Basic Principles, Procedures and Challenges
Basic operation principles of our dispensers:
The sample solution is transferred through the air towards the target as a small, individual droplet, a series thereof or as a liquid jet. The main advantage of non-contact dispensing is that the properties of the target surface do not affect the deposited volume. The surface is not touched by the dispenser: no damage, no contamination. Another benefit is that no down and up movement is required for sample transfer, which saves valuable time.
The wet area on the target (spot size) is determined by the surface tension of both the liquid and the target surface. If a moderate number of samples need to be dispensed onto many targets, non-contact microarray spotting is clearly the best method.
The sample solution is transferred from a pin or capillary to the target surface via short contact. The main advantage of contact-based microdispensing is the ease of parallel sample transfer, enabling high sample throughput.
If many samples are to be printed onto a moderate number of targets, contact-based microdispensing using many pins is the method of choice.
Low-volume dispensing adheres to the following procedure:
- Aspirate an aliquot of the sample solution.
- Pre-dispense some droplets in order to get the dispenser running and prevent initial effects.
- Move to the desired target and start dispensing.
- Move to the wash station and clean the dispenser tip from inside and outside.
- Proceed with the next sample.
If only one solution must be dispensed, it can be advantageous to feed it from a reservoir attached to the dispenser, instead of aspirating it. Our so-called Bulk-Dispenser does exactly that. The solution to be dispensed is filled into a 20 mL glass vial and transported from there through a connection capillary into the dispenser.
If a smaller vessel is preferred, e.g. a 0.6 or 1 mL Eppendorf tube, it can be placed inside the vial. If a lot more solution needs to be dispensed than can be aspirated, bulk dispensing is often an attractive choice. It saves time and space, and the sample solution is protected from solvent loss by evaporation.
Challenges during dispensing:
If the sample solution is exposed to ambient air, solvent loss by evaporation will increase its concentration. For aqueous solutions, raising the humidity above them is an efficient means to reduce solvent evaporation. Reducing their temperature is another.
However, it may not fall below the dew point. If it does, instead of evaporation, condensation of water from the ambient humidity becomes a problem, as it dilutes the samples. Therefore, the dew-point temperature is the best choice here, as evaporation and condensation are in balance with each other.
All our microdispensing instruments can be equipped with an accurate temperature and relative humidity sensor, a humidifier and a source-vial temperature control. Measured temperature and relative humidity are displayed and used to calculate the corresponding dew point. That value is then used to control the cryostat chilling the sample solutions.
Dust is a common source of contamination and it can cause problems during sample aspiration, like the clogging of the dispenser tip. Therefore, a clean environment is highly recommended. Our instruments are well sealed to accurately enable internal climate control (temperature and humidity) and prevent dust ingress. In addition, the production machines can optionally be equipped with a HEPA-filter module.
Electrostatic charges residing on non-conductive surfaces can generate strong electrical fields. As droplets ejected by microdispensers are charged, they can be deflected by such fields. This is a common problem when dispensing onto plastic targets, for instance, the bottom of a 96-well MTP or a channel of a lab-on-a-chip device. The smaller and lighter the droplets, the more severe the effects. For example, droplets with a volume of 100 pL can change their flight direction by 90° and then fly toward the sidewall of a well of a 96-MTP, instead of toward the bottom.
For that reason, eliminating all charges on plastic target surfaces before microdispensing is necessary. For that task, our instruments can optionally be equipped with one or more ionizers, depending on size. These devices generate an alternating flow (50-60 Hz) of positively and negatively charged air molecules, which are directed at the targets in order to quickly and efficiently neutralise all electrical charges residing upon them.