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
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.
- Cold on-column injection: Determination of Free and Total Glycerin in Pure Biodiesel (B100) by GC in Compliance with EN 14105
- Headspace analysis:
Rapid and Reliable Detection of Dissolved Gases in Water
Thermo Fisher Scientific
United States
