Signal Processing for a Portable Multi Sensor Gas Chromatograph
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Introduction The handheld measurement device DACHS was developed to detect low concentrations of trace gases. The main application can be found in major incidents concerning hazardous substances. The lightweight instrument combines broadband and selective gas sensors to distinguish a great number of substances. To separate mixtures of compounds a short multicapillary gas chromatograph column is applied in front of the detectors. The most selective detector is the Ion-Mobility-Spectrometer (IMS) that extends the time-domain of the GC run with another time dimension. Sensors There are four different types of sensors in the system: MOS-Sensors, that react very broadband to several substances, two electrochemical cells that are very selective to formaldehyde and phosgene and a PID with 10.6 eV ionization potential. These detectors generate single analog signals corresponding to the concentration. These time-domain signals are filtered by a savitzky-golay algorithm and differentiated to find the peaks within the chromatogram. The last type of sensors is the IMS. It repetitively generates spectra of the substance by separating molecule-classes within very short time. System Setup In figure 1 the gas path through the measurement device DACHS is shown. Starting at the left, the substance can get to the set of sensors on the straight way. The sampling and injection unit also provides the option to dilute the sample in this configuration of direct measurement. Utilizing the gas chromatograph, this unit loads the sample to the adsorbents and injects it to the column. The chromatographic column used in this system is only 34 cm in length and consists of 38 single capillaries. Due to this short length of separation, the signal logging has to be quite fast for chromatography. Signals The electrical change of the sensor, that is generated, when the detector is affected by a chemicals, is captured by a analog to digital converter. For the chromatography it is important to use a sufficient sampling rate of the AD- Converter so that every peak can be captured according to nyquist. For suppressing noise within the signals, that can cause incorrect recognition of peaks, a savitzky-golay filter is applied. It preserves the shape of the peaks in contrast to a moving average [Fig. 2]. Thus the System can be used in direct detection mode and in chromatography mode, there are different methods to identify substances. In direct mode it is only possible to distinguish them by different response characteristics of the sensors and the peak location in the ion-mobility-spectrum [Fig. 3]. The preceding chromatograph permits a substance discrimination also by its retention time. A differentiation of the signal leads to the detection of peak maxima. The sequence of data processing is shown in figure 5. A microcontroller based on ARM9 architecture handles the data acquisition and signal processing. The implemented algorithm additionally uses the temperature and flow of the GC for matching the retention times. Finally the resulting chromatogram and identified substance peaks are displayed [Fig. 4]. Jörn Frank, Hendrik Fischer, Gerhard Matz Hamburg University of Technology, Institute of Measurement Technology Hamburg University of Technology Institute of Measurement Technology Environmental Measurement Technology Harburger Schlossstrasse 20 21079 Hamburg, Germany www.tu-harburg.de/mt Signal Processing for a Portable Multi Sensor Gas Chromatograph Figure 1: Schematic of DACHS-Fluidics. Charcoal Filter GC Sampling and Injection Unit Main Pump Inlet Figure 2: Signal Path. Fluidics Substance Sensors Electr. Signal ADC Time Temperature Flow GC Parameter Signal Processing Filter Peakfinding Data- Linking Display Communikation Tenax Conclusion The processing method was first tested on a numerical computing environment and then implemented on the target system. The parameters for peak detection were adapted to the typical behavior of the DACHS sensors and electrical data acquisition procedure. The DACHS-System was tested with real chemical compounds. Some pure substances can be identified in direct mode with discrimination of different reactions on the detector channels. Compounds can be analyzed by chromatography, the retentions times are matched with an included database. . . . Sensor 1 Sensor 2 Sensor 7 IMS + IMS - . . . . . . Total 11 Channels Pattern Recognition signalheight Figure 3: Direct Measurement. Figure 4: Display of GC-Run. Raw Signal noisy Data Bunching harmonize sampling rates SG-Filter noise reduction Detect Peaks Skimming for shoulder peaks Integration Figure 5: Data Processing. Array of Sensors Direct Path to Array of Sensors
Transcript of Signal Processing for a Portable Multi Sensor Gas Chromatograph
Introduction The handheld measurement device DACHS was developed to detect low concentrations of trace gases. The main application can be found in major incidents concerning hazardous substances. The lightweight instrument combines broadband and selective gas sensors to distinguish a great number of substances. To separate mixtures of compounds a short multicapillary gas chromatograph column is applied in front of the detectors. The most selective detector is the Ion-Mobility-Spectrometer (IMS) that extends the time-domain of the GC run with another time dimension.
Sensors There are four different types of sensors in the system: MOS-Sensors, that react very broadband to several substances, two electrochemical cells that are very selective to formaldehyde and phosgene and a PID with 10.6 eV ionization potential. These detectors generate single analog signals corresponding to the concentration. These time-domain signals are filtered by a savitzky-golay algorithm and differentiated to find the peaks within the chromatogram. The last type of sensors is the IMS. It repetitively generates spectra of the substance by separating molecule-classes within very short time.
System Setup In figure 1 the gas path through the measurement device DACHS is shown. Starting at the left, the substance can get to the set of sensors on the straight way. The sampling and injection unit also provides the option to dilute the sample in this configuration of direct measurement. Utilizing the gas chromatograph, this unit loads the sample to the adsorbents and injects it to the column. The chromatographic column used in this system is only 34 cm in length and consists of 38 single capillaries. Due to this short length of separation, the signal logging has to be quite fast for chromatography.
Signals The electrical change of the sensor, that is generated, when the detector is affected by a chemicals, is captured by a analog to digital converter. For the chromatography it is important to use a sufficient sampling rate of the AD-Converter so that every peak can be captured according to nyquist. For suppressing noise within the signals, that can cause incorrect recognition of peaks, a savitzky-golay filter is applied. It preserves the shape of the peaks in contrast to a moving average [Fig. 2]. Thus the System can be used in direct detection mode and in chromatography mode, there are different methods to identify substances. In direct mode it is only possible to distinguish them by different response characteristics of the sensors and the peak location in the ion-mobility-spectrum [Fig. 3]. The preceding chromatograph permits a substance discrimination also by its retention time. A differentiation of the signal leads to the detection of peak maxima. The sequence of data processing is shown in figure 5. A microcontroller based on ARM9 architecture handles the data acquisition and signal processing. The implemented algorithm additionally uses the temperature and flow of the GC for matching the retention times. Finally the resulting chromatogram and identified substance peaks are displayed [Fig. 4].
Jörn Frank, Hendrik Fischer, Gerhard Matz
Hamburg University of Technology, Institute of Measurement Technology
Hamburg University of Technology
Institute of Measurement Technology
Environmental Measurement Technology
Harburger Schlossstrasse 20
21079 Hamburg, Germany
www.tu-harburg.de/mt
Signal Processing for a Portable Multi Sensor Gas Chromatograph
Figure 1: Schematic of DACHS-Fluidics.
Charcoal Filter
GC
Sampling and Injection Unit
Main Pump
Inlet
Figure 2: Signal Path.
Fluidics
Substance
Sensors Electr. Signal
ADC
Time
Temperature
Flow
GC
Par
ame
ter
Signal Processing
Filter Peakfinding Data-
Linking
Display
Communikation
Tenax
Conclusion The processing method was first tested on a numerical computing environment and then implemented on the target system. The parameters for peak detection were adapted to the typical behavior of the DACHS sensors and electrical data acquisition procedure. The DACHS-System was tested with real chemical compounds. Some pure substances can be identified in direct mode with discrimination of different reactions on the detector channels. Compounds can be analyzed by chromatography, the retentions times are matched with an included database.
. . .
Sensor 1
Sensor 2
Sensor 7
IMS +
IMS -
. . . . . .
Total 11
Channels
Pa
tte
rn
Re
cog
nitio
n
signalheight
Figure 3: Direct Measurement. Figure 4: Display of GC-Run.
Raw Signal noisy
Data Bunching harmonize sampling rates
SG-Filter noise reduction
Detect Peaks
Skimming for shoulder peaks
Integration
Figure 5: Data Processing.
Array of Sensors
Direct Path to Array of Sensors
