10.3.1 Gas flow-rate
With the coulometric detector, it is necessary to burn the eluted substances before they enter the coulometric cell. The composition of the gaseous mixture is determined by the manufacturer, who selects the composition of the reaction solution consider- ing the qualitative properties of the combustion products.
The amount of gases passed does not affect the detection mechanism but does influence the detector operation. The gaseous mixture is damp after passing through the solution, i.e., moisture is gradually evaporated in the reaction space, with conse- quent changes in the composition of the electrolyte, leading to an increase in the noise.
A continuous supply of electrolyte is especially necessary with continuous analyzers.
The electrolyte must be added at daily to weekly intervals with flow-rates of 0.5 to 1.5 I/min.
10.3.2 Construction of the detector
The eluted substances are converted into products with known properties in the combustion space of the coulometric detector. Combustion usually takes place in combustion tubes of various volumes (up to 200 ml). It is evident that when the volume of the combustion space is larger than the difference in the elution volumes of two successive substances, a mixture is again formed in the combustion space and the separation process is negated. In addition to combustion tubes, a pyrolyzer has also been used [16, 311, for example, for the determination of toxic substances in the atmosphere at the parts per billion level, but the results were similar to those obtained with combustion tubes. These designs are not advantageous and therefore combustion tubes have been replaced by the FID burner [37], which permits simultaneous record- ding of both universal and specific responses. In this way, the dead volume is consider- ably decreased and the risk of re-mixing the separated components is less.
The same criteria relate to the reaction space of the coulometric cell. A small volume is desirable, as concentration changes in a small volume are larger than in a large volume and consequently the changes in the indicating circuit of the coulo- metric detector are greater. The original device, employing an electromagnetic stirrer, had a volume of about 40 ml [lo]. In later constructions, the electrolyte was stirred by the passing gas. The electrolyte circulates in the reaction space, the volume of which was decreased to 5 ml [37,41]. By excluding the combustion tube and using a coulometric cell with a small reaction space (see Fig. 10.3), a low time constant and undistorted elution curves are obtained; with a detector effective volume of 5 ml and a gaseous mixture flow-rate of 300 ml/min, the coulometric detector delay is 1 sec.
A number of methods can be employed for indicating changes in a coulometric analyzer [48]. Potentiometry is generally used because of its advantageous signal- to-noise ratio. In addition, amperometry and biamperometry have been tested, but these methods invariably exhibited a lower signal-to-noise ratio than potentiometry.
A 7 k -
% - +
j * - _ _ --?+-- +%,
9
B SECTION 7 6 3 5
A-A
2 4 9 1
SECTION z ,8
FIG. 10.3. Scheme of a coulometric cell; 1 - body of the cell, 2 - reaction space., 3 - vent, 4 - inlet for the gaseous mixture, 5 - working electrode, 6 - indicating electrode, 7 - reference electrode, 8 - auxiliary electrode, 9 - frit.
The reaction taking place is followed using an indicator electrode whose potential changes correspond to concentration changes in the solution. Variations in potential in the indicating circuit are amplified and used for control of the generating current and consequently of the amount of substance added to the solution. The magnitude of the generating current is recorded and corresponds to the elution curve. The indication and generation spaces are separated by a capillary [15] or by glass frits and membranes [16, 371. The cathodic and anodic electrode processes need not necessarily affect one another; then it is unnecessary to separate the electrode spaces and it is possible to decrease the number of electrodes [18].
The electronic part of the coulometric detector (see Fig. 10.4) can be a source of distortion of elution curves. Most instruments operate with a proportional d.c.
current source powered by a 15 V source. With an electrolyte conductivity of 10-3Q-'
174
the maximum generation current is 15 mA. Thus more than lo-' mole/sec cannot be eluted from the column. This condition is not always fulfilled and the elution curves are flat at the top (see Fig. 10.5). The beginning of the elution curve corresponds to the beginning of the elution of the substance from the column, but the end of the curve does not necessarily correspond to the end of the elution.
+ 15V
P
i I I I I I I I
I L - - - - - I
b d
-15V -15V
FIG. 10.4. Scheme of the electronic part of the coulometric detector.
FIG. 10.5. Chromatogram of 2 p1 of the mixture of (CH,),SO (3) and C,H,CI (2) in acetone (1).
With continuous analyzers, the concentration level of, for example, sulphur dioxide is monitored, the concentration changes not exceeding one order of magnitude.
Therefore, situations in which the generating current is insufficient for re-establish- ment of the initial conditions do not occur.
10.3.2.1 Bius
It is impossible to vary a large number of parameters with the coulometric detector, similar to analyzers of airborne pollutants. The composition of the reaction solution is determined by the manufacturer and cannot be changed. However, the composition of the solution determines the sensitivity of the detector towards oxygen in the gaseous mixture. After filling the detector reaction space and introducing the gaseous mixture, the potential of the system increases and attains an equilibrium value. This value is reached within 10 to 15 min. After attainment of the equilibrium state, a value is set by a compensation potentiometer so that no electrolytic current passes through the circuit. Thus the zero position is adjusted in the indicating circuit and changes caused by the chemical reaction are then either positive or negative with respect to this value.
If the background current of the coulometric detector is large after adjustment of the compensation potentiometer, then the adjustment is incorrect and must be changed.
10.3.3 Temperature
As the reaction in the coulometric detector or analyzer proceeds in the liquid phase, the entering gases must be absorbed in the electrolyte. The solubility of gases, how- ever, depends significantly on temperature, decreasing with increasing temperature.
In the coulometric detector, the temperature of the entering gaseous mixture is approximately constant and relatively high.
With continuous-measurement coulometric analyzers, the temperature of the entering gaseous mixture depends on the time of day. At night, the ambient tempera- ture is lower than during the day and the results of the determination of SO, may vary accordingly. More recent devices, e.g., the Philips SO, Monitor contain a ther- mostat to remove this drawback.