In my last two installments in this series, I covered Hydrophobic Interaction Liquid Chromatography (link to blog post) and Sample Preparation: Three Techniques Reviewed (link to blog post); and here, I would like to provide you with some helpful tips and references on how to select the correct gas chromatography (GC) consumable for your experiment.
I hope that the content in this post proves to be useful to you and provides a frequent and handy reference. More resources that you can use are listed in the end of this post in the Additional Resources section.
Cone (tapered tip): The most versatile needle for autosampler use. Resists coring of the vial and inlet septum.
Bevel (sharp tip): Most commonly used for manual injection. The tip shape helps to reduce septum coring.
Side Hole (dome tip with a side hole for sample exit): Usually used for headspace and large-volume injections.
Blunt End or 90 ͦ(flat top): Used for injections that do not contain an inlet septum, such as Merlin MicroSeal.
Dual Gauge: The narrow gauge part is suitable for megabore on-column injection while the wider part is suitable for autosampler use.
Note: Gauge is a measure of the thickness of the needle. The higher the gauge, the thinner the needle. For example. 23 gauge is thicker than 26 gauge.
Liners are used as a vessel into which the injected liquid sample is vaporized into a gaseous state and then passed into the GC column.
If your Injection method is:
Split: The injection mode requires rapid vaporization and effective mixing of the sample. You will need a liner that is open ended, with a large surface area and volume with low activity.
Splitless: This injection mode requires the sample to be focused onto the column with minimal sample contact with reactive metal components. You will require a liner that is tapered with small volume and low activity.
PTV: The injection mode requires rapid heating and cooling, and fast transfer to the column. It is used for active compounds such as pesticides, and large-volume injections. You will need a liner that is small with good thermal properties and a Siltek highly inert coating.
Ferrules are used to seal the connection of the column or liner with the system; three types of ferrules are in use, as follows:
100% Graphite Ferrules are used for FID, NPD, and high–temperature applications. They provide an easy-to-use stable seal with high temperature limits and can be easily removed and re-used. Limitations include incompatibility with MS or oxygen-sensitive detectors, easy deformation due to their softness, and possible system contamination.
85% Vespel/15% Graphite Ferrules are used with MS and oxygen-sensitive detectors. They provide a long lifetime at high temperatures. Limitations include the inability to be re-used and the need to re-tighten after the initial temperature cycle.
SilTite Metal Ferrules are also compatible with MS and oxygen-sensitive detectors. They have a long lifetime, a high temperature limit with MS compatibility but cannot be re-used.
Septa are used to isolate the sample flow path from the outside world and they must be easily penetrated by the injector needle while maintaining internal pressure. Here are the types of septa you can use and their best uses.
BTO Septa have a maximum operating temperature of 400 ⁰C (only 330 ⁰C with 17 mm size) and very low bleed.
TR-Green Septa have a maximum operating temperature of 400 ⁰C and a long lifetime.
Marathon Septa have a maximum operating temperature of 400 ⁰C and high mechanical durability.
TR-Blue Septa have a maximum operating temperature of 200-250 ⁰C and are easy to penetrate for routine applications.
Vials are used to contain and deliver your sample into the GC system and the vial used in the experiment must be compatible with your sample characteristics and provide a tight closure for reliable quantification. Here are some recommendations for vial use for different types of samples and types of analysis.
Routine Samples: Vials made of clear glass with either a SureStop 9-mm screw cap or an 11-mm crimp cap provides reliable sample delivery.
Light-Sensitive Samples: Vials made of amber glass with either a SureStop 9-mm screw cap or an 11-mm crimp cap provides protection for light-sensitive samples.
Low-Volume Samples: Micro inserts or micro-sampling and high recovery vials with fixed inserts for reduced internal volume provide a way to maximize the volume of sample injected.
Trace Levels: Silanized glass vials or certified kits provide the lowest background.
Ultra Trace MS Analysis: MS-certified vials provide low particulate and low background. They are pre-cleaned to provide unmatched consistency and performance for mass spectrometry.
Columns allow the separation of sample components and changing column parameters can affect your component separation as follows:
Column Length (m) affects the efficiency of the sample separation. Doubling the column length increases the resolution by ~40%.
Internal Diameter (mm) affects the efficiency and the retention of the sample. The smaller the column I.D., the greater the efficiency and the better the resolution will be.
Film Thickness (µm) affects the retention of the sample. The thicker the film, the greater the retention. Thicker is more ideal for highly volatile compounds. The thinner the film, the sharper the peaks and lower the bleed.
Stationary Phase chemistry affects the selectivity of your sample compounds. Altering the stationary phase can affect the elution order and help separate closely or co-eluting peaks.
Here are some valuable resources that can help speed up the selection of the correct consumable in your gas chromatography experiments:
Karen Thorson is a marketing manager for chromatography consumables in the Chromatography and Mass Spectrometry Division at Thermo Fisher Scientific. As a former drug formulation chemist, Karen moved from biopharmaceutical research to roles in chromatography instrument development where her focus was to assist in the development of new technologies that would better assist customers in their research. She prides herself in keeping a strong focus on customer needs via technical webinars and seminar presentations on industry related topics, and shares new technology applications with customers on a regular basis. Karin received her B.S. in Chemistry from the University of Massachusetts Dartmouth (USA).
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