Frequently Asked QuestionsIf you have a question that is not covered here, send it to us (XRF@detora.com) and help us update our website.
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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.
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