Automatic analytical methods for environmental monitoring and control

1. 
   aspiration of air;
   
2. 
   aspiration of sample (S₁);
   
3. 
   aspiration of air;
   
4. 
   aspiration of washing solution;
   
5. 
   aspiration of air;
   
6. 
   aspiration of the next sample (S₂).
   

F
igure III.2.9.

1. 
   washing the aspirating probe internally and externally;
   
2. 
   removing or substantial dilution of the static, thin liquid film on the tubing inner walls formed by the small ‘delayed’ amount of the heading sample;
   
3. 
   establishing a liquid barrier between sample zones which drastically reduces the possibility of reaction zones contamination after the debubbler.
   

The sample throughput is defined as the number of samples processed per hour and it is one of the features whereby the performance of an SFA analyzer is evaluated. Taking this into account, a determination will be feasible only if the signal reaches the steady-state in a time sufficiently short to permit its recording or acquisition. This means that the t_in, sample aspiration time, and t_r, interval over which the sample resides in the system, should be minimized. However, they must not be to short in order to prevent physical and chemical equilibrium to be attained.

Carry-over is another factor that influences the sample throughput. The use of a washing solution considerably delays the sampling operation. Also, the long lag phase and half-washing times impose an increase in the sample volume to be aspirated and therefore the sample throughput is reduced.
Factors affecting SFA signal quality

There are three aspects of decisive importance that have repercussions on the quality of the SFA signals and hence on the analyzer performance. These three aspects are:

1. 
   sample dispersion – the term refers to the spread of an aspirated sample in a flow system, as a result of the static liquid film formed on the tubing inner wall that reduces the separation efficiency of the liquid slugs with air bubbles. The influence of a series of experimental variables on the dispersion has been studied and these variables were classified as:
   
   1. 
      system variables: tubing inner diameter; flow rate; residence time; segmentation rate (expressed as number of air bubbles circulating per second);
      
   2. 
      sample variables: viscosity; surface tension; molecular or ionic diffusion coefficient.
      
2. 
   sample-reagent mixing – the liquid slug (containing sample, reagent (diluents)) between two air bubbles must be homogenized. This physical equilibrium is attained during sample zones flow through the manifold to the detector, (t_r). There are two parameters that contribute to the slug homogenization:
   
   1. 
      the compressibility of the air bubbles gives rise to a turbulent flow regime which fosters mixing
      
   2. 
      the helically coiled tubing favors the radial diffusion through the centrifugal force additional to the sweeping effect of the flowing stream, which shortens the homogenization time.
      

3. 
   flow stability – reliable and reproducible results in SFA systems are obtained if the flow profile is stable. This means that the circulating liquid slugs should have the same length. The factors responsible for the slug length irregularities are: flow-rate inconstancy; peristaltic pump pulsations; temperature variation (which affects the compressibility of the air bubbles); impurities in the sample or reagent tubing, etc.
   

The most applications of the TECHNICON SFA analyzers are in the field of various parameters determination in biological fluids, in clinical laboratories. But these analyzers can be easily adapted to other needs in the areas such as: environmental, pharmacology, food, agriculture or chemical industry. Companies such as SKALAR design and make analyzers aimed at non-clinical applications.

The applications of SFA analyzers may be classified according to the type of the detector involved. Thus, 70-75% of all SFA methods used molecular UV –VIS absorption, followed by ISE potentiometry (10 -14%) and much less nephelometry, fluorimetry.

Figure III.2.10.a depicts an assembly used for the determination of ammonia in see and tap water. The sample is aspirated (eventually after filtration) in the system and mixed with EDTA (metal ion masker) and then with phenolate and hypochlorite streams to form the dye indophenol blue, whose color is intensified by a nitroprusside stream. After de-aeration, the absorbance is measured at 630 nm. In this manner, nitrogen can be determined over the range of concentration 0.02 - 2 mg/mL at a sample throughput of 60 samples/h.