undissociated AlCN (15),the sensitivity being reduced by over 60% when Nz is substituted for Ar as the sheath gas (11,14). “Chemical” loss of atomic vapor through the formation of involatile compounds (11) is the cause of incomplete atomization, and hence, “memory effects” of some elements (i.e., Mo and V).
CONCLUSIONS At atmospheric pressure, condensation a t the cooler extremities of the graphite tube and/or “chemical” loss are responsible for the greatest fraction of the total amount of atomic vapor lost. A t pressures above 1 atm, the major loss mechanism may change. Additional work is required to substantiate some of the conclusions drawn. In particular, a study of the distribution after atomization of radioactive tracer elements in the individual components of the furnace (end windows, tube extremities, interior surfaces of cones, etc.) for both pyrolytic-graphite-coated and uncoated tubes should allow the contribution of each loss mechanism to be assessed for easy-to-volatilize elements such as Cd. Additionally, such a technique may be used to determine whether mechanical expulsion of the atomic vapor occurs at high pressure. The loss mechanisms outlined for Mo and V should be compared with those obtained with a graphite tube lined with a metal foil (unreactive toward Mo and V) in order to determine the contribution of “chemical” loss to the total loss.
ACKNOWLEDGMENT The authors thank E. A. Flood for valuable discussion.
LITERATURE CITED B. V . L’vov, “Atomic Absorption Spectrochemical Analysis”, translated by J. H. Dixon, Adam Hilger Ltd., London, 1970. R . E. Sturgeon, C. L. Chakrabarti, I. S.Maines, and P. C. Bertels, Anal. Chem., 47, 1240 (1975). R. E. Sturgeon, C. L. Chakrabarti, and P. C. Bertels, Anal. Chem., 47, 1250 (1975). R. E. Sturgeon and C. L. Chakrabarti, Spectrochim. Acta, Part 8 , (in press). R. E. Sturgeon and C. L. Chakrabarti, Anal. Chem., 49, 90 (1977). J. 0. Hirschfelder, C. F., Curtiss, and R. B. Bird, “Molecular Theory of Gases and Liquids”, John Wiley and Sons, New York, N.Y., 1954, p 14. W.J. Moore, ‘ Physical Chemistry”, 4th ed., Prenticakll, Inc., Englewood Cliffs, N.J., 1972, p 157. C. S. G. Phillips and R. J. P. Williams, “Inorganic Chemistry”, Voi. 11, Oxford University Press, Oxford, 1966, p 22. L. Pauling, “The Nature of the Chemical Bond”, 3rd ed., Cornell University Press, Ithaca, N.Y., 1960, p 403. R . E. Sturgeon and C. L. Chakrabarti, Spectrochim. Acta, Part 8 , (in press). R. E. Sturgeon, ph. D. Thesis, Carleton University, Ottawa, Ontario, Canada, 1977. J. H. Runnels, R. Menyfiekl, and H. B. Fisher, Anal. Chem., 47, 1258 (1975). Y. Taimi and G. H. Morrison, Anal. Chem., 44, 1455 (1972). R. E. Sturgeon, C. L. Chakrabarti, and C. H. Langford, Anal. Chem., 48, 1792 (1976). B. V. L’vov, “Electrothermal Atomization-The Way Towards Absolute Methods of Atomic Absorption Analysis”, presented as an invited paper at the 3rd FACSS meeting and the 6th International Conference on Atomic Spectroscopy, Philadelphia, Pa., Nov. 15-19, 1976.
RECEIVED for review January 17,1977. Accepted April 5,1977. The authors are grateful to the National Research Council of Canada for financial support of this project. One of the authors (RES) is grateful to the National Research Council of Canada for a post-graduate scholarship.
Atomic Absorption Spectrophotometry Based on the Polarization Characteristics of the Zeeman Effect Hideaki Koizumi“ Naka Works, Hitachi Ltd., Katsufa Ibaraki, 3 72, Japan
Kazuo Yasuda Instruments Division, Hitachi Ltd., Nishikubosakuragawa, Minato- ku Tokyo, Japan
Mikio Katayama Department of Pure and Applied Sciences, College of General Education, The University of Tokyo, Meguro-ku, Tokyo, Japan
A new type of atomlc absorption spectrophotometer was developed by using the Zeeman effect and its polarlzatlon properties. A steady magnetic field was applied to a sample vapor in the direction perpendicular to the propagation of Incident light. The direction of polarlzatlonand the Intensity of light from a hollow cathode lamp were modulated wlth 100 Hz and 1.5 kHz, respectively. The polarlzatlon modulation makes both the background correction and the double beam measurement, and the Intensity modulation eliminates the signal caused by the emlsslon from a graphite atomizer. The dlfference of absorption was observed between light polarlzed parallel and perpendlcular to the field. The signal proportlonal to the atomic density can be obtained readily from the dlfference of absorption. The system is much more efficient than conventlonal Instruments In correctlng for high background absorbances. The present spectrophotometer could correct the background absorption up to 1.7, and Its baseline always had constant level. The present instrument can analyze, with high sensitlvlty, almost all of the elements that can be measured by conventional atomlc absorption spectrometry. 1106
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
A r.ew technique of atomic absorption spectrometry (AAS), such as the analysis of mercury by using the isotope shifted Zeeman effect, was proposed by Hadeishi e t al. (1, 2). In previous papers (343, we also reported an improved technique of AAS using the Zeeman effect for the background correction and the same research was made by a number of investigators including Hadeishi and Stephens (6-9). A magnetic field was applied to a light source and the components of Zeeman emission lines were used for an absorbing and a reference light, respectively. Recently, the authors reported another type of the Zeeman atomic absorption spectrometry (10) in which a steady magnetic field was applied to the sample vapor perpendicular to the direction of light beam emitted from a conventional spectral source, and absorptions of radiation, perpendicular (Pl)and parallel (Pll)to the field, were observed alternatively. Figure 1shows the emission line of the hollow cathode lamp and the spectral pattern of the normal Zeeman components of the sample element. When the incident light is polarized perpendicular to the
-
Emission 1
i
The differential absorption is expressed as following in either case of normal or anomalous Zeeman effect, when
T P// (Sample light)
Absorption line
lol = Ioll and KIB( H )= KsB ( H ) ,
log IOlexp - [anAKIA
+ PnBKIB( H ) ]
- log I O I i exp - [an* KliA( H I + pnBKIIB = -anA lKIA ( H )-
I Background
Frequency
o=
z
(1)
a 2A
!
(0) Perpendicular T
(H)l
e
r
Ill1
e
n
c
e light)
o.+
i -
/ , ; , I Background I
Frequency
Figure 1. Relationship between the emissian and the absorption line for each observation on the parallel and the perpendicularly polarized components of the emission line; directijon of the observation is perpendicular to the magnetic field applied to the sample vapor. (Cd line at 228.8 nm, magnetic field at 10 kG) magnetic field (Pi), it gives rise to the transitions of AM = fl (u*) of the sample atoms only if the energy difference between the relevant levels coincides with the energy of incident photon. The light polarized parallel to the magnetic field (PI,)causes the transition of AM = 0 (7).Although the absorption line is broadened and shifted by the Lorentz and the Doppler effects, only the Wavelength of the K component, a t a certain strength of the magnetic field, coincides with that of the emission line from the conventional light source. On the other hand, the wavelengths of both the u* components a t the same strength of the magnetic field are separated completely from the emission line. Therefore, it is evident that the emission line from a conventional hollow cathode lamp is absorbed by the atomic vapor when its direction of polarization is parallel to the magnetic field. When the polarization is perpendicular to the field, almost no absorption is observed, and if any, it may be very small.
Here, KllA(H) and KrB(H)are the coefficient of atomic absorption and background absorption for P I , respectively. KIA(H) and KiB(H) are those for P,, nA and nB are th.e number of atoms and apparent number of background absorbers. CY and ,8 are constants and 1011and Iol are incident intensities of the Pi1 and PI. The signal due to the differential absorption of Pi1 and Pl was obtained by a log convertor and a lock-in amplifier tuned a t 100 Hz, the frequency of rotation of the linear polarizer. As a result, the difference of absorption intensities between light Pll and F l is proportional to the atomic density. Even if the absorber atom coexists with any other molecules, any noticeable change in the differential absorption would not be produced in the vast majority of cases, because of their small g value and broad band absorption, as known from the experimental and theoretical analysis of molecular spectra (11). Although the directions of polarization are perpendicular to each other, both light beams have the same wavelength and the same light path. Thus, the background absorption is monitored at exactly the same wavelength as the atomic absorption line. This method of absorption measurement has also an advantage of the double beam technique. In this respect, i t is quite different from any other method of background correction (1-9,12-15). Furthermore it should be emphasized that “polarization” is the key of the present method. As the atomic absorption spectrophotometer based on this principle is constructed in our laboratory, we would like to report on the instrumentation and some experimental results.
INSTRUMENTATION Figure 2 shows the schematic diagram of the optics and the electronics of the present atomic absorption spectrophotometer. A picture of its main part is shown in Figure 3. Graphite Furnace Atomizer with a Permanent Magnet. The construction of a graphite furnace atomizer with a permanent magnet is shown in Figure 4. Figure 5 shows the cuvettes which are convenient to apply a strong and homogeneous magnetic field. The maximum sample loading capacities of the cuvettes A, B, Line Pass Filter
-
( 1 0 0 Hz)
Synchronous Rectlf ier
- Recorder
I
Superposing
Monochroma tor Phot omu 1tiplier
( I Permanent Magnet (11 kgauss)
Senarmon
milow Cathode Lamp
(rotating)
Flgure 2. Block diagram of the present atomic absorption spectrophotometer using the polarization and the Zeeman effect ANALYTICAL CHEMISTRY, VOL. 49, NO. 8,JULY 1977
1107
Absorption C e l l Pole P i e c e
Light Beam
---
-
de-
Cone Sample Cup
(A)
Figure ti. Construction of cuvette B
Figure 3. Main part of the present atomic absorption spectrophotometer (the graphite furnace atomizer,the permanent magnet, and the rotation linear polarizer)
U
Water Cooler
Figure 4. Construction of the graphite furnace atomizer and the permanent magnet
Figure 5. Three kinds of graphite cuvettes used and C were 20, 50, and 150 bL, respectively. The maximum temperatures of the cuvettes elevated were 2900,2700 and 2400 "C, respectively, when the electric power of 10 V-350 A was supplied to the cuvettes. The temperature of the cuvette was measured by an optical pyrometer calibrated at the melting point of Mo (2620 OC). Argon gas flowed at the speed of 2 L/min through the chamber of the atomizer to prevent the graphite surface from oxidization. At the speed of 0.5 L/min, another Ar gas flowed through the cuvettes from both sides to the center. One of its purposes was to reduce the background absorption. 1108
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8,JULY 1977
Cuvette B was designed to obtain a good reproducibility of measurement of various kind of liquid and solid samples. The construction of cuvette B is shown in Figure 6. Cuvette C is for solid or powder samples. Unlike the conventional graphite atomizer, the absorption cell of these cuvettes is separated from the part of sample atomization. A sample is atomized in the sample cup, which has lower temperature than the absorption cell, and the atomized vapor flowed and diffused in the veitical direction and passed through the absorption cell. Cuvette A, which is a modified Massmann type, was used for the elements requiring a high temperature for atomization. The shape of the graphite tube was slightly changed so as to give efficient atomization. Both sides of the central part of the cuvette was kept a t a higher temperature than the center by cutting both sides thin. Owing to large resistances, these cut ditches show higher temperatures than the central part. Unlike the cuvette used in the method of the Zeeman shifted emission line ( 5 ) , carrier gas was flowed through these cuvettes from both sides to the center to keep the sample vapor in the central part. Thus, the atomic vapor is kept in the central part where a homogeneous magnetic field is applied and the temperature is sufficiently high. The temperature of the cuvette, the heating time, and the rate of temperature rise were controlled by a programable power unit constructed in our laboratory. The cuvette was placed in a 9-mm gap between the pole pieces of a permanent magnet of 11 kG. It was possible to roughly change the field strength a t the absorption cell from 3 to 11kG by moving the magnet in the vertical direction. HI-MAG casted by Hitachi Metal Co. was used for the permanent magnet, and the Curie point of this metal was 890 "C. Therefore, the permanent magnet may be safely used up to the temperature as high as 550 "C. The pole pieces were also made of the same material to eliminate the effect caused by the difference of the coefficient of expansion. This magnet was designed for the permeance of 15 and the leakage coefficient of 5. The yoke was made of soft iron. Although the temperature of the surface of the pole pieces sometimes rises as high as a few hundred degrees because of the radiation from the cuvette, almost the whole part of the magnet was kept at about room temperature by circulating cooling water. Optics. The schematic diagram of the present Zeeman effect atomic absorption spectrophotometer is shown in Figure 2. A senarmon prism made of artificial quartz with optical contact was used for a linear polarizer. It was successfully used over the wavelength range of 180-1000 nm with the polarization ratio of higher than 99%. The prism was fixed on a shaft rotating at 3000 rpm by a synchronous motor. The phase of rotation was monitored by using an emission diode with a phototransister. A grating of 1440 grooves/mm and blazed at 230 nm in Littrow mounting was used in our monochromator. The effective aperture was F8 and the reciprocal dispersion was 2.25 nm/mm. The slit width was changeable from 0.1 to 1.0 mm. The photomultiplier (Hamamatsu TV. R 456) was used for a detector. The source image with the diameter of 4 mm is formed a t the absorption cell and also at the entrance slit of the monochromator. An iris with a diameter of 7 mm was fixed between the graphite cuvette and the monochromator to eliminate the extraordinary rays from the polarizer and the black body radiation from the graphite cuvette. Alignment of the optical system is not critical. Light Source. The conventional hollow cathode lamps (Hitachi HLA-3 and HLA-4) and multielement lamps, for example, Ca-Mg, Pb-Cd-Zn, and Ni-Fe-Cr-Mn-Cu were used
Table I. Detection Limits of Various Elements Wavelength, Elementa nm Ag A1 As Ba Bi Ca Cd
Sr
328.0 309.2 193.6 553.5 306.7 422.6 228.8 240.7 359.3 852.1 324.7 248.3 253.6 766.4 670.7 285.2 279.8 588.9 232.0 288.3 217.5 196.0 460.7
Ti
319.2
CO
Cr
cs
CU
Flgure 7.
Hg K Li Mg Mn Na Ni
Signals of each stage of the signal processing
for the light source. Modulation of the emission intensity of the hollow cathode lamp was employed to eliminate the signal due to the black body radiation from the atomizer. A current supplied t o the lamp was modulated with the sine-wave signal from a mechanical oscillator at 1.5 kHz. In the conventionalpulse lighting of a hollow cathode lamp, the ignition of the discharge was repeated with every pulse. While, in the present technique, the discharge was always maintained by a small dc current superposed on the sine-wave current. This technique made it possible to modulate the intensity of a hollow cathode lamp with high frequency even at 20 kHz. In the present instrument, the total current, Le., the dc and the sine-wave current, was variable from 1.5 to 20 mA, and the current maintaining the discharge was fixed at 1.5 mA. The sine-wave current was generated by a mechanical oscillator (Tuning Fork Oscillator, Seiko SL-02) at 1.5 kHz. Signal Processing. The block diagram of electronics is shown in Figure 2. The synchroscope traces of the signal are shown in Figure 7. The hollow cathode lamp was operated by supplying the modulated current. The modulated light from the lamp with a frequency at 1.5 kHz is absorbed by atomic vapor in the furnace. Since the light intensity is further modulated at 100 Hz with the rotation of the linear polarizer, the magnitude of the modulation at 100 Hz corresponds to the differential absorption between P , and PII.The signal from the photomultiplier was amplified by a preamplifier (Figure 7A), and entered into a mechanical band pass filter (Tuning Fork Filter, Seiko SL-W22). The central frequency and the flat region of the band pass filter were 1.5 kHz and f l l O Hz, respectively. The constituent of the signal with the frequency of 1.39 to 1.61 kHz could pass through the filter (Figure 7B). The signal was detected to obtain only the 100-Hz component corresponding to the atomic absorption (Figure 7C and D). The signal due to the black body radiation from the graphite cuvette was eliminated by this band pass filter. The 100-Hz component was transformed into logarithm (Figure 7E), and passed through a line pass filter at 100 Hz, and detected by a synchronous rectifier (Figure 7F). To improve the precision of logarithm conversion, the high voltage supplied to the photomultiplier was automatically controlled by a feed back loop so that the intensity of the signal due to P , was kept at a constant level. The response time of the total system was 0.1 s.
RESULTS AND DISCUSSION The absorption lines of elements are classified in several groups by the Zeeman patterns as described in the previous paper (10). For the elements in 2a and 2b of the periodic table, the principal resonance lines (lSo-lP1)show the normal Zeeman effect in which the absorption lines split into triplet. In this case, KLA(H)in Equation 1 is reduced t o nearly zero when the H is higher than 15 kG. As the frequency of the P component is independent of the magnetic field, K1lA(H) remains unchanged with the magnetic field. Therefore, K1lA(H) has almost the same value as the absorption coefficient with H = 0 when the magnetic field is above 15 kG. The principal
Pb Sb
Se
318.4 213.8 Quantities of metal at S/N = V Zn
a
Transition
Detection limit, G 9 x 10-13 2 x lo-" 1 x lo-" 8 x lo-" 3 x 10-9 4 x lo-" 3 x 10-13 2 x lo-" 9 x 10-12 3 x lo-" 1x
lo-"
x 10-l2 1 x 10-l0 2 x lo-" 2 x lo-" 1 x 10-13 3 x 10-lZ 2 x lo-', 3 x lo-" 4 x 10-12 3 x 10-l0 1 x 10-9 2 x 10." 4 x 10-9 4x 1x l o - " 2; magnetic field, 11 kG. 4
and the secondary absorption lines of elements in 4 and 2b show the triplet Zeeman pattern, but the shift is 1.5times as large as that of the normal Zeeman effect. For the elements in l a and Ib, both the principal doublet (2sl/z-2P3/2) and (2S1/2-2P1/2) show the anomalous Zeeman effect in which the P component also splits to doublet. Then, K1lA(H)-KLA(H) shows the maximum value at field strength in the region from 5 to 15 kG. Since the shift of P component of (2Sl/z-2P3p) transition is smaller than that of (2Sl,2-2Pl/z), the larger differential absorption between P11 and P, is obtained by using the former spectral line. In the case of Ag, the sensitivity at 3280.7 8, (2S1,z-2P3,z) line is 13 times as high as that of 3382.89 8, (2Sl/z-2Pl/z) line a t the magnetic field of 11kG, although the transition probability of the former is only 2.3 times as large as that of the later one. The absorption lines of transition elements, such as Cr lines (7S3-7P3)and (7S3-7P4), show generally the anomalous Zeeman effect in which not only the u but also the P component splits to fine structures. The lines at 3578.69 8, (7S3-7P4) is used most frequently in the conventional AAS because its transition probability is larger than that of the line a t 3593.5 a (7S3-7P3). However, the line of the latter gives the better sensitivity in the present method because the fine structure of P component of this line is more concentrated than that of the former. The detection limits for various elements were measured by using the permanent magnet of 11 kG. Table I shows the latest results of the measurement, the wavelengths of the light used for the observation, and their relevant electronic states. I t shows that high sensitivity for the analysis is obtained by the present method even in the case of either the complicated anomalous Zeeman effect or the large hyperfine structure. The sensitivity of the present instrument is almost the same as that of a conventional atomic absorption spectrophotometer with a similar type of a graphite furnace atomizer (16). Unlike the case of the conventional atomic absorption spectrophotometer, however, the detection limit of the present instrument were measured by employing both the background correction and the double beam optics. Therefore, the same order of ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1109
Flgure 10. Figure 8.
Analysis of Ag sample of high concentration
Magnetic f i e l d : 5 kgauss Sample : ag 0.1 ng
Figure 9.
Analysis of Ag sample of low concentration
detection limit and a stable baseline are obtained even when actual samples bring about large background absorption. When the magnetic field was applied to the light source as in the previous method, the measuring light, i.e., the a component of the emission line, was broadened by an anomalous Zeeman effect as much as the absorption line width. Thus, the linear range of measurement would be shortened in a relatively narrow region of sample concentration; while, in the present spectrophotometer, only the absorption line was broadened in the anomalous Zeeman effect. Therefore. the linear ranee of measurement was much improved even in 'the case of strong absorption. Figure 8 shows the measurement of Ag sample with comparatively high concentration by the present spectrophotometer a t the field strength of 11 kG. The split of the a component of the Ag resonance line a t 338.3 nm is very large as shown in the previous paper (10). However, no bending of the analytical curve could be observed even in the strong absorption reached to absorbance 1.0. I t was also possible to measure the Ag sample of low concentration with high sensitivity by using the line a t 328.1 nm a t the magnetic field of 5 kG as shown in Figure 9. The concentration of the former sample is about lo00times as large as that of the latter one. Thus, the present atomic absorption spectrophotometer has a wide dynamic range of measurement by choosing a suitable absorption line and a magnetic field strength. 1110
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
Analysis of Pb in NaCl solution of 10000 ppm
Furthermore, the precision of the background correction of the present spectrophotometer was examined. First of all, it was checked by using filters (fine mesh) which make no polarization and have transmittances of 10 and 3%. The corrected signal was observed when the filter was quickly inserted in the light beam and then pulled out after a few seconds. The quick change of the incident light induces the rapid change of the background absorption intensity. Even when the transmittance decreased to 3% (absorbance 1.5), no signal could be observed within an error of 0.002 in absorbance scale. We measured P b of an order of 0.1 ppm in an NaCl aqueous solution of 10000 ppm with good reproducibility and high sensitivity. Although NaCl has a strong absorption band around the wavelength of the P b resonance line, no signal are observed when the P b is absent. Figure 10 shows the direct analysis of P b in an NaCl aqueous solution of 10000 ppm. Although the sensitivity is a little decreased by the coexistence of NaC1, a good linearity of measurement can be obtained. Then, it is possible to analyze P b in NaCl solution by using the standards addition method. In the conventional method of the background correction, the detection limits are frequently reduced to 1/6-1/60 because of under- or over-correction if the sample has a large background absorption. On the other hand, the present method makes it possible to analyze an amount of element of an order of the detection limit listed in Table I in actual samples such as blood, urine, hair, and soil. The upper limit of the correction is not subjected to the optical alignment, but mainly to weakness of the absorbed light. Supplied voltage to the photomultiplier is usually 380-520 V. However, if the supplied voltage exceeds 950 V by a large background absorption, the noise level is much increased and an off-set of the electronic circuit appears on the signal. Therefore, the brighter lamp has the wider correction range. When a magnetic field is applied to the light source, the a and the u* components of the emission line have different wavelengths and also have different intensities by reason of self-absorption in the light source (17). The ratio of their intensities changes with the field strength and strongly depends upon the vapor pressure of metals in the light source. However, these undesirable effects are never caused in the present method because the intensity ratio of P I and Pi1 of the emission of the conventional light source is observed. It gives rise to the ideal condition of double beam optics, resulting in the unchangeable baseline. No baseline drift was observed in the present Zeeman atomic absorption spectrophotometer even when the intensity of the emission light increased with time several times as large as the initial
I
: I
1 1
,
'
l -
l
1
' *
l i '' a
~
1
l
1 I
l
1
,
l
I
1
!
Element : Cd Wavelength : 228.8 m Magnetic field : 11 kgauss
Comparison of the baselines between a conventional single beam atomic absorption spectrophotometer and the present instrument
Figure 11.
Figure 14.
Figure 12.
Signals before and after the band pass filter
condition. Figure 11 shows the baselines of a conventional single beam atomic absorption spectrophotometer and of the present Zeeman atomic absorption spectrophotometer recorded just after lighting the hollow cathod? lamp. The flat baselines are those obtained by the present instrument. This comparison was made by using the same hollow cathode lamps. The double beam optics of the present method is different from the conventional double beam technique in the respect that both the light beams pass through the sample vapor. Then, no signal due to the emission from the atomizer appears in the present method. It should be emphasized that the two beams have exactly the same path and the same wavelength, and the only difference between the two beams is the direction of polarization. Although the emission from the atomizer does not appear in the output signal in the present method, it reduces apparent sensitivity and a linear range of measurement, i.e., the bending of an analytical curve, and also gives rise to a considerable noise. However, it was completely eliminated at the entrance of the signal processing by the mechanical band pass filter.
Analysis of Cd in urine
Figure 12 shows the signal from the photomultiplier and the signal after passing through the band pass filter. The large part of the emission from the atomizer was intentionally taken into the monochromator to check the elimination capability. Even when the emission from atomizer was 10 times larger than the emission from a Cu hollow cathode lamp operated a t 10 mA, the emission from the atomizer could be eliminated. The Cu line at 324.8 nm is the brightest one in all the emission lines obtained by hollow cathode lamps. For the case of Ca analysis, it requires a high temperature of almost 3000 K to obtain the vapor of Ca atom. Although the apparent peak of the black body radiation at this temperature is close to the wavelength of the measurement of Ca (422.7 nm), it is possible to analyze by the present method without interference with the emission from the atomizer. Barium has a resonance line at 553.6 nm, and it requires a high temperature of about 3000 "C for atomization. At a temperature around 3000 "C, the surface of the graphite cuvette begins to sublimate, and the light is scattered by the small particles of carbon. The iris inserted between the cuvette and the monochromator is unable to cut scattered light of this kind, while the sine-wave modulation of the light source is effective in eliminating this kind of interference. However, movements of the scattering particles increase the noise level which reduces the detection limit of Ba. Figure 13 shows a repeated measurement by the present Zeeman atomic absorption spectrophotometer using the cup type cuvette. The reproducibility of 1.27% in coefficient of variance (C.V.) was obtained in the analysis of 1 ng P b in aqueous solution. The light beam was partially cut by the pipet when a sample was introduced in the sample cup of the ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1111
Elenent : Cu kavelength 324.8 nm Magnetic field : 11 kgauss Full scale Abs. 1.0
.
’
+
Cd concentrations of 0,0.5, and 1.0 ppb. These samples were directly introduced into the cuvette and measured without any ashing process. Carrier gas was flowed at the rate of 0.05 L/min to reduce the background absorption to less than 1.5. The signals of four samples, i.e., the dilute solution, Cd added solutions, and distilled water, were observed as shown in Figure 14. The measured Cd concentration in the original urine was 0.7 ppb. Figure 15 shows the process of analysis of Cu in serum. Serum was diluted to 50% concentration with distilled water, standard solution of Cu being added to provide samples of additional Cu of 0,0.5, and 1.0 ppm. No ashing process’was employed in this measurement. The small peak or fluctuation of the baseline is due to the surface reflection of bubbles of serum formed during the drying process. The measured Cu concentration in the original serum was 0.89 ppm.
ACKNOWLEDGMENT Flgure 15. Analysis of Cu in serum
cuvette. However, no signal appeared except a small fluctuation attributed to the change of polarization by the reflection on the surface of the pipet. The temperature of the absorption cell was higher than that of bottom of the cup because of their different heat capacity and electric resistance. However, the temperature of the cup was homogeneous and the rate of vaporization was independent on the position of the sample loaded in the cup. Thus, good reproducibility was obtained without the special care of sample introduction. Next, the precision of background correction was checked by actual samples. When 10 KLof urine was directly atomized without any ashing process, a strong background absorption over 1.0 was observed. However, no signal was observed by the present method on the various lines in the case where the analyte element was absent in the sample. Figure 14 shows the process of analysis of Cd in urine. Urine was diluted to 50% concentration with distilled water. A standard solution of Cd was added to the one part of the dilute solution of the sample and we made solutions of additional
The authors express their sincere gratitude to T. Hadeishi of the University of California and to K. Ohishi, K. Fukuda, K. Uchino, K. Takahashi, and K. Moriya of Naka Works of Hitachi Ltd. for their discussions and great help.
LITERATURE CITED (1) T. Hadeishi and R. D. McLaughlin, Science, 174, 404 (1971).
(2) (3) (4) (5) (6) (7) (8) (9) 10) 1 1) 12) 13) 14) 15) 16) 17)
T. Hadeishi, Appl. Phys. Lett., 21, 438 (1972). H. Koizumi and K. Yasuda, Anal. Chem., 47, 1679 (1975). H. Koizumi and K. Yasuda, Spectrochim. Acta, Parts, 31, 237 (1976). H. Koizumi and K. Yasuda, Anal. Chem., 48. 1178 (1976). T. Hadeishi, D. A. Church, R. D. McLaughlin, 8.D. Zak, M. Nakamura, and B. Chang, Science, 187, 348 (1975). T. Hadeishi and R. D. McLaughlin, Am. Lab., 7 (E), 57 (1975). T. Hadeishi and R. D. McLaughlin, Anal. Chem., 48, 1009 (1976). R. Stephens and D. D. Ryan, Talanta, 22, 655 and 659 (1975). H. Koizumi and K. Yasuda, Spectrochim. Acta, in press. G. Herzberg, “Molecular Spectra and Molecular Structure”, Van Nostrand Reinhold Co., New York, 1950. W. Slavin, At. Absorp. News/. 24, 15 (1964). S. R. Koirlyohann and E. E. Pickett, Anal. Chem., 37, 601 (1965). C. Ling, Anal. Chem., 39, 798 (1967). J. Kuhl, G. Marowsky, and R. Torge, Anal. Chem., 44, 375 (1972). Perkin-Elmer, HGA-2100 Report L-357, Dec. 1973. H. Koizumi and M. Katayama, unpublished work.
RECEIVED for review November 1,1976. Accepted March 29, 1977.
Design and Evaluation of a Random Access Vidicon-Echelle Spectrometer and Application to Multielement Determinations by Atomic Absorption Spectrometry Hugo L. Felkel, Jr. and Harry L. Pardue” Department of Chemistry, Purdue University, West La fayette, Indiana 47907
A silicon target vidicon tube has been coupled to an echelle grating spectrometer for simultaneous multielement determinations by atomic absorption. A minicomputer is interfaced to the vidicon detector to implement random access interrogratlon of selected detector elements and Is used for data processing and display. The resolution capabilities of the system are assessed for the spectral region from 3000 to 7000 A. An in-depth study of the parameters affecting the random access signal and detector linearity is presented. Quantitative data are reported for simutlaneous multielement determinations in synthetlc solutions using atomic absorptlon spectrometry.
I n a n early report from this laboratory, we described the 1112
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adaptation of a vidicon tube for rapid scanning spectrometry ( I ) . Subsequent to that report, numerous papers have appeared from this and other laboratories which described applications to molecular (2-5) and atomic (6-10) spectrometry. While these reports demonstrate beyond any question that the vidicon and other types of imaging detectors are viable tools for analytical spectrometry, they also emphasize some serious limitations of the devices. One of the most serious limitations results from the finite length of the detector which usually forces the user into a rather severe tradeoff between spectral resolution and spectral range (2,9, 11). Most vidicon applications reported to date have been limited to what we shall call one-dimensional scanning modes
