Changing the Carrier Gas
The following explains precautions and other information for switching the carrier gas from helium to nitrogen or hydrogen.
Carefully review the condition settings before switching the carrier gas to nitrogen.
Nitrogen gas (N2) is an inexpensive and safe gas. However, if nitrogen gas is used in a capillary GC system with the same analytical condition settings as for helium, then separation capabilities will decrease in most cases. If there are no adjacent peaks involved, then the carrier gas can be changed to nitrogen without changing the condition settings. If there are many adjacent component peaks and a certain level of separation is required, then the condition settings need to be reconsidered before using nitrogen gas.
Improving separation capabilities for nitrogen gas will require reconsidering the carrier gas linear velocity and temperature settings and will probably result in a longer analysis time. The vaporization state and detector sensitivity will probability change and the peak area percent might differ from when helium gas was used.
Separation with Helium Gas and Nitrogen Gas
Separation examples for helium and nitrogen gases at different linear velocities are shown below.
Using helium gas, the separation is virtually unchanged across the range from 20 to 47 cm/s linear velocity. With nitrogen gas, however, the separation deteriorates at 47 cm/s linear velocity. This occurs because the optimal linear velocity for separation is lower with nitrogen than with helium and the optimal linear velocity range for nitrogen is narrower than for helium.
Reviewing Analytical Conditions for Nitrogen Gas
If samples are analyzed with nitrogen gas using the same conditions as optimized for helium gas, then the separation capability will decrease due to different column efficiency (HETP). If high separation is unnecessary, then the same analytical conditions can be used in some cases. If high separation is required, however, it is recommended that analytical conditions be reconsidered, starting at a linear velocity in the 10 to 20 cm/s range where separation performance tends to be higher. (Slower linear velocity values will result in longer analysis times.)
That means the linear velocity will vary as the temperature increases when performing programmed temperature analysis, but Shimadzu GC units use constant linear velocity control functionality to provide optimal separation even when using nitrogen gas.
Average linear velocity (cm/sec)
In an increasing number of cases, hydrogen gas is being considered as a carrier gas for use in GC analysis. Hydrogen (H2) gas is easier to obtain and less expensive than helium gas and offers higher separation performance for a given linear velocity. Therefore, if it is used safely, it can significantly reduce running costs. On the other hand, hydrogen is a dangerous gas that can easily explode. The following explains the risks, precautions, and other information about using hydrogen gas safely.
- Be sure to use hydrogen gas safely, in accordance with manuals relevant to using respective devices safely. Also read the hydrogen gas precautions included in the gas chromatograph instruction manual.
- Changing the type of carrier gas used may require reviewing the specified analytical conditions.
- Because GC pretreatment systems also use other gases besides carrier gases, it might not be possible to use hydrogen as the carrier gas with some instruments.
|If a leak error occurs using an electronic flow controller and APC/AFC unit, even though the gas supply pressure is normal, stop using the system and request service from a Shimadzu representative.|
|For gas chromatographs with a manually specified hydrogen gas flowrate, if the flowrate or pressure is much higher (or lower) than normal, then check for gas leaks, including from the pressure control valve.
If no leaks are found, leakage does not stop, or normal performance is not restored after stopping the leak, stop using the system and request service from a Shimadzu representative.