Geoscience Reference
In-Depth Information
sample data or (2) using an auto-blanking function found in some manufacturer's software,
where the blank file is recorded before sample collection and applied to the data while
they are being collected. The first method allows for the recording of raw fluorescence
data. Readers should beware that if using the latter method, the data stream could be sub-
ject to complications if an ideal blank is not used, as it may be impossible to retrieve the
non-blanked values once data collection is completed. Regardless of the method selected,
blanks must be monitored carefully to ensure accuracy and to minimize systematic error.
6.4.3 Standards and Intensity Calibration
Throughout the literature there appear numerous findings of NOM fluorescence values
reported in volts or relative fluorescence. Although the scientific community gains from
these reports, the relative spatial and temporal trends in fluorescence values, it is impos-
sible to compare absolute intensities without calibration to a known standard. Calibration
of field sensor fluorescence intensity is routinely conducted using three standards. QS and
natural organic matter (NOM) samples are utilized for humic- and fulvic-like fluorescence
calibration, where NOM can be either humic acid standards or filtered sample water. The
third standard is tryptophan, which is used for calibration of protein fluorescence.
QS is a standard reference material (SRM) that is a strong absorber of UV light with a
fluorescence peak at ex/em = 347.5/450 nm. It has a high quantum fluorescent yield, par-
ticularly if diluted in weak acids and its peak fluorescence corresponds to regions of the
spectrum where organic matter components absorb and fluoresce. Just as with laboratory
instruments, a QS dilution series can establish the sensitivity and the linearity of a field
sensor, and the range of concentration should be appropriate for the environment which is
being sampled. Fluorescence intensity is converted to QSE (QS Equivalents) through the
following equation:
(
)
CC
−
sample
CWO
[
sample
]
=
(6.1)
QS Slope
where
C
sample
= sample raw counts,
C
cwo
= blank raw counts (if choosing to blank subtract
the data), and QS Slope = linear regression slope of QS dilution series versus fluorescence
counts. Analysts should also note that some manufacturer software automatically calcu-
lates the fluorescence data in QSE, so a dilution series need only to be conducted to confirm
manufacturer scale factors and to verify that instrument is maintaining its calibration or
the dynamic range of the sensor. Similarly, for sensors customized for protein detection, a
dilution series of tryptophan standards prepared in purified water or seawater can be used
for calibration (Tedetti et al.,
2010
).
NOM can also be used to calibrate field sensors and is used because no true standard
for DOM exists. This method is conducted using either discrete filtered samples or com-
mercially available humic acid standards. The latter method requires dissolution in purified
water and the generation of a dilution series (Breves and Reuter,
2001
), where the method
