The 10-32 threads are probably the only ones you will use with HPLC. However, you may occasionally see ¼ x 28 or M6 (abbreviated form for M6 x 1) fittings used in laboratory settings or for lower pressure connections. The ¼ x 28 size is based on English measurement units like the 10-32. The ¼ x 28 threads are further apart and slightly larger in diameter and usually they do not withstand higher pressures as well. The M6 fittings are metric threads that are actually 6 mm in diameter and 1 mm apart.
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