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Properties of Glass and Plastics


Type 1, Class A Borosilicate Glass

  • Composed primarily of silicone and oxygen, with trace amounts of boron and sodium
  • Most chemically inert composition of glass readily available for today's laboratories
  • Has an expansion coefficient of approximately 33x10-7°C

Type 1, Class B Borosilicate Glass

  • Composed of silicone and oxygen, trace amounts of boron, sodium and other elements
  • This mixture is more alkaline than Class A glass, but is adequate for many uses in the laboratory
  • All amber borosilicate glass is made of Class B unless otherwise specified
  • Has an expansion coefficient of 51x10-7°C

Properties of Plastics

  • Low-density polyethylene (L.D.P.E.) is a flexible and translucent material. This material is used to make snap caps, plug closures and selected inserts.
  • Polypropylene (P.P.) is a rigid and translucent material, which also comes in a variety of colors. Polypropylene is used for vials, selected closures, inserts and well plates.
  • Polymethylpentene (TPX) is a rigid transparent material, which has a relatively high melting point.

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Headspace Vial and Septa Issues


In order to achieve the best performance with headspace GC, careful attention should be paid to sample preparation and instrument setup. The vial and cap are a very important, and often overlooked, element in the headspace analysis preparation.

It is important to ensure the correct vial and cap are used!

Vial Size
Headspace vials are available in 6, 10 and 20ml sizes. Use a vial large enough to ensure adequate headspace or phase ratio without excessive dilution of the components of interest. Phase ratio is discussed later and should normally be at least 50% of the sample volume.

Bottom Profile
Headspace vials may be round-bottomed or flat-bottomed. Either type is suitable but round-bottomed vials tend to be stronger and may work more reliably when the autosampler moves the vials into and out of the incubator. Round-bottomed vials also tend to withstand higher pressures and are more suitable for elevated temperatures and applications such as derivatization.

Correct Crimping
Headspace vials must be correctly crimped. Ensure the crimper is not worn and is adjusted correctly. The cap should not turn after application to the vial. Find out more about correct vial crimping.

Septa Choice
Use septa suitable for the temperature of the system. Poor septa can result in bleed and so contaminate the headspace.


Cleanliness of Vials
Vials should be clean. Not all vials are the same and dirty or re-washed vials can cause ghost peaks or erroneous results.

 

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What is GC Headspace?


Simple Definition



'Headspace' is the gas space above the sample in a chromatography vial. Volatile sample components diffuse into the gas phase, forming the headspace gas. 

Headspace analysis is therefore the analysis of the components present in that gas.

Suitability

Headspace gas chromatography is most suited to the analysis of the very light volatiles in samples that can be efficiently partitioned into the headspace gas volume from the liquid or solid matrix sample. Higher boiling volatiles and semi-volatiles are not detectable with this technique due to their low partition in the gas headspace volume. Partitioning is covered in more detail later in this article.

Headspace gas analysis also lends itself to automation for quality control or sample screening. This is made possible by modern instrumentation, through which highly reproducible samples can be prepared in an efficient and accurate manner.

Complex sample matrices, which may be difficult to analyse directly or would otherwise require sample extraction or preparation, are ideal candidates for headspace since they can be placed directly in a vial with little or no preparation. This saves both time and money.

Applications

Headspace GC is used for the analysis of volatile and semi-volatile organics in solid, liquid and gas samples. The popularity of this technique has grown over recent years and has now gained worldwide acceptance for analyses of alcohols in blood and residual solvents in pharmaceutical products.

Other common applications include industrial analyses of monomers in polymers and plastic, flavour compounds in beverages and food products, and fragrances in perfumes and cosmetics. 

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