How to establish a gas chromatography analysis method - Database & Sql Blog Articles

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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

,

Sample source and pretreatment method

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Samples that can be directly analyzed are usually gases or liquids. Solid samples should be dissolved in a suitable solvent prior to analysis, and must be included in the sample.

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Components that cannot be analyzed (such as inorganic salts) can damage the components of the column. 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 is no sample in the sample

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For components that are directly analyzed, or where the concentration of the sample is too low, necessary pretreatments must be performed, such as adsorption, resolution, extraction, concentration, dilution, purification, derivatization, and the like.

2

,

Determine 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 are often chosen

FID

Detector containing an electronegative group (

F

,

Cl

Etc.) more substances with less hydrocarbon content are easy to choose

ECD

Detector; optional for sensitive detection or non-hydrocarbon components

TCD

Detector; selectable for samples containing sulfur and phosphorus

FPD

Detector.

For the liquid sample, the diaphragm pad injection mode can be selected. The gas sample can be a six-way valve or an adsorption thermal analytical injection method. Generally, the chromatogram is only configured with a diaphragm pad injection method, so the gas sample can be adsorbed.

-

Solvent analysis

-

The way the diaphragm pad was injected was analyzed.

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. Sample concentration does not exceed

10mg/mL

The injection volume of the packed column is usually

1-5uL

For a capillary column, if the split ratio is

50

:

1

When the injection volume is generally not more than

2uL

. 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 condition 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 also does not achieve the purpose of baseline separation, you should replace the longer column, or even replace the column with different stationary phases, because

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In the middle, the column is the key to the success of the separation.

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 two-column or multi-column retention index is qualitative.

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A more reliable method because the probability of different compounds having 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 validation

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.