Improved Performance in the Analysis of Drinking Waters and Wastewaters by EPA Method 200.8 with an SC-FAST System

The application of an SC-FAST introduction system to the analysis of natural and certified water samples is described. The SC-FAST system consists of an autosampler, a switching valve, a high efficiency PFA-ST nebulizer and Peltier-cooled cyclonic spray chamber to perform analysis by direct nebulization. The potential benefits of this introduction system are numerous and include increased throughput, reduced memory effects, increased stability, lower reagent consumption and less instrument maintenance. These parameters are evaluated as the system is applied to EPA Method 200.8. Results indicate that analysis is accomplished in 90 s with significantly improved washout compared to ICP-MS analysis by conventional introduction. Improved Performance in the Analysis of Drinking Waters and Wastewaters by U.S. EPA Method 200.8 with an SC-FAST System A P P L I C A T I O N N O T E IC P -M A S S S P E C T R O S C O P Y www.perkinelmer.com Authors Maura Mahar Kenneth Neubauer Zoe Grosser PerkinElmer, Inc. 710 Bridgeport Avenue Shelton, CT 06484 USA Introduction Summary of Method EPA Method 200.8 contains a lengthy, detailed description of procedures, both required and recommended. A summary of the experiments that must be performed to establish the performance of the instrument, along with a 10-step daily analysis procedure, can be found in Table 1. Instrument A PerkinElmer® SCIEX® ELAN® 9000 ICP-MS was used for the analysis of natural and certified water samples described in this work. Sample introduction was accomplished with an Elemental Scientific, Inc. sampler changer (SC)-FAST sample introduction system. Instrument conditions for the ICP-MS and FAST, as well as experimental parameters used throughout this work, are presented in Tables 2 and 3. The SC-FAST is a sample introduction system consisting of an autosampler, diaphragm vacuum pump, 6-port switching valve, merging stream manifold, high efficiency PFA-ST nebulizer, and a Peltier-cooled cyclonic spray chamber. The system is shown schematically in Figure 1. The contents of the sample loop, which is large enough to provide a steady state signal, are injected into an acid carrier stream that merges with the internal standard solution. The length of tubing connecting the output of the switching valve with the nebulizer are short so that so that the time between injection and measurement is minimized. The instrument response as a function of time for back-to-back sample injections is illustrated schematically in Figure 2. The FAST system has been designed to increase throughput and to decrease sample carryover compared to conventional sample introduction systems. The carrier and internal standard solutions provide a continuous flow of solution to the nebulizer, which creates a nearly constant steady-state situation. The constant plasma steady-state, combined with the short distance the sample travels to reach the nebulizer, allows sample uptake and stabilization to take place more rapidly (see Figure 2). Furthermore, during analysis the FAST system rinses the autosampler probe and moves it to the next sample vial. The FAST system thus completes the analysis of a sample (following Method 200.8 protocol) in 90 seconds (sample-to-sample), about half the time needed to perform the same analysis with conventional sample introduction. Increasing the sample throughput increases productivity and lowers costs, both laborand instrument-related. 2 Figure 1. Schematic of a FAST ICP-MS introduction system2. Table 1. Summary of Method 200.81. Established Initial Performance Data 1. Linear Range 2. Perform IDLs and MDLs 3. Analyze Quality Control Sample with Acceptable Performance Daily Analysis 1. Light Plasma, Allow 30 Minute Warm-up 2. Tune Instrument a. Per Manufacturer’s Instructions b. Use Tuning Solution Specified in 200.8 (10 ppb Be, Mg, Co, In, Pb) 3. Perform Mass Calibration Check, Adjust if Change >0.1 amu 4. Perform Resolution Check, Adjust if >0.75 amu at 5% peak height 5. Calibrate Using Blank and Standards a. Monitor All Masses Necessary for Interference Correction 6. Screen New Samples for Relative Levels and Presence of Internal Standards 7. Run Instrument Performance Quality Control Samples 8. Run Analytical Batch Quality Control Samples 9. Run Analytical Samples 10. Review Results of Quality Control Samples for PASS/FAIL Criteria As the sample solution is not in contact with the peristaltic pump tubing, washout times and memory effects are decreased. The sample is contained within the sample loop, which is made of chemically resistant Teflon®. As the volume of sample introduced into the nebulizer is decreased, the amount of salt that is deposited on the cones is also decreased. The FAST system uses pump a total flow rate between 300 and 400 μL/min, much lower than flow rates used with conventional introduction systems. These lower pump flow rates, combined with the shorter analysis time, reduce the amount of salt deposition on the cones, reagent consumption, and waste production, all of which lower maintenance operational costs. In addition to higher throughput and reduced memory effects, the FAST system allows for the online addition of internal standards, simplifying sample preparation and decreasing the opportunities for contamination. FAST method The FAST system is controlled through the ELAN software. Since the FAST system allows for direct nebulization using a fixed sample volume, a sample injection profile was first taken to determine an appropriate read delay and analysis window. The read delay and analysis window chosen for a 1 mL sample volume injected by a carrier moving at 0.5 mL/min were 20 s and 60 s, respectively. The timing parameters of the quantitative analysis were set to be within this read window. Therefore, the read parameters listed in Table 1 were chosen such that three replicate measurements of the twenty-six elements outlined in this method could be made in 60 s. For samples with the tendency to foam or outgas, 5 s rinse step was added to the end of each measurement to pump air through the probe tubing. Evacuating the loop also reduces hydrodynamic resistance when trying to rapidly load the subsequent sample. The FAST method consists of three steps. The first step is to load the sample loop. The injection valve remains in the “load” position while a sample solution fills the sample loop at 20 mL/min (via a diaphragm vacuum pump) at 3 to 4 times its volume. Excess solution from the sample loop is sent directly to waste. While the loop loads, carrier and internal standard solutions are pumped continuously into the nebulizer (see Table 1 for identity, concentration, and flow rate of carrier and internal standard solutions). The second step involves switching the valve to the “inject” position, allowing the carrier stream to push the contents of the sample loop into the nebulizer. All data collection occurs during this step. While data collection occurs, the sample probe performs a 4 s rinse at a rinsing station located at the end of the autosampler. The rinse station consists of 2 flowing reservoirs, which allows the user to program 2 individual rinse steps if desired. The injection valve is in the “inject” position during the rinse step which directs the rinse solution through the sample probe tubing and directly to waste. Therefore, a relatively aggressive rinse solution could be used without concern that it would pass through the nebulizer and affect subsequent measurements. The third step is to rinse and reload the sample loop. The valve is switched back to the “load” position where first air, and then the next sample solution are pumped through the loop. In this step, the next sample is used to rinse the loop of the previous sample before it is loaded into the loop for analysis. Standards All solutions were prepared using 18 megaohm-cm water and double-distilled nitric acid. All acid concentrations reported in this document are described as a relative (v/v) percentage. Reference materials for this work were obtained from High Purity Standards (Charleston, SC) and from NIST, (Gaithersburg, MD). Double distilled nitric and hydrochloric acids were purchased from GFS Chemicals, Inc. (Sidney, BC, Canada). 3 www.perkinelmer.com Figure 2. Profile for back-to-back sample injections of 1 ppb U.