Anal. Chem. 1981, 5 3 , 1187-1190
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Reduction of Matrix Ionization Interference in Laser-Enhanced Ionization Spectrometry Gregory C. Turk Center for Analytical Chemlstty, National Bureau of Standards, Washington, D.C. 20234
Despite many attractlve features, laser-enhanced ioniratlon (LEI) spectrometry has been dlfflcult to apply to the analysis of many complex samples because of a susceptibility to 88vere lonlzatlon interference. The interference Is a consequence of changes In the distribution of the electrlc field In the flame whlch occur when the concentratlon of charge species Is Increased by the unassisted thermal ionlratlon of matrix elements. The effect of these changes can be mlnlmired by aligning the laser beam as close to the surface of the cathode as posslble. However, prevlously used LEI electrode conflguratlons have utlllzed cathodes external to the flame, thus precludlng the posslbility of laser alignment near the cathode surface. A new water-cooled cathode has been designed whlch can be used directly Inside the flame, very close to the laser beam. Tolerance of LEI slgnal collectlon to a matrix of sodlum In an air-acetylene flame has been Improved from less than 300 Fg/mL to over 3000 pg/mL.
Previous publications from this laboratory (1-5)and elsewhere (6-8)have described the development of a new method of analytical atomic flame spectrometry called laser-enhanced ionization (LEI). This technique, a form of opto-galvanic spectroscopy (9, IO),utilizes direct electrical detection of the collisional ionization in a flame of excited-state analyte atoms produced by the absorption of laser radiation. The method has shown much promise, with detection limits for some elements at or near the part-per-trillion level, and has been successfully utilized for the analysis of some types of real samples (4). However, general application of LEI to the analysis of complex samples has been hampered by the method’s susceptibility to severe signal depression when the sample contains even moderate levels of elements which are partially ionized in the flame, most notably the alkali metals (4, 7 , B ) . This paper will discuss the mechanism of this interference and describe a simple modification of the LEI spectrometer which has greatly reduced susceptibility to matrix ionization interference. Direct electrical detection of enhanced ionization in the flame is accomplished by observation of changes in an electric currect passed through the flame by means of a pair of electrodes. Electric charges move in response to an electric field, which is the negative gradient of the electric potential. With a high negative potential at the cathode, and nominal ground potential at the anode, a potential “drop” and, hence, electric field, must exist somewhere in the intervening space, but not necessarily everywhere. The response of flames to external fields has been extensively studied and documented by Lawton and Weinberg (11). At the instant a field is applied to a flame, electrons and positive ions begin to move toward opposite electrodes with different drift velocities (u), given by v, = -pJ vi = piE where E is the electric field, p is the species “mobility”, and
the subscripts distinguish electrons (e) and positive ions (i). The mobility depends inversely on mass and is thus at least 2000 times greater for electrons than positive ions. Since electrons and ions are generated at the same rate in the flame, the greater electron extraction rate results in a buildup of net positive charge in the flame. This charge is localized around the cathode and continues to build up until the negative potential of the cathode is effectively neutralized beyond the net positive charge cloud. The net positive charge region around the cathode is called the “sheath” or “cathode fall”. The latter designation refers to the fact that virtually the entire potential difference between the cathode and anode is “dropped” across the sheath. Figure 1illustrates the calculated behavior of the potential and the field in three idealized flames of uniform temperature and composition between plane parallel electrodes, with a negative potential applied to the cathode (11). The dashed lines represent the behavior of the potential (upper line) and field (lower line) in the absence of a flame. The solid lines show the effect on the potential and field of three flames which differ in volume ionization rate, ri (ions produced per cm3/s). This rate is a function of the temperature and composition of the flame as well as aspirated ionizable species such as K, Na, etc. The sheath is the region of nonzero field and nonconstant potential. Between the anode and the edge of the sheath no field exists, and the potential takes on the value of the anode potential (zero, for a gounded anode). Thus, when the equilibrium electron/ion density of a flame is increased--say, by aspirating a high concentration of sodium into an air-C2H2 flame-the sheath size shrinks, for a given applied potential. This behavior is reasonable, since a smaller volume (at the greater positive ion density) is required to shield the cathode. Figure 2 illustrates the effect of increasing (negatively) the cathode potential, for a fixed volume ionization rate (i.e., flame condition). The sheath becomes physically larger, increasing the region of nonzero electric field. At the “saturation” potential, the sheath just extends across the entire space to the anode, resulting in a nonzero electric field everywhere in the flame. If one measures the current through the flame as a function of applied potential, the current will increase with potential as the sheath becomes larger and more charges are “sampled” until the saturation potential is reached. Beyond t b potential no further increase in current will occur until the electrical breakdown potential is reached. Ordinarily, for an LEI signal to be registered, it is necessary that the LEI active volume (the laser beam) be within the sheath. When ionization of the sample matrix occurs, the sheath region is compressed toward the cathode. If this compression occurs to such an extent that the laser beam is no longer in the sheath, then the LEI signal is lost. For this reason, it is desirable to keep the laser beam as close to the cathode surface as possible. Most of the LEI work to date has utilized an external split cathode configuration. In this configuration, the burner head serves conveniently as the anode, and a pair of rods or plates
This article not subject to U.S. Copyright. Publlshed 1981 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981 Potential ( V )
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Distance (cm)
Flgure 1. Spatial distribution of electric field and potential in a flame subjected to an applied potential of -750 V at three values of volume ionization rate: A,A’, 1 O I 1 s-’; B,B’, IOi2 cm-3 s-l; C,C‘, IOl3 cm3 s-’. Dashed lines indicate distributions in the absence of a flame.
n\x-1
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-
4000
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Figure 4. End-on view of (a)external split cathode configuration and (b) internal water-cooled cathode configuration: 1, split cathode plates; 2, laser beam; 3, primary reaction zone; 4, burner head; 5, water-
cooled cathode.
0
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Flgure 2. Spatial distribution of electric field and potential in a flame with a volume ionization rate of cm3 s-I at four applied potentials: A,A’, -750 V; B,B’, -1000 V; C,C’, -1250 V; D,D’, -1500 V.
just outside opposite sides of the flame serve as the cathode. The advantage of this geometry is that it is nonobtrusive. The earliest LEI experiments were done by using electrode wires (1mm diameter thoriated tungsten rods) inside the flame (1). Contamination and memory effects were observed, and deterioration of the electrode material caused signal amplitude drift. Electrode replacement was necessary after about 1 h of operation in an air-acetylene flame. The nonobtrusive split cathode configuration solved these problems, but since the cathode surfaces are outside the flame, alignment of the laser beam in close proximity to the cathode surface is not possible. Consequently this configuration, and any electrode configuration where the cathode is external to the flame, is particularly susceptible to severe matrix ionization interference. The problem is exaggerated with the split cathode rod configuration due to compression of the sheath by the high electric field strength a t the surface of a low-radius conductor, and as little as 30 pg/mL of sodium in an air-acetylene flame is enough to prevent collection of an LEI signal ( 4 , 7). This paper describes the performance of a new LEI electrode configuration utilizing a water-cooled cathode directly in the flame, allowing the laser beam to be aligned very close to the cathode surface, and thereby reducing the possibility of matrix ionization interfering with LEI signal collection.
EXPERIMENTAL SECTION The LEI spectrometer is illustrated in Figure 3. Except for modification of the cathode, the system is identical with that described in ref 4. The laser is a linear-flashlamp-pumped,pulsed
dye laser. Rhodamine 6G was the only dye used for the present work. The laser was operated at 15 pulses/s with frequency doubling of the output. Laser energies of 0.2 mJ per pulse, with a 0.8-ps pulse duration, a 0.05-nm bandwidth, and a 1.5-mm beam diameter were typical. The sample solutions are aspirated (-3 mL/min) into a premix air-acetylene flame with a 5-cm single-slot burner head. A potential of -1500 V is applied to the cathode, and the LEI signal current is measured from the anode. A RC filter ( R = 10 kQ, C = 100 pF) separates the LEI signal pulses from the dc background current due t o normal flame ionization. The pulses of signal current are converted to voltage pulses with a lo6 V/A current to voltage converter preamplifier (2). Further discrimination from flame current noise is obtained with a 10-kHz to 1-MHz active band-pass filter. Gated detection of the signal pulses is done with a gated integrator triggered by the laser beam with a 1-ps gate width. Final readout of the processed signal is on a strip chart recorder. For comparison, two electrode configurations were used. The first, shown in Figure 4a is the external split cathode plate configuration which has been used extensively in previous work (4). The plates are made from 0.6 mm thick stainless steel and are aligned 12 mm apart outside opposite sides of the flame. They are 25 mm high with the bottom edge 6 mm above the burner head. The burner head is utilized as the anode. The second electrode configuration is the water-cooled internal cathode, shown in Figure 4b. This cathode is constructed from in. 0.d. stainless steel tubing connected to a cooling water line. Again the burner head serves as the anode, and the cooled cathode is immersed directly into the flame, 10 mm above the anode. For better approximation of a parallel plate anode-cathode geometry, which yields a well-understood electric field distribution, the cylindrical cathode tube is slightly flattened to a 4 mm X 8 mm oval shape. The laser beam is aligned just below the cathode.
RESULTS AND DISCUSSION The water-cooled stainless steel cathode behaves very well in the hostile flame environment. No deterioration of the electrode has been observed after many hours of operation with a wide variety of solutions aspirated into the flame. Condensation of material on the cathode surface has been observed when solutions containing high levels of dissolved solids are aspirated. This condensation has shown no effect on the measurement of the LEI signal and is easily washed
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Figure 6. Percent recovery of an LEI signal for 50 pg/mL of Fe as a function of matrix Na concentration for the external split cathode (m) and the water-cooled internal cathode ( 0 ) .
302.1-nm excitation wavelength. The interference in question is the same for all analyte elements and is independent of analyte concentration. Similarly, any matrix element which is ionized in the flame will cause the interference, at concentrations depending on the degree of ionization. The 7.87 eV ionization potential of iron is high enough that in the absence of laser excitation, no appreciable ionization takes place, and therefore no ionization interference resulting from perturbation of the Saha ionization equilibrium is possible. At the higher levels of sodium used in this study, a spectral background LEI signal from the wings of extremely sensitive sodium transitions a t 285.3 and 330.2 nm can be detected at the iron excitation wavelength. At the 3000 pg/mL Na level the magnitude of the spectral background is 46% of the 50 pg/mL Fe signal. This can be corrected for by tuning the wavelength across the iron transition and subtracting the base line or by subtracting the signal from a matrix matched blank solution. For the present study, the latter method was more convenient. The reproducibility and limit of detection of the Fe signal are of course degraded by the presence of this background. For each of the sodium levels studied, signal recovery percentages were determined from the ratio of the net iron signal with sodium present to that without sodium present. The results are presented in Figure 6 for both electrode configurations. The interference observed with the external split cathode is consistent with previously reported results (4,8). As the concentration of sodium in the matrix is increased, an enhancement of the iron LEI signal is first observed, but when the sodium concentration reaches -300 pg/mL, a near complete loss of signal is encountered as the sheath moves out of the region of the flame irradiated by the laser. The results obtained by using the water-cooled internal cathode show a dramatic increase in tolerance to a sodiumcontaining matrix. At sodium concentrations above 300 pg/mL an enhancement is again observed, but no loss of signal is observed with as much as 3000 pg/mL of sodium present. Electrical arcing through the flame was observed when 5000 pg/mL of sodium was aspirated. The mechanism of the signal enhancement observed for both electrode configurations is not yet understood. It may be related to the increasing electric field strength in the sheath as it compresses when sodium is added. Further work regarding this effect continues. The concept of the cation sheath which compresses toward the cathode as the ion background concentration in the flame increases can be experimentally visualized by observing the LEI signal as the position of the laser beam between the anode and cathode is varied. This is shown in Figure 7 for 50 pg/mL
1190
Anal. Chem. 1981, 53, 1190-1 192
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A simple modification to the LEI spectrometer has been described which replaces the external split cathode plates with an internal water-cooled cathode. This modification reduces the susceptibility of laser-enhanced ionization to interference from the ionization of matrix species. In the case of sodium in an air-acetylene flame, tolerance is increased from a concentration of less than 300 pg/mL to over 3000 pg/mL. This is a major improvement in the LEI method for spectrochemical analysis which should improve accuracy and broaden the scope of the method to include a wide variety of complex sample materials.
ACKNOWLEDGMENT The author thanks J. C. Travis, T. C. OHaver, J. R. DeVoe, and W. G. Mallard for helpful discussions and H. B. Shubin for technical assistance. LITERATURE CITED
Beam Height Above Burner (mm)
Figure 7. Effect of laser beam posltion on the LEI signal for 50 pg/mL Fe with -1500 V applied to a water-cooled internal cathode: distilled
water matrix
(M), 1000 pg/mL
Na matrlx (0).
of iron with and without loo0 Ng/mL of sodium present using the water-cooled internal cathode. As the laser beam position is varied, changes in temperature, atom density, and electric field strength are encountered, all of which affect the LEI signal intensity. The change in electric field strength is most important since the beam must be in a position of some electric field in order to detect enhanced ionization in the microsecond time domain. A dramatic change in electric field strength occurs as the edge of the sheath is traversed. In the sodium-free matrix an iron LEI signal can be detected from any point above the reaction zone and below the cathode, indicating that the sheath extends from the cathode at least to the edge of the reaction zone, 3 mm above the burner head anode. In comparison, the iron LEI signal in the lo00 Hg/mL sodium matrix drops precipitously when the laser beam is more than 2 mm below the cathode surface, indicating that the edge of the sheath is encountered in this region.
Green, R. B.; Keller, R. A.; Schenck, P. K.; Travis, J. C.; Luther, 0. G. J. Am. Chem. SOC. 1976, 98, 8517-8518. Turk, G. C.; Travis, J. C.; DeVoe, J. R.; O'Haver, T. C. Anal. Chem. 1978, 50, 817-820. Travis, J. C.: Schenck. P. K.: Turk, G. C.: Mallard. W. G. Anal. Chem. 1979, 57, 1516-1520. Turk, 0. C.; Travis, J. C.; DeVoe, J. R.; O'Haver, T. C. Anal. Chem. 1979 .- . - , 51 - . , i.a- -m- - i.a-m - -. Turk, G. C.: Mallard, W. G.; Schenck, P. K.; Smyth, K. C. Anal. Chem. 1979. 51. 2408-2410. Van Dljk, 'CA.; ikemade, C. Th. J. Comb. Flame 1980. 38, 37-49. Green, R. B.; Havrllla, G. J.; Trask, T. 0. Appl. Spectrosc. 1980, 34, 561-569. Havrilla, G. J.; Green, R. B. Anal. Chem. 1980, 52, 2376-2383. Green, R. B.; Keller, R. A.; Luther, G. 0.; Schenck, P. K.; Travis, J. C. Appl. Phys. Len. 1976, 29, 727-729. Travis, J. C.; DeVoe, J. R. I n "Lasers and Chemical Analysis"; Hieftje, 0. M., Lytle, F. E., Travis, J. C., Eds.; Humana Press: Clifton, NJ, 1981; Chapter 6. Lawton, J.; Weinberg, F. "Electrical Aspects of Combustbn"; Claredon Press: Oxford, 1969; pp 319-322. Schenck, P. K.: Travis. J. C.: Turk, G. C.: O'Haver, T. C. J. fhvs. Chem., in press. Willls, J. B. I n "CRC Handbook of Spectroscopy"; Robinson, J. W., Ed.; CRC Press: Cleveland, OH, 1974; Vol. I, p 815.
RECEIVED for review February 12,1981. Accepted April 15, 1981.
Detection of Methanol in Wine by Microwave Spectroscopy Robert W. Kitchln, Robert E. Wlllls," and Robert L. Cook Department of Physics, Mississippi State University, Mississippi State, Mississippi 39762
Microwave rotational spectroscopy In the frequency range 26.5-40 GHz was used to detect methanol at various concentratlons. The presence of moderate amounts in a water and ethanol solutlon was quickly revealed by a low-resolutlon scan. The detection of amounts on the order of 100 ppm requlred the hlgh-resolution ldentlflcatlon of particular Ilnes. A quantltatlve estimate of the concentratlon of methanol In flve dtfferent wlnes was performed by comparing the area of a particular line to that of a standard methanol sample obtained under Identical experimental conditions.
The analysis of trace amounts of methanol in wine can present some problems with conventional analytical techniques (1). The present study was initiated to demonstrate the applicability of microwave rotational spectroscopy to this problem. It has been recognized for over 30 years that mi-
crowave spectroscopy could be employed for analytical purposes, since it is only the location and intensity of an absorption line belonging to a particular molecule which is required for chemical analysis. Moreover, similar molecules will usually have absorption lines at quite different frequencies. This nondestructive technique also allows mixtures to be analyzed in situ without prior sample preparation, or separation, and with a very small amount of sample. Reviews of this subject are available (2-4). In the case of methanol, this technique is particularly useful since the transitions of methanol are strong and the presence of water in large quantities presents no special problems because its rotational frequencies are so high that only one transition is observed in the region below 40 GHz. EXPERIMENTAL SECTION The microwave spectra were obtained with a stabilized Stark-modulated microwave spectrometer employing phase-sensitive detection. Rotational transitions in the 26.5-40 GHz region
0003-2700/81/0353-1190$01.25/0 0 1981 American Chemical Society
