be independently connected to a suitable pumping apparatus not shown in order to introduce a fluid independently into each of the m microchannels 36 in the system 10 the flow cell 34 is mountable onto the spr surface in two orientations one of the two orientations is shown in fig1 a and is referred to herein as u201c the probe orientation u201d the second orientation shown in fig1 b is referred to herein as the u201c target orientation u201d in each of the two orientations the microchannels are perpendicular to the microchannels in the other orientation each microchannel 36 is open on a side of the microchannel facing sensor surface 32 so that fluid flowing in the microchannel in either orientation contacts the spr surface in a rectangular region in the probe orientation shown in fig1 a fluid flowing in a microchannel contacts the sensor surface at a respective rectangular region 42 referred to herein as the microchannel 39 s u201c probe region u201d see fig1 b in the target orientation shown in fig1 b fluid flowing in a microchannel contacts the sensor surface at a respective rectangular region 43 referred to herein as the microchannel 39 s u201c target region u201d see fig1 a the probe regions and the target regions are thus perpendicular to each other regions of some microchannels 36 in the system 10 in fig1 a and 1 b are cut away to show microspots 58 formed at the crossover regions of the probe regions and the target regions.