Anal. Chem. 1989, 61, 163-169
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Introduction of Liquid Samples into the Inductively Coupled Plasma by Direct Insertion on a Wire Loop R. L. A. Sing' and E. D. Salin* Department of Chemistry, McGill University, Montreal, Quebec H3A 2K6, Canada
The use of a wire loop as sample support for direct insertlon of samples Into the Inductively coupled plasma results In very rapid volatlllzatlon of the sample yielding hlgh momentary concentrations of the analyte In the vlewlng zone. The shapes of the resulting transient signals for a number of elements (Ag, Cd, Cr, Mg, Pb, V, and Zn) are presented. Tungsten wire Is used for the majority of the study but contributes spectral overlaps that cannot be temporally resolved from the analyte peak In some cases. Tantalum provides better temporal resoiutlon but suffers from a short lifetime. Detection limits for the elements studied are in the picogram to subplcrogram range. I t Is demonstrated that the Cu concentratlon detection llmlts can be Improved a factor of at least 10 by multidrop krsertlons. Matrix effects were Investigated wlth Ca as matrix element. Increasing the concomitant Ca concentration affects the peak shape slgnllcantly. The peak heights vary consklerably but peak area remalns constant for 100 pg/L Cu In the presence of 0 to I O 4 mg/L Ca. For Cd a slight decrease In peak area occurs above I O 3 mg/L Ca.
Previous investigations of direct sample insertion as a method of sample introduction have been conducted almost exclusively with graphite as the sample support. Conventional dc arc graphite electrodes (I-@, specially machined graphite cups (6-8), and flat top graphite rods ( 9 , I I ) have been employed. The use of graphite as a sample support imposes certain limitations. In particular, the formation of refractory carbides prohibits the determination of certain elements, though the addition of volatile halide-forming agents can provide some relief from this problem (11). The graphite electrodes, which are employed in most cases have relatively slow heating characteristics. Barnett et al. (6) have shown, by machining special graphite supports of reduced mass, that significant improvements in the analytical performance could be achieved. This was attributed to the more rapid release of the sample into the plasma providing a higher momentary concentration of the analyte in the viewing zone. These authors also recognized that the surface characteristics of the graphite, in particular the porosity, played a role in the vaporization processes. In two previous articles in this journal, we reported preliminary results on the use of tungsten wire loops as sample support for direct insertion of desolvated liquid microsamples into the inductively coupled plasma (ICP) for atomic emission spectrometry (12) and for ICP source mass spectrometry (13). In this article, we provide details of a stepping motor direct sample insertion device (DSID), and we extend our previous preliminary investigations of the wire loop DSID for atomic emission spectrometry. EXPERIMENTAL SECTION Instrumentation. Two different direct sample insertion devices, one pneumatically driven and the other stepping motor *Author to whom correspondence should be sent. 'Present address: DB artement de Chimie, UniversiG de Montrbal, P.O. Box 6128 !tation A, Montrbal, Qubbec H3C3J7, Canada. 0003-2700/89/0361-0163$01.50/0
Table I. Instrumental Components and Operating Conditions Instrumental Components Plasma Therm Model HFP2500D with AMN2500E automatic matching network monochromator Jarrell Ash model 78-462 Czerny-Turnertype 1 m focal length 1200 g/mm holographic grating PMT 1P28 amplifier laboratory built data acquisition S-100bus based system 2-80 CPU 256 kB RAM dual 8-in. floppy disks CP/M operating system ADC resolution 12-bits ICP
power coolant gas auxiliary gas chamber gas viewing height insertion height drying height
Operating Conditions 1.75 kW 16 L/min 1 L/min 0.8 L/min 17 m m above top of load coil 2 m m below top of load coil 22 mm below top of load coil
driven, were used for the experiments described in this article. The pneumatically driven device was described briefly in our f i i t report (12). The stepping motor driven device, which has replaced the pneumatically driven device, is described in detail in the following section. The remainder of the experimental apparatus and operating conditions used for the majority of experiments are listed in Table I. Stepping Motor DSID. The stepping motor driven DSID is shown schematically in Figure 1(for clarity, the traveling shaft is shown in the raised position in the side view). The core of the system consists of the same concentric aluminum shafts (7,11) as used with the pneumatically driven DSID. The narrow shaft (11) is fixed and serves to guide the traveling shaft (7). The traveling shaft is linked to the belt drive system by a plastic mounting block (8). The belt drive system consists of a flexible chain belt (9) suspended between two sprockets (5,13). The upper sprocket (5)rotates freely on a bearing-mounted shaft while the lower sprocket (13)is mounted directly on the shaft of the stepping motor (12). An opaque flag assembly (10)is mounted on the belt on the opposite side of the traveling shaft and serves to trip an opto-switch (6)mounted near the top of the device's frame when the shaft is fully retracted (home position). The complete belt driven shaft assembly is constructed as a separate unit that is attached to the sampling area portion of the device. The samplingarea portion of the stepping motor DSID consists of two aluminum plates separated by 6-in. spacer rods (4) and a solenoid-driven assembly (2,3) to seal the base of the plasma torch (1)when the traveling shaft is retracted. The base of the plasma torch fits into the top of the solenoid assembly (3). The whole device is fixed to the plasma torch box by mounting cleats on the top plate. A sample chamber is not necessary with the stepping motor DSID since the sample support is cooled in the lower portion of the plasma torch. This is possible due to the enhanced movement control provided by the stepping motor DSID. The stepping motor DSID is controlled by a BIG STEPPER motion controller (Centroid Corp., State College, PA). This unit is capable of controlling up to four motors in independent or 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989
was fused onto the end of a short section of 3 mm diameter glass rod. The glass rod was either epoxied into the head of an 8-32 screw, which was later screwed into the tapped insert on the end of the traveling shaft, or was held in place by a collet attached to the end of the traveling shaft. Plasma Torch. The plasma torch used with both DSIDs consists of a Fassel type torch modified by the removal of the injector tube and by the extension of the auxiliary tube with 10 mm 0.d. quartz tubing. The internal diameter of the 10 mm 0.d. tubing is 8 mm, which permits passage of the 0.297 in. (7.5 mm) 0.d. traveling shaft. The length of the auxiliary tube extension is determined by the ICP unit being used. Chemicals. Standard solutions were prepared daily by serial dilution of 1000 mg/L stock solutions with deionized-distilled water. Stock solutions were either commercially available atomic absorption standards (BDH, Montreal, Quebec, Canada) or prepared from reagent grade chemicals according to ref 14. The tungsten and tantalum wires used as samples supports for the introduction of the liquid samples were 0.5 mm in diameter and were obtained from J.B.E.M. Services (Dorval,Quebec, Canada). Certificates of analysis for the wire materials were not available.
RESULTS AND DISCUSSION
F r o n t view
Side view
Figure 1. Schematic diagram of stepping motor driven direct sample insertion device. See text for descrlption.
concerted movement. The unit is microprocessor equipped and communicates via the RS-232 protocol. The unit accepts direct movement commands or can be programmed in BASIC to execute a series of movement and control procedures. For the DSID application, a BASIC program is used and communicationswith the data acquisition computer, for purposes of synchronization, are performed via the RS-232 interface. The BIG STEPPER is equipped with eight inputs for limit switch sensing. One such input is used for the “home”opbswitch. The solenoid mechanism of the DSID is directly controlled by this input. This ensures that the solenoid cannot, through improper programming, obstruct the path to the plasma torch when the traveling shaft is in movement. Sample Application. Sample application to the wire loops was performed in two ways, namely by manual pipetting of the 10-pL sample volume with an Eppendorf micropipet and via a laboratory-constructedautomatic sample deposition device. This device consists of a swing-in arm on which is mounted a PVC solution delivery tube. The arm swings into position above the sample support and solution is allowed to flow onto the support for a fixed time. At the end of this time, the arm swings back to the standby position where the solution continues to flow into waste. SoIution delivery is provided by a peristaltic pump. The volume of solution delivered was approximately 14 pL. The automatic sample application device was used for those experimenta requiring repeated insertions of a single solution. These include the reproducibility, the wire replacement, the multidrop insertions, and some optimization studies. The procedure for each individual insertion is described in our first report (12). Sample Supports. Metallic wire loop sample supports were employed for the insertion of liquid samples into the ICP. The wire was 0.5 mm in diameter and was formed into two side-by-side 2 mm diameter loops on the end of a 4-cm stem. The formed wire
Stepping Motor DSID Performance. The stepping motor based configuration was designed with a 2.5-cm sprocket. Using a stepping rate of 2000 steps, one can easily reach a velocity of 20 cm/s. This is sufficient for the insertion of liquid samples on wire loops and far faster than is necessary for solid sample introduction work with larger graphite sample carriers. The increased flexibility of the configuration now allows relatively complex drying programs to be used by carefully controlling the distance between the graphite sample carried and the plasma during the drying and ashing steps. While the pneumatic system (12)can be automated, the advantages provided by the apparent simplicity and speed are outweighed when one seeks both automation and flexibility. An important byproduct of the new design is the elimination of the sample chamber. In the pneumatic system, careful purging of the sample chamber was necessary during the cooling cycle and prior to insertion. Allowing oxygen to contact the wire during the cooling cycle resulted in very rapid deterioration of the wire. The present system, as mentioned above, allows convenient cooling of the wire in the lower portion of the plasma torch. The positive pressure of auxiliary argon flow will ensure that no oxygen leaks into the system. The open design provides convenient access to the wire for the operator of an automated system. There is actually sufficient room to allow a robot hand to remove and replace sample supports. One important evaluation criterion for any sample introduction system will be the precision that can be obtained with that system. With manual delivery of 10-pL sample volumes with an Epperdorf micropipet, relative standard deviation ranging from 3% to 15% were obtained for 10 consecutive insertions. In our experience, this level is typical of applications requiring manual dispensation of such volumes using micropipets. Reproducibility is significantly improved by the use of the automatic sample application device with the stepping motor DSID. To illustrate this, 250 consecutive insertions of the 100 pg/L copper solution were performed with automatic dispensing of the sample. Each insertion required 50 s to perform; therefore the experiment spanned a period of 3.3 h. Table I1 lists the short-term and long-term relative standard deviations of peak height and peak area obtained during this study. The short-term precision values are calculated for each group of 30 consecutive insertions (thus each group spans 25 min). The long-term value is for insertions at 25-min intervals over a 3.3-h period. The data of Table I1 attest to the excellent precision achievable with the stepping motor DSID and automatic sample application using atomic emission optical detection. We believe that the same
ANALYTICAL CHEMISTRY, VOL. 61, NO. 2, JANUARY 15, 1989
Table 11. Short-Term and Long-Term Reproducibility of the Signal Obtained for Insertions of 100 pg/L Cu Using Automatic Sample Application to the Wire Loop % red std dev
short term (25 min)
long term (3.3 h)
peak height
peak area
1.2 3.5 1.3 1.0 1.4 1.9 0.9 2.3
0.6 0.6 1.1 0.5 0.5 1.4 0.8 2.0
5.4
.
1.4
level of precision would be enjoyed by inductively coupled plasma mass spectrometry;however we have not yet confiied this by experiment. It is especially interesting to note that the precision levels reported in Table I1 rival those of nebulizer-based systems. However, one must keep in mind the relatively low salt concentrations in the evaluation solution. Sample Drying. The drying of the sample is accomplished by placing the wire loop in proximity of the plasma and allowing the convective, radiative, and inductive heating to desolvate the sample. The distance from the plasma, the forward power, and the auxiliary gas flow are the three parameters with the greatest influence on the time required to dry a given sample volume. The drying time was measured as a function of the distance from the top of the load coil (for 1.75 kW of power and 1.0 L/min of auxiliary flow). Water was used as a sample and was delivered with the automatic sample application device (approximately 14 ILL). A wide range of drying times can be selected by adjusting the drying height. A marked transition in drying time occurs in the vicinity of 24 mm below the load coil and the drying time appears to approach a limiting value of 4 s. For drying heights above 20 mm below the top of the load coil, the drying causes severe constriction of the plasma, great instability, a sharp rise in reflected power, and sputtering of the sample. The plasma is often extinguished when drying is performed closer than 15 mm below the top of the load coil. The drying time varies directly with auxiliary gas flow (between 0.8 and 1.2 L/min) and inversely with power (between 1.25 and 1.75 kW). The drying height was maintained at 22 mm for subsequent experiments since it provided a relatively short drying time (20-30 s) with no fear of sample sputtering or excessive reflected power. The effects of prolonged drying at the established drying conditions were investigated for both Cu and Cd using solutions of 100 fig/L. For Cu, no effect was observed on either the peak height or peak area for drying times extending from 20 to 240 s. For Cd, the peak height showed a slight variation (
