tometry, 3rd ed., American Society for Testing and Materials, Philadelphia, Pa.. 1969. R. N. Hager, Jr., Anal. Chem., 45, 1131A(1973). C . P.Thomas, Ph.D. Thesis, University of Maryland, 1972. J. Ingle, Anal. Chem., 46, 2161 (1974). M. S.Epstein, T. C. Rains, and 0. Menis. 15th Eastern Analytical Symposium and 12th National SAS Meeting, New York, 1973, Paper I f . J. Y. Marks, R. J. Spellman. and E. Wysocki, FACSS 2nd National Meeting, Indianapolis, Ind., 1975, Paper No. 202. G. R. Harrison, "MIT Wavelength Tables," MIT Press, Cambridge, Mass., 1969. M. Kirk, E. G. Perry, and J. M. Arritt, Anal. Chim. Acta, 80, 163 (1975). M. L. Parsons, 6.W. Smith, and G. E. Bentley, "Handbook of Flame Spectroscopy", Plenum Press, New York, 1975. T. C. O'Haver, 28th Annual Summer Symposium on Analytical Chemistry, June 1975, Knoxville. Tenn. P. N. Keliher, Res. lDev., 27(6), 26 (1976).
(35) G.Horlick and K. R. Betty, Anal. Chem., 47, 363 (1975).
RECEIVEDfor review December 11, 1975. Accepted March 19,1976. From a dissertation to be submitted to the Graduate School, University of Maryland, by A. T. Zander, in partial fulfillment of the requirements for the Ph.D. degree in Chemistry. The financial support provided by the National Science Foundation, Grant No. ESR-75-02667 (NSF-RANN), and partial support for one author (PNK) by the Villanova Faculty Research Program is gratefully acknowledged. This paper was presented at the 27th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 1, 1976. Paper Number 31.
Controlled Atmosphere Discontinuous Semiautomatic Dispenser for Carbon Rod Atomizer Vito Sacchetti, Gin0 Tessari,* and Giancarlo Torsi lstituto di Chimica Analitica, Universita di Bari, Via Amendola, 173-70 126 Bari, Italy
A semiautomatic dispenser for liquid solutions is described which makes possible a strict control of the atmosphere during the sampling operations. The dispenser was optimized with regard to the sampling mass reproducibility, by checking different capillary treatments. A typical relative standard deviation at l pl level of aqueous samples was 0.6 %.
When automatic procedures of sample introduction are sought for the carbon rod atomizer in atomic spectrometry three requirements should be met: i) small sample volumes (1-5 ~ 1 ) ii) ; critical sample positioning (narrow strip of graphite); iii) absence of containing walls. The concomitance of these three requirements rules out solutions such as those proposed by Pickford and Rossi ( I ) (too large a volume injected: 100 11.1) or by Molnar and Winefordner (2) (graphite tube acting as container for the nebulized sample). Maessen et al. ( 3 )used a commercial syringe (S.G.E.) with a device for accurste sample positioning in the case of an Argon-sheathed Mini-Massmann device open to the atmosphere. In the course of some investigations in this laboratory on the kinetics of atomic release from different substrates (4-7), it was found that in some cases the kinetic parameters of the metal release were critically dependent on the "inert" atmosphere surrounding the rod (8). The present investigation was carried out in order to work out a sampling device which, besides the above mentioned requirements: i) should permit a complete control of the atmosphere surrounding the rod, avoiding the opening of the atomization chamber after each measurement; ii) could be easily interfaced with a microcomputer in the fully automatized apparatus envisioned in this laboratory. The important features embodied in the device are: i) the use of a capillary of fixed volume to be filled by the sample solution; ii) the possibility of sampling different volumes of analyte solution by changing the piston tip, fitted with different capillaries; iii) the use of the same mechanical device for positioning the capillary and for generating the vacuum and the pressure needed to fill and to drain the capillary; and iiii) a variable buffer volume acting as a pressure regulator according to the volume and the viscosity of the solution to be sampled.
EXPERIMENTAL Sample Dispenser. A scale drawing of the device is presented in Figure 1. In Figure 2, a sketch is shown in which the actual dimensions have been altered to emphasize the parts which are critical to understanding how the device works. The hollow piston A, by its reciprocating motion, produces alternatively pressure or depression in chamber B. In Figure 2, the piston is shown in its extreme positions corresponding to: a) maximum pressure in chamber B and the tip of the capillary over the carbon rod; b) maximum depression in chamber B and the tip of the capillary dipping in the sample reservoir. The conditions prevailing in chamber B are transmitted to chamber A through the electrovalve C. The pressure (Figure 2a) is used for ejecting the sample on the carbon rod. Likewise, the depression is used for sucking the sample solution inside the capillary (MicrocapsDrummond Corp.), as usually ( 6 ) curved a t one end a t 90° by gentle heating. The solution volume inside the capillary is the dispensed quantity. The solution sucked in excess is collected a t the bottom of the piston (chamber A), and occasionally spilled through the stopcock F. The suction is interrupted by equilibrating, through electrovalve C, chambers A and B with the pressure inside the atomization chamber (dome). During the piston displacement, obviously chamber B is isolated, whereas the connection between chamber A and the dome is maintained. A buffer volume in parallel to chamber B has been added, with the aim of obtaining a pressure or depression difference between chamber A and the dome allowing: i) the suction of the minimum quantity necessary; ii) a gentle ejection of the sample drop avoiding any splashing. All the chambers and tubings were thoroughly washed with purified gas before a run of measurements, in order to have the same atmosphere inside the atomizer and the dispenser. In order to allow visual inspection in all the steps of the sampling process, the device was obtained from a block of transparent methacrylic material. The dispenser was screwed to a side window of the dome of a standard atomizer already described ( 3 , 6 ) .This coupling, as can be seen from Figure 1,forms a lever with a fairly long arm and, in order to make the applied force as small as possible, the device was driven by a light synchronous motor at 400 Hz.An endless screw was keyed to the shaft of the motor and coupled, through a helicoidal gear, to the circular rack moving the piston. T o suck the solution inside the capillary, the sample container D must be raised to such a height that the capillary tip dips in the liquid. As can be seen in Figure 1,the circular rack driving the piston, at the other extremity, acts on lever E raising the sample container D. This tight coupling between the motion of the capillary and the sample container secures the synchronism necessary to avoid the breaking of the fragile capillary. The force is applied to lever E through ball bearing L, to minimize the friction. The horizontal displacement of the rack is converted to a vertical displacement of the container by ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
1175
I
Ism
mieel
iX3
bra,‘
ezZa
teflon
methcrllic material
Figure 1. Schematic diagram of dispenser (A) Hollow piston, (6)high and low pressure chamber,(C)electrovalve,(D) sample Container,(E) lever, (F) stopcock, (G)graphite rod, (H)circular rack, (I) helicoidal gear, (K) endless screw, (L) ball bearing, (M) dome, (N) atomizer,(P) hollow screw
dermic syringe. A three-way stopcock inserted in the plastic tube permits the washing of the container and the changing of the analyte solutions with no air entering the dispenser. It was not the purpose, at this stage of the investigation, to optimize the device for minimum volume consumption and fast sample change. The amount of sample necessary to wash one time and load the container was 1 ml and the time for the operation less than 1 min. Apparatus. The optic and electronic measuring systems have already been described (3,6).The channel measuring the temperature was not used because this quantity was not important in the present investigation. The field stop, which strongly limits the view, necessary only in the kinetic measurements, was removed. A recorder was used to collect the data. Procedure for Absorption Measurements. The synchronous motor rotating a t 30 rpm, which drives the dispenser was manually switched, since a t this stage the fully automatic operation was not foreseen. Also the electrovalve for filling and draining off the capillary was manually switched. The remaining procedures have already been described (3,6). Mass Measurements. The drop was directly measured by letting the sample fall onto a pan of a Mettler MB/SA balance. The reported precision of the balance is 2 X g.
A
/,
,///////I
Figure 2. Enlarged sketch of chamber B where pressure or depression is originated
(a)Forward position, maximum pressure in chamber B; device ready tor ejectlon solution drop on the carbon rod (b) Backward position;maximum depression in chamber 0 device ready for sucking the solution
of
bending one arm of lever E. After fixing the position of the fulcrum, the angle, at which the lever arm was bent, was chosen to get the proper raising of the sample container when the tip of the capillary travels inside the edges of the same. A fine adjustment of this displacement was made possible by small changes in the position of the fulcrum and by changing the rest level of the container through the hollow screw P. When necessary, the level of the analyte solution is restored in the container by forcing the liquid through a plastic tube (silicone rubber) fitted a t the bottom of the container using a hypo1176
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
RESULTS AND DISCUSSION To avoid concomitant effects of the evaporating surface and of the transport properties of the atomizer, the behavior of the dispenser was first checked with a balance. Since the drop will not detach itself properly without treatment of the capillary, giving consequently erratic results, several runs of drop weighings were used to investigate the best way of treating the capillary with hydrophobic materials. The best results were obtained when only the front tip was treated with wax or Desicote (3,6)while the other extremity was left untreated. This can be easily accomplished by dipping the tip of the capillary in molten wax or in Desicote (Beckman). In order to avoid any clogging a small stream of gas was blown inside the capillary during the operation. In Figure 3a a typical histogram is reported for a run obtained with a waxed capillary of 101 volume and pure water. The calculated u for 50 drops is 0.6%. It can be pointed out that the mean value is significantly less than 1 mg as required by the capillary nominal volume of 1~ 1also , taking into account the temperature dependence of the water density. This effect is probably due: i) to a real decrease of the volume in the process of curving the tip; ii) to the creeping of the hydrophobic layer inside the
b
I I
EPt: 51
5
1
111 . .
I
0.90
WEIGHT (me)
CLASS ~ E Q U E N C E
Figure 3. Histograms relevant to a waxed capillary of 1 MI nominal
volume Sample: pure water. (2a) Frequency vs. weight of one drop: (2b) weight of a class of ten drops vs. the temporal class sequence
capillary. In Figure 3b the same results are presented in their temporal sequence taking the average value of classes of 10 drops. The trend depicted is fairly general with a dip after the first stages, followed by a slow increase with a sharp break just before the destruction of the hydrophobic layer, associated with difficulties in the drop detachment. If the hydrophobic treatment is properly made, several hundred drops can be obtained. The same results were obtained for whole blood (liquid with higher viscosity than water) and siliconized capillary of 1p1 nominal volume. The calculated u for 100 drops was 3.0%. This result was considered satisfactory not only for kinetic measurements but also for analytical purposes, and no further investigations were devoted to the study of the correlation between the precision of the drop mass and the physical and chemical properties of the liquid to be sampled. When coupled with the atomic spectrometer according to the procedures indicated in the experimental section, a u value of 3% was found for 50 drops of 1 111 nominal volume waxed capillary using 0.1 ppm lead acetate solution acidified with acetic acid. A sequence of curves which is part of this run is shown in the photo reported in Figure 4. I t must be noted that: i) the useful signal is the difference between the total absorption and the signal a t the foot of the transient; ii) no particular effort has been made to optimize the spectrometer (at this time used for kinetic measurements) for analytical purposes, except for the minor changes noted in the Experimental section. Thus, the sensitivity of the instrument for lead could be improved with respect to the value which can be derived from Figure 4. The worsening (roughly 5 times) of the precision between mass and absorption measurements was tentatively related to the spreading of the sample on the porous graphite surface. Also, the influence of the transport properties of the system in the gaseous phase cannot be ruled out. In order to increase the limit of detection, one can accumulate several drops on the graphite rod. The spreading on
'*' Flgure 4. Series of replicate measurements obtained with 1 PI nominal volume of Pb Ac 0.1 ppm HAC
+
the surface, which increases with the sample volume, produces analytical curves with lower and lower slope. This effect is clearly visualized in diagrams of Absorption vs. sample volume which generally is not linear. In order to overcome this drawback, the successive drop deposition is made only after drying the preceding drop. The curve obtained is linear up to 10 drops with an equation, for the conditions used y = (3.6 f 0.07) x
- (0.15 f 0.08)
(1)
where y is the recorder chart divisions and x the microliters. It must be noted that with this technique just one calibration curve is needed, also when different sample volumes are used. Of course this procedure can be used only with diluted solutions or when matrix effects are minimal.
ACKNOWLEDGMENT Thanks are due to Antonio Valentino of the mechanical shop for helpful discussions in the design of the dispenser.
LITERATURE CITED C. J. Pickford and G. Rossi, Analyst, 97, 647 (1972). C. J. Molnar and J. D. Wlnefordner, Anal. Chem., 46, 1807 (1974). F. J. M. J. Maessen. F. D. Posma, and J. Balke, Anal. Cbem., 46, 1445 (1974). G. Torsi and G. Tessari, Anal. Chem., 45, 1812 (1973). (5) S. L. Paveri-Fontana, G. Tessari, and G. Torsi, Anal. Cbem., 46, 1032 (1974). (6) G. Torsi and 0.Tessari, Anal. Chem.. 47, 839 (1975). (7) G. Tessari and G. Torsi, Anal. Chem., 47, 842 (1975). (8) G. Tessari and G. Torsi, to be published.
(1) (2) (3) (4)
RECEIVEDfor review October 22,1975. Accepted March 22, 1976. Work done under contract No. 74.00675.03 of Consiglio Nazionale delle Ricerche (C.N.R.) Roma.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 8, JULY 1976
1177
