes through the flame, where the light is absorbed, measured, and reported by the instrument. While flame AA is rather ubiquitous and inexpensive relative to other technologies, imprecise atomization, sample retention, and beam pathways yield less sensitivity than technologies described below.
Graphite Furnace Atomic Absorption Spectroscopy:
In comparison to flame AA, Graphite Furnace Atomic Absorption (GFAA), the sample is introduced directly into a graphite tube. The tube is then heated according to a programmed series of steps to remove liquid solvent and to atomize the remaining sample. Because the sample is introduced to the tube, and not a spray chamber as with flame AA, the entire sample is atomized, and the atoms are retained within the tube for an extended period. Light is passed through this tube and absorbance measured. As a result of this improved analyte control relative to flame AA, sensitivity and detection limits are significantly improved.
However, graphite furnace analysis times are longer than those for flame AA. In addition, fewer elements can be determined using GFAA based on atomization programs. However, the enhanced sensitivity of GFAA, and its ability to analyze very small samples, significantly expands the capabilities of atomic absorption as an analytical technique.
Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES):