The so-called retention time locking means that the retention time of a particular compound remains constant between different instruments and different columns (but the nominal stationary phase and the same phase).
I. Overview
The factors that determine the retention time are as follows:
1, the nature of the compound, the nature of the fixative solution.
2, operating conditions. That is, carrier gas flow rate, column temperature, capillary column specifications, and detector type.
Second, explain in detail
1, carrier gas control
For the same carrier gas, pressure and flow rate are the parameters that affect the retention time;
Different manufacturers of pressure gauges and flowmeters have different manufacturing precisions, so when the pressure and flow of different instruments show exactly the same, the flow rate in the column is not the same;
Now that there is an EPC, this problem has basically been solved; although the EPCs made by the same manufacturer cannot be strictly identical for each item, the difference is small, and this difference can be completely compensated by the automatic adjustment of the pressure.
2, temperature control
The retention time is extremely sensitive to temperature. In the past, mercury temperature was used to measure the column temperature, and the reproducibility was obviously problematic. The temperature is now controlled by thermistors and electronic circuits, and the accuracy is greatly improved. The temperature reproducibility between different instruments is satisfactory. Even with a temperature difference of 0.5°C, we can re-establish the retention time by adjusting the column pressure.
3, column specifications
Columns from different manufacturers, despite having the same nominal specifications, inevitably have non-uniform inner diameters, changes in the thickness of the fixed liquid film, and inaccuracies in the column length, which can cause retention time fluctuations.
The same column will change during use, such as re-installation or due to column contamination cut off 1 ~ 2cm, if the other operating conditions remain unchanged, the retention time can not be reproduced.
At present, the reproducibility of the column manufacturing of the same manufacturer is already quite high, and the chromatographic column of different manufacturers can't be fully reproduced, especially the polar column. However, if we use the same manufacturer's column, we can basically solve this problem. As for the change of column length, it can be compensated by adjusting the column front pressure.
4, the detector type
Conventional detectors: work under normal pressure;
MS: Work under high vacuum;
AED: Work above atmospheric pressure (eg, 10 kPa).
In this way, when comparing the results obtained with conventional detectors and MS or AED, the column pressure drop is different even if the precolumn pressure is exactly the same, and the retention time cannot be reproduced. With EPC, it is easy to keep the column pressure drop the same by adjusting the pressure.
Third, the summary
Through the above analysis, we can conclude that as long as the instrument's carrier gas and temperature control accuracy is high enough, as long as the column nominal specifications are consistent, we can compensate for small changes in operating parameters by adjusting the column front pressure to achieve retention times. This is the basis of RTL.
(Article source Hengtianli Instrument Network)
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