Flow Injection for Chemical Oceanography

Chemical Analysis for Oceanographic research  is a very challenging area, relying on a  cutting edge, robust analytical instrumentation, that  must be easily transferable  from laboratory on board of a research vessel, where it must function well, with minimum maintenance for long periods of time. Even more challenging is development of automated analyzers, that can be deployed, while functioning  well for days, or months at a time, without human intervention.

Automated assays, based on Flow Injection, mainly with spectrophotometric and fluorescence detection, have found numerous  applications for oceanographic assays worldwide.  They have been successfully used both for nutrient assays ( phosphate, nitrate, silica, ammonia) as well as for assays of trace elements ( Fe, Mn, Co and Zn).   For specific method descriptions please review the environmental methods page.

Assay of nutrients at micromolar  level has been based on  colorimetric protocols originally developed  in Flow Injection  format for serial assays of soil and water.  Since these reagent based assays, yield products  with molar extinction coefficients of 20,000 A/ mol/ cm, a typical  detection limit lies  at  1 microM  level of corresponding analytes,  with  a limiting instrumental response of  0.02A in 1 cm long light path cell. By using extended light path flow cells, as available with FIAlab 2500, automated systems, with fiber optic instrumentation and Ocean Optics spectrophotometers, valuable improvement  in detection limit is now achievable.

Assay of  trace elements is, however, far more challenging proposition, since desirable detection limit for assay of iron is less than 1 nM, for ocean regions deplete of Fe, while Fe replete regions are defined by Fe content higher than 10nM.  In order to reach  such a detection level, that is almost 1000 times lower than achievable by standard spectrophotometry, researchers use an ingenious combination of  preconcentration techniques with catalytic determination  of Fe(III) automated by means of  a Flow Injection technique.  (ref Measures C.I. et.al, Marine Chemistry  50 1995).  Use of FIAlab 2500, automated analyzer with fiber optic instrumentation, and 50 cm extended optical path, will allow running with the elimination (or at least the minimization) of the preconcentration process, and will improve economy of reagent and time consumption.

While Flow Injection is ideal for "batch" assays, for online systems there are, however, there are some disadvantages that should be considered.  First, reagents are pumped and used continuously, even during the start up periods, that may be quite  long, if the flow channels have to be thoroughly washed, in order to minimize blank values. Next, the peristaltic pump's tubing gradually, stretches, the flow rates change in time and thus must be periodically replaced..

Sequential injection, is a new generation of flow injection technology, that holds a promise for research and routine in chemical oceanography. The system uses computer controlled programmable flow that minimizes use of reagents and waste generation. The flow is programmed to suit individual steps of assay protocol, and colorimetric assays are carried out in stopped flow mode, that yields reaction rate measurements. The instrument microSI –“lab-on-valve” system, has only two moving parts – a multiposition valve and a syringe pump. It has a small weight, small footprint and thus easily transported to and on a marine vessel. 

The flow cell, integrated within the “lab-on-valve” module has selectable optical path – from 1 mm can be extended up  to  50 cm, and is connected by fiber optics to a light source ( Tungsten or LED) and to a Spectrophotometer. FIAlab software integrates all instrument functions, including microfluidic manipulations, spectra selection,  data collection, assay calibration and data transmission via e-mail. The assay may be developed in a laboratory, transferred on board within the same instrument, and even, after instrument has been appropriately packaged or housed – to be deployed.