Anal. Chem. 1999, 71, 5515-5521
On-Line Microextraction of Metal Traces for Subsequent Determination by Plasma Atomic Emission Spectrometry Using pH Peak Focusing Countercurrent Chromatography Eiichi Kitazume,* Tadako Higashiyama, and Nobuyoshi Sato
Faculty of Humanities and Social Sciences, Iwate University, Morioka, Iwate 020-8550, Japan Masahumi Kanetomo
Central Research Laboratory, Hitachi Ltd., Kokubunji, Tokyo 185-8601, Japan Takeshi Tajima and Seiichiro Kobayashi
Hitachi Tokyo Electronics Co., Kokubunji, Tokyo 185-8601, Japan Yoichiro Ito
Laboratory of Biophysical Chemistry, National Heart, Lung, and Blood Institute, National Institutes of Health, Building 10, Room 7N322, Bethesda, Maryland 20892
Metal ions were highly efficiently enriched by pH peak focusing high-speed countercurrent chromatography. The peak intensity for a 10-mL standard sample in the effluent stream was increased over 100-fold compared to conventional plasma atomic emission spectrometry. Ca, Cd, Cu, Mg, Mn, Ni, and Zn are chromatographically extracted in a basic organic stationary phase containing a complexforming reagent such as bis(2-ethylhexyl) phosphoric acid. After the sample solution is introduced into the column, metal ions remain around the sharp pH border formed between acidic and basic zones, moving toward the column outlet. Enriched metal ions are finally eluted with the sharp pH border as a highly concentrated peak into a volume of less than 100 µL. We evaluated this method for concentration efficiency in trace determination in tap water using different column diameters. Conventional trace determination of inorganic elements by nonflame atomic absorption spectrometry (NFAAS), inductively coupled plasma atomic emission spectrometry (ICP-AES), and inductively coupled plasma mass spectrometry (ICPMS) is facilitated by preconcentrating the sample solution, substantially improving detection limits. In 1983, we developed one-drop sample introduction for ICP to decrease its absolute detection limits by evaporating the sample on a heated wire filament.1 Two problems * Corresponding author: (tel/fax): 019-621-6825; (e-mail): SGB01551@ nifty.ne.jp. (1) Kitazume, E. Anal. Chem. 1983, 55, 802-805. 10.1021/ac990074x CCC: $18.00 Published on Web 11/09/1999
© 1999 American Chemical Society
blocked its practical application, however: (1) the lack of appropriate enrichment to reduce the sample volume to the microliter level because it is difficult to obtain concentrates even on a submilliliter order by conventional solvent extraction, for example, unless using a special ultramicroanalysis apparatus;2 and (2) restricted application to relatively nonvolatile elements due to limited thin wire filament durability. If we could effectively concentrate target trace elements to 0.1 mL or less from a large sample volume, practical detection limits for the trace analysis such as NFAAS, ICP-AES, and ICPMS would be greatly improved and adverse matrix influences eliminated. If trace elements could be enriched in a small-bore tube, on-line preconcentration interfaced with suitable analytical instrumentation would enable new ultratrace analysis. Flow injection analysis (FIA) combined with ICP-AES was reported using a miniature ion-exchange column that achieved a 100-fold increase of peak intensities of metal elements for a 30-mL sample solution;3 detection limits were over 20 times lower than for conventional ICP, while column lifetime was at least 20 h. High-speed countercurrent chromatography (HSCCC) is useful in separating natural and synthetic products.4 As with other CCC, it is free of problems using a solid support such as adsorptive (2) Korenman, I. M. Vvedenie v Kolichestvennyi Ul’tramikroanaliz; Gosudarstvennoe Nauchno-tekhnicheskoe Izdatel’stvo Khmicheskoi Literatury: Moscow, 1963. (3) Hartenstein, S. D.; Ruzicka, J.; Christian, G. D. Anal. Chem. 1985, 57, 2125. (4) Ito, Y., Conway, W. D., Eds. High-Speed Countercurrent Chromatography; John Wiley & Sons: New York, 1996.
Analytical Chemistry, Vol. 71, No. 24, December 15, 1999 5515
Figure 1. Flow diagram of instrumentation assembly.
sample loss and contamination. Highly efficient chromatographic separation is achieved using a multilayer coil in a centrifugal force field produced by column rotation. Despite these advantages, HSCCC has not been applied to the separation of inorganic elements until recently. Since the end of the past decade, HSCCC has been applied to preconcentration and separation of inorganic elements including rare earth elements and divalent and trivalent ions.5-8 Also separation of metal ions has been made by centrifugal partition chromatography (CPC), another liquid-liquid partition methodology that is a relative of HSCCC.9 Preconcentration and separation of inorganic elements from geological samples was also studied.10 We have demonstrated high enrichment by HSCCC with good recovery of Ca, Cd, Mg, Mn, Pb, and Zn at a concentration of 10 ppb each from 500 mL of the sample solution.11 The final concentrated sample volume, however, was several milliliters due to longitudinal diffusion as the sample band in the retained stationary phase. Additional band spreading occurred in the flow tube when the concentrated solution was eluted with an acid solution for subsequent analysis. Separation followed by enrichment based on displacement chromatography was studied by Talabardon et al., and a 10-fold concentration of transition metal ions was observed in the stationary phase.12 Diffusion is inherent to chromatographic separation, with sample band spreading in the column unavoidable in HSCCC as in conventional liquid chromatography. pH peak focusing countercurrent chromatography (pHPFCCC) is a unique technique based on neutralization between mobile and stationary phases.4,13 It has been applied to the (5) Zolotov, Yu. A.; Spivakov, B. Ya.; Maryutina, T. A.; Bashlov, V. L.; Pavlenko, I. V. Z. Anal. Chem. 1989, 335, 938-944. (6) Kitazume, E.; Bhatnagar, M.; Ito, Y. J. Chromatogr. 1991, 538, 133-140. (7) Nakamura, S.; Hashimoto, H.; Akiba, K. J. Liq. Chromatogr., A 1997, 789, 381-387. (8) Kitazume, E.; Sato, N.; Saito, Y.; Ito, Y. Anal. Chem. 1993, 65, 2225-2228. (9) Ma, G.; Freiser, H.; Muralidharan, S. Anal. Chem. 1997, 69, 2835-2841. (10) Fedotov, P. S.; Maryutina, T. A.; Grebneva, O. N.; Kuz’min, N. M.; Spivakov, B. Ya. J. Anal. Chem. 1997, 52, 1034-1038. (11) Kitazume, E.; Sato, N.; Saito, Y.; Ito, Y. J. Liq. Chromatogr., Relat. Technol. 1998, 21, 251-261. (12) Talabardon, K.; Gagean, M.; Marmet, J. M.; Berthod, A. J. Liq. Chromatogr., Relat. Technol. 1998, 21, 231-250. (13) Ito Y.; Shibusawa, Y.; Fales, H. M.; Cahnmann, H. J. J. Chromatogr., A 1992, 625, 177-181.
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separation and enrichment of organic compounds such as indole auxins, bromoacetylthyroxine and its analogue, dinitrophenyl amino acids, trans-retinoic acid, and diazepam. Neutralization is initiated at the mobile-phase front but advances through the column at a lower pace, forming a sharp border between basic and acidic zones. Trace impurities in the sample solution are concentrated at this narrow pH boundary in the column. This has great potential for on-line enrichment and subsequent analysis of trace inorganic elements by interfacing HSCCC with analytical instruments such as NFAAS, ICP-AES, and ICPMS. We demonstrate the feasibility of a HSCCC centrifuge in enriching several metal elements using pH-PFCCC. Under optimum conditions, an excellent enrichment factor of over 100 is achieved by on-line detection using a direct-current plasma atomic emission spectrometer (DCP-AES) as a detector. EXPERIMENTAL SECTION Apparatus. A Hitachi Tokyo Electronics countercurrent chromatograph (HSCCC-R1, prototype) was used to hold a singlemonolayer or multilayer coil separation column on the rotary frame 10.0 cm from the central axis of the centrifuge. The column was prepared from a single piece of 10-50-m-long, 0.3-1.6-mmi.d. poly(tetrafluoroethylene) (PTFE) tubing by winding it directly onto the holder hub (10-cm diameter). β, an important parameter governing hydrodynamic distribution of the two solvent phases in the rotating coil, ranged from 0.5 for the monolayer column to 0.58 for the multilayer column. β ) r/R, where r is the distance from the column holder axis to the coil and R is the distance from the column holder axis to the centrifuge axis. The coil capacity is 2-40 mL. The column holder revolves horizontally around the vertical axis of the centrifuge, and the column was rotated at 1200 rpm. The flow diagram of the experimental assembly is shown in Figure 1. A Tosoh CCPM prep pump (pump 1) was used to wash the injector valves 1 and 2 and the HSCCC column with water and ethanol and to pump water as a carrier for the standard solution in the injector valve 3. A Shimadzu LC-10Ai pump (pump 2) was used to pump the eluent (mobile phase) and a stream splitter to deliver an appropriate amount of effluent to the DCP via a peristaltic pump (pump 3). The DCP-AES is a SpectraMetrics
Table 1. Wavelength Used for Present Work element
line (nm)
element
line (nm)
Ca Cd Cu Mg
315.8 226.5 324.7 280.2
Mn Ni P Zn
257.6 341.4 213.6 206.2
Model SpectraSpan IIIB with 20 fixed-wavelength channels for observation of the elution profile. To facilitate monitoring element elution profiles, analog signals from the DCP were converted to a digital signal using a Keithley ADC 16 data acquisition analog input board on a PC. Data were calculated and plotted using Microsoft Excel. Wavelengths used for the present work are listed in Table 1. Reagents. To prepare the two-phase solvent system, an analytical reagent grade of ether, n-heptane, and tartaric acid were purchased from Kanto Chemical Co., Inc. (Tokyo, Japan) and bis(2-ethylhexyl) phosphoric acid (DEHPA) was purchased from Nacalai Tesque Inc. (Kyoto, Japan). Metal standards were obtained as 0.1 M nitric acid or hydrochloric acid solution (1000 ppm) from Kanto Chemical. To prepare the low-level concentration of the standard sample, ammonium tartrate for atomic absorption spectrometry (Kanto Chemical) was used. Water was purified by circulation, prepared by mixed bed resins from tap water, in a Millipore Super-Q System with a train of carbon, mixed bed resins. All other reagents were of analytical reagent grade. Preparation of Solvent Systems and Sample Solutions. Tartaric acid dissolved in deionized water was shaken in a separate funnel with an organic solvent, ether or heptane. After separating phases, a given amount of DEHPA and ammonia was added to the organic phase used as the stationary phase. At the first stage of the experiment, ether was mainly used for the organic phase. Hydrochloric acid was added to the aqueous phase used as the mobile phase. Standard sample solutions were prepared by diluting 1000 ppm standard solutions with 0.1 M tartaric acid, the pH adjusted by adding ammonia. Ammonium tartrate for atomic absorption spectrometry was used to measure the recovery for low-level Cu, Mg, Ni, and Zn to minimize procedural blanks. Tap water samples were prepared as 0.1 M tartaric acid solution by adjusting the pH with ammonia. Procedure. After washing all lines, including the injector valve and the HSCCC column, with water and ethanol using pump 1 (Figure 1), the HSCCC column was filled with water. Each experiment using a large-capacity column was initiated by filling the entire column with the stationary phase followed by sample injection using pump 2. For the 0.5-mm-i.d., 2-mL capacity column, the stationary phase was introduced into the rotating column by injector valve 2 (0.6 mL) and the sample solution by injector valve 1 (10 mL). The eluent was introduced using pump 2 while the emission signal for each element was continuously monitored by the DCP. Pump 3 was for conventional measurement and adjustment of DCP. Injector 3 (2 mL) was used to introduce the standard solution into the DCP after detecting enriched peaks. Each signal was stored by a PC to determine the concentration by comparison with the signal for the standard. At the first stage experiment using a larger bore column whose inner diameter exceeded 0.5 mm,
each experiment was initiated by filling the entire column with the stationary phase followed by sample injection using the pump. Column rotation was started, and the mobile phase was introduced into the column. Measurement of Distribution Ratio. Distribution ratio Kd of each metal element was obtained using a simple test tube method
Kd ) (AT - AL)/AL ) Cs/Cm
(1)
where AT is the total concentration of the sample in the lower phase before equilibration with the upper phase, AL is the concentration of the sample in the lower mobile phase (Cm) after equilibration, and AT - AL is the concentration of the sample in the stationary upper phase (Cs) after equilibration. RESULTS AND DISCUSSION Mechanism of Metal Enrichment By pH Peak Focusing CCC. The principle of extraction is based on chemical equilibrium in a two-phase solvent (Figure 2a). In a relatively basic environment, the metal ion in an aqueous phase forms a complex with the ligand DEHPA and partitions into the organic phase. In an acidic environment, the metal ion is released from the complex and transferred to the aqueous phase. Even if a large quantity of aqueous samples is introduced into the column, metal ions having high distribution ratio are enriched in the CCC column. After the column is filled with the basic (NH3) stationary phase containing the ligand (R), a sample solution is introduced into the column. If there are many types of metal ion in the sample, they make individual zones arranged in the column based on displacement chromatography.11 After the basic sample has passed through the column, the eluent acid is introduced. As the pH between two phases in the column reverses, the stationary phase is continuously neutralized with the mobile phase. The pH border, where neutralization has just finished and showing pH at an equivalent point, moves to the tail (outlet) from the head (inlet) of the column. pH border movement in the column is controlled by adjusting the base and acid concentration in each phase. Impurities in sample solutions are quantitatively trapped and enriched in the pH border at a specific condition of movement. The distribution ratios in acidic and basic zones are defined as Ka and Kb. VpH shows the volume of the eluent when the pH border exits the column (Figure 2c). KpH is assumed to be a temporary distribution ratio when a solute peak appeared at the retention volume of VpH. KpH is variable by changing the molar concentration between basic and acidic phases. If KpH is adjusted between Ka and Kb, impurities in the sample solution are quantitatively trapped and enriched in the pH border, e.g., for an ion (M+) (Figure 2b). A portion of the separation column contains the organic stationary phase in the upper half and the aqueous mobile phase in the lower half. By the effect of Archimedean screw force (under this force, all subjects different in density are driven toward one end of the coil, called the head), the mobile phase flowing through the column only partially displaces the stationary phase. As elution proceeds, hydrochloric acid in the mobile phase steadily neutralizes ammonia in the stationary phase, forming a narrow pH border between the basic front zone and the acidic rear zone (Figure 2b). The travel of this sharp pH border through Analytical Chemistry, Vol. 71, No. 24, December 15, 1999
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Figure 2. Mechanism of pH peak focusing countercurrent chromatography: (a) Chemical equilibrium of metal element; (b) cyclic movement of metal element trapped by the sharp pH border; 1, aqueous acidified mobile phase before neutralization; 2, high-pH, aqueous mobile phase (sample phase after the metals were extracted in pH border); 3, organic stationary phase with ammonia; 4, organic stationary phase saturated with acid in the mobile phase (after neutralization); (c) requirement conditions for peak focusing process.
the column (v) is determined mainly by the molar ratio between the base (NH3) in the stationary phase and acid (HCl) in the mobile phase, but is substantially lower than the flow rate of the mobile phase (V), i.e., v < V. If Knsthe distribution ratio of M+ at the point of the neutralizationsequals KpH, an M+ ion in the pH border moves at speed v. Then KpH may be defined as the distribution ratio of the metal ion moving at the center of the pH in the moving border. However, due to a large pH gradient on both sides of the pH border, the region of the pH border is too narrow to reach partition equilibrium so, practically speaking, the metal ion will move around the pH border by circulating. In Figure 2b, the metal ion (M+) in the acidic zone (position 1) quickly moves with the mobile phase passing through the pHborder into the basic zone (position 2), where it forms a metalligand complex (MR) and is transferred into the stationary phase (position 3). As the pH border moves forward, the complex is exposed to lower pH (position 4) where the metal is displaced by proton (H+) and released into the aqueous phase as its ionic form (M+) to repeat the above cycle. The metal element having Kn between Ka and Kb is always confined in a narrow region around the sharp pH border and finally eluted as a highly concentrated sharp peak in the pH slope at a specific KpH condition (peaks 2 and 3 in Figure 2c). The system eliminates longitudinal spreading of the sample band due to separation inherent in other liquid chromatography. If KpH exceeds Ka and Kb, the metal ion elutes earlier than the sharp pH border (peak 1), and if KpH is smaller than Ka and Kb, the metal ion elutes after the sharp pH border (peak 4). Peak trapping occurs only when KpH falls between Ka and Kb (peaks 2 and 3), regardless of the Kn. The two metal peaks (peaks 2 and 3) may be resolved within a narrow range if they have a substantial difference in Ka and Kb. 5518
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In practical concentration of trace metals from a large volume of sample solution, one must consider the following two types of sample introduction. Method 1: introduction of basic aqueous sample solution (volume V) into the rotating column previously filled with the basic stationary phase. The retention volumes of the sample (Vsample) and pH border (VpH) are expressed as
Vsample ) Vm + KbVs
(2)
VpH ) V + Vm + KpHVs
(3)
From these equations, the maximum sample volume extracted is given when Vsample ) VpH, i.e.,
Vmax ) (Kb - KpH)Vs
(4)
The maximum sample volume is increased by increasing Kb and Vs and decreasing KpH close to Ka. We used this sample introduction for a small-bore column. Method 2: introduction of basic aqueous sample solution into the column filled with the organic stationary phase before column rotation. Retention volumes of the sample and pH border are expressed as
Vsample ) Vm - V + KbVs
(5)
VpH ) Vm + KpHVs
(6)
where the sample volume does not exceed total column capacity, i.e., V < Vt. As above, the maximum sample volume is given when
Figure 3. Relationship between the flow rate of the mobile phase and retention of the stationary phase in the HSCCC column. Experimental conditions: revolutional radius, 10 cm; β, 0.5; column, one multilayer coil, 0.5 mm i.d. × 32 m (6.28 mL) or one multilayer coil, 0.3 mm i.d. × 60 m (5.85 mL); mobile phase, 0.1 M HCl saturated with ether; stationary phase, 0.22 M DEHPA and 0.20 M ammonia in ether.
Vsample ) VpH and similarly expressed as
Vmax ) (Kb - KpH)Vs
(7)
Although eq 7 is identical to eq 4, this method has the following complications: When V is increased over Vm, the stationary phase volume is decreased and the retained stationary phase is distributed from the head side. The portion of the tail is entirely occupied by the aqueous mobile phase, causing sample band broadening. As soon as column rotation starts, the two phases in the column quickly undergo countercurrent flow due to Archimedean screw force. The relative flow rate between two phases far exceeds the pumping flow of the mobile phase. This may cause inefficient extraction, resulting in excessive band broadening of components with a relatively low distribution ratio. We used this in a largebore column to study the experimental conditions for enrichment, because the column rounding a large-bore column can be operated at high flow rate of the eluent. The concentration of the extract trapped within the pH border is affected by the sharpness of the pH gradient or bandwidth of the pH border in the column, determined mainly by the internal diameter and partition efficiency of the column. The use of a smalldiameter column yields a highly concentrated extract in a small volume of the mobile phase. Figure 3 shows the relationship between the flow rate of the pump and retention (Sf, the retained stationary-phase volume (Vs) relative to the total column volume (Vt)) at different revolution rates. No sample was charged. If over several milliliters of the sample were introduced into the column, retention would be reduced to about half that shown here. Using large-bore tubing or decreasing the pumping rate was essential to ensure sufficient retention. The condition, which provides over 40% retention (Figure 3), was used to achieve good enrichment. If tubing of over 1-mm i.d. was used, retention exceeded 40% even for an eluent pumped at 1 mL/min at 800 rpm. Figure 4 shows the distribution ratio measured at different pH. The extraction profiles of Mn and Zn were almost the same as Cd. Profiles of Ca and Mg resembled Cu. Almost the same
Figure 4. Relationship between distribution ratio and pH of the sample. Experimental conditions: upper phase, 2 mL of 0.22 M DEHPA and 0.20 M ammonia in heptane; lower phase, 2 mL of each 10 ppm standard solution in 0.1 M tartaric acid with the pH adjusted by ammonia; after both phases were equilibrated in a test tube, each element in the lower phase was determined by DCP-AES.
extraction results were obtained using ether as the upper phase except for Ca, which was less extractable into the ether phase at low pH. Figure 5 shows enrichment for a 10-mL sample (pH 3.02). Cd and Zn were enriched well at this pH. The peak intensities of both elements were increased about 15-fold compared to conventional continuous introduction whose intensities for a 10 ppm standard solution are indicated. Cu was not enriched effectively despite the extraction result (Figure 5). The peak of Cu slowly appeared from 2 mL of the eluent and showed almost a constant level equivalent to the 10 ppm standard from 5 mL of the eluent except for a concentrated small peak at 14 mL. The sample volume was large compared to conventional chromatographic elution but rough estimation for the distribution ratio of Cu may be possible. If the peak of Cu appeared at 5 mL of the eluent, Kd is estimated as 0.138. In this experiment, as the 10-mL sample was introduced into the HSCCC column before elution started, Vt was estimated as 30 mL. Vs was estimated as 29 mL because the solvent front was observed at about 1 mL of the eluent. Despite showing over 4.0 for Kd (Figure 4), the estimated value calculated in Figure 5 was low, indicating incomplete extraction equilibrium for Cu in the HSCCC column under experimental conditions (Figure 5). Extraction equilibrium must be reached quickly to get higher peak intensities and higher recoveries. With increasing sample pH, Cu was concentrated more effectively and its peak height increased severalfold compared to that of the 10 ppm standard. Peaks eluted closer to each other with their width reduced to about 2 mL at around neutral pH, but peak intensity was not substantially increased, so small-bore columns were used to enrich a sample with higher concentration efficiency in much smaller volumes and higher recovery in sufficient extraction equilibrium with vigorous mixing in much smaller areas, in subsequent experiments. Figure 6 shows enrichment peaks for 10 ppm each of Zn, Cd, and Cu using a 1-mm-i.d. column. The emission intensity for the 100 ppm standard solution of Zn is also shown. Intensities for other elements were adjusted at almost the same level as that for Zn. The peak height of Cd was about 25-fold that of the 10 ppm standard solution, and the peak height of Cu also substantially increased. The peak width was below 1 mL for each peak. The Analytical Chemistry, Vol. 71, No. 24, December 15, 1999
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Figure 5. Enrichment profiles for 10 ppm standard solution. Experimental conditions: column, one multilayer coil, 1.6 mm i.d. × 20 m (40 mL); sample, 10 mL of each 10 ppm solution (pH 3.02) in 0.1 M tartaric acid; mobile phase, 0.1 M HCl saturated with ether; stationary phase, 0.22 M DEHPA and 0.20 M ammonia in ether; flow rate, 1.0 mL/min; revolution, 800 rpm; Sf, 60%; sample was introduced into the column before revolution was started. Table 2. Recovery of Standard Solutions sample
column
recovery of metal ions (%)
concn, ppm
vol, mL
pH
vol, mL
i.d., mm
ammonia concn, M
solvent
Cd
Cu
Mg
Mn
Ni
Zn
10 10 10 0.2 0.02 0.002
10 10 5 10 10 10
8.6 7.6 9.3 5.7 6.6 6.3
40 40 6 2 2 2
1.6 1.0 0.5 0.5 0.5 0.5
0.12 0.12 0.20 0.20 0.20 0.20
ether ether ether heptane heptane heptane
94 84 102 104 113 114
65 39 9 120 100 103
89 62 111 92 99 89
99 86 94 107 98 105
57 48 54 110 97 117
100 83 92 108 94
Figure 6. Enrichment profiles for 10 ppm standard solution. Experimental conditions: column, one multilayer coil, 1.0 mm i.d. × 50 m (40 mL); sample, 10 mL of each 10 ppm solution (pH 7.72) in 0.1 M tartaric acid; mobile phase, 0.1 M HCl saturated with ether; stationary phase, 0.22 M DEHPA and 0.20 M ammonia in ether; flow rate, 1.0 mL/min; revolution, 800 rpm; Sf, 28%; sample was introduced into the column before revolution was started.
recovery of each element remained insufficient. Cu recovery was especially low compared to that of other elements when ether was used as the stationary phase, as described later (Table 2). The Cu peak sometimes appeared at the solvent front of the chromatogram, so part of the Cu-DEHPA complex may stay in the interface between aqueous phase and ether. To concentrate metal elements into an extremely small volume, a small-bore column of 0.5-mm-i.d. tubing was used. The experi5520 Analytical Chemistry, Vol. 71, No. 24, December 15, 1999
Figure 7. Enrichment profiles for 2 ppb standard solution. Experimental conditions: column, one monolayer coil, 0.5 mm i.d. × 10 m (2 mL); sample, 10 mL of each 2 ppb solution (pH 7.10) in 0.1 M tartaric acid; mobile phase, 0.1 M HCl; stationary phase, 0.22 M DEHPA and 0.20 M ammonia in heptane; flow rate, 0.1 mL/min at enrichment stage and 1.0 mL/min at detection stage; revolution, 1200 rpm; Sf, 28%; sample and 0.6 mL of the stationary phase were introduced into the column line at the same time after revolution was started.
ment was done by introducing the ligand stationary phase and sample solution in the rotating column using method 1, above. Figure 7 shows an enrichment profile for 10 mL of each 2 ppb standard. Each metal was well concentrated and recovery was greatly increased. The emission intensity for each metal showed about a 100-fold increase but not equilibrated emission signals in
Table 3. Determination of Trace Metals in Tap Water (ppm) element
present methoda
DCP-AES direct methodb
Ca Cd Cu Mg Mn Ni Zn
6.67 0.0006 0.057 3.98 0.004 0.0027 0.045
7.18
