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Common Questions
for the SCUFA®
Submersible Fluorometer
· What is the sensitivity of
the SCUFA®?
The SCUFA® sensitivity
is defined in terms of the minimum detection limits of various analytes.
The SCUFA® can detect:
Analyte Minimum Detection Limit
Chlorophyll a 0.02ppb or 0.02 µg/L
Rhodamine WT 0.04ppb or 0.04 µg/L
Turbidity 0.05NTU
· Do I adjust the sensitivity with a gain knob or switch?
There is no manual sensitivity or gain
control mechanisms on the SCUFA® . There are three gain
settings (x100, x10, x1) on the fluorescence channel and two gain settings
on the turbidity channel that are controlled automatically.
With analog output, the user has control
over instrument range that will affect the gain settings available. For
example, by setting the 0-5V range to 0-20ppb, you will eliminate the
need for the x1 gain setting.
To achieve optimal performance, the appropriate
calibration standard should be used. An optimal standard will be a standard
with a concentration that represents 40-60% of the maximum concentration
you will experience in the field.
· Is SCUFAsoft compatible with
Macintosh computers?
No, the correct version of SCUFAsoft is
not compatible with Macintosh computers.
· How often do I need to calibrate
the SCUFA®?
For greatest accuracy, check calibration
before every deployment. Verify the need to calibrate by reading a stable,
known concentration standard, such as a solid secondary standard, immediately
after calibration and again before each deployment to see if the readings
have changed significantly. Recalibrate when there is a change in the
environment or when the accuracy becomes unacceptable for your study.
· What is the purpose of the
solid secondary standard?
The solid secondary standard is designed for recalibration in place of
primary standards and to check for instrument performance and drift. It
is very stable and does not require special storage conditions.
· What is the concentration of
the solid secondary standard?
There is no exact concentration for the
solid secondary standard. It is a relative concentration for both Chlorophyll
a and Rhodamine WT. You can easily adjust the fluorescence signal of the
solid standard and use it as a reference value for future calibrations
as well as check for electronic drift.
· What is the power consumption if using internal data logging?
When using internal data logging, the total
power consumption is dependant on the sampling rate set by the user. If
the sampling rate is set to an interval longer than 1 reading every 5
seconds, the unit will power down into sleep mode (60mA). The instrument
will consume 60mA when logging data.
· How long can the SCUFA®
stay submerged with anti-fouling screens?
The Copper Anti-fouling System is intended
to be used for fluorometer deployments of extended periods (>1 day).
The copper components should be installed prior to instrument calibration.
The copper components will slowly dissolve
in water and need to be monitored for wear. It is recommended that all
components be changed after two months of use.
· How do I attach the SCUFA®
to a CTD?
To integrate a SCUFA® with
a CTD, two pieces of hardware are required; an integration cable and a
mounting bracket. If possible, the integration should be conducted by
the CTD manufacturer. If this is not possible, you must contact the appropriate
cable/connector vendor to have an integration cable made that will allow
communication between the fluorometer and CTD.
An integration cable consists of two in-line connectors, locking sleeves
and a cable of specified length, usually 3-4 feet.
The in-line connector required for the
SCUFA® is an Impulse 8-pin, female connector (Impulse P/N:
MIL-8-FS), the locking sleeve is P/N MCDLS/F. The maximum cable length
is 50m.
The bulkhead connector on the SCUFA®
is Impulse P/N:MCBH-8-MS.
The wiring for the SCUFA bulkhead connector is as follows:
1 V Batt +
2 V Batt -
3 RS-232 ground
4 RS-232 T1 Out
5 RS-232 R1 In
6 V Out 1
7 V Out 2
8 ground
· How do I set the 0-5V Outputs?
Setting the 5V to a value greater than
the 0V activates analog calibration for the channel of interest; fluorescence
and/or turbidity. When analog output is activated, the Internal Data Logging
(IDL), if purchased, is automatically disabled and the IDL screen will
be faded out.
Activating the analog signal output should
follow instrument calibration. By calibrating first, you can then set
the 0V and 5V to calibrated values. For example, if you calibrated with
a 10ppb solution and know that you will not exceed 100ppb in the field,
you can set 5V to equal 100. By doing this you can optimize the resolution
and accuracy of your analog data and interpret your analog data with a
calibration coefficient. In this example, the calibration coefficient
would be 20 (5V=100ppb, 0.5V = 10ppb).
Once set, the analog output will be activated
upon the next power up as long as the unit is not connected to the portable
computer.
· What if I need a longer integration
cable?
We recommend using a deployment cable no
longer than 50m to avoid electrical signal decay. If a deployment cable
with an AC power supply and RS-232 connector is required, Turner Designs
offers 20m and 50m versions (P/N 2000-970 and P/N 2000-980).
Deployment cables to be used with a DC
power source and analog signal output should be purchased directly from
Impulse Enterprise or the appropriate distributor.
Please contact Impulse Enterprise for information
on their products and distributors.
Impulse Enterprise
8254 Ronson Road
San Diego, CA 92111
tel: 800-327-0971
fax: 858-565-1649
e-mail: impulse@impulse-ent.com
· What is the flow-through cap for?
The flow-through cap is an optional accessory
that can allow the SCUFA® to be used in a flow-through
mode. The cap is installed over the optics and has inlet and outlet ports
to connect with a plumbing system. A common use of the cap is in conjunction
with Sea-Bird CTD systems that can use a submersible pump to pump water
through all of the probes. Another use could be to use the cap on a ship
or in the lab with an external pump.
The cap is not necessary for use. The SCUFA®
has been designed as an open-optics unit, meaning it can operate successfully
with high levels of ambient light without the need for a pump.
· How can I use the SCUFA®
in the laboratory?
There are two ways that the SCUFA®
could be used to analyze samples in the laboratory. The optical head of
the SCUFA could be immersed into sample solution. The SCUFA®
should be held at least 2" off the bottom of the container. The second
option is to use the flow-through cap (P/N 2000-900) with an external
pump or syringe.
CHLOROPHYLL ANALYSIS
· How does the SCUFA®
detect and quantitate chlorophyll in water?
Chlorophyll a naturally absorbs blue light
and emits red light. The SCUFA® will detect chlorophyll
by transmitting an excitation beam of light in the 440nm (blue) range
and by detecting the light emitted by the sample in the 680nm (red) range.
· What is in vivo chlorophyll
analysis?
In vivo chlorophyll analysis is the fluorescent
detection of chlorophyll in living algal and cyanobacterial cells in water.
In this technique, the excitation light from the fluorometer passes through
the untreated sample water and
excites chlorophyll within the living cells of the algae present. Due
to the nature of light, cells and other dissolved and particulate materials
in the water will affect the excitation light before it reaches the chlorophyll
molecules. Examples of interfering materials include other plant pigments
and degradation products, dissolved organic matter, turbidity, and cell
morphologies. Therefore, in vivo analysis is a semi-quantitative tool.
In vivo numbers
should correlate well with each other but rarely can they be used as actual
chlorophyll a concentration measurements until correlated with extracted
chlorophyll a data.
· Could I calibrate with extracted
chlorophyll a?
No. Methanol, 90% acetone and other organic
solvents will react with the SCUFA's delrin housing. The SCUFA®
should not be used to measure extracted chlorophyll a. Please use a separate
fluorometer such as the Turner Designs 10-AU or TD-700 Fluorometers to
determine actual chlorophyll concentrations.
· What environmental factors
cause error in in vivo chlorophyll analysis?
Temperature has an inverse relationship
with fluorescence. In a vertical profile, as temperature decreases, the
fluorescence will increase independent of chlorophyll concentration. The
SCUFA® is equipped with temperature
compensation to automatically correct data for temperature effects.
Light history will have significant affects
on the fluorescence in algal cells. Cells will fluoresce more chlorophyll
per cell when in darker environments than in well lit zones. One way of
reducing the effects of light is to use the
flow-through cap when sampling natural waters. By using a flow-through
cap and an external pump, the algal cells will be dark-adapted before
entering the fluorometer, significantly reducing fluorescence error caused
by variations in the light history of the cells.
Dissolved organic matter (DOM), chlorophyll
degradation products, and turbidity can also affect fluorescence response.
If these factors are suspected to be significant it is worth conducting
a quick study to look at the effects by comparing the fluorescence from
filtered and non-filtered water samples from below the photic zone where
chlorophyll concentrations would be at a minimum.
FLUORESCENCE
· What variables will effect
the linearity of a sample?
Fluorescence intensity is typically directly
proportional (linear) to concentration. When a concentration is too high,
light cannot pass through the sample to cause excitation; thus very high
concentrations can have very low fluorescence (concentration quenching).
The fluorometer reading rises at a decreasing rate and eventually
begins to decrease, even though the concentration is still increasing.
Diluting a sample 1:1 or some other convenient ratio may check linearity.
If it is linear, the reading will decrease in direct proportion to the
dilution.
· How does photochemical decay
affect fluorescence?
Many fluorescent molecules can be bleached or destroyed by light (fading
of dyes in the sun). Ultraviolet light, especially, can cause certain
molecules to break down. Fluorescence readings decrease as the molecules
are
destroyed. Rate of destruction varies depending upon environmental factors,
temperature. Fluorescein, for example, is destroyed rapidly in sunlight.
Rhodamine WT, however, is adequately stable for field studies. For chlorophyll
measurements, samples and standards need to be kept in the dark until
read. All flow measurements should employ opaque delivery hoses to minimize
photochemical interference.
· What are the advantages of
fluorescence?
Sensitivity: Limits of detection
depend to a large extent on the properties of the sample being measured.
Detectability to parts per billion or even parts per trillion is common
for most analytes. Fluorometers achieve 1,000 to 500,000 times better
limits of detection as compared to spectrophotometers.
Specificity: Spectrophotometers
merely absorb light. Spectrophotometric techniques are prone to interference
problems because many materials absorb light, making it difficult to isolate
the targeted analyte in a complex matrix. Fluorometers are highly specific
and less susceptible to interferences because fewer materials absorb and
also emit light (fluoresce). And, if non-target compounds do absorb and
emit light, it is rare that they will emit the same wavelength of light
as target compounds.
Simplicity and Speed: Fluorometry
is a relatively simple analytical technique. Fluorometry's sensivity and
specificity reduce or eliminate the sample preparation procedures often
required to concentrate analytes or remove interferences from samples
prior to analysis. This reduction in or elimination of sample preparation
time not only simplifies, but also expedites the analysis.
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