How stable are the signals generated by the x-ray
fluorescence (XRF) technique?
The proportional counter used to detect fluorescent
signals in the DETORA on-line metals monitor has the tendency to drift slightly,
particularly with temperature changes. As a guard against this, a periodic
stabilization measurement is used to initiate automatic gain control adjustments
of the electronics. In the on-line monitor, alternating cycles between sample
accumulation and x-ray measurements allow time for such stabilization
measurements. During each sample accumulation cycle a fresh stabilization
measurement is made prior to each sample measurement so that proportional
counter drift has not been a problem.
What about
multi-element monitoring?
The XRF measurement technique used in the DETORA monitor
is of the energy dispersive (EDXRF) type. This means that each measurement
comprises a simultaneous scan of a range of x-ray energies. The range available
with each measurement is determined by the source energy (x-ray tube settings or
selection of an appropriate radioisotope) and by various electronic settings.
As iron is our most commonly requested metal from customers, a curium isotope
(Cm-244) is used for excitation. Iron emission occurs at 6.403 KeV and an
excitation energy of 1.5 to 2 times that energy will efficiently stimulate the
desired emission. Cm-244 emits radiation in the 12 to 14 KeV region and serves
as a suitable excitation source for iron and other elements in the general
vicinity of iron on the periodic table. Thus, the same excitation
conditions used for iron are useful for a range of elements including titanium,
chromium, nickel, copper, and zinc, as well as several heavier elements which
have low lying emission lines in this region such as lead and mercury.
The simultaneity of the EDXRF technique makes this all
possible. When an iron measurement is made, the x-ray energy spectrum obtained
with the iron measurement contains the information on other elemental emission
as well. Software modifications to the system plus appropriate calibrations
enable on-line, multi-element monitoring.
Though iron is the element of main concern and typically
constitutes 90% or more of the corrosion product inventory, many power plants
are asking about the possibility of analysis and monitoring of other metals.
For example, power plants with copper containing components (e.g., condenser
tubes) are aware of the adverse effects of copper corrosion, particularly with
respect to turbine damage. Thus, iron and copper monitoring is quickly becoming
a more common request.
What is the
detection limit of the DETORA on-line metals monitor?
The detection limit for this type of analyzer, which uses
a concentrating mechanism, is defined differently than for a direct detection
analyzer. The detection limit in our case refers to a minimum detectable amount
(MDA) of material accumulated under the probe rather than a concentration in the
liquid passing through the flow cell. This being the case, the accumulation of
the MDA, and increments thereof, may be achieved by manipulating two
controllable system parameters: time and flow rate.
Either, or both, of these may
be increased (within practical limits) to lower the apparent concentration
limit.
The inherent detection limit
can be shown mathematically, via the calibration data, to be about 5 micrograms
total iron on a filter surface. This is where time and flow rate come into
play. The fundamental relationship is as follows:
micrograms
collected =
(concentration) • (flow rate) • (time)
or,
A = C • F • T
If the amount collected is set
to the MDA, and the equation rearranged, the concentration limit may be defined
as follows:
If 5 micrograms is used as the
MDA, and some typical values are substituted for the flow rate (400 ml/min) and
time (20 minutes), the minimum detectable concentration under these conditions
can be solved:
Are any reagents
required to operate the On-line Metals Monitoring System?
DETORA’s instrument requires no reagents for operation.
No waste is generated. Once the liquid sample passes through the instrument, it
can be directed back into the sample stream, if desired.
What are the maintenance requirements?
Minimal maintenance is required to continually operate
DETORA’s monitor. A simple procedure is required to replace sample collection
filters prior to beginning a new monitoring session. A filter change takes
approximately five minutes.
Can differences between particulate and dissolved
metal fractions be detected and quantified?
DETORA’s unique system design can be used to detect both
particulate and dissolved forms of metals in liquid sample streams. The flow
cell can be fitted with either a standard 0.45 micron membrane filter or an ion
exchange membrane to trap dissolved (ionic) forms of metals. Using a
dual-channel system, the first flow cell would contain a standard membrane
filter to collect particulate, with the second flow cell containing an ion
exchange membrane to collect dissolved components. This allows simultaneous
detection of particulate and dissolved fractions of metals or corrosion
products.
What
can I do with the filter samples after the XRF measurement is complete?
In general, we recommend that samples be labeled and
archived for future reference. Filters can be subjected to x-ray diffraction to
obtain information about specific compound types. For instance, the oxidation
state and corresponding percentages of the iron compounds present in a sample
can be determined.
Can I
measure conductivity, or other chemistry parameters, in the same stream sent
through the on-line flow cell?
Yes. DETORA’s monitoring systems are expandable and can
accommodate data acquisition from other sensors. Dissolved oxygen and pH are
two other common parameters monitored, which can readily be incorporated into
the existing software and hardware of the system.
How can the On-line equipment be used for off line
tasks?
The heart of the on-line monitor is a fully functional XRF
elemental analyzer. During on-line operation, this analyzer is programmed
to perform individual analyses of the metal(s) accumulated on the filter or
ion-exchange membrane in the flow cell beneath it. The progressively higher
amounts of metals detected as time goes on, combined with the continuous record
of the flow totals through the flow cell, give the ppb concentration of the
sample stream.
When not on-line, as for example during the periodic
filter change, the XRF analyzer may be used for a variety of other laboratory
and/or plant tasks. XRF is truly a multi-purpose analytical tool.
Both liquid and solid samples may be analyzed. For
qualitative analysis, even samples with odd shapes or unusual surface properties
are readily examined for elemental content. For example, a screw, a piece of
wire, or other scrap of metal is easily identified as to alloy type in minutes.
Quantitative results depend only on the user’s ability to prepare or acquire
suitable standards for comparison with the samples.
Considering the versatility of XRF as an analytical
technique, it is difficult to conceive of a general laboratory, which could not
benefit from the addition of XRF capabilities to its repertoire.
This product
incorporates technology
developed for the
electric power industry
under the
sponsorship of EPRI,
the
Electric Power Research Institute.
