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Gas chromatography for the separation of elemental species

(16.12.2012)


Pros and cons of GC for speciation analysis
Gaseous sample introduction to the detection system is advantageous because it provides nearly 100% analyte transport efficiency, and energy requirements for atomization, excitation and ionization are moderate since energy for desolvation and vaporization is not necessary.

Avoiding thermal degradation of the analyte species during gas chromato-graphic separation and transport from the end of the column to the detection system and condensation at cold surfaces on the way to the detector are major challenges to efficient interfacing of GC with atomic spectroscopy. With respect to elemental species, this means that only thermally stable compounds, in which the element is covalently bound, can be separated by GC. A number of native organometallic compounds are volatile enough to be separated by GC. They include tetraalkyllead species (Men,Et4-nPb) (n = 1 + 4), methylselenium compounds (e.g. Me2Se, Me2Se2), some organomercury compounds (MeHg+, Me2Hg) as well as naturally occurring metalloporphyrins. They can either be readily purged with an inert gas or extracted into a non-polar solvent and subsequently chromatographed by thermal desorption, packed column or capillary GC.

Derivatization for chemical speciation analysis using GC

The majority of organometallic species of interest exist in quasi-ionic polar forms which  have relatively high boiling points and often poor thermal stability. To be amenable to GC separation they must be converted to non-polar, volatile and  thermally stable species. The derivative chosen needs to retain the structure of the element-carbon bonds to ensure that the identity of the original moiety is conserved. The most common derivatisation methods include: 
  1. conversion of inorganic and small organometallic ions into volatile covalent compounds (hydrides, fully ethylated species) in aqueous media;
  2. conversion of larger alkylmetal cations e.g. R,Pb(,-")+ to saturated non-polar species using Grignard reagents
  3. conversion of ionic species to fairly volatile chelates (e.g. dithiocarbamate, and trifluoroacetone) or other compounds.
The three principal methods are fairly versatile in terms of the organometallic species to be derivatised but may differ in reaction efficiency and ease of handling. Therefore the choice depends on the concentration, the matrix and the sample throughput required. Frequently, the derivatives are concentrated by cryotrapping or extraction into an organic solvent prior to injection onto a GC column.

The most often used derivatisation (chemical modification) techniques for GC in speciation analysis are:
  • Hydride generation is severely restricted by either the thermodynamic inability of some species to form hydrides, or by considerable kinetic limitations to hydride formation. Hydride generation with NaBH4, is prone to interference with transition metals which affect the reaction rate and analytical precision and artefact formnation has been observed for some organometal(1oid) species.
  • Derivatisation with Tetraalkyl(aryl)borates can overcome some of the limitations of hydride generation. Ethylation with the water soluble sodium tetra-ethylborate (NaBEt,) is the the most common derivatisation procedure, leading to thermally stable gas chromatographic species. For the differentiation of ethylspecies from inorganic species other reagents such as sodium tetrapropylborate or tetra-methylammonium tetrabutylborate have been proposed.
  • Derivatization with Grignard reagents is fairly versatile but requires an aqueous-free medium for the reaction to be carried out. Grignard reagents proposed for derivatisation in speciation analysis have included: methyl-, ethyl-, propyl-, butyl- and pentylmagnesium chlorides or bromides. Lower-alkyl magnesium salts are generally preferred due to the smaller molecular mass and, hence, the higher volatility of the resulting species which makes the GC separation faster with less of the column carryover problems associated with derivatised inorganic forms (which are often present in large excess).


Gas chromatographic separation techniques for elemental speciation analysis

The mobile phase in GC is usually helium which not only enables the quantitative transport of the sample to the detector, but also accounts for a low background contribution in the detector itself. Differences between the provided flow rate of the GC and requirements of the detector can be matched by an appropriate make-up flow. GC techniques can be divided into different categories according to:

  • the choice of the column used - packed, capillary, high temperature capillary, multicapillary;
  • the sample introduction principle - direct injection (split, splitless, on-column) of an organic solution, solid-phase microextraction (SPME), headspace-injection or purge-and-trap using cryofocussing


Choice of column
Packed column GC-ICP MS is a favorable technique to follow hydride generation purge-and-trap because of the easier handling of highly volatile species at temperatures below - 100°C. Capillary GC offers improved resolving power over packed column GC which is of importance for separation of the complex mixtures of organometallic compounds found in many environmental samples.


Detection systems
Different kinds of detection systems have been used in conjunction with GC separation for speciation analysis that differ in sensitivity and selectivity:
  • Atomic spectrometric detectors are element selective
    • Atomic emission spectrometry allows for the simultaneous detection of different elements,
      • Microwave induced plasma detectors (GC-MIP-AES) even allow for the determination of nonmetals (O, N, P, S, halogens)
      • Inductively coupled plasma (GC-ICP-AES)
    • Atomic absorption spectrometry (GC-AAS) is a sensitive single element detector
    • Atomic fluorescence spectrometry (GC-AFS) is mainly used for mercury and the hydride forming elements
  • Mass spectrometric detection systems are the most versatile detectors.
    • Electron impact mass spectrometry (GC-MS) is a very versatile detection system that requires careful optimization of the detection scheme in order to achieve the necessary selectivity for the identification of a special species.
    • Inductively coupled plasma mass spectrometry (GC-ICP-MS) is a very sensitive detector with highly selective element response and isotope capabilities.
  • Other detection principles
    • Flame-photometric detector (FPD) is based on the recording of the emission spectra produced by the thermal heating of the specimen in a burner flame. The combustion of substances whose molecules contain atoms of sulphur, phosphorus, nitrogen, selenium, tin, and various other elements gives excited species (for example S*, HPO*, etc.), which emit a characteristic spectrum.
    • Flame-ionization detectors (FID) are the most generally applicable and most widely used detectors with however, limited selectivity.  In a FID, the sample is directed at an air-hydrogen flame after exiting the column.  At the high temperature of the air-hydrogen flame, the sample undergoes pyrolysis, or chemical decomposition through intense heating.  Pyrolized hydrocarbons release ions and electrons that carry current.  A high-impedance picoammeter measures this current to monitor the sample's elution. In order to achieve appropriate selectivity, careful sample preparation is necessary.




Reviews related to gas chromatography for speciation analysis
Sample Preparation:

Yong Cai, Derivatization and vapor generation methods for trace element analysis  and speciation, in: Z. Mester, R. Sturgeon, Sample preparation for trace element analysis, Elsevier, 2003, 575-592. doi: 10.1016/S0166-526X(03)41019-2

M.P Pavageau, E. Krupp, A. de Diego, C. Pécheyran, O.F.X Donard, Cryogenic trapping for speciation analysis,in: Z. Mester, R. Sturgeon, Sample preparation for trace element analysis, Elsevier, 2003, 495-531. doi: 10.1016/S0166-526X(03)41016-7

Wuping Liu, Hian Kee Lee, Chemical modification of analytes in speciation analysis by capillary electrophoresis, liquid chromatography and gas chromatography, J. Chromatogr. A, 834/1–2 (1999) 45–63. doi: 10.1016/S0021-9673(98)00962-5

Alan G. Howard, (Boro)Hydride Techniques in Trace Element Speciation, Journal of Analytical Atomic Spectrometry, March 1997, Vol. 12 (267–272). DOI: 10.1039/A605050F

Detectors:

Stig Pedersen-Bjergaard, Gas Chromatography with  Atomic Emission Detection  in Environmental Analysis, in: R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry,  John Wiley & Sons Ltd., 2000 

Renee N. Easter, Joseph A. Caruso, Anne P. Vonderheide, Recent developments and novel applications in GC-ICPMS, J. Anal. At. Spectrom., 25/4 (2010) 493-502.  DOI: 10.1039/b924393n

Jorgelina C.A. Wuilloud, Rodolfo G. Wuilloud, Anne P. Vonderheide, Joseph A. Caruso, Gas chromatography/plasma spectrometry—an important analytical tool for elemental speciation studies, Spectrochimica Acta Part B, 59 (2004) 755– 792.  doi: 10.1016/j.sab.2004.03.009



 Tutorial material related to GC

Sheffield Hallam University, UK: Gas Chromatography
John V. Hinshaw, A Compendium of GC Terms and Techniques, LC-GC North America, 20/11 (2002) 1034-1040
Konrad Grob, The GC Separation Process: A simple model for non-mathematically minded chromatographers
IonSource.com: Introduction to Capillary Chromatography
IUPAC: Chromatography Nomenclature and Definitions
  Thomas G. Chasteen: Flash Animation: Gas Chromatography with Split/Splitless Injection
  Thomas G. Chasteen: Quicktime Movie: Gas Chromatography with Split/Splitless Injection
Thomas G. Chasten: Flash Animation: Temperature Programming in Gas Chromatography
 Thomas G. Chasten: Quicktime Movie: Temperature Programming in Gas Chromatography
Thomas G. Chasteen: Flash Animation: Cryogenic Trapping Gas Chromatography
Thomas G. Chasteen: Quicktime Movie: Cryogenic Trapping Gas Chromatography
Thomas G. Chasteen: Flash Animation: Solvent Focussing in Gas Chromatography
Thoms G. Chasten: Quicktime Movie: Solvent Focussing in Gas Chromatography
in German: Tobias Brinkert (ISAS, Dortmund): Grundlagen der Gaschromatographie
in German: Virtual GC - On-line Simulation eines GC instruments


Books on Gas Chromatography

Dean Rood, The Troubleshooting and Maintenance Guide for Gas Chromatographers, 4th ed., Wiley-VCH, 2007.

Bruno Kolb, Leslie S. Ettre, Static Headspace - Gas Chromatography, 2nd ed., John Wiley & Sons, ISBN 0-471-74944-3, 2006.

Robert L. Grob, Eugene F. Barry, Modern Practice of Gas Chromatography, 4th ed., John Wiley & Sons, New York, ISBN 0-471-22983-0, 2004.

K. Grob, Split and Splitless Injection for Quantitative Gas Chromatography, 4th ed., Wiley-VCH, Weinberg, ISBN 3-527-29879-7, 2001.

Alan J. Handley, Edward Adlard, Gas Chromatographic Techniques and Applications, Blackwell Publishing, Oxford, UK, ISBN 1-8412-7118-7, 2001.


GC Maintenance and Trouble-Shooting

Analytical Chemistry Resources: GC Troubleshooting
Agilent Technologies: GC Troubleshooting Guides 
Fraunhofer IVV (in German): GC troubleshooting

EVISA Database system

Journals Database: Journals related to Gas Chromatography
Company Database: Professional Organizations relelated to Chromatography
Instrument Database: GC Systems
 Instrument Database: GC Autosampler (Liquid samples)
Instrument Database: GC Autosampler (Headspace)
Instrument Database: GC Autosampler (GC/SPME)


EVISA link pages

Resources related to analytical sciences
Resources related to Chromatography
Resources related to quality assurance/quality control


 Other web resources:

Agilent Technologies: Gas Chromatography (Web portal)
Analytical Chemistry Resources: Gas Chromatography
Thermo Scientific: Complete Speciation Solutions
Thermo Scientific: Gas Chromatography (Web portal)



Further chapters on techniques and methodology for speciation analysis:

Chapter 1: Tools for elemental speciation
Chapter 2: ICP-MS - A versatile detection system for speciation analysis
Chapter 3: LC-ICP-MS - The most often used hyphenated system for speciation analysis
Chapter 4: GC-ICP-MS- A very sensitive hyphenated system for speciation analysis
Chapter 5: CE-ICP-MS for speciation analysis
Chapter 6: ESI-MS: The tool for the identification of species
Chapter 7: Speciation Analysis - Striving for Quality
Chapter 8: Atomic Fluorescence Spectrometry as a Detection System for Speciation Analysis
Chapter 9: Gas chromatography for the separation of elemental species
Chapter 10: Plasma source detection techniques for gas chromatography
Chapter 11: Fractionation as a first step towards speciation analysis
Chapter 12: Flow-injection inductively coupled plasma mass spectrometry for speciation analysis
Chapter 13: Gel electrophoresis combined with laser ablation inductively coupled plasma mass spectrometry for speciation analysis
Chapter 14: Non-chromatographic separation techniques for speciation analysis



last time modified: December 14, 2016



Comments
Photoionization detection
There are one or two papers describing this possibility (though none is very recent) and maybe a commercial instrument for arsenic speciation is (or shortly will be) available?
2016--0-3-  Julian Tyson
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2014--0-1-  tonyviet
Good
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2013--0-2-  Talaha






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