Flow injection analysis

The reliable injection principle of MP-SPR injects sample in a flowing carrier (buffer) stream allowing for precise kinetics.

How to avoid dilution

MP-SPR instruments are built as true flow injection analysis systems (similar to Biacore or HPLC), where the valve that switches between running buffer and the sample is as close to the flow-cell as possible minimizing dead-volume and dilution of sample. This is in contrast with SPR or QCM setups utilizing separate stand-alone pump or autosampler, where the distance from pump or valve to the measurement chamber causes the sample and the buffer to mix at the interface (dilution).

How to measure high quality kinetics

In order to get accurate kinetics, the liquid above the surface of the sensor has to be switched rapidly from buffer to sample. This is typically given as a rise time. For a full exchange of liquid in the channel above the sensor, the sample should be changed 3-5 times before reaching 100% sample concentration. This is faster achieved in small volume measurement chamber, such as MP-SPR.

Difference in fluidics between QCM and MP-SPR - see here.

Fluidistiikka_v2_logo_ontop.gif

Flow Injection Principle Step-by-Step

Here, an animation of a flow-injection analysis set-up is explained. In MP-SPR there are 2 or 4 channels depending on a model type. To increase understanding, this animation shows only a single channel.

In the center, there is a switching valve (here a 6-port valve). In the normal state of the valve, the running buffer (in the bottle on the left side) is pumped via a pump (here peristaltic pump, in MP-SPR Navi automated models syringe pumps are used) through the valve directly into the measurement channel under the SPR sensor and from there into waste bottle.

The sample is introduced through a separate port (here indicated via a syringe, in automated MP-SPR Navi instruments an automatic syringe pump is used) into the injection loop. The sample stays in the loops in the normal state of the valve and does not move anywhere.

Once the valve is switched into the injection state, the pump continues pumping. In the injection state of the valve, the flow of running buffer pushes the sample from the injection loop into the measurement channel above the SPR sensor and thus, allows for molecules from the sample to bind to the SPR sensor and cause a change in the measurement of plasmonics.

After a pre-set interaction time, the valve will switch back to the normal state. Hence, the measurement channel will be again filled with running buffer.

Note: Flow-injection is used for instance in chromatography instruments (GC, HPLC, UPLC). The carrier flow in HPLC is running buffer in MP-SPR. In HPLC, the sample is retarded depending on the affinity of the sample molecules to the walls of the column to obtain separation. In MP-SPR the whole sample ihas minimum interaction with the fluidic walls and is carried to the SPR sensor, which has a specific capture for analyte molecules in the sample.