The impetus of the Viking Mission to Mars led to the first miniaturization of a X-ray Fluorescence Spectrometer (XRFS). Two units were flown on the Viking Mission and successfully operated for two years analyzing the elemental composition of the Martian soil. Under a Bureau of Mines/NASA Technology Utilization project, this XRFS design was utilized to produce a battery powered, portable unit for elemental analysis of geological samples. This paper will detail design improvements and additional sampling capabilities that were incorporated into a second generation portable XRFS that was funded by the EPA/NASA Technology Utilization project. The unit, Environment Monitoring with Portable Analysis by X-ray (EMPAX), was developed specifically for quantitative determination of the need of EPA and and any industry affected by environmental concerns, the EMPAX fulfills a critical need to provide on-site, real-time analysis of toxic metal contamination. A patent was issued on EMPAX, but a commercial manufacturer is still being sought

Clark, Ben

Kelliher, Warren C.

Thornton, Mike

English

NASA Technical Reports Server

WATER QUALITY MONITOR
(EMPAX _s_nvmNT)
N92-22724
Warren C. Kelliber
NASA Langley Research Center
Hampton, VA 23665
Ben Clark and Mike Thornton
Martin Marietta Astronautics Group
Denver, CO 80201
ABSTRACT
The impetus of the Viking Mission to Mars led to the first miniaturization of a X-ray Fluorescence
Spectrometer (XRFS). Two units were flown on the Viking Mission and successfully operated for two years
analyzing the elemental composition of the Martian soil. Under a Bureau of Mines/NASA Technology Utilization
project, this XRFS design was utilized to produce a battery powered, portable unit for elemental analysis of
geological samples. This paper will detail design improvements and additional sampling capabilities that have been
incorporated into a second generation portable XRFS that was funded by EPA/NASA Technology Utilization
project. The unit, known as EMPAX (Environment Monitoring with Portable Analysis by X-ray) was developed
specifically for quantitative determination of the need of EPA and any industry affected by environmental concerns,
the EMPAX fulfills a critical need to provide on-site, real-time analysis of toxic metal contamination. A patent has
been issued on EMPAX, but a commercial manufacturer is still being sought.
INTRODUCTION AND BACKGROUND
In 1976, two miniaturized X-Ray Fluorescence Speca'ometers (XRFS) were landed on Mars and successfully
operated for over two years performing elemental analysis of Martian soil samples. These spectrometers were built
for NASA by Martin Marietta and were part of the instrumented landers of the Viking Mission to Mars Proj_t.
These spectrometers used proportional counters for X-ray detectors, radioactive isotope sources as the X-ray
generators and standard pulse height analysis for the electronics. This combination has served as the basis of all
national and international space missions flown to date.
In 1979, NASA and Martin Marietta built a portable battery operated version of this instrument and
delivered this prototype instrument to the Bureau of Mines for evaluation. Since then, several bench and portable
versions are commercially available with the proportional counters/isotope source combination.
In 1979, at the request of EPA, NASA and Martin Marietta embarked on a feasibility study to determine if
the portable X-Ray Spectrometer concept could be advanced to the point where sensitivities to a series of toxic
metals in the range of parts per million (ppm) to parts per billion (ppb) in water samples could be achieved.
Through the use of precipitating agents and collection of the precipitate on filtration membranes, sufficient
preconcentration was obtained to achieve the majority of the elemental sensitivities desired using an X-Ray tube as
the X-ray generator and a solid state, liquid nitrogen cooled Si(Li) detector as the X-ray sensor. Based on this
information, funding was obtained to develop and deliver two prototype instruments for evaluation by NASA and
EPA.
INSTRUMENT DEVELOPMENT AND DESIGN
In the development of a portable XRFS for the EPA, two decisions were made early in the program that
directly affected the final instrument design. One was to have separate modules for the basic components rather than
have them all incorporated into one combined instrument package and two was to use commercially available
components wherever possible. Since the instrument concept evolved in the 80's time period, it will reflect the
technology that was available during that time. With some of the 90's technology, selected components could be
changed or updated and this will be discussed in a later portion of this presentation. The instrument design was also
affected by the changing environmental concerns and issues at EPA in the 80's. The changing emphasis on water
quality and soil analysis capabilities varied sufficiently, that the instrument is equally adapt in its capacity to handle
either type of samp!e.
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The unit as presently configured consists of four modules: 1) Water Sample Module, 2) Portable Analyzer
Module, 3) Electronics Module and 4) Computer Module.
Water Sample Module
The water sample module is a unit designed by Martin Marietta to accept known volume water samples that
have had a precipitant (Dibenzyldithiocaflmmate) and a buffer (Potassium Hyckogen Phthalate) added and will perform
the filtration process by depositing the precipitate on a membrane filter that is housed for ease of handling in a
35mm slide mount holder. The analyzer module will fit onto the water module and once the filtration step is
complete (as indicated by filtration complete light), the operator has only to slide the 35mm holder from the
filtration position to the analyzer position to initiate sample analysis.
Portable Analyzer Module
The portable analyzer module or sensor head is a combination of commercially procured instruments that
were modified and incorporated in an enclosure designed by Martin Marietta. The detect_ is a Si(Li) solid state
detector cooled by liquid nitrogen from a reservoir (dewar) fl_t has sufficient capacity for 8-12 hours of operation.
When not being operated in the portable mode, the dewar is attached by hose to a larger liquid nitrogen dewar and is
automatically refilled when the liquid nitrogen is depleted. This detector and dewar system was procured from PGT,
Inc. Due to the high elemental sensitivities needed by EPA, it was deemed necessary to go to the high resolution
solid state detector rather than use the proportional counter tube type detector. Likewise, it was necessary to
eliminate the use of radioactive isotopes as the X-ray source and go to a miniaturized X-ray tube for both the safety
aspects as well as the need for the high intensity fluxes obtainable from a tube source. A 30 Kilovolt, molybdenum
target, X-ray tube was built for this unit by X-Tech, Inc. and included as a part of the analyzer module. The ttigh
voltage bias and electronics power is supplied from an array of lead-zinc "Gel-cell" batteries located in the back
section of the analyzer module.
El_tronics Module
The electronics module is the pulse height analyzer, used to accumulate the energy pulses from the detector
into an energy versus counts speclrum. Rather than attempting an extensive modification of the electronics used in
the portable unit for the Bureau of Mines, a commercially available unit, the Canberra Series 10 Multi-channel
Analyzer, was chosen instead which required only minor modification to interface with the analyzer and computer
modules. It is capable of storing up to 16 spectra before Iransfer to computer storage is required. The rechargeable
NiCad batteries power the unit and also provide detector bias and detector electronics power.
Computer Module
The computer module is used to accept and stove the spectra received from the electronics module and to
convert the spectral peaks to quantitative concentration values. The 80's saw considerable changes and increasing
capabilities in small portable computers and as a result our original choice of the only mini-computer available at
that time, the HP-85, rapidly became obsolete and so for our field tests, we switched to a Grid Case 2 portable unit.
This too is rapidly on its way to being replaced with more powerful units, but the point is that our software as well
as comparable commercial versions, will all work satisfactorily on 286 or higher number CPU machines.
The combination of the four modules comprise what we have designated as the "EMPAX" instrument
(EMPAX being the acronym for Environmental Monitoring with Portable Analysis by X-Ray). The EMPAX
instrument has been issued Patent No. 5014287.
FIELD DEMONSTRATION TEST
Two prototype units were built by Martin Marietta, one of which was delivered to EPA, Las Vegas
Laboratories in Nevada and the other to NASA, Langley Research Center in Virginia. Before delivery, both units
were involved in extensive field testing. To illustrate the potential usage of these units, the field trip to a Superfund
site in Aspen, CO, conducted in August 1988 will be described. The site was chosen for being in driving distance to
the Martin Marietta facilities in Denver, CO, and for having been already characterized for toxic pollutants which
438 ""
were chieflymine tailingsfrom operationofthemines inSmuggler Mountain. Working from dawn todustatthe
site,thecombined team ofNASA, MartinMariettaand EPA personnelcollectedover 120 spectratwhich timeall
theinstrumentsand personnelneededregeneration.A normal surveyofa sitesuch asthisone,involvestheanalysis
ofmany samplesina certifiedlaboratory,would takemany months toobtainanalysisresultsand would costup to
$I00,000. The EMPAX instrument,ina few days,couldalsoperformthesitesurvey,would providein-site
analysisresultsand thetotalinstrumentfabricationcostisestimatedatabout$80,000. Thus,theinstrumentisvery
cost effective compared to standard analysis costs.
Water samples were also available at this site and were taken from the nearby outcrop stream. The analysis
results surprised us with very high Z peaks that turned out to be Uranium. Since the water percolated through the
abandoned and flooded old mine shafts, by hindsight, the presence of Uranium is not too unusual but demonstrates
that XRFS because of its elemental scanning ability, will routinely detect the presence of unusual elements. Other
analysis techniques such as Atomic Absorption or Emission Spectroscopy (ICP) require the prcselection of elements
of interest and this would miss the presence of elements that may also be of interest.
Comparison of the XRFS analysis to that performed by standard laboratory procedures as stated in the EPA
evaluation report - "Precision and accuracy of the in-site measurements were within +10% of the true value when
compared to the samples analyzed in the laboratory." These results are considered outstanding by all parties
concerned and exceed expectations for the analytical capabilities of the EMPAX in most situations.
POTENTIAL DESIGN CHANGES
The prototype instrument design was preset several years ago and with the continuing advances in the field of XRFS
instrumentation, there are a number of changes that could be made to EMPAX to enhance its capabilities and
commercial applications.
Electronic Module
In many of the commercial laboratory XRFS, the function of the PHA has been incorporated on a computer
circuit board so as to use the computer terminal and keyboard functions as the PHA readout and controls. With the
newer laptop computers being able to add full size boards, it is quite possible to combine the functions of the
Canberra Series 10 Multichannel Analyzer into the Grid Case type computer and thereby eliminate one module
entirely. The power supply functions of the PHA could then be transferred to the analyzer module.
Water Sample Module
The present design uses a membrane filter to collect the precipitate and thus achieve up to a 100 to 1
increase in concentration of the elements to be analyzed. With the advent of new ion exchange type membranes, it is
possible to have the membrane filter act as the concentrator thus eliminating a time consuming filtration step.
Since the ion exchange membrane can also act as a filter, it can also serve to collect the precipitate while
concenlrating the cations that do not react with the preeipitanL By analyzing both the front and backside of the
membrane filter, one can increase the number of cations that can be analyzed.
Analyzer Module
When initially designed, the portable liquid nitrogen cooled detector was a major advance over the use of
proportional counter tubes. Now there are several commercially available units but the continual need of liquid
nitrogen as a coolant is an impediment that could be rectified by changing to a thermoelectrically cooled detector.
There are commercial available XRFS systems that use this means of cooling but power consumption is in order of
30 to 80 watts. The literature and experimental work by Martin Marietta has shown this power consumption can be
reduced to 12 watts or lower which is within the ability of using batteries as the power supply. This change would
eliminate the problem of obtaining or carrying liquid nitrogen to remote sites.
The prototype instrument lack of sensitivity to Cd, Sb and Ag is due to the choice of using Mo as the X-
ray tube target source. The L line energies of these elements are obscured by the Argon present in the atmosphere
and the K lines energies are too high to be excited by the Mo X-ray source. A possible simple means to enhance the
sensitivity to these elements is to use some of the gaseous nitrogen from the boil off of the liquid nitrogen coolant
439
and have it act as a purge over the sample to be analyzed. A more effective but more complex solution is to be able
to selectively change the excitation source energy. As part of this project, a higher voltage (50 Kv) miniaturized X-
ray tube was devised that provided dual energy outputs through the use of either a composite single target or a dual
target source. This development came too late to include in the prototype insu'uments but have since become
commercially available. --
COMMERCIALIZATION
The prototype EMPAX instruments were built mainly to demonstrate feasibility and be used for field
evaluation studies. During the design phase, consideration was made to make fabrication as simple as possible by
using commercially available components, however modification and redesign is still needed ff EMPAX is to be
commercially marketed. As currently designed, the EMPAX instrument will meet most but not all of the FDA
requirements for X-ray sources. Thus, some refinements plus any of the aforementioned enhancements must be made
before this instrument could become a marketable product.
There is more emphasis today on the use of XRFS for environment monitoring and has become a
recommended procedure by EPA. It is common now to use laboratory XRFS in vans for on-site analysis but the
EMPAX instrument has the additional capability of performing in-situ analysis, a major advantage over sample
removal and then perform the analysis.
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Element Line
Water Samples Soil Samples
Energy (keV) MDL, ppb* MDL. ppm
Cr Ka 5.414 100 1000
Ni Ka 7.477 20 300
Cu Ka 8.047 20 250
Zn Ka 8.638 I0 200
As Ka 10.543 10 150
Se Ka 11.221 10 140
Ag Ka 22.162 200 >1000
CA La 3.133 ND ND
Sb La 3.605 ND ND
Ba La 4.467 200 >1000
Hg Lb 11.823 20 80
T1 Lb 12.210 30 75
Pb Lb 12.610 20 70
ND = Not Detectable
*Assumes 100 ml sample volume containing
single element only
ppm = parts per million ppb = parts per billion
Table I. Mininum Detection Limits (MDL) for Priority Elements
Module Size (in) Weight (Ib) Power
Water Sample 1 8 x 1 8 x 1 0 40
Portable Analyzer 22.5xl 1 xl 2.5 34
Electronics 4.5x9x 11 12
Computer 1 5xl I x2.5 12
AC
4-Gel Cell Batt.
5-NiCd Bart.
Rechargeable Batt.
Detector LN2 cooled Si(Li)
X-ray Source - 30 Kv - Mo target - 0.2 ma
PHA - 4096 channels
0-600 volts bias power
Water Sample Module
Portable Analyzer Module
Electronics Module
Computer Module
Precipitant
Filter Membrane
Patent number
NASA I.aRC Contacts
EPA Las Vegas Contact
- Martin Marietta Design
- PGT Detector
- X-Tech X-ray Tube
- Martin Marietta Design
- Canberra Series 10 PHA
- Grid Case 2 Portable Computer
Dibenzyldithiocarbamate
- Rainin Nylon - 0.45 um pore size
5014287
Warren C. Kelliher (804) 864-4172
- Technology Utilization Office (804) 864-2482
Bill Engelmann (702) 798-2664
Table 2. EMPAX instrument information
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