Welcome to the September 2012 issue of the chromatography newsletter. This issue the topic is 'Choosing the Carrier Gas Type for GC'.

Dr Gerard Sharp...

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Choosing the Carrier Gas Type in GC

Introduction

With the rising cost of helium, it is a good time to review the choice of the carrier gas and the impact on the chromatogram. Some labs are now upwards towards $1000 a cylinder for helium and it is one of the most expensive consumables in running a GC lab.


Why the Different Choices in Carrier Gas?


The mobile phase in GC is usually called the carrier gas because unlike HPLC, it has no impact on the selectivity of the separation. The carrier gas simply pushes the components in the mixture down the column. We can't change the polarity of the carrier gas like we can with the HPLC mobile phase to obtain better separation. However, the type of the carrier gas we choose does impact on resolution. This is because the carrier gas affects the efficiency of the separation. Efficiency can be best thought of as making the peaks more narrow. The narrower the peaks, the better the resolution as can be seen from this diagram. In GC, by choosing the correct carrier gas type and by optimizing for the carrier gas velocity, we can minimize peak spreading and increase resolution. The most common choices of carrier gas are hydrogen, helium or nitrogen.

 

  • Nitrogen is cheap and non-explosive but not the best for the separation
     
  • Helium is efficient and non explosive
     
  • Hydrogen is the most efficient producing the best separation in the shortest time but is explosive
     


 About Gases and Diffusion

When the components are being pushed down the column and moving in and out of the stationary phase, there are forces which spread the molecules apart. The more spreading of the molecules (peak dispersion), the wider the peak becomes and the worse the resolution. One of these forces is known as longitudinal diffusion. A fancy name but not difficult to understand. When a gas is allowed to mix with another gas in a vessel, it will diffuse into the second gas until all parts of the vessel are filled and the concentration is equal in all parts of the vessel. For example, after opening a cylinder of argon in a sealed room, the argon will diffuse into the air and eventually lead to an equal concentration of the argon in all parts of the room. This force also occurs in the capillary column. The components in your mixture which have been converted into the gas phase, will diffuse into the carrier gas. The longer the components spend in the column, the more time they will have for this diffusion. So a low carrier gas velocity increases the width of the peak and decreases resolution.

The second type of gas diffusion in GC is known as resistance to mass transfer . As the components are being pushed down the column by the carrier gas, some of the molecules are moving into the stationary phase coated on the wall as other molecules are still in the gas stream. The faster the carrier gas, the further apart the molecules in the gas stream are pushed from the molecules in the stationary phase. This is because there is a lag time time for the molecules to partition into the stationary phase and back out again. The quicker they diffuse in and out, the less spreading of the molecules and the peak remains narrow. Hydrogen is the best because the molecules wil diffuse fastest while nitrogen is the worse.

As can be seen, these two terms - the longitudinal term and the mass transfer term contribute to the peak spreading but in opposite ways. For the longitudinal term, the faster the carrier gas, the better as this leads to minimal peak spreading. For the mass transfer term, the opposite is true. Lower carrier gas velocities lead to thinner peaks. So what carrier gas velocity is best. The answer lies with the type of carrier gas.

 

Carrier Gas Velocity

Helium, hydrogen and helium can produce peaks with the same resolution. So why pay so much for helium? Using nitrogen can produce narrow peaks and excellent resolution but it does this at the expense of time.

This slide shows the effect of changing the carrier gas velocity (u, cm/sec) on the efficiency. Efficiency here is measured by the height equivalent of a theorectical plate (HETP, mm) and the lower this value, the more theoretical plates and the greater the efficiency and resolution.

 

  • Nitrogen has an optimum velocity of 15cm/sec
     
  • Helium has an optimum velocity of 35 cm/sec
     
  • Hydrogen has an optimum velocity >40cm/sec


This is the main reason people use helium, it is over twice as fast as nitrogen for the same resolution. Hydrogen is even better and can be 3-5 times faster.

The carrier gas velocity can be easily set or measured. in modern GCs, this is done by entering the capillary column dimensions, the carrier gas type and the desired carrier gas velocity into the software. For older GCs, the carrier gas velocity can be measured by injecting an unretained component into the GC. Butane from a cigarette lighter is often used and using the retention time (in seconds) in the equation below.

Gas Velocity = Column Length (cm) / Retention time of unretained peak (secs)

The inlet pressure is then adjusted accordingly.



Helium, Hydrogen or Nitrogen?

For complex mixtures i.e. 20 or more components, helium or hydrogen is preferred. Hydrogen is the carrier gas of choice and is recommended for modern GC's which have an in-built safety feature in electronic pressure control which will shut down the system if there is a large leak. Helium is usually used for mass specs because of the higher risk of explosion but there has been a trend in the last couple of years towards hydrogen which is cost driven. Hydrogenand is safe if the recommended safety precaustions are followed.

Nitrogen is worth considering for simpler mixtures and when run time is not as critical.

eLearning

 

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