ANALYTICAL CHEMISTRY, VOL. 50, NO, 7, JUNE 1978
*
861
Determination of Chlorine and Bromine by Molecular Absorption of Aluminum Monohalides at High Temperature Kin-ichi Tsunoda, Kitao Fujiwara, and Keiichiro Fuwa” Department of Chemistry, University of Tokyo, Hongo, Bunkyo-ku, Tokyo, 113, Japan
The diatomic molecules of aluminum monochloride, AICI, and aluminurn monobromide, AIBr, give intensive absorption band spectra, respectively, at band head (bh) 261.4 nm and bh 279.0 nm In both the flame and carbon rod furnace. The best sensitivities observed are 0.12 ng for chlorine and 1.1 ng for bromine with carbon rod furnace. An excess aluminum salt Is applied first to the furnace before the sample solution is plpetted, so that these molecules are effectively formed inside the furnace. A certain amount of strontium and cobalt, or Strontium and Iron Is found to enhance the signals with the decrease In backgrounds for both elements. A 2-channel spectrometer Is used as an effective correction procedure of background absorption.
Molecular absorption spectrometry a t high temperature has been investigated and developed for the trace determination of nonmetallic elements such as sulfur, phosphorus, and halogens ( I , 2 ) . Such elements are difficult to determine by conventional atomic absorption spectrometry since their resonance lines lie in the vacuum ultraviolet. Recently, molecular absorption of aluminum monofluoride has been used t o determine subnanogram quantities of fluorine ( 3 ) . In addition, absorption of indium monochloride has enabled sensitive determinations of chlorine ( 4 ) . This approach has been extended to aluminum monochloride and aluminum monobromide since such species have been reported ( 5 )to give intense UV emission and absorption in discharge tube experiments. This paper describes the production of the monohalogen complexes in a carbon rod furnace and the utilization of the respective absorption spectra for the determination of nanogram quantities of chlorine and bromine. T h e serious background absorption problems in molecular absorption spectrometry are corrected by the use of a 2channel spectrometer (6). EXPERIMENTAL Apparatus. For the measurement of the absorption spectra, an atomic absorption spectrophotometer, AA-I Mark 11, and a 2-channel atomic absorption spectrophotometer, AA-8500 from Nippon Jarrell-Ash Co., Ltd., were used. For the sample dispersing devices, a 5-cm slot burner with the nitrous oxide-acetylene flame. and a carbon rod furnace (Nippon Jarrell-Ash FLA 100) were employed. A deuterium lamp of the thermal cathode type (Hamamatsu T V Co., Ltd.) was used as the continuous light source. The spectral bandwidth of the AA-I spectrophotometer could be varied between 0.03 and 0.16 nm, and 0.08 nm was used, unless stated otherwise. The spectral bandwidth of the AA-8500 spectrophotometer was 0.16 nm. Reagent. Both chlorine and bromine standard solutions were prepared by dissolving the respective sodium salts in distilled water. Metal ion solutions were prepared from the nitrate salts. Procedure. The absorption spectrum of AlCl was recorded by aspirating an aqueous solution of aluminum chloride (1 M) into the nitrous oxide-acetylene flame at the rate of 3 mL min-’. The flow rates of fuel and oxidant were maintained at 6 and 8 L min-*, respectively. The observation height in the flame was 8 mm above the burner. The measuring procedure and operating conditions for the carbon rod furnace are summarized in Table I. To obtain the
Table I. Experimental Procedure and Operating Conditions for Absorption Measurements with the Carbon Rod Furnace 1. Application of “aluminum solution”a Dry-I ( 0 . 0 1 M, 1 0 p L ) (20 A, 1 0 s) Ash-I ( 5 0 A, 1 5 s)
2. Application of chlorine or bromine solution (5 pL) Dry-I1 (20A, 1 0 s ) Ash-11 (50 A, 30 s) Atomization and measurement (200 A-280 A, 7
S)
Aluminum nitrate was dissolved in water. For analysis, cobalt nitrate (0.01 M ) and strontium nitrate (0.01 M), or iron(II1) nitrate (0.01 M ) and strontium nitrate (0.01 M) were added t o this solution. absorption spectrum of AlC1, “aluminum solution” (10pL) [the mixed solution of aluminum nitrate (0.01 M), cobalt nitrate (0.01 M), and strontium nitrate (0.01 M)] and chlorine solution, (5 pL, 10 pg mL-’ C1-) were separately transferred to the carbon rod furnace prior to ashing. The absorption intensities of AlCl and AlBr were independent of ashing current up to 90 A (1300 “C) after which the intensities decreased sharply. When the atomizing current reached 180 A (2200 “C), AlCl and AlBr absorption intensities became maximal and leveled off thereafter. In addition, AlCl and AlBr absorption intensities became constant for aluminum concentrations greater than 0.005 M. From these results, the measuring conditions shown in Table I were selected. The appropriate amount of metal cations, either a mixture of cobalt and strontium, or a mixture of iron and strontium were added to the aluminum test solution to increase both the sensitivity and the precision of analysis. Ash “I” was performed only for the measurement of AlBr absorption in order to eliminate interference from nitrate ions. A continuous wavelength scan was performed in order to obtain the absorption spectrum of AlCl in the flame. With the high temperature cuvette, the absorption spectra of AlCl and AlBr were measured at fixed wavelengths at intervals of 0.1 to 0.3 nm. The AlCl absorption peak at 261.4 nm was employed for chlorine determinations. For background correction, the absorption intensity at 260.1 nm, where AlCl absorption is minimal, was subtracted from that at 261.4 nm, using the 2-channel spectrophotometer. The AlBr absorption peak at 279.0 nm with background correction at 281.5 nm was used for bromine determinations. Preparation of Biological Samples. Standard Reference Material Orchard Leaves from National Bureau of Standards (SRM No. 1571), was chosen as an example of a biological sample for the determination of chlorine. Dried samples (90 “ C for 24 h), accurately weighed, were ashed with milk of lime (5 mL) in a porcelain crucible at 550 “ C for 10 h. The ash was dissolved in hot water and neutralized with nitric acid (0.1 M).After the precipitation was filtered, the solution was diluted to the required volume and then measured according to the method outlined above. RESULTS AND DISCUSSION Molecular Absorption S p e c t r a of AlCl a n d AlBr. The absorption spectra of AlCl in the nitrous oxide-acetylene flame (a) and in the carbon rod furnace (b) are shown in Figure 1.
0003-2700/78/0350-0861$01.00/0 6 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, J U N E 1978
862
a
A7 0.30 1
t
0.20
O' 258
259
260
261
262
263
2i4
-
2651 (nmj
Flgure 1. Molecular absorption spectra of AICI, (a) in N,0-C2H2 flame and (b) in carbon rod furnace. In (a),line A is the spectrum obtained with aluminum chloride solution (1 M) and line B is the background spectrum obtained with distilled water. In (b), the dotted line is the background spectrum obtained with aluminum solution (10 pL) [the mixed solution of aluminum nitrate (0.01 M), cobalt nitrate (0.01 M), and strontium nitrate (0.01 M)], and the solid line is the spectrum obtained with sodium chloride solution (5 pL, 10 pg mL-' of Cl-') added to the dried background solution A
1
0 0.08 0.16 0.32 S p e c t r a l Band W i d t h ( n m )
Figure 3. Effect of spectral bandwidths on the intensities of AlCl and AlBr absorption. (-0-) AlCl absorption intensity (5 pL, 5 pg mL-' of CI-). (-A-) AlBr absorption intensity (5 pL, 10 pg mL-' of Br-). The aluminum solution used was the same as that in Figure l b . Subtraction of the background absorption was performed and the net absorbance values were plotted
I 0,o
0.1
Absorbance 0.2
0.3
0.4
Al AI+% AI+Fe Al+CO AI+Ni A1 Co+ AI + Co+ A l + CO+ A I + Co+ A I + Co+ A I + Fe+ I C a t i o n s 0.01 M Flgure 4. Effect of cations on AlCl absorption intensity in carbon rod furnace. Aluminum solutions (10 FL) which contained each cation, were applied to and dried on the carbon rod furnace before the addition of chloride solution (5 pL, 5 pg mL-' of Cl-). Upper levels: aluminum solution 4- CI, lower levels: aluminum solution only (background absorption)
1
+
Flgure 2. Molecular absorption spectrum of AlBr in carbon rod furnace. Dotted line is the background spectrum obtained with 10 pL of the aluminum solution which is the same as used for AICI, and solid line is the spectrum obtained with sodium bromide solution (5 FL, 100 pg mL-' of Br-) added to the dried background solution In lower temperature flames, e.g., air-acetylene and airhydrogen flames, absorption was not observed. Both absorption spectra exhibit fine structure with peaks of varying intensity a t 261.4 nm, 261.7 nm, 262.0 nm, and etc. The band system is identical to that reported in high frequency discharge (7)and is attributed to the electronic transition, 'n-'Z. The strongest AlCl absorption peak was a t 261.4 nm for the flame compared to 261.7 nm in the carbon rod furnace. The difference in absorption was, however, small and the wavelength of 261.4 nm was adopted for chlorine determinations. For background correction, the absorption a t 260.1 nm was substracted from that a t 261.4 nm using the 2-channel spectrometer. Figure 2 shows the absorption spectrum of AlBr in the carbon rod furnace, where aluminum solution (10 pL) and bromine solution (5 p L , 100 pg mL-' Br-) were used. AlBr absorption was difficult to observe in the flame. The absorption spectrum also has fine structure with peaks at 279.0 nm, 279.7 nm, 280.6 nm, and etc. This band is identical with the UV-band system of AlBr and is due to the electronic transition, lII-IZ (8). The absorption peak a t 279.0 nm was selected for the determination of bromine, and the absorption a t 281.5 nm was substracted to correct for background absorption.
Effect of Slitwidth on AlCl and AlBr Absorption. The effect of slitwidth on the respective absorption intensities of AlCl and AlBr is shown in Figure 3. The absorption intensity of AlCl decreased with increase in slitwidth up to 200 pm (0.32 nm spectral bandwidth). In contrast, AlBr absorption shows little dependence on slitwidth. Effect of Cations on AlCl and AlBr Absorption in Carbon Rod Furnace. In AlF molecular absorption spectrometry with a carbon rod furnace, it has been already reported (3) that certain cations, e.g., Ni2+, Co2+, Sr2+when added to the aluminum solution increase significantly the sensitivity of the procedure and decrease the background absorption. The effect of such cations on AlCl and AlBr absorption in the carbon rod furnace was, therefore, also examined. Figure 4 shows the effects of various metal cations on AlCl absorption when added to the aluminum solution (10 pL)-chlorine solution (5 p L , 5 Fg mL-' C1-). The extremely weak absorption of AlCl can be observed when only aluminum nitrate solution is applied. The addition of Ni2+,Fe3+,or Co2+ to the aluminum solution decreased the background which
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978 A
863
Table 11. Interference of Cations on AlCl Absorptiona Cation
0.4
None added Na‘
K’ Mgz+ Ca2+ Mn” Fe 3+
0.3
0.2
Ni”
cu2+
0.1
0.0
nrn
Flgure 5. Background absorption spectra in carbon rod furnace. (-0-) Aluminum nitrate solution (0.01 M). ( - - A - - )Mixed solution of aluminum nihate (0.01 M), and strontium nitrate (0.01 M). (-0-) Mixed solution of aluminum nitrate (0.01 M), and cobalt nitrate (0.01 M)
100 92 94
85 90 100 100
85 105
a Aluminum solution [a solution of aluminum nitrate (0.01 M), cobalt nitrate (0.01 M), and strontium nitrate (0.02 M)] was used. Sample solution (5 p L ) containing C1 (5 p g mL-’) and each cation (0.005 M ) separately was
examined. Table 111. Interference of Cations on AlBr Absorption4 Cation
0.101
Relative absorption
None added Na’ K‘ Mg2+
Ca2+ MnZ+ Fe 3 + Ni 2t cu2+
Relative absorption 100 66
53 100 105 102 101
88 101
a Aluminum solution was the same as that described in Table 11. Sample solution ( 5 p L ) containing Br- ( 1 0 p g mL-’) and each cation (0.005 M ) separately was examined.
&
0
5 10 20 C o n c e n t r a t i o n o f Sr ( x 10-3 M ) Flgure 6. Enhancing effects of strontium on AiCl absorption intensity in carbon rod furnace. Aluminum solutions (10 fiL, 0.01 M) containing cobalt and various concentrations of strontium were applied to and dried on the carbon rod furnace before the addition of sample solutions. Sample solutions (5 fiL) are: (-0-) sodium chloride solution (5 fig mL” of GI-) and (-A-) sodium chloride solution (5 fig mL-’ of CI-) containing sodium nitrate (0.005 M). Subtraction of the background absorption was performed and the net absorbance-values were plotted is mainly due to the absorption of A10, but the sensitivity was hardly increased. La3+ and alkali earth metal ions, except Mg2+,caused a strong enhancement of the AlCl absorption. For AlBr absorption, essentially the same results were found. Further detailed studies were carried out on the effects of Co2+and Sr2+. Figure 5 shows the background absorption spectra using (1)A13+ (0.01 M) solution, (2) a mixed solution of A13+ and Sr2+ (0.01 M), (3) A13+ (0.01 M) and Co2+ (0.01 M) in the carbon rod furnace. The former two solutions yielded large background absorption, particularly in the range from 240 to 270 nm; this was considered to be due to absorption of A10. Background absorption was considerably reduced for Co2+addition. This is interpreted as being due to Co2+preventing the formation of A10. T h e enhancing effect of strontium on AlCl absorption is shown in Figure 6. Addition of strontium (0.005 M) increased the absorption intensity. When A13+ (0.01 M)-Co2+ (0.01 M) solution was used, AlCl absorption was strongly suppressed in sodium nitrate solution (0.005 M Na+). However, the addition of Sr2+(concentrations greater than 0.02 M) eliminated the influence of Na+ and AlCl absorption was almost completely restored, as is clearly shown in the figure.
From these results, it is evident that the addition of Fe3+ and/or Co2+ and Sr2+to aluminum solution is an effective procedure for decreasing background absorption and also for increasing the sensitivity. In this investigation, aluminum solution containing Co2+and Sr2+,or Fe3+ and Sr2+was selected and used in measurements of AlCl and AlBr absorption. I n t e r f e r e n c e of Cations a n d Acids. Tables I1 and I11 show the interferences of concomitant cations on AlCl and AlBr absorption; aluminum solution (10 fiL) (see the footnote in Table 11) was used in these measurements. Many cations do not interfere. AlBr absorption was, however, suppressed by alkali metal ions such as Na+ and K+ regardless of whether Sr2+and Co2+were present. The effect of H2S04,“OB, and H3P04on AlCl absorption, and those of H2S04,“OB, H3P04, and HC1 on AlBr were investigated. There is little interference up to a concentration of 0.01 M for all acids except in the case of HC1 for AlBr absorption. A t concentrations of HC1 greater than 0.001 M, AlBr absorption is appreciably reduced in proportion to the acid concentration. Background Correction with 2-Channel Spectrophotometer. As already mentioned in the previous section, the correction of background absorption due to A10 is an important requirement for accurate determinations of C1 and Br via AlCl and AlBr molecular absorption spectrometry. The background correction methods using the D2 lamp or Zeeman effects, which are normally employed in atomic absorption spectrometry, cannot be applied t o the present system. Background correction utilizing a 2-channel spectrophotometer was, therefore, performed. Using the continuous light source, channel “a” measured the absorbance ( A , ) a t the bandhead of the molecular absorption while channel “b” recorded the absorption (Ab) a t a wavelength where the molecular species exhibited minimal absorption, Le., in the latter case the
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
864
Table IV. Analytical Data on AlCl and AlBr Molecular Absorption Spectrometry (MAS) AlCl MAS
T?
-+
AA-8500,Q slit width, 100 pm
AA-I, slit width, 1 5 pm 1%-Absorption, ng Rel. std dev, at 2.5 ng of C1, % Dynamic range, ng
0.12 4
0.21
up to ca. 5
up to ca. 5
2
AlBr MAS b 260.lnm
tI
Figure 7. Chart profile of AlCl measurements with 2-channel background
correction. Chart a shows the AlCl absorption at 261.4 nm without background correction. Chart b shows the background absorption at 260.1 nm. Chart a - b shows the net absorption of AlCl obtained by subtracting absorption at 260.1 nm from the absorption at 261.4 nm a 279.0nm 1OngBr
AA-85OO,Q slit width, 100 pm
AA-I, slit width, 100 pm
a-b
1%-Absorption, ng Rel. std dev, at 25 ng of Br, % Dynamic range, ng
1.9 7
1.1
up t o ca. 100
up to ca. 100
3
The 2-channel spectrometer was used to correct for background absorption. Table V. Determination of Chlorine in Orchard Leaves (SRM 1571 of NBS)
blank Found, wglg (dw)‘
1 -
x1
Chlorine Recommended value, pglg (dwY
630 r 24 580 i 27
b 281.5nm I
a
(690)
dw = dry weight.
Figure 8. Chart profile of AlBr measurements with 2-channel background
correction. The method of presentation is the same as in Figure 7 background absorption was measured. The net absorption (Aa.b) was calculated to obtain the true absorption of the molecule. Figures 7 and 8 show the chart profiles of AlCl and AlBr absorption with and without background correction. P a r t “a” shows the chart profile in which the background is not corrected and part “a - b” shows t h a t in which the background is corrected. For both AlCl and AlBr, unless the 2-channel background correction is employed, the background A10 absorption is severe in spite of the addition of Co2+ or Fe3+. The background was, however, completely eliminated by using the 2-channel correction procedure. From these results, it is concluded t h a t the method is applicable for accurate determinations of chlorine and bromine.
Analytical Data on AlCl and AlBr Molecular Absorption Spectrometry. Table IV summarizes the analytical data of chlorine and bromine determination utilizing AlCl and AlBr molecular absorption. The highest sensitivity for chlorine (0.12 ng) was obtained with the AA-I instrument using a slit-width of 15 Fm (0.03-nm spectral bandwidth). A lower value of the relative standard deviation was obtained with AA-8500 than with the AA-I indicating the improved precision available for the procedure using background correction. Analytical Application of AlCl and AlBr Molecular Absorption. As an application of AlCl molecular absorption to C1 determination in the biological field, the content of chlorine in Orchard Leaves (Standard Reference Material, 1571 of the National Bureau of Standards, Washington, D.C.) was measured. The results are shown in Table V. The analytical conditions were the same as those mentioned in the previous sections. The values of chlorine obtained for Orchard Leaves were in good agreement with that recommended by N.B.S. The present methods were also applied to the determinations of chlorine and bromine in solutions containing trace
Table VI. Determination of Chlorine and Bromine in Solutions Containing Organo and Oxo Chlorine and Bromine CompoundsQ Compound o-Chlorobenzoic acid Sodium chloroacetate Sodium perchlorate Compound p-Bromobenzoic acid Sodium 2-bromoethanesulfonate Potassium bromate
Chlorine, ng Found Calculated 5.7 f 0.3 4.9 r 0.4 5.6 t 0.3
5.1 5.0 4.9
Bromine, ng Found Calculated 28.2 25.8
i t
1.3 0.9
25.5 25.6
26.4
* 1.0
24.3
Compounds were weighed, dissolved in distilled water, and an aliquot was analyzed. Aqueous solutions of sodium salts were used as the standard. levels of organo and oxo chlorine and bromine compounds. The results shown in Table VI are consistent with the calculated values. From the above results, the characteristics of the present methods can be summarized as follows: (1) High sensitivity, (2) Sample solution volumes are extremely low (-10 FL), (3) Simple and rapid operation, and (4) Applicable to chloride and bromide as well as the organo and oxo chlorine and bromine. Accordingly, the present methods utilizing molecular absorption of AlCl and AlBr afford high potential with respect to the trace analyses for chlorine and bromine.
ACKNOWLEDGMENT The authors are thankful to K. Matsumoto for her valuable helps and suggestions.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
LITERATURE CITED K. Fuwa and B. L. Vailee, Anal. Chem., 41, 188 (1969). H. Haraguchi and K. Fuwa, Anal. Chem., 48, 784 (1976). K. Tsunoda. K. Fujiwara, and K. Fuwa, Anal. Chem.. 49, 2035 (1977). E. Yoshimura, Y . Tanaka, K . Tsunoda, S.Toda, and K . Fuwa, Bunsekl Kaaaku. 26. 647 (1977). (5) R. %'. B,. Pearce and A: G. Gaydon, "The Identification of Molecular Spectra , 3rd ed., Chapman & Hall, London, 1976. (6) K. Tsunoda, et al., 38th Annual Symposium of Japan Society for Analytical Chemistry, June 3, 1977.
(1) (2) (3) (4)
(7) B. N. Bhaduri and A. Fowler, R o c . R . (1934). (8) H, G,Howell, proc, D C n r I n n A n n
865
SOC.London, Ser. A , 145, 321 Car
A
'IAR
COC 110'2Kl
RECEIVED for review January 5 , 1978. Accepted March 6, 1978. This work is supported by the Government Grant-in Aid No. 203015, and No. 211204, and u. s. Cooperative Project No. 6R023.
Determination of Ultra Trace Cadmium by Laser-Induced Photoacoustic Absorption Spectromery Shohei Oda," Tsuguo Sawada, and Hitoshi Kamada Department of Industrial Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo, Japan
Trace determination of cadmium was carried out by laserinduced photoacoustic absorption spectrometry. The heavy metal-tolerant fungus, Penicilium ochro-chloron, was decomposed and cadmium was measured after extraction into chloroform. The results obtained were in good agreement with those of atomic absorption spectrometry. The detection limit for cadmium was 0.02 ng mL-' Cd (14 ppt). This value was approximately two orders of magnitude lower than that for colorimetric and conventional flame atomic absorption spectrometry.
Absorption spectrometry utilizing the photoacoustic effect has become of major interest. With the photoacoustic absorption technique, it is possible to measure absorption spectra of opaque solids and liquids which previously had been extremely difficult using conventional transmission or reflectance spectrometry. Rosencwaig ( I ) and Adams et al. ( 2 )have been actively investigating possible applications of the technique. Lasers, used extensively as light sources in many areas of analytical spectrometry, have provided sensitive detection when employed in photoacoustic absorption spectrometry. Ultra-low gas concentration has been determined by this approach, while Kreuzer ( 3 )has discussed photoacoustic gas analysis in detail. Lahmann et al. ( 4 ) were the first workers to apply the laser-induced photoacoustic absorption technique t o analysis of liquids. They determined trace (3-carotene in chloroform with a n argon ion laser, and a detection limit of 9 X 1O'O molecules per cm3 (12 ppt) was achieved. The latter authors ( 5 ) also determined Mn0,- in aqueous solution with the same method. T h e detection limit obtained was about two orders of magnitude lower than that of the standard colorimetric method. In the present paper, the determination of cadmium in the heavy metal-tolerant fungus, Penicilium ochro-chloron, was carried out with this new technique. The fungus, important in the biochemical and medical fields, is used for studies concerning the mechanism of resistance t o heavy metals in vivo. Satisfactory results were obtained, compared with those of flame atomic absorption spectrometry.
EXPERIMENTAL Apparatus. A block diagram of the photoacoustic spectrometer is shown in Figure 1. The output beam of argon ion 0003-2700/78/0350-0865$01 .OO/O
laser (Spectra Physics Model 164-03), operating in a single line mode of 514.5 nm, was modulated at a given frequency by a light chopper and was directed into the sample cell through a collecting lens (f = 20 cm). The pressure fluctuation induced in the sample solution by absorbed radiation was detected by a piezoelectric ceramic (NPM, N-6 supplied by Tohoku Kinzoku Co. Ltd.). A lock-in amplifier/preamplifier (NF Co. Ltd., Model LI-574) was used to amplify the modulated output signal. The piezoelectric ceramic acts simultaneously as a sample cell and a pressure sensor. The middle part of the cell was of cylindrical piezoelectric ceramic (length 50 mm and inner diameter 24 mm) which was sealed on both sides by Pyrex tubes incorporating a stopcock and a quartz window. The cell was placed inside an airtight chamber which was secured to a vibration-free stand to prevent pressure fluctuations caused by vibrations from external sources. An atomic absorption spectrophotometer (Dainiseikosha Co. Ltd., Model SAS 7 2 5 ) using the air-acetylene flame was used for atomic absorption measurements. Reagents and Procedure. All reagents were of ultrapure grade or spectral grade and were used without further purification. Water was prepared by distilling (twice) deionized water. Cadmium was extracted into chloroform as cadmium dithizonate according to Saltzman's procedure ( 6 ) ,to separate cadmium from interfering metals. A stock solution of cadmium dithizonate (0.5 pg/mL Cd) was prepared by diluting cadmium solution (10 mL, 1 Fg/mL Cd) to 20 mL with chloroform. The calibration graph was then obtained for standard cadmium solutions which were prepared by appropriate dilution of the stock cadmium solution (0.5 Fg/mL Cd). Peniczlzurn ocho-chloron fungus, 0.3 g (dry weight), was decomposed by nitric acid (4 mL) and sulfuric acid (1 mL). The solution was adjusted to 25 mL with distilled water. The extracted solution was diluted with chloroform to provide a solution whose cadmium concentration was within the range of the standard solutions. The procedure was carried out for the blank solution. The photoacoustic signal intensity of the blank solution was subtracted from that of the cadmium solutions to obtain the cadmium concentration.
RESULTS AND DISCUSSION Kohanzadeh e t al. (7) reported that the laser-induced pressure fluctuations in a sample cell depend upon several physical constants of the solute and solvent. In very dilute solution, however, the photoacoustic signal intensity is considered to be proportional to the incident laser power, the absorptivity of the sample a t different wavelengths, and the concentration of the solute. In a given solvent, the photoacoustic signal intensity is proportional to the concentration C 1978 American Chemical Society
