How to clean the injection valve of liquid chromatograph?

In the process of injection, the injection valve must be very clean, otherwise better results can not be obtained. After the injection valve is switched to the injection position, it should be kept in this position for a period of time, so that the mobile phase can fully flush the sample quantitative tube, which can ensure high accuracy and does not need to flush the sample quantitative tube. This kind of washing controlled by chromatograph is obviously better than manual washing. Note: The volume of mobile phase required for cleaning is at least 10 times that of the incoming sample. When the sample quantitative tube is completely flushed, the rotary handle can be turned back to the sampling position (loading) or kept in the sampling position until the next sampling. When switching back to the sampling position, pull out the sample injection needle or micro sample injection needle from the injection valve. In order to prevent cross contamination, it is not necessary to clean the injection needle inlet of the injection valve after each injection. The injection valve is directly connected to the injection hole according to the patented design, so that the front end of the injection needle can be directly connected to the end of the sample quantitative tube, and there will be no other sample residual space. In this way, at the next injection, there will be no residual sample that entered the quantitative tube last time. However, during the process of inserting or pulling out the injection needle, there will be trace samples deposited in the sealed area of the needle. Accurate determination shows that this residue is 1nl- 10nl. This shows that 0.005% ~ 0.05% will remain in 20ul sample. After each injection, these residues can be removed by flushing the inlet of the injection needle. The process of cleaning the injection needle inlet is as follows: set the flow rate of mobile phase to 0.1ml/min ~1ml/min, connect the cleaning head of the injection needle inlet to a large syringe, and clean the injection needle inlet only at the injection position with large mobile phase. In this way, the liquid entering the injection valve bypasses the sample quantitative tube and is discharged from the sample overflow nozzle. This process can thoroughly clean the injection needle inlet, catheter, injection needle inlet tube and injection needle sealing ring. However, it is impossible to completely clean the surfaces of the above-mentioned components by the cleaning method using an inserted syringe. When the injection needle is inserted into the injection needle inlet of the injection position, the needle pushes a small amount of sample liquid in the injection needle sealing ring (left by the last cleaning of the injection needle inlet tube) into the sample quantitative tube. When the composition of the sample solution is different from that of the mobile phase used, strange peaks may appear on the highly sensitive detector by partially filling the quantitative tube. Therefore, it is best to clean the injection needle with mobile phase. If serious cross-contamination has been found, please check whether it is caused by the following reasons: 1. The length of the injection needle used is not enough (the needle length is at least 5cm), and the end of the injection needle cannot reach the stator surface. 2. The needle is not completely inserted into the inlet of the needle. 3. Contaminants at the inlet of the injection needle or worn fragments of the injection needle seal will prevent the end of the injection needle from contacting the stator surface. If cross contamination is not considered, it is best to flush the injection needle inlet every 10 or 20 times to ensure that the injection needle inlet tube is full of liquid. In the process of inserting or pulling out the injection needle, clean the injection needle or dilute the contaminated sample in this area, and at the same time, the injection needle inlet and the sample overflow outlet are filled with fluid, thus preventing air from entering the sample tube.