QUANTUM CASCADE LASER FOR DETERMINATION OF SOLUTES IN LIQUID by vsb11259

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									                          QUANTUM CASCADE LASER FOR DETERMINATION OF SOLUTES
                                            IN LIQUID PHASE
                                                    S. Schaden1 (stefan.schaden@tuwien.ac.at), M. Haberkorn1, J.
                                                      Frank1, W. Ritter1, M. Koelhed2, B. Karlberg2 and B. Lendl1
                                                          1   Institute of Chemical Technologies and Analytics – Vienna University of Technology,
                                                                                     Getreidemarkt 9, Vienna, 1060, Austria
                                                               2   Department of Analytical Chemistry, Stockholm University, Stockholm, Sweden


                                                                                                                                                           INTRODUCTION
QCLs have the potential to be applied in industrial relevant process analyzers. While most sensing applications of QCLs so far have been shown in gas phase, QCLs may also be advantageously applied for
determination of solutes in liquid phase. Room temperature operation, small size and high spectral density make them ideal light sources for dedicated absorption measurements also in aqueous phase. Unfortunately,
their high cost is hindering their application in industrial relevant sensors despite of their capability to solve relevant analytical problems e.g. in the food industry. It is the aim of this contribution to highlight this fact
on the example of the direct determination of dissolved CO2 in beverages. Similar applications covering other analytes such as glucose, ethanol or artificial sweeteners could be targeted in a similar way. In the second
part of the contribution the use of QCL as a new detection principle in capillary electrophoresis is shown.


                                                                                                                                                EXPERIMENTAL SET-UP
In both experiments QCLs from Alpes Lasers where used. The beam was collimated and focused on a flow cell using off-axis parabolic                                                                                                  1321 1236
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                                                                                                                                                                                                                                                9852



mirrors. A photovoltaic MCT detector with a response time of 3,5 ns was used. The detector signal was recorded using a boxcar averager
                                                                                                                                                                                                                                    6468 4878   4668




and stored using a home made software. For the determination of CO2 the QCL was controlled by the devices from the Alpes Lasers                                                                                                                               Computer Interface                           Boxcar Averager
Starter kit. For the connection of capillary electrophoresis with QCL based absorption spectroscopy a laser driver from Laser Components
- specially adapted on the Alpes Lasers Laser housing - was used.
                                                                                                                                                                                                                                                                                      Cooling
The CO2 determination [1] a DFB-QCL lasing near room temperature at 4,29 µm and a 119 µm flow-cell equipped with CaF2 windows
                                                                                                                                                                                                                          Driving Units                                                           M        M             Detector
have been used. Standards of different CO2 concentrations were prepared by purging defined ratios of CO2 and nitrogen through a gas                                                                                       [Alpes Lasers or                                              LH
wash bottle at a controlled temperature (20 °C). The bottles were connected to the flow cell via an automated pump system. The whole                                                                                      Laser Components]

optical system was purged with nitrogen to remove any gaseous CO2 from the optical path.                                                                                                                                                                                                               F                 F......Flow Through Cell
                                                                                                                                                                                                                             Temperature Controll                                                                        LH.....Laser Housing
The application of a FP-QCL for absorption spectroscopy in capillary electrophoresis is also demonstrated here for the first time. To                                                                                                                                                                                    M.......Mirror
overcome the problem of total IR absorption of the fused-silica capillaries that are normally employed in CE separations, a micromachined                                                                                                              Acrylic Plastic Box                        M        M
IR-transparent flow cell was developed. The cell consisted of CaF2 plates separated by a polymer coating (SU-8). The flow channel was
about 2 mm long, 150 µm broad and with an optical path of 60 µm. The cell was placed in an in-house made cell holder with special design
to connect the capillaries with commercial O-rings to the cell. Results of a separation of 4 analytes are reported.




                                     CARBON DIOXID DETERMINATION                                                                                                                                 CAPILLARY ELECTROPHORESIS
                          SAMPLE PREPARATION                                                                                                                                                 OPTICAL SET-UP                                                                                            CE SET-UP
                                             Peristaltic Pump
                                                                                                                                           Wash bottles containing
                           Selection Valve
                                                                                    Flow Through Cell
                                                                                                                                     distilled      water         were
                                                                                                                                     thermostatically controlled at 20°                                                                                                                            Cell Holder with Cell
                                                                                                                                     C. Different mixtures of N2/CO2
                                                                                                                                     were fed to the wash bottles.
                                                                                                                                     According to the CO2 contend of                                                                                                                                           13-16 kV
                                                                        1321 1236
                                                                        6544 7895
                                                                                    3682
                                                                                    8962
                                                                                                                                     the gas phase, a certain
                                                                                                                                     concentration of CO2 in water
                                                                        6543 4566   3215
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                                                                                                                   Waste
                                                                                                                                     was reached. The bottles were
                                                                                                                                     connected to a selection valve
                                                                                                                                     followed by a peristaltic pump.
     Thermostated
     Wash Bottles
                                                                                                                                                                                 The picture on the left shoes the set up. The laser beam was focused on flow cell with CaF2 windows.
                                                                                                                                                                                 The left side shoes a schematic drawing of the CE system. Capillaries with a inner diameter of 50 µm and
                                                                                                                                                                                 a 10 mM Borax buffer were used. Depending on the length of the capillaries voltage from 13 to 16 kV were
                      OPTICAL SET-UP                                                                                               RESULTS                                       applied.
                                                                                                         4
                                                                                                        3.8
                                                                                                        3.6                                                                                                                      The picture on the left
                                                                                                        3.4                                                                                                                      shoes the cell holder where
                                                                                           Signal [V]




                                                                                                        3.2
                                                                                                                                                                                                                                 the capillaries are conected
                                                                                                         3
                                                                                                        2.8            one tubing                                                                                                to the CaF2 cell.
                                                                                                                       connection was
                                                                                                        2.6
                                                                                                                       untight and had                                                                                           On the right side a
                                                                                                        2.4            to be fixed.
                                                                                                        2.2
                                                                                                                                                                                                                                 microscopic close-up is
                                                                                                         2                                                                                                                       given. The cell has an
                                                                                                              0            500           1000       1500    2000    2500
                                                                                                                                                                                                                                 aperture of ca. apr. 150 µm
                                                                                                                                                Time [s]                                                                         to make sure that the light
                                                                                                                                                                                                                                 is focused on the channel.
                                                                                     On the begin of the measurement the light beam was
  The optical part was covered with a box made                                       interrupted to record the baseline signal of the detector.                                                                                                                              5
  of acrylic plastic, which was purged at 15                                         The difference of the baseline and the signal obtained
  l/min with N2 so that the optics was free from                                     from a standard purged with pure nitrogen was used to                                         Absorption Values                                                                       4.95

  CO2.                                                                               calculate the absorption values. The decrease in the
                                                                                                                                                                                                                                                              Signal [V]




                                                                                                                                                                                  Concentration [g/l]   Absorbtion [AU]                                                     4.9
                                                                                     signal by changing the flow to solutions containing more                                         0,338             0,019     0,019
                                                                                     CO2 is well visible.                                                                             0,675             0,041     0,042                                                    4.85

                                                                                                                                                                                      1,013             0,060     0,061
                                                                                                                                                                                                                                                                            4.8
                                                                                                                                                                                      1,350             0,081     0,082
                3.8                                                                                                              First results of
                                                                                                                                                                                                                                                                           4.75
                3.6                                                                                                        direct             CO2                                                                                                                                 0              0.5             1           1.5            2       2.5
                3.4                                                                                                        measurement in beer.                            See more Details in:                                                                                                                time/(timeEOF) [ ]
   Signal [V]




                3.2
                                                                                                                           After 2 FIA peaks the
                 3
                                                                                                                           cell was completely                Direct determination of carbon dioxide in aqueous                                               Here a typical recording of a CE seperation is given.
                                                                                                                           filled with sample for             solution using mid-infrared quantum cascade lasers.                                             The analytes are Adenosine, Guanosine, Xanthosine
                2.8
                                                                                                                           around 4 minutes.                  Schaden, S.; Haberkorn, M.; Frank, J.; Baena, J. R.;                                            and Adenosine-5‘-monophosphate. All concentrations
                2.6
                                                                                                                           Than again 2 FIA                   Lendl, B.                                                                                       of the given seperation were 5 mM. The first peak is
                2.4                                                                                                        peaks            where                                                                                                             corresponding to the Electro-osmotic Flow (EOF).
                      0   2      4      6    8       10       12   14       16                      18            20
                                                                                                                           measured                           Applied Spectroscopy (2004), 58(6)                                                              The analytes Adenosine and Guanosine are not
                                                 Zeit [min]
                                                                                                                                                                                                                                                              completly baseline separated.
                                                                                                                                 CONCLUSIONS AND OUTLOOK
ACKNOLEDGEMENT                                                                       • New sensitive method for the direct determination of dissolved CO2 using mid-infrared quantum cascade lasers.
                                                                                       => the achieved sensitivity (39 mg/l) already demonstrates the potential of this technique .
 Financial support by the Austrian Science                                                                                                                                                                                                                                                                 5th Workshop on
 Foundation within the project P-15531 ÖCH                                           • Emission band of the laser for CO2 measurements was not in the absorption maxima:
                                                                                       => Lower limits of detection are possible.
 is gratefully acknowledged.                                                                                                                                                                                                                                                                     Quantum Cascade Lasers
                                                                                     • The coupling of QCL absorption spectroscopy with CE separation is demonstrated for the first time.                                                                                                       Tehcnology and Application
 All Photos made by Markus Schaufler                                                 • The quantum cascade laser can be easily exchanged :
                                                                                       => easy adaptation of the whole analyzer to different analytical problems                                                                                                                                               Freiburg 2004
                                                                                       => group specific detection in CE will be possible

								
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