2614
Anal. Chem. 1986, 58, 2614-2617
(16) Pavel, G.; Kliment, J.; Stoerle. S.; Suter, 0. Fresenius' Z . Anal. Chem. 1985, 321, 587-591. (17) Blaslus, J. Lehrbuch der analytbchen und praparativen anorganischen Chemie; Hlrzel Verlag: Stuttgart, 1979; p 282. (18) Ortner. H. M.: Kantouscher. E. Talsnts 1975. 22. 581-556. (19) Mahumdar, S.K.; We, A. K. Anal. Chem. 1980, 32, 1337-1339. (20) Savory, J.; Mushak, P.; Sunderman, F. w.; Estes, R. H.; Roszel, N. 0. Anal. Chern. 1970,4 2 , 294-297.
(21) Beyermann, K. Fresenius' Z . Anal. Chem. 1982, 190, 4-33. (22) Moshier, R.; Sievers, R. Ges Chromatographyof Metal Chelstes; Pergamon Press: Frankfurt/Main, New York, 1965.
RECEIVED for review February 10, 1986. Resubmitted July 2, 1986. Accepted July 11, 1986.
Alternative to Solid Sampling for Trace Metal Determination by Platform Electrothermal Atomic Absorption Spectrometry: Direct Dispensing of Powdered Samples Suspended in Liquid Medium Michel Hoenig* and Paul Van Hoeyweghen Institute for Chemical Research, Ministry of Agriculture, Museumlaan 5, B-1980 Tervuren, Belgium
A possible aiternatlve of solld sampllng Is the dlrect Introductlon of the llquld suspended powdered sample Into the E M S devlce uslng the autosampler. I n thb case, sample dilution Is easy and the use of the platform together with matrlx modlflers Is poe8ible. For cadmium and lead determlnatlonr the dmcrlbed procedure permlts the use of dlrect caHbratlon wlth shnple standard rdutlono: the worklng curve slopes are very clore for all the matrkw studled. Powdered plant US~UO,sodlmmt samples, and Iyophyllredanlmal tissues are suspended In a mixture of glycerlne, mahand, nllrlc acid, and adequate matrix modllkr. After the powdered sample b mlxed, the suspension Is stable for about 1 h. Rellablllty of thls procedure was confirmed by the determlnatlon of cadmlum and lead contents In eight standard reference materlals of the NBS and IAEA.
Compared to the analysis of dissolved samples, solid sampling followed by electrothermal atomic absorption spectrometry (EAAS) offers two advantages. First, it saves time and effort for samples that are difficult to dissolve. Second, the solid sampling may be particularly convenient when only small amounts of sample are available. However, direct analysis of solid samples by EAAS is initially handicapped due to some restrictive factors not present in the analysis of dissolved samples: 1. The greatest problem originates from sample heterogeneity which requires a substantial effort to obtain representative subsamples. 2. Determination of relatively high analyte concentrations is limited by the minimum representative sample weight, obtained with a current analytical balance, that can be introduced into the atomizer. 3. One of the major difficulties associated with solid sampling concerns the availability of appropriate calibration standards. 4. For multielement analysis this technique is particularly time-consuming compared to the work with solutions. 5. The interference effecta observed with the solid sampling are probably greater compared to the dissolved sample whose matrix is simplified as a result of the mineralization. 6. The good contact between the analyte and the graphite surface necessary for reproducible heat transfer from platform
to analyte and also for the possible analyte reduction prior
to the atomization is not ensured as satisfactorily as in the case of solutions. 7. The use of matrix modifiers, often required to achieve the efficiency of platform techniques, is problematic. 8. Sample introduction into the atomizer is less convenient compared to dissolved sample. Finally, for all the reasons mentioned above the precision obtained by solid sampling is generally less than that obtained with solution analysis. In spite of this discouraging situation, many researchers (see the recent review by Vollkopf et al. (1))consider that solid sampling facilitates analysis is some specific cases and may lead to consistent results. Moreover, Perkin-Elmer and Hitachi have elaborated commercially available devices for direct solid sampling using EAAS, which act like the L'vov platform. In 1974, Brady et al. (2,3)proposed an interesting method of solid sampling, the dispensing of water suspended powdered sample into the atomizer, using a micropipet. These authors obtained satisfactory results for lead and zinc determinations in plant samples and marine sediments. In this work, we investigated further this approach and found that it indeed offered advantages as compared to true solid sampling. The recent evolutions of EAAS using platforms, modifiers, autosamplers, and adequate signal processing contribute largely to routine applications of this solid sampling alternative.
EXPERIMENTAL SECTION
All work was performed on a Varian AA-1275 BD spectrometer equipped with a deuterium arc background corrector, GTA-95 graphite furnace, and programmable sample dispenser. Pyrolytically coated tubes and solid pyrolytic graphite platforms (both Le Carbone Lorraine, France) were used. The appropriate amounts of the powdered sample were weighed with an analytical precision 0.01 mg) in 2-mL polyethylene balance (Mettler H-64, microvials for autosampler. After addition of the liquid medium (discussed below and shown in Table 11),the samples were suspended and homogenized with a Mini-Mix stirrer (Vitatron Scientific) prior to the analysis. The suspended samples were dispensed on the preheated platform (130 "C) via the autosampler. Details on the electrothermal programs are given in Table I. A Hewlett-Packard 82905-A printer was used for plotting absorbance-time profiles. Signal processing was in the peak-height mode. Lead and cadmium hollow cathode lamps (Instrumentation Laboratory) were operated at 5 and 2 mA, respectively. All experiments were performed at 283.3 nm for lead and 228.8 nm
0003-2700/86/0358-2814$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
2615
Table I. GTA Parameters for Lead and Cadmium Platform Determinations dispension tube wall temp, "C
on platform
dry
charb
atomize
130
600
2200 (Pb) 1800 (Cd) 0.7 (Pb) 0.4 (Cd)c 2 0
ramp, s
40
800 (Pb) 800 900 (Cd) 900 3
hold, s Ar flow, L/min
3
5 3
2 0
clean
cool
2500 (Pb)
130
2300 (Cd) 0.2 (Pb) 0.3 (Cd)' 1 3
11
3
"The samples were dried in the 40 s heating ramp from 130 to 600 OC. With matrix modifiers. cMaximum heating rate. for cadmium with a spectral width of 1.0 nm in both cases. The complex liquid media were prepared with glycerine (87%, Merck pea.),methanol (Baker p.a.1, HN03 (65%, Suprapur Merck), and appropriate matrix modifiers: (NH4)H2P04(Merck p.a.) and Mg(N03)2 (Merck p.a.). The lead and cadmium standards were diluted from commercial standard solutions (Titrisol, Merck). Preparation of the liquid media and dilutions of the standards and the modifiers were performed with a Hamilton Microlab 1000 diluter or Gilson micropipets.
RESULTS AND DISCUSSION Homogeneity Problems of Suspended Material. With suspended solid samples it is necessary to ensure the stability of the particles in the liquid medium by following the reproducibility of the measurements with time. We have tested three different types of solid samples: ground plant tissue, lyophilized animal tissue, and ground marine sediment sample. The samples were first suspended in demineralized water. In this medium all samples showed rapid sedimentation. The P b EAAS determinations performed on aliquots of these suspensions showed a strong decrease of atomic signal with time. Ten minutes after sample stirring, P b absorbance signal decreased by 60% for sediment sample and between 10 and 20% for ahimal and plant samples, due to particle sedimentation. However, acceptable results can be obtained if the suspension is stirred periodically just before sampling. We tried to elaborate a liquid medium that would produce satisfactory reproducibilites for suspensions composed of different matrices such as animal tissue, plant tissue, and sediment samples. Preliminary tests have shown that the viscosity of glycerine kept the different types of particles in suspension for a sufficient period. Furthermore, the decomposition of the glycerine, accompanied by heavy smoke, is achieved during the ashing step and no significant background signal is generated during the atomization step. The background levels generated at the 283.3-nm P b line are very small for all matrices studied (Maximum observed: 0.050 AU for 5 pL of 10 mgmL-' sample, equivalent to 50 wg of actual sample weight). At the 228.8-nm Cd line, the background signals range between 0.15 AU ( 5 p L of aqueous-glycerine standard) and 0.27 AU for 5 pL of 4.3 mgmL-l estuarine sediment sample, equivalent to 22 pg of actual sample weight. These values are easily corrected by the deuterium device. For plant and sediment samples the best results were obtained with a 1:l ratio of glycerine-demineralized water. Larger amounts of glycerine generate irreproducibilities in aspiration of the suspension aliquot by the autosampler capillary. However, lyophilized animal tissue particles agglomerate in glycerine-water medium. Their dispersion was achieved by substitution of demineralized water by methanol. The composition of liquid media used in this work is given in Table 11. The amounts indicated are not very critical and small changes of these parameters do not influence the quality of the analysis. Determination Problems with Suspended Material. In the field of EAAS, introductions of the platform by L'vov (4) and of matrix modifiers by Ediger (5) were the greatest con-
Table 11. Composition of Complex Liquid Media amts in vol % Cd
Pb analysis glycerine H,O, demineralizedc (NHJH2PO4, 5 % sol (NHJH2PO4, 5 % 501 + Mg(NO3)2, 0.1 sol "OB, 65%
50 25
analysis 50
25
200
20b
%
5
5
"Equivalent to 50 pg of (NH4)H2P04for a 5-pL sample. bEquivalent to 50 pg of (NH4)H2P04+ 1 pg of Mg(NOB)2for a 5-pL sample. "or animal tissue analysis, absolute methanol was used instead of demineralized water (see text for further details). tributions to the reduction of matrix interference and to the equalization of the working curve slopes of different samples tested. The incorporation of matrix modifiers into the sample is more difficult for the solid sample than for the liquid sample. In the case of liquid suspended samples, the addition of matrix modifier is much more straightforward. W@have added the well-known matrix modifiers for lead and cadmium (6) to the initial liquid medium (Table 11). The amounts of matrix modifiers added were the same as those used for routine analysis of dissolved samples and standard solutions. To compensate for differences in viscosities when dispensing standards and samples, we also added glycerine to the aqueous standards. Reproducibility of the Procedure. The solid samples were weighed (typically 3-30 mg) in the autosampler microvials. After addition of 1-mL of liquid complex medium, the suspensions were homogeneized with the microstirrer. The autosampler capillary collects an aliquot of the suspension from the middle of the microvial (typically 5-10 wL) and dispenses it on the platform. These volumes represent an initial solid sample amount between 15 and 300 wg. The electrothermal program is about 90 s long and the rapidity of the entire procedure easily allows repetitive analysis. The analysis frequency is the same as with solutions and in any case faster than with actual solid sampling, which requires sample weighing for each atomization. Figure 1illustrates the reproducibility of the measurements during 1 h of continuous analysis for the three matrices studied. Samples were homogenized only once, just before the first dispensing. The reproducibilities obtained are similar to those obtained for the aqueous-glycerine medium standard. Accuracy of the Procedure. The matrix modifiers used allowed the use of higher ashing temperatures (900 "C instead 400 "C for Cd and 800 "C instead 550 "C for Pb). Their effect on the atomization process of lead is illustrated in Figure 2. In the absence of matrix modifier, the absorbance-time profiles show differences in atomization rate between simple and complex matrices (A). In the presence of ammonium phosphate modifier absorbancetime profiles are well superimposed for all the matrices studied (B). A similar situation was observed for the cadmium in the absence and presence of com-
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
-I
-
--
--
-- - -,
I
I
I
I
1
2
TIME,s
L
IB
,
005-
< -0-
40
20
60
800
TIME minutes
Flgure 1. Cadmium repetability test on different powdered liquid suspended samples (5 p L dispensing): (0)aqueous standard with glycerine (3 pgL-'), RSD,=,, = 2.10%; (D) animal lyophilized tissue, RSD,=,,, = 3.21%; (0) plant sample, RSD(,=,,, = 3.37%; (A)sediment sample, RSD(, =40) = 3.61 %.
18.92 19.28 19.28
0.8133 0.8305 0.8251 0.8310 0.8133 0.8282
mean slope re1 std dev, %
19.04 1.35
0.8229 0.93
I
I
1
2
I
TIME,s
suspended plant sample, (4) suspended animal tissue. using simple reference solutions is valid. Table IV presents the results obtained by direct calibration for different standard reference materials. The agreement with recommended values shows the validity of the approaches used in this work. The full analytical procedure provided relative standard deviations ranging from 0.45 to 13% depending on the magnitude of the measured concentrations. The detection limits (2a) seemed to be lower than 0.4 pg for Cd and 5 pg for Pb. With the experimental conditions given inTable I the sensitivities in peak-height mode are 0.25 and 4.5 pg for 0.0044 AU for Cd and Pb, respectively.
0.8190
n.d. n.d.
n.d. n.d. n.d.
2200
Flgure 2. Absorbance-time profiles for lead in the different matrices studied: (A) without matrix modifier, (B) with (NH4)H2P04modifier; (1) aqueous standard with glycerine, (2) suspended sediment sample, (3)
working curve slope, Amng-' cadmium lead 18.89 18.65 19.20
temperaiurc.0C
L 1
Table 111. Working Curve Slopes for All Matrices Studied"
aqueous standard citrus leaves orchard leaves spinach tomato leaves pine needles bovine liver fish homogenate estuarine sediment
tube 1.11
/
CONCLUSION With direct dispensing of liquid suspended powdered sample on the platform in the E T atomizer, the determination of lead and cadmium in environmental samples is rapid and easy. It is evident that the procedure described above may be extrapolated to other elements and matrices using adequate modifiers. We do not observed any excessive nonspecific absorbance signals. The background levels were not significantly higher than those observed when analyzing corresponding sample solutions. The integrated absorbance value is, in principle, more suitable for an appropriate quantification (7). This work shows that the measurement of the peak absorbance can also lead, under controlled atomization conditions, to consistent results.
The slopes were calculated from four-point standard addition. Each addition point was measured four times. "n.d., not determined.
bined ammonium phosphate-magnesium nitrate modifier. Reliability of the whole analytical approach was then tested by establishing standard addition curves for all matrices studied. Absence on nonspectral interferences was checked by comparison of calibration curve slopes obtained in simple medium and working curve slopes (standard addition) in all matrices studied. For lead and cadmium Table I11 shows that working curve slopes are very similar for all matrices, including simple medium. In such cases the direct calibration procedure
Table IV. Cadmium and Lead Content in Standard Reference Materials sample wt for 1 &L,* mg citrus leaves (SRM-1572) orchard leaves (SRM-1571) spinach (SRM-1570) tomato leaves (SRM-1573) pine needles (SRM-1575) bovine liver (SRM-1577) fish homogenate (MAA-2) estuarine sediment (SRM-1646)
24.0 14.5
6.5 29.6 4.3
cadmium found value," recommended pgg" ic u value, peg-' 0.028 f 0.003 0.101 f 0.009
0.304 f 0.020 0.062 f 0.008 0.420 f 0.030
aDirect calibration method: mean value from five injections of 5 actual samde weight is 1/200 of the weights indicated above.
sample wt for 1 m ~ ,mg *
lead found value: bgg-' i r~
recommended value, beg-'
10.1 3.2 3.1 19.5 21.8 3.8
1.42 f 0.15 6.62 f 0.03 11.4 f 0.9 0.380 f 0.032 0.505 f 0.045 29.2 i 1.6
1.2 f 0.2 6.3 f 0.3 10.8 f 0.5 0.34 f 0.08 0.58 h 0.07 28.2 f 1.8
0.03 f 0.01 0.11 f 0.01
0.27 f 0.04 0.066 ic 0.004 0.36 f 0.07 pL
suspended sample. *For 5-pL sample dispensed in the atomizer,
Anal. Chem. 1986, 58, 2617-2621
The accuracies obtained and the rapidity of the technique permit the analysis of solid samples on a routine basis. Registry No. Cd, 7440-43-9; Pb, 7439-92-1.
LITERATURE CITED (1) Vollkopf, U.; Grobenski, 2.; Tarnm, R.: Welz, B. Analyst (London) 1985, 110, 573-577. (2) Brady, D. V.; Montalvo, J. G.; Jung, J.; Curran, R. A. A t . Absofpt. News/. 1974, 13, 5-6.
2617
(3) Brady, D. V.; Montalvo, J. G.; Joseph, 0.; Glowacki, G.; Pisciotta, A. Anal. Chim. Acta 1974, 70, 448-452. (4) L'vov, B. B. Spectrochim. Acta, Part8 1978, 338, 153-193. (5) Edlger, R. D. At. Absorpt. News/. 1975, 1 4 , 127-130. ( 6 ) Savin, W.; Carnrlck, G. R.: Manning, D. C.; Pruszkowska, E. At. Spectrosc. 1983, 4 , 69-86. (7) Barnett, W. 6.; Bohier, W.; Carnrick, G. R.; Savin, W. Spectrochim. Acta, Part8 1986, 408, 1689-1703.
RECEIVED for review April 21, 1986. Accepted July 1, 1986.
Indirect Determination of Nitrogenated Drugs by Atomic Absorption Spectrometry Cristina Nerh* and Agusth Garnica
Departamento de Qulmica, Escuela Tgcnica Superior Ingenieros Industriales, Universidad de Zaragoza, Zaragoza, S p a i n Juan Cacho
Departamento de Q d m i c a Analitica, Facultad de Ciencias, Universidad de Zaragoza, Zaragoza, Spain
A new procedure for determlnatlon of alkalolds and other pharmaceuticaldrugs using the Dragendorf reagent is studied. The method consists of extracting an Ion pair between the organic base and the lnorganlc complex BII,- and measuring BI In the organic phase by atomlc absorption spectrometry at 223.1 nm. The opthnal experlmental condltlons pH, concentration of Bl14-, shaking time, phase ratlo, number of extractlons, and the range of calibratlon are studied In the determlnatlon uf dlphenhldramlne, papaverlne, amylocaine, bromhexine, sparteine, and avacan. The organlc phase used Is 1,2dichloroethane. The standard devlatlon of the method Is around depending on the substance analyzed. The new method allows the determlnation of binary mlxtures of alkaloids and drugs. The Interference of forelgn substances that accompany these drugs In pharmaceutlcal preparatlons Is studied. I t Is observed that the Ion pair dlphenhldramineBiI,- Is soluble In 1,Bdlchloroethane.
The indirect determination of organic products by atomic absorption spectrometry (AAS) has long been the subject of research. In 1967, at the 153rd National Meeting of the American Chemical Society, a paper was read on this subject and was later published (I). Since then, numerous publications have appeared in the literature on AAS and this aspect of indirect determination and have been compiled in the reviews by Kirkbright and Johnson in 1973 (2),Miller in 1978 (3), Roussellet and Thuller in 1979 ( 4 ) ,Kidani in 1981 (5), Ebdon in 1982
