XRF Sample Preparation MethodsProcedure - Mineral ProcessingMetallurgy ResearchTesting

8/17/2019 XRF Sample Preparation MethodsProcedure - Mineral ProcessingMetallurgy ResearchTesting

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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

https://www.911metallurgist.com/blog/wp-content/uploads/2015/09/making-an-xrf-sample-cip.pnghttps://www.911metallurgist.com/blog/wp-content/uploads/2015/09/xrf-sample-preparation-process-method.png


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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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By David Michaud | September 29th, 2015 | Categories: Tools of a Metallurgist | 0

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