Some Observations on the Effect of Molecular Structure Upon the Determination of the Selenium to Carbon Ratios in Various Organoselenium Compounds Using Gas Chromatography With Atomic Emission Spectrometric Detection RICHARD BOS AND NEIL W. BARNETT* School of Biological and Chemical Sciences, Deakin University, Geelong, V ictoria, 3217, Australia The performance of a commercially available gas early 1970s,7,8 was the ability of the technique to provide interelement ratios and therefore empirical formula information.chromatograph employing an atomic emission detector has been evaluated with respect to analytical figures of merit and The numerical ratio of any particular element to carbon in an unknown compound can be calculated from the element the determination of selenium to carbon ratios using a series of asymmetric and symmetric diorganyl diselenides and selective chromatograms of both the unknown and a reference compound, by employing the following expression:3,9,10 selenides.Three selenium atomic emission lines, at 196.09, 203.98 and 216.42 nm, were employed during the study. A temporally resolved secondary emission phenomenon was N¾E N¾C = R¾E R¾C × RC RE × NE NC (1) observed near the 196.09 nm line which was tentatively attributed to molecular emission from SeO. This emission where NC and N¾C are the numbers of carbon atoms in the impaired the analytical performance of the 196.09 nm atomic reference and unknown compounds, respectively; NE and N¾E emission line, with the best overall figures of merit being are the numbers of analyte atoms in the reference and unknown achieved at 203.98 nm.The accuracy of the selenium to carbon compounds, respectively; RC and R¾C are the carbon responses ratios was found to be adversely eected when the for the reference and unknown compounds, respectively; RE concentration of the analyte varied significantly from that of and R¾E are the analyte responses for the reference and the reference compound, especially when the Se 196 nm unknown compounds, respectively. emission line was used.Under optimal conditions, it was found The above expression assumes the absence of random instruthat the accuracy of Se5C ratios could be determined to within mental error as well as operation within the linear calibration ±10% in the majority of cases. The instrumentation was range of the analyte elements in question. Errors associated successfully used to separate and identify the products from with empirical formulae calculated for organosulfur9 and two separate series of organometallic exchange reactions, the organochlorine11 compounds using GC–MIP-AES have been first involving the eight symmetrical diselenides with found to be within ±5%, and for organotin compounds,12 themselves, and secondly these same diselenides with diphenyl ±10%.However, van Dalen et al.13 observed that the elemenditelluride. tal response in MIP-AES could be dependent upon molecular structure of the analyte species. Subsequent investigations12–19 Keywords: Gas chromatography; microwave-induced plasma; have revealed empirical formulae determined by GC–MIP- atomic emission spectrometric detection; organoselenium; AES may also be dependent upon molecular structure; organotellurium compounds additionally, carbon to hydrogen ratios determined in this way show sensitivity to concentration.11,14,20 More than three decades ago, McCormack et al.1 together In the present paper the evaluation of the performance of a with Bache and Lisk2 first demonstrated the feasibility of commercially available gas chromatograph using atomic emis- coupling a gas chromatograph to an argon microwave-induced sion detection21,22 (AED) is reported for the determination of plasma (MIP) in order to achieve element selective detection selenium to carbon ratios for various classes of organoselenium of the eluents via atomic emission spectrometry (AES).The compounds. high eciency of capillary gas chromatography (GC) facilitates the introduction of small bands of pure compounds into such a detector. This combination yields a technique with the EXPERIMENTAL separating power of chromatography enhanced by the selec- Instrumentation and Procedures tivity and sensitivity of AES.3 Advances in the development of microwave cavities, particularly the work of Beenakker4 who The GC–AED instrumentation was configured from an HP 5890 Series II gas chromatograph equipped with electronic designed a cavity which could sustain a helium plasma at atmospheric pressure, greatly facilitated the use of MIP-AES pressure control and an HP 7673 automatic sample injector.The chromatograph was interfaced to an HP 5921A atomic for element selective detection in GC. The excitation energy associated with the helium plasma is sucient to produce emission detector. Control and operation of the system was achieved using an HP 35920A pascal chemstation with atomic emission from all elements in the Periodic Table.5 The analytical applications of MIP-AES as an element selective GC–AED software.Instrumental operating parameters are summarised in Table 1. The chromatographic column and detector have been recently reviewed by Uden.6 Another advantageous feature of GC–MIP-AES, first highlighted in the conditions used with the GC–electron impact ionisation mass Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 (733–741) 733Table 1 Instrumental operating conditions for the GC-AED both peak shape and resolution. Initial pressures higher than 225 kPa resulted in very high flow rates, which caused the Injection port temperature 250°C dimethyl diselenide to be unresolved from the solvent front. Cavity temperature 250°C Water temperature 66°C Transfer line temperature 250°C Reagents and Standards Injection volume, mode 0.5 ml, splitless The homologous series of symmetrical dialkyl diselenides Column HP1 25 m×0.32 mm with 0.17 mm film thickness (dimethyl to dihexyl), dibutyl selenide, 1,3-dihydrobenzo- Electronic pressure program 225 kPa, (giving a flow rate of 110 ml [c]selenophene and 2,1,3-benzoselenadiazole, were synthesized min-1), 0.05 min, then decreasing according to established literature methods25–28 and were at 680 kPa min-1 to 65 kPa, purified by either double distillation using a GKR-1 and set to constant carrier flow of Kugelrohr micro distillation apparatus (Buchi Laboratoriums- 1.91 ml min-1 Technik AG, Flawil, Switzerland) at reduced pressure, or in Spectrometer purge 2 l min-1 N2 Solvent vent on, o 1.1 min, 2.2 min the case of 2,1,3-benzoselenadiazole, triple sublimation was Reagent gas Hydrogen used.The six asymmetric aromatic selenides (compounds A–F) and diphenyl ditelluride were available from colleagues within the School, while the diphenyl diselenide was commercially available (Fluka Chimica, Buchs, Switzerland).The spectrometer (HP 5972) were the same as those used for molecular structures of the organoselenium compounds GC–AED (Hewlett-Packard Australia, Blackburn, Victoria, employed in this study have been shown in Fig. 1. [N.b. It Australia). Nuclear magnetic resonance (NMR) spectra (77Se) must be remembered that working with inorganic and organic were obtained with a JNM-GX 270 MHz Fourier Transform selenium compounds may entail a serious risk of poisoning if NMR spectrometer (Jeol, Tokyo, Japan).they are handled without due care.29 In particular, if an excess Reagent and make-up gas conditions were as per manufac- of sodium borohydride is added during the synthesis of selen- turers recommendations. The purity of gases used were: nitro- ides, then the highly toxic gas hydrogen selenide could be gen 99.99, hydrogen 99.999, and helium 99.999% (Linde Gas, produced. The more volatile members of the dialkyl selenides Sydney, Australia), the helium was further purified using a and dialkyl diselenides are also exceedingly malodourous.] getter, Model PS2GC50R (SAES, Milano, Italy).The oxygen purity was 99.999% (BOC Gases Australia, Chatswood, Australia). The reagent gas added to the plasma during the RESULTS AND DISCUSSION simultaneous determination of carbon and selenium was Analytical Standards hydrogen. The oven temperature program for the determination of the The purity of the each of the selenium compounds used in the present study was estimated from five replicate C 193 chroma- analytical performance was 70°C initially (held for 0.3 min), then increasing at 25°C min-1, with the final temperatures tograms run for each of the individual analytes.The percentage purity of each compound was calculated by summing the being dependent upon the particular analyte compound. For the separation of diselenide exchange reaction products the average areas of all extraneous peaks obtained in the C 193 chromatograms and calculating the ratio of the analyte peak program was 70°C initially (held for 0.3 min), then increasing at 3°C min-1, to a final temperature of 250°C. For the area to the total area of all peaks present.With the exception of 1,3-dihydrobenzo[c]selenophene being at 95% m/v purity, separation of diselenide–ditelluride exchange reaction products the program was 70°C initially (held for 0.3 min), then increas- all other compounds used in this study exhibited purities better than 98% m/v.ing at 3°C min-1 up to a temperature of 150°C, then increasing at 6°C min-1, up to a final temperature of 300°C. The Five replicate injections of each of 11 standard solutions were used to construct individual calibration graphs for the remaining determinations were carried out at 70°C initially (held for 0.3 min), then increasing at 10°C min-1, with the compounds listed in Table 2. Owing to the availability of only small amounts of the compounds 1,3-dihydrobenzo[c]seleno- final temperature in each case being dependent upon the particular analyte compound.All solutions used for GC–AED phene, 3-phenylbenzo[b]selenophene, C, D, E and F (see Fig. 1), it was not possible to ascertain the analytical figures were made up in n-hexane unless otherwise specified; those for NMR experiments were prepared in deuterated chloroform. of merit for these species. Given the structural similarities between the selenophenes and between the compounds A–F, The atomic emission line wavelengths23 selected for this investigation were as follows: C I 193.0905, Se I 196.09, Se I it was reasonable to assume that their respective elemental responses and hence their analytical figures of merit would 203.98, Se I 216.42 and Te I 200.002 nm.For simplicity these atomic emission lines will be hereafter abbreviated to C 193, reflect this. The first ten analyte standards were made up in the concentration range 0.70–3000 mg l-1 of Se.The Se 196, Se 204, Se 216 and Te 200, respectively. Hydrogen was deliberately omitted as preliminary results indicated that 3-methylbenzo[b]selenophene and compounds A and B were made up in solutions of n-hexane and dichloromethane (1+1) hydrogen was found to be concentration dependent, and thus not reliable. Additionally, the determination of hydrogen (at in the concentration range 0.70–1500 mg l-1 of Se; the inclusion of dichloromethane greatly facilitated the solubility 486.13 nm) would have required an additional injection, thus doubling an already large data set, resulting in little gain.of these analytes. The slopes, linear correlation coecients (r2) and detection As this study employed the use of organoselenium compounds known to be thermally labile, pressure programming limits (DL), calculated for each of the resultant log–log calibration graphs have been listed in Table 2.Some typical using the electronic pressure control (EPC) was employed to reduce thermal decomposition during injection. The method log–log calibrations for each of the selenium atomic emission lines used are shown in Fig. 2. The DLs were determined from used was directly analogous to that recently documented by Vincenti et al.24 It was found that utilising splitless injection the individual calibration functions using a response equal to a signal-to-noise ratio of 351.The linearity, as measured by in conjunction with EPC resulted in a pressure split injection with an eective split ratio of 6.5:1. Using diphenyl diselenide the respective slopes of the calibrations (see Table 2) was clearly superior for the Se 204 line compared with the other and dimethyl diselenide (the analytes with the highest and lowest boiling points, respectively), it was found that an initial two emission lines. It should be borne in mind that the slopes and linear correlation coecients listed in Table 2 are those pressure of 225 kPa gave the best chromatograms, in terms of 734 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Fig. 1 Molecular structure of the various organoselenium compounds used in the present study. Table 2 Analytical figures of merit for selected organoselenium compounds. The slopes and correlation coecients (r2) were calculated from linear regression carried out on the individual log–log calibration graphs.The detection limits (DL, in mg l-1 of Se injected) were determined from each calibration function using a response equivalent to a signal-to-noise ratio of 351 Se 196 Se 204 Se 216 Analyte Slope r2 DL Slope r2 DL Slope r2 DL Dimethyl diselenide 0.80 0.972 0.06 0.99 0.996 0.1 0.89 0.998 1 Diethyl diselenide 0.77 0.979 0.05 0.93 0.997 0.08 0.78 0.999 0.9 Diisopropyl diselenide 0.80 0.977 0.05 0.95 0.997 0.1 0.82 0.998 0.9 Dipropyl diselenide 0.85 0.965 0.08 1.02 0.993 0.1 0.86 0.998 1 Dibutyl diselenide 0.81 0.977 0.06 0.94 0.995 0.1 0.77 0.999 0.9 Dipentyl diselenide 0.84 0.964 0.08 0.98 0.990 0.1 0.78 0.996 0.9 Dihexyl diselenide 0.85 0.969 0.08 0.98 0.992 0.1 0.81 0.980 1 Diphenyl diselenide 0.87 0.921 0.2 1.01 0.985 0.4 0.85 0.994 2 Dibutyl selenide 0.78 0.978 0.03 0.93 0.999 0.05 0.81 0.999 0.6 2,1,3-Benzoselenadiazole 0.85 0.952 0.1 1.02 0.984 0.1 0.85 0.993 1 3-Methylbenzo[b]selenophene 1.02 0.988 0.5 0.98 0.998 0.8 0.80 0.996 10 Compound A 0.97 0.987 0.5 1.02 0.993 0.8 0.85 0.998 10 Compound B 1.13 0.918 0.6 0.94 0.996 0.8 0.84 0.996 10 calculated from the line of best fit.The dimethyl diselenide dierence in the correlation coecients obtained for the calibration functions with either the Se 204 or Se 216 lines, those calibration plots in Fig. 2 clearly indicate some degree of curvature for all three atomic emission lines, with the Se 196 calculated from the Se 196 calibration functions were found to be considerably worse and covered a much broader range.line being most severely aected. However, there also appears to be some compound dependence superimposed upon the This result reflects the curvature exhibited by these calibrations (see Fig. 2). results in Table 2. For example, diethyl diselenide and dibutyl selenide produced some of the largest deviations from linearity The generally poorer linearity observed with the Se 196 line arises from a temporally resolved (secondary) emission, which on all three emission lines.Some considerable improvements in the linearity of calibrations using the Se 196 line were could be due to the presence of SeO in the plasma. This secondary emission became evident with all analytes at concen- observed with 3-methylbenzo[b]selenophene and compounds A and B. The latter observations reflect the narrower cali- trations between 500 and 700 mg l-1 of Se, with the eect being most noticeable on the Se 196 line (see Fig. 3).The bration range employed and the slightly inferior DLs achieved with these three analytes. While there is little or no significant secondary peak shown in Fig. 3 was not a closely eluting Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 735Fig. 4 Linear plot of peak area versus concentration for dibutyl diselenide using the Se 196 line which illustrates the dramatic eect of Fig. 2 Log–log calibration plots for dimethyl diselenide at each of the secondary emission upon the calibration function.the three selenium atomic emission lines. 50928 cm-1 (196.35 nm), with less intense bands occurring at 49128 (203.55) and 46068 cm-1 (217.07 nm).31 These three molecular emission band-heads are, respectively,+0.26, -0.55 and +0.65 nm from the Se 196, Se 204 and Se 216 nm atomic emission lines. The more pronounced secondary emission observed at the Se 196 line (see Fig. 3) is consistent with both the relative intensities of the SeO band-heads31 and the dispersion of the spectrometer employed in the GC–AED instrument (0.2 nm per pixel in the vacuum UV).22 The DLs listed in Table 2, for each atomic emission line, show no significant dierences within the homologous series of dialkyl diselenides.However, somewhat inferior detectability was realized with diphenyl diselenide, 3-methylbenzo[ b]selenophene, plus compounds A and B. These dierences in elemental response may reflect the various molecular environments of the selenium (see Fig. 1). For example, the varying carbon–selenium bond strengths as well as carbon Fig. 3 Typical example of the temporally resolved secondary emission to selenium ratios could aect the eciency of atomization observed while monitoring the Se 196 line, in this case the analyte was and excitation within the plasma. Carbon to element ratios dihexyl diselenide (#3000 mg l-1 of Se). have previously been found to be sensitive to samples of high relative molecular mass.15,32 The marginally better DLs achieved with the Se 196 line organoselenium impurity since no such peak was seen in any of the GC–EI-MS experiments.This finding was confirmed compared with the Se 204 line have also been previously reported by Timmins,33 who employed a tantalum electrother- using the wavelength snapshot facility on the GC–AED instrument, which indicated only background levels of carbon from mal vaporiser to introduce samples into a helium MIP. To the best of our knowledge, the analytical figures of merit for the the C 193 line at the appearance time of the secondary peak.The curvature observed with the Se 196 calibrations occurs Se 216 line reported here are presented for the first time. The DLs achieved in the present study are generally compar- due to the integration of both the primary and secondary peaks together at concentrations below (500–700 mg l-1 of Se) able to those obtained by earlier workers employing GC–MIPAES instrumention30,34–37 (see Table 3). While it is clear that and separate integrations above this range, thus causing deviation of the calibration function towards the concentration units of DL given in Table 2 (mg l-1 of Se injected) have far more relevance to the practising analyst, the conversions to pg axis (see Fig. 2) owing eectively to decreasing the response per unit of analyte concentration. This eect is more dramati- or pg s-1 in Table 3 were necessary in order to compare the present work with earlier studies.30,34–37 With the exception of cally demonstrated using a linear–linear calibration plot where this artifact of the integration caused by the secondary emission the work of Tsunoda et al.36 there are only minor dierences in the reported DLs for similar compounds at each of the two produces two separate calibration functions (see Fig. 4). The temporal resolution could result from the chemisorption of commonly used atomic emission lines. It should be noted that the analytical figures of merit reported in the present study selenium onto the internal walls of the silica discharge tube, followed by subsequent desorption of a selenium–oxygen com- were routinely achieved over several months with the commercially available instrumentation by adhering to the manufac- pound into the plasma.Estes et al.30 have reported similiar secondary emission problems with the determination of turers recommended operating parameters. An evaluation of the instrumental precision was carried organoboron compounds using GC–MIP-AES.They, likewise, postulated the adsorption of the analyte onto the discharge out using two organoselenium compounds, diphenyl diselenide and 2,1,3-benzoselenadiazole, at concentrations of 100 and tube prior to the desorption of a boron–oxygen species.30 The molecular emission spectrum of SeO has been studied in detail 120 mg l-1 of Se, respectively, for each of the three simultaneously monitored selenium atomic emission lines.Twenty- by Reddy and Azam31 using a microwave supported discharge, and they qualitatively reported a very strong band at five replicates of each standard solution were sequentially 736 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Table 3 Comparison of analytical figures of merit achieved for several organoselenium compounds using GC–MIP-AES Atomic DL Cavity type and emission plasma gas Analyte line/nm pg s-1 pg Reference Raytheon 7097–1001 2,1,3-Benzoselenadiazole 203.98 —* 40 34 G1 and 7097–500 G1 Argon Beenakker TM010 Diethyl selenide 203.98 5.3 62 30 Helium Hewlett Packard TM010 —* 196.09 4.0 —* 35 Helium —* Dimethyl selenide 203.98 —* 500 36 Hewlett Packard TM010 Diphenylselenide 196.09 —* 10 37 Helium Hewlett Packard TM010 Dibutyl selenide 196.09 8 15 Present Helium 203.98 12 25 study 216.42 100 200 2,1,3-Benzoselenadiazole 196.09 15 50 203.98 15 50 216.42 200 500 * Information not available.injected (0.5 ml). Using these two standard solutions relative range 1.0–1.2, whereas those for 3-phenylbenzo[b]selenophene standard deviations (RSD) could be determined at concen- were in the range 0.8–1.0 (with one exception). This result trations which varied from 50 times to 1200 times the achiev- suggests some type of systematic eect upon the calculation able DL (see Table 2). The results of the precision which may reflect the dierences in molecular size or structure determinations are summarised in Table 4.The slightly between the two analytes. However, no such systematic deviimproved precision attained with the Se 204 line compared ation was observed with the determination of the selenium with the Se 196 line could reflect the irreproducible nature of content of compounds A–F using four reference compounds the secondary emission. The inferior precision attained with (see Table 6). In this more extensive evaluation the Se 204 line the Se 216 line was consistent with the poorer detectability at exhibited a consistently more accurate performance compared this atomic emission line (see Fig. 2). Overall, the achievable with the Se 196 and Se 216 lines. The poorest estimation of precision at all three atomic emission lines was acceptable for the selenium content of compounds A–F was achieved using inter-element ratio determinations. the Se 196 line and 2,1,3-benzoselenadiazole as a reference. A detailed examination of the chromatograms employed for these determinations revealed the presence of a small but significant Determination of Selenium to Carbon Ratios secondary emission peak which was included in the peak The chromatographic experiments used to evaluate the ability integration.This explains the high selenium results for these of GC–AED to calculate elemental ratios were set up with compounds. The overall performance of GC–AED (with some selected compounds compared with a series of internal refer- exceptions) for the determination of selenium content was ence compounds.These combinations were made on the basis fairly good with no obvious relationship emerging between of chromatographic resolution and molecular diversity in order accuracy and the molecular structure of either analytes or to ascertain how the molecular structures of both the analyte reference compounds. and reference compounds might aect the calculation of The eight symmetrical diselenides (see Fig. 1) could be elemental ratios. The results calculated for selenium were readily separated from each other, and the chromatogram normalised to the number of carbon atoms in the analyte shown in Fig. 5 was obtained from the analysis of a mixed molecule, consequently only the calculated numbers of sel- diselenide standard solution plus 2,1,3-benzoselenadiazole enium atoms per molecule appear in the following tables. The (which was present as the reference compound) injected calculations were performed using eqn.(1) with the tabularised immediately after preparation. Given the ease with which the results being the average of five individual determinations for members of this homologous series could be separated from each analyte and each reference combination. each other, and the selected reference compound, a more The results listed in Table 5 show no obvious dependence detailed experiment was designed. Mixed diselenide stan- upon either concentration or the choice of emission line.dard solutions were prepared at various concentrations of Overall, the estimation of selenium content was, at worst, from 1 up to 2000 mg l-1 of Se each containing 2,1,3-benzo- ±20%, with the majority being within ±10% of the actual selenadiazole at approximately the same concentration as value. Interestingly, the choice of reference compound, based the diselenides. These standard solutions were then analysed upon a similar molecular environment of the heteroatom, does five times each with a view to determining the selenium not appear to be essential, since both the dibutyl selenide and content in a similiar manner to the compounds in Tables dihexyl diselenide performed, on average, equally as well as 4 and 5.As the suite of standard solutions took several hours did the 1,3-dihydrobenzo[c]selenophene. It was noteworthy to prepare, owing to the care required for the handling of that the results for 3-methylbenzo[b]selenophene were in the volatile and toxic compounds, the analyses were performed overnight using the programmable autosampler.Examination Table 4 RSDs (%) for 2,1,3-benzoselenadiazole (120 mg l-1 of Se) of the considerable number of resultant chromatograms from and diphenyl diselenide (100 mg l-1 of Se) obtained simultaneously at the C 193, Se 196, Se 204 and Se 216 atomic emission lines each of the three selenium atomic emission lines revealed that there were many more peaks present than there were analytes in the original standard solutions (see Fig. 6). Compound Se 196 Se 204 Se 216 Also, those peaks at the retention times previously determined 2,1,3-Benzoselenadiazole 1.3 0.8 3.6 for the individual diselenides, when integrated, gave area values Diphenyl diselenide 1.3 1.1 2.8 which translated to concentrations well below what was Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 737Table 5 Calculated selenium to carbon ratios for 3-methylbenzo[b]selenophene and 3-phenylbenzo[b]selenophene normalised to the number of carbon atoms in each of the two analyte molecules using various reference compounds at two concentrations of selenium Reference compound 1,3-Dihydrobenzo- Analyte compound Wavelength/nm Dibutyl selenide [c]selenophene Dihexyl diselenide 3-Methylbenzo[b]selenophene Concentration of analyte and reference compounds, 100 mg l-1 of Se — Se 196 Se1.2 Se1.0 Se1.1 Se 204 Se1.2 Se1.0 Se1.1 Se 216 Se1.2 Se1.0 Se1.1 Concentration of analyte and reference compounds, 500 mg l-1 of Se — Se 196 Se1.1 Se1.2 Se1.1 Se 204 Se1.1 Se1.0 Se1.0 Se 216 Se1.1 Se1.0 Se1.0 3-Phenylbenzo[b]selenophene Concentration of analyte and reference compounds, 100 mg l-1 of Se — Se 196 Se1.0 Se0.9 Se0.9 Se 204 Se0.9 Se0.8 Se0.9 Se 216 Se1.0 Se0.8 Se0.9 Concentration of analyte and reference compounds, 500 mg l-1 of Se — Se 196 Se1.0 Se1.1 Se1.0 Se 204 Se0.9 Se0.8 Se0.8 Se 216 Se1.0 Se0.9 Se0.9 Table 6 Calculated selenium to carbon ratios for compounds A–F (normalised to the number of carbon atoms in each of the six analyte molecules) using various reference compounds with both analytes and reference compounds at 500 mg l-1 of Se Reference compound used Analyte Dibutyl 2,1,3-Benzo- 3-Methylbenzo- Diphenyl Compound selenide selenadiazole [b]selenophene diselenide Se 196 — A Se1.0 Se1.5 Se0.9 Se1.2 B Se0.8 Se1.2 Se0.7 Se0.9 C Se0.9 Se1.3 Se0.8 Se1.1 D Se1.0 Se1.5 Se0.9 Se1.2 E Se0.9 Se1.4 Se0.8 Se1.1 F Se0.8 Se1.3 Se0.8 Se1.0 Se 204 — A Se1.0 Se1.0 Se1.0 Se1.2 Fig. 5 An Se 196 chromatogram obtained from a mixed diselenide B Se1.1 Se1.1 Se1.0 Se1.2 standard solution injected immediately after preparation: 1, dimethyl; C Se0.9 Se0.9 Se0.9 Se1.0 2, diethyl; 3, dipropyl; 4, 2,1,3-benzoselenadiazole; 5, dibutyl; 6, dipen- D Se0.9 Se0.9 Se0.9 Se1.0 tyl; 7, dihexyl; and 8, diphenyl. E Se0.9 Se0.9 Se0.8 Se1.0 F Se0.8 Se0.8 Se0.8 Se0.9 Se 216 — equilibrium exchange reactions of the type A Se1.2 Se1.2 Se1.1 Se1.3 R-Se-Se-R+R¾-Se-Se-R¾=2R-Se-Se-R¾ (2) B Se1.2 Se1.2 Se1.1 Se1.3 C Se0.8 Se0.9 Se0.8 Se1.0 As the mixed standard solutions contained eight symmetrical D Se1.0 Se1.0 Se0.9 Se1.1 diselenides, the maximum number of asymmetric exchange E Se0.9 Se0.9 Se0.8 Se1.0 products possible was 28, thus giving a total of 36 compounds.F Se0.8 Se0.8 Se0.7 Se0.9 The characterisation of these exchange products was obtained by analysing a series of nine mixed diselenide standard solutions.The first standard contained all eight symmetrical dis- expected from earlier calibration functions. However, the peak area for the 2,1,3-benzoselenadiazole was not diminished in elenides, whilst in the remaining eight standards each had a dierent diselenide omitted from the mixture. Therefore, by any of the selenium chromatograms. As the carbon and selenium chromatograms were all identical with regard to the comparing each of the resultant chromatograms from the latter eight standards with that containing all the symmetrical dis- number of peaks and their retention times, the additional compounds observed were, therefore, all organoselenium elenides, a process of elimination clearly identified the presence and molecular formula of the 36 exchange products (see Fig. 7). species. Decomposition of the individual analytes upon standing or during the chromatographic process could be ruled out These assignments were subsequently confirmed using GC–EI-MS.The random distribution of the relative peak on the basis of previous calibration studies. Nevertheless, all the diselenides were redistilled and re-assayed to confirm their areas in Fig. 7 probably reflects the complexity of the 36 compounds attempting to equilibrate with each other via a purity. Using these re-purified compounds another suite of mixed diselenide standard solutions were prepared and ana- myriad of exchange reactions.The characterisation of the exchange products resulting from lysed overnight with the same result as shown in Fig. 6. An examination of the organometallic literature38 revealed that the symmetrical diselenide mixture allowed the calculation of the selenium to carbon ratios for an extra 28 compounds. As the symmetrical diselenides exhibit a propensity to undergo 738 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12Table 7 Calculated selenium to carbon ratios for the seven symmetrical diselenides plus the 28 asymmetrical diselenides resulting from the exchange reactions of the eight parent diselenides, with dipropyl diselenide arbitrarily selected as the reference compound C5Se Compound ratio Se 196 Se 204 Se 216 Me-Se-Se-Me C1Se Se0.6 Se0.8 Se0.7 Et-Se-Se-Me C3Se2 Se1.0 Se1.7 Se1.4 Pri-Se-Se-Me C2Se Se1.0 Se1.0 Se1.0 Et-Se-Se-Et C2Se Se0.8 Se0.9 Se0.7 Me-Se-Se-Pr C2Se Se0.6 Se1.0 Se0.9 Et-Se-Se-Pri C5Se2 Se2.2 Se2.1 Se2.1 Et-Se-Se-Pr C5Se2 Se1.8 Se1.9 Se1.8 Pri-Se-Se-Pri C3Se Se0.7 Se0.9 Se0.8 Me-Se-Se-Bu C5Se2 Se1.8 Se1.8 Se1.8 Pri-Se-Se-Pr C3Se Se1.2 Se1.0 Se1.1 Pr-Se-Se-Pr C3Se (Used as reference compound) Bu-Se-Se-Et C3Se Se1.0 Se1.0 Se0.9 Me-Se-Se-Pe C3Se Se0.9 Se1.0 Se0.9 Bu-Se-Se-Pri C7Se2 Se2.5 Se2.0 Se2.3 Bu-Se-Se-Pr C7Se2 Se2.2 Se2.0 Se2.1 Et-Se-Se-Pe C7Se2 Se2.4 Se2.3 Se2.2 Hx-Se-Se-Me C7Se2 Se1.7 Se1.9 Se1.8 Pe-Se-Se-Pri C4Se Se1.2 Se1.1 Se1.2 Me-Se-Se-Ph C7Se2 Se2.6 Se2.2 Se2.3 Fig. 6 An Se 196 chromatogram obtained from a mixed diselenide Bu-Se-Se-Bu C4Se Se1.1 Se1.0 Se1.0 standard solution which had been allowed to stand for several hours Pe-Se-Se-Pr C4Se Se1.1 Se1.0 Se1.0 prior to injection.Et-Se-Se-Hx C4Se Se0.9 Se1.0 Se0.9 Et-Se-Se-Ph C4Se Se1.3 Se1.2 Se1.3 Hx-Se-Se-Pri C9Se2 Se2.4 Se2.1 Se2.3 with the earlier determinations the selenium content was Bu-Se-Se-Pe C9Se2 Se2.4 Se2.1 Se2.4 derived from eqn. (1) using the simultaneous responses from Hx-Se-Se-Pr C9Se2 Se2.1 Se1.9 Se2.0 Pri-Se-Se-Ph C9Se2 Se2.5 Se2.2 —* the simultaneously monitored C 193, Se 196, Se 204 and Se Ph-Se-Se-Pr C9Se2 Se2.5 Se2.3 Se2.3 216 lines.Dipropyl diselenide was arbitrarily chosen as the Pe-Se-Se-Pe C5Se Se1.2 Se1.1 Se1.1 reference compound and all results are listed in order of elution Bu-Se-Se-Hx C5Se Se1.2 Se1.0 Se1.1 in Table 7. Unlike the results shown in Tables 5 and 6, in these Bu-Se-Se-Ph C5Se Se1.3 Se1.0 —* experiments the Se 204 atomic emission line clearly out per- Hx-Se-Se-Pe C11Se2 Se2.4 Se1.9 Se1.8 formed the other two lines with all the determinations being Pe-Se-Se-Ph C11Se2 Se1.9 Se1.8 —* Hx-Se-Se-Hx C6Se Se0.9 Se1.0 Se0.8 within ±20%, and 29 of the 35 within ±10% of the true Hx-Se-Se-Ph C6Se Se1.4 Se1.1 Se0.7 value. This represents fairly reasonable performance consider- Ph-Se-Se-Ph C6Se Se1.4 Se1.2 Se1.0 ing the significant variations in the concentrations of the reference and analyte compounds based upon peak areas (see * Below the DL, where: Me is methyl, Et is ethyl, Pr is propyl, Pri Fig. 7). The Se 196 results exhibited the greatest deviations is isopropyl, Bu is butyl, Pe is pentyl, Hx is hexyl and Ph is phenyl. from the actual selenium values, with variations from -50 to +40%. This relatively poor performance most probably arises from the secondary emission eect exacerbated by the large Fig. 7 An Se 196 chromatogram showing all 36 resultant peaks (two co-eluting) from the exchange reactions between the eight symmetrical diselenides, where: 1, dimethyl; 2, ethylmethyl; 3, isopropylmethyl; 4, diethyl; 5, methylpropyl; 6, ethylisopropyl; 7, ethylpropyl; 8, diisopropyl; 9, butylmethyl; 10, isopropylpropyl; 11, dipropyl; 12, ethylbutyl; 13, methylpentyl; 14, butylisopropyl; 15, butylpropyl; 16, ethylpentyl; 17, hexylmethyl; 18, isopropylpentyl; 19, methylphenyl; 20, dibutyl; 21, pentylpropyl; 22, ethylhexyl; 23, ethylphenyl; 24, hexylisopropyl; 25, butylpentyl; 26, hexylpropyl; 27, isopropylphenyl; 28, phenylpropyl; 29, dipentyl; 30, butylhexyl; 31, butylphenyl; 32, hexylpentyl; 33, phenylpentyl; 34, dihexyl; 35, hexylphenyl; and 36, diphenyl.Journal of Analytical Atomic Spectrometry, July 1997, Vol. 12 739Fig. 8 An 77Se NMR spectrum of an equimolar mixture of dimethyl diselenide and diphenyl diselenide (collected 16 h after mixing) showing the two resonances arising from the exchange product at 291 and 440 ppm. dierences in concentration between some analytes and the reference compound. Fig. 9 A Te 200 chromatogram showing seven organylselenenyltellur- As noted previously, a mixed diselenide standard, if prepared ides which result from the reactions between the seven dialkyldiselen- and analysed rapidly, would give a chromatogram which ides and diphenyl ditelluride: 1, methyl; 2, ethyl; 3, isopropyl; 4, propyl; contained the same number of peaks as there were compounds 5, butyl; 6, pentyl; 7, hexyl; and 8, diphenyl ditelluride.in the solution. If, however, the mixed standard solution was allowed to stand for ca. 2 h prior to injection (especially on a warm day) the onset of the diselenide exchange reactions was chromatographic determination. The eight symmetrical dis- clearly evident with many small peaks appearing between the elenides used earlier were mixed with diphenyl ditelluride at symmetrical diselenides. Most of the previous investigations approximately equal concentrations (2000 mg l-1 as Se or Te) into diselenide exchange reactions have employed 77Se NMR and allowed to stand for 24 h at room temperature (#25°C).spectroscopy. A 77Se NMR experiment was performed in order A further eight solutions, each containing diphenyl ditelluride to establish the time taken for dimethyl diselenide and diphenyl plus seven of the diselenides, were prepared at the same time. diselenide to reach equilibrium with methylphenyl diselenide. Each of these exchange solutions was then analysed using the These two compounds were chosen because of the considerable GC–AED which monitored the Te 200 atomic emission line.dierence in their 77Se chemical shifts, with dimethyl diselenide The resultant Te 200 chromatogram from the solution contain- at 268 ppm and diphenyl diselenide at 463 ppm, with the ing the diphenyl ditelluride and each of the eight symmetrical resultant two resonances from the asymmetrical exchange diselenides is shown in Fig. 9. By comparing this chromatog- product being at 291 and 440 ppm. The experiment was ram with the other eight (each of which had one of the performed with approximately equimolar amounts (6.8×10-3 symmetrical diselenides missing) the identity of the peaks in mol) of each diselenide at room temperature (#25°C). The Fig. 9 could be assigned in an analogous manner to the peaks first spectrum (collected after 2 h) showed no extra peaks in Fig. 7. These assignments were subsequently confirmed using characteristic of the presence of the exchange product.Some GC–EI-MS. Interestingly, whilst all the symmetrical dialkyl 16 h after mixing, a subsequent spectrum showed that exchange diselenides exchange products with the diphenyl ditelluride had clearly started (see Fig. 8). Based upon further spectral could be readily detected, the exchange product with diphenyl investigations, the time taken for these two diselenides to reach diselenide was not found. The exchange mixtures were allowed equilibrium was approximately 36 h.Given the greater sensito stand for a further 14 d, after which time they were tivity and speed of analysis oered by GC–AED compared re-analysed with no diphenylselenenyltelluride being detected. with 77Se NMR, the former technique may prove to be a useful This particular exchange reaction has been studied by 125Te tool for studying such reactions, particularly those having and 77Se NMR spectroscopy and the formation of the exchange more than three species under investigation.product diphenylselenenyltelluride (Ph-Se-Te-Ph) is known to The present study is not the first to employ GC to examine form,45 a result which was confirmed using the same technique. the diselenide exchange reactions. Evans and Johnson,39 The non-appearance of this exchange product in the tellur- attempting to characterise organoselenium compounds of bioium chromatogram may be due to the lability of the logical importance using GC with electron capture detection, seleniumMtellurium bond in this particular exchange product, observed the asymmetrical exchange products of dimethyl, emphasising the requirement that analytes must be able to diethyl and dipropyl diselenides.More recently, Cai and withstand the chromatographic conditions employed. co-workers40–43 have reported the detection of exchange prod- However, the power of GC–AED for the investigation of ucts arising from the interaction of diselenides with disulfides, certain organometallic reactions at relatively low concen- these workers employed GC–AED instrumentation similar to trations has been ably demonstrated.that used in the current study. With a view to evaluating the applicability of the GC–AED further for following such exchange reactions a preliminary CONCLUSION investigation into the reaction between the symmetrical diselenides and diphenyl ditelluride was conducted. MacFarlane and The GC–AED method has particular applicability in the area of synthetic chemistry where the products and/or reactants MacFarlane44 employed 125Te NMR to study the exhange products of diselenides and ditellurides, and reported the contain hetero atoms and are suitable for GC analysis.The progress of reactions can be monitored along with the elemen- presence of various dichalcogenides, such as dimethylselenenyltelluride (Me-Se-Te-Me) and methylpropylselenenyltelluride tal ratios of the products.This technique is ideally suited to microscale synthesis, requiring less than a microlitre of dilute (Me-Se-Te-Pr). Diphenyl ditelluride was selected for the study on the basis of its ready availability, purity and suitability for solution per determination. 740 Journal of Analytical Atomic Spectrometry, July 1997, Vol. 1220 Wylie, P. L., Sullivan, J. J., and Quimby, B. D., J. High Resolut. The authors express their sincere gratitude to Jim Watson, Chromatogr., 1990, 13, 499.Peter Harrison and Rod Minett (Hewlett-Packard Australia) 21 Quimby, B. D., and Sullivan, J. J., Anal. Chem., 1990, 62, 1027. for arranging the loan of, and supporting the GC–AED 22 Sullivan, J. J., and Quimby, B. D., Anal. Chem., 1990, 62, 1034. instrumentation, and for the use of the GC–EI-MS equipment. 23 CRC Handbook of Chemistry and Physics, ed. Lide, D. R., CRC Our thanks go also to Carl Scheisser for provision of the Press, Boca Raton, FL, 77th edn., 1996, pp. 10–1 to 10–127. 24 Vincenti, M., Minero, C., Sega, M., and Rovida, C., J. High selenium compounds A–F, and to Jenny O’Connell for the Resolut. 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