Anal. Chem. 1993, 65,2590-2595
2590
High-Performance Flow Flame Atomic Absorption Spectrometry for Automated On-Line Separation and Determination of Cr(I I I)/Cr(VI) and Preconcentration of Cr(V1) Jozsef Posta,' Harald Berndt,' Shen-Kay Luo, and Gerhard Schaldach Znstitut fur Spektrochemie und angewandte Spektroskopie, Bunsen-Kirchhoff-Strasse 11, 0-44139 Dortmund, Germany
High-performance flow atomic spectrometry permits the fully automated separation of Cr(III)/ Cr(V1) species and subsequent flame AAS determination within only 1 min. An HPLC integrator at the output of the flame AA spectrometer renders possible the simultaneous signal processing of both oxidation states. Samples of drinking water, waste water, and extracts of soils were investigated. The relevant detection limits are 0.03 Cr(II1) or 0.02 pg/mL Cr(V1) (30 values, N = 25). The relative standard deviation amounts to 1% (N=10,l pg/ mL). With the same type of chromium speciation column (modified Cl8 type), Cr(V1) can be preconcentrated from drinking water and then determined online by flame AAS within 3.5 min. A detection limit of 0.5 pg/L (34 value) can be attained when a sample volume of 5 mL is used;the standard deviation is 3.1 (10 pg/L Cr(V1))or 1.5% (100 pg/L Cr(V1)).
-
INTRODUCTION In high-performance flow atomic spectrometry (HPF-AS) the aerosol generation is caused by hydraulic high-pressure nebulization (HHPN). This very efficient nebulization mode, which is widely used in technical fields (e.g., diesel oil injection, combustion jets of aeroplane turbines, and spray drying) has now, by the availability of the HPF/HHPN system (Knauerl Varian), also gained entry in the field of sample introduction in atomic spectrometry. The pressure is generated by an HPLC pump. As is common practice in HPLC, the sample is brought into a high-pressure carrier stream by means of a sample introduction valve. The liquid is then injected into the nebulization chamber by a pressure of -17 MPa (170 bar) (high-pressure injection) and consequently nebulized (flow rate 2.5 mL/min). The opening of the special nebulization nozzle measures only 20 pm. Dependent on the type of flame AA spectrometer, the aerosol yield amounts to more than 50% .l Using this nebulization mode, a high-performance flow system becomes a functional component of an atomic spectrometer.'$ High-pressure flow systems can be equipped with columns of large bed length and fine-grained packing material. An additional HPLC column placed between the sample introduction valve and the HHPN nozzle facilitates an interface-free coupling of the HPLC separation technique and atomic spectrometry. Due to the great number of theoretical plates of HPLC columns, a separation of oxidation t On leave at Varian GmbH, Alsfelder Str. 6, D-64289 Darmstadt, Germany. (1)Berndt, H.; Miiller, A. Freseniw J. Anal. Chem. 1993,345,18-24. (2) Berndt, H. Freseniw 2.Anal. Chem. 1988,331,321-323.
0003-2700/93/0365-2590$04.00/0
states and bonding forms becomes possible.3~4 As regards the on-line separation of Fe(II)/Fe(III) and the subsequent determination by flame AAS, the resulting detection power improved by -1 order of magnitude compared to that obtained by coupling with pneumatic nebulization.3 By equipping the system with two valves and a small ion-exchange or C18 RP column, on-line trace element preconcentrations/ matrix separations can be performed.516 An inert HPLC autosampler serves for automation of sample introduction. The toxic behavior of chromium and its compounds in different oxidation states is already described in numerous scientific papers of medicine, biology, and environmental and analytical chemistry. Surveys containing general information on the element and its behavior can be found in refs 7 and 8. In 1992, Sperling et al. gave a description of Cr(V1) and total chromium determination by electrothermal AAS after preconcentration. The paper contains detailed statements about chromium distribution as well as about former and recent analytical determination methods.9 In this paper, for the first time is described the rapid and automated separation and determination of Cr(II1) and Cr(V1) by HPF flame AAS using an HPLC integrator for signal processing. At the separation of Cr(III)/Cr(VI),flame AAS measurements yield two peak signals (two transient signals). Standard software of the flame AA spectrometer permits the measurement of only one transient signal at a time. This was the reason that hitherto the detection of both species after separation could only be handled in a rather complicated way. However, connection of an HPLC or GC integrator to the analog output of the atomic spectrometer offers a simple solution to this problem. Another excellent method for processing several transient signals in atomic spectrometry (speciation analysis) is offered by the availability of HPF-AS software (Knauer), which is quite similar in its structure to HPLC software. Additional investigations centered on the preconcentration of Cr(V1) traces. Samples of drinking water, waste water, and extracts of contaminated soil served as matrices.
EXPERIMENTAL DETAILS AND RESULTS General Procedures. (1) Apparatus. ( a ) Automated Separation of Cr(III)/Cr(VI). The following equipment was used: Varian flame AA spectrometer SpectrAA 400 (doublebeam spectrometer with external PC), additional 1-V analog output (modified by Varian, Darmstadt, Germany). Varian/ (3)Weber, G.;Berndt, H. Chromatographia 1990,29,254-258. (4)Weber, G.;Berndt, H. Znt. J. Enuiron. Anal. Chem., in press. (5)Ivanova, E.; Schaldach, G.; Berndt, H. Freseniw J. Anal. Chem. 1992,342,47-50. (6) Berndt, H.; MMer, A.; Schaldach, G. Freseniw J. Anal. Chem. 1993,346,711-716. (7)Berman, E.Toxic Metals and Their Analysis; Heyden: London; 1980;Chapter 10. (8)Bowen, H. J. M. Enuironmental Chemistry of the Elements; Academic Press: London, 1979. (9)Sperling, M.; Yin, X.; Welz, B. Analyst 1992,117, 629-635. 0 1993 Amerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 65, NO. 19, OCTOBER 1, 1993 HPLC pump
HPLC autosampler
5 cm chromium speciation column
2591
flame AA spe*rometer with HHP nebulization
m
P
PC (flame AAS)
L -
I
eluent reservoir
HPLC integrator
Flguro 1. HPF flame AAS arrangement for on-line separation and determination of Cr(II1) and Cr(V1).
Knauer HPF/HHPN system, basic system with original Varian HHP nebulization chamber plug, HPF/HHPN autosampler (inert HPLC autosampler), 100-pL sample loop (PEEK), chromium speciation column (5 cm, 4.6-mm inner diameter, modified C18 type, Knauer), Knauer Wissenschaftliche Geriite, D-14163 Berlin, Germany. HewlettPackard HPLC/GC integrator HP 3396 A. (b)Preconcentration of Cr(Vn. There was an additional sample introduction valve (PEEK) with a 5-mL sample loop. (2)Reagents. The following reagents were used tetrabutylammonium acetate (TBAA, Fluka 86849), acetic acid, phosphoric acid, methanol, and ammonium acetate (reagents of AFt grade). Cr(II1) stock solutions were prepared from Titrisol ampules. Cr(VI) stock solutions were made from K2Cr20, (Merck, Darmstadt, Germany). (3)HPFIHHPN Arrangement for Separation of Cr(IIn/ Cr(VI). Figure 1shows the apparative arrangement for the automated separation and determination of both oxidation states of chromium via flame AAS, functioning as an elementspecificdetector. In contrast to the commonly used coupling of HPLC with flame AAS, the exit of the HPLC column is not connected to a pneumatic nebulizer, but via a highpressure capillary with an HHPN nozzle. Thus, the highpressure flow system ends with the high-pressure nebulization (interface-freecoupling). In order to protect the Pt/Ir nozzle with an inner diameter of only 20 pm against blockage by particles, a 3-pm titanium sieve filter is positioned in front of the nozzle. At a flow rate of 2.5 mL/min, a pressure of -17 MPA (170 bar) is required for the nebulization. The HPLC pump produces a maximum pressure of 40 MPa (400 bar); thereby, a pressure of more than 20 MPA was available for chromatographicseparation. Using the Cr speciation column (5-cm bed length, 5-pm packing material), the total pressure at a flow rate of 2.5 mL/min was -25 MPa. The sample (100-pLsample loop) is introduced (time-controlled)into the carrier stream by a programmable HPLC autosampler, which also gives a read signal to the AA spectrometer. The delay time and the measuring time are determined (signal representation via PC of the AAS instrument) via the spectrometer software;it is also possible to attain an automated calibration using the standard addition method. The corrected AAS signal is additionally transferred to an HPLC/GC integrator via the analog output of the spectrometer. For flame AAS measurement, standard conditions for the determination of chromium were chosen (wavelength 357.8 nm, slit width 0.7 nm, slightly reduced aidacetylene flame, measuring height 8 mm above the burner slit). Separationand Determinationof Cr(III)/Cr(VI). (1) Investigations for Optimization of Separation. The separation of Cr(II1) and Cr(V1) by ion pair chromatography was initially followed as described in the literature. But unlike Syty et al.,l0 who used tetrabutylammonium phosphate, acetate was chosen in this case. Phosphoric acid served for (10)Syty,A.;Chriitensen,R.G.;Raim,T.C.J.Anal.Atom. Spectrom. ISS8,3,193-197.
adjustment of the pH value (3.0-3.2). The concentration of phosphoric acid in the solution was 3.7 X 10-3 mol/L. For separation, a flow rate of 2 mL/min is cited in the literature.10 For hydraulic high-pressure nebulization, a flow rate of 2.5 mL/min has proved optimal with respect to flame AAS working conditions (20-rm HHPN nozzle). Therefore, all subsequent investigations were carried out at this flow rate. In agreement with ref 10, the approximate time used for separation was 3 min. In order to optimize the separation step, the concentration of tetrabutylammonium acetate and phosphoric acid was varied. However, only insignificant improvementsresulted. Methanol is generally used for elution of complexed trace elements from C18 RP columns. In subsequent determinations of the trace elements by flame AAS, an additional advantage is given by methanol, which causes considerable improvement in sensitivity. Therefore, during further experiments,methanol was addedto the carrier (eluent). At fixed concentrations of tetrabutylammonium acetate (1X l(r M), ammonium acetate (1 X lo-' M), and phosphoric acid (4 X 10-3 M), the stepwise variation of the methanol concentration of 2.5 vol % differed between 0 and 40%. Whenlow methanol concentrations (
