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ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979 O30r
an environment at constant temperature. This method can be used to reduce the various matrix effects often observed when sampling a solution deposited on the wall. 2200'
u 0
020t
2700'
ACKNOWLEDGMENT We acknowledge numerous helpful discussions with A. Walsh and B. V. L'vov.
0
LITERATURE CITED
0001 0002
OW5
001
002
005
01
02
05
(1) B. V. L'vov, L. A. Pelieva, and A. I. Sharnopolskii, Zh. Prikl. Spectrosk., 27, 395 (1977). (2) B. V. L'vov, Specfrochim. Acta, Part 6, 33, 153 (1978). (3) R. E. Sturgeon, Anal. Chem., 49, 1255A (1977). (4) W. Slavin and D. C. Manning, Anal. Cbem., 51, 261 (1979). (5) D. C. Gregoire and C. L. Chakrabarti, Anal. Chem., 49, 2018 (1977). (6) 6. V. L'vov, Specfrochim. Acta, 17, 761 (1961). (7) R. Woodriff and G. Ramelow, Spectrochim. Acta, Part 6, 23, 665 (1968). (8) D. Littlejohn and J. M. Ottaway, Analyst (London), 103, 662 (1978). (9) H. Massmann, Specfrochim. Acta, Part B , 23, 215 (1968). (10) 6. V. L'vov and L. A. Pelieva, Zh. Anal. Chim. (Russian), 33, 1572 (1978). (11) J. Janssen, Compf. Rend., 71, 626 (1870). (12) R . Mavrodineanu and H.Boiteux, "Flame Spectroscopy", Wiiey & Sons, New York, 1965. (13) V. P. Garnys and L. E. Smythe, Anal. Chem., 51, 62 (1979). (14) D. C. Manning and W. Slavin, Anal. Chem.. 50, 1234 (1978). (15) W. Slavin, S. Myers, and D. C. Manning, unpublished results.
0
% MgCk2
Effect of atomizing temperature on MgCI, interference of absorption
Figure 7. TI
Plotted in Figure 7 is the T1 absorbance as the MgC1, is increased, a t three temperatures: 1800, 2200, and 2700 "C. The absorbance signal for T1 is larger a t lower temperatures because the residence time is longer in the furnace. However the chloride interference is more severe a t lower temperature, as predicted. Introducing a sample dried on a wire is a convenient method for ensuring that the sample is vaporized and atomized within
RECEIVED for review June 27,1979. Accepted September 4, 1979.
Reduction of Argon Consumption by a Water Cooled Torch in Inductively Coupled Plasma Emission Spectrometry Guy R. Kornblum, Wouter Van der Waa, and Leo de Galan" Laboratorium voor Analytische Scheikunde, Technische Hogeschool Delft, Jaffalaan 9, 2628 BX Delft, The Netherlands
By adding a water cooled jacket to the conventional three-tube quartz torch, the argon consumption In inductively coupled plasma (ICP) spectrometry has been reduced 10-fold. Two designs are described and evaluated. In comparison with the conventional high-flow ICP, the plasma generated in the novel torch design shows a reduced background Intensity and comparable excitation temperature, dynamic range, and freedom from interferences. Limits of detection decrease with the sample carrier gas flow and presently range from 0.01 to 1 mg/L for a carrier gas flow of 0.1 L/mln.
The present state of the art of the inductively coupled radiofrequency argon plasma has been reviewed (1, 2). A disadvantage of all commercial instruments and nearly all research units is the large consumption of argon gas. Whereas only 1 L/min is used to carry the nebulized sample solution into the plasma, typically between 10 and 20 L/min are required to shield the outer quartz tube of the plasma torch from the hot plasma gas. Occasionally, a third argon flow (plasma gas) is used, especially for nonaqueous solvents. This large consumption of argon not only adds to the running costs of the ICP, but it creates the practical problem of regular supply. Continuous operation requires one 50-L gas tank per day and has induced some laboratories to install large units with liquid argon. Several proposals have been made to decrease the need for argon. A rather drastic solution is to replace the main argon flow by nitrogen, but this requires a much higher rf power (>5 0003-2700/79/0351-2378$01,00/0
kW into the plasma), which is undesirable for obvious reasons. With the more popular medium power rf generator ( 1 2 kW) Dagnall, Smith, and West (3) succeeded in a partial replacement of the main argon flow, so that their ICP ran on 7 . 5 L/min Ar and 7.5 L/min N2in addition to 0.5 L/min of argon as the carrier gas. Scott et al. ( 4 ) optimized the design of the three-tube configuration and utilized a total argon flow of 10 L/min. A similar approach was followed by Allemand and Barnes ( 5 ) ,who in a later publication (6) reduced the main argon flow by applying an angular gas introduction system that creates a so-called swirl flow. In this way the main argon flow could be decreased from 18 to 10 L/min in a commercial instrument and from 12 to 5 L/min in their own research unit. In all these attempts the total argon consumption is reduced by no more than a factor of two with a lower limit of about 6 L/min. The torch design described in this work permits a reduction with another factor of three to 2 L/min. Torch Design. Apart from sustaining the rf plasma, the main function of the large argon flow used in present ICP torches is to prevent the outer quartz tube from melting. I t is, therefore, commonly referred to as the coolant gas. Consequently, a substantial reduction in argon consumption can be realized, if the cooling is performed by other means. In the proposed torch design, this is realized by fitting a water jacket around the torch. For obvious reasons the inlet point of the water supply cannot be located within or on top of the rf coil. Hence both the inlet and the outlet point of the cooling water must be situated a t the bottom end of the jacket. In order to force the water toward the hot plasma region inside the rf coil, where 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
a
2379
b
Flgure 1. Two designs of the water-cooled torch for the low-flow ICP. In both designs the conventional three-tube arrangement is maintained and some relevant radial and axial dimensions are given in mm. Also, in both designs the two inner quartz tubes are independent from the outer tube, which is equipped with a cooling jacket. I n design (a) the outer tube and cooling jacket are rigidly assembled into a single unit made from quartz. The water inlet and outlet points are on opposite sides of the two vertical partitioning walls, so that the cooling water is forced to pass the plasma region. In design (b) the cooling jacket consists of two independent tubular parts, that slide over each other and are held in position by plastic nuts and O-rings. The shaded partitioning tube is made from glass and the outer torch tube is made from quartz. Although positioning this design is a little more delicate, its construction is easier than of design (a)
cooling is most required, some partitioning wall must be placed inside the water jacket. Several modifications have been tried and the two most promising approaches are shown in Figure 1. T h e design in Figure l a is made entirely from quartz t o avoid elongated quartz-glass connections. In this design the position of the fused connection point of the vertical partitions to the cooling jacket was found to be critical. Multiple points or fusion along the entire length of the partition creates tensions that destroy the torch after ignition. A single connection point, as shown in Figure la, must be neither too close to the top of the torch nor too close to the bottom where the jacket is attached to the torch. The design shown, with the point of attachment halfway, operated reliably for four months (see below). A more recent design is shown in Figure 1b. Here the jacket consists of two pieces: a straight glass tube acts as the partition and the quartz jacket slides over it. The two systems are connected with two open nut caps, in which silicone rubber O-rings are mounted against water leakage. Here the concentricity of the design is critical and careful positioning is required, but once this is accomplished the configuration is quite stable. This design has been operative for several months. In both designs, the presence of lingering air or steam bubbles in the cooling water proved disastrous. In fact, an unnoted air bubble destroyed the torch shown in Figure l a owing to local overheating and melting of the quartz tube. I t is, therefore, essential to maintain a high flow of cooling water to rapidly carry away any bubbles. By using a waterflow of 2 L/min, we have been able to run the plasma daily for over 8 h without overheating. From the temperature increase of the cooling water of 5 "C, it is concluded that practically all of the 700 W rf input power is carried away by the cooling
water. Since the cooling water is observed to boil close to the plasma region, the outside wall of the torch tubo must be over 100 "C. With a thermal conductivity of quartz of 1.5 W. m-'.K-'(7), the inside temperature of the 1.4-mmthick quartz wall is estimated to be 800 "C. This is well below the softening point of quartz (1600 "C), so that the torch can be operated at significantly higher rf powers than utilized in the present study. Operation. The water-cooled torch has been tested by inserting it inside a 2-turn rf coil connected to a low-power rf generator (8). The efficiency and the stability of the power transfer to the plasma was markedly improved by coating the copper coil with a thin layer of gold. In this way oxidation was prevented of the coil's surface which carries the main of the rf current. The power input was held constant a t about 700 W. The spectra emitted by the plasma were observed either with a Hilger Large spectrograph or with a 0.5 m Jarrell-Ash monochromator and photoelectric detection. The water-cooled torch can be operated in the normal way, e.g., with a carrier gas flow of 1.25 L/min and a coolant gas flow of 18 L/min. If the latter flow is slowly reduced, the plasma approaches the quartz tube, which becomes red-hot but does not melt because of the water-cooling. At a main argon flow of 7 L/min, a hole is spontaneously generated in the plasma even in the absence of carrier gas. When in this situation a sample is introduced, atomic spectral lines are extremely weak. Indeed, calcium solutions display the characteristic red color of the CaO band, but the detection limit for the Ca(I1) line is no less than 1000 mg/L. However, upon a further reduction of the main argon flow to 2 L/min, the plasma comes off the quartz walls again and, in the absence of carrier gas, hangs as a white hot sphere inside the rf coil. When the carrier gas is introduced to pierce a hole through the plasma, an introduced calcium solution now exhibits the
2380
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14, DECEMBER 1979
Table I. Influence of Argon Flow Rates on the Detection Power of the Low-Flow ICP
Table 11. Limits of Detection, p g / L low flow
linear argon flow rates, L/min plas-
“coolant”
ma
carrier
7 2.2 0.9
0 0 0 0 .
1.25 0.53 0.25 0.25 0.16 0.12 0.10
0a
0 0.9
0.9
1.3
0.8 0.8
carrier limit of detec gas ve- tion Ca(I1)
locity, mi s 23
393.3 nni, mg/L
10
1000 10
18
2
18
1.5 0.1
24 8
0.04
12
0.01
’ Unstable plasma. ~
~
blue color arising from the atomic transition, and atomic and ionic spectral lines are strongly observed in the emission spectrum. The data in Table I clearly demonstrate that the low-flow ICP can be used to full advantage only if, simultaneous to the reduction of the main argon flow, the carrier gas flow is also drastically decreased. Apparently, the tenfold excess of main argon over carrier gas adhered to in the conventional ICP must be retained in the water-cooled low-flow ICP. Indeed, detection limits below 1 mg/L can be realized only if the carrier gas flow rate is reduced to 0.1 L/min. For future studies a suitable sample introduction system operating on such a low gas flow may be adapted from novel designs (9,101. In the present study we used a conventional cross-flow pneumatic nebulizer operating on 0.7 to 1L/min of argon and rejected the major portion of the carrier gas before introducing it into the plasma. In this way aerosol flow into the plasma is only 5-10 mg/min. T o place this figure in its proper perspective, it should be realized that the conventional ICP operating on a tenfold larger argon flow can sustain only up to 150 mg/min of sample solution. At higher sample introduction rates, desolvation is required to prevent extinguishing the plasma. Consequently an amount of 20 mg/min constitutes the upper limit permissible in the present low-flow ICP. On the other hand, if a sample introduction unit can be designed operating on 100 mL of argon/min and with an efficiency of 5 % , the uptake rate of the sample solution would be only 0.3 mL/min. Then, the present ICP would require only a very small amount of sample solution. T h e data on detection limits in Tables I and I1 are not corrected for losses due to the gas splitting system and refer to the real sample solutions used. The importance of careful optimization of the respective gas flows can be concluded from the data in Table I. A tenfold reduction of the carrier gas flow reduces the detection limit for calcium from 1000 to 1 mg/L. Curiously, a signal can actually be observed when only carrier gas is used, but the plasma is unstable in this situation. A further tenfold improvement in detection power is observed when the argon flow is divided over the two possible introduction points of the three-tube design. This means that argon is also introduced between the sample tube and the intermediary tube, which in the conventional ICP is formally referred to as plasma gas. To avoid confusion the conventional terms have been retained in this communication, although it is clear that the functions of the two main argon streams are different from those in the high-flow ICP. Optimum limits of detection have been obtained so far when the argon flow is distributed evenly over coolant and plasma gas. This behavior is not typical for calcium, but has also been observed for other elements. Indeed, if the “coolant” argon flow is maintained a t 1 L/min and the plasma argon flow is increased from 0 to 0.6 L/min, the intensities for ionic lines
element A1 I Ba I1 Ca I1 cu I Fe I1 La I1 hlg I1 Mn I1 Ti I LT I1
analysis line, nm 309.3 455.5 353.3 324.8
259.9 408.7 279.6 257.6 334.2 386.0
‘ Recalculated
9’ ‘ ‘
present study
conventional ICP ref 13‘
ref 14
2300 30 9 250
2
0.8
0.1 0.07 1
1000
D
0.04 0.0004 0.2 0.4 0.4
300 15 80
140 8000
3 0.7
0.7 3 30
0.01
0.08 0.1 -
for a time constant of 1 s.
of Ba, Ca, La and Ti are increased between two- and fivefold. At still higher plasma gas flows, the atomic and ionic line intensities remain constant or decrease slightly, but the continuum background intensity keeps decreasing quite sharply just as the molecular bands of OH a t 306 nm, NO (3rd positive) a t 237 and 248 nm, and NZ+(1st negative) a t 358, 391, and 428 nm. Consequently, the signal to background ratios increase even more. This contributes to the decrease in limits of detection bv more than an order of magnitude as shown in Table I. Performance. The following data should be regarded as preliminary, awaiting full scale optimization and further improvements of the torch design. Minor differences have been observed between the two torch designs shown in Figure 1; the data quoted below refer to the design of Figure l b . In addition to the respective gas flows, three parameters were found to be important for the sensitivity and limits of detection: the position of the torch tubes, the tip diameter of the carrier gas tube, and the observation height. Because the glass partition does not extend very close to the top of the water jacket, the flow rate of the cooling water is relatively slow a t the top. T o prevent overheating, the tip of the jacket must extend slightly above the rf coil; a value of 2 mm was found to be sufficient. If the jacket is lowered, the rf anode current starts to fluctuate and an audible noise is heard. The position and the tip diameter of the carrier gas tube determine the transfer of the sample into the plasma. In the conventional high-flow ICP run on a carrier gas flow of 1.4 L/min, a carrier gas tube with a 1.4-mm tip produces an optimum linear cold gas velocity of 10 m/s (11). In the present low-flow torch, this linear velocity is maintained if simultaneous to the decrease of the carrier gas flow to 0.1 L/min the tip diameter is reduced to 0.5 mm. The elevation of the carrier gas tube does not influence the background intensity, but the analyte signals are raised twofold a t an optimum separation of 35 mm between the tip of the carrier gas tube and the top of the torch. The aerosol now leaves the orifice some 24 mm below the visible plasma region located midway between the rf coils. The position of the intermediary plasma tube was observed not to be critical. As expected for the low-flow ICP, all intensities fall off much more rapidly with increasing observation height than in the conventional high-flow ICP. Initially, the background spectrum drops more rapidly but over 10 mm the signal-to-background ratio remains constant. Optimum signal-to-background and signal-to-noise ratios (detection limit) were observed a t 4 mm above the rf coil, which is only 2 mm above the top of the torch. Under these conditions the following data of analytical interest were observed. The background spectrum contains the usual argon lines and several molecular band heads.
ANALYTICAL CHEMISTRY, VOL. 51, NO. 14,DECEMBER 1979
111°
4 0
1
lo
loo
I
1000
Interferent concentration,mg/L
Figure 2. Interferences of some ions on the intensity of the calcium ion line at 399.3 nm; the vertical arrow bar indicates the precision of the data (3%)
Generally, however, the background spectrum is less intense than in the conventional ICP. This constitutes a marked advantage in terms of optical resolution required. The excitation temperature was determined as 4500 K with the two-line emission method using spectral lines of Fe(1) and Ti(I1). This value is fully comparable to the values found in conventional ICPs. Accordingly, most analyte spectra display strong ionic lines. Interferences of aluminum, sodium, and phosphate on calcium were studied up to 1000-fold excess. The results in Figure 2 show that the depression of the 1 mg/L calcium signal, although somewhat larger than in the conventional ICP, is not more than 25%. Because the optimization study was not directed toward minimum interference levels, this is considered quite acceptable. The dynamic range was found to be four orders of magnitude for calcium and copper. Limits of detection of a representative set of analytes are presented in Table 11. Obviously, our data compare poorly with the results obtained with present day conventional ICPs. Whereas the limits of detection in the high-flow ICP is around 1 pg/L or better, our data for the low-flow ICP range from 10 to 1000 pg/L. However, several remarks must be made a t this point. The data for the conventional ICP represent an improvement of three to four orders of magnitude over the initial data reported for this configuration (12). Also, the data reported by Boumans and de Boer (13) refer to ultrasonic nebulization and cannot be realized with commercial instru-
2381
mentation. The data of Fassel and Knisely (14) are more representative for the current state of the art of conventional high-flow ICP. Our preliminary results for a first-generation low-flow ICP fall short of these values by about two orders of magnitude. In fact, the limits of detection obtained with the present equipment are comparable to detection limits in flame atomic absorption spectrometry. There are several ways in which these values may be improved. The optical system presently employed is far from optimal. The dual observation system (spectrograph and monochromator) requires four mirrors and an unfavorable observation window. By far the greatest drawback of the present design, however, is the sample introduction system. As remarked earlier, splitless introduction of the sample aerosol (20 mg/min) would decrease the limits of detection fourfold. Consequently, optimization of all these factors could improve the limits of detection by a t least an order of magnitude. In conclusion, the present study has shown that an ICP run a t a substantially lower total argon flow is feasible. The stability and spectral properties of the plasma discharge are quite similar to that of a conventional high-flow design. Preliminary analytical data indicate low interference levels and weak background spectra. Limits of detection below 1 ppm are observed and might be reduced to a few ppb by an improved sample introduction system. This is presently under examination.
LITERATURE CITED (1)
S. Greenfield,
H. McD. McGeachin, and P. B. Smith, Talanta, 23, 1
(1976). (2) R. M. Barnes, Crit. Rev. Anal. Chem., 7 , 203 (1978). (3) R. M. Dagnall, D. J. Smith, and T. S. West, Anal. Chim. Acta, 54, 397 (1971). (4) R. H. Scott, V. A. Fassel, R. N. Kniseley, and D. E. Nixon, Anal. Chem., 46, 75 (1974). (5) C. D. Allemand and R. M. Barnes, Appl. Spectrosc., 31, 434 (1977). (6) I. L. Genna, R. M. Barnes. and C. D. Allemand, Anal. Chem., 49, 1450 (1977). (7) "Handbook of Chemistry and Physics", 47th ed., Chemical Rubber Company, Cleveland, Ohio, 1967, p E5. (8) G. R. Kornblum and L. de Gaian, Spectrochim. Acta, Part 6, 29, 249 (1974). (9) J. W. Walcott and G. Butler-Sobel, Appl. Spectrosc., 32, 591 (1978). (10) R. F. Suddendorf and K. W. Boyer, Anal. Chem., 50, 1769 (1978). (1 1) G. R. Kornblum, J. Wigman, and L. de Galan, Proc. XX Coll. Spectr. Int. Prague 1977, no. 8. (12) R. H. Wendt and V. A. Fassel. Anal. Chem., 3 7 , 920 (1965). (13) P. W. J. M. Boumans and F. J. de Boer, Spectrochim. Acta, Part 6. 30, 309 (1973). (14) V. A. Fassel and R. N. Kniseley, Anal. Chem., 46, 1110 A (1974).
RECEIVED for review April 30, 1979. Accepted August 27, 1979.
