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prior to modification of the method, but are greatly reduced by the recommended precautions. The procedures described permit accurate direct analysis against acidic standards of most soft tissues for Cr, Co, Mn, Mo, and Ni, eliminating the need for several procedures for different tissues or elements. Tissues can be analyzed for these metals if they contain particulate metal at least as high as 3000 bg/g, with a practical lower limit for metal in ionic form of approximately 0.03 bg/g. In digests of the TORT-1 material, concentrations were also obtained for vanadium, zinc, and magnesium that were in good agreement with the certified values, indicating that these procedures are probably applicable for other nonvolatile elements.
LITERATURE CITED ( 1 ) Clegg, M. S; Keen, C. L.; Lonnerdal, B.;Hurley, L. S . Biol. Trace €/ern.
Res. 1981, 3 , 237-244. (2) Desaulniers, R. E.; Sturgeon, R . E.; Berman, R . S . At. Spectrosc. 1985, 6. 125-127. (3) Williams, M. C.; Stallings, E. A.; Foreman, T. M.; Gladney, E. S. At. Spectrosc. 1988, 9. 110-114. (4) Agins, H. J.; Alcock, N. W.;Bansal, M.; Salvati, E. A,; Wilson, P. D.. Jr.; Pellicci, P. M.; Bullough, P. G. J . Bone Joht Surg. 1988, 70-A, 347-355. (5) Techniques inGraphite FurnaceAtomic Absorptjon q,ectrop,,otornetry: Perkin-Elmer Corp.: Norwalk, CT, 1985.
RECEIVED for review December 12,1988. Accepted February 14,1989. This work was supported by a grant from the Clark Foundation.
Diode-Laser-Based Optical Rotation Detector for High-Performance Liquid Chromatography and On-Line Polarimetric Analysis David K. Lloyd, David M. Goodall,* and Helen Scrivener Chemistry Department, University of York,Heslington, York YO1 5DD, England
A simple polarimeter with microdegree sensitivity based on a near-infrared (820 nm) semlconductor dlode laser is described, and signal to noise ratio analysis is discussed. With the use of small-volume flow cells (typically 8 pL) its utility as a high-performance liquid chromatography (HPLC) detector In the gradient elution separation of carbohydrates is shown. Coupled wlth a UV detector, the HPLC enantiomeric purity determination of DA-tryptophanmixtures wlthout chirai separatlon Is demonstrated. On a laboratory-scale system an appiicatlon from the sugar industry Is reported, monitoring molasses flow streams which are opaque at vislbie wavelengths.
INTRODUCTION Simple mechanical polarimeters have been constructed for over a century with sensitivities of the order of 0.01O. Modern instruments use ac modulation techniques to assist signal recovery. Some studies have investigated the performance of intensity-modulated polarimeters where the light flux through the sample is varied ( I ) . The most sensitive instruments modulate the axis of the polarization state of the light traversing the sample (2,3). Currently available commercial instruments use this technique. Light sources are generally gas discharge lamps, giving sensitivities of the order of O.O0lo (1 mdeg). The use of laser light sources allows a considerable improvement in performance. Argon ion lasers have been used in polarimeters designed as detectors in high-performance liquid chromatography (HPLC) (4) and for detection in biopolymer fast-reaction studies (5). Helium-neon lasers provide a more convenient source of light, and various designs have utilized them (6-8).Unfortunately many lasers, particularly gas lasers, exhibit a high degree of intensity instability, or flicker noise, a t low frequencies. This can severely degrade
detector performance below the theoretical shot noise limit. Thus the key to any optical layout involving gas lasers is to reject flicker noise (9);various schemes, using extremely low depolarization (4),high-frequency modulation ( I O ) , balanced photodetectors (8),multiple polarizers ( I I ) , or a combination of these techniques (12), achieve this aim. In this paper a somewhat different approach is described, utilizing a semiconductor diode laser. The inherently low flicker noise characteristics of diode lasers (13) have allowed the production of a simple polarimeter, without the need for an instrumental configuration designed to reject flicker noise, while maintaining a good sensitivity. Chromatography of chiral molecules provides the widest application for this type of instrumentation. Enantiomeric separation by HPLC is a rapidly expanding area of interest (14,15). Here a polarimetric detector may be of use during method development for chiral separations or during the development of new chiral stationary phases. Alternatively, enantiomeric purity determination may be performed by HPLC with partial or without chiral resolution by the coupled use of a polarimeter and another detector. Mannschreck et al. (16, 17) and Boehme et al. (18)used commercial polarimeters for such determinations. More recently Reitsma and Yeung have shown greatly enhanced performance in enantiomeric purity measurement of amino acids without chiral separation (19) using coupled optical rotation (OR) and UV or refractive index (RI) detection and in dansyl amino acid determination with OR/UV detection (20). There are various demands for chiral purity analyses. The largest will probably come from the pharmaceutical industry-over 50% of the top 200 prescribed drugs in the United States are chiral ( 2 I ) ,and often the two enantiomers may have widely differing biological activity (22). The dating of organic fossil remains by measuring the degree of racemization of amino acids present is an established geochronological technique (23). At present the ratio of L-isoleucine t o D-alloisoleucine is usually determined for dating because
0003-2700/89/0361-1238$01.50/0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
D
F
P C A L M
C
A
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PD
\@ Optical rotation detector block diagram: D, diode laser: F, lens; P, polarizer; CAL, calibrator; M, modulator; C, cell; A, analyzer; PD, photodiode; DC, dc power supply; PA, audio frequency power amplifier: LIA, lock-in amplifier. Figure 1.
separation is relatively easy by ion-exchange chromatography. Coupled OR/UV or OR/RI detection could allow the simultaneous measurement of the degree of racemization of many amino acids in one chromatogram without chiral separation. The food industry may also find application for more sensitive polarimeters. HPLC detection of carbohydrates is traditionally done by RI measurements. Laser polarimeters offer similar or better sensitivity than that of standard RI detectors with the advantage of allowing gradient elution to be performed. Conformational studies of biopolymers frequently use polarimetry as a probe of molecular conformation (24). Enhanced sensitivity in polarimetry will allow investigations at very low polymer concentrations where transitions are essentially intramolecular. Finally, there are applications involving process monitoring and control of chiral flow streams. Of particular interest is the use of diode-laser-based polarimetry in monitoring molasses flow streams which are opaque a t visible wavelengths (25). In this paper we shall discuss the construction and operation of a simple and robust diode laser polarimeter. Signal to noise ratio optimization will be considered. Typical applications are illustrated in the separation by gradient elution HPLC of glucose syrup, in the detection and enantiomeric purity determination of mixtures of D- and L-tryptophan, and in measuring the optical rotation of very dark cane sugar molasses flow streams in a laboratory-scale system.
EXPERIMENTAL SECTION The detector block diagram is shown in Figure 1. A collimated laser diode light source D (Phillips 515-CQL-A) provides up to 2 mW of light at 820 nm. It is used in the power control mode with feedback from an internal monitor photodiode to a commercial power supply (Canadian Instrumentation and Research, Ltd.). A 35 cm focal length lens F focuses the beam at the cell. Polarizers P is a standard commercial Glan-Taylor calcite prism (Rofin G15A). The polarization of the light is modulated through the Faraday effect by modulator M consisting of 1500 turns of 24-swg enameled copper wire wound on a Delrin former around a 5 cm long, 6 mm diameter Schott S F l l dense flint glass rod. A capacitance of 0.5 pF in series with the modulator coil gives a resonant frequency of 1.45 kHz. A 1-A root mean square (rms) sinusoidal modulation current at this frequency is supplied by an audio power amplifier module PA. This arrangement gives a modulation angle of up to 1'. A winding of 1000 turns, CAL, next to the modulator coil on the same core, driven by a direct current supply, DC, provides a calibration facility. One milliampere of current passing through this coil provides approximately 1 mdeg of rotation, which is used as a standard against which to compare sample rotations. Flow cells, C, used in this work were (i) a 1 cm path length, 8 pL volume tapered bore flow cell with a planoconvex lens as an input window (Applied Chromatography Systems, Ltd., Macclesfield, England) or (ii) a 2.5 cm long, 8 pL volume flow cell with a cylindrical bore, a central inlet port, and standard Z-type outlets (26). Either flow cell may be held in a manual micropositioning mount. After the cell the beam passes to the analyzer, A, a second Glan-Taylor prism similar to P. Coarse and fine angular adjustments are provided by a rotational mount (Photon Control,
\
I
/
Figure 2. Polarization modulation leading to amplitude-modulated, signal. Timedependent rotation B(t) away from polarizer/analyzer cross point is transhted onto the curve a,, sin20 to give the transmitted signal intensity 'P(t). Conditions: (a) no sample rotation: (b) sample rotation a.
Ltd., RM50). All optical components are secured to an optical breadboard (Photon Control, Ltd., LCB). The photodetector PD is a silicon photodiode, mounted directly behind the analyzer, with an active area of 60 mm2 and a responsivity of 0.45 A W-' at 820 nm (Centronics OSD60-5T). The photodiode is enclosed, and the analyzer entrance facet may also be shielded to reduce incident extraneous light. The photodiode is connected directly to the current preamplifier of a lock-in amplifier, LIA, (EG&GModel 5209) used for signal recovery. The internal oscillator of the lock-in amplifier provides a signal to drive the power amplifier and acts as reference. The resulting optical rotation signal is displayed on a chart recorder or computer integrator. The chromatography system was supplied by Applied Chromatography Systems, Ltd., and consists of a ternary gradient pump (Model 352) with a variable wavelength UV detector (Model 750/12). Sample injection is via a Rheodyne 7125 valve with a 20-pL loop. Chromatography of glucose syrup was performed on a 25 cm X 0.46 cm amino bonded column (Spherisorb S5 amino) by using isocratic and gradient elution with acetonitri1e:water. Tryptophan separations were made with a 25 cm X 0.46 cm C18 column (Spherisorb S5 ODS) and methanokwater as solvent. All solvents were of analytical reagent grade or better and were fdtered through a 0.45-pm membrane filter before use. To measure the optical activity of molasses or other opaque samples, the chromatographic flow cell is replaced by a 0.5 mm path length spectrophotometer flow cell. This is situated away from the beam focus. The focusing lens F may be dispensed with entirely, provided the beam passes adequately through all the optical components, Material is either injected directly into the cell for static measurements or pumped through by a peristaltic pump. Because of the high viscosity of molasses at room temperature, rotation measurements on flowing samples could only be made after dilution.
RESULTS AND DISCUSSION The principle of operation of the polarization-modulated polarimeter is illustrated in Figure 2. The intensity of light
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NO. 11, JUNE 1, 1989
transmitted by rotation away from the polarizer/analyzer cross point is given by Malus' law. Modulation by angle 0 about the cross point a t frequency f gives rise to an amplitudemodulated signal at frequency 2f a t the detector (Figure 2a). A sample rotation a away from the cross point varies the transmitted waveform, a major component a t frequency f proportional to N being introduced ( 3 ) (Figure 2b). The rms signal flux, a?,at frequency f is given by (27)
I
/
/'
where a0 is the light flux through the sample, and the rms modulation and depolarized light flux, aa,is +a
= (Po(.i2
+ 1 sin2 0 + y8 sin4 0)1:2
(2)
where is the system depolarization, the ratio of light transmitted with polarizer and analyzer crossed to that transmitted when they are parallel. For a polarimeter with a lamp light source, performance is likely to be shot noise limited. However, with the use of intense laser sources, flicker noise may also be significant (9). The shot noise limited situation has been considered by Goodall and Cross in ref 27, and signal to noise ratio (S/N) as a function of transmitted intensity, modulation angle, and depolarization is described by eq 6 therein. Flicker noise is a widespread but poorly understood phenomenon displaying a noise power spectrum proportional to f-*, where generally a = 1. At some cutoff frequency, f c , shot noise and flicker noise are equal, but below f, flicker noise predominates. If the flicker noise current density in n, then the flicker noise power density, n2, may be described by an equation of the form (28, 29) n2
=
P(P2(l/f)
0
(3)
K is a constant for each device or system. Thus the total flicker noise current in a frequency band f l to f 2 , N ( f l- f z ) , Log
is (28) N(f1 -
fz) =
K @ [In
(f2/fJ11/z
(4)
Total flicker noise and cutoff frequency vary between light sources. Semiconductor light-emitting diodes generally display almost shot noise limited performance. Semiconductor laser diodes often exhibit low flicker noise characteristics, while gas lasers tend to be inherently noisy devices (13). Shot, flicker, and detection electronics noise terms were combined as uncorrelated sources (9)to produce an expression describing system S/Nin terms of signal and noise currents
S/N =
[2eE@.,Af +
@.,E (E@aK)2 In { ( f+ A f ) / f ) + ien2I1/' (5)
where E is photodetector responsivity, Af is measurement bandwidth, e is electronic charge, and i,, is noise current due to the detection electronics. The terms in the denominator of eq 5 are due to shot, flicker, and photodetector/amplifer noise, respectively. Figure 3 uses eq 5 to calculate S/Nfor a situation with no noise contribution due to the detection electronics (ien= 0). It shows a contour plot (Fiure 3a) of S/N (t axis) as a function of depolarization (A) and modulation angle (0) for a system with laser power of 0.1 W, total flicker noise of 0.2% (10 Hz to 2 MHz), detector responsitivity of 0.065 A W-l, and detection bandwidth of 0.083 Hz (3-s time constant) a t a modulation frequency of 1.0 kHz. The S / N contours are numbered, and the contour intervals are given in the figure captions. For example, in Figure 3a, the contour interval is 3162, so contour 1 represents S I N = 3162, contour 2 represents S / N = 6324, etc., with S/Ntaken relative to a 1-mdeg signal. It can be seen that the maximum S / N is achieved with very low de-
e
Flgure 3. S/N as a function of depolarization and modulation angle. Contours are of SIN relative to a 1-mdeg signal calculated from eq 5 with = 0.1 W, E = 0.065 A W-', A f = 0.083 Hz, f , = 1 kHz, i,, = 0. Contour interval = 3162. Conditions: (a) S/N contour plot with 0.2% flicker noise (10 Hz to 2 MHz); (b) SIN contour plot with 0.01 % flicker noise (10 Hz - 2 MHz).
a0
polarization, and quite small modulation angles, around 100 mdeg or less. Figure 3b shows the effect of reducing the flicker noise to 0.01 % . There is now a much larger plateau of maximum S / N , which can be accessed by using larger modulation angles a t higher depolarizations. This level of flicker noise is attainable either by the use of external intensity stabilization with a gas laser system or by the use of a low-noise diode laser. The choice of photodetector and its associated electronics is important, as a t low light levels their noise contributions may be larger than the shot of flicker contributions. It is interesting to note that, from eq 1, the rms signal flux due to a 0.1-mdeg sample rotation, with a 2-mW laser and 0.1' modulation angle, is around lo-" W. Photomultiplier detectors offer the advantage of low-noise internal amplification. However, a t our laser operating wavelength of 820 nm photomultiplier responsivities are generally rather low, while Si photodiodes are approaching their maximum efficiency. The relative importance of the different photodetector responsivities and associated amplification noise must be weighed, taking into account the characteristics of each light source. This is illustrated in Figure 4, where contour plot a shows the S / N surface for a diode laser polarimeter with a 2-mW laser, flicker noise = 0.0075% (measured, 10 Hz to 14 kHz), photodiode responsivity = 0.45 A W-', and detector noise current i, = 3.75 X A. The effect of removing detector noise
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
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total noise laser noise background noise
1.o
S
-3
-2
-I Log
0
e
2.0
3.0
f
Figure 5. Measured noise at the lock-in amplifier input as a function
of frequency. S is the shot noise limit calculated from the measured light flux at the photodetector.
I"
-3
-2
-1
Log
0
e
Figure 4. Effect of adding detector noise term. Contours are of S/N relative to a 1-mdeg signal calculated from eq 5 with a,, = 2 rnW, E = 0.45 A W-', A f = 0.083 Hz, f , = 1 kHz, flicker noise = 0.0075% (10 Hz to 14 kHz). Contour interval = 447. Conditions: (a) noisy photodetector, i,, = 3.75 X lo-'' A; (b) noiseless photodetector.
contributions is shown in Figure 4b. Amplifier noise is dominant except at large signal levels (high 0, a). Depolarization has little effect. Thus the photomultiplier is the detector of choice for the gas laser system where flicker noise is rejected by using high-quality polarizers and low modulation, and consequently there is a small signal flux. The low-noise diode laser allows greater modulation angles and thus a larger signal flux so that the photodiode amplifier noise may be made small relative to the shot and flicker contributions. The high responsivity of the photodiode also results in reduced shot noise relative to the photomultiplier. System noise for our detector was measured by using the lock-in amplifier's noise facility, and on the lock-in output by using an oscilloscope. Figure 5 shows the photodetector noise current as a function of frequency. Laser power was 1.6 mW, and detection bandwidth 1.25 Hz. Background noise due to detector electronics contributions and external sources was determined with the laser off. Total noise was measured with the laser on. The shot noise level S indicated is calculated from the total photodetector current. With the assumption that all noise sources were uncorrelated, laser flicker noise was calculated. At low frequencies laser flicker noise predominates while above 3 kHz preamplifier voltage noise becomes significant due to the large photodidode capacitance. The optimum operating area is from 0.5 to 2.5 kHz. Here shot and flicker noise levels are similar with a total equivalent to an optical rotation noise of 4 pdeg rms (1-s time constant).
Recent experiments with a 30-mW, 820-nm diode laser (Spindler and Hoyer DC25F) have shown a noise level of 0.5 pdeg rms (1-s time constant) measured under the same experimental conditions. Comparison may be made to the performance of the latest argon-ion laser based systems (10, 12), with sensitivities normalized from published values to a 1-s detection time constant for each. These polarimeters use either high-frequency modulation to avoid flicker noise, giving a sensitivity of 0.5 pdeg rms (laser 20 mW at 488 nm) (IO),or multiple polarizers and a balanced detector arrangement, with a sensitivity of 0.2 pdeg rms (200 mW at 514.5 nm) (12). Due to the optical rotatory dispersion effect (30),for a given rotational sensitivity these systems operating at visible wavelengths enjoy an advantage in mass sensitivity of typically 2-3 times over that of our detector operating in the near-infrared region. During our measurements the depolarization was around giving an undesirably high background flux at the detector. It is only the low diode flicker noise and large modulation angle that allow a usable sensitivity to be obtained with such poor depolarization. Thus standard commercial polarizers may be used without selection for optimum quality. Also a certain amount of birefringence in the optical elements is acceptable, provided it does not vary and cause drift. There are several possible ways to improve sensitivity. Reducing depolarization to around should help considerably, as would an increase in laser power. Increasing the modulation angle beyond 1" is unlikely to prove helpful. Chromatography. Sugars. Separations were performed on glucose syrup samples (Globe glucose syrup 01132, CPC (UK), Ltd.) under both isocratic and gradient elution conditions. Since the solvent is not chiral, there should be no response from the OR detector due to the gradient. However, large, rapid changes in the RI of the eluent may defocus the laser beam. The resulting scattered light from the cell walls gives rise to spurious output signals. The two flow cells used here have proved superior to conventional z-type straight-bore cells in reducing effects due to RI changes. Both have volumes of around 8 pL. The 2.5-cm cell gives greater sensitivity but is rather more awkward to align than the 1-cm tapered cell. Large-bore cells of conventional design may be used to avoid the problem of scattering, but at the expense of possible loss of chromatographic efficiency. Previous gradient separations with OR detection have optimized solvents such that the RI change during the gradient is minimal (20). In this work typical gradients from 70:30 to 5050 acetonitri1e:waterproduced no base-line shift or increase in noise levels in comparison with the isocratic situation. The
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ANALYTICAL CHEMISTRY, VOL. 61,NO. 1 1 , JUNE 1, 1989
2 3
1
4
5
I
1
0
5
I
10 time/minutes
I
I
15
20
J
Figure 6. Chromatogram of glucose syrup with optical rotation detection: l, glucose; 2,maltose; 3, maltotriose and higher sugars, with numbers indicating degree of polymerization; column, Spherisorb S5 amino; solvent gradient 70:30to 50:50 aceonitri1e:water over 15 min; flow rate, 1 mL min-'.
I
L
1
0 5 timelmins
-
0
5
tlme/mins
Flgure 7. Chromatograms of D and L-tryptophan. Large signal at the
beginning of the D-trp trace is due to beam defocusing during elution at the void volume. Calibration peaks of 1.6mdeg appear after the trp signals. Conditions: column, Spherisorb S5 ODs;solvent, 5050 methanokwater; flow rate, 1 mL min-'.
total RI change during the gradient is quite small, 0.0022 RIU (refractive index unit). However, as explained by Knox et al. (31),it is the time-dependent change in RI that causes a liquid lens to form in the laminar flow region of a cell. With this gradient run over 15 min, the rate of change of refractive index = 2.7 X RIU min-'. A typical chromatogram of a 20-pL injection of 2% syrup is shown in Figure 6. The individual peaks are (1)glucose, 58 pg; (2) maltose, 48 l g ; (3) maltotriose, 42 pg, and higher oligomers with numbers indicating the degree of polymerization. Tryptophan. Injections of various quantities of D- and L-tryptophan (trp) were made. Figure 7 shows chromatograms of the two isomers, with an injection of 25 wg of each. The detection limit can be seen to be around 1pg on column. The large step functions are electronically applied calibration signals. There may be a severe disturbance due to beam defocusing during elution a t the void volume through the 2.5-cm flow cell. Careful alignment reduces this problem. To determine the sensitivity for enantiomeric purity determination, 50-pg injections were made of mixtures of D- and L-trp in various proportions from 100% L-trp to 5050 D-:L-trp. Plotting the detector response as a function of enantiomeric ratio gave a linear relationship (correlation coefficient r = 0.993). From the measured S/Nlevel it is estimated that,
using the OR detector in series with a UV detector, and ratioing the peak areas, enantiomeric purity of trp could be determined with an accuracy of f l % a t the 50-pg injection level. Determination to greater accuracy requires either a greater loading of material or a reduction in the detector noise level. Dark Solution Polarimetry. Initial investigations have been made into the use of this detector in measuring the rotation of visibly opaque sugar syrups. These studies were performed with the laboratory-scale system described, but performance, in particular the light throughput, is such that extension to a process-scale system is feasible. Undiluted 1500-ICU (international color units) molasses and undiluted and 10% solutions of 36 000-ICU and 45 000ICU molasses gave transmissions in the 0.5-mm flow cell greater than 80% at the laser wavelength. As the amplitude of the output signal is proportional to the transmitted intensity, care was teaken to compare the signal due to the sample rotation with the electronically applied calibration signal to calculate a true rotation value. With static samples of 45 000-ICU molasses satisfactory agreement is found between the rotation measured at 820 nm (109 mdeg) and the value of 118 mdeg estimated by extrapolation from measurements made on dilute samples at visible wavelengths using the Drude relationship (30). With sample being pumped through the cell in a laminar flow regime, artifacts occurred with varying flow rates, presumably due to flow-induced birefringence. These were eliminated by aligning the axis of the beam polarization with the direction of flow. This gave rotation measurements that were in agreement with the static results. Thus it can be seen that on-line monitoring is possible with diode laser polarimetry, in contrast to the present situation where samples have to be removed and diluted before measurement in a laboratory polarimeter.
ACKNOWLEDGMENT We thank Rowntree plc for supplying samples and columns, Tate and Lyle Group Research and Development for supplying samples and columns and for suggesting the molasses flow stream monitoring application, and Gary Bowden for assistance with the chromatography. LITERATURE CITED (1) Kankare. J. J.; Stephens, R. Talanta 1984, 3 7 , 689-692. (2) Gillham, E. J. J . Sci. Instrum. 1957, 3 4 , 435-439. (3) Billardon, M. Ann. Phys. (Paris) 1962, 7 , 234-267. (4) Yeung, E. S.;Steenhoek, L. E.;Woodruff, S. D.; KUO,J. C. Anal. Chem. 1980, 52. 1399-1402. (5) Goodail, D. M.; Lloyd, D. K. Gums Stab. Food Ind. 1985, 3 , 497-500. (6) Kuo, J. C.;Yeung, E. S.J. Chromatogr. 1981, 223, 321-329. (7) Kankare, J. J.; Stephens, R. Talanta 1986. 3 3 , 571-576. (8) Alexandrov, E. 8.; Zapasskii, V. S.Opt. Spektrosk. 1976, 4 1 , 855858; Opt. Spectrosc. (USSR) 1976, 4 1 , 502-504. (9) Yeung, E. S. Talanta 1985, 3 2 , 1097-1100. (IO) Bobbitt, D. R.; Yeung, E. S. Appl. Spectrosc. 1988, 4 0 , 407-410. (11) Zapasskii, V. S. Opt. Spektrosk. 1979, 4 7 , 810-812; Opt. Spectrosc. (USSR) 1979, 4 7 , 450-451. (12) Zapasskii, V. S.Opt. Spektrosk. 1982, 52, 1105-1108;Opf. Spectrosc. (USSR) 1982, 52, 667-669. (13)Dandridge, A,; Tveten, A. B.; Miies, R. 0.: Giallorenzi, T. G. Appl. Phys. Lett. 1980, 3 7 , 526-528. (14) Chromatographic Chiral Separations; Zief, M.. Crane, L. J., Eds.; Dekker: New York, 1988. (15) Wainer, I. W. TrAC, Trend. Anal. Chem. (Pers. Ed.) 1987, 6 , 125-1 34. (16)Mannschreck, A.; Mintas, M.; Becher, G.; Struher, G. Angew. Chem. Znt. E d . 1980, 19, 469-470. (17) Mannschreck, A,; Andert, D.; Eigelsperger, A,; Gmahl, E.; Buchner, H. Chromatographia 1988, 2 5 , 182- 188. (18) Boehme, W.; Wagner, G.; Oehme. U. Anal. Chem. 1982, 5 4 , 709-711. (19) Reitsma, B. H.; Yeung. E. S. J . Chromatogr. 1986, 362, 353-362. (20) Reitsma, 8.H.; Yeung. E. S. Anal. Chem. 1987. 5 9 , 1059-1061. (21) Wainer, I. W.; Doyle, T. D. Liq. Chromatogr. HPLC Mag. 1984, 2 , 88-92. (22) Ariens, E. J. I n Stereochemistry and Biological Activify of Drugs; Ariens. E. J., Soudjin, W., Timmermans, P. M. W. M . , Eds.; Blackwell: Oxford, U.K.. 1983;pp 11-32. (23) Sykes, G. A. Chemistry in Britain 1988, 24, 235-244. (24) Goodall, D. M.; Norton, I . T. Acc. Chem. Res. 1987. 2 0 , 59-65.
Anal. Chem. 1989, 6 1 , 1243-1248 (25) Kysllka, J.; Blchsel, S. E. In Proceedings of the 1984 Sugar Processing Research Conference, Oct. 16-18, New Orleans, LA; U S . Department of Agriculture, Agriculture Research Service ARS-49, 1986. (26) Felton, H. J . Chromatogr. Sci. 1969, 7 , 13-16. (27) Goodall, D. M.; Cross, M. T. Rev. Sci. Instrum. 1975, 4 6 , 391-397. (28) Clayton, G. B. Operational Ampifiers, 2nd ed.; Butterworths: London, 1979; p 76. (29) Alkemade, C. Th. J.; Snellman, W.; Boutllier, 0.D.; Pollard, B. D.; Wlnedorfer, J. D.; Chester, T. L.; Omnetto, N. Spectrochim. Acta 1978, 338,383-399. (30) Djerassi, C. J. Optical Rotatory Dispersion; McGraw-Hill: New York, 1960.
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(31) Knox, J. H.; Done, J. N.; Fell, A. F.; Gilbert, M. T.; Pryde, A,; Wall. R. A. High Performance Liquid Chromatography; Edinburgh University Press: Edinburgh, U.K., 1977.
RECEIVED for review July 5,1988. Accepted February 27,1989. This research was supported by the University of York’s Research and Innovation fund and by the British Technology Group, who have also made patent applications concerning this detector.
Relative Elemental Responses for Laser Ablation-Inductively Coupled Plasma Mass Spectrometry James W. Hager
S C I E X , 55 Glen Cameron Road, Thornhill, Ontario L 3 T 1P2,Canada
A method for determlnlng relative elemental response factors for laser ablation-Inductively coupled plasma mass spectrometry Is described. The model uses response factors determined from solution nebuilzatlon and modHles them based on the element-dependent volatillzation effIclencles, whlch can be calculated or determined empirically. Comparisons between observed and calculated relative responses for Qswltched and free runnlng laser ablatlon of steel, copper, and aluminum standards are presented. I n general, the agreement Is wlthln approximately f50 %. The implications of this approach for semiquantitatlve analysls of solids for which standards are not available are discussed.
INTRODUCTION The ability of focused laser radiation to volatilize virtually any material has provided the analytical chemist with a versatile method of direct solid sampling for subsequent analysis (1). Within the last several years laser solid sampling has been combined with analytical techniques such as atomic absorption (2),microwave-induced plasma atomic emision (3), dc plasma atomic emission (4-6), inductively coupled plasma atomic emission (7), and inductively coupled plasma mass spectrometry (ICP-MS) (49)with varying degrees of success. Because of the diversity of these atomic spectroscopic techniques and the wide range of laser characteristics employed, it has been difficult to assess the degree to which laser ablation competes with other techniques for direct solid analysis. A recent comparison by Arrowsmith (8)however suggests that laser sampling ICP-MS is characterized by comparable or better detection limits than several conventional techniques. The reasons behind the proliferation of laser ablation sampling are clear. Minimum sample preparation, a reduction of injected solvent, and microprobe capabilities have driven the development of this versatile technique. Furthermore, laser volatilization can be used to sample a wide variety of materials including conductors, semiconductors, superconductors, and dielectrics. Considering the good detection limits reported for laser ablation-ICP-MS and the range of solid types that can be sampled, it appears that laser solid sampling has secured a permanent place on the list of sample introduction techniques for atomic spectroscopy. There are, however, potential difficulties associated with laser ablation techniques. Previous investigators have shown
that the sensitivity of laser sampling with ICP-MS detection is dependent on the mode of operation of the laser (long or short pulse duration) and physical properties of the solid (8). A degree of elemental selectivity in the laser volatilization process, leading to relative elemental response factors differing significantly from unity, has also been observed (8). This presents the question: to what extent are the signals measured at the detector truly representative of the elemental composition of the solid? Ideally, one would like to be able to relate the analytical signals to the material properties, the elemental response factors determined from solution nebulization, and the laser sampling conditions in a comprehensive fashion. In this publication, a model of laser solid sampling is presented and used to explain some recent laser ablation ICP-MS experimental results. Such a model is potentially of great utility in helping to understand the important experimental parameters in the sampling step. Since the laser solid sampling in laser ablation-ICP-MS is separated from the atomization and ionization steps of the ICP, it is possible to characterize the efficiency and elemental selectivity of the sampling process itself and convolute these characteristics with those of the spectroscopic technique to obtain an overall picture of the analytical method. The goal is to obtain standardless full elemental analysis with a minimum of sample information.
THEORY The model described here is based on laser-induced heating of an opaque surface producing a phase change of the material. For even a simple treatment of such complex phenomena it is necessary to include properties of the laser radiation, the solid material, and the laser beam-solid interaction. A review of previous approaches can be found in ref 1. From the outset, it is assumed that the infrared laser light incident on the sample surface is not attenuation by any vaporized particulate matter or any laser-produced plasma. The absorbed energy is considered to be instantaneously converted into heat where the laser beam is incident on the sample. At infrared wavelengths this process occurs rapidly with respect to nanosecond or longer laser pulses (10). A further assumption is that local thermodynamic equilibrium is established during the pulse, consequently the concept of temperature is valid, and the usual heat flow equations can be applied. One can think of the laser as a heat source with a characteristic temporal and spatial extent with the amount of heat actually coupled into the solid being dependent on the
0003-2700/89/0361-1243$01.50/0 0 1989 American Chemical Society
