Number of theoretical plates (USP): (N, one of the column efficiency parameters of chromatography (referred to as column efficiency). N depends on the type and nature (particle size, particle size distribution, etc.), filling conditions, column length of the stationary phase, type and flow rate of the mobile phase, and the nature of the substance used to determine the column efficiency:
This formula is a formula derived from the chromatographic plate theory. Use one to measure the column efficiency of the chromatographic column, that is, under the same operating conditions, use the same sample to determine the N or H value of the chromatographic column. The larger the N value (the smaller the H value), the higher the column efficiency.
It is not difficult to see from the formula that the size of N is directly related to the retention time, peak width and column length. If you want to improve the column efficiency in the experiment, in addition to the factors of the chromatographic column itself (column length, inner diameter, film thickness, etc.) , the most direct way is to prolong the retention time and reduce the chromatographic peak width, and prolonging the retention time is the most commonly used and simplest way in the laboratory. Prolonging the retention time can be achieved by reducing the initial temperature and reducing the flow rate, but reducing the flow rate may cause the peak width of the target peak is increased to achieve the opposite effect, so it is usually chosen to lower the initial temperature and prolong the retention time to achieve the purpose of increasing the number of theoretical plates.
It is not difficult to see from the formula that the size of N is directly related to the retention time, peak width and column length. If you want to improve the column efficiency in the experiment, in addition to the factors of the chromatographic column itself (column length, inner diameter, film thickness, etc.), the most direct way is to prolong the retention time and reduce the chromatographic peak width, and prolonging the retention time is the most commonly used and the easiest way in the laboratory. Prolonging the retention time can be achieved by lowering the initial temperature and lowering the flow rate, but lowering the flow rate may increase the peak width of the target peak, thus achieving the opposite effect, so it is usually chosen to lower the initial temperature and extend the retention time to increase the number of theoretical plenates (USP) the goal of.
For example:
- Determination of benzene
Analysis result table
| Component name | Retention time | Capacity factor | Peak area | Area | |||
| Benzene | 3.72 | 5.95 | 22.20 | 100.00 |
Column System Evaluation Form
| Component name | Retention time | Capacity factor | Peak area | Area | |||||
| Area | 6.58 | 9.80 | 49.00 | 100.00 |
Analysis result table
| Component name | Retention time | Height | Peak area | Area | ||
| Benzene | 6.58 | 9.80 | 49.00 | 100.00 |
Column System Evaluation Form
| Component name | Retention time | Height | Peak area | Area | ||
| Benzene | 0.00 | 40040 | 0.00 | 1.05 |
It can be seen from the experimental results that the retention time of the same column is extended by 75% and the number of theoretical plates is increased by about 75% by reducing the initial temperature under the same other conditions.
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