Precautions for injection needles - Liquid phase
2024.09.12
\1. When using a manual injector for injection, the injection syringe must be cleaned with a needle wash solution before and after injection. The needle wash solution is generally selected to be the same solvent as the sample solution. The injection syringe must be cleaned with the sample solution for more than 3 times before injection, and the bubbles in the syringe must be removed. The injection needle must be a flat-head needle.
\2. When the sample volume in the sample bottle is small, the injection needle of the automatic injector cannot reach the liquid surface. You can lower the injection height of the injection needle. Be careful not to let the injection needle touch the bottom of the bottle when setting it, otherwise it is easy to puncture the injection bottle or bend the injection needle. For trace sample analysis, use a micro sample bottle or place a micro inner tube in the injection bottle.
\3. The sample to be injected should be a pure solution without particles and harmless to the liquid phase system. The sample should be checked under light for particles, turbidity or emulsification, and filtered with a 0.22μm filter if necessary.
\4. The sample bottle should be free of any contaminants and is usually a glass bottle; if the sample needs to be protected from light, the sampler lighting should be turned off and a brown sample bottle should be used.
\5. If the six-way valve is leaking, it may be due to wear of the stator or rotor, which can be solved by cleaning or replacing the six-way valve.
\6. If the needle seat is leaking, it may be due to a needle seat seal problem, so clean or replace the needle seat seal.
\7. If the sample has poor repeatability, it may be due to a sample needle leaking, so replace the rotor seal or stator seal.
Why can manual injection only select full-loop injection or half-loop injection of the quantitative loop? And the automatic sampler can set different injection volumes?
1) When manually injecting, full-loop injection is usually used, and the injection volume requires 2 to 3 times the volume of the quantitative loop. 2) When manually injecting, the sample solution fills the injection pipeline and the quantitative loop, and the excess sample solution enters the waste liquid. For non-full-loop injection, the injection volume is required to be less than half of the full-loop volume. For non-full loop injection, the injection volume is required to be less than half of the full loop volume because there are two solutions in the quantitative loop under non-full loop conditions, one is the mobile phase and the other is the sample solution. When the sample solution is injected into the quantitative loop, it mixes with the mobile phase and is diluted. Therefore, for non-full loop injection, the injection volume is preferably less than half of the full loop volume.
3) Manual injection requires operators to be skilled in injection techniques, and there is a greater possibility that human factors will affect the analysis results; the automatic sampler mainly relies on the metering pump to accurately absorb the sample volume, which can well solve the injection repeatability and accuracy problems, is not affected by human factors, and can also achieve batch operation.
Is the six-way valve for manual liquid injection the same as the injection valve of the automatic sampler?
1) Generally speaking, the principle is the same, and the difference is outside the valve. 2) For liquid chromatography, both manual samplers and automatic samplers use six-way valves for injection, but the dead volume of automatic injection is larger than that of manual injection, and there are more metering pumps and some connecting pipelines.
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