The challenges of field sensors for trace metal detection Chris Searle Product 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. Irritability Inflammation Thyroid Problem Mental Confusion Depression Parkinson MS Alzheimer Headaches Fatigue Diabetes Prostate 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
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 Atomic Absorption
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 ICP-OES and ICP-MS
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 unaffected by 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 Cell A V CE WE RE 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 analytes of interest. Measuring current during oxidation/reduction allows us to measure concentration. Zn Cd Pb Cu Potential
Anodic Stripping Voltammetry (ASV) A Deposition step Stripping step V WE WE CE WE RE Cd 2+ Cd 2+ Cd2+ Cd 2+ Cd 2+ Cd 2+ Cd 2+ + X + 2e - CdX Reduction CdX Cd 2+ + X +2e - 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.
Interpreting the data Plot of recorded data reveals a peak. The peak height is proportional to the concentration of ions in solution Voltammogram 12 10 I 8 6 4 2 0-1.2-1 -0.8-0.6-0.4-0.2 0 V
Typical Voltammogram for Multiple standard additions of Cd & Pb Cd Pb
Working electrode selection Variety of possible working electrode options Hanging mercury drop electrode (HMDE) Solid metal electrode Carbon-based (glassy carbon, graphite) Modified electrodes
HMDE 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
Silver, gold, platinum Good for ASV, less useful for CSV Adsorbed hydrogen layers Surface oxides Require electrochemical cleaning 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 Carbon electrodes Soft surface easily renewable
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
Introduction to Metalyser 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
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 Metalyser: What & How? Mounted within the handheld unit» Potentiostat» Data Processor 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
Advantages of Metalyser 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
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 nonheavy 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
Field-based analysis of heavy metals in soils at mg/kg levels Fast, reliable results Innovative portable digestion apparatus HM4000 Metalyser - Soils Robust, dustproof, waterproof, IP67- rated Simple user-friendly reagent system Affordable alternative to ICP-AES and XRF techniques
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 HM5000 Benchtop
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?