Industries Information

Atomic emission and optical emission spectrometers determine analyte concentration via a quantitative measurement of the optical emission from excited atoms.  Analyte atoms in solution are aspirated into the excitation region where they are desolvated, vaporized, and atomized by a flame, discharge, or plasma. These high-temperature atomization sources provide sufficient energy to promote the atoms into high energy levels. The atoms decay back to lower levels by emitting light. Since the transitions are between distinct atomic energy levels, the emission lines in the spectra are narrow. The spectra of samples containing many elements can be very congested, and spectral separation of nearby atomic transitions requires a high-resolution spectrometer. Since all atoms in a sample are excited simultaneously, they can be detected simultaneously using a polychromator with multiple detectors. This ability to simultaneously measure multiple elements is a major advantage of AES compared to atomic-absorption (AA) spectroscopy.

Atomic emission and optical emission spectrometers characterize all forms of emission from atoms. These are principally electrons, x-rays and optical photons. So strictly, therefore, AES should include the spectral measurement of all types of atomic emissions.

Optical emission describes emissions in the optical range, i.e. UV-Vis-IR. Such emissions are characteristically different from emissions at very short wavelengths (x-rays and gamma rays), and from emissions at long wavelengths (far-IR and radio waves). In particular, different types of instruments are used to detect them. Optical emissions are generally detected with optical spectrometers.

Thus OES is a part of AES, and all of the techniques of interest could be called AES, but since they all exclusively measure optical emissions with optical spectrometers, the use of OES is much more precise.

Atomic absorption spectrometers (AA) use light absorption to measure the concentration of gas-phase atoms. An analyte, usually a solid or liquid, is vaporized in a flame or in a graphite furnace with a temperature of 1000° - l 200° K. Atoms absorb ultraviolet or visible light and achieve higher energy levels. Absorption amounts determine the analyte concentration.

Atomic absorption spectrometers can measure analyte concentrations over a spectral range or a dynamic range. The spectral, or wavelength, range captures the dispersion of the grating across the linear array. This amount is also expressed as the “size” of the spectra on the array. The dynamic range, also known as the linear dynamic range or the linear range, is the range over which a response is a well-defined (usually linear) function of the analyte concentration. To vary the dynamic range, operators adjust instrumental parameters to, for example, decrease the absorption path length and sample volume.

Flame atomic absorption and graphite furnace atomic absorption (GFAA) are vaporization methods used with atomic absorption spectrometers. Flame atomic absorption, a common technique for detecting metals and metalloids in environmental samples, is based on how ground state metals absorb light at specific wavelengths. Applying a flame converts metal ions in a solution to an atomic state. Light of the appropriate wavelength is supplied and the amount of light absorbed is measured against a standard curve. Graphite furnace atomic absorption (GFAA) is a highly sensitive spectroscopic technique that measures each element sequentially. GFAA is useful when samples are very small, when very low levels of detection are required, and when matrices are dilute or volatile. Furnace parameter adjustments and matrix modifiers allow operators to optimize the analytical method for each element.

To convert radiant energy from a light source to electricity, atomic absorption spectrometers include a detector. Typically, the detector is a photomultiplier tube; however, some instruments use a solid-state detector. Photomultiplier tubes (PMTs) are photo-detectors with adjustable voltages that translate optical signals into electrical current. Increasing the PMT voltage increases the output signal for a given amount of light. Photodiodes are semiconductor devices used to detect light and generate an electrical current. Typically, photodiodes are used in forward scatter (FSC) detection.

Atomic absorption spectrometers allow operators to adjust sample temperatures and program fluorometers. Some spectrometers are self-calibrating, position lights automatically, or compensate for stray light emissions and various spectral interferences that bias analytical results. Intrinsically safe (IS) instruments do not release sufficient electrical or thermal energy to ignite hazardous atmospheric mixtures.

Atomic absorption spectrometers provide different adjustment and display options. Some spectrometers have an analog panel with dials and switches; typically, an analog meter or simple visual indicator displays data. Other instruments have a digital front panel with keypads and menus, and provide digital readouts or video displays. Some atomic absorption spectrometers also include application software or serial, parallel, or other computer interfaces.

Titration instruments are used to determine the concentration of dissolved substances. Titration is based on a complete chemical reaction between the analyte and a reagent or titrant of known concentration that is added to the sample. The analyte is the substance which a laboratory test is designed to measure. The reagent or titrant is the substance that produces a chemical reaction in order to determine the presence of the analyte. There are two basic titration methods: manual and automatic. Manual titration is performed with a burette, a piece of laboratory glassware that has volumetric graduations along its length and a tap or stopcock on its bottom. Automatic titration is performed with an auto-titrator, an electrically-powered laboratory instrument that can be interfaced to a personal computer (PC). Typically, automatic titration instruments are used for repetitive titrations.

Titration instruments differ in terms of addition and indication methods. There are two titrant addition methods: volumetric and coulometric. Volumetric devices add the titrant directly to the sample. Coulometric devices generate the titrant electromechanically. There are many different titration indication methods. Voltametry, a technique also known as the Karl Fischer water determination method, measures the concentration-dependent potential of a solution against a reference potential. Potentiometry, redox and precipitation measure the potential at a constant electric current. Photometry, complexometry, and turbidimetry measure the light transmission of a colored or turbid solution with a photometric sensor. With amperometry, the current flowing in a sample is measured at a constant potential. Titration instruments that measure conductivity include a conductivity meter while devices that measure temperature include a sensor.

Selecting titration instruments requires an analysis of product specifications, features, display types, and computer interfaces. Product specifications for titration instruments include concentration range and reaction time. Some instruments provide data storage and temperature-compensation features. Others measure solids or gases. In terms of display types, titration instruments with analog or digital front panels are commonly available. Analog user inputs include potentiometers, dials and switches. Digital front panels can be setup or programmed using a digital keypad or menus. Titration instruments that can be controlled or monitored via a computer interface use serial or parallel communications and may include application software. RS232, RS485 and universal serial bus (USB) are common serial interfaces. The general-purpose interface bus (GPIB) is a common parallel interface.

Solid phase extraction (SPE) is an extraction method that uses a solid phase and a liquid phase to isolate one, or one type, of analyte from a solution. It is usually used to clean up a sample before using a chromatographic or other analytical method to quantify the amount of analyte(s) in the sample. The general procedure is to load a solution onto the SPE phase, wash away undesired components, and then wash off the desired analytes with another solvent into a collection tube. Solid-phase extractions use the same type of stationary phases as are used in liquid chromatography columns. The stationary phase is contained in a glass or plastic column above a frit or glass wool. The column might have a frit on top of the stationary phase and might also have a stopcock to control the flow of solvent through the column. Commercial SPE cartridges have 1-10 mL capacities and are discarded after use. Separation techniques covers a wide range of instrumentation used to separate samples into their component parts for analysis and measurement. Centrifuges, capillary electrophoresis, laboratory filters, titration, Solid-phase extraction (SPE), and chromatography are major groupings within this family. Chromatography instruments are divided into two main areas, high performance liquid chromatographs (HPLC) and gas chromatographs (GC), and their related columns and detecting devices.

Laboratory filters are used to remove particulates from samples in laboratory-scale applications. They consist of a filter medium and housing or holder that constrains and supports the filter media in the sample’s path. There are several basic types of laboratory filters. Membrane filters are thin, polymeric films that contain thousands of microscopic pores. The size of the pores determines the size rating of the membrane. Typically, membrane filters are used in the quantitative separation or filtration of suspended matter from liquids and gases. Pre-filters are often placed upstream from membrane filters to reduce particulate loading and allow the membrane to operate more efficiently. Centrifugal filters are suitable for small-scale laboratory separations such as protein or nucleic acid desalting and concentration. These devices use centrifugal force to drive liquid through the filter. Increasing the centrifuge speed (G force) increases the pressure across the filter. Fouling is minimized by design features that cause the G force to reach the filter at an angle, sweeping accumulated molecules from the surface. Syringe filters consist of a filter element and housing assembly and are used in applications where a sample must be filtered before entering a syringe. Specialized and proprietary filters that are made from a variety of porous papers are also available.

Laboratory filters vary in terms of configuration, sample type, filter paper measurement type, filter medium material and housing material. Configurations usually consist of a filter element, a housing element, or both a filter element and housing assembly. Filters that are used with solid, liquid or gaseous sample types are commonly available. There are two filter paper measurement types: qualitative and quantitative. Qualitative filter papers have an ash content that is ten times higher than quantitative filter papers. Common filter medium materials include cellulose, cellulose acetate, nitrocellulose, and regenerated cellulose; ceramic, carbon, and glass fiber materials; polytetrafluoroethelene (PTFE), polyvinylidene fluoride (PVDF), and polyvinylidene chloride (PVDC); and polypropylene (PP), polysulfone (PSU), and polyethersulfone (PES). Housing materials for laboratory filters include acrylics, plastic acrylics, modified acrylics, and polypropylene (PP).

Selecting laboratory filters requires an analysis of both physical specifications and performance specifications. Physical specifications include sample size, pore size, filtration area, and filter shape. Sample size, the maximum amount that filters can accept, is especially important for centrifugal filters and syringe filters. Pore size indicates whether particles of a specific size are retained with an efficiency rate less than 100%, typically 90% – 98%. Rating methods vary widely among manufacturers. The effective filtration area (EFA) is the total, usable filter area. As a rule of thumb, the larger the filter area, the faster the flow rate at a given pressure differential and the larger the throughput volume prior to clogging. Most filters are circular or rectangular in shape. Performance specifications for laboratory filters include particle retention size, flow rate, porosity, hold-up volume, and maximum pressure. Flow rate determines the volume of a liquid that flows through the filter at a fixed pressure and temperature. Hold-up volume is the maximum volume of a sample that a filter can retain.