JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1991 VOL. 6 105 Temperature Programmed Static Secondary Ion Mass Spectrometric Study of Phosphate Chemical Modifiers in Electrothermal Atomizers* D. Christian Hassell Vahid Majidit and James A. Holcombe* Department of Chemistry and Biochemistry The University of Texas at Austin Austin TX 78772 USA Temperature programmed static secondary ion mass spectrometry is used to investigate surface chemical reactions of phosphate chemical modifiers used for the determination of Cd and Ag in electrothermal atomic absorption spectrometry. Cadmium-xyphosphorus reactions are initiated on the surface at dry cycle temperatures with further stabilizing reactions occurring on the surface in the char cycle temperature range. This leads to a delay in the atomization of Cd to a higher appearance temperature.However for the determination of Ag addition of phosphate results in an attenuation in the atomic absorption spectrometric signal intensity with no change in appearance temperature. Similar Ag-xyphosphorus surface reactions are not observed. Keywords Secondary ion mass spectrometry; electrothermal atomic absorption spectrometry; phosphate chemical modifier; cadmium; silver Various analytical techniques have been used in attempts to elucidate electrothermal atomizer surface reactions. These include electron microscopy,I X-ray crystallography2 and surface spectroscopies such as electron spectroscopy for chemical analysis' and A ~ g e r . ~ While these techniques have been useful in many mechanistic studies they are relatively insensitive to the sub-ppm analyte concentrations normally encountered in electrothermal atomic absorption spectro- metry (ETAAS).A further drawback is that reactions have not been studied during the actual thermal heating of the surface (i.e. during the drying or charring cycles) since the sample surfaces were usually cooled prior to analysis. Prod- ucts stable at these elevated temperatures might react to form new products when cooled or undergo a reaction upon contact with the atmosphere during transfer between instru- ments. Temperature programmed static secondary ion mass spectrometry (TPS-SIMS) was used in order to examine A B C D Translational rod Fig. 1 TPS-SIMS sample introduction system A Cu gasket; B conflat- flanged stainless-steel chamber; C polytetrafluoroethylene spacer; D.Furon seal; E port to rotary-vane roughing vacuum pump; F port to tur- bomolecular vacuum pump; G graphite sample platform; H thermocou- ple; I Ta strip heater; and J Cu electrodes * Presented at the Fifth Biennial National Atomic Spectroscopy Sympo- t Present address Department of Chemistry. University of Kentucky. $ To whom correspondence should be addressed. sium (BNASS). Loughborough UK 18th-20th July. 1990. Lexington. KY 40506 USA. surface reactions occurring during the thermal treatment. The technique combined high sensitivity with minimal surface perturbation hence permitting surface interrogation using analyte concentrations more closely associated with routine ETAAS. An investigation of the chemical role of phosphates used as chemical modifiers with ETAAS served both to gain insights into reaction mechanisms and to evaluate the utility of TPS-SIMS.Temperature programmed thermal desorption mass spectrometry (TP-TDMS) a method used to study gas- phase electrothermal atomization reaction products,s.6 was also utilized to support conclusions drawn from the TPS- SIMS data. Phosphates are often used for the determination of Cd Pb and Zn leading to a delay in atomization to higher tempera- tures and removal of more volatile interfering matrices during the thermal pre-treatment steps. Czobik and Matou- sek7 originally postulated the formation of a metal pyrophos- phate which then decomposed to release the free metal vapour. While the existence of a metal-oxyphosphorus com- pound was later corroborated by Bass and Holcombe6 and Wendl and Miiller-Vogt,2 it is unclear whether the stabiliza- tion occurs via a gas-phase reaction or on the graphite surface.Experimental Apparatus and Solutions The TPS-SIMS system consists of an extractor type ion gun for generation of primary ions (Leybold-Heraeus Model IQE 12/38). The secondary-ion optics include an electrostatic einzel lens pre-filter and a quadrupole mass spectrometer with pulse-counting detection and an effective mass range of 2-456 u (Leybold Vacuum Products Export PA USA). Turbomolecular pumps maintain a base pressure of 1.3 x Pa. The stainless steel ultra-high vacuum chambers transla- tional rod and heating-block assembly were designed and manufactured in the University of Texas Chemistry Depart- ment. The sample introduction system is illustrated in Fig.1. The heating block consists of a corrugated tantalum strip heater po- sitioned between two copper electrodes and a thermocouple temperature probe in contact with the pyrolytic graphite coated graphite sample platform (10 x 5 x I mm Stackpole/ Ultracarbon Bay City MI USA). The platform is secured to the heating block directly above the tantalum strip with a stain- less-steel screw and ceramic washer thereby ensuring that heating is predominantly by radiation rather than conduc- tion. The tantalum strip heater and thermocouple are connect-106 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1991 VOL. 6 ed to a feedback and proportional control circuit which permits programmable heating and cooling.Also shown in Fig. 1 are the two separate pumping stages which isolate the main ultra-high vacuum chamber from atmosphere. The precision ground stainless steel translational rod is sealed by spring-energized polymer seals (Furon Los Alamitos CA USA) such that when the heating block is trans- lated from atmosphere it first passes through the roughing- pumped chamber then through the turbomolecular-pumped chamber before being positioned for analysis in the main ultra-high vacuum chamber. This differentially pumped ar- rangement allows rapid sample introduction and pump-down to working pressure (<1.3 x 104 Pa) within approximately 15 min. Mass scanning and data collection are performed with software written in ASYST (Asyst Software Technologies Rochester NY USA) running on a PC-AT compatible com- puter in conjunction with a Keithley Series 500 data acquisi- tion and control system (Keithley Instruments Cleveland OH USA) which incorporates analogue to digital digital to analogue and pulse counting interface boards. The software permits full mass spectral scanning multiple individual ion monitoring (i.e.‘mass hopping’) selective scaling auto- matic gain and numerous data reduction and analysis capa- bilities. The TP-TDMS system consists of a quadrupole mass analy- ser with a 70 eV electron impact ionizer and a programmable peak selector (Uthe Technology International Sunnyvale CA USA). An Apple 11+ computer is used for data collection and control of the heating programme which is capable of reach- ing temperatures of 3000 K.This system has been described previously in greater detaiL6 Electrothermal AAS studies were performed on a Varian GTA-95 graphite tube atomizer and an AA-875 spectrometer interfaced to a PC compatible computer via a Keithley 570 data acquisition system. ASYST software was used for data collection and analysis. Vaporization was from the wall of a standard pyrolytic graphite coated graphite tube. The cadmium and phosphate stock solutions were prepared by dissolving ACS reagent grade Cd(N03)2 and NH4H,P04 in distilled de-ionized water. Working solutions were prepared by serial dilution of the stock solutions. Procedure For the TPS-SIMS studies 2 p1 aliquots of the aqueous solu- tions were deposited on a pyrolytic graphite coated graphite platform and allowed to dry at ambient temperatures.The sample platform was then secured to the heating block assem- bly and translated through the differentially pumped vacuum stages into the main vacuum chamber for analysis. The tem- perature range of the system used in this study was between ambient and 900 K thus permitting mechanistic studies in the dry and char cycle regimes. The heating rate was 1 K s-I. The primary Ar+ ion beam current was 1 nA at 3 keV using a 1 0 p m spot. The beam was rastered in the x-y plane to achieve a scan area of 4 mm2 thus ensuring a static mode of operation. ‘Static’ implies that at this low ion flux only the outermost surface layers are probed; furthermore this mode is considered to be non-destructive since total surface damage to the scan area due to primary ion collisions is minimized. For the TP-TDMS studies the sample was deposited on a pyrolytic graphite coated graphite platform and thermally treated in the pre-treatment chamber under nitrogen at atmo- spheric pressure.This chamber was evacuated to <I .3 x Pa before the sample was translated into the high-vacuum ana- lysis chamber and positioned below the quadrupole. When a pressure of 2.6 x 1W Pa was achieved the sample was ato- mized and subsequently detected by an electron multiplier located at the end of a quadrupole mass analyser. 1 .00 c 0.80 u) 3 * 4 0.60 : 0.40 - (0 C CII .- w .- - Q g 0.20 z 0 .I Cd+ I CdNO’ CdNO,’ Cd,’ Cd,O’ Cdo+ x 100 k+ 100 150 200 250 300 m/z Fig. 2 graphite coated graphite surface T = 298 K Secondary ion mass spectrum (positive) Cd(N03)? on pyrolytic c C I 0.80 - 2 0.60 - c a 0 C 0 v) .- 2 0.40 - E ; 0.20 - N (0 .- - z CdO+Cd p+ OIUC’ - 100 150 200 250 300 m/z Fig.3 Secondary ion mass spectrum (positive) Cd(NO3)? and NH4H2PO4 on pyrolytic graphite coated graphite surface T = 298 K Results and Discussion Fig. 2 is static SIMS spectrum of 260 ng of Cd [as Cd(N03)2] on a graphite surface at 298 K without any prior heating. While the Cd isotope peaks dominate the oxide and nitrate ions and their fragments are clearly evident. (The presence of dimer ions and associated oxides does not necessarily indicate the presence of dimers on the surface; these are often an arti- fact of secondary ion collisions between nearest neighbour surface or second-layer species.*) With the addition of 1200 ng of NH4H2P04 to the original aqueous solution (Fig.3) Cd(N0,)2 and its fragments are not detected and CdPO,+ species are evident. The CdO+ peaks are diminished to 25% of their former intensity. These observations clearly indicate the formation of a Cd-oxyphosphorus compound on the surface by the end of the desolvation step. Fig. 4 displays the signal from several Cd species monitored during sample heating. Between 340 and 410 K the Cd+ signal which could include a small contribution of daughter fragments in addition to the ionized free Cd decreases rapidly. In this local region the decrease of the Cd+ signal is accom- panied by an increase of the CdP02+ signal. This suggests a chemical coupling of these two species and a nearly complete conversion into a surface-bound CdPO species by 400 K.The increase in the CdPO+ intensity at still higher tempera- tures suggests interconversion of the CdPO species. The dis- similar thermal behaviour of CdPO+ and CdP02+ also indicates that these two are not simply daughter fragments of the same higher order CdP,O species on the surface. While changes in surface character or composition can alter the ionization cross-JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1991 VOL. 6 1 07 10.0 > 70.0 c1 .- v) Q c - 50.0 > .- c - 30.0 4 - ..--’ I I I I I I I Fig. 4 TPS-SIMS Cd(NO3I2 and NH4H2P04 on graphite surface heated at 1 K s-l. A Cd+; B CdP02+; and C CdPO+ 400 300 200 - v) 4- ‘E 100 2 I 2 0 - k lo00 .- E v) 800 c - 600 400 200 n 300 600 900 1200 1500 1800 2100 TIK Fig.5 TP-TDMS temperature programme of 300 K s-I. (a) Cd(N03)2 on graphite surface; and (h) Cd(N03)* and NH4H2P04 on graphite surface section of surface species and thus significantly affect the SIMS signal inten~ities,~ the dissimilarity between CdPO+ and CdP02+ suggests that such ‘SIMS matrix effects’ are not a logical first choice to explain the signal intensity variation with temperature. By using the TP-TDMS system the gaseous products also can be monitored during a temperature ramp (Fig. 5 ) . The Cd+ and CdN03+ peaks at 650 K in the absence of phosphate [Fig. 5 (a)] are probably due to crystal fracturing which is often ob- served while heating salts in a vacuum.6 The first PO2+ peak centred at 500 K in the presence of phosphate [Fig.5(6)] is the result of vaporization of excess of phosphate. The absence of CdN03+ peaks in the presence of phosphate substantiates the previous suggestions from the SIMS observations of rapid low- temperature conversion of any nitrate species into the more stable oxyphosphorus compound. The observed delay in peak atomization temperature from 970 K [Fig. 5(a)] to 1330 K [Fig. 5(b)] is consistent with the appearance temperature shift of the Cd atomic absorption signal in ETAAS. The P307+ peak is coin- cident with the Cd+ peak and indicates the decomposition of a higher order CdP,.O species which might exist on the surface prior to atomization; however it is possible that a relatively low ionization cross-section of such a large molecule combined 1 .oo r 0.80 cn 3 L C c1 CI 8 0.60 .- 0.40 A E 0.20 b .- - 0 z 0 1 I I 1 J 100 150 200 250 300 m/z Fig.6 SIMS spectrum (positive) AgN03 and NH4H2P04 on graphite surface T = 298 K 0.30 A 0 1 .o 2.0 3.0 4.0 t/S Fig. 7 ETAAS Ag absorbance profiles (328.1 nm 400 K s-I thermal ramp from 673 K) for A 0.1 ng Ag; and B 0.1 ng Ag and 100 ng Pod3+ as NH4H2P04 with the low transmission efficiency of the TPS-SIMS system at higher masses might preclude its detection on the surface. The static SIMS data suggest that relatively low temperature reactions (i.e. within the drying cycle region) ‘stabilize’ the Cd in the presence of phosphates although thermal pre- treatment beyond 500 K is required to remove the bulk of the unreacted phosphate in order to minimize gas phase chemical and spectral interferences.l o Silver has been reported as an element which is ‘stabilized’ by phosphates in a fashion similar to that observed for Cd.7-11 However the static SIMS studies of Ag with NH4H2P04 (Fig. 6) show no significant Ag analogues of the CdP,.O species. Repeating conventional ETAAS studies of Ag with NH4H2P04 has shown no shift in the appearance temperature but rather has demonstrated an attenuation of the Ag atomic absorption signal with the addition of NH,H,PO,; for Cd the shift to a higher appearance temperature with the addition of NH4H2P04 is not accompanied by such attenuation. Fig. 7 illustrates typical absorbance profiles for Ag with and without the addition of NH4H2P04 modifier at a thermal ramp-rate of 400 K SKI. Slower ramp-rates result in further attenuation which is consistent with gas-phase interference since the vapour temperature is not sufficiently high to promote dissoca- tion prior to diffusional loss.Thus contrary to ‘accepted dogma’ no chemical basis exists for the practice of using phosphate modifiers for the determination of Ag. Although the ammonium cation might help remove any interfering chloride matrix as NH,Cl(g) indiscriminate use of phosphate modifiers for the determination of Ag might actually reduce analytical sensitivity and accuracy.108 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY MARCH 1991 VOL. 6 In summary while a mechanism cannot be assigned based upon these data it is clear that Cd-oxyphosphorus reactions are initiated on the graphite surface at relatively low tempera- tures during or immediately following desolvation.Intercon- versions of CdP,O species continue to occur at char cycle temperatures. Finally based on both TPS-SIMS and ETAAS data phosphate chemical modifiers are relatively counter- productive for the determination of Ag. This work was supported by National Science Foundation grant CHE-8704024. References 1 Welz B. and Schlernmer G. and Ortner H. M. frog. Anal. Spec- trosc. 1989 12 1 1 1. 2 Wend] W. and Muller-Vogt G. J. Anal. At. Spectrom. 1988,3,63. 3 4 9 10 1 1 Sabbatini L. and Tessari G. Ann. Chim. (Rome) 1984,74,779. Wu S. Chakrabarti C. L. Marcantonio F. and Headrick K. L. Specwochim. Acta Part B 1986,41,65 1 Styris D. L. and Kaye J. H. Spectrochim. Acta Part B 1981,36,41. Bass. D. A. and Holcornbe J. A. Anal. Chem. 1987 59,974. Czobik E. J. and Matousek J. P.. Talanta 1977,24 573. Benninghoven A. Rudenauer F. G. and Werner H. W. Secmdury ion Mass Spectrometry Wiley New York 1987 p. 215. Benninghoven A. Rudenauer F. G. and Werner H. W. Secondary Ion Mass Specmmem-y Wiley New York 1987 p. 824. Ohlsson K. E. A. and Frech W. J. Anal. At. Spectrom. 1989,4,379. Slavin W. Carnrick G. R. Manning D. C. and Pruszkowska E. At. Spectrosc. 1983,4 69. Paper 0104009F Received September 4th I990 Accepted October 12th 1990