Advantages and Disadvantages of Using Discrete Versus FIA/SFA Technology in Automated Monitoring of Ions in Water

Transcription

1 Application Note Advantages and Disadvantages of Using Discrete Versus FIA/SFA Technology in Automated Monitoring of Ions in Water Keywords Automated Chemistry Analyzer CNSolution FS 3100 DA 3500 Discrete Analyzer FIA/SFA Flow Injection Analysis Flow Solution IV Segmented Flow Analysis Introduction Automated wet chemical analysis using flow injection analysis (FIA) or segmented flow analysis (SFA) has been well established in environmental laboratories since the 1970s due to flexible configurations, low detection limits, and in-line sample pretreatment capabilities. Discrete analyzers were successfully introduced into the environmental market approximately four years ago to further improve automation of wet chemistry applications and methods. Just as discrete analyzers replaced FIA and SFA systems in the clinical market over a decade ago, the same is beginning to occur for environmental applications. Discrete analyzers offer much greater sample capacity per hour, better ease of use, and significantly lower reagent consumption, a key point with increasing waste disposal costs. Both technologies are currently accepted in environmental laboratories, even though discrete methods are still in the process of obtaining full approval by various regulatory agencies. Each technology has its own advantages and disadvantages. This application note compares the advantages and disadvantages of each technology and presents data performance comparisons for discrete vs. FIA/SFA analyzers. It discusses new trends in automated wet chemistry, and describes how to select the right technology for many individual applications. Automated Chemistry Analyzer SFA FIA FIA/SFA Discrete/Robotic Analyzes a wide range of concentrations: ppt, ppb, ppm, and percent. Accomodates on-line distillation, dialysis, digest extraction, and separation New technology in environmental analysis with superior ease of use Presented at the 2005 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Orlando, FL February 27 March 4, 2005 Figure 1. Automated chemistry analyzers

2 What is FIA/SFA Technology? FIA involves injecting small segments of sample solution into a flowing carrier stream. The sample stream merges into the reagent stream to produce a compound that can be determined in a flow-through detector, typically a photometric or amperometric detector. SFA measures analyte quantities in a continuously flowing stream of sample or sample wash. Bubbles injected consistently throughout the stream ensure complete mixing and minimize sample band dispersion. FIA/SFA is the instrument that combines FIA and SFA processes into one system, gaining advantages from both technologies. Figure 2. Flow Solution IV and CNSolution FS 3100 What is a Discrete Automatic Chemistry Analyzer? A discrete analyzer automatically adds sample and reagent to a small cell, i.e. cuvette. It measures a reaction product while still in the cell or transfers it into a flow cell. It processes the sample in batches or individually. Key advantages include speed and use of only microliter amounts of sample and reagent. It exactly mimics the operations of traditional manual wet chemistry methods. Figure 3. DA 3500 Discrete Analyzer Advantages of Using Discrete Automatic Chemistry Analysis for Wet Chemistry Analysis Much less sample volume than any other method, benefiting laboratories with small sample volumes (soil paste extracts) Low reagent consumption (about 3,000 tests per 25 ml for the nitrite method); only uses the exact reagent amount required for each test compared to a continuously-flowing system; generates less waste Automatic dilutions without added hardware No baselines to watch, no air bubbles to add, no peak shapes or flows to monitor No bubble formation in the flow cell No pump tubes to change or maintain Switch methods automatically without changing hardware True walk away capability Enormous improvements in setup, operational time, and ease of use Stable calibration curves over long periods of time 2

3 Disadvantages of Using Discrete Automatic Chemistry Analysis for Wet Chemistry Analysis Presently, discrete analyzers can only do simple colorimetric chemistries Cannot achieve ultralow detection limits Time-consuming sample preparation steps such as distillations, digestions, and matrix removal or enhancement performed manually before testing by a discrete analyzer. Cannot perform complex chemistries such as on-line gas diffusion, dialysis, distillations, extractions, and digestions Table 1. Hardware differences between FIA/SFA and discrete analyzers Parameter FIA, SFA, or FIA/SFA system Discrete Analyzer Detectors Colorimetric detector with individual optical interference filter Amperometric detector ISE detector Flame photometric detector Colorimetric detector with multiple optical interference filters built into a filter wheel (8 12 wavelength) Or Spectrophotometer (27 30 wavelengths or scanning) ISE detector option Sample delivery system Multichannel peristaltic pump High-precision syringe or piston pump Heater Resistant-based heater Resistant or Peltier-based heater Dilution device Add-on dilutor Built-in dilution function Process Uses continual flow mode through a flowcell, parallel batch analysis uses a multiple channel system Individual cuvette, sequential batch analysis or random-access single samples Table 2. Selected discrete analyzer methods vs. FIA or SFA methods Parameter Discrete methods 1 FIA or SFA Methods 2 Range (mg/l) MDL (mg/l) Range (mg/l) MDL (mg/l) Ammonia Chloride mg/l Cyanide Nitrite Orthophosphate Phenol SiO TKN Discrete method generated on the DA Flow method generated on the Flow Solution IV (some based on seawater methods) 3

4 Table 3. Analysis cost comparison Parameter Discrete Flow Manual Typical cost per sample $0.10 $0.30 (reagents +) $0.20 $0.50 (reagents) $0.50 $2.00 (reagents) Typical reagent volume per sample ml ml ml Tests per liter of reagent Thousands Hundreds Tens Waste per sample Microliters Milliliters Deciliters FIA/SFA vs. Discrete Analysis by Application Discrete analysis technology is suitable for simple chemistries with medium or high concentration ranges. Discrete analysis has difficulty handling chemistries that require more than four reagents, high heat (>50 C), or extra device, such as a column, dialysis block, in-line distillation, digestion, extraction device, etc. Therefore, some FIA/SFA applications cannot be run on a discrete analyzer. Table 4. FIA/SFA applications not compatible with discrete analysis Analyte Ammonia in soil Anion surfactant Fluoride On-line cyanide Nicotine in tobacco Nitrate-nitrite in soil Orthophosphate in soil Penicillin Phenol Reducing sugars in tobacco Reducing sugars in wine Seawater chemistries Sulfate, colorimetric Sulfates in wine Total dissolved nitrogen Total Kjeldahl nitrogen in soil Total nitrogen in feed Tobacco starch Total acidity in wine Total sugar in tobacco Total dissolved phosphorus Total phosphorus in soil Volatile acidity in wine Issue Six reagents, dialysis, heat Phase separator ISE option, slow response On-line distillation Five reagents, cadmium column, dialysis Six reagents, dialysis, heat Nine reagents On-line distillation, heat Sub-ppb detection limit Cation exchange column On-line digestion Six reagents, dialysis, heat block Perchloric acid Two-step heating On-line digestion Five reagents, dialysis, heat On-line distillation Advantages of Using FIA/SFA for Automated Wet Chemistry Analysis Well-accepted technology for wet chemistry with regulatory approval Applicable to a broad range of compounds using a variety of simple photometric and electrochemical detectors Accurate and precise sample manipulation for high precision and low detection limits Wide range of useful manifold components incorporated in the analyzer for complex operations such as dilution, trace enrichment, gas permeation, dialysis, phase separation, and digestion 4

5 Disadvantages of Using FIA/SFA for Automated Wet Chemistry Analysis Higher reagent consumption and more waste due to continuously flowing reagents Baseline monitoring required Air addition for SFA Frequent pump tubes replacement required Hardware reconfigured when switching methods Trained and experienced personal needed for setup, operation, and troubleshooting Daily calibration required Separate autodiluter used for dilution, increasing capital costs System carryover On-line Distillation Phenol by SFA Provides Excellent Data Performance Distillation Head Distillation Reagent Air To Waste 505 nm 160 C Sample 4-AAP Resample FeCN Figure 4. Flow diagram for on-line distillation phenol by SFA Table 5. Analysis results for on-line distillation phenol by SFA 5-ppb Standard Average = 4.41 ppb Standard deviation = 0.14 ppb % RSD = 3.08% MDL = 0.43 ppb 5

6 Nitrate Nitrogen in Seawater by SFA Achieves ppt-level Results Figure 5. Nitrate plus nitrite and nitrate nitrogen in seawater by SFA, precision at 0.02 µm (<10% RSD) Table 6. Analysis results for nitrate plus nitrite and nitrate nitrogen by SFA Range µm Throughput 36 samples/hour Precision at 0.02 µm <10% RSD Precision at 40 µm <1% RSD Method detection limit µm Conclusion FIA/SFA has been the main technology used by environmental laboratories in the past 20 years. Discrete technology is a new technology in the environmental market since Because of its ease of use and low reagent consumption, regulatory resistance is rapidly decreasing and the technology has been well-accepted by environmental laboratories. However, both FIA/SFA or discrete analyzer have their own advantages and disadvantages. Selecting an analyzer depends on the individual application and the laboratories primary priorities. P.O. Box 9010 College Station, TX Tel: (979) FAX: (979)