Meniscus sample problem

Another drawback of the immersion cell concept is that the active area is badly defined, because of the meniscus formed at the electrolyte-air interface. The form of the meniscus greatly depends on whether the sample is hydrophilic or hydro-phobic, which again is a function of applied potential. This problem can be circumvented, if the active area of the sample is defined by a window in an inert layer, for example resist or CVD nitride, which is fully immersed into the electrolyte, as shown in Fig. 1.6 a. 

The dead-space above the sample in the dart-like tip is much reduced by the use of the type of micropipette which uses fine capillary tips. The Oxford Ultramicro-sampler is an example. Use of this device completely overcomes the solvent expulsion problem. The principal disadvantage from the point of view of electrothermal atomisation with a graphite furnace is that its maximum capacity is 5 pi. This may be too little for the sensitivity of some elements. The tip material provided with this syringe is still prone to droplet formation, but this can be replaced by PTFE capillary tubing of the correct dimensions, e.g. Polypenco size TW 24, which appears to overcome the problem completely. Good precision should be attainable with tips made from this material, provided the tips are cut across at 90° to the tube axis. Chamfered tips give rise to a variable position of the meniscus with consequent loss of reproducibility. 

The problems of adequate mixing of the gas and liquid phases to ensure saturation, pressure and temperature control and sampling and measurement of the gas dissolved at high pressure present greater difficulties than in apparatuses that operate at atmospheric pressure. These problems were solved in Smith and Gardiner apparatus (37), by a modern autoclave design and connections of stainless-steel, a magnetically driven bladed turbine stirrer, a modern temperature and pressure measurements and control, and a meniscus volume correction for the liquid in the buret measurement. In this apparatus, a volumetric method was applied to measure the gas and liquid volumes in buret system at atmospheric pressure. 

It is not necessary for STM to be operated in a vacuum and work can be undertaken at atmospheric pressure and under liquids. At atmospheric pressure, the drag from the meniscus effect of absorbed moisture can be a problem therefore, some workers prefer to immerse the sample in water. 

---