Anal. Chem. 1985, 57, 2153-2155
and concentrated as an amine by liquid-liquid or solid-phase extraction. It also appears that various other reagents could produce equally interesting E1 mass spectra with the desired characteristics, such as tert-butyldimethylsilyl ethers and esters frequently employed in GC/MS because their derivatives produce intense M+ - 57 ions (9,lO). ACKNOWLEDGMENT The stimulating technical discussions over the years with Phillip T. Funke are gratefully acknowledged by A.I.C. LITERATURE CITED (1) Funke, P. T.; Ivashkiv, E.; Malley, M. F.; Cohen, A. 1. Anal. Chem. 1980, 52, 1086-1089. (2) Cohen, A. I.;Devlln, R. G.; Ivashkiv, E.; Funke, P. T.; McCormlck, T. J . fharm. Sci. 1982, 71, 1251-1256. (3) Ivashkiv, E.; McKinstry, D. N.; Cohen, A. I.J. fharm. Sci. 1984, 73, 1 1 13-1 1 17.
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(4) Funke, P. T.; Malley, M. F.; Ivashkiv, E.; Cohen, A. I. J. fharm. Sci. 1978, 6 7 , 653-657. (5) Cohen, A. I.; Ivashkiv. E.; McCorrnick, T.; McKinstry, 0. N. J. fharm. Sci. 1984, 73, 1493-1495. (8) Cohen, A. I.; Deviin, R. G.; Ivashkiv, E.; Funke, P. T.; McCormick, T. J. fharm. Sci. 1984, 73, 1571-1575. (7) Black, H. T. AMrichimica Acta 1983, 76 (l),3-10. (8) Carlin, J. R.; Walker, A. W.; Dauies, R. 0.; Ferguson, R. T.; VandenHeuvel, W. J. A. J. fharm. Sci. 1980, 6 9 , 1 1 11-1 115. (9) Mawhlnney, T. P.; Madson, M, A. J . Org. Chem. 1982, 47,
(io)
3336-3339. Bazan, A. C.; Knapp, D. R. J. Chromatogr. 1982, 236, 201-207.
Allen I. Cohen* Mohammed Jemal The Squibb Institute for Medical Research P.O. Box 191 New Brunswick, New Jersey 08903 RECEIVED for review April 5 , 1985. Accepted May 20, 1985.
AIDS FOR ANALYTICAL CHEMISTS Differential Thermal Lens Liquid Chromatography Detector Teng-Ke Joseph P a n g and Michael D. Morris* Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109 Thermal lens spectroscopy and variants on that technique have drawn increasing attention for low absorbance solution phase absorption measurements. Thermal lens measurements appear especially promising for detection in liquid chromatographic (1-7) and flow injection analysis (8,9) systems since strong signals can be obtained with nanoliter working volumes (10). One-laser designs are attractive for chromatography, because they minimize initial alignment and stability problems. Systems which employ lock-in amplifier extraction of the thermal lens signal are especially appealing, since they combine excellent real-time performance with ease of use. Several such thermal lens systems have been proposed. They include extraction of the thermal lens signal at the harmonic of the modulation frequency (6),formation of probe beams as orthogonally polarized fractions of the same laser beam (7,11) and splitting a laser beam into heating and probe fractions and operating them at a slight crossing angle (8). Mobile phase flow fluctuations have been recognized as an important source of excess noise in thermal lens (I, 3,4,12) and photoacoustic (13) liquid chromatographic detectors. Thus, compensation for this effect is one means to improved performance. Cylindrical flow cells in which laser beam propagates perpendicular to the cylinder axis reshape the laser beam from a circular to an elliptical outline (6, 7). This geometry can be avoided in extracolumn detectors but is inevitable in oncolumn detector systems. Compensation for ellipticity is desirable. Full utilization of thermal lens detectors will usually require some means of solvent absorbance background correction. Water, which is a component of most LC solvent systems, is relatively opaque. The absorbance natural log scale minimum is about 1.5 X 10-4/cm in the 450-nm region. However, the abosrbance increases to about 5 x 10-3/cm at 250 nm and to 3 X 10-2/cm at 200 nm (14). The differential thermal lens technique devised by Dovichi and Harris (15) is a useful approach to compensation for all
of these effects. The series cell configuration efficiently uses laser power and introduces no extra alignment problems. However, this technique cannot be directly applied to LC systems. That application requires tightly focused lasers, with confocal parameters of less than 1mm. Thick cell walls and the supporting structures of the system make minimum cell separations far too great. Straightforward application of the ABCD law (16) demonstrates that any point at distance d before the input lens of a 1:1 telescope is translated the same distance d beyond the output lens. For a telescope comprised of two lenses of local length f, the translation is a total distance 2d + 2f. This translation is independent of d or f, which can take any convenient values. Thus, insertion of a 1:ltelescope between sample and reference cells provides a way to utilize the series differential thermal lens measurement technique in a chromatographic system. We have modified the Dovichi and Harris design by including a 1:l telescope between the sample and reference cells to translate the conjugate points far enough from the sample cell to allow convenient placement and adjustment of a reference cell. This modified system provides increased signal, increased signal/noise ratio and efficient solvent absorbance compensation. Although we have used a single laser thermal lens system, the principle is equally applicable to any system employing collinear heating and probe beams, provided that a relay telescope constructed of achromatic elements is employed. EXPERIMENTAL SECTION The thermal lens apparatus is a modification of the system used for second harmonic detection (6). A block diagram of the instrument is shown as Figure 1. An argon ion laser (Lexel85-1, 458 nm, P = 70 mW) was used. The telescope, matched achromatic 73 mm focal length lenses L2and L3,was inserted between the sample and reference flow cells. The cells were positioned approximately 90 mm from the telescope lenses. The sample cell was positioned approximately 30 mm beyond the focusing lens, L1. The final condensing lens,
0003-2700/85/0357-2153501.50/00 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 11, SEPTEMBER 1985
MI
N o absorbance cornpensotion
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n
M
SAMPLE CELL
REFERENCE CELL
ND
PHOTODlODf
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m
I 0
100
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,
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Flgure 1. Thermal lens detector: S.L., sampllng loop: M,, M,, mlrrors; L,, 30 mm focal length lens: L,, L,, 73 mm focal length lens: L,, 100
mm focal length lens; N.D., neutral density filter.
L,, was placed approximately 150 mm beyond the reference cell. Both cells were the straight-through capillary design used previously. A 0.6 mm diameter optical fiber served as limiting aperture and to relay the signal to the detector photodiode (EG&G,DT-25). A mechanical chopper (Laser Precision CTX-534, 6 aperture blade) modulated the laser at 315 Hz. A PARC 186A lock-in amplifier was used to extract (1s time constant) the thermal lens signal. The sample and reference cells were connected to 4.6 X 250 mm stainless steel chromatographic columns with bonded stationary phases on 10-pmparticles. The working column employed RP-18 on Lichrosorb. Both columns were supplied with mobile phase through a tee from the same pump. A needle valve was used to balance the mobile phase velocities at 1.0 mL/min. The mobile phase was 80% methanol (Burdick and Jackson). The test substrate was o-nitroaniline (Aldrich). Samples were introduced to the system through a 1-pL sampling loop. The system was aligned by positioning lens L1 and L4 in the laser beam at approximately their desired positions and centering them. Next, L2 and L3 were inserted in the optical train and adjusted to provide unit magnification. The thermal lens response was then optimized by translating L1 and with only the sample cell in the optical train. Then the reference cell was inserted in the train and adjusted so that a null signal was obtained when sample and reference cells were filled with solvent and, simultaneously, the dc photocurrent on the photodiode was maximized. Because it was necessary to use a 6 aperture chopper blade which generates a trapezoidal modulation of the laser beam, the second harmonic signal generated from this modulating function was subtracted from the observed signals. Independent measurementsof the second harmonic signal generated by the chopper were made periodically and at various values of photocurrent with no cells in the optical train in order to make the proper correction.
RESULTS AND DISCUSSION Figure 2 shows typical chromatograms taken with the single cell and with differential configurations. In the two-cell system the solvent absorbance signal is reduced from about 0.07 yA to about 0.0025 MA. Although the dc photocurrent is reduced from about 220 MAto about 170 MA,the thermal lens response increases from 0.075 MAto about 0.155 MA. Simultaneously the noise is reduced from about 0.004 FA to about 0.002 pA. The chopper (trapezoidal) contribution to the observed signal has been subtracted from the signals shown in Figure 2. A series of measurements verified that this chopper-induced signal was proportional to dc photocurrent. The chopper contribution to the second harmonic signal is 0.4176 FA per 100 MAof dc photocurrent. Attempts to reduce this signal by use of a two-aperture blade were unsuccessful, since the chopper was unstable when operated at the high rotation rates
Absorbance cornpensotfon
0 160 0 150
0 140 0 130 n 0120
6
t F p
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E 0060 0050
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Performance of differential thermal lens detector: flow condltlons, 1.0 mL/min, 8 0 % methanol: sample, 22.5 ng of o-nltroanlline; laser, Ar', 458 nm, 70 m W upper curve, one cell; lower curve, two cell differential configuration. Flgure 2.
necessary to generate a 315-Hz modulation. Solvent absorbance compensation ranges from 95% to 98%. In the example shown, the absorbance background is reduced from a value of about 93% of the peak chromatographic signal with a single cell to about 1.7% of the peak chromatographic signal with the differential configuration. This reduction represents a compensation of 98% of the solvent absorbance. Similar performance has been reported by Dovichi and Harris (13) for the basic differential thermal lens system. The %fold increase in thermal lens signal is a result of compensation for the strong cylinder lens effect of the capillary cell. As observed earlier (6, 7),the beam after the sample cell is elliptical, with the major axis parallel to the capillary. The presence of the second capillary cell cancels this effect and transforms the beam back to a circular shape. The corrected beam overflows the circular limiting aperture equally in all radial directions, doubling the detected fractional change in laser power. The signal/noise ratio is increased by a factor of 2.5-4 in the two-cell system. The reduced noise comes from partial compensation of flow fluctuation (1,3 , 4 , 6 , 7, 12, 13) effects. Very good alignment of both cells in the optical path is required to maximize the noise reduction effects. Both cells were constructed to the same design and affect mobile phase flow similarly. Flow patterns in the two cells are similar if mobile phase velocities are equal. Thus, flowinduced aberrations in the thermal lens should also cancel, and generate a quieter signal. A similar effect has been observed by Betz and Nikelly (17 ) in dual beam UV absorbance detectors. They also observed reduced noise when sample and
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reference cell flow rates were equalized. Unless cells can be perfectly matched and mobile phase velocities can be exactly equal, complete cancellation is not expected. The dc signal is reduced by 23% when the reference cell was inserted in the optical path. Reflection losses at the surfaces of the second cell account for an attenuation of 15%. The remaining 8% is attributed to the change in beam shape and cancellation of the extra focusing caused by the cylindrical sample cell.
LITERATURE CITED (1) (2) (3) (4)
Leach, R. A.; Harris, J. M. J . Chromafogr. 1981, 278, 15-19. Woodruff, S. D.; Yeung, E. S. Anal. Chem. 1982, 54, 1174-1178. Buffett, C. E.; Morris, M. D. Anal. Chem. 1082, 54, 1824-1825. Buffett, C. E.; Morrls, M. D. Anal. Chem. 1983, 55,376-378.
(5) Sepaniak, M. J.; Vargo, J. D.; Kettler, C. N.; Maskarinec, M. P. Anal. Chem. 1984, 56, 1252-1257. (6) Pang, T.-K. J.; Morris, M. D. Anal. Chem. 1984, 56, 1467-1469. (7) Pang, T.-K. J.; Morris, M. D. Appl. Specfrosc. 1985, 39, 90-93. (8) Yang, Y.; Hairrell, R. E. Anal. Chem. 1084, 56,3002-3004. (9) Leach, R. A.; Harris, J. M. Anal. Chlm. Acta 1084, 164, 91-101. (101 Carter. C . A.: Harris. J. M. Anal. Chem. 1984. 56. 922-925. iiij Y a n g , ’ ~Anal. . Chem. 1984, 56. 2336-2338. (12) Dovlchi, N. J.: Harris, J. M. Anal. Chem. 1981, 53,689-692. (13) Oda, S.; Sawada, T. Anal. Chem. 1081, 53,471-474. (14) Smith, R. C.; Baker K. S. Appl. Opt. 1081, 20, 177-184. (15) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1080, 52,2338-2342. (16) Yarlv, Amnon, “Introduction to Optical Electronics”, 2nd ed.; Holt, Rinehart and Wlnston: New York, 1976; pp 18-28. (17) Betz, J. M.; Nikelly, J. G. J . Chromafogr. Sci. 1983, 27, 478-479.
RECEIVED for review March 28,1985. Accepted May 6,1985.
Volumetric Dilutor: Design and Testing of a Passive Mixer John R. Wallace* and Russell A. Nye Denver Research Institute, Chemical and Materials Sciences, 2390 South York Street, Denver, Colorado 80208 The dilution of gas samples is an important analytical operation. For example, a stable, concentrated gas standard may be kept in a pressurized bottle and then diluted immediately prior to use to a less stable dilute mixture. Similarly, prior to analysis, saturated gas samples from chemical or petroleum plants may require dilution to lower the dew point below ambient temperatures or to decrease the analyte concentration to within the dynamic range of the analyzer. In many such applications it is necessary to produce a continuously flowing, diluted stream in order to meet the requirements of the analyzers and to prevent sample loss. However, the dynamic dilution of a gas sample is not easily accomplished using typical gas metering equipment. The calibration of rotameters, mass flowmeters, critical orifices, and orifice plates all depend on the composition of the gas, which in the case of process streams is often unknown and variable. In addition, such devices often contain restricting orifices which can easily become clogged and fouled by condensable material or entrained particles, or components which are easily coorroded by reactive gases ( I , 2). One method to avoid such difficulties is to repeatedly inject a fixed volume of the unknown gas into a flowing diluent gas stream. This method is independent of the composition of the unknown gas and can be calibrated against primary gravimetric standards. The principal difficulty with this method, referred to here as volumetric dilution, is that each injection results in a sharp spike in concentration, followed by a period of zero concentration. Thus, a mixing chamber is required downstream from the injection to average the concentration. The difficulty that arises from such a mixing chamber is best illustrated with a specific example: Consider a vqlumetric dilutor consisting of a 10-L mixing chamber into hhich is injected 1.0 mL of sample ten times per minute along with 1.0 L min-l of diluent at a constant rate. In this example the dilution factor is 100, so that if the original sample contains 10% by volume of the analyte, the diluted sample contains an average of lo00 ppmv (parts per million by volume). With each injection, the concentration increases as a step function by 10 ppmv (Le., 1% relative) and then decays exponentially until the next injection (referred to here as “ripple”). If the concentration in the sample suddenly changes, the concentration in the flask responds exponentially with a time constant of 10 min-’. Forty-six minutes are thus required to achieve 99% of the change. This response time can be 0003-2700/85/0357-2 155$01.50/0
shortened by decreasing the volume of the flask, but at the cost of increasing the ripple. Increasing the injection frequency would decrease the ripple but is limited by the longevity of the valve and the pneumatics of injecting the sample. A single mixing chamber thus results in a generally unsatisfactory compromise between ripple, response time, valve longevity, and size. It is thus the purpose of this effort to design a mixing device with improved performance. As shown below, dramatic improvement can be achieved by simply dividing the mixing chamber into a series of subchambers.
THEORY Consider a chamber of total volume V divided into N subchambers of equal volume vi = V / N . The sample of mass m is injected instantaneously at time t = 0 into the first chamber along with the diluent (assumed incompressible) at constant volumetric rate Q. The gas mixture flows serially from one chamber to the next. For this arrangement the time evolution of the concentration in the last chamber is described as a function of reduced time, 9 = tQ/V = a t / N (3)
Equation 1 is normalized in the sense that
Equation 1 has a maximum at 0 = 1 - (l/iV), and as N becomes large, it approaches a Gaussian peak with standard deviation l/Wlz. Consider now a series of injections each of mass m occurring with period 6t, or in reduced time with a period 7 = 6tQ/ V. After n + 1 injections, the time can be expressed as 0 = n7 + 8‘, where 9’ is the time since the last injection. Then the concentrationis given by summing eq 1,starting with the most recent injection. The result of this summation can be expressed as the variation about the average concentration, (C) = m/rV
where yi = N(0’ + i~). 0 1985 American Chemlcal Soclety
