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