Sequestration of volatile element hydrides by platinum group elements for graphite furnace atomic absorption

A~~ct-E~cient trapping ofthe volatib hydrides of As, Sb, Se, Bi and Sn cm Barnes of Pd and other phxtinm group metals (PGM) reduced in the graphi& furnace occurs at a relatively law te~~~at~e (200°C).Analytic4 ~gur~ of merit are improved over hyd~d~ trap~ng on the bare tube wall, ~n~~t~ons are established for mu~t~element sequestration. Absoiute limits ofde~tjon (36) range between 8 pg (Se) and 43 pg (As). The catalytic reactivity of PGM’s promotes low tem~rat~m deposition of the hydride by d~ss~a~ve chemisorption. Thermal desorption of alt analytes from the KM’s is first order. Arrhenius energies were used to characterize all anaiyte-substrate interactions.

HYDRIDEgeneration techniques can be used to ~j~~~~~~tl~ improve atomic speetrometric concentration detection limits for a number of elements [I]. Several approaches have been taken in conjunction with atomic absorption, based either on direct transfer of the generated hydride into an atomizer [Z-S] or its colhxtion and concentrative (@purgeand trap”) prior to introduction into the at~rni~ti~n cell [610]. In sbu trapping pr~edu~s which utilize the graphite furnace as both the ~u~centration medium and at~m~tion cell [!2-141 are ~~~i~ularly attractive for this purpose and currently provide the most sensitive atomic spectrometric methods available For detection of As, Se, Sb and Sn [IS]. Despite simplicity and ease of apphcation, this latter approach suffers from a number of disadv~tages [lt;l which inelude the need for a w~~~~~v~~o~ porous graphite struoture on the tube wag1for ~~~rn~rn tapping efficiency, high deposition t~~~tu~s for some elements (As, Sn) and little chance of irnp~rnentat~~~ of m~~tiei~ent collectiun (for subsequent sirnu~ta~~o~s m~~tielement dete~jnat~o~~ because of incompatible thermal trapping parameters. Alf of these problems can be overcome with use of Pd as a substrate onto which the analyte hydride can be chemisor~d~ This study was undertakes to evaluate the analytical ~~o~an~e and mechanism of ~questration of the hydrides of As, Se, Bi, Sb and Sn using several platinum group metals ~P~~~ as collection substrates, 2. EXPERIMENTAL

Analytical a~o~t~~~ m~~su~~ents were made using a ~e~kf~-~Imer modef !IooO system fItted with a Model HCA-500 graphite furnace and &man e&ct ~~k~ound correction. Perkin-Efmer hoffow cathode (Sn) end electrodefess discbarge lamps (As, Se} and Wea~~~~ou~ holfow cathode lamps (Pd, Pt, Ru, Rh, Pi, Sb) were operated at the manufacturers’ recommended currents and powers and the nominal bandpass of the spectrometer was set in accordance with the m~ufa~urers’ (Perkin-Elmer) suggestion. An~yte hydrides were generated in a custom made Pyrex cell into which sodium tetrahydro~rate was intr~u~d using a peristaltic aunt’ The evolved hydrides were staff from solution and swept into the preheated graphite furnace using an Ar purge gas. The cell assembly and sequence of operations used to generate and trap the hydrides have been described in detail earlier [lo]. Time resolved absorbance and temperature measurements in the graphite furnace were obtained with an HCiA-2200 furnace mounted in a modified V&an Techtron Model AA-5 spectrometer having a system response time of ea, 10 ms. This instrument has been described in detail elsewhere [Ia. A series 1100 automatic opt&f pyrometer &con, Nifes, IL, I.QLA+)was sighted throu& the sampfe

66f

R. E. STURGEONet al.

668

introduction hole of the graphite furnace and temperatures acquired assuming blackbody conditions. Scanning electron micrographs of samples and surfaces were obtained using a JEOL JSM-840A instrument equipped with a LINK AN10/85S energy dispersive X-ray system. Auger electron spectroscopy was used to characterize PGM substrates and analyte deposits. A Perkin-Elmer Physical Electronics Industries Scanning Auger Microprobe Model 590 was used for this purpose. A 5 keV probing electron beam was scanned over a 300 x 300 pm area. Samples were surface-sputter cleaned with a 1 keV Xe-ion beam rastered over a 1 x 1 mm area prior to analysis. 2.2. Reagents Stock solutions (1000 mg 1- ‘) of Se(IV), As(III), Bi(III), Sb(III), Sn(IV), Pd(IV), Pt(IV), Ru(IV) and Rh(IV) were prepared by the dissolution of high purity metals or their salts (Spex Industries, Metuchen, NJ, U.S.A.) in subboiling distilled HCl and HNO,. Working solutions were obtained through dilution of these stocks with high purity distilled, de-ionized water (DDW) containing ca. 5% (V/V) HCl. A 1% (m/V) solution of NaBH, (Alfa Inorganics) was prepared as needed by dissolution of 0.25 g pellets in DDW. Thin foils (0.25 mm) of high purity Pt and Pd (Johnson Matthey) were cut into chips (ca. 2.5 x 2.5 mm) and finely polished with successive application of 5, 1 and 0.25 pm diamond dust paste until a mirror-like surface was obtained. Arsine gas (113 ppm in N2) was obtained from Matheson gas products (Whitby, Ont., Can.). 2.3. Procedure Hydrides were generated from 10 ml aqueous solutions of 1 M HCl (Bi, Se, As, Sb) or 0.1 M HCl (Sn) using 1% NaBH, as described earlier [lO-141. Varying amounts of PGM (5 ng-40 pg) were introduced into the furnace in varying volumes using an Eppendorf pipette. The PGM sample was dried and thermally pretreated at 400°C prior to introduction of the analyte hydride onto the PGM sample residue. A generalized HGA furnace program utilized for analytical and diagnostic purposes is given in Table 1. For diagnostic studies, absorbance-time traces were obtained during atomization of all samples and PGM deposits and appearance temperatures and activation energies were derived from the leading edges of the signals. A heating rate of approximately 1700 K s-l was used throughout and an internal Ar purge gas of 650-1000 ml mitt-l served to isolate the analyte source functions. Measurements of hydride deposition temperatures were made with a small bead chromel-alumel thermocouple (room temperature reference junction). The head of the thermocouple was inserted through the sample introduction port of the graphite tube and held in firm contact with the opposite interior wall. As these temperatures of interest were steady-state values attained after heating for 6Os, the response time of the thermocouple was immaterial and only small errors were introduced due to the poor contact between the heated surface and the thermocouple bead. 3. RESULTS ANDDISCUSSION 3.1.

Analytical performance 3.1.1. Optimization. Figure 1 illustrates the effect of mass of Pd on the efficiency of collection and atomization of the hydride elements. An optimum amount was established for Table 1. Furnace program’ Temp.

Hold (s)

Internal purge (ml/min)

120 400

25 15

3M) 300

Hydride Deposition

200 200’ 200

9 29 89

200 100 200

Analyte Atomization

400 2600+ 100 2700

5 3 1

0 0 300

(“C) PGM

Addition

Ramp (s)

Generalized program for 10 ml sample volume. +Addition of 1% NaBH, at 4.0 ml/min.

l

669

Sequestration of volatile element hydrides 1.0

0.6

I

0.6

g

0.4 0.2

0

-2.0

-1.0 m

0

1.0

2.0

mpdI w

Fig. 1. Effect of mass of Pd on peak absorbance response for analyte hydrides in a new pyrolytic graphite coated tube. Deposition temperature was 200°C. 1 ng of As, Se, Sn, Bi; 0.5 ng Sb.

element, viz, Bi (0.5 pg), Se (1.25 pg), As (4 pg), Sn (3 pg) and Sb (4 lug) although all hydrides could be sequestered with a relative peak height response > 95% of maximum using 4 ~18Pd. In general, a drop in the efficiency of caption was noted as the mass of Pd was decreased below optimum values, presumably because the surface area of reduced Pd available for deposition became too small for rapid uptake of the generated analyte vapor plug. With increasing mass of Pd, the surface area available for analyte trapping should also increase, although growth of individual particles would take precedence over formation of new ones. The initial number of particles of Pd is likely determined by the number of active sites on the graphite substrate at which ion exchange-deposition occurs. Further growth at these initial nucleation centers would then proceed as the analyte solution evaporated. Beyond the optimum mass, analyte response decreased because of a change in the rate of desorption of analyte from the Pd surface. With more than 10-15 pg Pd on the surface, analyte absorbance-time profiles were delayed and distorted. The trends of individual elements within this general framework are modified by the specific analyte-Pd interaction energies. Figure 2 shows the effect of volume of Pd solution (for a constant 4 rugmass of Pd) on the peak absorbance response for 1 ng As deposited as ASH,. A significant enhancement in efficiency of collection is noted as the volume of Pd is increased from 2.5 to 20 fi. The greater solution volume wets a larger area of the graphite surface, thereby encountering a larger number of surface active sites at which ion-exchange and Pd nucleation can occur. This presumably results in formation of a greater number of smaller Pd particles on the surface which present a larger surface area for deposition of analyte hydrides. All subsequent studies utilized 20 ~1 injection volumes of PGM’s. The data presented in Fig. 1 reflect the signals obtained from analyte sequestered only on the Pd surface. Minimal contribution to the signal arose from analyte which may have

Fig. 2, Effect of volume of Pd soiution injected on peak absorbance response for 1 ng As as ASH,, Mass of Pd maintained at 4 pg, deposition temperature of 200°C.

R. E.

670

STURGEON

et al.

-&4-4-o-*4 h

0

e

800 DEPOSITION

800

TEMPEFWURE,

1000

K

Fig. 3. Effect of trapping temperature on relative peak absorbance response for analyte hydrides in a new pyrolytic graphite coated tube. Optimum mass of Pd used for each analyte (cf. Fig. 1).

simultaneously deposited on the graphite substrate because the 200°C deposition temperature used for this purpose was, with the exception of Sb and Se, well below that at which efficient deposition could be obtained for these elements on a clean pyrolytic graphite surface [16]. For Sb, that fraction of analyte sequestered by the graphite wall was easily removed with a 1000°C thermal pretreatment. Further, for both Sb and Se, the enhanced sensitivity (z 3-fold} and the higher analyte appearance temperature in the presence of Pd (> 350 IC) minimized any contribution to the measured signal arising from desorption of a fraction of the analyte from graphite. Figure 3 illustrates the effect of deposition or trapping temperature on the relative efficiency of collection of the analyte hydrides on Pd in a new pyrolytic graphite coated tube, The optimum mass of Pd was used in each case. Temperatures as low as 470 K suffice to efficiently sequester all of the analytes. This is well below the minimum trapping temperatures of 780 K for As, 950 K for Sn and 570 K for Bi established for the deposition of these elements on graphite [16]. This relatively low temperature suggests that the hydrides undergo a catalytic dissociation on the Pd, rather than a thermal decomposition, since this temperature lies in the range of reported d~om~sition tem~ratures of these hydrides [l&22]. These latter temperatures refer to the auto-catalytic deposition of the analyte on thin films of the metal. Additional evidence suggestive of a catalytic dissociation process was the observation of no affinity for ASH, by similar amounts of Au, Ni or Cu deposited into the furnace. Pretreatment of these residues at temperatures up to 1200°C in an attempt to reduce any metal oxide and enhance trapping efficiency was unsuccessful. In addition to Pd, other PGM’s were useful for trapping the analyte hydrides at low temperature. Table 2 compares the relative response (peak height absorbance) obtained for each of the analyte hydrides when sequestered on Pd, Pt, Rh and Ru. In each case, the optimum mass of Pd earlier established for each analyte (cf. Fig. 1) was utilized for each PGM. The unique catalytic properties of the PGM’s make them generally useful for this purpose but it is clear that Pd exhibits superior ~rfo~an~. This is perhaps related to its affinity for H,. Table 2. Relative response PGM’s’

with

Element

Pd

Pt

Rh

Ru

As Se Sn Bi Sb

1.0 1.0 1.0 1.0 1.0

0.87 0.92 0.87 0.71 0.91

0.74 0.96 0.31 0.42 0.60

0.40 0.74 0.10 0.76 0.30

‘Peak height absorbance quantitation.

671

Sequestration of volatile element hydrides Table 3. Standard deviation of absorbance signal components and absolute analytical blanks* 10

Element

Baseline

AS

0.00132 (8) 0.08013 (10) 0.00145 (10) 0.0014 (12) O.OOB29 (10)

Se Sn Bi Sb

unloaded furnace 0.00214

(9) o.ooo5o w 0.08233 (10) 0.0013 (7) o,ooo53 (10)

Total blank

Absolute blank (ng)

0.~282 (8) O.ooo56 (8) 0.80320 (7) 0.0013 (11) 0.0028 (8)

0*150*0.014

Pd+ furnace

(8) 0.00058 (10) 0.00193 (10) 0.0016 (7) 0.00046 (10)

0.027 f 0.003 0.110~0.012 0.023 f 0.011 0.116+O.OOQ

* Number in parenthesis is number of replicate s&nals contributing to computed 6.

3.1.2. A~lyt~al ~Zan~s.Table 3 summarizes the absolute blanks for each element in addition to the standard deviation of the absorbance signals obtained for each constituent step comprising the overall blank signal. These latter values consist of the “baseline signal” from the source lamp (lamp noise); signals obtained during the heating of the unloaded furnace (lamp and furnace noise); signals obtained during atomization of the Pd and, finally, the total experimental blank. These individual data are useful for identifying the major sources of noise contributing to the measured analytical blank. Application of an F test at the 95% probability level revealed the following atomization of Pd is never a significant contributor to the overall noise; for Bi, fluctuations in the source lamp intensity determine the overall detection limit; for Se and Sb the heating of the blank or unloaded furnace is the greatest ~nt~butor to overall noise and, for the latter element, reagent contamination (use of a KI reductant) introduces the greatest source of error and hence determines the LOD. For As and Sn, no single dominant source of noise could be identified which would determine the overall LOD. It is clear from the above that improvements in the LOD of the technique are likely to be obtained for Bi if a more stable/intense source such as an EDL is used and for Sb if cleaner reducing reagents can be obtained or the present ones (KI) purified. 3.1.3. Figures ofmerit. Analytical figures of merit are summarized in Table 4. Data pertain to peak height absorbance measurements. Absolute sensitivities compare well with those obtained by direct introduction of aqueous solutions of the analyte with Pd modifier (cf. Section 3.1.4). Limits of detection are based on a 30, criterion. Relative LOD’s are calculated on the basis of a 10 ml sample volume. These values can be lowered by increasing the sample aliquot to a maximum of 50 ml (constr~ned by the present cell volume). Absolute detection limits reported here are superior to those available using other hydride generation atomic spectroscopic or GC techniques [IS].

Table 4. Figures of merit for desorption from Pd*

Element As Se Sn Bi Sb

Sensitivity (Pg)

(pg)

23&l 23;tl 16fO.S 34&l 14~2

43 8 37 32 29

LOD (pg/mf)

*A, response, 10 ml sample volume.

4.3 0.8 3.7 3.2 2.9

Precision, W@ x LOD) 4W) 2025) 200) 30% 8(20)

R. E. STURGEON et al.

672

Precision is given as the % relative standard deviation of replicate determinations of 1 ng quantities of the elements (amounts 20-125 fold larger than the LOD). The linear range of calibration curves using resonance lines and gas stop was in the range l-3 ng. This upper limit was not determined by availability of surface sites on the Pd residue, as verified by the observations that up to 1 pg of As could be predeposited onto the Pd without influencing the response from the subsequent deposition of 1 ng Se. 3.1.4. System performance. Figures of merit reported in Table 4 are limited to peak height absorbance because of the superior performance obtained with this measurement parameter as compared to signal integration. Table 5 reports the ratios of peak height and integrated absorbance sensitivities and detection limits when Pd is used to sequester the analyte hydrides. Peak height parameters are superior by approximately Cfold. The furnace program detailed in Table 1 was utilized in all cases with sample atomization from the tube wall. Use of the L’vov platform to increase the integrated absorbance was successful for most of the elements; absolute sensitivity improved by 34% for Sb, 90% for Bi and 29% for Sn. This enhancement was insufficient to recommend its use in favor of peak height quantitation. Table 5 also presents estimates of the overall efficiency of the system for the generation, transfer, collection and atomization of the analyte hydride. These data are derived from a comparison of the characteristic masses obtained by integration of signals arising from hydride deposition with those resulting from direct injection of the same mass of analyte and Pd modifier in the form of an aqueous solution into the furnace. Integrated absorbance measurements were used for this purpose because the signal characteristics for some analytes (Sb, Sn) were significantly different for aqueous and hydride deposits of the analytes. For both Sb and Sn, it was necessary to introduce the Pd modifier into the furnace and ensure it was in a reduced state (by reaction with HZ) prior to pipetting the aqueous analyte solution on top of it. Failure to achieve this severely degraded the characteristic mass. It is clear from these data that the overall efficiency of the system is near quantitative. Table 6 highlights the enhancement in system performance which occurs when the analyte Table 5. System performance--Pd

Element

Efficiency (%)

Sensitivity

LOD

&/QA)

&/QA)

100 79 95 100 73

4.0 4.1 4.1 3.6 4.9

3.0 4.8 3.1 1.2 4.8

AS

Se Sn Bi Sb

*Furnace program as in Table 1. A, = peak height recording, QA = integrated signal.

Table 6. Performance enhancements, Pd: graphite*

Element As Se Sn Sb

LOD

Sensitivity

Precision+

(Pd/C)

(Pd/C)

Pd

C

(K)

1.0 8.8 1.2 6.9

1.0 3.1 2.8 3.6

4 2 2 8

2 5 5 5

600 365 235 630

A T&p

*Ratio of figures of merit for A,,, Pd: graphite. +%, at 1 ng level except 0.5 ng for Sb. * Increase in appearance temperature when hydride sequestered onto Pd surface.

Sequestration of volatile element hydrides

673

hydrides are sequestered onto a Pd surface rather than a bare graphite surface. Significant improvements in sensitivities for the Pd-based system likely arise as a result of the matrix modification effect of Pd in preventing losses of volatile precursor species such as dimers and dioxides L-231,coupled with the general shifting of the transient signals to higher tube wall temperatures, thereby promoting more efficient dissociation of any molecular species as well as limiting vapor loss processes to diffusional rather than convective-expansion removal. 3.2. Physico-chemical characterization Apart from characterizing the analytical utility of this technique, it is of interest to address the mechanism of interaction of the PGM’s with the analyte hydrides at elevated temperature. A growing body of information is becoming available concerning the mechanism of (inter)action of Pd modifier with a number of analytes [23-251 which suggests that, in a reduced state, Pd likely forms thermally stable species with the analyte and/or inhibits formation and losses of volatile analyte precursors. In the present work, advantage is taken of the catalytic activity of the PGM’s to scavenge analyte hydrides at low temperature. This action is probably linked with the ability of these metals to participate in hydrogen abstraction reactions. Both Pd and Pt are also capable of absorbing large volumes of molecular hydrogen and palladium hydride phases have been suggested. The observation that reduced Ag, Au, Cu or Ni are incapable of sequestering the analyte hydrides further supports this assumption. Additionally, excess H, and water vapor accompanying the generation of the analyte hydrides are not essential participants in the trapping process. Reduced Pd is capable of trapping As from a stream of dry ASH, in N,. Macroscopic studies using a polished chip of Pd metal revealed formation of a black film on the heated surface (ZOOC) following exposure to dry ASH, in Nz. The same effect was obtained by exposing the surface to H,, suggeting that the ASH, dissociatively chemisorbs onto the Pd surface and the abstracted Hz is incorporated into the Pd metal. This conclusion finds a parallel in the work reported by AL-DAHER and SALEH [26] who studied the interaction of arsine with evaporated metal films. Rapid, dissociative chemisorption onto Pd was reported even at - 80°C. 3.2.1. Atomization of PGM’s. Figure 4 is an electron micrograph illustrating the distribution of a 4 pg residue of Pd which resulted from the deposition of a 20 ~1 volume of a

Fig. 4. Electron micrograph (50000 x magnification) of the residue from a 20~1 volume of a 200 &ml solution of Pd placed on a graphite platform and thermally pretreated at 400°C.

R. E. STURGEONet al.

674

200 pg/ml solution of Pd (IV) onto a graphite platform. The platform was located within the graphite furnace and subjected to a 130°C drying and 400°C thermal pretreatment. Clearly visible are numerous submicron-sized particles of reduced Pd metal. The oxidation state was inferred by Auger analysis due to the absence of any signal from oxygen during sample sputtering. Isolation relevant to the elucidation of the mechanism of atomic vapor formation may be obtained from an examination of the vapor source or supply function [27-291. The supply function may be ex~rimentally isolated from the gross signal transient (which represents the convolution of both the supply and removal functions) by making the removal function much larger than the supply function. The equivalent time constant for the supply function, z,, is defined by [27]: _ _ Co

S(t)dt 2, =

f

0

Smax

NO =-& ”

4nsx

(1)

1

%J

where S(t) is the supply function describing the rate of introduction of atoms into the observation volume and S,, is the maximum value of this function for introduction of a total of No atoms of analyte. Ex~~men~ly, t, is characterized by the ratio of the integrated-to-yak height absorbance, or “normalized” peak area [27]. Figure 5 shows that it is possible to isolate the supply function for Pd (using the furnace heating program detailed in Table 1) if the convective flow rate of Ar through the furnace exceeds about 450 mlfmin. As a result, all subsequent experiments requiring isolation of the supply function were completed using an internal purge gas flow rate in the range 0.65-l l/min. Desorption of analyte from the heated surface may be described by an equation of the form [30]: da - vd’exp(-E,,JRT) z-= where G is the surface coverage, t is the time, v is a pr~x~nenti~ factor, n is the order of release (generally 0 < n < 2) and E, is the activation energy for desorption. The kinetic order of a desorption process provides some information as to the mechanism of vaporization, including insight into the physical dispersion of the analyte on the graphite surface and the relative interaction between the analyte and the substrate [28]. First order desorption may indicate the presence of significant analyte-substrate interaction (large E,), as in the case of dispersed atoms or submonolayers. If there is massive coverage, n=O, indicative of the presence of bulk species or 3-dimensional particles. Monolayered islands or hemispherical droplets or caps can result in fractional orders of l/2 and l/3 or 2/3 for spheres if vaporization rates are governed by availability of edge sites or total surface area. Figure 6 shows that desorption of Pd from the graphite surface appears qualitatively to be a 2/3 order process (cf. Ref. [28]), characterized by a minimal late shifting in the temperature

0.2 -

0

I 0.2

I . 0.6 AROONFLOW, Lfmln .

I 0.4

1

i 0.8

*

1

Fig. 5. Normalized peak area [Eqn (l)] for desorption of Pd as a function of convecti’ve Ar flow rate. Furnace program as given in Table 1.

~~~ira~onofvolatile

element hail

575

ofthe peak of the d~~~~

tract as the surface ~~~e~~ is inert alozlg with B ear& sh~ti~g iir the ap~~~t~~e* This smts the prsRnoe of spherical strmmreson the sarfmsa eonebsin whit% is consistent with the physhl abet

evidence depicted in ‘Fig 4. In addition to Pd, a 2/3 ccrrderdesorption process was &KIwidewed for atomizatim af R and Rh, as shown in Fig,7. Ru was not studied. Micmgraghs of 8%residues were similar to thase noted for Pd, shown in Fig. 6.

676

R. E. fh.JRGEON

et al.

Activation energies for desorption (E,) of 2-4 pg amounts of Pd, Pt, Rh and Ru were obtained using data from the early portions of the leading edges of the signals according to the procedure outlined by STURGEON et al. [31]. These results, along with appearance temperatures ( Tapp), the thermodynamic heats of vaporization of the bulk elements (AH,,,) calculated at temperatures midrange of those used for the E, determinations, as well as the estimated order of desorption, are summarized in Table 7. With the exception of Ru, activation energies are in good agreement with the heats of vaporization of the elements. This follows from the observation of bulk particles being present on the graphite surface. These conclusions, i.e. atomization of the PGM’s is a simple reduced metal vaporization phenomenon (for these sample masses), as well as the measured TBppvalues for these metals are in reasonable agreement with those reported by ROWSTON and OTTAWAY [32]. 3.2.2. Desorption of sequestered hydrides. Figure 8 is a micrograph showing the topology of a 1 pg deposit of As as ASH, on the surface of a polished chip of Pd metal. The chip was placed in the furnace directly opposite the sample dosing hole, heated to 200°C and ASH, (from the generator cell) introduced in the usual manner. The deposit is massive and spongelike but, clearly, non-uniform coverage has resulted, suggesting preferred sites for deposition

Table 7. Atomization of platinum group metals

Element

T:W (K)

E! (kcal/mol)

AH,,, (kcal/mol)

n

Pd Pt Rh Ru

1375 2060 1935 2170

96+5 132k6 136k7 170&5

93(1785 K) 130(2270 K) 132(2240 K) 153(2380K)

213 l/3-2/3 213

* Stop flow, 4 pg samples. t 1 l/min Ar, 2 pg sample.

Fig. 8. Electron micrograph (5000 x magnification) showing residue from a 1 fig deposit of ASH, at 200°C on the qurface of a polished chip of Pd metal.

Sequestration of volatile element hydrides

677

or nucleation and growth. Decomposition of analyte hydrides is a catalytic process and, in practice, once a deposit is initiated on a surface site, more analyte may preferentially deposit onto this nucleus [16]. Extrapolation of this observation to ng quantities of analyte suggests that either a low surface density, highly dispersed submonolayer deposit will occur or a small number of nucleation centers will produce islands or caps of analyte on the PGM surface. The former would appear more likely and is supported by the observation of first-order desorption kinetics for the hydrides from all PGM surfaces studied. Figure 9 shows the desorption profiles for varying amounts of As on both Pt and Pd substrates. The characteristic first-order behavior is manifested in the constancy of peak temperature with changing surface coverage as well as the early shift in TBppas the latter is increased. Table 8 summarizes data from a series of experiments similar to that illustrated by Fig. 9. As the analyte mass and, hence, surface coverage is increased, the Tapp decreases by more than 500 K whereas the temperature at the peak of the desorption pulse remains constant (within experimental error). These observations suggest that the analyte is present on the Pd surface in submonolayer or highly dispersed coverage and that the As interacts strongly with the substrate. Figure 10 shows similar data for desorption of Se, Bi and Sn from Pd. In all cases, desorption of the analyte hydrides from each of the PGM’s is clearly a first-order process. Activation energies for release of As from a Pd surface are summarized in Table 8. These data were derived from the slopes of the Arrhenius plots [33] shown in Fig. 11. Initial desorption energies are constant to at least 50 ng sample masses (well above the normal

0.5

1.0

1.5 TIME, s

@I

-0.5

1.0

1.5

2.0

TIME, s

Fig. 9. Desorption profiles for ASH, as As from (a) Pd and (b) Pt substrates. Furnace program as given in Table 1. Internal purge gas flow of 1 I/min Ar. Analyte mass: 5,10,20,40,80,160 and 320 ng.

R. E.

678

STURGEON

et al.

(4

0

0.5

1.0

1.5

TIME, 8

0

0.a

1

0 05

1.0

1.5

TIME,s

ZO

Fig. 10. Desorption profiles for (a) Se(b) Bi and (c) Sn from a Pd surface. Furnace programas given in Table 1. Aaalyte mass: Se 2,5,10,20,40,80,150 ng; Bi 10,20,30,40,60,100 ng; Sn 5, IO, 20,40,60, 80 ng. Internal Ar purge gas flow: 0.35 l/min for Se, 1 l/min for Bi, 0.65 l/min for Sn.

analytical range of this t~hnique). Above this loading, fiti decreases to a somewhat lower, but constant, value. It is possible that the E, value below 50 ng reflects the desorption energy of As from the Pd surface and above 50 ng the lower E, may be related to the desorption of As off of As, since multilayers of analyte may have been laid down on the Pd. This interpretation is supported by the observation that the E, values for masses > 50 ng appear to converge to the low mass limit at higher temperatures, as the multilayers would he stripped off, exposing the remaining As which is bound only to the Pd. An alternative explanation may lie with the formation of several binary alloys between Pd and As, as illustrated by the data in Table 9 [34]. As the surface composition of the Pd-As

Sequestration of volatile element

hydrides

679

Table 8. Desorption of ASH, (as As) from Pd Mass As (ng) 5

tQ 25 50 200 200 400

i-!&W (K)

7L.k (K)

zo3Q 2985 1895 1810 1760 1625 1500

2340 2295 2310 2335 2380 2360 2350

(iu$mol, 107 102 I04 105 105/x? 73 78

* t I/tin Ar, 4 pg Pd. t Estimated precision, &-30 K.

4.4

4.6

4.3

5.0

5.2 lo4

5.4

IT,

5.6

5.8

6.0

I

i’

Fig, 11. Activation energy plots characterizing release of As from ASH, deposits on 4 JIMreduced Pd. Ar purge gas flow of 1 l/min. Furnace program as in Table 1.

Table 9. Identified binary alloys’ Pd Pd,As Pd,As PdAs,

Pt RAS,

Ru RuAs RuAs,

Rh Rh,As RhAs RhAs, RhAsz

* From Ref. [34].

melt changes (with As loading), the release energetics of As from the different phases may change and, at minimal loading, may correspond to release of As from a Pd, As phase. Desorption energies are specific to the particular As-PGM system, as summarized by the data given in Table 10. For comparative purposes, the graphite system is also included [16-J. Activation energies were derived using Smets method. In all cases, desorption was confirmed as being fhst-order. Specific As-PGM interactions are also evident from a comparison of the signal overlaps between As and ~n~~dual PGh4’s* as shown in Fig_ 12. Release of the bulk of the Pd from the graphite surface begins approximately go0 K earlier than release of As from the Pd surface. Release of both elements occurs at the same temperature for Pt whereas release of As from Ru occurs 300 K lower than release of Ru from graphite.

Table 10. Dasorption of ASH, (as As) From PGM’s’

680

PGM Pd Pt Rh Ru Grapl&e

T.ppW) stop flow 1 l/min 2150 2050 2040 1865 1550

1840 1700 1355 1100 -

J% (kcal/inOl)

105&3 g99+3 B&3 g9+4 86&Z

*2-g ng As stop low, SO-100 ng at 1 t/min, 4 pg PGM, Furnace program as in Table 1.

Fig. I2 Ikxzption

of ASH, as As from (a) Pd, (b) Pt and fc) Ru. Fumaw program as @en in Table 1. Internal purge gas stop.

Sequestration of volatile element hydrides

681

Further evidence of such specific As-Pd interaction is given in Fig. 13, which compares the absorption profiles for Pd in the presence and absence of a relatively massive amount of As. Release of Pd is delayed by 160 K in the presence of the As and presumably reflects the formation of a less volatile Pd-As’ species. Results of Auger microanalysis of ASH, deposits on polished chips of high purity Pd and Pt placed in the furnace revealed that oxygen was associated with the arsenic deposit. Xe-ion sputter cleaning of each sample prior to data collection removed adsorbed oxygen in about 25 s. Analysis of sputter-cleaned Pd and Pt produced no evidence of oxygen whereas ASH,-loaded chips showed a signal for oxygen, the intensity of which paralleled that for As as the surface was sputtered. It would appear that a [Pd-As-O] species is formed on the surface. The identity of this species may be similar to that conjectured by STYRIS [23] to account for the mechanism of stabilization of As by palladium modifier. Although in-depth studies were confined to an examination of the As-Pd system, the data presented in Table 11 also reveal that specific analyte-PGM interactions occur in the other systems as well. For comparative purposes, it should be noted that E, values obtained with aqueous samples in the presence of Pd were similar to the hydrides for As (92 f 4 kcal/mol), Se (55 f 1 kcal/mol) and Bi (58 f 3 kcal/mol). For SnCa9)an E, of 200 + 15 kcal/mol was obtained, distinctly different from the SnH,-Pd system.

0.6

0.6 t a

0.4

9 0.2

0

1.0

0.5

TIME,

I.3

s

Fig. 13. Absorption profile of Pd with/without addition of 1 pg As. Furnace program as given in Table 1, internal purge gas stop.

Table 11. Desorption of analyte hydrides from PGM’s Bit

Se’

PGM Pd Pt Rh Ru graphite

Internal purge

Tapt, (K)

stop flow stop flow stop flow stop flow stop

1825 1465 1495 1270 1645 1255 1825 1465 1460

E. (lccal/mol)

59*3 49*3 39+3 62k3 87&3

TSPP (K) 14150 1380 910 835 845 845 855 790 1140

2 ng Se stop flow; 60 ng Se at 0.75 l/min; 1.25 pg PGM. t 2 ng Bi stop flow; 100 ng Bi at 1 I/min; 0.5 pg PGM. *2-5 ng Sn stop flow; 100-140 ng Sn at 0.65 I/min; 3 pg PGM. 61200 K thermal pretreatment to desorb Bi from graphite. l

SAlEI

44:7-C

Sn*

4 (kcal/mol)

60*5 41_+2 45&4 40&3 60&6

TWP (K) 2225 2150 2095 1590 1775 2070 1815 1620 1590

4 (kcal/mol)

380& 10 57+5 76&3 59&2 74&4

682

R. E. STURGEONet al. 4. CONCLUSIONS

Rapid chemisorption of analyte hydrides on PGM’s coupled with the high thermal stability of the products makes their analytical application attractive. In particular, Pd offers the highest appearance temperatures and relative response over other PGM’s and strengthens its position as a potential universal modifier for GFAAS. PGM’s are present on the graphite surface as bulk 3-D spherical structures of reduced metal which desorb with a 2/3-order dependence on surface coverage. Specific analyte+substrate interactions lead to a first-order desorption for all analyte hydrides sequestered by the PGM’s studied. Acknowledgement-This paper was presented at the First Rio Symposium on Furnace Atomic Absorption Spectrometry, Rio de Janeiro, 18-23 Sept. 1988.

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[27] [28] [29] [30] [31] [32]

[33] [34]

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