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Schematic representation of the chromatographic process. Adapted from Harold M. McNair, James M. Miller, Basic Gas Chromatography , John Wiley&Sons, New York,1998. Reproduced courtesy of John Wiley&Sons, Inc.

A typical chromatogram

[link] shows a chromatogram of the analysis of residual methanol in biodiesel, which is one of the required properties that must be measured to ensure the quality of the product at the time and place of delivery.

Chromatogram of the analysis of methanol in B100 biodiesel, following EN 14110 methodology. Reproduced courtesy of PerkinElmer Inc. (http://www.perkinelmer.com/)

Chromatogram ( [link] a) shows a standard solution of methanol with 2-propanol as the internal standard. From the figure it can be seen that methanol has a higher affinity for the mobile phase (lower K c ) than 2-propanol (iso-propanol), and therefore elutes first. Chromatograms ( [link] b and c) show two samples of biodiesel, one with methanol ( [link] b) and another with no methanol detection. The internal standard was added to both samples for quantitation purposes.

Instrument overview

Components of a gas chromatograph system

[link] shows a schematic diagram of the components of a typical gas chromatograph, while [link] shows a photograph of a typical gas chromatograph coupled to a mass spectrometer (GC/MS).

Schematic diagram of the components of a typical gas chromatograph. Adapted from http://en.wikipedia.org/wiki/Gas_chromatography
Image of a Perkin Elmer Clarus SQ 8S GC/MS. Reproduced courtesy of PerkinElmer Inc. ( (External Link) ).

Carrier gas

The role of the carrier gas -GC mobile phase- is to carry the sample molecules along the column while they are not dissolved in or adsorbed on the stationary phase. The carrier gas is inert and does not interact with the sample, and thus GC separation's selectivity can be attributed to the stationary phase alone. However, the choice of carrier gas is important to maintain high efficiency. The effect of different carrier gases on column efficiency is represented by the van Deemter (packed columns) and the Golay equation (capillary columns). The van Deemter equation, [link] , describes the three main effects that contribute to band broadening in packed columns and, as a consequence, to a reduced efficiency in the separation process.

These three factors are:

  1. the eddy diffusion (the A-term), which results from the fact that in packed columns spaces between particles along the column are not uniform. Therefore, some molecules take longer pathways than others, and there are also variations in the velocity of the mobile phase.
  2. the longitudinal molecular diffusion (the B-term) which is a consequence of having regions with different analyte concentrations.
  3. the mass transfer in the stationary liquid phase (the C-term)

The broadening is described in terms of the height equivalent to a theoretical plate, HEPT, as a function of the average linear gas velocity, u . A small HEPT value indicates a narrow peak and a higher efficiency.

Since capillary columns do not have any packing, the Golay equation, [link] , does not have an A-term. The Golay equation has 2 C-terms, one for mass transfer in then stationary phase (C s ) and one for mass transfer in the mobile phase (C M ).

Questions & Answers

how do you translate this in Algebraic Expressions
linda Reply
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
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Damian Reply
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abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
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Sravani Reply
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Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
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In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
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how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
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I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
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Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
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