ultracentrifugation or some other means before the samples are used. Otherwise, the large amount of solid in the brine may produce interferences in ashed samples. Calibration should be checked frequently when using this method-at least two times a week. Variations in the thickness of the dialysis film and other factors could result in considerable error if this is not done. SICKEL DETERMINATION.The technique for nickel determination is the same as for vanadium, except that the nickel intensity ratio must be measured. Some typical results for determination of nickel in petroleum and petroleum distillates are given in Table VII. Interferences usually are not encountered
for distillate and crude oil analyses. For some other types of samples, correction factors for other elements will have to be determined experimentally. Otherwise errors of several per cent of the amount present can result. LITERATURE CITED
(1) Agazzi, E. J., Burtner, D. C., Critt.en-
den, D. J., Patterson, D. R., ANAL. CHEM.35,332 (1963). (2) Dwiggins, C. W.,Jr., Zbid., 33, 67 (1961). (3) Dwiggins, C. W., Jr., “Applications of X-Ray Spectrography to Analysis of
Organic Materials with Emphasis on Petroleum Products,” ACS Eastern Analytical Symposium, New York, N. Y., Nov. 15,1961. (4) Dwiggins, C. W.,Jr., U. S. Bur. Mines, Rept. Invest. 6039 (1962).
( 5 ) Dwiggins, C. W., Jr., Dunning, H. N., ANAL.CHEM.31. 1040 (1959). (6)Zbid., 32, 1137’(19603. ’ (7) Dwiggins, C. W., Jr., Lindley, J. R., Eccleston, B. H., Ibid., 31, 1928 (1959). (8)Hale, C. C., King, - W. H., Jr.. Ibid.. 33, 74 (1961). (9) Jones, R. A , , Ibid., 33, 71 (1961). (IO) Kang, C. C., Keel, E. W., Solomon, E.,Ibid., 32,221 (1960). (11) Miller, D. C.,Norelco Reptr. 4, 2 I,1- 9.57) - - . ,. (12) Rowe, W. A , , Yates, K. P., ANAL. CHEM.35,368(1963). (13) Shott, J. E.,Garland, T. J., Clark, R. 0..Ibid.. 33. 506 (1961).
RECEIVED for review February 25, 1964. Accepted April 16, 1964.
Mass Spectra of N-Substituted Ethyl Carbamates CARL P. LEWIS O h Research Center, O h Mathieson Chemical Corp., New Haven, Conn.
b The spectra of 25 N-substituted ethyl carbamates have been determined and correlated with structural features. Interactions between the N substituents and the carbamate grouping give rise to a number of intense, characteristic peaks. Fragmentation paths responsible for many of these peaks have been elucidated so that mass spectrometry may b e used for the identification of compounds of this type.
B
(3) has pointed out that the mass spectrometric fragmentation of an organic molecule depends upon the mutual interaction of all of the atoms within a molecule and not upon the fragmentation of an isolated functional group. Nevertheless, fragmentations characteristic of a functional group do occur, and these must be recognized before the modifying effects of the remainder of the molecule can be appreciated. This was particularly true in the case of carbamates (urethanes). Their spectra were too complex to permit the assumption of sample analogies. Not until the numerous rearrangements and fragmentations of this functional group had been characterized was it possible to carry out a satisfactory structural examination of such compounds. The characteristic fragmentations of N-substituted ethyl carbamates are presented below. Much of the IEMANN
1 Present address, The Johns Hopkins University, School of Medicine, 725 N. Wolfe Street, Baltimore, Md.
1582
ANALYTICAL CHEMISTRY
corroborative evidence for the suggested cleavage paths has been previously published in a detailed report describing the behavior of ethyl N-phenylcarbamate and ethyl N-ethylcarbamate upon electron bombardment (4). The volatility of carbamates, the small amount of sample required for an analysis, and the wealth of structural information contained in a spectrum make mass spectrometry an excellent analytical approach for carbamates as well as for compounds which can be easily converted into carbamates. EXPERIMENTAL
The compounds utilized in these studies were either prepared from ethyl chloroformate and the corresponding primary or secondary amine or purchased from Aldrich Chemical Co. Solids were purified by recrystallization from hexane. Liquids, including several of the purchased ones, were purified by vacuum distillation or gas chromatography. The following compounds are considered in the present report : ethyl N-methylcarbamate, Figure 1 (purchased) ethyl N-n-propylcarbamate, Figure 2 (purchased) ethyl N-n-butylcarbamate, Figure 3 (prepared) eth 1 N allylcarbamate, Figure 4 (purcEmed ) ethyl N-cyclohexylcarbamate, Figure 5 (purchased) ethyl N-benzylcarbamate, Figure 6 (prepared) ethyl N-p-tolylcarbamate, Figure 7 (prepared) ethyl N-o-tolylcarbamate, Figure 8 (prepared)
ethyl N-m-tolylcarbamate, Figure 9 (prepared) ethvl N-2,6-dimethylphenylcarbamate, Figure 10 (prepared) ethyl N-p-chlorophenylcarbamate, Figure 11 (purchased) ethyl N-p-nitrophenylcarbamate, Figure 12 (purchased) ethvl N-a-naphthylcarbamate, Figure 13 (prepared) ethyl N-a-naphthylcarbamate, Figure 14 (prepared) ethvl N,N-diphenylcarbamate, Fimre 15 (purchased) ethvl N-methvl-N-phenylcarbamate, Figure 16 (prepared) ethyl N-ethyl-N-phenylcarbamate, Figure 17 (prepared) ethyl .hi-n-butvl-AT-phenylcarbamate, Figure 18 (purchased) ethvl N-n-allyl-N-phenylcarbamate, Figure 19 (purchased) ethyl Y,Y-dimethylcarbamate, Figure 20 (prepared) ethyl N,N-diethylcarbamate, Figure 21 (purchased) ethyl N,N-dipropylcarbamate, Figure 22 (prepared) ethyl N-piperidinocarboxylate, Figure 23 (Durchased) ethyl IN-piperaxinocarboxylate, Figure 24 (purchased) ethyl morpholinocarbamate, Figure 25 (purchased) Deuteration experimentq, similar to those described in the earlier publication concerned with the fragmentation of ethyl N-phenylcarbamate and ethyl N-ethylcarbamate ( 4 ) , were performed, as necessary, to elucidate the composition of many ions. In these experiments, deuterium was exchanged for active protons by simply saturating the inlet y y t e m with D20 before the introduction of the undeuterated sample. An 80 to 90% conversion of N-H into N-D could be achieved in this way. The spectrum of the pure deuterated
58(lV)
FIG.1
*'
29
FIG 7
CH$JHCOOCH2CH3
NHCOOCH2CH3 MW 179
MW 103
M
C H J C H 2 C H 2 C H 2 NHCOOCH2CH
CH2, C H C H 2 N H C O O C H 2 C H 3 MW 129
4' I Fiaure 1 . FiGure 2. Figure 3. Figure 4. Figure 5. Figure 6.
Mass sDectra of ethyl N-methylcarbamate Ethyl i-n-propylcarbamate . Ethyl N-n-butylcarbamate Ethyl N-allylcarbamate Ethyl N-cyclohexylcarbamate Ethyl N-benzylcarbamate
Figure 7. Ethyl N-p-tolylcarbamate Figure 8. Ethyl N-o-tolylcarbamate Figure 9. Ethyl N-rn-tolylcarbamate Figure 1 0. Ethyl N-2,6-dimethylphenylcarbamate Figure 1 1 . Ethyl N-p-chlorophenylcarbamate Figure 1 2. Ethyl N-p-nitrophenylcarbamate
VOL. 36,
NO. 8, JULY 1964
1583
Figure Figure Figure Figure Figure Figure
1584
1 3.
14. 15. 16. 17. 1 8.
Mass spectra of ethyl N-cy-naphthylcarbamate Ethyl n-P-naphthylcarbamate Ethyl N,N-diphenylcarbamate Ethyl N-methyl-N-phenylcarbamate Ethyl N-ethyl-N-phenylcarbamate Ethyl N-n-butyl-N-phenylcarbamate
ANALYTICAL CHEMISTRY
.
Figure 1 9. Figure 2 0 . Figure 21. Figure 22. Figure 23. Figure 24;
Ethyl N-allyl-N-phenylcarbamate Ethyl N,N-dimethylcarbamate Ethyl N,N-diethylcarbamate Ethyl N,N-dipropylcarbamate Ethyl N-piperidinocarboxylate Ethyl N-piperazinocarboxylate
t r9
compound was then obtained by subtracting the contribution of the remaining undeuterated sample from the total record. A11 spectra were obtained with a Consolidated 21-103C mass spectrometer equipped u-ith a heated inlet system of our own modification ( 5 ) . Instrumental conditions were as follows. ionizing current of 10 pa.; ionizing voltage of 70 volts; ion source temperature of 270' C.; heated inlet temperature of 100" C.
TABLE I.
a
-
42
FIG. 25
n
OJCOOCH2CH3 MW 159
56
130(IU)
eaY) I
(IVI
lh
I
Figure 25.
1
I
I
J
I
Mass spectra of ethyl morpholinocarbamate
SCHEMATIC FRAGMENTATION OF N-SUBSTITUTED ETHYL CARBAMATES
Except as noted, Ri and RZ (also R3 and Rd)
aliphatic, aromatic, or H. often accompanies the fragmentation. Known to occur in some cases ( 4 ) but undetectable in the presence of other paths. A probable mode of fragmentation but undetectable in the presence of the paths indicated by metastable peaks Rz = H. Rz = aliphatic. R1 = RCH?; R = aliphatic (larger than methyl) possessing an available proton. Path XV preferable t,o S when both are possible. Or an analogous olefin elimination from R,when possible. =
* The symbol m* signifies that a metastable peak
c
f
0
i
VOL. 3 6 , NO. 8, JULY 1964
1585
RESULTS
The spectra of 25 N-substituted ethyl carbamat'es are shown in the accompanying figures. A11 were obtained a t what may be considered to be an optimum inlet temperature, 100" C. Lower temperatures are not sufficient to volatilize some of the compounds; higher C.temperatures--e.g., 150-200" cause extensive thermal decomposition of others. Most of the monosubstituted ethyl carbamates tend to decompose into ethyl alcohol and the corresponding isocyanate. However, a t 100" C., the rate of this decomposition is slow, and the spectra are quite reproducible except for those few peaks to which the decomposition product's contribute. The analytical usefulness of the data is unhampered by a slight thermal instability. Table I schematically summarizes the cleavages and the rearrangements which appear to be responsible for the observed ions. The various modes of fragmentation are identified by Roman numerals and, in each of the figures, the intense peaks corresponding to these modes have been so labeled. The paths were determined by a consideration of metastable peaks (when present), the spectra of monosubstituted ethyl carbamates after exchanging the acbive proton for deuterium, and the known fragmentation of ethyl N-phenylcarbamate and ethyl N-ethylcarbamate. The use of deuteration experiments and metast,abIe peaks in the elucidation of fragmentation pat,hs has been described in the previous publication (4). Many of the specific conclusions reached a t that' time may nom be viewed in light of the more generalized schema shown in Table I. For example, it, was mentioned that, although ethyl N-phenylcarbamate and ethyl N-ethylcarbamate did not shox a metastable peak accompanying an internal elimination of CO,, the compound ethyl N-butyl-Nphenylcarbamate did show such a peak a t mass 101 (corresponding to path XV). I t is now recognized that the CO, elimination (paths X or XV) often produces a detectable metastable peak and that ethyl iV-phenylcarbamate and ethyl ;L'-ethylcarbamate were unusual in this respect. Other fragmentat'ions which generally are accompanied by a metast'able peak also emerged during the present study. These, too, are indicated in Table I. With perhaps the exception of the skeletal rearrangements of X and XV, the suggest,ed modes of cleavage find many analogies in the mass spectrometric, literature which has been so comprehensively reviewed by Beynon ( 2 ) and Biemann ( 3 ) . Fragmentations similar to those of esters and amines are particularly pertinent ( 4 ) . Such analogies, as well as the adequacy with 1586
ANALYTICAL CHEMISTRY
which the 17 given paths explain the spectra of those compounds which have been investigated, serve to substantiate the suggested routes. DISCUSSION
A significant feature in the spectra of ethyl carbamates is the predictable array of intense peaks a t high mass values and a t the molecular ion (M). While some paths cannot be followed by certain compounds, peaks corresponding to many of those paths which are available are often present in a single spectrum. Seven of the paths (111, IV, V, VII, VIII, X I I , and XVI) account for the largest peak above m/e 50 in 20 of the 25 given spectra. In one compound or another, no less t,han 12 (111-VIII, X, XII, and XIV-XVII) of the 17 paths may be seen to be capable of producing either the most intense peak or the second most intense peak above m/e 50. Such a variety reflects upon the fact that the functional group is not an isolated entity; it' is strongly influenced by the iT-substituents. The remaining five paths (I, 11, IX, XI, and X I I I ) rarely, if ever, yield intense peaks, but they are usually detectable in the appropriate compounds. Molecular Ion. As may be seen from t'he figures, the molecular ion is obvious in the spectra of simple ethyl carbamates. Recombination peaks cannot be detected. T h e relative intensities of ( M + l ) and ( M f 2 ) + peaks agree with those to be expected from known isotopic ratios. The size of the molecular ion peak of an ethyl carbamate depends upon the nature of the LV-substit'uents. Carbamates with Ar-aromatic substituents generally yield more intense molecular ion peaks than do those with iV-aliphatic substituents. For example, compare Figures 1-5, 20-22 with Figures 7-10, 17, 18. The molecular ion intensity is relatively low either when the N substituents render the ethyl carbamate grouping more labile or when the N-substituents, themselves, are particularly susceptible to cleavage. Both of these effects would appear to be operative in certain compounds, such as those IT-aromatic carbamates which have a highly inductive group at'tached to the ring. This may be seen as an inverse relationship between the polarity of the molecule and the intensity of the molecular ion in the spectra of ethyl N-p-tolylcarbamate (Figure i ) , ethyl N-p-chlorophenylcarbamate.(Figure 11), and ethyl AT-p-nitrophenylcarbamate (Figure 12). A compound with a particular weakness in the AT-substituent, is exemplified by ethyl S-n-butylcarbamate (Figure 3). In this case, the carbon chain of t,he butyl group is easily broken by cleavage 0 to the carbamate nitrogen, and the molecular ions (and +
most other ions) are not plentiful due to the predominance of the following sequence of events: C4HsKHCOOCZHLi -+ (145) +CHzNHCOOC2HE + (102) +CHzNHC&Hs (58) +CHzNHZ (30) The 17 major fragmentation pathways producing ions other than the molecular ion will now briefly be considered. PATHI. The loss of the terminal methyl from the ester moiety appears in none of the figures, but it could be detected as a very low peak in many of the original iecords. PATHII. ;1 peak resulting from the elimination of ethylene from the ethyl ester moiety is weak in most of the given spectra. I t is more intense in the case of methyl- or aromatic-substituted compounds, for they do not possess an easy mode of fragmentation elsewhere in the molecule. PATH III. The loss of 29 mass units by path I11 is essentially absent in the spectra of .V-aromatic carbamates and small in the spectra of A'-aliphatic carbamates. However, it appears to be unexpectedly large in the spectra of certain compounds which possess structural peculiarities preventing them from being categorized as either typically aliphatic or aromatic. Four such compounds are ethyl N-allylcarbamate (Figure 4), ethyl Nbenzylcarbamate (Figure 6)> ethyl N piperidinocarboxylate (Figure 23) and ethyl morpholinocarbamate (Figure 25). The reasons for this increased tendency to lose 29 mass units is not apparent. Ethyl cleavage and stabilization of the resulting carbonium ion by ring closure could be suggested as an explanation, but this would not be equally satisfactory in all four cases. Furthermore, it is known that the loss of 29 mass units from other ethyl carbamates does not always result from a simple cleavage of the terminal ethyl group ( 4 ) ; rather, as indicated in Table I, ethylene plus a proton from elsewhere in the molecule may be eliminated. In view of this type of behavior, and the known ease of carbonium ion rearrangements, an explanation for this structurally significant mode of fragmentation must await further study. h peak a t 29 mass units, which is from the terminal ethyl group ( 4 ) , is generally the base peak in ethyl carbamate spectra. Consequently, cleavage of the molecular ion may take place a t the ethyl-oxygen bond. If it does so,
the charge is not retained by the larger fragment represented in path 111. PATHIV. h peak corresponding to path IV, the loss of the ethyoxy end group, is present t o a minor extent in the spectra of almost all ethyl carbamates. Only in methyl or dimethyl N-substituted ethyl carbamates has it been observed to be more intense than 10 to 20y0 of the base peak. PATHv. This pathway, representing the loss of COOCzH6, is important in those compounds which do not have aliphatic chains of more than two carbon atoms in length substituted onto the nitrogen atom. When such an N-aliphatic chain is present, other pathways are predominant. PATHVI. Small peaks a t ( M - 7 4 ) + and/or ( M - 75) may be seen in almost all ethyl carbamate spectra. Reasonably large peaks corresponding to these masses may be seen in the spectra of the highly aromatic compounds ethyl N-a-naphthylcarbamate (Figure 13), N-p-naphthylcarbamate (Figure 14), and ethyl N,N-diphenylcarbamate (Figure 15). The sequential operation of paths IV, V, and VI is usually indicated b y metastable peaks when V and VI are of sufficient frequency. PATHVII. This pathway represents the loss of the elements of ethylene and carbon dioxide. Metastable peaks have not been detected; thus, the peak is shown as being derived from the molecular ion in Table I. Path VI1 provides an important diagnostic peak for structural analysis because it is restricted to a limited class of compounds. Monosubstituted ethyl carbamates in which the nitrogen atom is linked to an aromatic ring give an intense peak corresponding to this mode of fragmentation. Other monosubstituted ethyl carbamates do not do so (ethyl A;-allylcarbamate, Figure 4, provides the only observed exception to this statement). Disubstituted ethyl carbamates appear to fragment by path VI1 only if one of the substituents is aroma ic and the other is methyl or aromatic (or allyl). PATH VIII. Isocyanates a-e apparently formed by both thermal decomposition and electron bombardment (4). Aroma+ic stabilization of the isocyanate group makes path VI11 a prominent mode of cleavage (and/or decomposition) for N-aromatic carbamates. This, too, is an important diagnostic pathway in structural analysis. It is unavailable to N-disubstituted carbamates. PATHIX. A relatively insignificant mode of cleavage is represented by path IX. It is, of course, limited to those compounds with the appropriate structure. Examples may be found a t mass 132 in the spectra of ethyl Nbenzylcarbamate (Figure 6) and the ethyl N-tolylcarbamates (Figure 7-9).
I n these cases, a proton is lost by path IX after the isocyanate is formed by path VIII. I n the spectrum of ethyl N-ethylcarbamate ( 4 ) there is a small peak ?t mass 56 which corresponds to the following sequence :
These peaks appear to be artifacts produced by thermal decomposition of the parent compound and subsequent fragmentation of the resulting isocyanate; they, like the isocyanate peak, increase
PATHXI. Very small peaks may be seen in some spectra a t one and/or two mass units below the peak resulting from path X. These peaks are believed to be related to path X as shown in Table I. PATHS XII to XVI. When the structure of an X-substituent is such that there is a likelihood of cleavage within it p to the nitrogen atom of the ethyl carbamate moiety, path XII, and the subsequent four paths ( X I I I to XVI), are among the most important modes of cleavage. These paths may be illustrated with the following schematic diagram of the fragmentation of ethyl N,N-dipropylcarbamate (molecular weight of each ion is indicated in parentheses) :
+
CHaCHzCHz
\N-COOCH~CH~+
CHaCHzCH2
\,N-COOCH~CH~ +CH2/
-
XVI
r-
(144)
/?rT-COOCHzCH3
(116)
+CH2’ CH3CH2CH2 \NH +CH/
.
(72)
.
+CH/ in intensity when the carbamate is allowed to stand in the heated inlet system. Despite this origin, they are useful in structural analysis. PATHx. An unusual skeletal rearrangement, which appears to be unique to carbamates, is given as path X in Table I. This mode of cleavage has been discussed in detail in the report describing the spectra of ethyl N-phenylcarbamate and ethyl N-ethylrarbamate (4). It may be represented in the following way:
0
I1 c--0 R1
\N
Rz/
PPI I
CH2CHI-c
”‘>N-
CH2
+
Rz In general, ethyl carbamates which cannot readily undergo cleavage in R1 or R2 0 to the nitrogen atom of the carbamate (including the case when R1 or R2 is methyl or allyl) display a relatively intense peak due to this rearrangement. Path X is clearly represented in Figures 4, 7-11, 13-16.
Peaks corresponding to all of these ions may be observed in Figure 22 (aths XI11 and X I V cannot be distinguished in this case). There are few other contributions to the spectrum. Comparable processes may be seen t o be operable in other spectra when p cleavage is possible. Paths X I I I , XIV, and XVI represent nothing more than the secondary elimination of a neutral molecule from a preexisting ion. The literature contains numerous examples of such eliminations ( 2 , 3). On the other hand, the skeletal rearrangement of path XV appears to be unique to carbamates. It is comparable to path X, and it, also, has been prev ously described in some detail ( 4 ) . PATHXVII. Cleavage between the ethyl carbamate nitrogen atom and the substituted R group is a common event. A peak is rarely, if ever, found a t the mass corresponding to the remaining carbamate moiety; thus, either the charge is retained exclusively by the substituted group, or R+ arises from other fragments by a secondary cleavage. OTHERPATHS. Almost every peak in the accompanying spectra may be attributed either to one of the 17 paths VOL. 36, NO. 0, JULY 1964
1587
summarized in Table I or to a characteristic cleavage m the remainder of the molecule Cleavage not directly induced by an obvious functional group must always be taken into account during the interpretation of mass spectra. Such cleavage may so dominate the spectrum that the presence of the functional group may not even be detected. Tendencies along this line may be seen in the spectra of ethyl S-cyclohexylcarbamate (Figure 5) , ethyl S-p-nitrophenylcarbamate (Figure 12), and ethyl N-piperaainocarboxylate (Figure 24). In these cases, the more intense ions do not
reflect the carbamate structure. Instead, as may be readily verified by examining the spectra of cyclohexylamine, p-nitroaniline, and piperazine (1). respectively, they are representative of characteristic fragmentations occurring ,within the S-substituted group. In view of this type of behavior, it would be futile to attempt to give a set of rules by which carbamates may be recognized by mass spectrometry. Let it simply be stated that ethyl carbamates often behave as indicated in Table I. The presence or absence of particular paths, as well as peaks not corresponding to one of the
given paths, may be interpreted in terms of structure. LITERATURE CITED
( 1 ) Am. Petrol. Inst., “Catalog of lfass Spectral Data,” API Research Project 44,New York, S . Y. (2) Beynon, J. H., “Mass Spectrometry and Its Applications to Organic Chemistry,” Elsevier, Kew York, 1960. ( 3 ) Biemann, K., “Mass Spectrometry, Organic Chemical Applications,” McGraw-Hill, New York, 1962. ( 4 ) Lewis, C. P., ANAL. CHEM.3 6 , 176 11964). ( 5 ) Lewis, C. P., Hoberecht, H. D., Zbid., 35, 1991 (1963). RECEIVED for review January 27, 1964. Accepted April 9, 1964.
Activation Analysis by Absolute Gamma-Ray Counting and Direct Calculation of Weights from Nuclear Constants FRANCESCO GIRARDI, GIAMPAOLO, GUZZI, and JULES PAULY Servizio Chimica Nucleare, Centro Comune
di Ricerche Euratom, lspra (Varese), ltaly
b A method based on neutron activation and gamma-ray spectrometry has been studied by irradiating samples of different elements, measuring the activities by means of a calibrated gamma-ray spectrometer, and calculating the weights of elements from the activation formula. The values obtained were compared with the weights of the irradiated samples. Then accuracy and precision were evaluated. The results show that random errors generally are not greater than those obtained by using the relative method, but that systematic errors may reach 20%, although in 1 1 cases out of 13 they were lower than 10%. Different causes of error have been evaluated. Calibration of the gamma ray spectrometer and neutron flux measurements do not cause any relevant error. Uncertainties in nuclear data taken from literature, especially those on decay schemes and activation cross sections, may be responsible for most systematic errors. If the nuclear constants are known and the precision required is not better than 2 0 ~ o , ~ t h e method can be applied to trace analysis by neutron activation and the experimental procedures considerably simplified.
Only a few examples of actual analysis (Z) are reported in the literature. The drawbacks of the direct method have been point’ed out by, among others, Taylor and Havens (22) in 1956. These included poor knowledge of t’he nuclear constants and difficulties in measuring exactly neutron fluxes and disintegration rates. On. the other hand t’he method presents some advantages over the more generally adopted relative procedure-i. e., diminution of the experimental work and elimination of errors due to inhomogeneity of the neutron flux within the irradiation capsule (19). In recent years many nuclear constants have become known with a better precision, and t’he techniques and instrumentation of nuclear detection have been greatly improved. These improvements were checked to determine whether the direct method was now competitive with the relative one. In this work the direct method has been applied to 13 elements which are among those most commonly determined by activat,ion analysis. Known quantities of elements were irradiated in Ispra I reactor; the weights were then calculated from the measured activities and compared with the actual weights. I n order to have the maximum accuracy, the different quantities appearing in the activation formula were considered, and an attempt was made to obtain the best values by studying the literature data on nuclear constants. The disintegration rates have been determined by absolute gamma spectrometry using a geometrical arrange-
T
H E DIRECT METHOD of activation analysis, based on the determination of weights by means of the activation relationship has always been considered as a semiquantitative method.
1588
ANALYTICAL CHEMISTRY
ment which is generally employed for activation analysis by the relative method. The determination of the efficiency of the detector us. gamma ray energy was made by the absolute method proposed by Lazar, Davis, and Bell (16) after It was demonstrated that errors introduced by the presence of scattering material between source and detector in our geometrical conditions were negligible. EXPERIMENTAL
Principle of the Method. The disintegration rate, A , of a radioisotope formed by neutron activation is related to the weight, T I ’ , of the t’argetelement by means of the actitation formula
d
U B Q W ~-( I =
Jf
(1)
where u
0
= =
4 X
=
N
=
=
‘11 =
T t
= =
activation cross section natural isotopic abundance of the target nuclide neutron flux decay constant of the radioisotope formed Avogadro’s number at’omic weight of the target element irradiation time decay time
I n Equation 1, U , 8, 41;, X,and X can be obtained from the literature. T and t are experimental parameters which can be measured accurately without difficulty in most cases.
