Method of Analysis by Mass Spectrometry 5.
E. J . JOH\ SEh, Monsunto Chemical C o m p a n y , Texas C i t y , Texas
.i new method of mass spectrometric analysis is proposed in which the usual pressure measurement of the sample prior to expansion is not necessary. Peak ratios are used throughout and the method is claimed to be capable of general application except in the analysis of ternary isomeric mistures.
T
IIE original and gtm,rally used iiiet hod of mass apcctromet ric analysis Icqaires that the prcwurc of the sample introduced into the samplc hanclling system be knoirn (3, ?, 8). This providt.2 it nifans of rclfrxrring all ion currents (pealis) measured to an accurate prclssure standard. The use of the “pressure standard” method to analyze a liquid mixture having a vapor pressure near, or lew than, 1.0 min. of mercury poses the problem of measuring such a pressure accurately to better than 10.01 nim. Such a measurement is not easily made hy any of the ordinary techniques, although apparatiis for such measurements has been described ( 2 ,4, 5 6 ) .
The cyclohexane values found in the two most dilute samples secm to indicate that, the original benzene used to prepare the mixtures contained about 0.03% cyclohexane as an impurity. The average error of the above determinations appears to be *5.0% except fur very low values of cyclohexane. The precision is good. ~ S ~ L Y S IOF S MIXTURES OF TOLUEYE METHY LCYCLOHEXANE
Avn
The toluene used was a “heart cut” from a sample of nitration grade toluene. The methylcyclohcuane used was also a “heart cut” from good quality commercial methylcyclohexane. The standard mixture here contained 2.364% methylcyclohexane in toluene. The data are given in Table 11. The last two analyses indicate impurities in the major component (toluene) of 0.07 t o 0.09% methylcycloheuant~. Thc average error here is again about *5 to 6%. These two cases arc evaniples of the w r y simplest kind of analyses by the ratio method wherein neither of the peaks used is interfered with by the other component.
METHOD
A method involving the use of peak ratios where pressure is no factor and peaks of analytical significance are all referred t o some definite large peak in the mass spectrum of the mixture has been developed. The general principle can be illustrated by the following discussion. Assume that a binary hydrocarbon mixture has been espanded into the instrument,. No loss in generality will result in this simple case, since the method can obviously be extended to cover the analysis of more complex systems. Designate the two components as a and b. Choose a peak due t o b, iM;,which is substantially independent of a (though not necessarily) and a peak, 3.2f.t, which is. if possible, one of the largest peaks in the composite (or mixture) spectrum. It is easy to show that if Pa and Pb are, respectively, the n-eight per cent concrnt,ration of a and b iu the mixture, then
(z)
I n the case of the benzene-cycloheuane mixtures, the peak ratio
was used ;-here 84+ is the height of the parent mass peak of cyclohexane only and 78+ is the height of the parent mass peak due t o benzene. Cyclohexane does actually contribute a very small addition t o the total 78+ peak but, because of the magnitude of the total 78- peak a t low concentrations of cycle hexane, this correction is negligible. The same situation arises in the toluene-methylcyclohexane mixtures for the peak ratio
(s),
but the methylcyclohexane contribution t o the total 92+ is much too small t o consider. nhcre z.11; (C) andjJf; (C) are the i- and j-peaks in the composite spectrum for a and b corrected for mutual effect-that is zV;(C) is the i-peak in the mixture corrected for the effect of component a and represents only that part of the peak due to component b. The constant, Rh,can be determined by measuring this ratio for a given synthetic mixture of a and b and, from a knowledge of the spectra of purc a and b, the corrected peaks can be found.
-IY&LYSIS OF ETHYLBENZENE FOR T R 4 C E S O F DIETHYLBENZENES
The analysis of ethylbenzcnr for small amounts of the dicthylbenzenes has been reported by Washburn and cn-workm-a 18).
There are some valid reazons for expecting somewhat better accuracy by the ratio method. The errors normally associated with a pressure measurement of the sample prior to expansion are not present. Temperature fluctuations in the sample-handling system affect all components equally. Peak ratios are less likely to be affected by electrical fluctuations in the ionization chamber than individual peaks. For qimple mixtures, the speed of operation is increased. A few specific examples will illustrate the accuracy and precision of the method.
Table I.
.inal?sis of Known \Iixtures of Cjclohevane in Benzene
Cyclohexane Present
Cyclohexane Found
% h y zLezght
% h?) i l e l g h t
60 48 40 20 0 11 0 06 0 01 0 0 0 0
ANALYSIS OF BENZENE FOX SMALL AMOUNTS O F CYCLOHEXANE
0 0 0 0 0 0 0
0 63
63 44
0 0 0 0 0 0
39 21 11 09 04
44 39 21 13 09 04
Difference
7%
70
+5 0 -8 3 -2 5 +5 0 0 0 +50 0 400
+5 0 -8 3 -2 5 +5 0 A5 5 +50 0 400
+
+
Table 11. inalysis of Known Rlixtures of Methylcyclohexane and Toluene MetPylcyclohexane Present
The benzene used t o prepare the standard mixture and the synthetic samples was a “heart cut” from a 30-plate fractionation of nitration grade, acid-washed 1O benzene. The cycldhexane, Eastman Kodak No. 325-X, was used without further purification. All mixtures were prepared by weighing to the nearest 0.1 mg. and all concentrations are expressed in weight per cent. A standard mixture containing 1.271S cyclohexane was used for the calibration (Table I).
%
305
b y uetght
Methylcyclohexane Found
7ob y
II
eagiit
or
Difference
%
306 Because of the mass spectra of each of these pure compounds, the 119+ peak of the diethylbenzenes and the 106+ peak of ethylbenzene were chosen and the ratio was used in the analysis. In this example, the total 106+peak is made up largely from ions due to ethylbenzene, but enough of the composite 106+ peak is due to a contribution from the diethylbenzenes to make a correction necessary. If the ratio __ for pure diethylbenzene
(Is:)
.
ANALYTICAL CHEMISTRY
Table 111. Analysis of Known 3Iixtures of Ethylbenzene and Diethylbenzene Diethylbenzene Present
Diethylbenzene Found
% b y weight
70 b y weight
Difference
70
42
( ~ ~ ~ ~ )
is measured beforehand and designated by
(%+) ,
then the
DEB
corrected 106 peak is +
(106%
=
(106’)T -
(s) DEB
(119+lT
(2)
In Equation 2, (106*)~and (119’)~ designate the total ion current a t masses 106 and 119, respectively. I t is this corrected (106+), ( 1 0 6 + ) xvhich ~ must be used in the ratio in or-
(s)c
der to obtain correct values of the diethylbenzene content. A standard mixture of 1.927% diethylbenzene in highly purified ethylbenzene was prepared as well as five other mixtures of lower diethylbenzene content. The analysis of these mixtures is summarized in Table 111. The average difference here is only *3.4% and the data a t low percentages indicate that the loxver limit of detection (defined as the percentage a t which the error is *loo%) is close to 0.00370diethylbenzene by weight. Similar data have been obtained on the ternary mixture of traces of ethylbenzene and isopropylbenzene in styrene where the the styrene comprises 99.0% 01 more of the total. ANALYSIS OF HIGH-PURITY STYRENE FOR ETHYLBENZENE AND o-XYLENE
The problem of the analysis of high purity styrene for ethylbenzene and o-xylene required some preliminary work before any answers could be given. It was suspected that the 91+ peak in the styrene spectrum was due not to styrene but to the impurities usually present. This required proof, however, since some spectra have been observed to contain “forbidden masses” which probably arise by isomerization of the excited molecule prior to ionization ( 3 ) . I n order to determine whether or not 100.07, styrene exhibits a peak a t mass 91, samples of styrene of various purities (determined by freezing point meawrements) were introduced into the
best analysis, R, should differ as much as possible from R,. Evidently, if R, is this same ratio for a mixture of a and b, then lf
R,
> Rb,
Ro
> R m > Rb
(3)
As the concentration of b in the mixture increases, R, increases from R, to Rb The method of application in this example 1s obvious. The ratio method as described here is not easily applicable to the analysis of ternary isomeric mixtures in the general case. I n some conceivable cases, in which two of the three isomers do not contribute much to a given mass, a method of successive approuimations could be used. These methods of calculation have been used only in trace analysis to date and have not been extenslvely employed in the higher concentration ranges. For this reason, and because some ion currents were too small for accurate recording on the usual mass spectrometer recording mechaniqms, a galvanometer deflection method was used in conjunction with a precision type K potentiometer. This allowed a 50,000-fold range of ion currents to be measured to an average accuracy of less than 1.0%. DISCUSSIOh
I t is not to be inferred that the ratio method of calculating certain mass spectrometer analyses can replace the standard methods (1, 7 , 8) of calculation but rather that it can be advantageously used to speed up routine analyses of simple mixtures and, in some cases, to augment the older methods. In an analysis of a multicomponent mixture by this method, a series of equations illustrated below is obtained.
PI = alp2 P, = asp? P , = a,P?
(&:)
mass spectrometer and the ratios measured. These ratios were plotted against the amount of total impurities present in each sample and extrapolated graphically to zero impurity. All the points fell on a straight line n-hichextrapolated, within experimental error, to zero ratio a t zero impurity, proving that pure styrene yields no 91 ion under present spectrometer operating conditions. The purest styrene used in the extrapolation contained 99.857, styrene and the crudest assayed 98.91%. Between these limits, 11 points xere plotted. The ratio was used +
(s)
to detect and determine total ethylbenzene and xylenes in styrene with much the same ease and accuracy as in the other cases described above. The 91 peak was selected for the ethylbenzene analysis because it is the largest peak in the ethylbenzene cracking pattern. +
C P &= 100 1
These equations can be solved easily, since the determinants for this system contain many zeros. In the case described above all components are referred to component 2. The system could be arranged to involve reference to any one (or more) reference compounds. The last equation in (4)will always be required.
SCOPE AND APPLICATION
The ratio method can be applied to binary mixtures of isomers: normal and is-, cis- and trans-, ortho- and meta-, or any other isomeric binary type in which the mass spectra are sufficiently different to allow an analysis to be made. A pair of peaks (not necessarily adjacent) in the mass spectrum of one of the pure components is chosen in such a way that the change of this peak ratio per unit concentration change in a mixture of the two compounds is as large as possible. The ratio of these peaks for the pure isomer, a, is represented by R. and the ratio of these same two peaks for pure isomer b is Rb. For the
LITERATURE CITED
(1) Brewer, A. K., and Dibeler, V. H., J . Research Nat2. Bur. Stand-
ards, 35, 125 (1945). (2)
(3) (4)
(5) (6)
(7) (8)
Germann, F . E. E., and Gagos, K. A , , IND.ENG.CHEM.,ANAL. ED., 15, 285 (1943). Hipple, J. 9.,private communication. LeRoy, D. J., IND. ENG.CHEM., A N ~ LED., . 1 7 , 6 5 2 (1945). Pearson, T. G., 2.p h y s i k . Chem., A156,86 (1931). Stillman, M. H., Bull. B U T Standards, . 10,372 (1913). Washburn, H. IT., Wiley, H. F., and Rock, S. hl., IXD.ESG. CHEM.,ANAL.ED..15, 541 (1943). Washburn, H. W., Wiley, H. F., Rock, S.XI., and Berry, C. E., Ibid., 17, 74 (1945).
