which GCOrganic or inorganic; abundant or in trace amounts; in solid, liquid, or gas form—most sample types and reasonably-volatile analytes can be analyzed using a choice of gas chromatography (GC) (link to GC overview page) approaches.

Outstanding results and flexibility do not require great expense or complexity. Today’s GC provides a wide selection of technologies (link to product page), each suitable for a variety of analyte types, sample matrices, and workflows. Innovative equipment featuring instant-connect injectors and detectors (link to product page) makes it easy to tailor a cost-effective system for the methods you need, with the option to expand or reconfigure easily as your needs evolve.

 

What are the choices?

Gas chromatography setups differ in three areas:

  • Sample introduction: First, the chemical properties of your analytes—primarily volatility and fragility—along with the nature and complexity of the sample matrix will indicate the best means for getting the sample into the chromatograph.
  • Separation: Sample abundance, complexity, and analyte concentration and polarity help guide the choice of carrier gas, GC column(s), and single versus multi-dimensional separation.
  • Detection and analysis: Your analysis goals along with analyte characteristics like thermal conductivity and ionization will make some methods of detection preferable to others.

In Part One of this blog post we’ll look at gas chromatography sample introduction approaches to address different sample types. Future posts will explore choices in separation columns, and delve into the many methods of GC detection.

 

Which Approach for Which Samples?

Packed versus Capillary Column Inlets

Many general GC applications can use regular columns packed with adsorbant media. Inlets for packed columns are fairly straightforward: they protect the column from nonvolatiles in the sample, and direct all sample flow to the column.

The picture is more nuanced for capillary GC inlets, due in part to the need to limit sample and analyte amounts, but also to support many varied and demanding applications. The list below compares five GC sample introduction approaches that address the most common analysis challenges.

 

Combination Split/Splitless Inlet Approach

Goal: General or trace GC analysis

Example Application: General and trace analyses

Strengths:

  • Rugged and easy-to-use
  • Good for high-concentration analytes (split) or trace levels (splitless)
  • Easy to automate

Limitations:

  • Not for heat-sensitive analytes
  • Best for samples with limited range of analyte boiling temperatures
  • Splitless (trace analysis) requires careful method optimization

 

Cold On-Column Injection Approach

Goal: Careful handling for heat-fragile molecules; also trace analysis and wide boiling temperature ranges

Example Application: Biodiesel

Strengths:

  • Avoids sample discrimination and degradation
  • High analytical accuracy, precision and reproducibility

Limitations:

  • Limited injection volumes
  • Requires careful optimization of column temperature, solvent and injection rate
  • Can expose column to more contaminants

 

Programmable Temperature Vaporizing (PTV) Injection Approach

Goal: Analysis of heat-sensitive analytes or trace level analysis in “dirty” matrices

Example Application: PAHs in soils, pesticides in environmental water and food matrices

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Strengths:

  • Very flexible
  • Large injection volumes
  • Avoids analyte discrimination due to differences in boiling temperature

Limitations:

  • More complex to use

 

Headspace Analysis Approach

Goal: Easy determination of very light volatiles in solid, liquid

Example Applications: Dissolved gases in water, residual solvents in pharmaceuticals, blood alcohol

Strengths:

  • Minimal sample prep, even for complex matrices
  • Readily automated

Limitations:

  • Not suitable for analytes with higher boiling temperatures

 

Dynamic Headspace Approach (Purge and Trap, Thermal Desorption)

Goal: Sensitive detection of specific trace level analytes and/or removal of unwanted compounds from samples

Example Application: Trace level VOCs

Strengths:

  • Minimizes detection limits
  • Can help clean sample matrix
  • Highly compound-specific

Limitations:

  • Highly compound-specific
  • Small fractionation effects possible during pre-concentration

 

Gas Sampling Valve approach

Goal: Detection of trace compounds in gaseous matrix

Example Applications: Greenhouse gases, TOGA, natural gas, refinery gas

Strengths:

  • Usually for very small molecules
  • Can be used for at-line analysis

Limitations:

  • Non compound-specific
  • Used for trace analysis of gaseous compounds

 

Stay tuned for part 2 of this post, with more detail and example applications.

 

Additional Resources

See the following application notes for example applications using several of the methods discussed above.