Green Lasers and Fluorescein (or: Get rid of that 530/30!)
roederer at drmr.com
Sun Jan 20 17:09:20 EST 2008
This message is solely intended for those of you who are outfitting a
new instrument, or retrofitting an existing one, and are thinking
about green or yellow-green lasers.
I recently visited some laboratories that are acquiring new LSR II
instruments, and was told that BD is recommending that they equip the
instruments with "yellow-green" lasers (~560 nm) rather than green
lasers (532 nm). The stated reason for this is that the green laser
has been observed to result in deteriorating fluorescein (FITC)
The big reason to use a green laser is because of the significantly
better detection of PE and PE tandems. This comes about for two
reasons -- one is the increased excitation efficiency at 532 nm
(compared to 488 nm), and the second is the availability of high-power
lasers (200 watts). In multicolor experiments, we find as much as 10-
fold increased sensitivity on these channels using a high-power green
laser compared to low-power blue laser. (The benefit over a high-
power blue laser is still significant, albeit less so).
It is possible that the green-yellow laser may provide this advantage
as well; I'm not sure because we haven't done the comparison.
Certainly, if you get a 560 nm laser at less than 200 watts, I would
expect it to perform less well than the 200 watt 532 nm laser.
So why the recommendation for the switch to a the green-yellow laser?
Because of the poor FITC results. But changing lasers is not the
solution -- changing filters is! Most FITC measurements are made with
a 530/30 nm filter -- a filter that is nearly centered on the green
laser. Thus, the high powered green laser provides all sorts of stray
light that contaminates this measurement (off the blue laser), and
leads to high background. There are two solutions to this: one is a
"notch" filter that selectively blocks the 532 light; the other is to
use a shorter bandpass filter.
First, why are manufacturers supplying a 530/30 filter for FITC?
FITC's emission peak is ~512 nm -- the 530/30 is only collecting less
than half of the tail of the FITC emission! We should use filter that
is much closer to the peak. The only reason 530/30 filters are still
supplied is purely historical inertia. Years ago, when filter
technology was less advanced, a filter bandpass needed to be far from
a laser line to block the laser light -- i.e., anything closer to the
488 than a 530/30 let through some of the blue light. But filter
technology is vastly better these days; we can come much closer to the
laser line and still block it out.
Currently, we use a 515/20 filter for FITC. Not only does it
efficiently block the 488 nm line, it also efficiently blocks the 532
nm line. AND... it collects more FITC fluorescence than the outdated
As for the notch filter solution: don't use it! Why take less than
half of the available fluorescence, and eliminate a large fraction of
it, simply to block the green laser light! It's a silly solution that
costs much more than the relatively inexpensive 515/20 nm filter
(which can be ordered from any filter company).
There are reasons to get a yellow-green laser (for example, to detect
certain fluorochromes or fluorescent proteins). But don't get it
because the green laser makes FITC look worse -- that's not a valid
Finally .... if you get a high power green laser (or even if you get a
yellow-green laser), then you should use a low-power blue laser (NOT
the high power blue laser). The high-power blue laser helps for
detecting PE and PE tandems, but it does not help with FITC, and it
hurts significantly with PerCP. If you use the green laser for PE
etc., then get the cheaper low power blue laser: you will get as good
(or better) results.
PS, the information about the green laser, PE tandems, and the use of
alternative FITC filters is fully described in Perfetto et al.,
Cytometry A, 71, 73-9 (2007).
More information about the Cytometry