Troubleshooting Gas Chromatography: Reduced Peak Size (Loss of Sensitivity)

We are frequently asked about issues with reduced peak size in gas chromatography (GC). There are so many potential causes for reduced peak size that an inexperienced GC user may not know where to begin in the troubleshooting process. Sensitivity problems can be grouped by way of their presentation within the chromatogram:

All peaks sizes (heights and or areas) decrease–retention times do not change.

As always, it’s best to start with the most obvious possible causes.

• If operating in split mode, check the inlet split ratio in the acquisition method and correct, if required.

• If splitless mode with pressure pulse, check the inlet pulse pressure and duration in the acquisition method.

• Check that the inlet and detector temperatures have been set correctly within the acquisition method.

• Check that the sample vial contains sufficient liquid and use separate vials containing the same sample liquid for repeated injections to rule out the possibility of sample loss through a compromised septum (especially when analytes are highly volatile), and confirm the problem is reproducible.

• Check that the autosampler syringe plunger is free within the barrel and does not leak. Observe an injection cycle and confirm that sample is being aspirated from the vial and that it the correct volume.

• Check and replace the inlet septum as required, and check that the correct liner has been installed.

• Check that the sample preparation or dilutions are correct as per the sample preparation method.

• With flame based ionizing detectors, check that the fuel gas ratios are appropriate and that all flow rates are as they should be (using a flow meter, checking one gas flow at a time), and check all applied voltages according to the manufacturers specification.

• With MS detectors in selected ion mode, check the masses and dwell times of each ion within each SIM group to ensure that the cumulative ion count is matched to the acquisition method.

• With MS detectors in scan mode, check the MS tune and verify that the repeller or accelerator voltage has not increased dramatically (indicating a dirty ion source), that the electron multiplier or MCP voltages have not increased dramatically (indicating a worn-out detector), and that the ionization energy is correctly set (typically 70 eV for electron ionization).

• Check the detector attenuation range and set appropriately, as a secondary check. When the detector attenuation is incorrectly set, the signal-to-noise ratio for a given peak will not reduce, but the chromatogram will appear ”smaller.”
   

All peaks sizes (heights and areas) decrease. Retention times shift, no evidence of loss of efficiency (peak broadening)

• Check that the correct column dimensions have been entered into the data system and that the correct column is installed, paying attention to the stationary-phase film thickness.

• Check that the carrier gas is flowing at the correct volumetric flow rate using a calibrated flow meter.

• Check that the carrier gas flow programming method is correct by selecting either constant pressure or constant flow operating modes.  In the latter, the carrier gas pressure will be ramped to achieve constant linear velocity of carrier through the column as the oven temperature increases and the carrier gas viscosity reduces.

• Change the inlet septum to overcome any leaks that may be occurring during the injection phase.

All peaks sizes (heights and or areas) decrease–peaks are broadened, retention time shift may occur.

• Check that the correct column has been installed and column dimensions and carrier gas flow rate are appropriate.

• Check the column logs; if the column is old or has been used with dirty sample matrices, suspect that the column efficiency might have been reduced. 

• Run a column test mix and compare with the result obtained originally.

• If loss of column efficiency is suspected, trim 0.5–1 meters of column at the inlet end.

• Verify that the column has been installed to the correct distance within the GC inlet and detector. For flame-based and ionizing detectors, check that the gas flows are correct paying particular attention to any make-up gas flow rates.

• When all peak heights or areas reduce and the peaks broaden, the most obvious cause is a loss of efficiency within the chromatographic system.  This phenomenon should also lead a reduction in signal-to-noise ratio for analyte peaks.

Tony Taylor is the Chief Science Officer of Arch Sciences Group and the Technical Director of CHROMacademy. Direct correspondence to: LCGCedit@mmhgroup.com.