New Concepts and Sensing Materials for Thermal Conductivity Detectors in Gas Chromatography

NEW CONCEPTS AND SENSING MATERIALS FOR THERMAL CONDUCTIVITY DETECTORS

NEW CONCEPTS AND SENSING MATERIALS FOR THERMAL CONDUCTIVITY DETECTORS IN GAS CHROMATOGRAPHY

Adrian Bot, Rodica Turcu, Izabella Peter, Viorel Cosma, Vasile Surducan

National Institute for Research & Development of Isotopic and Molecular Technologies, POBox 700, 3400 – Cluj-Napoca, România.

We report two new detector designs, which improve the sensitivity of TCD in order to make it suitable for trace analysis and with capillary columns:

- a half bridge "classic" thermal conductivity detector consisting by two microcells – sample measuring and carrier reference – with conducting polymers based temperature-sensitive resistors;

- a new concept of differential thermal conductivity detector based on pyroelectric effect, consisting by two cavities in a single block, each with a polyvinylidene fluoride (PVDF) pyroelectric transducer.

The thermal conductivity detector (TCD) was the first detector used in gas chromatography and, despite the general dissatisfaction with the problem of his relative low sensitivity, it still remains, after more that 50 years, one of the most commonly used detectors, even though more sensitive and specialized detectors have been developed. The advantage of the TCD lies in the detection of gases such as CS2, H2S, SO2, CO, NO, NO2 and CO2 in gas-solid chromatographic analysis on packed columns. Some of the advantages of TCD are its simplicity, stability, versatility and low cost; one of his best features is the ease of quantitative analysis.

The internal volume of the thermal conductivity detector is relatively large, thus the necessity to use high flow rate of column effluent (15÷50 ml/min); this represents an incompatibility with the new modern opened capilary columns, wich operates at lower flow rate [1, 2].

The thermal conductivity detector with polypyrrole ribbon filaments

The TCD consists of a metallic block containing a cavity through which the gas flows. A heated element is positioned upon the thermal conductivity of the gas. For practical considerations a differential method is usually used that requires two cavities and two heated elements. Only carrier gas is passes trough one cavity and the column effluent through the other. The two most commonly used detector transducers are resistive wires (metallic filaments) and thermistors (beads of metallic oxide). Their operation is similar except that filaments have a positive coefficient of resistance and thermistors have a negative coefficient of resistance . The choice between thermistor or filament is usually based on working temperature considerations - thermisors for ambient or sub-ambient and filaments for higher temperatures. Once the decision has been made to use filaments, the selection is based on the corrosiveness or oxidation characteristics of the materials to be analyzed. Filaments are fabricated from a variety of metals, the most common being tungsten; other materials are nickel, rhenium-tungsten and gold platted tungsten.

Based upon our experience in obtaining and manufacturing for different applications of conducting polymers, we have designed a new TCD with polypyrrole ribbon filaments used as temperature-sensitive transducers.

Table 1.

Physical properties of some materials used as temperature-sensitive transducers

Physical properties / Tungsten / Nickel / Thermistor / Polypyrrole
Density 10-3 δ [kg m-3] / 19,3 / 8,9 / 3,5 / 1,6
Specific heat 10-2 cp [J kg-1 K-1] / 1,33 / 4,44 / 7,5 / 11
Thermal conductivity λ [W m-1 K-1] / 173 / 90 / 30 / 0,3
Electrical resistivity 10-8 ρ [Ω m] / 5,4 / 6,9 / ≈ 60.000 / ≈ 12.500
Temperature coefficient α [K-1] / 0,0048 / 0,0068 / - 0,16 / - 0,0039

The Table 1. presents some of the physical properties of films made by naphtalensulfonate doped polypyrrole [3, 4] compared with actually used materials. As we can see, the polypyrrole film has a relative low temperature coefficient, which represents a disadvantage, but the assembly of properties allow appropriate design solutions which guaranties the good performances of the detector.

a. b.

Fig.1. The detection cell with polypyrrole ribbon temperature-sensitive resistors (a) and the electrical scheme of the detection circuit (b).

The Figure 1.a. presents a transverse section of the TCD block (1) with the two cavities - one for the column effluent and the other for reference gas. The temperature –sensitive elements are made by polypyrrole film – 0,015x1,4x16 mm ribbon (3); the resistance legs are made by gold plated Kovar Ø0,6 mm (7) and are electrical insulated from the cell block by Teflon passage rings (4), which assure also the pneumatic insulation of the cavities – the cell must be perfect gastight.

The two polypyrrole ribbon filaments (as good as possible electrically and mechanically matched) are connected into two arms of a Wheatstone Bridge (Fig.1.b.); two conventional resistors (R) comprise the remainder of the bridge. When the thermal conductivity of the gas in one of the cavities changes as a result of the sample being eluted from the column, the temperature and the resistance of the detector element in that cavity change and an imbalance of the bridge appear. An instrumentation amplifier (Ainst) amplifies the small signal. The readout can be a potentiometric recorder or a DAQ system. The matching in/out is done by the digital resistive attenuator (Ratten). With the same gas passing trough both cavities the network is balanced by the offset potentiometer (Roff) so the electrical output is zero.

Polypyrrole films (PPY) were obtained by electrochemical polymerization in galvanostatic conditions. Among the different types of ions used for in-situ doping polymerization of PPY we selected the following organic ions which results in stability and good mechanical properties of the polymer: p-toluensulfonate (TS-) and naphtalensulfonate (NS-). The ions concentration in the synthesis solution was varied in the range 0.01-0.1 M. The electropolymerization was carried out on stainless steel electrodes by using current densities in the range 0.11- 4 mA/cm2. The as-synthesized PPY films were peeled off from the electrode, washed and dried. Flexible freestanding PPY films with good mechanical properties and thickness in the range 10-20m were obtained. The electrical conductivity of PPY films measured by the standard four contacts methods by using painted silver contacts has values in the range 37- 92 S/cm.

Fig.2. The slope of the electrical resistance vs. temperature for some polypyrrole films.

A better stability of the electrical properties was obtained for PPY doped with NS- ions as compared with TS- doped ones, due to the compactness of the resulting structure for NS- doped polymer. The temperature dependence of the electrical resistance for PPY films doped with both types of ions (TS and NS) shows a reversible behavior in the temperature range -100  +120 C; at higher temperatures (140<T<250 C) irreversible changes of the polymers properties or the films degradation could appear.

From the figure 2 one can see that the slope of the resistance vs. temperature dependence is higher for the samples doped with NS- ions as compared with TS- doped ones. This means that the activation energy of the interchains charge transport process is higher for NS doped PPY. This fact can be attributed to the structural differences between the two doping ions (NS and TS respectively) which strongly influence the PPY morphology and consequently the interchains distances. The control of the resistance vs. temperature variation can be done mainly by two synthesis parameters: the nature and the concentration of the doping ions. In order to obtain a strongly variation of the resistance vs. temperature for PPY films, the synthesis should be performed with lower concentrations of NS- doping ions.

Table 2.

Estimated performances of polypyrrole ribbon filaments compared with metallic filaments and thermistor beads

Performance / Tungsten filament
Ø0,025mm / Nickel filament Ø0,033mm / Thermistor
bead
Ø1,2mm / Polypyrrole
ribbon 0,015x1,4x 16 mm
Weight (without legs) m [μg] / 3,45 / 2,36 / 2,20 / 0,504
Lateral surface Sl [mm2] / 26,5 / 32,1 / 3,9 / 42
Rezistance @ 25OC R [Ω] / 40 / 25 / 8000 / 100
Resistance variation at 1mW power input ΔR1mW/R [%] / 0,0105 / 0,00649 / 0,0965 / 0,00704
Power consumed at 100OC P100 [W] / 0,89 / 1,16 / 0,48 / 0,41
Power losses trough legs ΔP100 [W] / 0,18 / 0,12 / 0,08 / 0,02
Excellence coefficient of sensitivity
Sl x ΔR1mW / 0,278 / 0,208 / 0,280 / 0,296
Excellence coefficient of power consumption 1 / (P100 +ΔP100) / 0,93 / 0,78 / 1,78 / 2,32

We have presented in the Table 2 some of the estimated performances of the ribbon polypyrrole filaments compared with metallic wires and thermistor beads; we have defined two excellence coefficients – one of sensitivity and the other of power consumption – which allow a better comparison between the new design and the classic TCD's.

The new device can operate at cell temperatures between –100  +115OC and we estimate an increase of sensitivity more than ten times compared with hot-wire filaments or thermistor beads made TCD. Moreover this detector has a few advantages related to the low temperature differential between sensitive resistors and cell block and low power consumption, which allow a major decrease of cavity volume, thus the using of lower column effluent flow rate (0,5÷2 ml/min).

The conducting polymer has a superior chemical resistance and doesn't degrade to different chemical agents – especially corrosive or oxidizing conditions.

The thermal conductivity detector with pyroelectric transducers

One of the most sensitive thermoelectric transducers at this moment is the pyroelectric sensor, still it has two disadvantages. First, all the pyroelectric materials are also piezoelectric, thus a major source of electric noise due to mechanical vibrations. Second, the pyroelectric sensor is a dynamic transducer, i.e. it can measure only a change of temperature not a stable one.

Considering the distribution of samples in the column effluent a dynamic process (usually a Gaussian distribution), we have designed a thermal conductivity detector based on pyroelectric effect, consisting by two cavities in a single block (Figure 3) each with a polyvinylidene fluoride (PVDF) pyroelectric transducer (8).

Fig.3. The detection cell with polyvinylidene fluoride pyroelectric transducer(in this section on can see only one of two identical cavities with transducers)

The temperature of the base block of the detector (2) is controlled with a principal thermoelectric module (10); the temperature difference between the wall of the microcells (4) and the substrate of the pyroelectric sensors is done with a second thermoelectric module (3). The Peltier elements assure the best temperature stability, which is the essential condition for the measuring performances of this detector. The temperature of the base and the cell blocks are measured with the integrated semiconductor sensors (7). The heat pumped by the thermoelectric modules is evacuated by the forced air convection cooled finned heat sink (1). The assembly of the detector is thermally insulated with the neoprene foam cover (9).

When a component of the analyzed sample flow trough the measuring cell, the difference between its thermal conductivity and that of the carrier gas produce a variation of heat transfer, and, as a consequence, a temperature variation of the pyroelectric sensor which will generate an electrical charge (Figure 4).

Fig.4. The theoretical electrical response – voltage mode and current mode - of the pyroelectric PVDF transducer (25μm, 0,2cm2) at a chromatographic mode thermal excitation (four components with Gaussian distribution)

The resulted small current signal (tens of pA to nA) is voltage converted trough an electrometric amplifier. The reference cell serves to compensate the piezoelectric noise and the parasite signals due to the residual variation of the detector temperature and the variation of the flow rate. After the analog digital conversion, the electric signal is numerical processed and integrated in order to obtain the real chromatogram.

This pyroelectric thermal conductivity detector (PYTCD) can operate at two temperature ranges: –35  +70OC and +50  +125OC, depending upon the class of thermoelectric modules used. The PVDF sensors are gold-sheathed and the detector has a good chemical resistance. We estimate an increase of sensitivity more than hundred times compared with usual TCD's, at a temperature differential of just a few degrees. The thermal slew rate is very good and the volume of cell cavities is minimal - tens of μl - thus this detector can operate at very low flow rate of column effluent.

Conclusions

To create an appropriate image of the technical performances of the new thermal conductivity detectors, we have presented in the Table 3. some of the estimated characteristics, compared with usually metallic wires filaments and thermistor beads based TCD's [5, 6, 7, 8]. On can see the improvements of the sensitivity and linearity, but mostly the major decreasing of the internal volume.

Table 3.

Compared performances of the four types of thermal conductivity detectors

Technical characteristics / Metallic wire filaments / Thermistor beads / Polypyrrole ribbon filaments / Pyroelectric transducers
Working temperature range
[OC] / 50÷450 / -100÷50 / -100÷115 / -35÷70
50÷120
Sensitivity / 10-6 / 10-7 / 10-7 / 10-8
Linearity / 10-4 / 10-2 / 10-4 / 10-5
Thermal time constant [sec] / 0,2 / 0,5 / 0,1 / 0,01
Internal volume [ml] / 4 / 0,25 / 0,088 / 0,018
Gas flow rate [ml min-1] / 15÷60 / 2÷8 / 0,5÷2 / 0,25÷1

All those performances make the new thermal conductivity detectors – with polypyrrole ribbon filament and with pyroelectric PVDF transducers - suitable for trace analysis and use with new modern capillary opened columns.

References :

1. David, D. J., Gas Chromatographic Detectors, John Wiley & Sons Inc., New York,

1974, p. 14-41;

1. Grob, R. L., (editor), Modern Practice of Gas Chromatography, John Wiley & Sons

Inc., New York, 1995, p. 281-291;

1. Coldea, A., Turcu, R., Bot, A., Brie, M., The Temperature Dependence of the

Electrical Conductivity of polypyrrole Films Doped with Different

Counterions, Romanian Reports in Physics, Vol.52, Nos. 1-2, 2000,

p. 127-138;

1. Turcu, R., Brie, M., Frandas, A., Pruneanu, S., Optical Studies on Free-standing

Polypyrrole Films by the Photopyroelectric Method, Appl. Phys., B62,

1996, p 499-502;

1. Lechner-Fish, T. J., Yang, X., A Microvolume Thermal Conductivity Detector for

Online GasAnalysis, Sensors Application Buletin 042, Daniel Industries

Inc. USA, 2000;

1. Yu, C., Micro-Machined Thermo-Conductivity Detector, U.S. Patent 6,502,983 B2,

USA, 2003;

1. * * * Thermal Conductivity Elements for Gas Analysis, TCD Filaments and

Thermistor Beads Data Sheets, GOW-MAC Instrument Company USA, 2002;

1. * * * FinniganThemal Conductivity Detector, Product Bulletin No. 802, CE

Instruments – ThermoQuest Corporation USA, 1997.