Table I. Determination of Polyacrylate in Simulated Boiler Water Medium: 100 ml simulated boiling water, sludge free Polyacrylate: Goodrite K-702 (free acid) Absorbance Calculatedn Added Added polyacrylic Maracell-E, at 415 mp, polyacrylic 1-inch tube acid, ppm Error, ppm ppm acid, ppm 1. 0 ... 0,054 0.5 -0.5 2.0 ... 0.221 2.1 +o. 1 3.0 ... 0.292 2.7 -0.3 4.0 ... 0.442 4.1 +o. 1 5.0 ... 0.520 4.8 -0.2 6.0 ... 0.642 6.0 0.0 7.0 ... 0.734 6.8 -0.2 8.0 ... 0.906 8.4 +0.4 9.0 ... 1.002 9.3 +O. 3 9.8 -0.2 10.0 ... 1.052 4.8 -0.2 5.0 ... 0.520 5.2 +0.2 5.0 20 0.557 4.8 -0.2 5.0 40 0.513 Calculated from formula: Polyacrylic acid (ppm) = 9.3 X absorbance. 4
interferes, and because it is the pH of the cupric nitrate reagent solution. In this way the pH of the sample can be adjusted at room temperature, then heated, and the copper added without further significant change in the eventual p H value. Assays were run at p H 3.9 in the simulated boiler water with added polyacrylic acid levels of from 1.0 to 10.0 ppm. A linear relationship was found, with polyacrylic acid content (ppm) being 0.31 times the copper found in the precipitate (expressed as ppm with respect to the sample). Calculated
in this manner, the mean error for the ten determinations above was 0.23 ppm. (See Table I.) Boiler water treatments sometimes contain modified lignosulfonates. Under the conditions of the assay the lignosulfonate (20-40 ppm added) coprecipitated with cupric polyacrylate, rendering the floc dark brown. However, the copper content of the floc was not affected. Dissolution with ammonia gave a solution of brown color, but this color was not transferred into the isoamyl alcohol layer. Nevertheless, care must be taken not to carry significant amounts of the aqueous layer into the organic extract, since the color of lignosulfonate is similar to that of the copper complex. The copper content of precipitates obtained in 2% NaCl solutions were somewhat lower than those obtained in simulated boiler water at the same pH. The change in copper content with p H was similar. Flocculation in 2-4% NaCl solutions is somewhat faster than in boiler water. A few qualitative tests were performed with polymethacrylic acid (molecular weight 9000) in sea water. The lower molecular weight diminishes the rate of flocculation, and the lower limit for visible floc formation within 1-2 hours is around 1 ppm. The precipitate has the same appearance as cupric polyacrylate, however. ACKNOWLEDGMENT
The authors are grateful for the help of Harry P. Gregor, whose suggestion started this investigation.
RECEIVED for review May 21, 1968. Accepted August 7, 1968.
A New Type of Interference in Atomic Absorption or Emission Measurements with the Premixed Nitrous Oxide-Acetylene Flame S. R. Koirtyohann a n d E. E. Pickett Unicersit): of Missouri, Columbia, Mo. 65201 THEPREMIXED nitrous oxide-acetylene flame was introduced by Willis (I) for atomic absorption determinations of oxideforming metals. It quickly became a popular flame because of efficient atom formation ( 2 ) and relative freedom from chemical interferences (3). The high temperature and the premixed configuration make this flame equally valuable as a source for emission measurements (4, 5), where some important spectral interferences are also rfduced. It is possible, for example, t o detect barium at 5535 A by emission in the presence of 100,000 fold excess of calcium, in spite of the well known molecular band interference (6). A new type of interference due to the presence of mineral acids in the sample solution was noticed, however (6),and is described in more detail in this paper. (1) J. B. Willis, Nature, 207, 715 (1965). (2) S. R. Koirtyohann and E. E. Pickett, XI11 Colloquium Spectroscopicum Internationale, Ottawa, 1967. (3) M. D. Amos and J. B. Willis, Spectrochim. Acta, 22, 1325, (1966). (4) M. D. Amos, “The Element: Techical News Notes,” No. 17, Aztec Instruments, 1967. (5) E. E. Pickett and S. R. Koirtyohann, Spectroclzim. Acta, 23B, 235 (1968). (6) S. R. Koirtyohann and E. E. Pickett, ibid., in press. 2068 *
ANALYTICAL CHEMISTRY
EXPERIMENTAL
For most of the work, the burner was essentially the same as that used by Willis ( l ) , consisting of a Perkin-Elmer nebulizer and spray chamber and a burner head of our manufacture. The flame was formed at a slot 0.5 mm wide by 50 mm long. A Jarrell-Ash 0.5-m Ebert Spectrometer was mounted with an optical bar which supported all external optical components. The Jarrell-Ash 82-375 ac electronics were used in conjunction with a 10-mV strip chart recorder for all intensity measurements. For atomic absorption measurements, the usual two-lens system in which the source is imaged midway through the flame and again on the spectrometer slit was used. Perkin-Elmer hollow cathode lamps were powered by a Hilger Model F A 41.301 supply. A light chopper placed between the source and the flame provided for discrimination against flame emission. For emission measurements, the chopper was placed between the flame and the spectrometer and the source was turned off; no other change was needed. For a few of the measurements, a Perkin-Elmer nitrous oxide burner head was used on the above instrument and on a Perkin-Elmer Model 303 Atomic Absorption Spectrophotometer. Reagent grade chemicals and deionized water were used throughout. Most solutions contained 500 ppm (micro-
a.
h d.
b
c.
d
._
AI 3951
f . .Ol ,o! ,1 .3 1.0 3.0 h C I C e CPP.
hl
Figure 1. Effect of perchloric acid concentration on emission intensities
a
All solutions contained 1 ppm of the test element and 500 ppm of potassium
=
0
Burner slot parallel to optical axis
0 Burner slot perpendicular to optical axis
Slot Parallel, with Additive
RESULTS AND DISCUSSION
Figure 1 shows the change in emission signals for a number of elements as a function of the perchloric acid concentration. The upper curve in each case was made with the long axis of the burner slot parallel to and centered below the optical axis. The intensities shown are fully corrected for any flame background and reagent contamination. For each element tested there is an increase in response up t o about IMHCIO, followed by a rather rapid decrease at higher concentrations. The decrease would be expected because of reduced nebulizer efficiency, but the observed enhancement is harder to explain. When operating in the emission mode, it was possible to turn the burner head through 90 degrees and operate with the long axis of the slot perpendicular to the optical axis. About a 10-fold loss in intensity resulted, but the amplifier gain was increased t o compensate. The entire width of the flame was thus sampled rather than its length. The results appear as the lower curves in Figure 1. In this configuration, low concentrations of perchloric acid had no significant effect; only the expected decrease in signal at high HCIOl concentration was observed. Observations similar to those in Figure 1 were made for several other acids and salts. The results were similar, with an
Slot Perpendicular, without Additive Slot Perpendicular, w t l h Additive
Figure 2. Effect of acids and salts on emission and absorption response
4,
1 ppm Ca, emission,4227 0 and 0.5MH2S04 b. 1 ppm Ca, emission, 4227 A, 0 and 0.3MH3PO4 C. 1 and 20 ppm Ca, atomic absorption, 4227 A, 0 and 0.3M U.
grams per milliliter) of potassium as the chloride to suppress ionization and the test elements were added at the 1-ppm level. Unless otherwise stated, all measurements were made on a flame supplied with 9.2 liters/min (30 psig) of nitrous oxide and 4.5 liters/min of acetylene. A portion of the flame 5 mm in height starting 1 mm above the primary reaction zone was normally sampled. The flame showed a red zone about 3 mni in height. Both ends of the long flame were somewhat out of focus at the spectrometer slit, and light from as far as 1.2 mm outside the designated zone at the ends of the flame could have been included by thef/lO optics. Measurements with a ribbon filament lamp showed, however, that very little light from more than 1.0 mm outside the zone focused o n the slit was included and this only a t the extreme ends of the flame.
Slot Parallel, w l n o u t Additive
d . 1 ppm Ba, emission, 4554 A (Ba II), 0 and 0.3M H3PO4 (no added K) e . 1 ppm Li, emission, 6708 A,oOand 0.3MH~P04 f. 1 ppm Li, emission, 6708 A, Perkin-Elmer Burner Head, 0 and 0.3MH3PO4 0 and 0.3M Neutral. g. 1 ppm Li, emission, 6708
A,
11.
ized to pH 3.0 with "!OH 1 ppm AI, atomic absorption, 3093 A, Perkin-Elmer Model 303 Atomic Absorption Spectrophotometer, 0 and 0.3M H3POd
initial enhancement in signal when the long axis of the flame was parallel with the optical axis, followed by suppression at higher concentrations. Representative data at selected concentrations are presented in Figure 2. Figures 2a and 26 show that the results from sulfuric and phosphoric acids for calcium emission are quite similar to those for perchloric acid shown in Figure 1. Figure 2c shows the essentially identical results obtained when atomic absorption rather than flame emission was used for calcium determinations in the presence of phosphoric acid. Note that the usual chemical suppression of calcium response due to sulfate or phosphate is totally absent in this flame. Other parts of Figure 2 show that similar results were obtained from an ion line ( d ) , when the Perkin-Elmer nitrous oxide burner head was used rather than the home-made one ( e f ) , when the acid was neutralized (g), and when the PerkinElmer burner head was used on a Model 303 Atomic Absorption Spectrophotometer (12). No data were taken with the slot perpendicular to the optical axis in the last case because the low absorbance prevented accurate readings. In all, we have made measurements on calcium, strontium, barium, barium ions, lithium, sodium, potassium, aluminum, and gallium by flame emission while calcium, zinc, and aluminum were measured by atomic absorption. Matrix materials included sulfuric, phosphoric, and perchloric acids, as well as ammonium phosphate and sodium chloride. Enhanced VOL. 40, NO. 13, NOVEMBER 1968
2069
emission and absorption were consistently observed near the center of the flame a t moderate concentrations of the acids (0.3-1M) and rather high concentrations of the salts (1-2z) even though chemically dissimilar elements were tested. Nitric acid produced a smaller enhancement of 2-3z. Hydrochloric acid and organic materials, such as sucrose and glycerol, produced no effect. The enhancements are not changed significantly by variations in fuel or oxidant flow but are reduced as one views higher in the flame, and they disappear about 15 mm above the primary reaction zone. Because emission and absorption results are similar, the atomic populations are being altered rather than the excitation process or self absorption. The change is associated with the nitrous oxide flame rather than the burner type because sodium emission and zinc absorption were unaffected by perchloric and phosphoric acids when an air-acetylene flame was used on the same burner. In every case the enhancement disappeared or was drastically reduced when the flame was turned with the slot perpendicular to the optical axis. The total number of atoms measured across the width of the flame remains relatively unaffected by the acids. The emission near the edges of the flame must, therefore, be lowered in the presence of the acid. Figure 3 shows the results obtained when the burner slot was aligned with the optical axis and then displaced across the axis causing various portions of the flame to be imaged on the slit. The presence of perchloric acid caused enhanced emission in about the central 1 mm of the flame but lowered it outside this area. Results similar to thost in Figure 3 were obtained for calcium and potassium (4044 A) emission in the presence and absence of sulfuric acid. Thus, we are faced with a new type of interference which involves the spatial distribution of the sample within the flame. The cause for the altered distribution is not yet known, but one reasonable postulate involves the rate of diffusion of the salt particles outward from the center of the flame. In the presence of a high boiling acid, or of a nonvolatile salt, the particle remaining after solvent evaporation will be relatively heavy until the solute can evaporate. This heavy particle will diffuse outward less rapidly than the lighter one formed in the absence of the acid or salt, resulting in a higher concentration of sample near the center of the flame at its base. After the solute is vaporized, it would have no effect on diffusion rates and its effect on sample distribution would tend t o disappear higher in the flame, in accordance with the observations. The enhancements were about the same using the two nitrous oxide burner heads available to the authors. The magnitude of the effect could, however, depend on details of the
2070
e
ANALYTICAL CHEMISTRY
AO.0 M HCIOI
sr 4607 8,
0 0. 5 M HCIO4
-
.-> "?
w c
-
I
C
w >
.-m.. ._ DI
a
0
3
0
3
Displacement, mm
Figure 3. Horizontal profile of strontium emission with and without added perchloric acid
burner head construction and be quite variable from instrument to instrument. Several authors have reported enhanced absorption due t o the presence of various matrix materials in the nitrous oxide-acetylene flame (3, 7-9). Competition for the available oxygen was suggested as a cause (7), but a mechanism similar to that reported here also may have played a part. RECEIVED for review June 27, 1968. Accepted August 15, 1968. Contribution from the Missouri Agricultural Experiment Station.
(7) S. L. Sachdev, J. W. Robinson, and P. W. West, Anal. Chim Acta, 37, 12 (1967). ( 8 ) J. B. Headridge and D. P. Hubbard, ibid., 151. (9) D. C. Manning and L. Capacho-Delgado, ibid., 36, 312 (1966)
