Liquid Waste Analysis

Garry Smith, XRF Application Specialist

SciMed XRF, a division of Scientific and

Medical Products Ltd

About Us

• SciMed represent Rigaku (RESE and

ART) and Seiko (SIINT) XRF ranges

• Over 20 years history of working with XRF

• A wide range of UK installations across

many types of applications

Summary

• What are the difficulties with liquid waste

samples and how does the sample

introduce error into the test result?

• Examples of errors induced on analysis:

– Suspended solids and settling

– Immiscible liquids and separation

• What are the practical solutions?

What are the difficulties?

• Liquid waste samples are rarely “ideal” as

they are rarely homogeneous.

• May contain solids which settle over time.

• May be mixtures of immiscible liquids

which separate over time.

An Example

Example photos of multiphase sample before,

during, and after separation and settling.

Why is this a Problem?

• For any analysis, the measured sample

must be representative of the bulk material

being tested.

• With XRF we are not always measuring

everything in the sample cell.

• If components separate during analysis

they may not be measured accurately.

Critical Depth

• Fluorescent photons from analytes are re-absorbed by the sample.

• Absorption follows the Beer-Lambert law: I = I0e

-μρx

where: μ = mass absorption coefficient ρ = matrix density x = path length

Critical Depth

• Critical Depth is defined as the depth of sample from which 99% of the fluorescent photons are re-absorbed.

• To calculate we can re-arrange Beer-Lambert to express path length, x, where:

Io = 100 I

Critical Depth

• The equation simplifies to:

Critical depth, x(mm) = 46.605 μρ

• Critical depth increases with increasing photon energy.

• Critical depth decreases with increasing average atomic number and density of the sample.

Critical Depth

Liquid Sample in Cell

Incident X-rays

Cl

Cl

Fluorescent photon to detector

Fluorescent photon re-

absorbed by sample

Critical depth of Cl Kα

Critical depth of Zn Kα

Zn

Critical Depth

Line Energy (keV) Typical Critical Depth (mm)

Oil Water

S Kα 2.31 0.36 0.12

Cl Kα 2.62 0.50 0.17

Cr Kα 5.41 4.33 1.44

Zn Kα 8.64 16.3 5.42

Pb Lα 10.55 27.9 9.74

Typical critical depths of selected XRF lines in oil and water

Example 1 – Effect on Peak Intensity of Analyte by

Settling of Sediment (Analyte in Solution)

Overlay view of spectra for S in heavily sedimented oil measured at 2

minute intervals, showing attenuation of peak as sediment settles. The

analyte is concentrated in the solution.

Example 1 – Effect on Peak Intensity of Analyte by

Settling of Sediment (Analyte in Solution)(cont.)

Overlay view of corresponding spectra for Zn in in the same sample.

However there is virtually no change in intensity with time. The higher

energy of Zn Kα has greater critical depth in the sample. Intensity is

less variable with distribution throughout sample.

Example 1 – Effect on Calculated Concentration of

Analytes by Settling of Sediment (Analyte in

Solution)

150

160

170

180

190

200

210

220

230

240

250

3000

3500

4000

4500

5000

5500

6000

0 2 4 6 8 10 12 14 16 18

Me

as

ure

d Z

n (

pp

m)

Me

as

ure

d S

(p

pm

)

Time after mixing (min)

Measured S and Zn Concentration vs. Time for Waste Oil

Example 2 – Effect on Peak Intensity of Analytes

by Partitioning of Liquids (Aqueous Analyte)

Overlay view of spectra for Cl measured at 2 minute intervals after

mixing, showing enhancement of measured peak as aqueous

component containing chloride separates to bottom of liquid cell.

Example 2 – Effect on Peak Intensity of Analytes

by Partitioning of Liquids (Oil Analyte)

Overlay view of corresponding spectra for Cr measurement. Cr is

concentrated into the oil component and is attenuated as this layer

separates to the top of the liquid cell.

Example 2 – Effect on Calculated Concentration of

Analytes by Partitioning of Liquids (Analytes in Both

Phases)

150

170

190

210

230

250

45000

50000

55000

60000

65000

70000

75000

80000

85000

90000

0 2 4 6 8 10 12 14

Me

as

ure

d C

r (p

pm

)

Me

as

ure

d C

l (p

pm

)

Time after mixing (min)

Measured Cl (aqueous), and Cr (oil) vs. Time in Oil / Aqueous Mixture

Measured Cl (ppm) Measured Cr (ppm)

Summary

• Effect on measured intensity may be attenuation or enhancement depending on circumstances.

• Rate is dependent on the specific sample, e.g. particle size, viscosity, etc.

• Amount of error varies with analyte – higher energy lines (usually heavier elements) affected less than lower energy lines (usually lighter elements).

In the case of real samples, these effects

are in practice impossible to predict.

What are the Solutions?

• Ensure analysis is completed before

phases separate (approximate analysis)

• Separate phases to remove unwanted

components or analyse separately

• Stop phases from separating

– Use solid binders to immobilise components

Aqueous Samples

• Aqueous samples containing solids can be dealt with quite easily:

– Allow sample to settle or centrifuge sample and pipette supernatant liquid (dissolved components only).

– Filter sample and analyse separate components as required.

Note – can also be applied to organic liquids.

Immiscible Liquids

• Oil / hydrocarbon samples may contain

water.

• While it may be possible to use emulsifier

to stabilise, no universal solution for all

sample types.

• More robust solutions:

– Separation

– Immobilisation using binder

Separation of Liquids - Centrifuging

• Centrifuging can provide a simple approach to separating aqueous component from oils and hydrocarbons

• Also removes solids

• Component required for analysis can simply be pipetted after centrifuging

Reference: ASTM D4294 – Sulphur in Petroleum and Petroleum Products by Energy Dispersive X-Ray Fluorescence Spectrometry.

Immobilisation Using Binder

• Separation of liquids or solids not always a

practical solution (especially for waste oils)

• Both immiscible liquids and solids can be

immobilised by mixing the sample with a

powder binder material

– Graphite powder

– Activated alumina

Graphite Powder

• Fine graphite powder provides an inert and spectrally pure carrier material for oil / hydrocarbon samples

• Typically mix 10g sample with 4-6g graphite to produce a slurry

• Transfer to liquid sample cell and measure

Reference: ASTM D5839 – Trace Element Analysis of Hazardous Waste Fuel by Energy Dispersive X-Ray Fluorescence Spectrometry.

Activated Alumina

• Activated alumina provides a stable carrier

material for aqueous / organic solvent samples

• Typically mix 5g sample with 15g activated

alumina to produce a slurry

• Transfer to liquid sample cell and measure

Reference: ASTM D6052-97 – Preparation and

Elemental Analysis of Liquid Hazardous Waste by

Energy Dispersive X-Ray Fluorescence.

In Closing…

• Multiphase liquid samples can introduce significant errors into analysis results if not treated appropriately

• These errors are a result of the potential for phases to separate during measurement and the varying critical depth of different analytes.

• A number of potential solutions to remove or neutralise sources of error. Standard methodology often incorporates these precautions.