The valve in the transfer line before the separator does not close completely, 2-5x of the material always leaks into the separator. Thus the decane peak in the previous figure is diminished, but not eliminated from the TIM chromatogram. The sharp peak on the back side of the leakage peak is due to residual decane in the dead volume of the transfer line just upstream from the separator valve. When the venting is terminated, this material is swept into the MS, causing a small, sharp peak. This can be eliminated by shortening the line leading from the tee to Vf. Figure 4 shows the TIM chromatograms of benzoic acid, containing a trace of p-methyl benzoic acid, in acetone. In both cases the solvent was vented; in A the benzoic acid was not vented; in B, it was vented. In the latter case, the benzoic
acid was completely excluded from the MS, the mass spectrum of p-methyl benzoic acid showing no detectable benzoic acid background. No solvent spike was observed because sufficient time had elapsed for all solvent to bleed from the transfer line dead volume. This interface has provided troublefree operation for almost a year under a wide variety of GC conditions, temperatures from 60 to 320 "C; packed and open tubular columns, both wall and support coated; flow rates from 4 to 60 ml/min; helium and hydrogen carrier gas; polar as well as nonpolar materials, and with and without venting. RECEIVED for review August 23, 1971. Accepted January 7, 1972.
Simple Device for Compensation of Broad-Band Absorption Interference in Flameless Atomic Absorption Determination of Mercury Ron L. Windham Jefferson Chemical Company, Inc., Austin Laboratories, P.O. Box 4128, Austin, Texas 78765
THE VERY LOW detection limit obtained with the flameless atomic absorption technique for the determination of mercury, along with its adaptability to any atomic absorption spectrophotometer, has led to wide use of this analytical method. A number of papers (I, 2) have been published dealing with applications and various modifications of the method first described by Poluektov et al. (3), and later refined by Hatch and Ott (4). The process of vaporizing atomic mercury from solution and passing the vapor into an absorption cell rather than the usual atomic absorption flame greatly increases the opportunity for interference from broad-band absorption. This type of absorption can arise from volatile organic contaminants which might be present. Proper sample treatment prior to the flameless determination of mercury can eliminate this problem. However, such treatment is not always performed, nor are all treatments carried to completion. Mercury determinations performed I outinely on water effluents often utilize a standard aliquot of acidic permanganate. Failure to test for the complete destruction of organic materials can lead to vaporization of volatile organics, resulting in broad-band absorption and erroneously high values for the calculated mercury content. Mercury determinations at this laboratory have on occasion produced absorbance peaks indicating a mercury content in excess of that expected or known to be present. Such observations on samples of wet-digested organic materials caused us to question whether the observed absorbance was really mercury due to sample contamination, or was the result of broad-band absorption interference from incomplete destruction of organic material. The background correction device described here will identify the type of interference (1) D. C. Manning, A t . Absorption Newslett., 9, 97 (1970). (2) A. L. Malenfant, S . B. Smith, and J. Y . Hwang, Twelfth Annual Eastern Analytical Symposium, New York, N . Y . , Nov. 19, 1970, available from Instrumentation Laboratory Inc.,
Lexington, Mass.
(3) N. S. Poluektov, R. A. Vitkun, and Y . V. Zelyukova, Zh. Anal. Khim., 19,873 (1964). (4) W. R. Hatch and W. L. Ott, ANAL.CHEM., 40, 2085 (1968). 1334
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
H ,-way
t,,,,on
glass wool
Stopcock
Figure 1. Palladium chloride absorber for mercury
present and possibly eliminate the need to repeat a long wetdigestion step. The problem of interference due to broad-band absorption has been recognized by other workers (5,6), and various methods have been used to compensate for this interference. The use of a blank containing the same background works well, but is impractical in that it requires a knowledge of the type and amount of contaminant present. The use of a nearby non-absorbing line is difficult in the case of mercury because of the absence of such a line in the mercury hollow cathode. A line of another metal can be used, but requires a double-beam instrument or the changing of hollow cathodes during the determination. A continuum source such as a hydrogen lamp can be used in place of a non-absorbing line, but suffers the same inconvenience. Kahn ( 5 ) and Manning (6) have described a compensation method which utilizes a deuterium arc device built into the instrument optical system. This device has the advantage of being useful for background compensation in the determination of any metal by atomic absorption spectrometry. Other investigators have resorted to the separation of mercury from any background by amalgamation with gold (7)or ( 5 ) H. L. Kahn, At. Absorption Newslett., 7, 40 (1968). (6) D. C. Manning, ibid., 9, 109 (1970). (7) W. W. Vaughn and J. H. McCarthy, U.S. Geol. Surv., Prof. Pap,, 501-DD, 123 (1964).
to hood
0.16~
4,
air inlet
I
I
0.08 m
a 0.04
peristaltic pump
0.00
A
B
C
D
E
Figure 3. Absorbance peaks using palladium chloride absorber for mercury stirrer
Figure 2. Vaporization system for flameless determination of mercury silver (8). After amalgamation, the mercury is driven from the amalgamator with heat. This method works well, but also requires time and some special equipment. James and Webb (9) have described a double-beam UV instrument that splits the mercury-containing vapor stream into two cells and two detectors. One vapor stream is passed through a tube containing PdClz on glass wool. Mercury is absorbed from the stream because of its oxidation to the Hg2+ state, leaving only the background to pass through the second detection cell. The instrument then corrects for the background absorption present in the original vapor stream. This paper describes a simple technique utilizing a palladium chloride on glass wool mercury absorber which can be assembled and attached to a cycling flameless vaporization system in only a few minutes. Its use is simple, requiring only the turning of a stopcock, and it requires only about one minute additional time for each sample. EXPERIMENTAL
Palladium Chloride Absorber for Mercury. Glass wool is dipped into a 1% solution of PdC12 in water and is then air dried at room temperature. A small plug of the treated wool is placed in a 2-in. glass or plastic tube. A polypropylene connecting tube is very convenient for this purpose. This absorber is attached below a 2-way Teflon (Du Pont) stopcock as shown in Figure 1. Tygon tubing and glass Y-shaped connecting tubes are used as needed. Attachment of Absorber to Vaporization System. Figure 2 shows the flameless vaporization system incorporating the mercury absorber. This system may be operated as an open or closed system by opening or closing the inlet, exit, and cycle stopcocks. It also contains a sample flask bypass. An ice trap is used for water removal in place of a magnesium perchlorate absorber. The palladium chloride absorber for mercury is placed in line between the water trap and the sample cell. Apparatus. This system was used with a Perkin-Elmer Model 303 atomic absorption spectrophotometer with mercury hollow cathode lamp operated at the 2536.5-A spectral line. Reagents and Sample Preparation. The solutions and reagents used were essentially as described by Hoover et a f . (IO). Fifteen milliliters of nitric acid was added to each (8) G . W. Kalb, At. Absorption Newslett., 9, 84 (1970). (9) C. H. James and J. S. Webb, Trans. Znst. Mining Met., 13, 633 (1964). (10) W. L. Hoover, J. R. Melton, and P. A. Howard, J. Ass. Ofic. Agr. Clzem., 54, 860 (1971).
Each peak represents 0.10 pg Hg present with the following background producing compounds: A, none; B, 2 ml of acetone; C, 4 ml of acetone; D, 1 ml of methyl isobutyl ketone; E, 5 ml of hydrochloric acid. The decrease in absorbance due to removal of mercury is indicated in peak C as is the broad-band background absorbance (bg)
of five 250-ml flasks, designated A through E, containing about 75 ml of deionized water along with an aliquot of mercury standard containing 0.10 pg of mercury. The following background producing compounds were then added as indicated: A, none; B, 2 ml of acetone; C, 4 ml of acetone; D, 1 ml of methyl isobutyl ketone; E, 5 ml of HC1. These solutions were then diluted to 150 ml with deionized water. Procedure. Mercury was determined in each of the above samples by the following procedure. A base line was established using an acid blank. The sample container was attached to the vaporization apparatus. The inlet and exit stopcocks were closed and the cycle stopcock was opened. The flask bypass stopcock was set to direct the flow through the sample flask, and the absorber stopcock was set to direct the flow around the PdC12 absorber for mercury. Ten milliliters of stannous chloride reducing solution was added through the sample port. The peristaltic pump was turned on and the vaporization process was allowed to reach equilibrium (about 1 minute). The bypass stopcock was turned to bypass the sample flask. The recorder was activated for 5 sec to record the absorption peak and then deactivated. The recorder pen was thus stopped at the peak maximum. The flow was then diverted through the PdC12 absorber tube by turning the stopcock, and the mercury vapor was absorbed from the vapor stream by the PdC12. This removal of mercury required about 1 min depending on the amount of mercury present. The recorder was again activated for 5 sec. The pen reading then decreased an amount proportional to the mercury originally present in the vapor stream. The absorption peak recorded after absorption of mercury represented absorption due to volatile organic compounds interfering by broad-band absorption. The vapor stream was then diverted around the PdC12 tube by turning the stopcock. Stopcocks were then turned to open the air inlet and vent-tohood exit, and the cycle stopcock was closed. The sample flask and system was then flushed with air. The recorder was then reactivated. The pen reading then descended to the original base line and the system was ready for a second sample. This entire process required only 4 min. RESULTS AND DISCUSSION
Figure 3 shows the absorbance readings obtained using the PdC12 absorber for mercury. Sample A showed an absorbance of 0.06 for 0.10 pg of mercury with no broad-band background absorbance present. With samples B, C, D, and E, each containing 0.10 pg of mercury as did sample A, erroANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
1335
neously high absorbance peaks were obtained. However, when the mercury was absorbed from the vapor stream by the PdClz on glass wool, the absorbance decreased by 0.06 absorbance unit, which corresponds to the 0.10 pg Hg present, and to the absorbance of sample A in the absence of organic contaminants. No premature reaction of mercury with the PdC12 due to back-diffusion into the absorber was observed. However, the use of a pinchcock between the PdC12 absorber and the Y-junction will prevent this possibility.
The lifetime of the PdCh absorber is dependent on sample load, but as many as ten samples have been observed to cause no decrease in absorbing ability. This method for background compensation is convenient for mercury determinations whenever a broad-band background interference is present or suspected. It appears to be reliable and specific for mercury. RECEIVED for review September 29, 1971. Accepted December 14,1971.
Simplified Determination of Rate Constants by Scanning Calorimetry R. N. Rogers University of California, Los Alamos Scientific Laboratory, Los Alamos, N .M . 87544 METHODS FOR THE DETERMINATION of rate constants by use of a differential scanning calorimeter (DSC) at constant temperature have been reported by Duswalt (I) and Dorko, Hughes, and Downs (2). Duswalt’s method involves the measurement of the decomposition energies of samples held for extended times at a test temperature in the DSC. The method is laborious, and it incorporates two possible sources of error. Inaccuracies in the average and differential temperature calibrations of the DSC can cause errors of several degrees in the test temperature, and measurements of decomposition energies of reactions involving gaseous products can be very erratic. However, it is the only method capable of yielding data at temperatures too low for direct rate measurements. The method proposed by Dorko et al. requires a partial integration of the rate curve for the determination of each reactant fraction, a. They then make the classical plot of log a us. time for the determination of rate constants. When a planimeter is used for integration, this method is also laborious, and a planimeter introduces considerable error into the method. Two facts make it possible greatly to simplify many isothermal DSC operations. First, many organic decomposition reactions, e.g., the thermal decomposition of organic explosives, take place at measurable rates only in the liquid phase, and a decomposition in a homogeneous liquid phase that is subject to measurement must be first order (3). Second, the DSC yields rate data directly, and Keenan and Dimitriades ( 4 ) have shown how use of direct rate data can simplify kinetics work. The DSC deflection above the base line, b, is directly proportional to the rate of energy evolution or absorption of the sample, dq/dt, which in turn is directly proportional to the rate of the reaction, daldt. Therefore, ab
=
pdq/dt
=
da/dt
=
k(l
- a)
+ In (1 - a)
(2)
For a first-order reaction, (1) A. A. Duswalt, in “Analytical Calorimetry,” R. S. Porter and J. F. Johnson, Ed., Plenum Press, New York, N.Y., 1968, p 313. (2) E. A. Dorko, R. S. Hughes, and C. R. Downs, ANAL.CHEM., 42, 253 (1970). (3) R. N. Rogers and L. C. Smith, Thermochim. Acra, 1, 1 (1970). (4) A. G. Keenan and B. Dimitriades, Trans. Faraday Soc., 57,
1019 (1961). 1336
1500
t ( recl
I800
2100
Figure 1. A simplified first-order plot of the differential scanning calorimeter data for the decomposition of cupferron tosylate at 114OC. Recorder deflection from the base line, b, is measured in millimeters -In (1
- a)
=
kt
+c
(3)
where c is a constant. Substituting Equation 3 into Equation 2 and combining the constants gives the following expression:
(1)
where CY and p are proportionality constants and k is the rate constant. Hence, In b = In k / a
1260
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
lnb
=
C - kt
(4)
Therefore, rate constants for first-order reactions can be obtained directly from a plot of In deflection us. time. When reactant fractions are required for other treatments of the data, integration can be performed by use of Simpson’s Rule. For example, prepared program No. 09100-70003 for the Hewlett-Packard Model 9100 programmable calculator, using deflections and times, gives partial integrals for every second deflection measurement. The Simpson’s Rule integration, using closely spaced deflection measurements, has been found to be as accurate as a planimeter, and is considerably more convenient.
