Nano flow HPLC (<0.1 mm ID, flow-rate < uL/min) is essentially not different from classical HPLC (4.6 mm ID column  ~1 mL flow rate). However,  it is now widely recognised that the sensitivity of detection in HPLC by UV, ESI-MS and other concentration dependent detectors is improved by reducing the interior diameter of the column. This holds only if equal amounts of a sample is introduced into the column. The gain in sensitivity follows directly from the smaller volume i.e the higher concentration in which analytes elute from the column. The gain is proportional to the square of the ID ratio of the two columns.
The table compares the relative sensitivity of  different columns of different ID. Going from the conventional column of 4.6 mm ID down to a 0.01 (10 um), the gain can be as large as  over a factor of 200,000 times.  High sensitivities are particularly relevant in cases where sample amount is the limiting factor. Draw backs of nano flow LC are (1) Nanoscale columns allow a very limited sample volume injection  and (2) difficulties associated with performing high performance separations at the nano flow scale. The problem of limited sample volume injections can be solved by the use of a solid phase extraction column. SPE columns allow relatively large injection volumes ( e.g 5-50 uL) in a reasonable time interval (1-10 min). Maintaining the chromatographic integrity in nanoflow LC requires a.o the elimination of dispersing volumes, both upstream and downstream the analytical column.

gain ~ (d1/d2)2

Column Flow Relative
ID rate Sensitivity
(mm)> (uL/min)

(-)


4.6 1000 1
2 200 5.3
1 50 21
0.5 12 84
0.2 2 529
0.1 0.500 2,116
0.075 0.250 3,762
0.050 0.120 8,464
0.025 0.030 33,856
0.010 0.005 211,600
Instrumental demands for high performance nanoflow HPLC include:
(i) the HPLC pump must be capable of delivering a linear gradient at the flow-rate of the column (see pump hold-up volume)
(ii) the solid phase extraction trap should not affect the separation efficiency (see precolumn void volumes)
(iii) peak broadening in the ESI interface should be absent or minimal (see post column void volumes).