Automatic analytical methods for environmental monitoring and control

The detector signal reflects the degree of spreading and dilution that has occurred during the zone sample transportation through FIA system and its typical shape is generally tailed rather than symmetrical. Thus, the mixing between the carrier and the sample solution is always incomplete, but because the mixing pattern for a given experimental setup is perfectly reproducible, FIA yields reproducible results.

It is important to recognize that FIA methods are dynamic in the sense that neither physical equilibrium (complete mixing) nor chemical equilibrium (the analytical reaction goes to completion) are prerequisite and there are concentration gradients in the system during the sample transportation through the detector.
Dispersion of sample zone

Certain analyses in FIA that are used to determine original sample composition, such as: pH determinations, conductance measurements, atomic absorption determinations, require that the sample solution be transported through the flow-cell as an undiluted zone and in a higher reproducible manner. On the other hand, for the majority FIA systems described in literature, the species being monitored have been generated by on-line chemical reaction. The prerequisite for performing such analysis is that during transport through the system, the sample zone is mixed with reagents and sufficient time is allowed to produce a desired compound in an easily detectable amount. Additionally, sometimes the sample must be diluted, so that the resulting signal can be accommodated within the dynamic range of the detector, or in the case of trace analysis, the excessive dilution must be avoided to obtain a maximum sensitivity. All these diverse requirements can be fulfilled by manipulating the dispersion of the sample: a) how much of the original sample solution is diluted on its way toward the detector, and b) how much time must elapse between sample injection and analytical signal recording.

J. Ruzicka and E. Hansen introduced the concept of dispersion coefficient, D, that has been defined as the ratio of concentrations before and after the dispersion process has taken place in the element of fluid that yields the analytical readout.
D = C⁰/C^max = H⁰/H
where C⁰ is the initial concentration of the sample before it is diluted, C^max is the maximum concentration of the sample that flows through the flow-cell. In most FIA methods the analytical readout is based on the measurement of the peak height H, and therefore the dispersion coefficient may be expressed as a ratio between the heights of the initial (H⁰) and diluted (H) sample. Similarly, the dispersion coefficient for the reagent is:
D_R = C⁰_R/C^max_R
But D may be calculated in any point on the ascendant or descendent part of the FIA signal. Taking into account that the FIA signal is characterized by an infinite number of C^g values, where C^g is the concentration at any point on the gradient, D may be defined as:
D^g = C⁰/C^g

The values of D^g range from infinity (C^g = 0) to unity (C^g = C^max). Thus, D = 1 indicates no dilution of sample at the center of the injected sample zone and D = 3 indicated a dilution factor of 3. As a function of D, the FIA systems may be classified in FIA systems using limited dispersion (1<D<3); medium dispersion (3<D<10); and large dispersion (D>10). FIA systems in which the analytical determination is based on chemical reactions must be designed in such manner to provide D of 3 or more, allowing adequate mixing of sample with the carrier reagent with a moderate loss in sensitivity due to the dilution.

There is a tendency to regard D as a characteristic of the manifold, but it should be remembered that factors that affect the diffusion of the injected sample such as its size, viscosity of the carrier stream, etc., will affect the D value, too.

A considerable amount of experimental works summarized that D is influenced by:

1. 
   sample volume. D increases with the volume injected because a large volume of sample leads to a large zone and to a high sensitivity.
   
2. 
   length of reactor, tubing between injection valve and detector. D increases with the squared root of reactor length. But the reactor must be long enough to allow the reaction product development.
   
3. 
   flow rate. D increases with the flow rate, but for faster flow rates a loss in sensitivity can occur since there is less time for the chemical reaction to develop.
   

The instrumentation for FIA is simple, as it is show in Figure III.2.10. The requirements for designing a FIA system include a pulse-less, easily controlled flow, a reproducible injection of the sample; a detector equipped with a flow-cell and readout devices.

Flow injection analyzers are today commercially available as integrated units comprising valve(s), pump(s), detector, auto-sampler and computer controlled/data station from a number of manufacturers in a number of countries. Their distinguishing feature is their automated sample-handling capacity, hundreds of samples being analyzed for different analytes per unit of time.
Table III.2.1. Typical parameters for a FIA system of medium dispersion.

Parameter	Value
Injected sample volume (L)	10 – 200
Carrier flow rate (mL/min)	0.3 – 2.5
Reaction coil length (cm)	10 – 200
Tubing inner diameter (mm)	0.3 – 0.8
Flow-cell volume (L)	8 – 40
Sample throughput (h-1)	60 – 360
Time for one determination (sec)	10 – 60

The syringe-drive, pressurized-gas, and peristaltic pumps are some means of propelling the carrier stream(s). The advantage of the syringe-drive pumps is the pulse free flow, but periodic refill is necessary. The most used and suitable device for propelling the liquid flows is the peristaltic pump, because it may accommodate several channels whereby, according to individual tube diameters, equal or different pumping rates may be obtained. A schematic diagram of a peristaltic pump is presented in Figure III.2.13.

The popularity of the peristaltic pump can be attributed to its design, which uses an electric motor to turn a set of rollers. The rollers compress and release a flexible tube as they pass across the tube. The liquid will follow the rollers until the tube is no longer compressed and by this time a 2nd or even 3rd roller is compressing the tube, preventing flow back, pushing the initial dose of liquid out of the pump. By a repetitive operation as the rollers rotate a pumping movement is created, which has an element of pulsing as a standard. By the squeezing of the tube the rotor creates suction lift and outlet pressure. This squeezing action creates a vacuum, which then draws fluid through the tubing to achieve the pumping action. Because the flexible tubing is the only wetted part, maintenance and cleanup are simple and convenient.

Peristaltic pumps are never completely pulse free. A well-constructed pump must: a) stop and start instantaneously, allowing the precise control of all moving streams for stopped-flow or intermittent pumping functions; b) have many closely spaced rollers that rotates rapidly and thus the tubes are compressed frequently for short periods of time, generating a pulsation of high frequency and a low amplitude. The Gilson Minipulse and the Ismatec Minipump are examples of pumps that allow a stepwise regulation of the flow rates, generate nearby pulse-free flows, start and stop instantly. A new alternative is the use of individual, computer-controlled peristaltic pumps, which make it possible to run each analytical channel under full computer control and to select individual methods to be run on a batch of samples.
Sample injection

In order to save reagent, to increase the residence time with a minimum sample dispersion, and to accomplish zone sampling ingenious flow manifolds have been designed with different devices for sample introduction. Such devices have evolved from the first device equipped with a syringe, as described by J. Ruzicka and E. Hansen, to a device equipped with an automatic injection with multiple injection sections, culminating in the widespread employment of six port valves, which are in fact rotary injection valves. The valve injection mode approximates the plug injection and is a more facile way of inserting well-reproducible sample volumes into the carrier without disturbing its motion. Figure III.2.14 shows a rotary injection valve and its operating mode.

The loading and injection steps employed by displacing a movable part between two resting positions are a common feature of these devices. Air bubbles and pressure surges must be avoided during the injection because they will modify the pattern of the flow in FIA system, affecting dispersion and precision. The dimensions of the sampling loops define the volume of the injected sample.
Flow lines, reactors, connectors

Pump tubes are available in different materials and have two bridges for fix them around the mobile part of pump. These bridges are often color-coded to designate the delivery rate. The choice of the peristaltic tube material depends on the nature of the solvent.