Anal. Chem. 1963, 55, 1626-1629
1626
100
0
50 (ppm)
Figure 1. 13C NMR spectrum of tert-butyl peroxide, fert-butyl hydrcperoxide, and fert-butyl alcohol. The spectrum was acqulred by using a 10-mm sample containing an Internal D,O capillary for lock. Sixty-four scans were accumulated by using a 90' 13C pulse and full relaxation of the magnetization during the relaxation period. Spectral assignments are given in Table I.
Table 11. Analytical Results of a Typical Sample by 13C NMR
molecule tert-butyl hydroperoxide tert-butyl peroxide tert-butyl alcohol dioxane
mol % calcd from mol % quaternary calcd from C methyl C
mol % expected from sample prep
58 (k3.6)
60.9 (k0.6)
61 (k1.2)
1.4 (i0.7)
1.5 (k0.3)
6.9 ( k 2 . 1 )
6.6 (i0.7) 6.5 (i0.07)
1.6 (k0.02)
31.0 ( k 0.3)
One of the principal advantages of this technique is its inherent sensitivity. Since the signal acquired in a Fourier transform NMR spectrometer increases with the number of scans, n, and the noise acquired increases with n1J2,the signal-to-noise ratio is proportional to n1/2. Thus, a very weak signal can be observed by continually acquiring and coadding free induction decays (FID). In the case of the sample presented in Table 11, a 0.5 mol % component of any species could be detected with comparable precision by acquiring about 600 FID's. The precision of any measurement is dependent upon several operational parameters. First, one must consider the signal-to-noise ratio which is dependent upon the sample concentration, the sample volume, the number of each type of molecular group per molecule (the butyl methyl carbon is three times more abundant than the quarternary carbon), number of acquisitions, and homogeneity. Second, the digital resolution of the spectrometer, expressed in hertz per point,
must be adequate to determine the intensity and shape of the resonances of interest. For example, a resonance with a line width of 0.15 Hz (fwhm) will probably not be recorded properly if the digital resolution is 0.15 Nz per data point. From a practical standpoint, the best accuracy with which one can expect to measure a spectral intensity or integral is about *0.5%. The percentage error will vary inversely with the signal-to-noise ratio. Instrumental factors such as digital resolution and homogeneity can add nonstatistical errors into the analysis. Quantitative results are obtained by adding a known volume of dioxane as an internal reference. Typical results, which appear in Table 11, have been found to be in excellent agreement with that expected from sample preparation. In the illustrated example, complete and reliable absolute quantitative results from the proton NMR spectra were not possible due to the problem of resonance overlap, In order to get comparable results by using proton NMR, a high field superconducting NMR spectrometer would be required.
CONCLUSIONS The use of 13C NMR of peroxides, hydroperoxides, and alcohols results in spectra which have much better spectral resolution than can be obtained by using proton NMR at comparable fields. The spectra can be analyzed in a relative or absolute sense with excellent reliability to reveal the presence of trace quantities of the various species with the only expense being a relatively small amount of spectrometer time. Since chemical reactions involving one of the three species may be perturbed by the presence of impurities, the ability of 13C NMR to detect and quantify small quantities of these species is a particularly important advantage. The results have been determined to provide reliable and accurate information which is often difficult to obtain by chemical or other instrumental techniques. Registry No. tert-Butyl hydroperoxide, 75-91-2; tert-butyl peroxide, 110-05-4; tert-butyl alcohol, 75-65-0. LITERATURE CITED Malr, R. D.;Hall, R. T. "Treatise on Analytical Chemistry"; Korthoff, I. M., Elving, P. J., Eds.; Wlley: New York, 1959; Part 11, Vol. 14. Malr, R. D.;Graupner, A. J. Anal. Chem. 1964, 36, 194. Swern, D.;Clements, A. H.; Long, T. M. Anal. Chem. 1969, 47, 412. Ward, G. A.; Malr, R. D. Anal. Chem. 1969, 47, 538. Carrlngton, A.; McLachlan, A. D. "Introduction to Magnetic Resonance"; Harper and Row: New York, 1967. Levy, G. C.; Llchter, R. L.; Nelson, G. L. "Carbon-13 Nuclear Magnetic Resonance Spectroscopy"; Wiley: New York, 1980. Vold, R. L.; Waugh, J. S.; Klein, M. P.; Phelps, D. E. J. Chem. fhys. 1968, 48. 3831. Aldrich Chemlcal Co. Technlcal Bulletin Aldrichimica Acta 1979, 72, 63.
RECEIVEDfor review November 29, 1982. Accepted March 28, 1983.
Modified Concentric Glass Nebulizer for Reduction of Memory Effects In Inductively Coupled Plasma Spectrometry Mlchael H. Ramsey," Michael Thompson, and Barry J. Coles Applied Geochemlstty Research Group, Department of Geology, Imperlal College of Science and Technology3 Prince Consort Road, London S W7 ZBP, England
The concentric glass nebulizer manufactured by J. Meinhard Associates (type TR-30-A3) is widely used for injecting solutions into the inductively coupled plasma (ICP). The internal liquid volume of this nebulizer is about 0.08 mL and 0003-2700/83/0355-1626$01.50/0
uptake rates commonly used lie in the range 0.5-1.5 mL mi&. A simplistic interpretation of these figures would suggest that the nebulizer system cleanout time should be of the order of 3-10 s plus an additional time for the analyte to clear the spray 0 1983 Amerlcan Chemlcal Soclety
ANALYTICAL CHEMISTRY. VOL. 55. NO. 9. AUGUST 1983
I
Type C
I
Polyethylene, uptake tube Polyethyiene~uptake \\
Type D
rubber cement I
22 gauge Ptllr capillary
Flgue 1. The Meinhard nebulizer and four different methods of cocnecting n to the sampie uptake capi~lary. In types A. E. and C the uptake capillary has been connected by means of a shm length of silicone rubber tubing. In type D a platinumlifflium capillary has been bonded whh silicone rubber cement.
chamber. However, some degree of mixing always takes place between the main stream of the blank solution and pockets of test solution which are more-or-lesa isolated in dead spaces in the nebulizer and its connections. This mixing causes a roughly exponential decay of analyte concentration from the time when the test solution is replaced by a blank, rather than a sharp cutoff. Meinhard (I) quoted a cleaning time for his nebulizers of 20-40 s. Wohlers (2) recognized an expontial relationship for cleanout and found that the Meinbard nebulizer tested took 30 s to fall to 1%of the original level and 60 s for 0.01%. Mixing effects of the nebulized sample can also occur within the spray chamber, especially if the design is unnecessarily large or includes dead volume. Dobb (3) found this to be the primary source of memory on his system and quoted a time in excess of 400 s for the signal to reach true background. He proposed, therefore, a mathematical correction for memory effect. As users tend to exploit the long calibration range of the ICP, the decay of the analyte concentration to the level of the blank noise may be inordinate if the time constant of this decay is large. Thus many ICP users employ signal stabilization times of up to 60 s to ensure cleanout of the test solution before beginnii the integration for the next. We have found that signal stabilization times and cleanout time can be substantially improved by a simple adaption of the Meinhard nebulizer which virtually eliminates liquid dead volume. We believe that it is preferable to reduce the duration of memory effects rather than to correct for them mathematically.
1OOmm
1627
-
Flgure 2. Two spray chambers of the Scott double pass type. The larger version has substantial dead volume (shaded)which is not efficiently swept by the gas flow.
Table I. Signal Onset and Stabilization Times for Four Methods of Nebulizer Connection Measured with the Small Spray Chamber nebulizer connection onset type time, s
rise to stabilization time, s
total stabilization time, s
A B
8.5 8.0
8.5 11.0
C
4.5
9.5
17.0 19.0 14.0
D
4 ~ 0
4.5
8.5
EXPERIMENTAL SECTION A Bausch and Lomb (ARL) 34000 ICP instrument was used in this study. The response (R,)produced by an individual analyte at time ( t ) was monitored hy connecting particular channels to a fast-response chart recorder. Nebulizers were supplied hy Meinhard Associates (type TR-30-A3) and connected to the flexible polyethylene sample uptake tube (350 mm X 0.5 mm id.) in various ways (Figure 1). Method A shows a connection commonly employed. Method B was designed to represent a had connection technique and method C was one of the better ways of making the connection with flexihile tubing. Method D is our adaptation with no dead space. This was constructed hy bonding a length of platinum/iridium capillary (95% Pt, 5% Ir,30 mm X 0.71 nun 0.d. X 0.41 nun i.d. 22 gauge) in contact with the central g h capillary of the nebulizer with silicone rubber cement. The nebulizer was not pumped and the flow rate was fixed at 0.8 mL min-' (at 30 psig) for all configurations, by means of the small bore uptake tube. Spray chambers were of the Scott double pass type ( 4 ) in two different designs (Figure 2). RESULTS AND DISCUSSION The signal stabilization time was studied by recording an instrumental response after a solution containing the analyte at a concentration of lo5times its detection limit was quickly substituted for pure blank solution (1M hydrochloric acid). Cleanout was monitored by reversing the exchange of solutions. Signal stabilization was characterized by two features: the onset time and the time required for the signal to reach 99% of its final value (Figure 3). Results obtained with the nebulizer connected in each of the four ways with the smaller spray chamber are shown in Table I. Improvement in the
1628
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
0
A
1.[
I
C
0.1
?. 5
‘I 0 I
m
0 0
/-onset
Time
time-
0.01
/-Stabilkation
time-
Flgurr 3. Schematic representation of signal change when analyte solution is substituted for blank, showing time of changeover (A), the onset of signal rise (B),and the signal reaching 99% of Its final value (C). 10 0 001
20
10
30
Time(s1
Flgure 5. Effect of spray chamber on cleanout characteristics. The larger, less efficient chamber has a slower cleanout rate for the same nebulizer (type D).
-
0.1
F B
g
g
s 0.01
0.001
Flgure 4. Fall of relative analyte signal (R,IR,) with time ( t )for different nebulizer connections in the small spray chamber. Data for the type B and C connections tend toward a double exponentlal curve.
onset and stabilization of the signal are clearly related to the smaller internal volume of the nebulizer with type D connection. Results for the cleanout study with the small spray chamber are shown in Figure 4. All nebulizers showed a sudden drop from 100% signal (Ro),the onset, about 1 s after the small bubble segregating the analyte solution from the blank passed out of the nebulizer. This was followed by an initial linear fall of log (R,/Ro)with time, and with a similar slope for all nebulizer connections. The nebulizer with type D connection continued this linear trend until the signal fell within the range of the noise of the blank signal. Nebulizers with type B and C connections showed a different behavior, tending toward smaller slopes (and less efficient cleanout) after the signal had fallen to about 1% of its original value. The nebulizer with a type A connection showed characteristics intermediate between these two types. The larger spray chamber had obvious dead-volumes which were imperfectly swept out by the nebulizer gas. The nebulizer
with the platinum/iridium insert (type D) again gave a uniform slope but with a smaller value of -d(log R,)/dt (Figure 5). We interpret these results as follows. The onset of cleanout is clearly related to the internal volume of the nebulizer with its uptake tube and to the internal volume of the spray chamber. The initial steep exponential decay of signal (demonstrated by a constant value of d(1og R,)/dt) is related to a fast process, the cleanout of the spray chamber. Thus the slope is smaller for the larger and less efficient spray chamber. Where there is dead volume a t the rear of the nebulizer, as in connections of type B and C, cleanout of this liquid volume follows a relatively slower exponential fall in analyte concentration. This is shown by the tendency of the graph of log R, vs. t to adopt a smaller negative slope after the signal has fallen from the original value by about 2 orders of magnitude. Thus the cleanout of the nebulizer system seems to be a combination of two exponential processes with different time constants, one related to sweeping the spray chamber with the nebulizer gas and the other related to the sweeping out of the dead volume of the nebulizer by the blank solution. Where there is effectively no dead volume in the nebulizer, as with type D connection, there is no break in the curve, and cleanout down to blank noise is considerably faster. Feeding of the nebulizer by means of a peristaltic pump, at the same uptake rate of 0.8 mL min-l, does not change these cleanout characteristics. Once the signal has stabilized, the analyte precision, sensitivity, and detection limits were unaffected by the variations in nebulizer connections or spray chamber design. The precision for a large batch of samples with large variation in analyte levels will improve with the modified type D connection because of the absence of unsuspected memory effects. The type B connection cleanout, for example, extrapolates to over 300 s for a sample IO6 times the detection limit.
CONCLUSIONS The suggested modification to a commercial concentric glass nebulizer permits a reduction in the total time required to analyze a sample to about 45 s and hence an increase in the
Anal. Chem. 1983, 55, 1629-1631
cost effectiveness of ICP. The 45 s is made up of 9 s preflush, three 5-9 integrations, and a 21-s cleanout period before the next sample. The minimum time between integrations for two different solutions would therefore be 30 8 , long enough to allow an analyte at a concentration IO6times its detection limit to cleanout entiirely by using the proposed nebulizer connection. We have also shown that the normal method of charactering nebulizer cleanout, i.e., measuring the time sequired for the signal to fall to a fixed proportion of the original level (normally 1%) may be seriously misleading if a double exponential cleanout process governs the performance of t,he system.
1629
The same general principles would apply to any nebulizerspray chamber system with dead space in the liquid or gas flow paths.
LITERATURE CITED (1) Meinhard, J. E. ICP I n f . News/.1978, 2 (No. 5), 163. (2) Wohlers, C. C. ICP I n f . News/.1977, 3 (No. 2), 37.
(3) Dobb, D. E. Presented at 1982 Pittsburgh Conference, Atlantic City, NJ, paper No. 534. (4) Scott, R. H.; Fassel, V. A.; Knlsley, R. W.; Nixon, D. E. Anal. Chem. 1974, 46, 75.
RECEIVED for review January 12, 1983. Accepted April 22, 1983.
Determination ad Boron by ][on-Pair Liquid Chromatography with 1,8-Dihydroxyntrphthalene-3,6-dlsulfonic Acid Shoji Motomizu," Ihiuo Sawatani, Mitsuko Oshlma, and Kyoji T6ei Department of Chemistry, Faculty of Science, Okayama University, Tsushima-naka, Okayama-shi, Japan
The analytical chemistry of boron is very important in the fields of nuclear-reactor materials, industrial metallurgy materials, pharmacy, and agriculture. Furthermore, boron is one of a few common elements whose simple and less time-consuming determination must be developed. The reaction of boric acid with 1,8-dihydroxynaphthalene-3,6-disulfonic acid (chromotropic acid) was first described by Andress and Topf (I),and a method was developed by Kuemmel and Mellon (2) for the spectrophotometric determination of boron in an aqueous solution. Lapid et al. (3) reported the method for the deterimination of boron on the basis of the fluorescence intensity of the boron complex with chromotropic acid in an aqueous solution. During the course of study of extractive spectrophotometry of boron with chromotropic acid and quaternary ammonium salt, we observed that boric acid reacted with chromotropic acid to form a 1:2 complex (BR2", where R4- is deprotonated chromotro~picacid) and the extractability of the ion pair formed between BRl- and the quaternary ammonium cation is larger than those formed between HzR2-or HR3- and the quaternary ammonium cation (4, 5 ) . Though boron-chromotropic acid complex shows a scarcely different spectrum from those of chromotropic acid and its deprotonated ones, the merits of chromotropic acid as the organic reagent for boron are as follows: (1)pH range for complex formation is wide, (2) the complex formed is very stable, (3) the reaction time for complex formation is short, (4) coexisting ions scarcely interfere the complex formation or interfering metal lions are masked with EDTA, and ( 5 ) chromotropic acid is easily available. In this work, the authors studied the spectrophotometric detection of boron after separation of the boron-chromotropic acid complex and excess of chromotropic acid by means of reversed-phase high-performance liquid chromatography. EXPERIMENTAL SECTION Apparatus. The high-performance liquid chromatographic system consisted of a UV-8 Model I1 variable-wavelength detector connected to a flow cell (8 pL) (Toyo Soda), a Toyo Soda Model HLC-803D pump, a Rheodyne Model 7125 injection valve (100 pL loop), a Yanaco System-1100 computing integrator (Yanagimot0 Co. Ltd.), and a strip chart recorder. Column. A TSK LIS-410K (ODs type, 5 pm) (Toyo Soda) 4 mm i.d. X 100 mm column was used. Tlhe column was kept in a thermostatically controlled water bath (40 "C).
Mobile Phase. The mobile phase consists of 1.1 X M tetrabutylammonium bromide (TBA-Br) (Tokyo Kasei Co. Ltd., analytical reagent grade) and phosphate buffer (pH 8, 5 X M) in a mixture of 48 vol % methanol and 52 vol % deionized water. The solution was filtered with a 0.45-pm membrane filter and degassed before use. Reagent Solution A. This consists of 0.15 M chromotropic acid disodium salt (Dojindo Lab.) and 0.2 M EDTA disodium salt in deionized water. When this is stored in a refrigerator, this is stable for at least 1 week. Reagent Solution B. This consists of 1M TBA-Br and acetate buffer (pH 4.8, 1 M) in deionized water. Preparation of a Steel Sample Solution. About 0.5 g of a standard steel sample (JSS 159-3: certified boron content 0.0013%) was weighed into a 100-mLsilica beaker. Ten milliliters of 2.5 M sulfuric acid was added and then about 5 mL of 30% m/v hydrogen peroxide solution was gradually added. After the sample dissolves, the solution is heated on a hot plate (40-50 "C) for a few minutes until effervescence ceases. The solution is then diluted to 100 mL with deionized water. Ten milliliters of a steel sample solution is transferred into a polyethylene beaker. One milliliter of 1 M trisodium citrate is added and the mixture is neutralized to pH 4.8 with sodium hydroxide solution. It is diluted to 50 mL with deionized water. A IO-mL portion of this solution was used for boron determination. Recommended Procedure. Ten milliliters of sample solution was transferred to a test tube made of polypropylene. Onehalf-milliliter portions of both reagent solutions A and B were added. The solution was mixed throughly and allowed to stand for more than 10 min at room temperature. After this is filtered with a 0.45-pm membrane filter attached to a 2-mL syringe made of polypropylene, it is injected on to the column. Detection was made on a UV detector at 350 nm, and signals were recorded, and simultaneously the area of the peak was calculated by the computing integrator. Boron content existing as boric acid was calculated from the calibration curve prepared by using the peak height or peak area. RESULTS AND DISCUSSION Formation of Boron-Chromotropic Acid Complex. Effect of pH and concentration of buffer solution. The effect of pH on the formation of boron complex was examined by using acetate buffer. The peak height and area were almost constant a t pH region from 4 to 5. The concentration of acetate buffer (pH 4.8) was examined. The peak height and area were almost constant from 0.01 M to 0.3 M.
0003-:?700/83/0355-1629$01.50/00 1983 American Chemical Society
