The challenges of field sensors for trace metal detection

Chris SearleProduct Manager

Trace2o Ltd

What is a heavy metal and why is it necessary to determine concentration?

• Tends to be categorised as Metallic elements with either;

high atomic number, high atomic weight or high density

• Many are toxic even at very low concentrations, for example;

arsenic, cadmium, mercury, chromium, thallium, lead

• Heavy metals bio-accumulate in the food chain – plants, fish, aquatic life, animals

• Accumulation in the body causes serious health problems

Human Health

Some of the symptoms and

diseases caused by heavy

metal poisoning.

Mental ConfusionDepressionParkinson

MS Alzheimer Headaches

FatigueDiabetes

Prostate Problem

IrritabilityInflammation

Thyroid Problem

The monitoring and clean-up of metal

pollution is thus very important.

Sources of HM Contamination

• Mining

• Smelting

• Military activities

• Effluent discharge

How are Heavy Metals traditionally analysed?

• Laboratory methods

• AAS

• ICP-MS

• ICP-OES

• Expensive instruments

• Outsource? But long lab turnaround time…

• Samples need to be collected, stabilised, transported

Atomic Absorption

• Flame AAS

• Cheaper instrument, but poor LODs

• As – 150ppb, Hg – 300ppb

• Single element technique

• Seconds for determination

• Approx £15-20k system cost

• Cannot be left unattended!

Atomic Absorption

• Graphite furnace AAS

• Better LODs but more expensive

• Approx £20-40k system cost

• Still a single element technique

• Minutes for determination

ICP-OES and ICP-MS

• ICP-OES – higher LODs than GFAA

• ICP-MS – VERY low LOD

• Multi element technique

• Minutes for determination

• ICP-OES - £40-70k system cost

• ICP-MS - £95-150k system cost

Portable techniques

• Photometry

• Ion-selective electrodes

• X-ray fluorescence

• DNA-based biosensors

• Stripping voltammetry

Photometry

• Limited range of parameters

• Complicated sample prep

• Affected by turbidity

• Very high LODs

• But – low cost

• Reliable

Ion Selective Electrodes

• Limited range of parameters

• Expensive to replace

• Huge interference

• Unreliable

• But – unaffectedby turbidity

X-Ray Fluorescence

• Only for suspended fraction in aqueous samples

• Useless for dissolved components

• Expensive

• H&S concerns

• But – easy, fast

• Non-destructive

DNA-based biosensors

• Limited range of parameters

• Limited detection ranges

• Expensive cost per test

• May require filtration

• But – easy to use

Stripping voltammetry

• Proven technique (since 1922)

• Wide detection range

• Wide range of parameters

• Low LODs

• Low cost per test

• Little interference

• Turbidity not a problem

Principle of Operation

• Stripping Voltammetry (Anodic and Cathodic)

• Electrochemical Cell

– Comprises 3 Electrodes (Working, Ref, Cnt/Aux)

– Temperature Measurement

– High precision Stirrer

• Automatically applied potentials

Electrochemical CellA

V

CE REWE

• Working electrode (WE) is where the cell reaction takes place

• Reference Electrode (R) provides reference point for cell potential

• Counter Electrode (C) electrochemically inert, allows current to flow

Redox Potentials

• Utilise known redox potentials of metal ions to selectively reduce/oxidise analytesof interest.

• Measuring current during oxidation/reduction allows us to measure concentration.

Zn Cd Pb Cu

Potential

Anodic Stripping Voltammetry (ASV)

CdX → Cd2+ + X +2e-Cd2++ X + 2e- → CdX

WE

Cd2+

Depositionstep

Cd2+ Cd2+

WE

Cd2+

Strippingstep

Cd2+ Cd2+

Reduction Oxidation

1. A potential lower (more –ve) than redox potential is applied to the cell, the analyte is reduced and deposited onto the electrode

2. The potential is then increased anodically, above the redox potential (more +ve), which oxidises the analyte and strips them from the electrode surface.

3. As analyte is oxidised, it gives off electrons and an increase in electrical current occurs.

4. The measured current provides a value which directly correlates to the concentration of metal in the solution.

A

V

CE REWE

• Plot of recorded data reveals a peak.

• The peak height is proportional to the concentration of ions in solution

0

2

4

6

8

10

12

-1.2 -1 -0.8 -0.6 -0.4 -0.2 0

Voltammogram

V

I

Interpreting the data

Typical Voltammogram for Multiple standard additions of

Cd & Pb

CdPb

• Variety of possible working electrode options

• Hanging mercury drop electrode (HMDE)

• Solid metal electrode

• Carbon-based (glassy carbon, graphite)

• Modified electrodes

Working electrode selection

• Hanging mercury drop electrode (HMDE)

• Static but renewable surface

• Popular for lab work

• Extremely reliable

• Unsuitable for field use

• Liquid mercury handling

• H&S concerns

HMDE

• Silver, gold, platinum

• Good for ASV, less useful for CSV

• Adsorbed hydrogen layers

• Surface oxides

• Require electrochemicalcleaning

• Very expensive substrate

Solid metal electrode

• Many possibilities

• Pyrolytic graphite electrode (PGE), edge plane vs. basal plane

• Carbon paste electrode

• Carbon composite electrodes

• Glassy carbon electrode (GCE)

• Very low cost

• Soft surface – easily renewable

Carbon electrodes

• Bare electrodes are not always ideal surfaces

• GCE can be modified with metal films

• Hg film, Au film, Bi film

• Can also be modified with nanomaterials

• Carbon nanotubes

• Gold nanoparticles

Modified electrodes

• Developed to provide low cost field analysis• Traditionally Lab based Method• Uses well established voltammetric technique• Comparable with ICP and AAS• Targeted at Natural water samples • No pre-treatment required• More complex samples can be analysed

after pre-treatment• Glassy carbon electrodes modified with Hg

or Au film

Introduction to Metalyser

Metalyser: What & How?

Mounted within the handheld unit

» Potentiostat

» Data Processor

The Metalyser uses the technique of Stripping Voltammetry and has 2 main components:

- the Sonde

- the Handheld unit

Mounted within the Sonde

» Electrodes

» Temperature probe

» Stirrer

• Response varies considerably with temperature, so must be measured• Stirrer used partly to apply convective conditions, partly to ensure homogeneity

of the solution.• Applied potentials and stirrer motor automatically controlled

• Low cost Analysis• Linear data gives confidence• Rapid results• Portable field analyser• Analysis possible without sample prep.• No fume extraction required• False results easily identifiable• Turbid samples can be analysed• Ease of use

Advantages of Metalyser

Limitations of Metalyser

• Interference from Organics

• Interference from other metals

• Relatively limited range of metals compared with laboratory techniques

Factory Calibration ?

• Response is linear but sensitivity will change for different sample types due to sample matrix effects (pH, conductivity, temperature)

• Electrode surface also affects sensitivity• Thickness of the film• Sensitivity reduces with number of samples and

time.• Can calibrate on one sample and test several

consecutive samples from the same source• Most accurate method is standard addition

– Effectively calibrated for every sample

HM1000 Portable

• Most popular instrument

• Automatic analysis

• Step-by-step instructions

HM2000 Deluxe

• 2 instruments – voltammetric and photometric

• Increased concentration range from ppb to ppm

• Increased metal range (heavy metals and non-heavy metals)

HM3000 Field Pro• Advancement on the HM1000 instrument with

tablet PC

• 2 boxes (instrument and PC and reagent box)

• Lower detection limits due to ability to increase deposition times and perform linear regression on multiple data points.

• Sonde clamp included for laboratory-type analysis in the field

HM4000 Metalyser - Soils

• Field-based analysis of heavymetals in soils at mg/Kg levels

• Fast, reliable results

• Innovative portable digestion apparatus

• Robust, dustproof, waterproof, IP67-rated

• Simple user-friendly reagent system

• Affordable alternative to ICP-AES and XRF techniques

HM5000 Benchtop

• Compact, lightweight benchtop trace metal analyser

• R&D usage

• Powerful bespoke control and analysis software

• Wireless connectivity

• Compact variable-volume electrochemical cell

• Integrated stirrer and temperature sensor

• Fully computer-controlled

HM5100 Benchtop

• Benchtop analyser with pre-installed tests• Designed for routine analysis• Remote control via Bluetooth (PC required)• Complete with Windows control software• Linear regression• Capable of running EPA methods

• Could be used as a replacement AA instrument –cheaper to buy, cheaper to install and cheaper to run.

UV Digester HMUV1825

• Dual wavelength digester for rapid break down of organics, which improves the sensitivity and accuracy of the results

• Timer allows for increased efficiency in the lab allowing unattended digestion

• Available as benchtop or portable model

Questions ?