Anal. Chem. 1962, 5 4 , 1659-1663
Table VI. Distribution (if A % Obtained by Comparing 200 Interference Coefficients Measured om 2/11/80 and 5/5/81 no. of comparisons A % level included 0-9.9 10.0-14.9 15.0-19.9 20.0-24.9 25.0-32.9 33.0-40.0 a
115 34 12 22 12
5 200 (total)
%of
total ( 200)a
58 17 (75) 6 (81) 11 (92) 6 (98) 2 (100)
maximum %error for 25% 50% correction correction 0-2.5 2.5-8.7 3.7-5.0 5.0-16.3 6.3-8.3 8.3-10.0
0-5.0 5.0-7.5 7.5-10.0 10.0-12.5 12.5-16.5 16.5-20.0
Figures in parentheses are cumulative percentages
tween the 2/80 and 5/81 calibrations (88 interferences) was only 13% (10.4% minus the eight Ca, and. Pt interferences). The average of the 88 R!3Ds (each calculated from 18 interference coefficients mea,sured during the 15-month period) was near lo%, about 4% greater than the expected short term precision. Table V contains a more complete comparison of the 2/80 and 5/81 interference calibrations. The percent differences between the pairs of interference coefficients for each analytical channel are arranged in columns to better reveal the effects of misalignment or other changes. :By far the greatest change was noted in interferences affecting the Cawand Pt channels. Fortunately, these interferences are not very important (Caw is used only for Ca concentrations above 20 wg/mL where interference corrections of this magnitude have little effect). The Ca, i3nd Pt exit slits had both become misaligned as noted above. The slight misalignment of the Ni, Sb, and W slits had no apparent effect on interference calibration. All together, 200 interference coefficientB (excluding those for Caw and Pt) were measured on 2/11/80 and again on 5/5/81. The percent differences (A%) were calculated for the 200 data pairs. Table VI shows the distribution of these percent differences and the maximum error which can be incurred in the analytiaal results for 25% and 50% interference correction at each level of A%. Clients receiving our ICPAES reports are advised that the most reliable data have had less than a 25% interference correction applied to them. Results correcbd for 25% to 50% contributions from interferences are considered usable, but less accurate and precise. Table VI shows that about 75%
1659
of the coefficients programmed for interference correction in 2/80 had changed by less than 15% by 5/81. The maximum analytical error resulting from those changes is 3.7% for a 25% correction and 7.5% for a 50% correction. In reality, analytical error would be considerably smaller due to compensation where corrections were performed using more than one coefficient. Over 90% of the coefficients changed by less than 25% over the 15-month period. Even these coefficients inflicted less than 6.3% error in results corrected by 25% and less than 12.5% error in results corrected by 50%. It is concluded that the accuracy of interference calibrations for ICPAES polychromators can be adequately maintained over many months using the intensity ratio calibration technique for reproducing plasma conditions. However, future calibrations will not be carried beyond 6-8 months. This practice will avoid the loss in sensitivity due to photomultiplier fogging and provide for more frequent realignments. The interference monitoring program will continue to provide useful information on the long term stability of our interference calibrations.
ACKNOWLEDGMENT The author is grateful to Nem Bryan, 111, for performing the measurements used for this study. LITERATURE CITED (1) Uchida, H.; Uchida. T.; Iida. C. Anal. Chlm. Acta 1980, 776, 433-437. (2) Wallace, G. F. At. Spectrosc. 1981, 2 , 61-64. (3) McLaren, J. W.; Berman, S. S. Appl. Spectrosc. 1981, 35, 403-408. (4) McLaIen, J. W.; Berman, S. S.; Boyko, V. J.; Russell, D. S. Anal. Chem. 1981, 53, 1802-1806. (5) Boumans, P. W. J. M. Spectrochim. Acta, Part 6 1980, 356,57. (6) Boumans, P. W. J. M. “Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectrometry”; Pergamon Press: Oxford, New York, 1980. (7) Mermet, J. M.; Trassy, C. Spectmchim. Acta, Part6 1981, 366, 269. (8) Larson, G. F.; Fassel. V. A. Appl. Spectrosc. 1979, 33, 592-599. (9) Reeves, R. D.; Nikdel, S.; Winefordner, J. D. Appl. Spectrosc. 1980, 34,477-483. (10) Larson, G. F.; Fassel, V. A.; Winge, R. K.; Knisely, R. N. Appl. Spectrosc. 1978, 30, 384-391. (11) Lee, J.; Pritchard, M. W. Spectmchlm. Acta, Part 8 1981, 366, 591-594. (12) Ediger, R. D.; Fernandez, F. J. At. Spectrosc. 1980, 1 , 1-7. (13) Boumans, P. W. J. M. Spectrochim. Acta, Part 6 1976, 376, 147- 154. (14) Botto, R. I. I n “Developments in Atomic Plasma Spectrochemical Analysls”; Barnes, R. M., Ed.; Heyden and Son: Philadelphia, PA, 1982; p 141. (15) Bournans, P. W. J. M.; de Boer, F. J. Spectrochim. Acta, Part 6 1975, 306, 309-334.
RECEIVED for review December 23,1981. Accepted April 20, 1982.
Four-Rod Carbon Rod Atomizer for Atomic Absorptlon Spectrometry Darryl D. Slemer CPP-602, Exxon Nuclear Idaho Co., Idaho Falls, Idaho 83401
Matrix interferences have been and continue to be the most serious practical drawback to the analytical application of thermally pulsed graphite! furnace atomic absorption spectroscopy (GFAAS). The Massmann and the “Mini Massmann” [or carbon rod atomizer (CRA)] furnace designs were originally adapted for commercialization by instrument manufacturers primarily lbecause these designs are compact, relatively cheap to manufacture, and simple to operate-not because they provide better analytical results on “real” samples
than do the more complex Woodriff or L’vov furnace designs ( I , 2). In point of fact both the Woodriff and L’vov furnaces were shown to be superior for most real sample applications because the analytical responses obtained with them were much less affected by vagaries in matrix composition ( I , 3, 4). However, an advantage that the CRA and the Massmann designs possess as compared to the L’vov and Woodriff furnaces is that some degree of sample “cleanup” can often be performed “in situ” by employing a carefully selected
0003-2700/82/0354-1659$01.25/00 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
“ASHING” heating cycle or “matrix modification” step prior to the actual GFAAS determination. In many instances this pretreatment compensates for these furnaces’ deficiencies as “atom cuvettes” (Le., short atomic residence times and low effective gas-phase temperatures). In this writer’s opinion, the most significant recent advances in GFAAS technology have been the incorporation of temperature feedback control circuitry in furnace power supplies and the application of the “L’vov platform” to Massmann-type furnaces ( 5 , 6 ) . The “L’vov platform” eliminates many gasphase matrix effects because its relatively low heating rate prevents volatilization of the analyte until the temperature of the atomization tube is several hundred degrees higher than would be the case if no platform were present. Therefore, the volatilized analyte experiences conditions more closely approximating those extant in the Woodriff and L’vov furnaces; i.e., relatively constant and high gas-phase temperatures throughout the time that sample vapor is in the tube. The conventional Varian Techtron CRA furnace design (Model 63/90 series) features a very short atomization tube (9 mm) pinched between two graphite rods. The temperature gradient along the length of this tube is very much smaller than is the case with Massmann furnaces. However, the short residence time of volatilized material within the tube ( T ~ with ) respect both to the time that it takes to evaporate that material (TJ and to the limited rate at which the tube can be heated, causes the volatilization temperature to be, perforce, the “atomization” temperature for each increment of analyte released from the graphite surface throughout each experiment. In the case of a volatile analyte such as lead, those atoms released at the beginning of the transient signal experience temperatures no higher than perhaps 1100 to 1300 K before they escape the confines of the tube-temperatures which are too low to effectively dissociate many volatile lead compounds. To correct this problem, volatilized material should be held within the atomizer tube until it reaches a higher (and preferably stable) temperature. However, there is insufficient room in the relatively small tube to accommodate a L’vov platform of useful size. Lawson and Woodriff (7) recently developed a “Y” shaped rod to use with a CRA which permitted the heating of a relatively long atomization tube at the ends instead of from the center as is the case with most other present furnace designs (7). The thermal lag between the center and the hotter ends of the tube served to give an effect similar to that of a L’vov platform. This writer suggested the use of four rods instead of two in the CRA workhead to achieve the same end several years ago (8). This paper describes the characteristics of a CRA atomizer workhead so modified and compares it both to the conventional system and to Lawson and Woodriff s approach.
EXPERIMENTAL SECTION The CRA power supply used for this work was an early Model 63 design modified to incorporate the temperature controlling circuitry described in an earlier paper (9). The glass light pipe previously used to direct light from the hot atomizer tube to the phototransistor temperature sensing transducer was replaced with a 17 mm long graphite rod with a small (0.79 mm) axial hole through it (the tip of a “vacuum cup” spectroscopic electrode). This modification gives the temperature controller a narrower light acceptance angle permitting more accurate control of the temperature of a limited portion of the atomizer tube’s length. A Model 63 CRA atomizer head was modified as shown in Figure 1 by drilling extra holes in the nickel-plated brass rod support blocks and by making some saw slits in order to permit enough “flexibility” in the metal to clamp the rods. The hard plastic spacer normally used to electrically isolate the left-most block in Figure 1 was replaced with one made of 4 mm thick neoprene rubber and the holes around the insulated screws serving to fix that block to the atomizer base were enlarged somewhat.
Figure 1. M63 atomizer modifications: (A) plated brass end block, (B) new rod holes are drilled with a no. 14 drill (4.63 mm), (C) standard optical path, (D) modified versions’s optical path. This was done to permit the block to twist slightly under pressure in order to make rod alignment less critical. The knurled aluminum cylinder used as a thumbscrew to secure the rods into place in the standard CRA workhead was replaced with a steel nut brazed onto the same threaded rod in order to permit more positive tightening with a wrench. A Model 90 workhead can likewise be modified to accommodate the extra rods if the additional holes drilled are situated above the cooling water passage. The ”long” atomization tubes were fabricated by first cutting a standard round-tube atomizer tube of the type normally used with the Instrumentation Laboratories IL 455/555 Massman-type furnace into two equal pieces and then drilling a 1.6 mm hole in the center of each for sample introduction. A Dremel “Moto tool” used first with a thin cutoff wheel and then with a drill bit served for the machining required. The resulting tubes are 18 mm long and possess internal and external dimensions of 4.8 mm and 6.2 mm, respectively. Four standard CRA rods were used with the larger tubes. The ends of these rods were machined with a round file of the appropriate diameter in order to match the curvature of the larger diameter atomization tubes. In practice, short pieces of broken CRA rods are inserted into unused rod holes in order to prevent distortion of the rod support blocks when the rods are clamped into place with the wrench. The atomic absorption spectrometer utilized combined the optical components of a Varian Techtron AA6 spectrometer with the homemade analog signal processing circuitry described in a previous paper (10). A 50 ms low-pass time constant was used in the phase sensitive detector and a 100-ms filter constant was used in the offset-gain circuitry following the logarithmic amplifier stage. One thousand data points were collected during each 4-5 atomization experiment by a Hewlett-Packard (HP) 3447 DVM controlled by a HP9825 computer. The plotter, optical pyrometer used to measure atomizer temperatures, and other components used were the same as described in the prior papers (8-10). A Westinghouse lead hollow cathode lamp was used at an average current level of 5 mA. A Varian hydrogen continuum lamp was utilized to monitor nonatomic background absorbance signals present at the 283.3-nm wavelength of the lead resonance line used for the experiments. A homemade inert gas controller was placed in series with the standard gas box containing the flowmeters. This controller contains a solenoid operated by an integrated circuit timer which, when triggered by a signal from the CRA power supply at the beginning of the atomization cycle, turns on an argon gas flow of 10 L/min for a period of 30 s. Use of this controller greatly reduces argon consumption. Three artificial matrix solutions were prepared. The first two were pure MgC1, or CuCl, salt solutions at the same concentrations investigated by Lawson and Woodriff (7). They reported very serious signal suppressions (295%)with these concomitants in the unmodified CRA. These matrix effects were either totally (with 0.48 pg of MgC12)or partially eliminated (33% suppression with 2.4 pg of CuC1,) with their “Y” rod, end-heated furnace. The third matrix studied represented a typical ”hard” water sample
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982 1661
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Table I. Matrices Studied CuC1,.2H,O in water, 2.4 pg of CuC1,a~b CuC1,/5 pL aliquot, MgC1,.6H,O water, 0.48 pg of MgClZa M[gC1,/5 pL aliquot pure salts + nitric acid in water, hard water b , c 3.8 fig of total solids/5 N L
;;;;
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c
a MgC1, and CuCl, concentrations are same as those used in ref 6. CuCl, and hard water matrices had some 2 ppm Fe; 70 ppm Na; 55 reagent-blank lead in them. ppm Mg; 174 ppm Ca; 7 gpm NH,; 309 ppm C1; 122 ppm SO, ; 38 ppm PO, ; 1 mL o f concentrated HNO,/L.
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11300
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8C
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/F
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acidified with nitric acid. Table I lists the composition of these solutions at the concentrations actually injected into the furnace. In actual practice, the matrix solutions were prepared in a lo-fold more concentrated form and diluted with or without added lead stock solution immediately prior to performing a series of experiments.
RESULTS AND DISCUSS1ON For a better appreciation of both the qualitative and quantitative nature of the effects that changes in atomizer configuration, mode of power application, and the presence of matrix concomitants have on the response of the AAS spectrometer, the use of figures instead of tabulated data has been used. Each of the following figures represents a complete, representative, experiment performed under each set of experimental conditions. Figure 2 depicts the absorbance signals seen when 5-pL aliquota of both matrix-free and hard-water concomitant 0.1 ppm lead solutions were atomized using the standard two-rod, short-tube atomizer configuration and a power supply setting of ”4”. The homemade temperature controlling circuitry (9) was not used for this experiment. The temperature of the center of the atomizer tube as a function of time as measured by the optical pyrometer is shown on the same figure along with both the total and nonatomic absorbance signals of the covolatilized matrix. No “ashing” power cycle was used with either this or any of the following experiments as the object of this work was to compare the performance of the different furnace configurations as cctomizers, not as selective volatilization-separation devices. Figure 3 gives similar daka obtained when the homemade temperature feedback cointroller is used with the same, standard, rod-tube configuration. Two temperature traces are shown because with the rapid heating rates safely achievable when the controller is used (about 1.5 K/ms with
a -
“’L 00
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2 3 4 Seconds Flgure 4. Four-rod configuratlon-same temperature setting (84 % recovery): (A) total absorbance-no matrix I = 0.290, (B) total absorbance-with hard water matrix I = 0.311, (C) total absorbance-matrix alone I = 0.074, (D) nonatomic absorbancematrlx alone I = 0.018, (E) temperature at center of tube, (F) temperature at end of tube. 1
new, tight, rods), an appreciable difference exists between the rates of heating at the center (where the rods clamp the tube) and the ends of the tube. With lower rates of power application (as in the prior experiment) this difference is very small. Figure 4 gives the responses observed when the same solutions are run using the four-rod, long-tube CRA configuration with the same atomizer power supply settings as in Figure 3. The temperature sensing probe of the CRA power supply was aimed a t one end of the atomizer tube-not at ita center-because it was desired that the ends of the tube be at a constant, controlled temperature when the analyte entered into the light path. If the probe is directed at the center of the tube, the ends of the tube will not be isothermal during the time that volatilized material is within the tube. These figures illustrate the following important points. First, the lead volatilizes a t temperatures which are affected by the presence of concomitants. Second, clear resolution of concomitant absorbance peaks (nonatomic absorbance peaks) from those of the lead is not achieved, even when relatively low heating rates with a short, almost isothermal (spatially), atomizer tube are used (see Figure 2). Third, raising the rate of heating of a given atomizer configuration tends to reduce matrix interference because the highest concentration of lead atoms (the absorbance peak value) is attained at a somewhat higher atomizer tube temperature (compare Figures 2 and 3). Fourth, even with the most abrupt temperature ramp rate possible ( 1.6 K/ms), it is not possible to reach a moderately N
1682
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
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high and stable atomization tube temperature (e.g., 1800 K) with the short-tube, two-rod atomizer configuration before most of the lead atoms have left the light path (Figure 3). Fifth, the several hundred degree temperature lag between the center and ends of the longer, end-heated, atomizer tends to greatly reduce matrix effects (a 16% as opposed to a 55% signal suppression) because much of the light path within the atomizer is both much hotter and isothermal during the passage of the entire cloud of volatilized analyte from the center to the ends of the tube. Sixth, and fially, the longer tube is a better “atom cuvette” than is the short tube. The peak atomic absorbance signal observed with the longer tube is approximately twice as high even though the cross sectional area of the tube is (4.8/3)2 times greater. This indicates that the overall “efficiency”of the atomizer (Le., the ability to both produce and to contain free analyte atoms within the light path in a single “slug”) is almost five times better than is achieved with the short tube. This advantage is even more pronounced with a “matrix” present. None of these observations is particularly surprising. Both theoretical and experimental justification for predicting the results shown in Figures 2-4 have been available in the literature for years (2, 5 , 8, s 1 2 ) . Examination of Figure 4 indicates that the ends of the long, end heated, atomizer tube are isothermal (at 1800 K) well before an appreciable fraction of the lead signal appears. This indicates that it should be possible to heat the ends to a still higher temperature before the lead analyte volatilizes from the tube’s center. Raising the temperature would be expected to further reduce the signal suppressing effect of the concomitants. Figure 5 depicts the results of that experiment. The maximum temperature setting of the controller was increased from 1800 K to 2320 K but the other experimental parameters were left unchanged. As the figure indicates, the suppressive effect of the hard water matrix totally disappears when this is done. Figure 6 depicta the absorbance signals observed when lead was atomized both with and without CuClz or MgClz concomitants introduced at the same levels investigated by Lawson and Woodriff. The net integrated atomic absorbance signals for the three different cases in a random block experiment of 24 different atomizations (eight trials with each concomitant and eight with no matrix) were identical with three significant figures. The lead signal due to the appreciable reagent blank present in the CuC12 matrix was subtracted from the gross signals seen with it present. At the 5% significance level, the null hypothesis (i.e., no difference) would be accepted with a relative maximum error limit of 2%.
1100
D‘
4900
Seconds Flgure 5. Four-rod conflguration-higher temperature settings (103% recovery): (A) total absorbance-no matrix I = 0.134, (B) total absorbance-with hard water matrix I = 0.169, (C) total absorbance-matrix alone I = 0.024, (D) nonatomic absorbancematrix alone I = 0.002, (E) temperature at center of tube, (F) temperature at end.
I“”‘
e
1
2
3
900
4
Seconds Flgure 6. Four-rod configuration-lead with and without CuCI, and MgCi,: (A) no matrix I = 0.136, (B) CuCI, matrix I = 0.160, (C) MgCI, matrix I = 0.142, (D) CuCI, matrix alone I = 0.021, (E) temperature at center, (F) temperature at end. The observed total elimination of matrix effects was expected for MgC12 but not for CuC1, because Lawson and Woodriff’s essentially similar “Y” rod, end-heated, CRA still evinced a serious signal supression with the copper salt present (7). To explain the superior performance of the four-rod system, an explanation of the relevant differences between the two experimental approaches is in order. First, the atomizer tube used for this project has a lesser wall thickness than that used by Lawson and Woodriff as well as a considerably greater inner diameter (4.8 as compared to 3 mm). This gives the tube poorer thermal conductivity. The rod spacing used for this work was 18 mm between the outermost extremes which left a gap of more than 9 mm between the inner edges of the rods. The inner spacing of the “Y” rod contacts used by Lawson and Woodriff was only on the order of 6 mm. These differences in tube construction and rod placement tend to cause a greater disparity between the rates of end as opposed to center heating with the four-rod system. Lawson and Woodriffs atomizer tube possessed heating rates of 1.9 K/ms at the ends and 1.1K/ms at the center as compared to rates of 1.6 K/ms and 0.32 K/ms (see Figure 5) in this work. In practice this caused the volatilized lead species to experience higher gas-phase temperatures in the experiments performed with the four-rod design. The other major difference in the experimental approach lies in the fact that in this project a temperature controller was used which served to keep the bulk of the length of the atomization tube temporally isothermal during the time that volatilized analyte species are within the tube. Lawson and Woodriff’s power supply was not feedback controlled and the temperature of the entire tube varied continually throughout the entire “atomization” cycle. This fact left the actual atomization temperatures still dependent upon the volatilization temperature, i.e., dependent on a factor which is strongly matrix dependent. Figure 7 shows the determination of a much more refractory element, cobalt, in both the hard-water and matrix-free solutions. The temperature controller was set at 2800 K for this experiment. A similar absence of matrix related analytical response perturbation is noted for this metal as was the case for lead. The discussion and data given above support the following conclusions: First, a great deal of improvement in the analytical utility of the CRA atomizer can be easily (and relatively cheaply) accomplished by going to the four-rod, long tube, tube-end-sensed-temperature-feedback-controlled system. In actual analytical practice of this facility the only instances when the conventional furnace configuration has proven to
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
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The atomizer discussed in this paper is a first attempt modification of an existing device and certainly does not represent the ultimate potential of this approach to reducing matrix effect problems. A second generation atomizer workhead optimized to efficiently utilize this principle should incorporate a much more efficient inert gas flushing system in order to better protect the graphite working parts from oxidation by entrained air. It would also be desirable to include a second temperature sensing transducer (directed at the center-not at the end(s)-of the atomizer tube) to control the power supply during “ASHING”.
LITERATURE CITED Seconds Figure 7. A 0.5-119 sample of cobalt whh and without hard water matrix ( 9 8 % recovery): (A) total absorbance (solid) no matrix Z = 0.253, (B) total absorbance (dotted)wkh matrix Z = 0.249, (C) total absorbance of matrix alone I = 0.002, (D) temperature of center, (E) temperature at end. be superior are when an absolutely maximum volatilization temperature is required (e.g., for molybdenum determinations in matrix-free solutions) A more powerful power supply transformer would eliminate this limitation of the four-rod configuration. Second, the four-rod syintem is more practical than the “Y” rod approach of Lawson and Woodriff (7) because the atomization power supply transformer in the unmodified CRA 63 power supply is adequate to provide sufficient power to it for most applications and because the atomizer workhead need not be dismounted and t h m dismantled to replace burned-out rods. I
(1) Woodriff, R.; Rameiow, G. Spectrochim. Acta, Part 6 W88, 2 3 6 , 665. (2) L’vov, B. V. “Atomlc Spectroscopy”; Israel Program for Scientific Translations: Jerusalem, 1969. (3) Hageman, L.; Nlchols, J. A.; Vlswanadham, P.; Woodriff, R. Anal. Chem. 1979, 5 1 , 1406. (4) Hageman, L.; Mubarek, A,; Woodriff, R. Appl. Specfrosc. 1978, 33. 226. (5) L‘vov, B. V. Spectrochim. Acta. Part 6 1978, 336, 153. (6) Slavin, W.; Manning, D. C. Anal. Chem. 1979, 5 1 , 261-265. (7) Lawson, S. R.; Woodriff, R. Spectrochlm. Acta, Part 6 1980, 356, 753. (8) Slemer, D. D. 1979 Piitsburgh Conference, Cleveland, OH, March 5-9, 1979; paper no. 157. (9) Slemer, D. D. Appl. Spectrosc. 1979, 33, 613. (10) Slemer. D. D.; Baldwin, J. M. Anal. Chem. 1880. 5 2 , 295. (1 1) Fuller, C. W. “Electrothermal Atomization for Atomic Absorption Spectrometry”; Analytical Sciences Monographs No. 4 The Chemical Society: London, 1977. (12) Chakrabarti. C. L.; Hamed, H. H.; Wan, C. C.; LI, W. C.; Bertells, P. C.; Gregolre, D. C.; Lee, S. Anal. Chem. 1980, 5 2 , 167.
RECEIVED for review November 5,1981. Accepted May 13, 1982.
ADDENDUM Review: Mass Spectrometry A. L. Burlingame, Anne Dell, and David H. Russell (Anal. Chem. 19(12,54, 363R-409R). The following reference, cited on page 394R, was inadvertently omitted in the list of Literature Cited: (T54) Smith, K. M.; Brown, S. B.; Troxler, R. F.; Lai, J. J. Tetrahedron Lett. 1980,21, 2763-2766.
Chemical Ionization in Fourier Transform Mass Spectrometry Sahba Ghaderi, P. S. Kulkarni, Edward B. Ledford, Jr., Charles L. Wilkins, and Michael L. Gross (Anal. Chem. 1981, 53, 428-437). Coupling of Capillary Gas Chromatograph and Fourier Transform Mass Spectrometer Edward B. Ledford, Jr., Robert L. White, Sahba Ghaderi, Charles L. Wilkins, and Michael L. Gross (Anal. Chem. 1980, 52, 2450-2451). These works were partially supported by a U.S. Environmental Protection Agency Grant (R807251010).
