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atom, or one chlorine and one bromine atom. It is noted that the most abundant parent ion is [p + 4]+ whereas the most abundant daughter ions are the daughter ions from the parent ion, [ p + 21') in the cases of one chlorine atom and one bromine atom loss. Shown in Figure 5 is the CA spectrum of the molecular ion, [p 2]+, of 2,5,2'-trichlorobiphenyl,which exhibits loss of one and two chlorine atoms in the generation of the two doublets a t m / z 223, 221, 187, and 186. The isotopic abundances of the daughter ions are similar to the calculated ones shown in Figure 1, where n = 3, m = 0. The isobutane chemical ionization mass spectrum of 1,2dibromo-3-chloropropane shows that the [M - C1]+ ions are the highest mass ions and the CA spectra of the [M - C1]+ ions are shown in Figure 6. The CA mass spectrum of the monoisotopic parent ion, p+, a t m / z 199 shows three singlet daughter ions at m / z 119,93, and 39 which are generated by the losses of HBr, C9H2Br,and 2HBr from the parent ion, respectively. The similar daughter ions from the parent ion,
+
[p + 2]+, are expected to show two doublets and one singlet from Figure 2 where n = 0, m = 2. This expectation is confirmed experimentally as shown in Figure 6. Registry No. 2,5,2'-Trichlorobiphenyl,37680-65-2; 1,2-dibromo-3-chloropropane, 96-12-8.
LITERATURE CITED (1) Beynon, J. H. "Mass Spectrometry and Its Applications to Organic Chemistry"; Eisvier: New York, 1960. (2) Kondrat R. W.; Cooks R. G. Anal. Chem. 1978, 50,81A-92A, and references therein. (3) McLafferty F. W. Acc. Chem. Res. 1980, 13, 33-39, and references therein. (4) Todd P. J.; Barbaias M. P.; McLafferty F. W. Org. Mass Specfrom. 1982, 17, 70-80. (5) Tou J. C.; Zakett D.; Caldecourt V. I n "The MS/MS Applications to Chemical Problems In Tandem Mass spectrometry"; McLafferty F. W., Ed.; Wiiey: New York, in press. (6) Mukhtar E. S.;Griffiths I . W.; Harris F. W.; Beynon J. H. Int. J . Mass Spectrom. Ion Phys. 1981, 3 7 , 159-166.
RECEIVEDfor review July 9,1982. Accepted November 1,1982.
CORRESPONDENCE Exchange of Comments on the Measurement of Aerosol Transport Efficiencies in Atomic Spectrometry Sir: Recently, Smith and Browner (1)have reported on the measurement of aerosol transport efficiences in some nebulizer-chamber assemblies. Of the three direct methods used with ICP systems the cascade impactor and the membrane filter technique gave mutually consistent results (1.4%), but the silica gel technique yielded a much higher value (5.3%). On this basis Smith and Browner rejected the silica gel data and analyzed possible causes for systematic errors with this technique. Since the silica gel technique was taken from one or our earlier publications ( 2 ) )we feel compelled to comment on Smith and Browner's conclusions, the more so, since the efficiences we measured with the silica gel technique for a similar nebulizer-chamber system ( 2 )were in the range expected for ICP systems (1-1.4%) and agree with the lower data of Smith and Browner. In adapting the silica gel technique to their systems, Smith and Browner made a major change to our design. Instead of measuring in the normal operation practice of the ICP (Le., with an argon flow of 1 L/min) an additional air flow of 2 L/min is pumped through the silica gel adsorption tube, probably to improve the resemblance between the silica gel technique and the other two techniques. However, the silica gel collects solvent, whereas the cascade impactor and the membrane filter collect solute. Obviously, the auxiliary air flow influences the two collection processes in a different way and, indeed, Smith and Browner argue that 60% humidity of the air (15 mg/L) might explain a major part of the observed deviation (30 mg/min out of 39 mg/min). This could have been verified with a blank run, but although a "high" blank is reported, this observation is not quantified. Experiments in our laboratory indicated a substantially lower value for laboratory air, 10 mg/L air, but this can easily be explained by a climatical difference between Smith and Browner's laboratory and our laboratory. Whatever the value, it is clear that this systematic error occurs only when addi-
tional air is pumped through and, hence, could not have faulted our results. This conclusion does not apply for the second source of systematic error advanced by Smith and Browner. A possible saturation of carrier gas argon with water might invalidate all silica gel data if the water arose from the wet walls rather than from the sample spray. At a room temperature of 20 OC the contribution would be 20 mg/L argon. The first question to answer is whether an argon flow of 1L/min can become saturated with water by a single passage through a wet Scott chamber. The general expression of mass transfer inside a tube with a constant wall concentration is $ = kA (c,~I - E)
(1)
where q5 is the mass flow (g/s), k the coefficient of mass transfer (m/s), A the wall surface (m2),c,d the constant water concentration at the wall (20 mg/L at 20 "C), and the mean concentration in the tube. For the coefficient k handbooks on technology (3) yield the following expression for laminar flow in relatively wide tubes, which reflects the present situation
k = 1.62(~D~/clL)~/~
(2)
where u is the gas velocity (m/s), D is the diffusion coefficient (m2/s),and d and L are the tube diameter and length (m), respectively. If we consider the Scott chamber as a system of two tubes with a length of 10 cm and a width of 1 cm and 4 cm, respectively, we find for an argon flow of 1L/min and a diffusion coefficient of 0.2 cm2/s that k is equal to 0.18 cm/s for the wide tube and 0.70 cm/s for the narrow tube. Substitution of these data in eq 1shows that after passage through the chamber the water content of argon could be raised from 0 to 19 mg/L. In agreement with this estimate, measurement of the water concentration of dry argon passed through a wet Scott chamber yielded a value of 15 mg/L with the silica gel
0003-2700/63/0355-0372$0 1.50/0 0 1983 American Chemical Society
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technique. I t thus appears that the argument of Smith and Browner is confirmed by theoretical calculation and experimental measurement. In reality, the situatilon is less clear, because there are two processes, which enhance E in eq 1,without affecting the total analyte concentration of the spray, namely, initial wetting during the nebulization process and water evaporation from spray droplets. Since c is increased, mass transfer of water from the chamber wall irn decreased. Only experiments with labeled water (DzO) coudd solve this uncertainty. At present, we can only say that nebulizer efficiencies measured with an argon flow of 1 L/min contain an uncertain contribution between 0 and 15 mg/min. In our previous publication (2) we measured a collection of 30 mg/min for a solvent uptake of 2 mL/min. As a resdt the true efficiency lies in the range of 0.7 to .4%. With either the filter collection or the cascade impactor technique this range would be substantially smaller (6% instead of 35%). A more accurate result might be obtained by humidification of the argon prior to the nebulization and subtraction of a bllank value, but the possibility of an influence of the water vapor on the droplet size distribution by preventing droplet evaporation should be taken into consideration, since this would influence the aerosol transport efficiency too. In our previous publication we also reported data for a Babington nebulizer operating on only 0.1 L/min of argon. It is clear from eq 2 that the lower argon flow diminishes the transfer coefficient, but even when this effect is ignored,
saturation of the argon would only contribute 1.5 mg/min of water. With the reported collected amount of 10 mg/min for a liquid uptake rate of 1mL/min, the efficiency of the Babington nebulizer system is in the range of 0.85 to 1.00%. We conclude that Smith and Browner have identified a possible cause of systematic error when the silica gel adsorption tube is used to measure aerosol transport efficiencies a t high argon flows. Obviously, the use of an additional air flow must be avoided or corrected for by suitable blank data. When used properly, the silica gel method may provide valuable data for nebulizer efficiencies especially a t low argon flows. It remains, of course, an extremely simple technique.
Sir: The comments of Ripson and de Galan ( I ) , regarding transport of solute and aolvent in inductively coupled plasma nebulizer/spray chamber systems, raise several important and interesting points. In essence, Ripson and de Galan’s model for aerosol transport reduces schematically to that shown in Figure 11. This model presumes that water vapor collected a t the exit to the spray chamber arises from two sources only, namely, from evaporation of the aerosol which passes through the spray chamber and reaches the collection trap and from solvent evaporation from the chamber walls. We would suggest that other sources of water vapor can make a major contribution to the total water vapor loading in the gas and that a more complete model of the evaporation process is that shown in Figure 2. In this model, it is assumed that water vapor can come from two additional sources, namely, from the liquid jet, a t the moment of disruption by the air stream and from the evaporation of larger (>5 pm) droplets in the air stream, prior to their loss on tho chamber walls. However, the crucial consideration in examining the validity of silica gel collection as a means for estimating transport efficiences (6,) is the following: Does vapor collected on the trap arise predominantly from evaporation of the same aerosol which is caught on the trap, or does it come from some other source? If a major portion of the vapor comes from some other source, then the t, values measured by silica gel trapping will always have a positive lbias. The origin of the bias is that additional water vapor in the gas stream, which is not directly related to aerosol transport properties, will register on the trap as if it were. The magnitude of the bias will depend on the ratio of vapor to aerosol loading in the gas stream. It can be seen that the silica gel ]procedure always measures soluent rather than solute transport properties as discussed earlier (3). By contrast, it should be noted that the IUPAC definition of en (2,3),refers explicitly to solute transport measurement. In terms of analytical signals, solute transport is the relevant parameter, unless the influence of solvent on flame or plasma conditions is to be examined.
Ripson and de Galan have shown by both theoretical and experimental treatments (1)that water film on the walls of the spray chamber acting alone has the capacity to nearly saturate the argon stream. Subsequently, however, they suggest that in many instances this might only be expected to give rise to a relatively minor uncertainty in estimating e, values. The question regarding the magnitude of the positive bias introduced by the presence of extraneous water vapor in the aerosol stream can only be resolved experimentally. At present, theory is too poorly developed to provide quantitative predictions. Furthermore, while the use of DzO could indicate the possible role of evaporation of water from the chamber walls, it cannot determine whether vapor arises predominantly from evaporation of one part of the aerosol or another. The situation would also be complicated by the rapid mixing of water from the nebulizer and that originally on the chamber walls. However, it is clear that if experimental t, solvent collection values, obtained by using silica gel collection, differ significantly and consistently from experimental cn solute collection values, measured by filter collection, then the solvent vapor must originate predominantly from other than the collected aerosol. In comparing their approach to silica gel collection with our own (3), Ripson and de Galan (1)raise a valid point of difference between the two procedures. The use in our measurements of additional dilution air, containing water vapor, adds a further source of positive bias to the measurements. However, from the data presented in the paper (3) it can be readily seen that subtraction of the dilution air blank alone cannot account for the discrepancy between tn values based on water collection and those based on solute collection. We have therefore repeated a number of silica gel trap measurements, exactly duplicating the published procedure of Ripson and de Galan ( 4 ) , with the one exception that we nebulized a 100 pg/mL Mn solution rather than deionized water. This was in order that we might make both solvent
t
LITERATURE CITED (1) Smlth, D. D.; Browner, R. F. Anal. Chem. W82, 54,533-537. (2) Ripson, P. A. M.; de Galan, L. Spectrochim. Acta, Part B 1081, 36, 71-76. (3) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. “Transport Phenomena”; Wlley: New York-London, 1960.
Peter A. M. Ripson Leo de Galan* Laboratory for Analytical Chemistry Technische Hogeschool Delft P.O. Box 5029 2600 GA Delft The Netherlands RECEIVED for review June 14, 1982. Accepted October 25, 1982.
0003-2700/83/0355-0373$0 1.50/0 0 1983 American Chemlcal Society
