Anal. Chem. 1981, 53, 2224-2228
2224
Table 111. Arsenic Content of NBS Reference Materials, pg/g (Dry Weight) no. of NESS certified value SRM samples present studya 1577, bovine liver 1575, pine needles 1571, orchard leaves 1566, oyster tissue a
10
0.056 i 0.003
0.055
6
0.215 * 0.006
0.21
f
i
0.005
0.04
6
10.43 i- 0.22
10 f 2
6
13.17
13.4 f 1.9
i.
0.34
Each value represents the mean
* standard deviation.
or more times as much tin as arsenic. By use of the conditions described in the Experimental Section, sample blanks were determined to contain 2.5 ng of arsenic. Also, a calibration curve linear to 10 ng of arsenic (Figure 6) was obtained. The digestion procedure used was chosen over wet digestion because residual acids (HN03, €&SO4, or HC1Q4)from the latter procedure apparently interfere with arsenic determination by hydride generation methods (9). The cellulose was added during the digestion procedure to reduce sample foaming and, thus, to allow for faster drying of the sample. After dry combustion, arsenic will be present as As(V). Nonetheless, the samples were measured against an As(IIX)
st,andard because, with experimental conditions used, hydride generation from an As(1II) standard was identical with that from an As(V) standard. Recovery of 40 ng of arsenic as As203 in samples digested by the described procedure was determined to be 99.8 f 1.1%. Using our procedure, we found sensitivity and absolute detection limits (IUPAC) of the method were 0.11 ng and 0.14 ng, respectively. We also determined the arsenic content of various NBS standards. As shown in Table 111, the values found agreed with the certified values.
LITERATURE CITED (1) Fowler, 8.A. I n "Toxicology of Trace Elements"; Goyer, R. A., Mehlman, M. A,, Eds.; Hemisphere Publishing Co.: Washington, DC. 1977; Chapter 3. (2) Nielsen, F. H. I n "Advances in Nutritional Research", Vol. 3; Draper, H. H., Ed.; Plenum Press: New York, 1980; Chapter 6. (3) Evans, W. H.; Jackson, F. J.; Dellar, D. Analyst(Landon) 1979, 104, 16-34. (4) Chapman, J. F.; Dale, L. S. Anal. Chim. Acta 1979, 111, 137-144. (5) Brodie, K. G. Am. Lab. (Fairfield, Conn.) 1979, 7 1 , 58-66. ( 6 ) Peats, S. At. Absopt. News/. 1979, 18, 118-120. (7) Knudson, E. J.; Christian, G. D. Anal. Lett. 1973, 6, 1039-1054. (8) Braman, R. S.; Justen, L. L.; Foreback, 6. C. Anal. Chem. 1972, 4 4 , 2195-2199. (9) Kang, H. K.; Valentine, J. L. Anal. Cbem. 1977, 49, 1629-.1832.
RECEIVED for review June 1, 1981. Accepted July 31, 1981. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U.S. Department of Agriculture and does not imply its approval to the exclusion of other products that may also be suit,ahle.
Determination of Metal Chelates uctively Coupled Plasma Atomic Emission Spectrometry nd Applications to Biological Materials Marilyn S. Black" Consultant, 3304 Indian Valley Trail, Atlanta, Georgia 3034 1
Mlchael B. Thomas and Richard F. Browner School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332
Inductively coupled plasma (ICP) is used as a selectlwe and sensitive detector for the analysis of /3-diketonate complexes of Fe, Cu, Zn, Mn, AI, and Cr when solutions of the chelate complexes in xylene are dlrectly lntroduced by pneumatic nebulization. Operating parameters for ICP sample introductlon of the organic solutions are discussed and detection limRs In the ng/mL range are presented for the metals as their trlfluoroacetylacetonate complexes in xylene and are compared to those obtained with aqueous solutions of the metals. Techniques and quantitatlve data are presented, compared, and discussed for the determination of these metals in various blologlcal materials including human blood serum, human skin, and NBS reference materials, bovine liver end orchard leaves. Techniques presented include slmultaneous of AI, Fe, Cu, and Cr from acid digests and nebulization of the resulting metal chelate complexes in xylene, direct reaction of the chelate ligand [H(tla)] with Fe, Cu, Pn, AI, Mn, and Cr In the biologlcal materials and nebulization of the resulting trifluoroacetylacetonate complexes in xylene, and direct nebulization of acid digests of the biological materials.
Inductively coupled plasma atomic emission spectrometry (ICP-AES) as a trace analysis technique is experiencing a period of rapid growth with extensive research effork toward instrumental optimizations and applications. Its applicability to the analysis of a broad range of samples-biological, geological, environmental, and agricultural-has been demonstrated (1-6). The analytical advantages of ICP-AES, including low detection limits, wide linear operating ranges, minimal chemical interferences, and simultaneous multielement capability render the technique excellent for trace and ultratrace determinations of metals in a variety of matrices. The most common form of sample introduction for ICPAES is pneumatic or ultrasonic nebulization of aqueous solutions. This requires a sample in a relatively nonviscous, aqueous form and necessitates, in most cases, extensive sarnple pretreatments prior to nebulization. These pretreatments can include acid digestions, fusions, dry ashings, and sample dilutions, all of which can be time-consuming m well as a source of sample contamination. These procedures can also require large amounts of samples since the pretreatment procedures
0 3981 American Cheinical Society ~003-2700/81/~353-22241$01.25/0
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
entail considerable dilution of the original sample. There have been some preliminary studies of direct pneumatic nebulization of organic samples with minimal sample preparations. These trave included lubricating oils diluted with organic solvents for wear metal determinations (7, 8), diluted biological fluids for trace metal determinations (9), and solvent extracted metals from geological materials (10). In general, solvent extraction of metal chelate complexes has been recognized as an effective separation and preconcentration step in analytical procedures and has been applied to flame atomic spectrometric (11) and gas chromatographic (12, 13) techniques. Solvent extraction of volatile metal 0diketonate complexes has been used extensively in the gas chromatographic analysis of metals and has been applied to numerous matrices (14, 15). This separation and analysis technique has worked extremely well for some metals with very low detection limits and with excellent accuracy and precision. Solvent extraction of metal P-diketonate complexes has typically been applied to previously digested or fused sample materials, although some studies have illustrated the feasibility of directly chelating some metals from the matrix with minimal sample pretreatments (16, 17). Several spectroscopic detectors for the analysis of metal chelate complexes have been described, including a mass spectrometer (18),a microwave excited emission detector (191, and an atomic absorption spectrometer (20). In a previous publication, Black and Browner (16) presented the direct vapor-phase introduction of volatile metal chelate complexes to an ICP spectrometer along with data to support the ICP as a selective and sensitive detector for the analysis of volatile metal &diketonate complexes. In this current study we discuss the pneumatic nebulization of metal chelate complexes in an organic solvent and present quantitative data for the solvent extraction of various metal trifluoroacetylacetonates from digested biological materials. Additionally, data on the direct reaction of trifluoroacetylacetone with metals in biological materials are given.
EXPERIMENTAL SECTION Instrumental Methods. The manually controlled Plasmatherm ICE' system previously described (16) was used in addition to a Plasmathem Model TN-5500 cross flow nebulizer fitted to a concentric glass spray chamber for liquid sample introduction. Plasma torch coolant and auxiliary argon flows are maintained by a system of pressure gauges, precision needle valves, and rotometers. Precise control of the nebulizer argon was critical and was maintained with a Matheson Model 8240 h e m mass flow controller, Sample feed to the nebulizer was controlled with a Buchler peristaltic pump (Buchler Instruments, Fort Lee, NJ) using solvent resistant Tygon tubing (Norton, Co., Akron, OH). Operating parameters of the ICP were initially chosen to provide maximum plasmla stability while aspirating xylene solutions of the metal trifluoroacetylacetonate complexes. Stabilily of the plasma with organic solvents is in general less than that obtainable with aqueous solutions; however, an increase in operating pawer of the radiofrequency (rf) generator, the addition of auxiliary gas to the plasma, and fine control of the nebulizer gas flow rate produced a stable plasma that could be operated for extensive periods of time (up to 6 h in this study) without deterioration of stability and without observable carbon buildup on the torch. The operating parameters given in Table I were used for studies of all the trifluoroacetylacetonate complexes. This common set of operating conditions was carefully chosen to provide a stable plasma and good signal-to-noise(S/N) ratios for those elements of interest. Optimization of the nebulizer gas flow rate was critical. Although its increase resulted in better plasma stability, owing to better penetration of the organic solvent into the central channel (or core) of the plasma, its increase resulted in a decrease of S / N ratios. However, nebulizer flow rates less than 0.8 L!min resulted in the plasma being extinguished. At these low flowrates, significant green C2 emission mound the outer
2225
Table I. ICP Operating Parameters operating power coolant gas flow auxiliary gas flow nebulizer gas flow aspiration rate observation height wavelengths
1.85 kW 14.5 L/min 1.5 L/min 1.2 L/min 0.75 mXI/min 1 6 mm above the coil Fe, 259.9 nm Cu, 324.7 nm Al, 308.2 nm Cr, 267.7 nm Zn, 213.9 nm Mn, 257.6 nm ~-
edges of the plasma was observed thus indicating inefficient penetration of the organic solvent into the plasma core. Reagents. Reagent grade chemicals were used throughout except where otherwise indicated. All acids and hydrogen peroxide were Ultrex quality (Baker Chemical Co.), xylene was Nanograde quality (Fisher Scientific Co.), and water was doubly distilleddeionized. The ligand trifluoroacetylacetone[H(tfa)]was obtained commercially (PCR Chemicals, Gainesville, FL) and distilled prior to use. Distilled H(tfa) was stored in a refrigerated, silanixed borosilicate glass bottle and was used within 2 weeks of distillation. Metal chelates were obtained from earlier studies or prepared and purified by previously reported techniques (2, 21). Standard solutions of metal celates were prepared by dissolving weighed amounts of pure metal chelates into preleached (with H(tfa)/ xylene solutions),silanized volumetric flasks. Additional standards were prepared from these stock solutions by volumetric dilutions. Calibration curves were prepared from these standards for all quantitative determinations. Acid Digestions. Blood serum, NBS bovine liver (SRM 157'0, and human skin (Plantar Stratum Corneum) were digested with 2:1 mixtures of concentrated nitric and sulfuric acids in a reflux system consisting of a reflux condenser fitted onto a 25-mL or 50-mL round-bottom digestion flask. A Variac controlled heating mantle was used to efficiently heat the digestion contents. From 25 to 75 mg of skin, from 1 to 5 mL of blood serum, and -250 mg of bovine liver, respectively, were accurately weighed into a digestion flask and sufficientacid mixture was added to oompletely cover the material to be digested. After initial bubbling subsided, the digestion mixture was heated to boiling and refluxed for 3 h. Additional aliquots of 12 N HNOBwere added to wash down the sides of the reflux condenser. The digests were cooled, neutralized with saturated NaOH and volumetrically diluted to 25 mL or 50 mL with distilled-deionized HzO. NBS orchard leaves (SRM 1571) were digested according to a previously published procedure using -260 mg of sample (20). Chelation Procedures. Solvent extractions of Fe, Gu, Al, and Cr from acid digests of NBS orchard leaves and Fe from human skin were performed in the following manner. Five-milliliter aliquots of the digested samle were pH adjusted to 4.5-5.0 and an equal volume of 0.5% (v/v) H(tfa) in xylene was added in a silanized borosilicate reacti-flask with a Teflon capped lid (Pierce Chemical Co., Chicago, IL). The contents were heated and mixed at 60 "C in an ultrasonic bath for 45 min. The flasks were cooled to room temperature and shaken by hand for 5 min. The organic phase was separated from the aqueous phase and aspirated directly into the ICP. Direct chelation reactions between H(tfa) and metals in the biological materials were carried out in sealed ampules following techniques and procedures similar to those previously published (16,17). For bovine liver and human skin analyses, -250 mg and -25-75 mg of sample, respectively, were weighed into IO-mL glass ampules and 3 mL of neat H(tfa) was added. The ampule was sealed and placed for 5 min in an ultrasonic bath after which it was heated in an oven for 4 h a t 100 "C. After being heated, the ampule was opened, and the contents were quantitatively transferred to a 25-mL flask and diluted with xylene. The flask was shaken by hand for 5 min and the organic solvent was transferred to a beaker and directly aspirated. When b l a d serum was directly reacted with H(tfa), 1 mL of the sample was placed with 1 mL of 0.5% H(tfa) in xylene (v/v) in an ampule and the reaction was carried out as described above. When the ampule
2226
ANALYTICAL CHEMISTRY, VOL.
53, NO. 14, DECEMBER 1981 Table 111. Simultaneous Solvent Extraction of Fe, Cu, A1 and Cr as Trifluoroacetylacetonates from Acid Digests of NBS Orchard Leaves (SRM 1571) and Determination of Continuous Nebulization-ICPa % recovery
by solvent ICP value extractionmetal Mg/g ICP Fe 296 f 1 2 99 cu 11.6 f 0.4 97 AI C 140 i 8 nd Cr 2.6 f 0.3 2.4 f 0.6 92 a Values reported as mean i SD (five determinations). Not deterNBS certificate of analysis. Not given. mined.
NBS value, Pglg 300i: 20 12 f 1
__T I Y E
Figure 1. ICP-AES responses of 50 ng/mL Fe as Fe(tfa), (259.9 nm) and 20 ng/mL Cu as Cu(tfa), (324.7 nm) in xylene.
Table 11. Comparison of Detection Limits by ICP-Continuous Nebulization of Aqueous Solution of Metals and Metal Trifluoroacetylacetonates in Xylene metal metal in chelate in wavelength, H,O, xylene, metal nm ngld ng/d 259.9 0.5 1 Fe I1 324.7 0.5 0.5 cu I Zn I 21 3.9 2 2 308.2 4 3 A1 I 267.7 3 5 Cr I1 257.6 0.2 0.5 Mn I1 was opened, the organic solvent was directly aspirated from the ampule. All glassware used in these direct chelation reactions was preleached with H(tfa) prior to use. This was extremely critical in removing contaminant metals. R E S U L T S AND DISCUSSION I C P Characteristics of Metal-Chelate Complexes. By careful optimization of the ICP operating conditions, a very stable plasma could be maintained during the pneumatic nebulization of metal /3-diketonate complexes dissolved in xylene. The spectroscopic ICP technique was sensitive and allowed determinations in the nanogram per milliliter range for the organically bound metals in the chelate complex. Typical responses of Fe(tfa)a and Cu(tfaI2 in xylene corresponding to 50 and 20 ng/mL, respectively (as the metal), are shown in Figure 1. Reproducibility of responses was within 2-3% relative standard deviation (RSD) for all the metal trifluoroacetylacetonates studied; these precision data were obtained from 10 replicate determinations of each metal with concentrations 20-40 times above the detection limit. Responses were quantitated by measuring the mean of the recorder deflection. Spectral interferences from the organic matrix were minimal for the metal chelates of interest. The major spectral background originating from the H(tfa) in xylene was atomic C emission and the C2 and CN molecular band emissions, all of which did not interfere with the metals and respective wavelengths studied. ICP detection limits for Cu(tfa)z,Fe(tfa),, Zn(tfaIz,Al(tfa),, Cr(tfa), and Mn(tfa)a in xylene are given in Table I1 along with detection limits obtained for the free metals in aqueous solution. These limits, defined as the concentration of metal required to produce a net signal twice that of the standard deviation of the background signal, were determined with the previously described experimental system and conditions. Observation height was varied within f 2 mm to obtained the optimum S / N ratio for each element. The detection limits of the trifluoroacetylacetonates of Fe, Mn, and Cr were higher (approximately by a factor of 2) than those of the free metals in aqueous solution whereas the detection limits for Cu, Zn, and A1 metal chelate complexes were comparable to those of their aqueous counterparts. This effect could be explained
by weaker emission of the metal I1 ion lines (see Table I) within the organic matrix. The presence of xylene in the plasma could effectively lower the plasma excitation temperature since energy is absorbed in dissociation of the organic molecules. In general, the net intensity signals of the metal trifluoroacetylacetonates in xylene increased with increased viewing height when the other plasma operating conditions were held constant. Optimum S / N ratios were obtained within 15-18 mm above the coil with normal plasma conditions; above 18 mm, increased background intensity began to degrade the S / N ratios. Applications to Biological Materials. Solvent extraction is often an excellent selective separation and preconcentration step in analytical techniques. The applicability of combined solvent extraction of Fe, Cu, Al, and Cr (as trifluoroacetylacetonates) from acid digests of NBS orchard leaves was evaluated. The results are presented in Table 111, where the values obtained by solvent extraction-ICP analysis compare well with those values certified by NBS. Percentage recovery of the reported mean NBS value as determined by solvent extraction-ICP is given for each element. Since a certified value for A1 was not given, a 120 pg/mL aqueous solution of AlC13 was prepared and solvent extraction was performed according to the same experimental procedure. An average 93% recovery was obtained for three independent solvent extraction-ICP determinations of Al. The direct reactivity of H(tfa) with some metals in biological materials has been presented in previous publications (16-18). The ability to directly chelate metals out of biological materials without prior acid digestions or other complicated sample pretreatments offers significant analytical advantages. This approach reduces manipulative preparation steps and thus can minimize sample contamination and loss of sample. In addition, it can save considerable analysis time and permit the use of smaller amounts of original sample. All of these factors are critical when analyzing biological materials for trace metals, A direct chelation-ICP analysis was conducted on NBS bovine liver and the results are presented in Table IV. Results are given for metal determinations of an acid digest of the bovine liver as well as determinations of each metal as the trifluoroacetylacetonate in the organic extract of the direct reaction procedure, Recoveries for each procedure are given with respect to the certified NBS mean value. For each of the six elements, recoveries for ICP analysis of the acid digest were excellent and slightly higher than those obtained with the direct extraction procedure. However, the recoveries for the direct extraction procedure were good and analytically acceptable. The precision data given (SD) was obtained from 12 determinations of each metal (three separate reactions and four analyses of each resulting solution). Relative standard deviations (RSD) ranged from 1.8% for Fe to 10% for Al and Cr by direct ICP analysis of the acid digest and from 4.4% for Cu(tfa)z to 11%for Cr(tfa), by the direct chelation-ICP
ANALYTICAL CHEMISTRY, VOL. 53, NO. 14, DECEMBER 1981
2227 I
Table IV. Determination of Some Metals in NBS Bovine Liver (SRM 1577)by ICP-Continuous Nebulization of Acid Digests and Directly Extracted Metal Trifluoroacetylacetonatesa metal free metal in acid % trifluoroacet ylacetonate % metal ABS value,b pg/g digest, pg/g recovery in xylene recovery Fe cu Zn
268 i 8 193 f 10 130 f 1 3
Al
C
Mn Cr _
1.0.3 f 1.0 0.088 f 0.012
Values reported as mean _
266 f 5 187 f 4 130 f 4 8.2 f 0.8 9.5 f 0.'7 0.092 f 0.009
_
-
~
f
99 97 100 nd 92 104
259 f 12 180 f 8 120 f 6 8.0 f 0.6 9.2 f 0.9 0.072 i: 0.008
NBS certificate of analysis.
SD (12 determinations).
Table V. Determination of Iron in Human Skin (Plantar Stratum Corneum) by ICP-Continuous Nebulization Using Different Sample Preparation Techniques" Fe found, sample preparation sample introduction pg/g
Values reported as mean
f
nd 89 82
Not determined.
-
acid digestion Fe3+in aqueous digest 166 f 4 acid digestion/solvent Fe(tfa), in xylene 164 f 6 extraction of Fe(tfa1, direct reaction with Fe(tfa), in xylene 168 f 10 H(tfa) a
Not given.
97 93 92
SD (10 determinations).
Figure 2. Chromium responses (267.7 nm) of a standard Cr(tfa), solution and extracted Cr(tfa), from human blood serum obtained by continuous nebulization ICP-AES.
analysis. The obtained precision is good and within the ranges typically found for trace metal determinations in biological materials. Table V presents data obtained on the determination of Fe in human skin (Plantar Stratum Corneum). Analyses were conducted by all three techniques previously described ICP analysis of an acid digest of the skin; solvent extraction-ICP analysis of an acid digest so that Fe as Fe(tfa)3was determined, and direct extraction-ICP analysis of the skin with Fe determined (asFe(tfa)% The results found by all three techniques were in excellent, agreement with precisions of the techniques running from 2 to 6% RSD. Fe availability in skin is presently being studied in this laboratory in relation to certain pathological skin disorders. The specific skin analyzed in this study was abnormal with an F'e level ranging approximately 5 times higher than the normal value. Another biological material of importance with respect to trace metal levels and pathological disease states is human blood serum. Determinations of Fe, Zn, Cu, and Cr were made on a referenced blood serum sample obtained from the Center for Disease Control (CDC) of Atlanta, GA, using ICP techniques. Results are given in Table VI for the analysis of an acid digest of the blood serum and for analysis of the directly extracted metal trifluosoacetylacetonates from the sample. Average vidues obtained from round robin CDC analyses are given in the first column (22). The metal concentrations obtained from ICP analysis of the acid digest were in good agreement, with the average CDC values as were the concentrations determined from the direct extraction of the metals
as trifluoroacetylacetonates. Recoveries of the directly extracted metals ranged from 88% for Fe to 97% for Zn when compared to the CDC average values. Chromium determination was not feasible with the acid digest, since its concentration in the digest was below the detectable limit. However, the direct reaction of H(tfa) with the blood serum successfully extracted Cr as Cr(tfa), in detectable concentrations. The chromium level in the original serum was considerably diluted by the acid digestion but the 1:ldirect reaction/extraction procedure did not entail dilution of the original sample. The response obtained from the ICP determination of Cr as Cr(tfa), is shown in Figure 2 along with a response from a 10 ng/mL Cr(rfa), in xylene standard. Blood serum has been previously analyzed by ICP without prior sample digestion, but for direct nebulization, the neat serum must be diluted by a factor from 2 to 10. This dilution, in effect, reduces the concentration levels of the elements in the serum to be analyzed and consequently increases the detection limits of that technique. The availability of metal chelate analysis by continuous nebulization ICP-AES offers a different approach for the determination of certain trace elements in biological materials. Direct reaction and solvent extraction procedures can be used to minimize sample preparation steps and to serve as efficient preconcentration and separation stages, while ICP-AES permits precise analyses with good detection capability of the
Table VI. Analysis of Human Blood Serum by ICP-Continuous Nebulization of Acid Digests and Directly Extracted Metd Trifluoroacetylacetonates in Xylene concns found, pg/mL, mean f SDa extracted metal
a
Fe Zn cu Cr Six determinations.
CDC value
acid digest
0.68 0.62 0.81
0.65 0.60
f f f
0.05 0.06 0.05
%
in xylene
recovery
0.60 f 0.05 0.58 f 0.06 0.76 f 0.06 0.009e
88 93 94
0.04 0.05 0.8f t 0.05
C
Reference 22.
trifluoroacetylacetonate
Not given.
f f
nd Below detection limit.
e
Average of two values.
2228
Anal. Chem. 1981, 53, 2228-2231
resulting metal chelate complexes.
ACKNOWLEDGMENT The authors gratefully acknowledge William Artis and his staff, School of Dermatology, Emory University, Atlanta, GA, for their collaboration in studying Fe in human skin and the Center for Disease Control, Nutritional Biochemistry Division, Atlanta, GA, for supplying the serum samples.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9)
Boumans, P. W. J. M. Opt. Pura Apl. 1978, 1 1 , 143. Barnes, R. M. Anal. Chem. Fundam. Rev. 1976, 48, 106R. Fassel, V. A.; Kniseley, R. N. Anal. Chem. 1974, 46, I I I O A , 1155A. Dahlquist, R. L.; Knoll, J. W. Appl. Spectrosc. 1978, 32, 1. Barnes, R. M., Ed. "Applications of Inductlveiy Coupled Plasmas to Emission Spectroscopy"; Franklln Instltute Press: Philidelphia, PA, 1978. Abe;crombie. F. N.; Silvester, M. D.; Cruz, R. B. Adv. Chem. Ser. 1979, No. 772, 10. Fassel, V, A.; Peterson, C. A,; Abercromble, F. N.; Knlseley, R. N. Anal. Chem. 1976. 48, 516. Merryfield, R. N.; Loyd, R. C. Anal. Chem. 1979, 51, 1965. Kniseley, R. N.; Fassei, V. A,; Butler, C. c., Clin. Chem. ( Winston-Sa/em, N.C.) 1973, 19, 807.
Motooka, J. M.; Mosier, E. L.; Sutley, S.J.; Viets, J. G. Appl. Spectrosc. 1979, 33, 456. Cresser, M. S. "Solvent Extraction in Flame Spectroscopic Analysis"; Butterworths Press: London, 1978. Moshier, R. W.; Sievers, R. E. "Gas Chromatography of Metal Chelates"; Pergamon Press: New York, 1965. Gulchon, (3.; Pommier, C. "Gas Chromatography in Inorganics and Organometallics"; Ann Arbor Science Publisher: Ann Arbor, MI, 1973. Rodriquez-Vazquez, J. A. Anal. Chim. Acta 1974, 73, 1. Burgett, C. A. Sep. Purif. Methods 1976, 5 , 1. Black, M. S.; Browner, R. F. Anal. Chem. 1981, 53, 249. Hansen, L. C.; Scribner, W. G.; Gilbert, T. W.; Sievers. R. E. Anal. Chem. 1971, 43, 349. Wolf, W. R.; Taylor, M. L.; Hughes, B. M.; Tiernan, T. 0.; Sievers, R. E . Anal 14'12 44 616 . Chem _ Black, M. S.; Sievirs, E.Anal. Chem. 1976, 48, 1872. Wolf, W. R. J. Chromatogr. 1977, 134, 159. Fay, R. C.; Piper, T. S. J. Am. Chem. SOC. 1963, 85, 500. Carter, R. J. "Summary Report: Trace Metals Survey I"; U.S Department of Health, Education and Welfare. Center for Disease Control: Manta, GA, 1977.
d.
.
RECENEDfor review July 7, 1981. Accepted August 24,1981. M.B.T. acknowledges his support by the National Science Foundation Undergraduate Reasearch Program, 1980. This work was supported by the National Science Foundation under Grant No. CHE-8019947.
Wavelength Modulation in Photoacoustic Spectroscopy S. L. Castleden,' G.
F. Kirkbright," and D. E. M. Spillane
Department of Instrumentation and Analytical Science, The University of Manchester Institute of Science and Technology, P.0. Box 88, Manchester M60 100, United Kingdom
A simple modification of a photoacoustic spectrometer to allow for the direct generation of first- and second-order differential spectra is described. Uncorrected spectra are presented for a number of samples; the apparent enhancement in the resolution of the system and the ability to locate with increased preclsion the position of absorptlon band edges and maxima and minima are demonstrated.
In recent years photoacoustic spectroscopy (PAS) has become established as a useful technique for the qualitative (1, 2) and quantitative (3, 4) examination of condensed phase samples. At present the conventional photoacoustic spectrometer employing a continuum source cannot be regarded as a high-resolution instrument owing to the requirements for a high-energy throughput which necessitates the use of a relatively large monochromator band-pass. Although it is clear that taking the differential of a spectrum does not result in an increase in the actual informing power of the system, the observation of an apparent enhancement in spectral resolution will be expected as a result of the change in the presentation of the information obtained. A number of experimental methods exist in spectroscopic practice whereby differentiation of signal amplitude with respect to wavelength may be obtained ( 5 , 6 ) . These may be classified into three broad categories, namely, digital storage of the zero-order spectrum followed by computation of the nth order derivative, analogue differentiation of the spectrum, and techniques which employ some form of wavelength modulation to produce differential spectra directly. Digital signal processing is ultimately the most satisfactory and flexible method for the generation of a differential signal; Present address: D e p a r t m e n t of Chemistry, I m p e r i a l College, L o n d o n SW7. U.K.
it is relatively straightforward to obtain higher order differentials with provision for the careful control of smoothing functions. However, digital signal processing normally requires a considerable investment in hardward and software as well as a relatively long development time. Analogue differentiation, although simple and inexpensive, can result in differentiation of artifacts caused by temporal variations in source intensity and in the degradation of signal-to-noise ratios caused by the differentiation of detector noise. The latter problem is particularly relevant to photoacoustic spectroscopy in which the system is detector-noise limited. The production of differential spectra using wavelength modulation may be achieved by performing a relatively minor modification to a conventional photoacoustic spectrometer without the degradation in signal-to-noise ratio experienced with analogue differentiation systems. Conventional PAS relies on the periodic interruption of the incident radiation for the production of a signal. This may be achieved by using electronic modulation of the source or by mechanical chopping using a rotating sector. However, wavelength modulation will provide an alternative method of producing a periodic signal. If the absorption coefficient of the sample is not constant with wavelength, the periodic variation of the wavelength of the incident radiation will result in the production of a periodic photoacoustic signal. Thus large changes in absorption coefficient will result in large photoacoustic signals and small changes lead to small amplitude variations. It is evident that obtaining the P.A. signal by wavelength modulation is directly analogous to taking the first-order differential of the conventional photoacoustic spectrum. Therefore, wavelength modulation is expected to provide a simple means for the generation of differential photoacoustic spectra while simultaneously removing the requirement for a periodic interruption of the incident radiation.
0003-2700/81/0353-2228$01.25/00 1981 American Chemical Society
