Anal, Chem. 1995, 67,2308-2313
UV Photometer Based on Radioluminescence with Applications in HPLC and Process Control Wnter Rinke* a d Christian HaMg Forschungszentmm Katismhe, Postfach 3640,0-76021Katismhe, Gennany
A new W photometer based on radioluminescence was dqveloped. The light source consists ofa luminous a y a b l which is irradiated by a radioactive element. ’Ihia W source has a lifetime of more than 10 years. A special absorption cell with high light throughput together with a fiber rod was used. Although radioluminescence has a rather weak intensity,it was possible to use an inexpensive and robust silicon photodiode. ’his photometer is especially suited for process control applications, explosive environments, and HPLC because there are no movable parts and because of its good stability. The minimum detectable absorbance, limited by noise, amounts to 1.2 x AU, and the peak form of our special HPLC cell is comparablewith or better tfum those obtained with conventional HPLC cells. The aim of this work was to develop a new W photometer with a W light source having a lifetime of several years. Such instruments are used, for example, as pure photometers or W detectors in high-performance liquidchromatography (HPLC) Because of the long lifetime of the radiation source, the instrument described in this paper is especially suited for process control applications. Common W photometers always use deuterium, mercury, or high-pressure xenon lamps as W radiation sources, which have two major disadvantages. Fiit, the lifetime of these lamps varies between 3 months @Z and Xe lamp) and 1year (Hg lamp). This is too short for process control applications, where the lamp is running all day long. Furthermore, all these lamps show a limited short- and long-time stability, which requires twc-beam optics for compensation. Even if the lamp with the longest lifetime is used (Hg), the application fields are small because only one emission line (254 nm) has a high intensity. Phosphorcoated Hg lamps are not an alternative,firstly because the emission wavelength is longer than 270 nm and secondly because the degradation is by far stronger compared with that of pure Hg lamps. These disadvantages can be avoided if the radioluminescence of special crystals is used a closed radioactive element emits electrons, which are absorbed in a crystal, creating electronhole pairs. This recombination leads to a luminescence in the ultraviolet spectral region. The lifetime of this ‘light source” is determined by the radioactive decay time and can amount to some years or more. The process of radioluminescence is not new. It was used, for example, in the first part of this century in watches containing phosphors and radioactive elements. Furthermore, all glass dosimeters are based on this ptinciple. Some similarity also exists
.
2308 Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
ib television systems containing a phosphorcoated tube, which in this case is activated by an electron gun. The first realization of radioluminescence in analytical instruments was done by K Jones and D. J. MalcomeLawes.1-3 Because of the low W luminescence intensity,they had to employ a photomultiplier tube as optical detector. By using a completely different optical design, we were able to get a robust analytical instrument for process control without using a fragile photomultiplier tube and with good stability. This is very important here, because this detector should be used as part of a process HPLC system.’ BACK~ROUWL)
RadiatimSourceandrumineacent cryscale. The radiation source should ideally emit only electrons but no y-rays to simpw the radiation shielding. In order to get a high W intensity, the activity must be as high as possible, and the electron energy spectrum should have a lot of highaergy particles. On the other hand, the maximum electron energy should be below a few megaelectronvolts in order to keep the bremsstrahlung low. The decay-time should be between 5 and 30years to get a long lifetime and a high specific activity. Last, but not least, the possible nuclides should have a low price and should be easily available. These conditions can be fulfilled by only a few elements, for example, nickel-63, tritium, and strontium-90. Because nickel43 and tritium have low bnergies, we decided to use strontium-90, which has a high benergy, no y-rays, and a satisfactorily long decay time.5 The specifications for good luminescent crystals are (a) high W intensity by electron activation, (b) low UV absorption, and (c) long-term stability. The geometry of the crystal is, in general, less important. However, for crystals that show some W absorption, there exists an optimum thickness, because total absorption of the electrons requires a large thickness, whereas W absorption requires a low thickness in order to get a good optical transmission. Optical Design. The design of the flow cell is the most important part of this new photometer for applications in HPLC. Usually, a HPLC cell has an optical length of 10 mm and an inner diameter of 1mm, so that the cell volume amounts to 10 pL. In HPLC, the volume of the cell has to be low in order to get sharp peaks. The disadvantage of such a HPLC cell lies in its flow ~~~~
~~~
~~
(1) Jones, IC;Malcolme-Law=, D. J. J. Chmmatogr. 1985,329,25-32. (2) Jones, IC; Malcolme-Iawes, D. J. J. Chem. Sac., Famday Trans. 2 1986, 82,665-677. (3) Jones, IC;Malcolme-Iawes,D. J. J. Chmmotogr. 1988,441,387-393. (4) SteinmfiUer, D.I ~ m t aAnalytical l ~ Chmishyand Gmputer Technology (InCom); GlT-Verlag: Dannstadt, 1990; pp 214-224. (5) Horen, D. J.; Ewbank, W. B. Nuclear h e 1 Schemes A = 45 through A = 2 5 7 h m Nuclear Data She&; Academic Press: New York, 1973.
0003-27W/95/0367-239.W/0 Q 1995 American Chemical Society
sensitivity by refractive index changesa7and its low light lransmio sion because of its low solid angle to accept light. A simple consideration shows a possible way to improve such a cell to get a better throughput. The well-known Lambert-Beer law may be written in the form
Table I.Lumlnercent Propertler of Some CyStalS
crystal
CaFz SrFz Azo3
BaFz
Ma204
Caw04 CdWO4 where a is the absorptivity, c,i, the minimum detectable concentration (limit of detection), b the optical length of the cell, and A i , , the minimum detectable absorbance. In order to get a low limit of detection (crmn) ,the length b should be as large as possible, if A k is assumed to be independent of 6. This is the usual way to design absorption cells. On the other hand, the limit of detection is also determined by the noise, which determines A h . If this noise is limited by photon shot noise? which may be reached by good UV detectors?Jo Ad,, is inversely proportional to the square root of the photon flux on the detector; the electrical signal current, Z, of the photodiode is proportional to the photon flux S, and the noise, SZ, of this current is proportional to the square root of the flux S. This means that the signal-to-noise ratio is proportional to the square root of the flux S. Because the absorbance, A, is d e h e d as the logarithm of the transmittance T @ = -log 2),any variation in T,and therefore in I,leads to a minimum detectable absorbance,
Amin= 0.436Z/Z= kS-1/2 where k is a proportionality constant. The light flux S on the detector is proportional to the solid angle Q (S = mQ; m = constant) the cell can transmit. In the simplest optical arrangement without any lenses, Q
w A/b2 = V/b3
where A is the cross section of the HPLC cell, which is assumed here to be smaller than the constyt area of the detector, and V is its volume. For the following approximation, it is important that the cell volume is assumed to be constant (typically 10 pL) in order to get small peaks in HPLC, but A may be varied to get an optimum light throughput. Returning now to the equation for the limit of detection and expressing S by Q, c h becomes
cmh= (~b)-'k(mVb-~)-'/~= db'I2, d = constant
This simple estimation shows two important thiigs: the limit of detection is proportional to b-l, as usual, and it is proportional to because of the lengthdependent miniium detectable a b sorbance. Therefore, the limit of detection cmhdecreases with
bt3I2
b1I2. (6) White, P. C. Analyst 1984, 109,677-697. (7) Stevenson, R L. Liquid Chmmutopaphy Detecton; Marcel Dekker: New York, 1983. (8) Grum, F.; Becherer, R J. Optical Rudiation Measurements, Vol. 1;Academic Press: New York, 1979 (9) Wiese, A; Teitz. IC; Brombacher, V.; Hbchele, G.; Kuderer, H. HewlettPackurd J. 1990,April, 36-43. (10) Yeung, E. S.DetectonfirLiquid Chromatography;JohnWiley & Sons: New York, 1986, Chapter 2.
LiYF4
Znw04 diamond LiFz
MgFz Si02 BaB204 CaC03 KBr
KCI MgO NaCl
21-02
intensity
wavelength region (nm)
high (35 nA) high (35 nA) high (20 a high (15 nA) high (12 nA) medium (8 nA) medium (8 nA) medium (4 nA) medium (4 nA) medium (3 nA) low (1nA) low (1nA) low (1nA)
230-330 240-340 370-450 210-350 220-270 350-450 350-450 300-450 350-450 350-450 200-350 200-300 200-300
no emission no emission no emission no emission no emission no e+ssion no ermssion
200-450 200-450 200-450 200-450 200-450 200-450 200-450
Of course, this is a very crude calculation with many a p proximations,but it shows the way to design a cell which is better suited for low-level applications: the optical length b of this cell should be short and the diameter large. However, the length b has to be larger than a miniium value, which depends of other noise terms, such as electronic noise. This length-independent noise leads to an optimum cell length, which can only be calculated by a better theory, but this goes beyond the scope of this work. Besides electronic noise and photon shot noise, in photometers further noise arises from fluctuations of lamp intensitylOJ1 and small changes of the refractive index in the ~ell.6.~ However, these noise sources do not cause a disturbance in the case described herein, because this light source is quite stable due to the radioactive decay. Furthermore, our cell is insensitive to changes in flow speed and has only to be thermostated in order to eliminate noise caused by remaining refractive index changes. However, these temperature effects are considerably lower than those in conventional photometers using hot discharge lamps. EXPERIMENTAL SECTION
Samples. In order to build a UV lamp based on luminescence with a high intensity and a large spectral emission bandwidth, we examined a lot of crystals, as shown in Table 1. In many cases, we bought crystals of the same type by different manufacturers, because we expected differences depending on impurities. Absorption and Emission MeasurementSetup. We measured the absorption spectra with a commercial dual-beam spectrophotometer (Perkin Elmer, Lambda 9). Because some crystals had a small diameter (diamond (C),for example, was only 4 mm), both measurement beam and reference beam included the same optical aperture. Before a measurement was started, a background correction with both stops was done. In addition to these measurements, all crystals were irradiated for 24 h with the strontium-90 source. Immediately after this excitation, the absorption spectrum was recorded again. In order to measure the luminescence spectra of our crystals, we used a modifled HPLC detector (Shiadzu SPDGA). This (11) Christian, G. D.; Callis,J. B. Trace Analysis; JohnWiley & Sons: New York, 1986.
Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
2309
I
I
4 I
a
b
I
3a
Figure 1. Design of the HPLC cell, top and side view. 1, Inlet and outlet; 2, quartz plates 20 mm x 20 mm; 3, quartz spacer with opening 3a; 4, drillings; b, optical path length; 0,optical diameter of the cell.
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Uwelength (nn)
Flgun 3. Absorption spectra of spinel 1 with (broken line) and without (solid line) preceding electron excitation. Flgure 2. Optical layout of the UV photometer.
instrument is a variable wavelength spectrometer consisting of an entrance slit, a concave collimating mirror, and a diffraction grating. Its absorption cell was replaced by an extra exit slit, the photodiode by a photomultiplier tube (Hamamatsu R 1527), and the original deuterium lamp by a combination of a strontium-90 point source (SIF 33, Amersham, 370 MBq = 10 mCi) and changeable crystals. The output signal of the fist ampliser of this instrument was connected to an external analog-digital converter, which was situated in a personal computer. This PC also created some digital signals to control the wavelength scanning via the back-side interface. The instrument spectral response was measured with the help of a deuterium lamp and a calibrated high-pressure mercury lamp, and all luminescent spectra were corrected for this response. The measured wavelength region was 200-450 nm. Chromatographic Equipment. We used naphthalene in methanol as a test substance, injected by a Rheodyne injection valve (7520) and a 0.5 pL loop. The HPLC pump (Gynkotek 300 C) was used with a flow rate of 0.8 mL/min at a pressure of 48 bar. The column was a 5 pm Cl8 type, and the eluent was a methanol-water mixture (91). Construction of the Prototype. The strontium-90 source was bought from Amersham (SIF 1177). It has an activity of 3.7 GBq (100 mCi) and an active diameter of 5 mm. This source is completely closed and allows temperatures up to lo00 K Other experiments were done with a 1Ci tritium source, but the results were worse compared with those with strontium-90. The reason is the low ,!?-energy of tritium, which could not be compensated by its much higher activity. The h a l design of our flow cell is shown in Figure 1. It consists of two fused quartz plates, each 2 mm thick and with a 20 mm base length, melted together with a special opaque quartz 2310 Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
1 0
R S
8 4
e . @ , 200
,
,
,
,
,
,
,
,
,
300
1
400
,
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,
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wsvelenqth (nm)
Flgun 4. Absorption spectra of calcium fluoride with (broken line) and without (solid line) electron excitation.
plate used as a spacer. This spacer determines the optical path length b by its thickmess. We tested different cells with nearly the same internal volume of 10 pL but with spacers varying between 0.2 and 2 mm thickness, corresponding to an optical diameter between 8 and 2.5 mm. The in- and outlets are small stainless steel tubes. This design not only gives a high optical throughput, but it also has no dead volume and the liquid flows
.
200
.
,
.
.
.
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.
.
.
.
.
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.
.
.
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.
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400
Wavelength Flgure 5. Luminescence spectra of seven different sapphires.
rectangular to the optical axis. As we will show later, this will give excellent peak forms because of the optimized flow profile. During our investigations we noticed an increased noise when we reduced the distance between the B-source and the detector, dependent on the type of detector. As a result, the final solution utilized a fiber-optic rod of 8 mm diameter to guide the light and to increase the distance. Figure 2 shows the complete design. All optical elements are arranged as close together as possible. In the middle part, where the light guide is located, a metal plate serves to divide regions which are located in an explosive environment from nonexplosive regions. This often is necessary in process control. As photodetector, we used a silicon photodiode with an active area of 100 mm2. The signal current of this photodiode is large compared to the cathode current of the photomultipliertube used by Malcolme-Lawes and Jones.' However, it is still small compared to typical signal currents of UV photometers using an electric discharge lamp. Therefore, the preamplifier has to be able to handle currents in the picoampere range without drift and noise. We used two different circuits. One was the first part of a Shimadzu SPD6A HPLC detector, which utilizes the OPA lllBM integrated circuit from Burr Brown. The other version was a modiiied FID amplifier from a Siemens gas chromatograph. Both circuits work as current-to-voltage converters with a typical feedback resistor of 10 GQ and a parallel capacitance of 15 pF to reduce the noise. All optical parts are surrounded by iron and lead to reduce the bremsstrahlung which is created inside the radiation source. The construction also completely elimiiates any high-energy
electrons. It must be emphasized that this detector may not be used without this shielding. RESULTS AND DISCUSSION
Absorption Measurements. All crystals have a sufficient W transmission down to 200 nm except ZrOz, diamond, CaWO4, CdWO4, and ZnWO4. Figure 3 shows the absorption spectra of one spinel (spinel 1) with and without excitation with strontium-90. The absorption bands at 260 and 195 nm are produced by impurities, especially by iron ions. Another spinel (spinel 2) shows a similar absorption spectrum, but without the peak at 260 nm and with only a little change after B-excitation. The increased transmission below 280 nm after B-irradiation of spinel 1can be understood by reduction of Fe3+to Fe2+.12J3The low Fe impurity content is essential for a good conversion. The absorption spectra of seven different sapphires were measured. All show slight differences at the absorption band at 205 nm, which is produced by an F center which varies with the manufacturing process. All the tested BaF2 crystals show not significantly different absorption spectra, and these spectra did not change, even with ,&irradiation. Figure 4 shows the absorption spectra of calcium fluoride with and without electron excitation. Two specimens show increased (12) white, G. S.; Lee, IC H.; Crawford, J. H. Bys.Stat. Solidi 1977,A42, K139K141. (13) White, G. S.;Jones, R V.; Crawford, J. H. 1.Appl. Phys. 1982, 53, 265270.
Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
2311
x . U 3
i
E
CI H E
Wavelength
Figure 6. Luminescence spectra of the most interesting crystals: 1, BaF2; 2, MgA1204; 3, CaF2; 4, SrF2; 5, A1203.
absorption at 200 and 250 nm after &irradiation, which was also found by other workers14 and which is not well understood. Furthermore, the a-absorption band at 370 nm is created. In contrast to these types, we also found one which did not change the normal UV absorption spectrum. Emission Measurements. F i e 5 shows the emission spectra of seven different sapphires. 'The differences in the luminescent bands at 275 and 410 nm between the various crystals can be explajned by different impurity contents, leading to different amounts of quenching and lattice distortion. Figure 6 show the emission spectra of the most interesting crystals. The luminescent performance of all spinels is different. The chemical analysis of these crystals shows a correlation between luminescent intensity and iron content, which is the dominant impurity. The luminescent spectrum of barium fluoride consists of two bands at 215 and 310 nm. Luminescence at 215 nm is the result of a cross-over transition (2p F - 5p Ba2+),and the emission at 310 nm is produced by excitons.15 The height and form of these spectra are influenced little by the choice of the manufacturer. All tested calcium fluoride crystals show nearly the same luminescent spectrum. However, only one crystal has a good long-term stability; the other crystals degraded within 1 day. This can be explained by the production technique, which leads to different impurity levels. Other crystals not shown in F i i e s 5 and 6 show that no or little luminescence or emission takes place in the visible spectral region. Table 1summarizes all results and gives the photomultiplier anode current in parentheses.
Measuremats with the Prototype. To test the long-term stability of the whole instrument, the best crystals of BaF2, CaF2, and spinel were irradiated by direct contact with the &source (3.7 GBq = 100 mCi) between 1500 and 2400 h. Bal3 showed no degradation, CaFi displayed some gray color without influencing luminescence intensity, and spinel developed a brown color after 500 h and gave a higher (!) intensity. A further investigation should reveal the influence of temperature. Heating of the silicon photodiode alone without the &source showed an exponential increase of the dark current, in agreement with the physics of photodiodes. However, this effect has a minor influence, because this drift is not the dominating one. The dominating effect comes from the crystal itself, because the luminescent intensity is a strong function of temperature, which is well known, too. This drift leads to a change in absorbance of 2.4 x AU/K Therefore, this UV photometer has to be thermostated. This is not a problem here, because it should work within a process HPLC system,which is already thermostated. The interference filter showed no measurable effect on temperature. chromatolgaphicTeele. We investigated linearity, reproducibility, and peak form of this new UV photometer used as HPLC detector. Iinearity measurements were done at 215 nm with a 0.5 mm cell for up to 100 ng of naphthalene. The result is a straight line, and the limit of detection ( 3 4 amounts to 0.9 ng. The reproducibility was measured by repeating an injection 10 times with the same concentration. F w e 7 shows the results. (15) Schotanus, P.;van Eijk, C.;Blasse, G.; den Hartog, H. Bjs. Stat. Solidi
(14) Heath, D.F.;Sacher, P.k Appl. Opt. 1966,5.937-943.
2312 Analytical Chemistry, Vol. 67, No. 13, July 1, 1995
1988,B148,K77-KS1.
1000 500 ng A
+-1,6% h
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h V
A "
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Flgure 7. Reproducibility, absorbance in of measurements.
I
lo4 AU against number
Especially important is the measurement of the peak form, because an unconventional cell type was used. Our measurements show that there is no evidence for tailing or fronting up to a cell length of 1 mm. W Intensityand Noise. It is very interesting to analyze the signal currents and minimun detectable absorbance of this detector, firstly to compare it with conventional HPLC detectors and secondly for further improvements. This experiment was done under the following conditions: j3-source, %r, 3.7 GBq (100 mCi); crystal, CaF2; filter, 260 nm, h h m 34 nm, T = 31%detector, silicon photodiode, 100 mm2, 0.12 A/W (260 nm); distance, 50 mm (crystal photodiode). Under these conditions, a signal current of 330 PA was measured, and the minimum detectable absorbance
\
amounts to 9 x 10-5 AU at 2 Hz bandwidth. If shot noise as the physical limiting noise were the dominating term, the resulting AU. minimum detectable absorbance would be 4 x Further experiments were done to measure the noise as a function of the cell length b. Cells with a relatively long length of 2 mm have a small optical diameter of 2.5 mm, and therefore the transmitted intensity was low and the noise high. Short cells (0.2 mm) with 8 mm optical diameter, on the other hand, showed the highest intensity, but there was an additional constant electronic noise, which degraded the limit of detection. As the final result, a path length between 0.5 and 1 mm was the optimum. For most of the experiments, we used BaFz and a wavelength of 215 nm. In this case, the minimum detectable absorbance was AU. To compare this value with that of commercial 5x HPLC detectors, we first have to calculate the rms values from our peak-to-peak values. This gives a minimum detectable absorbance of -1.25 x AU. Second, we have to take into consideration that the limit of detection is also determined by the path length. If the peak form were the same for both types of detectors, the upper path length would be 1 nun. So,a typical, commercial HPLC detector (10 mm cell) must have a minimum AU to show the same limit detectable absorbance of 1.25 x of detection. This minimum detectable absorbance is higher than that of actual HPLC detectors but indeed much lower than one would expect from the low luminescence intensity. ACKNOWLEWMENT
We thank Dr. N. Zeug and the Siemens Co. for supporting this development. Received for review October 5, 1994. Accepted March 20, 1995.@ AC9409820 @
Abstract published in Advance ACS Abstracts, May 1, 1995.
Analytical Chemistty, Vol. 67, No. 13, July 1, 1995
2313
