The ideal injector
We often get asked questions on what to consider when talking about the ″ideal injector”. Let's first look at what we would consider to be important when talking about injectors and then compare different types of injectors against those parameters:
- Injection accuracy and reproducibility: seems to be very obvious, but is often hidden with other features of an instrument.
- Time required to perform the injection: this is very important for luminescence assays with so-called flash-kinetics such as aequorin or acridinium esters. In these assays the light emission reaches its peak just after several hundred milliseconds, and more than 90% of the light is emitted after only 2 to 3 seconds.
- Reagent consumption: what is the total dead volume from the reagent container to the injector tip? How many priming sequences are required to achieve a homogeneous mix of 99% at the tip? This is extremely important to guarantee that the same reagent concentration has been injected into A1 through H12 for proper results. Furthermore, the more sequences, the higher the reagent consumption and the associated reagent costs will be.
- Maintenance costs: can a frictionless operation be achieved to avoid mechanical failure and costly maintenance?
- Design & additional functions: how is the injector being built? What are the resulting shear forces of the injector? This is an important consideration when working with living cells, e.g. in Aequorin-based Calcium assays.
Syringe injectors vs JET injectors
As you can see in the following all injectors are not created equal. Look at Table 1 for a detailed comparison. Starting with a high-level view two different designs are used based on either a syringe or a bellows. What these injectors have in common is that they use a stepper motor and a 3-way solenoid valve to direct the flow of the liquid.
|Syringe-based Injector||JET Injector|
|Injection accuracy & reproducibility|
But potential accuracy issues when injecting small volumes. Accuracy for 10µl can be as low as just 90%
98% over the entire volume range.
|Time required per injection||Many syringe type injectors cannot perform ultra-fast injections with adequate mixing to capture the first 150ms of a kinetic assay.||Ultra-fast injection enabling the measurement of the first 150ms of a kinetic assay.|
High dead volume.
Need to be primed up to 5 times their volume to achieve a 99% homogeneous mix at the tip.
Low dead volume.
Recovers up to 60% of the reagent in the injector line. A single priming sequence is sufficient to achieve a 99% homogeneous mix at the tip.
Syringe based injectors are very expensive (typically >$3,000 each).
They need to be replaced as they tend to wear out over time due to friction.
Frictionless operation for extended lifetime.
Typically, more than 3 million injections without mechanical failure.
|Design & additional functions||Material mix that can produce stronger shear forces.||100% Teflon resulting in negligible shear forces enabling injection of cell suspensions.|
The syringe-based injector is by far the most common type of injector being used today. This type of injector contains a syringe and a Teflon coated barrel. The piston moves within the glass syringe which can cause friction, resulting in increased maintenance costs.
Most microplate readers from Berthold Technologies use up to 3 or 4 software-controlled JET Injectors which have many advantages that are relevant to many types of assays. A sophisticated Prime Mode reduces the reagent consumption while ensuring homogeneous filling and JET Injectors are built for extreme longevity and fast injection times to guarantee very efficient mixing.