In actual work, when we get a sample, how do we determine the qualitative and quantitative, it is the key to establish a complete gas chromatograph analysis method. The following Lu Chuang analysis introduces 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 a suitable solvent before analysis, and the sample must be free of components (such as inorganic salts) that cannot be analyzed by the 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 can be confirmed, it can be analyzed directly. 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, including some pre-separation methods such as various extraction techniques, concentration and dilution methods, and purification methods.
2, determine the gas chromatograph 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.
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 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.
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 gas chromatograph 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.