Classification of gas chromatograph detectors

Understand the classification of detectors, and grasp the performance characteristics and working principle from the whole. According to the performance characteristics and working principle of the detector, it is divided into two classification methods.

First, classified by performance characteristics

Observe the detector performance from different angles, with the following classifications:

1. Destruction of the sample or not

During the detection process, if the molecular form is destroyed, it is a destructive detector such as FID, NPD, FPD, MSD, and the like.

The component is a non-destructive detector during the detection process, such as still retaining its molecular form. Such as TCD, PID, IRD, etc.

2, according to the relationship between response value and time

The response of the detector is the cumulative amount of components at that time, which is an integral detector such as a volume detector. In the current gas chromatography analysis, such detectors are generally not used.

The response of the detector is the instantaneous amount of the component at that time, which is a differential detector. All detectors described in this book are of this type.

3. According to the response value, the concentration or quality

The response of the detector depends on the concentration of the components in the carrier gas and is a concentration-sensitive detector, or simply a concentration detector. The relationship between its response value and the carrier gas flow rate is that the peak area decreases with increasing flow rate and the peak height remains essentially unchanged. When the amount of the component is changed and the carrier gas flow rate is changed, only the speed of the component passing through the detector is changed, that is, the half width is changed, and the concentration is unchanged. Such as TCD, PID, etc. All non-destructive detectors are concentration detectors.

When the response value of the detector depends on the amount of components entering the detector per unit time, it is a mass (flow) sensitive detector or simply a mass detector. The relationship between its response value and the carrier gas flow rate is that the peak height increases with increasing flow rate, while the peak area remains substantially unchanged. When the amount of the component is constant, the carrier gas flow rate is changed, that is, the amount of components entering the detector per unit time is changed, but the total amount of the components is not changed, such as FID, NPD, FPD, MSD, and the like.

4, according to the size of the response of different types of compounds

When the detector's response value to different types of compounds is basically equivalent, or when the RRF value ratio of each type of compound is less than 10, it is called a general-purpose detector such as TCD, PID, and the like.

A selective detector is used when the detector has a RRF value that is ten times greater than the other class. Such as NPD, ECD, FPD, etc.

Second, according to the working principle (test method) classification

Classification according to the performance characteristics of the detector is very beneficial for grasping a certain performance of the detector, but many detectors have various performances. There is no inherent rule for a detector to fall into. If classified according to the working principle or detection method, because a detector has only one working principle, it is relatively clear, and there are certain rules to follow, which is easier to grasp.

From the point of view of the working principle, the detector uses the difference in physical or (and) chemical properties of the component and the carrier gas to detect the presence of the component and the amount thereof. These differences are multifaceted: the difference between the composition and the physical constant of the carrier gas, such as thermal conductivity, density, etc., is called the physical constant detection method; the difference between the light emission and absorption of the component and the carrier gas is used. Detection, called photometric detection, etc. Many of the above methods are relatively mature detection methods in analytical chemistry, such as photometry, electrochemical methods and mass spectrometry. After nearly two decades of development, they have been used for gas chromatography. These devices have become a detector in gas chromatographs. Therefore, the current gas chromatograph detector has been lined up. Table 1-1 shows the common gas chromatograph detectors classified by test method.

Table 1-1 Common Gas Chromatography Detector Classification Table

Some documents also divide the detector into two categories: bulk property detector and solute property detectlor. The former is the change of some overall physical properties of the mobile phase before and after the component enters the detector, as shown in Table 1-1. The latter is to measure some properties of the solute (ie, components) that are not available in the mobile phase (or very small), such as trapped electrons (ECD), emission spectra (AED, FPD), etc., ie, in Table 1-1. 2~5 method. Therefore, the classification of Table 1-1-1 is basically consistent with this classification.