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Application of a Wire Loop Direct Sample Insertion Device for Inductively Coupled Plasma Mass Spectrometry Sir: A great deal of interest has been expressed recently with the inductively coupled plasma (ICP) as an ionization source for mass spectrometry (MS) (1-10). In most cases conventional sample introduction systems have been used, though applications of electrothermal sample introduction have been reported ( I , IO). In our laboratory at McGill University, we have developed a unique sample introduction system for ICP atomic emission spectrometry (AES) called the wire loop direct sample insertion device (DSID) (11). The sample is applied to a tungsten (or other material) double loop, dried just below the plasma, and then quickly inserted into the core of the plasma along its central axis. With optical detection this system provides detection limits of 0.2 and 0.8 ng/mL for Cu and Zn corresponding t o 2 X and 0.8 X W2g for the 1O-bL sample size used. These detection limits are 10 to 100 times better than those reported by others using electrothermal vaporization sample introduction techniques for ICP-AES (12-15). The drastic improvement appears to be due to the very rapid evolution of the sample from the wire surface with a resulting momentary high concentration of the analyte in the viewing zone. Recently ICP-MS with electrothermal sample introduction has shown improvements compared to conventional nebulization (IO). The application of the wire loop DSID to ICP-MS could then potentially result in further improvements in detection limits. Our interest in this device stems from an ever increasing need at the Ontario Ministry of the Environment for improved detection limits, particularly in water samples. The initial results from our work are very encouraging and are presented below.
Table I. Operating Conditions for DSID-ICP-MS
EXPERIMENTAL SECTION A commercial ICP-MS (Sciex ELAN Model 250) was used for the experiments. The ELAN is controlled by software written in the C language and executing on a 68000 CPU based computer. Two generations of the software distributed by Sciex were used during the experiments. The first generation did not permit rapid enough data collection to obtain time profiles. While the second was sufficiently rapid, it was difficult to synchronize with the sample insertion. This is particularly important because the signal half width is approximately 0.1 s. The ICP unit of the ELAN is a Plasma Therm 2.5-kW 27-MHz generator and autotune match box with a four-turn coil run horizontally. The ICP-MS wire loop DSID was prototyped using a conventional Plasma Therm three-turn coil system in our laboratory at McGill University. This wire loop DSID is operated horizontally and is similar to the vertical unit described elsewhere (11). While gravity can be used to withdraw the wire from the plasma in the vertical pneumatic delivery system, either manual retraction or house vacuum driven retraction is necessary in the horizontal unit. A 10-pL volume of the sample is deposited onto the wire loop using an Eppendorf pipet. The sample chamber is then closed and the wire loop is moved to a position approximately 10 mm away from the plasma where drying takes place. The progress of the drying step can be monitored by observing the reflected power as described previously (11). During the drying step, the pneumatic driving gas is applied. At the completion of the drying, the stop block maintaining the wire at the drying distance is removed, and the wire is propelled into the plasma. All standards were prepared by dilution of Spex liquid standards with deionized distilled water.
insertion of the sample with the acquisition and ensure the integration of the whole transient signal. The plasma distance from the sampling orifice, the insertion distance, the plasma power, the auxiliary gas flow rate, and the ion optics were optimized, in that order, for a maximum signal to background ratio. The signal was the integrated value for the 2-s period during which the sample was inserted, and the background is the value for the subsequent 2-s period. Figure 1shows a plot of the signal to background ratios as a function of plasma distance from the orifice. The optimum distance is very well-defined and corresponds to the position where the sampling orifice is located at the tip of the plasma plume. Radial translations of the plasma would be necessary to identify the path taken by the analyte and to identify positions where the signal to background levels could be substantially higher. The optimization of the various parameters allowed an increase of signal to background ratio from 1500 at 100 ppb a t the initial settings, which were similar to those used with the optical DSID and conventional ICP-MS configurations, to a value of 10000 at 100 ppb for the optimal conditions for Mn with DSID-ICP-MS. The conditions found in the optimization and used for the remainder of the experiments are listed in Table I. Figure 2 shows a typical time profile for the DSID-ICP-MS combination as obtained with the second generation software. This profile is quite similar to that obtained for the wire loop DSID used for AES as shown in Figure 3. Both show the decrease in the signal intensity that accompanies the insertion of the wire into the plasma followed shortly by the narrow analyte transient signal. Table I1 provides the detection limits ( S I N = 3) and precision at high concentrations. These were obtained with the second generation software, which allowed the capture of the
RESULTS AND DISCUSSION The univariate optimization of a number of operating parameters was performed for manganese. The signal at m / z 55 was monitored using the first generation software a t 2-s integration times since it was possible to synchronize the 0003-2700/86/0358-0975$01.50/0
orifice distance insertion distance plasma power coolant gas auxiliary gas nebulizer gas ion optics BBA BBB (photon stop) BBC ring lens sample volume
-1 mm from tip of plume 9 mm from top of load coil 0.95 kW 14 L min-'
2.0 L min-' 0.75 L min-' -6.1 V (30)" +4.7 V (48) -9.1
v (45)
-6.9 V (12) 10 PL
Values in parentheses indicate digipot settings on Elan. Table 11. Detection Limits and Precision for DSID-ICP-MS
element
DSID-ICP-MS detctn limit," rglL
precisionb
Mn
0.09
6
As
Pb Cd Li
0.5 0.03 0.05 0.075
Ag
0.02 0.027
13 7 5 13 5 9
cu
ICP-MS detctn limit: rg/L 1 50 0.8 2 2 1 0.5
OThree times std dev of blank. *Percent relative standard deviation for five insertions at 100 ppb. 'Values quoted by Sciex.
0 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
1200001
-10
-5
0
5
Distance (mm)
Flgure 1. Signal to background ratio as a function of the distance of the sampling orifice from the tip of the plasma plume.
Time (s)
Figure 3. Characteristic response for 10 ppb Cu by DSID-ICP-MS.
4000-5000 counts but is most probably due to the presence of 5-20 ppm of iron in the tungsten wire. The DSID-ICP-MS combination shows considerable promise and we hope to report shortly on more complete studies of optimal operating conditions, dynamic ranges, matrix effects, and the degree of oxide formation. Registry No. W, 7440-33-7;Mn, 7439-96-5;As, 7440-38-2;Pb, 7439-92-1; Cd, 7440-43-9; Li, 7439-93-2; Ag, 7440-22-4; Cu, 7440-50-8.
LITERATURE CITED
I
0.0
0.7
(s) Figure 2. Characteristic response for 10 ppb Cu by DSID-ICP-AES. Time
time profiles provided great care was taken to synchronize with the sample insertion. Table I1 also lists the detection limits obtained by using conventional nebulization on the ICP-MS system. The DSID results show an improvement of 40 over conventional nebulization data. The detection limits for the DSID-ICP-MS system rival those for the ETV-ICP-MS combination recently reported (IO). Through further optimization of various parameters and proper interfacing with the software, the DSID system with mass spectrometry detection could show the same improvements in detection limits compared to ETV sample introduction as seen with optical detection. The linear dynamic range for cadmium appears to be 4 orders of magnitude. One of the major difficulties with ICP-MS when using conventional nebulization is the significant background levels caused by the formation of oxides due to the presence of the water vapor (3, IO). With the DSID system, as with the ETV system, the sample is desolvated well before its introduction into the plasma. The background spectrum with DSID sample introduction is therefore greatly simplified and spectral overlap problems are severely curtailed. This is exemplified by iron, m / z 56, which is overlapped by argon oxide with the same mass. With conventional nebulization, the background count rate is from 10000 to 30000 counts/s whereas with DSID sample introduction the preinsertion background is only 800-1000 counts/s. The postinsertion level is approximately
Gray, Alan L.; Date, Alan R. Analyst(London)1983, 708, 1033-1050. Houk, Robert S.; Thompson, Joseph J. Biomed. Mass Spectrom. 1983, 10, 107-112. Douglas, D. J.; Quan, E. S. K.; Smith, R. G. Spectrochim. Acta, Part 8 1983, 388,39-48. Date, Alan R.; Gray, Alan L. Spectrochim. Acta, Part 8 1983, 388, 29-37. Gray, A. L.; Date, A. R. Int. J. Mass Spectrom. Ion Phys. 1983, 4 6 , 7-10. Houk, Robert S.; Thompson, Joseph J. Am. Mineral. 1982, 67, 236-243. Date, Alan R.; Gray, Alan L. Analyst(London) 1981, 706, 1255-1267. Gray, A. L.; Date, A. R. Dyn. Mass Spectrom. 1981, 6 , 252-266. Houk, R. S.;Fassel, V. A.; Svec, H. J. Dun. Mass Spectrom. 1981, 6 , 234-25 1. Arrowsmith, P.;Boorn, A.; Douglas, D.; French, J. 8 . ; Park, C. J. Plttsburgh Conference and Exposition, New Orleans, LA, 1985, paper 335. Salin, Eric D.; Sing, R. L. A. Anal. Chem. 1984, 56, 2596. Aziz, A.; Broekaert, J. A. C.; Leis, F. Spectrochim.Acta, Part8 1982, 378,381. Aziz, A.; Broekaert, J. A. C.; Leis, F. Spectrochim. Acta, Part B 1982, 378,369. Swaidan, H. M.; Christian, G. D. Anal. Chem. 1984, 56, 120. Ng, K. C.; Caruso, J. A. Anal. Chim. Acta 1982, 753,209-222.
D. W. Boomer M. Powell Ontario Ministry of the Environment Trace Contaminants Section Toronto, Ontario M9W 5L1, Canada
R. L. A. Sing E. D. Salin* Department of Chemistry McGill University 801 Sherbrooke Street West Montreal, Quebec H3A 2K6, Canada
RECEIVED for review September 9,1985. Accepted December 3,1985. The majority of this work was funded by a grant from the Ontario Ministry of the Environment, Project 176 PL. R. L. A. Sing acknowledges the financial support of the Natural Sciences and Engineering Research Council of Canada.
