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A pneumatic solution nebulization system producing dry aerosol for spectroscopy
Spectrochimica Acts, 1968, Vol. 23B, pp. 553 to 555. Pergamon Press. Printed in Northern Ireland
TECHNICAL NOTE A pneumatic solution nebulization system producing dry aerosol for spectroscopy* (Received
6 November
1967)
Ab~tr&---An efficient sample introduction system was developed for atomic spectroscopy of aqueous solutions. It is made up of a pneumatic nebulizer, a heated chamber for solvent evaporation, and a chilled condenser for solvent removal. Features include low gas flow rate, high efficiency, and elimination of the solvent. Overall efficiency is 35 %. A NEED arose in our research [l] for an efficient sample introduction system for aqueous sample solutions, to be used with an r.f. plasma torch. Our requirements were: low gas flow rate; high solution uptake rate (i.e., high solution-to-gas flow ratio); high efficiency (defined aa the fraction of the aspirated analyte entering the flame); elimination of the solvent; and convenience. After consideration of several types of sample introduction systems, we felt that the pneumatic type was most promising in meeting all of our requirements. The system design shown in Figure 1 proved satisfactory.
ASPIRATOR
15cm
4
FRIEORICHS, CONDENSER
HEATING TAPE (6'~42: 288W)
~110DlfIED)
ARCllN
50 psi
TO TORCH
‘\%A DRAIN (UNDERWATER 1 Fig. 1. Sample introduction
system.
The nebulization system employs a pneumatic sprayer, a heated chamber, and a condenser for removal of solvent vapor. Heated spray chambers were occasionally employed in flame * Contribution from the National Bureau of Standards, not subject to copyright. [l]
C. VEILLON
and
M. MARQOSEES,
Spectrochim. 553
Acta m,
503 (1968).
554
Technical note
photometry [2] to improve the efficiencyof nebulization, but the practice is uncommon today. A sample introduotion system employing 8 heated chamber and a condenser for removal of sample vapor is available commercially for use with chemical fhwnee(Beckman Instruments, Fullerton, California). The OOmmerCi81 unit W88 tried, but it proved unsuitable for the plasma torch due to the high gae flow rate necessary for aspiration and efficient nebuhzation. The system describedhere differs from the commercial unit in several important respects, although the general principlesof operation 8re similar. +
+
0.78mm ORIFICE STAINLESS
THREE SCREWS
STEEL
O-80TANTALUM
1.20’
APART
TIPS
ROUNDED
d. NOT
TO
SCALE
THREE 120’
2-56rSCREWS APART
+--‘Pt
- RU TUBING 22
GAGE
Fig. 2. Sprayer. The efficientnebulizersprays a very fme mist into the he8ted chamber which is operated at just sufiicientheat input (approx. 280 W) to completely evaporate the solvent from the aerosol when sprayed continuously. To maintain a constant temperature within the heated chamber, water is aspirated continuously between samples. The approximate dimensionsof the heated chamber are 5 cm dia. x 15 cm long. This size and con6guration gave the best signal-to-noise ratio and/or operating convenience of the several tried. The resulting water vapor and dry particles are then conducted through a modified Friedrichs condenser cooled to 10°C. This removes almost 811of the water vapor from the clerosoland surprisinglylittle of the dry particles. Removal of the water vapor was verified by the complete absence of OH band spectra in an argon plasm8 discharge[l] and more stable operation of the discharge. The measured overall eftlciencyof the sample introduction system is 35 o~-extremely high efficiencyfor 8 chambertype system [3]. Compared to the spray chamber used alone (unheated), the analytical sensitivity was increasedapproximately lo-fold. No drift in nebulizationrate with time was noticed during several months of use of the system. The efficiency of the sample introduction system was measured 8s the percentage of the metal in the aspirated sample solution emerging from the condenseras a dry spray (i.e., metal out/metal in): 10 ml of 100 c(g/ml Li solution were aspirated; the condensatewas collected and added to washings of the condenser,heated chamber, 8nd nebulizer; this solution was diluted to 100 ml and compared by flame emission spectrometry with Li standard solutions of 5 and 10 ,ug/ml. The solution was found to contain 0.6 pg/ml Li, which gives an efficiencyof 36 %. This indirect measurement of the efficiencywas neoessarybecause trapping the dry aerosol p8rticles emerging from the condenser was found to be extremely difficult. For example, passing the dry aerosol through fritted glass, cold trapping (even at liquid N, temperature) or bubbling through a water column quite visibly failed to stop all of the particles. The aerosol can be conducted over long distanceswith no signifkant loss. [2] R. HERRand C. T. J. ALKEMADE, Flame Photometry, translated by P. T. QIL~ERT,JR., p. 115. Interscience (1903). [3] J. B. WILLIS,Spectrochint.Acta 23A, 811 (1967).
‘h&nice1
note
665
The nebulizer itself is shown in Figure 2. It is Op0r8ti at 8 rd8tivdy high pressure Of 3.5 kg/cm2 (50 psi) argon and has 8 high solution aspiration-to-gas flow ratio. For example, the argon flow rste through the orifice at this pressureis 1.7 l/n&, which aspirates water at 8 rate of 2.3 ml/min. This performance,combined with the 36 % over-all &ciency of the chamber system me8ns th8t the available sample introduction rate is equivalent to about 0.8 ml/min. The nebulizer is constructed of stainless steel and is demountable for cleaning. The or&e is machined in tantalum discs for interchangeabilityand corrosionresistance. Three screwscenter the platinum capillary (90 okPt-10 % Ru) in the orifice. When used with the r.f. plasma torch [l], the flow from the sample introduction system is diverted and the plasma initiated using dry argon. The system flow is then fed in and the initiatingflow stopped. When the aspiratortubing is removed from the solutionbeing nebulized, an abrupt flow increase in the plasma tube occurs, due to air being sucked into the chamber through the aspirator. This introduction of moleculargases will often extinguish the discharge. This ca,n be prevented by momentarily restricting the aerosol tubing leading to the plasma torch while the aspirator tubing is not in the solution. This sample introduction system should have many other applications than the one for which it was designed. The high efficiency,low gas flowand solvent removal capabilitiesshould make the system very useful with high-temperatureflames [e.g., Hz/F,, O,/(CN),], other types of elect,ricaldischarges (e.g., plasma jet) and graphite-tube furnaces [4, 61. Acknowledgmen&--We thank Mr. JAMESS. JONESof NBS for construction of the nebulizer, and Mr. ENRICODELEONIBUS of NBS for construction of the glass chambers. Spectrochemical Analysis Section, National Bureau of Sltandurds Washington, D.C. 20234
CLAUDEVEILLON* MARVINMARUOSHES
* Present address: Department of Chemistry,University of Houston, Houston, Texas 77004. [4] G. RAMELOWand R. WOODRIFF,Spectrochim. Acta, in press. [5] H. MASSMANN, Spectrochim. Acta 23B, 215 (1968).
TECHNICAL NOTE A pneumatic solution nebulization system producing dry aerosol for spectroscopy* (Received
6 November
1967)
Ab~tr&---An efficient sample introduction system was developed for atomic spectroscopy of aqueous solutions. It is made up of a pneumatic nebulizer, a heated chamber for solvent evaporation, and a chilled condenser for solvent removal. Features include low gas flow rate, high efficiency, and elimination of the solvent. Overall efficiency is 35 %. A NEED arose in our research [l] for an efficient sample introduction system for aqueous sample solutions, to be used with an r.f. plasma torch. Our requirements were: low gas flow rate; high solution uptake rate (i.e., high solution-to-gas flow ratio); high efficiency (defined aa the fraction of the aspirated analyte entering the flame); elimination of the solvent; and convenience. After consideration of several types of sample introduction systems, we felt that the pneumatic type was most promising in meeting all of our requirements. The system design shown in Figure 1 proved satisfactory.
ASPIRATOR
15cm
4
FRIEORICHS, CONDENSER
HEATING TAPE (6'~42: 288W)
~110DlfIED)
ARCllN
50 psi
TO TORCH
‘\%A DRAIN (UNDERWATER 1 Fig. 1. Sample introduction
system.
The nebulization system employs a pneumatic sprayer, a heated chamber, and a condenser for removal of solvent vapor. Heated spray chambers were occasionally employed in flame * Contribution from the National Bureau of Standards, not subject to copyright. [l]
C. VEILLON
and
M. MARQOSEES,
Spectrochim. 553
Acta m,
503 (1968).
554
Technical note
photometry [2] to improve the efficiencyof nebulization, but the practice is uncommon today. A sample introduotion system employing 8 heated chamber and a condenser for removal of sample vapor is available commercially for use with chemical fhwnee(Beckman Instruments, Fullerton, California). The OOmmerCi81 unit W88 tried, but it proved unsuitable for the plasma torch due to the high gae flow rate necessary for aspiration and efficient nebuhzation. The system describedhere differs from the commercial unit in several important respects, although the general principlesof operation 8re similar. +
+
0.78mm ORIFICE STAINLESS
THREE SCREWS
STEEL
O-80TANTALUM
1.20’
APART
TIPS
ROUNDED
d. NOT
TO
SCALE
THREE 120’
2-56rSCREWS APART
+--‘Pt
- RU TUBING 22
GAGE
Fig. 2. Sprayer. The efficientnebulizersprays a very fme mist into the he8ted chamber which is operated at just sufiicientheat input (approx. 280 W) to completely evaporate the solvent from the aerosol when sprayed continuously. To maintain a constant temperature within the heated chamber, water is aspirated continuously between samples. The approximate dimensionsof the heated chamber are 5 cm dia. x 15 cm long. This size and con6guration gave the best signal-to-noise ratio and/or operating convenience of the several tried. The resulting water vapor and dry particles are then conducted through a modified Friedrichs condenser cooled to 10°C. This removes almost 811of the water vapor from the clerosoland surprisinglylittle of the dry particles. Removal of the water vapor was verified by the complete absence of OH band spectra in an argon plasm8 discharge[l] and more stable operation of the discharge. The measured overall eftlciencyof the sample introduction system is 35 o~-extremely high efficiencyfor 8 chambertype system [3]. Compared to the spray chamber used alone (unheated), the analytical sensitivity was increasedapproximately lo-fold. No drift in nebulizationrate with time was noticed during several months of use of the system. The efficiency of the sample introduction system was measured 8s the percentage of the metal in the aspirated sample solution emerging from the condenseras a dry spray (i.e., metal out/metal in): 10 ml of 100 c(g/ml Li solution were aspirated; the condensatewas collected and added to washings of the condenser,heated chamber, 8nd nebulizer; this solution was diluted to 100 ml and compared by flame emission spectrometry with Li standard solutions of 5 and 10 ,ug/ml. The solution was found to contain 0.6 pg/ml Li, which gives an efficiencyof 36 %. This indirect measurement of the efficiencywas neoessarybecause trapping the dry aerosol p8rticles emerging from the condenser was found to be extremely difficult. For example, passing the dry aerosol through fritted glass, cold trapping (even at liquid N, temperature) or bubbling through a water column quite visibly failed to stop all of the particles. The aerosol can be conducted over long distanceswith no signifkant loss. [2] R. HERRand C. T. J. ALKEMADE, Flame Photometry, translated by P. T. QIL~ERT,JR., p. 115. Interscience (1903). [3] J. B. WILLIS,Spectrochint.Acta 23A, 811 (1967).
‘h&nice1
note
665
The nebulizer itself is shown in Figure 2. It is Op0r8ti at 8 rd8tivdy high pressure Of 3.5 kg/cm2 (50 psi) argon and has 8 high solution aspiration-to-gas flow ratio. For example, the argon flow rste through the orifice at this pressureis 1.7 l/n&, which aspirates water at 8 rate of 2.3 ml/min. This performance,combined with the 36 % over-all &ciency of the chamber system me8ns th8t the available sample introduction rate is equivalent to about 0.8 ml/min. The nebulizer is constructed of stainless steel and is demountable for cleaning. The or&e is machined in tantalum discs for interchangeabilityand corrosionresistance. Three screwscenter the platinum capillary (90 okPt-10 % Ru) in the orifice. When used with the r.f. plasma torch [l], the flow from the sample introduction system is diverted and the plasma initiated using dry argon. The system flow is then fed in and the initiatingflow stopped. When the aspiratortubing is removed from the solutionbeing nebulized, an abrupt flow increase in the plasma tube occurs, due to air being sucked into the chamber through the aspirator. This introduction of moleculargases will often extinguish the discharge. This ca,n be prevented by momentarily restricting the aerosol tubing leading to the plasma torch while the aspirator tubing is not in the solution. This sample introduction system should have many other applications than the one for which it was designed. The high efficiency,low gas flowand solvent removal capabilitiesshould make the system very useful with high-temperatureflames [e.g., Hz/F,, O,/(CN),], other types of elect,ricaldischarges (e.g., plasma jet) and graphite-tube furnaces [4, 61. Acknowledgmen&--We thank Mr. JAMESS. JONESof NBS for construction of the nebulizer, and Mr. ENRICODELEONIBUS of NBS for construction of the glass chambers. Spectrochemical Analysis Section, National Bureau of Sltandurds Washington, D.C. 20234
CLAUDEVEILLON* MARVINMARUOSHES
* Present address: Department of Chemistry,University of Houston, Houston, Texas 77004. [4] G. RAMELOWand R. WOODRIFF,Spectrochim. Acta, in press. [5] H. MASSMANN, Spectrochim. Acta 23B, 215 (1968).