ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
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2147
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Comment on the Ability of the Representation Space Transformation to Preserve Data Structure Sir: Lin and Chen have recently presented a new algorithm for displaying multivariate chemical information on a three-dimensional coordinate system, which they call Representation Space Transformation (1). They claim that this method of nonlinear mapping would fulfill the criterion of preserving the proximity relationship between information points and each of three pairs of reference information. But this assertion, which is formally expressed by Equation 4, is in contradiction to the definition of the representation coordinates expressed in Equation 6. This fact is easily perceived by calculating the distances from an arbitrary information point to one pair of reference information points, e.g., X_ and X+:
Equation 3 is also in error as the distances of X_ to Y+ and for example are not equal in general, and hence the condition for the vanishing of the y-component of the representation point of X._ according to Equation 4 is not fulfilled. Consequently the bounds of the three axis of the representation space are not identical with the representation points corresponding to the reference information! These facts do not mean that the proposed algorithm has to be rejected, but its ability to preserve data structure has to be interpreted in a slightly different way. As Equation 4 is correct for hl 0 only, one may state that the distance ratio preservation criterion is fulfilled by the points ((Pf¡¡)), defined by the individual components of the representation point vectors along the three axis (e.g. ((Px¡)) = ((*¿,0,0))), with regard to pairs of reference information marked by the same letter R. Equation 4 of course has to be reformulated in this Y_
=
sense.
where h¡
=
y¡2 + z2
A mapping method meeting criterion 4 would consist in calculating the intersection of three ellipsoids. It may easily be shown that there does not have to exist a real solution and if one exists it is not unique. Even under the above restriction the Representation Space Transformation has proved to be useful according to our
and
experience.
LITERATURE
CITED
(1) Chi-Hsiung Lin and Hwa-Fu Chen, Anal. Chem., 49, 1357 (1977),
Helmut Drack Instituí für Analytische Chemie unci Mikrochemie der Technischen Universitát Wien Getreidemarkt 9 A-1060 Vienna, Austria for h¡ 0, Equation 4 does not hold. The same result is valid for the other pairs of reference information and an arbitrary choice of distant exponent µ.
Received for review April
17, 1978.
Accepted September
5,
1978.
Breakdown of Organomercurials Sir: The recent paper by Agemian and Chau (1) concerning total mercury determination has prompted us to draw attention to the bromate-bromide technique (2) for breaking down organomercurials in water. Here, the oxidation of bromide by acid brómate produces bromine in the sample. The reaction is unaffected by waters of high chloride content since the Br2/Br~ couple has a half cell oxidation potential of 1.087 V which is lower than that of the CL/'CL couple (1.358 V). Residual bromine is removed prior to the cold vapor determination with 1-2 drops of hydroxylamine reagent. We tested the method on seawater samples (19000 mg/L CL) from the outer Thames Estuary which were spiked to 8.3 Mg/L with mercury as methylmercuric chloride. The bromination treatment and recovery experiments were performed as described previously (2) and mercury signals were compared with a standard calibration line produced by the addition of inorganic mercury standards to seawater. As
a
1-min bromination time and 2 ml, of bromate-bromide reagent, recoveries of organically bound mercury in excess of 96% were obtained. We infer from this that the bromination pretreatment could be used as an alternative method to the UV oxidation procedure of Agemian and Chau. In addition, with the bromate-bromide technique, it is possible to brominate samples at the time of sampling. Preliminary results suggest that the acid bromate-bromide reagent will preserve the inorganic mercury formed in solution. We found that samples of distilled water, seawater, and final sewage effluent, spiked to 2 Mg/L with inorganic mercury in glass flasks and treated with 2 mL of the brominating reagent, showed no significant loss of mercury after 30 days' standing. Finally, similar to the UV oxidation procedure, the bromination pretreatment removes the sulfide interference from the cold vapor mercury signal. Following bromination with a
result, employing
0003-2700/78/0350-2147S01,00/0
c
1978 American Chemical Society
2148
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978 (2) B. J. Farey, L. A. Nelson, and M. G. Rolph, Analyst (London), 103, 656
mL of reagent, sulfide concentrations as high as 24 mg/L shown not to interfere with the recovery of inorganic and organically bound mercury added to distilled water. The interference from higher concentrations of sulfide can be eliminated by using larger quantities of brominating reagent but some depression of the subsequent mercury signal may be obtained (2). 3
(1978).
(as Na2S) were
Brian J, Farey L. Andrew Nelson*
Metropolitan Water Services Thames Water Authority 177 Rosebery Avenue London, ECIR 4TP, U.K.
LITERATURE CITED
Received for review -July 24, 1978. Accepted September
11.
1978.
(1) H. Agemian and A. S. Y. Chau, Anal. Chem., 50, 13 (1978).
Limitations of Synchronous Luminescence Spectrometry in Multicomponent Analysis Sir: Synchronous excitation spectrofluorimetry was introduced by Lloyd (1) and developed as a technique for fingerprinting complex forensic samples (2). It has also been applied to the characterization of oil pollution samples (3). More recently, simultaneous variation of the excitation and emission wavelengths at a predetermined wavelength separation ( ) has been used to identify specific polyaromatic hydrocarbons (PAH) in a synthetic mixture (4). An effort to utilize this approach in our laboratory immediately revealed that only a fortuitous combination of substances, concentrations, and fluorescence quantum efficiencies will provide reliable qualitative and quantitative data. Conventional fluorimetric procedures are required to reveal interferences that may mask the presence of one or more components in a mixture. In the event that masking is not complete, these same interferences would preclude any quantitative measurements.
i n+m
(b)
(XI)
EXPERIMENTAL Apparatus. A laboratory-constructed spectrofluorimeter was
used to obtain all spectra. The components used are described in Table I. The monochromator stepping motors were operated
300 Wavelength
individually by controlling their respective scan drivers in the internal mode or simultaneously using an external pulse to both
400
500
(nm!
RESULTS AND DISCUSSION
Figure 1. Synchronous fluorescence spectrum of mixture of poly= 3 nm. (a) Five-component PAH aromatic hydrocarbons (PAH). mixture. I. Perylene, 0.06 gg/mL; II. Anthracene, 1.0 gg/mL; III. Phenanthrene, 40.0 gg/mL; IV. Fluorene, 24.0 gg/mL; V. Pyrene, 40.0 gg/mL. (b) Four-component PAH mixture. I. Perylene, 0.06 gg/mL; II. Anthracene, 1.0 gg/mL; III. Phenanthrene, 40.0 gg/mL; IV. Fluorene, 24.0 gg/mL
The synchronous fluorescence spectrum from a mixture of five randomly selected PAHs is shown in Figure la ( s 5 nm was found to be optimum in terms of spectral discrimination and resolution; $ < 4 nm resulted in severe stray > 5 nm resulted in a loss in selectivity). light problems and Peak identification was based on spectra derived from pure solutions of the individual compounds. It should be noted that the largest peak (II + III) is due mainly to an anthracene concentration of 1 gg/mL with only a small contribution from 40 gg/mL of phenanthrene which has its strongest peak at the same wavelength. The secondary phenanthrene peak also has some contribution from pyrene. It is evident that qualitative surveys can be in error because of wavelength coincidences and/or weak fluorescence signals. Most notable in Figure la is the lack of any fluorene peak at =300 nm, where a strong signal was observed for a pure
solution of this compound. An emission spectrum of the mixture (used to obtain Figure la) obtained with a fluorene excitation wavelength resulted in a pyrene emission spectrum with a small phenanthrene component, making it apparent that any fluorene emission was being efficiently masked. The synchronous spectrum of a mixture containing no pyrene is shown in Figure lb and the strong fluorene signal obtained confirmed that the depressive interference was due primarily to pyrene. The extent to which the pyrene concentration affects the fluorene emission intensity is presented in Table II. It is apparent that as long as the ratio of concentrations of fluorene to pyrene exceeds ~36, the error in the fluorene signal will be less than 10%. Concentrations of the PAH components other than those in Table II or Figure 1 gave similar results and conclusions.
scan
drives.
Chemicals. All of the compounds used were commercially available and were used without further purification.
0003-2700/78/0350-2148$01.00/0
©
1978 American Chemical Society
