signment for the peaks may be inferred from the literature presented. The most characteristic peak for polyfluorinated compounds is m / e 69 and is due to CF3+ (6). This and other prominent peaks commonly associated with fluorinated hydrocarbons are present (Figure 2, m / e 119 and 169). Fragments reported to be associated with N-nitrosodimethylamine ( 7 ) were also present: m / e 27, 28, 29, 41, 42, 43, 53, 5 5 , 58, and 59 (Figure 2). Peak m / e 42 has been reported (8) to be representative of N-nitrosodimethylamines in the nonderivatized compound and is due to the H2C=N+=CH2 fragment. Fragment m / e = 44,assigned the structure CHaN+-CH3, could be the result of homolytic cleavage of the molecular ion 0
It
CHI
\
O-C-R
N++N
/ CH3
/
. \
O-C-R
ll
0 to yield the fragment H3C-N+-CH3 shown in Figure 2 or N20+. The procedure is well documented (9). Although we have tentatively assigned to the peak at m / e 43 H3CN2,it is equally plausible the peak might be CHsNCH2. Although mass spectral data of halogenated compounds leave much to be desired from a standpoint of clearly revealing the actual molecular weight and structure of the derivatized N-nitrosodimethylamine, the presence of fluorine and nitrosamine fragments are clearly indicated, and they are evidence that halogenation of the nitrosamine did occur. N-nitrosodiethylamine, N-nitrosopyrrolidine, and N-nitrosopiperidine (7) T. Fazio, J. H. Damico, J. W. Howard, R. H. White, and J. 0. Watts, J . A g r . Food Chem., 19,250 (1971). (8) H. Budzikiewicz, C. Djerassi, and D. H. Williams, “Mass Spectrometry of Organic Compounds,” Holden and Day, San Francisco, 1967, p 523. (9) J. Collins, Bull. SOC.Roy. Sci. Liege, 23, 201 (1954).
were not collected and analyzed by mass spectra; therefore the labeling of these compounds (Figure 1) must be considered tentative; however, the retention times observed for the HFBA derivatives of these amines (Figure 1) seem reasonable on the basis of the known differences in boiling points of the non-derivatized compounds. The response of the EC detector to the fluorinated nitrosamine was about 6 thousand times the response of the flame ionization detector, as indicated by analysis of N-nitrosodimethylamine with both detectors on the same instrument with the same column. To obtain 2 1 3 of a full-scale response from the flame detector, one millimole of sample was derivatized, and a 2-111 injection of the sample (100 pl total) was used at an attenuation of 8. All other parameters were the same as for the analysis with the EC detector. Procedures reported (7) for detection of known underivatized N-nitrosodimethylamine with a modified thermionic detector-mass spectrometer combination required that a minimum of 50 ng/5 p1 injection of sample be present before identification could be made, Based on calculations made from data obtained by using HFBA-Py derivatives of N-nitrosodimethylamine, an EC detector, and 0 . 2 4 injection of sample, the minimum amount of N-nitrosodimethylamine detectable (about scale) with a 5-111 injection of sample should be about 17 ng. In addition, the use of HFBA-Py derivatives and a 3 z OV-1 column permits analysis of other much higher boiling nitrosamines. The results reported here indicate that some of the biologically important nitrosamines form electron capturing derivatives when reacted with fluorinated anhydrides in the presence of pyridine. Data presented elsewhere (IO) show that these derivatives have been used successfullyto analyze nitrosamines in urine samples.
RECEIVED for review January 21, 1972. Accepted May 12, 1972. Use of trade names is for identification only and does not constitute endorsement by the Public Health Service or by the U. S. Department of Health, Education, and Welfare. (10) J. B. Brooks, W. B. Cherry, L. Thacker, and C. C. Alley, J. Inf. Dis., in press.
Electrolytic Sample Preparation and Its Application to Atomic Absorption Rapid Determination of Magnesium in Cast Iron A. H. Jones and W. D. France, Jr. Chemistry Department, Research Laboratories, General Motors Corporation, Warren, Mich. 48090
ELECTROLYTIC CONSTANT CURRENT techniques have been used for many years in corrosion research, in the associated areas of metallographic polishing and etching ( I ) , and only to a limited extent in analytical chemistry (2, 3). Because such proce(1) S. W. Dean, Jr., W. D. France, Jr., and S. J. Ketcham, in
“Handbook on Corrosion Testing and Evaluation,” W. H. Ailor, Ed., John Wiley & Sons, New York, N.Y., 1971, p 171. (2) Silvio Barabas and Sydney G. Lea, ANAL.CHEM.,37, 1132 (1965). (3) Abraham Aladjem, ibid., 41,989 (1969). 1884
dures can accelerate metal dissolution in a corrosive environment, they represent one means of rapidly preparing solutions of metal samples for instrumental analysis; by comparison, conventional procedures can be time-consuming. For example, the ASTM method (4) for determining magnesium in cast iron by atomic absorption includes the preparation of (4) “1971 Annual Book of ASTM Standards,” Part 32, American
Society for Testing and Materials, 1961 Race Street, Philadelphia, Pa., p 878.
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REGULATED POWER SUPPLY
Table I. Comparison of Values and Precision Data for the Electrolytic and ASTM Methods of Sample Preparation Magnesium, Electrolytic Sample method" ASTM method 0.059 1 0.062 0.058 0.059 4 0.065 5 0.060 6 0.060 7 0.059 8 0.062 9 0.058 10 0.058 0.060 Av ' 0.0023 Std dev Std dev 3.8 = Based on 15.8 mg of iron in sample solution. 2 3
0.058
0.061
0.059 0.058 0.059 0.0010 1.7
GRAPHITE ROD CATHODE -+
PLASTIC CELL-I AND COVER
sample chips or particles which are then weighed and dissolved in acid. The solution is heated to dryness, and a known amount of acid solution is added and diluted to volume prior to analysis. Total elapsed time for a single determination is about one hour. The electrolytic procedure described in this paper was designed to minimize the sample preparation time. The basis for this is Faraday's law-the weight of metal dissolved during electrolysis is directly proportional to the quantity of electricity passed through the cell; this weight can be calculated from the following mathematical relation: Weight of metal dissolved (g)
AUTOMATIC ON-OFF TIMER (30 SEC)
0.060 0.060 0.059 0.059 0.060
-
[Atomic weight (grams)][Current (amperes)][Time(sec)] [Number of electronsl[Faraday Constant] Thus, the amount of sample in solution depends on the current and time. For a cast iron electrolyzed for 30 seconds at 2 amperes with 100% efficiency, the theoretical amount of iron dissolved is 17.4 mg. EXPERIMENTAL
Test Samples. Test samples were in the form of pins that had been drawn from molten iron into evacuated borosilicate glass tubes with 3-mm inside diameters. After the pins were removed from the tubing, a specimen 6.5-cm long was secured in the chuck of a flexible-shaft motor. The specimen was rotated and held against a rotating grinding wheel (about 20 seconds) until approximately 0.013 mm of metal surface was removed from the last 2.5 cm of the specimen. Preliminary experiments indicated the need for removing the surface of these specimens to obtain repeatable results. Reagents and Standard Solutions. The choice of a suitable electrolyte was critical because of its influence on uniform dissolution of the cast iron; also a solution with good conductivity was required. After experiments with other electrolytes, hydrochloric acid (1 : 4) was found to be satisfactory. This electrolyte offered the further advantage of providing a matrix that had been proved suitable for the determination of magnesium in cast iron by atomic absorption spectrophotometry (AAS). Two standard magnesium solutions containing 0.25 and 0.50 kg of magnesium per milliliter, respectively, in 1.2M hydrochloric acid were used to calibrate the atomic absorption spectrophotometer. The equivalent concentrations of these solutions in terms of percentage concentration of
-
*-CAST
-_I--
--
IRON SPECIMEN ANODE
I 20
m l of HCI (1:4)
Figure 1. Electrolytic apparatus and circuitry for the rapid preparation of sample solutions for atomic absorption determinationof magnesium in cast iron magnesium in the cast iron were used to set the spectrophotometer to read concentrations directly. For a 16-mg sample, the concentrations represented 0.031 3 and 0.0625 % magnesium, respectively. Electrolytic Dissolution Apparatus. The apparatus designed for this method, which is shown schematically in Figure 1, is similar to that used in corrosion research ( I ) . The circuitry includes a regulated power supply that is capable of supplying a constant current of 2 amperes at 150 V dc, an on-off interval timer for exact control of the 30-second electrolysis time, and a plastic electrolysis cell that consists of a graphite rod cathode and the cast iron pin as the anode. The electrolyte, 20 ml of HC1 (1 :4), is stirred continuously with a magnetic stirrer and a small Teflon-coated magnetic stirring bar during the electrolysis and subsequent analysis to produce a homogeneous solution for aspiration. Atomic Absorption Analysis. An atomic absgrption spectrophotometer capable of resolving the 2852.1 A magnesium line and equipped with a magnesium hollow cathode tube was used for the determination of magnesium. The instrument was calibrated to read directly in concentration. After the 30-second electrolysis, the electrolytic cell cover and the electrodes were removed, and the capillary to the AAS burner was inserted into the cell. The solution was atomized in the flame of the air-acetylene pre-mix burner. The percentage concentration of magnesium was recorded from a calibrated digital readout. RESULTS
Because the success of this method depends on a constant sample weight for each electrolytic dissolution, ten sample pins from a heat of cast iron were processed to determine the variation in the amount of iron dissolved, which itself was determined by atomic absorption. For the ten samples, the average value was 15.8 mg with a coefficient of variation of 3.5. (This amount of iron represents about 91 efficiency, as calculated from Faraday's law.) This average value was considered to be suitable for use throughout this investigation.
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Table 11. Determination of Reliability of the Electrolytic Method of Sample Preparation over a Range of Concentration Magnesium, Electrolytic Sample methoda ASTM method 1 0.002 0.002 2 0.001 0.003 3 0.028 0.028 4 0,029 0.030 5 0.046 0.043 6 0.052 0.049 7 0.051 0.052 8 0.053 0.056 9 0.057 0.058 10 0.058 0.061 11 0.062 0.066 12 0.068 0.071 13 0.073 0.078 14 0.080 0.091 a Based on 15.8 mg of iron in sample solution.
For the determination of magnesium, Table 1 shows a comparison of values and precision data for the electrolytic and ASTM methods of sample preparation. The same ten samples were first analyzed by the electrolytic method and then by the ASTM method ( 4 ) . The average value for the electrolytic method, 0,0602 magnesium, agrees well with 0.059 magnesium determined by the ASTM method. In all cases the results obtained on samples prepared by the rapid electrolytic method are within 0.005x of the results obtained by the ASTM method. The standard deviation of the ten “differences” is 0.0016x magnesium. A set of 14 sample pins was used to determine the reliability of the electrolytic method over a range of magnesium concentrations, using the ASTM values as a criterion. The results are shown in Table 11. These data indicate that the electrolytic method is reliable in the range of interest, i.e., 0.02 to 0.08 magnesium, where the data vary from the ASTM values by no more than 0.005 2 magnesium. The deviations are less at lower concentrations. The standard deviation of the 14 differences is 0.0028; when the highest value in the series is excluded, the standard deviation is only 0.0015 2 magnesium. The one value in Table I1 which is outside the desired limit of 0.005 is at the highest magnesium value of 0.091 2,which is beyond the range of interest. The sample available did not permit replicate determinations at the highest magnesium concentration. Very infrequently, gross deviations occur within the specified range, Such deviations are assumed to result
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from inhomogeneity that probably occurs due to slight differences in the way the sample is drawn from the cast iron melt. The adherence to a standard procedure for taking sample pins should eliminate these rare occurrences. DISCUSSION
Electrolytic sample solution preparation is a rapid procedure that is particularly useful when it is essential to obtain an analytical result with a minimum of elapsed time. In the determination of magnesium in cast iron, the elapsed time from receipt of sample to the time the result was obtained was only 1.5 minutes. This included sample handling (30 seconds), sample grinding (20 seconds), electrolytic dissolution (30 seconds), and atomic absorption analysis (10 seconds). It is presumed, of course, that all necessary arrangements for the operation of the electrolytic dissolution apparatus and the atomic absorption spectrophotometer have been made ahead of time. The 1.5 minutes of elapsed time can be compared quite favorably to the ASTM atomic absorption procedure, where the emphasis is on precision and approximately one hour of elapsed time is required to analyze a single sample. However, in batch work the electrolytic dissolution technique loses much of its advantage because of the increased efficiency of the ASTM procedure with a large number of samples. Although cast iron pins were used to illustrate this sampling procedure, there is no fundamental reason why other metals (and sample configurations) could not be prepared in suitable solutions. Obviously, selective dissolution and localized attack must be controlled to provide a representative sample solution, but the availability of information from electrochemical corrosion research studies minimizes this problem area. The electrolytic dissolution technique described in this paper provides a rapid (1.5-minute) means of preparing a known quantity of sample in solution without the necessity of weighing. Combined with an analytical technique such as atomic absorption, this process seems particularly applicable for “on-line” analyses of metals when analytical data are required in a minimum of elapsed time. ACKNOWLEDGMENT
The authors express their appreciation to E. F. Ryntz, Jr., for supplying the cast iron pins and for encouraging this work. We also wish to thank R. L. Spencer for his assistance in the preparation of the experimental apparatus. RECEIVED for review March 16, 1972. Accepted May 12, 1972.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
