The sampling effect of air microbial samplers is influenced by various factors and needs to be comprehensively controlled from multiple aspects such as instrument performance, operating standards, environmental conditions, and sample processing. The following are the key influencing factors and their analysis:
1、 Performance of the instrument itself
Sampling flow accuracy
Impact: Excessive or insufficient flow rate can lead to sampling volume deviation, directly affecting the calculation of microbial concentration (such as colony forming unit CFU/m ³).
Example: If the nominal flow rate is 28.3 L/min, but the actual flow rate is too low, microorganisms in low concentration environments may be missed.
Countermeasure: Regularly calibrate the flow rate with a soap film flowmeter to ensure an error of ≤ ± 5%.
Sampling head design and aperture
Principle: The size of the sampling head aperture determines the particle size range that can capture microorganisms (such as the impact sampler relying on inertial impact, the smaller the aperture, the smaller the captured particle size).
Risk: Blockage or unreasonable design of the aperture may lead to a decrease in the sedimentation efficiency of large particles of microorganisms, or small particles may escape with the airflow.
Maintenance: Clean the sampling head after each sampling to avoid residual particles affecting subsequent sampling.
Collect media (culture medium/filter membrane)
Type selection: Different microorganisms need to be matched with specific culture media (such as nutrient agar for bacteria and malt extract agar for fungi), and medium failure or compositional deviation can lead to missed detections.
Humidity control: If the culture medium is too dry, it will reduce the survival rate of microorganisms. If it is too wet, it can easily cause the spread of bacterial liquid and affect the colony count.
Storage conditions: The culture medium should be refrigerated (2-8 ℃) and stored away from light to avoid spoilage.
2、 Operating standards and human factors
Sampling height and position
Environmental differences: 0.5-1.5 meters above the ground is the human respiratory zone, which is the standard sampling height; But the clean room needs to sample above the work plane according to standards (such as ISO14644).
Avoid interference: Keep away from doors, windows, ventilation openings, or areas where people frequently walk to prevent uneven sample distribution caused by airflow disturbance.
Sampling time and frequency
Duration control: A single sampling time that is too long may cause the culture medium to dry out or microorganisms to overgrow (especially in high concentration environments), and adjustments should be made based on the estimated concentration (such as extending to 30 minutes for low concentrations and shortening to 10 minutes for high concentrations).
Repeatability: Collect at least 3 parallel samples in the same area to reduce accidental errors.
Aseptic operation
Pollution risk: Failure to disinfect the surface of the instrument (such as the sampling head and culture dish) before sampling, or touching the sampling area with hands during operation, may introduce exogenous microorganisms.
Standardized process: Use 75% alcohol to wipe the sampling head, wear sterile gloves, quickly change the culture medium to avoid prolonged exposure.
3、 Environmental conditions
temperature and humidity
Temperature: High temperature (>30 ℃) may accelerate the drying of the culture medium, while low temperature (<10 ℃) may inhibit microbial activity. It is recommended to control the sampling environment temperature at 15-25 ℃.
Humidity: High humidity (>80% RH) can easily lead to the spread of bacteria, while low humidity (<30% RH) can cause dehydration and inactivation of microorganisms. The ideal humidity is 40%~60% RH.
Airflow and Air Pressure
Turbulence effect: Strong airflow (such as fans and air conditioning vents) can alter the distribution of microorganisms, leading to uneven sampling; Negative pressure environments (such as biosafety cabinets) require adjusting the sampling flow rate to compensate for the pressure difference.
Pressure correction: In high-altitude areas, the actual sampling volume needs to be converted based on pressure (formula: actual volume=nominal volume x (local pressure/standard atmospheric pressure)).
Background pollution
Cleaning before sampling: If the environment has just been disinfected (such as UV irradiation or chemical fumigation), wait for at least 30 minutes to avoid disinfectant residue inhibiting microbial growth.
Cross contamination: When multiple samplers are used simultaneously, the distance between them should be kept at least 1 meter to prevent mutual interference of sampling airflow.
4、 Sample transportation and processing
transport conditions
Timeliness: After sampling, it needs to be sent for testing within 2 hours. If there is a delay, it needs to be refrigerated (2-8 ℃), but fungal samples need to be stored at room temperature (to avoid low temperature inhibition of growth).
Seismic protection: Avoid severe vibrations during transportation to prevent bacterial displacement caused by collisions between the culture medium and sampling head.
culture conditions
Temperature and time: Bacteria are usually cultured at 35-37 ℃ for 48 hours, while fungi are cultured at 25-28 ℃ for 5-7 days. Insufficient time can lead to incomplete colony formation.
Aerobic/Anaerobic Environment: Some microorganisms require specific gas environments (such as CO ₂ incubators), and failure to meet these conditions can result in missed detections.
5、 Other factors
Microbial characteristics
Particle size distribution: Airborne microorganisms mostly exist in the form of aerosols (particle size 0.1-10 μ m), and different samplers have different capture efficiencies for different particle sizes (such as a six stage sieve impact sampler that can separate microorganisms of different particle sizes).
Survival ability: Certain microorganisms (such as viruses and spores) require special treatment (such as immediate inactivation after sampling or the use of nutrient enhanced culture media).
Calibration and maintenance cycle
Regular verification: Conduct comprehensive calibration of the instrument (flow rate, sampling head efficiency, culture medium performance) at least once every six months, and check the sealing of the gas path after long-term use.
By systematically controlling the above factors, the accuracy and reliability of air microbial sampling can be significantly improved, providing effective data support for environmental monitoring (such as hospitals, food factories, laboratories).