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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
relative and the wet precision as 1.3% relative (0.02 SD a t the 1.5% level), then the 95% CL for a single comparison of the average of three plasma determinations and one wet determination is 2.8% relative (= 2.0(0.8*/3 1.3'/1)'''). For seven such comparisons the limit is 1.1% relative. The two glasses in Table VI-B could be included in this calculation without changing the results. Thus the bias of 3.2% relative is significant. Its explanation probably lies in the known volatility of boron trifluoride which could be formed and released during the fusion treatment employed by the wet titrimetric method. This explanation is also consistent with the low analytical totd results (> [H'] and thus the recombination be32 tween the electrons and helium ions is the dominant depletion 5.3 5.4 33 6.70 reaction for the electrons. From Equations 4, the steady-state 34 6.22 concentrations are 4.4 4.5 5.78 35 5.36 36 (64 [H+l = h ~ [ H e * l [ H ~ l / ( k ~ C+e k3[021) l 3.8 3.7 4.98 37 4.63 5.5 38 5.8 and 3.1 3.1 4.30 39 [el = (kz[He*l[H21+ k ~ [ H e l ) / k ~ [ H e + l (6b) 41 3.71 4.6 5.0 43 3.20 3.9 3.7 Applying the steady-state approximation to the buildup of 1.8 1.8 3.3 45 2.76 3.3 the hydrogen excited state, 1.45 47 1.5 2.38 2.8 2.6 d [ H * ] / d t = k,[H+][e] - [H*]/T = 0 1.05 1.0 1.77 2.0 2.0 (7) 51 0.70 0.69 1.31 55 where 1 / is~the atom emission rate. From Equations 6a, 6b, 0.55 0.54 1.2 1.13 57 1.2 and 7, the observed Balmer CY intensity signal, S, is then 0.46 0.46 1.0 1.0 59 0.978 S = a [ H * ] = a&[He+][e] = 0.80 61 0.844 0.86 0.29 0.31 0.727 0.73 0.75 63 0.51 0.50 67 0.541 0.43 0.45 69 0.467 0.35 0.402 0.36 71 where a is an instrumental factor. In order to fit Equation 0.347 0.30 0.30 73 8 to the 0.2 and 0.6% oxygen fraction curves in Figure 3, 0.22 0.20 77 0.258 Equation 8 was put in the following simplified form: 0.17 0.223 0.18 79 0.13 0.166 83 0.13 (9) S = (ab[HzI2 + a [ H z l ) / ( l + ~ [ H z l ) a = 0.24 X a = 0.66 X where lo-'' photon lo-'' photon cm3/molecule cm3/molecule. a = cu~k~k~k~[He*][He]/(k~k~[He+][O~] klKo[He]) b = 1.88 X b = 2.64 X lo-" cm3/ lo-'' cm3/ b = kz[He*]/ko[He] molecule. molecule. c = 0.47 X and c = 1.29 X lo-" cm3/ lo-" em3/ molecule. molecule The hydrogen concentration in both curves followed a 6.1 X a Underlined Doints were used to determine a. b. and c. 1013 eXp-.074t(min) molecules/ cm3 dependence on exponential time, t. Values of a, b, and c which fit Equation 8 to the two is increased.) For example, oxygen can undergo a charge oxygen curves were obtained by solving Equation 9 for three transfer reaction with the helium ion (19), points on each of the two curves. The three selected points He+ O2 O+ 0 H e (10) covered the range of points on each curve. For the 0.2% oxygen curve, a = 0.66 X lo-" photon cm3/molecule, b = 2.64 and Penning ionization, X lo-" cm3/molecule, and c = 1.29 X lo-" cm3/molecule. For the 0.6% oxygen curve, a = 0.24 X lo-" photon cm3/molecule, He* O2 02+ H e e (11) b = 1.88 X lo-" cm3/molecule, and c = 0.45 X 10-l' cm3/ in addition to oxygen electron reactions. Reactions such as molecule. The signal, S, is in arbitrary units of photon counts. Reaction 10 which remove the helium ion electron trap, could In Table I are shown values of the other points calculated from be responsible for the constancy of the 1.4 power dependence Equation 9 and the a , b, and c values for each curve. with increasing oxygen concentration. Agreement between the calculated points and the experiWith increase in the helium plasma pressure, the electron mental ones is quite good for both curves. concentration increases via the Penning Ionization Reaction The difference between the values of a, b, and c for the 0.2% (Reaction 2) and increase in [He], thus diminishing the oxygen curve and the 0.6% oxygen curve can be related to importance of the depletion of protons by the change transfer their oxygen concentration dependence. The ratio of a at 0.2% Reaction 3, Le., oxygen fraction to a a t 0.6% oxygen fraction is 2.75, almost identical to 2.87, the ratio of c a t 0.2% oxygen fraction to c k,ko[HeI + k,~,[He*I[Hzl >> h3k4[O21 [He+] at 0.6% oxygen fraction. The close proximity of the two ratios reflects the identical inverse oxygen concentration of both a This increase in electron concentration has also been detected and c . Furthermore the two ratios are almost inversely by ion probe measurements (20). The hydrogen emission, S, proportional to the relative oxygen concentration. The value then approaches a linear dependence on [H,] as shown in of b shows a corresponding increase of only 1.4. Figure 1 at 40 and 101 kPa. In the absence of oxygen, S At oxygen fractions >0.6%, reactions involving the oxygen depends linearily on hydrogen concentration as shown in molecule become important and the kinetics become more Figures 1 and 3. complex. (Above 1% fraction, a color change from yellow to The disappearance of the deuterium anomaly at higher blue is observed for the plasma as the oxygen concentration plasma pressures can also be explained by the increase in the
+
-
-
+
+
+
-
-+
+ + +
+
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
electron concentration. A hydrogen background emission resulting from the presence of impurities in the helium carrier gas is always observed. Upon the addition of oxygen to the carrier gas, this background decreases. When deuterons are produced in the plasma upon passage of deuterium through the MPD, the deuterons undergo a charge transfer with the oxygen,
D+ + O2
-
D
+ O,+
(12)
This reaction effectively decreases the oxygen concentration, thus increasing the hydrogen background emission which is observed as a GC peak. At high pressures, the increasing electron production diminishes the importance of Reaction 1 2 and thus the deuterium anomaly disappears. A corresponding hydrogen anomaly upon passage of hydrogen through the MPD does not appear because of the absence of deuterium background emission.
CONCLUSION In MPD analysis, reactions between the analyte and scavenger gas in the plasma have been ignored with the exception of Serravalla and Risby's investigation (15). They found that small amounts of oxygen had to be added to the helium plasma gas to generate atomic chromium emission in the determination of chromium in chromium P-diketonates. Since the clean-up gas is usually as concentrated as the elutant in the plasma, reactions such as charge transfer may, as shown in this investigation, seriously affect the MPD response. The kinetic scheme based on proton and electron generation by Penning Ionization, proton and electron recombination, and proton to oxygen charge transfer in the plasma successfully accounts for the nonlinearity of the hydrogen emission in the presence of oxygen and for the appearance of a hydrogen emission peak upon passage of deuterium through the plasma. As predicted by the kinetic scheme, both effects decrease with increase in the plasma pressure and disappear in the absence of oxygen in the plasma. The variation of the hydrogen response with change in oxygen fraction from 0 to 0.6% follows the variation predicted by the
kinetic scheme. Modeling the kinetics of the plasma is not only helpful in understanding the analyte response of the MPD but can predict improvements of the response. For example, the optimum MPD operating pressure for the determination of hydrogen to deuterium isotope ratios is 101 kPa where the hydrogen response is linear and the deuterium anomaly is not present.
ACKNOWLEDGMENT The author acknowledges the advice with the kinetics which Pierre Ausloos and L. Wayne Sieck of the Chemical Thermodynamics Division have provided in this study.
LITERATURE CITED A. J. McCorrnack, S.C. Tong, and W. D. Cook, Anal. Chem., 37, 1470 (1967). C. A. Bache and D. J. Lisk, Anal. Chem., 37, 1477 (1965). W. Braun, N. C. Peterson, A. M. Bass, M. J. Kurylo, J . Chromatogr., 55, 237 (1971). K. M. Aldous, R. M. Dagnail, 8. L. Sharp, and T. S. West, Anal. Chim. Acta, 5 4 , 233 (1971). R. M. Dagnall. T. S. West, and Paul Whitehead, Anal. Chem., 44, 2074 (1972). W. R. McLean. D. L. Stanton, and G. E. Penketh, Analyst(London),98, 432 (1973). J. P. J. van Dalen, P. A. de Lezenne Coulander. and L. de Galan, Anal. Chim. Acta, 94, 1 (1977). F. P. Schwarz, Anal. Chem., 50, 1006 (1978). F. P. Schwarz, W. Braun, and S. P. Wasik, Anal. Chem., 50, 1800 (1978). C. J. M. Beenakker, Specfrochlm. Acta Part 6 ,32, 133 (1977). C. J. M. Beenakker. Spectrochim. Acta Part 6 ,31, 483 (1976). J. E. Lovelock, Anal. Chem., 33, 162 (1961). H. Schluter, Z . Nafurforsch. A , 18. 439 (1963). P. Brassern and F. J. M. J. Maesser, Spectrochim. Acta Part 6 ,30, 547 (1975). F. A. Serravalla and T. H. Risby, Anal. Chem., 47, 2141 (1975). E. E. Ferguson, Phys. Rev., 128, 210 (1962). P. F. Fenneliy, R. S.Hemsworth, H. I. Schiff, and D. K. Bohrne, J . Chem. Phys., 59, 6405 (1973). W. T. Huntress. Jr., private communication, JPL, Pasedena, Calif., 1978. F. C. Fehsenfeld, A. L. Schrneltekopf, P. Golden, H. I . Schiff, and E. E. Ferguson, J . Chem. Phys., 44, 4087 (1966). R. Avni and J. D. Winefordner, Spectrochim. Acta, Part 6 ,30, 281 (1975).
RECEIVED for review March 5 , 1979. Accepted May 16, 1979 This work has been supported by the Office of Environmental Measurement a t NBS.
Determination of Lithium by Optically Monitored Stable Isotope Dilution D.
F. Brost, J. M.
Brackett, and K. W. Busch"
Depatiment of Chemistry, Baylor University, Waco, Texas 76703
A high accuracy atomic absorption procedure for the determination of lithium is described. The method, which utilizes conventional commercially-available atomic absorption instrumentation, is based on the principle of stable isotope dilution. The procedure is applied to serum lithium determinations and gives an average relative error of about 0.7% for samples in the psychotherapeutic range (5-10 ppm). Analysis time is relatively short since no sample pretreatment is required and the necessity of repeated instrument calibration is eliminated.
In recent years, lithium salts have been extensively prescribed for the treatment of a variety of mental disorders ranging from deep depression to violent hysterical mania. 0003-2700/79/0351-1512$01 .OO/O
Since serious side effects, even death, can be induced by lithium levels in excess of the therapeutic range, frequent and reliable monitoring of serum concentrations is absolutely essential ( 1 ) . Lithium, along with other serum electrolytes, is usually determined in the clinical laboratory by conventional flame photometry. Unfortunately, this technique is subject to considerable systematic error from the multitude of general and specific interferences afforded by the serum matrix. Although several methods are routinely employed either to compensate for these interferences through the preparation of suitable standards (2-4), or to minimize their effects through chemical sample treatment (5-7), the possibility of unsuspected matrix effects makes it difficult to assess the reliability of analytical results obtained on unfamiliar and poorly characterized samples. As a result, the accuracy of serum B 1979 American Chemical Society
