Anal. Chem. 1982, 5 4 , 2133-2134
detector is at least 2 orders of magnitude more sensitive than the R1 detector, and second, it responds to a much narrower class of compounds. The latter is a large advantage when the interpretation of a complex chromatogram is required. All of the compoundri listed in Table I are volatile and thus may also be determined by gas chromatography. Recent work has concentrated on nonvolatile compounds that can only be determined by liquid chromatography. In collaboration with Frei and Brinkman we have achieved detection limits of approximately 10 ng for a variety of digitalis glycosides separated by reversed-phase HPLC. Applications of the PRF detection scheme to these and other nonvolatile compounds such as saccharides will be demibed in future articles.
ACKlVO WLEDGMEIVT We are especially thankful to Kenneth Sigvardson and Tad Koch for helpful discussions.
2133
LITERATURE CITED (1) Iwaoka, W.; Tannenbaum, S. R. IARC Scl. Pub/. 1979, 14, 51. (2) Twltchett, P. J.; Willlams, P. L.; Moffat, A. C. J. Chromatogr. 1978, 149, 683. (3) Scholten, A. H. M. T.; Frei, R. W. J. Chromatogr. 1979, 176, 349. (4) Scholten, A. H. M. T.; Brinkman, U. A. Th.; Frel, R. W. Anal. Chim. Acta 1980, 114, 137. (5) Schoiten, A. H. M. T.; Welling, P. L. M.; Brinkman, U. A. Th.; Frei, R. W. J. Chromatogr. 1980, 199, 239. (6) Gandelman, M. S.; Blrks, J. W. J. Chromatogr. 1982, 242, 21. (7) Wllklnson, F. J. Phys. Chem. 1962, 66, 2569. (8) Bridge, N. K. Trans. Faraday Soc. 1980, 56, 1001. (9) Turro, N. J. "Modern Molecular Photochemistry"; Benjamin-Cummlngs: Menlo Park, CA, 1978; pp 362-380.
RECEIVED for review February 2,1982. Accepted July 9,1982. The authors gratefully acknowledge the support of the National Science Foundation (Grant No. CHE-7915801). J.W.B. thanks the Alfred P. Sloan Foundation for a Research Fellowship.
Noise Reduction in Liquid Phase Photoacoustic Spectroscopy M. R. Fisher Department of Chemistty, University of Nebraska, Lincoln, Nebraska 68588
N. S. Nogar" Group CNC-2, MS G738, /-os Alamos National Laboratory, Los Alamos, New Mexico 87545
Cell geometry has been shown in the past to have a significant affect on both signal and noise levels in photoacoustic spectroscopy, PAS (1). A number of different cell designs have been used in the past for piezoelectric detection of liquid phase PAS signals. These include placing a scirew-mounted transducer in direct contact with the analyte solution (2, 3 ) , fabricating the entire sample cell from a piezoelectric material (4,5),and placing a transducer in contact with a quartz vessel which contains or is attached to the sample of interest (6-8). This latter arrangement has a number of advantages over other designs. It allows detection without contact between sample and transducer, a serious consideration when dealing with incompatible materials. In addition, sample change is expedited, and the prolblem of formation of microscopic air bubbles on the transducer face is eliminated. However, poor signal reproducibility inay be a probleim (8) with this arrangement. It is this latter problem that we address.
EXPERIMENTAL SECTION The excitation source used was a nitrogen llaser (NRG 0.7-5-200) capable of delivering l-mJ, 6-11s pulses at repetition rates up to 60 Hz. Two spherical Suprasil quartz lenses, 1000 and 250 mm focal lengths, were used to focus the beani into the cell. The detector employed was a piezoelectric pressure transducer (Celesco LG-65, Canoga Park, CA) which was screw mounted into the side of an aluminum C-clamp. This transducer exhibits a 60-kHz mechanical resonance. A spring mounted on the opposite side of the C-clamp applied pressure to the sample cell and ensured good acoustic contact of the cell with the transducer using mineral oil as the coupling fluid. A standard Suprasil quartz fluorescence cuvette, 5 cm in height, was used in addition to a modified cell. The modified cell was made by reducing the height of a standard cell to 2 cm. Signal processing has bisen described previously (9,lO) and will only be outlined here. Tlhe signal was passed through an impedance matched pre-amp, Celesco LG-1344 (40 dB, 1000 mil, 20 Hz-100 kHz) and then through a Tektronix 7A18 plug-in, used
as an amplifier. Recording electronics consisted of a transient digitizer (Biomation 805) interfaced to an &bit microcomputer (North Star Horizon) (IO),where signal averaging could be used to compensate for shot-to-shot fluctuations of the nitrogen laser. The data acquisition cycle is triggered by a photodiode monitoring the laser pulse. Hard copies of the data were obtained by outputting to an X-Y recorder (Omnigraph 2000). Doubly glass distilled water was used as the sample for these experiments.
RESULTS AND DISCUSSION It has been our experience (8)that the poor signal reproducibility often seen with the above apparatus is due to the presence of low-frequency acoustical noise which is asynchronous with the laser pulse. Alternation of the cell geometry offers a simple way of controlling this problem, without electronic filtering. Figure 1 shows the acoustic signal generated when the N2 laser is directed through a standard cuvette containing doubly distilled water. The relatively large low-frequency excursions seen in a nominally nonabsorbing sample are due to detection of room acoustic noise. While this noise is random in phase relative to the laser generated signal, and can thus be reduced by signal averaging, there are a number of difficulties associated with this procedure. First, the acoustic noise is often comparable to, or greater than, the sample generated signal, thus making averaging very timeconsuming. Second, the finite dynamic range of the detection system @bit storage) limits the extent to which signal can be retrieved from noise (11-13). Figure 2 shows the result of preforming the same experiment with a cuvette reduced in height to 2 cm. A dramatic decrease in low-frequency noise is obvious. In a parallel series of experiments using Pyrex glass tubing in place of the cuvette, the acoustic noise was seen to scale directly with length of the tube over the range 1-10 cm. Actual PAS signals were found to be independent of cell height. Further, the noise level for a given cell height was found to depend only weakly on the
0003-2700/82/0354-2133$01.25/0 @ 1982 American Chemical Society
Anal. Chem. lg82,5 4 , 2134-2136
antenna picking up vibrations from the surroundings. This pickup can be related to the sympathetic resonance seen when a vibrating tuning fork is brought close to a nonvibrating tuning fork of the same frequency. Reduction of cell height increases the resonance frequency, an effect similar to the relationship of frequency to height of organ pipes (14). The resonance frequency for the shorter cell is apparently outside the dominant room noise frequency spectrum. In summary, low-frequency acoustical noise can be dramatically reduced in liquid PAS by reducing the height of the sample container. This simple expedient can greatly enhance the signal-to-noise ratio observed in PAS. I
I
I
25
50
75
ACKNOWLEDGMENT
TIME ( m s l
We greatly appreciate the technical assistance of D. M. Fasano and several helpful comments from N. H. Dovichi and R. A. Keller.
Figure 1. PAS signal generated from a 1-mJ N, laser pulse. The sample of doubly distil!ed water is contained in a 5 cm tall curvette.
LITERATURE CITED
I
25
(1) Patel, C. K. N.; Tam, A. C. Rev. Mod. Phys. 1981, 53, 517-550. (2) Tam, A. C.; Patel, C. K. N. Appl. Opt. 1979, 18, 3348-3358. (3) Fisher, M. R.; Nogar, N. S.;Schuster, S . M. And. Blochem. 1981, 113, 112-117. (4) Lahman, W.; Ludewlg, H. J.; Welling, H. Anal. Chem. 1977, 4 9 , 549-55 1. ( 5 ) Oda, S.; Sawada, T.; Kamada, H. Anal. Chem. 1978, 50,865-867 (1978). (6) Volghtman, E.; Jurgensen, A.; Winefordner, J. D. Anal. Chem. 1981, 53, 1442-1446. (7) Tam, A. C.; Patel, C. K. N. Opt. Lett. 1980, 5,27-29. (8) Fisher, M. R.; Nogar, N. C., unpublished results. (9) Fisher, M. R.; Fasano, D. M.; Nogar, N. S.Appi. Spectrosc. 1982, 36, 125-128. (10) Fasano, D. M.; Nogar, N. S.Chem. Blomed. Envlron. Instrum. 1981, 1 1 , 331-339. (11) Alkemade, C. Th. J.; Snelleman, W.; Boutlller, G. D.; Pollard, B. D.; Wlnefordner, J. D.; Chester, T. L.; Omenetto, N. Spectrochim. Acta, Part 8 1976, 338,383-399. (12) Boutiller, G. D.; Pollard, B. D.; Winefordner, J. D.; Chester, T. L.; Omenetto, N. Spectrochim. Acta, Part 8 338,401-415. (13) Alkemade, C. Th. J.; Snelleman, W.; Boutiller, G. D.; Wlnefordner, J. D. Spectrochim. Acta Part 8 ,1980, 358,261-270. (14) Sears, F. W.; Zemansky, M. W. “University Physics”; Addison-Wesley: Palo Alto, CA, 1964.
I
75
50
TIME (rns)
Figure 2. Same as Figure 1, except that the cell height has been reduced to 2 cm. Vertical scale is the same In both figures. A
photodiode was used to monitor the laser pulse in both cases. The pulse energies used to generate these traces varied by less than 5 % . level to which that cell was filled. Relative standard deviation in blank signal was 5% for the short cell, compared to -60% for the standard cell. We rationalize these results in terms of acoustic resonances in the sample cell. Apparently the cuvette cell acts as an
RECEIVED for review April 19,1982. Accepted June 25, 1982. This work was supported by the U.S. Department of Energy under the auspices of Los Alamos National Laboratory.
Anhydrous Sodium Thiosulfate as a Primary Iodometric Standard A. A. Woolf School of Chemistty, University of Bath, Bath, BA2 7AY, United Kingdom
Whereas sodium thiosulfate pentahydrate is not recommended as a primary iodometric standard because it tends to lose water inhomogeneously under ambient conditions, Tomlinson and Ciapetta ( I ) demonstrated that the anhydrous salt, which only rehydrates slowly, is a suitable standard. This anhydrous material has not been widely accepted, probably because of the inconvenient preparation. Popular analytical texts seem to change their allegiance between editions. Thus in Vogel(2) the anhydrous sodium salt of the second edition was replaced by BaS2O3.H20 in the third edition, and the latter had disappeared from the fourth edition but has reappeared in the fourth edition of Skoog and West (3). 0003-2700/82/0354-2134$01.25/0
The barium salt seems a poor alternative since it is too insoluble to prepare 0.1 M solutions, besides requiring large amounts of material in its preparation (4). We have now found that the crystalline pentahydrate, or even concentrated aqueous solutions, can be dehydrated with hot methanol in a matter of minutes to a loose powder of the anhydrous form. This material, which can be handled in the open for hours without appreciable rehydration and is rapidly and highly soluble with only slight heat evolution, can serve as a primary standard. A considerable saving in operating time €or preparing standard solutions of thiosulfate is possible since the preparation of and titrations with standard potassium iodate 0 1982 American Chemical Society
