Choosing and Fitting HPLC Capillaries and Connections

Capillaries and fittings decide how sharp peaks stay in HPLC and whether a connection holds. This guide summarizes documents from IDEX Health & Science, Waters, Thermo Fisher Scientific, Shimadzu and Agilent. All figures come from these sources and apply to the products described there.

Stainless steel, PEEK or fused silica

IDEX describes PEEK as biocompatible, resistant to most common solvents, flexible and easy to cut. It can replace stainless steel in most liquid analytical systems. Fused silica with a polyimide coating is used in micro and nano HPLC.

The table shows pressure figures from the IDEX catalog. For stainless steel, the figure is the safe continuous working pressure, which sits below the burst pressure.

The weakest part limits the pressure of the whole path. Thermo quotes 1,310 bar for its stainless steel Viper capillaries, 345 bar for PEEK and 1,517 bar for MP35N. Shimadzu lists 20 MPa for the PEEK male nut at the column connection and 130 MPa for the nut with its matching ferrule for 1.6 mm tubing.

Inner diameter, volume and peak broadening

IDEX considers 1/16 in outer diameter tubing with 0.010 in inner diameter usually safe throughout an analytical system. The pressure drop stays negligible, and the bore limits band broadening. For Shim-pack Scepter columns, Shimadzu names PEEK tubing with 0.25 to 0.3 mm inner diameter. For UHPLC it names stainless steel tubing with 0.1 to 0.2 mm. Keep the run from injector to column as short as possible.

Volume rises with the square of the diameter. The values come from Upchurch Scientific. The per meter column is converted from the per centimeter value.

Fifty centimeters of 0.010 in tubing hold about 25 µL, and 0.005 in holds about 6 µL.

Excess volume in the flow path broadens peaks and lowers resolution. Thermo lists voids in fittings, long or wide tubing and a poorly designed injection system as causes. IDEX names delayed run times, wider peaks, poorer separation, carryover and split peaks as effects. IDEX advises matching the tubing bore to the through hole in the instrument. According to Shimadzu, stronger tailing of early peaks than late ones points to dead volume, so check the connections.

Ferrules, fittings and vendor specific systems

Metal compression fittings

Waters gives a fixed procedure for stainless steel tubing. Pass the tubing through screw and ferrule and seat it fully in the port. Tighten the screw by hand until it stops and mark it. Add three quarters of a turn with a wrench while watching the mark. Then remove screw and ferrule and check that the ferrule is secure. On reinstallation, tighten by hand and add a quarter turn with a wrench. Waters lists the article for ACQUITY UPLC, ACQUITY Arc and Alliance. Do not transfer these turn counts to other systems.

IDEX names a ferrule in the wrong position on the tubing wall as a frequent leak cause.

Fingertight and polymer

According to IDEX, polymer fingertight fittings work in the same ports as their all metal counterparts. The choice depends on chemicals, sample, pressure and temperature. IDEX warns that with 25 µm PEEK tubing some fittings can close the bore, especially wrench tightened ones.

Ferrules are not freely interchangeable. For the three piece fingertight fitting on the active preheater, Waters states that ferrules are not interchangeable between product generations. Older ACQUITY instruments do not accept the captive ferrule.

At the column connection Shimadzu requires hand tightening. Do not overtighten with a wrench, and hold the end fitting, not the column body, when mounting. Parts with an integrated ferrule can damage the frit if tightened too hard. Shimadzu recommends the ferrule free Nexlock for this case. Flush the line with mobile phase before connecting.

Ferrule free systems

The Thermo Viper fitting needs no ferrule and seals at the tubing tip with a PEEK seal. Thermo rates it fingertight up to 1,500 bar, with a thread for 10-32 ports. With conventional fittings Thermo sees a possible extra volume of 100 to 500 nL from misplaced ferrules. A slipped capillary distorted peaks at only 600 bar.

According to the Vanquish Neo manual, the capillary is inserted into the port, tightened by finger, then finished with the torque screwdriver until it clicks. Tightening and loosening happen only at zero pressure and only with fingers and the supplied tool.

For the capillary fittings on a detector flow cell body, Agilent states a maximum torque of 0.7 Nm.

Cutting and preparing tubing

IDEX supplies PEEK tubing in five foot pieces, rough cut to about five feet and one inch. Make a clean trim cut before use, especially with small inner diameters. A razor blade works, and a tubing cutter gives a better cut. Cutting stainless steel roughens the tube end. IDEX therefore recommends precut tubing and advises passivating tubing you cut yourself. Flat, burr-free ends matter for low dead volume connections.

Fixing leaks

For Viper capillaries, Thermo describes this procedure at zero pressure. Remove the capillary, clean the ends with a lint free tissue wetted with isopropanol and reinstall it. If the connection still leaks, fit a new capillary. For metal ferrules per Waters, check that the ferrule sits firmly. Shimadzu names a loose end fitting as a possible leak point.

Matching parts from the catalog

Fit is checked before ordering against the instrument nameplate and part number.

• The 5021-1820 is, per the catalog name, a flexible capillary of 0.12 mm inner diameter and 105 mm length without fittings.
• The 0100-1543 is, per the catalog name, a PEEK fingertight fitting with ferrule.
• The 0100-2441 is, per the catalog name, a PEEK ZDV union with fittings.
• The 6040.2325 is a stainless steel Viper capillary of 0.13 mm inner diameter and 250 mm length.
• The 6041.5616 is a PEEK Viper capillary of 0.13 mm and 150 mm.
• The 00301-22912 is red PEEK tubing of 0.005 in inner diameter and 1/16 in outer diameter.
• The 220-91246-02 is also red PEEK tubing of 0.005 in, on a ten foot roll for cutting to length.
• The 00101-18202 is a PEEK LC union with a 0.010 in bore.
• The kit 700003139 is, per the catalog name, a reusable fingertight fitting with 10-32 thread from Waters.
• The WAT271023 is a two piece ferrule for 1/16 in tubing.

FAQ

Which inner diameter is common for analytical HPLC?

IDEX considers 0.010 in at 1/16 in outer diameter usually safe across the whole system. Shimadzu names 0.25 to 0.3 mm for PEEK and 0.1 to 0.2 mm for stainless steel in UHPLC. Your instrument manual decides.

How tight should a fitting be?

That depends on the vendor. Waters gives three quarters of a turn past hand tight for metal ferrules, Thermo the click of the torque screwdriver, and Shimadzu hand tightening only at the column connection. Agilent states at most 0.7 Nm at the detector cell. Follow the specification for your system.

How do I recognize dead volume at a connection?

Typical signs are broader peaks, poorer separation, carryover and tailing of early peaks. Check seating and cut of the tube ends and the matching ferrule. If the effect remains, replace the capillary.

Do you need one of the parts mentioned? We deliver the matching part with a quote on the same day, and express shipping is possible.