Direct Current Plasma Atomic Emission Spectrometry (DCP-AES)

Fundamental Principles and Instrumentation

DCP-AES is based on the principles of atomic emission spectroscopy. The key elements of the equipment include the plasma source, the sample introduction system, the optical spectrometer, and the detector. 

The DCP is produced, as opposed to the inductively coupled radiofrequency plasma, of ICP, by a direct current arc maintained between two or three electrodes, which are usually thoriated tungsten. In a common three-electrode inverted Y setup, two blocks of anode and one block of cathode form two arcs of the plasma, which converge at a common overlapping point. Sample excitation takes place in this small, intense (approximately 8000-10,000 K) and stable plasma. Depending on the specific type of sample, the sample introduction system is typically a pneumatic nebulizer and spray chamber that transfers the liquid sample into a fine aerosol and energy to carry the sample to the base of the plasma via a central carrier gas flow, typically argon. The emitted light is then measured, diffracted with a diffraction grating in the optical spectrometer, and the intensity is measured by a photoelectric detector, a photomultiplier tube (PMT), or, in modern systems, a charge-coupled device (CCD).

Comparison with ICP-AES and Other Techniques

DCP-AES is midway between other plasma methods in terms of ability and price. Its major benefit against ICP-AES is that it consumes much less argon gas (1-2 L/min of DCP compared to 12-18 L/min of ICP) and thus, much less cost is incurred when operating. An ICP is also much more stable and much less subject to the matrix effects of easily ionized elements (EIEs) than DC plasma. Moreover, solutions that contain a high concentration of dissolved solids or organic solvents can be more easily dealt with using DCP instruments as compared to many ICP systems. Nonetheless, DCP-AES possesses a number of various drawbacks, which resulted in the prevalence of ICP-AES. The most important is reduced sensitivity and increased detection limits of most elements, which are usually by a factor of 5-10. DCP plasma is relatively smaller and colder, resulting in low atomization and excitation efficiency of refractory elements. It is also affected more by spectral interferences caused by high background emission by the electrodes and molecular bands (e.g., by CN and NO), which may make analysis more difficult. The geometry of the DCP renders it less ideal in the axial view, as opposed to ICP, which is a radial source with a central channel, and this reduces its sensitivity.

Key Applications

Although DCP-AES has been replaced by ICP in most labs, there are still a few distinct application niche areas where it has managed to survive. It was very strong and had low running costs, hence being better suited to high-throughput environmental analysis, such as determining major and minor components (e.g., Ca, Mg, Na, K, Fe, Si) in water samples. It was also applied in geochemical and mining industries for exploration and ore grade control. Its capacity to take high dissolved solids was utilized in the analysis of both brines and industrial process streams. Also, it has been used in the petrochemical industry to analyze wear metals in oils and fuels.

Advantages of DCP-AES

DCP-AES has several strong points that helped it establish its niche in analytical chemistry, especially in the cost-sensitive and high-throughput markets. The greatest advantage is its economic nature of operation, since it only consumes up to 1 to 2 liters per minute of argon gas as opposed to the 12 to 18 liters per minute of argon gas that the Inductively Coupled Plasma (ICP) systems demand. This leads to a very low running cost, and it therefore becomes a good choice in a laboratory with a high volume of samples. Moreover, the DC plasma is well known for its great stability and analytical soundness. It is very tolerant of complex sample matrices, such as those containing high total dissolved solids and organic solvents, both of which may be unstable or cause plasma extinction in ICP systems.

Limitations of DCP-AES

The drawbacks of DCP-AES are severe, even though it has its benefits and has contributed to the general substitution of this method in most contemporary laboratories by ICP. The least significant disadvantage is that it has a lower ability to analyze than ICP-AES, with most of the elements having a detection limit that is 5 to 10 times lower than that of ICP-AES. This limits its use in the ultra-trace level analysis in other areas, such as environmental or clinical research. It also has more serious spectral interferences, mainly because of the high background emission by the tungsten electrodes and molecular bands by species such as cyanogen (CN), which make the identification of spectral lines more difficult and may necessitate more complex background correction algorithms.

Conclusion

DCP-AES is a mixture method of analysis in history, which brought about a stable and cost-effective method of analysis of multiple elements. Although better sensitivity, larger elemental coverage, and reduced spectral interferences of Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) have established it as the method of choice in contemporary analysis laboratories, the concept and the use of DCP-AES are still worth noting. Awareness of its ability and weaknesses has a good context for the development of analytical chemistry and draws attention to how certain technical characteristics have been used to address specific analytical problems.