JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL. 1 55 Improvement of a Pneumatic Nebuliser for Atomic Absorption Spectrometry* Barry T. Sturman Varian Techtron Pty. Limited, 679 Springvale Road, Mulgrave, Victoria 3 7 70, Australia The importance of the liquid transport characteristics of atomic absorption nebulisers is discussed and a simple method for determining these characteristics reported. This method was used to optimise the pumping characteristics of a commercial pneumatic nebuliser. Compared with the original nebuliser, the optimised version is significantly less sensitive to variations in the level of sample liquid (approximately a factor of five improvement). The effects of the nebulising gas pressure and sample uptake rate are presented and the improved analytical performance of the new nebuliser is shown.Keywords: Nebulisers; flame atomic absorption; pumping characteristics The precision and accuracy of flame atomic absorption spectrometry depend critically on the performance of the nebuliser and spray chamber. A fundamental requirement is that the rate of supply of solution to the nebuliser remains constant for all the standards and samples in an analysis. In commercial flame atomic absorption instruments it is current practice for the pneumatic nebuliser to also act as a pump, transporting liquid from the sample vessel through a length of flexible sampling capillary tube and the nebuliser capillary into the venturi where the primary nebulisation takes place. If the nebuliser does not perform adequately as a pump, the uptake rate may vary with the level of sample liquid relative to the nebuliser.As the absorbance typically varies with the uptake rate, this can result in solutions of the same concentra- tion giving different absorbances, depending on the level of liquid in the vessel. While much work has been reported on the characterisation of aerosols produced by atomic absorption nebulisers,l the pumping characteristics of these nebulisers have received less attention.2-4 O’Grady et a1.2 evaluated three methods for the measurement of “nebuliser suction ,” and found that under typical operating conditions the suction (the pressure differ- ence between the end of the capillary in the venturi and the atmosphere) varied with the liquid uptake rate permitted by the dimensions of the sample capillary tube.This paper reports a simple and convenient procedure for determining the variation of the pressure difference deve- loped by the nebuliser with uptake rate. This procedure has been used to optimise the pumping characteristics of a commercial pneumatic nebuliser , resulting in significant improvements in analytical performance. Theory It is well established that at the flow-rates typical of atomic absorption nebulisers, the relationship between the rate of liquid flow in a narrow tube and the pressure difference between the ends of the tube is given by Poiseuille’s Equation xR4P 800qL * . * ‘ * Q=- * (1) where P = pressure difference in Pa; q = viscosity in poise; R = radius in mm; L = length in mm; and Q = liquid flow-rate in rnl s-1. As shown by O’Grady et ul.,* it cannot be assumed that the pressure difference P across the capillary of a pneumatic * Presented in part at the 36th Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, New Orleans, LA, USA, March 1985. nebuliser is independent of the uptake rate. To optimise the pumping characteristics of atomic absorption nebulisers it was necessary to determine the relationship between the pressure difference developed by the nebuliser and the uptake rate. For a given capillary tube and a sample of viscosity q it is convenient to define . . . * (2) 8OOq L a=- nR4 * * ’ ’ so from equation (1) P Q=- . . . . . . * (3) a The quantity a is the resistance to flow resulting from the tube dimensions and sample viscosity, and is subsequently referred to as the hydrodynamic resistance.To calculate the hydrodynamic resistance for a nebuliser, it is necessary to know the viscosity of the sample liquid and the dimensions of both the capillary in the nebuliser and the sampling capillary. With the capillary diameters used in this study, pressure losses at the joint of the two capillaries were insignificant and the total hydrodynamic resistance was calculated as the sum of the resistance of each of the two capillaries. The viscosity of the distilled water samples was obtained from the temperature of the samples using standard published tables.5 Having determined the hydrodynamic resistance and measured the uptake rate, one can calculate the pressure difference developed by the nebuliser from equation (3).Repeating the procedure with a number of sample capillaries of different dimensions then reveals the relationship between uptake rate and pressure drop for the nebuliser. Experimental Nebulisers The design and construction of the Varian nebulisers used in this work has been discussed previously.6 The nebulisers are available as an adjustable uptake model, in which the position of the nebuliser capillary in the venturi can be adjusted by the user, and as a fixed uptake model in which the position of the nebuliser capillary in the venturi is not adjustable.6 The improvements in performance reported here were obtained by optimising the position of the capillary in the venturi, as will be explained later. Liquid Uptake Rate Measurements To measure uptake rates unaffected by hydrostatic head effects we used the apparatus shown in Fig.1. As indicated,56 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL. 1 Burette- R calibrated Water 7 1 I absorption c( NehtiliSer Ca pi1 la ry Fig. 1. Apparatus for measuring the uptake rate of the nebuliser 60 I I 0 I 2 3 4 5 6 7 8 9 1 0 Uptake rate/rnl min-’ Fig. 2. Relationship between uptake rate and pressure difference developed by the nebuliser for two nebuliser settings: A, original setting for the “fixed uptake” version6; and B, new maximum pressure drop setting the modified calibrated flask was located so that the gradua- tion mark was at the same height as the platinum - iridium capillary in the nebuliser. For each measurement, the flask was initially filled with distilled water to a level above the graduation mark.Timing commenced as soon as the water level reached the graduation mark and water was added from the burette to keep the level constant during the timed period. At the end of the timed period the burette was turned off and the volume of liquid added was read. Uptake rates were varied by using different lengths of 0.6 mm i.d. and 0.4 mm i.d. polyethylene tubing. The mean diameters of these capillary tubes were calculated from the mass of water contained in a known length. The pressure difference developed by the nebuliser at zero uptake rate was measured by connecting a U-tube mercury manometer directly to the platinum - iridium capillary. Atomic Absorption Measurement Analytical studies were carried out using a Varian SepctrAA 40 atomic absorption spectrometer equipped with a Varian PSC-56 sample changer. Operating conditions were as set by the instrument software.The glass impact bead position and burner orientation were adjusted to obtain maximum absor- bance for each element. Results and Discussion The original capillary setting for the Varian fixed uptake rate nebuljser is shown by Howarth et al. in Figure 2,A of their paper.6 The plot of the pressure difference developed by the nebuliser versus uptake rate for this nebuliser is shown in Fig. 2. The pressure difference developed by this nebuliser did not vary greatly with uptake rate. At the recommended uptake rate of 4.5-5 ml min-1 the pressure difference developed by the nebuliser was close to 9.5 kPa.The pressure difference resulting from a hydrostatic head change of 10 cm of water is 0.98 kPa, which is a significant fraction of the total pressure difference developed by the nebuliser. Clearly, it would be advantageous to increase the pressure difference at normal uptake rates so that the pressure differences resulting from changes in the liquid level in sample vessels were a smaller fraction of the total pressure drop across the nebuliser capillary. The position of the capillary in the venturi throat was adjusted until the pressure drop was at a maximum. This setting is shown by Howarth et al. in Figure 2,B of their paper.6 The relationship between pressure drop and uptake rate for this modified nebuliser is shown in Fig. 2.The pressure drop developed at uptake rates in the range 3-5 mi min-1 was at least a factor of three greater than with the former setting of the nebuliser, and the pressure drop decreased very much more rapidly with increasing uptake rate. It was found that the relationship between pressure drop P and uptake rate Q shown in Fig. 2 could be described by where fi and y are constants for a given nebuliser driven at a specified nebulising gas pressure and Po is the pressure drop at zero uptake rate. For a given hydrodynamic resistance a, by definition from equation (3), so from equation (4) P=P,-cjQ-yQ2 . . . . . . (4) P = aQ (5) or Note that when P and y = 0 equations ( 5 ) and (6) are equivalent to Poiseuille’s equation. From equation (6), ignoring the physically meaningless negative solution, Q = ye’+ (P+ a)Q-P()=O .. . . (6) . . . . ( 7 ) [(P + a> + 4Ypo1°.5 - (fi + a) 2Y Equations (5) and (7) proved to be very useful for establishing nebuliser parameters. Equation ( 5 ) was used to find the hydrodynamic resistance required to obtain a specified uptake rate, and hence to select the required dimensions for the sampling capillary for the nebuliser, given the dimensions of the platinum - iridium capillary in the nebuliser. Equation (5) can also be used to predict the effect of changes in sample viscosity on uptake rate. As indicated in equation (2), in a given system the hydrodynamic resistance cx is proportional to the sample viscosity 7 . By differentiating equation ( 5 ) it is easy to show that the fractional change in uptake rate for a given fractional change in resistance is If the pressure drop is independent of uptake rate, fi and y = 0 and d& da .. * . . . . . -- _ - - Q a (9) When S and y are positive, as they are for the modified nebuliser (Fig. 2, line B) <1 Po - PQ - YQ’ Po + and from equations (8) and (9) it is clear that the fractional change in uptake rate for a given fractional change in sample viscosity is less than it would be if the pressure drop developed by the nebuliser were independent of uptake rate.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986. VOL. 1 57 Table 1. Hydrostatic head effects in the analysis of a 5 mg 1-1 copper solution, as explained in the text Average error (Yo) = (mean result for 5-mi tubes - mean result for full tubes) x 100 mean result for full tubes Nebuliser Average error, Yo O1d“fixed” .. . . . . -5.4 0ld“variable” . . . . . . -2.4 New . . . . . . . . -0.6 Table 2. Analysis of 5 mg I-’ copper in 5% sodium chloride solution, as explained in the text mean reported result - 5 mg 1-1) 5 mg 1-1 Error = ( Relative Samples standard Nebuliser analysed deviation, YO Error, Yo Old “fixed” . . 15 1.1 -0.42 New . . . . 26 0.7 -0.24 Equation (7) can be used to predict the effect of hydrostatic head changes on uptake rate. The pressure drop at zero uptake rate is given by Po = PA - P N where PA is the external pressure (i.e., atmospheric pressure) and PN is the pressure in the nebuliser venturi at the exit of the capillary tube (at zero uptake rate). From equation (lo), it is obvious that any change in atmospheric pressure PA will result in the same change in Po.For the purposes of calculation, hydrostatic head effects may be treated as changes in atmospheric pressure and hence as changes in Po. For a given system the hydrodynamic resistance a is constant and and y are assumed to be constant for small changes in Po. With constant a, B and y, and differentiating with respect to Po, . . . . . . (10) Calculations of the expected hydrostatic head effect with the new nebuliser setting indicated a significant improvement over the old setting. To investigate this experimentally we took ten test-tubes filled with a 5 mg 1-1 copper solution and placed them in a Varian PSC-56 sample changer alternated with ten test-tubes containing only 5 ml of the same test solution.Copper was used as the test element because the absorbance (at uptake rates near 4 ml min-1) was known to be significantly altered by changes in the uptake rate. The hydrostatic head difference 1 .o 0.9 0.8 0.7 & 0.6 9 2 0.5 n 0.4 0.3 0.2 0.1 0 0 1 2 3 4 5 6 7 8 9 1 0 Uptake ratelm1 min-’ Fig. 4. Effect of uptake rate on the absorbance in the air - acetylene flame for: A, copper ( 5 mg 1-1); B, chromium ( 5 mg I - I ) ; and C, magnesium (0.2 mg 1-1). Uptake rates were altered by using different lengths or diameters of polyethylene sampling capillary 0.6 I 0.5 8 0.4 C ((1 + 2 a 0.3 0.2 0.1 0 1 2 3 4 5 6 7 8 9 1 0 Uptake rate/ml min-’ Fig. 5. Effect of uptake rate on the absorbance in the nitrous oxide - acetylene flame for: A, calcium (1.5 m 1-1 + 1 g 1-1 of potassium); B, aluminium (50 mg 1-1); and C, silicon 800 mg 1-I).Uptake rates were altered by using different lengths or diameters of polyethylene sampling capillary between a full tube and one with only 5 ml of sample was 12.5 cm of water (1.23 kPa). This was very close to the maximum hydrostatic head difference that might occur in the use of this sample changer. The spectrophotometer was calibrated using a single standard of 5 mg 1-1 of copper in a full test-tube and the 20 “samples” were analysed automatically. The test was carried out three times, with the same nebuliser set up first as the fixed uptake version as indicated by Howarth et al. in Figure 2,A of their paper,6 then as the adjustable uptake version (Figure 2,C in reference 6) and finally with the new maximum pressure drop setting.The uptake rates of all three configurations were very similar (3.8-3.9 ml min-1). This was achieved either by choice of appropriate capillary dimensions or (with the variable nebuliser) by adjusting the uptake rate control. Results of these tests are shown in Table 1. A worthwhile improvement in the accuracy of the analysis was achieved with the new nebuliser setting. A potential probelm with any alteration of a nebuliser is a possible increase in the tendency of the nebuliser to clog when solutions containing high levels of dissolved solids are analysed. To investigate this, we carried out an automated analysis of a test solution of 5% mlV sodium chloride solution58 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, FEBRUARY 1986, VOL.1 containing 5 mg 1-1 of copper, using the nebuliser in the old fixed uptake configuration and with the new maximum pressure drop setting. The variable uptake version is not recommended for samples with high levels of dissolved solids. The single calibration standard was the 5 mg 1-1 of copper in 5% sodium chloride test solution in a full test-tube. In each instance the automated analysis was carried out using a series of test-tubes filled with the solution, eliminating any hydros- tatic head effects. The sample number at which there was any visible disruption of the flame by clogging of the burner was noted. Analysis was continued until the reported result was below the lowest result reported before the flame was visibly disrupted by clogging.The number of samples analysed and the standard deviation of the results are shown in Table 2, together with the average percentage error. In both the old and the new configuration the nebuliser did not clog. The duration of the test was limited by the clogging of the burner slot with salt deposits. It was pleasing to find that the new nebuliser version not only failed to clog but also processed more samples with marginally better precision and accuracy than was obtained with the nebuliser set in the old configuration. Having found useful improvements in analytical perfor- mance with the new nebuliser configuration, we decided to develop a commercial version. We carried out detailed studies of the effect of oxidant gas pressure and of solution uptake rates on the performance of the new nebuliser.The effect of oxidant gas pressure on pressure drop and uptake rate with a given hydrodynamic resistance is shown in Fig. 3. A maximum occurred close to 200 kPa oxidant gas pressure. Analytical studies showed that maximum signal to noise ratio was also obtained at ca. 200 kPa oxidant gas pressure, and this pressure was used for all subsequent work. The uptake rate of the new nebuliser is controlled by the hydrodynamic resistance of the capillary system. The effect of varying the uptake rate (by using different capillaries) on the absorbance of some elements measured in the air - acetylene flame is shown in Fig. 4. While the absorbance of the relatively easily atomised element copper continued to increase with increasing uptake up to at least 8 ml min-1, the more refractory element chromium showed no increase in response above 5 ml min-1 and a decrease in response above 7 ml min-1.Use of high uptake rates is undesirable because of increased clogging of the burner and increased risk of interferences, and, as shown here, may not even lead to an increase in the absorbance signal. The levelling out and eventual decrease of absorbance at higher uptake rates could perhaps be the result of overloading the flame with water and so decreasing the efficiency of atomisation. Another contri- buting factor could be a change in the droplet size distribution of the aerosol at higher uptake rates, resulting in a greater proportion of large droplets reaching the flame. As shown in Fig. 5 , the effect was even more severe with elements measured in the nitrous oxide - acetylene flame.With the three elements shown, the effect was least for calcium , which is relatively easily atomised and less sensitive to flame conditions. The effect was very pronounced for silicon, which is difficult to atomise and extremely sensitive to flame conditions. These tests, and others investigating the effect of uptake rate on precision, led to the conclusion that an uptake rate of between 4 and 5 ml min-1 was the most appropriate for general analytical use. This uptake rate was achieved by using a 238 mm x 0.38 mm i.d. polyethylene sampling capillary. Other uptake rates could of course be obtained by using a longer (or narrower) capillary for a decreased uptake rate and a shorter (or wider) capillary for an increased uptake rate. For the reasons stated previously, we do not recommend the use of uptake rates above 5 ml min-1. At the recommended uptake rate (4-5 ml min-1) the sensitivity and precision of the modified nebuliser were very similar to those reported for the original “fixed” version.6 Conclusion We have developed a simple and convenient procedure for characterising the liquid transport or pumping properties of atomic absorption nebulisers. Use of this procedure has led to the modification of a commercial pneumatic nebuliser, resulting in worthwhile improvements in analytical perfor- mance. The author wishes to thank Ted Rothery for suggesting this approach to measuring the pumping characteristics of nebu- lisers and for helpful discussions. References 1. 2. 3. 4. 5. 6. Browner, R. F., and Boorn, A. W., Anal. Chern., 1984, 56, 786A, 87.5A, and references therein. O’Grady, C., Marr, I. L., and Cresser, M. S . , Analyst, 1984, 109, 1085. O’Grady, C. E., Marr, I. L., and Cresser, M. S . , Analyst, 1984, 109, 1183. O’Grady, C. E., Marr, I. L., and Cresser, M. S . , Analyst, 198.5, 110, 431. Weast, R. C., Editor, “CRC Handbook of Chemistry and Physics,” CRC Press, Boca Raton, FL, 1982, p. F40. Howarth, H., McKenzie, T. N., and Routh, M. W., Appl. Spectrosc., 1981,359, 164. Paper J5/8 Received June 17th, 1985 Accepted August 15th, 1985