Direct-current plasma
Direct-current plasma (DCP) is a type of plasma source used for atomic emission spectroscopy that utilizes three electrodes to produce a plasma stream.Skoog, D. A.; Holler, F. J.; Crouch, S.R. Principles of Instrumental Analysis, 6th ed., Brooks Cole, 2007; pp. 258-259. {{ISBN|9780495012016}}. The most common three-electrode DCP apparatus consists of two graphite anode blocks and a tungsten cathode block arranged in an inverted-Y arrangement. An argon gas source is situated between the anode blocks and argon gas flows through the anode blocks.{{Cite web |url=http://www.chemistry.nmsu.edu/Instrumentation/DCP.html |title=NMSU: DC Plasma |access-date=2012-04-24 |archive-date=2012-01-11 |archive-url=https://web.archive.org/web/20120111011714/http://www.chemistry.nmsu.edu/Instrumentation/DCP.html |url-status=dead }} The plasma stream is produced by briefly contacting the cathode with the anodes. Temperatures at the arc core exceed 8000 K. This three-electrode arrangement is illustrated in Figure 1.
Applications
The applications of DCP are comparable to inductively coupled plasma (ICP). Some applications include, but are not limited to:
- identification of boron in tissues and cells{{Cite journal |doi = 10.1021/ac00009a010|pmid = 1858981|title = Determination of boron in tissues and cells using direct-current plasma atomic emission spectroscopy|journal = Analytical Chemistry|volume = 63|issue = 9|pages = 890–893|year = 1991|last1 = Barth|first1 = Rolf F.|last2 = Adams|first2 = Dianne M.|last3 = Soloway|first3 = Albert H.|last4 = Mechetner|first4 = Eugene B.|last5 = Alam|first5 = Fazlul.|last6 = Anisuzzaman|first6 = Abul K. M.}}
- analysis of trace metals in cows{{Cite journal | doi=10.3168/jds.S0022-0302(92)77977-6| pmid=1401369| title=Determination of Markers in Digesta and Feces by Direct Current Plasma Emission Spectroscopy| journal=Journal of Dairy Science| volume=75| issue=8| pages=2176–2183| year=1992| last1=Combs| first1=D.K.| last2=Satter| first2=L.D.| doi-access=free}}
- synthesis of carbon nanofibers{{cite journal |last1=Melechko |first1=A. V. |last2=Merkulov |first2=V. I. |last3=McKnight |first3=T. E. |last4=Guillorn |first4=M. A. |last5=Klein |first5=K. L. |last6=Lowndes |first6=D. H. |last7=Simpson |first7=M. L. |title=Vertically aligned carbon nanofibers and related structures: Controlled synthesis and directed assembly |journal=Journal of Applied Physics |date=15 February 2005 |volume=97 |issue=4 |pages=041301–041301–39 |doi=10.1063/1.1857591|bibcode=2005JAP....97d1301M }}
Figure 2 shows DCP being used to grow carbon nanofibers.
Comparison to inductively coupled plasma (ICP)
DCP incurs several key disadvantages in comparison to ICP. In addition to the lower sensitivity, spectra generated by DCP generally present fewer spectral lines. DCP samples are often incompletely volatilized due to the relatively short amount of time spent in the hottest region of the plasma. Furthermore, DCP requires more regular upkeep than ICP, because the graphite electrodes wear out after a few hours and must be exchanged
However, DCP is not without a few advantages over ICP. The amount of argon needed for DCP is much less than that needed for ICP. Also, DCP can analyze samples that have a higher percentage of solid in solution than can be handled by ICP.
References
{{Reflist}}