Chromatography

Tenax TA

Further information

• hydrofobic
• low inherent artifacts (<1 ng)
• inert - suitable for labile compounds
• Tenax GR (graphitised form) is the best for PAHs and PCBs
• efficient desorption
• sorbent maximum temperature must not be exceeded as sorbent will breakdown and vapourise and may contaminate your flowpath
• 35/60 mesh is the recommended mesh size for sorbent tubes and traps
• recommended to re-pack tubes after 100 thermal cycles

Carbograph 2TD

Further information

• hydrofobic
• minimal inherent artifacts (<0.1 ng)
• some activitity due to trace metals in the charcoal raw materials used for sorbent production
• friable - susceptible to fines formation when mechanically shocked. Avoid dropping tubes containing these sorbents
• compressable - overpacking can result in high impedance and adverseley affect pump sampling flows
• 40/60 mesh is the recommended mesh size for sorbent tubes and cold traps
• recommended to re-pack tubes after 200 thermal cycles

Carbopack C

Further information

• hydrofobic
• minimal inherent artifacts (<0.1 ng)
• some activitity due to trace metals in the charcoal raw materials used for sorbent production
• friable - susceptible to fines formation when mechanically shocked. Avoid dropping tubes containing these sorbents
• compressable - overpacking can result in high impedance and adverseley affect pump sampling flows
• 40/60 mesh is the recommended mesh size for sorbent tubes and cold traps
• recommended to re-pack tubes after 200 thermal cycles

Carbotrap C

Further information

• hydrofobic
• minimal inherent artifacts (<0.1 ng)
• some activitity due to trace metals in the charcoal raw materials used for sorbent production
• friable - susceptible to fines formation when mechanically shocked. Avoid dropping tubes containing these sorbents
• compressable - overpacking can result in high impedance and adverseley affect pump sampling flows
• 40/60 mesh is the recommended mesh size for sorbent tubes and cold traps
• recommended to re-pack tubes after 200 thermal cycles

Sorbent selection

Selection of the correct sorbent or series of sorbents for sampling and release of the analytes of interest is one of the most important factors when developing a valid and robust thermal desorption method. Tube conditioning and capping are also critical issues. This site includes summary information on some of the most commonly used sorbents.

Sorbent list

Note: sorbent tubes should typically be conditioned using higher temperatures and faster gas flows than those selected for analysis, provided this does not mean exceeding the temperature limit of the material used.

Sorbent tube storage

Conditioned or sampled sorbent tubes should always be capped using ¼" brass Swagelok type screw caps fitted with combined PTFE ferrules (p/n C-CF020) as recommended by international standard methods for thermal desorption. We recommend the tubes to be tightened using a Markes CapLok™ tool (p/n C-CPLOK) or using conventional spanners / wrenches. The Cap-Lok tool was invented by scientists and makes it easier to cap tubes in the lab or field.

The CapLok tool also prevents over-tightening and distortion of the PTFE ferrules. Capped conditioned or sampled tubes should always be stored in as clean an atmosphere as possible. As an additional precaution batches of clean or sampled tubes can be wrapped in uncoated aluminium foil and placed in clean, nonemitting, air-tight containers such as unused paint cans, or freezer-grade food storage containers during transportation or extended storage.

It is not necessary to store capped tubes (blanks or sampled) in refrigerated conditions - unless the sampled tubes contain more than one sorbent. In this case, refrigeration is recommended to minimise risk of middle volatility analytes migrating from weaker to stronger sorbents during storage and thus resulting in incomplete recovery during analysis. If refrigeration is used, the caps must be retightened using the Cap-Lok tool once they have reached their storage temperature. Refrigerated tubes must also be removed from the freezer/refrigerator and left to equilibrate with the laboratory temperature before the storage caps are removed for analysis. If the tubes are uncapped while they are still cold, humidity from the lab air can condense inside the cold tube causing subsequent analytical difficulties.

Be aware that that the air within many general-purpose laboratory fridge / freezers is highly contaminated with volatiles from other samples or from the refrigeration system itself. Note also that if tubes are to be transported in such a way as to be exposed to very cold temperatures i.e. in an aircraft hold, by rail/ road overnight during cold weather, it is advisable to follow the above retightening procedure by cooling the tubes prior to shipment and retightening the caps.

Reference: This information is a part of Thermal Desorption Technical Support document TDTS Note 5 by Markes International.

Injector port issues

The injection port of a gas chromatograph is where the liquid sample is vaporized and transported onto the column by the carrier gas. A disposable glass liner is used in the injection port to limit sample degradation and improve vaporization. The glass liners in it’s simplest form is a straight piece of cylindrical glass but they are much more complex in most instances. The glass liner design can be altered or adjusted to optimize sample vaporization, similarly poor liner choice can be a major source of errors in the analysis. It is a crucial part of the whole chromatography process and needs to be understood fully to enable optimum separations.

Factors which need to be taken into account are the temperature of the injection port, carrier gas flow, injection technique, sample size, split ratio and the design of liner used. The incorrect choice of liner can result in poor quantitation, sample flashback, peak tailing and mass discrimination just to name a few. Therefore choosing the correct liner design critical for your analysis.

5 attributes of an effective inlet liner

• The liner design should minimize mass discrimination by ensuring complete vaporization of the sample before it reaches the column entrance.
• The volume of the inlet liner must be larger than the volume of vaporized sample and solvent.
• The liner must not react with the sample. This is especially important for polar solutes where the liner should be deactivated.
• Addition of quartz wool increase the vaporization surface area for the sample and promotes efficient mixing of the sample and carrier gas.
• The position of the quartz wool should be optimized corresponding to the needle depth in the liner.

Different liner design

Quartz wool or no quartz wool?

The use of quartz wool in a liner has been of much debate and there are many advantages of using wool. The quartz wool acts as a crude filter for the analytical column and minimizes the chances of any particulate or non-volatile material from reaching the column. If the quartz wool is positioned so that the needle tip is in the centre of the wool mass, the wools large surface area helps in the efficient vaporization of the sample. The quartz wool also promotes mixing in the liner, which has a short sample residence time. The needle tip is wiped on the wool as the needle is withdrawn, allowing all the sample to be vaporized. To prevent sample degradation the wool in the liner has to be deactivated. The quartz wool used in SGE liners is fully deactivated in situ, it is still recommended not to use wool if analyzing low level pesticides such as DDT or Endrin.

Taper at the bottom

A taper at the bottom of the liner acts as a feed-in for the capillary column and reduces the amount of dispensed liquid hitting the bottom of the injector. If the quartz wool is packed loosely, then the taper prevents the wool moving out of the liner which might happen during high pressure injection techniques eg. pulsed splitless injection.

Restrictions, baffles, cups, complex forms, etc. in the liner without quartz wool

All these complex forms of liner can promote vaporization and mixing of the formed vapors which will minimize mass discrimination. One of the key functions of the inlet liner is for representative transfer of the sample from the syringe into the capillary column. If a reduced quantity of high molecular weight components are transferred then this is known as high molecular weight mass discrimination. Liner design and quartz wool can both promote mixing and minimize mass discrimination.

Discrimination - liner type

Taper at the top

The taper at the top can be used to minimize the effect known as Flashback. This occurs when an excessive amount of liquid is injected into the liner and the volume of the vaporized gas is larger than the volume of the liner. The gas can then escape back out of the liner into the inlet lines and cause contamination. The taper at the top reduces this effect by acting as a partial lid on the liner.

Inner diameter

The inner diameter will obviously determine the volume capacity of the liner. Therefore for a liquid volume greater than 2 µl, the liner ID should be as large as possible. Remember, when the liner ID is halved, the subsequent liner volume is reduced to one quarter of the original. The other important consideration for liner ID is the velocity of the carrier gas through the liner. A smaller ID will result in a higher velocity of gas and this means faster analyte transfer and therefore sharper peaks, especially for early eluting components. The transfer rate is more critical in splitless injection because the liner flow is equal to the column flow and is low. A smaller ID can have a dramatic effect of peak shape as shown below.

Deactivation

Deactivation is more critical during splitless injection than split injection. In splitless injection, the split vent is usually closed for about 1 minute, resulting in a low liner gas flow. A low gas flow will result in slow transfer and the residence time of analytes within the liner is increased. Therefore for thermally labile and heat sensitive compounds, the interaction time of the analyte with the inner surface of the glass liner is increased and this enhances breakdown. The effect is not nearly as pronounced in split injection because the residence time in the liner is very short.

SGE's effective liner solution

The SGE FocusLiner^TM addresses all attributes of an effective liner by using a simple but effective design by where the quartz wool is held in the correct position by means of two tapered sections in the liner. The tapered sections are located to ensure the needle tip penetrates the quartz wool at the optimum position every time allowing the needle tip to be wiped consistently. As seen below the %RSD values for active compounds have been determined when injecting into liners with the quartz wool in different positions. From this you can see that the FocusLiner^TM provides the most accurate and reproducible results when compared with a glass frit liner and a liner with quartz wool located in the middle.

For more information on SGE Focusliners^TM please contact us and remember that no matter how good the deactivation of the liner, its design, or the number of samples injections, it will need to be changed on a regular basis to achieve optimum chromatography.

forte BPX5

(5% Phenyl Polysilphenylene-siloxane)

• Low polariry phase, an MS-Premium, low bleed column with a maximum temperature up to 370°C.
• Popular column used for a wide variety of applications.

Applications

• Drugs
• Solvent impurities
• Pesticides
• Hydrocarbons
• PCB congeners or (e.g.) Aroclor mixes
• Essential oils
• Semivolatiles

forte HT5

(5% Phenyl (equiv.) Polycarborane-siloxane)

• Low polatiry phase.
• Unique high temperature phase suited for simulated distillation and other petroleum applications.

Applications

• Simulated distillation
• Petroleum application

forte HT8

(8% Phenyl Polysiloxane-carborane)

• Non-polar phase.
• Unique high temperature phase suited for the analysis PCB congeners.
• Ideal for ECD and MSD systems.

Applications

• PCB congeners

*proprietary