Table I. Field Desorption Mass Spectra of Onium Salts Heating current, Compommd
mA
Et,N+BH,-
18
I!-BUJN+I'
20
PhMe,N+I'
24
MePh, P'Br'
24
Et Ph, P'Br'
22
$2-
PrPh,P+Br-
28
11-
BuPh,P'Br-
31
Me;PhP+picrate
25
Compound I
19
Ph, A s T 1 -
G45
Re1 int
h p l a n a tion
hlass
130 131 242 243 136 137 277 278 291 292 305 306 319 320 153 154 427 428 445 112 114 306 307 382 383 384 385
100 8 100 17 100 15 100 12 100 20 100 22 100 26 100 10 100 28 21 9 4 30 6
C C + 1 C C + 1 C C + 1 C C+1 C C + 1 C C + 1 C C + 1 C C + 1 C-H C-H +'1 C + 17 Ph-"Cl Ph-"Cl C-Ph
8 100 34 4
C- 1 C C + 1 C +2
C-Ph
+
The triethyloxonium salt also had the cation as its base peak. There was a peak corresponding to CzA+ a t 0 mA heating current, and one corresponding to the pyrolysis product diethyl ether (mass 74) a t higher currents. The peaks clustered around rnle 131 correspond to the uptake of another CzH4 unit. The ion may merely be an oxonium ion with ethylene solvating the charge; other representations are possible if polymerization of ethylene occurs: ( C 2Hg )20
C&.
C2H,
+
+ *
C,H@(C~HS)~
CdHg-e-
but there is no evidence to support either structure. We conclude that for all classes of onium salts tested by FDMS the base peak in all spectra is the cation and that FDMS provides an easy determination of the cationic weight.
ACKNOWLEDGMENT We thank Gordon Wood and H.-R. Schulten for preprints and Richard Buck for allowing us to use the MS-702 mass spectrometer.
LITERATURE CITED 1
decompositions; the methyl iodide peak at mass 142 was present. The base peak corresponded to dimethyl sulfoxide, the product of thermal degradation besides methyl iodide. The other major peak, at mass 93, corresponds to the cation. These spectra were obtained using a carbon emitter and heating currents between 8 and 50 mA. The two oxonium salts studied, using carbon emitters with sample adsorbed from acetonitrile solvent, which had been dried over molecular sieve, were trimethyloxonium hexafluorophosphate, (CH3)3OfPF~-,and triethyloxonium hexafluorophosphate. A typical spectrum for the former, obtained a t 19 mA, again has the cation as the base peak. There is a peak corresponding to dimethyl ether, a likely pyrolysis product, and very small peaks in the mass range of protonated methanol.
(1) H.-R. Schulten and F. W. Rollgen, submitted to J. Am. Chem. Soc. (2) J. R. Hass. M. C. Sammons, M. M. Bursey, B. J. Kukuch. and R. P. Buck, Org. Mass Spectrom., 9, 952, (1974). (3) D. A. Brent, D. J. Rouse, M. C. Sammons. and M. M. Bursey. Tetrahedron Left., 4127 (1973). (4) M. C. Sammons, M. M. Bursey, and D. A. Brent, Biomed. Mass Spectrom., 1 , 169 (1974). (5) G. W. Wood and P.-Y. Lau, Biomed. Mass Spectrom., 1 , 154 (1974). (6) H.-R. Schulten and H. D. Beckey, Adv. Mass Spectrom., 6 , 499 (1974). (7) H . 4 . Schulten. H. D. Beckey, E. M. Bessel, A. B. Foster, M. Jarmin, and J. H. Westwood, J. Chem. Soc., Chem. Commun., 416 (1973). ( 8 ) H.-R. Schulten and D. E. Games, Biomed. Mass Spectrom., 1 , 120 (1974). (9) H.-R. Schulten, Biomed. Mass Spectrom., 1, 223 (1974). (10) J. H. Beynon. A. E. Fontaine, and B. E. Job, Z.Naturforscb.,TeilA, 21, 776 (1966). (11) H.-R. Schulten and H. D. Beckey, Urg. Mass Spectrom., 6 , 885 (1972). (12) R . M. Wightman, D. M. Hinton, M. C. Sammons, and M. M. Bursey, lnt. J. Mass Spectrom. /on Phys., in press. (13) H. D. Beckey, lnt. J. Mass Spectrom. /on Phys., 2, 500 (1969). (14) A. M. Aguiar, K. C. Hansen. and J. T. Mague. J. Org. Cham.. 32, 2383 (1967). (15) C. K. White, Ph.D. Dissertation, The University of North Carolina. 1974. (16) J. R. Hass, Ph.D. Dissertation, The University of North Carolina, 1972. (17) E. B. Owens, "Mass Spectrometric Analysis of Solids", A. J. Ahern, Ed., Elsevier Publishing Company, New York, NY. 1966, pp 84-86. (18) G. W. Wood, J. M. McIntosh, and P.-Y. Lau. J. Org. Chem., 40, 636 (1975).
RECEIVEDfor review December 2, 1974. Accepted February 5, 1975.
Identification of the Isomeric Phthalic Acids by Mobility and Mass Spectra F. W. Karasek and S. H. Kim Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada
The isomeric phthalic acids and their derivatives have been increasingly used in various industries, particularly as plasticizers in the polymeric materials used for packaging of food and medica1 products. It is reported that the phthalate compounds have been, detected in cooking fat 1166
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE
1975
( I ) , vegetables, fruits, fish, meat, milk, butter, cheese ( 2 ) , and human plasma (3, 4 ) . Although the toxicity of these compounds is low ( 5 ) , sensitive and reliable methods of analysis for these compounds are useful for detecting their possible transfer from the plastic containers to the human
~~
~
~~
~~
~
Table I. Ion -Mobility D a t a
I
Reduced mobility, Species
Compound'
I /I
CWH
KoCmZ/V secb
Positive ions
(c,H,o)H+
B,C
(c,H,o~)H+ (C,H,03)H'
D A , W
(c,H,o,)NO:
(C8H403)NO (C7HGO2 + H20)NO'
(C*H,o, )H' (C 8 ~ , )NO' ~ 4 Negative ions (C8H@,-H@)(C,H,O,) [(C8H601)2-H20]-
D A D B,C A,B,C
A B
B
1.91 1.82 1.77 1.74 1.64 1.63 1.57 1.52 1.77 1.58 1.15
A = Phthalic acid; B = isophthalic acid; C = terephthalic acid; D = benzoic acid. * K O =: (l/x) (6.55P/760T) where x = drift time (secj, P = pressure (Torr), T = K, and 6.55 is a factor incorporating cell length (6 cm), electric field gradient (250 V/cm), and correction to 273 K . Reported values are measured a t 150 "C. All K O values had a standard deviation of f 0 . 0 2 .
body. Column chromatography/gas chromatography ( 3 ) , gas chromatography and thin layer chromatography ( 4 ) , mass spectrometry (6-9), chemical ionization mass spectrometry (Io), negative ion mass spectrometry (111, and gas chromatography (12) are some of the techniques that have been used for the identification of phthalic acid, isophthalic acid, terephthalic acid and their esters. Plasma chromatography (PC) shows promise as a method for qualitative and quantitative trace analysis of organic compounds and is particularly useful when employed to detect gas chromatographic effluents (13-18). Previous results show that organic compounds can be selectively detected in 10-10 g or less quantities through their mobility spectra, and that the positive mobility spectra can be compared to chemical ionization mass spectra ( 1 3 ) . The PC mobility spectra of the aromatic dicarboxylic acids have been reported previously as a part of general studies of the applicability of PC technique to detection of GC effluents ( 1 4 ) . The purpose of this paper is to explore the method of identification of isomeric phthalic acids by PC and to compare these results with those obtained by both electron ionization (EI) and chemical ionization (CI) mass spectrometry. EXPERIMENTAL T h e Beta VI chromatograph (Franklin GNO Corp. West Palm Beach, FL 33402) used in this study has been described previously (13-18j. Experimental conditions for this study were: PC tube and inlet temperature, 150 "C; carrier gas flow rate, 120 cm3/min; drift gas flow rate, 380 cm3/min; electric field, 250 V/cm; injection and scan gate width, 0.2 msec; time base, 20 msec; recorded scan time, 2 min; ambient pressure, 724-736 Torr. The carrier and drift gases were nitrogen (Linde, high purity 99.996%) passed through individual stainless steel traps of 2.25-1. capacity packed with Linde Molecular sieve 13X to remove impurities. T h e phthalic acid and isophthalic acid reagents are Baker reagent grade. The terephthalic acid is B.D.H. Chemical Co. reagent. Samples were prepared as g/ml solution in ethanol and introduced to the plasma chromatograph via a clean platinum wire onto which 0.1 microliter of solution had been dispensed and the solvent allowed to evaporate prior to introduction into the sample inlet of the P C tube. Chemical ionization data using CHI reactant gas were obtained with the Hewlett-Packard H P 5930A duodecapole GC/MS unit operating with the H P 5933A dualdisc data system.
-: 5
6(
10
m/s
1 0
150
170
lit Figure 1. Normalized intensity vs. mass of the negative El mass spectra (from Ref. 7 7) and normalized Intensity vs. reduced mobility of negative PC spectra for isomeric phthalic acids. Data measured at 4 5 0 O C
RESULTS AND DISCUSSION Isomeric phthalic acids are separated in the form of esters by GC (12) and also are usually identified by E1 mass spectrometry in the forms of acids or esters (6-11). The separation of these isomeric acid mixtures by GC is difficult. Also, because spectral patterns are very similar except for a low intensity molecular ion for phthalic acid, it is difficult to distinguish benzoic acid from phthalic acid and isophthalic acid from terephthalic acid by E1 mass spectrometry a t low trace concentrations. The positive and negative mobility spectra of benzoic acid, phthalic acid, isophthalic acid, and terephthalic acid were observed by PC. The reduced mobilities of the ion species observed in this study are shown in Table I. The relation between assigned masses of Table I and reduced mobility ( K O )values agrees reasonably well with the correlation curve established previously for oxygenated aliphatic and aromatic compounds (13). No negative mobility spectra are observed for benzoic acid and terephthalic acid, while phthalic acid and isophthalic acid show strong negative peaks. Phthalic acid exhibits an intense single negative peak at KO = 1.77,which is coincident with the most prominent peak of its positive spectrum and appears to be the (M - H2O)- ion. Isophthalic acid exhibits an intense peak at a KO of 1.58, which is assigned as the M- ion formed by associative electron capture. Another weak peak at a KO of 1.15 appears to be a dimer ion (M2 - HzO)-. When the concentration is low, this peak is very weak as expected for dimer ions. Figure 1 compares the normalized plots of relative intensity vs. mass obtained by negative ion mass spectrometry ( 1 1 ) and normalized plots of relative intensity vs. reduced mobility ( K O ) of the species observed in the PC negative ion mobility spectra for the isomeric phthalic acids. The negative ion mass spectrum of phthalic acid is easily distinguishable from the negative ion mass spectra of iso- and terephthalic acids by observation of the prominent peak at mass 148 formed by loss of H20, which does not appear in the spectra of the iso- and terephthalic acids. This ion is undoubtedly the negative molecular ion of phthalic anhydride ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
1167
coon
1,
0
coow
I
: l x j L
A=
cww
mle PC
40
60
80
,MU+
,c
100
m /e
120
140
160
coon @COO"
I
180
1
COOR
2.20
210
2.00 1.90 1.80 1.70 M o s i i i i v K. i.ma/v...I)
1.60
1.50
USDUCED
Figure 2. Comparison of the positive El mass spectra (from Ref.
3,
CI mass spectra (from Ref. 79),and reduced mobility of positive PC spectra for benzoic acid and phthalic acid. Data measured at -150 OC
Figure 3. Comparison of the positive El mass spectra (from Ref. 7). CI mass spectra (from Ref. 79), and reduced mobility of positive PC spectra for the iso- and terephthalic acids. Data measured at 4 5 0 OC
formed by heating the acid by the instrumental temperature of 150 OC. Some difference is also observed between iso- and terephthalic acid because of the relative intensities of the negative molecular ions; isophthalic acid shows a relatively less intense molecular ion than terephthalic acid. The negative PC mobility spectra for these isomeric phthalic acids show clear differences throughout the three isomers (ortho-, meta- and terephthalic acids); phthalic acid exhibits a single prominent peak due to the (M H20)- ion at KOof 1.77 and isophthalic acid shows a prominent peak a t KO of 1.58 which is assigned as the M- ion. Terephthalic acid exhibits no negative mobility spectrum. The positive mobility spectra of the compounds investigated have strong and characteristic peaks as shown in Figures 2 and 3. The positive mobility spectra of benzoic acid, iso- and terephthalic acids show the MH+ ion as the most prominent ion. This is a typical peak found for polar compounds. For phthalic acid, protonation occurs followed by dehydration to form the (M - H20)Hf ion, an occurrence also observed in CI mass spectrometry. Figure 2 compares the normalized plots of relative intensity vs. mass from E1 and CI spectra to the relative intensity vs. reduced mobility plots from PC spectra for benzoic and phthalic acids. Figure 3 gives these plots for iso- and terephthalic acids. All the spectra obtained by E1 and CI mass spectrometry and by PC show some differences between phthalic acid and iso- or terephthalic acids. Also, positive CT and PC spectra can easily differentiate between benzoic acid and phthalic acid, with the PC spectra being useful a t very low concentrations. It is difficult to distinguish between the two isomers, isophthalic and terephthalic acids, using the positive spectra from any of the three techniques, E1 and CI mass spectrometry and plasma chromatography alone. However, the negative PC spectra easily differentiates between all three isomers, since the mobility spectra of phthalic and isophthalic acid are quite different and terephthalic acid produces no negative PC spectrum. Although negative ion E1 mass spectra show some differences 1166
ANALYTICAL CHEMISTRY, VOL. 47, NO. 7, JUNE 1975
in observed intensities for iso- and terephthalic acids, it appears that negative PC spectra are most useful to distinguish between these two isomers.
ACKNOWLEDGMENT The authors gratefully acknowledge the assistance of J. A. Michnowicz of the Hewlett-Packard company who obtained the CI mass spectra for this study using the H P 5930A/5933A GCIMSlComputer system. LITERATURE CITED (1) E. G. Perkins. J. Am. OilChem. Soc.. 44, 197 (1967). (2) M. Morita, H. Nakamura, and S. Mimura, Tokyo Toritsu Eisei Kenkyusho Kenkyu Nempo, 24,357 (1973). (3) J. T. Piechocki and W. C. Purdy. Clin. Chim. Acta, 48, 385 (1973). (4) R. J. Jaeger and R. J. Rubin. Science, 170, 460 (1970). (5) D. Calley. J . Autian, and W. L. Guess, J. Pharm. Sci., 55, 158 (1966). (6) F. W. Mchfferty and R. S. Gohlke, Anal. Chem., 31, 2076 (1959). (7) J. H. Bevnon. B. E. Job, and A. E. Williams. Z.Nafurforsch. 209. 883 (1965). (8)Carl Djerassi and Catherine Fenselau, J. Am. Chem. Soc., 87, 5756 (1965). (9) F. Benoit, J. L. Holmes, and N. S. Ikaacs, Org. Mass. Specfrom.. 2, 591 (1969). (IO) H. M. Fales. G. W. A. Milne, and R. S. Nichobon. Anal. Chem., 43, 1785 (1971). (11) A. Ito. K. Matsurnoto and T. Takeuchi, Org. Mass Spectrom., 7, 1279 (1973). (12) 0.Mlejnek and L. Cveckova, J. Chromatogr.. 82, 377 (1973). (13) F. W. Karasek, Anal. Chem., 46, 710A (1974). (14) F . W. Karasek and S. H. Kim, d Chromfugr., 98, 257 (1974). (15) F. W. Karasek, 0.S. Tatone, and D. M. Kane. Anal. Chem., 45, 1210 (1973). (16) F. W. Karasek and 0.M. Kane. Anal. Chem., 45, 576 (1973). (17) F. W. Karasek and M. J. Cohen, J. Chromatogr. Sci., 9, 390 (1971). (18) F. W. Karasek and D. M. Kane, J. Chromafogr. Sci., IO, 673 (1972). (19) J. A. Michnowicz, Hewleit Packard, Palo Alto, CA, personal communication, June 1974.
RECEIVEDfor review August 23, 1974. Accepted February 5 , 1975. The research for this paper was supported by the Defence Research Board of Canada, Grant Number 9530116, and the National Research Council of Canada, Grant Number A5433.
