18!00
Anal. (>hem. 1981, 53, 1899-1902 (8) Newcomb, M.; Cram, D. J . Am. Chem. SOC.1977, 97, 1257-1259. (9) Nakamura, H.; Takagl, M.; Ueno, K. Tdanta 1979, 28, 921-927. (10) Heo, G. S.; Bartsch, R. A.; Schlobohm, L. L.; Lee, J. 0. J. Ow. Chem. 1981, 48, 3574-3575. (11) King, E. J. J . Am. Chem. SOC.1980, 82, 3575-3578. (12) Lamb, J. D.; Iaatt, R. M.; Robertson, P. A.; Christensen, J J. J . Am. Chem. SOC. 1980, 702,2452-2454. (13) !$Ore,W. E., Amman, D.; Blsslg, R.; Pretsch, E.; Slnion, W. In Progress in Macrocyclic Chemlstry"; Izatt, R. M., Christensen, J. J., Eds.; Wlley-Intersclence~New York, 1979; Uol. 1, p 9. (14) Flett, D. S.; Jeycock, M. J. "Ion Exchange and Solvent Ilxtraction";
Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker: New York. Vol. 3, Chapter I.
1978;
RECEIVED for review April 13, 1981. Accepted July 7, 19811. This research was supported by the Department of Energy (Contract DE-AS0~-80ER-10604)and the Texas Tech university center for Energy Research (postdoctoral ship to J.S.).
Scanning Laser Mass Spectrometry for Trace Level Solute Concentration Profiles R. J. Conzemius," F. A. Schmidt, and H. J. Sirec Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Iowa 5001 1
Scanning laser mass spectrometry (SLMS) is shown to support solld-state etudles of mlgration of trace level c;olutes In sollds. SLMS possesses the spatial chemlcal analysis capabllltles necessary for these studles. Nuclldes present In the solid speclmen at less than 10 parts-permlllion atomic (ppma) are measured accurately wlth ordinary faraday Ion detectors. Spatlal resolutlon for these studies is on the order of 25-50 pm. Quantlficatlon Is demonstrated wlth standards where a relative deviatlon of a mean calibration factor 11s 1.8%. Scanning samples are achleved by sequentlai steppilng or by a dynamic measurlng technique. Several dlfferent solutes and solld matrlces are measured concerned with actual solid-state experiments invoivlng electric moblllty and chemical dtffuslon.
Table I. SLMS Experimental Parameters Spectrograph-Constructed in the Ames Laboratory accelerating potential 20 kV magnetic field 2-8 kG as required
lo-* torr lo-' torr
vacuum: analyzer ion uource object slit
1000 Mm
Lasel-Holobeam Model lamp excitation spot diameter energy per pulse pulse width pulse repetition rate computed power density
255 QT 19 A 12 pm 0.001 J -100 Ils 1000 Hz - l o 9 W cm-?
-
Ion Detectors Knowledge of the movement of metallic solutes in solids is important in the utilization of materials in advanced energy systems, purification of imetals, and in the study and understanding of solid-state theory. Measuring behavioral properties such as thermal diffusivity and electric mobility requires special analytical techniques. High analytical senciitivity i s needed because solute concentrations must be kept low, well below their solubility limits. Reasonably high spatial resolution is needed to permit flexibility in choosing expcrimental parameters such as time, temperature, and specimen preparation for the solid-state experiment. The most straightforward means of evaluating the result of the solid-state experiment i s to slice the specimen into sections followed by dissolution and application of an appropriate classical analytical procedure. More recently, instrumental techniques such as auger electron spectroscopy, low-energy electron diffraction, Rutherford ion backscattering, secondary ion mass spec1rometry, and others have been used (I) as analytical tools to evaluate results of such solid-state experiments. This paper describes a new instrumental technique, scanning laser mass spectrometry (SLMS), which has excellent potential for supporting these types of solid-state studies. The SLMS technique has been described (2)and a recent review (3) presents the general application to solids of the laser ion source in mass spectrometry. Here we describe the specific application of the measurement of diffusivity and electrotransport mobility of trace level solutes in metal systems. The advantages of SLMS over other instrumental techniques (I) lie in its higher sensitivity, minimal matrix effects, and insensitivity to eurface effects. 0003-2700/8 1/0353-1898$01.25/0
scan type
total beam monitor operational - amplifier step continuous
-
gain 10 input resistance (a) input capacitance (F) range (v. full 10. scale)
analyte signal vibrating reed continuous
step
100 1O8
m
IO-*
2x
10.
0.3
1 10"
1
lo-"
0
0.03
EXPE.RIMENTAL SECTION General experimental parameters for the SLMS are given in Table I. Apparatus. Laser. The laser system, a Nd-doped yttrium aluminum garnet witlh an acoustooptic Q switch, has been described (2). Ion Source. The ion source was the same as described previously (2) with some minor exceptions: The y and z ion-beannadjust electrostatic lenses in the ion accelerator have been removed. The total distance from the anode chamber to the main object slit of the mass spectrometer was reduced from 40 to 20 cm. The object slit width was increased from 17 to 1000 pm. Mass Spectrometer. The mass spectrometer used here was constructed at the h e s Laboratory (4).It employs an ion optical scheme (modified Mattauch-Herzog ( 5 ) ) similar to the spectrometer used previously (2). An advantage of this spectrometer over the previous one lies in its externally adjustable slit system allowing more versatile electrical ion detection which was utilized for all the experiments reported here. The spectral analyte signal was detected with a faraday cup and amplified by a Cary Model 401 MR vibrating reeld electrometer. The total ion beam tran0 1981 American Chemical Society
1900
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
52 50 46 44 42
t
\
*\ \
-
f
m 40-
5
3
9
? t
383634-
5 32cc
c
r
302826-
I
RELATIVE DOSITION ALONG SPECIMEN
24-
Flgure 2. Representative concentration profile of Zr in thorium obtained by the continuous scan with SLMS.
IO
t weld junction
2
.
I I 1 . 1 I 1 I I 1 1 IJI I +_**I “0 2 4 6 8 10 I2 I4 6 8 2 0 22 24 26 28 X 3 2 W 3638
DISTANCE ALCUG SPECIVEN IN MV
Flgure 1. Representativeconcentration profile of Co in yttrium obtained by the step scan with SLMS.
versing the spectrometer was monitored with a grid located at the entrance to the magnetic field. The grid intercepts -15% of the beam and the signal is amplified by an operational amplifier circuit (6) constructed in the Ames Laboratory. Reagents. All sample specimens were prepared in the Ames Laboratory. Exact procedures for preparation and characterization of such specimens have been described (7). The materials were as follows: pure thorium; thorium doped with approximately 100 ppmw (parts-per-million by weight) of Mo, Zr, Re, and W (each dopant was in individual specimens);four thorium specimens doped with 24,54,82, and 109 ppmw Mo; pure hafnium; hafnium doped with 15 ppmw Si; pure yttrium; and yttrium doped with approximately 500 ppmw of Fe, Co, and Ni (each dopant was in individual specimens). The specimens were fabricated as rods 2.5 mm diameter with lengths from 5 to 13 cm dependent upon the requirements of the solid-state experiments. Procedure. Two different schemes were used for measuring the concentration profile of the dopants along the length of each rod specimen. Step Scan. Analytical information is obtained at specific steps along the axis of the rod specimen. During each step the laser beam is rastored perpendicular to the rod axis (1/2 mm at 15 Hz) and simultaneously along the rod axis (0.02-0.05 mm at 2 Hz). Thus, during each step the laser beam erodes a rectangular trough 0.5 mm X (0.02-0.05) mm. The actual width and depth of the trough depend upon the length of time needed for the experiment and normally both were of the order of 0.05 mm. The time required was usually about 20 s with 2 X lo4laser shots per step. The analytical data were obtained by measuring simultaneously the charges accumulated on the faraday cup and on the total ion beam monitor. The ratio of these signals provides the relative analyte response for a step. The specimen was then moved a predetermined distance (i.e., along the rod axis by a stepping motor with 1pm resolution) to the next step and the measurement was repeated. A representative set of data obtained in such a manner is shown in Figure 1 for a concentration profile of Co in yttrium. Continuous Scan. In this mode of operation the specimen is moved continuously. The laser beam is rastored only perpendicular to the axis of the rod. The rod is moved at a constant rate along its axis over the entire length of the desired concentration profile. Input resistors are used in place of capacitors on the electrometers. The electrometer outputs are connected to an analog divider circuit whose output drives the y axis of an x-y recorder. The x axis is swept at a constant rate consistent with
the rate of specimen movement in order to place the entire concentration profile on a single x-y plot. A reproduction of an x-y plot taken in this manner is shown in Figure 2. The five “spikes” on the plot are due to short time intervals when the laser was turned off momentarily while the specimen continued to move. During this time an undefined analog divider output occurs since both the faraday cup and monitor ion signals drop to zero. The plotted data can be related to the distances along the specimen using these spikes as calibration points since the specimen has analogous sharply defined sections which have not been eroded by the laser beam. Quantitation. Since preparation of accurate trace level solute standards is difficult and very expensive this was done for only one solute system, viz., Mo in Th. The concentration levels of the four Mo standards were verified by using a spectrophotometric technique. Analyses of the standards by SLMS were by the step scan technique.
RESULTS AND DISCUSSION Quantitation Calibration. Table I1 gives the results of analyzing the Mo standards. Column 1 shows the dates of the SLMS analyses where each entry represents a separate loading of the standard into the instrument between other samples. Each determination is the average of four to six individual measurements. Column 2 gives the results of the spectrophotometric analyses. The results are repeated in the table to identify the standard levels used for the SLMS measurements given in column 3 as relative values. The ratio of the values in column 3 to those in column 2 gives the calibration factor shown in column 4 for each measurement. Statistical data at the bottom of the table indicate good agreement for the spectrophotometric and SLMS techniques. The method was tested with standards containing approximately 20, 60, 80, and 110 ppmw of Mo in thorium. A plot of the calibrated SLMS results vs. the spectrophotometric results shows a linear relationship. Quantitation Application. A thorium specimen containing 103 ppmw Mo and measuring 12.7 cm in length was butt welded to a pure thorium rod 2.5 cm long. This shorter segment served as an adapter in an electrotransport purification experiment in which the composite rod was heated to 1500 “C with an applied electric field of 0,140 V cm-’ for 30 h under a vacuum of 8 X torr. In order to obtain a valid test of the theoretical electrotransport equations it was necessary to perform the experiment at this temperature and for a time period such that the solute concentration profile could be measured accurately (>1 ppmw with the faraday cup detector) by SLMS for a t least 75% of the length of the rod. The rod was then cut into three segments in order to allow insertion into the SLMS ion source housing. These segments, labeled A, B, and C, were analyzed by the step scan technique with the standard specimen being remeasured between each segment loading. The measurements on the segments were then quantified by using the instrument calibration factor
4NALYTICAL CHEMISTRY, VOL. 53, NO. 12. OCTOBER 1981
Table 11. Calibration of SLMS with Mo Standard
date
spectrophotometric result"
9/22
54 109 24 82 54 109 24 82 54 109 24 82 54 109 24 82 54 109 24 82 54 109 24 82
9/23
9/24 AM
9/24 PM
9/25
9/26
SLMS ion signal/ spectrophotometric result
SLMS ion signal at %lo' 0.584 1.140 0.290 0.894 0.5475 1.0835 0.2235 0.6845 0.5655 1.112 0.2293 0,828 0.4435 1.0595 0.2345
0,0108 0.0105 0.0121b 0,0109 0.0101 0,0099 0.0093 0.0083 0.0105 0.0102 0.0096 0.0101 0,0082 0,0097 0.009s 0.0096
0.786
0.432 1.0965 0.273 0.7995 0.529
0.0101 0.0114 0,0098 0,0097 0.0099 0.0103 0.0096
1.080
0.246 0.790
0.00993 t 0.00093, n = 24 0,00992 * 0.00073, n = 22
meanb mean uncertainty
ob/n'l' = 0.00016 mean uncertainty = 0.00992 * 0.00016 or (0.00016/ 0.00992) X 100%= 1.6% re1 uncertainty
* ppmw error estimated to be t 2% relative with a minimum of * 1ppmw. Without entries > t o from mean. , , ,
, , , , , , , , , , , ,.
e
.
.,;. a ;.
..... .......,.... . :.
C
--
n
Table 111. Electrotransport Mobilities and Diffusion Coefficients for Various Elements in Thorium and in Yttrium matrix Th
Bolute temp, "C Re 1520
electrotransport mobility x lo5
diff coeff
x 106 em' s-'
cm' V-' s-'
4fi. ~ . 40.
18
.
9.8 1.7 0.089 31. 17.
10.
0.17 6.1
9.1 4.5
13.
0.0080b
mean
500
1901
YOION
s
c
7
Flgure 3. plot of Mo concentration profile in thorium. Steady-state electrotransport experiment for trace level solution in a refractory metal.
from Table II. The quantified concentration profde is shown in Figure 3 where the Mo concentration is plotted against the position of the measurement along the rod. There were a total of 110 measurements (steps) along the rod which permits accurate smoothing of the concentrationprofile. The absolute error in the smoothed pmfde drawn through these data points is estimated to he less than 20% relative. The structure of the profile in section C is normal in this type of experiment due to pile-up of the solute a t the end of the specimen which is approximately 50 "C cooler than the main portion of the specimen. Continuous Scan Application. The continuous scan mode was used for measuring concentration profiles of Mo,
7
I POSITION ALONG ROD 1250 yrn ceer step1
Flglm 4. (A) Pkture of erosions due to SLMS step scan of SI in Hf (upper). (E) Fkd of resultant data. Relathn, SLMS Si signal vs. distam along specimen (lower).
Re, W,and Zr in an experiment to determine their electric mobilities, diffusivities, and effective valences in hody-centered-cubic (hcc) thorium (7).However, these determinations do not require absolute concentrations for the profile measurements because the units of concentration cancel in the mathematical calculations. Accordingly, measurements of these concentration pmfdes were made hy using the continous scan technique. A typical plot of concentration vs. distance by the continuous scan is shown in Figure 2 where the solute was Zr. Table 111hts electrotransport mobilities and diffusion coefficients a t a given temperature. A complete discussion of these data including their change as a function of temperature may be found elsewhere (7). Step Scan Applications. The electrotransport of silicon in hafnium was measured by the step scan technique. Figure 4A is a picture of a hafnium specimen after the SLMS measurements showing the erosions due to the laser beam at each step. The relative Si signal resulting from each SLMS measurement is plotted in Figure 4B directly below the picture. The electric mobility of Si in hcc Hf resulting from these measurements was determined to be 6.4 X lo* cm2 V-' s-' a t 1810 "C. Also hy use of the Grube method (7),in which an error function plot of these data is made, the diffusion coefficient of Si in Hf was determined to be 1X 10.' cm28.'. The electric mohilities and thermal diffusivity for Co, Ni, and Fe in yttrium were determined from data measured by the step scan technique and is shown in Table 111. In yttrium these elements are fast diffusers which require measurements over long distances as indicated in Figure 1. The longer distances are necessary to permit accurate determination of the electric mobilities which are discussed elsewhere (8).
CONCLUSIONS Scanning laser mass spectrometry has excellent analytical capabilities for measuring concentration profdes for trace level solutes in metal systems. The analytical sensitivity could he improved if needed hy using an electron multiplier as an ion detector. The good spatial resolution and flexible scanning techniques meet the requirements of solid-state experiments.
1902
Anal. Chem. 1981, 53, 1902-1906
When standards are available, quantitation is straightforward and gives a linear response function. Due to the high cost for producing such standards only one set was used here. The experience at other laboratories (3) indicates that similar results would be obtained for solutes in other metals and that the concentration range covers a very large range. Computerization of the technique will permit high sample throughputs thus partially offsetting the cost for the special instrumentation. The technique has been shown to be a valuable tool in determining transport parameters such as electric mobility, thermal transport, and diffusivity of solutes in metals especially in those cases where the solid solubility is low.
ACKNOWLEDGMENT The authors acknowledge the assistance of M. Beck, D. Rehbein, and I. Okafor for preparation of the specimens and conductance of the solute mobility experiments. Also acknowledged is C. Ness who aided in the mass spectrometric measurements.
LITERATURE CITED (1) Hail, P. M.; Morabito, J. M.; Poate, J. M. Thln SolM Films 1976, 33,
107-134. (2) Conzemius, R. J.; Svec, H. J. Anal. Chem. 1978, 50, 1854-1860. (3) Conzemius, R. J.; Capelien, J. M. Int. J . M s s Specfroin. Ion Phys. 1980. 34. 197-271. (4) Foss,’G. Ph.D. Thesis 1981,Iowa State University. (5) Mattauch, J.; Herzog, R. Z.Phys. 1934, 89, 786. (6) Fergerson, L. B.; Conzemius,
R. J.;
Svec, H. J. Talsnta 1970, 77,
762-766. (7) Schmidt, F. A,; Carison, 0. N.; Beck, M. S.; Rehbein, D.K.; Williams, D. E. IS-4722,“Electrotransport of Solutes in Refractory Metals”, 1979. (8) Okafor, I.; Carison, 0. N. unpublished work, Ames Laboratory, Iowa State University, Ames, IA.
RECEIVED for review April 8, 1981. Accepted July 2, 1981. Ames Laboratory is operated for the U.S.Department of Energy by Iowa State University under Contract No. W7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Science, Contract NO.WPAS-KC-03-02-03-3.
Method Validation for the Determination of Tetrachlorodibenzodioxin at the Low Parts-per-Trillion Level M. L. Gross,” l u n g Sun, P. A. Lyon, S. F. Wojinski, and D. R. Hilker Department of Chemistty, University of Nebraska-Lincoln,
Lincoln, Nebraska 68588
A. E. Dupuy, Jr. USEPA, OPTS/OTS, EED, Field Studies Branch, Toxicant Analysls Center, Bay St. Louis, Mississippi 39529
R. G. Heath* USEPA, OPP/OTS, HED, Health Effects Branch, 401
M St., S. W., Washlngton, D.C. 20460
This statistically deslgned study Is directed at determining the precislon and accuracy obtalnable in the quantitation of 2,3,7,8-tetrachlorodlbenzodioxin (TCDD) in standard solutlons and In fortlfled beef adlpose tissue. The TCDD was extracted after dlgestion of the adlpose tissue in ethanollc potasslum hydroxlde, and the resultlng solution was cleaned up using a concentrated sulfuric acid wash and short column iiquld chromatography. The analysls was conducted wlth packed column gas chromatography interfaced to a hlgh-resolutlon mass spectrometer operating at a mass resolution of 10 000-12 000 (10 % peak width deflnltlon). Quantltation was achieved by employlng an internal standard method which Involved 2,3,7,8-TCDD labeled wlth ”CI. The results were submitted to comprehensive statlstical analysis in order to determlne “best estimates” of concentratlons In actual samples and to express the reliability of such estimates In terms of statlstical confldence Ilmlts.
Attention has been focused recently on the need for evaluation of data taken in environmental analysis (I). One aspect of evaluation is a quality assurance program which should include proficiency testing. This is a report of a blind study of the proficiency achievable for the analysis of 2,3,7,8tetrachlorodibenzo-p-dioxin. The study also included comparison of replicate results with those obtained in other laboratories. Those results are available in another report (2).
This study was undertaken to measure the accuracy and precision with which 2,3,7,&tetrachlorcdibenzodioxin (TCDD), when added to beef fat at low parts-per-trillion concentrations, could be extracted and quantified using packed column gas chromatography-high-resolution mass spectrometry (GCHRMS). Method validation also included quantitation of equivalent amounts of TCDD in standard solutions. The extraction and analysis employed in this study stem from the pioneering work of Baughman and Meselson (3)and later O’Keefe, Meselson, and Baughman (4) who employed high-resolution dual ion monitoring with direct probe introduction of the sample. The disadvantage of this procedure has been overcome by scientists at Dow Chemical by developing packed column GC for sample introduction to the mass spectrometer (5). Although this approach is specific for the general class of TCDDs, it does not permit separation of all TCDD isomers. However, some can be distinguished (6); for example, 2,3,7,8-TCDD can be resolved from 1,3,6,8-TCDD using the gas chromatography employed in this study. To meet the needs of environmental monitoring at the low parts-per-trillion range, particularly for samples relating to certain types of combustion (7-11),isomer specific methods have been developed. One approach is to use capillary column GC coupled with either low (8,12,13) or high-resolution (14) mass spectrometry. A single column can be used to separate 2,3,7,8-TCDD from all the other 22 isomers (13). A second approach involves a combination of high-pressure liquid chromatography and packed column GC/low-resolution MS
0003-2700/81/0353-1902$01.25/00 1981 American Chemical Society
