Anal. Chem. 1986, 58, 1143-1140 (9) W e n , R. G.; Thornerson, D. R. Analyst (London) 1080, 705. 1137-1156. ( I O ) Dedina, J. Fortschr. Atomspektrom. Spurenanal. 1084, 7 , 29-47. (1 1) Meyer, A.: Hofer, Ch.; Knapp, 0.;Tag, G. Fresenlus ' 2.Anal. Chem . 1081, 305, 1-10,
(12) Andreae, M. 0.; A s m e , J.-F.; Foster, P.: Van't dack, L. Anal. Chem. 1081, 5 3 , 1766-1771. (13) Branch, C. H.; Hutchinson, D. Analyst(London) 1085, 770, 163-167. (14) Drasch, G.;Meyer, L. V.; Kauert, 0. Fresenlus' Z . Anal. Chem. 1080, 304, 141-142. (15) Lee, D. S. Anal. Chem. 1982, 5 4 , 1682-1686. (16) Brovko, I. A.; Tursunov, A,; Rlsh, M. A.; Davlrov, A. D. Zh. Anal. Khim. 1084, 39, 1768-1772. (17)Sturgeon, R. E.; wiiiie, s. N.; Berman, s. s. Anal. chern.1085, 5 7 , 2311-2313.
1143
(18) Reamer, D. C.; Veillon, C.; Tokousbalides. P. T. Anal. Chem. 1081, 53, 245-248. (19) Slu, K. W. M.; Berman, S. S. Anal. Chern. 1083, 5 5 , 1603-1605. (20) Siu, K. W. M.; Berman. S.S. W a n t s 1084, 3 7 , 1010-1012. (21) Reamer, D. C.; Veillon, C. Anal. Chem. 1981, 53, 1192-1195. (22) de Oliveira, E.; McLaren, J. W.; Berman. S.S.Anal. Chern. 1083, 55,
2047-2050. (23) Vijan. P. N.; Leung, D. Anal. Chlm. Acta 1080, 720. 141-146. (24) Welz, 6.; Melcher, M. Analyst (London) 1084, 709, 569-572; 577-579. (25) Welz, 6.; Melcher, M. Vom Wasser 1084. 62, 137-148.
I~C!EIVF,D for review September 17,1985. Accepted November 12, 1985.
Comparison of Furnace Atomization Behavior of Aluminum from Standard and Thorium-Treated L'vov Platforms Thomas W. Brueggemeyer* and Fred L. Fricke Elemental Analysis Research Center, U S . Food and Drug Administration, 1141 Central Parkway, Cincinnati, Ohio 45202
A standard pyrolytlc graphite L'vov platform was compared to one treated by soaking In a 10% thorlum nitrate solution. The half-wldhs d fwnace atomk absorptbn peak profiles for Ai were reduced by as much as a factor of 4 by the modlfled surlace, and the maxlmum pennWble drarrlng temperature was extended by more than 800 O C . The peak area preckh (1.5% relative standard devlatbn for 200 pg of AI) and sensltivHy were comparable to those obtained with untreated platforms. Thls behavlor was stlli observed afler 500 flrlngs of a treated platform at 2450 O C .
Improvements in trace-metal analyses using graphite furnace atomic absorption spectrometry (GFAAS)have been hindered by the lack of fundamental knowledge concerning the high-temperature chemistry taking place in the furnace itself. Due to this uncertainty many analysts have resorted to an empirical approach in their attempts to modify the behavior of analyte in the furnace. Such research necessarily has a trial-and-error appearance-various reagents and conditions examined until success is reached. This has been particularly true in the area of graphite atomizer surface modification. The treatment of graphite with various metals or solutions of their salts has produced new surfaces exhibiting properties markedly different than those of the original graphite. The chief pattern that has emerged is the fact that most metals used to modify graphite are known to form metal carbides. It is not known whether the carbide-forming metals not reported in the literature as surface modifiers have yet to be tried or have been found ineffective. The most commonly reported advantage of a metal-treated surface is an increase in analyte sensitivity (1-8). I t should be pointed out that in some instances various authors explicitly mentioned that peak heights were used in preference to integrated peak areas or did not mention which mode of quantification was used. It is possible in some of these cases that the reported sensitivity improvement for peak heights was in fact a peak sharpening, leaving integrated area un-
changed. Theory (9) predicts that integrated peak area is the better indicator of the extent of free atom formaton. It should also be pointed out that most of the reported sensitivity enhancements were in comparison to nonpyrolytic graphite tubes, which have now been largely supplanted by pyrolytically coated graphite. In some instances the advantages indicated might not have been realized over the newer graphite. The benefits of metal-modified graphite surfaces have extended beyond signal enhancement. It was reported by Norval et al. (10) that a tungsten- or tantalum-treated surface resulting from sputtering of the metal led to an improved resistance of the graphite surface toward oxidizing acids. A similar result was found by Sotera et al. (11) for a borontreated surface. Vickrey et al. found that a zirconium carbide surface allowed organolead (12) and organotin (13) compounds to be determined by using aqueous standards in the furnace while this could not be accomplished on normal graphite. It was reported by Thompson et al. (14) that a La treatment of graphite led to reduced matrix interferences in the determination of P b and Cd, while Hodges (15) stated that a phosphate-induced molecular absorption signal was greatly lowered through the use of a molybdenum-treated surface. In view of the somewhat tentative nature of the proposed mechanisms for the atomization of metals from graphite surfaces, it is understandable that little work has been reported dealing with the modification of these mechanisms upon going to metal-treated surfaces. Muller-Vogt and Wend1 explained that for both Si ( 4 ) and Sn (16) a role of the metal-treated surface may be to inhibit the carbon-induced formation of volatile metal suboxide species. Greater sensitivity and charring stability are thus afforded. Wahab and Chakrabarti (17, 18) discussed the mechanism for Y atomization from a graphite surface modified by Ta, La, and Zr, in addition to investigating atomization from a metal foil surface. The most frequently made mechanistic statement concerning the use of metal carbide atomization surfaces is that the metal additive inhibits the formation of analyte-carbide refractory compounds by preferentially forming these carbides itself. When the analyte is not a known carbide former, however, the role of the surface modifier becomes less clear.
This article not subject to U S . Copyrlght. Published 1986 by the American Chemical Society
1144
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
Perhaps the reducing properties of the surface toward condensed-phase analyte are altered. Salmon and Holcombe (19) have shown that the number of available active sites on a graphite surface affects absorbance peaks for certain elements believed to atomize through reduction by carbon. It is also possible that the mode by which surface modification affects analyte atomization is physical rather than chemical in nature-surface porosity or thermal conductivity alterations, for example. The work to be described in this paper deals with a thorium treatment for pyrolytic graphite L'vov platforms and the subsequent effect on the atomization of aluminum. The work stemmed from the examination of various matrix modifiers for use in aluminum determinations. Manning et al. (20) showed that magnesium nitrate was effective in both stabilizing Al during a charring step and in delaying its atomization. Since thorium oxide is even more refractory than magnesium oxide, it was investigated as a potential modifier. The changes in analyte behavior to be described were observed initially when thorium(1V) nitrate was being added before each firing as a matrix modifier. The continuation of such behavior after thorium nitrate additions were stopped showed that a permanent surface change had occurred. The work to be discussed was a systematic comparison of two types of L'vov platforms-untreated solid pyrolytic graphite platforms and identical platforms that had been treated with thorium nitrate solution. The effects caused by the Th treatment were examined with regard to several analytical parameters: peak size and shape, charring stability, precision, and detection limits. EXPERIMENTAL SECTION Apparatus. All data were taken with a Model 5000 atomic absorption spectrophotometer and Model HGA 500 furnace system (Perkin-ElmerCorp., Norwalk, CT) equipped with Zeeman effect background correction. A Perkin-Elmer Model AS-40 autosampler was used for sample introduction and a Perkin-Elmer Data System 10 for displaying peaks and calculating heights and areas. A factory-calibrated optical pyrometer (Modline 11,220 series, Ircon Corp., Skokie, IL) was focused through the injection port of the furnace tube to allow the monitoring of charring and atomization temperatures. Interchangeable temperature modules in the pyrometer circuitry allowed temperatures from approximately 1000 to 3000 "C to be measured. The pyrolytically coated graphite tubes (Perkin-Elmer 0290-1822) contained grooves for holding the solid pyrolytic graphite L'vov platforms (Perkin-Elmer 10932). Reagents. Argon (99.995% purity, Wright Bros., Cincinnati, OH) was used throughout the study as the furnace purge gas. A commercial standard of 1000 pg/mL aluminum in 2% nitric acid (Spex Industries, Metuchen, NJ) was diluted with 1%v/v nitric acid, doubly distilled from vycor (G. F. Smith Co., Columbus, OH) in a 100-mL polypropylene volumetric flask to give a 1 pg/mL stock solution. On a daily basis this stock solution was diluted with 0.5% v/v nitric acid to the 0.01 pg/mL A1 standard used in the study. This standard was made directly in an autosampler cup (Perkin-Elmer 0290-2089)using a procedure similar to that described by Alcock (21). Deionized,distilled water with a specific resistivity of 18 MQ-cm was obtained from a commercially available system (MiUiporeCorp., Bedford, MA). The thorium(I\r) nitrate used for treating the L'vov platforms was Puratronic grade (Alfa Products, Danvers, MA). CAUTION: Thorium is a mildly radioactive element and, while used in small amounts in this type of work, must nevertheless be treated with caution. Procedure. The 309.3-nm A1 line was used with a low slit setting and a 0.7-nm spectral bandwidth. A hollow cathode lamp current of 20 mA was used throughout the study. Unless otherwise indicatd, 6 s of absorbance data was obtained by the spectrophotometer beginning 1s before atomization and sent to the data station. The first 1 s was used for zeroing the base line and the remaining 5 s for visual display and calculations. The peak display software was modified to calculate the height, area, appearance
Table I. Furnace Parameters Utilized ramp time, hold time, argon flow, S S mL/min
step
temp, "C
drying charring stabilization atomization clean out
130 1000' 1000
10 10 1
2450a
OQ
30 40 20 5
2600
1
3
300 300 300
10n 300 Indicates parameters individually varied during study.
time, and the time of absorbance maximum while the next run was taking place. The Zeeman magnet was programmed to initiate operation 10 s prior to atomization. The furnace program used in the study is shown in Table I. One parameter at a time was selected for study and varied through a range of values while all the others were held at the values given in the table. Duplicate runs were made under each set of conditions. The optical pyrometer was utilized to measure the atomization and charring temperatures. For these two furnace steps, the temperatures reported in the course of the study refer not to the nominal furnace settings but rather to the pyrometer-monitored values. The autosampler was used to dispense AI standards (0.5% nitric acid) onto the platform. Except during the construction of calibration curves or where indicated otherwise, 20 pL of 0.01 pg/mL standard (200 pg of Al) was injected. New solid pyrolytic graphite platforms were soaked in 10 mL of aqueous thorium nitrate solutions of the following weight percentages: 0% (control), 0.1%, 1%, l o % , and 40%. The unstirred solutions were maintained at ambient temperature for 20 h in covered polystyrene disposable beakers. The platforms were then removed from solution and transferred to individual glass beakers and dried at 100 "C in a muffle furnace for 30 min. After insertion into a pyrolytically coated graphite tube (using Perkin-Elmer insertion tool B0112-657),each platform was further dried in the graphite furnace at 150 " C using a 100-8ramp time and a 100-5 hold time. A 1500 "C conditioning step with a 100-5 ramp time and 30-s hold time was then run 3 times. T o complete pretreatment for each platform, deionized water was run 4 times using the program shown in Table I. RESULTS AND DISCUSSION Temperature Measurement. There is considerable controversy as to whether the inside of a graphite furnace tube closely enough approximates an ideal blackbody to permit accurate pyrometric temperature monitoring (22). When a L'vov platform inside the furnace is heating up-and still a t a reduced temperature relative to the walls-the measured results may be in error. However, temperatures recorded after the platform temperature has stabilized should be more accurate. In this study the purpose of reporting the temperatures measured via optical pyrometry rather than simply giving furnace settings is to compensate for long-term temperature drift that can occur as graphite contact rings age. Normally, measured temperature and nominal furnace setting agreed within 100 "C. Platform Pretreatment. Figure 1shows the absorbance profiles for 100 pg of Al atomized from four different platforms using the program shown in Table I. The same graphite tube was used for all four runs shown in the figure. There was no observable difference in the signal from a platform soaked in water only and one used without any treatment, which indicates that water soaking alone did not contribute to the sharpening of the peaks. The integrated areas of the four peaks agreed within 15% despite the obvious differences in peak height and shape. Particularly noteworthy is the symmetry of the sharpest peak. It can be seen that the aluminum peaks begin at essentially the same point in time regardless of the amount of thorium deposited on the platforms. A platform soaked in 40% w/w thorium nitrate solution gave a profile similar to the one from the 10% solution. However,
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
0.;
1145
helphl
B-a 0.0 Y
x
:: 8 m
,-
-
,
,--
0.1
-
I
3
OJ Time
C s e c >
Comparison of absorbance profiles for 100 pg of AI atomized from four different platforms at 2450 O C . Platforms soaked 20 h in the indicated solutions.
Flgure 1.
this platform caused poor signal reproducibility and upon removal from the tube was seen to lack the surface uniformity observed on the other platforms. After being taken through the program in Table I, the thorium-treated platforms were essentially the same black color as standard pyrolytic graphite platforms but possessed a less glossy surface. As a check on these results, the control platform (soaked in deionized water alone) was reinserted into the tube after the other platforms and rerun. An identical profile resulted. This demonstrated that platform differences rather than changes in tube condition were responsible for the oberved signal behavior. A single platform soaked in 10% thorium nitrate was used for all the remaining studies, because it produced the greatest change in A1 atomization behavior combined with good signal reproducibility. Platforms soaked in 10% thorium nitrate solutions seemed to exhibit slight Al contamination as a result of their treatment. This signal generally became quite small (peak area less than 0.005 AU-s) after no more t h m 20 firings. It was also observed that both peak sharpness and signal precision improved after the Th-treated platform aged somewhat. For these reasons, both the standard solid pyrolytic graphite platform and the Th-treated platform were fired 50 times with deionized water using the program in Table I before the comparison studies were begun. Such an extensive break-in would not normally be required. All the studies were completed for one platform before proceeding with the other. Because a new tube was used for each platform, and the various segments of the study completed in the same order for each platform, tube and platform aging should have been very similar for both at any given stage of the comparisons. Atomization Temperature Study. In the atomization temperature study a comparison was made between the standard and Th-treated platforms in terms of signal size and shape as a function of atomization temperature. All other parameters were kept constant at the values given in Table I. It can be seen in Figure 2 that for 200 pg of A1 with low atomization temperatures, larger integrated areas were obtained with the Th-treated platform than with the standard one. At an atomization temperature of 2350 "C,the areas were equivalent for the two platforms. At higher temperatures the areas became larger for the standard platform by a factor of about 30%. With either platform, a leveling of the area vs. temperature curves was seen at higher atomization temperatures. However, the platform that had been treated in 10% thorium nitrate exhibited less temperature dependency
~
z 100
zaoo
zroo
2600
A T O M 12A T I O N TE M PE R A TUR E
I
( 'C
Flgure 2. Peak heights and areas as functions of atomization temperature for 200 pg of AI. Boxes represent peak helghts; circles denote peak areas (AU-s). STANDARD PLATFORM
Th-TREATED PLATFORM
0-31
h
I: 0
n
8-31
4 Q
n
0. 3
+,-,-,----,
e
:
0 ~
!
5 0 l - i m e
:
!
t
5
Absorbance profiles for 200 pg of AI at three atomization temperatures from standard and treated platforms. Figure 3.
throughout the range examined when peak areas were considered. The results seen when peak heights are examihed are more interesting. Peaks were significantly higher at all temperatures with the Th-treated platform. When this platform was used the peak height stopped increasing above about 2450 O C , while the heights obtained from the standard platform were still increasing at 2700 "C. Figure 3 shows actual peak profiles from the atomization study. The fact that both heights and areas are more dependent upon atomization temperature when using the standard platform is apparent. Also obvious is the fact that the aluminum peaks resulting from atomization off the standard platform change shape considerably with increasing temperatures-something which happens to a lesser extent for the Th-treated platform. A parameter that can be used to estimate peak widths is the area-to-height ratio (AIH). The value of A / H is equal
1146
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
='or
\o
1;
0 S E C O N D RAMP
\
\ \
-=.
'-0
-nI
I
t
0
6
0 s t a n d l r d plaltorrn
- _ _ _ _--_ ----___
-----4 ..............-......... ......................
aioo
p :
1 SECOND RAMP
Arealheight ratio (half-width) of 200-pg AI peaks as a function of atomization temperature. Flgure 4.
to the half-width for a triangular peak. Figure 4 shows that A / H is considerably less for peaks from the Th-treated platform than for those from the standard platform. At 2160 "C there is a 5-fold decrease in half-width when using the treated platform. At 2700 "C, this factor has been reduced to 3-largely because peaks from the standard graphite have continued to sharpen with increasing temperature. It is noteworthy that the peaks from the Th-treated platforms (lower curve, Figure 4) show little tendency to become narrower as the temperature of atomization is increased. If the mechanism for A1 atomization from this modified surface is such that the atom formation process is very rapid relative to atom removal processes, then the absorbance profile is expected to approach the shape of the atom removal function (23). Should this be the case, further increases in the rate of atom formation due to higher temperatures would have a minimal effect on the calculated peak width, because the diffusion of atoms from the furnace would be the limiting process. Any further peak sharpening would be related to the temperature dependence of atomic diffusion coefficients; i.e., A1 atoms should exit the furnace more rapidly at higher temperatures. It should also be pointed out that 1 s or more is required for the platform to heat from the stabilization temperature of 1000 " C to the final atomization temperature. In the Perkin-Elmer Model 500 furnace, maximum power heating is used to reach the preset atomization temperature, at which point a lower voltage is automatically applied to maintain this temperature. If release from the Th-treated platform surface is rapid, the aluminum may already be in the gaseous atomic form before the platform temperature stabilizes. Therefore, the use of still higher temperatures for atomization would not affect the rate of atom formation, but only the residence time of atomic vapor in the system. However, assuming a slower rate of atom formation from a standard nontreated platform, it is certainly conceivable that atom formation would not be complete when the temperature of the platform levels off. Therefore a higher setting of the atomization temperature would cause aluminum still on the surface to experience enhanced vaporization kinetics. This could explain why A1 peak heights continue to increase with temperature on the standard surface, yet reach a maximum at 2460 " C on the Th-treated surface (see Figure 2). Heating Rate and Flow Study. In the next segment of the study, the effects of furnace heating rate and purge gas flow rate upQn the size and shape of the absorbance profiles were examined. Again, the standard platform was untreated solid pyrolytic graphite, and the treated platform was the one
,'.\,
--
T ~ - ~ ~ * * l P. lal t f D l r n
................................. 0- ---/---
Time
Absorbance profiles for 200 pg of AI atomized at 2450 "C at two heating rates and three argon flow rates: (- - -) 0 mL/min, (-) 30 mL/min, 100 mL/min. Flgure 5.
( e e e)
soaked in 10% thorium nitrate. Two furnace heating ramps were examined, with the final atomization temperature being approximately 2450 "C in either case. The 0-s ramp setting actually requires about 1s to heat the furnace from 1000 to 2450 "C. This heating ramp abruptly levels at the preset atomization temperature when the built-in pyrometric feedback system switches the appIied voltage to a lower level. The 1-s ramp setting uses a single voltage and does not utilize the feedback system. The temperature asymptotically approaches its final value. As can be seen in Figure 5, this lower rate of heating causes a 2-3-9 peak delay relative to the 0-s ramp setting (note the difference in values on the time axis-the numbers represent the elapsed time after the start of the atomization step). This peak delay seems unrelated to purge gas flow rate, but appears lengthier on the standard platform. It is also evident that the lower rate of heating leads to a decrease in heights and areas at all flow rates and for both surfaces. As might be expected, the 1-s ramp setting causes peak broadening relative to the 0-s setting regardless of the flow or the nature of the platform. Figure 5 illustrates that on both surfaces and at both heating rates peak heights and integrated areas are reduced in size as the flow of argon purge gas is increased. It is also seen that the increase in flow lowers the A / H ratio, or sharpens the peak. Both the lowering of signal and the narrowing of peaks are accounted for by the shortened residence time of aluminum atoms in the furnace. In addition, a lower gas-phase temperature is expected as flow is increased. When the 0-s ramp setting was used (upper two plots in Figure 5), the peaks at 100 mL/min were about 40% as large as those at 0 mL/min with either of the two platforms. However, when the slower rate of heating was employed, an increase in purge flow to 100 mL/min removed nearly all the signal. It is likely that the slower heating of the platform surface combined with a more rapid flow of gas causes a loss of vaporized analyte in molecular form before actual atom formation can occur. With a higher heating rate or with a lower purge gas flow, analyte-containing molecules would still be within the confines of the furnace when a temperature sufficient for their dissociation to atoms was reached. Such an idea is consistent with the mechanism
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1147
Table 11. Precision and Detection Limits platform thorium untreated treated
level, pg of A1 mode
N
200
height av % RSD"
\ \ \
detection limitb
\
b I IO00
ooo
1400
CHAllllNO 1 I U P L l A l U R l
*eo0
I 1800
1%)
0.198 AU 2.1%
0.073 AU.5
0.046
1.8%
1.5%
height av % RSD area av % RSD height area
0.0107 AU
0.0268 AU 6.4% 0.0061 AU-s
6.8% 0.0074 10.2%
AU*S
5.8 Pg 5.7 Pg
AU.5
6.8% 1.8 Pg 4.8 Pg
Percent relative standard derivation. *Defined as the amount of analyte giving a signal equal to 3 times the standard deviation of 10 blank firings (each 20 p L of 0.5% nitric acid).
Figure 6. Peak area for 200 pg of AI atomized at 2450 "C as a function of charring temperature.
proposed for aluminum atom formation by Sturgeon et al. (24) in which the analyte leaves the graphite surface as an oxide molecule, A1203,and subsequently decomposes to yield aluminum atoms. Exactly how the thorium treatment of a pyrolytic graphite surface might be affecting such a mechanism is not entirely clear, beyond the fact that the rate of atom formation is evidently increased. Char Study. The next part of the comparison between the standard platform and the one soaked in 10% thorium nitrate involved varying the charring temperature while holding all other furnace parameters a t the values given in Table I. The purpose of a stabilization step a t 1000 "C between charring and atomization was to ensure that the atomization ramp would not change as charring temperatures were varied. Thus, changes seen in peak size, shape, and appearance time could be attributed solely to charring temperature and not to atomization ramps beginning at various temperature levels. Figure 6 shows the resulting atomization signal (integrated peak areas) after 200 pg of A1 was charred at various temperatures for 40 s. Clearly, A1 is afforded significantly more thermal stability on the Th-treated platform. A charring temperature of 1600 "C, for example, caused almost complete loss of A1 from the standard platform and yet little or no loss from the treated platform. The area maximum seen with the Th-treated platform between 1400 and 1500 "C was observed on a number of occasions. When peak heights were plotted against charring temperatures, a local minimum occurred a t this same temperature. Thus, peaks resulting from this charring temperature were broader-larger area, smaller height-than peaks from higher or lower temperatures. This observation cannot be readily explained. It is interesting that A1 is more thermally stable on the Th-treated surface, yet when released during atomization, appears in such a sharp peak. The upper half of Figure 5 shows that the onset of atomization occurs at about the same time (and presumably the same temperature) for either platform. Since the peaks from the standard platform are broad, it would appear that a strong interaction existed here between analyte and the surface. However, A1 is lost from the standard platform at a far lower charring temperature than from the Th-treated one. Seemingly, the greater thermodynamic stability of A1 on the treated platform is accompanied by very rapid kinetics of release, but not before a sufficiently high temperature is reached. Because identical tubes were used with the two platforms (new, commercially available, pyrolytically coated graphite) the observed differences must stem from platforms only and not from differing interactions with the tube walls. Therefore, while an atom transport mechanism that includes interaction of vaporized analyte with
AU
av % RSD
area 20
0.076 2.8%
"1
I
' 1
-!I
STANDARD PLATFORM
Th-TREATED PLATFORM
2
0 Time
C s e c >
Figure 7. Absorbance profiles for 20 pg of AI and a 0.5% nitric acid blank using standard and treated platforms: 2450 "C atomization temperature.
the tube walls may be involved (25),it cannot explain the differences observed between the two platforms. The fact that modification of the standard solid pyrolytic graphite platform by thorium treatment caused large changes in both charring stability and peak shape would seem to relate to the idea of carbon involvement in the A1 atomization mechanism as described by L'vov and Savin (26). A faster surface-effected reduction of A1203should lead to sharper peaks. On the other hand, the fact that atomization kinetics differ for the two surfaces does not preclude the gas-phase dissociation of A1203molecules as the route to free A1 atoms (24). The rate of desorption of aluminum oxide from the platform could be a function of the microstructure and chemical nature of the surface-both of which could be modified by thorium treatment. Precision, Detection Limits, and Linearity. The precision levels obtained for A1 atomization from the two platforms were compared a t the 200-pg and 20-pg levels. Table I1 summarizes the results. Relative standard deviations were slightly better for the Th-treated platform using either heights or areas, and detection limits somewhat improved also. Figure 7 shows the atomization profiles that resulted when 20 pg of A1 (20 pL of a 1.0 ppb solution) was injected onto the two platforms. While integrated areas were similar, the greater sharpness of the peak from the Th-treated platform causes it to standout more sharply above the blank signal. In
1148
Anal. Chem. 1988, 58, 1148-1152
theory at least, peak sharpening gives better detection limits by permitting the use of a narrower integration window encompassing a lesser noise contribution. Calibration curves were constructed for each of the two platforms. With the standard platform the slope for peak areas was 0.000 39 AU.s/pg for a linear range of 0-1000 pg. With the platform treated in 10% thorium nitrate, the slope was 0.000 28 AU.s/pg over a linear range extending up to 500 pg. For both platforms the correlation coefficients were 0.999. The earlier onset of curvature for the Th-treated platform can be understood in terms of peak shape. For a given integrated area, absorbance peaks produced from the treated platform are about 3 times as high as those from a standard platform. Tall, narrow peaks reach an absorbance level exhibiting nonlinearity at a lower analyte level than do low, broad peaks. For either platform, the use of Zeeman effect background correction with its attendant rollover tendency restricts the linear range to a peak height of about 0.5 AU. The longevity of the Th-treated platforms was tested by monitoring the absorbance signal from 20 pL of 0.01 pg/mL A1 in 0.5% nitric acid using the program in Table I. After 500 firings the platform appearance had very definitely degraded in terms of surface regularity. In spite of this, the relative standard deviation was approximately 5% for 500 firings and often as low as 1% for 10 consecutive runs-even after the onset of platform deterioration. Peak half-widths ( A / H ) dropped from 0.39 s to 0.31 s during the 500 runs, showing that peaks were actually becoming sharper as a function of platform aging. The comparison of aluminum atomization from standard and Th-treated L’vov platforms demonstrates the chemical modification of a pyrolytic graphite platform surface can markedly affect a number of parameters. Most striking is the effect on peak shape due to enhanced rate of atom formation. Sharper peaks should prove particularly beneficial when AI is being determined at low analyte levels. Also of potential analytical value is the enhanced charring stability obtained on the Th-treated surface. Such a feature could allow higher
charring temperatures when interfering components are present in a real sample.
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RECEIVED for review October 21,1985. Accepted December 26, 1985.
Light-Scattering Method Combined with Production of Fine Particles by Photochemical Reaction for Determination of Silane in Air Hiroaki Tao,* Akira Miyazaki, and Kenji Bansho National Research Institute for Pollution and Resources, Yatabe, Ibaraki 305, Japan Flne particles of siilca are produced by irradlatlng the mixture of sllane and ozone with light from 200 to 320 nm. A mechanism that Involves the formation of silica nuclei by oxldatlon of sllane with ozone in the gaseous phase and the growth of them by a heterogeneous reaction of silane with the O,(’A) or O( ‘D) species, which occurs on the surface of the nuclei, Is proposed. By combination of the productlon of fine particles with the light-scattering method, the determination of sllane below 5 ppmv, the threshold llml value recommended by the American Conference of Governmental Industrlal Hygienists, can be easily carried out with only 1 mL of sample.
Silane has been recently used for epitaxial and thin-film deposits in the semiconductor industry, and it is predicted
that the amount will increase much more in the future. Since silane is toxic and spontaneously flammable in air, a rapid and sensitive monitoring is desired. However, there have been only a few monitoring instruments or sensors developed so far, Le., a chemiluminescence detector with ozone oxidation, a galvanic cell with a gas-permeable membrane, and an indirect atomic absorption spectrometer utilizing reduction of mercury oxide with silane followed by the mercury determination (I). Moreover, these methods do not necessarily satisfy the requirements for a monitor such as high sensitivity, rapid response, and simple operation. In a previous paper (2) the authors have described a sensitive method for volatile hydrides such as phosphine and arsine that is based on the production of fine particles by photochemical reaction with oxygen followed by light-scattering detection of the particles. In the method, for example,
0 1986 Amerlcan Chemical Soclety 0003-2700/86/0358-1148$01.50/0
