Generation of a square wave inductively coupled plasma scanning mass spectrometry signal using electrothermal vaporization sample introduction

Abstract

A device was designed to convert the typical skewed gaussian shaped signal from an inductively coupled plasma mass spectrometer (ICP-MS) using electrothermal vaporization (ETV) into a square wave signal. The basis for such a transformation is centered on the assumption that the aerosol particles produced from ETV are aerodynamically small, and thus, have diffusive properties similar to gases, which makes them easy to mix and resistant to loss by settling. The ETV square wave signals were generated through the use of a stainless steel cylinder equipped with a motor driven piston assembly, which was used to trap the transient ETV signal and deliver a uniform density aerosol to the ICP-MS. The length of the signal was controlled by the rate of travel of the piston during expulsion of the sample aerosol. A constant flow rate was achieved and maintained using a pressure sensor feedback to the motor driving the piston. The system was evaluated by quantitative full mass scans on a variety of signal shapes including square wave signals, normal ETV signals, and broadened ETV signals generated with the use of a single bead string reactor (SBSR). The square wave signals yielded precisions for quantitative full mass scans of better than 10% RSD for most metals compared to approximately 15% for SBSR broadened signals and 40% for normal signals. The improved precision was attributed to an increased duty cycle, but the increase did not become significant until more than 100 masses were monitored due to the relatively small amount of “temporal overhead time” (fly-back time, amplifier settling time, etc.) relative to data collection periods. Because the ETV generated aerosol consists of aerodynamically small particles, there was only a minimal decrease (<2–11%) in the integrated signal area as a result of using the square wave generator.

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1 Introduction

The coupling of electrothermal vaporization (ETV) with an inductively coupled plasma mass spectrometer (ICP-MS) provides the benefits of analyzing microsamples, slurries, solids, and samples with complex matrices. The signals generated by the ETV are transient in nature, lasting approximately 1–3 s. Because of this rapid transient event, quantitative full mass scans are nearly impossible with sequential scanning spectrometers (e.g., quadrupole). In order to perform quantitative full mass scans with acceptable precisions (e.g., <10%) on a typical ETV firing, the sampling rate must be high enough to define the transient peak, but low enough to acquire an acceptable number of counts. Recent papers have addressed this issue in greater detail.^1–3 The balance necessary in selecting the sampling rate is further complicated if the temporal location of the signal relative to that of data collection is not reproducible.³

It has previously been shown that the transient signal could be lengthened significantly through the use of either a single bead string reactor (SBSR)⁴ or a transient extension (TEx) flask.⁵ A SBSR is simply a short length of tubing (35 cm long × 1 cm id) packed with 4 mm glass beads and placed in-line between an aerosol-generating source and the ICP-MS. Moenke-Blankenburg and coworkers⁴ broadened the signals for a single laser ablation event up to 5 s using a SBSR. One potential advantage of using the SBSR is the production of turbulent flow, which creates more gaussian-shaped signal profiles, as opposed to the exponentially decaying signal from the TEx flask. A gaussian signal can be sampled less frequently than a comparably wide, skewed gaussian signal because signals containing higher frequency components (e.g., sharp rising or falling edges) must be sampled at a faster rate than those containing lower frequency components.³

Langer and Holcombe⁵ manipulated the signal shape by placing the TEx flask (100 mL round bottom flask) in-line between the ETV and ICP-MS to promote the multi-element analysis capabilities of ETV-ICP-MS by stretching the ETV signal to nearly 20 s and thus were able to perform a full mass scan on a single ETV firing. While the SBSR and TEx flask allow for a slower sampling rate and the possibility of full mass scans, the signal magnitude still changes with time so multiple scans over a single waveform are required if quantitative analysis is to be conducted.

Changing the shape of the ETV transient signal into a longer duration square wave avoids this requirement because the signal intensity remains constant over the life of the transient. Thus, multiple sequential scans are not required to account for a continually changing signal, and the duty cycle is maximized by performing only one scan. The duty cycle is maximized because the fixed temporal overhead paid on each scan (i.e., jump and flyback time) is only experienced once, as can be seen in eqn. (1):³

where n_m is the number of masses being monitored and t_d, t_j, and t_f are the dwell, jump (amplifier settling time between masses) and fly-back times, respectively.

Maximizing the duty cycle also maximizes the total number of counts that can be acquired because longer dwell times can be used at the expense of fewer jumps and flybacks. This is especially important in situations where a large number of masses are monitored (e.g., full mass scans) and/or relatively low concentrations are used, due to the dependence of statistical noise (e.g., shot noise) on the total number of accumulated counts .^6,7 In other words, a greater number of counts collected (N) per m/z provides improved precision, assuming counting statistics is a limiting factor. It is important to note, that statistical noise is proportional to the square root of the sum of the counts collected for the analyte, background, and that of the detector. Therefore, it is possible for an adverse effect to occur if the background and/or detector shot noise were extremely high with respect to the analyte signal and the same signal magnitude were stretched over a longer period of time, (i.e., S/N would decrease by increasing the signal duration).

Resano et al.² clearly demonstrated the possibilities for multiple mass monitoring with respect to precision, sensitivity, and detection limits of a transient event. More recently, Venable et al.³ quantitatively addressed the precision and sensitivity issues associated with attempting quantitative measurements with a sequential scanning ETV-ICP-MS system. From their analysis, it is clear that optimal sensitivity (e.g., maximum duty cycle) results from the longest scan time, which implies the smallest number of scans within the total measurement time. This is most easily achieved if the signal magnitude is constant with time and, in the limit, a single scan can be used. This can be approximated if the transient waveform is a square wave.

2 Experimental

2.1 Reagents

A stock 200 ppb multi-elemental solution was made fresh daily from a 10 ppm multi-element standard (Solutions Plus Inc., Fenton, MO) and commercially available single element standards. Appropriate serial dilutions were then made to create 50 and 100 ppb stock solutions. The final multi-element solution was composed of the following elements: Ag, Ba, Be, Bi, Ce, Cd, Co, Cu, Eu, Er, In, Li, Mg, Mn, Ni, Pb, Rb, Ru, Sr, Tl, Y, and Zn. The stock solutions were diluted to final volume with 1% HNO₃ made from 70%, redistilled to 99.999% pure HNO₃ (Aldrich, Milwaukee, WI). High purity Ar was used for the plasma and carrier gases (Praxair Inc, Austin, TX).

2.2 Instrumentation

Two different ICP-MS systems were used in this study; a Varian Ultramass and a PerkinElmer Elan 500 (with Elan 5000 software upgrade). The purpose of using the two systems was two fold: (1) it ensured the versatility of square wave generator with respect to different ICP-MS systems and (2) the PerkinElmer is unable to achieve dwell times under 1 ms, which was deemed necessary to perform quantitative analysis of the SBSR broadened and normal ETV signals. Sample introduction was performed using a modified Varian GTA-95 electrothermal atomizer, which has been previously described.⁵ Sample volumes of 20 µL were delivered to the ETV by a Varian ASD-53 autosampler. After sample introduction, the GTA-95 triggered a pneumatically actuated dosing hole plug. The temperature program used for the ETV is shown in Table 1.

Table 1 Varian GTA-95 temperature program

Program step	Temp./°C	Ramp time/s	Hold time/s
1	∼90	10	0
2	200	15	10
3	3000	1	5
4	∼50	14	0

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Fig. 1 shows an overall view of the square wave generation device that was placed in-line between the ETV and ICP-MS system. The square wave generator is centered around a stainless steel cylinder (3.53 cm id, 4.20 cm od, 12.95 cm length). The total cylinder operating volume with the piston and piston shafts taken into account is 97 cm³. Fig. 2 shows a closer view of the piston and end cap design. The Delron^® piston achieves a seal with the cylinder using two o-rings. The piston is tapped to accommodate a threaded (0.64 cm × 20.32 cm) piston shaft from each side. Furthermore, the piston is equipped with a spring-loaded copper plate on each side, which is held in place via locking clips on each piston shaft. The cylinder end caps are also made of Delron^® and o-ring sealed to the cylinder and piston shafts. As can be seen from the side view, the end caps are fitted with electrical contacts (modified 1/8 npt male brass pipe plugs), inlet and outlet fittings (1/4 in. barb to 1/8 npt fittings), and double o-rings in the center for the piston shaft. The inlet fitting was retrofitted with a flow restriction plug (0.14 cm diameter hole). The flow restrictor was placed in the inlet fitting to create turbulent flow for the influent gas as a means of encouraging good aerosol mixing within the cylinder.

Fig. 1 Schematic of square wave generation device detailing the major components: (a) 24V/4000 RPM DC motor (Servo Systems Co., Montville, NJ) with 10:1 gearbox, (b) variable 8–400 oz-in. slip clutch (PIC Design Co., Middlebury, CT), (c) 10 thread/in in-line linear actuator, (d) aluminum shaft mounting block, (e) piston and cylinder assembly, (f and g) two-way solenoid operated valves (Burkert, Irvine, CA), (h) pressure sensor (Honeywell Micro Switch, 164PC01D37), and (i) a 133 MHz Pentium computer containing a commercially available DIO card (National Instruments, PCI 6503) and software written in Labview™.

Fig. 2 Detail schematic of cylinder, piston, and end cap.

Another important design feature was incorporated into the end cap so that flow could be achieved from the inlet to outlet port when the piston had been driven to the end of the cylinder. There is a notch in the end cap that is needed to purge any remaining aerosol from the connecting tubing prior to loading the cylinder with the next sample; therefore eliminating the possibility of a carryover effect. The end of piston travel is determined when electrical contact between an end cap and a spring-loaded copper plate attached to the piston has been made.

A slip clutch has been placed between the gearbox and in-line linear actuator to protect the reduction gears in the gearbox in case the piston is accidentally driven into the end of the cylinder. A screw drive linear actuator containing 10 threads per inch was placed between the motor and cylinder assembly. The minimum time required for the piston to traverse the length of the cylinder for the current motor is approximately 6 s and can be increased by reducing the motor speed.

Two three-way solenoid-operated valves act to direct gas flow to the correct side of the cylinder while redirecting the discharge of the aerosol from the other side to the ICP. A pressure sensor is positioned to measure the pressure at the outlet of the ETV. The sensor output is used to control the motor speed via the feedback circuit shown in Fig. 3. The output of the pressure sensor input is feed into an inverting op amp (IC1) and then compared with a reference voltage that is set using a variable 5 kΩ resistor (R1) by summing amplifier (IC2). Altering the set point of R1, changes the rate of gas/aerosol expulsion from the cylinder and alters the duration of the final square wave. The output of IC2 controls the gate voltage of a power MOSFET, which in turn controls the amount of current flowing through the 24 VDC motor. The 100 kΩ variable resistor (R2) was set to 25 kΩ, which controls the rate at which the summing amplifier gives feedback to the MOSFET. The pressure sensor has a response time of 1 ms and contains an on-board signal conditioning circuit to output a linear signal (1–5 V). The pressure sensor has an operating range of 0–18.6 torr and was used in the differential mode with one of its ports left open to the atmosphere.

Fig. 3 Pressure sensor feedback circuit with R1 (5 kΩ variable resistor), R2 (100 kΩ variable resistor), IC1 (MC1741), IC2 (MC1741), and 125 W n-channel FET (NTE2397).

2.2.1 Square wave generator timing. Referring to Fig. 1, the system is initialized at the start by positioning both the inlet and outlet valves (f, g) to side A, as well as moving the piston to the end of travel on side A. At this point, the dead volume at the end of side A is being purged by Ar through the end cap purge channel to the ICP-MS system. For ETV sample introduction, valve g is positioned to block flow coming out of side A and connect side B to the ICP. As a consequence, the piston moves towards side B and the aerosol is loaded. The piston continues to move until electrical contact between the piston's copper plate and end cap of side B is made. The time required to reach side B is flow rate dependant (i.e., the faster the flow rate the faster the speed). The motor stops once it reaches side B and valve f repositions. Thus, side A is now loaded with the sample aerosol from the ETV and the dead volume at the end of side B is being purged. On the next ETV heating cycle, valve g is positioned to capture the sample aerosol from the ETV in side B as the motor moves towards side A. While the next sample is being introduced into side B, the aerosol in side A is being sent to the ICP, thus producing a nearly square wave signal.

2.3 Procedures

Plasma and mass spectrometer conditions were tuned and optimized using a nebulizer and the 100 ppb stock solution mentioned previously. Relevant plasma parameters are listed in Table 2.

Table 2 Plasma settings

Instrument parameter	Varian Ultramass setting	PerkinElmer Model 500 setting
Plasma power	1.2 kW	1.1 kW
Plasma flow	13.0 L min^−1	15.0 L min^−1
Auxiliary flow	1.20 L min^−1	2.0 L min^−1
Sampling depth	7 mm	7 mm

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A 20 µL sample aliquot of the 100 ppb multi-element solution was used with all ETV experiments unless otherwise noted. This sample solution was introduced into the ETV and the vaporized aerosol captured by the cylinder. The total gas flow to the plasma was maintained at 1.2 L min⁻¹ with the use of a makeup gas after the square wave generator.

3 Results and discussion

3.1 System characterization

3.1.1 Effect of flow rate on square wave generation. Only 5 masses (⁵⁹Co, ¹¹³In, ¹¹⁵In, ²⁰⁶Pb, and ²⁰⁸Pb) were monitored in this particular experiment in order to minimize the scan time so that the time-dependent shape of the signal waveform could be well defined. Fig. 4 shows the ¹¹⁵In square wave signals obtained using Ar flow rates through the ETV of 200, 400, and 800 mL min⁻¹. All other metals tested exhibited similar results to ¹¹⁵In. The shot-to-shot precision was ∼5% or less for all flow rates tested, which is comparable to that seen for normal ETV firings and probably governed by precision of sample dosing. The leading and trailing edges of the square wave are not sharp, most likely due to the effect of broadening caused by laminar flow in the transport tubing from the cylinder to the plasma.⁸ This results in a time varying signal at the beginning and end that cannot be used if a constant signal is required. However, the intensity is fairly constant over the transient once the maximum is reached. At the minimum flow rate of 200 mL min⁻¹, this constant intensity is maintained more than 16 s. The signal duration at a carrier gas flow rate of 200 mL min⁻¹ is at least 5 times that of a typical ETV signal obtained using a carrier gas flow rate of 1.2 L min⁻¹ and twice as long as the time varying signal obtained using a 200 mL min⁻¹ carrier gas flow rate without the square wave generator. As expected, the average steady state signal intensity is directly proportional to the flow rate.

Fig. 4 ¹¹⁵In square wave signals obtained using ETV flow rates of 200, 400, and 800 mL min⁻¹ with the use of makeup gas flow to provide a constant 1.2 L min⁻¹ gas flow to plasma.

3.1.2 Analyte loss. The extent of analyte loss either through settling in the cylinder or adsorption to the cylinder wall was determined by comparing the area under the curves for the ETV square wave signal after the aerosol was allowed to reside within the cylinder for times varying from 10 to 180 s before delivering the aerosol to the plasma. Fig. 5 shows the integrated signal observed as a function of aerosol retention time in the cylinder. In comparison to the minimal hold time of 10 s, an analyte loss of 2–11% (n = 5 firings) was observed after 180 s for the different metals monitored. It is not clear what is responsible for the observed element-to-element variation. The reduction in peak area at 180 s cylinder hold time was significantly less than the 35 ± 4% seen by Langer and Holcombe⁵ with the TEx flask using 120 s stop flow conditions. Several reasons may be attributed to this difference. First, the plasma system maybe less perturbed by the square wave generator (i.e., central channel flow is always maintained to the torch). Secondly, the exact location of the particles in the TEx flask experiment is not known due to lack of a valving network; therefore, not all particles maybe trapped in the round bottom flask. Based on the minimal losses, it can be inferred that the particles produced from the ETV are aerodynamically small which is consistent with several previous studies, which concluded that most of the particles are significantly less than 100 nm in diameter.^8–11

Fig. 5 Loss in signal areas obtained from 20 µL injections of a 100 ppb multi-element solution as a function of retention time in cylinder.

3.2 Full mass scan with quantitative analysis

In spite of recent papers to the contrary,^2,3 one of the biggest limitations cited for conventional ETV-ICP-MS systems employing scanning mass spectrometers has been the inability to perform quantitative full mass scans on a single ETV firing. However, by manipulating the ETV transient signal shape and duration, it is possible to use the sequential scanning, multi-element analysis capabilities of ICP-MS systems to their full potential.

In order to document the benefits of signal manipulation of ETV transient signals, quantitative full mass scans were performed for a variety of signal shapes including a normal ETV signal, a broadened ETV signal using an SBSR, and the square wave signals discussed in this paper. The broadened and normal ETV signals were collected to show the effect of the increased duty cycle obtained from the generation of square wave signal profiles. It is important to note that the central channel flow rate was maintained at 1.2 L min⁻¹ through the use of a make-up gas when using the square wave generator and SBSR. The flow rate through the ETV was maintained at 400 mL min⁻¹ for both the square wave generator and SBSR, with 800 mL min⁻¹ used as the make-up gas to achieve 1.2 L min⁻¹. The normal ETV signal was conducted at 1.2 L min⁻¹ through the ETV to the torch, which is the typical flow rate employed for routine analyses. In total, 248 masses (n_m = 248) were monitored for each experiment and the data were collected in peak-hopping mode. Dwell times were 25, 0.60 and 0.125 ms for the square wave signal, the SBSR broadened ETV and the normal ETV signal, respectively. Most importantly, only 5 to 6 full scans were collected over the transient signal for the SBSR broadened peak and only 2 to 3 full scans were collected for the normal ETV signal using the aforementioned dwell times. On the condition that 248 masses are to be monitored, dwell time selection, for the SBSR and normal ETV signals, was determined through eqn. (1). In eqn. (1), the number of masses (n_m) is known (248), as are t_f (20 ms) and t_j (2.6 ms). Scan times were based on the element with the smallest peak half width and peak width ratio to be quantified as that described by Venable et al.³ It is important to note that in the paper by Venable et al.³ the number of masses to be monitored is the dependent variable and calculated with respect to a required precision. In the present study, the number of masses is fixed at 248 and the scan time is calculated based on minimizing the error associated with variations in peak appearance times and data collection synchronization. For the SBSR broadened signal, a scan time of 0.81 s ensured an error no larger than 2% would occur as a result of variations in peak appearance times and data collection synchronization. However, an optimal scan time producing an error less than a maximum of 10% could not be achieved for the normal ETV signal due to quadrupole scanning limitations (i.e., the smallest dwell time achievable is 0.050 ms). The difference in scan time incurred between a 0.050 ms dwell time and that of the 0.125 ms dwell time used was insignificant and at least 2 to 3 full scans were collected during the transient. Furthermore, the amount of actual counts collected in the transient would be reduced even further than that shown in Table 3. A dwell time of 0.125 ms fostered a total scan time for 248 masses of 0.69 s. The square wave signal is constant and only one scan is required. Therefore, a dwell time of 25 ms was chosen for 248 masses (i.e. one point per mass across the transient). A dwell time of 25 ms was chosen for the square wave signal in order to produce a total scan time of 6.88 s, which ensured that only the flat portion of the square wave was sampled.

Table 3 Comparison of total counts and precision between the square wave signal, the broadened ETV signal using a “single string bead reactor” (SBSR), and the normal ETV signal

Isotope	Square wave/counts	Square wave % RSD	Broadened ETV/counts	Broadened ETV % RSD	Normal ETV/counts	Normal ETV % RSD
^7Li	906	0.6%	379	10.3%	44	9.9%
^9Be	779	4.8%	486	3.3%	19	0.0%
^24Mg	3186	4.3%	1552	5.9%	179	5.3%
^25Mg	428	3.7%	199	6.8%	23	6.5%
^55Mn	1669	0.4%	659	3.6%	57	14.4%
^58Ni	891	3.3%	425	8.9%	19	45.9%
^59Co	1557	1.6%	513	4.5%	38	10.9%
^60Ni	366	3.3%	164	13.3%	10	21.5%
^63Cu	1402	7.8%	554	6.0%	84	35.8%
^66Zn	282	5.5%	162	11.0%	0	173.2%
^68Zn	233	2.1%	124	16.1%	2	89.2%
^85Rb	1400	2.6%	707	5.2%	32	22.8%
^87Rb	766	2.4%	363	10.3%	23	42.8%
^88Sr	2291	3.9%	1065	4.8%	56	17.2%
^89Y	1154	7.5%	357	7.3%	33	39.5%
^102Ru	414	7.5%	198	15.1%	6	36.7%
^104Ru	247	9.5%	127	12.4%	5	34.6%
^107Ag	451	5.5%	223	7.1%	4	73.2%
^109Ag	424	6.2%	209	10.9%	3	77.6%
^112Cd	103	7.4%	68	12.6%	4	96.1%
^114Cd	123	6.5%	79	14.5%	4	90.1%
^113In	170	7.5%	94	11.3%	3	62.4%
^115In	2674	6.6%	1164	3.3%	8	36.7%
^136Ba	162	6.4%	83	8.6%	8	25.0%
^137Ba	240	10.9%	119	5.8%	3	86.6%
^138Ba	1464	7.5%	760	3.5%	34	32.1%
^140Ce	789	2.3%	296	6.5%	11	66.8%
^142Ce	103	6.7%	41	10.9%	2	34.6%
^151Eu	1057	2.9%	509	6.8%	18	27.5%
^153Eu	1245	1.7%	579	4.0%	24	35.6%
^166Er	643	2.6%	356	4.8%	20	25.1%
^168Er	493	7.8%	267	6.0%	14	8.1%
^170Er	289	5.3%	153	5.1%	6	16.7%
^205Tl	750	4.9%	504	7.7%	3	94.4%
^206Pb	1414	7.2%	481	6.7%	7	24.7%
^207Pb	1091	8.2%	396	8.5%	4	41.7%
^208Pb	2693	6.5%	942	3.3%	15	53.3%
^209Bi	1481	6.9%	895	6.5%	9	48.4%

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In routine ETV analyses with a smaller number of monitored masses, the dwell times are generally not less than 1 ms, yielding 10 to 20 points per peak. The number of counts accumulated for a variety of masses and the associated precision for 5 replicates are shown in Table 3 for the analysis of a 50 ppb multielement solution. Although the total number of masses monitored was set at 248 (n_m = 248), only those listed in the Experimental section were quantified. The other masses were monitored but since no significant amounts were present, no quantification was attempted.

As can be seen in Table 3, the total number of counts collected using a typical ETV transient signal is low and, thus, subject to large uncertainties due to statistical counting noise. Error is further propagated by an insufficient amount of data points collected during the transient.^2,3 By statistical analysis, the number of counts collected for the square wave signal was, on average, 60 times than the typical ETV signal. The duty cycle for each mass using a normal ETV signal was calculated to be 0.02%. However, SBSR broadened ETV signals yielded significantly more counts for most isotopes studied and % RSD values dropped to approximately 15%. The enhanced precision values can be directly attributed to the 3.5 fold increase in the duty cycle to 0.07% and the corresponding decreased significance of statistical noise. Finally, the square wave generated signal generated approximately twice the number of counts of the SBSR broadened signal. The duty cycle for the square wave generated signal was ∼0.36% and yielded % RSD values of 10% or less for most metals. Although the duty cycle for the square wave generator was about 5 times that of the SBSR, the average increase in registered counts were only about 2 times that of the SBSR. This results from data monitoring only during the time period when a flat signal in the square wave generator existed. The front and the back edge of the signal (before and after the plateau) represents about half of the total counts contained in the signal coming from the piston-cylinder assembly. In particular, a total scan time of 6.88 s is ∼45% of the total square wave. Clearly, the ability to minimize the temporal overhead (i.e., jump and flyback times) caused by multiple scans on a single ETV firing is beneficial when conducting quantitative analysis from a full mass scan. Further optimization of the signal to a more perfect square wave logically produced significant improvements in the signal and RSD; albeit, a signal improvement of only an additional factor of about two was expected and observed.

It is important to note that in a few cases the precision appears to be better with the SBSR signal than the square wave signal. However, the difference in precision is not greater than 2% in most of those cases. Furthermore, in these instances all isotopic precisions with the exception of ¹³⁷Ba were under 10% for both methods concerning those cases. Overall the precision can be seen to have improved through the use of a square wave signal. Based on the 38 isotopes quantified, 97.4% are under <10% RSD for the square wave signal as opposed to only 68.4% for the SBSR. Of these analytes, 45.9% and 34.6% are <5% RSD for the square wave and SBSR, respectively.

4 Conclusion

The manipulation of ETV transient signal shapes using a square wave generator permitted acquisition of quantitative data from full mass scans on single ETV firing. The ability to mix and store the aerosol with minimal sample loss also supports the postulate of sub-micrometer diameter aerosol particles leaving ETV. The need to perform only one scan per ETV firing maximized the duty cycle and, in turn, led to a higher number of accumulated counts and better precisions. It should be noted that minimal signal manipulation using an SBSR also provided good quantitative results (% RSD < 15%) for a full mass scan on a single ETV transient event.

Furthermore, the square wave generator may also be used to eliminate or minimize the space charge effects witnessed in samples heavily laden with a matrix or carrier¹² by introducing the matrix into the ICP over a longer time. The square wave nature of the signal may also prove useful when accurate isotope ratios are sought using the ETV introduction mode. Since all masses cannot be monitored simultaneously with a scanning instrument, isotopic ratio information may be compromised when working with a transient signal. The square wave generator eliminates this potential problem as a result of the constant intensity signal achieved over the course of the measurement.

Particularly exciting is the possibility of coupling this device to the “multiplexed ETV system”¹³ to allow for quantitative analysis using full mass scans at an increased sample throughput of ca. 100–150 samples h⁻¹.

Possible limitations of the square wave generator include (a) a loss in sensitivity (ca. 2–11%) if single ion monitoring were employed simply because of small losses in the square wave generator apparatus; and (b) the inability to temporally separate elements due to their differences in vaporization temperatures.

Acknowledgements

This research was supported, in part, by grant CHE9986047 from the National Science Foundation.

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