Anal. Chem. 1990, 62, 1543-1545
1543
Linearity Considerations for a Near-Infrared Laser Diode Intracavity Absorption Spectrophotometer James Hicks and Gabor Patonay* Georgia State University, Department of Chemistry, Atlanta, Georgia 30303
INTRODUCTION Lasers are almost ideal illumination sources for spectroscopic applications that require high-power monochromatic radiation ( 1 , 2 ) . Several applications of lasers can be cited to illustrate their general utility for analytical measurements (3-6). For example, laser intracavity spectrophotometry has proven to be a very sensitive method for absorbance measurements (6). Laser diodes, which are a source of coherent, monochromatic light, are also finding increasing use in analytical chemistry (7,8). Their most important advantages are low power consumption, low cost, and fast response (9). In an earlier paper, a simple and inexpensive near-infrared laser diode intracavity spectrophotometer (LDIS) was reported (IO). This instrument uses the built-in PIN photodiode of a laser diode for measurement of the analytically useful signal. In this configuration, the analyte was placed in the extended optical cavity. This single-beam laser diode spectrophotometer used a 780-nm laser diode and could detect absorbencies better than many commercial double-beam spectrophotometers. Although the operation and the feasibility of the instrument has been demonstrated (IO, I I ) for detecting near-infrared (NIR) dyes, no theoretical explanation of the behavior of the system has been given. In this note, we present a theoretical description of the operation of the near-infrared laser diode intracavity spectrophotometer developed in our laboratory (IO). The optical arrangement is described using a doublecavity gain-loss approximation. Use of this model permits evaluation of the system and determination of linearity limits. This theory is compared to experimentally derived data.
THEORY A mathematical description of the laser diode intracavity absorption process is different from that of a dye laser (12). However, due to the broad-band absorbance of most analytes at room temperature, the mode competition is negligible. We are concerned here with the PIN diode signal as a function of the analyte absorbency. The mathematical model that follows applies to systems in which the absorption of the analyte in the external cavity is low, preferably less than 0.1 absorbance units. In our previously described configuration (IO), the back facet mirror serves as the back mirror of the extended optical cavity. With this double-cavity gain-loss configuration, the reflectance of the front mirror of the laser diode must be taken into consideration. It has been shown (9) that if there is no external optical feedback present, the slope efficiency of the laser diode can be expressed as
7 = yi
+ (Yb + y f ) / 2
where yi is the internal logarithmic loss and yb and yf are the respective logarithmic losses per pass due to back and front facet mirror transmission. If an external mirror is placed facing the front facet of the laser diode so that the laser beam is reflected back to the active layer (IO),the apparent logarithmic loss per pass due to front facet mirror transmission will decrease, resulting in increased laser output and slope efficiency. This will result in an increased back radiated output as measured by the built-in PIN photodiode. Since y = -In R, where R is the reflectance of the facet mirrors, we can determine the change in back radiated output due to the external mirror. For LDIS, we can write y = a1 - (In
Rb + In R f ) / 2
(3)
wherea is the semiconductor attenuation coefficient arising from internal losses and 1 is the length of the semiconductor cavity. If we assume that the external mirror reflects 99% of the emitted light back to the front facet of the laser diode (assuming 1%loss due to transmission loss of collimating optics, cuvette wall, etc.), the apparent value of y would be larger. The apparent Rf will be
R f = Te2T?Rex
(4)
where T,,is the transmittance of the external cavity, Re, is the reflectivity of the external mirror, and Tf is the transmittance of the front facet of the laser diode. In our case, Re, 1 (practically perfect mirror); then
R f = Te,2Tf2
(5)
where T,, is determined by the external optics (laser diode exit window, focusing lens, cuvette, and anal*). For practical purposes, we can assume that this value for LDIS is determined by the analyte. From eq 3, we obtain y = a1 - (In R b ) / 2 - In
(Te,Tf)
(6)
Since the back facet of the laser diode is highly reflective for medium-power or antireflective (AR) coated laser diodes (&, > 0.9), we get -In
Rb
= -In (1 - T b ) E Tb
(7)
From eq 6, we obtain y = al T b / 2 - In (T,,Tf) =
+
a1
where P, is the output power through one mirror, V is the power supply voltage, I is the power supply current, qi is the internal quantum efficiency, e is the electron charge, ym is the logarithmic loss per pass due to mirror transmission (y = -In (1- T),where T i s the transmission), and y is the total loss per pass (9). We should note that (hv/eV) is usually slightly less than unity due to a small voltage drop in the diode bulk resistance. For out NIR laser diode intracavity configuration, we can define a total loss per pass y as
(2)
+ T b / 2 - In Tf + In T,, (8)
For low analyte concentrations (absorbancies less than 0.05-0.1), we can write -In Te, = 1- T,,. In eq 8, a,1, Tband In Tf are constant for a given laser diode, and we can combine all constants in eq 8 into a single constant, p,+ We shall call the Pd term the characteristic laser diode loss. Thus, eq 8 becomes y = Pd
+ Tex
(9)
The expression for the output power through the back facet mirror is
* Author to whom correspondence should be addressed. 0003-2700/90/0382-1543$02.50/00 1990 American Chemical Society
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ANALYTICAL CHEMISTRY,
VOL. 62,NO. 14,JULY 15, 1990
where I is the operating current and In is the threshold current for the laser diode (9). From eqs 9 and 10, we obtain the following expression of the output power through the back facet mirror of the laser diode:
SOLUTION
TANK
gALV
NASTE SOLVENT
For low concentrations and with use of the first element in the Taylor expansion series, eq 11 becomes LASER BEAP
The slope efficiency of the laser diode is then obtained from (12) as the first derivative:
FLOW-THROUGH CUVETTE C E L L
Figure 1. Schematic diagram of the flow-through system. 800
There are several interesting inferences that can be derived from eqs 12 and 13. One consequences is that the output of the LDIS is essentially linear over the concentration region where absorbance is low because at low absorbancies T,, = 1 - 2.3A or T,, = 1 - 2.3cbc, where A is the absorbance, E is the molar absorptivity, b the path length, and c is the concentration. In addition, if the back-radiated light output power is used as an analytical signal, the sensitivity of the method should be dependent on the (I - Ith) difference, i.e., how far we operate the laser diode from the threshold. The sensitivity of the method can be determined from dPb/dT,,: dpb
s=--
dTex
Vihv I - I t h 2
e
Tb ( ~ d
1)'
(14)
If the slope efficiency change is utilized as the analytical signal, the sensitivity can be determined from dqa/dTex:
Another and maybe the most important practical consequence is that we can measure p b directly using the built in PIN diode. Since p b is directly determined by Tex(the analyte transmittance for low absorbancies), the output signal of the PIN diode will be a linear function of the absorption (and the concentration) of the analyte. Using eq 14, we see that the sensitivity of the method will be higher if the laser diode is operated at higher laser intensity, i.e., the I - Ith value is larger. Obviously, the absolute maximum rating of the laser diode determines the maximum driving current. On the other hand, if the change in slope efficiency is used as an analytical signal, the sensitivity of that method will not be dependent on the operating current of the laser diode, as is shown in eq 15. Also, as is evident from eq 14, individual laser diode parameters determine the sensitivity of the LDIS. For example, the lower the characteristic laser diode loss ( p d ) or the higher the maximum operating current (I)of the laser diode, the higher the sensitivity of the LDIS. These and other previously discussed factors provide many advantages for the use of antireflective (AR) coated laser diodes. The AR coated laser diodes have special X/4 coatings to minimize front facet reflections. Finally, we should note that according to eqs 10-15 the development of shorter wavelength output laser diodes may be advantageous for LDIS applications.
EXPERIMENTAL SECTION The optical arrangement used in these studies has been previously described (IO). The 30-mW 780-nm laser diode (LT024MFO) used in this study was obtained from Sharp. A gradient index lens was used to collimate the laser beam. The
-
ID
I
I
229 *
'
4
b
4
9 030 9 CO00'
03044
00086
00129
CONC
00'72 ( Y 10-4)
00215
00257
00100
Figure 2. Linearity limit of a laser diode intracavity spectrophotometer. (Molar absorptivity for HDITCP: 1.69 X lo5,which results in 0.08 AU
linearity limit). beam was directed through a 1-cm semimicro flow-through cell (59 FL, NSG Precision Cells, Inc.) to a reflecting mirror as previously described (IO). This arrangement uses optical feedback generated by the reflected laser beam. However, only a portion of this light enters the laser chip, while the other portion is reflected by the front face of the laser diode chip. Maximum sensitivity can be achieved, according to eqs 14 and 15, if the optical feedback is maximized, Le., pd is minimized. The effect of optical feedback may be determined by the change in the threshold current. Since the threshold current is representative of the extent of the optical feedback, it is used to optimize the system. The Kodak laser dye HDITCP (1,1',3,3,3',3'-hexamethylindotricarbocyanine perchlorate) was chosen to characterize the system. In our system, shown in Figure 1, the flow-through cell was fed via a gravity feed reservoir mounted on the optical breadboard. In this manner, mechanical noise generated by use of a pump is eliminated. Several HDITCP solutions of different concentrations were prepared freshly in acidic methanol. Methanol used in this study was of ACS grade. Stock solutions were prepared by disM solution. Samples solving HDITCP in methanol to make a were prepared by pipetting appropriate amounts of dye stock solution into 50-mL volumetric flasks and diluting with acidic methanol. Further dilutions were prepared by using diluted solutions. Absorption spectra of less diluted solutions were taken on a Varian DMS 200 UV-vis spectrometer to determine molar absorptivities and check dye stability.
RESULTS AND DISCUSSION The performance of the system was evaluated by generating calibration curves using 12-15 solutions of HDITCP a t different concentrations. A linear calibration curve was obtained for concentrations less than 5 X lo-' M. Significant deviation from linear behavior is observed above this concentration in accordance with theory discussed above (Figure 2). Measurements were performed while solution was flowing through the cell at about 0.5 mL/min. This flow rate was maintained to minimize bleaching of the dye by the strong laser radiation.
Anal. Chem. 1990, 62, 1545-1547
Although HDITCP does not show bleaching under laser diode radiation, the flow through system is more advantageous if other, more sensitive dyes are used. Previous work with NIR laser dyes has shown the possibility of bleaching, especially in aqueous solutions (7). The flow operation of this system permits cleaning and solution replacement so that the cell need not be removed from the optical path between measurement of different solutions. Thus, optical alignment of the system is easier. The data presented in Figure 2 are indicative the multipass effect, Le., a more than 50% change is observed in the PIN diode signal at C0.06 absorption units. The highest absorbance that could be detected without compromising linearity was 0.08 AU, in good agreement with the theoretical calculations. The ( I - Ith)dependence of the sensitivity of the method was studied earlier. It was found (11)that the sensitivity of the LDIS does depend on how far the system is operated from the threshold current as predicted by eq 12. This indicates that individual laser diode parameters can influence sensitivity. Naturally, the absolute maximum rating of the recommended electrical characteristics of a particular laser diode will determine the maximum operating current and therefore the sensitivity. In conclusion, we have shown that a simple laser diode intracavity device that may be simply constructed will operate reliably and produce a linear output signal without using an external detector. Further studies will be necessary to determine the dependence of average effective optical path length on the reflectance of the front face mirror of the laser diode chip. It is expected that the best results can be obtained by using antireflective coated laser diode chips (eq 8 refers
1545
to losses from front facet mirror reflectance). These results may prove useful in HPLC, where NIR dyes could be used to label compounds and selectivelydetect them in the presence of a complex matrix. This process could be effectively used to minimize background interference. NIR dyes and laser diodes may also prove to be useful in the determination of biologically active molecules if NIR dyes are used as labels, where the inherently low interference of the NIR spectral region should improve detection because of higher detection power and higher selectivity. Higher detection power arises from the laser intracavity detection method and the high molar absorptivity of NIR dye labels. Higher selectivity is a direct result of the low interference of the NIR spectral region.
LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
Harris, J. M.; Dovichi, N. J. Anal. Chem. 1980, 52,695A. Bennett, H. S.; Forman, R. A. Appl. Opt. 1977, 76, 2834. Tran, C. D. Anal. Chem. 1988, 6 0 , 182. Kawabata, Y.; Imasaka, T.; Ishibashi, N. Talanta, 1986, 33, 281. Bialkowski, S. E.; He, 2 . F. Anal. Chem. 1988, 6 0 , 2674. Harris, T.; Mitchell, J.; Shirk, J. S. Anal. Chem. 1980, 52, 1701. Sauda, K.; Imasaka, T.; Ishibashi, N. Anal. Chem. 1986, 58, 2649. Imasaka, T.; Tsukamoto, A.; Ishibashi, N. Anal. Chem. 1989, 67, 2285. Svelto, 0. Principles of Lasers; Plenum Press: New York, 1989. Unger, E.; Patonay, G. Anal. Chem. 1989, 67, 1425. Hicks, J.; Patonay, G. Anal. Instrum. 1989, 18, 213. Harris, T. D.; Mitchell, J. W.Anal. Chem. 1980, 52, 1706.
RECEIVEDfor review December 13,1989. Accepted March 26, 1990. This work was supported in part by a grant from the National Science Foundation (CHE-8920456). Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Device for Subambient Temperature Control in Liquid Chromatography Lane C. Sander* and Neal E. Craft National Institute of Standards and Technology, Center for Analytical Chemistry, Gaithersburg, Maryland 20899 Although regulation of column temperature in liquid chromatography is less common than in gas chromatography, temperature remains an important separations parameter. In reversed-phase liquid chromatography, as with gas chromatography, absolute retention varies invenely with temperature (In (123 However, unlike gas chromatography, retention in LC is more conveniently adjusted by altering the mobile-phase composition. For this reason, the potential benefits of temperature control in liquid chromatography have gone largely unnoticed. Separations are commonly carried out under ambient conditions, although greater reproducibility usually results when isothermal conditions are maintained (1). In addition, the use of elevated column temperature has been reported as a way of increasing separation efficiency (2), particularly for large molecular weight solutes (3). Recently, changes in column selectivity that occur at subambient temperatures have been reported ( 4 ) . Enhanced shape recognition was observed among isomeric polycyclic aromatic hydrocarbons (PAHs) a t reduced temperatures. Changes in selectivity with temperature also have been observed for other classes of solutes, including carotenoid (5) and steroid isomers (6). The variation in column selectivity with temperature is a universal effect that is not specific to column brands or phase type (4). In general, enhanced specificity for isomeric mixtures is usually possible for separations carried out a t reduced temperatures.
l/n.
Commercial instrumentation for column temperature regulation typically consists of an insulated metal block in which elevated temperatures may be achieved by resistive heating. Other designs include forced hot air ovens and column "jackets" through which a fluid is recirculated by a controlled-temperature bath. The latter method has the potential for subambient operation and provides excellent, uniform temperature control. The disadvantages of this approach include difficulty in changing columns, need for a bulky external refrigerated bath, and hazards associated with use of solvents other than water (for temperatures below 0 "C). The thermoelectric design described below and illustrated in Figure 1 eliminates these disadvantages and permits column cooling to approximately -25 "C. In addition, the device can be used for column heating (to approximately 80 "C) by reversing the polarity of the power supply. The column cooler has added benefits of portability and reduced cost compared to refrigerated bath systems. The thermoelectric cooling plate assembly was fabricated to specification by Thermoelectrics Unlimited, Inc. (Wilmington, DE). The unit consists of water cooled thermoelectric heat exchangers attached to an aluminum top plate measuring 0.32 X 3.8 X 30.5 cm X 1.5 X 12 in.). To this plate is attached an aluminum block 2.5 X 3.8 X 30.5 cm (1 X 1.5 X 1 2 in.) machined to accept standard length (25 cm) LC columns (see Figure 1). Shorter column lengths could also
This article not subject to US. Copyright. Published 1990 by the American Chemical Society
