2. The Thermal Conductivity Detector (TCD) - Eawag-Empa Library

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2. The Thermal Conductivity Detector (TCD) - Eawag-Empa Library

2. The Thermal Conductivity Detector (TCD)

2.1 DETECTION MECHANISM

The thermal conductivity detector is among the most commonly used measuring

devices in gas chromatography for monitoring substances separated in the column.

This detector measures changes in the thermal conductivity of the carrier gas, caused

by the presence of the eluted substances.

The thermal conductivity, If, of component S is determined by the gas density, e,

the mean molecular velocity, ij, the mean free path, 2, and the specific heat at constant

volume, Cv:

It follows from equation (2.1) that the thermal conductivity is a function of the size

of the molecules, their mass and the temperature, as

where D is the diffusion coefficient. The dependence of the thermal conductivity on

the molecular masses of substances S and C is given by the relationship

Thermal conductivity values for some compounds are given in Table 2.1.

The thermal conductivity of binary mixtures is given by the thermal conductivities

of the components and their molar fractions, x [46]:

where A and B are constants and .yS = 1 - sc.

By solving equation (2.4) for a change in the conductivity of a binary mixture [22],

an expression that quantitatively describes the non-linear variation of the conductivity


40

with the concentration is obtained (see Fig. 2.1)

TABLE 2.1

THE THERMAL CONDUCTIVITY OF SOME COMPOUNDS

w N

Compound KS x lo3 [W/cm. K] Compound KS x lo3 [W/cm. K]

1710

1470

303

240

239

24s

23 I

216

164

7 0

FIG. 2.1. Thermal conductivities of some binary mixtures.

145

86.2

154

173

152

111

190

170

For very low concentrations of the eluted substance, all the terms in the expanded

Taylor series (equation (2.5)) can be neglected except for the first, thus giving a linear

relationship between the concentration of the eluted substance and the change in the

conductivity of the binary system.

Measurement with the TCD is based on monitoring changes in the electric con-

ductivity of the filament, caused by variations in its temperature during passage of


the gaseous mixture through the measuring cell. A temperature gradient is established

due to transfer of thermal energy by the medium. Under stationary conditions, the

amount of heat transferred, Q, is proportional to the thermal conductivity of the

flowing gaseous mixture and to the difference in the temperatures of the filament, T,,

and the cell walls, T,:

During the design of practical measuring devices, it was found that the overall

amount of heat transferred is not given by the thermal conductivity of the medium

alone, but that the molar heat capacity and other factors also play a role. The follow-

ing processes contribute to the overall heat change, i.e., to the measured signal:

- the thermal conductivity of the medium;

- forced convection of the gaseous mixture;

- free convection and diffusion of the gas;

- the thermal conductivity of leads and connections;

- thermal radiation.

The participation of the thermal conductivity and of forced convection of the gas

in the overall heat transfer can be distinguished only with great difficulty. Forced

convection represents heat transfer coupled with mass transport under the dynamic

conditions present in the gas chromatograph and amounts to about 25% of the

overall change. This part of the transferred heat is proportional to the volume velocity

of the gas, u, and to its heat capacity, C,:

Hence the TCD signal depends on the flow-rate of the carrier gas.

The contribution of free convection corresponds to energy transfer in the concen-

tration gradient and is negligible compared with the other factors.

Heat transfer by the leads and electrical connections is determined by their cross-

section, length and thermal conductivity. Therefore, it is desirable that the leads to

the sensor should be as short as possible and have as small a cross-section as possible.

It has been found that this parameter does not have a large influence on the overall

change in the TCD signal in devices with heated filements, but that its importance

increases when thermistors and transistors are used as sensors:

Heat radiation depends on the surface area of the measuring element, on its

temperature and on its material quality. The literature gives values of less than 4%

for the contribution of thermal radiation to the overall heat transfer.

41


42

2.2 TCD SIGNAL

Measurement with the thermal conductivity detector is based on monitoring changes

in the resistance of the sensor, R,. A current I passes through the sensor and the

thermal equilibrium in the measuring cell, through which a gaseous mixture with

a thermal conductivity of K passes, can be expressed by the equation

12Rf = J d(T, - T,)

where a is the geometric constant of the measuring cell and J is Joule’s constant.

It follows from the experimental arrangement that part of the thermal energy is lost

during the passage of the gaseous mixture through the measuring cell and is manifested

by a decrease in the sensor temperature, T,, as the temperature of the cell walls, T,,

can be considered to be constant because of their high heat capacity. The sensor

resistance is a linear function of the temperature:

R, = RP(1 + XT,)

(2.10)

and therefore the measured changes in the resistance are given by the relationships

and

AR, = aRP. AT,

AK

AT, = - (T, - T,)

K

(2.11)

(2.12)

The signal of the thermal conductivity detector, SrcD, is proportional to the change

in the voltage of the measuring bridge, AE:

srcD N AE -f --

AR

- AR a(T, - T,)

(2.13)

R, K (1 + aT,)

From equation (2.13) it follows that the signal depends on a number of parameters,

the most important being

- the voltage of the measuring bridge, E;

- the geometric constant of the measuring cell, a;

- the properties of the construction material, expressed by a;

- the temperatures of the sensor and the cell walls, T, and T‘, respectively;

- the thermal conductivity of the carrier gas, gc, and its flow-rate;

- the thermal conductivity of the eluted substance, AK.

As has already been mentioned, the TCD signal is not given by changes in the

thermal conductivity of the gaseous mixture alone; forced convection of the material,

which has a certain heat capacity, is also important. It is obvious that this contribu-


tion will become smaller with increasing participation of diffusion in the mass trans-

port in the measuring cell. Therefore, the term containing heat capacities is most

significant with flow-through cells. Bearing this in mind, the following expression has

been derived for SJCD [28, 291:

43

(2.14)

where constants A and 6 are determined by the effective collision cross-sections and

by the molecular weights of the eluted substances and of the carrier gas, where

0 < A < 1 < 6, and u, is the molar flow-rate.

The signal of the thermal conductivity detector is therefore strongly dependent on

the molecular weights of the eluted substance and the carrier gas. It generally holds

that the thermal conductivity decreases with increasing molecular weight, whereas

the heat capacity increases. If hydrogen is the carrier gas, then all eluted substances

will have lower thermal conductivities and higher heat capacities. The difference in

the thermal conductivity term in the proportionality (2.14) increases with increasing

molecular weight of the eluted substance and is always negative; the difference in the

thermal capacity term increases and is positive. Hence the two processes have opposite

effects and under certain circumstances their sum is equal to zero.

2.2.1 TCD background current

The background current of the thermal conductivity detector is given by the compo-

sition of the carrier gas, by its flow rate and by the detector temperature. As all

practical designs employ a compensation bridge circuit, it is pointless to specify

background current values. The TCD noise is caused by fluctuations in the detector

temperature and in the carrier gas flow-rate [12]. For this reason, the measuring and

the reference branches of the thermal conductivity detector are usually connected in

a bridge circuit. This circuit is indispensable with flow-through and semi-diffusion

cells, while diffusion cells yield results that are independent of the flow-rate and thus

they have the lowest noise level.

2.2.2 TCD response

The TCD response is the sum of the signals, SJCD, over the substance elution time.

It follows from equations (2.13) and (2.14) that the relative magnitude of the signal

depends on the character of the eluted substance; it decreases with increasing mo-

lecular weight. A criterion frequently applied during evaluation of the effect of the

structure of the eluted substance on the signal of the thermal conductivity detector

is the relative molar response (RMR) [2, 3, 471. The RMR values are proportional

to the various measuring sensitivities for various substances caused by their different


44

thermal conductivites. The literature gives the dependences of STCD on the molar and

weight percent content of a substance. When light carrier gases are employed, then

the proportionality of the signal to the weight per cent of the substance is usually

TABLE 2.2

THE RELATIVE MOLAR RESPONSE TO ORGANIC OXYGEN-CONTAINING

COMPOUNDS AND TO ALKYL-AROMATIC HYDROCARBONS [16]

M - molecular weight

Organic compounds

Number of carbon

atoms

Equation for RMR

n-Alkyl-aromatic c6-c10 0.741.M + 17.0

hydrocarbons

Primary alcohols c1-c4 0.672.M + 25.4

Esters RCOOR’

Methyl ketones

R = Co-C,;

R’ = Ci-C4

C,-C2

0.630.M + 25.9

0.688.M + 24.6

n-Aldehydes

n-Ethers

c1-c6

c2-c4

0.631.M+ 21.9

0.473.M + 49.5

used. In any event, the use of relative responses is to be recommended for quantitative

measurements, as they depend very little on experimental conditions such as the

bridge voltage, cell temperature and flow-rate. The RMR values for homologous

FIG. 2.2. The concentration dependence of the TCD response to some hydrocarbons,

N, - carrier gas, 1 - pentane, 2 - heptane, 3 - octane [29].

series of hydrocarbons, alcohols, etc., are given in Table 2.2 [16]. These values are

obtained using hydrogen as the carrier gas and only a narrow range of molecular

masses is covered. When the values are compared with those given in the literature


[25, 341, substantial differences are found. The validity of the published relative re-

sponse rates must be judged carefully, as data concerning the measuring cell geometry

and the experimental conditions are sometimes not specified. RMR values differing

by as much as 18% between flow-through and semi-diffusion cells have been found

[40]. It should be emphasized that, when using nitrogen as the carrier gas, even the

significance of empirical RMR values is doubtful. Under these conditions, direct

calculation of the concentration from the temperature dependences of il in the

carrier gas employed [SO] has only limited validity.

The molecular weight of the eluted substance significantly affects the character and

magnitude of the JCD response. As the thermal conductivity decreases with increasing

molecular weight, while the heat capacity increases, these two parameters have oppo-

site effects and lead to conversion of negative peaks into positive peaks. This conver-

sion is important with high-molecular-weight carrier gases, e.g., nitrogen. Fig. 2.2

shows that the change in the signal polarity is attained earlier with increasing mole-

cular weight.

TABLE 2.3

PARAMETERS CHARACTERIZING THE THERMAL CONDUCTIVITY

DETECTOR WITH VARIOUS SENSORS

Parameters ~.

Sensor

heated filament thermistor transistor

Sensitivity [mV/mole] 4 x 104 4 x 105 2.5 x 106

Linear dynamic range 5 4.4

Noise [pV]

The lower detectable amount

f 3 [I1 & 10 56

[PPnlI

Proportionality constant

5 0.2 0.012

ImVIKI 6 2900

The basic parameters of the thermal conductivity detector depend on the exper-

imental conditions used and therefore they are discussed in greater detail in the

following paragraphs. Generally, it can be stated that the thermal conductivity

detector is a universal measuring device with a wide linear dynamic range. However,

the minimum detectable amounts are sometimes large and then other detectors are

preferable. Some parameters characterizing the thermal conductivity detector are

listed in Table 2.3. The linearity and linear dynamic range must be given in log-log

coordinates because of the exponential form of equation (1.23).

45


46

2.3 EFFECT OF EXPERIMENTAL PARAMETERS

ON THE MAGNITUDE AND SHAPE OF THE TCD SIGNAL

2.3.1 Carrier gas

In measurements with the thermal conductivity detector, hydrogen, helium, nitrogen,

argon [45], carbon dioxide and mixtures of various gases, e.g., air or nitrogen with

5-10% of hydrogen or helium [43], are employed as carrier gases. The suitability

of various carrier gases can be evaluated with reference to the discussion related to

equation (2.14).

If hydrogen or helium is used as the carrier gas, the difference in the thermal

conductivities is always large and always negative. When the eluted substance is an

inert gas with a low molecular weight, then the term containing the heat capacities

plays almost no role and the TCD value is given chiefly by the change in the thermal

conductivity of the gaseous mixture.

When nitrogen is used as the carrier gas, the changes in the conductivity caused by

the presence of an eluted substance are small, but the difference in the heat capacities

increases significantly. It is evident that the magnitude of the TCD signal is inversely

proportional to the molecular weight of the eluted substance and generally SL, is

greater than SLY, as the thermal conductivity and heat capacity terms in equation

(2.14) are comparable for nitrogen.

The purity of the carrier gas effects the magnitude of the TCD signal [20]. If

a sample is injected into a carrier gas containing impurities, then, owing to changes

in the partial pressures, the impurities are replaced by the eluted substance and the

resulting change in the signal is small. For this reason, pure carrier gases should be

employed, although identical sensitivities for measurement with pure and impure

carrier gases have sometimes been reported [45].

The pronounced dependence of the signal of the thermal conductivity detector on

the gas flow-rate follows from the detection mechanism, involving removal of heat

by forced convection of gases in the measuring cell. As already mentioned, this

effect decreases with increasing participation of diffusion processes in the measuring

cell; the dependence of the TCD signal on the flow-rate is negligible with the diffusion

type of cell, expecially when a light carrier gas is employed [38]. It follows from

equation (2.14) that the thermal conductivity term is independent of the flow-rate.

The change in the signal with increasing flow-rate will be greater the greater is the

(CP, - CP~) value, i.e., the greater is the molecular weight of the eluted substance.

This change, leading to peak inversion, will naturally be greater and more frequent

when nitrogen is used as a carrier gas because of the similarity of the conductivity

and heat capacity terms in equation (2.14).

A decrease in the signal is not always observed during a change in the carrier gas

flow-rate; in fact, over a certain range of flow-rates, there is virtually no signal change.

This is due to the fact that the elution peak becomes narrower with increasing flow-


ate and hence the fdet/d t ratio increases with increasing detector volume. Consequently,

the mean concentration of the eluted substance in the effective volume of

the detector increases and a maximum concentration is attained.

The dependence of the response on the flow-rate, which is regulated either before

or after the column, is depicted in Fig. 2.3, from which it follows that the two

dependences have the same shapes, i.e., the products of Cs and of the integration time

are identical.

t

t ' I-

I

FIG. 1.3. The dependence of the JCD response on the flow-rate of the gaseous mixture;

A - flow-rate varied before the column, B - flow-rate varied after the column, before

the detector inlet [18].

From these dependences, it follows that the type of carrier gas used is of basicimpor-

tance. It is advisable to avoid the use of nitrogen and to work with hydrogen or helium.

When using heavier carrier gases, the relationship between the response and the concen-

tration may be markedly non-linear in the region of chromatographically significant

concentrations, resulting in distortion and inversion of chromatographic peaks.

2.3.2 Construction of the JCD

The measurement of thermal conductivity is carried out with a sensor, the resistance

of which is strongly dependent on the temperature of the medium (Fig. 2.4). The

sensor itself is at a temperature 7;, which is higher than the temperature of the walls

47


48

of the measuring cell T,. The sensor temperature is constant with stationary heating

conditions and a constant flow-rate of an unchanging gaseous mixture. A change in

the composition of the flowing gases is reflected in a change in the sensor temperature,

causing a change in the sensor resistance, R,, thus providing an electrically treatable

signal. At present, heated filaments, thermistors and transistors are employed as

sensors.

FIG. 2.4. The dependence of the sensor resistance on temperature; a - heated filament,

b - thermistor, c - transistor.

The heated filament was the first sensor to be used in the thermal conductivity

detector. The dependence of its resistance on temperature is linear over wide temperature

range (up to 400" C). This advantage is partly offset by its relatively low

sensitivity to temperature changes (see Table 2.3.).

In addition to heated filaments, thermistors with negative thermal coefficients are

used as sensors. The thermistor resistance is an exponential function of temperature

and the maximum temperature used is about 100 "C; above this value, the sensitivity

of the thermistor towards ,temperature changes is very low. If the thermistor resistance

equals RP at a particular standard temperature, To, (usually 25 'C), then the resistance

at temperature T, is given by the relationship.

R, = RP exp [ - (A - 31 (2.15)

A transistor was first used as a sensor in the thermal conductivity detector in 1972

[31], its use being based on the direct conversion of changes in the sensor temperature

into an electric signal. As transistors cannot be heated directly, they are maintained

at a temperature T, by an external source. The steady-state thermal equilibrium can

be expressed in terms of the transistor collector current, I,:

I, E = 0 . R (T, - T,) (2.16)


The sensor temperature, T,, changes during the elution and the change is manifested

in a change in the collector current, AI,, which is measured directly. A bridge circuit is

not used and this method leads to a considerable increase in sensitivity.

2.3.2.1 Seiisor heating voltage

In contrast to the heated filament detector, the dependence of the SrcD obtained

with a thermistor on the voltage exhibits a maximum. The optimum value of the

heating voltage is given by the relationship

E,,, - J (RO . 6. Rc . T,) (2.17)

Under constant thermistor parameters, i.e., standard-state resistance, Ro, and ma-

terial constant, 6, and with a given carrier gas characterized by a thermal conductivity

Kc, the optimum temperature of the supply voltage depends solely on the temperature

of the detector walls (see Fig. 2.5).

FIG. 2.5. The dependence of the relative

magnitude of SrCD on the heating voltage

and the detection block temperature;

a, b - thermistor, c - heated filament.

49

Fig. 2.6. The dependence of STCD on the

carrier gas quality; 1 - nitrogen,

2 - hydrogen.

A change in the carrier gas results in a change in the optimum heating voltage.

As follows from the introductory section, carrier gases with lower molecular weights

absorb more heat and therefore a larger heat supply is necessary in order to maintain

the same sensor temperature and a similar sensitivity. This dependence is depicted

schematically in Fig. 2.6.

2.3.2.2 Sensor pararneters

The signal of the thermal conductivity detector with a heated filament depends on the

properties of the filament, especially on its specific resistance, e, and its thermal


50

coefficient, a. The signal is related to these values by the relationship

STCD - alJe

The values of a, e and STCD for some materials are summarized in Table 2.4.

TABLE 2.4

THE SPECIFIC RESISTANCE, THERMAL COEFFICIENT

AND THE RELATIVE RESPONSE OF SOME MATERIALS

USED AS TCD SENSORS

Material a. 103[K-'] p [Q. mm2/m] sTCD

Platinum 4.0 0.106 13

Pt-Ir 90-10 1.2 0.24 2.5

80- 20 0.8 0.31 1.5

Tungsten 4.54 0.058 19

Nickel 4.91 0.072 18

(2.18)

One of the most commonly used materials is platinum, which, however, has poor

mechanical properies; the filament diameter is therefore generally rather large (about

0.02 mm). Alloys of platinum with iridium or rhodium have better mechanical proper-

ties but yield smaller signals then platinum sensors. After platinum, tungsten is the

most commonly used filament material. Its relative response is comparable with that

of platinum and it also has very satisfactory mechanical properties, so that very thin

filaments (down to 0.006 mm) can be used. However, it is oxidizeh by atmospheric

oxygen at temperatures above 500 "C. Nickel filaments are occasionally used in

corrosive media, but their use is limited by their poor mechanical properties [8, 9,

13, 15, 36, 441.

A number of workers have tried to prolong the life-time of sensors, decrease the

noise level and prevent drift of the background current. In addition to passivation of

the W-Re filament at 330 "C by formation of oxides [8], the surface of the filament

was gilded [17] and treated with HF [15] or CH,Cl, [36] vapour. These modifi-

cations led to an increase in the detector stability. The measuring filament has also

been covered with a palladium catalyst [13], increasing the sensitivity for both

a- and p-olefins.

Thermistors are characterized by the values of Ro and the material constant. The

thermistor resistance varies within a wide range, from 1 to lo6 0. In general, the

detector can be used at higher temperatures if a high-resistance thermistor is em-

ployed, but a recorder with a high input impedance must be used. As the sensitivity

of the thermistor to temperature changes is highest at low temperatures, thermistors

with low resistances (of the order of lo3 to lo4 62) can be employed.

The thermistor material constant corresponds to the energy required to increase


the temperature by AT,. Hence it is evident that, with increasing material constant,

the detector time constant will increase, thus rendering the results less reliable.

2.3.2.3 Cell geometric constant

The amount of heat removed from the surface of a heated filament is proportional to

its surface area (see equation (2.6)). For a cylindrical body, this geometric constant

is given by the relationship

51

(2.19)

where 1 is the length of the heated filament, rc is the internal radius of the cell and rf

is the external radius of the filament. It follows from equations (2.6) and (2.19) that

STCD increases with increasing length and radius of the heated filament and is inversely

proportional to the cell radius. These dependences must be evaluated correctly, as it

is impossible simultaneously to lengthen the filament and make the cell volume smaller.

If the filament is lengthened, the cell must be larger and the detector volume increases;

this leads to an increase in the detector time constant. Existing commercial detectors

represent a compromise among the above requirements. The effect of the geometric

constant on the magnitude of the TCD signal is represented schematically in Fig. 2.7.

0 M 15 -a

I

m

FIG. 2.7. The dependence of the FIG. 2.8. The dependence of STCD on the

TCD signal on the geometric constant

at various temperatures of the heated

temperature of the filament and of the cell

walls; the difference (Tf - T,) is plotted

filament. on the x-axis.

When a thermistor is employed as the sensor, it is assumed to be spherical and the

geometric constant is expressed by the relationship

4xrrrc

a = - - 4nrt

rc - rf

(2.20)

Therefore, the signal of a detector employing a thermistor is directly proportional to

its radius.


52

2.3.2.4

Temperatures of the sensor and the cell walls

From the principle of the thermal conductivity detector, it follows that T, must

generally be larger than T, (when a heated filament is used), so that (T, - T,) 2

I - 200 "C. The detector signal is directly proportional to the difference between the

temperatures of the heated filament and the cell walls, from which follow important

conclusions concerning the adjustment of the experimental conditions.

At a selected detection block temperature, SrCD is proportional to the temperature

of the heated filament, i.e., to the heating intensity (Fig. 2.8); on the other hand, at

a given temperature of the heated filament, i.e., with constant heating, STCD is

inversely proportional to the detection block temperature. In practice, it is most

advantageous to maintain the temperature of the heated filament as high as possible

(taking care not to burn the filament) and the detection block temperature as low as

possible (avoiding condensation of eluted substances in the cell).

When working with columns with programmed temperatures, the possibility of

a decrease in the TCD signal must be borne in mind if the detection block is connected

to the column thermostat. During an increase in the temperature of the column

thermostat, the temperature difference (T, - T,) decreases and consequently STCD

also decreases.

The above rules also hold for thermistor sensors. It has been found experimentally

that the highest measuring sensitivity is attained for a small difference between the

sensor and cell temperatures, (T, - T,) = 35 - 50 "C, while the sensor temperature

should not exceed 100 "C. STCD is strongly dependent on the detection block temperature:

STcD 81.5R1l2T-2 f c (2.21)

Therefore, it is preferable to maintain the detection block temperature as low as

possible.

2.3.2.5

Time constant of the TCD

The magnitude of the time constant depends on the effective volume of the detector.

The requirement that the time constant of the measuring device should be as low as

possible, leading to the smallest possible distortion of the elution curves, is partially

in opposition to the requirements regarding the magnitude of the detector signal

concerning, for example, the length of the heated filament or a large thermistor radius.

The cell time constant is expressed by the relationships

and

vdet = ' * 'dst

7 = 0.632td,,

These relationships are valid, however, only for cells in which transport occurs


exclusively through convection. A number of thermal conductivity designs have been

proposed in which both diffusion processes and mass convection towards the sensor

surface participate. According to this criterion, cells are classified as flow-through,

semi-diffusion and diffusion. The shapes of these cell types are depicted in Fig. 2.9.

FIG. 2.9. Various shapes of thermal conductivity cells; a - flow-through,

b - diffusion, c - semi-diffusion.

The expression for the time constant of flow-through cells reflects the significant

dependence of their signal on the gas flow-rate. Therefore, manostats are placed

before the cells in order to stabilize the flow-rate and the pressure [12]. The smallest

distortion of the shape of the elution curve is achieved with a low time constant;

designers of thermal conductivity cells thus attempt to make the flow-through cell

volume as small as possible. Cells with volumes of 20 pI [21], 2.6 ,d [32] and even

1 pL1[23] have been described.

TABLE 2.5

TIME CONSTANTS OF VARIOUS CELL TYPES

IN THERMAL CONDUCTIVITY DETECTORS

Cell 7 [sec] Notes

Flow-through 0. I- 1 most frequently used, most

sensitive to all changes

Diffusion 10-20 unsuitable for modern

measuring requirements

Semi-diffusion o1 /r2 > 1 up to 10 properties of the flow-through cell

CI/U2 J. 1 up to 20 properties of the diffusion cell

53


54

It is obvious that the time constant will increase with increasing participation of

diffusion in the transport process. The time constant of a semi-diffusion cell can be

expressed by the relationship

1

z = 0.632t,,, -

2-- V

111

where trl is the gas flow-rate through the measuring branch (see Fig. 2.9). The time

constants of the cell types discussed are listed in Table 2.5. In addition to the time

constant of the measuring cell, the sensor time constant, determined by its mass and

heat capacity, must also be considered. Thus the sensor requires a certain time to

record a change. The time constant of a heated filament is given by the relationship

As the heat capacity of the filament is proportional to its volume, i.e., mC, - $1, the

requirement of high TCD sensitivity (see equations (2.9) and (2.19)) leads to an

increase in the sensor time constant. The heat capacity of the thermistor approxima-

tely equals the material constant, 6, and varies around 1 sec for most sensors of this

type. In a provisional arrangement, the transistor sensor had a large time constant

of 10 sec [31], which the authors felt could be decreased to 0.6 sec.

In some designs, the heated filament is sealed in glass in order to decrease the

catalytic action of the heated filament on the thermal decomposition of substances

and to prevent corrosion of the sensor. Similarly, materials other than glass have also

been employed, e.g., fluorinated plastic [5]. These modifications lead to an increase

in the lifetime of the sensor and make its use at higher temperatures possible, but the

TABLE 2.6

THE TIME CONSTANT OF THE THERMAL CONDUCTIVITY DETECTOR

Cell

Sensor

heated filament

filament in thermistor transistor

Length of cell

- sensor [cm]

2 10 2 0.2 0.2

Diameter of cell

- sensor [cm]

0.5 0.002 0.1 0.2 0.2

Gas flow-rate [ml/min] 30

Time constant [sec] 0.45 0.01 4 1 9.6


time constant of the sensor and consequently that of the whole detector are increased

considerably due to an increase in the heat capacity of the measuring element. Cell

time constants and those of individual sensor types are given in Table 2.6, from

which it follows that a heated filament has the lowest time constant. It should be

used in combination with a flow-through cell whenever theoretical or quantitative

empirical conclusions are to be made on the basis of the measured results.

2.3.2.6 Measuring circuits

If a heated filament or thermistor is used as a sensor in the thermal conductivity

detector, it is connected in a Wheatstone bridge for compensation measurements.

When a voltage E is brought to a bridge consisting of the sensor resistance, R,,

and a reference sensor resistance, R,, then, for the current passing through the circuit

(Fig. 2.10)

_-

.I

U

E = I(R, + R,)

A signal corresponding to the voltage change due to a change in the sensor resistance

can be measured at the bridge output.

f f

FIG. 2.10. Scheme of the

FIG. 2.1 1. Scheme of the TCD circuit with a constant

TCD constant-current circuit. sensor temperature.

The above principle is used in both constant-current and constant-voltage circuits,

with either two or four sensors. A single thermistor in a voltage-powered bridge has

also been employed [7]; however, the only advantage of this circuit was its simplicity,

the JCD parameters not being improved. In addition to the measurement of d.c.

voltage and current, square-wave JCD operation has also been proposed. Constant-

current bridges yield a broader linear dynamic range than constant-voltage bridges.

Recently, a thermal conductivity detector with a constant sensor temperature was

introduced [49]. In this circuit, a single variable resistor is connected in the bridge

(Fig. 2.11). The uncompensated output voltage is amplified and fed back to the bridge

55


56

through a transistor controlling the current passing through the bridge. In this

way, the current is instantaneously adjusted so that the sensor resistance, R,, and

consequently its temperature, T,, do not change.

It has been found experimentally that a device with a constant-temperature

sensor has a number of advantages over constant-voltage or constant-current circuits.

The lowest detectable amount is decreased ten-fold, the linear dynamic range is

broader by one order of magnitude and the detector time constant is decreased.

Concentrations of 10% by volume were measured in this way without a change in the

detector linearity.

The pronounced improvement in the JCD parameters when a constant temperature

is used can be explained by the fact that, owing to the constant sensor temperature

during the elution, the entire elution curve is measured under constant conditions

with constant sensitivity. With constant-voltage or constant current circuits, the

sensor temperature gradient follows the concentration gradient of the eluted substance.

With increasing amounts of eluted substances, the measuring sensitivity decreases

owing to a decrease in the sensor temperature.

2.4 APPLICATIONS OF THE TCD

The thermal conductivity detector is one of the most commonly used detectors in gas

and liquid-solid chromatography [41]. In addition to a number of reviews [17, 19,

TABLE 2.1

EXAMPLES OF THE APPLICATION OF THE THERMAL CONDUCTlVITY

DETECTOR

Substance determined Material analyzed The lowest detect- Ref,

able amount

Permanent gases in air

Permanent gases in chlorine

Permanent gases

in high purity ethylene

N2, 02, c02

in exhaled air

NO, in air

0 in organic compounds

N in organic compounds

Metal halides

PCI,

Chlorinated hydrocarbons

Alkanes, alcohols, aliphatic acids

Olefins, branched olefins

‘BH18; C10H22;

0.002 1.11 10, 23,

35,43

I1

17a

30

3a

27

1 1.1g 39

1.9 x mole 5

8.7 X mole 5

15

37

13

1.4; 2.7; 3.5ppm 32


24, 33, 34, 42, 481, the excellent treatment of the TCD in the book by Jentzsch and

Otte [18] should be mentioned. Some examples of TCD applications are given in

Table 2.7. When the TCD is compared with other detectors, for example the discharge

detector [l], E CD [5] or FP D [14], better parameters are usually found for the TCD.

Comparison of the TCD with the FID [4, 35, 371 depends on the structures of the

eluted substances. The TCD is, of course, more suitable for inorganic gases such as

CS,, COS, H,S and SO, [35]. During measurement of the signal ratio, S"/SrCD,

n

FIG. 2.12. Analysis of air using a TCD with a transmodulator [23].

values greater than unity were obtained for alkanes, and values smaller than unity for

oxygen-containing compounds [37]. It can be presumed that the thermal conductivity

detector will find further use in analyses of inorganic gases, of silylated samples, etc.

[26]. A thermal conductivity detector with a palladium transmodulator [23] gave

excellent results in analyses for rare gases in air (Fig. 2.12).

2.5 LITERATURE

I. Arnikar H. J., Rao T. S., Karmarkar K. 13.: J. Chrornatogr. 38, 126 (1968)

2. Barry E. F., Rosie D. M.: J. Chrorwtogr. 59. 269 (1971)

3. Barry E. F., Jr.: Chenz. Abslr. 76, 50 486c (1972)

3a. Beskova G. S., Filipov V. S.: Zaaod. Lab. 38, 154 (1972)

4. Bogoslovskii Yu. N., Razin V. L.: Zlz. Fiz. Khim. 45, 2490 (1971)

5. Brazhnikov V. V., Sakodynskii K. I.: Clzern. Absrr. 74, 94 013j (1971)

6. Castello G., DAmato G.: J. Chromatogr. 32, 625 (1968)

7. Chowdhury B., Karasek F. W.: J. Chroniatogr. Sci. 8, 199 (1970)

8. Cieplinski E. W., Spencer S. F., Illingsworth W. L.: US Pat. 3,537,914 (Nov. 3, 1970)

57


58

9. Delew R. B.: J. Chromatogr. Sci. 10, 600 (1972)

10. Ediz S. H., van Swaay M., McBride H. D.: Chem. Instruni. 3, 299 (1972)

11. Gerdes W. F.: US Pat. 3,474,661 (Oct. 28, 1969)

12. Goryunov Yu. A,, Zalkin V. S., Okhotnikov B. P., Rotin V. A., Rozanova L. I., Maksimov

B. G.: USSR Pat. 371 511 (Feb. 22, 1973)

13. Guillot J., Bottazzi H., Gyuot A., Trambouze Y.: J. Gas Chromatogr. 6, 605 (1965)

14. Gutsche B., Herrmann R.: 2. Anal. Chem. 249, 168 (1970)

15. Hachenberg H., Gutberlet J.: Brensf. Chem. 49, 242 (1968)

16. Hara N., Katsuda M., Kato H., Hasegawa K., Ikebe K.: Chem. Abstr. 68, 65 5D4r (1968)

17. Hartmann C. H.: Anal. Chenz. 43, 113A (1971)

17a. Jadrijevic V., Deur-Siftar D.: Chromafogruphia 7, 19 (1974)

18. Jentzsch D., Otto E.: Detektoren in der Gas Chromatographie, Akademische Verlaggeselschaft,

Frankfurt a. M. 1970

19. Johns T., Stapp A. C.: J. Chromatogr. Sci. 11, 234 (1973)

20. Karp S., Lowell S.: Anal. Chern. 43, 1910 (1971)

21. Kiefer M. E.: Ger. Offen. 2.222 617 (Nov. 30, 1972)

22. Lindsay A. L., Bromley L. A,: Ind. Eng. Chem. 42, 1508 (1950)

23. Lovelock J. E., Simmonds P. C., Shoemake G. R.: Anal. Chem. 43, 1958 (1971)

24. McNair H. M., Chandler G. D.: J. Chromatogr. Sci. 11, 454 (1972)

25. Monfort J. P.: Chim. Anal. (Paris) 53, 646 (1971)

26. Morrow R. W., Dean J. A., Shults W. D., Guerin M. R.: J. Chromatogr. Sci. 7, 572 (1969)

27. Nikolaeva N. A., Dolgopolskaya P. I., Rezler R. Yu.: Chem. Abstr. 70, 111 448s (1969)

28. Novik J., WiEar S., Janak J.: Coll. Czech. Chem. Commun. 33, 3642 (1968)

29. Novak J., Janiik J.: Coll. Czech. Chem. Commun. 35, 212 (1970)

30. Noviik J., JaniEek M.: Chem. Listy 65, 739 (1971)

31. Otte E., Gut J.: Chromatographia 5, 246 (1972)

32. Pecsar R. E., De Lew R. B., Iwao K. R.: Ana. Chem. 45, 2191 (1973)

33. Richmond A. D.: J. Chrornatogr. Sci. 9, 92 (1971)

34. Rosie D. M., Barry E. F.: J. Chromatogr. Sci. 11, 237 (1973)

35. Schaeffer B. A.: Anal. Chenz. 42, 448 (1970)

36. Seibel A. C., Johns T.: US Pat. 3,533,858 (Oct. 13, 1970)

37. Sokolov D. N., Golubeva L. K.: Zauod Lab. 35, 143 (1969)

38. Tamura H., Hozumi K.: Chem. Abstr. 72, 128 311f (1970)

39. Thuerauf W.: 2. Anal. Chem. 250, 11 1 (1970)

40. Vermont J., Guillemin C. L.: Anal. Chem. 45, 775 (1973)

41. Versino C., Fogliano L., Giaretti F.: Riu. Combust. 21, 389 (1967)

42. Villalobos R.: Chem. Eng. Progr. 64, 55 (1968)

43. Volkov E. F., Rabinovich S. I., Breshchenko V. Ya.: Ref. Zh. Khim. 1969, 5 G 14

44. Wade R. L., Cram S. P.: J. Chromatogr. Sci. 10, 622 (1972)

45. Walsa J. T., McCarthy K. J., Merritt C., Jr.: J. Gas Chromatogr. 6, 416 (1968)

46. Wassiljewa A.: Physik. 2. 5, 737 (1904)

47. Watanabe S., Nakasato S., Kuwayama H., Sasamoto Y., Shihaishi S., Seino H., Nagai T.,

Negishi M., Hayano S.: Yukagaku 22, 102 (1973)

48. Winefordner J. D., Glenn T. H.: Aduan. Chromatogr. 5, 263 (1968)

49. Wittebrood R. T.: Chromatographia 5, 454 (1972)

50. Zalkin V. S.: Zauod. Lab. 36, 129 (1970)

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