How to establish a gas chromatographic analysis method

Gas chromatography analysis method establishment steps
In practice, when we get a sample, how do we characterize and quantify, it is critical to establish a complete analysis method. Here are some general steps:
1. Source of sample and pretreatment method
Samples that can be directly analyzed by GC are usually gases or liquids. Solid samples should be dissolved in a suitable solvent before analysis, and the samples must be free of components (such as inorganic salts) that cannot be analyzed by GC, which may damage the column. The components. In this way, when we receive an unknown sample, we must know the source to estimate the components that the sample may contain, as well as the boiling range of the sample. If the sample system is simple, the sample components can be vaporized for direct analysis. If there are components in the sample that cannot be directly analyzed by GC, or if the sample concentration is too low, the necessary pretreatment must be performed, such as adsorption, resolution, extraction, concentration, dilution, purification, derivatization, etc.
2, determine the instrument configuration
The so-called instrument configuration is what is used to analyze the sample, what sample device, what carrier gas, what column and what detector.
The detector type should generally be determined first. Hydrocarbons often choose FID detectors, substances with more negative electron groups (F, Cl, etc.) and less hydrocarbon content are easy to choose ECD detectors; low sensitivity to detection, or contain non-hydrocarbons For components, the TCD detector can be selected; for sulfur and phosphorus samples, the FPD detector can be selected.
For the liquid sample, the diaphragm pad injection mode can be selected. The gas sample can be sampled by a six-way valve or an adsorption thermal analysis. Generally, the chromatogram is only configured with a diaphragm pad injection method, so the gas sample can be sampled by adsorption-solvent analysis-diaphragm pad injection. The way to analyze.
Selecting a suitable column according to the nature of the component to be tested generally follows a similar compatibility rule. Select a non-polar column when separating non-polar materials and a polar column when separating polar substances. After the column is determined, the working temperature of the column is determined according to the difference of the distribution coefficient of the components to be tested in the sample. The simple system adopts the isothermal method, and the complex system with large difference of the distribution coefficients is analyzed by the program temperature heating method.
Commonly used carrier gases are hydrogen, nitrogen, helium, and the like. The molecular weight of hydrogen and helium is often used as a carrier gas for packed column chromatography; the molecular weight of nitrogen is large, which is often used as a carrier gas for capillary gas chromatography; and gas chromatography mass spectrometry uses helium as a carrier gas.
3. Determine initial operating conditions
When the sample is ready and the instrument configuration is determined, an attempted separation can begin. The initial separation conditions are determined at this time, including injection volume, inlet temperature, detector temperature, column temperature, and carrier gas flow rate. The injection volume is determined by sample concentration, column capacity, and detector sensitivity. When the sample concentration does not exceed 10 mg/mL, the injection volume of the packed column is usually 1-5 uL, and for the capillary column, if the split ratio is 50:1, the injection volume generally does not exceed 2 uL. The inlet temperature is primarily determined by the boiling point range of the sample, taking into account the column's operating temperature. In principle, the inlet temperature is somewhat higher, generally close to the boiling point of the highest boiling component of the sample, but lower than the easily decomposable temperature.
4, separation conditions optimization
The purpose of the separation condition optimization is to achieve the desired separation result in the shortest analysis time. When changing the column temperature and carrier gas flow rate to the point of baseline separation, you should replace longer columns and even replace columns with different stationary phases, because in GC, the column is the key to separation success.
5. Qualitative identification
The so-called qualitative identification is to determine the attribution of the peak. For simple samples, they can be characterized by standard substance controls. That is, under the same chromatographic conditions, the standard sample and the actual sample are separately injected, and based on the retained value, it can be determined which peak on the chromatogram is the component to be analyzed. When characterizing, it must be noted that different compounds may have the same retention value on the same column. Therefore, it is not enough to use only one retained data for the identification of unknown samples. The double-column or multi-column retention index is more reliable in GC. Method because the probability that different compounds have the same retention value on different columns is much smaller. Gas chromatography mass spectrometry can be used to characterize on-line conditions.
6, quantitative analysis
It is necessary to determine what quantitative method is used to determine the content of the component to be tested. Commonly used chromatographic methods are nothing more than peak area (peak height) percentage method, normalization method, internal standard method, external standard method and standard addition method (also called superposition method). The peak area (peak height) percentage method is the simplest, but the least accurate. This method is optional only if the sample consists of homologues or is only for rough quantification. In contrast, the internal standard method has the highest quantitative accuracy because it is quantified with a response value relative to a standard (called an internal standard), and the internal standard is added to the standard sample and the unknown sample, respectively. It can offset the error caused by fluctuations in operating conditions (including injection volume). As for the standard addition method, a standard product to which a test substance is added is added to an unknown sample, and then quantitatively calculated based on the increase in peak area (or peak height). The sample preparation process is similar to the internal standard method but the calculation principle is entirely from the external standard method. The standard addition method should be between the internal standard method and the external standard method.
7, method verification
The so-called method verification is to prove the practicability and reliability of the developed method. Practicality generally refers to whether the instrument configuration used can be purchased as a commodity, whether the sample processing method is simple and easy to operate, whether the analysis time is reasonable, and whether the analysis cost can be accepted by the peers. Reliability includes quantitative linear range, detection limits, method recovery, repeatability, reproducibility, and accuracy.

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