Increases in GC output can often be achieved through relatively minor adjustments in operating parameters

This Mind Map outlines some thought processes designed to help identify potential performance-enhancing changes.

GC is an important analytical and preparative technique, requiring skill and experience to operate effectively. However, no matter how skilled or experienced the operator, GC output can often be improved by simple adjustments in the set-up or selected operating parameters. The following MIND MAP offers some suggestions for how GC output can be improved relatively easily. The different components and settings that are likely to have the biggest infl uence on output are discussed, together with suggestions for modifi cations to improve overall performance.

Increase My GC Output

Vary Experimental Conditions

Effective GC depends on the appropriate selection of components and conditions.

• • Column size
  • Stationary phase
  • Mobile phase

Injection Technique

Several injector types are available. Good injection technique is essential for effi cient GC output and should:

Direct Injection

A sample is injected, fl ash vaporized, and passed directly to the column. However, it can be diffi cult to reliably inject small sample volumes.

Split Injection

Only a fraction of the injected sample passes to the column; the rest goes to waste.

Split-Splitless Injection

The amount of analyte injected can be varied using software before an injection is made. This is called the split ratio and is a function of the ratio of gas fl ow through the GC’s injector, as controlled by pressure/gas flows in the injector.

• • allow accurate and reproducible injections of small amounts of representative samples
  • induce no change in sample composition
  • not exhibit discrimination based on differences in boiling point, polarity, concentration or thermal/ catalytic stability
  • be applicable for trace analysis as well as for undiluted samples. 

Multidimensional GC

Multidimensional gas chromatographs have been developed for more effi cient separation than can be achieved with a single column.

When refining GC technique, the most appropriate detector or detectors will depend largely on the expected application, and anticipated sample throughput. 

Comprehensive Two-Dimensional Chromatography (GCxGC)

As each “fraction” emerges from the fi rst GC column, it is cryofocused (condensed and concentrated with a burst of very cold air) and then applied to a second, orthogonal column, with a narrow internal diameter and fast fl ow rate. The result is both higher, sharper peaks, and enhanced chromatographic separation.

Heart Cutting

Selected sections of the sample that emerge from one column are introduced into a second, orthogonal column (one whose separation medium differs from that of the first column.

Improve Sample Preparation

Sample preparation is an essential step in gas chromatography and can be the most errorprone and labor-intensive task in the process. A number of techniques for GC sample preparation are available, and GC output can be greatly enhanced by selecting the most appropriate method and accurately following it. Most sample preparation techniques for GC are based on variations of extraction theory whereby the sample solvent, temperature, pressure, phases or volume may be altered.

Static Headspace Technique

Volatile analytes are equilibrated in a closed vial at a specifi ed temperature and pressure. A gas-tight syringe is used to transfer the headspace sample into the gas chromatographic injection port.

Dynamic Headspace Technique

Analytes are swept or purged onto an adsorbent platform and then thermally desorbed into the gas chromatograph.

Microwave-Assisted Extraction (MAE) (or microwave-assisted solvent extraction (MWE))

Similar to ASE. The extraction container must be microwave transparent, while the solvent used may be either microwave absorbing or non-microwave absorbing. Ultrasonic vibrations may be used to assure good contact between sample and solvent in ultrasonic extraction (USE).

Accelerated Solvent Extraction (ASE)

Used for solid and semi-solid samples. Elevated temperatures and pressures used in these techniques cause hydrogen bonds and dipole interactions to be reduced, and surface wetting is increased.

Solid-Phase Extraction (SPE)

This technique is used to concentrate analytes from gaseous or liquid samples. The adsorbed analytes can be eluted with a solvent or thermally desorbed.

Solid-Phase Microextraction (SPME)

Used for gaseous and liquid samples. A fused-silica polymer coated fi ber is exposed to the stirred sample. The shielded fi ber is then inserted into the injection port of the gas chromatograph.

Stir Bar Sorptive Extraction (SBSE)

A dynamic variation of SPME in which a spinning glass-covered magnetic bar is used to adsorb analytes, which can be removed by thermal desorption in the gas chromatographic injection port.

Invest in New Equipment

Any of the components of the GC system, or the entire system, may need to be replaced at some time. At this stage, the principal decision is whether to buy a brand new system or component, or a refurbished version. Refurbished equipment can often have similar specifi cations to newer models but may be available for a greatly reduced capital outlay. Many refurbished models also carry a guarantee.

Buy New Equipment

For current models and latest technology.

Buy Refurbished Equipment

For cost savings, while still enjoying much of the functionality of newer models.

Instigate User Training

GC is a complex technique requiring skill and experience to derive the best outcomes. If output is falling below what is expected, consider instigating staff training, particularly if users are new or inexperienced. Staff training is an essential part of GLP compliance. It may also contribute to Continuing Education.

Online

Some courses are available online for users to follow at their own convenience. Particularly suited to theoretical training. Considerable resources are also available online.

In-House

Some training companies offer in-house training courses where users may be trained on the equipment available. This is a particularly appealing option if several users need to be trained at one time.

Events

Conferences and meetings are a useful way to expose staff to the rigors of GC.

GC Detector

Many detectors are available, offering different types of selectivity. Selective detectors respond to a range of compounds with a common physical or chemical property and specifi c detectors respond to a single chemical compound. Consider varying the GC detector for improved output.

Mass Flow Dependent Detectors

The signal is related to the rate at which solute molecules enter the detector. The sample is usually destroyed. The response of a mass flow dependent detector is unaffected by the make-up gas.

• • • Flame Photometric (FPD):
  Selective for sulfur phosphorus, tin, boron, arsenic, germanium, selenium, chromium, using hydrogen and air as support gases. Detectability is around 100 pg.
  • • Hall Electrolytic Conductivity:
  Selective for halide, nitrogen, nitrosamine and sulfur, using hydrogen and oxygen as support gases.
  • • Flame ionization (FID):
  Selective for most organic compounds, using hydrogen and air as support gases. Detectability is around 100 pg.
  • • Nitrogen-Phosphorus: Selective for nitrogen and phosphorus, using hydrogen and air as support gases. Detectability is around 10 pg.

Concentration Dependent Detector

The signal is related to the concentration of solute in the detector, and does not usually destroy the sample. Dilution with make-up gas will lower the detector’s response.

• • Thermal Conductivity (TCD):Universally selective, with a detectability of around 1 ng.
  • Electron Capture (ECD): Selective for halides, nitrates, nitriles, peroxides, anhydrides and organometallics. Detectability is around 50 fg.
  • Flame Photometric (FPD): Selective for aliphatics, aromatics, ketones, esters, aldehydes, amines, heterocyclics, organosulfurs and some organometallics. Detectability is around 2 pg.