Digestion and Dissolution
Since it is desirable to automate the entire assay protocol, including sample preparation steps, both sample dissolution and digestion have been proposed to carry out in flow based systems, both in FI and SI format.
Yet regrettably, only batch digestion is, in our experience, practical and reliable, resulting in 100% decomposition yield. Even flow based digestion of soluble compounds ( e.g. polyphosphates), should be carefully reconsidered, since not all compounds can be mineralized with 100% efficiency during the short time, when a sample moves downstream, even at such a slow flow rate that only 6 samples can be analyzed per hour.
On-line Digestion
While the FIAlab 2500 can be easily be configured into an automated analyzer designed to decompose organic compounds for assay of Total N content, it is not practical to use such an instrument. Besides the use of additional components: additional peristaltic pumps, UV heater/photoreactor and heating coils, pumping aggressive chemicals under elevated temperature is neither advisable, for safety reasons, nor practical. Corrosive vapors will soon damage the instrument and its electronics. Other issues are long startup, labor intensive shutdown, large consumption of aggressive chemicals and maintenance of a complex flow system. Since complete digestion requires time, typical sampling rates are less than 10 samples/ hour.
Batch Digestion, and Dissolution
Decomposition and digestion in the batch mode can be carried out automatically with 100% efficiency, using small volumes of acids at a well controlled temperature for a precisely selected period of time. Batch digestion is very efficient since it safely processes a large number of samples simultaneously. Once dissolved, samples can be loaded into an autosampler, split for assays of different analytes, and analyzed at a high sampling frequency by FIAlab 2500 or FIAlab 2600.
The AIM600 Automated Programmable Digestion System
The AIM600 Programmable Digestion System performs acid digestion of large numbers of samples under controlled conditions. The digestion block heats samples at a controlled rate in the presence of one or more acids to the required temperature. The AIM600 then holds the samples at that temperature until digestion is complete. Under these conditions, the acid breaks down the samples to release the component elements for subsequent analysis.
Typical applications include:
TKN and Total Phosphorus Digestion
Mercury Digestion
COD digestion
Metal dissolution
The AIM600 Digestion System
The AIM600 is available as a 50 place unit for 75ml or 100ml digestion tubes or a 28 place unit for 250ml digestion tubes. The system comprises the block digester unit, microprocessor controller, tube rack/draft shield, a set of digestion vessels and a cooling stand that supports the tube rack above the block. The entire unit has been designed for durability and robustness. The block has a stainless steel covering to provide a greater acid resistance. Similarly, the tube rack and cooling stands are also manufactured from the highest quality stainless steel. Following are application notes for TKN and Total Phosphorus Digestion of water samples. Similar application notes are available for soil and plant materials.
Total Kjeldahl Nitrogen
Total Phosphorus
FIAlab standard method for the Total Kjeldahl Nitrogen (TKN) assay using FIAlab-2500/2600/2700 system.
Principle:
Samples are first digested in AIM600 digestion system with a sulfuric acid process prior to FIA assay, converting Kjeldahl nitrogen to the ammonium cation and measured as ammonia using the salicylate method.
The salicylate method is a variation of the Berthelot-Phenate method but does not require the use and disposal of toxic phenol. The salicylate method involves a three-step reaction sequence. The first reaction step involves the conversion of ammonia to monochloroamine by the addition of chlorine. The monochloroamine then reacts with salicylate to form 5-aminosalicylate. Finally, the 5-aminosalicylate is oxidized in the presence of sodium nitroferricyanide to form a blue-green colored dye that absorbs light at 650 nm.
FIAlab standard method for total phosphorus assay using the FIAlab-2500/2600/2700 system. Variations of this method for lower/higher concentration samples exist. Please contact FIAlab for additional details.
Samples are first digested in AIM600 digestion system prior to FIA assay, converting total phosphorus to reactive phosphorus. This can be then measured through the standard reactive phosphorus assay.
Soluble phosphorus is typically found in three forms: 1) Orthophosphate 2) Organo-phosphates 3) Condensed phosphates (metaphosphates, pyrophosphates, and polyphosphates). In this method all three forms can be analyzed but only reactive phosphorus can be directly determined without any pretreatment. Reactive phosphorus is simply hydrolyzed orthophosphates. Orthophosphates react directly with molybdate anions to form a yellow-colored phosphomolybdate complex. This complex is then reduced by ascorbic acid to create a molybdenum blue species that has a broad absorbance range from 700nm to 900nm, making this method suitable for a variety of light sources and detectors.