Instruments4Chem

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What Is Flow-Injection Analysis (FIA) ?

Flow injection is a technique used to perform chemical analysis in a flowing stream of liquid, and is well suited to remote sampling. The user's flow manifold allows various introduction/reaction/mixing steps to be carried out in a well defined sequence.

The diagram below shows a stylised schematic of a very simple FIA manifold; here the type of detector (D) is deliberately not specified. Commonly, it would be colorimetric – perhaps employing a light emitting diode (LED) as the light source. Let us assume that the carrier is a colourless liquid and a sample (a coloured dye) is injected via an injection valve (more on this shortly). If the LED in the colorimetric detector has a complementary colour to that of the dye, a significant drop in intensity will be recorded by this detector as the injected sample passes through. Some data showing this appears later on this page.

This example is purely illustrative - in order to perform a useful chemical analysis an FIA manifold utilises specific chemistry that is initiated by mixing the sample (previously held in a sample loop) with one or more reagent streams. Typically, a chemical reaction then takes place downstream in the mixing coil, resulting in a transient absorption. By calibrating the system with a series of standards the FIA manifold can be used to measure the concentration of target analytes.

Sample Injection


A 6-port valve is commonly employed to load a sample loop with analyte. Once loaded, this analyte can be injected to enable an analysis to be carried out, as described below.
With the 6-port valve in the LOAD position (left), the carrier C and analyte A streams are maintained as separate flows. A continuous flow of carrier is pumped P through the flow-through detector cell D and out to waste W while an injection loop L is filled with sample that is continually recycled back to analyte reservoir A.

When the valve is switched into the INJECT position (right), the path of the carrier stream is diverted to pass through and pick up analyte held in the sample injection loop. The sample thus enters the carrier stream and is transported on to the detector.

The Propeller Flow Injection Analyzer (FIA)

The Propeller FIA is designed to greatly simplify control and data-logging in flow-injection analysis experiments. To make a complete flow-injection analysis system one adds a peristaltic pump, a flow manifold and a source/detector such as VODS.

Two mini-DIN connectors are visible on the right hand side of the PCB - one supplies logic signals to a solenoid valve that controls the sampling loop’s load/injection cycling and the other connects to the user’s source/detector system.

Connectors


Two mini-DIN6 connectors visible on the right hand side of the PCB are used to make external connections to the FIA board. The connector at the bottom (J2) brings in signals from one of two detectors. In addition, 3 power rails (V+, 5V and 3.3V) are brought out to this connector. V+ comes directly from the plug-pack input at left, while the other two supply rails are supplied by on-board voltage regulators.

The top connector on the right hand side of the PCB has 3.3V logic signals to control a 6 port switching valve fitted with a microelectric actuator. A good source of valves/actuators for this application is VICI. Voltage for a peristaltic pump and a pump speed control are also available on this mini-DIN connector.
Pin
Colour
Function
1
Brown
GND
2
Red
TSL235 input
3
Orange
TSL25x input
4
Yellow
3.3V
5
Green
5V
6
Blue
V+
Pin
Colour
Function
1
Brown
GND
2
Red
Solenoid valve A
3
Orange
Solenoid valve B
4
Yellow
Pump Speed
5
Green
NC
6
Blue
V+ (pump motor)
mini-DIN6 (J2)
mini-DIN6 (J5)

Operation


The FIA PCB accepts the digital output signal from a light-to-frequency converter such as the TSL235 from AMS. This inexpensive detector chip can be installed into a VODS (versatile optical detection system) unit as described elsewhere. By adding an LED and flow-through cell the FIA/VODS combination provides the user with an extremely flexible, yet low cost colorimetric FIA detection system.

Alternatively, a light-to-voltage (LTV) converter such as a member of the TAOS TSL25x family can be installed into a VODS and connected to the FIA PCB. This type of detector is well suited to fluorescence or chemiluminescence detection.

The FIA PCB has an on-board P8X32A-Q44 Propeller chip as controller and a 32k EEPROM for program storage (24LC256), plus a USB interface to a host PC (FT245RL). It also has an on-board voltage regulator so that an externally connected potentiometer can control the speed of a peristaltic pump in the user’s system.

When a light-to-frequency converter is employed as the detector, the output frequency is measured on a dedicated Propeller input pin. If a light-to-voltage converter is being used, the optical signal is measured by a software ADC using two of the Propeller’s I/O pins. The two possible detector inputs enter the PCB via mini DIN connector J2.

The outputs of two additional pins on the Propeller are each connected to transistors, providing TTL logic levels to control the load/inject cycling of a 6-way switching valve and VICI Micro-Electric Two Position Valve Actuator – these are brought out on mini DIN J5.

A LabVIEWTM front panel records the detector output signal and display FIA-“grams” in real-time, in addition to giving the user control over the load/inject timing.

To summarize, the functions of the Propeller FIA are :

(a) to provide user control over peristaltic pump motor speed
(b) to allow switching between load and inject modes of a solenoid valve
(c) to record an optical signal in real time from two different types of detector
Stacks Image 1155
An FIA breadboard system, containing a multi-channel peristaltic pump, a 6-port injection valve, a gas diffusion cell and perspex mixing manifold, and two different detectors, one colorimetric and the other using a sensitive photomultiplier for chemiluminescence detection.

What does an FIA System look like ?


A complete FIA system typically consists of:

a. Reagent and waste reservoirs
b. A peristaltic pump for reagent delivery

A peristaltic pump forces fluid through plastic bridge tubing by peristalsis, due to the action of multiple rollers forced against taught tubing. This action produces a continuous and relatively pulse-free flow of fluid, whose flow-rate depends on the rotation rate of the rollers and the inside diameter of the tubing. The rollers are generally long enough that multiple tubes can be operated simultaneously. The peristaltic pump I typically use is a Watson Marlow Alitea model 040.DH1C.04L - this unit can accommodate up to four tubes.

c. A sample injection valve with a sample injection loop of known volume
d. One or more reaction (mixing) coils

As noted earlier, some experiments require a chemical reaction to take place prior to sample detection. Additional tubing can be added downstream from position 6 on the six-way valve prior to the flow-through detector cell to allow for these reactions to occur. Inclusion of a mixing coil is common, and like the injection loop, its volume can be altered by manually interchanging coils of different lengths. The length of a mixing coil determines the extent of reaction, and so time should be spent to identify the optimum mixing coil length for each experiment.

e. A flow-through detector cell