Oil chromatography analyzers mainly use differences in boiling points, polarities and adsorption properties of substances to separate mixtures. After the sample to be analyzed is vaporized in the vaporization chamber, an inert gas (i.e., carrier gas, also called mobile phase) is brought into the chromatographic column. The column contains a liquid or solid stationary phase. Each component in the sample tends to be in the mobile phase, forming a distribution or adsorption equilibrium between the stationary phases.

GDC-9560B Power system oil chromatography analyzer

With the flow of carrier gas, sample components are repeatedly distributed or adsorbed/desorbed during the movement. The components with high concentration in the carrier gas flow out of the chromatographic column first, and the components with high concentration are distributed in the stationary phase. After flowing out. After the components flow out of the chromatographic column, they enter the detector for measurement.

How to improve the sensitivity of gas chromatography without increasing sample concentration?

“Sensitivity” refers to the size of the electrical signal (voltage: mV, current: mA) generated when a certain amount of components passes through the detector.

Improving instrument sensitivity is an eternal topic in analytical chemistry. As far as oil chromatography analyzers are concerned, instrument manufacturers and analysts are always trying to manufacture highly sensitive instruments and develop highly sensitive methods. It is this demand that drives the development of instruments. The development of instruments has led regulatory manufacturers to put forward higher detection sensitivity requirements. This interaction is cyclical. So how to increase sensitivity without increasing sample concentration?

Cause Analysis:

Reasons for reduced sensitivity may include:

1. Use shunt injection.

2. The sample does not completely enter the chromatographic column.

3. The instrument is contaminated, especially the inlet and chromatographic column, which causes the system noise to increase and masks the response of the compound.

4. Compounds are easily adsorbed by active sites at the injection port and have a matrix effect.

5. The sensitivity of the selected detector is reduced.

solution:

There are several ways to improve detection sensitivity:

① Splitless injection: The so-called splitless injection refers to closing the solenoid valve of the split air path during sample injection to allow all samples to be injected into the chromatographic column. This improves sensitivity and eliminates transfer discrimination. The sensitivity of splitless injection is significantly higher than that of split injection, but the background signal will also increase. Commonly used for environmental analysis (such as monitoring of trace contaminants in water and atmosphere), detection of pesticide residues in food, and clinical and pharmaceutical analysis.

②Choose a detector with high specificity and high sensitivity: This is also a common method used by chromatographers to improve sensitivity. For example, ECD is used to analyze halogen-containing compounds, NPD is used to analyze nitrogen and phosphorus compounds, and FPD is used to analyze sulfur and phosphorus compounds. Of course, more sensitive universal detectors such as mass spectrometry detectors (MSD) can also be used.

③ Maintain the instrument and reduce system noise: Instrument system noise usually comes from two aspects: one is the instrument itself, such as detector noise, circuit noise, stationary phase loss, etc.; the other is the sample matrix, such as the injection solution containing many impurities. Picking up high background noise can cause contamination of the inlet and column system. The former can be suppressed by using selective detectors and low-loss chromatographic columns; the latter requires sample purification, such as solid phase extraction (SPE) or other methods to eliminate sample matrix interference, and timely replacement of liners, cleaning systems, etc.

④ Use a deactivated liner at the inlet: The active sites on the liner may adsorb the component to be measured, causing chromatographic peak tailing, reducing sensitivity, and affecting reproducibility. When splitless injection or when analyzing weakly polar compounds, the use of deactivated liners with high inertness, low loss, and good thermal stability can often significantly improve the sensitivity of active species, which has been difficult to solve in previous analyses. Suitable for analysis of complex matrices and active substances.

⑤ Use chromatographic columns with no pollution and low fixative loss: Chromatographic columns are used for a long time and the fixative loss is serious, causing the exposed carrier to adsorb the sample. Some carriers will become more and more numerous. Due to the existence of active sites, the components to be analyzed will directly contact the surface of the carrier during the analysis process and cause adsorption. At the same time, loss of stationary phase increases the chromatographic baseline, thereby reducing sensitivity. .

⑥ Use matrix to prepare standard solutions to improve sensitivity: In GC analysis, matrix effects are often encountered, seriously affecting the accuracy and precision of the results. For example, organophosphorus pesticides with smaller molecular weights are more susceptible to matrix effects and exhibit matrix-enhancing effects. Due to the presence of the matrix, the active sites on the inlet liner and chromatographic column are covered, reducing the number of active sites and substances to be measured. The opportunity of contact enhances the signal of the substance to be measured. Therefore, a matrix extract with the same composition as the sample solution should be used as a diluent for the compound as much as possible to improve the sensitivity of the test compound.