~
Table II. Examples of Mass Measurements
Reference peak 613.96473~ 865.9581
Mass ratio 1.255709 1.410430 1.013857 1.057735 1.071596 1.115476 1.230938 1.346417
Measured mass
Theoretical mass
770.9610 865.9542 877.9581 915.9541 927,9572 965.9555 1065.9407 1165.9407
770.9598 865,9581 877.9581 915,9549 927.9549 965.9517 1065.9453 1165.9389
A , ppm 1.6 4.3 0.0 0.9 2.5 3.9 4.3 1.5
AvA,ppm: 2.4 a
From heptacosaperfluorotertiarybutylamineused as an external reference standard,
stable ion at mje 686.4, as shown in Table I. The highest mass ion with only one N atom occurs at m / e 376, due probably to a similar process subsequent to the loss, by 0cleavage, of the second sidechain. As a completely fluorinated material, perffuorotriheptyltriazine does not yield ions that interfere with hydrocarbon ions. It is a liquid at room temperature, and may be easily introduced in any mass spectrometer. It may be used for the precise mass measurement of ions of m / e values up to 1600 with the Nier peak matching system which allows one to measure accurately peaks of mass 40 per cent higher than that of the reference peak, and up to m / e 1185 with a computerized data acquisition system, which requires that ion standards bracket sample ions.
This standard, together with the now commercially available insertion probes, should be of considerable aid in the mass spectral analysis of very heavy molecular weight materials. In these laboratories it was used for the mass measurement of organic (not halogenated) materials up to molecular weight 1200. The mass spectrometer was an Associated Electrical Industries, Ltd. model MS 9 high resolution instrument. ACKNOWLEDGMENT The perfluoroheptyltriazine used in this work was prepared and furnished by courtesy of Peninsular Chemresearch, Inc., of Gainesville, Fla. RECEIVED for review April 12, 1968. Accepted May 31, 1968.
Ultrasonic Nebulizer for Easily Changing Sample Solutions J. M. Mermet and J. P. Robin Service de Chimie IndustrieIIe et AnaIytique, Institut National des Sciences AppliquPes, 20, Avenue A . Einstein-VILLEURBANNE (%&e) France
PRODUCING AN AEROSOL raises some problems which have been solved by using a pneumatic nebulizer. The use of ultrasonic spraying has been discussed by several authors (1-14), for general and medical research and for spectroscopic purposes (flame photometry, atomic absorption spectrophotometry, induction-coupled plasma, etc . . . ). This spraying process has numerous advantages, particularly increased output due to the good quality of the fog.
(1) K. Bisa, K. Dirnagl, and R. Esche, Siemens Z., 8, 341 (1954). (2) H. Dunken, G. Pforr, and W. Mikkeleit, Z . Chem., 3, 196 (19631. ( 3 j Zbik, 4,237 (1964). (4) H. Dunken, G. Pforr, W. Mikkeleit, and K. Geller, Spectrochim. Acra, 20, 1531 (1964). (5) E. L. Gershenzon and-0. K. Eknadiosyants, Soviet Phys.Acousr. (English Transl.), 10, 127 (1964). (6) P. Herzog, 0. P. Nordlander, and C. G. Engstrom, Acra Anaesthesiol. Scand., 8, 79 (1964). (7) H. C. Hoare and R. A. Mostyn, ANAL.CHEM., 39,1153 (1967). (8) H. C . Hoare, R. A. Mostyn, and B. T. N. Newland, Anal. Chim. A m , 40, 181 (1968). (9) W. J. Kirsten and G. 0. B. Bertilsson, ANAL.CHEM., 38, 648 (1966). (10) R. H. Wendt and V. A. Fassel, ibid., 37,920(1965). (11) C. D. West and D. N. Hume, ibid., 36, 412 (1964). (12) C. D. West, ibid., 40, 253 (1968). (13) J. Spitz and G. Uny, Appl. Opt., 7 , 1345 (1968). (14) M. E. Ropert, Mithodesphysiques d'analyses, (GAMS), 4,231 (1968).
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ANALYTICAL CHEMISTRY
Pneumatic spraying, in the case of aqueous solutions, has a yield of about 10%. That is, only 10% of the nebulized solution constitutes a fine aerosol which can leave the cell where it is generated and, for instance, enter a flame. Indeed, whatever the type of spraying, it has been ascertained that the diameter of the droplets going through the flame is approximately identical. Such is the case with ultrasonic spraying when a suitable frequency is used. This explains the stability of the fog and also the good spraying yield. Consequently, analyses can be made with smaller quantities. It is also possible to get a spray rate independent of the gaseous flow rate. This may be advantageous for the study of some physical parameters of flames or plasmas. At first, we studied the works of West and Hume ( I I ) and Wendt and Fassel (IO) in order to design an accessory which would feed an H F induction-coupled plasma. Our aim was to devise an apparatus that would allow samples to be changed without interrupting the plasma discharge. Several kinds of ultrasonic generators are available; they operate with or without a liquid transmitter. After some testing, we chose the first type which appeared to us as the most suitable to solve our problem. To introduce and remove the sample, several authors (7, 12) have worked with this kind of generator by changing spraying cells. For our part, we think that this method has some drawbacks, especially in building cells of identical characteristics. Therefore, we use a motionless vessel combined with a reversible pump.
t
T‘
G
I
J
I
70
90
80
b
100
HEIGHT I N m m
Figure 1. Schematic diagram of the set (see text for the explanationsof letters)
EXPERIMENTAL
Apparatus. An ultrasonic generator (Macrosonics Corp. model 125 FF) delivering about 25 W of power and whose frequency is allowed to change, after measurements, in the 0.67-1 MHz range, is used (see Figure 1, F). It is connected with a piezo-transducer, E , placed in water as a liquid transmitting bath, D. The choice of a fixed spraying cell allowed us to design it so that the running distance to the plasma was as short as possible, and the condensation losses by collisions were reduced. For this purpose, we have preferred a vertical and direct outlet of droplets, L, and a side inlet, A , of carrier gas. The cell, B, is made of borosilicate glass, except its bottom which is separated from the transmission liquid by a 12 p thick Mylar membrane, C, sealed with Araldite whose useful diameter is about 30 mm. The height between transducer and membrane is adjustable. This membrane can be used €or several months without perceptible alteration. The reversible pumping system consists of a peristaltic pump, H , connected with 3 tubes of 1-mm i.d. which have their openings close to the membrane surface (only one tube is shown in Figure 1). The pump offers the following advantages: it has a very small dead volume and it is self-filling. Because it is powered by an electric reversible motor, it is easy to introduce an amount of liquid automatically and to evacuate the remaining portion at the end of the test. As the solution fails to wet the Mylar membrane, the capillary tubes can evacuate nearly all the remaining liquid by suction. The same process is used to clean the cell. Another evacuation system for the remaining solution has been used by Spitz and Uny (13): their membrane is slightly inclined so that the liquid gathers at the low point; one capillary tube is then sufficient. We have added a device, principally for concentrated solutions, which opens into the outlet spraying tube, K. It is fed by the same pump with a second tube connected as shown in Figure 1. The liquid for cleaning is simultaneously pumped out, thus giving very efficient operation.
Figure 2. Aerosol flow rate us. height between transducer and membrane
According to Ropert (13, size distribution gives a lo%er diameter. We have: 22% above 3.5 p ; 50% from 3.5 to 1.2 p ; and 28% below 1.2 p. The quality of this fog explains the excellent output. Apparatus Parameters. The amount of spray seems to depend on four main parameters : height between membrane and transducer, generator power, carrier gas flow rate, and temperature of the transmitting liquid.
(15) M. E. Ropert, Centre d’Etudes NuclCaires de Fontenay aux Roses, France, personal communication, 1967.
A
1.0
0.5
I
80
90
100
% MAXIMUM POWER
RESULTS AND DISCUSSION
Frequency and Droplet Size. The frequency of the generator varies between 0.67 and 1 MHz. We operated with a fixed frequency of 0.83 MHz where there was maximum spraying. From the relationship between the droplet size and the frequency, there was an average diameter of 4.4 p .
Figure 3. Aerosol flow rate us. generator power at different carrier gas flow-rates 1. 3.7liter/& 2. 3 . 0 liter/min 3. 2 . 4 liter/min 4. 1.7liter/min 5.
0 . 8 literimin
VOL. 40, NO. 12, OCTOBER 1968
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/ *
L ITER/MI N
Figure 4. Aerosol flow rate us carrier gas flow-rate at different fractions of the maximum ultrasonic generator power 1. 100% 2. 9 5 . 5 3. 91.0
4. 8 6 . 5 z 5. 82.0 6. 77.0
HEIGHTBEWEEN MEMBRANE AND TRANSDUCER. Tests have been done for heights between 60 and 110 mm with the flow rate of the aerosol carrier gas at 2.4 liters/minute and the generator operating at maximum power. We can see that over 90 mm, the spray rate becomes weak (Figure 2). Thus we chose to operate at 70 mm. The height of the liquid above the membrane does not influence output during a period of time longer than that re-
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ANALYTICAL CHEMISTRY
quired for a test. This allows the solution to be fed into the cell at intervals. GENERATOR POWER. The ultrasonic generator power can be set from zero to maximum power (about 25 W). Aerosol production is perceptible only if sufficient power is used. The experiment has been done with several different flow rates. It was found that the aerosol rate increases with power, with a tendency toward saturation (Figure 3). When studying the fog flow rate by opacity measurements, we observed that the aerosol production stability also increases with power. CARRIER GASFLOW RATE. At lower flow rates, we have approximately the same quantity of aerosol whatever the power. The deviation of the results becomes important when the gas flow rate increases (Figure 4). Note that we are limited in the higher rates (in our case, about 4 liters/minute) by the destruction of the fountain made by ultrasounds at the liquid surface. This limit principally depends upon the volume of the vessel. For different volumes, we evidently have different values, but a curve of the same shape results. Again, we see that stability increases as flow rate decreases. TEMPERATURE OF TRANSMITTING LIQUID. The influence of the temperature of the transmitting liquid cannot be considered negligible (the temperature of the liquid to be sprayed is not recorded). In this test (maximum power and a flow rate of 2.4 liters/minute), the variation of the aerosol flow rate reaches 1% per degree at about 25 "C. Because the transducer itself is a source of heat, we had to regulate the bath temperature in order to obtain reproducible readings. There is no advantage in increasing this temperature because of the risk of condensation losses on cold walls. It is preferable to work at room temperature. For each particular case there obviously is an optimum arrangement. For our problem the available aerosol flow rate was always superior to what we needed.
RECEIVED for review April 8, 1968. Accepted June 21, 1968. Work supported by the Centre National de la Recherche Scientifique, France.
