Changing the Carrier Gas

Helium is normally used as the carrier gas for GCMS analysis. However, helium cannot be used in some situations; for those analyses, hydrogen can be used. Shimadzu's gas chromatograph mass spectrometers (GCMS) are compatible with hydrogen carrier gas, allowing hydrogen to be used as a carrier gas for the GC unit. Hydrogen gas (H2) is easier to obtain and less expensive than helium gas, and can maintain high separation performance for a given linear velocity. If used safely, it can also significantly reduce running costs.

GCMS-TQ series, GCMS-QP2020 series, and GCMS-QP2010 series models include a carrier gas automatic shutdown function. An electronic flow controller (AFC), used to control the carrier gas flowrate, shuts off the carrier gas supply and automatically stops the GC unit if the column inlet pressure or the total flowrate does not reach the corresponding setting value within given parameters, assuming a major carrier gas leak or other problem has occurred. The carrier gas supply is also shut off automatically if the vacuum pump stops due to a failure.

Features for using hydrogen as a carrier gas are listed below. Verification of using hydrogen as a carrier gas in GCMS-TQ series and GCMS-QP2010 series systems is also described.

• By monitoring the pressure and flowrate at the injection port, the oven temperature is decreased, and carrier gas supply is shut off if the pressure and flowrate do not reach specified values within a certain time.
• If a carrier gas control problem occurs, the electronic flow controller automatically shuts off the carrier gas supply.
• If the vacuum pump stops due to a failure, the carrier gas supply is automatically shut off.
• Shimadzu GCMS systems are tested for hydrogen leaks inside the oven to confirm that the oven design does not permit hydrogen to collect inside the oven. Due to the high diffusion coefficient of hydrogen, it will not collect inside the oven.
• Shimadzu GCMS systems are explosion tested using hydrogen. Given analytical conditions normally used, hydrogen gas only causes a small explosion sound when intentionally ignited inside the column oven. Also, after a vacuum is pulled, fully loosening the knob on front door of the MS unit can prevent hydrogen from collecting inside the MS unit, under normal operating conditions.
• Shimadzu GCMSsolution workstation software can be configured with hydrogen as the standard carrier gas and migrated methods can be reviewed easily by specifying the constant carrier gas linear velocity mode.

Be sure to do the following when using hydrogen gas.

Precautions for Using (Handling) Hydrogen Carrier Gas Safely

Hydrogen is a dangerous gas that can easily explode. A Safety Manual was prepared that lists the possible risks associated with using hydrogen gas in a gas chromatograph mass spectrometer and indicates considerations for ensuring the systems are used safely.

>> Using hydrogen carrier gas safely in GCMS-QP2010 series, GCMS-QP2020, and GCMS-TQ8030/8040 systems  (228 KB)

Considerations That Could Affect Analysis

It will increase noise in terms of sensitivity (S/N). For S/N values using hydrogen gas in respective systems, refer to product specifications. Equivalent separation results can be obtained by using an optimal column and creating a method. Most compounds can be analyzed, but hydrogen is not an inert gas, so it can dissociate at the injection port in some cases.

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.