© Ultrafast Systems LLC /Vernier Software & Technology
22
Figure 7
: Time and voltage (raw data) and absorbance (calculated)
Viewing the trace in Figure 6, you see that at the left side (for about 400 ms before time zero) the
voltage level is stable with an average of 1.335 V. This corresponds to the intensity of the transmitted
beam prior to the firing of the flash lamp. We define this voltage as I
0
. The table in Figure 7 displays a
portion of the data plotted in Figure 6. Note that the time goes from negative to positive, as on Figure 6,
and that the first 10 voltage values are ~1.335, our I
0
value as shown in Figure 6. During the flash the
voltage dips to 0.024 volts and eventually, as seen clearly on Figure 6, it returns to the initial level. This
tells us that a species that absorbs light at the observation wavelength is generated by the flash, and thus
the resulting solution transmits less of the monitoring beam.
Except under certain conditions, the transmitted intensity is not directly proportional to
concentration of the generated transient. We need to process the raw data to obtain absorbance (A(t)),
which is proportional to concentration through the relationship
0
( ) log
( )
( )
tr
I
A t
c t l
I t
Equation 13
where
I
tr
is the transmitted voltage at a given time,
ε
is the molar decadic extinction coefficient of the
generated transient, and
l
is the optical path length. Most spreadsheet software allows you to write a
simple transform, with I
0
= 1.335 V in the example shown here, to convert the data in column 2 (‘Signal’)
to that in column 3 (‘
Δ
A’). Logger
Pro
allows you to do this by creating a calculated column. After having
done this, you can generate and display an absorbance
vs
. time graph.
