Smart sensors for BTEX
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Transcript of Smart sensors for BTEX
The Use of Smart Materials with Infrared Spectrometry for
Determination of hydrocarbons Fiona Regan National Centre for Sensor Research (NCSR) School of Chemical Sciences Dublin City University
Outline
Sensors and their applications Principles of ATR-FTIR spectroscopy Examples of materials for sensing Analytes determined using polymer-based
sensors
Sensor applications
Environmental Halogenated hydrocarbons Petroleum hydrocarbons Pesticides Pharmaceuticals
ATR spectroscopy
Infrared light propagating in a crystal of high refractive index is internally reflected.
Some of the light penetrates into the sample region outside the crystal in the form of an evanescent wave.
Analyte absorption spectra can therefore be recorded
Principle:
Evanescent wave
ZnSe crystal
Radiation from IR source To
detector
CHC
Evanescent wave
Aqueous phase containing CHC
~5 µm
Background
Mid-infrared spectroscopy is a well established and powerful technique for off-line analysis.
It relies on the IR absorption characteristics of many chemicals in the “finger print” region of the electromagnetic spectrum.
IR analysis is limited in aqueous systems due to strong water absorption.
ATR spectroscopy can overcome many of the limits of MIR spectroscopy in aqueous solution.
Evanescent field sensing (EFS) EFS with optical fibres is an extension of the
established spectroscopic measurement ATR. Routinely applied to the measurement of aqueous
systems / analytes. IR radiation is coupled to an ATR element (fibre/
crystal) which is non-absorbing and has a higher refractive index than the surrounding medium.
The medium is a thin film or the absorbing analyte.
Polymer-ATR spectroscopy Why use a polymer?
Removal of background water absorption To enable the detection of weaker signals at very low
concentrations of analyte. Overcome by coating the internal reflection element
(crystal or fibre) with a hydrophobic polymer. The polymer also serves to enrich the analytes within
the penetration depth.
Role of the polymer
PIB film
ZnSe crystal From IR source
To detector
CHC
Evanescent wave
Aqueous phase
Penetration depth
Fibre optic evanescent wave sensor - FEWS
Polymer selection criteria
No, or only weak, intrinsic polymeric IR bands in the region of interest;
Substances to be analysed must be reversibly absorbed in the film;
The time constant for the enrichment process should be low; The polymer should be easily prepared and be chemically
inert with respect to the analyte components; The polymer material must be resistant against water and
organic compounds; Must adhere well to the internal reflection element.
Example 1: Teflon AF
window
Characteristic C-Cl IR bands
Compound Max λTCE 935 cm-1
TeCE 913 cm-1
Cf 767cm-1
CB 740 cm-1
1,2-DCB 748 cm-1
1,2,4-TCB 812 cm-1
Simultaneous determination using Teflon film
Regan et al. Vibrational spectroscopy 14 (1997) 239-246
Teflon sensor reproducibility
Limit of detection- TeCE 250 ppb
0
50E-05
.001
950 900 850 800 750
Abs.
Wavenumbers cm-1
913 cm-1
FEWS measurement, 500 co-added scans
FEWS - Simultaneous analysis of 6 chlorinated hydrocarbons
0
.005
.01
950 900 850 800 750
Wavenumbers cm-1
Abs
TCE
TeCE
TCE TCB TeCE
Cf
DCB CB
10 min enrichment time, 32 scans, 60 ppm each standard.
Example 2: Plasticised PVC
Pesticide Determinations
Walsh et al. Analyst 121 (1996) 789-792
Plasticiser types
A. Adipic acid derivatives B. Azelaic acid derivatives C. Epoxy derivatives D. Lauric acid derivatives E. Mellitates F. Palmitic acid derivatives G. Phthalic acid derivatives H. Sebaic acid derivatives I. Stearic acid derivatives J.Oleic acid derivatives K. Linoleic acid derivatives L. Isophthalic acid derivatives M. Isobutyrate derivative
BTEX contamination
Determination of BTEX compounds
Example 3: Gas-phase studies
Multi-component analysis by sparging
Sparging @ 50oC, 0.02 L/min, 6 min enrichment. (Concentrations from 2-12 ppm)
Investigation of reproducibility using a 2% PIB film
0
0.05
0.1
0.15
0.2
0.25
0 20 40 60 80 100 120 140 160
Time (seconds)
Abso
rban
ce (A
U)
MCB
CF
TeCE
Sparging @ 50oC, 0.02 L/min. 50 ppm each.
Analysis of solvent residues in pharmaceuticals using sparging
Chloroform
Tablet sample batch analysis for chloroform residues Tablets crushed, dissolved in water, sparged
Sparging @ 18oC, 1 L/min
Outcomes & Potential
Basic research project è applied technologies
Novel mater ia ls f o r en r i chment o f hydrocarbon species present in industrial samples
Laboratory-based infrared ATR technology and this will lead to development of a laboratory-based fibre-optic or a planar waveguide approach to developing è novel sensing device.
Materials may also find uses for application to other parameters.
The area of materials development for sensors is under exploited and is an area of great potential.
Materials can be designed for particular analytes e.g. using sol-gels or molecular imprinting where even greater potential of selectivity can be realized.
Develop a modular system for monitoring water quality, The materials è probes for monitoring solvent
residues They may also find uses in occupational monitoring The cost of the materials will be small. The complexity of the devices to which they are
applied will influence the cost of the final sensor. The sensor design in the longer term will be determined
by the need of the user. Alarm level devices may be desirable è thus complex
instruments may not be necessary.
Acknowledgements
Fiona Walsh Kathleen O’Malley Eoin O’Donoghue