Ja. Dusenberry et Sl. Frankel, INCREASING THE SENSITIVITY OF A FACSCAN FLOW CYTOMETER TO STUDY OCEANIC PICOPLANKTON, Limnology and oceanography, 39(1), 1994, pp. 206-209
Existing limitations on the sensitivity of commercially available flow
cytometers do not permit the detection of extremely dim picoplankton
cells, particularly Prochlorococcus marinus in near-surface oligotroph
ic ocean waters. This problem was overcome by making some simple modif
ications to a FACScan (Becton Dickinson) flow cytometer to increase it
s sensitivity by about fivefold. The fluorescence and side scatter sen
sitivity was increased by replacing the existing focusing lens and bea
m-steering mirror with a 3 x expansion telescope and a slightly shorte
r focusing lens, resulting in a smaller laser spot size at the interro
gation point. Forward scatter sensitivity was increased by replacing t
he existing photodiode with a photomultiplier tube, which has a backgr
ound noise level about an order of magnitude lower. The fluorescence s
ensitivity of the modified instrument, measured with a quantitative fl
uorescence microbead standards kit, is 74 molecules of equivalent solu
ble fluorochrome for fluorescein. The instrument can now detect extrem
ely dim P. marinus from open-ocean surface waters.