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What is x-ray fluorescence and why is XRF sample preparation
is important for correct XRF analysis.
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This is a little introduction to some of our instruments; from
left to right we have the X-MET7000 series these hand held
analyzers are great for their versatility and portability, the X-
Supreme in the middle is used for bulk analysis of liquid,
powder and solid sample and the X-Strata 980 on the right is
used to measure thickness and composition it is commonly
used in printed circuit boards or the plating industry.
XRF works on the atomic level x-rays are produced from the
spectrometer and excite the analyzed volume within the
specimen. The incoming x-ray ejects an electron, when this
happens the atom becomes unstable and form an electron from
an outer shell fills the vacancy.
Once this happens it generates the characteristic x-ray
equivalent to the energy difference between the inner and
outer shell. Each element will have a characteristic x-ray of a
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specific energy and this is how one can determine what
elements are present in the sample.
The detector selects these characteristic x-rays and displays
them as a spectrum which is a visual representation of the
composition of the sample by looking at the x axis one can
see the energy range goes from 5keV to 10keV and as the
energy increases the atomic weight of the element increases,
this is because the electrons of the heavier elements are bound
tighter, on the y axis there are counts which is the number of
x-rays one can see that iron is present in a large quantity
compared to the other elements because of the high number of
counts, another important aspect of the spectra is the
background level, this is the level of at the base. one can see
that the background increases around 8keV, it is important tolimit the effect of the background if you want accurate
analysis, this can be done by using a primary filter this
component will be covered briefly in the next slide or one can
also expect for the changing background by using correction
once the calibration model is created.
This is the setup in a typical XRF instrument the x-ray source
can either be a radioactive isotope or an x-ray tube. The x-rays
excite the sample and the detector interprets the energy and
amount of characteristic x-rays. In this example we also have a
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primary and secondary filter; primary filters are typically used
to lower the background level which is caused by scattered x-
rays, secondary filters work by filtering out a specific element
these types of filters are only required if the detector doesn’t
have the resolution separate the adjacent beaks.
Once the setup calibration standards is measured one can then
create an Empirical calibration the standards of known
concentration are measured and the intensity is stored. A linear
regression can then be created which follows a straight line
model {Y=MX+E} where {M} is the slope and {E} is the {Y}
intercept more often than not the simple straight line slope
will not create the best model because there are other
influences from adjacent elements and also other matrix effect.
Proper sample prep along with correction factors for anyinterference will ensure the best possible calibration model
this way when the intensity of the unknown is detected it can
be compared to the intensity of the storage standards to give a
calculated value.
Another way of creating a calibration is using FP or
Fundamental Parameters. This slide shows the Sherman
Equation which is a typical FP equation, these equations can
become quite complex but they can produce a result without
using any standards at all. FP uses absorption co-efficient or
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each element and the measured intensities along with other
parameters to calculate a result. One can often make a FP
calibration more accurate by adding a standard called a type
standard.
One type of calibration is not better than another they have
different advantages. If standards are available then FP is a
better option. FP is typically more versatile and can cover a
large range of concentration in many different elements and
Empirical Calibration when using proper standards and sample
prep techniques will give more accurate XRF results. Empirical
Calibrations are also better when trace ability back to a
standard reference material is critical.
Now that the basic principles of XRF and the different type of
calibrations have been discussed briefly the details and sample
prep and sampling will make more sense. What is sampling?
Sampling is what we are most interested in for our analysis it is
important that the sample be representative of the bulk
material.From the sample you then create a specimen, the
specimen is what is actually placed in the instrument for
analysis finally within the specimen there is an analyzed
volume since the final results are only based-off of therelatively tiny analyzed volume one can see how important it is
to practice sound sample prep techniques.
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The following graph shows the depth that can be analyzed by a
typical XRF instrument for certain elements, one can see that
the thickness varies by elements but also varies by other
factors such as density and the absorption coefficient of the
sample. There is a maximum thickness and also a minimum
thickness that shows the limits of the XRF. On the high end we
can see that the x-rays do not penetrate very deep into a
sample at this maximum level we find that when the sample
becomes thicker the instrument does not detect a higher level
of expectation in other words the count rate doesn’t increase.
This concept is referred to as intimate thickness, in this
example if we look at chromium after about 25µm the sampleis infinitely thick because the detector does not see any
difference in the sample. The depth in which the x-rays
penetrate multiplied by the area of the x-ray beam is the
analyzed volume.
Here is an example of how the XRF analysis can be impacted
by the analyzed volume in this example two particles of
aluminum oxide are within the analyzed volume its imaginary
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line is drawn representing the second analyzed volume. There
are five particles of aluminum oxide this is an example of a
specimen that may not be ready for analysis due to the particle
size effect grinding and pressing can reduce the particle size
effect.
The same picture can also be used to show the mineralogical
effect of the sample, this time it is assumed that both the
green and blue particles represent silicon dioxide but here they
are in two phases of crystal structures. There are many
structures of silicon dioxide for this example of alpha course
and beta course is used these two structures have different
physical properties which may lead to different sensitivities,
this mineralogical effect is small compared to the particle size
effect, grinding and pressing can reduce the particle size effect
but the only way of eliminating the mineralogical effect is
through creating fusion beads, if fusion beads aren’t an option
it is important to make sure that the standards used for
calibration are made up of a similar structure as the actual
unknown. These standards may sometimes be difficult to
acquire so that is why it’s always a good idea to have a backup
set of standards that are actual production samples which are
measured by a different form of analysis this will give you
more confidence in your results.
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Grinding plays a vital role in reducing the effects of particle
size, grinding works by making all the particles in the specimen
a consistent size the picture here shows Ring & Puck mill in
general these mills can be made of hardened steel, agate or
tungsten carbide the choice of material will depend on cost
and type of contamination you potentially introduce to the
specimen. Tungsten carbide mills for example have been
shown to reduce cobalt at a five PPM level to a 7g silicatan
sample to four minutes of grinding. Grinding times will also
vary sample to sample, the testing should be done in order tosee if the grinding time shows them as adequate. Grinding time
can be increased in specified intervals and retested; the
optimum grinding time can be timed when the analysis does
not change significantly when more grinding time is added.
Higher samples will generally require more grinding time, a
grinding time of 5 minutes is typical for raw mill cement ad
finish cement in 4 minutes is typical for silica sample. The
following two slides show a real world example of the impact
of grinding.
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Here we have 12 samples of fertilizer and three specimens of
each, the samples were received in a loose powder form, we
can see that the average standard deviation for these 10
measurements was .228%.
This slide shows the same 12 samples after being ground in a
Tungsten Carbide Puck Mill, 20g of sample weighed up and the
samples were grounded for 2 minutes, after grinding the
average standard deviation dropped down to .171% that’s an
improvement in the repeat ability of 25%. After the sample is
ground to the appropriate size it can be measured as is or it
can be pressed into a pellet. Pressing pellets generally gives
better results than running a loose powder.
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Pellets are generally pressed at 15 to 20 tonnes per square
inch, which produces a flat smooth, dense sample for testing
once the pellets are held at this pressure for about 30 seconds
the pellets can be carefully removed. The pellets should now
be around 3mm thick, if the sample is thinner than this
consider using more sample the opposite is true if the sample
is much thicker than 3mm again consistency is key so the
sample needs to be weighed before pressing.
When creating a pressed pellet sometimes a Binder is used.
Binder holds the material together to make a more resilient
pellet. Binders are typically free of contaminating element
because it contains a very light matrix, elements that can’t be
detected by XRF.
The binder needs to be stable under a radiation and backing
conditions. The different types of binders can vary from liquid
to solid, starch and cellulose are some common examples of a
type of binder material. Since binder contains such a light
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matrix it usually makes little difference in the absorption of x-
rays in a sample containing the average atomic weight matrix
it can however increase the background due to scattering
which may reduce sensitivity to a light element this is why it’s
important to only use the minimum required amount of binder
if any.
different types of xrf binder
When making a press pellet one should use about 7g of sample
the binder should be added after the initial grinding and then
ground for an additional 30 seconds, this two-step method of
grinding was proved to produce highly resilient pellets.Agglomeration or caking may occur if the binder is added in
the beginning of the process, agglomeration can also occur if
the incorrect amounts of binder is added to the sample, the
correct amount of binder should be about 5 to 10% of the
sample weight, it is extremely important to use the same
amount of binder in each standard for calibration and in each
unknown any deviation from this ratio may decrease accuracy
for analysis. Here one can see some pictures of the typical
setup for pressing pellets, on the right there is the Dye set thesample is placed in between the two discs then the plungers is
inserted on top and the whole assembly is carefully placed in
the press.
Here one can press and hold the sample at 20 tons of force for
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about 30 seconds to reveal a smooth pellet the dye must be
cleaned thoroughly between each pellet to reduce
contamination and for ease of operation. The dye can be
cleaned with plastic dish washing source, do not use anything
abrasive or metal for cleaning as this can harm the dye surface
and affect the consistency of the pellets. As discussed in order
to reduce the effects of in-homogeneity and particle size effect
you can grind and press the pellet.
When the accuracy and precision of results is most important
consider making Fusion Beads. The beads are made of a small
ratio of sample to flux, a ratio is critical to analysis and willaffect both the speed of the reaction and the way the sample is
absorbed by the flux. The ratio sample to flux can range
dramatically but the first and a commonly proposed method
uses a ratio of 1 to 100.
The different thing is, is inside the sample alpha and beta
course if we go back to our previous example will undergo a
chemical reaction to create a more uniformed or completely
non-crystalline glass white structure that is ideal for analysis.The chemical reaction involves heating the sample and flux to
a temperature of 800 to 1000°C in a crucible. Crucible material
can also vary but then an alloy of platinum plus 3 % gold is
hardly wetted, this reduces cleaning time and contamination.
Taking fusion beads leads to better accuracy and precision over
the pressed pellet but in should be mentioned that since the
sample is diluted the detection of trace elements may not be
possible.
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Both products are commonly embedded with sample spinner in
order to rotate the sample during analysis, the sample spinner
is used when measuring powder, pellet, or fuse bead samples.
The sample spinner effectively increases the spot size as well
as helps to minimize the effects of in-homogeneity in the
sample service, once the calibration is created the sample
spinner is setup to run automatically.
XRF is essentially a comparative method so it is very important
that standards of the unknown are measured in a consistent
way. The direct analysis of a sample is certainly possible and
easy to perform especially with a handheld XRF, using ahandheld has its advantages as it eases the load on the lab and
also leads to quicker decisions. Measuring samples in the field
will often require some sample prep, this type of sample prep
can be as simple as ensuring that you are measuring on a flat
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smooth surface each time it is also important to measure many
test points gathering enough data for accurate results, looking
at the sample of rocks one can see the different test points in
an average of four measurements will certainly give the analyst
a better picture of what is in the bulk material over a single
measurement.
Drill core analysis and mine mapping are commonly done using
handheld analyzers. Miners can use the data collected to
determine how deep to dig or to find the next location for a
potential expansion of the mine. FP calibrations are used when
little sample prep is required like in the rock and core sample
seen here. The option to create Empirical calibrations with
more sample preparation techniques is available with more
accuracy or a lower limit of detection is needed. Grindingsamples, pressing pellets or creating fusion beads will all be
potential options for this type of material.
The X-met analyzer also has the ability to positively identify
different alloy. There is an extensive grade library within the
instrument and the operator has the choice of using alloymode which automatically selects from a list of stored
calibrations, some of the alloys include nickel, cobalt,
aluminum, stainless steel and many others. Alloy mode selects
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simple FP and stored empirical calibrations which are traceable
back to standards. Sample preparation for these types of
metals are simple; sanding, polishing, grinding and cutting can
all be done with the ultimate goal of producing a flat smooth
homogenous surface without contamination this is most
important when measuring aluminum alloys since this is a light
element.
When measuring coat weight on paper it is important to use a
paper stage. One typical calibration is silicon on paper which is
used as a relief liner for stickers and labels. The paper stage is
made up of metal which works to reduce the background by
providing a heavy matrix to absorb the x-rays; the paper is
placed on a paper stage with the coated side out so that itfaces the detector and x-ray tube. It is it important that the
same base paper be used for all standards and a blank or
sample without any coat weight be used as an instrument
correction, this way if the base paper does change a new blank
can be cut and measured to prep for the difference, usually
seen as a difference in background.
When a large amount of fluid is available for analysis and the
concentration level is too small to quantify certain
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concentration techniques can be done. The nuclear power
industry uses filters like these to test for soluble and insoluble
metals in the coolant water this technique has also been used
to extract and test for zinc and led an air. Pour and dilute liquid
samples into a standard liquid power cup will not work unless
they are detectable amount of analytes but by using filters
these analytes can be concentrated and easily detected. the
concentration of analytes in the fluid can then be backcalculated by using the total volume of liquid passed through
the filter. In order to use this calculation it is assumed that the
entire sample is excited because it is contained in a thin film.
Since most oils contain a high enough concentration to
measure directly they can be analyzed simply by pouring oil in
a sample cup prepared with sample film, different factors can
affect the accuracy of the oil analysis. Oils will have a matrix
made of light elements like carbon and hydrogen which tend
to scatter x-rays and produce more background which reduces
the sensitivity, these effects can be exaggerated when the
viscosity or density of this oil matrix is varied, there are
corrections that can be applied to compensate for this changein back ground but it is important to make sure that the
standards being used for calibration are similar to the actual
unknown samples. Care should also be taken to shape the
standards or measure the samples quickly to avoid settling.
Another example of a matrix effect can be illustrated by
measuring used and new oils in the same calibration. A new oil
will have unused fuel additives but a used oil will have the
additive, plus decomposition product in small metal particles,
in order to prepare this used oil sample for analysis it may be
required to filter the oil or user a center fuse to remove any
solids. Matrix effects can also be reduced by diluting the
sample but this may reduce the detection of trace element
these sample film is also required and does reduce sensitivity.
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Sample film is another important part of sample prep for
liquids and powders. Sample film is a transparent layer of
protection needed to shield the detector and x-ray tube from
the sample; the sample film needs to be cleaned and free of
contamination. In the oil industry this is especially important
as customers are usually dealing with smaller concentration in
tight limits, the sample film also needs to be robust and
resistant to petrol chemicals and solvents, and all these factors
are weighed to determine the correct sample film for the
application.
These are some of the examples of the films that are most
common; Poly 4 is the highest transmission film that is used, it
is important in the wear metal application when measuring
light elements like sodium, magnesium or aluminum. Theseelements are found in PPM levels in oils are especially
sensitive to film tightness and film thickness. When a thick film
is used it absorbs more count which leads to an instrument
that is not able to measure low levels. Mylar is a robust film
that is used for the 4th row elements in which the cleanliness
and thickness of the film are less important, sometimes film
has special properties that are more conducive to better
analysis than others. Poly M is used for sulfur determination
because there is no contamination from silicon. Silicon cannegatively affect the results due to the proximity of sulfur on
the periodic table.
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This just fairs as an illustration for the different types of filmone can see that after about 6keV which is iron, the type of
film used really doesn’t matter at all after 6keV all the film
type shown here have about the same transmission. The type
of film used is most important around 1keV which is the
sodium level and one can see a very big difference from
Kapton film shown in black and the polycarbonate film shown
in orange. The transmission percentage is about 60% at 1keV
for Polycarbonate and there is barely any transmission at all for
the Kapton of the same energy level.
When making a sample cup for liquid or powder analysis
generally a second layer of film is used this secondary window
is used as a backup in case there is any leak from the main cup.
The main cup is then placed inside the secondary cup, it is
important that the film on both cups be free of any wrinkles or
contamination, once the sample is measured the inner of the
main cup can be disposed of and replaced with a new
disposable inner. The film on the secondary cup must only be
changed if contaminated or wrinkled.
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This is how the liquid powder cup sits in the sample carouselthere is a thin line about half way up the inner, even though
the analyze volume is much thinner than this disc it is
important to be consistent when filling the cup because of the
packing density and other factors. Here we see that the main
cup is within the secondary window and this is placed inside
the sample carousel. The x-rays have to travel through two
layers of film so it is even more crucial that the correct film be
used.
This diagram shows the sample preparation techniques
covered in this presentation, there are many factors when
deciding on the right amount of sample preparation for your
needs. You must weigh out costs of the equipment and
materials with the accuracy desired. If you are new to using
XRF start with less sample prep and move on to more
advanced if your application is more critical. Thank you for
taking the time to attend this webinar. I will now hand it over
to Jordan so that he can answer or he and I can answer some
questions.
When do we need to use XRF Sample Prep Binders? Binder can
be used when the sample is soft or binds together more easily,
so gypsum might be an example of a sample type that you
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wouldn’t have to use binder, typically with Lax tone or line
samples we do use binder and you can also tell because when
you create the press pellet if it does crack or the surface does
flake off that will certainly impact analysis and we try to
minimize that.
Can you use more samples than 7g for making pellets?
What’s the maximum amount of this that you can use, and is it
dependent on the pellet size?
It certainly is dependent on the pellet size; you basically want
to strive for a pellet that is 3mm thick. So after you grind the
sample and you press the pellets you can then see how thick
the pellet is an estimate how much more sample you need to
add so that the pellet can become 3mm thick.
What is a type XRF standard? Type standard is any sample that
can be used to increase the accuracy of your results, so after
you have a calibration line you use the type standard to
measure a sample that might not be measuring accurately. So
an example would be if we set up a calibration for oils and we
were typical running a 5W30 oil and it ran accurately, if you
put a different type of oil say a 10 W30 or transmission fluid it
might not run accurately because now you’re comparing two
types of oils that may have a different weight or viscosity, so
you use a type standard to adjust for that difference.
What causes background in the spectrum? Background is
generally caused by; we call them to tube line, so basically
energy from the x-ray tube, there’s a filament and that causes
tube lines in backgrounds but background is also just caused
by constant scattering of white elements. So any time you
measure a sample containing a white matrix, like oil which is a
hydrocarbon or plastic it will generally produce more
background.
When measuring light elements such as aluminum and silicon
is there of big difference between using a fine powder
compared to a pressed pellet? There is a pretty big difference, I
don’t know if I could quantify that difference but generally for
the light elements press pellets will give you better accuracy
over a loose powder and that’s because it’s much more dense
and you just have better transmission.
When you’re measuring sulfur and oil with a thicker film such
as Mylar how well could the limit of detection be for sulfur,
using a thicker film compared to let say poly 4? Around five
PPM might be your limit of detection.
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Related Mineral Processing Articles:
By David Michaud | September 29th, 2015 | Categories: Tools of a Metallurgist | 0
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Laboratory Methods of
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Sample Preparation
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Sample Preparation
Flowchart
Preparation of
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