Hi I'm Gerard Sharp. Welcome to the August 2011 issue of the monthly newsletter. This month the Tech Tip is on GC Inlet Liner Deactivation.

Dr Gerard Sharp...

Ardent Scientific has released a new GC eLearning module titled 'Fundamentals of GC'. Visit the site to launch some of the free content. The module is suitable for new users as well as anyone wanting to update their fundamental knowledge of GC. There's many interactive animations and voice overs explaining the content as you go along. Click here to view. Enjoy!


 

Tech Tip - GC Inlet Liner Deactivation
 

Introduction

The GC inlet liner is the disposable chamber in the GC used for vaporisation before the capillary column in a split / splitless inlet system. As part of the sample pathway, it is important the sample does not bind to the inlet glass surface and decompose before transfer to the column. These days GC inlet liners are mostly supplied deactivated. This term refers to a chemical treatment applied to the liner glass surface which makes the liner suitable for analysis of acidic, neutral, basic and thermally labile compounds. Many labs recycle their inlet liners by cleaning and re-deactivating them in-house.

 When Deactivation Matters
 

 It depends whether you are doing split or splitless analysis because the conditions in the liner are completely different. In split analysis, most of the flow (and therefore the sample) is split out the split vent and only a small proportion enters the capillary column. The proportions of each is termed the split ratio. If the split ratio is 100:1 and the flow through the column is 1.0 mL/min, then the flow out the split vent is 100mL/min. A typical volume of an inlet liner is anywhere from 0.5 - 2.0 mL depending on the type of instrument and inlet. Since the flow through the liner is typically high in split mode (100 + 1.0 mL/min in this case), the residence time of the compounds after vaporisation is short. This is the part that affects the liner deactivation. Because the time the compounds spend in the liner is so short, deactivation has neglible affect on the performance of the liner. In fact, non-deactivated and deactivated liners will produce the same chromatography in split mode.

 In splitless injection mode, the split vent valve is closed usually for about a minute for the initial part of the run. Under these conditions, the flow into the liner has only one exit point - the capillary column. If the flow through the column is set at 1.0mL/min then this will be the same flow through the liner. If the volume of the liner is 1.0mL, it will take up to one minute to transfer the contents of the liner (your vapour sample) into the column. The residence time will now be up to one minute which gives the compounds a lot more time to interact with the inlet liner surface. Under these conditions of splitless injection, deactivation becomes important.

 The top chromatogram in the slide above shows analysis of a test mix in splitless mode using a non-deactivated liner. Compared with the bottom chromatogram (in split mode with a non-deactivated liner), peaks have either disappeared or have a reduced response because of the extra time the sample has to interact with the inner surface of the liner.

 How Deactivated Liners are Tested

The most stringent test for inlet liners is based on the USEPA 8270 method and tests for breakdown of thermally labile compounds such as DDT and some phenols. These compounds are well known for either degrading or 'sticking' to the inner surface of the liner if deactivation is not complete. This slide shows the difference between a non-deactivated and a deactivated liner in splitless mode. As can be seen, the two phenols are missing from the top chromatogram when the non deactivated liner is used in the analysis. A very thermally labile compound such as DDT degrades to other compounds in the liner. Not all compounds are affected though.

The analysis of aliphatic hydrocarbons is very robust and using a non-deactivated liner has no effect on response, even in splitless mode. Pentadecane (C15) is often used as an internal reference because of this.

If you suspect a compound is being adsorbed in the liner. You could try analysing in split mode with 100 times the concentration and a split ratio of 100: 1 and then compare peak area responses with a splitless analysis. They should be similar if no break down or adsorption is occuring.

 Ways to Minimise Compound Degradation in a Liner

The use of quartz wool in a liner is often preferred for many reasons including faster vaporisation and the wool acts as a filter for the column. The presence of wool in the liner however does promote breakdown and / or irreversible adsorption of compounds such as some of the phenols, drugs and organochlorine pesticides. Some labs therefore prefer not to use wool for these applications and a liner with a taper or restriction at the bottom is preferred. If you still prefer to use wool, it is better to deactivate after the wool has been inserted in the liner as the very act of poking the wool into the liner will cause the wool fibres to break apart and increase the amount of activity in the wool. Fused silica wool, although it is a very pure glass, is also very brittle and breaks apart when inserted into the liner so is not ideal for use in inlet liners. Quartz wool is preferred.

The smaller the liner volume, the faster the transfer rate of compounds into the column because reducing the liner diameter and volume will increase the carrier gas velocity through the liner. The downside to this is that the sample volume used has to be carefully considered to ensure there is no overload as the liquid sample expands into a gas.

 Pulsed splitless injection can be used with GCs equipped with electronic pressure / flow control. This mode allows the use of an initially high inlet pressure to more quickly transfer compounds into the liner thus reducing the compound residence time. After about one minute, the inlet pressure is brought back to the optimum used for chromatographic resolution.

 Tips for Minimising Compound Degradation in Liners:
 

  • Use deactivated liners in splitless mode
  • A smaller volume liner will have an increased carrier gas velocity
  • thereby reducing the compound residence time and compound degradation
  • Pulsed splitless injection will also decrease the compound residence time
  • For very low level (ppb) analysis, replace the liner into a cold injector
  • Don't use quartz wool for very low level analysis


How to Deactivate Your Liner

 Most labs recycling their liners will have to clean them first to remove charred organic material and then deactivate. Cleaning may be done by using a strong acid or burning off the residue in a muffle furnace. Small pieces of septa may need to be removed using a small bottle brush or a pipe cleaner. The muffle furnace is preferred because this achieves two functions. The charred residue is completely oxidised to carbon dioxide and water and the surface of the liner is primed and ready for deactivation. Heating the liner in the muffle furnace will also dehydrate the surface and open up cyclic groups on the surface (called siloxane bridges) and provide free silanol groups. It is these free silanol groups which react with the deactivating reagent. Chromic acid is not recommended as it will leave a residue of chromium on the surface which can chelate with some compounds. Besides, it is a dangerous reagent to use.

 Many labs use 5-10% DMDCS (dimethyldichlorosilane) in dry toluene to deactivate the liner. Deactivation can be either be carried out at room temperature in the liquid state or the whole liner can be enclosed in a container with a small volume of reagent, heated and the deactivation carried out in the gas phase. In the liquid state, the liner is immersed in the DMDCS solution for 15-30 minutes then removed, rinsed with methanol and dried. A disadvantage of DMDCS is that it very quickly reacts with moisture in the air and decomposes to produce hydrogen chloride. The HCl produced can be easily detected by placing a piece of Litmus paper over the liquid DMDCS. The reagent therefore must be carefully stored under dry conditions (purge the headspace with dry nitrogen) and the reaction carried out in a dry environment. It is better to buy small bottles because of the short shelf life of the reagent. The reaction product of DMDCS will not adequetely deactivate the liner.


eLearning

 

Chromatography eLearning is yet another way to enhance your chromatography knowledge. While not ever being able to replace a classroom course, eLearning does offer advantages on cost and rich interactive content.  

Also, unlike a classrom course you can learn at your own pace and return to the content time after time.

Ardent Scientific offers varying levels of membership for viewing eLearning content. The Single Membership starts at $330 / year and corporate memberships are also available. View here for full details.

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