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Comparison of an indirect and a direct method for measuring the efficiency of nebulizer systems
0.%4-8547/%6 33.00+0.00 0 1986. pnaamotlRcssLtd
spCemchii&aAcf.. Vol.418,No 3,pp.291-294,1986 Rinkd m GreatBritain
RESEARCH NOTE
Comparison of an indirect and a direct method for measuring the efficiency of nebulizer systems (Received 4 January 1985; in
1.
revised
form 24 June
1985)
INTRoD~~~N
DURING
the last few years several papers have been published dealing with nebulizer characterization. References [l-7] show that the measurement of the efficiency of nebulizer systems is a rather controversial matter. The controversy is whether a direct or an indirect efficiency measurement yields a correct result. Mainly three methods are used for direct measurements. The filter collection method has been applied by OLSONet al. [8] and SMITH and BROWNER [I]. The latter authors have also performed measurements with a cascade impactor. The third method is a silica gel collection system as described by RIPWNand DE GALAN[9]. The indirect methods make use of the volume or mass of the waste [l, lo]. Comparisons of the data obtained by direct and indirect methods have been reported [l, lo]. The prime aim of this work was to make an experimental comparison between a direct and an indirect method for efficiency measurements under controlled and identical conditions. 2.
EXPERIMENTAL
2.1. Apparatus
The equipment used for the comparison has been described in Table 1 of Ref. [lo]. The only difference is that the aerosol chamber WG made by Wiklunds was not used.
2.2. Measuring methods used The indirect measuring method used is identical with that described in Ref. [lo]. The direct method is similar to the filter method used in Ref. [l] but with some modifications and improvements: see Fig 2 in Ref. [l]. The temperature of the ambient air was kept at 20.0-20.5 “C and the temperature of the liquid aspirated varied within the range 20.0-20.5 “C. The temperature of the liquid did not change more than 0.2 “C during a series of experiments. Liquid was aspirated before any measurements were performed in order to obtain a stable temperature and liquid flow pattern in the aerosol chamber. The liquid used for the measurements was a 1000 pg ml-’ Mn solution. One of the reasons for using a 10 times higher concentration than SMITHand BROWNER [ 1) did was that it was desirable to obtain the same measuring time for both methods (2-3 min). Other reasons will be dealt with in Section 4. The amount of anaiyte uptake was calculated by multiplying the volume uptake with the concentration of the solution. The volume uptake was obtained by weighing the uptake and correcting it for the density of the solutior~ The amount of analyte leaving the aerosol chamber with the aerosol was measured with a Bker collection technique. A single polycarbonate membrane filter, 47 mm in diameter, and having a pore sire of 0.4m (N&eopore Corp.) was used with a collection air flow of at least 3Olmin-‘, maintained with a vacuum pump (SSB, Leybold-Heraeus GMBH) The filter holder was used with an adapter of TeBon having an inlet diameter of 8 mm. This adapter, having a smaller inlet diameter than the one employed in Ref. [l], was used to obtain a better collection e5ciency of the aerosol, because of higher suction (higher gas velocity). The collection e5ciency of the aerosol was better than 99.7 %. The collection efficiency was determined by collecting completely desolvated aerosols, i.e. aerosols having a mean diameter smaller than a normal aerosol. The desolvation system had an efficiency of at least 95%. The collection efficiency was obtained by comparing the sum of analyte for the collected aerosol and the washings of the [l] [2] [3] [4] [S] [6] [7] [8] [9] [lo]
D. D. SMITH and R. F. BROWNER,Anal. Chem. 54,533 (1982). P. A. M. RIP~ONand L. DE GALAN, A&. Chem. 55, 372 (1983). R. F. BROWNERand D. D. SMITH,Anal. Chem. 55, 373 (1983). G. L. MOORE, P. J. HUMPHRIES-CUFFand A. E. WATSON,ICP Inform. Newslett. 9, 763 (1984). R. F. BROWNER,ICP Inform. Newslett. 9, 778 (1984). G. L. MOORE and A. E. WATSON, ICP hform. Newslett. 10,259 (1984). R. F. BROWNERand D. D. SMITH,ICP Inform Newslett. 10, 532 (1984). K. W. OLSON, W. J. HAAS,JR and V. A. FASSEL,Anal. Gem. 49,632 (1977). P. A. M. RIBBON and L. DE GALAN, Spectrochim. Acta 36B. 71 (1981). A. GUSTAVCJON, Spectrochim. Acta 39B, 743 (1984). 291
292
Research note
desolvation system with the total analyte uptake. This procedure is based on the assumption that the washing of the desolvation system is 100% effective. Experiments in our laboratory have shown this assumption to be valid. The filter and the support were washed with nitric acid (1% by vol.) and the washings were analysed with atomic absorption spectrometry (AAS). Passing laboratory air through the filter for three minutes gave an AAS result which did not significantly deviate from zero. Results will be given as x f Sx where x is the mean value, SXthe standard deviation of the mean (S/J) n and n is the number of experiments. The precision of this direct method was determined with nebulizer TR-3&B3 operating at a gas flow of 11 min-’ and aspirating the Mn solution at a temperature of 20.15”C. The efficiency value obtained was l.OO&O.Oly0 (n = 16). Statistical F-test was used to ascertain a possible difference between measured values. 3. RESULTS
The analyte transport efficiencies (8,) obtained with the direct method as well as the indirect values (q) obtained from Ref. [lo] are given in Table 1. The working reference levels for the nebulizers are those giving a gas flow of 1 I min- ’ except for the MAK 200 where the level is a pressure drop of 200 psi (13.8 bar). 4. DISCUSSION Since the aerosol collection efficiency is high and the standard deviation is low, it can be assumed that the present direct method for measurement of analyte transport efficiency has good precision and accuracy. If a better precision is needed, a longer measuring time can be used. It could seem questionable to make a comparison of the two methods using solutions of different concentrations of analyte, 1000 and 0 pg ml- ’ of Mn in deionized distilled water, respectively. The reasons for choosing two different solutions are stated here and will be further discussed below. The idea was to establish a possible difference between the methods. This would be facilitated by choosing a large difference in analyte concentration. Further aspects were the following. (1) The time for a measurement should be equal for the two methods (2-3 min). (2) The analyte content should be high enough to give an acceptable concentration of the analyte in the sample for the AAS determination in view of (1). (3) The analyte content should be high enough to make the “shift in” process negligible. When a droplet having an initial diameter larger than the cut-off diameter (d,) of the aerosol chamber, evaporates to such an extent that its diameter will be less than d, before any impact with the aerosol chamber walls occurs then it is “shifted in “; that is, the droplet initially belongs to the part of the aerosol going to waste but because of evaporation it will pass the aerosol chamber. The “shift in” process will become more important the lower the salt content of the aspirated solution. Comparison of the data obtained by the indirect method with those obtained by the direct one (Table 1)clearly shows that the direct values are lower. Using the dual concentric aerosol chamber with the various nebulizers will give indirect values that are as a whole 82 % high, ranging from 50 to 130 %. BROWNER [3,5] states that when using an indirect method based on mass measurement of solution passing to drain a positive error of at least 300 % would arise. The results presented here do not give a positive bias of more than 130 % with a measuring time as short as 2-3 min compared with 30 min used by Browner. The efficiency value presented by SMITHand BROWNER [ 11,viz. 1.1 f 0.1% for a typical ICP nebulizer system plus torch seems somewhat high as compared with the present direct values. This discrepancy can have two causes. First, the concentric nebulizer (a Meinhard T-23@A3) used in Ref. [l] was fed with a flow of 1 ml min- l from a peristaltic pump. Consequently a higher efficiency than normal should have been obtained owing to the lower liquid flow according to Ref. [ 111. Secondly, it is evident from Refs [12,13] that droplets larger than d, have most probably been shifted in because Smith and Browner used a solution with a Mn content of only 1OOpg ml-’ as compared to 1000 pg ml- 1 in this work. The differences between the indirect and direct values in Table 1 are all significant at least at the 95 “/, level with one exception. The exception is the nebulizer T-230-A3 and the reason is the higher standard deviation for that efficiency value. One reason for the difference in efficiency values measured with indirect and direct methods respectively has been dealt with by RIPSONand DEGALAN[2] and BROWNER and SMITH[3]. The water vapour saturating the nebulizing gas will come from two sources: the generated aerosol and the liquid at the walls of the aerosol chamber. The source for saturating the gas is important only in the initial stage of the saturation. If the greater part of the water vapour comes from the aerosol, it could be subject to [l l] A. GUSTAVSSON, Ad. Chem. 55.94 (1983). [12] J. PORSTENDORFER, J. GEBHART and G. R~BIG,J. Aerosol Sci. 8, 371 (1977). [13] M. S. CRESSER and R. F. BROWNER, Spectrochim. Acta 35B, 73 (1980).
293
Research note
severe redistribution. This is not in accordance with Figs 1 and 2 in Ref. [13], because when deriving the curves in the figures it is assumed that the gas containing the aerosol droplets has a humidity of loOo/, The evaporation rate will be higher if the gas has a humidity of less than 100 %. The droplets wig be evaporating egashasahumidityof100%ie.aftertheinhial~EvaporaaonhngtoFiiland2inRef[13]onlywhenth anon from a droplet will-after the initial stage-lead to a net condensation of vapo~r into the bulk, because the gas cannot be “over-humidified”. The direction of the water transport-from droplets to the bulkis clear from the fact that the droplets have a higher vapour pressure than the bulk. In the case, that we have an aerosol generated with a solution having a certain salt content, the situation will be further complicated because droplets will not evaporate completely but reach an equilibrium diameter. Throughout the remainder of this paper we will not consider what happens to the aerosol during the initial stagebefore reaching 100 y0 humidity-due to lack of theory and knowledge. If we assume-to begin with-that the generated aerosol does not contribute to the saturation of the nebulixing gas, then the 18.5mg water vapour per 1nebulixing gas will be contributed only by the liquid on the walls of the aerosol chamber. This is an incorrect assumption but we will correct for it later on. If we convert the direct values in Table 1 with this water amount, we obtain the figures in the central column of Table 2. These values differ by + 13 % to - 27 % (with a mean of - 0.8 %) from the indirect values in Table 1. The most interesting values are those for the T-250-A3 and the TN-l nebulixers because they are 13 and 27 % lower (significant at the 82 and 92 % levels, respectively) than the indirect values (explanation below). The converted values in the central column of Table 2 need not be corrected for “shift in” according to Fig. 2 in Ref. [ 131 because of the high salt content. However, the values must be corrected for the evaporation of the aerosol droplets leaving the aerosol chamber since the evaporated solvent goes into the bulk of the solution in the aerosol chamber. Most of the droplets leaving the aerosol chamber are not completely desolvated but reach an equilibrium diameter. It can be. shown after studying Refs [ 12, 131that we can obtain a good estimate of the amount of water going into the liquid bulk in the aerosol chamber if we assume that droplets smaller than 0.7 pm are completely desolvated and larger ones are not desolvated at all. It is possible by using the model presented by GIJSTAVSSON [l l] and the 0.7~pm value as a cut-off diameter to obtain a good estimate of the influence of the evaporation Table 1. Nebuhzer efficienciesfor the various nebulixers at their working reference level
Nebuhxer
Uptake* (ml mitt-‘)
Indirect (q)* Direct (a,,) (%) (%)
T-230-A3 T-250-A3 T-230-Al
3.10
1.22
0.80
3.14 1.14
1.56 2.80
0.78 1.66
T-220-B3 TR-3CLB3 m-1 MAK 200
2.62 2.87 3.62 1.37
1.56 1.65 1.70 2.23
0.92 1.00 0.73 0.95
*Values from Table 2 in Ref. [lo].
Table 2. Converted efficiency values Analyte transport efficiency (E,)
Plus chamber evapodtion
Plus chamber evaporation minus aerosol evaporation
Nebulizer
(%)
(%)
T-230-A3 T-25@A3 T-230-A 1 T-220-B3 TR-3@B3 TN-l MAK 200
1.36 1.36 3.17 1.62 1.70 1.24 2.30
1.16 1.16 2.89 1.33 1.46 1.00 2.06
294
Research note
on the efficiency. After performing this correction, the values in the right-hand column of Table 2 were obtained. The efficiency values for the cross-flow nebulixers have been corrected with the mean efficiency contribution of the concentric nebulixers of droplet size d 0.7 pm. This is because the model [ill is not valid for cross-flow nebuliiers. The right-hand values in Table 2 differ by between + 3 and - 41% (with a mean of - 15 %) from the indirect values in Table 1. The difference for the T-250-A3 and the TN-l are ~~i~~tly lower at the 98.5 and 98.8 % levels. The trend, which is statistically si~ifi~t, shows that the directly measured efficiencies with correction for evaporation are smaller than the values obtained with the indirect method. The discrepancy can be due to the fact that the indirect values are too high and/or the direct ones are too low. The direct values can be low as a result of collision of droplets with diameters < d, with larger ones and/or the walls of the aerosol chamber. If the larger droplets--hit by smaller ones-have a diameter > d, after the collision then we obtain a lower efficiency. This also implies that larger droplets with a diameter < d, can have an analyte content larger than the solution aspirated. If this happens, it will have an effect on the results of studies of aerosol ionic redistribution [l4]. A likely reason for the T-2%A3 and TN-1 nebulixers giving significantly low efficiencies in comparison with the other nebulixers is that they bring about a larger turbulence in the inner tube of the aerosol chamber as a consequence of pressure shock waves due to the large pressure fall and the construction (cross-flow), respectively. The MAK 200, being of the cross-flow type, also creates shock waves due to a large pressure fall but the initial diameter of the aerosol chamber is larger than for the SC-2; so fewer collisions will occur. The indirect values can of course be high due to “shift in” of larger droplets. But the “shift in” phenomenon should affect all the nebulixers in the same way and cannot explain why only T-250-A3 and TN-l should be significantly low. Consequently, the fact that the directly measured and converted values are lower than the indirectly measured values is most probably attributable to collision of smaller droplets with larger ones and/or the walls of the aerosol chamber. The above arguments demonstrate that both direct and indirect methods have their advantages and drawbacks and that they excellently complement each other. Therefore the choice of the one method or the other or a combination of the two depends on the phenomenon one wishes to study. 5. CONCLUSION Carefully performed indirect efficiency measurements on nebuliier systems with low efficiencies, e.g. pneumatic nebulixers used with ICPs, will give values typically SO-130 % higher than those obtained with direct methods. This difference can be attributed to the following. (1) The nebulixing gas will be saturated with the solvent, thus making indirect values larger than direct ones. The solvent vapour will come from the bulk of the solution in the aerosol chamber and from the generated aerosol. The origin of the solvent vapour is only important in the initial stage before the gas is saturated. (2) Small droplets having diameters < d, will be retained in the aerosol chamber owing to collisions with other droplets and the walls of the aerosol chamber. (3) Droplets with diameters > d, can be “shifted in”, i.e. they evaporate to such an extent that their diameter will be d d, before they impact with the walls of the aerosol chamber. %JMMARY
A direct method for measuring the efficiency of nebulixer systems is described and ex~~men~lly compared with an indirect method under similar conditions. Results for various cross-flow and concentric nebulixers are presented. It is shown that the present direct method has equal or better accuracy and precision than other direct methods. The analyte transport efficiency of cross-flow and concentric nebulixers found for ICP systems are typically 0.7-1.7x. Carefully performed indirect measurements will give 50-l 30 y0 higher e5ciency values than direct measurements. The reasons for this difference are discussed. It is concluded that the choice of a direct or an indirect measuring method is dictated by the phenomenon one wishes to investigate. Acknavledgements-The author thanks FOLKEINGMAN for stimulating discussions manuscript and the Swedish Natural Research Council for financial support.
Department of Analytical Chemistry Royal In~tit~te of Tech~ol~~y S-100 44 Stock~lm Sweden
and comments regarding the
ANDERS GUSTAV~~ON
[14] J. A. BOROWIEC,A. W. BOORN, J. H. DILLARD, M. S. CRESSER,R. F. BROWNERand M. J. MATTESON,Anal. Chem. 92, 1054 (1980).
spCemchii&aAcf.. Vol.418,No 3,pp.291-294,1986 Rinkd m GreatBritain
RESEARCH NOTE
Comparison of an indirect and a direct method for measuring the efficiency of nebulizer systems (Received 4 January 1985; in
1.
revised
form 24 June
1985)
INTRoD~~~N
DURING
the last few years several papers have been published dealing with nebulizer characterization. References [l-7] show that the measurement of the efficiency of nebulizer systems is a rather controversial matter. The controversy is whether a direct or an indirect efficiency measurement yields a correct result. Mainly three methods are used for direct measurements. The filter collection method has been applied by OLSONet al. [8] and SMITH and BROWNER [I]. The latter authors have also performed measurements with a cascade impactor. The third method is a silica gel collection system as described by RIPWNand DE GALAN[9]. The indirect methods make use of the volume or mass of the waste [l, lo]. Comparisons of the data obtained by direct and indirect methods have been reported [l, lo]. The prime aim of this work was to make an experimental comparison between a direct and an indirect method for efficiency measurements under controlled and identical conditions. 2.
EXPERIMENTAL
2.1. Apparatus
The equipment used for the comparison has been described in Table 1 of Ref. [lo]. The only difference is that the aerosol chamber WG made by Wiklunds was not used.
2.2. Measuring methods used The indirect measuring method used is identical with that described in Ref. [lo]. The direct method is similar to the filter method used in Ref. [l] but with some modifications and improvements: see Fig 2 in Ref. [l]. The temperature of the ambient air was kept at 20.0-20.5 “C and the temperature of the liquid aspirated varied within the range 20.0-20.5 “C. The temperature of the liquid did not change more than 0.2 “C during a series of experiments. Liquid was aspirated before any measurements were performed in order to obtain a stable temperature and liquid flow pattern in the aerosol chamber. The liquid used for the measurements was a 1000 pg ml-’ Mn solution. One of the reasons for using a 10 times higher concentration than SMITHand BROWNER [ 1) did was that it was desirable to obtain the same measuring time for both methods (2-3 min). Other reasons will be dealt with in Section 4. The amount of anaiyte uptake was calculated by multiplying the volume uptake with the concentration of the solution. The volume uptake was obtained by weighing the uptake and correcting it for the density of the solutior~ The amount of analyte leaving the aerosol chamber with the aerosol was measured with a Bker collection technique. A single polycarbonate membrane filter, 47 mm in diameter, and having a pore sire of 0.4m (N&eopore Corp.) was used with a collection air flow of at least 3Olmin-‘, maintained with a vacuum pump (SSB, Leybold-Heraeus GMBH) The filter holder was used with an adapter of TeBon having an inlet diameter of 8 mm. This adapter, having a smaller inlet diameter than the one employed in Ref. [l], was used to obtain a better collection e5ciency of the aerosol, because of higher suction (higher gas velocity). The collection e5ciency of the aerosol was better than 99.7 %. The collection efficiency was determined by collecting completely desolvated aerosols, i.e. aerosols having a mean diameter smaller than a normal aerosol. The desolvation system had an efficiency of at least 95%. The collection efficiency was obtained by comparing the sum of analyte for the collected aerosol and the washings of the [l] [2] [3] [4] [S] [6] [7] [8] [9] [lo]
D. D. SMITH and R. F. BROWNER,Anal. Chem. 54,533 (1982). P. A. M. RIP~ONand L. DE GALAN, A&. Chem. 55, 372 (1983). R. F. BROWNERand D. D. SMITH,Anal. Chem. 55, 373 (1983). G. L. MOORE, P. J. HUMPHRIES-CUFFand A. E. WATSON,ICP Inform. Newslett. 9, 763 (1984). R. F. BROWNER,ICP Inform. Newslett. 9, 778 (1984). G. L. MOORE and A. E. WATSON, ICP hform. Newslett. 10,259 (1984). R. F. BROWNERand D. D. SMITH,ICP Inform Newslett. 10, 532 (1984). K. W. OLSON, W. J. HAAS,JR and V. A. FASSEL,Anal. Gem. 49,632 (1977). P. A. M. RIBBON and L. DE GALAN, Spectrochim. Acta 36B. 71 (1981). A. GUSTAVCJON, Spectrochim. Acta 39B, 743 (1984). 291
292
Research note
desolvation system with the total analyte uptake. This procedure is based on the assumption that the washing of the desolvation system is 100% effective. Experiments in our laboratory have shown this assumption to be valid. The filter and the support were washed with nitric acid (1% by vol.) and the washings were analysed with atomic absorption spectrometry (AAS). Passing laboratory air through the filter for three minutes gave an AAS result which did not significantly deviate from zero. Results will be given as x f Sx where x is the mean value, SXthe standard deviation of the mean (S/J) n and n is the number of experiments. The precision of this direct method was determined with nebulizer TR-3&B3 operating at a gas flow of 11 min-’ and aspirating the Mn solution at a temperature of 20.15”C. The efficiency value obtained was l.OO&O.Oly0 (n = 16). Statistical F-test was used to ascertain a possible difference between measured values. 3. RESULTS
The analyte transport efficiencies (8,) obtained with the direct method as well as the indirect values (q) obtained from Ref. [lo] are given in Table 1. The working reference levels for the nebulizers are those giving a gas flow of 1 I min- ’ except for the MAK 200 where the level is a pressure drop of 200 psi (13.8 bar). 4. DISCUSSION Since the aerosol collection efficiency is high and the standard deviation is low, it can be assumed that the present direct method for measurement of analyte transport efficiency has good precision and accuracy. If a better precision is needed, a longer measuring time can be used. It could seem questionable to make a comparison of the two methods using solutions of different concentrations of analyte, 1000 and 0 pg ml- ’ of Mn in deionized distilled water, respectively. The reasons for choosing two different solutions are stated here and will be further discussed below. The idea was to establish a possible difference between the methods. This would be facilitated by choosing a large difference in analyte concentration. Further aspects were the following. (1) The time for a measurement should be equal for the two methods (2-3 min). (2) The analyte content should be high enough to give an acceptable concentration of the analyte in the sample for the AAS determination in view of (1). (3) The analyte content should be high enough to make the “shift in” process negligible. When a droplet having an initial diameter larger than the cut-off diameter (d,) of the aerosol chamber, evaporates to such an extent that its diameter will be less than d, before any impact with the aerosol chamber walls occurs then it is “shifted in “; that is, the droplet initially belongs to the part of the aerosol going to waste but because of evaporation it will pass the aerosol chamber. The “shift in” process will become more important the lower the salt content of the aspirated solution. Comparison of the data obtained by the indirect method with those obtained by the direct one (Table 1)clearly shows that the direct values are lower. Using the dual concentric aerosol chamber with the various nebulizers will give indirect values that are as a whole 82 % high, ranging from 50 to 130 %. BROWNER [3,5] states that when using an indirect method based on mass measurement of solution passing to drain a positive error of at least 300 % would arise. The results presented here do not give a positive bias of more than 130 % with a measuring time as short as 2-3 min compared with 30 min used by Browner. The efficiency value presented by SMITHand BROWNER [ 11,viz. 1.1 f 0.1% for a typical ICP nebulizer system plus torch seems somewhat high as compared with the present direct values. This discrepancy can have two causes. First, the concentric nebulizer (a Meinhard T-23@A3) used in Ref. [l] was fed with a flow of 1 ml min- l from a peristaltic pump. Consequently a higher efficiency than normal should have been obtained owing to the lower liquid flow according to Ref. [ 111. Secondly, it is evident from Refs [12,13] that droplets larger than d, have most probably been shifted in because Smith and Browner used a solution with a Mn content of only 1OOpg ml-’ as compared to 1000 pg ml- 1 in this work. The differences between the indirect and direct values in Table 1 are all significant at least at the 95 “/, level with one exception. The exception is the nebulizer T-230-A3 and the reason is the higher standard deviation for that efficiency value. One reason for the difference in efficiency values measured with indirect and direct methods respectively has been dealt with by RIPSONand DEGALAN[2] and BROWNER and SMITH[3]. The water vapour saturating the nebulizing gas will come from two sources: the generated aerosol and the liquid at the walls of the aerosol chamber. The source for saturating the gas is important only in the initial stage of the saturation. If the greater part of the water vapour comes from the aerosol, it could be subject to [l l] A. GUSTAVSSON, Ad. Chem. 55.94 (1983). [12] J. PORSTENDORFER, J. GEBHART and G. R~BIG,J. Aerosol Sci. 8, 371 (1977). [13] M. S. CRESSER and R. F. BROWNER, Spectrochim. Acta 35B, 73 (1980).
293
Research note
severe redistribution. This is not in accordance with Figs 1 and 2 in Ref. [13], because when deriving the curves in the figures it is assumed that the gas containing the aerosol droplets has a humidity of loOo/, The evaporation rate will be higher if the gas has a humidity of less than 100 %. The droplets wig be evaporating egashasahumidityof100%ie.aftertheinhial~EvaporaaonhngtoFiiland2inRef[13]onlywhenth anon from a droplet will-after the initial stage-lead to a net condensation of vapo~r into the bulk, because the gas cannot be “over-humidified”. The direction of the water transport-from droplets to the bulkis clear from the fact that the droplets have a higher vapour pressure than the bulk. In the case, that we have an aerosol generated with a solution having a certain salt content, the situation will be further complicated because droplets will not evaporate completely but reach an equilibrium diameter. Throughout the remainder of this paper we will not consider what happens to the aerosol during the initial stagebefore reaching 100 y0 humidity-due to lack of theory and knowledge. If we assume-to begin with-that the generated aerosol does not contribute to the saturation of the nebulixing gas, then the 18.5mg water vapour per 1nebulixing gas will be contributed only by the liquid on the walls of the aerosol chamber. This is an incorrect assumption but we will correct for it later on. If we convert the direct values in Table 1 with this water amount, we obtain the figures in the central column of Table 2. These values differ by + 13 % to - 27 % (with a mean of - 0.8 %) from the indirect values in Table 1. The most interesting values are those for the T-250-A3 and the TN-l nebulixers because they are 13 and 27 % lower (significant at the 82 and 92 % levels, respectively) than the indirect values (explanation below). The converted values in the central column of Table 2 need not be corrected for “shift in” according to Fig. 2 in Ref. [ 131 because of the high salt content. However, the values must be corrected for the evaporation of the aerosol droplets leaving the aerosol chamber since the evaporated solvent goes into the bulk of the solution in the aerosol chamber. Most of the droplets leaving the aerosol chamber are not completely desolvated but reach an equilibrium diameter. It can be. shown after studying Refs [ 12, 131that we can obtain a good estimate of the amount of water going into the liquid bulk in the aerosol chamber if we assume that droplets smaller than 0.7 pm are completely desolvated and larger ones are not desolvated at all. It is possible by using the model presented by GIJSTAVSSON [l l] and the 0.7~pm value as a cut-off diameter to obtain a good estimate of the influence of the evaporation Table 1. Nebuhzer efficienciesfor the various nebulixers at their working reference level
Nebuhxer
Uptake* (ml mitt-‘)
Indirect (q)* Direct (a,,) (%) (%)
T-230-A3 T-250-A3 T-230-Al
3.10
1.22
0.80
3.14 1.14
1.56 2.80
0.78 1.66
T-220-B3 TR-3CLB3 m-1 MAK 200
2.62 2.87 3.62 1.37
1.56 1.65 1.70 2.23
0.92 1.00 0.73 0.95
*Values from Table 2 in Ref. [lo].
Table 2. Converted efficiency values Analyte transport efficiency (E,)
Plus chamber evapodtion
Plus chamber evaporation minus aerosol evaporation
Nebulizer
(%)
(%)
T-230-A3 T-25@A3 T-230-A 1 T-220-B3 TR-3@B3 TN-l MAK 200
1.36 1.36 3.17 1.62 1.70 1.24 2.30
1.16 1.16 2.89 1.33 1.46 1.00 2.06
294
Research note
on the efficiency. After performing this correction, the values in the right-hand column of Table 2 were obtained. The efficiency values for the cross-flow nebulixers have been corrected with the mean efficiency contribution of the concentric nebulixers of droplet size d 0.7 pm. This is because the model [ill is not valid for cross-flow nebuliiers. The right-hand values in Table 2 differ by between + 3 and - 41% (with a mean of - 15 %) from the indirect values in Table 1. The difference for the T-250-A3 and the TN-l are ~~i~~tly lower at the 98.5 and 98.8 % levels. The trend, which is statistically si~ifi~t, shows that the directly measured efficiencies with correction for evaporation are smaller than the values obtained with the indirect method. The discrepancy can be due to the fact that the indirect values are too high and/or the direct ones are too low. The direct values can be low as a result of collision of droplets with diameters < d, with larger ones and/or the walls of the aerosol chamber. If the larger droplets--hit by smaller ones-have a diameter > d, after the collision then we obtain a lower efficiency. This also implies that larger droplets with a diameter < d, can have an analyte content larger than the solution aspirated. If this happens, it will have an effect on the results of studies of aerosol ionic redistribution [l4]. A likely reason for the T-2%A3 and TN-1 nebulixers giving significantly low efficiencies in comparison with the other nebulixers is that they bring about a larger turbulence in the inner tube of the aerosol chamber as a consequence of pressure shock waves due to the large pressure fall and the construction (cross-flow), respectively. The MAK 200, being of the cross-flow type, also creates shock waves due to a large pressure fall but the initial diameter of the aerosol chamber is larger than for the SC-2; so fewer collisions will occur. The indirect values can of course be high due to “shift in” of larger droplets. But the “shift in” phenomenon should affect all the nebulixers in the same way and cannot explain why only T-250-A3 and TN-l should be significantly low. Consequently, the fact that the directly measured and converted values are lower than the indirectly measured values is most probably attributable to collision of smaller droplets with larger ones and/or the walls of the aerosol chamber. The above arguments demonstrate that both direct and indirect methods have their advantages and drawbacks and that they excellently complement each other. Therefore the choice of the one method or the other or a combination of the two depends on the phenomenon one wishes to study. 5. CONCLUSION Carefully performed indirect efficiency measurements on nebuliier systems with low efficiencies, e.g. pneumatic nebulixers used with ICPs, will give values typically SO-130 % higher than those obtained with direct methods. This difference can be attributed to the following. (1) The nebulixing gas will be saturated with the solvent, thus making indirect values larger than direct ones. The solvent vapour will come from the bulk of the solution in the aerosol chamber and from the generated aerosol. The origin of the solvent vapour is only important in the initial stage before the gas is saturated. (2) Small droplets having diameters < d, will be retained in the aerosol chamber owing to collisions with other droplets and the walls of the aerosol chamber. (3) Droplets with diameters > d, can be “shifted in”, i.e. they evaporate to such an extent that their diameter will be d d, before they impact with the walls of the aerosol chamber. %JMMARY
A direct method for measuring the efficiency of nebulixer systems is described and ex~~men~lly compared with an indirect method under similar conditions. Results for various cross-flow and concentric nebulixers are presented. It is shown that the present direct method has equal or better accuracy and precision than other direct methods. The analyte transport efficiency of cross-flow and concentric nebulixers found for ICP systems are typically 0.7-1.7x. Carefully performed indirect measurements will give 50-l 30 y0 higher e5ciency values than direct measurements. The reasons for this difference are discussed. It is concluded that the choice of a direct or an indirect measuring method is dictated by the phenomenon one wishes to investigate. Acknavledgements-The author thanks FOLKEINGMAN for stimulating discussions manuscript and the Swedish Natural Research Council for financial support.
Department of Analytical Chemistry Royal In~tit~te of Tech~ol~~y S-100 44 Stock~lm Sweden
and comments regarding the
ANDERS GUSTAV~~ON
[14] J. A. BOROWIEC,A. W. BOORN, J. H. DILLARD, M. S. CRESSER,R. F. BROWNERand M. J. MATTESON,Anal. Chem. 92, 1054 (1980).