Organic 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
- Rugged and easy-to-use
- Good for high-concentration analytes (split) or trace levels (splitless)
- Easy to automate
- 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
- Avoids sample discrimination and degradation
- High analytical accuracy, precision and reproducibility
- 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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- Very flexible
- Large injection volumes
- Avoids analyte discrimination due to differences in boiling temperature
- 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
- Minimal sample prep, even for complex matrices
- Readily automated
- 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
- Minimizes detection limits
- Can help clean sample matrix
- Highly compound-specific
- 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
- Usually for very small molecules
- Can be used for at-line analysis
- Non compound-specific
- Used for trace analysis of gaseous compounds
Stay tuned for part 2 of this post, with more detail and example applications.
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
- PTV: Eliminate Invisible High Boiling Matrix in GC and GC/MS by Using PTV Backflush Injection Technique for Increased Productivity and Reliability
- Headspace analysis:
Rapid and Reliable Detection of Dissolved Gases in Water
- Purge and trap: Enhanced Method Detection Limits for irm-GC/MS of Volatile Organic Compounds Using Purge and Trap Extraction
- Thermal desorption: Determination of Volatile Compounds in Automotive Interior Materials by Thermal Desorption GC-MS
- Gas Sampling valve: An Innovative, Reliable, Easy Set-Up for the Analysis of Permanent Gases via PDD and Gas Sampling Valve