(4) K. Kwock, R. Venkataraghavan, and F. W. McLafferty, J . Arner. Chem. Soc., 95, 4185 (1973). (5) C. E. Costello, H. S. Hertz, T. Sakai, and K . Biemann, Clin. Chem., 20, 255 (1974). (6) T. D. Sterling and S. V. Pollack, Ann. N.Y. Acad. Sci., 161, 632 (1969). (7) B. S. Finkle. D. M. Taylor, and E. J. Bonelli, J. Chromatogr. Sci., 10, 312 (1972). (8) N. C. Law, V. Aandahl, H. M. Fales, and G. W. A. Milne, Clin. Chim. Acta, 32, 221 (1971). (9) P. Toft, B. A. Lodge, and M. B. Simard, Can. J. fharm. Sci., 7, 53 (1972). (10) H.Budzikiewicz. in "Biochemical Application of Mass Spectrometry," G. Wailer, Ed., Wiley-lnterscience, New York, N.Y.. 1972, p 251. (11) J. R. Chapman and E. Bailey, Anal. Chem., 45, 1636 (1973). (12) E. M. Chambaz, G. Defaye, and C. Madani, Anal. Chern.. 45, 1090 (1973). (13) E. Gelpi, W. A. Koenig, J. Gilbert and J. Oro. J . Chrornatogr. Sci., 7, 604 (1969). (14) B. S. Middleditch and D. M. Desiderio, Anal. Biochem., 55, 509 (1973) and earlier work cited therein. (15) 8 . S. Samuelsson, E. Granstrom. D.Green, and M. Hamberg, Ann. N. Y . Acad. Sci., 180, 138 (1971). (16) U. Axen, K. Green, D. Horlin, and B. S. Samuelsson, Biochern. Biophys. Res. Comrnun., 45, 519 (1971). (17) J. N. Damico, R . P. Barron, and J. M. Ruth, Org. Mass Spectrom., 1, 331 (1968). (18) T. R. Kanter and R. 0. Mumma. Residue Rev., 16, 138 (1966). (19) R. Bonnichsen, C. G. Fri, B. Hedfjali, and R. Ryhage, Z. Rechfsmedizen, 70, 150 (1972). (20) T. L. Isenhour, Anal. Chem., 45, 2153 (1973). (21) G. Ramirez, R. C. Dinio. and H. C. Pribor, Comput. Bid. Med. 2, 39 (1972). (22) M. Lipkin, R. L. Engle, Jr., B. J. Davis, K. V. Zworykin, R. Ebald, M. Sendrow, and C. Berkley, Arch. lnt. Med., 108, 124 (1961). (23) R. L. Reece and R. K . Hobbie, Amer. J . Clin. Pathol., 57, 664 (1972).
nonexistent correlation between those masses found a n d t h e library spectrum of methaqualone. In contrast, t h e reverse search extracts only t h e ten relevant masses from t h e spectrum in Figure 2 for a comparison. Without using t h e background subtract capability, t h e reverse search found t h a t eight of t h e ten intensities selected were within its allowable range yielding a HIT QUA1,ITY of 8-120. T h e background intensities a t masses 7'7 a n d 251 added t o t h e selected masses caused them t o be discarded as big positive deviations. T h e remaining mechanical functions can be automated (sample injection, solvent bypass valve). I t is t o be expected t h a t , under the supervision of a mass spectrometer specialist, such a computerized GC/MS system will become automatic and capable of processing a large number of routine samples without operator intervention. In addition, the ease of operation will make qualitative and yuantitative answers t o routine problems accessible t o a wide range of users without requiring them t o understand either mass spectrometers or mass spectra. ACKNOWLEDGMENT T h e assistance of Norris Huse a n d Royce Howard of Dupont Instruments is gratefully acknowledged. I also thank Mario Werner for many helpful discussions. LITERATURE CITED
RECEIVEDfor review April 18, 1974. Accepted September 9, 1974. This paper was presented in part a t the 21st Annual Conference on Mass Spectrometry and Allied Topics, American Society for Mass Spectrometry, San Francisco. Calif., 1973.
(1) R. G. Ridley in "Biochemical Applications of Mass Spectrometry,'' G. Waller, Ed.. Wiley-lnterscience. New York, N.Y., 1972, Chapter 6
(2) L. E. Wangen, W. S . Woodward, and T. L. Isenhour. Anal. Chem., 43, 1605 (1971). (3) S R. Heller. Anal. Chem., 44, 1951 (19721.
Negative Chemical Ionization Mass Spectrometry-Chloride Attachment Spectra Harvey P. Tannenbaum,' J. David Roberts, and Ralph C. Dougherty D e p a r t m e n t of Chemistry, Florida State University, Tallahassee, Fla. 32306
This paper explores the analytical potential of negative chemical ionization (NCI) mass spectrometry using methylene chloride as the reagent gas. The NCI mass spectrum of methylene chloride is dominated by CI-, HCIz-, and CH2C13- ions. Negative chemical ionization with this reagent gas results in chloride attachment to the substrate as the primary chemical ionization mode. The importance of chloride attachment and the sensitivity of the technique both increase with increasing ability of the substrate to form strong hydrogen bonds. The selectivity of the ionization makes this technique attractive for examining nonhydrogenbonding substrates like ethers for traces of alcohol or acid impurities. Molecule anions resulting from resonance capture and fragment anions that were the result of disassociative capture were also observed in the spectra of specific substrates. Formation of molecule anions under these conditions appears to correlate with molecular electron affinities.
'
Present address. E 1 I h P o n t d e Nemours & Company, T e x tile Fiber\. 1007 91arket Street. Wilmington, Del 19798
Negative chemical ionization mass spectrometry is an obvious extension of chemical ionization mass spectrometry (1-4) and nonreactive gas enhancement of negative ion mass spectra ( 5 - 7 ) . T h e bulk of the literature reports of negative ion mass spectra (8-16) have been concerned with spectra of compounds which readily form anions under NCI conditions. These compounds include haloalkanes (8-10), organometallics ( I O ) , nitroalkanes (12-14 ) , arid pesticidal compounds of t h e carbonate ( 1 5 ). organophosphate ( I C s )and , chlorinated hydrocarbon types ( 1 5 , 1 6 ) . In most of these cases, t h e spectra were not the result ot' chemical ionization in the usual sense. T h a t is, the spectra were dominated by ions that resulted from resonance capture or disassociative capture of t h e thermalized electrons in the NCI plasma, and the abundance of ions that resulted from chemical reaction of reagent gas ions was generally low. Chemical ionization with anions is a substantially "milder" form of ionization than corresponding reactions between cations and molecules. This is because the bonds t h a t form between anions and molecules with few excep-
A N A L Y T I C A L C H E M I S T R Y , V O L . 47, N O . 1 . J A N U A R Y 1975
49
I
I
Figure 1. NCI mass spectrum of methylene chloride (1 Torr,
Exploitation of the analytical potential of NCI mass spectrometry must be preceded by evaluation of the characteristics of various NCI reagent gases and the NCI reactivity of various substrates to the ions formed in these gases. We have chosen methylene chloride as the reagent gas in our initial studies. Methylene chloride is easily purified, it has a vapor pressure of more than 1 Torr a t room temperature, and it is a convenient source of substantial quantities of chloride anions for NCI experiments. This paper presents a study of the analytical potential of chloride attachment NCI mass spectrometry.
190 OC)
EXPERIMENTAL tions are much weaker than corresponding bonds to cations; thus, little energy is released on bond formation to drive subsequent fragmentation reactions of the ion. This comparison is best illustrated by examining molecular proton affinities and anion affinities. T h e former are generally in the range of 150-250 kcal/mol ( 1 7 ) while the latter are generally in the range of 5-25 kcal/mol(18). T h e second reason for the “softness” of negative chemical ionization stems from the intrinsic nature of negative ion mass spectra. If a gas phase, odd electron negative ion has sufficient internal energy to undergo fragmentation reactions, it will usually have sufficient energy to undergo autoneutralization, i . e . , electron ejection. T h a t is, the activation energy for reaction must be lower, usually a lot lower, than the electron affinity of the neutral. In the case of even electron anions formed by anion attachment, high internal energies will generally result in disassociation into the original neutral anion pair. Even electron ions that are formed by disassociative capture or by fragmentation of a molecule ion generally have low internal energies and, in the cases where the internal energy is sufficient for subsequent fragmentation, autoneutralization can again be important. Negative chemical ionization mass spectra have been reported for polycyclic chlorinated pesticides ( 1 6 ) and aliphatic hydrocarbons ( 1 9 ) .T h e NCI (isobutane) mass spectra of polychloro insecticides were dominated by the chloride anion, which is formed by disassociative capture, and its reaction products. In virtually every case, the ( M C1)ion was the most intense ion in the spectrum above mle 100. Hydroxide was the reactive ion used in the study of petroleum hydrocarbons ( 1 9 ) . The (M OH)- anion was shown to be the only anion derived from the hydrocarbons in these spectra.
Negative ion mass spectra were recorded with an AEI MS-902 mass spectrometer equipped with an SRIC chemical ionization source (20) and operating a t -8 kV in the negative ion mode. S a m ples were examined by direct probe inlet using source temperatures appropriate t o the physical characteristics of compounds. Methylene chloride a t approximately 1 Torr ion source pressure was used as t h e source of chloride ions. T h e source pressures were monitored by a Mensor quartz monometer connected t o the source through a discharge supressor tube (21). T h e negative ion plasma was generated by electron impact using 490-volt electrons with a regulated emission current of 0.25 mA. All compounds examined were commercial samples and were used without further purification.
RESULTS AND DISCUSSION The High Pressure Negative Ion Mass Spectrum of Methylene Chloride. T h e NCI mass spectrum of methylene chloride is illustrated in Figure l. T h e ions observed a t 1 Torr and 190 OC correspond to C1-, HClZ-, and CH*C1:3-. T h e chloride ion is formed by disassociative capture of a thermalized electron in the plasma, Equation 1. CH,C1, + es-
-
C1-
+
CHIC1.
(1) T h e chloride can then attach to methylene chloride to give the S N 2 complex ion, CH2ClS- ion, Equation 2 (22, 23) (2) There are two possible mechanisms for formation of HC12-. T h e first is a complex disassociative capture, Equation 3; the second is unimolecular fragmentation of the CHZC13ion, Equation 4. CH2C1, + C1-
+
C H Q , + es-
-
-
+
CHZC1,-
HC1,-
+
.CH
CH2C1,HC1,- + :CHC1 (4) Of these mechanisms, the unimolecular decomposition of
1
I
2
3
4
5
6
7
8
9 1 0
/I
12
I3
PRESSURE (torr) Figure 2. Variation of relative intensities in the methylene chloride NCI mass spectrum at 100 and 210 OC
50
(3)
ANALYTICAL CHEMISTRY, VOL. 47, NO. 1, JANUARY 1975
14
1 5 1 6
CH.Cl:j-, Equation 4, seems the most likely on chemical grounds. T h e probable importance of this mechanism is also indicated by t h e fact t h a t t h e HC12- and C1- appear t o be in pseudo-equilibrium (22, 2 3 ) . T h e pseudo-equilibrium is most easily explained if HC12- is formed by path 4 and destroyed by disassociation t o C1- and HC1. T h e relative abundances of t h e ions in t h e methylene chloride NCI mass spectrum were sensitive to both t h e pressure and t h e temperature of t h e CI source. T h e variation of intensities over a pressure range of 0.1 t o 1.5 T o r r a t 100 and 210 “C is illustrated in Figure 2. At pressures less than 0.75 Torr and a source temperature of 100 “C, there was a substantial variability in t h e relative abundances of t h e three major ion clusters. Pressures of the order of 0.75 Torr or higher, and a source temperature of 210 “ C were found to provide a reasonably constant abundance of t h e various ions (see Figure 2), with chloride being t h e most abundant ion. T h e optimum operating conditions for observing chloride ion-molecule association reactions, when employing methylene chloride as the reagent gas, appear t o he pressure near 1 Torr and a source temperature near 200 “C. It is, however, possible t o ohtain reproducible chloride attachment spectra a t lower source t,emperatures. Under these conditions, attachment of HC12- or CH2C13- can also occur; thus, with the exception of thermally unstable samples, relatively high source temperatures were used. At low temperatures and relatively high source pressures, i t was possible t.o observe t h e C1:j- cluster at mle 105. We were unable t o observe C1:l- attachment t o substrates in t h e source. These ions were universally low in intensity, and they were not present at source temperatures above 150 “C. T h e same is true for t h e (HClZCH2C12)- and (C:H&l-CH?ClS)- ions. We have consequently ignored these ions in t h e figures and t h e following discussion. Survey of Methylene Chloride NCI Mass Spectra of Organic Compounds. T h e organic compounds examined in this survey were selected so as t o provide a representative cross section of commonly occurring organic functional groups. T h e NCI-methylene chloride mass spectra of organic compounds which exhibited (M C1)- as t h e base peak are shown in Table I. I n each case in Table I, introduction of pg quantities of t h e sample in t h e source was sufficient to suppress t h e methylene chloride spectrum to t h e extent t h a t the (M C1)- ion for t,he substrate was t h e base peak in t h e spectrum. T h e ions list,ed in Table I are those for which the intensities were greater than 1% of the base peak. In all cases, t h e mass spectra were very easy t o interpret and utterly devoid of complex fragmentation patterns. T h e characteristic chlorine isotope patterns proved t o he of considerable utility in t h e interpretation of t h e spectra. All of t h e compounds listed in Table I have “active hydrogens.” In fact, all b u t aniline a n d urea are considered acidic by t h e standards of solution chemistry. Presumably in each of’ t h e cases in Table I, t h e chloride-molecule complex is held together by a relatively strong hydrogen-bond. Although t h e range of acidities of t h e compounds in Table I is quite large, each of these molecules competes effectively with methylene chloride for t h e chloride ions in the source. T h e heat of association of methylene chloride with chloride has been determined to be -15.5 f 0.3 Kcal/ mol (22, 2 3 ) (AGzoso, 1 = -8.9 Kcal/mol (22, 2 3 ) . Chloride inn attachment can be observed for substances with lower heats of association t h a t this, e g . , methyl chloride has a heat of association with chloride of -8.6 f 0.2 Kcal/ mol (22, 2 3 ) ;however, the analytical utility of chloride attachment in these cases is dubious. Chloride ion att,achment can be relatively unimportant in certain carboxylic acids because of t h e formation of mo-
+
+
Table I. Methylene Chloride NCI Mass Spectra of Organic Compounds Exhibiting [M Cl] as the Base Peaka-Hydrogen Bonding S y s t e m s
+
Relatite abmidancr, (2.2’1
(M t Cl).(M
Compound
Aliphatic and aromatic carboxylic acids Benzoic acid Cyclohepten-5 -carboxylic acid Cyclopropyl carboxylic acid 11-Fluorobenzoic acid 11-Methoxybenzoic acid p -Nitrobenzoic acid Amides Octadecanamide Urea Amino acids 2 -Aniino-2 -methylbutyric acid Glycyl -L-tyrosine Aromatic amines and phenols Aniline Phenol Hydroquinone
100 100 100
~
131-
6
... , ,
.
100 100 100
12
100 100
...
...
14
33
100 100
7.5
100 100
7.5
100
5.6 53 4
0 Ions from the methylene chloride 3pectrum are not included in the listing; source pressure -1 Torr. temperature -200 ‘ C . intensities are relative to the most intense ion in the entire spclctrum, namely, [M + Cl] . ”The relative intensities of‘ these ion> *ire highly temperature and suhstrat~-concentrationdependent. ‘:‘hey may he eliminated in all cases h y increasing tht, sourcr tempera-
ture.
lecular anions. For example, t h e NIC-methylene chloride spectrum of 1-naphthoic acid contained only three ions, C1- (GO%), Me-(100%) and (M C1)- (1.5%); of these t h e chloride ion attachment peak was t h e smallest. This could be due t o decomposition of t h e (M C1)- to give Me- and C1.. In view of probable relative electron affinities, it is more likely t h a t t h e majority of t h e gaseous naphthoic acid molecule ions are formed by resonance capture, and those molecules are thus removed from t h e chloride ion attachment equilibrium. T h e electron affinity of 1-naphthoic acid is not known; however, t h e electron affinity of 1-naphthaldehyde is approximately 0.6 e\’ (24 ) while t h a t of t h e chlorine atom is 3.6 eV (25). T h e ability to form hydrogen bonds is not a sine qua non for chloride ion attachment, nor does hydrogen bonding ability guarantee t h a t chloride ion attachment will occur under the conditions used in this study. At low source temperatures, it is possible to observe C1- and HCle- attachment to n-nonylamine a n d C1- attachment to secondary amines like diethyl amine; however, at source temperature of 200 “C. chloride attachment t o aliphatic amines was generally insignificant. Aliphatic alcohols generally form stronger hydrogen bonds than their amine counterparts, C1)- ions in t h e methylene and t h e abundance of (M chloride NCI spectra of aliphatic alcohols was correspondingly larger than that, in amines (Table 11). Alcohols also show a greater degree of attachment than those substrates t h a t attach chloride by carbon honding (nucleophilic association). By “carbon honding” we mean association of a nucleophile with a Lewis Acid center cm carbon, P.$.. carbon bonding occurs in the S N transition ~ state (14 j . 1 , and in chloride association with carbonyl groups, 2. T h e analogy with “hydrogen bonding” is obvious and direct.
+
+
+
A N A L Y T I C A L C H E M I S T R Y , VOL. 4 7 , N O . 1 , J A N U A R Y 1975
51
"0
H
Table 111. Compounds Which Did Not Exhibit Chloride Attachment in Methylene Chloride NCI Mass Spectraa Aliphatic hydrocarbons Hexatriacontane Triacontane Olefins Cyc lohexene Acet y 1ene Phenyl acetylene Aromatic hydrocarbons Naphthalene Anthracene Tetracene T e r t i a r y amines Triethyl amine 1.4 -Diazabicycloactane Nitrile 1 2 -Hexylnitrile 0 Source pressure 1 'Torr. temperature -200 "C.
1
'6
c1-- ;c R':
1
bR
2
T h e group of compounds that form chloride ion complexes by carbon bonding include aldehydes, ketones, esters, ethers, nitroaromatics, and alkyl halides (Table 11). With the exception of molecules with high electron affinities, it seems unlikely that chloride ion attachment spectra will have substantial analytical utility for direct characterization of these classes of materials. This is because t h e heats of chloride association with t h e carbon bonding substrates will be on t h e order of t h e heat of association of chloride with methylene chloride; t h e relative concentrations of the substrate and methylene chloride in t h e source are such that C1- and CHZC13- will generally carry t h e bulk of the ion current for spectra involving chloride attachment
-
Table 11. Methylene Chloride NCI Mass Spectra of Organic Compounds Exhibiting Varying Amounts of [M C1]- a
+
Relative Intensit),
(M +
Compound
c1j-
a
-
(2M t
(M
c1)-
HClj-
(at.-)
H y h o g e n Bonding Substrates
Aliphatic alcohols 1 -Propanol / o r / -Butanol Aliphatic a mines i i -Nonyl amine i i -Hexyl amine
6.3 0.65 0.05 0.01
+
Carbon Bonding Substrates
Aldehydes Ben z a Id e h yde Propionaldehyde Octanaldehyde Ketones Benzophenone 4 -Heptanone 2 -0ctanone Esters Et h y 1 butyrate Ethers Anis o 1e Propyl ether Tetrahydrofuran X i t r oar oma tics p -Nitrotoluene Nitrobenzene p -Nitro-sulfonate e s t e r s ii -Butyl nosylate 2 -Norbornyl nosylate Alkyl halides and chlorinated hydrocarbons 12 -Butyl bromide 11
-Hexyl iodide
+
+
1.6 0.5 0.3
4.5
45
22
... 4.2
+
0.1 1.2
... 0.1
2
100 100
0.55
100
16
18
0.005
100
100
(Br-, 100; CzH,Br-, 8.2) (I-, 100; CZHdI-. 1.1)
0.1 A Id r in 7.7 a Ions from the methylene chloride spectrum are not included in the listing; source pressure -1 Torr, temperature 200 "C, with the exception of n-butyl bromide and n-hexyl iodides, intensities are relative t o 35C1- which was the base peak.
52
a
t o carbon bonding molecules. Furthermore, t h e relative concentrations of methylene chloride and substrate cannot in this case be accurately controlled so the appearances of the spectrum will vary with time and substrate concentration. T h e methylene chloride NCI mass spectra of compounds with significant molecular electron affinities (24 ) (nitrobenzene and nitrotoluene, EA > 0.4 eV, benzophenone and the nosylates EA > 0.8 eV) were dominated by molecule anions and the HC1 adduct of the molecule anion. These were the only compounds for which HCl attachment was observed. T h e intensity of the (M HC1).- ion appeared t o be dependent on t h e concentration of HC12- in the source. In cases where the concentration of HC12- was low the ( M HCl).- ion was also of low intensity. It is, however, unlikely that the (M HCl).- ions are formed by direct reaction of HCla- with the molecule because of the electron affinity of chlorine radicals is a t least 2 e\' higher than the molecular electron affinities of any of the compounds in Table 11. T h e methylene chloride NCI mass spectra of alkyl halides were dominated by the halide anions, X-, which were formed by disassociative rapture. Low intensity ions for halide attachment (M C1)- and CH2Cl?X-, and cleavage, X-, were also observed. T h e NCI mass spected pesticides (2, 2 6 ) , e g . , aldrin, are best obtained without methylene chloride in the source. T h e chlorinated pesticides are an ample source of chloride ions, and since these molecules are carbon bonding substrates, methylene chloride serves only to diminish the intensity of the pesticide spectrum by competition for the available chloride. I n cases where the pesticides are present in minute amounts, as in pesticide residue screening applications, it is necessary to use methylene chloride as the chloride source to obtain a typical chloride attachment spectrum. A number of organic compounds were examined which under these conditions did not exhibit ions other than those attributed t o methylene chloride. These compounds are listed in Table 111. T h e polynuclear, aromatic hydrocarbons in Table I11 all showed varying amounts of molecule anion formation? but the molecule anion intensity was always trivial (j
~
(a!
Conlp""?'l
('
--
A
c1)-
(h!
...
2 -0ctanoi I-Propanol / i , ri - Bilt a no 1
+ c1)- (65 -)
70 69 80
6.3 0.65
1.1
l ' i ' l ' l '
lntensities a r e relarive t o C1 = 100.
00
_____
-.
20
40
1
I ' 1 ' 1
1 ' 1 '
loo
120
140
Table V. Tracc Impurities Observed by Chloride 1011 Atta~hme~lt K80.
4 -Heptano:ie
-mC60.
Butyric acid Propionic acid Octanoic acid Heptanonic acid F o r m i c acid
Oc t a lde hyde
Ei!iyl butyrate
c
g-
2 Oct:inonc>
Benzaldehyde Pi.1 I!)i:Jlla'd?hpde Xiiilinc
+-
+
+
r
,
,
I
,I
'G
Figure 3. Mass spectra for a commercial sample of 4-heptanone
chI(irirle ions hy ion-dipole and/or ion-induced dipole interactions. The fact that we did not ohserve these ions in the spectra indicates that the heat of association of chloride with these m~leciilesmust he less t h a n 8-9 Kcal/mol (22, . In cases where the vapor pressure is high enough. it sh!)~iltI be possible t o ohserve ( M + C1)- ions for these structural types by nhiaining the spectra a t low temperatures. Temperature Effects. Temperature effects were import q n t in the specire of virtiially all of t h e carbon bonding stihstrntes as well .?s those substrates t h a t form relatively weak hvdrogen ho.ids. T h e magnitude of the decrease in relativr, intensity d ; h r ( M ( 2 - ion in the spectra a t aliphatic alcohols with increasing source temperature is illustrated in Tahle IV; in each case the probe tip was inside t h e source. 1lecreasing the 51) u r w ternperatwe always increased t h e rehiivc prominenw of ( M HCI?)-. (2M CI)-, and (2M - HI-- ion?. In t h e cases of alcohols, amines, and carbon bonding substrates, sciurce temperat~ureplayed a vital role i n t h e appearanre of the spectrum. T h e methylene chloride Nc'T mass spectra (if the compounds in Table I. which all form relatively strong hydrogen bonds (27) with chloride, wc'rp relative!? insensitive t o temperature effects. Increasing t h r soiirce temperature always increased the relative a.hrind;ince of C1k ccimpared t o t h e rest of the spectrum and especiall?. t h e ( M C1)- ions. Increasing temperature also fabors the formation of ( M C1)- ions as compared t o (2M - Hi- ions i n the ipectra where t h e deprotonat,ed dimers were prnminent a t low tempepatures. Potential Analytical Iltility. It, is obvious from the disciissicsn ahovr that t ~ h eresponse of a given compound t o rnethylene chloride NCI m spectrometry is strongly dependent on the compound's structure. This means that methylene chloridr N(?I mass spectra would not be satisfactory in applications like general structure determina-
+
40.
,
Pileno1
+
>,;dj
+Clg
(CH,CH2COOH+C I
c? 20-
Butyric acid Crotonic acid Phenol Butyric acid Octmoic acid Benzoic acid Propionic acid
A I1 i s ol e P T i ip:y 1 c 111 e r
--
(CH&H&H&OOH
CI-
t,inn. T h e seiectivity of the methylene chlnride mass spectrometry should, however, be useful in t h e det,ection of trace levels of impurities t h a t form strong hydrogen honds in t,he presence of large quant,ities of nonhydrogen honding suhst,rates. This situation is often encountered, c.g., acids are often impurit,ies in hulk samples of aldehydes or ketones. Figure 3 ( a ) illustrates t h e electron impact, mass spectrum o f a commercial sample of 4-heptanone. T h e spectrum appears entirely normal, and does not indicate t h e presence of impurities. T h e methylene chloride NC1 mass spectrum of t h e same sample is shown in Figure 3 ( h ) . Chloride association with 4-heptanone dnes not occur under these conditions; however, t h e spectrum shows relatively intense ions which corresponded to association of propionic and butyric acids with chloride. Table V lists other compounds and inipurities detected in them hy methylene chloride NCI mass spectrometry. Impurit>ydtitection hy this technique is rapid and molecularly selective, if., vou immediately knnw the molecular weights of the acidic impurities in a sample. Detection sensitivities in the ppm range can be obtained, and there is reason 1.0 helieve that impurity levels of 1:lO" should be d~tec~t.ahle by chloride attachment NCI mass spectrometrv. For example, direct probe evaporation of I O ~1 of a hydrocarhon ext,ract of chicken fat which contained 0.9 ppm dieldrin gave a prominent ion corresponding to chloride attachment t o dieldrin (28 1. In the case of qtrong hydrogen hnnding impurities, t h e cletect,ion limit should he siihstantially lower.
CONCLUSIONS Methylene chloride NCI mass spect,ronetry has been found to he particularly sensitive to organic cornpounds which contain acidic protons and far less sensitive t o other organic compounds. T h e amount 01' chloridr att,achment,is somewhat sensitive t o temperature in t h e soiirce. With these limitations in mind, the use of methylene chloride NCI mass spectra i'or trace analysis of specific organic (:ompounds may he significantly faster and simpler than other procedures.
A N A L Y T I C A L C H E M I S T R Y , V 3 L . 37, NO. 1 . J A N U A R Y 1975
53
LITERATURE CITED (1) F. H. Field, Accounts Cbem. Res., 1, 42 (1968) and references therein. (2) F. J. 8iros, R C. Dougherty, and J. Dalton, Org. Mass Specfrom., 6, 1161 (1972). (3) H. M. Fales, F. W. Milne, and T. Axenrod, Anal. Chem., 42, 1432 (1970). (4) H M. Fales, H. A . Lloyd, and G. W. Mi!rie. .J. Amer. Chem. Soc., 92. 1590 (1970). ( 5 ) R. C Dougherty and C . R. Weisenberger, J. Amer. Chem. S O C . 90, 6570 (1968). ( 6 ) R. C. Dougherty. J Chem. fbys.. 50, 1896 (1969). (7) C. Cottrell, R. C. Dougherty, G. Frankel, and E. Peochold, J. Amer. Chem. Soc., 91, 7545 (1969). (8) E. M. Chait, W. B. Askew. and C. 8.Matthews. Org. Mass Specfrom., 2, 1139 (1969). (9) W. T.Naff and C. D. Cooper. J. C k m . Phys., 49. 2784 (1968). (10) J. C. J. Thyme, Chenr. Commun. 1075 (1968). (11) R. S . Gohlke, J. Amer. Chem. Soc., 90, 2713 (1968). (12) J. T.Larkins, J. M. Nicholson, and F. E. Saalfeld. Org Mass Specfrom.. 5, 265 (1971). (13) K. Jaker and A. Henglein, 2. Nafurforsch. A, 22, 700 (1967). (14) S. Tsuda. A . Yokohata, and M. Kawai, Bull. Chem. SOC.Jap., 42, 607 (1969). (15) P. C. Rankin, J. Ass. Offic. Anal. Cbem., 54, 1340j1971). (16) R. C. Dougherty, J Dalton, and F. J. Biros. Org Mass Spectrom., 6 , 1171 (1972).
(17) E . M. Arnett, F. M. Jones 111, M. Tnagepera. W. G. Henderson, D. Holtz, J. L. Beauchamp. and R. W.Taft, J. Amer. Cbem Soc., 94, 4724 (1972). (18) R. Yamidagni and P. Kebarle, J. Amer. Chem. Soc., 94, 2940 (1972). (19) C. Kajoas and R. Tummler, Org, Mass Specfrom., 2, 1049 (1969). (20) D. Beggs, H. M. Fales. G. W.A. Milne, and M. L. Vestal, Rev. Sci. hsfrum,-42, 1578 (1971). (21) J. H. Futrell and L. H. Wojcik, Rev. Sci. lnstrum., 42, 244 (1971). (22) R. C. Dougherty, J. Daiton, and J. D. Roberts, Org. Mass Specfrom., 8, 77 (19741. (23) R. C . Dougherty and J. D. Roberts, Org. Mass Specfrom., 8, 81 (1974). (24) R . N. Compton and R. H. Heubner, Advan. Radiaf. Cbem., 2, 281 (1970). (25) R. S. Berry and C. W. Reimann, J. Chem. fhys., 38, 1540 (1963). (26) R. C. Dougherty, J. D. Roberts and F. J. Biros, Anal. Chem., 47, 54 (1975). (27) R. Yamidagni and P. Kebarle, J. Amer. Cbem. Soc., 93, 7139 (1971). (28) R. C. Dougherty, A. Bergner, P. Levonowich, and J. D. Roberts, to be published.
RECEIVEDfor review .4pril 2, 1974. Accepted September 11. 1974. This work has been supported by a grant from t h e National Science Foundation. A preliminary account of this work has appeared: R. C. Dougherty, J. Dalton, and J. D. Roberts, 19th Annual Conference on Mass Spectrometry and Applied Topics, Atlanta, Ga., 1971, Paper J. 4.
Positive and Negative Chemical Ionization Mass Spectra of Some Aromatic Chlorinated Pesticides R. C. Dougherty and J. D. Roberts Department of Chemistry, Florida State University, Tallahassee, f l a . 32306
F. J. Biros' Primate and Pesticides E f f e m Laboratory, Environmental Protection Agency, Perrine, f l a . 33 157
The positive (CI) and negative (NCI) isobutane chemical ionization mass spectra of twelve aromatic, chlorinated pesticides, metabolites, and degradation products have been determined. The compounds investigated included six polychlorinated diphenylethanes (DDT-type), three polychlorinated diphenylethylenes (DDE-type), and three diphenylmethanol derivatives ( e.g., Kelthane). The CI spectra are considerably less complex than the corresponding electron impact mass spectra reported previously by other investigators. For the diphenylethanes, the base peak corresponded to elimination of CI from the molecule ion. The CI spectra of the diphenylethylene compounds were dominated by the molecule ion and the quasi-molecule ion, (M -k H)'. This class of pesticides also exhibited ion-molecule attachment peaks corresponding to ( M C3H7)+ and (M C4H9)+, as well as significant ions corresponding to chloride elimination ( M - CI)+. Rearrangement ions were observed in the spectrum of DDMU. The most intense ion in the CI spectra of the diphenylmethanol compounds corresponded to [M - OH]'. The NCI spectra with isobutane as enhancement gas were exceptionally simple. The most abundant ion for all compounds, except DDMU and methoxychlor, was (M CI)-. Low intensity dimers and other attachment ions were also observed in some cases. Fragmentation was noted only for oxygenated molecules including methoxychlor and chlorobenzilate. Potential applications to pesticide residue analysis are discussed.
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i Present addresi I S F:n\ironinental Protection Agency, 401 M Street. S W . LVashlngton, D C 20.160
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In previous publications ( 2 , 21, we have discussed the positive (CI) and negative (NCI) methane chemical ionization mass spectra of a series of polycyclic chlorinated insecticides. T h e purpose of the present investigation was to employ both chemical ionization techniques, with isobutane as reagent gas, in the study of a series of aromatic chlorinated insecticides and degradation products. T h e overall objective of this study is the development of rapid and reliable procedures for the qualitative and quantitative measurement of pesticide residues and other toxins in environmental substrates. T h e range of applicability of the technique should include substrates like air, water, soil, and human or animal tissue. By use of both CI and NCI spectia, the reliability and redundancy of the analysis should be increased to the point that additional purification, e.g., GLC introduction, would not be necessary for minimally cleaned u p extracts of environmental substrates. T h e approach described here is a direct extension of the CI ( 3 ) and GLC-CI ( 4 ) examination of body fluids and other substrates for drugs and related materials. This report discusses the general features of the isobutane CI and NCI spectra of the D D T class of pesticides. When combined with previously published data ( I , 2 1, this information should provide a basis for the analytical procedure suggested above. For the polycyclic chlorinated insecticides ( I 1, the most significant single feature of their CI mass spectra was the high intensity of ( M - Cl)' fragments which are probably formed by chloride abstraction, Equation 1.
A N A L Y T I C A L C H E M I S T R Y , VOL.. 47, NO. 1, J A N U A R Y 1975
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