656
Anal. Chern. 1980. 5 2 , 6 5 6 - 6 6 2
-
M i- H202 + M-H202 M-OZH
+
Luminol
OH'
hl-O~-tum~noi Complex kZlBr-
% M-02H M-02-Luminol
Complex
k 4 decomposltlon OH
r
l*
selective, a t least partially, if a suitable promotor can be chosen. For many metal ions, e.g., Ni(I1) and Cu(II), which are less sensitive in the luminol CL analysis without additional modifications, promotion by a metal co-catalyst is certainly another area worth exploitation.
ACKNOWLEDGMENT The authors thank Daniel E. Bause for many helpful discussions and some preliminary measurements. Thanks are also due John F. Endicott (Wayne State University) for a preprint of his paper (14)and Mark R. Spear for his technical assistance. LITERATURE CITED Seitz, W. R.; Hercules, D. M. I n "Chemiluminescence and Bioluminescence", Cormkr, M. J., Hercules, D. M., Lee, J.. Eds.; Plenum: New York, 1973; pp 427-449. Isacsson, U.; Wettermark, G. Anal. Chim. Acta 1974, 68, 339-362. Seitz, W. R.; Suydam, W. W.; Hercules, D. M. Anal. Chem. 1972. 4 4 , 957-963. Seitz, W. R.; Hercules, D. M. Anal. Chern. 1972, 44, 2143-2149. Bause, D. E.; Patterson, H. H. Anal. Chem. 1979, 5 1 , 2288-2289. Burdo, T. G.;Seitz, W. R. Anal. Chem. 1975, 47, 1639-1643. MacDonald, A.; Chan, K. W.; Nieman, T. A. Anal. Chem. 1979, 51, 2077-2082. Roswell, D. F.; White, E. H. I n "Methods in Enzymology", Deluca, M. A. Ed.; Academic Press: New York, 1978; pp 409-423. McCapra, F. Quart. Rev. 1966, 20, 485-510. White, E. H.: Zafiriou, 0.; Kagi, H. M.; Hill, J. M. J . Am. Chem. SOC. 1964, 86, 940. Maskiewicz, R.; Scgah, D.; Bruice, T. C. J. Am. Chem. Soc. 1979, 101, 5347-5354. Wildes, P. D.; White, E. H. J . Am. Chem. SOC.1973, 95, 2610-2617. Chock, P. B.; Dewar, R. B. K.; Halpern, J.; Wang, L-Y. J . Am. Chem. SOC.1969, 91, 82-84. Wong, C-L.; Switzer, J. A.; Endicott, J. F.; Balakrishrnan, K. P. submitted for publication. Chang, C. A.; Patterson, H. H.; Mayer, L. M.; Bause, D. E., submitted for publication in Anal. Chem. Neary, M. P.; Seitz, W. R.: Hercules, D. M. Anal. Len. 1974, 7, 583-590. Delumyear, R. G.; Hartkopf, A. V. Anal. Chem. 1976, 48,1402-1405.
Figure 5. A proposed mechanism for the halide effect of the metal ion catalyzed chemiluminescence reaction
using a promoter or an inhibitor (quencher) in chemical analysis is no longer uncommon. Trace amounts of zirconium, cerium, and thorium have been determined based on the quenching effect of the reaction between luminol and H z 0 2 catalyzed by Cu(I1) ions (2). The halide enhancement of the CL signal for Cr(II1) and Fe(I1) determination has certainly great analytical implications. For example, a procedure for the analysis of free chromium(II1) ion in seawater has been developed which involves diluting the seawater to eliminate Mg2+interference and then adding bromide ion to enhance the CL signal (15). Coupling the luminol CL method with liquid chromatography has been suggested as an attractive means of separation and detection for certain transition metals a t trace levels because the CL method is not selective (16,17). Owing to the discovery that different halide enhancement is observed for metal ions a t different conditions, the system may be made
RECEIVED for review October 15, 1979. Accepted January 10, 1980. The work upon which this publication is based was supported in part by funds provided by the Office of Water Research and Technology (No. B-D16-ME), U.S. Department of the Interior, Washington, D.C., as authorized by the Water Research and Development Act of 1978.
Room Temperature Phosphorescence of Nitrogen Heterocycles Adsorbed on Silica Gel Charles D. Ford and Robert J. Hurtubise" Chemistry Department, The University of Wyoming, Laramie, Wyoming 8207 1
The room temperature phosphorescence (RTP) of benro[ 4qulnoline adsorbed on several silica gel samples was Investigated by luminescence, reflectance, and infrared spectroscopy to obtaln a better understanding of the analytical condltlons needed for strong RTP. The results showed that silica gel chromatoplates containing a polymeric binder with carboxyl groups were the best samples for inducing strong RTP from benro[llquinollne. The polymer itsell was essentlal for inducing strong RTP. It was postulated that benro[f]quinoline was adsorbed flatly on the surface with the carboxyl groups anchorlng benro[ l]quinollne via hydrogen bonding with T electrons. 0003-2700/80/0352-0656$0 1.OO/O
Lloyd and Miller ( 1 ) have commented in a recent publication on the earliest known report of room temperature phosphorescence (RTP). In more recent years, RTP has been shown to be a useful analytical technique (2-23). Current interest in R T P was stimulated by the reports of Schulman and Walling (3, 4 ) . Furthermore, some workers in the area of R T P have sought to explain the adsorbate-solid surface interactions which are necessary to give RTP from compounds adsorbed on solid surfaces (15-17, 19, 20). The results of a study on R T P by Schulman and Parker (16) indicated that hydrogen bonding via hydroxyl groups on the support was essential in fixing the compound rigidly, 0 1980 American Chemical
Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980
657
Table I. Silica Gel Brands Tested as RTP Supports for B[flQ brand
description
RTP (neutral)b
RTP (acid)c
A1 backed TLC chromatoplate moderate strong moderat e strong glass backed TLC chromatoplate moderate strong plastic backed TLC chromatoplate moderate strong glass backed (IIPTLC) chromatoplate moderate strong plastic backed N-HR (TLC) chro matoplat e Brink mann plastic backed Sil-G (TLC) none none chromatoplate S &Sd glass backed (TLC) chromatoplate none moderate Applied Science Labs glass backed Permakotes I (TLC) none weak. chro ma to plat e none EM Silica Gel 40, column chromatography none none EM Silica Gel 60, column chromatography none EM Silica Gel 100, column chromatography none llolle MN Silica Gel 60, column chromatography none weak MN none weak Kieselgel G O for TLC chromatography Mallinckrodt none none SilicAH TLC-7G chromatography a All work done with aluminum backed EM chromatoplate unless stated otherwise in the text. R [ f l Q spotted from ethanol. B[flQ spotted from 0.1 M HCl ethanol. Schleicher & Schuell. e Macherey, Nagel & Co --_ __ __ EMa EM EM EM Brin kman 11
~
thereby preventing nonradiative decay of the triplet state. The results of our study on the RTP of the phthalic acid isomers adsorbed on dried silica gel chromatoplates also supported hydrogen bonding interaction ( I 7). Von Wandruszka and Hiutubise (25)have shown, for p-aminobenzoic acid and other compounds adsorbed on sodium acetate, the strong adsorbate-solid surface interaction which resulted in IZ'I'P usually came from hydrogen bonding and formation of t h e sodium salt of the adsorbed species. Several workers have employed sodium halides, AgN03, or lead and thallium salts as sources of heavy atoms to enhance the RTP of compounds adsorbed on solid surfaces (7-10,12, 19). Bower and Winefordner (19) studied the effect of sample environment on the RTP of polycyclic aromatic hydrocarbons (PAH) on filter paper. From their results, they postulated Ag+ could be considered bonded to the PAH with the K electrons of the molecule and also to the free hydroxyl groups of the support (19). I n this manner, the analyte molecules could be held rigidly to the filter paper and thus exhibit RTP. Niday and Seybold (20) discussed the possibility that packing the support matrix with salts or sugars imparted the matrix with rigidity and greater resistance to O2penetration, both of which would serve t o enhance RTP. Recently, we reported RTP from nitrogen heterocycles on dried silica gel chromatoplates (23). We found acid solutions of t h e compour,ds spotted on EM Laboratories and Brinkmann N-HR silica gel chromatoplates gave strong RTP. Other braiids of commercial chromatoplates or column chromatographic silica gels were less suitable surfaces for the RTP of nitrogen heterocycles under the same conditions. We give here a n explanation for t h e observed RTP and propose a n adsorbatesolid surface interaction mechanism for the RTP from the nitrogen heterocycles. EXPERIMENTAL Apparatus. All quantitative RTP data were obtained with a Schoeffel SD3000 spectrodensitometer equipped with a phosphoroscope accessory. Experimental details were described previously (23). Fluorescence emission spectra and quantitative fluorescence data were obtained with the spectrodensitometer with the inlet and exit slits set at 2 mm and 3 mm, respectively. Reflectance spectra were obtained point-by-point at 10-nm intervals with the spectrodensitometer. The slit settings for the reflectance work were as above. Infrared spectra were obtained with a Reckman model 10 IR spectrophotometer. Because of the low transmittance of the silica pellets, a beam attenuator was employed in the reference beam. Reagents. Ethanol was purified by distillation as described by Winefordner and Tin ( 2 4 ) . HTP procedures were described
previously (17,23). Benzomquinoline (BMQ) and phenanthridine were recrystallized from ethanol arid water; 4-azafluorene was recrystallized from n-hexane, ethanol, and water. All other compounds were reagent grade and used as received. Table I lists the brands of commercial silica samples employed in this work. Aluminum backed EM silica gel chromatoplates were employed throughout most of the work unless otherwise noted in the text. Procedures. Acid Pretreatment of Chromatoplates. For the data in Table 11, separate silica gel chromatoplates (EM) were soaked for 10 s in water solutions of the acids. The acid treated chromatoplates were dried 0.5 h at 110 "C prior to use as RI'P supports. The RTP relative intensity values in 'Tables I1 IV are not directly comparable because different gain settings were used with the Schoeffel spectrodensitometer to obtain the data. Reflectance Spectra. Reflectance spectra were obtained for 1 pg of B[flQ adsorbed on the silica gel chromatoplates under various conditions. The Schoeffel spectrodensitometer, in the double beam mode, was zeroed for each wavelength with blank adsorbent in the reference and sample beams prior to each measurement. If the chromatoplate was plastic backed, six layers of Whatman No. 1 filter paper were placed under the chromatoplate to ensure infinite layer thickness (25). For the reflectance spectra obtained on powders, the adsorbent was crushed and spread evenly on top of six layers of filter paper. The thickness of the powder layer was about 2 mm. Infrared Data. Silica gel samples were pressed into self-supporting pellets in the following manner (26). The samples were crushed in a Wig-L-Bug, and then 14 mg were placed in a small glass container. The silica gel samples were treated with either 0.10 mL of ethanol or 0.10 mL of 0.1 M HC1 ethanol solution and then dried for 0.5 h at 110 "C in an oven prior to pellet formation. After drying, the samples were transferred to a Beckman die and the die was placed on a Carver hydraulic press. The pressure on the die was increased to about 24000 psi over an interval of 2 min. The die was left in the press at this pressure for 10-15 min. The pressure on the die was released slowly and the silica pellet was placed in an infrared pellet holder, and the infrared spectrum was obtained. RESULTS AND DISCUSSION Silica Gel Samples Tested as Solid Surfaces. BMQ was selected as the model compound in this study because it was found to be a strong phosphor on EM Laboratories silica gel chromatoplates (23). We reported earlier that B[flQ did not exhibit RTP on some commercial brands of silica gel (23). Table I lists those brands of silica gel which were tested as surfaces for B m Q and indicates the relative magnitude of the R T P signal observed for B[flQ, Table I shows E M Laboratories chromatoplates and Brinkmann N-HR silica gel chromatoplates were the best surfaces to induce strong R T P signals for B[flQ. However, B[flQ exhibited moderate to weak RTP
658
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980
Table 11. RTP Relative Intensity Values of 100 ng B[f]Q Spotted on Acid Treated EM Silica Gel Chromatoplates
a
aqueous acid, 0.1 M
relative intensitya
no acid H*SO, HC10, HC1 HNO,
1.0 1.6
>-
IA
L I
I
\
3.4
4.2 5.4
Average from duplicate spots.
Table 111. RTP Relative Intensity Values of 100 ng B[f]Q Spotted from 0.1 M Acid-Ethanol Solutions onto EM Silica Gel Chromatoplates 0.1 M acid
no acid HCl HBr HI a
.-...-.
--A.
.... ,... Q
relative intensity” 1.0
12 29 none detected
Average from duplicate spots.
on other brands and types of silica gel (Table I). Acid Studies. As reported previously, significant increases in the R T P of nitrogen heterocycles adsorbed on E M silica gel chromatoplates were observed when the compounds were spotted from 0.1 M HCl ethanol solutions (23). I t was determined t h a t silica gel chromatoplates could be pretreated with 0.1 M aqueous solutions of either HC104, HN03, or HCl and enhanced R T P was observed from B[flQ spotted from an ethanol solution. T h e acid pretreatment data is given in Table 11. Pretreatment of the EM chromatoplate with each acid except H 2 S 0 4gave an enhanced R T P from B[flQ. This indicates the acid affects the chromatoplate in a manner which strengthens the adsorbate-solid surface interaction, and the effect is not limited t o HC1. T o determine what effect HBr and HI had on the RTP of B[flQ, 100 ng of the compound were placed on an E M chromatoplate from ethanol solutions which were 0.1 M in these acids. T h e R T P data are given in Table 111. The data in Table I11 show a 2.4-fold increase in the R T P of B m Q spotted from HBr-ethanol solution compared to B[flQ spotted from HC1-ethanol solution. The results suggest, in addition to the acid effect, some of the R T P enhancement can be attributed to the heavy atom, Br-. The R T P lifetime of the HBr spot of B[flQ was noticeably shorter than the HC1 spot of B[flQ on an E M silica gel chromatoplate. Also, the fluorescence of the compound from HBr solution on the chromatoplate was less intense than the compound spotted from HCl solution. The use of HBr for the enhancement of the RTP of nitrogen heterocycles should be further studied and developed to its fullest analytical potential. As Table I11 indicates, no R T P was detected from the HI spot of BWQ. The stability of this acid in ethanol was poor. Upon standing a short time, the HI solution appeared brown. P r o t o n a t e d vs. U n p r o t o n a t e d Forms. We reported earlier the low temperature phosphorescence (LTP) of phenanthridine in 0.1 M HC1 ethanol was about twice the L T P intensity of phenanthridine in ethanol (23). We found B[flQ followed a similar trend. This gave a measure of the enhancement of phosphorescence of the protonated form of B[flQ and phenanthridine in an HC1-ethanol solution. T o determine t o what extent the R T P enhancement could be attributed to adsorbing B[flQ on the silica gel chromatoplate in the protonated form, the hydrochloride of the compound was prepared by passing HC1 gas through an ether solution of B[flQ. The product of the reaction was shown to be the hydrochloride by solution fluorescence spectroscopy. The
I
2
3
4
5
-..-..*...
.... ...,.
6
7
... .
8
--A-
..._..-..a
9
IO
A g /spot
Figure 1. RTP relative intensity vs. amount of B[ f ] Q (O),eazafluorene (A), and isoquinoline (0)spotted from 0.1 M HCI ethanol solutions onto EM silica gel chromatoplates. Each data point represents t h e average of duplicate spots
average R T P relative intensity values for equimolar amounts of B[flQ, and B[flQ-HCl spotted from ethanol, and B[flQ spotted from 0.1 M HC1 in ethanol onto an EM chromatoplate were 5.4, 5.6, and 62.9, respectively. These data suggested the R T P enhancement was more than the result of adsorbing the protonated species on the chromatoplate, and that HC1 and the other acids probably interacted with the chromatoplate in some manner to allow stronger adsorbate-solid surface interactions. S u r f a c e Area Considerations. Snyder (27) described a method of calculating molecular areas of compounds adsorbed on silica gel and aluminum oxide. Von Wandruszka and Hurtubise (15) have shown that the maximum in a plot of RTP relative intensity vs. the amount of phosphor adsorbed on sodium acetate corresponded to monolayer coverage by the phosphor. They employed Snyder‘s method to calculate the surface area of sodium acetate with phosphorescence data from p-aminobenzoic acid, and their calculated area corresponded very well with a B.E.T. surface area value for sodium acetate (15). Figure 1 gives plots of R T P vs. amount of B[flQ, isoquinoline, and 4-azafluorene, spotted on E M silica gel chromatoplates from 0.1 M HC1 ethanol solutions. The phosphorescence maxima for the three compounds occurs a t 1.4, 1.2, and 0.60 pg, respectively. Assuming flat adsorption of the compounds on the chromatoplate, and using a calculation similar to von Wandruszka and Hurtubise (15), surface area values for EM silica gel chromatoplates were 18 m2/g with B[flQ, 19 m2/g with isoquinoline, and 8 m2/g with 4-azafluorene. These calculated surface area values for the E M silica gel chromatoplate are not close to the manufacturer’s surface area value of 550 m2/g (28). This seemed to suggest that only a few sites on the EM silica gel chromatoplate were available for strong interaction with the adsorbates. Reflectance Spectroscopy. Von Wandruszka and Hurtubise (15) used reflectance spectroscopy in their investigation of the interactions responsible for the R T P of p-aminobenzoic acid on sodium acetate. Reflectance spectra of B[flQ and B[flQ.HCl spotted from ethanol and B[flQ spotted from 0.1 M HC1 ethanol were obtained on several silica chromatoplates, NaC1, and M N silica gel for column chromatography. The reflectance spectra of these compounds on NaCl were not well defined; however, the reflectance spectra of the neutral compound and hydrochloride were very similar. This implied a certain similarity in the ground state for both species on NaC1.
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980
A
3
280
320
360
WAVELENGTH
---- ,
400
(nrn)
Flgure 2. Reflectance spectra of B [ f ] Q spotted from ethanol (...), B[ f]Q-HCl spotted from ethanol (- - -), and B[ f ] Q spotted from 0.1 M on MN silica gel for column chromatography. HCI ethanol solution (--) collected at 10-nm intervals
Data
Examination of the ultraviolet absorption spectra of the neutral compound and hydrochloride in ethanol showed the hydrochloride spectrum to be very similar to the spectrum for the neutral compound. This suggested the hydrochloride was in equilibrium with a relatively large fraction of its neutral counterpart in ethanol solution. This seems reasonable since BMQ has a pK, of 4.75 in a water-thanol solution (29). Thus, i t appears when a n ethanol solution of t h e hydrochloride is spotted onto NaCl both the neutral compound and hydrochloride are spotted onto the surface. The reflectance spectrum of B[AQ from 0.1 M HC1 ethanol solution on NaC1, though not well defined, showed enhanced absorbance. T h e enhanced absorbance was presumably due to the protonated species adsorbed on the surface; however, there could he other factors contributing t o the enhanced absorbance. Figure 2 gives t h e reflectance spectra of B[flQ and B[flQ.HC1 spotted from ethanol and B[flQ spotted from 0.1 M HCl ethanol solution onto M N silica gel for column chromatography. The spectra in Figure 2 are much more defined than those of the compounds on NaCl. The neutral sample
of B[flQ exhibited reflectance maxima a t 270 and 330 nm. The hydrochloride sample exhibited the same maxima as the neutral compound, indicating a fraction of the neutral form adsorbed from solution. B[flQ from HC1 solution gave a n increase in absorbance as well as a red shift in the spectrum. T h e maxima for this spectrum were 275 and 350 nm. This indicated the spectrum of the compound from 0.1 HC1 ethanol solution represented the B[flQH+ species. This is supported by the absorption spectrum of BWQ in 0.012 M HC1 ethanol solution which gave absorption peaks a t 280 and 360 nm. Figures 3a and 3b give the reflectarice spectra of BMQ and B[flQ-HCI spotted from ethanol and B[flQ spotted from 0.1 M HCl ethanol solution onto EM and Brinkmann N-HR silica gel chrornatoplates, respectively. As with the spectra for the samples on M N silica gel (Figure 2), the reflectance spectra of B[flQ and B[flQ.HCl are almost identical. This indicated a relatively large fraction of the neutral compound was adsorbed from the hydrochloride solution. Also the spectrum of B[flQ from 0.1 M HCl ethanol solution on the chromatoplates exhibited enhanced absorbance and a red shift indicative of the adsorbed protonated species (Figures 3a, 3b). According to Bayliss and McRae (XI), a red shift for solution absorption spectra is observed when the dipole moment of the solute increases during a n electronic transition. This is the case for B[flQ. Schulman (31) has pointcid out that the nonbonded electron pair of pyridinic nitrogen does not satisfy the sqmmetry requirements for conjugation with the x system, making t h e pyridinic nitrogen atom a poor electron donor. Thus, the electronegativity of the nitrogen atom is the main property affecting the polarization of electronic excitation. This allows the nitrogen atom to accumulate x-electronic charge in its vicinity as a result of electronic excitation. This effect is very pronounced as a result of hydrogen bonding or protonation and a red shift is observed in solution spectra. Thus the information obtained from the reflectance spectra strongly suggest that the protonated form of B[flQ is adsorbed on t h e silica gel surface from acid solution. Fluorescence Spectroscopy. T h e fluorescence of B[flQ spotted from 0.1 M HC1 ethanol solution onto silica gel chromatoplates showed a significant enhancement compared with B[flQ spotted from ethanol solution on the Same chromatoplate. Furthermore, the fluorescence emission spectra of the acid R[flQ spots on the chromatoplates exhibited a 5 t o 8 nm red shift compared to the ethanol B[rlQ spots. T h e increased fluorescence intensity o f t h e acid B[flQ spots and red shift in the emission spectrum were probably due to the protonated form of BMQ adsorbed on the chromatoplate, and
a
240
200
320
659
b
360
400 WAVELENGTH
(nrn)
Figure 3. Reflectance spectra of B[ f ] Q spotted from ethanol (...), B[ f]Q.HCI spotted from ethanol (---), and B[ f ] Q spotted from 0.1 M HCI ethanol (-) onto (a) an EM silica gel chromatoplate and (b) a Brinkmann N-HR silica gel chromatoplate. Data collected at 10-nm intervals
660
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980 100
IO0
b
T
I 0
0
2000
I800
1600
1400
WAVENUMBER
Infrared spectra of ethancl treated (-), (b) EM silica gel from a chromatoplate Figure 4.
1800
2000
1600
1400
(cm-')
and 0.1 M HCI ethanol treated (---) silica gel samples. (a)MN silica gel for column chromatography,
100
100
b
% T
0 2'
0
1800
1600
1400
WAVENUMBER
(cm'
Infrared spectra of ethanol treated (-) and 0.1 M HCI ethanol treated a chromatoplate, (b) Brinkmann N-HR silica gel from a chromatoplate Figure 5.
an increased adsorbate-solid surface interaction between B[flQ and the acid-treated silica chromatoplate. Infrared Spectroscopy. Because the acid studies indicated a change on the chromatoplate with acid treatment, infrared spectroscopy was employed to examine silica gel samples after ethanol or acid treatment. Figure 4a gives the infrared spectra of MN silica gel samples for column chromatography. The band a t about 1870 cm-' is an overtone band typical of silica gel and the band a t 1630 cm-' is attributed t o a water deformation band (32). Note, there is essentially no difference in the infrared spectra of the acid-treated silica sample compared to the ethanol-treated silica sample (Figure 4a). Figure 4b gives the infrared spectra of EM silica gel samples scraped from an aluminum backed chromatoplate. T h e ethanol-treated sample exhibited broad bands a t 1560 and 1720 cm-' in addition to the 1870 cm-l band and an ill-defined band a t 1630 cm-'. Upon acid treatment, the 1560 cm-' band disappeared and the 1720 cm-' band became more
I800
2000 I
(---)
1600
1400
1
silica gel samples. (a) Brinkmann Sil-G silica gel from
prominent (Figure 4b). The changes in the infrared spectrum on acid treatment are consistent with carboxylate anions being converted to carboxylic acid groups (33). This aspect will be discussed later. Figure 5a gives the infrared spectra of silica samples from a Brinkmann Sil-G chromatoplate which proved to be a poor surface for RTP. In fact, no R T P was observed from B[flQ adsorbed on this surface, and only a very weak low temperature phosphorescence signal was observed from B [flQH' adsorbed on this support. This indicated that a quenching mechanism was probably responsible for the absence of R T P from compounds adsorbed on Brinkmann Sil-G chromatoplates. Notice, the infrared spectra show essentially no change between ethanol treatment and acid treatment for this brand of silica gel chromatoplates (Figure 5a). Figure 5b gives the infrared spectra of a silica sample from a Brinkmann N-HR chromatoplate which yielded enhanced R T P from B[flQ after acid treatment. Like the infrared
ANALYTICAL CHEMISTRY, VOL. 52, NO. 4, APRIL 1980
Table IV. RTP Relative Intensities of 600 ng of B [ f ] Q Spotted on Samples Containing Varying Percentages of Polyacrylic Acid in M N Silica Gel and NaCl %
polyacrylic acid 0 5
10
20 30 100
relative intensity with MN silica gela 1.0
relative intensity with NaCP 0.0
21
42
43 49 31 2.3
56 67
48
2.3
Average from duplicate spots. spectra of the E M silica chromatoplate (Figure 4b), this sample exhibited a 1560 cm-' band which disappeared on acid treatment, and a 1720 cm-' band which became more defined on acid treatment (Figure 5b). Apparently, this brand of Brinkmann chromatoplates also contains carboxylate anions which are converted to carboxyl groups with acid treatment. T h e E M patent indicated the use of the sodium salt of polyacrylic acid as a binder in t h e manufacture of silica chromatoplates (28). T h e Brinkmann patent mentioned several binder materials but the information given was not specific about t h e manufacturing process for N-HR silica chromatoplates (34). Binder Studies. T h e infrared data presented above indicated that the form of the binder E M employs changes upon acid treatment (Figure 4b). Also, enhanced R T P was observed only when the chromatoplate was treated with acid. For these reasons polyacrylic acid was examined as a possible support for R T P . Polyacrylic acid was mixed with MN silica gel 60 for column chromatography to give mixtures containing varying percentages of polyacrylic acid. B[flQ exhibited a weak RTP when adsorbed on M N silica gel (Table I). These mixtures were placed in depressions in a blackened brass plate ( 1 3 ) . On each of these silica gel samples, 600 ng of B[flQ were L spotted from 0.1 M HC1 ethanol solution with a ~ - F Hamilton syringe. After drying, the R T P relative intensity of B[flQ on each sample was obtained. The data are given in Table IV. From the data in Table IV, it can be seen the R T P signals increased almost linearly to 10% polyacrylic acid and decreased above 20% polyacrylic acid. T h e data in Table IV also indicated 100% polyacrylic acid is not a good surface for inducing R T P . T h e above seems to indicate that a certain percentage of polyacrylic acid is important in achieving the strong adsorbate-solid surface interaction which induces enhanced RTP. T o determine if polyacrylic acid could be mixed with other inorganic solids and remain a suitable R T P surface, mixtures of polyacrylic acid and crushed NaCl were prepared. NaCl was chosen because no R T P was observed from B[flQ adsorbed on NaC1. The RTP intensities were measured for 600 ng of B[flQ spotted from 0.1 M HCl ethanol solution. T h e R T P data paralleled those of the polyacrylic acid-MN silica gel mixtures, in that t h e phosphorescence increased to 20% polyacrylic acid and decreased above 20 % polyacrylic acid. However, the intensity of the R T P of B[flQ adsorbed on polyacrylic acid-NaC1 mixtures was about twice t h a t of the compound on polymer-MN silica gel mixtures when compared a t 5% polyacrylic acid (Table IV). Since polyacrylic acid itself was not a good surface for inducing strong R T P but polyacrylic acid-silica gel and polyacrylic acid-NaC1 mixtures were, silica gel and NaCl seemed to play an important role in the R T P of B[fJQ. Deanin (35) noted that modulus in polymers increases when the polymer
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is mixed with an inorganic powder of higher modulus. Generally modulus measures the resistance to deformation of materials when external forces are applied (361. The powder serves t o restrict the ability of the polymer to rotate and migrate. With polyacrylic acid and silica gel or NaC1 mixtures, it seems the silica and NaCl serve to restrict the movement of the polymer matrix, thereby lending the sample matrix greater rigidity and allowing greater R T P signal intensities from adsorbed compounds. These ideas appear to complement those of Niday and Seybold on matrix packing (20). We have no specific explanation yet for the enhanced R T P with the NaCl mixtures compared to the silica gel mixtures. Proposed Adsorbate-Solid Surface Interaction. Silica Gel without Binder. It is generally accepted that adsorption of compounds on silica gel occurs via hydrogen bonding between the adsorbate and the silanol groups of the silica surface (37-39). Snyder (39, 40) indicated that generally aromatic compounds are adsorbed flatly on silica gel (i.e., parallel to the surface experienced by the adsorbate). It seems that only a flatwise adsorption mechanism would afford the compound the rigidity necessary for RTP. Therefore, any discussion of an adsorption mechanism which follows assumes flatwise adsorption of the adsorbate on the solid surface. Because weak R T P was observed from B[flQ spotted from 0.1 M HCl ethanol solution on MN silica gel without organic binder, it seems reasonable to conclude t h a t t h e R T P observed in this case is a result of hydrogen bonding between B[f]QH+ and the silanol groups. The acid serves to protonate BMQ, but because it must be added in excess of the stoichiometric amount of B[flQ to obtain a signal, another role of the acid is suggested. Deanin (41) discussed the Si-0-Si bond and its lability to acids. I t therefore seems possible that the excess acid would attack siloxane groups to produce more silanol groups which would then be available for hydrogen bonding with the T electrons of the nitrogen heterocycle. Apparently, the hydrogen bonding between the silanol groups and B [flQH' holds the molecules rigidly enough for weak R T P to occur. Silica Gel with Binder. When acidic polymers or their salts, such as polyacrylic acid, are used as binders for commercial silica gel chromatoplates, another adsorbate-solid surface interaction is implicated. The results of the acid studies, the luminescence, reflectance, infrared, and binder studies clearly indicated that enhanced R T P signals were obtained only when the binder was in its acidic form. Furthermore, the presence of many carboxyl groups dispersed throughout the silica established sites for strong hydrogen bonding interaction. For E M chromatoplates, polyacrylic acid is present in the range of 0.1-10% (28). Infrared results with mixtures of polyacrylic acid and silica gel without binder indicated the amount of binder a t about 5% by weight. Any reasonable proposed mechanism of interaction between B[flQH+ and E M silica chromatoplates, which contain an acidic polymer as its binder, must account for possible interactions between the adsorbed compound and the binder. The most logical adsorbate-solid surface interaction would be to propose hydrogen bonding between the T electron systems of the adsorbate and the carboxyl groups of the binder. Since the acidic polymer is a stronger acid than the silanol group, the carboxyl groups would be expected to form stronger hydrogen bonds with the T ring system of B[flQH+ and therefore hold the compound more rigidly to the surface. For these reasons, it appears BlflQH"' exhibited stronger R T P adsorbed on polyacrylic-NaC1 mixtures than on pure silica gel. Because the reflectance data and fluorescence data indicated B[flQ was adsorbed on the chromai,oplate in the protonated form, it seems probably that the >>NH+ entity could form hydrogen bonds with the carbonyl oxygen of the carboxyl group.
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Anal. Chem. 1980, 52, 662-666
As additional evidence for the hydrogen bonding mechanism, the hydrocarbon analogue of BMQ, phenanthrene, was placed on a 5% polyacrylic acid-NaC1 mixture from ethanol solution. After drying, the phenanthrene exhibited a strong R T P . Phenanthrene also exhibited a moderate R T P on E M silica gel chromatoplates when spotted from 0.1 M HC1 ethanol solution. These results suggested the excess acid converted carboxylate groups to carboxyl groups which then interacted strongly with phenanthrene. Because phenanthrene has no heteroatom, i t seems reasonable to conclude the main interaction responsible for the enhanced R T P observed from this compound is hydrogen bonding between the carboxyl groups of the binder and the R electron system of phenanthrene.
de Lima, C. G.; de M. Nicola, E. M. Anal. Chem. 1978, 50, 1658-1665. Bower, E. L. Y.; Wlnefordner. J. D. Anal. Chim. Acta 1978, 102,1-13. Niday. G. J.; Seybold, P. G. Anal. Chem. 1978, 50, 1577-1578. Bower, E. L. Y.; Winefordner, J. D. Anal. Chim. Acta 1978, 701, 319-332. (22) Bower, E. L. Y.; Winefordner, J. D. Appl. Spectrosc. 1979, 33, 9-12. (23) Ford, C. D.; Hlrrtubise, R. J. Anal. Chem. 1979, 51, 659-663. (24) Winefordner. J. D.; Tin, M. Anal. Chim. Acta 1964, 31. 239-245. (25) Frei, R. W.; MacNeil, J. D. "Diffuse Reflectance Spectroscopy in Environmental Problem-Solving"; CRC Press: Cleveland, Ohio, 1973; p 5. (26) Majors, R. E.; Hopper, M. J. J . Chromatogr. Sci. 1974, 72,767-778. (27) Snyder, L. R. "Principles of Adsorption Chromatography"; Marcel Dekker: New York, 1968; pp 199-202. (28) Bruckner. K.; Halpaap, H.; Rossler, H. U.S. Patent No. 3 502 217. (29) Favaro, G.; Masetti, F.; Mazzucato, U. Spectrochim. Acta, PartA 1971, 27, 915-921. (30) Bayliss, N. S.; McRae, E. G. J . f h y s . Chem. 1954, 58, 1002-1006. (31) Schulman, S. G. In "Modern Fluorescence Spectroscopy", Volume 2, Wehry, E. L., Ed.; Plenum Press: New York, 1976; Chapter 6, pp 245-246. (32) Peri, J. B. J . f h y s . Chem. 1966, 70,2937-2945. (33) Silverstein, R. M.; Bassler, G. C.;Morriil, T. C. "Spectrometric Identification of Organic Compounds", 3rd ed.; John Wiley and Sons: New York, 1974; pp 99-102. (34) Radmacher, E.; Wollenweber, P. U.S. Patent No. 3 922 431. (35) Deanin. R. D. "Polymer SVucture, Properties and Applications"; Cahners Books: Boston, Mass., 1974; pp 384-392. (36) Nielsen, L. E. "Mechanical Properties of Polymers and Comosites"; Marcel Dekker: New York. 1974; p 39. (37) Kiseiev, A. V.; Lygin, V. I. "Infrared Spectra of Surface Compounds"; John Wiley and Sons: New York, 1975; pp 123-137. (38) Little, L. H. "Infrared Spectra of Adsorbed Species"; Academic Press: London, 1966; pp 234-243. (39) Snyder, L. R. J . f h y s . Chem. 1968, 72,489-494. (40) Snyder, L. R. J . f h y s . Chem. 1963, 67,2622-2628. (41) Deanin, R. D. "Polymer Structure, Properties and Applications"; Cahners Book-s: Boston, Mass., 1974; p 40. (16) (19) (20) (21)
LITERATURE CITED Lloyd, J. B. F.; Miller, J. N. Talanta 1979, 26, 180. Roth, M. J . Chromatogr. 1967, 30, 276-278. Schulman, E. M.; Walling, C. Science 1972, 778,53-54. Schulman, E. M.; Walling, C. J . f h y s . Chem. 1973, 77,902-905. (5) Paynter, R. A.; Wellons, S. L.; Winefordner, J. D. Anal. Chem. 1974, 46,736-738. (6) Wellons, S. L.; Paynter, R. A.; Winefordner, J. D. Spectrochim. Acta, Part A 1974, 30, 2133-2140. (7) Seybold, P. G.; White, W. Anal. Chem. 1975, 47, 1199-2000. (8) White, W.; Seybold, P. G. J . Phys. Chem. 1977, 81, 2035-2040. (9) Vo-Dinh, T.; Yen, E. L.; Winefordner, J. D. Anal. Chem. 1976, 48, 1186-1 188. (10) Vo-Dinh, T.; Yen, E. L.; Winefordner, J. D. Talanta 1977, 24, 146-148. (11) Vo-Dinh, T.; Walden, G. L.; Winefordner, J. D. Anal. Chem. 1977, 49, 1126-1130. (12) Jakovljevic, I. M. Anal. Chem. 1977, 49,2048-2050. (13) VOn Wandruszka, R. M. A.; Hurtubise, R. J. Anal. Chem. 1976, 48, 1784-1788. (14) Von Wandruszka, R. M. A,; Hurtubise, R. J. Anal. Chim. Acta 1977, 93, 331-333. (15) Von Wandruszka, R. M. A.; Hurtubise, R. J. Anal. Chem. 1977, 49, 2164-2 169. (16) Schulman, E. M.; Parker, R. T. J . fhys. Chem. 1977. 81, 1932-1939. (17) Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1978, 50, 610-612. (1) (2) (3) (4)
RECEIVED for review November 9,1979. Accepted December 31, 1979. This work was supported partially by the Department of Energy's Laramie Energy Technology Center under Contract No. DE-A520-79-LC01761 to the Rocky Mountain Institute of Energy and Environment.
Localization of Light Emission in Microporous Membrane Chemiluminescence Cells David Pilosof and Timothy A. Nieman School of Chemical Sciences, University of Illinois, Urbana, Illinois 6 180 1
A transparent cell was constructed and mounted on an X-Y positioning assembly to permit movement of the cell relative to a slit placed in front of the photomultiplier tube (PMT) detector. This arrangement allowed observation of the profile of the chemiluminescent (CL) emission intensity as a function of the distance from the microporous membrane surface and from the flow cell solution inlet. The Co(I1)-luminol reaction was used for this characterization. CL emlssion intensity profiles were examined as a function of the pressure applied to the reagent compartment, the analyte flow rate, and the volume of sample injected into the flow stream. Maximum emission intensity is generally observed to occur approximately 1 mm from the membrane and approximately 13 mm from the cell inlet.
A microporous membrane chemiluminescence (CL) cell consists of a microporous membrane separating a pressurized reagent reservoir from an analyte stream ( I ) . The light emitted in the zone contiguous to the membrane is monitored by 0003-2700/80/0352-0662$01.00/0
a detector placed in front of the cell. This method provides certain advantages over other common solution chemiluminescent techniques: (a) a high economy of reagents is attained owing to t h e low flow rate through the membrane, usually in the range of a few microliters per minute; (b) comparing this rate to a reasonable analyte flow rate, 1-20 mL/min, it can be concluded that the reagent is diluted several orders of magnitude by the analyte stream, resulting in negligible contamination and reducing the degree of self-absorbance ( 2 ) ;(c) the reagent delivery system is simple and easy to construct with no moving parts (excluding valves) required; (d) the light emission occurs mainly within a narrow layer near the membrane, allowing minimization of the total cell volume; (e) the system can be easily adapted to multicomponent analysis by allowing the analyte solution to flow along a series of membrane CL cells, each of them optimized for a given analyte. A transparent cell was constructed and mounted on an X-Y positioning assembly to permit movement of the cell relative to a slit placed in front of a photomultiplier tube ( P M T ) detector. This arrangement allowed us to observe the profile 0 1980 American Chemical Society
