Anal. Chem. 1986, 58,965-969 (16) Soderburg, R. H. I n “Measurement and Monitoring of Non-Criteria (Toxic) Contaminants in Air”; Frederick, E. R., Ed.; Publishers Choice Book Mfg. Co.: Mars, PA, 1983;pp 489-499. (17) GoMen, C.; Sawicki, E. I n t . J. Environ. Chem. 1975, 4 , 9-23. (16) Sawicki, E.; Belsky, T.; Friedel, R. A.; Hyde, D. L.; Monkman, J. L.; Rasmussen, R. A,; Rlpperton, L. A.; White, L. D. Health Lab. Scl. 1975, 12, 407-414. (19) Seifert. B.; Steinbach, I.Z . Anal. Chem. 1977, 287. 264-270. Lee, F. S-C.; Schuetzie, D. I n “Handbook of Poiycycllc Aromatic (20) Hydrocarbons”; Bjorseth, A., Ed.; Marcel Dekker: New York, 1983; Chapter 2. (21) Fitch, W. L.; Everhart, E. T.; Smith, D. H. Anal. Chem. 1978, 50, 2122-2126. (22) Gimmarise, A. T.; Evans, D. L.; Butler, M. A.;Murphy, C. B.; Kiriazides,
D. K.; Marsh, D.; Mermeistein. R. I n “Polycyciic Aromatic Hydrocarbons: Physical and Biological Chemistry”; Cooke, M., Dennis, A. J., Fisher, G. L., Eds.; Batteiie Press: Columbus, OH, 1982;pp 325-334. (23) Ramdahi, T.; Alfheim, I.; Bjorseth, A. I n “Mobile Source Emissions Including Poiycyciic Organic Species”; Rhodia, D.,Cooke, M., Haroz, R. K., Eds.; Reidei: Boston, MA, 1963;pp 277-297. (24) Griest, W. H.; Caton, J. E.; Guerin, M. R.; Yeatts, L. B.; Higgins, C. E. I n “Polycycllc Aromatic Hydrocarbons: Chemistry and Biological Effects”; Bjorseth, A., Dennis, A. J., Eds.; Batteiie Press: Columbus, OH, 1980;pp 819-826. (25) Tomkins, 8. A.; Reagan, R. R.; Maskarinec, M. P.; Harmon, S. H.; Griest, W. H.; Caton, J. E. I n “Polynuclear Aromatic Hydrocarbons: Metabolism and Measurement”; Cooke, M., Dennis, A. J., Eds.; Battelle Press: Columbus, OH, 1983;pp 1173-1167. (26) Harrison, F. L.; Bishop, D. J.; Mallon, B. J. Environ. Sci. Techno/. 1985, 79, 186-193. (27) Later, D. W.; Lee, M. L.; Bartie, K. D.; Kong, R. C.; Vassiiaros, D.L. Anal. Chem. 1981, 53, 1612-1620.
965
(26) Gordon, R. J. Atmos. Envifon. 1974, 8 , 189-191. (29) Grosjean, D. Anal. Chem. 1975, 47, 797-605. (30) Kubtschek, H. E.; Williams, D. M. Mutat. Res. 1980, 77, 267-291. (31) Chrisp. C.; Hobbs, C.; Clark, R.; Kubitschek, H. E. I n “Pulmonary Toxicology of Respirable Particles”; Sanders, C. L., Cross, F. T.. Dagie, G. E., Mahaffey, J. A,, Eds.; NTIS, Publication No. CONF-791002,1980; pp 431-452. (32) Hagemann, R.; Virelizler. H.; Gaudin, D.;Pesneau, A. I n “Chemistry and Analysis of Hydrocarbons in the Environment”; Aibaiges, J., Frei, R. W., Merian, E., Eds.; Gordon and Breach Science Publishers: New York, 1963;pp 299-308. (33) Chester, C.; Pionke, H. B.; Daniel, T. C. I n “Pesticides in Soil and Water”; Guenzi, W. D., Ed.; Soii Science of America: Madison, WI, 1974 p 463. (34) Nuiton, C. P.; Haiie, C. L.; Redford, D. P. Anal. Chem. 1984, 56,
598-599. (35) Snyder, L. R. J . Phys. Chem. 1963, 67, 234-240. (36) Snyder, L. R. J . Phys. Chem. 1963, 6 7 , 2622-2627. (37) Griest, W. H.; Tomkins, B. A. Scl. TotalEnviron. 1983, 36, 209-214. (38) Renkes, 0. D.; Waiters, S. N.; Woo, C. S.; Iies, M. K.; D’Siiva, A. P.; Fassel, V. A. Anal. Chem. 1983, 55, 2229-2231. (39) Snyder, L. R. I n ”Chromatography”; Hettmann, E., Ed.; Reinhoid: New York, NY, 1967;p 59.
RECEIVED for-review September 20,1985. Accepted December 16,1985. This work was performed in the laboratories of the
U.S. Department of Energy under Contract W-7405-Eng-82. The work was supported by the Office of Health and Environmental Research, Office of Energy Research.
Determination of Thioglycolic Acid and Dithiodiglycolic Acid in Mineral Flotation Systems Megan Mclean, Stan Van Wagenen, Donna Wiedemann, and Quintus Fernando* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Srini Raghavan Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721
When aqueous solutions of thiogiycoilc acid are equilibrated with sphalerlte, a zinc sulfide mineral, a large fractlon of the thiogiycoiic acid Is either adsorbed on the sphalerlte surface or oxidlzed to dlthiodiglycollc acid. The total concentration of thiogiycoilc and dithloglycoilc acld In solution has been determined by molecular emlssion cavity analysis (MECA). The fraction of the thloglycoilc acid that is not adsorbed on the mineral surface and remains In solution has been determined by a coulometric titration In which iodine Is eiectrogenerated in situ and the end point located by an amperometric method. Attempts to determlne the thioglycoilc acid that was adsorbed on the mineral surface directly by MECA gave unreliable results. This has been attributed to the wide varlatlon In the surface area as well as the surface chemical compositlon of small samples (1-2 mg) of the mineral that must be used In the sample cup In MECA. Thloglycolic acld also leaches traces of metal Ions from the mineral surface. The concentration of rlnc(1 I ) in solutlon reflects the extent of leaching that has occurred.
The use of aqueous solutions of sulfur compounds to achieve selectivity in sulfide mineral flotation has been in vogue for more than five decades. For example, the selective flotation of molybdenite from concentrates containing molybdenum and copper sulfides is currently carried out industrially with
the aid of aqueous solutions of sodium sulfide or ammonium sulfide or mixtures of these two compounds. Another example is the use of aqueous sulfite solutions to depress sphalerite during the flotation separation of copper sulfide from zinc sulfide. The mineral processing industry has been in search of suitable reagents to replace aqueous sulfide solutions, because the massive dosage requirements of inorganic depressants te.g., 10-40 lbs of sodium hydrosulfide/ton of copper molybdenum concentrate) have resulted in severe odor and disposal problems. Short-chain organic compounds such as mercaptocarboxylic acids and mercapto alcohols have been considered as possible replacements for aqueous sulfide solutions. Promising results were obtained in some initial work that was carried out with short-chain mercapto compounds such as @-mercaptoethanol(HSCH2-CH20H),and mercaptoacetic acid (thioglycolic acid, HSCH&OOH), (1,2). The maximum potential of these compounds, however, has not been realized mainly because there is a lack of fundamental information on the mode of interaction of these mercapto compounds, either with minerals or with collector-coated minerals. The interaction of thioglycolic acid, the first member of the aliphatic mercaptocarboxylic acids, with chalcocite has been investigated in the pH range 3-10 (2). The analytical technique that was employed in these studies was the classical iodimetric titration with a standard iodine solution and a starch indicator (3). Serious difficulties were experienced with
0003-2700/86/0358-0965$01.50/00 1986 American chemical Society
966
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
the starch end point when thioglycolic acid solutions that. had been contacted with sulfide minerals were titrated iodimetrically. The results indicated that the uptake of thioglycolic acid was about 5500 times greater than the amount that was needed for a monolayer coverage of the mineral surface. I t was postulated, therefore, that a large amount of the thioglycolic acid was oxidized to the dithiodiglycolic acid on the mineral surface. The concentration of this oxidation product that was present in solution was determined by the reduction of the dithiodiglycolic acid with zinc metal in NaOH to the thioglycoiic acid, followed by the iodimetric determination of the total concentration of thioglycolic acid in solution. This determination was also plagued by difficulties with the starch end point and gave erratic results. Alternative methods that have been proposed for the determination of thioglycolic acid, for example, a spectrophotometric method ( 4 ) and a method based on the use of an ion selective electrode (5), were found to be unsuccessful under the conditions that were employed in these experiments. We have investigated the interaction of thioglycolic acid with a zinc sulfide mineral, sphalerite, and we have employed two independent analytical techniques for the determination of thioglycolate species in aqueous solution. The concentrations of HS-CHyCOOH and HS.CH&OO- were determined by a coulometric titration in which iodine was electrogenerated in situ, and the end point was determined amperometrically (6). The difficulties that were previously encountered with the starch end point were thereby eliminated. The total concentration of the sulfur containing species, HS.CH&OOH, HS.CH,.COO-, and -00C.CH2.S-S.CH2.C00-, was determined by a flame emission technique based on the emission intensity of the molecular sulfur species, S2*,that was produced in the flame (7,8). Hence, we were able to obtain the concentration of the disulfide, -00CCHz.S-S.CH2C00-, in the aqueous solution by difference. With the aid of these two analytical techniques we have deduced quantitatively the distribution of thioglycolic acid between the solid phase, sphalerite, and the aqueous phase. We have also demonstrated that these two analytical techniques can be used on a routine basis for obtaining quantitative data on the equilibrium distribution of organic sulfur compounds between solid sulfide mineral phases and aqueous solutions. EXPERIMENTAL SECTION Reagents. Mercaptoacetic acid (95%)obtained from Aldrich Chemical Co. was used without further purification. Stock soM) were freshly prepared every day and lutions (-2 x standardized by an iodimetric titration with a standard solution of KI03 (9). Standard solutions of thioglycolic acid were prepared by appropriate dilution of the stock solution and used to obtain calibration curves with the coulometric method and with the molecular emission method. The sphalerite used in this work had a mesh size of -2OO/+325, and its composition was Zn, 61.0%;Fe, 5.41%;Cu, 0.22%;and
s, 33.34%.
Coulometric Titration with Electrogenerated Iodine. The coulometric titration cell consisted of three compartments (Figure 1) (10, 11). The central compartment contained 25 mL of 0.08 M KI, a platinum foil generator electrode, a rotating platinum indicator electrode, and a platinum auxiliary electrode for the amperometric indicator circuit. The central compartment was connected by means of two side arms to two side compartments containing saturated KCl solutions. Sintered glass frits and KC1-saturated agar plugs in the side arms separated the solutions in the side compartments from the solution in the central compartment. A silver wire placed in one of the side compartments and the platinum foil generator electrode in the central compartment were connected to a Sargent coulometric current source that provided a constant current of 40 PA. A silver wire in the second side compartment and the rotating platinum indicator electrode together with the platinum auxiliary electrode were connected to a Princeton Applied Research electroanalyzer (PAR
COULOMETRIC CURRENT
POLAROGRAPHIC ELECTRODE
COUNTER ELECTRODE (Ag WIRE IN SAT. KC I I
REFERENCE ELECTRODE (Ag WIRE IN I SAT. KC I I
PLATINUM GENERATING ELECTRODE
ROTATING .PLATINUM INDICATOR ELECTRODE
" I
AGAR PLUGS SATURATED WITH KCI COARSE GLASS FRITS
Flgure 1. Electrochemlcal cell for the coulometrlc generation of iodine and amperometrlc detectlon of the end point. The solution was stirred continuously wlth a magnetic stlrrer.
Model 174). A constant potential of +200 mV, in the limiting current plateau of the 12/13- redox couple, was applied to the rotating platinum indicator electrode, which was maintained at a constant speed of 600 rpm with the aid of a Sargent synchronous motor. Adsorbed impurities on the surface of the platinum indicator electrode were removed by immersion of the electrode in concentrated "03 followed by electrochemical reduction of the electrode surface. Pretreatment of the electrode in this manner ensured that the electrode response was reproducible. The coulometric titration was carried out as follows: a measured volume of solution containing the thioglycolic acid was added to the central compartment containing 25 mL of the KI solution, and the solution was stirred continuously with the aid of a magnetic stirrer. The current in the indicator circuit was recorded as a function of time during the time that iodine was electrogenerated in the central compartment. The current-time plot showed a sharp increase when all the thioglycolic acid was oxidized to the disulfide by the electrogenerated iodine. The amperometric end point was readily located, because the oxidation reaction was extremely rapid. Molecular Emission Cavity Analysis (MECA). The total concentration of all sulfur-containing species in aqueous solution was determined by measuring the emission intensity of molecular sulfur that is produced in a cool hydrogen-diffusion flame. The equipment that is used in this work consists of a sample introduction device and a modified emission spectrometer, which have been described in detail previously (12-14) (Figure 2). The emission intensity of molecular sulfur was measured at 384 nm with a monochromator (GCA McPherson Model EV-700-2),with a slit width of 1.000 mm (spectral bandwidth = 2.0 nm). A photomultiplier tube served as a detector, and its output was monitored with an integrator (Hewlett-Packad 338OA) and a chart recorder (Linear Model 252A). A 6-pL sample containing the thioglycolic acid was added to the silica cup at the end of the sample rod. The cup was rotated into a cool nitrogen-diluted hydrogen-diffusion flame. The optimum flow rate of hydrogen was 1.1 L min-' and 2.1 L m i d for nitrogen. The inside of the sample cup was wiped clean and thoroughly rinsed with distilled water before each determination. RESULTS AND DISCUSSION Thioglycolic acid reacts rapidly and stoichiometrically with iodine to form the disulfide, dithiodiglycolic acid. This oxidation reaction is the basis for the classical determination of a large number of mercapto compounds, in which a solution of the mercapto compound is titrated with a standard solution of iodine to the starch end point. Solutions of thioglycolic acid that have been equilibrated with minerals are contaminated with high concentrations of anions and cations that have been leached from the minerals, and under these conditions erratic results are obtained with the starch indicator.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
m
MONOCHROMATOR
ENTRANCE
\
I
Table 11. Percentage of Thioglycolate Species in 30 mL of a 1.0 X M Aqueous Solution of Thioglycolic Acid Equilibrated with 1.0 g of -200/+325 Mesh Sphalerite
time, min
determn by MECA
coulometric determn
5 15 30 45 60
90.1 (3.310 83.6 (8.4) 80.8 (5.2) 75.8 (8.9) 73.0 (2.8) 73.2 (7.8) 76.4 (2.3)
19.9 (2.5)O 19.2 (1.1) 17.2 (1.1) 16.0 (0.7) 13.1 (1.8) 7.75 (2.4) 2.82 (3.1)
120 180
"The values in parentheses are the percent relative standard deviations.
COIL
OPTICAL RAIL CARRIAGES
OPTlCAL RAIL
967
I
v)
t
b
2 3
>.
a a a
t m LL
a
r-> v)
Table I. Coulometric Titration of Thioglycolic Acid with Iodine Electrogenerated at 40 WAin 25 mL of 0.08 M KI
mol of thioglycolic acid x 10'
time to reach end point, s
re1 std dev: %
1.86 1.97 4.13 4.38 8.26 8.76 10.9
45.1 69.5 94.6 128 185 235 284
2.7 0.4 1.3 1.7 0.7 1.0
0.3
" Three to five determinations were used to calculate the relative standard deviation. The coulometric generation of iodine in the presence of thioglycolic acid and the amperometric detection of excess iodine at a rotating platinum indicator electrode circumvents the difficulties with the starch end point that have been reported in previous work (2). We have shown that the presence of any impurities that are leached by the thioglycolic acid from sulfide minerals have no effect on the amperometric detection of the end point. We have also been able to determine routinely 7.5 X lo-' M thioglycolic acid solutions with a relative standard deviation of e270 (Table I). This concentration is a 1000-fold lower than the thioglycolic acid concentration that was determined iodimetrically with the starch end point. A lower concentration of thioglycolic acid may be determined by modifying the design of the cell in which the coulometric titration is performed, to accommodate a platinum rotating disk indicator electrode in a very small volume of solution. The coulometric titration technique was used to determine the concentration of thioglycolic acid in solutions that had been equilibrated with sphalerite. Thirty milliliters of a M) was added standard solution of thioglycolic acid ( = to a series of capped tubes containing 1.00 g of the solid sphalerite. The pH of each of the solutions was adjusted to between 6.0 and 7.5 by the addition of NaOH, and the heterogeneous mixtures in the test tubes were agitated vigorously for 3 h. During this 3-h period, tubes were removed at periodic intervals (=15 min) and the contents centrifuged until a clear supernatant solution was obtained. An aliquot of the supernatant solution was analyzed coulometrically, and the concentration of the thioglycolic acid in the aqueous phase was determined. It was found that equilibrium between the solid sphalerite and the aqueous solution of the thioglycolic
z w
Ez
0 v)
Figure 3. MECA peak obtained from: (a) 1.749 X lo4 M thiogiycolic acid soluton before contact with sphalerite and (b) thioglycolic acid solutlon after equilibration with sphalerite. The concentration, obtained from a calibration curve, was 1.75 X M.
acid was attained in less than 60 min and that -90% of the thioglycolic acid that was present initially in the solution was either adsorbed on the surface of the sphalerite particles or converted to the disulfide (Table 11). Any metal complexes of thioglycolic acid that were present in solution as a result of the leaching action of thioglycolic acid would be titrated by the coulometrically generated iodine and would. be accounted for in the -10% of the thioglycolic acid that remained unreacted in solution. The concentration of zinc(II), as well as any other transition-metal ions in solution, is a quantitative measure of the extent to which the sphalerite surface has been leached. The coulometric titration with electrogenerated iodine can be used only for the determination of unreacted thioglycolic acid in this system; the dithiodiglycolic acid that is formed is not oxidized further by iodine. Bromine, however, is a stronger oxidizing agent and, like iodine, can be electrogenerated in situ, with 100% current efficiency ( 1 0 , I I ) . Attempts to carry out coulometric titrations with electrogenerated bromine met with limited success, mainly because the reaction of bromine with thioglycolic acid occurs in a complicated and stepwise manner. In addition to the disulfide, sulfonyl compounds, sulfones, and sulfonic acids are formed and the reaction rates and products are governed by a variety of solution parameters (15). The concentrations of thioglycolic acid and its disulfide in solution were determined, therefore, by an independent method that is based on the emission intensity of molecular sulfur that is formed from these compounds in a cool hydrogen-diffusion flame. This method, known as molecular emission cavity analysis, has been widely used for the determination of sulfur compounds in solution and in solids (12-14, 16). The MECA results obtained with the thioglycolic acid-sphalerite system are discussed below. Figure 3 shows the MECA peaks obtained with a standard solution of thioglycolic acid, before and after the solution was
988
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
Table 111. Determination of Thioglycolic Acid by MECA"
concn of thioglycolic acid, M 1.09 x 4.38 x 1.75 x 7.88 x
I
peak area
re1 std dev, %
X
65.87 21.59 6.31 1.52
10-3 10-4 10-4 10-5
3.5 10
3.8
a
wt of solid sphalerite, mg
peak height, cm
5.0 X lo4
3.23 2.02 2.26 2.64 1.09
54.2 75.5
82.3 41.8 78.5 56.1 27.5
5 x 10-5
0.63 0.48 0.60 0.92 1.33
39.5 72.4 68.3 52.0 52.0
2.5 x 10-4
1.17
1.20 0.95 1.29 1.20
52.9 86.7 94.4 75.5 71.4
2.5 x 10-3
1.04 0.95 0.98 1.06 1.22
82.3 76.5 97.9 68.3 93.8
1.0 x 10-2
0.88 1.56
153 97.9 61.2 127 76.5
TIME (SECONDS)
equilibrated W i t h sphalerite. I he emission peaks nom the thioglycolic acid and the dithiodiglycolic acid overlap, and it is not possible to differentiate between the uncomplexed thioglycolic acid, its oxidation product, and any metal chelates of thioglycolic acid that are present in solution. The MECA technique, therefore, gives the total concentration of the thioglycolic acid species (uncompiexed, oxidized, and complexed) that is present in solution. Peak areas rather than peak heights were found to be more reproducible and were used to construct a calibration curve that could be used to determine thioglycolic acid concentrations between 8 X M (Table 111). M and 1 X In the equilibration experiments in which a standard thioglycolic acid solution was equilibrated with sphalerite, the results obtained with the coulometric titration indicated that N 10% of the initial thioglycolic acid remained in solution, either as thioglycolic acid or in the form of its metal complexes. The results obtained by the MECA technique with the of the initial thioidentical solutions indicated that ~ 7 5 % glycolic acid remained in solution. The difference between these two values (i.e., =65% of the initial thioglycolic acid concentration) is the percentage of the thioglycolic acid that is oxidized to the disulfide under the experimental conditions that are employed in this work. Previous work has shown that it is possible to determine the concentrations of sulfur compounds in solids by the MECA technique (12, 13). The sulfur peak obtained with solid sphalerite is shown in Figure 4a. This is attributable to the surface sulfur atoms that are lost when the solid sphalerite is placed in the sample cup and introduced into the flame. Figure 4b shows the sulfur peak that is obtained with the solid sphalerite that is equilibrated with a solution of thioglycolic acid. Attempts were made to obtain a quantitative estimate of the concentration of thioglycolic acid that is adsorbed on the surface of the sphalerite. The results were not reproducible because the surface area and the surface composition of the sphalerite particles varied considerably (Table IV). It
91.8 62.2
4.14 2.70 1.02 0.77 0.37
30
Flgure 4. (a) MECA peak obtained with solid sphalerite (-200/+325 mesh) and (b) MECA peak obtained wlth solid sphalerite (-200/+325 mesh), after equilibration with a solution of thloglycob acid.
122
1.0 x 10-5
b
0
-
concn of thioglycolic acid soln, M
12
'Six microliters of the thioglycolic acid solution was added to the silica sample cup. The emission intensity in the hydrogen-diffusion flame was monitored at 384 nm. The Hzflow rate was 1.1L mi&, and the N2flow rate was 2.1 L min-'.
w
Table IV. MECA Determination of Thioglycolic Acid Adsorbed on Solid Sphalerite (-200/+ 325 Mesh) after Equilibration with Aqueous Solution of Thioglycolic Acid'
1.18
1.70 1.32
a Twenty-five milliliters of the thioglycolic acid solution was eauilibrated with 0.2 E! of solid mhalerite in each exDeriment.
should be possible, however, to obtain quantitative data on the extent of adsorption of mercapto compounds on solid zinc sulfide by employing the MECA technique with synthetic preparations of the solid ZnS that have uniform particle size and surface composition. The surface area of the sphalerite sample that was measured by using a multipoint BET technique and krypton gas as the adsorbate was 1.0 m2/g (17). The data in Table IV can be used to obtain an alternative estimate of the surface area of the sphalerite on the assumption that each thioglycolic acid molecule occupies an area of 25 A2 and that only a monolayer is formed on the sphalerite surface. At each concentration of thioglycolic acid employed in this series of experiments, the sum of the peak heights divided by the total weight of sphalerite gives an indication of the extent of thioglycolic acid adsorbed per milligram of solid sphalerite. When this ratio reaches a limiting value, it can be assumed that there is monolayer coverage. The data shown in Table IV indicate that monolayer coverage occurs a t thioglycolic acid concenM. In each experiment 25 mL trations greater than 5.0 X of the thioglycolic acid was equilibrated with 0.2 g of sphalerite. Hence the surface area of the sphalerite is 0.9 m2/g of sphalerite. From the MECA results in Table 11,75% of the thioglycolic acid remained in solution. An additional estimate of the surface area of the sphalerite can be made by assuming that 25% of the initial concentration of thioglycolic acid (1.0 X M) was adsorbed on 1.0 g of sphalerite after equilibration in 30 mL of the thioglycolic acid solution. This gives a surface area of 1.1 m2/g, which is in reasonable agreement with the values that have been calculated above.
Anal. Chem. 1986, 58,969-972
Registry No. Thioglycolic acid, 68-11-1;dithiodiglycolic acid, 505-73-7; sphalerite, 12169-28-7.
LITERATURE CITED sect. C , in (1) ~ ~ D.; Ragh~ S, Trans. ~ Inst. i Mln.~ ~ ~ press. (2) Raghavan, S.; Unger, K. Trans. Insf. Min, Mete//. Sect. C 1983, 9 2 , 95. (3) Walker, G. T. Manuf. Chem. 1953, 2 4 , 376. (4) Walker, 0. T.; Freeman, F. M. Manuf. Chem. 1955, 26, 13. (5) Tseng, P. K. C.; Gutkencht, W. F. Anal. Chem. 1975, 47, 2316. (6) Oganesyan. L. 6.; Kon’kova, N. F. J . Anal. Chem. USSR (fngl. Trans/.) 1960, 12, 1296. (7) Beicher. R.; Bogdanski, S.L.; Townshend, A. Anal. Chlm. Acta 1973, 6 7 , 1. (8) Belcher, R.; Bogdanski, S. L.; Knowies, D. J.; Townshend, A. Anal. Chim. Acta 1975, 79. 292.
969
(9) Leussing, D. L.; Kolthoff, 1. M. J . Nectrochem. SOC. 1953, 100, 334.
(IO) O’Dom, G.; Fernando, Q. Anal. Chem. 1965, 3 7 , 893. (11) O’Dom, G.; Fernando, Q. Anal. Chem. 1986, 38, 844. (12) Schubert, S. A.; Clayton, J. W., Jr.; Fernando, Q. Anal. Chem. 1979, 51, 1297. (13) Schubert, S. A.; Clayton, J. W., Jr.; Fernando, Q . Anal. Chem. 1980, , 52. 963. (14) Tzeng, J.-H.; Fernando, Q. Anal. Chem. 1982, 5 4 , 971. (15) Oae, S. “Organic Chemistry of Sulfur”; Plenum: New York, 1977. (16) Burguera, M.; Bogdanskl, 8. L.; Townshend, A. CRC Crit. Rev. Anal. Chem. 1960, 185. (17) Gregg, S. J.; Sing, K. S. W. “Adsorption, Surface Area and Porosity”; Academic Press: New York, 1982.
Received for review October 3,1985. Accepted December 6, lgg5* We are grateful to the Arizona Mining and Resources Research Institute for financial assistance.
CORRESPONDENCE Consecutive Laser-Induced Photodissociation as a Probe of Ion Structure Sir: Photodissociation hae become an important means for studying the spectroscopy, structure, and thermochemistry of ions in the gas phase (1-8). A widely used method involves storing ions in a static magnetic ion trap, such as an ion cyclotron resonance cell, and detecting the fragment ions that are produced by photon absorption. Dunbar and co-workers have shown that ion photodissociation spectra are highly characteristic of the structure of an ion (4). Most ion photodissociation experiments have utilized visible and infrared lasers, but we are interested in investigating ion photodissociation induced by ultraviolet (UV) light. Our first experiments showed that oligopeptide ions are efficiently fragmented with 193-nm light from an excimer laser (9). The photofragment ions were detected by Fourier transform mass spectrometry (FTMS). An advantage of the excimer laber for these experiments is that UV radiation is strongly absorbed by the carbonyl and aryl groups in polypeptides. At 193 nm each photon deposits 6.42 eV specifically into electronic excitation of the ion, and absorption of even one W photon may be sufficient to cause fragmentation. In this paper we describe a new technique in which parent ions stored in an FTMS analyzer cell are photodissociated by a first laser pulse to produce daughter ions, and a particular daughter ion of interest is then isolated in the analyzer cell and subsequently photodissociated by a second laser pulse. The general scheme is parent
hu
daughter
hv
granddaughter
(1)
This scheme is similar to MS/MS/MS experiments except that UV radiation instead of collisional dissociation is used to fragment the ions (10,ll).We are interested in investigating whether photodissociation with UV radiation is more efficient than collisional dissociation for fragmenting high-mass ions. High-mass ions are not efficiently fragmented by collisional dissociation because of the increasing amount of energy that a large molecule can accommodate before dissociating (the number of vibrational modes increases) and the decreasing amount of energy that can be transferred during a collision (center of mass effect) (12-14).For the two examples presented, bromobenzene and Gly-Phe-Ala, the photodisso0003-2700/86/0358-0969$01.50/0
ciation yields for daughter ions are as high as 25%, and it appears that consecutive laser-induced photodissociation may become a practical and powerful method for elucidating ion structures.
EXPERIMENTAL SECTION A block diagram of the apparatus used in this work is shown in Figure 1. The excimer laser is a Lambda Physik Model EMG-103MSC operated on the line of ArF* at 193 nm. When the laser is triggered, a 20-11s pulse of UV radiation enters the vacuum chamber through a magnesium fluoride window. The beam passes through a 2.5-cm-diameter hole in the front plate of a cubic trapped ion analyzer cell (5.5 X 5.5 X 5.7 cm) and intersects ions that are trapped inside the analyzer by a 1.2-T magnetic field. The beam exits the cell through a hole of the same size in the back plate. This hole is covered with electroformed copper mesh (31liies/cm). We found that the mesh was nedessary because without it the mass resolution is severely degraded. The laser beam is not strongly focused and has a cross section of approximately 1 X 2 cm as it passes through the cell. Ions formed by electron ionization or laser photodissociation are stored inside the analyzer cell and detected by Fourier transform mass spectrometry (15-18).FTMS is a pulsed detection method that provides high sensitivity and high mass resolution. In addition, a complete mass spectrum can be obtained after each laser pulse. Basically, FTMS operates by accelerating ions at their cyclotron frequencies and detecting the coherent ion image current signals that are induced in the plates of the analyzer cell. Fourier analysis of the ion image current signals yields a mass spectrum. An IonSpec Model 2000 FTMS data system, shown in Figure 1, was used to control the analyzer cell, detect the ions, plot the data, and trigger the laser. The data system is based on an IBM Instruments CS-9002 computer with 1M byte of system memory. The FTMS signal is digitized and stored in a 64K byte static random access memory. A floating point array processor performs the Fourier transform calculation, and a new mass spectrum is displayed every 4 s. The pulse sequence utilized for our consecutive laser-induced photodissociation studies is shown in Figure 2. Initially, a quench pulse is applied to one of the side plates to remove all ions from the analyzer cell. The electron beam is then pulsed for 50 ms to ionize the sample. All ions formed by the electron beam are trapped in the analyzer cell by the combined effects of the magnetic and electrostatic fields. The parent ions of interest are 0 1986 American Chemical Society
