Carbon Adsorbents

By: Shyam Verma, AnalytiX Volume 9 Article 4

High-purity/quality unique adsorbents at economical prices
Shyam Verma, Market Segment Manager, Reagents & Chemicals shyam.verma@sial.com

Adsorbent carbons are frequently used in analytical techniques including chromatographic separations. Sigma-Aldrich®/Supelco®’s carbon technology development efforts have been critical to the advancement of chromatography and sample preparation applications. Supelco offers over 85 different carbon products including custom materials, ranging in particle size from 1–1000 microns and surface areas from 1–1500 m2/g. These adsorbent carbons include:

  • non-porous graphitised carbon blacks (GCB)
  • carbon molecular sieves
  • highly porous activated carbons and charcoals

Graphitised Carbon Black (GCB)

Generally, these are non-porous and non-specifi c adsorbents that exhibit high surface homogeneity. The high purity of these adsorbents ensures effective desorption of the analyte of interest. These adsorbents are suitable for gas-solid chromatography, even for eluting polar compounds. GCBs exhibit hydrophobic surface characteristics and, therefore, can be effectively used in trapping organic compounds in humid streams. Their hydrophobic nature minimises sample displacement by water, so accurate samples can be obtained despite levels of high levels of humidity. Trapped compounds can be desorbed by a solvent or thermal desorption at 100 % desorption effi ciency. These adsorbents offer excellent thermal stability, ensure minimal bleed at thermal desorption temperatures, and prevent high pressure-drop.

Carbotrap B, -C, and -F trap a wide range of airborne organics (C4-C5 hydrocarbons) to polychlorinated biphenyl and other large molecules

  • Carbotrap B adsorbent: surface area = 100 m2/g, useful in monitoring airborne C5-C12 compounds.
  • Carbotrap C and -F adsorbents: surface area = 10 and 5 m2/g, respectively, useful for trapping and effi ciently releasing larger molecules (C9-C30).

Carbon Molecular Sieve (CMS)

A carbon molecular sieve (CMS) also called molecular sieve carbon, is a specialised carbon that has a tailored porosity for selective adsorption. These materials are primarily used for collecting very small molecular-sized compounds (C2-C5). The size and shape of the analyte molecule, and the size and shape of the pore entrances in the CMS particle, determine how well the analyte is adsorbed and desorbed.

As our CMSs are prepared from high-purity polymers, resulting material is a high-purity carbon that allows effective adsorption of an analyte, and its effi cient desorption for quantifi cation. Carbosieve S-III and Carboxen CMS have upper temperature limits of at least 400 ºC.

  • Carbosieve S-II (mostly used in GC columns) is recommended for analysing mixtures of permanent gases (H2, O2, Ar, CO and CO2) and C1-C2 hydrocarbons (methane, ethane, ethylene, acetylene). Maximum operating temperature is 225 °C with oxygen-free carrier gas.
  • Carbosieve S-III is a spherical adsorbent with a surface area of ~ 820 m2/g and pore size of 15– 40 Å. It is an excellent adsorbent for trapping small airborne molecules, such as chloromethane. The pure carbon framework permits thermal desorption of analyte molecules without loss.
  • Carbosieve G (mostly used in GC columns) is recommended for analysing C1-C3 hydrocarbons. Maximum operating temperature is < 200 °C with oxygen-free carrier gas.

Carboxen

These CMS materials are hydrophobic in nature, thus ensuring accurate sampling at high humidity levels.

Carboxen-563 and Carboxen-564:

  • our preferred versions of Ambersorb® XE-340 and Ambersorb XE-347.
  • show higher capacity (breakthrough volume) for many volatile organic compounds (VOCs).
  • useful for analysing water quality or adsorbing airborne compounds.
  • Carboxen-564 is effective for monitoring many C2-C5 VOCs.

Carboxen-569:

  • 20/45-mesh material with no Ambersorb equivalent.
  • high hydrophobicity and higher capacity for organic molecules.

Carboxen-572: most effi cient for purge & trap, and air sampling applications.

Carboxen-1000:

  • large surface area enables effective and efficient adsorption and desorption.
  • suitable for low volume sampling of very volatile compounds, such as vinyl chloride.
  • primarily used in narrow bore tubes that are desorbed directly into the chromatographic column.

Carboxen-100: trap and retain small compounds.

Carboxen-1003: large surface area for effi cient adsorption/desorption and good hydrophobicity.

Carboxen-1012: highly-activated with large micropores; effective for aqueous phase adsorption of organic compounds, or for air sampling of C4-C6 compounds.

Carboxen-1016: analyte group of permanent gases.

Carboxen-1018: narrow (~6–7 Aº) micropores for adsorption/ desorption of small analytes such as, ethane, acetylene, ethylene and the C3 hydrocarbons. Hydrophobic.

Carboxen-1021: for air sampling of small molecules.

Activated Carbons and Charcoals

Activated coconut charcoal has been used extensively as a general purpose adsorbent due to its ability to adsorb/desorb a wide range of volatile analytes.

Carbon Adsorbent Sampler Kits

Choosing the right adsorbent or combination of adsorbents can often be diffi cult. Selecting a suitable adsorbent means an adsorbent that can retain a specifi c analyte, or group of analytes, for a specifi c sample volume, and also able to release the analyte(s) during the desorption process.

By using one of the Supelco Carbon Adsorbent Kits, the method developer obtains a cost-effective process to evaluate several carbon adsorbents when designing adsorbent-based applications and products. Once the appropriate material has been identified, we will work with you to produce larger quantities to your specifi cations.

Carbon Adsorbent Selection Guide

For multi-bed tubes, use the weaker adsorbent in front of the stronger adsorbent. For example, use Carbopack C in front of Carbopack B.


Note: Analyte size relative to n-Alkanes. Consider all atoms, not just Carbon. For example, even though 1,2-Dichloroethane is a C2, the two Chlorine atoms give it a relative size between C4 and C5.

Note: Analyte size relative to n-Alkanes. Consider all atoms, not just Carbon. For example, even though 1,2-Dichloroethane is a C2, the two Chlorine atoms give it a relative size between C4 and C5.

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