EXPERIMENTAL
Reagent. Pyromellitic dianhydride, 0.5.V. The pyromellitic dianhydride, 109 grams, is dissolved in 525 ml. of dimethyl sulfoxide, and then 425 ml. of pyridine are added. Procedure. Fifty milliliters of 0.5M pyromellitic dianhydride solution are pipctted into a glass-stoppered 250-ml. flask. h sample c.ontaining 0.010 to 0.015 equivalent of alcohol or amine 14 weighed and added to the reagent. The flask is placed on a steam bath and the .topper is wetted with pyridine and loosely seated in the flask. The contents are heated for 15 to 20 minutes (30 minutes for polyglycols). A 20-ml. portion of water is added and the heating is continued for 2 minutes. The mixture is cooled to room temperature and titrated with 1K sodium hydroxide to the phenol-
phthalein end point. A blank in which only the sample is omitted is treated in the same manner. DISCUSSION A N D RESULTS
The results in Tables I and I1 show that the values obtained using DMSO as the solvent are practically identical to those obtained using the original PMDA method. The reaction proceeds smoothly, and the mixture remains clear. Crystals of pyromellitic acid do appear on cooling after the reaction is complete, but these redissolve on neutralization, and the solution is again clear at the end point of the titration. The hydroxyl values for polyglycol ethers in Table I1 demonstrate that results obtained for these materials by the PMDA and phthalation ( 1 ) methods
are equivalent. The PMDA method has the advantage that it requires significantly less time. The PMDA reaction was complete in 30 minutes, but the phthalation reaction required 2 hours' reflux a t the boiling point of pyridine. LITERATURE CITED
Elving, P. J., Warshowsky, B., ANAL. CHEM.19, 1006 (1947). ( 2 ) Siggia, S., Hanna, J. G., Culmo, R., (1)
Zbid., 33, 900 (1961).
ROBERT HARPER
Olin Mathieson Chemical Corp. Brandenburg, Ky. SIDNEY SIGGIA J. GORDON HANNA Olin Mathieson Chemical Corp. New Haven, Conn.
Background Corrections in Long Path Atomic Absorption Spectrometry SIR: d method has recently been described for providing a much longer effective absorption path than was previously used in atomic absorption analysis (1). The longer absorption path, which is obtained by passing the hydrogen-oxygen flame from a Beckman burner through a Vycor tube, gives 10 to 100 times greater sensitivity than the more conventional slot or multiple port burner for several elements. We have used the method for the determination of calcium, copper, lead, magnesium, manganese, mercury, silver, thallium, and zinc in a variety of samples of interest in biology and agriculture. The higher sensitivity will frequently allow chemical separations to be avoided and is especially valuable when working with very small samples. Applications of this method to the analysis of tissue ash have been described and sensitivity data given for 13 elements ( 2 ) . At these higher sensitivities, absorption by matrix salts a t the wavelength of a n elemental resonance line can cause significant errors. More detailed information on the absorption by matrix salts and a method for correcting for the effect are given here. EXPERIMENTAL
Apparatus. A Beckman Model D E spectrophotometer with photomultiplier attachment was used. This instrument was modified to reverse the direction of light passage through the monochromator and mounted with a n optical bench t o support the external components. h Beckman hydrogm lamp and hydrogen lamp power supply were used for the continuous
ultraviolet source. The Hilger Model FA 41.301 supplied power for the hollow cathode lamps. A simple, demountable hollow cathode lamp, which was described by Werner et al. ( S ) , has been used for atomic absorption work ( 2 ) , and makes a versatile, low cost source. This lamp is operated with continuous pumping as commercial argon is bled into it through a needle valve to maintain the desired pressure. Impurities are removed by the flowing argon so that a tedious cleanup of the lamp is not required. The source for a given element is prepared by placing a small amount (50 to 100 mg.) of the metal or a suitable salt into the water-cooled brass cathode. The lamp is operated at a current of 80 to 100 ma. for a few minutes to sputter the added element over the cathode surface. The current is then reduced to the desired operating level (10 to 40 ma.) and the argon pressure adjusted to give maximum intensity of the line being used. The lamp is ready to use after a 10- to 20-minute stabilization period. A cathode is prepared for each test element and can be used repeatedly without further addition of metal. The brass cathode alone is used as a copper and zinc source. Magnesium is the one element for which the commercial sealed lamps provide a more stable source than the demountable ones which we have been able to prepare. PROCEDURE
Figure 1'4 shows the arrangement of the source, burner, and tube schematically. I t is a simplification of the arrangement used by Fuwa and Vallee ( 1 ) . The source (C) is imaged about
midway through the tube by lens L and again on the spectrometer slit by Lp. Both lenses are diaphragmed to about 6-mm. diameter openings to reduce the effect of reflection and emission from the tube walls. The tube is 40 cm. long (longer tubes cannot be mounted conveniently on our instrument), about 10-mm. i.d., and is constricted to about 6-mm. diameter at the end to reduce the amount of air entrained with the flame. The tube (2') is insulated with asbestos ( I ) to prevent condensation of salts on the walls. One stream of air cools the tube wall a t the point where the flame first strikes, and a second air stream, directed vertically past the end, protects the lens. All components are supported on spectrograph bench riders. The tube rests on asbestos blocks for thermal insulation from the remainder of the support. RESULTS
Emission from the hot tube walls has not been a serious problem, even though there is no electrical discrimination against it. The energy emitted depends on the condition of the tube wall as well as the wavelength. Yew tubes can be used to determine calcium a t 4227 A,, but the emission becomes greater as salts react with the tube walls. Tubes that have been in prolonged use give noticeable emission a t all wavelengths longer than 3000 A. Some emission can be tolerated if the zero setting of the instrument is made with a shutter in front of the lamp while solvent is aspirated into the tube. The use of a light chopper and a.c. amplification would eliminate the problem, but for work a t the short VOL. 37, NO. 4, APRIL 1965
601
Alr
0.6
A
NaCI (IO mg./ml. NoN03 A KCI
0.5
I.
-*O
A.
8.
1
Side view, hollow cathode source only Viewed from above, hydrogen lamp and hollow cathode source
wavelengths where many absorption lines lie, it is not essential. Figure 2 shows the absorption spectra that were obtained when several aqueous solutions were aspirated into the flame and tube arrangement just described] while a hydrogen lamp was used as the light source. The 100% setting was made while aspirating water into the tube. These spectra were plotted from point-by-point measurements and some fine structure features could have been missed. The flame was operated on 4.0 liters per minute of oxygen (14 p.s.i.) and 12 liters per minute of hydrogen. It was sufficiently fuel-rich to reignite at the tube exit. The reducing flame was used because it gives the best results for elemental absorption. Distilled and deionized water was used for all solutions. Reagent grade salts and redistilled sulfuric acid were used as solutes. DISCUSSION
The origin of these spectra is being investigated, However, at the present time the following conclusions can be stated: the absorbance is approxi-
Zn added, rg./ml.
0.04 0.00
H~SOI concn. 0 .OOM
0 04 0 04 0 04
Table II.
Cd added, w/ml. 0 040 0 000 0 020 0 000 0 020
602
0.2
0.1
2M
210
230
240
0,202 0.185 0.233 0 272 0 386 0 468
Figure 2.
ANALYTICAL CHEMISTRY
260
mately proportional to the solute concentration] depends on both the anion and cation, shows very little fine structure that can be resolved on our instrument, and follows no simple relationship with wavelength. The analyst is thus faced with a background absorption which depends on the composition of the sample solution and upon which the various elemental absorptions are superimposed. Background absorption does not ordinarily need to be considered in less sensitive atomic absorption methods. The monochromatic emission from the hollow cathode lamp does not enable one to scan wavelengths in the vicinity of the line to obtain a background correction as is commonly done in emission spectral
Zn fourid,. Uncorrected Corrected
0.002 0.185 0.185 0 066 0 172 0 280
0.000 0.0092 0 0390 0 0402 0 0353
0.0354 0.0447 0 0518 0 0722 0 0872
Absorbance Cathode Hydrogen Cd found, lamp lamp Uncorrected Corrected 0 142 0 151 n 221 0 131 0 205
0 0 0 0 0
004 149 151 133 135
0 0 0 0
270
280
290
300
- inp
Absorption spectra of solutes in long path flame
Determination of Cadmium in Solutions of Sodium Salts 2288 A.; spectrometer slit width, 0.06 mm.
Na concn. 0 0 3mg./ml (NaC1) 3 mg./ml (NaCI) 3 mg /ml. (NaN03) 3 mg /ml (NaN03)
250 Wovelength
Absorbance Cathode Hydrogen lamp lamp
0.3M 0.3M 0 1M 0 3M 0 5M
n ni
0 H2S04 (0.5 M o l d
0.3
Determination of Zinc in Solutions of Redistilled Sulfuric Acid 2138 A.; spectrometer slit width, 0.3 mm.
Table 1.
No)
(IO mg./ml. K)
0.4
” Source and tube arrange-
L
Figure 1 . ment
3
No)
(IO mg.;/ml.
042 063 037 058
0 0 0 0
000 020 000 020
methods. Matrix absorption and that due to an impurity of the test element in the matrix are indistinguishable without scanning. I t is possible to correct for this interference by using two light sources, the cathode lamp for measuring total (elemental and background) absorption at the appropriate wavelength for the test element, and a hydrogen lamp for measuring the background absorption at adjacent wavelengths. A lamp arrangement to permit convenient use of two light sources is shown schematically in Figure 1B. The plane, first-surface mirror M can be positioned to reflect light from the hydrogen lamp H through the tube and block that from the cathode lamp. It swings out of the way for normal cathode lamp operation. The supporting arm of the mirror is held against a metal stop (not shown) to ensure reproducible positioning. Both light paths are the same length so the two sources are imaged identically. An iris diaphragm (not shown) is positioned in front of the hydrogen lamp to allow approximate matching of the two intensities. An exact match is not necessary. The spectral slit width of our instrument is much wider than the absorption lines for the elements we have used. This is shown by the fact that little absorption can be detected from zinc solutions, for example, a t 2138 A. using the continuous source. This allows one to measure the total absorption and the background absorption without changing the wavelength setting of the monochromator. The absorbance of a sample solution is first measured using the appropriate wavelength and hollow cathode source.
The mirror is then positioned to use the hydrogen lamp and the absorbance measured again at the same wavelength. The difference between the two ahsorbances is the amount contributed by the test element. Table I gives the results from the determination of ainc in solutions of redistilled sulfuric acid, and Table 11, those from cadmium in sodium salts. These combinations were chosen because of particularly severe background interferences. The results clearly demonstrate that it is possible to correct for the absorption by matrix salts if both a cathode lamp and a continuous light source are used, and that absorption of the continuous radiation by the test element will need to he considered only in eases requiring the highest accuracy.
The corrections will be valid only if the background absorption at the precise wavelength of the elemental line is the same as the average absorption in the wavelength interval passed by the monochromator. Fine structure in the background (perhaps unresolved) could cause errors. In determining magnesium in the presence of large amounts of sodium, for example, the ground state sodium line a t 2852.8 A. would ahsorb part of the continuous radiation passed by the monochromator set for the Mg 2852.1-A. line and cause the correction to he too large. Corrections made in this manner are for absorption by matrix salts and not for any effect the salts might have on the slope of the working curve of the test element. However, the present method permits the use of the standard
additions method to correct for such effects. Standard additions cannot be used in the presence of an unknown amount of background absorption. LITERATURE CITED
(1) Fuwa, K., Vdlee, B. L., ANAL.CHEM. 35, Y42 (1963). (2) Koirtvohann. S. R.. Feldman. C..
copy,” ‘Vol. 3, J. g.. Forre&, ed., Plenum Press, New York, 1964. (3) Werner, G. K., Smith, D. D., Dvenshine, S. J., Rudolph, 0. B., iMcN~.lly, J. R., J. O p t . Soe. Am. 45, 203 (19.55).
S. R. KOIRTYOHANN E. E. PIcuEn
University of Missouri Department of Agricultural Chemistry Columbia, Mo.
Rapid, Quantitative Determination of Tertiary Amines in Long Chain Amine Oxides by Thin Layer Chromatography SIR: In the manufacture of long chain tertiary amine oxides, the main irnpurity is the starting amine. A number of procedures ( 1 , 8 , 7 ) have already been developed to determine the amount of unreacted starting material. However, a simple process control of this product was required and even though most methods are acceptable, they either lose precision in the lower concentration ranges or are too time consuming. In recent years, thin layer chromatography (TLC) has been making rapid strides toward being a precise analytical tool (3, 4, 9) besides being a most valuable qualitative analytical aid. Information can he obtained from standard TLC plates, usually within 30 to 45 minutes. However, by adopting the quantitative method of Purdy and Truter (8)to smaller plates prepared on 1.5- x 3-inch glass microscope slides, the time necessary t o chromatograph a sample can be reduced to a few minutes and an accurate quantitative analysis can he made. Quantitation is based on a linear relationship existing between the square root of the area of a comvonent after chromatographing and the iogarithm of the weight of samc,le applied to the plate. This relationship is valid for specific weight ranges of different chromatographed components. In the case of long chain tertiary amines, we found this range to be approlximately 200 pg.
The ultraviolet source used is a hand mineral lamp. A IO-pl. syringe i s used for spotting the plates and a 250-ml. indicator spray bottle for spraying the plates after development. Reagents. Silica Gel G was obtained from Research Specialities, Richmond, Calif. Chloroform, methanol, and isopropanol are C.P. grade, while ammonium hydroxide and 95% ethanol are reagent grade. The indicator, 2‘7‘dichlorofluorescein, wae obtained from Eastman Organic Chemicals. All water is deionized before using. Distilled, dimethyl “coco” amine was used in the preparation of standard solutions. Gas and thin layer chro-
EXPEIRIMENTAL
Figure 1. Separation of tertiary amine from amine oxide
Apparatus. G lass microscope slides (1.5 x 3 inches) :are used in preparing the thin layer plates. Eight-ounce glass jars are use d as chromatographing tanks.
component “sored ,OI“.”t
front I, *e
tSrtl0.y
amine
Component which moved only D short distonce from origin i s the amine oxide
matography and wet analysis showed the material to be 100% tertiary amine. Developing Solvent. The developing solvent system is one similar to that recommended by Mangold (6). It is prepared in the followin8 manner: SO’% chloroform equilibrated for 1 hour with 20% concentrated ammonium hydroxide, The organic layer is separated and measured and taking its volume as 97’%, 3% methanol is added. Standard Curve. The microscope slides are coated with Silica Gel G adsorbent by dipping them once into a slurry of 50 grams of Silica Gel in 150 ml. of chloroform (6). They are then air-dried for 5 minutes and any remaining adsorbent on the reverse side of each plate is removed with a tissue. Six different concentrations of the tertiary amine are prepared in 10-ml. volumetric flasks using isopropanol as a solvent. These amine solutions represent the following percentages of unreacted tertiary amine in a 5-gram sample of tertiary amine oxide diluted in a similar fashion: 0.28%, 0.73Y0, l.39%, 2.13%, 4.32%, and 8.28%. Five-microliter sliquots from these standard solutions are transferred to the microscope slide plates u$ing a microsyringe. All plates are spotted 1 cm. from the bottom of the plate. A few milliliters of the developing solvent are placed in the S-ounce glars jars that serve as chromatographing tanks and the plates are inserted into the tanks. The solvent front is allowed to travel to within 1 ern. of the top of the plate before the d a t e is removed. The time required f i r the development of earh chromatogram ranges from 3 to 5 minutes. All standards are run m quadruplirate. Upon removal from the tank, the plates are sprayed with a 0.057, soloVOL. 37, NO. 4, APRIL 1965
* 603
