contamination and variability which can affect the base line. Leak check connections and replace seals if needed.

b) Impurities in the carrier gas can be removed by replacing the supply and/or installing inline filters.

2. Inlet or detector contamination

a) Contamination from continued use or dirty samples can build up in the inlet and on the detector. Routine maintenance should be carried out and parts replaced if they show signs of damage.

3. Column contamination or stationary phase bleed.

a) Contamination on the column or excessive bleed from the stationary phase can affect the base line. The column should be conditioned to reduce the bleed and contamination removed by trimming and rinsing the column.

4. Septum coring or bleed

a) As the septum in the inlet ages, it can become brittle which leads to coring and bleed issues. It should be replaced regularly as part of a maintenance regime.

b) Septum coring can lead to a build-up of particles in the inlet liner. These particles can affect the transfer of the target analytes on to the column and can be a source of contamination. The liner should be checked for particles and replaced if needed.

5. Leak or poor-quality gases.

a) Contamination can be introduced to the system through leaks in the supply or poor-quality gases. All gas lines should be checked for leaks. The purity of the gas supply should be checked to ensure that it is adequate. If necessary, install gas filters to remove moisture and potential contamination.

6. Variable carrier gas or detector gas flows.

a) Variability in the flow of carrier gas or gas to the detector could result in baseline issues such as spiking. This can be resolved by checking the system and fixing any leaks and ensuring the flow and pressure controllers are functioning correctly.

7. Detector not ready.

a) Instability in the baseline could be due to the detector not having enough time to stabilise before the analysis is started. Load the appropriate method and allow enough time for detector temperatures and flows to equilibrate before starting the analysis.

Changes in response

Unexpected changes in the response of the target analytes can affect sensitivity, detection limits and accuracy of quantitation. 1. Sample

A change in the response of the target analyte may be due to issues with the sample. The following should be checked.

a) The sample concentration is as expected. b) The sample preparation procedure has not changed.

c) The sample has not decomposed and is being analysed within its shelf life. 2. Syringe

a) The syringe may have become partially blocked so is not injecting the required amount of sample on to the system. Check the syringe is working properly and replace it if needed.

b) The autosampler should also be checked to ensure it is functioning correctly.

3. Electronics.

a) A change in the analyte response may be due to the detector settings. Verify the signal settings and adjust them if needed. 4. Dirty or damaged detector.

a) Contamination from continued use or dirty samples can build up on the detector effecting the response of the target analyte. Routine detector maintenance should be carried out and parts replaced if they show signs of damage.

5. Flow/temperature settings wrong or variable

a) If the flow rates and temperatures are not set correctly in the method or are varying, the detector response will be not at its optimal. Verify the flow rates and temperatures are steady, then adjust the settings and/or replace any parts as required.

6. Adsorption/reactivity

a) Contamination in the inlet or front of the column can lead to active sites where the target analytes may be irreversibly absorbed or undergo a chemical change such as decomposition. This can be overcome by removing the source of contamination and using properly deactivated liners and columns.

7. Leaks

a) A leak in any part of the system can affect the amount of sample reaching the detector. Check the system for leaks and repair any connections as needed.

8. Change in sample introduction/injection method.

It is possible that the acquisition method may have been inadvertently changed and over written.

a) Verify what injection technique was initially used and change back to the original technique. b) Check split ratio is set correctly. c) Verify that the splitless hold time is correct.

Poor peak resolution

Poor resolution between closely eluting peaks may affect the accurate quantitation of target analytes. 1. Non-selective stationary phase

a) The column may not have the power to resolve the coeluting compounds. The resolution may be improved by choosing a column with a more appropriate stationary phase and column dimensions.

2. Poor efficiency.

a) The method conditions may not be the most efficient for resolving closely eluting compounds. Resolution can be improved by optimising the carrier gas linear velocity and the GC oven program.

3. Sample overload.

a) Injecting too much sample on to the column will reduce the resolution between closely eluting compounds due to the peaks becoming broader. The sample concentration or amount on column can be adjusted by increasing the split ratio or decreasing the injection volume.

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