How can you calibrate a gas chromatograph for accurate measurements?

1 External calibration

External calibration is the simplest and most common method of calibrating a GC. It involves preparing a series of standard solutions with known concentrations of the analytes of interest and injecting them into the GC. The GC will generate a signal that is proportional to the amount of each analyte in the solution. By plotting the signal versus the concentration, you can obtain a calibration curve that shows the relationship between them. You can then use this curve to calculate the concentration of unknown samples based on their signal.

2 Internal calibration

Internal calibration is a more advanced and accurate method of calibrating a GC. It involves adding a known amount of an internal standard to each sample and standard solution before injecting them into the GC. The internal standard is a compound that is similar to the analytes of interest but does not interfere with their separation or detection. The GC will generate a signal for both the analytes and the internal standard. By dividing the signal of each analyte by the signal of the internal standard, you can obtain a normalized ratio that is independent of variations in injection volume, flow rate, or instrument performance. By plotting the ratio versus the concentration, you can obtain a calibration curve that shows the relationship between them. You can then use this curve to calculate the concentration of unknown samples based on their ratio.

3 Factors affecting calibration

Calibration of a GC can be affected by several factors, such as the quality and purity of the standard solutions and the internal standard, the accuracy and precision of the injection system, the stability and reproducibility of the GC system, and the linearity and sensitivity of the calibration curve. To ensure accuracy, you should use certified reference materials or high-purity solvents and reagents to prepare them and store them properly. Additionally, you should use a suitable syringe or auto-sampler to inject each solution with minimal variation or loss. Furthermore, it is important to check and maintain the GC components regularly and follow manufacturer's instructions. And finally, you should use enough points to cover the expected range of concentrations when creating a calibration curve, while also checking the correlation coefficient and residual error to assess its fit and quality.

4 Calibration frequency

The frequency of calibration for a GC depends on various factors, such as the type and purpose of the analysis, the performance and usage of the GC, and any regulatory or quality requirements. Generally, it is recommended to calibrate a GC before starting a new analysis or changing any parameters or components of the GC system. Additionally, you should calibrate at regular intervals based on the frequency and duration of the analysis or the number of samples processed. Furthermore, calibration should occur whenever you observe any drift or deviation in the signal or results, as well as after performing any maintenance or troubleshooting on the GC system.

5 Calibration verification

Calibration verification is the process of confirming that the calibration of a GC is still valid and reliable, which involves injecting a known standard or quality control sample into the GC and comparing the result with the expected value. If the result is within the acceptable range or tolerance, the calibration is verified; otherwise, it should be repeated or corrected. Calibration verification should be done after completing a calibration or correction, at regular intervals during analysis or between batches of samples, and whenever you suspect any problem or error in the GC system or results. As a field service engineer, it is essential to have accurate measurements from a GC. By following these steps and tips, you can ensure that your GC is working properly and producing reliable results, as well as improve your efficiency and productivity by avoiding unnecessary rework or troubleshooting.

6 Here’s what else to consider

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