Can I initialize the
sensor in a liquid?
During sensor Initialization, an
Analysis Error warning appears in the
Analysis Display Box. What's wrong?
The "Analysis Error" warning
message is only relevant when the program
is analyzing the signal from the sensor
for the surface plasmon resonance: i.e.,
when it's in a liquid. During the
"initialization in air"
routine, the program is, in effect,
looking at the signal in air (which
should have a value of unity over the
entire range of the sensor.... hence, the
plot area will show a flat line around
1.00). But, in air there is no
"dip" in the signal for the
program to analyze and it spits out an
"analysis error" warning.
Please ignore this warning message during
air initialization.
Later versions of the software do not
display this warning during
initialization.
During Initialization, the LED level
goes to it's highest value of 15.
Should I be worried?
In general, this is not something to
worry about unless....
1) The photodiode array (in the sensor)
is not functioning
2) The LED is not functioning
3) The sensor is only loosely stuck in
its socket
These potential problems can be checked
using the Data/View Raw Signal
menu command. This allows you to
directly view the signal coming from the
sensor. It is a valuable debugging
tool. When using this command, a
window pops up and immediately begins
displaying the sensor signal (updated
about once a second). You can vary
all of the sensor control
variables. Here are some things to
look for:
A) Is there a reasonable signal coming
from the sensor or is the signal railed
at the top or bottom of its range (0 to 5
volts).
B) Can you vary this signal by changing
the LED intensity.
If you can see a signal and vary it
with the LED intensity then the
photodiode array and the LED are both
working.
If there is a signal but the LED
doesn't vary it, try exposing the sensor
to room light and seeing of the signal
changes... if so then the photodiode
array works.
I'm using gold-coated glass slides on
my sensor and can't get an SPR curve?
Assuming that the sensor electronics
are working properly, two problems might
be causing you to not see an SPR curve.
1) The glass slide was put on the sensor
upside down. This is a surprisingly
easy thing to do. The gold side
must in contact with the liquid.
2) The index matching oil (between the
sensor and the glass) has crept over the
active surface of the gold and is
blocking the water from touching the
gold. The probability of this
happening is a function of your glass
mounting skills. Flow IsoPropyl
Alcohol over the glass slide to remove
the oil. You will need to
re-initialize, though.
Note, when mounting slides onto the
sensor try to use as little index
matching oil as possible.
I'd like to try putting my own gold
(Au) film on my sensor. What film
thickness should I use?
We initially coat our bare sensors
with 10 Angstroms of Cr to aid adhesion
of the gold to the sensor surface.
Finally, we deposit 500 Angstroms of Au
onto to the sensor. Other metals
such as silver and copper can be used
although they are more chemically active
than gold.
Note, to remove the old gold from your
sensor hand polish the thin gold surface
with a fine polishing compound such as
0.3um or 0.05um alumina slurry.
How does the sensor calibation work?
The sensor works by looking at the SPR
curve over a range of angles of
incidence. The map between pixel
position and angle of incidence should
be automatically set by the geometry
of the sensor. So, we know this
relationship exactly.
Alas, though, the sensor elements are
hand placed during fabrication so there
will always be some deviation from the
expected sensor performance. The
resonance position of water (or any other
liquid of known refractive index) is used
to "slide" the calibration
curve of index versus pixel position
along the pixel-axis.
The calibration equations between
pixel position and angle of incidence (or
refractive index) can be viewed in the
calibration form: Options/Calibration.
The equations are hidden at the bottom of
the form; expand the form to full-screen
size to view them.
Glitches appear in my data while I'm
flowing liquids over the sensor.
Fluidics through the sensor can be
tricky. In general, refractive
index is sensitive to temperature
changes: about -1 part in 10,000 per
degree C. When flowing liquids
through the sensor, the incoming fluid is
a one temperature and the sensor is at
another temperature. Eventually,
equilibrium is reached and measurements
can begin.
If the flow is altered (e.g., stopped)
the sensor will begin to warm (or cool)
the liquid and the refractive index will
change. For example, when pumping
liquids over the sensor surface glitches
often occur if pumping momentarily
stops. Peristaltic pumps (with
their non-uniform flow rate) can also
with a glitch problem.
Can I use the Spreeta sensor in air?
The current range of the Spreeta is
for refractive indices of 1.30 to
1.40. Special sensor designs are
possible to shift this range to higher or
lower index values: even to 1.00
(air). If you are interested in
this, please contact us via SPRHELP.
How is the data smoothing and
differentiation done?
The algorithms for both smoothing and
differentiating the data involve simple
formulas that add in distant points (with
various weights) to get the result.
See these references for details:
Smoothing and Differentiation of
Data by Simplified Least Squares
Procedures, A. Savitzky and M.
J. E. Golay, Analytical Chem. V36 No8,
July 1964, pp 1627-1639
and
Comments on Smoothing and
Differentiation of Data by Simplified
Least Squares Procedures,
Analytical Chem. V44 No11, Sept. 1972, pp
1906-1909
There are so
many Analysis methods to choose
from. How do I pick the right one?
There are,
indeed, many Analysis methods to choose
from and each one is valuable although
some will be rarely used.
I imagine that
most people will use the First
Moment Method because it is fast,
simple, and low noise. When looking
at small changes in refractive index over
time, it works marvelous. One
drawback, the minimum point of the SPR
curve determines the refractive
index. While the first moment
mirrors the minimum, it isn't actually
the minimum. For example, if the
SPR curve were to "shift" up or
down the first moment method would see
this as an index change.
Two analysis
methods do look at the SPR curve
minimum: Polynomial Fit About
Minimum and Zero Of
Derivative.
Both of these involve fitting data
about the approximate SPR minimum point
and using that fit to get a more exact
minimum location. Because the
analyses are more complex, they tend to
take more time. They can be subject
to noise problems, though, since only a
few data points are used in the
analysis. Shifting the data by even
one point can lead to very different
results. The algorithms attempt to
soften this problem but it will always be
present.
We suggest that the kit User try all
of the analysis methods and find the one
that suits them best.
Can I initialize
the sensor in a liquid?
Yes, you can
initialize in a liquid. Sometimes
initializing in air is awkward or
impossible. A liquid will do just
fine as long as the refractive
index of the initializing liquid must be
sufficiently different from that of your
target liquid. A difference
of ±0.04 RIU is sufficient. For
example, if the target liquid is water
then Isopropyl Alcohol or 10% Sucrose
solution can be used for initialization
rather than air.
The resulting
SPR curve will look different.
Rather than a simple dip in the SPR curve
(indicating the target liquid's
refractive index) there will be a bump
(indicating the initializing liquid's
refractive index).
