Anal. Chem. 1995, 67, 2216-2223
Generation of Cadmium Atoms at Room Temperature Using Vesicles and Its Application to Cadmium Determination by Cold Vapor Atomic Spectrometry A. Sans-Medel,**tM. C. Vald6s-Hevia y Tempnmno,t N. Bordel Garcia,* and M. R. Fem%mlezde la Campat Departments of Physical and Analytical Chemistry and Physics, Univer.s@ of Oviedo, c/ Julian Claverla No. 8, 33006 Oviedo, Spain
The only metal recognized as able to form a monoatomic vapor at room temperature is mercury (“cold vapor“ generation). This property has been widely used in many analyticaltechniques, particularlyin atomic spectroscopy, to increase the efficiency of the transport of the metal to the measurement cell. In fact, cold vapor atomic absorption spectrometry (CV-AAS)has become the most common method for the determinationof trace and ultratrace levels of mercury in the most varied samples. Evidence is shown here demonstratingthat cadmium appears also to be able to form cold atomic vapor under appropriate reaction conditions. Experimental results showthat some types of volatile cadmium species (most probably hydrides) can be produced by reduction with sodium borohydride of Cd2+in an aqueous solution of vesicles of didodecyldimethylammonium bromide. Such volatile species are able to transport the metal to the atomic absorption measurement cell, where CdO cold vapor has been detected. The absorption profile of the absorbing species at room temperature has been measured in a typical absorption quartz cell by using a spectrograph/ charged coupled device arrangement, Results demonstrate the presence of monoatomic cadmium vapor in measurable concentrations in the absorption cell, the msyplitude of the AAS signal behg proportional to the metal concentration in the aqueous solution. This vesicular volatile species generation facilitates cadmium transport to the atomizer cell, thereby improvingby about 20 times the detectability of cadmium by the cold vapor inductivelycoupled plasma atomic emission spectrometry technique (detection limit, 1 ng/mL Cd). Moreover, a highly sensitive determination of the metal based on these facts can be carried out by CV-AAS(detection limit,0.08 ng/mL), and sensitivity can be further improved by working at lower temperatures for volatile species generation and transport to the absorption cell. It is recognized worldwide that mercury is the only metal proven to be able to form at room temperature a monoatomic vapor with a measurable partial pressure. This unique property has been known since the beginning of this century, when Wood’ +
4
Department of Physical and Analytical Chemistry. Department of Physics.
2216 Analytical Chemistry, Vol. 67, No. 73, July 7 , 7995
and others2measured the absorption of the intercombination line of mercury at 253.65 nm by Hg vapor produced at temperatures around 20 “C. The volatility of this HgO “cold vapor“ (CV) was utilized in 19683 to develop an extremely sensitive cold vapor atomic absorption spectrometry (CV-AAS) technique for the determination of low levels of this metal in rocks and other pure metals. Since then, CV-AAS4 has become the most commonly used method for determining ultratrace levels of mercury. No metal other than mercury seems to be capable of evaporating a concentration of its atoms which can be measured by atomic techniques at room temperature. However, generation of other volatile molecular species (e.g., hydrides) and final atomization (e.g., in a heated quartz tube) is a well-known alternative approach in atomic spectrometry to increase the efficiency of transport of the analyte or determinant to the measurement cell. Moreover, it has been shown5that such generation of volatile species for atomic spectrometry can be facilitated by resorting to “organized media”. In fact, the analytical potential of using micelles and vesicles to improve volatile species chemical generation, e.g., mercury CV or hydrides, has been demonstrated in our laboratory for Hg, As, Pb, and Cd.5-I0 Analytical sensitivity and selectivity achieved with hydride generatiodatomic detection systems could be improved by the use of surfactants for the following reasons. (a) They may concentrate reactants at a molecular level and so change thermodynamic and kinetic reaction constants; thus, the analytical sensitivity can be substantially changed, and perhaps improved, in an appropriate medium. (b) Surfactants may solubilize, in a selective manner, analytes and reactants (e.g., sodium borohydride) in organized “aggregates”; thus, the special microenvironment existing in or on these aggregates may change the reactions (interferences) observed (1) Wood, R. W. Phil. Mag. 1912,23, 696-701. (2) Hughes, A L. L;Thomas, A R Phys. Rev. 1927,30,466-472. (3) Ronald, W.; Ott.W. L. Anal. Chem. 1968,40,2085-2087. (4) Fujita, M.; Takanabe, E. Anal. Chem. 1983,55, 454-457. (5) Sanz-Medel, A; FernAndez de la Campa, M. R; Valdes-Hevia y Temprano, M. C.; Aizpun Femandez, B.; Liu, Y. M. Talunta, 1993,40, 1759-1768. (6) ValdesHevia y Temprano, M. C.; Femindez de la Campa,M. R; Sanz-Medel, A J. Anal. At. Spectrom. 1993,8,847-852. (7) Sanz-Medel, A; ValdesHevia y Temprano, M. C.; Bordel Garcia, N.; FemPndez de la Campa, M. R Anal. PYOC.1995.32,49-52. (8) Aizpun Fernandez, B.; Valdes-Hevia y Temprano, M. C.; Fernhdez de la Campa, M. R; Sanz-Medel, A; Neil P. Tulunta 1992,39,1517-1523. (9) ValdesHevia y Temprano, M. C.; Aizpun FemAndez, B.; Fernandez de la Campa, M. R; Sam-Medel, A Anal. Chim. Acta 1993,283,175-182. (10) Liu, Y. M.; Fernandez Sanchez, M. L.; Blanco GonzAlez, E.; SanpMedel, A /. Anal. At. Spectrom. 1993,8, 815-820.
0003-2700/95/0367-2216$9.00/0 0 1995 American Chemical Society
in the bulk aqueous phase. In other words, analytical selectivity could be favorably changed in suitable organized media. In a previous paper? we investigated in detail the continuousflow generation of volatile Cd species, using NaBH4 as the reduction agent, as a means of gaseous sample introduction for inductively coupled plasma atomic emission spectrometry (ICPAES). In order to achieve such generation, several organized molecular assemblies have been tried, including micelles of different charge and vesicles.6 The most promising analytical results were obtained with didodecyldimethylammoniumbromide @DAB) vesicles as the organized medium for Cd2+reduction. In a recent publication, we reported on the formation of cadmium atoms at room temperature by resorting to the use of cationic vesicles of DDAB showing their potential to achieve a sensitive CV-AAS determination of the metals7 In this paper, both the transport mechanism of the volatile Cd species to the atomizer and the nature of those species reaching the measurement cell are investigated by absorption measurements. Experimental evidence is given indicating that room temperature atomic absorption of the resonance radiation of cadmium (at 228.8 nm) by the volatile species formed takes place in the quartz absorption cell. In other words, Cd cold vapor generation has been demonstrated via reduction of Cd2+with NaBH4 in vesicular media. Factors affecting CdO generation, possible transport mechanisms, and its use to develop highly sensitive CV-AAS techniques for the determination of this toxic metal are discussed in detail. EXPERIMENTAL SECTION
Instrumentation and Analytical Procedures. A Philips Model PU7000 inductively coupled plasma atomic emission spectrometer was used for detection by ICP-AES. A Perkin-Elmer Model 2280 atomic absorption spectrometer was used for detection by AAS. A Perkin-Elmer Model 3030 graphite furnace atomic absorption spectrometer was used for measurements by ETA-AAS. Butch measurements of volatile species formed were canied out using a homemade F i e la) thermostated hydride generator to generate the volatile species. This device is based on a threenecked flask, where the cadmium sample is dissolved in HCl of appropriate concentration to secure a final concentration of 0.4 M in the acid for reaction and 1 x mol/L DDAB vesicles; one entrance is for connecting to the quartz cell of the atomic absorption instrument, the second entrance is to pass a low argon flow to purge the gases to the cell from the flask, and the last entrance, closed by a septum, allows addition of tetrahydroborate solutions by syringe. Continuous Bow getteration measurements of volatile species were carried out using the system shown schematically in Figure lb.6 The cadmium sample, dissolved in 2 M HC1 and 1 x mol/L DDAB vesicles, is continuously pumped through one of the channels of a peristaltic pump at a rate of 0.75 mL/min to merge with a 4% m/v solution of NaBK (flow rate, 0.75 mL/min) in 1 x 10+ mol/L DDAB vesicles. The merging solution feeds the gas-liquid separator to allow the volatile species to go to the quartz cell or the ICP. The liquid phase is continuously removed to waste (Figure lb). The Perkin-Elmer Model 2280 atomic absorption spectrometer, used for absorption measurements, was operated under optimum conditions suitable for the Cd 228.8 nm line (Cd hollow cathode
Ar
a)
1tetnbydro
I ~ ~ l a l l cadmium lc
bowte
-
1
n
7cm
quartz tuba
Cd+EkI+DDAB
n ICP-AES
Perirtaldc pump Ar nebulizer
Figure 1. (a) Batch CV system. (b) Continuous CV system.
lamp), and all these measurements were background corrected using a deuterium lamp and 228.8 nm setting. Instrumental Setup for Absorption Profile Measurements. The absorption spectrum between 200 and 235 nm of the Cd absorbing species in the quartz cell was measured at room temperature by illumination with a continuum source (deuterium lamp) and detection with a charged coupled device (CCD; EG&G Princeton Applied Research, Princeton, NJ). Spectral dispersion (Figure 2) was secured by using a spectrometer of 0.5 m focal length with an entrance slit of 25 pm and two different grating capabilities: a ruled grating of 600 grooves/mm (0.2 nm fwhm resolution) and a holographic grating of 2400 grooves/" (0.03 nm fwhm resolution). The complete diagram of the instrumental setup employed to obtain such spectra is shown in Figure 2. The same instrumentation was used to measure the absorption profiles of cadmium and mercury cold vapor for comparative purposes. Reagents. Stock standard solutions of loo0 mg/mL cadmium0 and mercury0 (Merck) were used. Working solutions were freshly prepared daily by diluting appropriate aliquots of the stock solutions in ultrapure water. Sodium tetrahydroborate(I1I) solutions were prepared by dissolving NaBh (Carlo Erba) in ultrapure water (Mii-Q, Millipore Corp., Bedford, MA), stabilized in 0.1%m/v sodium hydroxide solution. Solutions were prepared weekly and filtered before use. Vesicles of didodecyldimethylammonium bromide @DAB) were prepared by dissolving the surfactant powder (Eastman Kodak Co., Rochester, NY) in water and sonicating at room temperature at a power of 60 W for about 12 min, using the tip of a high-intensity ultrasonic processor (Sonics and Materials Inc., Danbury, CT) . This procedure will be referred to as "sonication" in the text. Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
2217
PERISTALTIC PUMP
GAS-LIQUID SEPARATOR
\
-! TH 7cm .Oxin)
11-
TO W A ~ T E
~
SPECTROGRAPH CCDDETECTOR QUARTZ TUBE 15cm (i.d. 1.2cm)
PRINTER
-
COMPUTER
INTERFACE
Flgure 2. Instrumental setup for absorption profile measurements.
All mineral acids used were of analytical reagent grade, and ultrapure water was used throughout.
I
I
I
8
- 7
RESULTS AND DISCUSSION
- 6
A few years ago, the formation of a volatile species of cadmium by treating Cd2+solutions with N&& in an organic medium of dimethylformamide was described." Unfortunately, the nature of such species was not investigated in detail. Experimentsin our laboratory, using several organized media of different nature and NaB& for reduction, verified the formation of a volatile Cd species. This Cd volatile species was responsible for the ICP-AES signals observed.5~6 However, no conclusive evidence of the nature of the volatile Cd species could then be provided nor has been provided so far, to the best of our knowledge. Preliminary efforts in our laboratory to characterize by GC/ MS the Cd compound reaching the ICP-AES were unsuccessful.6 Different experiments (e.g., leaching of tubing with diluted HCl) showed that Cd was deposited in the tubes used to "trap" the volatile species formed, preventing eventual MS detection. The Mect of Connecting Tubing Length. The above preliminary experiments, unsuccessful at duly characterizingthe Cd species formed, pointed to a prompt decomposition of the volatile compound (presumably a hydride) originally formed. If decomposition takes place, it has been reported in the inorganic literature that cadmium hydride decomposes quickly to CdO and hydrogen at temperatures around 0 OC;lZthe longer the connecting tubing to the atomizer, the longer the time available for volatile species to decompose at the 18-20 "C laboratory temperature. According to this supposition, Cdo coming from
- 5
(CdHz)"nshble e Cdo + Hz
(1)
should deposit on the tubing, and ICP-AES signals for the metal should decrease with longer connecting tubings. (11) Cacho, J.; Belt&, I.; Nen'n C. j . Anal. At. Spectrom. 1989,4, 661-663. (12) Barbaras, G. D.; Dillard, C.; Finholt, A E.; Wartik, T.; Willbach, IC E.; Schlesinger, H. I. J. Am. Chem. SOC.1951,73,4585-4590.
2218 Analytical Chemisfty, Vol. 67,No. 73,July 7, 7995
1
1
I
I
I
10
20
30
40
50
w
6
- 4
g
- 3
2t(
- 2
2
- 1
0' 0
co
E7 a
E'
'0 60
Reaction coil length (cm) Figure 3. Effect of reaction coil length on the Cd signal measured by volatile species generation (from DDAB) - ICP-AES. (a) Total intensity of the ICP-AES signal. (b) SignaVbackground ratio. (c) Intensity of the background.
The observed results for such an experiment are given in Figure 3, which shows that the observed ICP-AESsignal does, in fact, decrease dramatically with increasing tubing length connecting the flask to the nebulizer/spray chamber F i e lb) (deposition of the metal on the tubing was again proved). C a d " Cold Vapor Formation. In a further effort to characterize the species responsible for the cadmium signals observed, we investigated the possibility of detecting CdO and CdHz in reaction 1. For analytical measurements of CdO, a batch vapor generator was designed (Figure la), and the cadmium volatile species generated were transported to a classical quartz cell of the conventional AAS instrument. Working at room temperature for generation, calibration AAS graphs (using increasing concentrations of Cd2+in the sample vessel) were obtained independently with a heating flame and flameless in the quartz tube. Parallel experiments were carried out using the same instrumentation for the determination of Hg2+ (CV-AAS typical measurements). The results obtained have been plotted, in a comparative manner, in Figure 4,parts a (Cd) and b (Hg), and are crucial to
bl
0.25
A
-
Table I.Analytlcai Performance Characteristicsfor Cadmium by Cold Vapor Generation AAS
batch
I
/
flame DL5 (ng/mL) RSDb (%) .. SlOpeC
p" 0.15 U Om2I
Asloped a
0.22 2.0 14 x 10-3 1
continuous
flameless 0.08 1.8
39 x 10-3 2.8
flame
flameless
2.9 2.1
1
1.5 10-3
4
1
2.6
1.8
10-3
Detection limit, 3~/slope. Precision (RSD, %), relative standard
deviation for v p l e s . a t 5 ng/mL of Cd in batch mode and 10 ng/mL
of cadmium in contmuous mode (n = 10) levels. CSlope of the calibration line (mL/ng-l). Slope increase (flameless/flame) . v-
0
1
2
3
4
5
6
7
8
Conc. Cd (ng/ml)
0.12
0
1
A
I 30 40 60 70 80 0
10
20
50
Conc. Hg (ng/ml) Figure 4. Calibration graph by batch CV-AAS: (a) cadmium and (b) mercury 0 , flameless; X , with flame.
understanding the transport mechanism of volatile Cd to the absorption cell by comparison with Hg2+results. Figure 4a shows clearly the presence at room temperature in the absorption cell of Cd atoms able to absorb their own resonance radiation (228.8 nm) coming from the Cd hollow cathode lamp. The sensitivity of the Cd calibration graph is much better without the flame than with the flame (heating the quartz tube). Figure 4b shows similar results for the parallel studies carried out on cold vapor generation of mercury. The effect of heating the quartz tube with the flame is analogous for both analytes: when the b e is on, the gases are heated in the tube. On heating, and with burning the H2 excess, the gases expand, diluting the analyte and shortening its residence time in the beam path of the AAS instrument. Therefore, the slope of the calibration graphs decreases with the flame as compared to room temperature operation (cold vapor absorption). S i a r experiments were performed using on-line continuousflow generation AAS detection. The results obtained showed exactly the same type of behavior illustrated in Figures 4 (for both, Cd and Hg), except that continuous operation produced a clear loss of sensitivity. In Table 1, we have summarized the obtained analytical performance characteristics for optimized7cold vapor determination of Cd by quartz tube AAS, using both batch (Figure la) and continuous (Figure lb) operation, both with a flame and flameless (in the latter case, a membrane drying tube from PS Analytical, Seven Oaks, OK, was used at the exit of the gas-liquid separator to prevent background from moisture and water droplets reaching
the absorption cell at room temperature.13 It can be seen in Table 1 that extremely low levels of Cd can be analyzed, using either batch or continuous operation, by pure cold vapor quartz tube AAS measurements with good precision (2-3%). The technique can be used for the real analysis of Cd in tea infusions and lichens.14 In brief, those results demonstrate that the CV-AAS determination of cadmium is possible because measurable amounts of monoatomic CdO vapor exist in the quartz absorption cell during measurement (in a similar manner to the well-known CV-AAS of mercury). In the light of the above conclusions, a further question arises: Is the observed cold vapor generation of cadmium identical in nature with that of mercury? Intluence of Temperature on the Generation Reaction. In an attempt to answer the above question, the influences of temperature on the generation and transport to the absorption cell of cold vapors of Cd and Hg were investigated separately. The sample vessel (Figure la) was thermostated at different temperatures ranging from 0 "C to 60 "C, and the room temperature AAS signal for 100 ng/mL Cd was monitored at increasing thermostat temperatures in the flask. Similar experiments were carried out for 100 ng/mL Hg. The results observed have been plotted in Figure 5 for both metals, for comparative purposes. As can be seen, the effect of temperature on the cold vapor generation of HgO is nearly opposite to what is observed for the cold vapor generation of C d for mercury, the higher the temperature of generation in solution, the higher the vapor pressure of HgO directly formed and transported to the cell as such species (signal increases with temperature). However, the observed shape of the curve shown in Figure 5 for Cd exhibits the opposite behavior. In other words, the transport for Cd is probably mediated by the volatile unstable species (i.e., probably CdH2, which decomposes12with increasing temperatures). The Vesicular EiTect. The presence of an organized medium proved to be essential for efficient formation of the observed Cd volatile species in the reaction vessel. In order to ascertain the actual role of vesicles in such Cd cold vapor generation, three series of Cd ultdltration experiments were carried out using ICPAES detection: in the kst series, a Cd2+solution was added to DDAB vesicles already formed by sonication; in the second one, Cd2+and NaB& solutions were dissolved independently in DDAB (13)Aizpun FernAndez. B.; Femicndez de la Campa, M. R; SanaMedel, k J. Anal. At. Spectrom. 1993,8,1097-1102. (14) Valdbs-Hevia y Temprano, M. C. Ph.D. ?he&, Universidad de Oviedo, Spain, 1994.
Analytical Chemistty, Vol. 67, No. 13, July 1, 1995
2219
I
I
Table 2. Efficiency of the Cadium Species QenerationlVolatilization
sample 1 2 3 4
5 6 ZfSD
Cd found by ETA-AAS (ng/mL)
Room Temperature 5.4 4.5 4.8 4.9 5.2 5.3 5.0 f 0.3
efficiency“, (%)
46 55 52 51 48 47 49.8 iz 3.4
T=O”C U
0
40 50 60 Temperature ( “ C ) Figure 5. Temperature effect on (a) Hg signal by batch CV-AAS with the Perkin-Elmer instrument and (b) Cd signal by batch CV-AAS. 10
20
30
solutions and were sonicated at the same time as the surfactant solution (in order to “trap” the reagents inside the vesicles on their formation by sonication); finally, a third reference solution of pure aqueous Cd2+ (of the same concentration) was also prepared. For all three solutions, one fraction was directly analyzed for Cd by CV-ICP-AES,G and the other fraction was ~ltrafiltered’~ and the Cd2+content in the ultraflltrate determined by ETA-AAS. The results obtained for the three types of solutions containing 30 ng/mL Cd2+ are summarized as follows: when sonication of DDAI3 solutions is carried out in the presence of Cd2+,-50% of total cadmium added remains in the vesicles (which do not pass through the ultraliltration membrane), as the ultrafiltered Cd was 46.7%. This sonication procedure produced the greatest CV-ICP-AESemission signals for Cd. Conversely, when Cd2+solution was added to already formed (sonicated) vesicles, most of the added Cd2+seemed to remain outside the vesicles in the bulk aqueous solution (93%of total Cd2+added was ultrafiltrable). The observed CV-ICP-AESemission signals for this type of solution were 2.5 times lower. Pure aqueous solutions of Cd2+ treated with NaBH4 in the conventional mode without vesicles, thus, with 100%of Cd ultraliltrable, did not produce any CV-ICPAES signal for Cd. Obviously, a quantitative description of the above data is rather difficult due to the rather uncontrolled volatile compound decomposition. However, the above results support the concept that the concentration of reactants at a molecular level in the vesicles is responsible for the production of a reaction; in this case, presumably the generation a cadmium hydride compound, which does not appear to take place in the bulk aqueous solution. The efficiency of generation of volatile cadmium at different temperatures was also calculated (measuring the residual Cd by ETA-AAS in the vesicular solutions after borohydride addition for ICP-AES analysis and relating those values to total metal added). The results obtained Vable 2) showed that the efficiency of analyte volatilization was 49.8%f 3.4 (n = 6) at room temperature and 75.0%It 3.7 (n = 6) at 0 “C. Therefore, this confirmed the results shown in Figure 5: lower generation temperatures favor the stabilization of volatile compound responsible for the transport of the metal. Thus, by lowering the temperature of the generation and transportation of the volatile hydride, the already high (15)Wrobel, K; Blanco GonzPlez, E.; Sanz-Medel,A. Trends Elem. Med. 1993, 10.97-103.
2220 Analytical Chemistry, Vol. 67,No. 13, July 1, 1995
1 2 4 4 5 6 4fSD
2.8 2.8 2.5 2.0 2.8 2.1 2.5 f 0.4
72 72 75 80 72 79 75.0 f 3.7
a Efficiency, [Cd added - (10 x Cd found)/Cd added] x 100; Cd added = 100 ng/mL Residual solutions were diluted 10 times for analysis by ETA-AAS.
sensitivity of the CV-AAS or CV-ICP determination of Cd could be increased even further to that shown in Table 1 at room temperature &e., detection limits better than 80 pptr Cd can be achieved). Evidence of Room Temperature cadmium Atoms. Ab sorption Profile of the Absorbing Species. All the AAS measurements given were background corrected. However, even with background correction, the presence of a net absorbance does not disprove that a molecule is absorbing the light from the hollow cathode lamp; it just may show that, as deuterium correction is being used at 228 nm, the profile of the molecular absorption band is narrow compared with the band pass of the monochromator. Therefore, in order to ascertain that the absorber is monoatomic cold vapor, we obtained the absorption profile of the absorbing species at the laboratory temperature using a continuum source (a deuterium lamp) for illumination. The light (see Figure 2) passes through the quartz tube where the absorber is present, and the emerging light is dispersed in a spectrograph of 0.5 m focal length and detected with a CCD detector, which allows us to see simultaneously the profiles of the absorption in a 10 nm range in real time when the spectrograph is used with the 2400 grooves/” grating. Parallel experimentswere carried out, using the same instrumentation, to measure the absorption profile of cold vapor of mercury. Comparative results observed are illustrated in Figure 6. As shown in Figure 6, both Cd and Hg show a similar absorption profile: a narrow absorption at 228.80 nm for cadmium (with a half-width of 0.03 nm, Figure 6a) and a similar line for mercury at 253.65 nm (with a half-width of 0.03 nm, Figure 6b). The absorption profiles obtained demonstrate that the Cd absorber is also a monoatomic cadmium vapor. These measurements show also that on increasing the concentration of the sought elements in the sample solution, the intensity of the atomic absorption observed increases accordingly (allowing for analytical AAS determinations, as indirectly shown also by results in Figure 4 and Table 1). Using the instrumentation shown in Figure 2, we observed a linear relationship between atomic absorbance measured at 228.8 nm and Cd2+concentration in the sample.
b)
2
,
223
,
,
,
ne 238 wwu" (W
2x
23:
81
I
8'
x8
231
i5+
356
259
Wavekngth (nm)
Figure 6. Absorption profile of (a) monoatomic cadmium cold vapor with (1) 0, (2) 2, (3) 5, and (4) 10 ppm cadmium, and (b) monoatomic mercury cold vapor with (1) 0, (2)5, and (3) 10 ppm mercury.
To further c o b this approach, we tried to analyze cadmium with the conventional AAS instrument, using a nearby line of another element for illumination. By using a hollow cathode lamp of nickel (resonanceline at 232.0 nm), we measured possible AAS signals for increasing Cd2+concentrationin the sample solutions. Results showed that the absorbance measured at 228.8 nm was a straight line (A = 0.012 0.00396 X,I = 0,9991,X = cadmium concentration) with Cd2+ concentrations from 0 to 100 ng/mL, while measuring at 232.0 nm provided a constant absorbance (A = 0.01), even for a 500 ng/mL concentration of the element in the sample. This confirms once more that the absorption profile of the absorber species is very narrow and that at 3.2 nm outside the Cd resonance line, neither absorption is observed, nor is this absorption related to Cd2+concentration in the samples. Possible Molecular Absorptions. Absorption results o b served with the CCD detector showed that for Cd, there is also a certain band absorption (in these CCD measurements, no background correction was used); such absorption is clearly visible on comparing blanks and samples in Figure 6a, and it is not observable in Hg" measurements (Figure 6b). For instance, for Hg, in the region around 253.65 nm, e.g., 252 nm, the emission of the continuum Dz lamp was always 77 x 103 AU (emission arbitrary units) after the HgO vapor was passed or even for the blank (without Hgz+in the sample solution). However, for Cd in the region around 228.8 nm, e.g., 226 nm, the source emission with the blank (no Cd2+)was 225 x 103 AU if 5 ppm CdZ+is volatilized and measured Figure 6a3), an absorption band is noticed (emission zz 190 x 103 AU), and on volatilising 10 ppm Cd2+,a further decrease of signal at 226 nm is observed.
+
In order to verify the assumption (according to eq 1)that the absorbing molecular species is a hydride of cadmium, absorption bands of such Cd volatile species were obtained in our CCD instrumental setup and were compared with the absorption bands obtained for arsine and selenium hydride (it is well known that those hydrides exhibit absorption bands between 200 and 240 nm.16-19 The experimental system shown in Figure 2 was employed, but the grating of the spectrograph used for this experiment was of 600 grooves/", so that the CCD detector allowed simultaneous observation of the profiles of the absorption in a 50 nm range in real time. The hydrides were generated using working 1ppm standards of arsenic and selenium in 2 M HCl, pumping them continuously through one of the channels of the peristaltic pump to merge with 1%m/v N a B h (stabilized in 0.5%of NaOH) introduced through other channel. The hydrides formed should reach the quartz cell (Figure 2) after passing through the gas-liquid separator. The light of the D2 lamp should be absorbed by the hydrides along the quartz cell. The band spectra obtained for the different elements assayed are shown in Figure 7. The "net" absorption bands (numbers 3 in Figure 7) are the result of subtracting the spectrum obtained for samples (1ppm) (numbers 2) from the spectrum obtained for the corresponding blank (numbers 1). In the three cases studied, the absorption bands have similar shapes; the maximum of the band for the selenium is shift toward higher wavelengths in relation to the corresponding maximum of a r ~ i n e . ' ~The J ~ observed intensity of the absorption band of the molecule of Cd is lower than those of the others, which can be explained by the unstability of this molecule: only a small part of the supposed hydride of Cd produced should be available for absorption in the quartz cell, owing to the decomposition of the hydride in accordance with reaction 1, probably enhanced by ultraviolet light photodecomposition in the cell. In Figure 7c, the atomic absorption of the Cdo (228.8 nm) does not appear because of the poor resolution employed. In order to verify that there was CdO in the quartz cell, for the same generation experiment we used the 2400 grooves/" grating in the spectrometer. The spectra obtained under these higher dispersion conditions are shown in the Figure 7d. As can be seen, both the absorption line of CdO and the molecular absorption,probably attributable to a Cd hydride (as is the case for As and Se), appeared on comparing the blank Figure 7dl) and the Cd sample F i r e 7d2) spectra. Other volatile molecules which could be present in our experimental conditions at measurable concentrationsinclude Hz, CdH, Cdz, and CdCl. According to available information on molecular spectra,2O these others should have their absorption bands at wavelengths far away from the 228.8 nm regions, with the exception of Cdz molecule.2O It seems that the latter molecule exhibits a broad band with a maximum at 228.8 nm, and so it could also slightly contribute to the molecular absorption observed in that spectral region (Figure 7d). (16) Sanz,J.; Gallarta, F.; Galbhn, J. Anal Chim.Acta 1991,255, 113-120. (17) Sanz, J.; Gallarta, F.; Galbhn, J.; Castillo, J. R Analyst 1988,113, 13871391. (18) Sanz, J.; Ortega, L. A; Galbhn,J.; Castillo, J. R Microchem. 1. 1990,41, 29-40. (19) Sanz,1.;Gallarta, F.;Galbh, J.; Castillo,J. R FrseniusZ. Anal. Chem. 1988, 330, 510-515. (20) Pearce, R W. B.; Gaydon, A. G. nte Identification of the Molecular Spectra, 4th ed.; Chapman and Hall: London, 1976 p 132.
Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
2221
220
210
235
210
WAVE1,ENCTH (mn)
220 (nm)
235
WAVELWOTH
25
20 .-
-2
" 15-
??
2 5
200
210
220 W A V F L W T H 0"
235
224
I------
228
226WAVELPIOTH
230
(nm)
Figure 7. Absorption spectra for several hydrides: (a)As, (b) Se, and (c) Cd. (1) Blank, (2) 1 ppm sample, (3) net absorption band. Resolution of the experimental instrumentation is 0.2 nm. (d) Spectra obtained with Cd. Resolution of the experimental instrumentation is 0.03 nm.
FINAL DISCUSSION AND CONCLUSIONS Cold vapor generation of monoatomic CdO can be accomplished by reduction of Cdz+ solutions with NaBH4 from an organized medium. However, the volatile species generated and transported to the measurement cell is probably not CdO. Most likely, the mechanism for generation of the Cd volatile compound is described by the following reactions: BH,-
+ 3H20 + H+- H3B0, + 8H*
(2)
-
(3)
m*+ Cd2+,,)
(CdH2),01,ti,)
+ 2Hz(excess)
+ 2 H+
as for other better known volatile hydrides (H* being "nascent" hydrogen). As (CdHz) is volatile, this should be the transportation mechanism for cadmium at the temperature of the laboratory. However, at room temperature, during transport, the formed volatile hydride would decompose according to reaction 1, and the formed CdO, not volatile, would deposit all along the connecting tubing. This decomposition is more efficient the higher the temperature (see Figure 5). In any case, the great excess of HZ formed in the decomposition of BHd- in the acid medium of reaction will prevent the complete decomposition of the volatile hydride (see equilibrium 1). Thus, at least a certain amount of volatile cadmium willSnally reach the absorption cell. Once there, according to equilibrium 1,a substantial proportion of the volatile 2222 Analytical Chemistry, Vol. 67,No. 13, July 1 , 1995
Cd species transported seems to be able to form instantly monoatomic Cdo (probably favored by photodecomposition brought about by the exciting UV light), as it absorbs resonant radiation of Cd at room temperature. The peculiar ability of CdO atoms to survive in cold environments (at comparatively long distances, of the order of centimeters, from a rod glowing at 2500 "C, where those atoms were produced) has already been described.2l Moreover, Crabi et aLZ2used a combination of a graphite furnace and an inductively coupled plasma to study the thermal vaporization of cadmium, and they concluded that the free cadmium atoms had an extremely long lifetime near room temperature. Similarly, Khtor et used a combined quartz furnace and flame system for the specific element detection of atomic cadmium vapor that evolved inside the furnace from the thermal decompositionof Cd(N0h and CdC12. They showed that free cadmium atoms still remain in a significant amount for atomic absorption detection without flame at a considerable distance from the furnace where they were formed. They suggested that about one-ninth of the material reaching the observation zone is atomic cadmium vapor. (21) Dean, J. R ; Snook, R D.;J. Anal. At. Spectrow. 1986,1 , 461-465. (22) Crabi. G.; Cavalli, P.; Acilli, M.; Rossi, G.: Omenetto, N. At. Spectrosc. 1982, 3, 369. (23) KAntor, T.;Benir. L. J. Anal. At. Spectrom. 1986,1 , 9-17.
Concerning our work here, there are two points which deserve consideration. (1) The significantly different vapor pressures of Cd and Hg Torr, while Hg has metals (e.g., at 150 “C, Cd has 1.28 x 2.807 point to different behaviors in their respective cold vapor atomic absorption process, even if a true atomic absorption is demonstratedfor both metals (see F i r e 6). Our results show that their CV generation behavior is totally different: while Hg is transported to the atomizatiodmeasurement cell as the cold vapor of the HgO (very volatile), it seems that in the case of Cd, CdHz is more likely the volatile species responsible for the metal transport. This generation and transport has been possible via the use of an adequate “organized medium”, vesicles, able to favorably manipulate the hydride formation reaction in solution. The chemical differences between vesicles and micelles have been stressed elsewhere.25 (2) Survival of CdO atoms in a cold environment,as proposed in this work, is not exactly the same type of phenomenon as reported in refs 21-24. Those three papers referred to electrothermal formation of Cd atoms, which then survived for a while before cluster formation, condensation, or deposition occurred. Our proposition here is that we have a chemical formation of atoms according to the following decomposition reaction: [CdHZ]unstable
Cdo(monoatomic vapor)
+ HZ
(in a way somewhat similar to the well-knowngeneration of atomic hydrogen, “status nascendi”, by reduction of hydrogen ions, as in equilibrium 2). This decomposition process should obviously (24) Weast, R C.; Astle, M. J. Eds. CRC Handbook of Chemistry and Physics. 60th ed.; CRC Press Inc.: Boca Raton, E, 1980; p D-198. (25) Sanz-Medel, A; Aizpun, B.; Marchante, J. M.; Segovia, E.; Fernhdez, M. L;Blanco, E. j . Chromatogr. A 1994, 683, 233-243.
be governed by kinetics (considering the high tendency of CdO atoms, instantly formed, to react, condense, and deposit eventually on the tube walls) and the possibility of photodecomposition processes induced by the UV excitation light. Notwithstanding the different mechanism of atom formation, we believe that the peculiar ability of CdO atoms to survive a cold environment21-23 still holds here and could be responsible for the appearance and transient presence of a substantial concentration of monoatomic CdO cold vapor. Of course, this cold vapor would not be thermodynamically stable. However, the monoatomic Cdolifetime, its ability to survive, seems to be long enough to be measured. This provides the basis for a most useful AAS measurement of Cd at room temperature. In brief, what we propose is that in this case, the atomizer is a chemical reaction which on its own, or assisted by UV exciting light, provides the energy for atomization at room temperature. If the kinetics of such formation reaction, relative to those of the subsequent reactions of reactive CdO atoms, are favorable, we could be able to measure surprisingly high concentrationsof such transient species. In fact, we have proved here (see Table 1)that for cadmium this measurement is so sensitive and precise that CV-AAS determinationswill no longer be exclusively confined to mercury. ACKNOWLEWMENT Financial support from DGICYT (Spain) through project PB910669 is gratefully acknowledged. Received for review December 19, 1994. Accepted March 20, 1995.e AC941227D @
Abstract published in Advance ACS Abstracts, May 1, 1995.
Analytical Chemistry, Vol. 67,No. 13,July 1, 1995
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