Dispensing needle for a fraction collector
Described is a dispensing needle for a fraction collector. The dispensing needle includes a conduit having a fluid channel to conduct a chromatographic flow, an interior wall that defines the fluid channel, an exterior surface and an endface through which the chromatographic flow is dispensed. The dispensing needle also includes a coating of a hydrocarbon material or a fluorocarbon material that is bonded to the endface. The coating is also bonded to at least a portion of the exterior surface that is adjacent to the endface and at least a portion of the interior wall that is adjacent to the endface. The coating operates to reduce a droplet volume of a liquid dispensed from the endface that may remain at the tip of the dispensing needle. Consequently, the concentration variation in a collected fraction due to a missing droplet or extra droplet is reduced.
1. A dispensing needle for a fraction collector, comprising:
a conduit having a first fluid channel having a first cross-sectional area and a dispensing end;
a needle tip disposed at the dispensing end of the conduit and having an exterior surface, an interior wall defining a second fluid channel in fluid communication with the first fluid channel, and an endface disposed at a dispensing end of the second fluid channel, the second fluid channel having a second cross-sectional area that is less than the first cross-sectional area, wherein a flow rate of a liquid through the second fluid channel is greater than a flow rate of the liquid through the first fluid channel; and
a hydrophobic coating of a fluorocarbon material covalently bonded to the endface of the needle tip, to at least a portion of the exterior surface of the needle tip adjacent to the endface and to at least a portion of the interior wall adjacent to the endface, wherein the hydrophobic coating of the fluorocarbon material operates to reduce a droplet volume of the liquid dispensed from the endface.
2. The dispensing needle of claim 1 wherein the coating of a fluorocarbon material is a self-assembled monolayer of a fluorocarbon material.
3. The dispensing needle of claim 1 wherein the coating of a fluorocarbon material is bonded by a monopodal attachment.
4. The dispensing needle of claim 1 wherein the fluorocarbon material has a carbon chain length of at least six.
5. The dispensing needle of claim 1 wherein the fluorocarbon material has a carbon chain length that is less than the carbon chain length of a stationary phase in the chromatographic flow.
6. The dispensing needle of claim 1 wherein the conduit is formed of a metal, a ceramic or a glass.
7. The dispensing needle of claim 6 wherein the conduit is formed of one of titanium, titanium alloy or fused silica.
8. The dispensing needle of claim 1 wherein the hydrophobic coating of a fluorocarbon material comprises a bi-layer comprising a first applied layer comprising a first hydrophobic material covalently bonded to the endface, at least the portion of the exterior surface and at least the portion of the interior wall, the bi-layer further comprising a second applied layer disposed on the first applied layer and comprising a second hydrophobic material that is different from the first hydrophobic material.
9. The dispensing needle of claim 8 wherein the second hydrophobic material is more hydrophobic than the first applied layer.
10. The dispensing needle of claim 1 wherein the endface is perpendicular to a longitudinal axis of the second fluid channel.
11. The dispensing needle of claim 1 wherein at least one of the first and second cross-sectional areas is a circular cross-sectional area.