In practical work, when we get a sample, how can we determine qualitatively and quantitatively, and it is the key to establish a complete analysis method. Here are some general steps:
1. Source of sample and pretreatment method
The sample that can be directly analyzed by the GC must be a gas or a liquid. The solid sample should be dissolved in an appropriate solvent before analysis, and the sample must not contain components that cannot be analyzed by the GC (such as inorganic salts), which may damage the column. The components. In this way, when we receive an unknown sample, we must understand the source, so as to estimate the sample may contain components, as well as the boiling range of the sample. If you can confirm the sample can be directly analyzed. If there are components in the sample that cannot be directly analyzed by GC, or if the sample concentration is too low, necessary pretreatments must be performed, including the use of some pre-separation means such as various extraction techniques, concentration and dilution methods, and purification methods.
2, determine the instrument configuration
The so-called instrument configuration is what sample injection method is used to analyze the sample, what carrier gas, what column, and what detector.
3, determine the initial operating conditions
When the sample is ready and the instrument configuration is determined, a tentative separation can begin. At this point, the initial separation conditions, including sample volume, inlet temperature, detector temperature, column temperature, and carrier gas flow rate, were determined. The amount of sample injected should be determined based on sample concentration, column capacity, and detector sensitivity. When the sample concentration does not exceed mg/mL, the injection volume of the packed column is usually 1-5uL, while for the capillary column, if the splitting ratio is 50:1, the injection volume generally does not exceed 2uL. The inlet temperature is mainly determined by the boiling point range of the sample, and the use temperature of the column is also considered. In principle, the inlet temperature is higher, and it is generally better to approach the boiling point of the highest boiling component in the sample, but below the easily decomposable temperature.
4. Optimization of separation conditions
The purpose of the separation condition optimization is to meet the required separation results in the shortest analysis time. When changing the column temperature and carrier gas flow rate can not achieve the purpose of baseline separation, longer columns should be replaced, or even different stationary phase columns, because in the GC, the column is the key to separation success.
5, qualitative identification
The so-called qualitative identification is to determine the attribution of chromatographic peaks. 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 injected separately, and according to the retention value, it can be determined which peak on the chromatogram is the component to be analyzed. Qualitatively, it must be noted that different compounds may have the same retention values ​​on the same column. Therefore, it is not sufficient to use only one retention data for the characterization of unknown samples. Qualitative analysis with two-column or multi-column retention index is more reliable in GC. The method, because the probability that different compounds have the same retention value on different columns is much smaller.
6, quantitative analysis
To determine the quantitative method used to determine the content of the test component. Commonly used chromatographic quantitative methods are nothing more than the 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. The method is optional only if the sample consists of homologues or if only for a rough quantification. In contrast, the internal standard method has the highest quantitative accuracy because it is quantified with the response value of a standard (called an internal standard), and the internal standard is added to a standard sample and an unknown sample, respectively. This offsets the errors due to fluctuations in the operating conditions (including injection volume). As for the standard addition method, a standard product is added quantitatively to an unknown sample in 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 of quantitative accuracy should be between the internal standard method and the external standard method.
7, the method of 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 is all available as a commodity purchase, 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 peers. Reliability includes quantitative linear range, detection limit, method recovery, repeatability, reproducibility, and accuracy.
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