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Characterization of a membrane interface for sample introduction into atom reservoirs for analytical atomic spectrometry
Specrrochimica Acta, Vol. 438. No. 8. pp. 917-922, Printed in Great Britain.
0584-8547/88 S3.00 + .oO Fwgamon Ras pk.
1988.
Characterization of a membrane interface for sample introduction atom reservoirs for analytical atomic spectrometry
into
ANDERSGUSTAVSSON Department
of Analytical Chemistry,
Royal Institute of Technology,
S-100 44 Stockholm,
Sweden
(Receioed 10 November 1987; in revised form 9 February 1988) Abstract-This paper describes and characterizes a new interface for organic solvent sample introduction into atom reservoirs for analytical atomic spectrometry, especially for inductively coupled plasmas. The unoptimized analyte transport efficiencies were in a range between 45 and 65 % and the solvent removal efficiencies were SO-100 y! using Freon and chloroform as solvents. The interface gives-for inductively coupled plasmas-a higher analyte transport efficiency at optimum solvent load in comparison with normal nebulizer systems. The interface is easy to optimize since all gas and liquid flows can be varied independently. The interface provides for optimum coupling of analytical liquid flow techniques to instruments for analytical atomic spectrometry.
1.
INTRODUCTION
ORGANICsolvents constitute a major problem in analytical atomic spectrometry, especially for the inductively coupled plasma (ICP) technique. The increased use of the ICP has therefore resulted in an increased interest in and a need for new nebulizers and nebulizer systems. The reason is that the ICP and conventional nebulizer systems are more easily disturbed by a high solvent load and samples with a high salt content, than, e.g. an instrument for flame atomic absorption spectrometry (FAAS). There is a growing interest in coupling analytical liquid flow techniques, e.g. techniques involving flow injection analysis (FIA) or high performance liquid chromatography (HPLC), to techniques for analytical atomic spectrometry. Metal specific detection can be obtained for the HPLC-technique, and the FIA-technique provides for sample pretreatment and sample work-up. The problems associated with samples having a high salt content have been minimized by using different kinds of Babington nebulizers, e.g. the V-groove nebulizer[l]. The jet impaction[2], glass frit[3] and total consumption[4] nebulizers have been used to connect the ICP to different analytical flow techniques, e.g. FIA and liquid chromatography (LC). Despite all efforts, the nebulizer systems of today have for water an analyte transport efficiency (E)of only approximately 1%. Increasing the efficiency will give problems with too large aerosol droplets and an increasing solvent load, especially for the ICP. GUSTAVSSON[~, 61 has used the idea of looking upon the nebulizer system as an interface between the sample and the analytical instrument. Reference [5] describes work where a jet separator is used for obtaining an acceptable solvent load in an ICP at an analyte transport efficiency of approximately 35 %. Organic as well as inorganic solvents can be used; however still better solvent removal efficiencies-especially for the ICP technique-would be most advantageous. The interface seems to offer particular advantages for the coupling of analytical low-flow techniques, e.g. minituarized FIA and microbore LC, to the ICP. The aerosol chamber used in the jet separator interface has been used as an interface for FAAS instruments[6]. The FAAS interface showed better compatability with the FIA and HPLC techniques and gave better detection limits and characteristic concentrations than the nebulizer systems provided by the manufacturers. The interface also gave the possibility to fully optimize the coupling and the flame conditions since all gas flows could be independently varied. The object of this work was to develop a membrane sample introduction interface, designed especially for removing organic solvents from aerosols. It should remove organic solvent more efficiently than the jet separator and also give an acceptable analyte transport efficiency. The interface should also provide the possibility for optimal connection of the FIA or HPLC techniques to instruments for analytical atomic spectrometry. 917
918
ANDERS GUSTAVSSON 2. PRINCIPLEOF OPERATION
Membrane separators for interfacing a gas chromatograph to a mass spectrometer have been amply reviewed by, e.g. MCFADDEN[7]. The idea behind the new interface was to use a membrane separator to remove the solvent vapour from the aerosol. The sample solution is efficiently converted into a desolvated aerosol in a heated aerosol chamber. The desolvated aerosol+onsisting of solid aerosol particles, argon gas and solvent vapour-is conveyed over a silicon membrane on a support and the support side of the membrane is evacuated, as explained in detail in Section 3. The organic solvent vapour is highly soluble in the silicone polymer membrane and permeates through the membrane to the vacuum side and is thus removed. The argon carrier gas is less soluble in the silicone polymer and passes to a greater extent through the separator to the atom reservoir. Desolvated aerosol particles are efficiently carried through the separator since they are small. The rate of permeation of a gas through a polymer membrane is a function of its solubility (S) in and diffusivity (D) through the membrane, and of the membrane area (A) and thickness (L) [7]. The solvent removal efficiency (qs,,,) can be expressed in the following way, qsol= 1 -exp (-AWWQ,))
(1)
where Q, is the gas flow rate. The solvent removal efficiency shows a considerable temperature dependency. In order to obtain a high diffusivity of the solvent vapour, the temperature should be high; but in order to have a high solubility, the temperature should be low. This means that for each solvent an optimum temperature exists depending on its volatility. The highest solvent removal efficiency for organic solvents is obtained by working at a low temperature, because the solubility is the more important parameter of the two [7]. The argon permeation on the other hand is decreased by working at low temperature. 3. EXPERIMENTAL 3.1. Apparatus The interface shown in Fig. 1 consists of three parts: the nebulizer (A), the aerosol chamber* (B) and the membrane separator (C). The organic solutions were fed to the all-glass concentric nebulizer (Meinhard, TR-30-B3, No. 2857) with a peristaltic pump (Gilson Minipuls 2). The aerosol chamber used for the experiments was the same as the A-type used in Ref. [S]. A 250-W heating tape was used for heating the chamber. The membrane separator is schematically shown in Fig. 2. (A) is the main body of
Fig. 1. Schematic
drawing
Fig. 2. The membrane
of the interface. A = nebulizer, separator and D = vacuum
separator. support,
B = aerosol outlet.
chamber,
A = body of the separator, B = separator and E = vacuum outlet.
C = membrane
half, C = membrane
D = membrane
*Spray chamber is the UIPAC recommendation but aerosol chamber is regarded by the author as being a better chaise. The reason is that an aerosol is generated by a nebulizer and passed through a chamber. Consequently, the chamber should be called an aerosol chamber.
Characterization of a membrane interface
919
the separator. The two separator halves (B) are separated 5 mm by the main body. The supports (C) were made of sintered high density polyethylene (Vyon F, Porvair Ltd, Norfolk). The membranes (D) were made of silicone rubber and had an active area of 40 * 400 mm each. The minimum membrane thickness that can be fabricated with the current technique is 5 pm. Equation (1) was used for estimating preliminary values of the membrane area and thickness. The cross section for the flow in the separator area is rectangular (5 * 40 mm). The vacuum to the membranes was supplied by a vacuum pump (Leybold-Heraeus, SSB) through the tubes (E). 3.2. Measurement procedures The analyte transport efficiency was measured with the same procedure as used in Ref. [5]. The solvents used for the experiments were chloroform (p.a. Merck) and Freon TF (Du Pont). The solvent removal efficiency was calculated as one minus the quotient of the organic solvent concentrations in the aerosol downstream and upstream of the separator. The concentrations were determined with a gas chromatograph (Perkin-Elmer Fll) equipped with a flame ionization detector. Samples of the aerosol were sucked into a 20 ~1 sample loop on an injection valve (Altex 210). The samples in the loop were transported from the valve to the gas chromatograph with argon as carrier gas.
4. RESULTS AND DISCUSSION 4.1. The aerosol
chamber
The aerosol chamber has been described and characterized earlier [S], but there is still another property that should be described. Figure 3 shows the analyte transport efficiency of the aerosol chamber as a function of the setting of the heat regulator. The regulator could be varied between 0 and 10 (10 being maximum). The temperature scale in Fig. 3 is a rough measure of the gas temperature for the gas leaving the aerosol chamber. A solvent independent temperature was obtained by supplying the aerosol chamber with no solvent uptake and gas flow of 11 min- ’ argon for the nebulizer and 6 1min - 1 air for the extra gas. The gas and liquid flows for the experiments shown in Fig. 3 were chosen so as to increase the problem of slow desolvation of the aerosol droplets. The temperature dependence of the analyte transport efficiently is due to incomplete desolvation of aerosol droplets. If an aerosol droplet hits the wall of the aerosol chamber, it will wet the wall and consequently be retained in the chamber. An aerosol droplet which has been desolvated will not be retained because it will bounce on the dry wall. The best analyte transport efficiency is obtained if the heating is concentrated to the aerosol inlet of the aerosol chamber. A heat setting larger than 7-8 gives optimum performance for the gas flow rates used. If a higher analyte transport efficiency is needed the gas flow rates have to be optimized. The validity of using a heat setting of 3 when working with Freon was tested using gas flow rates of 0.5 and 1.0 lmin- ’ for the nebulizing and extra gas, respectively. An efficiency of 88.7 % was obtained, which should be compared with 88.4 % obtained in Ref. [S] using chloroform with a heat setting of 8 and the same gas flow rates. The long term stability of the
l-leaf
setting, 0.“.
2z-----
373
Terrperature,
K
Fig. 3. The analyte transport efficiency as a function of the setting of the regulator for the heat tape. The solvent used was chloroform and the nebulizing/extra gas flow rates were 0.4/0.4 Imin- I, respectively. See the text for information on the temperature scale.
920
ANDERSGUWAVSSON
aerosol chamber performance is obviously sufficient, as a time of 1.5 yr has elapsed between the tests. 4.2. The separator and interface This section will concentrate on sample introduction into the ICP as it is more difficult to introduce samples into the ICP in comparison with other techniques for analytical atomic spectrometry. The reason for having an analyte transport efficiency for water of only 1 y0 for common ICP nebulizer systems is to keep the solvent load of the plasma at a sufficiently low level. The idea behind this membrane interface is the same as the one used in Ref. [S], namely, to convert the sample efficiently into a desolvated aerosol in a flow of argon and then to remove the major part of the solvent vapour by a separator. An improved performance in comparison with the interface described in Ref. [S] will be obtained if the analyte transport efficiency can be further increased at an acceptable or lower solvent load level. The solvent removal efficiency needed depends on the uptake rate and the solvent tolerance of the ICP. The chloroform results have been included to facilitate the comparison with actual ICP work. The effect of a chloroform plasma load has been studied by MADSEN et al. [8,9]. In Ref. [8] they concluded-among other things-that the maximum tolerable solvent plasma load for chloroform using their system was > 10.7 mg s-i and that the optimum load was 3-5 mg s- ’ for a nebulizing argon flow rate of 0.7 1min- i. They also concluded [9] that it is the number of solvent molecules introduced into the plasma rather than the solvent mass which determines the plasma performance. In this work the solvent removal efficiencies for Freon were calculated using the assumption that the same number of molecules as for chloroform should be removed. To obtain an optimum solvent load for the uptake rates 0.5, 1.0, 1.64 and 2.0 mlmin-’ using chloroform/Freon we would need a solvent removal efficiency of 68,84,90 and 94/53,76,85 and 88 %, respectively. A solvent plasma load level-corresponding to the 10.7 mg s- ’ of chloroform-would be obtained using solvent removal efficiencies of only 79 and 68 y0 for chloroform and Freon, respectively, at an uptake rate of 2 ml min- ‘. The amount of argon gas lost is 0.2 to 0.3 1min- ’ , corresponding to heat settings of 3 and 6, and a separator temperature of roughly 20 and 35°C. The loss ofcarrier gas will increase if the separator is loaded with a solvent. This increase in loss was 0.1 to 0.15 1mini depending on the solvent used. Chloroform gives a higher loss than does Freon. This is probably due to a larger swelling of the membrane using chloroform. The membranes used for these experiments-8-12 pm thick-showed no significant difference in gas loss characteristics. Thus, the total loss of carrier gas was in the range 0.3-0.45 1min- ‘. Table 1 shows the solvent removal efficiency obtained using a membrane thickness of 12 pm. The argon gas flow rate is related to the outlet of the separator because of the loss in the separator. The solvent vapour contributes considerably to the total gas flow rate at the inlet of the separator, but it is almost negligible at the outlet. The solvent removal efficiencies for chloroform are more than sufficient for an optimum solvent load. For Freon an almost optimum solvent load was obtained at an uptake rate of 1.64 ml min- ’ (maximum pump Table 1. The solvent removal efficiency as a function of the argon gas flow rate and liquid uptake rate for Freon and chloroform. The separator temperature was 20°C Argon gas flow rate from the separator (lmin-r) 0.5 0.5 0.5 1.0
1.0 1.0 *Condensation
Solvent removal efficiency ( %) .Uptake rate (ml min- ‘) 0.5
1.0 1.64 0.5 1.0 1.64
Freon
Chloroform
92.6 94.0 94.1 79.4 81.6 81.9
99.7 99.7 _* 97.7 97.9 98.0
occurred in the inlet to the separator.
Characterization of a membrane interface
921
speed), approximately 82% instead of the need 85%. Operating the separator at a temperature of 13°C gave an efficiency of 88 %, i.e. it provides an optimum solvent load at an uptake rate of 2 ml min- ‘. On the other hand, using an 8-pm thick membrane at a separator temperature of 20°C gave a solvent removal efficiency of approximately 87 “/d,which is almost sufficient for an uptake rate of 2.0 ml min - ’ . The observed increase in solvent removal efficiency when increasing the uptake rate is due to the enhanced driving force-the concentration gradient-of the permeation. The values in Table 2 were obtained using a heat setting of 3. It is clear from the table that the results for chloroform are not optimum. This is because’of wet droplet deposition, and for an argon flow rate of 0.5 1min- ’ also of condensation in the separator inlet. Using a heat setting of at least 8 would give a better performance, but that could damage the membrane support due to too high a temperature. An analyte transport efficiency of 44.1 Y0and a solvent removal efficiency of 98.7 y0 were obtained using a heat setting of 6, chloroform as solvent (1 mlmin-“) an d an argon gas flow rate of 0.5 1min- l at the outlet of the separator. The reason for not obtaining the same efficiency as for Freon (46.3 %) is that the gas flow rates were somewhat different. The decreased solvent removal efficiency is due to the higher separator temperature. All the experiments in Table 2 were performed with the membraI~es in a horizontal position. When the separator was operated with the membranes in a vertical position using Freon an analyte transport efficiency of 64.5 % was observed instead of 63.1 o/ The difference which is statistically significant-at a level better than 99.9 %---is due to a longer distance for gravitational settling. Membranes have been used for weeks without any significant change in performance. 4.3. Areas of application The areas of application are those where a sample consists of an analyte in an organic solvent. Using a membrane separator interface will benefit the ICP technique because the solvent load is kept at an acceptable level for many applications and the analyte transport efficiency is increased by a factor of approximately 50 in comparison with normal nebulizer systems for the ICP. A gain of approximately 100 in aerosol analyte concentration is obtained in comparison with normal nebulizer systems if the interface is operated at an outlet argon gas flow rate of 0.5 1min- ‘. The gain in aerosol analyte concentration is of interest since the ICP-from a theoretical point of view [S]-is a concentration sensitive emission source. An interesting application is the coupling of the FIA technique to the ICP by the aid of the membrane interface, because a FIA extraction system can provide on-fine sample work-up and pretreatment. An improved single step FIA extraction system [5] gives matrix removal and concentrates the analyte by a factor of 10-I 5. The organic extract is introduced into the ICP with the interface. A FIA-extraction-membrane interface-ICP system seems to be advantageous for simultaneous multielement trace analysis, in view of the gain obtained by the FIA extraction and the interface. The analysis of samples by ICP mass spectrometry (ICP-MS) is severely influenced by the sample matrix [lo]. A FIA extraction-membrane interface-ICP system should improve analyses by ICP-MS because of the matrix removal and also by the gain obtained from the FIA extraction and the interface. It was shown in Ref. [6] that the aerosol chamber used provided for optimum coupling of the FIA and HPLC techniques to FAAS. As the same aerosol chamber is used in the
Table 2. The analyte transport efficiency as a function of the argon gas flow rate at an uptake rate of 1 ml min. ’ and a heat setting of 3 Analyte transport efficiency (%) Argon gas flow rate from the separator (Emin-‘)
Chloroform aerosol chamber
interface
aerosol chamber
interface
1.0 0.5
64.8 55.2
41.8 15.9
91.3 86.6
63.1 46.3
Freon
922
ANDERSGuSrAvSSON
membrane interface, there is much reason to assume that this interface will provide optimum coupling to an ICP since the coupling properties are determined by the aerosol chamber. The membrane interface can also be used for FAAS and flame emission techniques when the organic solvent content of the aerosol must be decreased.
5. CONCLUSIONS
The following conclusions can be drawn when the characteristics of the membrane interface are compared with those of normal nebulizer systems: (i) The membrane interface provides higher analyte transport efficiencies. (ii) The solvent load is kept at an acceptable level. (iii) The interface gives no waste. (iv) The aerosol chamber provides for optimum coupling of the FIA and HPLC techniques to the interference. (vi) It is easy to optimize the interface since all gas and liquid flows can be varied independently of each other. Acknowledgements-The author thanks FOLKEINGMANand SVERKERBLOMBERG for stimulating discussions and comments regarding the manuscript, J.E. Meinhard Associates for loan of nebulizers and the Swedish Natural Science Research Council for financial support.
REFERENCES [l] L. Ebdon and M. R. Cave, Analyst 107, 172 (1982). [2] M. P. Doherty and G. M. Hieftje, Appl. Spectrosc. 38, 405 (1984). [3] M. Ibrahim, W. Nisamaneepong and J. Caruso, .I. Chrom. Sci. 23, 14 (1985). [4] K. E. Lawrence, G. W. Rice and V. A. Fassel, Anal. Chem. 56, 289 (1984). [S] A. Gustavsson, Spectrochim. Acta 42B, 111 (1987). [6] A. Gustavsson, Spectrochim. Acta 42B, 883 (1987). [7] W. McFadden, Techniques ofcombined CC/MS: Application in Organic Analysis, Chap. 5. John Wiley, New York (1973). [8] F. J. M. J. Maessen, G. Kreuning and J. Balke, Spectrochim. Acta 41B, 3 (1986). [9] G. Kreuning and F. J. M. J. Maessen, Spectrochim. Acta 42B, 677 (1987). [lo] Inductioely Coupled Plasmas in Analytical Atomic Spectrometry, Ed. A. Montaser and D. W. Golightly, Chap. 11. VCH Publishers, New York (1987).
0584-8547/88 S3.00 + .oO Fwgamon Ras pk.
1988.
Characterization of a membrane interface for sample introduction atom reservoirs for analytical atomic spectrometry
into
ANDERSGUSTAVSSON Department
of Analytical Chemistry,
Royal Institute of Technology,
S-100 44 Stockholm,
Sweden
(Receioed 10 November 1987; in revised form 9 February 1988) Abstract-This paper describes and characterizes a new interface for organic solvent sample introduction into atom reservoirs for analytical atomic spectrometry, especially for inductively coupled plasmas. The unoptimized analyte transport efficiencies were in a range between 45 and 65 % and the solvent removal efficiencies were SO-100 y! using Freon and chloroform as solvents. The interface gives-for inductively coupled plasmas-a higher analyte transport efficiency at optimum solvent load in comparison with normal nebulizer systems. The interface is easy to optimize since all gas and liquid flows can be varied independently. The interface provides for optimum coupling of analytical liquid flow techniques to instruments for analytical atomic spectrometry.
1.
INTRODUCTION
ORGANICsolvents constitute a major problem in analytical atomic spectrometry, especially for the inductively coupled plasma (ICP) technique. The increased use of the ICP has therefore resulted in an increased interest in and a need for new nebulizers and nebulizer systems. The reason is that the ICP and conventional nebulizer systems are more easily disturbed by a high solvent load and samples with a high salt content, than, e.g. an instrument for flame atomic absorption spectrometry (FAAS). There is a growing interest in coupling analytical liquid flow techniques, e.g. techniques involving flow injection analysis (FIA) or high performance liquid chromatography (HPLC), to techniques for analytical atomic spectrometry. Metal specific detection can be obtained for the HPLC-technique, and the FIA-technique provides for sample pretreatment and sample work-up. The problems associated with samples having a high salt content have been minimized by using different kinds of Babington nebulizers, e.g. the V-groove nebulizer[l]. The jet impaction[2], glass frit[3] and total consumption[4] nebulizers have been used to connect the ICP to different analytical flow techniques, e.g. FIA and liquid chromatography (LC). Despite all efforts, the nebulizer systems of today have for water an analyte transport efficiency (E)of only approximately 1%. Increasing the efficiency will give problems with too large aerosol droplets and an increasing solvent load, especially for the ICP. GUSTAVSSON[~, 61 has used the idea of looking upon the nebulizer system as an interface between the sample and the analytical instrument. Reference [5] describes work where a jet separator is used for obtaining an acceptable solvent load in an ICP at an analyte transport efficiency of approximately 35 %. Organic as well as inorganic solvents can be used; however still better solvent removal efficiencies-especially for the ICP technique-would be most advantageous. The interface seems to offer particular advantages for the coupling of analytical low-flow techniques, e.g. minituarized FIA and microbore LC, to the ICP. The aerosol chamber used in the jet separator interface has been used as an interface for FAAS instruments[6]. The FAAS interface showed better compatability with the FIA and HPLC techniques and gave better detection limits and characteristic concentrations than the nebulizer systems provided by the manufacturers. The interface also gave the possibility to fully optimize the coupling and the flame conditions since all gas flows could be independently varied. The object of this work was to develop a membrane sample introduction interface, designed especially for removing organic solvents from aerosols. It should remove organic solvent more efficiently than the jet separator and also give an acceptable analyte transport efficiency. The interface should also provide the possibility for optimal connection of the FIA or HPLC techniques to instruments for analytical atomic spectrometry. 917
918
ANDERS GUSTAVSSON 2. PRINCIPLEOF OPERATION
Membrane separators for interfacing a gas chromatograph to a mass spectrometer have been amply reviewed by, e.g. MCFADDEN[7]. The idea behind the new interface was to use a membrane separator to remove the solvent vapour from the aerosol. The sample solution is efficiently converted into a desolvated aerosol in a heated aerosol chamber. The desolvated aerosol+onsisting of solid aerosol particles, argon gas and solvent vapour-is conveyed over a silicon membrane on a support and the support side of the membrane is evacuated, as explained in detail in Section 3. The organic solvent vapour is highly soluble in the silicone polymer membrane and permeates through the membrane to the vacuum side and is thus removed. The argon carrier gas is less soluble in the silicone polymer and passes to a greater extent through the separator to the atom reservoir. Desolvated aerosol particles are efficiently carried through the separator since they are small. The rate of permeation of a gas through a polymer membrane is a function of its solubility (S) in and diffusivity (D) through the membrane, and of the membrane area (A) and thickness (L) [7]. The solvent removal efficiency (qs,,,) can be expressed in the following way, qsol= 1 -exp (-AWWQ,))
(1)
where Q, is the gas flow rate. The solvent removal efficiency shows a considerable temperature dependency. In order to obtain a high diffusivity of the solvent vapour, the temperature should be high; but in order to have a high solubility, the temperature should be low. This means that for each solvent an optimum temperature exists depending on its volatility. The highest solvent removal efficiency for organic solvents is obtained by working at a low temperature, because the solubility is the more important parameter of the two [7]. The argon permeation on the other hand is decreased by working at low temperature. 3. EXPERIMENTAL 3.1. Apparatus The interface shown in Fig. 1 consists of three parts: the nebulizer (A), the aerosol chamber* (B) and the membrane separator (C). The organic solutions were fed to the all-glass concentric nebulizer (Meinhard, TR-30-B3, No. 2857) with a peristaltic pump (Gilson Minipuls 2). The aerosol chamber used for the experiments was the same as the A-type used in Ref. [S]. A 250-W heating tape was used for heating the chamber. The membrane separator is schematically shown in Fig. 2. (A) is the main body of
Fig. 1. Schematic
drawing
Fig. 2. The membrane
of the interface. A = nebulizer, separator and D = vacuum
separator. support,
B = aerosol outlet.
chamber,
A = body of the separator, B = separator and E = vacuum outlet.
C = membrane
half, C = membrane
D = membrane
*Spray chamber is the UIPAC recommendation but aerosol chamber is regarded by the author as being a better chaise. The reason is that an aerosol is generated by a nebulizer and passed through a chamber. Consequently, the chamber should be called an aerosol chamber.
Characterization of a membrane interface
919
the separator. The two separator halves (B) are separated 5 mm by the main body. The supports (C) were made of sintered high density polyethylene (Vyon F, Porvair Ltd, Norfolk). The membranes (D) were made of silicone rubber and had an active area of 40 * 400 mm each. The minimum membrane thickness that can be fabricated with the current technique is 5 pm. Equation (1) was used for estimating preliminary values of the membrane area and thickness. The cross section for the flow in the separator area is rectangular (5 * 40 mm). The vacuum to the membranes was supplied by a vacuum pump (Leybold-Heraeus, SSB) through the tubes (E). 3.2. Measurement procedures The analyte transport efficiency was measured with the same procedure as used in Ref. [5]. The solvents used for the experiments were chloroform (p.a. Merck) and Freon TF (Du Pont). The solvent removal efficiency was calculated as one minus the quotient of the organic solvent concentrations in the aerosol downstream and upstream of the separator. The concentrations were determined with a gas chromatograph (Perkin-Elmer Fll) equipped with a flame ionization detector. Samples of the aerosol were sucked into a 20 ~1 sample loop on an injection valve (Altex 210). The samples in the loop were transported from the valve to the gas chromatograph with argon as carrier gas.
4. RESULTS AND DISCUSSION 4.1. The aerosol
chamber
The aerosol chamber has been described and characterized earlier [S], but there is still another property that should be described. Figure 3 shows the analyte transport efficiency of the aerosol chamber as a function of the setting of the heat regulator. The regulator could be varied between 0 and 10 (10 being maximum). The temperature scale in Fig. 3 is a rough measure of the gas temperature for the gas leaving the aerosol chamber. A solvent independent temperature was obtained by supplying the aerosol chamber with no solvent uptake and gas flow of 11 min- ’ argon for the nebulizer and 6 1min - 1 air for the extra gas. The gas and liquid flows for the experiments shown in Fig. 3 were chosen so as to increase the problem of slow desolvation of the aerosol droplets. The temperature dependence of the analyte transport efficiently is due to incomplete desolvation of aerosol droplets. If an aerosol droplet hits the wall of the aerosol chamber, it will wet the wall and consequently be retained in the chamber. An aerosol droplet which has been desolvated will not be retained because it will bounce on the dry wall. The best analyte transport efficiency is obtained if the heating is concentrated to the aerosol inlet of the aerosol chamber. A heat setting larger than 7-8 gives optimum performance for the gas flow rates used. If a higher analyte transport efficiency is needed the gas flow rates have to be optimized. The validity of using a heat setting of 3 when working with Freon was tested using gas flow rates of 0.5 and 1.0 lmin- ’ for the nebulizing and extra gas, respectively. An efficiency of 88.7 % was obtained, which should be compared with 88.4 % obtained in Ref. [S] using chloroform with a heat setting of 8 and the same gas flow rates. The long term stability of the
l-leaf
setting, 0.“.
2z-----
373
Terrperature,
K
Fig. 3. The analyte transport efficiency as a function of the setting of the regulator for the heat tape. The solvent used was chloroform and the nebulizing/extra gas flow rates were 0.4/0.4 Imin- I, respectively. See the text for information on the temperature scale.
920
ANDERSGUWAVSSON
aerosol chamber performance is obviously sufficient, as a time of 1.5 yr has elapsed between the tests. 4.2. The separator and interface This section will concentrate on sample introduction into the ICP as it is more difficult to introduce samples into the ICP in comparison with other techniques for analytical atomic spectrometry. The reason for having an analyte transport efficiency for water of only 1 y0 for common ICP nebulizer systems is to keep the solvent load of the plasma at a sufficiently low level. The idea behind this membrane interface is the same as the one used in Ref. [S], namely, to convert the sample efficiently into a desolvated aerosol in a flow of argon and then to remove the major part of the solvent vapour by a separator. An improved performance in comparison with the interface described in Ref. [S] will be obtained if the analyte transport efficiency can be further increased at an acceptable or lower solvent load level. The solvent removal efficiency needed depends on the uptake rate and the solvent tolerance of the ICP. The chloroform results have been included to facilitate the comparison with actual ICP work. The effect of a chloroform plasma load has been studied by MADSEN et al. [8,9]. In Ref. [8] they concluded-among other things-that the maximum tolerable solvent plasma load for chloroform using their system was > 10.7 mg s-i and that the optimum load was 3-5 mg s- ’ for a nebulizing argon flow rate of 0.7 1min- i. They also concluded [9] that it is the number of solvent molecules introduced into the plasma rather than the solvent mass which determines the plasma performance. In this work the solvent removal efficiencies for Freon were calculated using the assumption that the same number of molecules as for chloroform should be removed. To obtain an optimum solvent load for the uptake rates 0.5, 1.0, 1.64 and 2.0 mlmin-’ using chloroform/Freon we would need a solvent removal efficiency of 68,84,90 and 94/53,76,85 and 88 %, respectively. A solvent plasma load level-corresponding to the 10.7 mg s- ’ of chloroform-would be obtained using solvent removal efficiencies of only 79 and 68 y0 for chloroform and Freon, respectively, at an uptake rate of 2 ml min- ‘. The amount of argon gas lost is 0.2 to 0.3 1min- ’ , corresponding to heat settings of 3 and 6, and a separator temperature of roughly 20 and 35°C. The loss ofcarrier gas will increase if the separator is loaded with a solvent. This increase in loss was 0.1 to 0.15 1mini depending on the solvent used. Chloroform gives a higher loss than does Freon. This is probably due to a larger swelling of the membrane using chloroform. The membranes used for these experiments-8-12 pm thick-showed no significant difference in gas loss characteristics. Thus, the total loss of carrier gas was in the range 0.3-0.45 1min- ‘. Table 1 shows the solvent removal efficiency obtained using a membrane thickness of 12 pm. The argon gas flow rate is related to the outlet of the separator because of the loss in the separator. The solvent vapour contributes considerably to the total gas flow rate at the inlet of the separator, but it is almost negligible at the outlet. The solvent removal efficiencies for chloroform are more than sufficient for an optimum solvent load. For Freon an almost optimum solvent load was obtained at an uptake rate of 1.64 ml min- ’ (maximum pump Table 1. The solvent removal efficiency as a function of the argon gas flow rate and liquid uptake rate for Freon and chloroform. The separator temperature was 20°C Argon gas flow rate from the separator (lmin-r) 0.5 0.5 0.5 1.0
1.0 1.0 *Condensation
Solvent removal efficiency ( %) .Uptake rate (ml min- ‘) 0.5
1.0 1.64 0.5 1.0 1.64
Freon
Chloroform
92.6 94.0 94.1 79.4 81.6 81.9
99.7 99.7 _* 97.7 97.9 98.0
occurred in the inlet to the separator.
Characterization of a membrane interface
921
speed), approximately 82% instead of the need 85%. Operating the separator at a temperature of 13°C gave an efficiency of 88 %, i.e. it provides an optimum solvent load at an uptake rate of 2 ml min- ‘. On the other hand, using an 8-pm thick membrane at a separator temperature of 20°C gave a solvent removal efficiency of approximately 87 “/d,which is almost sufficient for an uptake rate of 2.0 ml min - ’ . The observed increase in solvent removal efficiency when increasing the uptake rate is due to the enhanced driving force-the concentration gradient-of the permeation. The values in Table 2 were obtained using a heat setting of 3. It is clear from the table that the results for chloroform are not optimum. This is because’of wet droplet deposition, and for an argon flow rate of 0.5 1min- ’ also of condensation in the separator inlet. Using a heat setting of at least 8 would give a better performance, but that could damage the membrane support due to too high a temperature. An analyte transport efficiency of 44.1 Y0and a solvent removal efficiency of 98.7 y0 were obtained using a heat setting of 6, chloroform as solvent (1 mlmin-“) an d an argon gas flow rate of 0.5 1min- l at the outlet of the separator. The reason for not obtaining the same efficiency as for Freon (46.3 %) is that the gas flow rates were somewhat different. The decreased solvent removal efficiency is due to the higher separator temperature. All the experiments in Table 2 were performed with the membraI~es in a horizontal position. When the separator was operated with the membranes in a vertical position using Freon an analyte transport efficiency of 64.5 % was observed instead of 63.1 o/ The difference which is statistically significant-at a level better than 99.9 %---is due to a longer distance for gravitational settling. Membranes have been used for weeks without any significant change in performance. 4.3. Areas of application The areas of application are those where a sample consists of an analyte in an organic solvent. Using a membrane separator interface will benefit the ICP technique because the solvent load is kept at an acceptable level for many applications and the analyte transport efficiency is increased by a factor of approximately 50 in comparison with normal nebulizer systems for the ICP. A gain of approximately 100 in aerosol analyte concentration is obtained in comparison with normal nebulizer systems if the interface is operated at an outlet argon gas flow rate of 0.5 1min- ‘. The gain in aerosol analyte concentration is of interest since the ICP-from a theoretical point of view [S]-is a concentration sensitive emission source. An interesting application is the coupling of the FIA technique to the ICP by the aid of the membrane interface, because a FIA extraction system can provide on-fine sample work-up and pretreatment. An improved single step FIA extraction system [5] gives matrix removal and concentrates the analyte by a factor of 10-I 5. The organic extract is introduced into the ICP with the interface. A FIA-extraction-membrane interface-ICP system seems to be advantageous for simultaneous multielement trace analysis, in view of the gain obtained by the FIA extraction and the interface. The analysis of samples by ICP mass spectrometry (ICP-MS) is severely influenced by the sample matrix [lo]. A FIA extraction-membrane interface-ICP system should improve analyses by ICP-MS because of the matrix removal and also by the gain obtained from the FIA extraction and the interface. It was shown in Ref. [6] that the aerosol chamber used provided for optimum coupling of the FIA and HPLC techniques to FAAS. As the same aerosol chamber is used in the
Table 2. The analyte transport efficiency as a function of the argon gas flow rate at an uptake rate of 1 ml min. ’ and a heat setting of 3 Analyte transport efficiency (%) Argon gas flow rate from the separator (Emin-‘)
Chloroform aerosol chamber
interface
aerosol chamber
interface
1.0 0.5
64.8 55.2
41.8 15.9
91.3 86.6
63.1 46.3
Freon
922
ANDERSGuSrAvSSON
membrane interface, there is much reason to assume that this interface will provide optimum coupling to an ICP since the coupling properties are determined by the aerosol chamber. The membrane interface can also be used for FAAS and flame emission techniques when the organic solvent content of the aerosol must be decreased.
5. CONCLUSIONS
The following conclusions can be drawn when the characteristics of the membrane interface are compared with those of normal nebulizer systems: (i) The membrane interface provides higher analyte transport efficiencies. (ii) The solvent load is kept at an acceptable level. (iii) The interface gives no waste. (iv) The aerosol chamber provides for optimum coupling of the FIA and HPLC techniques to the interference. (vi) It is easy to optimize the interface since all gas and liquid flows can be varied independently of each other. Acknowledgements-The author thanks FOLKEINGMANand SVERKERBLOMBERG for stimulating discussions and comments regarding the manuscript, J.E. Meinhard Associates for loan of nebulizers and the Swedish Natural Science Research Council for financial support.
REFERENCES [l] L. Ebdon and M. R. Cave, Analyst 107, 172 (1982). [2] M. P. Doherty and G. M. Hieftje, Appl. Spectrosc. 38, 405 (1984). [3] M. Ibrahim, W. Nisamaneepong and J. Caruso, .I. Chrom. Sci. 23, 14 (1985). [4] K. E. Lawrence, G. W. Rice and V. A. Fassel, Anal. Chem. 56, 289 (1984). [S] A. Gustavsson, Spectrochim. Acta 42B, 111 (1987). [6] A. Gustavsson, Spectrochim. Acta 42B, 883 (1987). [7] W. McFadden, Techniques ofcombined CC/MS: Application in Organic Analysis, Chap. 5. John Wiley, New York (1973). [8] F. J. M. J. Maessen, G. Kreuning and J. Balke, Spectrochim. Acta 41B, 3 (1986). [9] G. Kreuning and F. J. M. J. Maessen, Spectrochim. Acta 42B, 677 (1987). [lo] Inductioely Coupled Plasmas in Analytical Atomic Spectrometry, Ed. A. Montaser and D. W. Golightly, Chap. 11. VCH Publishers, New York (1987).