Colloidal palladium—a promising chemical modifier for electrothermal atomic absorption spectrometry

SPECTROCHIMICA ACTA PART B

ELSEVIER

Spectrochimica Acta Part B 52 (1997) 1293-1304

Colloidal palladium a promising chemical modifier for electrothermal atomic absorption spectrometry Anatoly B. Volynsky*, Viliam Krivan Sektion Analytik und HOchstreinigung, Universiti~t Ulm, D-89069 Ulm, Germany

Received 4 November 1996; accepted 24 January 1997

Abstract

The efficiencies of chemical modifiers based on palladium for electrothermal atomic absorption spectrometry (ETAAS) were compared. The main attention was devoted to the comparison of two types of colloidal palladium with a mixture of palladium nitrate and magnesium nitrate or a mixture of ammonium dihydrogenphosphate and magnesium nitrate. The modifiers were applied to model solutions of the analytes Se, Cd, Te and Sn containing sodium sulphate, sodium chloride and glucose as matrix components. In general, no significant difference in effectiveness was found between the polymer-stabilizedcolloidal palladium and other chemical modifiers studied. Colloidal palladium stabilized with polyvinylpyrrolidone(Mr ~ 360 000) has proved to be as effective as colloidal palladium stabilized with the surfactant lauryldimethylcarboxymethylammonium betaine. However, concentrated solutions of the former modifier can be prepared easily and kept for a long time at room temperature. The increase in the palladium particle size in solution from 2-10 nm to 20-2000 nm resulting from storage for 16 months did not significantly influence the effectiveness of the modifier. Colloidal palladium can be considered as a prospective aid to the investigation of processes in graphite atomizers. © 1997 Elsevier Science B.V. Keywords: Electrothermal atomic absorption spectrometry; Magnesium modifier; Modifier; Palladium colloid; Palladium

modifier

1. I n t r o d u c t i o n

Palladium chloride was the original chemical form of palladium [11 used as chemical modifier for electrothermal atomic absorption spectrometry (ETAAS). Later, other forms of palladium were proposed (see Fig. 1) on the basis of both empirical and theoretical approaches.

* Corresponding author. Present address: V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, 117975 Moscow, Russia. ~Presented at the 2nd European Furnace Symposium, SanktPetersburg, Russia, 26-30 May 1996.

Meanwhile, elemental Pd and/or palladium oxide can be considered as the active forms of palladium modifier [ 11 ]. If palladium nitrate is used for analysis of samples containing no chloride ions, palladium oxide has formed at 158°C [12]. However, in reality, this situation is rare. Normally, the solutions to be analysed contain sufficiently high concentrations of chloride ions to enable transformation of the palladium cation into the tetrachloropalladate(II) anion. This anion and its salts are quite stable in solution [13] and under heating, respectively. For example, Na:PdCI4 melts without decomposition at 430°C [14]. The two main ways of transformation of palladium modifiers into their active form are via the thermal

0584-8547/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved PII S0584-8547(97)00011-6

1294

Anatoly B. Volynsky, Viliam KrivwdSpectrochimk'a Acta Part B 52 (1997) 1293-1304

PdC12

~ , ~

Shan and Ni, 1979 [1] ]

Pd(NO3)2 + ascorbic acid ] Shan et al., 1985 [3] ~

]

Niskavaara et al" 1985 [4]/

/ /

Pd(CHaCOO)2

I

Pd(NO3)2 Shah et al., 1985 [3]

~ ~

Niskavaara eial" 1985 [4]

~

Pd(NO3)2 + Mg(NO3)2

Pre-reduced Pd [ Grobenski et al., 1985 [2] /

PdC12 + reductants

[[ SchlemmerandWelz'198615] ~ " ]"I

]

~achandSchrader'~ -

[ Pd Ircoating ~

[Pd(NH3)4]CI2+ (NH4)2C204 Sachsenberg et al., 1993 [8]

Schuttler et al., 1992 [7]

(NH4)2[Pd(C204)2I

Colloidal Pd 1 Volynsky and Krivan, 1996 [10]

Bhattaeharyya et al., 1993 [9]

Volynsky and Krivan, 1996 [10] Fig. 1. Surveyof palladium modifiers [ 1- 10]. pre-reduction of palladium(II) in the graphite tube at 1000°C before introduction of the sample [2,15] and the application of a mixture of palladium salt with reductants (ascorbic or citric acid, hydroxylammonium chloride, etc.) [6,16]. A somewhat similar approach is the use of elemental iridium as a permanent modifier [7]. However, the pre-reduction of the modifiers increases the analysis time by 30-40%. The main disadvantage of mixtures of palladium compounds with reductants appears to be their inconstancy of action. Sometimes, no significant difference between the effect caused by a palladium salt alone and by its mixture with a reductant was found [17]. Although the addition of a reductant usually improves recovery of the analyte, this does not quite reach 100% [16,18,19]. A mixture of palladium nitrate and magnesium nitrate, proposed by Schlemmer and co-workers [5,20], is therefore more widely used as a chemical modifier, although the reasons for its high effectiveness are not yet well understood. Recently, two new forms of palladium modifier have been proposed [10]. One of them is

(NH4)2Pd[(C204)2]-2H20. Because of the chelate effect, the Pd[(C204)2] 2- anion is more stable than tetrachloropaltadate(II) [21]. Thus, unlike palladium nitrate and acetate, the Pd[(C204)2] 2 anion retains its form in chloride solutions. When heated in an inert atmosphere, palladium oxalates decompose at 100-200°C, forming elemental palladium [22,23], which is stable under these conditions even in the presence of a large excess of sodium chloride or other metal chlorides [24]. The other new form of palladium modifier for ETAAS is colloidal palladium. When this modifier is applied, the analytes may start to interact with elemental palladium even during the drying stage. Preliminary investigations showed that this modifier overcomes very effectively the negative influence of high concentrations of sodium chloride on the absorption signal of selenium [10]. In this work, the effectiveness of two forms of colloidal palladium has been compared with that of a mixture of palladium nitrate and magnesium nitrate or a mixture of ammonium dihydrogenphosphate and magnesium nitrate for a number of analytes in the presence of different interferents.

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1295

Toshima [25]. To 4 ml of PdC12 solution containing 50 mg Pd in 1 mol 1-I HCI were added 1 ml of water, 20 ml of an aqueous solution of 2.5 mg ml -~ PVP and 25 ml of C2H5OH. The solution was refluxed for 30-60 min (on an electric hotplate) in a flow of argon. Colloidal palladium stabilized by LDCAB was synthesized by the reduction of palladium acetate with hydrogen in the presence of the stabilizer [26]. The modifier was kept in the solid state and was suspended in water immediately before use. The mass content of palladium in the solid colloid was about 6%. The total volume of the solutions pipetted into the atomizer was 20-30/A. Peak areas and peak heights were used for evaluation of atomic and background absorption, respectively. Each experiment was carried out with at least two replicates. As a rule, the reproducibility of the measurements was better than 5%. Experimental conditions and instrumental parameters used are given in Table 1. The temperature programmes used generally coresponded to those recommended by Perkin-Elmer. However, in every case, the absence of analyte losses during the pyrolysis stage was checked and, if necessary, the pyrolysis temperature was adjusted.

2. Experimental

2.1. Reagents Reagents of "pro analysi" grade or higher purity (Merck, Darmstadt, Germany, or Fluka, Buchs, Switzerland) were used. Lauryldimethylcarboxymethylammoniumbetaine (LDCAB) was purchased from WITCO Corp. (Lake Success, NY). Polyvinylpyrrolidone (PVP) K-90 (Mr ~ 360 000) "for molecular biology" was purchased from Fluka. The stock standard solutions of the analytes (Merck) were diluted before use with 0.2% HNO3. All other solutions were prepared with doubly distilled water. The stock palladium modifier solution contained 10.0 mg ml -j Pd as Pd(NO3)2 in ~ 15% HNO3. The mixed palladium nitrate and magnesium nitrate modifier contained 1 mg m1-1 Pd and 0.6 mg ml -l Mg(NO3)> The argon used was of 99.998% purity (Linde, Unterschleissheim, Germany).

2.2. Apparatus Measurements were performed with a Perkin-Elmer Model 4100ZL atomic absorption spectrometer equipped with a THGA graphite furnace and an AS-70 autosampler. Background correction was based on the longitudinal inverse Zeeman effect. Micrographs of the particles of colloidal palladium were made by using an electron transmittance microscope, Model EM 10A (Carl Zeiss, Oberkochen, Germany).

3. Results and discussion

2.3. Procedure

3.1. Colloidal palladium with different types of stabilization

Polymer-stabilized colloidal palladium was synthesized according to the procedure of Hirai and

Colloidal solutions of palladium have been known since the beginning of the 20th century [27]. Recently,

Table 1 Instrumental parameters and experimental conditions used Element

~nm Bandwidth/nm Light source Pyrolysis temperature/~C Pyrolysis time/s (ramp/hold) Atomization temperaturePC (gas stop) Atomization time/s (ramp/hold)

Se

Cd

Te

Sn

196.0 2.0 EDL, 5W (Perkin-Elmer) I 100 10/60 1900 0/7

228.8 0.7 EDL-2, 230 mA (Perkin-Elmer) 600 10/30 1400 0/5

214.3 0.2 HCL, 14 mA (Orion) 1000 10/30 1800 0/7

286.3 0.7 EDL-2, 310 mA (Perkin-Elmer) 1200 10/60 2200 0/7

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Anatoly B. Volynsky, Viliam Krivan/Spectrochimica Acta Part B 52 (1997) 1293-1304

Table 2 Some characteristics of Pd modifiers used Modifier Characteristic

Pd/Mg(NO3)2

Colloidal Pd, stabilized with PVP "~esh .

Concentration of Pd/(#g t*l ~) Concentration of stabilizer/(#g ~1-~) Particle size/nm

.

.

.

Colloidal Pd, stabilized .with LDCAB

old"

1~

I

1

-

1

1

-

3.0-7.5

20-2000

1

15.7 100-1000

Concentration of Mg(NO3)2 was 0.6/zg tzl ~.

they have been intensively studied, mainly as prospective catalysts [26]. Despite the very small size of the metallic particles ( ~ 1-100 nm), in the absence of stabilizers colloidal solutions are not stable. There are two main techniques for their stabilization. With polymer compounds as stabilizers, the colloidal particles are surrounded with a shell formed by the molecules of the polymer. In this case, stabilization of the colloid is achieved as a result of steric effects. When surfactants are used for stabilization, the agglomeration of the metallic particles is prevented by electrostatic forces. In either case, the protecting shell is in dynamic equilibrium with the solution that makes the metallic particles approachable by the analytes. Some characteristics of the palladium modifiers used are presented in Table 2. The preparation of colloidal Pd stabilized with the polymer PVP is very simple and can be easily carried out in any chemical laboratory. The diameter of the particles in a freshly prepared colloidal palladium solution stabilized with PVP was in the very narrow range of 3.0-7.5 nm (see Fig. 2(a)). A fraction of the palladium existed as aggregations of 5-15 particles. When kept under an inert atmosphere at room temperature, these colloidal solutions were stable for at least 16 months. Thus, a mass ratio of PVP to palladium in the solution of 1:1 is sufficient for effective stabilization of the colloid. However, during storage, the size of the particles increased significantly, reaching 20-2000 nm (see Fig. 2(b)). Evidently, the large particles are formed from the small ones by agglomeration, which starts immediately after or even during synthesis (see Fig. 2(c)).

Although the mass content of palladium in the solid sample Pd-LDCAB was only 6%, some relatively large particles could not be transferred into suspension. At room temperature, a solution containing 1 mg m1-1 of colloidal Pd stabilized with LDCAB (see Fig. 2(d)) is stable for several hours only. The need to use gaseous hydrogen for the reduction of Pd(II) during the synthesis represents a further disadvantage of this palladium colloid. The application of other surfactants, such as tetraalkylammonium halides or the corresponding sulphonate derivatives [26,28], allows a significant decrease in both the mean size of the palladium particles and the mass ratio of stabilizer to palladium. However, halogen- and sulphur-containing surfactants may themselves act as interferents. Therefore we did not study systems of this kind. 3.2. Behaviour of selenium in the presence of sodium

chloride Without modifier, losses of selenium may start even at 100-200°C [29,30]. When palladium modifiers are applied, the interference-free determination of selenium is possible in the presence of 10 /zg of sodium chloride [17,20] (the data were obtained for HGA-400 and HGA-600 atomizers). Application of pre-reduced palladium as a chemical modifier in a THGA atomizer allows the determination of selenium even in the presence of 200/zg of chloride ion (330/zg of NaC1) [10]. The influence of larger masses of this interferent has not yet been studied. Although some characteristics of the Pd modifiers used differ significantly, the influence of their mass on the behaviour of Se in the presence of 200/zg of C1ion was surprisingly similar (see Fig. 3). For the

Anatoly B. Volynsky, Viliam Krivan/Spectrochimica Acta Part B 52 (1997) 1293-1304

-

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t

~o

|

8 18

~;,- 100 n m

.~:

, 200 n m

d

# A

a

.~2p m

1 pm

Fig. 2. Electron micrographs of colloidal Pd particles: (a) "fresh" Pd-PVP, magnification 100 000; (b) "old" Pd-PVP, magnification 5000; (c) "old" Pd-PVP, magnification 57 000; (d) Pd-LDCAB, magnification 10 000.

Pd-LDCAB modifier, the sensitivity of selenium was about the same for 2-10 t~g of palladium. However, for a larger mass of modifier, a slight increase in the integrated absorbance was observed (see Fig. 3). For the subsequent comparative investigations, we applied 15 #g of Pd-LDCAB, 15 t~g of "fresh" or "old" Pd-PVP and 5 #g Pd/3 /zg Mg(NO3)2 (corresponding to the recommendations of Perkin-Elmer

[31]).

In the absence of a matrix, the maximum sensitivity for selenium (m0 = 30 pg) was achieved with Pd/Mg(NO3)2. However, for this modifier, the background absorption sometimes exceeded 2.0 even in the presence of 200 t~g of C1- ion. This disadvantage of Pd/Mg as compared with other palladium modifiers was also noticed by Shan and Wen [17]. The maximum background absorption was observed with applied modifier amounts of 2 - 5 t~g Pd/1.2-3 t~g

Anatoly B. Volynsky, Viliam Krivan/Spectrochimica Acta Part B 52 (1997) 1293-1304

1298

o,3 /

~

(c)

J

F-W-

~

0,1

o

1'o

is

M o d i f i e r mass, lag

Fig. 3. Influence of the mass of Pd moditiers on the absorption signal for 2 ng of Se in the presence of 200 t~g of C1- (as NaCl): (a) Pd/Mg(NO3)2; (b) "fresh" Pd-PVP; (c) "old" Pd-PVP; (d) Pd-LDCAB.

Mg(NO3)2 (see Fig. 4). Although the background decreases with increasing Pd mass, it remains essentially higher than that for colloidal palladium. Furthermore, with the Pd/Mg(NO3)z modifier, a depression of the Se absorption signal (by about 15%) was observed in the presence of 50/~g of C1ion compared with that measured without a matrix. Thus, with this modifier the integrated Se absorbance was independent of the matrix mass only in the range 50-200 ~g of C1- ion. Application of Pd-LDCAB modifier allows Se to be determined in the presence of as much as 500 ~g of

It 5

o

3.3. Behaviour of cadmium in the presence of sodium sulphate

~(a)

-

1-

01!

• ...... 0

_

~

Cl- ion. However, an increase in chlorine mass above 350 #g led to an enhancement of the selenium absorption signal by 11%. As is evident from a comparison of the data in Table 3, approximately the same background absorption was observed in the presence of the Pd/Mg(NO3)2 and 200 #g C1- as in the presence of the Pd-LDCAB modifier and 500 #g C1-. The best results for selenium in the presence of NaC1 were obtained with the Pd-PVP modifiers. In addition to a relatively small variation of the Se absorption signal with changing mass of C1- ion in the range 0-500 #g (see Table 4), the average background absorption in the presence of 500/~g of CI- ion was only 0.70 for "fresh" and 0.39 for " o l d " modifier (see Table 3). Ageing of the Pd-PVP modifier led to a significant decrease in the background absorption, however, and simultaneously a slight decrease in sensitivity and reproducibility for selenium was also observed. The values of the characteristic mass for Se in the presence of 500 t~g of C1- ion with colloidal Pd modifiers were higher by only 18-29% than those obtained for pure selenium solution (28 pg [32]). In all combinations of modifiers and NaC1, the blank values remained below the integrated absorbance over 0.005 s. Although Pd-LDCAB proved to be a more effective modifier than Pd/Mg(NO3)2, we did not consider the former modifier in further studies, mainly because of the instability of its solutions, which significantly complicates their practical application.

~.

,~

i

i

i

/

4

8

12

16

20

Palladium mass, lag Fig. 4. The background absorption on the Se 196.0 nm line in the presence of 200 #g of CI- (as NaCI): (a) with Pd/Mg(NO~)2; (b) with "fresh" Pd-PVP.

Even in the presence of palladium modifiers, the maximum applicable pyrolysis temperature for cadmium does not exceed 800-900°C [17,20,33]. In several instances, this temperature is not high enough to volatilize sample matrices that give rise to serious problems concerning background absorption and chemical interferences. In the determination of Cd under STPF conditions [HGA-600 atomizer, Pd/ Mg(NO3)2 as chemical modifier], the maximum tolerable mass of sulphate ion (as potassium sulphate) was found to be 5 t~g [20]. An almost 10-fold increase in the tolerable mass of this interferent was reached with a mixture of palladium nitrate and ammonium nitrate as modifier [33].

Anatoly B. Volynsky, Viliam Krivan/Spectrochimica Acta Part B 52 (1997) 1293-1304

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Anatoly B. Volynsky, Viliam KrivapdSpectrochimica Acta Part B 52 (1997) 1293-1304

Table 5 Integrated absorbances for Cd, Sn and Te obtained with different modifiers in the presence of sulphate and glucose as interferents (n = 6) Integrated absorbance/s Analyte/interferent 30 pg Cd/250 tzg of SO4(as NazSO~) 3 ng Sn/300/~g of SOj (as Na2SO4) 1.5 ng Te/2.25 mg of glucose

15/zg Pd/9 #g Mg(NO3)2

15/~g "fresh" Pd-PVP

15 #g "'old" Pd-PVP

50 #g NH4H2POJ3 #g Mg(NO3)2

0.087 + 0.004

0.088 +~ 0.003

0.103 + 0,003

0,120 + 0.009

0.101 + 0.006

0.110 -+ 0.003

-

0.178 + 0.017

0.166 -+ 0.013

0.180 + 0.002

-

For comparison with the P d - P V P modifiers, a mixture of 50 tzg NH4HzPO4/3 /zg Mg(NO3)~ [31] in a volume of 15/A was used. For the system under investigation, this mass of the modifier was the optimum (see Fig. 5). The optimum mass of P d - P V P modifiers was 15 /zg Pd. With the NH4H2PO4/Mg(NO3)2 modifier, the integrated absorbance for Cd in the presence of 150 /zg of sulphate ion as Na2SO4 is constant at pyrolysis temperatures in the range 300-500°C; at 600°C, the sensitivity for Cd decreases by --~ 6%. Evidently, cadmium phosphates formed during the drying stage are stable only up to 500°C in the presence of large quantities of sodium sulphate. For the P d - P V P modifier, a slight decrease in the Cd absorbance was observed even at 300°C, reaching

0,12

o,os-

o

~o,~

1~0 Volume,

1'5

20

lul

Fig. 5. Influenceof the volume of modifiersolution on the integrated absorbance for 30 pg of Cd in the presence of 150/~g of SO4- (as Na2SO~): (a) with 3.33 mg ml ~ NH4H2POfl0.2 mg ml -I Mg(NO3)2; (b) with 1 mg ml -r of "old" Pd-PVP; (c) with 1 mg ml-I of "fresh" Pd-PVP,

about 10% at 600°C. During drying and at the beginning of the pyrolysis stage, chemisorption on palladium particles was considered to be the first step of analyte stabilization [34]. The subsequent dissolution of the analyte in the modifier body during the pyrolysis stage may be accompanied by a partial loss of the analyte as a result of evaporation from the modifier surface [35]. However, in the presence of a large surplus of matrix (molar ratio Cd:Na2SO4 ratio 1:6 x 108), losses below 10% may be accepted as tolerable. Thus, a pyrolysis temperature of 600°C was chosen for further experiments with all modifiers. Generally, for the determination of cadmium in the presence of Na2SO4, the NH4H2PO4/Mg(NO3)2 seems to be slightly superior to P d - P V P as the modifier (see Tables 4 and 5). The background absorption in the presence of 250 ~g of SO42- ion was higher with the P d - P V P modifiers (1.2-1.3) as compared with that for NH4H2PO4/Mg(NO3)2 (0.9). The respective values of the characteristic mass of cadmium were 1.5 pg and 1.3 pg. As the P d - P V P modifiers are applicable to a large number of analytes, they become clearly superior to NH4H2PO4/Mg(NO3)2 modifier when, in addition to Cd and Pb [31], other elements are to be determined by multielement ETAAS. 3.4. Behaviour o f tin in the presence o f sodium sulphate According to thermodynamic calculations [36], the formation of stable gaseous SnS at the atomization stage may significantly interfere with the determination of tin in the presence of sulphur. However, when a graphite platform is applied (HGA-600 atomizer), a

Anatoly B. Volynskv Viliam Krivan/Spectrochimica Acta Part B 52 (1997) 1293-1304

1301

3.5. Behaviour of tellurium in the presence of glucose

0,2

~"0,15

0,1"a

0,05-

0 0

100 200 Sulfate mass, lag

300

Fig. 6. Behaviourof 3 ng of Sn in the presenceof Na2SO4: (a) with Pd/Mg(NO:02; (b) with "'old" Pd-PVP, successful determination of tin may be carried out in the presence of up to 50/~g of sulphate ion (as K2SO4; Pd/Mg nitrates used as chemical modifier) [20]. Evaporation from the graphite wall without significant changes in the other experimental conditions decreases the permissible amount of the interferent 5-fold [171. We optimized the experimental conditions for tin in the presence of 150/zg of sulphate ion. Although the maximum pyrolysis temperature was 1300°C for both the Pd/Mg(NO3)2 and the Pd-PVP modifier, we carried out pyrolysis at 1200°C for 60 s, as this temperature proved to be sufficient for almost complete volatilization of the matrix. The optimum mass of palladium was 15 /xg [plus 9 /zg Mg(NO3) 2 for the mixed modifier], 3-fold higher than that recommended by Perkin-Elmer, With 8/zg and 15 ~g of "fresh" colloidal Pd modifier, the characteristic masses of tin in pure solution were found to be 77 pg and 107 pg, respectively, compared with the value of 55 pg reported by Perkin-Elmer [32]. Thus, the increase in modifier mass causes a decrease in the sensitivity for tin. However, by this means, the marked interference of sodium sulphate (with 150 ~tg of sulphate ion, without modifier, the integrated absorbance of 3 ng Sn was close to zero) was successfully suppressed (see Table 4). The sensitivity for tin was slightly better with Pd/Mg(NO3)2 compared with colloidal Pd, but, with varying SO24- ion concentration, it changed to a greater extent for the former modifier (see Fig. 6 and Table 4).

Organic substances may drastically decrease the sensitivity for Se, As and, especially, Te (see references in [37]). Terui et al. [38] reported that the peak height absorbance for As and Se decreases by about one order of magnitude in the presence of 0.1-0.01% of ascorbic acid. The integrated absorbance for tellurium is depressed 10-fold in the presence of 1% of ascorbic acid (these data were obtained under STPF conditions, but without modifier, using the HGA-600 atomizer [39]). The most probable reason for these phenomena is the formation of relatively stable volatile compounds of semi-metals with the products of the thermal decomposition of organic compounds [40]. Optimization of the experimental conditions for tellurium was carried out in the presence of 1.5 mg of glucose. Modifier amounts of 15 #g of colloidal Pd and of 15 ~g of Pd/9/~g Mg(NO3)2 and a pyrolysis temperature of 1000°C were found to be optimum. All the Pd modifiers studied eliminated the influence of up to 1.5 mg of glucose on the Te absorption signal; larger masses of glucose caused a slight decrease of the Te integrated absorbance. However, with Pd modifiers tellurium may be determined in the presence of at least 2.25 mg of glucose (see Table 4). The background absorption values in the presence of glucose were approximately the same for all three Pd modifiers studied. Although magnesium nitrate is widely used for the dry ashing of organic materials [41 ], we could find no advantage of a mixed Pd/Mg(NO3)2 modifier over colloidal Pd for the determination of Te in an organic matrix. Probably the ashing function of Mg(NO3), is not of importance for its application as a chemical modifier for ETAAS. 3.6. Comparison of Pd modifiers

Generally speaking, the difference among the palladium modifiers studied is quite small. Although the sensitivity for Se and Sn was slightly higher with Pd/Mg(NO3)2 in comparison with the other modifiers (see Table 5), this modifier was less effective in eliminating the matrix interferences (see Table 3). In our previous work [10], we showed that colloidal Pd was more effective for the determination of selenium

1302

Anatoly B. Volynsky, Viliam Krivan/Spectrochimica Acta Part B 52 (1997) 1293-1304 O,6

(b)

A

0,4 W

0,2-

0

-

0

1

2 Time, s

3

0,4

)

B

O,3-

0,2-

0,1-

o

1

2 Time, s

3

Fig. 7. Absorptionsignals for 3 ng of Sn in the presence of (a) 15 t~g of"old" Pd-PVP; (b) 15 ~g of "fresh" Pd-PVP; and (c) 15 #g Pd/9 #g Mg(NO~)2. A, solution without matrix; B. in the presence of 300 tzg SO4- as Na2SO4. in the presence of NaC1 than PdClz or Pd(NO3)z. The comparison of those results, which also were obtained using a THGA atomizer, with our new data allow us to conclude that an addition of Mg(NO3)2 actually improves the effectiveness of the Pd modifier. The most probable reason for this is the formation and subsequent hydrolysis of MgCI2.6H20, promoting the removal of chlorine from the graphite tube. However, the rate of thermohydrolysis depends to a great extent upon the experimental conditions applied [42]. This might be the reason why sometimes no enhancement of the effectiveness of the Pd modifier

by addition of Mg(NO3)2 was observed [17]. Colloidal Pd, on the other hand, starts to interact with the analytes in the solution and this enhances the robustness of this modifier. The action of all Pd modifiers is based on the formation of solid solutions and/or stoichiometric compounds of low volatility between the analyte and Pd [11,43]. However, in the presence of the Pd/ Mg(NO3)2 modifier, the absorption signals were broader and lower than those obtained in the presence of colloidal Pd (for an example, see Fig. 7). Slavin et al. [44] assumed that the function of the added

Anatoly B. Volynsky, Viliam Krivan/Spectrochimica Acta Part B 52 (1997) 1293-1304

magnesium is to embed the analyte in a matrix of MgO, delaying vaporization of the analyte until the magnesium oxide is vaporized. The application of different methods of synthesis allows variation of the particle size of the resulting colloidal solutions over a wide range [25,26,28]. Therefore, colloidal palladium represents a well suited model system for investigation of the influence of the particle size on the processes in graphite atomizers. The particle size was the only parameter differentiating between the "fresh" and " o l d " PVP-stabilized palladium modifiers used in this study. Recently it was assumed [8,16] that the size and distribution of the Pd particles on the graphite surface may influence the efficiency of the modifier. However, only small differences between the shapes of the absorption signals, especially for pure solutions of the analytes (for example, see Fig. 7(A)), and between the modifier efficiencies were observed for "fresh" and " o l d " Pd-PVP modifiers. Consequently, the influence of the size of Pd particles on the properties of the modifiers seems to be quite limited for the systems studied. Sometimes, with smaller sized colloidal Pd particles, the minimum applicable mass of the Pd modifier decreases. For example, the integrated absorbance for Cd remained approximately constant in the ranges 5-20 /~g of "fresh" Pd-PVP and 12-20 /zg of " o l d " Pd-PVP (see Fig. 5). For "fresh" Pd-PVP modifier, constant integrated Se absorbance was observed even for 2-20 ~g of Pd; for " o l d " Pd-PVP modifier this range was 5-20/~g (see Fig. 3). This phenomenon can be explained by a significantly larger surface area of the smaller particles, allowing more efficient chemisorption of the analyte. However, in practical terms, the influence of the age of the P d - P V P modifier on its effectiveness is negligible (see Tables 4 and 5).

4. Conclusion With a THGA atomizer, the use of Pd modifiers allows the maximum masses of interferents to be increased by more than one order of magnitude as compared with HGA atomizers used under STPF conditions [17,20]. However, for matrix masses in

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the range 200-500 txg of C1- or SO]- ions, the mass of Pd/Mg(NO3)2 modifier recommended by PerkinElmer has to be increased by a factor of 3. The effect of the optimized mass of this mixed modifier was about the same as that of colloidal palladium. However, for several reasons, colloidal Pd can be considered as an interesting chemical modifier. As the form of colloidal Pd is not affected by the sample matrix, it seems to be a more robust modifier in comparison with other forms of palladium modifier. As it is effective for different types of analytes in various matrices, colloidal Pd appears to be promising, especially for multielement ETAAS. The procedure for preparation of colloidal Pd is very simple. Colloidal Pd particles of different sizes, which can be easily prepared, could be of interest for the investigation of processes in graphite atomizers.

Acknowledgements Professor H. B6nnemann, Max-Planck-Institut for Kohlenforschung (M01heim, Germany), is acknowledged for providing us with a sample of P d LDCAB. The authors also thank Mr W.H. Fritz, Sektion Elektronenmikroskopie, University of Ulm, for the transmittance electron micrographs. This work was supported by the Alexander von Humboldt-Stiftung (Bonn, Germany) by providing a research fellowship to Dr A.B. Volynsky.

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