New, computer-controlled microwave discharge emission

New, Computer-Controlled Microwave Discharge Emission Spectrometer Employing Microarc Sample Atomization for Trace and Micro Elemental Analysis L. R. Layman’ and G. M. Hieft/e2 Department of Chemistry, Indiana University, Bloomington, Ind. 4740 1

A low power microwave-exclted plasma atomic emission source has been developed whlch overcomes dlfflcultles encountered with prevlous systems In the effective introductlon of samples into the plasma. To solve these difficuities, a novsi mlcroarc sample atomizer has been designed. This high voltage, low current, pulsating dc discharge efficiently vaporizes pre-dried, discrete volumes of sample solution Into the plasma where their atomlc emission can be efficiently excited. The combined source (microwave piasma and mlcroarc sample atomizer) is shown to possess many desirable features, including sensitlvlty (picograms for many elements), precislon (f5 %), stabllity, low background, and mlnlmal Interference effects. Axial viewing of the plasma and electronic volume measurement of samples are used to Increase precision In the overall determlnatlon. Computer control of the Instrument adds versatility and convenience of operation to the system and enables the coliectlon and storage of larger amounts of data than manual operation.

Determination of the elemental composition of samples is a common problem in most analytical laboratories. Of the methods available for elemental analysis, atomic spectrometry is one of the most powerful. In atomic methods, the sample is ideally decomposed into its constituent atoms, each of which is then probed spectrometrically, to provide a qualitative and quantitative determination of the sample’s elemental composition. In practice, atom formation devices (atom cells) do not often attain this ideal, since many elements form refractory compounds which are not decomposed, causing interferences to be noted in the output signal. In addition, the atom cell often adds a noisy background signal to that from the sample atoms, inhibiting the detection of trace amounts of many elements. Although flame spectrometry, particularly atomic absorption, is the most commonly employed atomic spectrometric technique, the chemical flame is recognized as being far from an ideal atom cell. Flames, such as the nitrous oxideacetylene flame, which are hot enough to decompose refractory compounds, often have such high background emission in certain spectral regions that detection limits are relatively poor for elements whose most sensitive spectral lines lie in those regions. Recently, much work has been done to develop stable, low background electrical discharges to replace the chemical flame as an atomic emission source. Plasma jets (1-6), inductively coupled plasmas (7-21 ), and microwave discharges (22-45), all have been investigated for this application with varying degrees of success. Among these, the lowpower microwave plasma appears to offer intriguing possi-

Present address, Department of Chemistry, Pacific Lutheran Universitv. Tacoma. Wash. Author to whom correspondence should be addressed. 194

bilities as a high-efficiency atomic emission source, but has received limited attention because of several fairly serious difficulties which have been encountered. The microwave plasma is an electrical discharge maintained by the coupling of an oscillating electric field to electrons and ions contained in a closed volume, usually a quartz tube. The most common!y used microwave frequency is 2450 MHz, and the microwave power is usually coupled to the plasma by means of a tuned-cavity resonator which contains the plasma cell. The advantages of the microwave plasma include great sensitivity and a high effective electronic excitation temperature ( 2 9 ) , making it useful for the determination of most elements. It has the additional advantage for m,any laboratories that no flammable gases need be handled, n o dangerous flames are used, and no exhaust system for spent gases is required. Because the plasma power is relatively low (less than 100 watts), cooling problems are minimized. The only consumable item is high purity argon, and this a t a flow rate of less than one liter per minute. This makes the low-power microwave plasma less expensive to operate than flames or high-powered radio frequency plasmas. The reasons for the limited application of the microwave plasma revolve primarily around the difficulties encountered in the introduction of samples into the plasma. Because of these difficulties, the most successful use of the source has been as an element-selective detector for gas chromatography (22-34), where samples are introduced as vapors in an inert carrier gas flow. In this application, of course, the plasma is not required to desolvate or to vaporize the sample. By contrast, introduction of solutions of metal salts into a microwave plasma has frequently led to problems of several sorts, which arise primarily from two properties inherent in the plasma. First, compared to most flames the plasma has little energy available for the vaporization of solid or liquid particles. Second, the plasma is quite easily perturbed or even extinguished by molecular vapors, such as those from a volatilized solvent. Unfortunately, water is particularly bad in this respect, having an absorption band a t the commonly used 2450-MHz operating frequency. In efforts to overcome these problems, two kinds of systems have been developed in the past for the introduction of metal salt solutions into the plasma. These systems consist either of a nebulizer, similar to that ordinarily used with flames (35-41), or of a heated cell thermal atomizer (42-45).The nebulizer systems depend on the plasma itself to vaporize the small solid particles of sample. Those nebulizer systems which do not have separate solvent evaporator-condenser systems also depend on the plasma to desolvate the sample droplets. These nebulizer-based sampling systems are affected by both of the previously mentioned plasma limitations and thus require special- plasma cell configurations and careful tuning of the microwave coupling-cavity to provide useful results.

ANALYTICAL C H E M I S T R Y , VOL. 47, N O . 2, FEBRUARY 1975

--

I i3rdare1

Controller

I II

\I

COMPUTER

Plasma Cell

The heated cell atomizers, which also possess some limitations, often employ resistively heated filaments or rods made of platinum or carbon to atomize samples. The atomized sample from such a source is then introduced into the plasma for excitation. While solvent can be separated easily and completely from the solute in these devices, the determination of some atomic species is precluded by the limited operating temperature a t which the atomizer support material can be used (e.g., the melting point of platinum is 1772 “C). Consequently, thermal atomizers have been used with greatest success for the determination of relatively volatile compounds. Although both nebulizer and thermal atomization methods have been found useful in specific applications, the low energy they provide for sample atomization has resulted in some rather pronounced interelement interference effects. In large part, these interferences are caused by changes in sample vaporization efficiency or rate with changes in sample composition. In an attempt to solve these problems, we have developed a microwave discharge system which incorporates a novel “microarc” sample atomizer. With this atomizer, many of the problems previously described have been minimized or eliminated. The microarc sample atomizer removes the solvent from the sample of interest before presenting the plasma with the sample in the form of individual atoms. Because the success of this system is due in large part to the microarc atomizer, understanding its nature and operation are critical to the understanding of the system. The microarc is essentially a high voltage, low current atmospheric pressure pulsating dc discharge which atomizes discrete samples of metal salts quickly and completely. The discharge combines high temperature and ion sputtering to vaporize even refractory samples, so that matrix interferences are minimized. Like conventional atomic absorption flameless atomizers, the microarc atomizer can work with microliter sample solution volumes and can separate the processes of solvent evaporation, sample decomposition, and atomic vapor production, so that solvent interference can be eliminated and the generated atomic vapor can be introduced into the plasma efficiently.

Momchromator and Detector

‘vv)

Printout and Display

Analog Integrator

The microarc has several advantages over the common resistively heated atomizers. Unlike the systems commonly employed in atomic absorption, it requires no high current (tens or hundreds of amperes), but uses a current of milliamps to vaporize and atomize the sample. Power supply requirements are thereby simplified. Also, the microarc is particularly attractive for use with the microwave plasma, because it works very well in the slowly flowing argon atmosphere required by the plasma. In addition, the microarc is inexpensive to operate because its low power dissipation, about 20 watts, makes expensive cooling procedures unnecessary, and the inexpensive tungsten electrode which it employs, lasts for several thousand determinations before replacement. Thus, the microarc appears to be a highly attractive atom source for use with the low-power microwave plasma. The microwave plasma and microarc atomizer are controlled by either a computer or analog timing controller. Accurate control of the timing of various instrument functions was necessary for stable operation of the system. The computer offers many advantages in versatility and ease of operation, but the system has been designed to stand alone as well. As a system, it offers ease of operation, great sensitivity, good precisioq low operating costs, and remarkable freedom from interferences.

EXPERIMENTAL The complete experimental system consists of a microwave generator coupled to a plasma cell, the microarc with its power supply, a sample dispenser, sample volume measuring electronics, optical elements with detector and readout, and a control system. The block diagram in Figure 1 shows the interconnections among these units while the following paragraphs describe the functions of the various blocks individually. Microwave Plasma System. The microwave plasma used in this study is powered by a 100-watt microwave generator (Model HV15A, Scintillonics, Inc., Ft. Collins, Colo.) which is stabilized by a 500-watt Sola constant voltage power transformer. The 2450-MHz output from the generator is coupled to the plasma by means of a 3/4-wave Broida

ANALYTICAL CHEMISTRY, VOL. 47, NO. 2, FEBRUARY 1975

195

I

,!I'

Out (Vent)

Anoj!j;go"

(Cathode)

(Imrn)

Microarc SUPPIY

Figure 2. Microarc and plasma cell, showing the microarc electrode configuration

type cavity (46),which was found to be more stable and to require less frequent tuning than the %-wave Evenson cavity initially used. The microwave cavity with the enclosed quartz plasma cell is shown in Figure 2. The plasma is supported by a flow of high purity argon gas (Matheson UHP grade) at atmospheric pressure a t a flow rate of 300 ml per minute, and a linear velocity of about 8 cm/sec in the plasma cell. The interaction time for a sample atom with the plasma is about 200 msec. The plasma is initiated by a three-inch spark produced between the spark electrode, shown in Figure 2, and the grounded microarc sample electrode. The spark generator used for this purpose consists of a rectifier-filter power supply (150 volts dc) which charges a 1-microfarad capacitor, a pulse transformer with a 1OO:l turns ratio (Model TT-65-12, U.S. Scientific Instruments, Inc., Watertown, Mass.) and a silicon controlled rectifier (SCR) which discharges the capacitor rapidly through the primary winding of the transformer. The SCR can be controlled manually or by the timing circuitry. The pulse produced by the spark generator reaches an open circuit potential of about 12,000 volts. The plasma and the microarc atomizer are contained within a quartz cell constructed of 8 mm 0.d. tubing. The cell and plasma are positioned to enable axial viewing of the plasma through a flat quartz window affixed to one end of the cell. The use of large diameter tubing and axial viewing has made it possible to employ one cell indefinitely, with the only required maintenance being cleaning of the cell tubing every three months of hard use to maintain high efficiency. Axial viewing of the plasma provided several advantages over the more common radial viewing configuration ( 4 7 ) . Axial viewing, we have found, provides a better signal-tonoise ratio than radial viewing, because instabilities of the plasma are not as noticeable from the end of the plasma as from the side. Thus, both background and sample emission signals are more stable. Also, because the entire plasma is visible to the monochromator entrance slit, atoms reside in the optical path for a longer time than would be possible with side viewing, thereby increasing the integrated signal for a given number of atoms. Finally, axial viewing enables the cell window to be placed sufficiently far from the plasma and atomization area to prevent fogging of the window by etching or sample deposition, minimizing light losses. The presence of unswept or dead volume within the cell between the microarc and the plasma can cause rather severe problems. Dead volume lowers the sensitivity of the system by trapping some of the analyte atoms, and causes large instabilities in the signal by introducing turbulence into the flowing argon stream. In addition, an unswept vol196

ume can trap solvent vapor which then slowly bleeds into the plasma, causing lowered sensitivity and stability. The dead volume of the present cell was minimized by using the smallest diameter tubing consistent with practical operation, as noted below, and by making the cell design as simple as possible. The diameter of the tubing used to contain both the microarc and the plasma is also important. Tubing of too small a diameter causes the walls of the cell to overheat, melting hose connections. Also, the walls of small tubing cells seem to be greatly affected by the action of the plasma, becoming etched quickly and tending to become coated with a deposit of tungsten and sample material. As it builds up on the cell walls, this coating causes the plasma to become unstable and results in an increasing loss of sample vapor. In contrast, cell tubing of too large a diameter allows the plasma discharge to wander excessively, adding noise to the signal. Large tubing also increases the cell dead volume, as noted above. The 8-mm 0.d. tubing used in this study was chosen as the best compromise among these considerations. A sidearm on the plasma cell (Figure 2) allows the microarc to be placed very close to the plasma itself (within l cm). This proximity enables sample vapor, mixed with the argon plasma support gas, to pass immediately into the plasma (aided by an argon jet described below) to minimize condensation or recombination of the liberated atoms. The actual time of transport from the microarc to the plasma is about 1 millisecond. Physical interferences caused by the presence of refractory compounds are thereby reduced substantially, and sampling efficiency is enhanced. Microarc Atomizer. The "microarc" is actually a high voltage, current controlled (less than 100-mA peak) atmospheric pressure glow discharge. This discharge was developed because it possesses both high energy density and low power consumption. By concentrating its energy into a micro-sized arc, the discharge can provide efficient atomization while using little total power. Both the microwave plasma and the microarc atomizer utilize this principle of high energy density and low total power for optimum small sample determination ( 4 7 ) .The microarc is inherently less expensive to build and operate than a high current arc, and yet provides more reproducible vaporization of microgram or smaller samples. Unlike conventional glow discharges (48), the microarc is operated a t atmospheric pressure in flowing argon, making expensive vacuum equipment unnecessary. It is powered by an unfiltered half or full wave rectified ac (60 Hz) supply of 1300 volts peak, which is shown in schematic form in Figure 3. This supply consists of a high voltage (900 V rms), medium current (100 mA rms) power transformer (TI),a high voltage rectifier diode


I I

I

I

I I I

I

I

I

I I

I I

I

I

I

,I

I

,

A Figure 3. Schematic diagram of the microarc power supply

(4000 V PIV, 500 mA, model HV-4000, Sarkes Tarzian, Bloomington, Ind.), and a group of current limiting resistors between 10 and 40 kilohms. As shown in Figure 4, the voltage across the microarc electrodes rises during each half cycle toward the peak supply value until the gap breaks down at approximately 600 volts. When breakdown occurs, the voltage falls immediately to 330 volts, which remains constant (in a manner similar to a glow discharge) until the supply voltage falls below this level a t which time the arc ceases conduction. Thus, each current pulse is independent of the last one except for its influence on the electrode temperature. This on-off cycle prevents an arc channel from forming to a single point on the electrode and thus helps keep the discharge uniform over a large area of the electrode. The microarc current pulses affect the sample in two ways: by heating the sample and the electrode, and by causing cathodic sputtering. We found sputtering to be the more important of these two effects by monitoring sample atomic emission and filament temperature simultaneously. The sample was almost completely atomized before the filament became hot enough to have thermally evaporated the sample material. In addition, the presence of significant amounts of tungsten vapor even at relatively low filament temperatures is indicative of the importance of the sputtering effect. This efficient sputtering-based atomization process enables even refractory samples to be quickly and completely atomized. The microarc sample electrode (cathode) is a modified hairpin-shaped loop of 0.25 mm (AWG No. 30) tungsten wire, as shown in Figure 2. This shape is wel! suited to holding by surface tension the small samples (less than 10 microliters) used in this system. The small size of the electrode also promotes very rapid and concentrated heating and sputtering of the sample. The microarc anode is a stainiess steel syringe needle (18 gauge), through which the argon carrier gas enters the sample cell. This flowing argon stream has a linear velocity in the needle of about 10 metershecond and serves to cool the anode so that anode vaporization is minimized, and also serves to stabilize the microarc discharge. Because the stream flows directly past the cathode, vaporized samples are carried quickly (within 1 msec) into the plasma immediately above. Sample Introduction. In the present device, sample solutions can be applied to the cathode either manually or automatically. In the manual mode, a 1- to 10-microliter syringe is used to deliver arid transfer the desired volume to the microarc electrode. The automatic sampling system employs a motor driven syringe (50 microliters) to dispense repetitive samples of a given solution to the electrode under hardware or computer control. Aliquots of sample from 0.1 to 40 microliters can be selected.

Time(msec )

Figure 4. Voltage-time and current-time waveforms at the microarc electrodes

Sample Volume Measurement. The quantitative transfer of measured microliter volumes of liquids is complicated by surface tension effects. In our work, it was found that transfer of samples of one microliter or less from a syringe needle to the tungsten electrode generated quantitative errors of up to 25%. T o reduce this rather large imprecision, a method was devised to measure the volume of the sample droplet after its transfer to the electrode. This method has been described in more detail elsewhere (49). Briefly, the procedure involves indirectly measuring the total heat of vaporization (proportional to total volume) of the dispensed solvent from the time required for complete evaporation of the drop, using a constant heat input. This evaporation time is then proportional to the volume of solvent in the sample. Signal Handling. The desired optical radiation is isolated by a monochromator (Model EU-700, Heath Co., Benton Harbor, Mich.) and detected by a 1P28 photomultiplier tube operated at 700 volts by a suitable power supply (Model EU-42, Heath Co., Benton Harbor, Mich.) An operational amplifier (Model 215, Princeton Applied Research, Princeton, N.J.) converts the photocurrent to a proportional voltage which is then fed either to the 10-bit A/D converter interfaced to the computer, or to an analog integrating system, used without the computer. Integration, either analog or digital, is necessary because the total sample quantity is proportional to the time integral of the emission-time curve for the duration of the emission pulse. Timing and Control of Events. The timing of the various functions described above-sample dispensing, drying, initiation of the plasma, striking of the microarc, and the collection and integration of the data-is an important factor in obtaining good precision with this system. Variations in this timing cause the several components of the system to drift out of calibration; Table I outlines the timing of the individual steps. To control all of the various functions, two alternative systems were designed and tested. In one system, shown in schematic form in Figure 5, an analog timing module was employed, which consists of a series of unijunction relaxation oscillators of selectable period. Each oscillator produces a series of output pulses starting a t a time T , (Ti= ~ ( R ~ ( C TTi, in ) ; seconds, RT, in megohms, and CT, in microfarads) after it is energized. The sequence of events is initiated by energizing the first oscillator in the series. this oscillator produces a After a delay of one period (TI), pulse which causes a silicon controlled rectifier (SCR) to conduct. Once triggered, the SCR will continue to conduct until it is reset. The SCR, in turn, closes a relay which per-


*

197

Microwave Generator

Spark Generator

Table I. Timing Sequence of Operations Carried Out During Automatic Operation of Spectrometer

LJ

Reset

Operation

" To Next -'Ooiliator

i

I

Oscillator Unit

Figure 5. Schematic diagram of the hardware controller Only one oscillator is shown. The actual number of oscillators used is dependent on the complexity of the timing cycle desired. The minimum cycle would require six oscillators (SCRbi = GE-CGG, Q1 = 2N2712,WT1 = 2N4893)

Approximate time

Dispense sample 4 . 5 sec/microliter Evaporate solvent 10 sec/microliter4 Remove solvent from cell 30 secb Start and stabilize plasma 2 sec 0.5-5 sec Strike arc-vaporize sample Cool down 10 sec Restart plasma 2 sec Restrike a r c 2 sec Cool down 10 sec a Depends on current used and volatility of solvent. Minimum time to ensure reliable results with water as solvent.

Table 11. Operating Parameters of Plasma Emission Spectrometer

r

I

2

3

4

S

Tim. l..Cl

Figure 6. Typical elemental emission-time curves obtained from the plasma. Emission measured is that from magnesium at 2852 A (a) Emission signal from 1 ng sample of magnesium; (b)signal from 1 ng of magnesium but with 1 @g platinum added as a refractory matrix: (c) background emission from the plasma

forms the desired control function (e.g.,turns on the microwave generator), and energizes the next unijunction oscillator in the series. After one period of this next oscillator, the next control function is performed, the following oscillator is started, and so forth down the chain. The pulse from the last oscillator in the series is used to reset all of the SCRs in preparation for the next operator command to initiate another cycle. Although this hardware system performs well, its versatility is limited by the requirement that each added timing cycle requires that a new oscillator be built, connected, and adjusted to the proper timing period. For routine work, this would not be a problem, but our studies required frequent changes in the timing cycle, in order to ascertain optimum experimental conditions. The second control system, built around a PDP-12/40 computer, proved more versatile than the hardwired unit and thus better suited to our studies. In the computer, timing functions are software-controlled so that new timing cycles can be added or old ones modified simply by entering via teletype new control parameters defining the period and controlled function for each new step. (Programs available from the authors.) System Operation. Before analyses can be made with this apparatus, it is necessary to flush the plasma cell with argon and allow five minutes for the magnetron tube in the microwave generator to warm up. During this time, chosen experimental parameters can be typed into the computer, or set on the hardware controller, and sample or standard solution loaded into the auto-sampler. The desired experi198

Parameter

Value

Microwave power Argon flow rate Arc voltage Arc current Monochromator slit

50 watts 300 ml/min 1300 V peak (open-circuit) 50 mA peak

20 pm

X

12-mm height

ments can then be performed under computer or hardware control. Finally, the results are analyzed and the final values printed on the teletype (or, with the hardware system, outputted on a strip chart recorder). The operating parameters of the various parts of the system used in this work are summarized in Table 11. In a typical analysis, the following sequence of events occurs. Although the sequence can be controlled by either the computer or hardware system, this discussion will refer specifically to the computer-based operation. On command from the computer, a sample is dispensed from the autosampler and manually placed on the cathode of the microarc. The solvent is evaporated, with the time required (about 10 seconds per microliter for aqueous solutions) being noted by the computer. After completion of solvent evaporation, 30 seconds are allowed for the solvent .vapor to be blown through the cell, the microwave generator is turned on, and the plasma is initiated with a spark and allowed to stabilize for two seconds. Finally, the microarc is struck. As the sample is atomized into the plasma, the resulting emission signal is measured and integrated by the computer. The emission signal generally lasts between 100 milliseconds and 2 seconds depending on the element and the matrix of the sample. A typical emission-time curve is shown in Figure 6a. After the sample is completely vaporized, the plasma and the microarc are turned off for a fivesecond cool-down time. They are then restarted exactly as before, but without a new sample, to provide a background emission signal (Figure 6c). The integrals of the two emission signals, sample and background, are subtracted to obtain the net sample emission integral. All three integral values are typed out by the computer, and the raw emission-time curves are displayed on the computer oscilloscope. The time required for drying the sample drop (which is proportional to sample volume) is also typed out. All the data, raw and calculated, are also stored on magnetic tape, completing the experiment. When multiple samples are to be taken, the experimental cycle is automatically repeated as many times as desired, with each complete experiment taking about one minute. Calibration of this system can be easily accomplished using only one standard solution. Because the emission sig-


Table 111. Detection Limits Previous Present workb Minimum Concen- volume tration needed, Mass, Literature Concentration, ml pg reference nglml Mass, pg Element n g l m i

c

Ag

B

3

2CO

4CC

600

A r F l o w Rate

8SC

Ca Cd Cu

.000

fmlim PI

Hg

Figure 7. Effect of argon support gas flow rate on the net integrated emission signal from 1 ng of calcium nal which is measured is proportional to the total mass of analyte, a calibration curve can be constructed using different volumes of one concentration, rather than a fixed volume of several different concentrations. Points on the emission signal calibration curve are converted from mass values to concentration units by dividing them by the time required to evaporate one unit of solvent. This result is then proportional to true concentration. This useful feature saves the considerable time needed to prepare and run a series of standard dilutions of a stock standard solution. Periodic maintenance of the entire spectrometric instrument consists merely of changing the tungsten filament electrode approximately every six months, depending on frequency of use, and cleaning the inner walls of the cell with pipe cleaner every three months to prevent the buildup of any significant deposits of sample material. R E S U L T S A N D DISCUSSION The chosen argon flow rate of 300 ml per minute produced an optimum signal-to-noise ratio for all samples over the range of sample sizes capable of being handled by the microwave plasma. Figure 7 illustrates this flow rate effect on the detected emission signal from a sample containing 1 ng of calcium. At low flow rates, the sample material has a greater likelihood of condensing on the cell walls before entering the plasma, resulting in a lower detected signal. At high flow rates, the residence time of each atom in the discharge is significantly reduced, also causing a reduced signal to be observed. Detection limits measured with this system for a number of elements are listed in Table 111. In Table 111, the detection limit has been defined as that mass of material which gives a net sample integral (sample integral minus background integral minus the net integral for a blank) equal to two standard deviations of the net blank integral. These detection limits were determined by measuring the signalto-noise ratio with a sample size of approximately ten times the detection limit and calculating the sample size which would have generated a signal-to-noise ratio of 2. Those detection limits listed under “Previous” were chosen as among the best from previous works using either a high power (1-2 kW) radio frequency plasma or a low power microwave plasma. Minimum volume needed for a determination for each work was calculated from the response time of the detector used, and the nebulization rate quoted in each paper, or in the case of Reference 52, the volume was stated as 100 microliters. In most cases, the detection limit expressed in mass units is lower in this work than in any previously reported study. It should be noted that all detection limits from the present study shown in Table I11 were determined under the same experimental conditions, so that in actual analyses no re-optimization is necessary to determine different elements. This feature, of course, simplifies simultaneous and

Li Mg

Na Pb

(52)

0.16

1.6

(50)

0.33

3.3

120

(21) (40)

30

(51)

1.0 0.28 0.16 0.62 0.10

10. 2.8 1.6 6.2 1.o

0.1 0.1

0.1 0.1

0.5 0.4“

0.7 0.3 0.3

350

0.1

200 90 60 90 600

0.1 2.0

0.3 0.2

0.3 0.3 0.3 0.3

10 10

(52) (51) (51) (51) (51)

0.045

0.45

0.001 0.01 2.0 0.38 3.8 Pt . 110 1100 Zn 0.6” 0.3 180 (40) 0.92 9.2 a Low power microwave plasma, others are 1-2 kW RF plasma. System is mass-sensitive; concentration values calculated using a 10-p1 sample volume, although in practice volumes down to 0.1 pl can be employed.

0.3

...

.. ...

I . .

sequential multielement analyses. Note that the detection limit for platinum is much worse than that for any of the other elements tested. This poor result is caused by a spectral interference between tungsten and the platinum lines, as discussed later. The limiting factor in the determination of calcium was the presence of a relatively high concentration of Ca metal in the tungsten microarc filament, which generated high, variable blank and background readings. The precision obtainable with this instrument, using automatic volume measurement and correction, is about f 5 % relative standard deviation in a single determination. Of this error, about 1%is due to photomultiplier shot noise, with most of the remaining 4% probably arising from plasma variations caused by changes in the microwave generator output and in the coupling of the microwave power to the plasma. The automated volume measurement technique has reduced the error in sampling to about 1%.With manual syringe sample injection and no automated volume measurement, the relative standard deviation increases to between 15 and 25%. Most of this additional error arises from losses during the transfer of the measured drop of liquid from the needle to the filament, indicating the importance of the automated volume correction. Methods for further improving the precision are still being investigated. A typical calibration curve obtained using this instrument shows an extended linear range. Non-linearity occurs near the detection limit as a result of ionization effects, although these can be simply eliminated by the introduction of an easily ionized element (such as an alkali) into the sample matrix, as discussed in the next section. Self-absorption appears to be absent in this system, as indicated by an extended linear range a t high solution concentrations. The observed linear range for most elements extends from a few picograms (the detection limit) to near one microgram. The limitation at the high end of the concentration range is overloading of the plasma itself. An extended linear range of this kind is especially useful in simplifying calibration for a number of elements in a multielement detection system. Interferences. Several possible interferences have been investigated with this system. Examples were chosen from each of the major types of commonly encountered interference effects: chemical (53, 5 4 ) , physical (55, 5 6 ) , and spectral (57). One additional interference effect, found to be important in this method, involves quenching of the plasma by large masses of sample material.

ANALYTICAL C H E M I S T R Y , VOL. 47, NO. 2, FEBRUARY 1975

199

Table IV. Effect of Refractory Matrix on the Emission Signal from 1 n g of Magnesium Time Normalized required for integrated atomizaemission signal tion, msec

Sample and matrix

MgC1, only MgC1, i1000 ng Pt MgC1, + O . 1 ng A1 MgC1, + 1 ng A1 MgC1, + 10 ng A1 MgC1, + 100 ng A1 MgC1, + 1000 ng A1 MgO +Limestone MgC1, + 100 ng A1 + 100 ng La MgC1, + 100 ng A1 + 1000 ng La

1.00 10.04 0.98 10.04 0.98 * O . 04 0.84 k0.07 0.86 i 0.07 0.9010.0’7 0.20~0.10 0.96 10.05 0.96 * O .06 0.3510.15

50 500 75 100 150 250 500 200 200 300

Table V. Effect of Various Anions and pH on the Emission Signal of Magnesium a n d Calcium ( 1 ng M g or Ca i n all cases)

Sample and matrix

pH

Time required for atomization, msec

Normalized integrated emission signal

7 1.00 8 1.005 2 1.012 2 1.005 2 0.995 Mg(NO3)z MgC1, + 0.1 mole ratio 0.97 MgC1, + 1 mole ratio P04-3 1.01 MgC1, 10 mole ratio PO,-3 0.99 MgC1, 100 mole ratio Podq3 1.01 MgC1, + 1000 mole ratio 0.79 CaC1, + 1 mole ratio PO,-^ 1.02 CaC1, + 10 mole ratio P04-3 0.97

MgC1, Mgs (Po4 z Mg(C10,), MgSO4

+ +

i 0.025 iO.025 10.025 f 0.025 hO.025 10.05 10.05 *0.05 10.05 f 0.10 10.05 1 0.05

50 75 50 50 50 50 75 100 150 300 75 150

Table VI. Spectral Interference of Tungsten on the Most Sensitive Lines of Platinum. D a t a Taken from M.I.T. Wavelength Tables Pt line

W line

i

Intensity

wavelength, A

Intensity

2659.454

500

2830.295

600

2 10 12 10 20 3

2929.794

200

2659.906 2659.189 2659.69’7 2829.825 2830.10 2830.288 2929.99 2930.146 2997.789 2998.287 3064.937

wavelength,

2997.967

200

3064.712

300

3

6 10 2 7

Several refractory matrices, including platinum and aluminum salts and limestone, were used to determine the extent of vaporization interference problems to be expected with the microarc atomizer. Of these matrices, as shown in Table IV, only aluminum caused a measurable depression of a measured magnesium signal. This effect was considerably less (50%) for sample solutions of the same composition. Also, the effect seems independent of the amount of aluminum present over a large range although no reason for this independence has been found. When lanthanum was added to the magnesium-aluminum sample solution as a 200

P

r o 0

Samph lntiqral

(0.1nq Rb)

I

I

1

IO Mass of K

I

L

IO0

1000

lnq)

Figure 8. Effect of ionization suppressant (potassium)on the signal from 0.1 ng of an easily ionized analyte (rubidium)

releasing agent, a noticeable reduction but not complete elimination of the interference was noted. Larger amounts of lanthanum caused the plasma to be overloaded, as described more fully below. While the presence of a refractory matrix had little effect on the total integrated emission signal, it did alter the rate of vaporization of the sample from the filament surface. This behavior can be seen in Figure 6, where the emission signals from 1 ng of magnesium alone (a) and with 1 microgram of platinum added ( b ) are recorded. This variation in sample vaporization time introduces a complication into analyses performed with this method; the integration time must be adjusted to include all of the sample peak, without extending so far that significant noise is added to the signal. This adjustment can be accomplished either with electronic level sensing, or with software logic in the computer. Chemical interferences can be due to several factors (53, 5 4 ) , including the influence of anions, p H effects, or the formation of stable oxides or other molecular species. Table V shows that the system presented here is essentially free from anion and pH effects, both of which cause problems in chemical flames. In addition, the low detection limit for boron, and the lack of interference of phosphate on the calcium signal indicate that the formation of stable oxides will not be a problem as it is in most chemical flames. Ionization is another commonly occurring problem in atomic spectrometry (56). In this kind of interference, the introduction of atoms of an easily ionized element into the atom cell can suppress the ionization of the analyte and lead to an enhancement effect. In this study, ionization interference was investigated by determining the degree of enhancement of a rubidium emission signal which was caused by the addition of potassium. Figure 8 shows that there is indeed a significant ionization effect in the case of rubidium, although only 30 ng (or 30 ppm in solution) of another easily ionized element (in this case potassium) is required to essentially eliminate the problem. Few real samples have less than this amount of other ionizable elements, so that ionization would not be a problem in most cases. Spectral interferences can be a problem with any emission source (57). While the argon plasma has a much simpler and less intense background emission spectrum than do most flames, arcs, or sparks used for emission work, tungsten (vaporized from the microarc cathode) has a rich atomic emission spectrum. Fortunately, of the elements determined in this work, only platinum has suffered a spectral interference from the tungsten emission. As seen in Table VI, each of the five most sensitive lines for platinum coincides with a line from the turigsten. This interference is


Table VII. Determination of Five Elements in Blood Plasma Using Simple Aqueous Standards Concentration, u g l m l Element

Mg Ca

Li K Na

Given by manufacturer

2111.0 94+2.0 4.6+0.3

172 +3.1 320 i.27.8

Found

20.5 i 1.0 92 +4.0 4.8 *0.2 168k5.0 312 i.10

Dilution used

Net Sornple Integrol

1oox 1oox lox lOOX

looox

responsible for the relatively poor detection limit measured for this one element. The use of a different element for the microarc filament (e.g., tantalum) might significantly improve the platinum detection limit, although spectral interferences with other elements would no doubt be thereby introduced. Spectral interferences between elements present in the sample matrix would be no different here than with other atomic emission sources. Because real samples are usually complex mixtures of several compounds, they often exhibit several of the previously discussed interference problems a t once. T o test the immunity of this analytical system to these interferences, three very different real world samples were analyzed. One sample was the NBS limestone sample (No. l a ) previously mentioned, which suffered no significant interference effects. As a second test, an electrolysis mixture was analyzed for mercury. This analysis was complicated by the fact that the mixture contained a 5000-fold excess (wt/wt) of tetrabutyl ammonium perchlorate (TBAP) used as a supporting electrolyte and that the solvent was dimethyl formamide (DMF) rather than water. A known concentration of mercury in this mixture was analyzed using simple aqueous mercuric chloride solutions as standards. The results showed that the mercury signal was unaffected by the presence of the TBAI’ matrix or by the nature of the solvent (DMF). The known concentration of mercury was 7.64 f 0.05 kg/ml in the DMF, and that calculated by the analysis data was 7.4 f 0.3 gg/ml. .In the analysis of freeze-dried blood plasma, certainly a complex mixture, a standardized sample (Moni-Trol I, Dade Division, American Hospital Supply Corp., Miami, Fla.) was diluted and analyzed using simple aqueous standards prepared from a reagent grade salt of each metal. No sample preparation other than dilution with deionized water was used. The results, presented in Table VII, show that the value obtained in each case is within the range listed by Dade as acceptable for atomic absorption or flame photometric analysis. Because of the great linear range of this method, all constituents could have been determined using only a single lOOX dilution, although actual analyses were performed on different days, with convenient dilutions in each case. A quenching effect of large samples on the plasma, observed in earlier measurements, was evaluated quantitatively as shown in Figure 9. The stability of the plasma, and thus its excitation ability, is perturbed when a sufficient amount of foreign material is introduced into it. The threshold of plasma overload is seen to depend on the nature of the disturbing atoms, although the property which determines the strength of the effect for any given element has not yet been determined. For all systems tested, the threshold was at least 3 micrograms of sample material. This limitation, along with the detection limit, effectively places a lower limit on the concentration as a fraction of total solids which can be determined. While the exact limi-

L

0 001

0.01

01 ToIoI

Moss o f S o l i d s

I

IO

Lug)

Figure 9. Effect of total sample mass on the normalized signal from a unit mass of analyte. In all cases, the volume of sample solution was 1 kI (A)Signal from 1 ng of Mg (as MgCI2) with added mass of K2HP04. ( (0) Emission signal from 0-10 pg of Rb, added as RbCI. ( X ) Emission signal from 1 ng Mg (as MgCIz) with added mass of AI(N03)3. (0) Signal observed from 1 ng Mg (as MgCIz), with a fixed mass of AI and a variable mass of La added: 100 ng AI added as Al(N03)~:La added as Lac12

tation depends on the element of interest and the nature of the matrix, it is generally on the order of 1 part per million by weight of the solid matrix. This limitation, although not severe, may well be overcome by several experimental modifications. For example, increasing the microwave power should increase the tolerance of the plasma to foreign material. An impedance matching network between the microwave generator and the coupling cavity would have the same effect. Increasing the carrier gas flow rate would help, simply by diluting the sample, although detection limits would be consequently worsened. Thus, while this plasma overload difficulty is not insurmountable, it is the most significant limitation yet found for this apparatus. CONCLUSIONS Because of the proven sensitivity of the method described, it is expected to be of importance in several areas. In clinical analysis, for example, the ability to determine several trace elements in a few microliters of sample is of great significance. Forensic science also has need of such capabilities. On the other hand, extremely dilute samples can be determined simply by evaporation of a larger sample on the microarc electrode. This approach could prove valuable in pollution studies. Also, the relative freedom from interferences would minimize necessary sample pretreatment in both clinical and environmental determinations. The microarc itself requires further investigation. In different experimental configurations, the microarc would appear to be an attractive atom reservoir for atomic emission, absorption, or fluorescence, without the use of an external excitation medium (ie., the microwave plasma). Its high available energy density, the possibility of employing an inert atmosphere, and its low initial and operating costs all combine to suggest the microarc as a competitive atom source worthy of further investigation. Multielement analysis is another area to be explored with this device. The source described here is especially useful in this area because i t requires no adjustment of experimental conditions to determine individual elements. Compromises common to multielement determinations in flames can thereby be avoided. Also, the high sensitivity of the technique enables even sequential determination of


201

several elements to be performed on very small total sample quantities. This approach would be simpler and less costly than a simultaneous determination, but would be accomplished at the expense of some analysis time. In a sequential analysis scheme, the only variable which would have to be changed from one element to another is the wavelength setting of the monochromator. In summary, the instrument presented here enables elemental analysis on a micro or trace scale. It combines the properties of very high sensitivity, high excitation energy, low total power, low set-up and operating cost, and instrumental simplicity. In this system, the microarc atomizer has solved many of the problems of sample introduction by providing high energy vaporization of metal salts and relative freedom from interference by solvent vapor and matrix effects. We strongly believe that the potential of the system indicated in these initial studies will be fully realized in the detailed investigations which are now proceeding.

LITERATURE CITED (1) S. Greenfield, I. L. Jones, and C. T. Berry, Analyst (London), 89, 713 (1964). (2) M. Marinkovic and T. J. Vickers, Appl. Spectrosc., 25, 3 (1971). (3) W. A. Loseke and E. L. Grove, Appl. Spectrosc.. 26, 527 (1972). (4) I. Atsuya and H. Goto, Anal. Chim. Acta, 65, 303 (1973). (5) J. F. Chapman, L. S. Dale, and R. N. Whittem, Analyst (London), 98, 529 (1973). (6) B. Zmbova and M. Marinkovic, Taibnta, 20, 647 (1973). (7) C. D. West and D. N. Hume, Anal. Chem., 38,412 (1964). (8) R. H. Wendt and V. A. Fassel. Anal. Chem., 37, 920 (1965). (9) R. H. Wendt and V. A. Fassel, Anal. Chem., 38, 337 (1966). (IO) C. Veillon and M. Margoshes, Spectrochim. Acta, Part 8, 23, 503 (1968). (11) G. W. Dickinson and V. A. Fassel, Anal. Chem., 41, 1021 (1969). (12) I. Kleinmann and V. Svoboda, Anal. Chem., 41, 1029 (1969). (13) D. Truitt and J. W. Robinson, Anal. Chin?.Acta, 49, 401 (1970). (14) D. Truitt and J. W. Robinson, Anal. Chim. Acta, 51, 61 (1970). (15) C. D. West, Anal. Chem.. 42, 811 (1970). (16) R. M. Dagnall, D. J. Smi!h, T. S. West, and S. Greenfield, Anal. Chim. Acta, 54, 397 (1971). (17) G. F. Kirkbright, A. F. Ward, and T. S. West, Anal. Chim. Acta, 62, 241 (1972). (18) S.Greenfield and P. B. Smith, Anal. Chim. Acta, 59,341 (1972). (19) J. C. Souilliart and J. P. Robin, Analusis, 1, 427 (1972). (20) G. F. Kirkbright. A. F. Ward, and T. S. West, Anal. Chim. Acta, 64, 353 119731. > - -, (21) R. H. Scott, V. A. Fassel, R. N. Kniseley, and D. E. Nixon, Anal. Chem., 46, 75 (1974). (22) A. J. McCormack. S. C. Tong, and W. D. Cooke, Anal. Chem., 37, 1470 (1965). (23) C. A. b c h e and D. J. Lisk, Anal. Chem., 37, 1477 (1965). (24) C. A. Bache and D. J. Lisk, Anal. Chem., 39, 786 (1967). (25) H. A. Moye, Anal. Chem., 39, 1441 (1967). (26) R. M. Dagnall, S.J. Pratt, and T. S. West, Talanta, 16, 797 (1969). (27) R. M. Dagnall, S. J. Pratt, T. S. West, and D. R. Deans, Talanta, 17, 1009 (1970). (26) E. Y. Y. Lam, P. Gaspar, and A. P. Wolf, J. Phys. Chem., 75, 445 1197 11. (29) W. Braun, N. C. Peterson, A. M. Bass, and M. J. Kurylo, J. Chromatogr., 55, 237 (1971). (30) P. M. Haupt and H. Compaan. Analusis, 1, 27 (1972). I . - .

202

. I -

R. M. Dagnall, T. S. West. and P. Whitehead, Anal. Chim. Acta, 60, 25 (1972). R. M. Dagnall, T. S. West, and P. Whitehead, Anal. Chem., 44, 2074 (1972). W. R. McLean. D. L. Stanton, and G. E. Penketh, Analyst (London), 98, 432 (1973). R. M. Dagnall, T. S. West, and P. Whitehead, Ana/yst (London), 98, 647 (1973). H. Goto, K. Hirokawa, and M. Suzuki, Fresenius' Z.Anal. Chem., 225, 130 (1967). M. Yamamoto and S. Murayama. Spectrochim. Acta, Part A, 23, 773 (1967). S. Murayama, H. Matsuno, and M. Yamamoto, Spectrochim. Acta, Part 8, 23, 513 (1968). K. Fallgatter, V. Svoboda, and J. D. Winefordner, Appl. Spectrosc., 25, 347 - 119711 > H. Kawaguchi, M. Hasegawa, and A. Mitzuike. Spectrochim. Acta, Part B, 27, 205 (1972). (40) F. E. Lichte and R . K. Skogerboe, Anal. Chem., 45, 399 (1973). (41) M. Sermin, Analusis, 2, 188 (1973). (42) J. H. Runnels and J. H. Gibson, Anal. Chem., 39, 1399 (1967). (43) H. E. Taylor, J. H. Gibson, and R. K. Skogerboe, Anal. Chem., 42, 1569 (1970). (44) K. M.'Aldws. R. M. Dagnall, B. L. Sharp, and T. S.West, Anal. Chim. Acta, 54, 233 (1971). (45) H. Kawaguchi, M. Hasegawa, and A. M i u i k e , Bunko Kenkyue, 21, 36 (1972). (46) F. C. Fehsenfeld, K. M. Evenson, and H. P. Broida, Rev. Sci. Instrum., 36, 294 (1965). (47) F. E. Lichte. Ph,D. Thesis, Colorado State University, Fort Collins, Colo.. 1973. (48) P. W. J. M. Boumans, Anal. Chem., 44, 1219 (1972). (49) L. R. Layman and G. M. Hieftje, Anal. Chem., 46, 322 (1974). (50) E. E. Picket and S. R. Koirtyohann, Analyst (London). 97, 860 (1972). (51) P. W. J. M. Boumans and F. J. deBoer, Spectrochim. Acta, Part 8, 27, 391 (1972). (52) D. E. Nixon, V. A. Fassel, and R. N. Kniseley, Clin. Chem., 19, 807 (1973). (53) C. Th. J. Alkemade in "Flame Emission and Atomic Absorption Spectrometry, Voi. I-Theory,'' J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York. N.Y., 1969, p 101. (54) T. C. Rains in "Flame Emission and Atomic Absorption Spectrometry, Vol. I-Theory, "J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York, N.Y., 1969, p 349. (55) S. R. Koirtyohann in "Flame Emission and Atomic Absorption Spectrometry, Vol. I-Theory," J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York, N.Y., 1969, p 295. (56) I. Rubeska in "Flame Emission and Atomic Absorption Spectrometry, Vol. I-Theory,'' J. A. Dean and T. C. Rains, Ed., Marcel Dekker. New York, N.Y., 1969, p 317. (57) B. E. Buell in "Flame Emission and Atomic Absorption Spectrometry, Vol. I-Theory," J. A. Dean and T. C. Rains, Ed., Marcel Dekker, New York. N.Y.. 1969, p 267. I.

RECEIVEDfor review May 1,1974. Accepted September 23, 1974. Partial support for this work was provided by P H S grants GM 17904-02 and GM 17904-03, and is gratefully acknowledged by the authors. This material was presented in part a t the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March, 1973, a t the Colloquium Spectroscopicum Internationale XVII, Florence, Italy, September 1973, and a t the International Conference on Atomic Spectroscopy, Toronto, Ontario, Canada, October 1973. Taken in part from the Ph.D. thesis of L. R. Layman, Indiana University, 1974.


New, computer-controlled microwave discharge emission

Recommend Documents