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Anal. Chem. 1982, 5 4 , 2029-2034
Table 11. Analytical Ereesults of Phosphate Concentration in Lake Kasumigaura concn of P, ng/mL sampling ordinary station colorimetrya this method 1
2 4
b b b b b
6 1 8
3
6 7 11
1 NDC
12
IVDC
0.221 t 0.232 f 0.103 f 0.116 t
15 15 12 15
a From ref 10. Concentration of phosphorus is too high to determine by the present method. ND = not detected.
to: 5.2 pg of P / m L of water; 1.32 pg of P / m L of 1:2 acetonewater mixture; and 4.41 pg of P/mL of 1:lacetone-water mixture in the respective calibration curves. This would indicate that the deviation in the color development process mainly determines the minimum detectable amount of phosphorus. Application to Determination of Phosphorus in Lake Water and Seawater. The present method was applied to standard seawater (CSK standard, Sagami Central Research Center) and lake water (the Lake Kasumigaura, sampling on Nov 25,1981). The analytical results with seawater for two CSK standard solutions of 500 pg of P/mL were 495 f 10 pg of P/mL and 485 f 10 pg of P/mL. The results for the lake water are listed in Table 11, along with results obtained by an Autoanalyzer (Technicon, type 11). This result attests to the efficacy of the present method for detection of phosphorus at levels too low to be measured by conventional colorimetry.
ACKNOWLEDGMENT colorimetric equipment is available.
Calibration for Phosphorus. Calibration curves for phosphorus (orthophosphate) were made for various wateracetone mixtures. After the color development for each concentration of phosplhorus, a respective amount of acetone was added to the samples. The obtained calibration curves are shown in Figure 5. Although the 1:1 mixture of acetone and water is the most diluted sample, the thermal lensing signal in this solution lis the most intense. In the case of a pure water system, the calibration curve was linear up to 500 pptr(pg of P/mL) of phosphorus. Conversely, 1:l and 1:2 mixtures of acetone-waiter maintain a linear curve up to 200 pptr. The steepest calibration curve was obtained in the case of a 1:2 mixture of acetone and water. The detection limits (twice the standard deviation of five repetitions of color developments for blank solutions) were as follows: water, 72.7 pg of P/mL; 1:2 acetone-water mixture, 5.0 pg of P/mL; 1:l acetone-water mixture, 13.9 pg of P/mL. The deviation of the signal for 50 laser shots corresponded
The authors thank Akira Otsuki for presenting the results obtained by the Technicon autoanalyzer in Table 11.
LITERATURE CITED (1) Gordon, J. P.; Leite, R. C. C.; Moore, R. S.; Porto, S. P.; Whinnery, J. R. J . Appl. Phys. 1965, 36, 3-6. (2) Dovichi, N. J.; Harris, J. M. Anal. Cbem. 1979, 51, 728-731. (3) Dovlchl, N. J.; Harris, J. M. Anal. Chem. 1980, 52, 2338-2342. (4) Dovichi, N. J.; Harris, J. M. Anal. Chem. 1081, 53, 106-109. (5) Imasaka, T.; Mlyaishl, K.; Ishibashi, N. Anal. Chim. Acta 1080, 155, 407-410. (6) Miyaishi, K.; Imasaka, T.: Ishlbashi, N. Anal. Chim. Acta 1981, 124, 381-389. (7) Haushalter, J. P.; Morris, M. D. Appl. Spectrosc. 1980, 445-447. (8) Fujiwara, K.; Uchikl, H.:Shimokoshi, F.; Tsunoda, K.; Fuwa, K.; Kobayashi, T. Appl. Spectrosc., in press. (9) Koyama, T.; Handa, N.; Sugimura, Y. "Lake and Sea-Water Analyses"; Kodansha: Tokyo, 1972; pp 49. (IO) Otsuki, A., Natlonal Instltute of Environmental Science, Ibaraki, Japan, 1981, unpublished data.
RECEIVED for review March 1,1982. Accepted June 18,1982. This work was partially supported by Grant No. 56030049 from the Ministry of Education, Japan.
Secondary Ion Mass Spectrometry with Cesium Ion Primary Beam and Liquid Target Matrix for Analysis of Bioorganic Compounds William Aberth," Kenneth M. Straub, and A. L. Burllngame Mass Spectrometry Resource, Department of Pharmaceutical Chemistry, University of California, San Francisco, California 94 143
A cesium ion gun has been designed to produce the prlmary beam for a secondary loin mass spectrometry (SIMS) source on a Kratos MSBOS mass spectrometer. A liquid target matrix is used for the anallysis of a wide variety of bloorganlc compounds. The source can be operated simultaneously with a standard Kratos Xeo faist atom bombardment (FAB) gun. A comparison between fast Cs' and Xeo primary beam sputtering efficiency on the yield of molecular ion species indicates that an improvement in sensitivity of at least a factor of 3 can be achieved wXth the C s' beam. I n addltion, no charging of the liquid matrix target Is observed when a primary ion beam is used.
The development of "soft" ionization techniques for mass
spectrometry which have the ability to produce gas-phase molecular ions without prior evaporation of the sample is in the process of revolutionizing our ability to detect, identify, and characterize the structures of thermally and chemically labile substances. This process began in 1969 when Beckey demonstrated that an underivatized carbohydrate (glucose) could be successfully ionized from the surface of a field ionization anode, producing molecular ion species without thermal decomposition of the sample. This technique was termed field desorption (FD) (1). The subsequent development of plasma desorption mass spectrometry (PDMS) by Macfarlane and co-workers further stimulated interest in the analysis of both highly polar and higher molecular mass (>ZOO0 daltons) substances and showed that ionization directly from the solid was experimentally feasible using time-of-flight mass spectrometry (2,s). Subsequent improvements in the range and
0003-2700/82/0354-2029$01.25/00 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
performance of mass analyzers have established that molecular weight and structurally relevant fragmentation data on polar, thermally labile substances in the range 2000 to at least 6000 daltons can be obtained by both FD and 2szCftechniques (4). In 1976, Benninghoven and Sichtermann reported on the secondary ion mass spectra (SIMS) of a number of amino acids obtained by bombarding the solid sample with argon ions (5). Similar results for a variety of organic compounds have been reported by a number of other researchers (6,7).Despite these initial successes, the "molecular SIMS" technique, wherein a solid sample target is bombarded with a primary ion beam (usually argon ions), generally gives low intensity secondary ion beams and transient signals. Recently, Barber, Surman, and co-workers reported on the development of "fast atom bombardment" (FAB), where the sample is supported in a glycerol or related viscous fluid matrix and bombarded with a fast (6-8 keV) neutral argon beam (8-12). The mass spectra of a wide variety of polar, biologically relevant molecules up to 3500 daltons were shown, with signal-tonoise ratios equivalent to those observed in conventional electron impact mass spectra. The success of FABMS led to the immediate commercialization and rapid dissemination of neutral atom sources which have been fitted to a variety of ion optical systems. Clear and impressive results using FABMS have been reported on such research problems as peptide sequencing (13),oligosaccharide analysis (14),bile salt analysis (15), and the structural characterization of polar, thermally labile substances including nucleotides, sulfate esters, and polypeptide antibiotics (16,17). However, in the initial publications describing FABMS, the developers of the technique reported that the need for a neutral primary beam was prompted by the observation that charged primary beams resulted in surface charging of the sample and subsequent loss of the secondary ion beam (8,10, 11, 16). In addition, the difficulty of steering a charged primary beam into the high-voltage ion source region of modern sector instruments (8) and a limited mass range capability (13) were also cited as a reason for using a neutral atom primary beam. Some of these claims have been recently restated and the phenomenological accuracy of the FAB terminology has been brought into question (18). In this report we describe results obtained with a liquid matrix SIMS source which employs a Cs+ primary ion beam. This ion source permits concurrent use of a commercial fast atom gun (Kratos, Inc.) as the primary bombarding beam for comparative purposes. Results have been obtained with the Cs+ source on a wide variety of polar, thermally labile substances of differing molecular size and properties which establish that for particles of similar kinetic energy and mass (e.g., XeO vs. Cs+), the charge or neutrality of the incident beam is of no consequence for the production of secondary ion mass spectra for samples supported in a viscous liquid matrix. In addition, the surface charging phenomenon described by others has not been observed to affect either the intensity or lifetime of the secondary ion mass spectra obtained. For a given sample, the maximum intensity of the secondary ion sample beam obtained using the Cs+ ion gun is at least 3 times greater than that obtained with the Xeo fast atom gun.
EXPERIMENTAL SECTION Mass analysis was performed on a Kratos MS-50s mass spectrometer equipped with a 23-kG magnet and negative ion switching. The instrument is capable of scanning up to m/z 3000 at 8 kV. Typical operating conditions were as follows: scan rate, 30 s/decade; dynamic resolution, M /AM 3000. The spectrometer was interfaced to our laboratory's LOGOS-II/Xerox Sigma 7 data system, permitting real-time assignment of masses (19). A standard Kratos FAB gun and ion source were used to obtain the neutral primary beam FAB spectra. The gun utilized xenon gas to produce an 8-keV beam of Xe atoms. A gun source current
L"""r
Figure 1. Schematic of source mounting of FAB (Xe') and SIMS (Cs') primary Ion guns: (1) beam centering plates; (2) source block: (3) sample target; (4) Insulator on direct insertion probe.
of between 40 and 45 pA was typically used. The primary Cs+ ion gun produces a 6-keV Cst beam focused to a spot size of between 1 and 2 mm diameter at the target surface. The Cs+ ions are produced by evaporation from a surface coated with cesium alumina silicate. Beam intensity can be independently controlled by varying the temperature of the evaporation surface. A Cs' current of about 1pA, produced at a temperature of 1000 "C, was generally used. The ion gun incorporates an extractor plate and Einzel lens for primary beam focusing. The entire structure is 3.4 cm long and is mounted within the mass spectrometer source housing. Further details of the gun design and operating characteristics will be given in a separate publication. Figure 1is a schematic showing the mounting arrangement of the FAB and Cs+ guns for producing comparative spectra. The sample matrix is placed on a copper probe tip with a surface plane at an angle of 40" with respect to a plane perpendicular to the MS beam center line. The Cs+ primary beam is directed at an angle of 20" with respect to the probe tip surface and the Xeo beam strikes this surface at an angle of approximately 40". Although the FAB primary beam is normally directed 20" to the target plane, we found that the larger striking angle had no significant effect on the efficiency of secondary ion production. Both Xeoand Cs+primary beams could be directed at the sample target with no additional adjustments to the instrument except for a 15" rotation of the target about the MS axis. Samples were prepared by either dissolving or making a slurry of the solid in glycerol: Alternatively, 1 p L of a solution of the sample (1-10 pg/pL) dissolved in water was added to about 3 pL of glycerol on the probe tip and the excess water pumped away in the vacuum lock of the mass spectrometer. Calibration was achieved by using either Ultramark 1621 (PCR, Gainesville, FL) or KI dissolved in glycerol. Peptide and lipid samples were obtained from Sigma (St. Louis, MO). Nucleotides were obtained from Collaborative Research (Waltham, MA). Steroid sulfates were provided by C. H. Shackleton (this laboratory).
RESULTS As described above, the size and placement of the Cs+ ion gun within the ion source permits concurrent operation of the source with the commercial fast atom gun (mounted externally on the ion source housing). In switching between the FAB and SIMS modes, it was observed that the source tuning (Y and Z lens, AV, beam centering) was essentially unchanged. Since the quantity of sample and mode of preparation are not variables, a direct comparison of the nature of the spectra from the two modes of operation for a given sample can be made. Some retuning of the mass spectrometer is required when switching between positive and negative ion modes. A series
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
I 1
I
Flgure 2. SIMS (positive ion) spectrum of
2031
1319.7 MHt
met-lys-bradykinln (Met-Lys-Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg). Major sequence ions are labeled.
a-MSH S I M S ICs’) -
-+-l--YL1670 m/r
Molecular Ion (MH’) of His-Phe-Arg-Trp-Gly-Lys-Pro-Val-”,). Figure 3.
a-MSH (Ac-Ser-Tyr-Ser-Met-GluCalculated mass (CITH1
N,,S) is 1664.8.
of examples where the E’AB or SIMS mass spectra were recorded on the same sample will serve to demonstrate the similarity of the two techniques. Figure 2 shows the positive ion SIMS spectra of the undecapeptide met-lys-bradykinin. This example is included since the FAB spectrum of this compound has been reported in the literature by others (20). It is evident that there is little difference between the fragmentation patterns of the FAB and SIMS spectra. Similar results were obtained with other model peptides over the mass range 1OOO-2900daltons. Figure 3 is an oscillograph tracing of the molecular ion region of the oligopeptide a-mblanocyte stimulating hormone (a-MSH; MH+ at m / z 1665), demonstrating the resolution and signal-to-noise level obtained with the cesium ion gun. A comparison of the spectra obtained by FAB or SIMS (both positive and negative ions) on a nucleotide, 5’-deoxyguanosine monophosphate is shown in Figure 4. In the positive ion mode, intense signals corresponding to a protonated molecular species (MH+, m / z 348), monosodium cationized species (MNa+, m/z 370), disodium cationized species (MNa2+,m/z 392), and trisodium cationized species (MNa3+, m / z 414) are observed for both SIMS and FAB. In the negative ion mode, intense signals (approximately 10 times those observed in the positive ion spectra) were observed for (M - H)-(m/z 346), the monosodium anion [(MNa - H)+,m/z 3681, and the disodium anion [(MNa2 - H)+, m / z 3901. Figure 5 shows the positive ion FAB and SIMS spectra obtained on a phospholipid, phosphatidyl choline (dipalmitoyl ester). An intense molecular ion is observed at m / z 734 (the choline group has a 1+charge on the quaternary nitrogen as drawn). Prominent fragment ions are present at m/z 550,496, 480, and 224, corresponding to loss of either the choline or
Figure 4. Comparative SIMS and FAB spectra (positive and negative ion mode) of 5’deoxyguanosine monophosphate (5’-dGMP).
fatty acid moieties (mlz 496 and 480 are due to cleavage adjacent or a to the carbonyl group). The peak at m / z 224 corresponds to loss of both fatty acid moieties. Glycerol cluster ions (mlz 93, 185, 277, etc.) are observed in both the SIMS and the FAB spectra. Loss of the secondary ion beam after 10-15 min with both Cs+ and XeO primary beams is most often due to evaporative loss of the liquid matrix. Application of an additional 2-5 p L of glycerol to the sample restores the secondary ion beam to its initial intensity. In the example shown in Figure 5, a change in the relative intensities of the signals at m / z 734 vs. 496,480, and 224 did not occur even after a total bombardment time of 30 min. Similarly, increased fragmentation is not observed with a larger primary ion beam flux; rather, the intensity of the secondary ion beam increases proportionately. Sample damage thus appears to be minimal, provided a liquid matrix is employed. We have occasionally observed the enhanced formation of ions corresponding to MCs+ (132 mass units higher than MH‘). This probably reflects an increase in the concentration of cesium within the sample matrix as a result of prolonged ion bombardment. Additional examples of data obtained using the Cs+ primary ion beam are shown in Figures 6-8. The negative ion SIMS spectrum of the dinucleoside phosphate deoxyadenosyl(5’3’)thymidine (d-APT) is shown in Figure 6. An intense molecular ion (M - H)- occurs at m / z 554, along with a less intense sodium cationized ion at m / z 576. Fragment ions corresponding to loss of the pyrimidine or purine moieties
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
24
I
OH
24
Figure 5.
rxeo,
Comparative SIMS and F A 6 spectra (posltlve ion mode) of phosphatidylcholine, dipalmltoyl ester. m9 I
M-Hi 554
occur at m f z 428 and 419, respectively. Cleavage of the 3'or 5'-ester bonds results in the ions at m/z 330 and 321. Both of these sets of fragment ions are observed in the B/E scan of m f z 554, showing that they are produced by gas-phase umimolecular decomposition of the quasi-molecularion. The peak at m/z 646 most likely corresponds to a glycerol-sample cluster (M + glycerin - H)-.The SIMS spectrum of a diphosphate nucleotide ester (5'-dGDP) is shown in Figure 7. A series of ions corresponding to (M - H)-and to the mono-,
di-, and trisodium cationized species occur at m f z 426,448, 470, and 492. Loss of a phosphate moiety leads to the fragment ions at m / z 346,368, and 390, while loss o f the purine base gives the ion at m f z 298. A final example is shown in Figure 8 for the negative ion SIMS spectrum of a steroid sulfate diester, 50-pregnane3a,20a-diol disulfate. The base peak in the spectrum, m / z 501, corresponds to the sodium cationized molecular ion (MNa - H)-. Other significant ions are (M - H)- a t m / z 479, and
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
Figure 7. Negatlve ion S I M S spectrum of B'deoxyguanosine diphosphate (5'dGDP).
fragments corresponding to hydrolysici of one sulfate group ( m / z 399) or loss of the elements of sulfuric acid (m/z 381).
DISCUSSION The results shown iin Figures 2-8 clearly demonstrate that the use of a neutral atom beam as the primary sputtering source does not have any inherent advantage over the use of a charged beam. The similar yields in riecondary ion current, and the observation that retuning of the ion source focusing controls is not required when switching between the two modes of operation, show that charging of the sample matrix by the primary ion beam is riot significant. This may be due to the charged nature of the selvedge region which would tend to equilibrate potential differences between the sample matrix and the surrounding support structure. Regardless of whether a neutral or ionic primary beam is used, the target surface potential value must be reasonably close to that of the probe tip for effective instrument performance. We have observed with liquid targetri that when either the Cs* or Xeo primary beam is turned on there is an initial intense secondary ion signal which diminishes in a few seconds to a stable signal of perhaps one-fifth of the beginning amplitude. A possible explanation for these results is that the high sample liquid-vacuum interface concentration on the liquid surface initially produces an intense secondary ion signal. As sputtering progresses, the surface concentration
2033
decreases and the secondary signal then reflects the rate a t which the sample solute diffuses to the liquid surface being sputtered. To date we have recorded the SIMS and FAB spectra of a wide variety of samples, including peptides, carbohydrates, nucleotides, and drug conjugates (sulfates, glucuronides), and have obtained essentially identical results with the two techniques. A Cs* primary current of about 1 pA heats the target probe tip at about the same rate as the XeO beam but produces a three times greater secondary signal. This is partly due to the tighter focus that can be achieved with the charged primary beam. Easy control of the deflection and the ability to control the focusing of a charged primary beam are advantages of this technique. Potential applications include the use of a split target where the charged primary beam could be alternately deflected between sample and reference compounds. Another advantage of the cesium ion gun is that it does not contribute to the ion source pressure. This may be important with certain types of spectrometers where ion source pumping capacity is limited. Also, the ion beam intensity can be independently controlled over several orders of magnitude by adjusting the source heating power. The Cs' source, however, has a limited lifetime which is dependent on its temperature (and hence beam intensity). Our present source has operated for over 300 h and continues to give consistent service. We expect the source to yield several hundred more hours of use. In any case, the Cs+ emitter is relatively inexpensive and can be readily replaced.
CONCLUSIONS We have carried out a series of experiments which establish that the charge or neutrality of the incident energetic atomic beam effecting energy deposition has no significant effect on the secondary ion mass spectra produced from labile, polar organic substances dissolved in a viscous liquid matrix. With adequate attention to ion source design, the use of a primary ion beam with a liquid matrix target (21) may result in superior performance for structural studies of biological compounds in magnetic sector instruments. In light of these results, the use of the term FAB to describe the state of the sample and its matrix on the electrode surface (viscous liquid as opposed to solid) is misleading. SIMS is the established, preferred acronym for describing the phenomenological process of sputter ionization (22). For the particular application to liquid matrix targets, it would be more appropriate to modify the term SIMS to specify this, i.e., LIQUID-SIMS.
E 501
Figure 8. Negative ion S'IMS spectrum of 5&pregnane-3a,20a-diol disulfate.
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Anal. Chem. lg82, 5 4 , 2034-2038
ACKNOWLEDGMENT We wish to thank Fred C. Walls for his expert technical assistance.
Proc. Jpn. SOC.Med. Mass Spectrom. 1981, 6 , 33-40. (16) Barber, M.; Bordoii, R. S.; Sedgwick, R. D.; Tyler, A. N. Nature (London) 1981, 293, 270. (17) Rlnehart, K. L.; Gandioso, L. A.; Moore, M. L.; Pandey, R. C.; Cook, J. C.; Barber, M.; Sedgwick, D.; Bordoli, R. S.; Tyler, A. N.; Green, 8. N. J . Am. Chem. SOC. 1981, 103, 6517-6520. (18) Garrison, B. J.; Wlnograd, N. Science 1982, 216. 805. (19) Kambara, H.; Walls, F. C.; McPherron, R.; Straub, K. M.; Burlingame, A. L. 27th Annual Conference on Mass Spectrometry and Allied TopIce, Seattle, WA, 1979; Paper MPM13, pp 184-185. (20) Barber, M.; Bordoii, R. J.; Sedgwlck, R. D.; Tyler, A. N.; Whally, E. T. Biomed. Mass Spectrom. 1981, 8 , 337. (21) The glycerin matrlx SIMS spectra of kanamycins A, B, and C obtained wlth an Ar' primary beam have recently been published. See Haroda, K.; Suzuki, M.; Tokeda, N.; Tatematsu, A,; Kambara, H. J . Anfiblot. 1982, 3 5 , 102. (22) Krauss, A. R.; Krohn V. E. I n "Mass Spectrometry"; Johnstone, R. A. W., Sr. Reporter; Burllngton House: London, 1981; Vol. 6, Specialist Periodical Reports, pp 118-152.
LITERATURE CITED (1) Beckey, H. D. Int. J . Mass Spectrom. Ion Phys. 1969, 2 , 500. (2) Macfarlane, R. D.; Torgerson, D. F. Science 1978, 191, 920. (3) McNeal, C. J.; Macfarlane, R. D. J . Am. Chem. SOC. 1981, 103,
1609. (4) Burllngame, A. L.; Dell, A., Russell, D. H. Anal. Chem. 54, 1982, 363R. (5) Benninghoven, A.; Jaspers, D.; Slchtermann, W. Appi. Phys. 1976, 11,35. (6) Kambara, H.; HiShlda, S. Org. Mass Specfrom. 1981. 16, 167. (7) Day, R. J.; Unger, S.E.; Cooks, R. G. J . Am. Chem. Soc. 1979, 101, 501. (8) Barber, M.; Bordoli, R. J.; Sedgwlck, R. D. I n "Soft Ionization Mass Spectrometry"; Morris, H. R., Ed.; Heyden: London, 1981, pp 137-152. (9) Barber, M.; Bordoll, R. S.; Sedgwick, R. D.; Tyler, A. N. J . Chem. SOC.,Chem. Commun. 1981, 325. (IO) Surman, D. J.; Vickerman, J. C. J . Chem. Res., Synop. 1981, 170. (11) Surman, D. J.; Vlckerman, J. C. J . Chem. SOC.,Chem. Commun. 1981, 324. (12) Barber, M.; Bardoli, R. S.; Elliott, 0. J.; Sedgwlck, R. D.: Tyler, A. N. Anal. Chem. 1982, 5 4 , 645A. (13) Wiillams, D. H., Bradley, C.; Bojesen, G.; Santikarn, S.; Taylor, L. C. E. J . Am. Chem. SOC. 1981, 103, 5700. (14) Forsberg, L. S.; Dell, A.; Waiton, D. J.; Ballou, C. E. J . Biol. Chem. 1982, 257, 3555. (15) Whitney, J.; Lewis, S.; Straub, K. M.; Thaler, M. M.; Burlingame, A. L.
RECEIVED for review May 7, 1982. Accepted July 14, 1982. Presented in part a t Euchem '82 Conference on Ion Beams, Gregynog, Wales, March 29-April 2, 1982. This work was supported by National Institutes of Health Division of Research Resources Grant RR00719/RR01614. Purchase of the negative ion unit for the Kratos MS-50s was made possible by a grant from the Academic Senate, University of California, San Francisco, 1981.
Thermal Lens Spectrophotometry Based on Pulsed Laser Excitation Kenji Morl, Totaro Imasaka, and Nobuhiko Ishlbashl" Faculty of Englneering, Kyushu University, Hakozaki, Fukuoka 8 12, Japan
A pulsed dye laser Is used as a selective exciting source In thermal lens spectrophotometry. The concentratlons of Cu( I I ) are determlned wlth porphyrln compounds as color reagents, whlch have B relatively sharp Soret band wlth a very large molar absorptivity. On the basis of metal Ion exchange, the direct determlnatlon of Cu(I1) In an aqueous solutlon Is carrled out uslng lead( I I ) tetrakls(N-methyl-4pyrldy1)porphlnetetra-p-toluenesulfonate(TMPyP). The analytical curve Is llnear In the range of (0-2.4) X lo-' M. Analysls Is also done based on solvent extraction Into benzene with meso -tetraphenylporphlnetrlsulfonic acid sulfate (TPPS). The solvent provldes a large blank slgnal because of Its large two-photon absorption cross sectlon ('A,, 'B,"). The present system enables the detection of 1 X lo-' M porphyrin in chloroform, whlch corresponds to an absorptlvlty of 4.7 X The enhancement factors calculated from optlcal parameters are provlded and the advantages of pulsed laser excltatlon are dlscussed.
-
Lasers have been recently used in several analytical applications. In fluorimetry the emission intensity is proportional to the intensity of an exciting source. The laser has usually a large output power so that it can be used advantageously as the exciting source. Sensitivity of conventional spectrophotometry is not determined by its source intensity, but by its source stability. Direct application of the lasers 0003-2700/82/0354-2034$01.25/0
for a light source in conventional spectrophotometry provides few advantages with respect to sensitivity. However, the signal intensity is proportional to the excitation intensity in some spectrophotometry and the use of the laser source enables detection of very small absorption by making the most of its capability. Under irradiation of the strong laser with a Gaussian beam profile, local heating takes place along a laser beam. It leads to a gradient of refractive index in the direction of radius and produces a thermal lens ( I ) . In thermal lens spectrophotometry the signal intensity is proportional to the power of the heating laser source (2); therefore this method is promising for determination of ultratrace samples with no fluorescence. Dovichi and Harris have reported the determination of Cu(I1) with EDTA using a single beam system, including a He-Ne laser as a heating source ( 3 ) . A dual beam system, consisting of a strong heating source and a stable probe beam, has been reported to have great advantages ( 4 ) . This dual beam system promises to provide a considerable enhancement factor (a relative sensitivity to conventional spectrophotometry) and to detect the sample at trace levels (2,5). Dovichi and Harris have also used the Arf laser as the heating source and have shown that the noise level could be reduced down to A = 7 X by measuring transient decay of the thermal lens signal (6). Recently thermal lens spectrophotometry based on image detection is reported to be quite useful for a reliable and accurate measurement of the thermal lens effect (7). This system allows the determination of Fe(I1) in an M and the deteraqueous solution at the level of 2 X Q 1982 American Chemical Society
