1/4-28, 10-32 or M6: which thread do you use for HPLC fittings?
The complete guide to thread types, pressure zones and material selection in analytical systems
In analytical instruments such as HPLC, GC and mass spectrometers, three thread formats are used, each with its own application and pressure zone: 10-32, 1/4-28 and M6. Choosing the wrong fitting risks leakage, dead volume or damage to expensive components. This article explains what the numbers mean, which format belongs where, and how to make the right choice.
⏱ approx. 8 minutes reading time
How do I read a thread designation?
The designations you see on fittings describe two things: the diameter of the thread and the number of threads per inch. Here is how to read them:
| Designation | Diameter | Threads per inch (TPI) | Thread system |
| 10-32 | 4.83 mm (0.190 inch) | 32 | UNF (Unified Fine Thread) |
| 1/4-28 | 6.35 mm (0.250 inch) | 28 | UNF (Unified Fine Thread) |
| 5/16-24 | 7.94 mm (0.313 inch) | 24 | UNF (Unified Fine Thread) |
| M6 | 6.00 mm | approx. 23 (metric) | ISO metric |
| 1/8-27 NPT | 10.3 mm | 27 | Tapered NPT (National Pipe Thread) |
| G1/8 (BSPP) | 9.7 mm | 28 | Parallel BSP (British Standard Pipe) |
The three pressure zones in an HPLC system
An HPLC system has three functionally different zones, each with its own pressures and associated fitting choices. Understanding these zones is essential for choosing the right fittings.
Zone 1: low-pressure zone (solvent reservoir to pump inlet)
This is the suction side of the pump. The pressure here is low or even slightly negative. The fittings in this zone are usually 1/4-28 flat-bottom fittings for tubing with an outer diameter of 3.2 mm (1/8 inch). PEEK fittings are the standard here: they are easy to tighten by hand, chemically resistant and inexpensive to replace.
Zone 2: high-pressure zone (pump outlet to downstream of the column)
This is the most critical zone. The pump outlet, the injector and the connection to the HPLC column are exposed to the full system pressure, which can reach 600 bar and higher with UHPLC. The standard fitting in this zone is 10-32 with a coned sealing surface, for capillaries with an outer diameter of 1.6 mm (1/16 inch). With standard HPLC, PEEK fittings are acceptable; with UHPLC, stainless steel fittings and ferrules are necessary.
Zone 3: low-pressure zone (downstream of the column to the detector and waste)
After the column, the pressure drops sharply. Fittings in this zone are comparable to zone 1. Most users choose 10-32 type PEEK fittings here, matched to the connections of the detector.
Rule of thumb for practice
10-32 is the high-pressure fitting for capillaries. 1/4-28 is the low-pressure fitting for larger tubing. M6 is used by European instrument manufacturers (including Agilent, Waters and Shimadzu). Mixing formats is not permitted, even if they sometimes fit.
Which format does each manufacturer use?
This is one of the most practical questions when purchasing fittings. The major manufacturers of analytical instruments are not always consistent, and some manufacturers use several formats within one system.
| Manufacturer / brand | Common connection | Note |
| Waters HPLC | 10-32 (high pressure), 1/4-28 (low pressure) | Own frit system for some columns |
| Agilent (HP) | 10-32 and M6 depending on the model | Check per HPLC generation |
| Thermo Fisher | 10-32 and 1/4-28 depending on the module | Various proprietary connector standards |
| Shimadzu | M6 and 10-32 | European instruments more often use M6 |
| IDEX / Upchurch | 1/4-28 and 10-32 (wide range) | Standard supplier of PEEK fittings |
| Valco / VICI | 10-32 (high pressure) | Widely used in GC systems |
| Bio-Rad | 1/4-28 | Typical for low-pressure liquid chromatography |
PEEK or stainless steel: material selection for fittings
In addition to the thread type, the material of the fitting determines the maximum working pressure and chemical resistance.
| Property | PEEK | Stainless steel |
| Maximum pressure | Approx. 350 to 400 bar (depending on type) | 700 bar and higher (UHPLC) |
| Chemical resistance | Excellent, but not for concentrated sulphuric acid and some halogen acids | Very good, but not for halogen acids and strong acids |
| Biological inertness | Good, no metal ion leaching | Metal ions can be released at low pH |
| Reusable | Yes, several times | Yes, but ferrules are permanently deformed on first installation |
| Ease of installation | Hand-tightened, tool-free | Requires a wrench; care needed with tightening torque |
| Costs | Lower | Higher |
| Recommended for | Standard HPLC, biological samples, ion applications | UHPLC, high pressure, organic solvents |
Dead volume: the invisible enemy of chromatographic separation
Dead volume is the extra liquid volume in a connection that does not contribute to the separation, but broadens the peak and degrades the resolution. In HPLC, dead volume is the most underestimated cause of poor chromatographic performance.
Dead volume is caused by:
- A poor connection between the capillary and the fitting sealing surface (the seal is not in the right place)
- Capillaries that are too long, with an unnecessarily large internal volume
- Using reducers or adapters where a direct connection is possible
- Ferrules that are not correctly matched to the capillary dimension
Peak broadening or a peak with a shoulder in the chromatogram is almost always a sign of dead volume. Reduce it by making all connections as short, precise and well-fitting as possible. Check the sealing surface visually during assembly.
Never combine NPT and UNF
1/8-27 NPT (tapered pipe thread) and 1/4-28 UNF (parallel) sometimes look similar in size but are not interchangeable. NPT seals on the thread itself (tapered); UNF seals on the sealing surface. Forcing them leads to damaged ports and leakage that cannot be repaired
