Atomic-Emission Detection Makes Debut

Analytical chemists in a broad range of fields must routinely separate and analyze complex chemical mixtures into individual’ species, some of which are present in extremely low levels. As the demand for research and industrial analysis continues to increase, rapid and accurate qualitative and quantitative analysis of such mixtures is essential. A technique of choice in the analysis of volatile chemical species is gas chromatography (GC), which separates chemical species on the basis of their differential affinity between a mobile gaseous phase and a stationary solid or liquid phase. After GC separation, each species can be analyzed by a variety of detection methods, including ultraviolet, infrared, flame photometric, fluorescence, and atomic-emission. Comparison of experimental chromatograms with known standards allows the chemist to identify and quantitate the presence of specific chemical species. Hewlett-Packard Co., based in Palo Alto, Calif., has developed the first automated benchtop atomic-emission detector (AED) for gas chromatography. AED is based on the principle that each element following excitation to an altered electronic state subsequently emits a characteristic wavelength of light upon return to ground state. With AED, the compound is broken into individual atoms with excited states by a high-energy plasma. Determination of the wavelength and intensity of light emitted by the individual atoms allows the detection and quantitation of any element with the exception of helium. Improvements in the 1980s in plasma stability, optics, and data handling have made GC-AED a commercially viable technique. The HP 5921A detector consists of three major hardware components: a microwave-induced plasma (MIP) light source, a spectrometer, and a photodiode array (PDA). The microwave helium-induced plasma light source is generated in a water-cooled disc tube contained in a newly designed reentrant cavity. The cavity is waveguide-coupled to a magnetron supplying microwave power. This arrangement provides for a stable atomic-emission source that does not require tuning of microwaves. An elliptical mirror in the spectrometer focuses light (160-800 nm) from the plasma onto a slit. The light is then dispersed according to wavelength by a diffraction holographic grating. Each wavelength is focused to a point on a focal curve. The PDA, a multichannel optical sensor, can be positioned at any point along the focal curve to determine wavelengths and, thus, the elements to be detected. Any group of elements that have emission lines within the span of PDA can be detected simultaneously, with a maximum of four elements. This provides a great advantage over a photomultiplier tube, which can monitor only one wavelength at a time. The plot of the detector output at a particular wavelength over time produces a chromatogram. Each peak on the chromatogram represents a different element present in the mixture. By allowing the user to determine elemental composition the HP 5921A AED complements other detection methods that identify chemical species based on different chemical principles yet lack the ability to selectively detect a specific element. Hewlett-Packard claims that the AED is the first device of its kind capable of detecting picogram levels of elements, including oxygen, nitrogen, sulfur, carbon, silicon, hydrogen, chlorine, and bromine. Applications include petroleum and petrochemical processing, environmental monitoring, and chemical manufacturing. Detection limits range from 0.5 pg/sec for carbon to 100 pg/sec for fluorine with selectivities of 10,000 for most elements when compared with carbon. Simultaneous detection of up to four elements per injection is possible, as well as automatic sequencing through any number of elements per sample by changing wavelength and subsequent reinjection. The response factor for the same element in different compounds is constant to within 10%. In addition, nearly constant response factors allow the element ratio of an individual compound to be determined to within 90% accuracy. Detector column compatability ranges from a 100u I.D. fused silica capillary column to a 1/4” I.D. packed column. The system’s computer workstation provides for total automation of analysis and customized data presentation via macroprogramming capabilities. The system, priced at $88,840, includes the HP 592 lA atomic-emission detector, HP5890A gas chromatograph, computer controller with HP 35920A GC/AED software, HP 7673A automatic injector, and HP-ThinkJet printer. Hewlett-Packard V. Richard Sheridan is a postdoctoral associate in pharmacology, and Wendy Wilson Sheridan is associate director of foundation and corporate relations, both at Fox Chase Cancer Center, Philadelphia

Atomic-Emission Detection Makes Debut

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