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Modifiers

Modifiers are added to the mobile phase to play with its properties. As mentioned a few times previously, CO 2 supercritical fluid lacks polarity. In order to add polarity to the fluid (without causing reactivity), a polar modifier will often be added. Modifiers usually raise the critical pressure and temperature of the mobile phase a little, but in return add polarity to the phase and result in a fully resolved sample. Unfortunately, with too much modifier, higher temperatures and pressures are needed and reactivity increases (which is dangerous and bad for the operator). Modifiers, such as ethanol or methanol, are used in small amounts as needed for the mobile phase in order to create a more polar fluid.

Advantages of supercritical fluid chromatography

Clearly, SFC possesses some extraordinary potential as far as chromatography techniques go. It has some incredible capabilities that allow efficient and accurate resolution of mixtures. Below is a summary of its advantages and disadvantages stacked against other conventional (competing) chromatography methods.

Advantages over hplc

  • Because supercritical fluids have low viscosities the analysis is faster, there is a much lower pressure drop across the column, and open tubular columns can be used.
  • Shorter column lengths are needed (10-20 m for SFC versus 15-60 m for HPLC) due to the high diffusivity of the supercritical fluid. More interactions can occur in a shorter span of time/distance.
  • Resolving power is much greater (5x) than HPLC due to the high diffusivity of the supercritical fluid. More interactions result in better separation of the components in a shorter amount of time.

Advantages over gc

  • Able to analyze many solutes with no derivatization since there is no need to convert most polar groups into nonpolar ones.
  • Can analyze thermally labile compounds more easily with high resolution since it can provide faster analysis at lower temperatures.
  • Can analyze solutes with high molecular weight due to their greater solubizing power.

General disadvantages

  • Cannot analyze extremely polar solutes due to relatively nonpolar mobile phase, CO 2 .

Applications

While the use of SFC has been mainly organic-oriented, there are still a few ways that inorganic compound mixtures are separated using the method. The two main ones, separation of chiral compounds (mainly metal-ligand complexes) and organometallics are discussed here.

Chiral compounds

For chiral molecules, the procedures and choice of column in SFC are very similar to those used in HPLC. Packed with cellulose type chiral stationary phase (or some other chiral stationary phase), the sample flows through the chiral compound and only molecules with a matching chirality will stick to the column. By running a pure CO 2 supercritical fluid mobile phase, the non-sticking enantiomer will elute first, followed eventually (but slowly) with the other one.

In the field of inorganic chemistry, a racemic mixture of Co(acac) 3 , both isomers shown in [link] , has been resolved using a cellulose-based chiral stationary phase. The SFC method was one of the best and most efficient instruments in analyzing the chiral compound. While SFC easily separates coordinate covalent compounds, it is not necessary to use such an extensive instrument to separate mixtures of it since there are many simpler techniques.

The two isomers of Co(acac) 3 in a racemic mixture which were resolved by SFC.

Organometallics

Many d-block organometallics are highly reactive and easily decompose in air. SFC offers a way to chromatograph mixtures of large, unusual organometallic compounds. Large cobalt and rhodium based organometallic compound mixtures have been separated using SFC ( [link] ) without exposing the compounds to air.

Examples of cobalt and rhodium based organometallic compound mixtures separated by SFC. Adapted from Compounds by I Bruheim, E Fooladi, E. Lundanes, T. Greibrokk, J. Microcolumn Sep. , 2001, 13 , 156.

By using a stationary phase of siloxanes, oxygen-linked silicon particles with different substituents attached, the organometallics were resolved based on size and charge. Thanks to the non-polar, highly diffusive, and high viscosity properties of a 100% CO 2 supercritical fluid, the mixture was resolved and analyzed with a flame ionization detector. It was determined that the method was sensitive enough to detect impurities of 1%. Because the efficiency of SFC is so impressive, the potential for it in the organometallic field is huge. Identifying impurities down to 1% shows promise for not only preliminary data in experiments, but quality control as well.

Conclusion

While it may have its drawbacks, SFC remains an untapped resource in the ways of chromatography. The advantages to using supercritical fluids as mobile phases demonstrate how resolution can be increased without sacrificing time or increasing column length. Nonetheless, it is still a well-utilized resource in the organic, biomedical, and pharmaceutical industries. SFC shows promise as a reliable way of separating and analyzing mixtures.

Bibliography

  • D. R. Gere, R. Board, and D. McManigill, Anal. Chem. , 1982, 54 , 736.
  • E. Klesper, A. H. Corwin, and D. A. Turner, Org. Chem. , 1962, 27 , 700.
  • M.N. Meyers, J. Giddings, Anal. Chem. , 1965, 37 , 1453.
  • K. Miyazawa, T. Ishiguro, and H. Oda, FPC production . Daicel Chemical Industries, LTD. Myoko, Japan (2007).
  • L. Toribio, C. Alonso, M. J. del Nozal, J. L. Bernal, and J. J. Jimnez, J. Sep. Sci. , 2006, 29 , 1363.
  • M. Caude and D. Thiebaut, Practical Supercritical Fluid Chromatography and Extraction , Harwood Academic Publishers, Amsterdam (1999).
  • M. D. Palmieri, J. Chem. Educ. , 1989, 66 , A141.
  • M. Saito, J. Biosci. Bioeng. , 2013, 115 , 590.
  • M. Yoshioka, S. Parvez, T. Miazaki, and H. Parvez, Supercritical Fluid Chromatography and Micro-HPLC , VNU Science Press, The Netherlands (1989).
  • R. M. Smith, Supercritical Fluid Chromatography , Royal Society of Chemistry (1988).
  • T. L. Chester and J. D. Pinkston, Anal. Chem. , 1990, 62 , 394R.

Questions & Answers

anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
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Damian Reply
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there is no specific books for beginners but there is book called principle of nanotechnology
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Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
NANO
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
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SUYASH Reply
for screen printed electrodes ?
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s. Reply
of graphene you mean?
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or in general
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in general
s.
Graphene has a hexagonal structure
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