A leak detector comprises a detector head which is to be scanned across a surface to detect a signal gas eminating from the surface. The opposite side of the surface from the head is supplied with a signal gas under pressure so that signal gas passes through leaks in the surface. Detection of signal gas by the head should then constitute detection of a leak. A curtain 1 of barrier gas is provided around a sample inlet 2 to protect the inlet from spurious signal gas. Orifices 36 provide narrow high velocity jets 38 which impinge on the surface S within curtain 1 and break up jets such as 34 of signal gas which escape under the curtain 1 without allowing the signal gas to reach the sample inlet.