Report
Automating Analytical Separations in Radiochemistry
Sequential injection techniques offer a versatile approach to automating chemical separations.
T
hough known primarily for automating simple chemical analyses, flow injection (FI) and sequential injection (SI) techniques provide a very versatile fluid-handling approach that can be used to automate chemical separations (1-3). We have been particularly interested in automating the separations that are re-
J a y W. G r a t e Oleg B. Egorov Pacific Northwest National Lciboratory
quired in radiochemical analyses by using SI methods to select and deliver samples, reagents, and eluents to a separation column. h/ven the material used to pack the column can be delivered to the column body using SI methods, creating the option of renewing the column material handsfree for each separation and analysis. In many cases, separation techniques are integrated with on-line detection instrumentation (radiometric or mass spectrometric) and, as this Report will discuss, this approach provides an automated analysis
technique for radioisotopes in nuclear waste. The sequential injection separation format we describe might also be applied to other analyses requiring preconcentration or separation by solid-phase extraction, extraction chromatographic, or ion-exchange methods prior to analyte quantification, and could serve as the basis for an automated separation workstation in such diverse applications as environmental analysis, bioanalytical chemistry, and medical isotope generation. Sequential injection separation
To put SI separation methods in perspective, it is useful to first consider the conventional approaches to analysis using FI and SI techniques. In classical flow injection analysis (FIA), a liquid sample is injected into a continuous nonsegmented flowing
Analytical Chemistry News & Features, December 1, 1998 779 A
Report stream of liquid using a multichannel peristaltic pump and an injection valve as shown in Figure la (i). The sample zone progresses through a reactor to the flow cell of a detector. Reagents incorporated into the flowing stream generate reaction products that can be detected. FIA has been useful in automating serial assays for many application areas, including environmental measurements, agriculture, biochemistry, pharmaceuticals, and clinical analysis. Preconcentration and separation steps can also be incorporated into an FI system, typically through the use of a minicolumn containing a sorbent material (2). In such procedures, the desired analytes are collected on the sorbent material from one solution and then eluted in another. With few exceptions (4), such column separations have been carried out with continuous forward fluid flow. Recently, FI methods have been supplemented with SI techniques using a reversible fluid drive (e.g., a precision digital syringe pump) and a multiposition selection valve to pull zones of various solutions and reagents into a holding coil, as shown in Figure lb (3,5). The stacked zones in the holding coil are then pushed by the pump and carrier liquid through a reactor and into the detector. SI systems are mechanically simpler and more adaptable than FI systems and have the advantage that experimental parameters—such as zone sizes, number of zones, and flow rates—can be varied via computer control rather than by physically reconfiguring the system (e.g., installing a larger injection loop). In the field of separations, SI breaks the continuous forwardflow paradigm of FI and conventional chromatography by using programmed flow reversals. SI offers a flexible fluid-handling approach, particularly for column procedures that may require multiple eluents. Eluents samples and standards can all be nested around a multiposition valve and sefiiientially selected for delivery to the column An SI separation system with a packed column between the multioosition valve and the detector is shown in Figure l r In a conventional SI analysis, submilliliter volumes of reagent and sample solu780 A
tions are stacked in the holding coil prior to delivery to a reactor or detector downstream. Traditional column separations, however, typically require milliliter quantities of multiple solutions to wash the column and elute species of interest. A conventional SI system, with typical flow rates of 0.5 to 1 mL/min, can take several minutes just to pull these solution volumes into the system. As a result, solution loading can become a significant portion of the overall separation and analysis time. Higher flow rates cannot be used because substantial O f f SS11 T"f*
drops in long narrowbore holding coils result in bubble formation as solutions outgas To achieve higher throughput for procedures involving milliliter solution volumes in SI systems, a modified solution-handling approach is necessary. Replacing the narrow-bore (0.5-0.8 mm i.d.) holding coil with wider bore (1.6 mm i.d.) tubing allows flow rates so up to 11 mL/min without outgassing when loading solutions into the system. A new problem arises with the wider bore tubing, however: Severe dispersion and mixing occur between adjacent solution zones. Separating the selected solution from carrier water in the holding coil using an air segment prevents dispersion problems provided that each solution is pulled into the holding coil and pushed out toward the column before selecting the nPYt Qnlution (fi\ Employincr a wiHp hnre holding coil air segmentation quential loading and deliverv of solutions rather than zone stacking renresents a separation-optimized sequential injection a
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fluid-handling method. It provides rapid manipulation of milliliter quantities of discrete solution compositions and prevents undesirable gradients that might degrade elution peak shapes. Radiochemical analysis We are particularly interested in using SI techniques to automate separations and analyses of radionuclides in nuclear waste (7). Production of nuclear weapon materials has left the United States with a legacy of nuclear waste accumulated in underground storage tanks (Nuclear waste analysis, p. 782 A), as well as environmental contamination. Improved radiochemical analysis methods would substantially reduce the
Analytical Chemistry News & Features, December 1, 1998
costs associated with characterization of wastes and contaminated sites (8). With the exception of a limited number of radionuclides that can be analyzed directly by high-resolution gamma spectroscopy—such as 134Cs, 137Cs, 154Eu, 555Eu, 60 Co, 54Mn, 125Sb, and 241Am—nondestructive radiochemical analysis is not possible. Chemical separation steps are required when detection by radiometry or MS cannot distinguish among various radioisotopes that are present simultaneously in a sample. Individual, group, or radionuclide/ matrix separations represent important and often critical steps of an overall radiochemical analysis scheme Radiometric detection of beta emitters (by liquid scintillation or gas proportional counting) has limited isotope discrimination capabilities because beta particles from a single isotope are emitted with a range of energies, leading to spectra that overlap with other beta-emitting radioisotopes. Alpha particles, on the other hand, are emitted with characteristic energies and alpha emitters can be analyzed by alpha spectroscopy with good energy resolution—typically on the order of 50 keV. A number of important radionuclides howhave unresolvable alpha energies including 241 Am/ 238 Pu and 237 Np/ 234 U Moreover detection by alpha spectroscopy requires extensive sample purification because effects erev resolution Detection of alpha particles hv standard linrriri scintillation count inghaslimiterl en r r sol tion Alternatively, radionuclides can be detected by inductively coupled plasma mass spectrometry (ICPMS). Whereas radiation counting is a more sensitive detection method for short-lived, high specificactivity radionuclides, atom counting is more sensitive for radionuclides with long half-lives and hence low-specific activities (9,10). ICPMS offers selectivity on the basis of its ability to discriminate between ions of different m/z. Nevertheless lowresolution quadrupole-based ICPMS is subject to interferences involving isobars and molecular ions as well as spectral interferin which the peak from isotope at high concentration tails into the peak of another isotope at an adjacent mass number (10-13) Relevant examples for radio-
cases, individual steps must be repeated. For example, the classical method for analyzing 90 Sr involves repeated precipitation as the nitrate from fuming nitric acid. The conventional procedure for " T c analysis requires a combination of ion exchange and solvent extraction. Actinide separations also typically require repeated precipitation, ion-exchange, and/or solvent extraction steps. These analytical separations are typically timeconsuming, labor-intensive, involve worker to radiation and generate significant quantities of secondary waste Extraction chromatography Methodology to simplify and improve radiochemical separations using selective or semiselective extractants impregnated on solid supports has recently been developed by Horwitz and co-workers (19-23). Typically, an organic liquid complexant or ion exchanger, or a solution of a complexant in a suitable diluent, is loaded into the pores of macroreticular polymer beads. These beads are packed in a column and used to separate species from aqueous solutions by extraction chromatography (24). The separation involves liquid-liquid extraction in which the organic phase is immobilized. In a typical procedure a sample solution is loaded on the column under conditions in which the analyte of interest is strongiv and selectively retained in the immobilized organic phase while unretained sample matrix constituents
Figure 1 . Schematic diagrams of typical FI and SI systems. (a) FIA, (b) SI analysis with stacked zones in the holding coil, and (c) SI column separation.
chemical analysis include 99 Tc/"Ru, 151Sm/151Eu, 238U/238Pu, 241Pu/241Am, 91 Zr 16 O/ 107 Pd, 238U1H/^39Pu, 232 Th 1 H/ 233 U, and 238 U/ 237 Np. In addition, a separation step prior to introducing a sample into the ICPMS is desirable so that the sample always enters the system in a consistent matrix; matrix effects influence quantification (14-16). The use of continous-forward-flow FI to automate simple extraction chromatographic radionuclide separations with ICPMS detec-
tion waa first tescribed in neparate eapers sb Hollenbach et al. and Aldstadt et al. (17,18)) In both reports, on-line separation and preconcentration resulted in improved detection limits relative to direct sample introduction. Chemical separations in conventional radiochemical analysis have been carried out by a variety of classical and chromatographic methods, including precipitation, liquid-liquid extraction, and ion exchange. Often, sequential combinations of these methods are used and, in some
s cies are removed with a wash solution The mobile nhase composition is then changed to PTearly reduce the retention of selected species, eluting the analyte for subsequent quantification. This approach takes advantage of decades of knowledge on solvent extraction separations of radionuclides (22, 25,26). In most laboratories, extraction chromatographic separations are implemented by manually adding samples and solutions to the top of an open column and collecting fractions. In this respect, the extraction chromatographic separation format closely resembles solid-phase extraction (27-29). Modern extraction chromatographic sorbents generally feature greater selectivity, higher capacity, and improved kinetic performance than conventional ion-exchange
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Report
These samples are taken with a handauger method. After removing aliquots for physical characterization and volatile organic carbon analysis, the remainder of each sample is mechanically homogenized. Composites of the core are also prepared from proportional amounts of each of the homogenized segments. Aliquots of the homogenized segments or subsegments and the composite are transferred to an appropriate laboratory (hot cell glove box or fume hood) where the samples are processed for analysis Sample preparation methods include water leaching fusion (both KOH in a nickel crucible and Na. O2 in a zirconium crucible) followed by acid dissolution and acid digestion Aliquots of these processed samCollecting a sample at the Hanford site.
ples blanks spikes and Hnplicates am then distributed to the analytical
Nuclear waste analysis Though the cold war is over, the waste remains. The production of nuclear weapons in the United States has left a legacy of stored nuclear waste and radiochemically contaminated sites. The processing of stored wastes into stable waste forms and the remediation of the contaminated sites will require characterization procedures throughout all phases. Consequently, the demand for radiochemical analysis is expected to increase in the future. Education in radiochemistry and radiochemical analysis however has decreased substantially in the United States
contain liquid wastes, slurries, sludges, and/or salt-cakes. At the Hanford site, solid or liquid samples are retrieved from the tanks by core sampling, supernatant (commonly called bottle-on-a-string), and auger sampling. A core sample is a 19-in. segment of solid and/or liquid from the tank material. To sample the entire depth of the tank, successive 19-in. segments are obtained. The number of segments in the core samples may range from 1 to 22. Core samples are obtained using either a push-mode or rotarymode sampling technique
Nuclear wastes from nuclear weapons production in the United States are stored primarily at the Hanford site (Washington), Oak Ridge National Laboratory (Tennessee), the Savannah River site (South Carolina), and at the Idaho National Engineering and Environmental Laboratory. The majority of the most difficult high-level wastes, in terms of both volume and Curies of radioactivity, are stored at Hanford and Savannah River. At Hanford 177 waste storage tanks hold 203 000 m3 of high level waste with 210 000 000 Ci of radioactivity Savannah River 51 tanks that hold 130 000 m3 of waste with
Each of these segments is transported to the laboratory in a shielded cask, which is placed in a hot cell when the sample is removed. The core sampling process maintains the stratification of the tank. If detailed stratification data is needed, the sample can be divided into quarter- or half-segments. Supernant samples are also obtained at differing tank depths using weighted bottles. The bottles are stoppered when lowered into the tank. Once it reaches the desired depth the stopper is removed and the bottle is filled with liquid Auger samples are limited to solid and/or liquid samples
470 000 000 Ci of radioactivity Tanks
on the surface of the waste (
