Ultrasound-Enhanced In-line
Raman Sensing of Solid Samples
in Liquid Matrices
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K. Wieland , H. Rasoulimehrabani , S. Tauber , C. Gasser , S. Radel , B. Lendl
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Institute of Chemical Technologies and Analytics, TU Wien, Getreidemarkt 9/164 UPA, 1060 Vienna, Austria
[email protected]
WORKING PRINCIPLE
INTRODUCTION
INTRODUCTION
Raman spectroscopy is a non-invasive method that offers analytespecific information. Compared to Infrared spectroscopic
techniques, it is less water sensitive and thus offers a very attractive
tool for in-line monitoring of aqueous systems in process analytical
technology (PAT). [1]
Ultrasonic standing wave fields have been used for particle
manipulation in ultrasound-enhanced attenuated total reflection
mid-infrared spectroscopy as in-line probing method in bioreactors.
The axial acoustic radiation force acting on the particles is applied to
push the analyte against the surface of the ATR element and thus into
the evanescent field. [2] Here, we go one step further by combining
the Raman probe with a custom-made ultrasound accessory to
improve the limit of detection (LOD) compared to conventional
Raman spectroscopy. Therefore, three different starch
concentrations in aqueous solution (0.1, 1.0 and 10.0 g/l) were
investigated and compared, respectively. Subsequently, the
crystallization process in H2O:C2H6O 80:20 solution as a more
complex chemical system was chosen for detailed investigation.
Raman in-line probe
(Kaiser Raman RXN1 Analyzer
785 nm)
Transducer
emitting sound
at approx. 2 MHz
Spectrometer, Detection Unit,
Data preprocessing
Raman sensor
Concentrated particles
in nodal points of the
ultrasonic standing waves
Particles in
liquid matrix
Raman sensor
Custom-made
ultrasound accessory
Transducer
STARCH PARTICLES IN AQUEOUS SOLUTION
OH
Conventional Raman Spectroscopy
O
H
O
H
H
α1
O
H
H
4
H
O
O
H
4
H
OH
H
O
α1
OH
H O
OH
O
H
4
O
H
O
H
OH
H
Raman spectrum of highly concentrated starch suspension
OH
H
α1
OH
H
α1
OH
H
H
H
4
OH
4
OH
Raman Shift [cm-1]
478
770
867
941
1083
1127
1263
1340
1381
1461
2912
H
O
OH
H
H
75 % amylose
O
H
O
OH
H
OH
OH
α1
O
OH
O
H
O
H
α1
H
H
O
4
H
4
OH
H
O
H
O
α1
OH
OH
OH
H
H
4
OH
H
O
H
O
α1
OH
H
4
OH
O
H
OH
H
H
OH
25 % amylopec n
O
α1
H
4
OH
O
H
H
α1
OH
OH
H
OH
O
H
OH
H
4
α1
O
H
OH
H
OH
OH
O
H
H
H
4
O
H
OH
H
O
H
α1
OH
H
4
O
H
O
O
6
OH
H
4
α1
OH
H
H
O
OH
O
α1
OH
H
H
4
O
H
OH
OH
H
α1
OH
H
O
Band assignment [3]
C-C-C bend; C-O torsion
C-C-O bend
C-C-H, C-O-C bend.
C-O-C, C-O-H bend; C-O str.
C-O, C-C str; C-O-H bend.
C-O, C-C str; C-O-H bend.
C-C-H, O-C-H, C-O-H bend.
CO str, O-C-H, C-O-H bend.
C-O-H bend.
CH, CH2, C-O-H bend.
CH str.
While there are hardly any Raman bands of starch visible in the spectra obtained with conventional Raman
spectroscopy (see figure above) the spectroscopic fingerprint of the starch particles are perfectly visible
once the ultrasound has been turned on (see figures below).
Ultrasound-Enhanced Raman Spectroscopy
CRYSTALLIZATION OF ACETAMINOPHEN
Conventional vs. Ultrasound-Enhanced Raman Spectroscopy
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US on
US off
The effect of US-Enhanced Raman spectroscopy
- 110 g/l Acetaminophen in H O:C H O 80:20
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2
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- Turning ultrasound on and off at constant temperature
- Single spectrum recorded every 5 seconds
Intensity changes with ultrasound on and off:
- Aromatic ring bending of Acetaminophen
- C-C-O stretching vibration of Ethanol
Financial support was provided by the Austrian research funding association (FFG) under the scope of the COMET program within the
research project „Industrial Methods for Process Analytical Chemistry – From Measurement Technologies to Information Systems
(imPACts)” (contract # 843546).
[1] Beer, T. R. M. De, Baeyens, W. R. G., Ouyang, J., Vervaet, C. & Remon, J. P. Raman spectroscopy as a process analytical technology tool for the understanding and the quantitative in-line monitoring of the homogenization process of a pharmaceutical suspension. 1137–1144 (2006). doi:10.1039/b605299a
[2] Koch, C. et al. Ultrasound-Enhanced Attenuated Total Re fl ection Mid-infrared Spectroscopy In-Line Probe: Acquisition of Cell Spectra in a Bioreactor. (2015). doi:10.1021/ac504126v
[3] Almeida, M. R., Alves, R. S., Nascimbem, L. B. L. R., Stephani, R., Poppi, R. J., & De Oliveira, L. F. C. (2010). Determination of amylose content in starch using Raman spectroscopy and multivariate calibration analysis. Analytical and Bioanalytical Chemistry, 397(7), 2693–2701. http://doi.org/10.1007/s00216-010-3566-2