Problem:
1. Analysis of the causes of the problem of "reagents not being evaporated dry" and possible consequences
(1) Possible cause 1: unreasonable rotation rate and insufficient evaporation area
When the rotation speed is too low (such as <30rpm), the liquid forms a thick film on the bottle wall and the evaporation surface area is small; when the rotation speed is too high (>150rpm), it is easy to cause liquid splashing or turbulence, and the effective evaporation surface is insufficient.
(2) Possible cause 2: Improper heating temperature setting: evaporation rate imbalance
When the temperature is too low, high-boiling point solvents (such as water, DMF) evaporate slowly; when the temperature is too high, the solvent may boil violently, the sample may decompose, or the steam may exceed the condensation capacity and be pumped away by the vacuum pump.
(3) Possible cause 3: Improper setting of circulating condensing water
When the condensing water temperature is too high (such as tap water > 25℃ in summer) or the flow rate is insufficient, the high-boiling point solvent vapor cannot be effectively condensed, and part of the vapor is lost with the vacuum system; scaling of the condenser tube will also reduce the heat transfer efficiency.
(4) Consequences of reagents not being evaporated dry
1) Residual solvents lead to inaccurate sample concentration, affecting subsequent analysis (such as drug residue detection errors), solvent effects, etc.;
2) Long-term residual high-boiling-point solvents may contaminate the instrument, and the target in the sample may have side reactions.
3) The target decomposes and the recovery rate decreases.
2. Analysis of the causes of the "boiling" problem and possible consequences
(1) Possible cause 1: The heating temperature is too high, exceeding the boiling point of the solvent under the current vacuum degree, causing the liquid to boil violently;
(2) Possible cause 2: The sample contains solid particles or viscous substances, and the local heating is uneven;
(3) Possible cause 3: The amount of liquid is too much (more than 1/3 of the volume of the evaporating flask), and the liquid shakes violently when rotating;
(4) Possible cause 4: The vacuum degree fluctuates or suddenly decreases, causing the boiling point of the solvent to rise sharply and then boil violently.
(5) The possible consequences of boiling: the sample splashes onto the condenser or receiving bottle along with the solvent, resulting in sample loss or contamination, thereby reducing the recovery rate; the splashed liquid may enter the vacuum pump, damaging the equipment or causing safety hazards (such as the risk of explosion of organic solvents).
Treatment methods:
1. Treatment methods when the reagent cannot be evaporated
(1) Reduce the rotation speed of the evaporating flask
For conventional solvents (methanol, chloroform), set it to 50-80 rpm; for high-viscosity solutions, it can be increased to 80-100 rpm, avoiding too low or too high.
(2) Increase the temperature
Adjust according to the boiling point and vacuum degree of the solvent. For example, when evaporating water, it can be set to 60-80℃ + high vacuum (10-20 mmHg); for heat-sensitive samples, a combination of low temperature (40-50℃) + high vacuum is used.
(3) Reduce the condensing water temperature
For high-boiling point solvents (such as DMF), use 5-10℃ low-temperature circulating water to ensure condensation efficiency; clean the condensing tube regularly to remove scale.
2. What to do when boiling occurs
(1) Immediately lower the heating temperature, stop rotating, and slowly increase the temperature after the liquid stabilizes;
(2) Reduce the amount of liquid to less than 1/3 of the volume of the evaporating flask to avoid violent shaking of the liquid;
(3) Add zeolite or a stirrer (for non-viscous systems), or filter solid particles in the sample in advance;
(4) Control the rate of vacuum increase to avoid sudden vacuuming that may cause boiling (especially for solutions containing bubbles).