Behaviour of a desolvation system based on microwave radiation heating for use in inductively coupled plasma atomic emission spectrometry

SPECTROCHIMICA ACTA PART B

ELSEVIER

Spectrochimica Acta Part B 52 (1997) 1201-1213

Behaviour of a desolvation system based on microwave radiation heating for use in inductively coupled plasma atomic emission spectrometry 1 Luis Gras, Juan Mora, Jos6 L. Todolf, Vicente Hernandis, Antonio Canals* Departamento de Qu[mica Anal[tica, Universidad de Alicante, E-03071 Alicame, Spain

Received 26 April 1996; accepted 23 November 1996

Abstract

The present paper describes the preliminary results obtained with a desolvation system for inductively coupled plasma atomic emission spectrometry that incorporates a heating unit based on microwave (MW) radiation. This system has been called Microwave Desolvation System (MWDS). The results have proved that MW radiation can be considered as a good choice for aerosol heating in a sample introduction system. MW radiation seems to be a more uniform way of aerosol desolvation than conductive/convective heating (i.e. lower radial temperature gradients), the degree of vaporization of the droplets is less dependent on the liquid flow rate (Q0, and also the background noise associated with the vaporization of droplets is reduced. As regards the results obtained with MWDS, in comparison with a conventional desolvation system (CDS), they are very dependent on Qi. When heating is applied, the amount of analyte that leaves the heating step increases by 30-60% with the MWDS, irrespective of Qi, whereas for the CDS this increase is very high (up to 300%) at low Ql values (0.4 ml min-I), but almost negligible at high Q~ values (2.4 ml min-~). In agreement with this, the analytical figures of merit are favourable to the CDS at low flow rates, and to the MWDS at high liquid flows. Under all the conditions studied, the amount of solvent that leaves the condensation unit are lower for MWDS than for CDS. © 1997 Elsevier Science B.V. Keywords: Aerosol heating; Microwave radiation; Microwave desolvation system; Inductively coupled plasma atomic emis-

sion spectrometry

1. I n t r o d u c t i o n

In Atomic Spectrometry, samples are usually introduced as aerosols, generated from appropriate solutions of these samples. This way, a large solvent load is introduced together with the analyte. A fraction of this solvent reaches the atomization cell as a vapour while the rest remains in liquid form. In spite of the

* Corresponding author. Presented at First Mediterranean Basin Conference on Analytical Chemistry, Crrdoba, Spain, November 1995.

great effort that has been devoted to the study of the interactions between plasma and analyte on one hand, and plasma and solvent on the other, some contradictory facts remain to be fully understood. The presence of solvents affects the geometry and the excitation characteristics of the plasma. The influence of the solvent load depends on the solvent nature and on its physical form (i.e., liquid or vapour) when it reaches the plasma. Two types of interference, spectral and non-spectral, are due to the solvent load. Solvent dissociation gives rise to species that modify the plasma background emission, such as OH

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(especially in the case of water), pyrolysis products of organic solvents etc., the molecular bands of which may coincide with the atomic lines, thus affecting blank subtraction [1[. In Inductively Coupled Plasma Mass Spectrometry, oxygenated and/or hydrogenated species originated from solvent molecules (ArO +, O +, MO +, MOH + etc.) [2,3] may interfere with the determination of some elements. As regards non-spectral interferences, there is a power consumption in the plasma associated with the process of vaporization, atomization, ionization and/or excitation of the solvent molecules that reduces the efficiency of analyte atomization [1,4,5]. Moreover, the solvent load, especially in liquid form, contributes to both signal and background noise due to the random fluctuations that its presence causes on the local characteristics of the plasma [6]. In addition, low in the plasma, the still incompletely desolvated droplets can affect the ionatom equilibrium of the already atomized sample, thus enhancing atomic emission and reducing ionic emission [7]. When the solvent is introduced as a vapour, the negative effects caused by its presence are considerably reduced [4]. Solvent atomization causes atomic and electronic density to increase. This, in turn, leads to a reduction in the volume and to an increase in the brightness of the plasma [1]. Several studies have been carried out to evaluate the effect of the solvent on the characteristics of the plasma (ne, Text, T~.... etc.), but the results are sometimes contradictory [8-15]. In many cases, the differences among the behaviours reported are due to the lack of information on the amount of solvent that has been introduced in the plasma. Anyway, two points seem to be clear: firstly, the physical form of the solvent plays an important role in the absolute emission intensity and on the stability of the signal; secondly, a high solvent load lowers the thermal capacity of the plasma, thus reducing the analytical signal, and can even lead to plasma extinction. With the aim of improving sensitivity in Atomic Spectrometry, a great effort is currently being devoted to the development of more efficient nebulizers that allow the analyte transport rate to increase. When these nebulizers are used, large solvent loads are introduced together with the analyte. The result is that improvements in sensitivity are not as great as expected from the point of view of analyte transport improvement, for the above-mentioned reasons.

Therefore, any study about more efficient systems for the introduction of liquid samples should have the following goals: (1) to produce primary aerosols as fine as possible, better if the aerosol generation is independent of the carrier gas flow; (2) to achieve high analyte transport rates (Wtot); (3) to remove most of the solvent in order to achieve low solvent transport rates (Sto0. Obviously, these goals are not independent of each other, and a compromise is usually required. Globally speaking, the development of nebulizers and spray chambers that are mainly related to goals (1) and (2) has attracted more interest than the development of desolvation systems which mainly affect goal (3). In order to accomplish the third goal, several desolvation systems have been proposed. Most of them consist of a first heating step in which the solvent is totally or partially evaporated from the aerosol droplets, and a second step in which solvent vapour is removed from the aerosol stream. This way, at the exit of the desolvation system the aerosol would be composed, ideally, of dry particles and carrier gas. Vapour removal is usually carried out by condensation on cold surfaces [16-22], which is a simple way of doing it. However, its effectiveness is hampered because of nucleation [23,24], since a given fraction of the droplets formed by nucleation are not removed and, hence, this contributes to Stot. Nucleation extent can be reduced by carrying out condensation in two steps, at two different temperatures. This way, oversaturation is always kept at a much lower level than with just one condenser. Vapour removal from the aerosol stream can also be done without condensation through membrane extraction [25-28]. Membrane extraction reduces nucleation and avoids contact between the aerosol and the liquid film of condensed solvent that flows down on the inner walls of the condenser. However, it shows a limited capability for vapour removal (i.e. its vapour removal rate is not as high as that of condensation systems). Besides, in both systems the reduction in Stot is accompanied by a more or less noticeable reduction in W~,,t. Aerosol heating, which precedes vapour removal, can take place in the spray chamber [16] or in a glass extension tube placed just at the exit of the spray chamber [3,14]. Heating the spray chamber is more efficient, in terms of improving the analyte transport rate, since most aerosol loss takes place there.

L. Gras et aL/Spectrochimica Acta Part B 52 (1997) 1201-1213

Usually, the walls of the spray chamber or the extension tube are kept very hot by means of a heating tape wound around it. Temperature is controlled by means of a thermocouple or a similar device [3,16]. Heat is transferred to the aerosol droplets by a conduction/ convection mechanism that gives rise to steep temperature gradients, as well as turbulences, that are detrimental to a smooth solvent vaporization. In addition, the fact that the wall temperature is much higher than that of the aerosol, on average, contributes to increased background and signal noise owing to pressure fluctuations caused by the sudden and violent evaporation of droplets that impact against the hot walls [29[, thus deteriorating the analytical performance of the whole process. Moreover, the analyte contained in these droplets remains adhered to the walls, causing Wtot to decrease, whereas memory effects become more significant. The relative importance of all these unwanted effects increases with the size of the droplets that impact against the walls. Aerosol heating can also be done by absorption of radiation, such as IR [30,31] or U V - V I S [18,32], instead of convection/conduction. Nevertheless, as in most of the cases radiation is also absorbed by the spray chamber (or the extension tube), a simultaneous contribution of convection/conduction occurs, thus increasing the efficiency of the global process. Aerosol radiative heating is quicker than resistive heating and reduces, but does not completely eliminate, the drawbacks of the latter. In chemical laboratories, microwave (MW) radiation is commonly and successfully used in several fields, such as MW spectroscopy [33], sample treatment [34] or reaction kinetics [35]. In principle MW radiation could also be applied to aerosol heating as a good choice for conventional systems. In addition, several materials are almost transparent to MW radiation (PTFE, quartz, etc.). Therefore, the spray chamber (or extension tube) can be manufactured using one of these materials, thus avoiding or greatly reducing the problems associated with overheating of the walls, since they would remain relatively cold as compared to the aerosol. Nevertheless, there is some controversy concerning the MW absorption by droplets. Some time ago, Bemdt [36] patented a desolvation system based on MW aerosol heating. Recently, Tyson et al. [37-39] have revisited the same idea. These authors have

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indicated that from the fundamental theoretical point of view MW does not couple efficiently to droplets and, hence, MW radiation is an unsatisfactory desolvation method. One reason invoked to explain these results was that aerosol drops are unable to absorb MW radiation since they have a much lower size (a few microns) than the wavelength used (cm). In our opinion, MW radiation is absorbed by individual water molecules placed inside a MW field containing the appropriate frequencies, irrespective of the physical state of the sample (solid, liquid or gas, in small portions or in large aggregations). The only conditions are that the field should contain the resonant frequencies corresponding to the rotational transitions of the molecule and that the field intensity should not be zero. Therefore, the absorption process will always take place when the sample is in gaseous form as well as when it is present as a mist (aerosol). There are some references [40-42] which describe the attenuation (absorption) of MW signal by clouds and atmospheric water vapour in remote sensing. Trying to explain the unsuccessful results, a speculative explanation could be as follows: In spite of the fact that the patent by Berndt [36] does not include neither experimental data nor results, it seems to us that such a system would be unable to properly work, since in FAAS the typical gas flow is very large (1215 1 min-l). This implies that: (1) the dwell time spent by a single drop in the irradiated zone is much lower than in ICP, about 10 times lower, so that the final liquid temperature would also be lower; (2) the combustion gases do not absorb MW radiation, so that heat is transferred from droplets to gas, thus lowering the temperature of the drops. In summary, water evaporation seems very unlikely to us with such a system, due to the high gas flow which is typical in FAAS. Trying to explain the results by Tyson [39] could be even more speculative since no experimental details have been published in periodical journals. However, it seems surprising that, with 30% of the analyte transported out of the spray chamber located in the microwave field, they conclude that microwaves do not couple very efficiently to water droplets. No other results on microwave desolvation have been published up to now. The present work is aimed at evaluating the behaviour of a desolvation system that includes an aerosol heating step based on MW absorption for use in

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Inductively Coupled Plasma Atomic Emission Spectrometry (ICPAES). Hence, this desolvation system has been called Microwave Desolvation System (MWDS). To our knowledge, this is the first time that successful results are obtained with MW radiation as an aerosol desolvation system although data do not allow one to explain unequivocally the mechanism of the effects observed.

(model AR-35-C2, Glass Expansion Pty., Australia) was used in all the experiments. Argon was used as the nebulizing gas when the ICPAES emission signal had to be measured and air was used in the remaining experiments (i.e., drop size distributions and transport measurements). Gas flow was varied from 0.70 to 1.20 l min -j by means of a calibrated flowmeter (Cole-Parmer Ins., Chicago, IL, USA). Liquid flow was varied from 0.4 to 2.4 ml min -~ by means of a peristaltic pump (model Minipuls 3, Gilson, Villiersle-Bel, France). All the experiments, with the exception of emission intensity measurements, were done using a solution containing 100/~g m1-1 of Mn. The partially evaporated aerosol leaves the spray chamber placed inside the MW oven through a silicone rubber tubing, the length of which was 30 cm with an inner diameter of 0.8 cm. A conventional desolvation system (CDS) based on resistive heating was used for comparison. This system incorporated the same spray chamber, but it was heated by means of a heating tape (265 W, 150 cm long, J.P. Selecta, Barcelona, Spain) wound around it. Heating was controlled by means of a contact thermometer that kept the spray chamber wall temperature at 130 °C. Solvent vapour was removed, in both cases, by means of a two-step condensation unit consisting of two Liebig condensers (33 cm long, 1.2 cm inner diameter) coupled in series, the first one kept at 25 °C and the second one at 1 °C, so as to reduce the extent of the nucleation processes. A thermostated bath (model F3-K, Haake Mess-Technik, Kalsruhe, Germany) was used to control the temperature of the second condenser. The heating unit and the condensation unit were connected, in both cases, by

2. Experimental Fig. 1 shows a scheme of the experimental setup of the MWDS [43]. The aerosol is generated in a spray chamber placed inside the cavity of a domestic microwave oven, in the position of maximum radiation intensity, where rapid heating and evaporation takes place. The aerosol, consisting of totally or partially desolvated droplets, solvent vapour and carrier gas, leaves the spray chamber through silicone rubber tubing connected to the condensation step, where most of the solvent vapour is removed. The resulting almost dry aerosol is then introduced into the base of the plasma torch. A domestic MW oven (model W-2235, Balay, Zaragoza, Spain), with a power of 890 ___ 30 W, was used as the microwave source. The power setting was always kept at its maximum value (i.e., without time chopping). The spray chamber was made of Pyrex glass (single-pass type, 14 cm long and 3 cm inner diameter). (Caution: A beaker containing about 500 ml of water was also permanently placed inside the oven so as to absorb most of the M W radiation, since, otherwise, the magnetron could have been damaged). A concentric pneumatic nebulizer

1 I

:i

dry aerosol

f

Fig. 1. Experimental setup of the MWDS.( 1) Microwaveoven,(2) spraychamber, (3) nebulizer, (4) condensationunits, (5) condensedsolvent vent, (6) sample, (7) spray chamberdrain, (8) nebulizing gas, (9) peristaltic pump.

L. Gras et aL/Spectrochimica Acta Part B 52 (1997) 1201-1213 Table I ICPAES operating conditions Outer gas flow (1 min i) Intermediate gas flow (1 min i) Nebulizer gas flow Liquid flow Incident Power (W) Reflected power (W) Integration time (s) Observation height above load coil (mm) PMT gain

16 1.7 Variable Variable 1000 < 5 0.2 8~ 4

Optimum for all the arrangements.

means of a piece of silicone rubber tubing measuring 20 cm long and with an internal diameter of 0.8 cm. Aerosol drop size distributions were obtained by means of a laser Fraunhofer diffraction system (model 2600c, Malvern Instruments, Worcestershire, UK) using a lens with a focal length of 63 mm that allows the measurement of drop diameters from 1.2 to 118 #m. The software employed was B.0D. A modelindependent algorithm was used to transform energy data into drop-size distribution. The instrument was previously calibrated by means of a calibration reticle (model IM 062, Malvern Instruments, Worcestershire, UK). Measurements were taken at a distance of 35 mm from the lens and 5 mm from the exit of the tubing used to convey the hot aerosol to the condensation unit. Analyte and solvent transport rates (Wto t and Stot, respectively) were determined by direct methods [44,45]. Stot was measured by nebulizing the solution, collecting the tertiary aerosol in U-tubes containing dried silica gel and weighing the amount of solvent collected. Wtot was measured by nebulizing the solution and trapping the exiting tertiary aerosol with glass Table 2 Elements, wavelengths and spectral bandwidths Element

Wavelength(nm)

Spectral bandwidth (rim)

Zn I Ni u Co n Cd I Mn II Fe ]l Cr II Cu i Ag I

213.856 221.647 228.616 228.802 257.610 259.940 283.563 324.754 328.068

0.2 0.1 0.2 0.1 0.1 0.2 0.2 0.2 0.2

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fiber filters (Type A/E, 47 mm diameter, 0.3 #m pore size, Gelman Sciences, Ann Arbor, MI, USA). Collected aerosols were washed from the filters into volumetric flasks with 1.0% (w/w) nitric acid. The analyte content in each volumetric flask was measured by FAAS. Emission intensity was measured with an ICPAES spectrometer (model ICP 2070, Baird, Bedford, MA, USA). Table 1 lists the instrumental conditions. A solution of 1/zg ml -I of Mn was used for the measurement of the signal and a 1 #g ml -l multielemental solution (ICP multielemental standard solution IV, Merck, Darmstadt, Germany) was used for the determination of the limits of detection (LOD). Table 2 lists the elements and lines employed. LODs were calculated according to the 3Sb criterion, Sb being the standard deviation from twenty replicates of the blank. For the sake of comparison, the results obtained with a standard sample introduction system, consisting of the same concentric pneumatic nebulizer adapted to a double-pass Scott spray chamber without desolvation system, have been included.

3. Results and discussion

3.1. Drop-size distributions of the aerosols leaving the heating unit Measuring the drop-size distribution of hot aerosols presents some difficulties. In the first place, nucleation, which just starts when heating ends, causes the distribution mean size to increase [46]. In the second place, when drop size distribution is measured by means of a laser Fraunhofer diffraction system, local temperature gradients in the measurement zone may give rise to random refraction of the beam, called beam steering, that is accounted for by the system as an apparent increase in the number of large drops [47]. In spite of these problems, a correlation between the distribution parameters of the emerging aerosol and the processes that take place inside the heated spray chamber is to be expected. Table 3 shows the effects of gas and liquid flows, Qg and Q1, on three distribution parameters of the aerosol at the exit of the heating units: (1) the median of the volume distribution, Ds0; (2) the fraction of the aerosol liquid volume contained in droplets smaller

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Table 3 Effect of Qg and Qi on the aerosol drop size distribution parameters at the exit of the heating units ~ Qg (I min -I)

With aerosol heating

Room temperature QI (ml min -I) CDS

MWDS

CDS

MWDS

Dso

Vi2

VC

Dso

VI2

VC

D~o

Vi2

VC

Dso

Vi.2

VC

(#m)

(%)

(ppm)

(~m)

(%)

(ppm)

(~m)

(%)

(ppm)

(~m)

(%)

(ppm)

0.70 0.85 1.00 1.20

2.4 2.4 2.4 2.4

7.96 7.24 6.47 5.60

7.5 8.8 10.1 11.8

274 592 739 749

5.40 5.64 5.47 5.37

12.6 12.0 12.4 12.7

2t8 469 584 652

7.83 7.07 6.75 5.70

8.0 8.6 8.9 11.3

462 418 405 372

7.65 6.97 6.57 6.23

9.2 10.6 13.0 14.0

193 253 283 368

0.70 0.85 1.00 1.20

0.4 0.4 0.4 0.4

6.10 5.29 4.70 4.29

9.2 10.9 12.0 14.2

478 652 735 753

6.15 5.08 4.84 4.31

6.8 13.3 13.4 12.4

473 543 620 684

6.77 6.46 5.62 5.46

9.4 10.3 12.4 15.7

195 152 117 54

7.90 6.91 5.91 6.45

7.2 10.1 10.8 13.6

355 399 359 378

1.00 1.00 1.00 1.00 1.00

0.4 0.9 1.4 1.9 2.4

4.70 4.87 5.43 6.03 6.47

12.0 12.0 11.2 10.5 10.1

735 763 754 741 739

4.84 4.81 4.97 5.43 5.47

13.4 13.3 13.0 12.5 12.4

620 756 703 678 584

5.62 5.57 5.62 6.42 6.75

12.4 11.9 11.2 10.0 8.9

117 475 518 553 605

5.91 5.66 5.78 6.02 6.57

10.8 11.5 12.9 14.4 13.0

359 362 349 302 283

a See text for explanation of the employed acronyms.

than 1.2/xm, Vl2; and (3) the volume concentration, VC [48], defined as the fraction of volume that is actually occupied by liquid aerosol in the measurement zone of the laser beam, expressed in ppm. These parameters were measured at the exit of both heating units, when heating was on as well as when it was off. In this point it is worth pointing out that the MWDS includes an additional piece of silicone rubber tubing 30 cm long which is not included in the CDS. At room temperature, Ds0 decreases when Qg is increased, whereas V~2 and VC increase. These trends are similar for both systems and for the two liquid flows tested, 0.4 and 2.4 ml min -j. These results can be discussed on the basis that, as Qg is increased: (1) the primary aerosol gets finer; (2) the initial velocity of the drops gets higher; and (3) the carrying capability of the gas stream is also enhanced. The first factor is expected to increase the amount (proportion) of aerosol that escapes the spray chamber [45], i.e., to increase the VC value. The second factor would contribute to lowering the cut-off diameter of the spray chamber, de, since impaction losses would become more likely, whereas the third factor would act in the opposite direction. The final result shows that, when Qg is increased, the exiting aerosol becomes

more abundant (i.e., VC increases), as expected from points (1) and (3). Moreover, the obtained aerosols are finer (i.e., Ds0 decreases and V~.2 increases); therefore, it seems that the second factor predominates over the third one. As regards the effect of Qj at room temperature, Table 3 shows that, as expected [49], the aerosol gets coarser (i.e., Ds0 increases and VIz decreases) as Q1 is increased, whereas the amount of aerosol (i.e., VC) remains more or less constant even though the amount of primary aerosol (i.e., Q0 is increased. These results can be discussed on the basis that, when Qi is increased: (1) the primary aerosol gets coarser; (2) the initial velocity of the drops decreases, so that impaction losses become a little less likely; (3) the carrying capability of the gas stream does not change. In principle, VC should increase in parallel with QI. However, this is counterbalanced by the fact that the primary aerosol gets coarser, so that a smaller fraction of it will leave the spray chamber. The final result is that VC does not significantly vary with Q1. Besides, the second factor is expected to enlarge the cut-off diameter of the spray chamber. This forecast is confirmed by the tendency of the exiting aerosol to get coarser as Qi is increased.

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Table 4 Effect of Qg and Q~ on transport parameters for the desolvation systems tested Qg

Q~

Room temperature

With aerosol heating

At the exit of the heating step

At the exit of the heating step

At the exit of the condensation unit

CDS

CDS

MWDS

CDS

Stot Wto, Stol Wtot Stol Wtot (,o.gs -I) ( n g s -I) (p.gs -]) ( n g s -I) (/~gs i) ( n g s -t)

Slot Wtot (/zgs i) (ngs -I)

Stot Wtot Stot Wtot (~gs t) ( n g s -~) ( # g s -I) (ngs -I)

(1min t) (ml min-I) MWDS

MWDS

0.70 0.85 1.00 1.20

2.4 2.4 2.4 2.4

438 607 830 840

34 46 57 66

396 549 761 771

31 40 53 61

2660 3340 3800 4240

30 45 56 78

3240 3950 4820 5940

35 56 70 81

145 252 315 419

8 12 16 20

96 162 235 301

13 26 36 45

0.70 0.85 1.00 1.20

0.4 0.4 0.4 0.4

428 590 670 781

31 47 50 64

392 510 654 745

29 42 47 64

1420 2510 3450 3700

114 136 159 154

2930 3520 3900 4190

38 57 69 84

ll2 130 182 211

62 91 125 116

87 93 123 127

18 26 30 30

1.00 1.00 1.00 1.00 1.00

0.4 0.9 1.4 1.9 2.4

670 823 841 852 830

50 57 58 59 57

654 766 795 777 761

47 56 54 52 53

3450 4080 4250 4090 3800

159 95 87 59 56

3900 4300 4850 5150 4820

69 86 87 86 70

182 348 357 357 315

125 80 20 18 16

123 198 238 246 235

30 37 39 35 36

On comparing the results for both systems, it appears that the cut-off diameter for the CDS is larger than for the MWDS, since exiting aerosols are finer for the latter. In the same direction, VC is higher for CDS than for MWDS. This result is not surprising, since the MWDS includes an additional piece of silicone rubber tubing 30 cm long which does not exist in the CDS. Table 3 also shows that heating usually results in an increase in Ds0. This striking behaviour can be explained as follows. The spray chamber has a given cut-off diameter at room temperature. Hence, drops with diameters that are larger than dc have little chance of escaping the spray chamber. When heating is applied, some of these drops lose a part of the solvent, reduce their diameter and enhance the possibility of getting out of the spray chamber. Though all the drops undergo size reduction because of solvent volatilization, the diameter reduction rate for the small drops is higher than for the large ones [46]. As a consequence of these two facts, one should expect an increase in Ds0 when heat is applied. This result is somewhat more pronounced for MWDS than for CDS.

The influence of heating o n VI. 2 is less defined. When heating is applied, all drops undergo a size reduction, on the one hand, which is relatively more significant for the smallest drops. This would cause VL2 to decrease. However, on the other hand, some drops the initial diameter of which was slightly larger than 1.2 ~m reduce it to a value lower than this, thus contributing to an increased V1.2. Hence, the overall influence of heating on V].2 is not easy to predict. The experimental results show that there is little influence of heating on V].2, irrespective of the type of heating employed. In general terms, heating the aerosol gives rise to a decrease in VC values. The extent of this reduction is about 40% for the MWDS, irrespective of the flow values employed. For the CDS, however, VC reduction is much more significant at low than at high liquid flows. This behaviour can be accounted for by taking into consideration some differences between both heating systems. With the CDS, as Qi is increased, the amount of energy (heat) absorbed per unit mass of nebulized liquid decreases, as does the aerosol temperature. With the MWDS, the amount of energy absorbed per unit mass of nebulized liquid is about

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the same, irrespective of QL (it is worth remembering that a relatively large volume of water is permanently placed in the MW oven). With this system, the energy used for solvent evaporation and for heating carrier gas and chamber walls is generated by the solvent itself (i.e., in the form of aerosol and on the spray chamber walls) on absorbing MW radiation, since the MW radiation directly absorbed by the chamber walls and by the carrier gas is negligible. Hence, the larger the volume of liquid aerosol in the MW radiation zone, the larger will be the amount of heat generated and the more effective the aerosol heating. In agreement with this, the aerosol temperature increases as Q~ is increased. In summary, CDS works better at low QI values, whereas MWDS works better at high Qt values.

3.2. Transport parameters As regards aerosol transport, analyte transport rate (Wtot) and solvent transport rate (Stot) are the relevant magnitudes to be measured so as to compare the behaviours of CDS and MWDS. They have been measured at two different points: (1) at the exit of the heating steps; (2) at the exit of the condensation unit.

3.2.1. Transport parameters at the exit of the heating steps First of all, Wtot and Stot have been measured at room temperature for both heating systems, at the end of the heating step. The results, shown in Table 4, are in good agreement with the VC data in Table 3. Thus, for both systems, analyte and solvent transport rates increase as Qg is increased, whereas they hardly vary with Qi. Besides, MWDS provides lower Wtot and Stot values than CDS, due to the longer and n arrover connecting tubing used in the first system which implies a smaller cut-off diameter for MWDS than for CDS. Table 4 shows that, when heating is applied, Stot increases with Qg, as happens at room temperature. The effect of Qi on Stot is less significant than that of Qg, showing in both cases a smooth maximum at intermediate Qi values. In all cases Stot is higher for MWDS than for CDS, though this was not so at room temperature, due to the greater heating efficiency of the former. Compared with the results obtained at room temperature, heating increased Stot very significantly, about 7 - 8 times for the MWDS and about 5

times for the CDS. Obviously, a significant contribution to this increase is due to solvent evaporation from the liquid film on the walls of the spray chamber. The behaviour of Wtot is different for both systems. For the MWDS, heating increased Wtot values by 3060%, in all cases except at Qg = 0.70 I min -L and Qi = 2.4 ml min -l, and their variations with Qg and Q~ are similar to those found at room temperature. For the CDS, however, the increase is very small at high liquid flows but very important at low liquid flows, especially at 0.4 ml min -~ (about 3 times). That is to say, Wtot varies greatly (decreases) as Ql is increased. This tendency is completely different to that shown at room temperature. As regards the variation of Wtot with Qg, this is similar to that found at room temperature. However, the variation of Wtot with Qg is relatively more important at high than at low liquid flows, even if the absolute values of Wtot are much higher in the latter case. This clearly different behaviour can be attributed to the fact that CDS heating is very rapid at low Qt values, so that most of the droplets are vaporized before impacting against the walls. As Qt is increased, heating efficiency decreases, so that droplets are not completely vaporized before they reach the walls. Therefore, these droplets do not contribute to Wtot, causing a decrease.

3.2.2. Transport parameters at the exit of the condensation unit Table 4 includes the values of Stot and Wtot at the exit of the condensation unit. One can notice that Stot increases as Qg is increased, as happened at the exit of the heating unit, but now Stot values are much lower, as expected. As regards the behaviour of both heating systems, MWDS shows lower Stot values than CDS at the exit of the condensation unit, although the opposite situation was found at the entrance. This probably means that the solvent transport rate in vapour form, S,.~p, is higher for the MWDS than for the CDS, since in a condensation unit vapour is more easily removed than liquid drops. In turn, this would probably mean that evaporation from the walls is stronger for the MWDS. As regards the effect of Qg on Wtot, from the results shown in Table 4 it appears that: (1) analyte transport increases with gas flow, as expected; (2) on comparing both heating systems, Wtot values for MWDS are

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1.2

nor

A 1.0 0.8 0.6 0.4 0.2 0.0 0.6

1.2

0.8

1 Qg (l/min)

1.2

I nor B

1.0

i

0.8 0.6 0.4 0.2 0.0 0.6

'

0.8

1 Qg (I/min)

1.2

Fig. 2. Normalized net emission intensity of manganese (lnor)as a function of the nebulizer gas flow rate (Qg), at variable liquid flow rate: (A) 0.4 ml min i; (B) 2.4 ml min -I. (1) MWDS, (2) CDS, (3) standard system (double-pass Scott spray chamber without desolvation system). higher than for CDS at Qi = 2.4 ml min -~, whereas at 0.4 ml min -I the opposite situation holds; (3) Wto t values at the exit of the condensation unit are significantly lower than at the exit of the heating unit; (4) in many cases the analyte transport rate is even lower than that found at room temperature, without a condensation unit. The results described in point (2), together with the V C values with aerosol heating shown in Table 3 (since these values are more or less proportional to the solvent transport rate in liquid

form, Sliq) , support the fact that at high liquid flows, droplets are more concentrated for M W D S than for CDS, and the opposite holds at low liquid flows. In other words, at QI = 2.4 ml min -l, solvent evaporation from the drops is stronger for M W D S , whereas at Qj = 0.4 ml min -~, this happens for CDS. The results described in points (3) and (4) might be attributed to droplet growth by nucleation, followed by settling losses. Nucleation is easier for large particles [46], that are relatively more abundant for CDS at high

1210

L. Gras et al./Spectrochimica Acta Part B .52 (1997) 1201-1213

liquid flow and for MWDS at low liquid flow (see also V~2 values in Table 3 with aerosol heating). The influence of Qi o n Sto t and Wto t at the exit of the condensation unit is also shown in Table 4. As regards Stot, at liquid flows higher than 0.9 ml min 1, there is little variation with Qi for both heating systems, as happened at the exit of the heating unit. Besides this, Sto t values for MWDS are lower than for CDS, in spite of the fact that the opposite situation was found at the exit of the heating unit. Something similar happened on studying the effect of Qg on Sto~, as shown above, and it can be explained in the same way. As regards Wtot, a different behaviour can be observed for each of the heating systems, in clear parallelism with the results obtained at the exit of the heating unit. On one side, the variation of Wto t with Qi when working with the MWDS is within the experimental error, whereas, on the other side, Wto t values for CDS are very high at low liquid flows, but they steeply decrease as Qj is increased. In all cases, a noticeable loss of analyte occurs at the condensation unit for both heating systems. With the MWDS the percentage of analyte lost in the condensation unit is about 55%, irrespective of Q~. For the CDS this percentage ranges from about only 30% at low liquid flows to about 70% at high liquid flows. This behaviour can be explained, again, in terms of nucleation. At low liquid flows, the decrease in the V C values due to heating is more pronounced for CDS than for MWDS, whereas at high liquid flows the opposite behaviour is found. In addition, VL2 values for CDS are higher than for MWDS at low liquid flows, and lower at high liquid flows. These observations support the assumption that at low liquid flows droplets (or particles) are smaller and are more concentrated for CDS than for MWDS, i.e., at low liquid flows CDS is more effective than MWDS in solvent vaporization from the drops. Hence, under these conditions, nucleation (and the aerosol losses associated with it) should be more significant for MWDS than for CDS. Similarly, as liquid flow is increased heating efficiency remains constant for MWDS (since a large volume of water is permanently placed inside the MW oven), but decreases for CDS. The result is that the droplets exiting the MWDS are smaller than those exiting the CDS at high liquid flows. Therefore, under these conditions, losses due to nucleation should be more significant for CDS.

3.3. Emission intensity in I C P A E S

Fig. 2 shows the influence of Qg on the normalized net emission intensity (related to the maximum value in each case) at two different liquid flow rates, 0.4 ml min n (Fig. 2(A)) and 2.4 ml min q (Fig. 2(B)). For the sake of comparison, the signals obtained in both cases with the standard sample introduction system (i.e., concentric pneumatic nebulizer coupled to a double-pass Scott chamber without desoivation system) have been included. The graphs show quite similar shapes, with intensity maxima at 0.85-1.00 1 rain <. This expected behaviour is the overall result of several factors that operate in opposite directions when Qg is increased: (1) Wtot increases with Qg, thus contributing to an increased signal; (2) Stot increases with Qg, also, this factor being detrimental to the excitation characteristics of the plasma and, hence, to the signal; (3) the residence time of the atoms/ ions in the viewing zone of the plasma decreases, and this causes the signal to decrease. At low liquid flows (Fig. 2(A)), the signal obtained with CDS at low gas flow is much higher than with MWDS, as expected from their analyte transport rates, but they become similar when gas flow is increased. In general, the ratio [(lnor)MWDsl(lnor)CDS] is higher than the ratio [(Wtot)MWDsI(Wtot)CDS]. This behaviour is probably caused by the higher &ot values associated with CDS in comparison with MWDS. In addition, Fig. 2(A) shows that desolvation greatly enhances emission intensity in relation to a conventional sample introduction system. The gain in emission intensity is much higher than the gain in the analyte transport rate, which, in fact, does not exist in some cases. Again, this observation is probably related to the lower solvent load. At high liquid flows (Fig. 2(B)) the behaviour of the signal is similar to that found at low liquid flows, but here the values for MWDS are always higher than for CDS. These results are again in good agreement with the results on transport, since MWDS shows higher W~ot values and lower Stot values than CDS. Fig. 3 shows the effect of Q~ on the emission intensity for the three sample introduction systems studied. As Q~ is increased, emission intensity decreases for all three introduction systems employed, very slowly for the MWDS and the standard system, and more quickly for the CDS. Again, these results can be easily

121 l

L. Gras et al./Spectrochimica Acta Part B 52 (1997) 1201-1213

1.2

nor

2

1.0 0.8 0.6

i

0.4 0.2 0.0 0.2

J

,

i

,

J

I

~

,

,

J

0.7

I

,

~

,

1.2 QI (ml/min)

~

I

~

,

1.7

, , I , , 2.2

Fig. 3. Normalizednet emissionintensityof manganese(l,or) as a functionof liquid flow rate (Q0, at 1.01 min-I nebulizergas flow rate. (1) MWDS, (2) CDS, (3) standard system (double-passScott spray chamber without desolvationsystem). explained from those related to transport (Table 4). Therefore, CDS provides a much higher signal than MWDS at low liquid flows, whereas MWDS is superior to CDS at high liquid flows. As a rule, the standard system gives rise to lower signals. Finally, Fig. 4 and Table 5 show the limits of detection for several elements under different sets of conditions. The limits of detection obtained through

200

150

desolvation are lower than with the standard system. At high liquid flows (Fig. 4) MWDS gives rise to the lowest LODs for all the elements, up to seven times lower than CDS, as a consequence of its high sensitivity and good background stability in these conditions. At low liquid flows (Table 5), however, the LODs obtained with CDS are 1 - 3 times lower than with MWDS.

LOD (ng/ml) [~lStandard system E3CDS IMWDS

100

50

Mn

Ag

Cd

Co

Cr

Cu

Fe

Zn

Ni

Element

Fig. 4. Limitsof detection (LOD), obtainedwith the three systemstested at 1.0 1min-I nebulizergas flowrate and 2.4 ml min-I liquidflow rate.

L. Gras et al./Spectrochimica Acta Part B 52 (1997) 1201-1213

1212

Table 5 Limits of detection, LOD, in ng ml -~ obtained with both desolvation systems at different liquid flow rates ~ Element

Mn Ag Cd Co Cr Cu Fe Zn Ni

CDS

MWDS

0.4 b

2.4 b

0.4 b

2.4 b

1 13 3 4 6 5 2 4 9

3 45 12 23 35 34 10 18 73

1 15 6 9 8 9 7 9 28

1 6 11 16 5 8 6 13 28

LOD calculated as three times the standard deviation from 20 readings of the blank. Qg = 0.85 1 min -r. b Qi (ml rain i).

4. Conclusions The present paper describes the preliminary results obtained with a desolvation system for ICPAES that incorporates a heating step based on the absorption of microwave radiation. The results have proved that, in spite of the evident limitations of this first design, MW radiation can be considered a good choice for aerosol heating in a sample introduction system. MW radiation seems to be a more uniform way of aerosol desolvation than conductive/convective heating (i.e., lower radial temperature gradients), the degree of vaporization of the droplets is less dependent on the liquid flow rate (Q1), and also the background noise associated with the vaporization of droplets is reduced. In our opinion, the results obtained with the MWDS can be easily improved, especially in terms of the analyte transport rate. Several points related to this issue are currently being studied in our laboratories: (1) the MW cavity is being modified in order to place the nebulizer outside the cavity, which would allow acidic solutions to be used (in the first design acidic solutions, which absorb MW radiation much more readily than plain water, give rise to pulsating nebulization due to partial premature vaporization of the solvent, since the nebulizer and a given length of the sample capillary are placed inside the cavity); (2) the MW power control device is being changed so that the applied power could be varied, since in its

present configuration this is not possible (the current 'power control' device is just an on/off temporizer); (3) selective membranes will be used in the vapourremoval step, replacing one or both condensers. Although it does not affect the validity of the results, some degree of incertitude persists about whether aerosol drops contribute or not to MW absorption. Our impression is that drops do really absorb because, otherwise, applying MW radiation would only c a u s e Sto t to increase, but not Wtot, since evaporation just from the walls does not contribute to the analyte transport, it only contributes to the transport of solvent. However, we are aware that this reason is not an unequivocal proof for solving this controversy. Finally, we hope that this paper will renew the interest in microwave desolvation in Atomic Spectrometry. Additional experiments would be required to assess the exact mechanism of this type of desolvation.

5. Glossary of terms and symbols CDS d~ Dso

/nor Q~ Qi MWDS

Stot Sliq S~ap VC Vi2 Wtot

Conventional desolvation system. Cut-off diameter of the spray chamber. Volume median diameter. Normalized net emission intensity. Nebulizing gas flow rate. Liquid flow rate. Microwave Desolvation System. Total solvent transport rate. Solvent transport rate in liquid form. Solvent transport rate in vapour form. Volume concentration. Fraction of aerosol liquid volume contained in droplets smaller than 1.2 ~m. Total analyte transport rate.

Acknowledgements The authors wish to express their appreciation to the DGICYT (Spain) for the financial support of this study (PB92-0336).

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L. Gras et al./Spectrochimica Acta Part B 52 (1997) 1201-1213

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