Global Brands, Local Support

X-Ray Fluorescence Spectrometry

​

Energy-Dispersive vs Wavelength-Dispersive

​

John Austin, XRF Applications Specialist

Overview

​

• X-Ray Spectrometry – Overview
• Hardware:
• ED-XRF
• WD-XRF
• Typical Applications

​

​

3

Mechanism of Fluorescence

1. Incident X-ray photon overcomes inner shell electron binding energy

Inner shell electron ejected from atom (photoelectron)

K-series X-ray photons

L-series X-ray photons

hν

2. Inner shell electron ejected from orbital creating unstable intermediate ion

3. Outer shell electrons decay into vacant orbital, releasing potential energy as fluorescent X-ray photon

1

2

3

3

How does XRF Work?

• Spectrometer generates a beam of primary X-ray photons, which are directed at the sample
• Primary X-rays interact with the sample in a number of different ways
• Some of these interactions cause fluorescence of secondary X-ray photons which are characteristic of the element they originated from

Moseley’s Law - Illustration

Typical EDXRF Configuration - Direct

• Mechanically very simple with few moving parts and very short optical path
• Typically low power and low cost
• Filters help to reduce background, but signal : noise typically not as good as WDXRF or Indirect EDXRF
• Separation of different photon energies by detector gives moderate peak separation (resolution)

X-Ray Tube

Sample

Detector

peak

BG

Tube Filter

Typical EDXRF Configuration - Indirect

• Mechanically more complex with optical path longer than Direct EDXRF
• Typically moderate power and moderate cost
• Secondary excitation reduces background, giving excellent signal : noise ratio
• Separation of different photon energies by detector gives moderate peak separation (resolution)

X-Ray Tube

Sample

Detector

Secondary Targets

Typical WDXRF Configuration

• Mechanically complicated with many moving parts and longer optical path
• Usually higher power, higher cost systems
• Exceptionally low background noise, hence excellent signal : noise ratio
• Physical separation of different wavelengths results in excellent peak separation (resolution).

2θ angle

I

Analysing crystals

X-Ray Tube

Sample

Detector(s)

Focusing

peak

BG

Tube Filter

Hardware Summary

Analysing crystals

X-Ray Tube

Sample

Detector(s)

Focusing

X-Ray Tube

Sample

Detector

Secondary Targets

Sample

Detector

Direct ED

Indirect ED

Sequential WD

Tube Filter

Tube Filter

ED Detectors and Resolution

FWHH Mn Kα in eV

• Low Res: Gas filled Proportional counter

ca. 750 eV

• Medium Res: Si Pin Diode

ca. 240 eV

• Hi Res: Si(Li) Detector

ca. 150 eV + LN2

• Hi Res: Si Drift Detector (SDD)

ca. 150 eV

• WD: (for comparison)

ca. 20eV (depends on element & crystal)

EDXRF Spectrum

Example spectrum: 100ppm metals in oil showing relationship between atomic number and fluorescent energy for Ti Kα - Fe Kα.

WDXRF Analysing Crystals

WDXRF Analysing Crystals

WDXRF Analysing Crystals

Cr-Kβ1

Cr-Kβ1

Mn-Kα

Mn-Kα

Significant Cr overlap onto Mn-Kα peak!

Cr overlap eliminated

Trading off resolution and sensitivity

150 kcps net

75 kcps net

WDXRF Spectrum

Example spectrum: 100ppm metals in oil showing relationship between atomic number and fluorescent energy for V Kα - Zn Kα.

ED and WD XRF Spectra

Line Overlaps: Empirical Calibrations

Example: Mo Ll interference on P Kα

Note the Sample “HM 18”, containing 3.17% Mo and 0.008% P, falling completely out of the calibration curve.

Before correction:

​

• No linear fit between concentration and intensity for P due to line overlap from Mo Ll
• P cannot be accurately quantified in presence of varying Mo concentration

​

W_P = CI_P + D

​

Line Overlaps: Empirical Calibrations

Note: the line overlap coefficient should be negative. Since line overlap interference is always positive, the correction should always be a subtraction.

After correction:

​

• Error in result of P regressed against concentration of Mo to calculate line overlap correction coefficient
• All P data now fits linear regression

​

W_P = CI_P + D + (fLO_Mo,P x W_Mo)

The Impact of Resolution – ED vs WD

Energy-Dispersive XRF

​

• Lower cost
• Can be portable (hand held XRF)!
• Limited resolution
• Resolution still plenty good enough for a broad range of (simple) applications
• Limited light element sensitivity (typically Na-U)
• Simultaneous analysis of many components at once - good for e.g. rapid screening applications
• Flexible scatter correction approaches for estimation of light element content (e.g. ester content in oils).

Wavelength-Dispersive XRF

​

• Higher cost
• Excellent resolution
• Excels in determination of trace heavy elements (e.g. lanthanides) due to reduced overlaps
• More sensitive to light elements (as low as Be)
• Ability to achieve very high count rates due to higher tube power and selectivity – good for low measurment uncertainties and/or more rapid measurements
• Ultimate tool for flexibility – R&D applications

Some typical applications – ED vs WD

Energy-Dispersive XRF

​

• Screening of waste (e.g. oils for recycling)
• Field work (portable XRF used widely in e.g. metal scrapyards, geological surveys, RoSH testing, etc)
• Checking for trace level contaminants in simple matrices such as clean water, oil, sands
• Online process monitoring systems
• Small spot coating thickness analysers (gold platings on circuit boards, jewellery, etc)
• Often used as backup systems for a main WD
• ED-Specific Standards, e.g.
• ASTM D4294 - S in petroleum products
• ASTM D6052 – Liquid Hazardous Waste

​

Wavelength-Dispersive XRF

​

• High throughput environments (e.g. cement works) with short turnaround times
• Research and development
• Applications where overlaps pose serious issues (e.g. Ta and W in Ni superalloys, low P in metals, geological trace element analysis)
• Requirement to analyse light elements (e.g. Beryllium Copper, Boron in Glass)
• WD-Specific Standards, e.g.
• ASTM D2622 – S in petroleum products
• ASTM E2465 – Ni-Based Alloys
• IP593 – Fuels from Waste Mineral Oils

​

Instrumentation from SciMed

NEX DE & DE VS

Supermini 200

NEX QC range

Micro-Z ULS

Rigaku NEX XT and OL range

Bowman Micro-XRF Range

NEX CGII

ZSX Primus IVi

Simultix15

Simultaneous WD-XRF

Rigaku ED-XRF

Rigaku WD-XRF

Rigaku Online XRF

Rigaku NEX LS

XRF Sample Preparation from SciMed

www.scimed.co.uk