424 (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36)
Anal. Chem. 1985, 57, 424-427 Karlberg, B.; Thelander. S. Anal. Chim. Acta 1978. 9 8 , 1. Nord, L.; Karlberg, B. Anal. Chim. Acta 1981, 125, 199. Kawase, J.; Nakae. A.; Yamanka, M. Anal. Chem. 1979, 5 1 , 1640. Nord, L.; Karlberg, B. Anal. Chim. Acta 1983, 145, 151. Ogata. K.; Tanabe, S.; Imanari, T. Chem. Pharm. Bull. 1983, 3 1 , 1419. Taylor, M. S. Proc. Anal. Div. Chem. Soc. 1978, 342. Rosman, K. J. R.; Jeffery, P. M. Chem. Geol. 1972, 8. 25. Kelly. W. R.; Moore, C. B. Anal. Chem. 1973, 4 5 , 1274. Headridge. J. E.; Sowerbutts, A. Analyst (London) 1972, 9 7 , 442. Cantwell, F. F.; Sweileh, J. A. Anal. Chem., in press. Sweileh, J. A.; Cantwell, F. F. C a n . J . Chem., submitted. Yoza, N.; Ohashi, S. Anal. Lett. 1973, 6, 595. Thornburn-Burns, D.; Glockling, F.; Harriott, M. Analyst (London) 1981, 106, 921. Koropchak, J. A.; Coleman, G. N. Anal. Chem. 1980, 5 2 , 1252. Messman. J. D.; Rains, T. C. Anal. Chem. 1981, 53, 1632. Jones, D. R., IV; Tung, H. C.; Manahan, S. E. Anal. Chem. 1978, 4 8 , 7.
(37) Fukamachi, K.; Ishibashi, N. Anal. Chim. Acta 1980, 119, 383. (38) Yoza, N.; Aoyagi. Y.; Ohashi. S. Anal. Chim. Acta 1979, 1 1 1 . 163. (39) Treit, J.; Nielsen, J. S.; Kratochvil, B.; Cantwell, F. F. Anal. Chem. 1983, 55, 1650. (40) Ruzicka. J.; Hansen, E. H. Anal. Chim. Acta 1978, 9 9 , 37. (41) Yamada, H.; Uchino, K.; Koizumi, H.; Noda, T.; Yasuda, K. Anal. Lett. 1978, A 1 1 , 855. (42) Fernandez. F.; Giddings, R. A t . Spectrosc. 1982, 3 , 61. (43) Boumans, P. W. J. M.; Lux-Steiner, M. Ch. Spectrochim. Acta, Part 6 1982, 376,97. (44) Gilbert, T. R.; Penney. B. A. Spectrochim. Acta, Part B 1983, 388, 297.
RECEIVED for review May 31,1984. Accepted October 17,1984. This work was supported by the Natural Sciences and Engineering Research Council of Canada and by the University of Alberta.
Use of Air-Cored Solenoids for Zeeman Background Correction Guo Tie-Zheng and Roger Stephens* Trace Analysis Research Centre, Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3
Design parameters are discussed for air-cored solenoids capable of generating pulsed magnetic fields at the strengths required by Zeeman-corrected AA spectrometers. Such coils are small and light enough to be mounted on an optical rail while their field volume is sufficient to avoid serious size ilmitations on the source or atomizer. The coil described here provides a combination of peak magnetic field strength and repetition rate which allows adequate signal to noise ratios to be maintained without requiring excessive power dissipation.
Zeeman background correction has been used successfully in AAS, either by direct means (1) or in the guise of magnetooptic rotation ( 2 ) . One of the most satisfactory variations of the technique is that described by de Loos-Vollebregt and de Galan (3-5), in which an alternating magnetic field is applied across the atomizer. In this case the background correction is highly effective unless the interfering species shows a Zeeman effect of its own ( 6 ) ;at the same time the degredation of signal to noise ratios, caused by anomalous Zeeman splitting and by the intensity losses associated with the introduction of polarizing components in the optical train, can be minimized. One disadvantage associated with the use of an alternating magnetic field is that the technique requires a careful design of the magnet and of its power supply if suitably high frequencies are to be attained, because of the large inductance of the electrical load. Conversely, if the modulation frequency is kept low, then the ability of the system to correct for fast transients is reduced, and in addition sensitivity to source flicker noise may become apparent. A second disadvantage, common to all Zeeman systems, is that their magnets are large, heavy pieces of apparatus which are not readily moved or adjusted and which can interfere with the operators’ choice of atomization system if inverse Zeeman correction is to be employed.
The use of air-cored solenoids offers a possible way to deal with both of these problems. However the associated power supply must be capable of delivering a heavy current a t very high power levels, and water cooling of the solenoid is essential ( 3 ,requirements which lessen the attractiveness of such an approach. These objections can be overcome by pulsed operation at a suitably low duty cycle. The use of air-cored solenoids driven by high current pulses allows useful fields to be generated with a very modest power supply and with a magnet assembly which is simple to build and which is small and light enough to allow its position to be adjusted as easily as that of any other component in the optical system. The present work was carried out to exame the feasibility of this approach.
THEORY The following equations describe the behavior of multilayer air-cored solenoids:
R = 8aNa/d2 L = 2 K a 2 P a 2 / ( 1X where H (T) is the magnetic field strength a t the center of the coil, n (m-l) is the packing density of the winding, i (A) is the current, 21 (m) is the length of the coil, ao,a x ,and a (m) are the inner, outer, and mean radii of the coil, R ( Q ) is the resistance of the coil, u ( Q m) is the conductivity of the wire, d (m) is the diameter of the bare wire, L (H) is the inductance of the coil, and K is the Nagaoka constant. Equation 1 is obtained by integration of Amperes’ law (8): eq 2 and 3 are of a standard form (9);the value of the Nagaoka constant, K, in eq 3 is given by 1/K = 1 + 0.45a/l + 0.32(a1 - a,)/a + 0.42(a1 - a o ) / l (9). Electrically the field coil can be treated as a resistance and inductance in series, driven by a voltage pulse of finite duration. A t the frequencies used here capacitance does not
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Figure 2. Block diagram of the electronics.
0
12
TIME (rns)-
Figure 1. Timing diagram for the apparatus, showing the main wave (A), the phase shifted magnetic pulse (B), and the switching pulses to the amplifier (C). Curve D is the current calculated from eq 1-5; X gives experimental points.
appear to play a major part. Thus if V is the voltage appearing across the coil at time t then
V = iR
+ L di/dt
where R and L are given by eq 2 and 3. Therefore
L ( V ) = RL(i)+ L L (di/dt)
(4)
where L represents the Laplace transform. Putting V = Vo sin wt, t o describe a sinusoidal pulse of circular frequency w and amplitude Vo,and solving eq 4 for i leads t o
L(i)= VOW p + R + 1 L(w2+ R 2 / L 2 )p 2 + w2 L ( p 2 w 2 ) R / L + P
+
Taking the inverse transform t o solve for i gives 1 =
+R - L-1VOW - L-1l L(w2 + R 2 / L 2 ) p2 + w2 L p2
+ w2 +
L-’ WL = -VO sin wt - - cos w t R 1+ w2L2/R2 R
1
R/L +P
+
Equations 1-5 allow the current which flows through a given coil as a result of an applied voltage pulse t o be calculated, as well as the magnetic field which the coil produces, once the nature of the applied pulse is known. The equations also allow the power dissipation in the coil t o be determined. Hence the maximum pulse repetition rate can be estimated for a given temperature rise in the coil once t h e rate of cooling is known. These parameters provide a basis for optimizing the design of the coil needed t o produce a particular magnetic field strength.
EXPERIMENTAL SECTION Field coils were wound with 180 turns of 18 gauge magnet wire on a tubular glass former of 4.2 cm i.d., to give a coil of 2.5 cm length, 8.1 cm o.d., and 400 g weight. Coils were driven from the 110 V 60 Hz mains through an SCR. The SCR was triggered a t selected zero crossing points of the mains cycle (Figure I), and automatically shut off when the current in the coils returned to
zero approximately one-half cycle later. This procedure minimized current surges and the associated electrical interference. Half-cycle pulses were normally selected at a repetition rate of 1 Hz. Current pulses up to about 150 A were obtained in this way without difficulty from a standard wall socket on a 15-A line. The voltage loss at the coil under peak load was 21 V (i.e., from 156 to 135 peak V). No cooling was employed other than natural convection. In the present work it was convenient to use a flame atomizer and a longitudinal magnetic field located at the source. A suitable magnetic field was generated by two of the field coils described above, clamped together to give a unit of 5 cm overall length, and connected electrically in parallel. This type of arrangement allows considerable versatility, since the field strength can be increased if necessary by adding more coils or decreased by changing to a series connection pattern, while the field geometry and location can be readily changed by separating the two windings and using them as a pair of Helmholtz coils. The measured values of the resistance, inductance, and peak magnetic field strength for the double coil were 0.72 Q , 3.5 mH, and 0.61 T when driven as described by isolated half-cycle pulses of the ac mains. The corresponding values calculated from eq 1-5 are 0.65 a, 3.0 mH, and 0.60 T. When in use the field coils were mounted at the end of the optical rail of a Varian AA5 spectrometer. Hollow cathode lamps were inserted through the tubular form to occupy their normal position in this instrument; the remainder of the optical system remained unchanged. The lamps were driven by a pulsed dc supply which was synchronized with the current pulse through the field coils (Figure 1C). The lamp pulses (a, b, and c) were generated respectively a t the start of the magnetic cycle, a t the peak field, and at the end of the cycle. Thus pulse b (Figure IC) comprises the u+, u- components of the longitudinal Zeeman effect, which can be displaced beyond the corresponding atomic absorption profile without need for polarizing optics, while pulses a and c occur essentially a t zero field strength. Hence pulses a and c serve as sample signals; pulse b serves as a reference. The alternating samplereferencesample pattern improves the flicker rejection (from either source emission or background absorption) of the apparatus since electrical signals with a time dependence below the second derivative are blocked. A full description of the signal processing characteristics of Zeeman correction with ac magnetic fields has been given by de Loos-Vollebregt (20). Care is needed with the present arrangement to allow for the phase shift between the drive voltage across the coils and the resultant magnetic pulse. The phase shift is contained in eq 1 and 5 and is illustrated in Figure 1B. It should be noted that the magnitude of this phase shift is modified by the transient nature of the drive. This is shown by the last term in eq 5, w L / K exp(-Rt/L), which goes to zero as t becomes large. Thus the pattern observed in Figure 1B applies to a succession of isolated half-cycle pulses; it differs from the pattern observed for a single pulse made up of several complete ac cycles, such as might be used with an electrothermal atomizer. The pulses a-c in Figure 1C also controlled a gated amplifier which passed the photomultiplier signals corresponding to pulse b, and those corresponding to pulses a or c, to separate integrators. The outputs of the two integrators represented the “field on” and “field off” signals which were compared to give a corrected output. A block diagram of the electronics is shown in Figure 2.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
426
cu
*E
Zn
Fe
/
.
C
-
I
Table I. Summary of Operating Characteristics
___/--
1 0 4 cI SONE lnlensily
-
Flgure 3. Ten-minute stability plots for Varian-Tectron Cu, Fe, and Zn lamps. Curves A, B, and C were obtained with the magnetic field on, field off (pulsed drive to the lamps), and field off, normal modulated drive, respectively.
‘-A
: \
Figure 4. Relative noise at the corrected output vs. (monochromator slit width)”2, slit width in micrometers. The curves are drawn for the function noise = l/(slit width)”*. Symbols 0 , X , and 0 are experimental points for Cu, Fe, and Zn, respectively. Curve A is with the magnetic field off: curve B is with the field on.
RESULTS AND DISCUSSION The sequential observation of pulses a-c is too slow to allow the present apparatus to function as an efficient double beam instrument, since high-frequency source flicker is not necessarily blocked. If this inefficiency is not to cause degredation of the output signal to noise ratio, then the source used must maintain a stable output for a minimum of one magnetic cycle. The following data were obtained to determine how well hollow cathode lamps could meet this requirement under the present operating conditions. Stability plots for various lamps are shown in Figure 3. In these cases the most apparent effect of applying the magnetic field is an increase in lamp drift. Further information was obtained by measuring the root mean square noise of the Zeeman corrected output as a function of the monochromator slit width (w),the photomultiplier gain (9) being adjusted to maintain a constant signal level ( s ) a t the output of the preamplifier. The method works as follows. If n is the photon count during the integration time of the instrument, then the shot noise level is -gn1/2 (11). Flicker noise which originates, for instance, from an instability of the plasma modulates n directly; Le., flicker noise is proportional to g n (11). Recognizing that n 0: w for a monochromatic source and that s = gn, then a noise vs. slit width curve is observed which falls ~ noise is limiting; if flicker noise is limiting, as l / ~ ’if /shot then the curve is independent of w. The observed results, given in Figure 4, show that flicker noise appears at large slit widths; it is particularly marked for Cu, with a shot-flicker transition occurring a t a slit width of about 30 pm. In view of these results, and assuming that the performance of the lamps used here was typical, it was concluded that some influence from source flicker was apparent on the Zeeman corrected output but that it was unlikely to reduce the signal to noise ratio by more than a factor of 2 or 3 in the worst case,
Ag line, nm 328.1 detection limita continuous AA 0.02 pulse AA 0.03 Zeeman corrected 0.05 K field onb 970 K field offb 5200 pulse lamp current, mA 19 continuous 2 relative photon flux‘ 0.3
Cr
Cu
Fe
Zn
357.9 324.8 248.3 213.9 0.05 0.06 0.1 160 1400 19 3 0.9
0.01 0.02 0.08 1900 4500 19 3 0.2
0.02 0.03 0.04 2800 4100 75 5 0.3
0.01 0.03 0.02 8900 16000 37 5 0.25
‘Concentration of solution in r g / m L giving a signal to RMS noise of 3:l upon aspiration. * Absorbance/molar concentration in solution. Ratio of the average photon flux (s-’) in the pulse mode t o that in the continuous mode.
Figure 5. Temperature-time curves for the magnet. Symbols X , v, 0, and 0 , represent repetition rates of 2, 1.3, 1, and 0.75 Hz, respectively.
even a t large slit widths. This conclusion was in accordance with the detection limits observed with the apparatus. These values, and associated performance data, are summarized in Table I. The effect of pulse repetition rate on the temperature of the coil assembly is shown in Figure 5 . Temperatures were measured at the inner surface, between the glass former and the first layer of the coil, the region of highest temperature accessible t o a probe. The results show that Newton’s law of cooling is operative, with a constant of 0.13 W/OC.
CONCLUSIONS The use of air-cored solenoids is felt to be a viable way to generate the magnetic fields needed by Zeeman corrected AA spectrometers. The approach has advantages in terms of its ac frequency response, simplicity of construction, and flexibility in use. Such apparatus can generate sufficiently high magnetic field strengths for satisfactory Zeeman splitting to be obtained at a high enough repetition rate to avoid a serious reduction of detection limits through degradation of the signal to shot noise ratio. The main disadvantage of the method is that the magnet must operate at a low duty cycle if excessive power consumption and forced cooling are to be avoided. This means that pulsed operation of the source is unavoidable, and synchronization of the electronics must be adjusted to match the type of duty cycle which is employed. LITERATURE CITED (1) de Galan. L., Trends Anal. Chem. 1982, 1 , 203-205. (2) Kitagawa. K.; Suzuki, M.; Aoi, N.; Tsunqe, S.Spectrochim. Acfa, Part 8 1981, 368,21-34 (3) de Loos-Vollebregt, M. T C , de Galan, L Spectrochim Acta, Part B 1980.35B 495-506 (4) de Loos-Vollebregt, M. T. C : Van Uffelen, J W. M.: de Galan, L Spectrochim. Acta, Part B 1982, 3 7 8 , 527-531.
Anal. Chem. 1985, 57,427-431 (5) de Loos-Vollebregt, M. T. C.; de Galan. L. Spectrochim. Acta, Part B 1982. 378,659-672. (6) Massmann, H. Talanta 1982, 29, 1051-1055. (7) SteDhens. R. Talanta 1979. 26. 57-60. (8) Page, Leigh: Adams. Norman I. "Principles of Electricity"; Van Nostrand: New York, 1958; Chapter V I I . (9) Welsby, V. G. "The Theory and Design of Inductance Coils"; MacDonald: London, 1960; Chapter 3. (10) de Loos-Voilebregt, M. T. C., Ph.D. Thesis, Delft University, 1980. (11) Allkemade. C. Th. J.; Snelleman, W.; Boutilier, S. D.; Pollard, B. D.;
427
Winefordner, J. D.; Chester, T. L.: Omenetto, N. Spectrochim. Acta, Part B 1978. 338. 383-399.
RECEIVED for review August 30, 1984. Accepted October 22, 1984. The authors are indebted to the Natural Sciences and Engineering Research Council of Canada for support of this work.
Decomposition of Marine Biological Tissues for Determination of Arsenic, Selenium, and Mercury Using Hydride-Generation and Cold-Vapor Atomic Absorption Spectrometries Bernhard Welz* and Marianne Melcher Department of Applied Research, Bodenseewerk Perkin-Elmer & Co. GmbH, D - 7770 Uberlingen, Federal Republic of Germany
Three decomposition procedures for marine biological tissue samples were Investigated for the subsequent determination of arsenlc, selenium, and mercury by udng hydrldegeneration and cold-vapor AAS, respectively. (I) Decomposltlon with nitrlc acid under pressure in a PTFE bomb resulted in low values for arsenlc and selenium but was adequate for the subwquent determination of mercury. (il) Decomposition wlth nitric, sulfuric, and perchiorlc acids gave the highest values for arsenlc and selenium, whereas mercury was partly lost under these condttions. (ill) Combustlon in a stream of oxygen could be applled for ail three elements and gave results that were in good agreement wlth the mean values of an Intercallbration. Pressure decomposition with nitric acid is recommended for mercury, followed by a sulfuric and perchloric acids treatment for the subsequent determination of arsenlc and selenium. Detection ilmlts under routine conditions are 0.3 mg/kg for arsenlc, 0.2 mg/kg for selenium, and 0.005 mg/kg for mercury.
Arsenic, mercury, and selenium are frequently determined in fish and other marine biological tissues. Arsenic and mercury are well-known for their toxicity, and these elements are often accumulated in marine organisms. Selenium, on the other hand, has not only a toxic effect but is an essential trace element for some animals as well as for humans ( I ) . In addition, an antagonistic effect exists between selenium and mercury. Hydride-generation AAS for arsenic and selenium and cold-vapor AAS for mercury are among the most sensitive techniques that are available in most routine laboratories. Another advantage of both techniques, when used properly, is the absence of background absorption interferences. Several transition elements, namely copper or nickel, however, can cause severe interferences in the determination of arsenic and selenium (2-4) unless appropriate measures are taken. The range of interference-free determination of these elements can be increased by one to several orders of magnitude when the hydride generation is carried out in 5 M hydrochloric acid instead of the usually applied 0.5 M acid (3, 5 ) . A second, generally applicable possibility to increase the range of in-
terference-free determination of the hydride-forming elements is dilution. Interferences in the hydride-generation technique do not depend upon the analyte-to-interferent ratio but upon the concentration of the interfering element in the solution for measurement ( 5 ) . Interferences by volatile nitrogen oxides, encountered when samples are oxidatively dissolved in nitric acid, are reported ( 6 ) ,and the addition of sulfanilamide ( 7 ) has been proposed to overcome this interference. Mutual interactions of the hydride-forming elements have been found by several authors, and the influence of selenium on the arsenic determination appears to be the most pronounced one. Addition of a small amount of copper has been proposed to control this interference (8). Both hydride-generation and cold-vapor AAS require dissolution and mineralization of the samples, and a common decomposition procedure for the subsequent determination of all three elements would be most desirable. The main problem in sample digestion for the determination of mercury is the volatility and mobility of this element. Kaiser et al. (9) propose a wet digestion with chloric and nitric acids which works without losses of mercury when a special long-necked digestion flask is used. Pressure decomposition with nitric acid in a closed P T F E vessel has also been proposed (10, 11) and is frequently applied because of the short decomposition times and the low risk of losses of mercury. The most widely applied decomposition procedure for marine biological tissue, however, is an acid digestion using nitric, sulfuric, and perchloric acids in different ratios (12-17). Egaas and Julshamn (18) used a nitric acid/sulfuric acid/vanadium pentoxide digestion procedure for the determination of selenium and mercury in fish products. Julshamn et al. (19) investigated three decomposition procedures for marine samples using (i) nitric and perchloric acids under moderate pressure, (ii) nitric acid/hydrogen peroxide, also under moderate pressure, and (iii) sulfuric acid/nitric acid/vanadium pentoxide and found that all results for selenium are comparable using both hydride-generation and graphite furnace AAS. Fiorino et al. (12) found good agreement of their arsenic and selenium values obtained by hydride-generation AAS with independent procedures when they used a nitric, sulfuric, and perchloric acids digestion. Ihnat et al. (14, 20), however, obtained poor within-laboratory and interlaboratory precision in a collabo-
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