Capillaries
Capillaries are used to connect individual components such as injectors, pumps and detectors and transport eluents and sample solutions. A wide range of different materials and diameters are available to ensure optimum suitability for your application. Typically, capillaries made of stainless steel or PEEK are used in HPLC, as these materials are characterised by good chemical resistance and high pressure stability. If a capillary does not have to withstand such high pressures, softer plastics such as nylon can also be used.
Technical Data
Which capillary material is suitable for what?
Stainless steel
- Good mechanical properties
- Pressure-stable for up to 1200 bar
- Not suitable for all samples and methods (bio applications difficult!)
- Preferred choice for UHPLC
- Deformation possible during cutting (caution!)
PEEK
- Made of plastic (polyetheretherketone)
- Suitable for organic applications
- Pressure-resistant up to 400 bar
- Easy to cut
Stainless steel with PEEK layer
- Also known as PEEK-lined stainless steel
- Outside: Stainless steel
- Inside: PEEK
- More robust than pure PEEK
- Suitable for organic applications
- Similar pressure stability as pure stainless steel
Other metals
- Copper - Standard material for GC applications
- Nickel - Alternative material for GC
- Titanium - Possible replacement for stainless steel, but acid labile and brittle
- MP35N - Nickel-cobalt-based alloy, extremely robust and highly corrosion-resistant
Other plastics
- ETFE and PTFE - trade name Tefzel, fluropolymers, chemically inert, less pressure-resistant than PEEK
- Nylon - Lightweight, flexible and resistant to breakage, but only suitable for low pressures
- Polyurethane (PU) - Softer and more flexible than nylon, only suitable for low pressures
PEEKsil / Fused Silica
- Combination material made of PEEK and silica
- Exterior: PEEK wall
- Inside: Layer of fused-core silica
- For use with capillary columns
- Fulfils the purpose of a precolumn
Which diameter makes sense when?
Capillaries for chromatography have a standard outer diameter of 1/16'' (1.59 mm) and a variable inner diameter. The size of the inner diameter determines the properties and intended use of a capillary. Large inner diameters enable a high flow rate at comparatively low pressure and rarely clog. A small diameter, on the other hand, is essential in critical areas of your system in order to obtain the narrowest possible peaks in the chromatogram.
Low pressure parts
- Range up to the pump
- Sample does not flow through here
- Comparatively low pressure
- Softer plastics can be used (e.g. PTFE or nylon)
- Dead volume has no influence on analysis
- Large inner diameter for maximum flow
- Standard: 1.0 mm inner diameter
Dwell volume
- Area from pump to injector
- Not flowed through by sample
- Point at which solvents are mixed
- Dwell volume delays the arrival of the eluent mixture
- Consequence: Isocratic region before a gradient reaches the column
- Minimise dwell volume if method with gradient is used
- For purely isocratic methods the dwell volume is less critical
Extra column volume
- Parts the sample solution flows through (injector to detector)
- Minimise extra column volume
- Capillary diameter that is too large broadens peaks, especially with UHPLC
- Standard: 0.17 mm or 0.25 mm inner diameter (corresponds to 23 µl/m or 50 µl/m dead volume)
- Use filters to prevent clogging
- For higher Performance at elevated Pressures smaller diameters may be needed
- Larger inner diameter may be necessary for preparative columns with high flow rates
Cutting capillaries
- Use of special tools recommended
- Cutting behaviour of metal and plastic is very different
- Plastics (e.g. PEEK) can be cut with a blade
- Do not cut steel the same way due to deformation
- Cutting tool for steel scores the outside, capillary can then be broken off
Have you picked out a capillary? Here you will find matching fittings.
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