Autonomous Batch Analyzer (ABA) for trace ammonium measurement in natural waters

In recent decades analytical chemists have devoted a considerable effort to automating classical wet chemistry. The instruments manufactured for analysis of a large number of samples can be categorized into two main groups—batch and continuous flow analyzers. In a batch analyzer each sample is processed in an individual container prior to measuring the analytical signal by a detector. The inherent advantages of a batch analyzer are that it is easy to achieve complete mixing of the sample with the reagents and there is minimal cross-contamination between samples. Moreover, the analytical procedure and chemical reagents in classical manual wet chemistry methods are in most cases readily adaptable to automated batch analyzers. However, there are inherent disadvantages: one, the complexity of their moving parts, which are both expensive to manufacture and subject to relatively rapid wear; and two, the need to employ multiple analyzers when high sampling rates are required. In continuous flow analyzers, samples are aspirated into a flow stream. They are then mixed with reagents in a fixed volume ratio proportional to the flow rate and a reaction product forms in the solution which flows through a continuously recording signal detector.The movement of the liquids (samples and reagents) is driven by a peristaltic pump. The great advantage of continuous flow analyzers is their high sample throughput and high precision; this precision results in lower limits of detection. Moreover, samples can be readily pretreated online, including extraction, filtration, dialysis, digestion and distillation, prior to the introduction of the reagents. That said, continuous flow analyzers have inherent disadvantages. These include much higher reagent consumption, system carryover (cross-contamination), frequent tube replacement and the necessity of major hardware reconfiguration when changing analytical approaches. As a result, experienced personnel are needed not only for daily calibration but also for routine operation and maintenance. Last, it has not proven easy to adapt some classical wet chemical methods for continuous flow analysis. 

At present, flow injection analysis (FIA) and gas-segmented continuous flow analysis (SFA) are the most prevalent automated wet chemical analyses. In 1990, sequential injection analysis (SIA) was introduced as an alternative to continuous flow analysis. SIA is based on a single multi-position valve coupled to a single bidirectional pump. SIA has lower reagent consumption/waste generation and much lower maintenance requirements. There is also no need to reconfigure the manifold to employ different chemistries because all required fluid-handling steps can be carried out through the same multi-position valve. Even more recently, syringe pumps have been utilized in lab-on-valve (LOV), hybrid flow analyzer (HFA), and multi syringe flow analysis systems. We developed a new system we call an autonomous batch analyzer (ABA). The ABA is similar to batch analyzers with respect to mixing efficiency and adaptability with respect to classical manual wet chemistry but is far simpler, less expensive and easier to operate. The method provides a sample throughput similar to SIA, but has inherently higher sensitivity because of complete and rapid mixing. Moreover, there are no carryover problems. Last, the ABA can produce a calibration curve by auto-dilution from a single stock standard solution with the same accuracy as traditional manual calibration methods. The ammonium analyzer we developed is based on the ABA method. With its simpler design, the system is robust, flexible, inexpensive, and requires minimal maintenance. The sampling frequency is 8 h-1 and the limit of detection is 1 nM. Reproducibility is 0.6% (n=10) at an ammonium level of 200 nM. In addition, the system produces a calibration curve by autodilution from a single stock standard solution with the same accuracy as traditional manual calibration methods. Representative field data and comparisons with standard EPA methods confirm the utility of the ABA. 

AutonomousBatchAnalyzer (ABA) for ammonium determination:
Std, Standard; R1, Phthaldialdehyde (OPA) solution; R2, Sulfite in formaldehyde
solution; FL, Fluorescence detector; and DIW, Deionized water.

 

Publication:

(1) Amornthammarong, N.; Zhang, J.-Z.; Ortner, P.B. Anal. Methods, 2011, 3, 1501-1506.

(2) Amornthammarong, N.; Zhang, J.-Z.; Ortner, P.B.; Stamates, J.; Shoemaker, M.; Kindel, M.W. Environ. Sci.: Processes Impacts. 2013, 15, 579 - 584.