Chemical ionization mass spectrometry of hydrofluorocarbons, hydrofluorocarbon ethers and perfluoroalkenes

ELSEVIER

Journal of Fluorine Chemistry 80 (1996) 81-85

Chemical ionization mass spectrometry of hydrofluorocarbons, hydrofluorocarbon ethers and perfluoroalkenes T. Isemura *, R. Kakita, A. Tamaoki, S. Yonemori Asuhi Glass Co., Ltd. Research Center, 1150 Hazuwu. Kanagawa-ku, Yokohama, 221 Japan Received 2 January 1996; accepted 20 May 1996

Abstract Positive and negative chemical ionization (CI) massspectrometryis studied for hydrofluorocarbons (HFCs) , hydrofluorocarbon ethers (HFB.) , and perfluoroalkenes(PFCs) using various kinds of reagentgas.While no quasi-molecularion was observedunder electron impact ionization for saturatedHFCs, [M-F] + is detectedunder CI conditions using methaneas a reagentgas. Mechanisms for the generationof [M-F] + are discussed.Furthermore,nitrogen monoxide can be usedas a reagentgasto observe [M + NO] + for many HFCs and HFEs. In negative mode chloroform is also available to generate[M + Cl] for HFCs and HFEs containing -CHF- groups. Keywordy:

Chemical ionization; Hydrofluorocarbon;

Hydrofluorocarbon ether; Reagent gas

1. Introduction Hydrofluorocarbons (HFCs) and hydrofluorocarbon ethers (HFEs) have mainly been developed as alternatives to chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons ( HCFCs) because of the need for ozone layer protection and are used in various fields as solvents, washes and foaming agents. Whereas the absence of chlorine makes the ozone depleting potential almost zero, the number of carbons present must be 3-6 to keep the boiling points similar to the corresponding CFCs, which have 1-2 carbons. Accordingly, the number of atoms in a molecule is increased, so it becomes difficult to determine the chemical structures by massor NMR spectroscopy, which have been widely applied in the case of CFCs. CC/MS is the preferred technique to analyze the chemical structures of minor components in matrices, or samples containing various kinds of impurities, where it is invaluable to know a molecular weight from the mass spectra. However, electron impact (EI) mass spectra of HFCs and HFEs rarely show a molecular ion [M] + or a quasi-molecular ion such as [M-F] +, because they are too unstable and break down into smaller fragment ions before detection. The chemical ionization method has been widely utilized for several decades, and many theoretical studies have been reported [ 11. In the case of halogenated compounds, some * Corresponding author. 0022-l 139/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved PIlSOO22-1139(96)03499-9

reports have been published for the negative CI method taking advantage of the electronegativity of chlorine or fluorine atoms [ 2-41. A few studies of positive and negative ionization of fluorine compounds [ 5-91 have been reported; however, there are few reports for CI of CFCs or HCFCs, still less for HFCs and HFEs. In this article, CI mass spectra of several HFCs and HFEs are investigated under positive and negative modes using various kinds of reagent gases. The peaks on each spectrum are assigned to the corresponding structures. An optimum measurement condition is established for a certain type of compounds. Furthermore, CI reactions among samples, reagent gases, and electrons are discussed in the case of an efficient generation of [M-F] + in the positive mode for HFCs and HFEs.

2. Experimental

details

HFCs and HFEs were purchased from PCR inc. (USA) and used without purification. They were diluted to approximately 1% solution by 1,3-dichloro- 1,1,2,2,3-pentafluoropropane (HCFC225cb, CF,ClCF,CFHCl). 1 ~1 of each solution was introduced into the HP-5890A or 5890 series II gas chromatography (GC) with a sample split of 100: 1. The GC was equipped with a 60 m capillary column (0.25 pm film thickness of middle polar liquid phase) and coupled with an JEOL SX- 102A double focusing mass spectrometer (MS)

Table 1 Representative positive Ct mass spectra of PFCs. HFCc, and HFEs in some reagent gases Sample \ reagent gas

CH, ml:

Rl

331 (M-F) 13l(C,F,)

100 131 II 33l(MmF)

CF2 = CFCFzCF = CFZ iv=212

193(MmF)

100

( CF.3) #ZFCF = CFCF i

28l(MmF)

PFCs CF \CF2CF2CFzCF2CF = CFZ

M=350

M= 300 HFCs CF,CFZCF2CF,H M=220 CF,CFZCF2CF2CHzCHF2

M=284 CF,CFZCF,CFzCHICH, M=248 CF,CF>CFHCFHCF, M=252 CH ,CF&?H,CH j

193 l?I(C~F,) 93(CF,) 23I(M-CF;)

M=248 CH,CHZCH2CH20CFZCF2H

RI

NH; ml;

RI

NO ml,

RI

100

131 162(C,F,)

100 12

131 69(CF<)

100 13

131 93 162(MpCF>) L

100 92 73

143(M-CF,) 131 93 2x1

n 12 100 62 44 100 7X 26

69(CF,) 231

20l(MmF) I3l(C,F,)

100 c 20

26S(MmF) 245 ( M-F,H ) 195( MmCF,H 1 229( M-F) 209 ( MmFzH ) 1X9( M-F,H,) 233( M-F)

100 9S(C,F,HL) 36 I lS(C,F,H,) 10 195 100 79(C&H,) 66 209 27 189 100 h

100 X9 35 100 41 12

9s 131 (C,F,) 24s h

100

100 13

E

100

131 229 109 h

75(MmFt

M=94 HFES CFZ = CFCF#CHFCF,

,-C,H,,, ml:

229( M-F) 13ltC,F,) S7( C,H,)

7.5 6S(M-CZH,)

100 131 97 229 109(C,F,O) 100 56(C,H,)

30 IO 100

M=l74 CF,CH,0Cl12CF, M= 182 CF,HOCH,CF, M= IS0

183(M+H) 163(M-F) 113(C,F,H) 13l(M-F)

100 74 24 100

I83 163 I13 113(M-37) Sl(CFzH) 82(CLF,H)

100 92 52 100 27 19

113 163 69(CF,) h

(EI) ml2

RI

131 69 181 100 143 93 212 75 131 100 65 SO 181 30 281

100 71 20 100 89 79 100 52 40 100 66 12 100 48 25 100 30 25 100 48 35 100 40 13 100 34 16 100 90 82 100 75 23 100 98 80

100 115 89 Sl(CFLH) 12 314(M+NO) ’ 79 Sl(CF,H) 278(M+NO) a 69(CF,) 233 282(M+NO) ’ 104(M-F+C?H,) 65 124(M+NO)

100 92 3 100 19 3 100 93 2 100 56 28

51 69 101 51 115 69 79 51 59 69 51 183 65 79 64

100 131 16 IOl(CZF,H) 16 229” 57 204(M+NO) 56 100 113 36 212(M+NO) 30 83(C,F,H,) Xl(M-CF,) 83(C>HzF;) iXO(M+NO)

100 18 8 100 92 61 100 92 84 100 57 31

131 101 51 56 41 57 113 83 61 83 51 81

* At most three mqor fragment ions for which RI 2 IO arc listed except for these mns. h No definite peaks are detected. ’ Not measured.

through a direct interface. Typical MS operating parameters are follows: 300 FA electron beam at 200 cV, an ion source pressure when the reagent gas was introduced: 1.5 X IO--” torr (0.5 X IO-’ torr without the gas introduction). The GC carrier gas helium (He) and the reagent gases such as methane (CH,), isobutane (K,H,,,), ammonia ( NHX), 1,1,1,2-tetrafluoroethane (HFC 134a, CF,CFH,), carbon tetralluoride (CF,), nitrogen monoxide (NO), sulfur hexafluoride (SF,) wcrc the commercially available grade with the analytical quality. The GC oven was initially set at - 20 “C for 5 min, and ramped linearly at 5 “C min ’ to 50°C and at 15”Cmin~’ to 200 “C. The temperature was set at 240 “C in the GC injector and 260 “C for GC/MS

interface, while the ion source of MS was initially IO0 “C.

set at

3. Results and discussion

Representative positive and negative CI mass spectra of PFCs, HFCs, and HFEs in some reagent gases are summarized as lists of at most three major fragment ions for which the relative intensity (RI) 2 10 in Table 1, respectively. The results are classified and discussed below.

Table 2 Reprensentative negative Cl mass spectra of PFCs. HFCs, and HFEs in some reagent gases r-Cd I<, ml:

RI

CHCI 1 ml?

RI

PFCs CF$ZF2CF2CF~CF2CF = CFZ

350(M)

M=350

23 I ( CsF,)

100 SO

350(M) 231

CF, = CFCF,CF = CF2

212(M)

100

401(2M-F,H+CI)

100 80 50 100

300(M) 262( M-F,)

100 21

300(M)

100

319(M +Cl)

100

79(GF,H,) 209(M-F2H) 287(M+Cl)

100 28 100

Sample \ reagent gas

381(C,F,,) M=212

(CF,)>CFCF=

CFCF,

M=300

HFCs CF,CF2CF2CF2CH$ZHFz M=284

CF,CF2CFLCF,CHLCH, M=248 CF,CF>CFHCFHCF,

,I 212(M-F,H-) 29l(M+~,~;?) 23 I (M-FH,) ,I

100

HFEs CF, = CFCFzOCHFCF,

1I7(OCHFCF,)

100

M=248

169(C,F,)

M=252

CH,CFZCHZCH 1

32

17

a

M=94

33l(M+83)

CH3CH2CH2CHLOCFZCF,H M=

174

CF,CH20CH,CF, M= 182 CF,HOCH&?F, M=

117( OCFICFZH) 97 ( OCF = CFI ) 17S(M+H)

283(M+Cl)

100

209(M+Cl)

100

217(M+Cl)

100

62 SO

100 67 7

d

0

150

At most three major fragment ions for which RI > IO are listed. a No definite peaks are detected.

3. I. Positive CI MS using reagent gases of the acid-base reaction type The use of CH4, i-&H,,,, and NH,, as acid-base reaction type reagent gases generally results in the formation of a protonated molecular ion [M + H] ’ with proton-accepting compounds. However, [M-F] + ion generation is predominant instead of proton transfer when CH, is used as a reagent gas for PFCs or HFCs, as shown in Table 1. Generation mechanisms of the [M-F] + peak are discussed later. Using i-C,H,o or NH, as reagent gases, mass spectra of each sample show a scattered pattern, and at the same time the sensitivity is much lower than that for the CHd CI mass spectra. EI and CH, CI mass spectra are shown for CF,CF,CF,CF,H in Fig. 1. A quasi-molecular ion is not seen in the EI spectrum which has the largest mass number peak at m/c = 150 corresponding to [ M-CF,H] +, while the CI spectrum shows an [M-F] + peak at m/z = 201, easily leading to the molecular weight (220). In the case of C4F,CH2CHF,. HF elimination occurs to generate an ]M-F,H] ’ peak at m/z 245 from the [M-F] + peak at m/z 265. Some HFEs, CF, = CFCF,OCHFCF, and CF,HOCH,CF,, show an [M-F] + peak in the CH, CI spectrum, while

l001

(a>

;’

i !I [M-F]+;a'

100-l

100

200

m/z

300

Fig. 1. Mass spectra of CF,CFZCF2CFZH by (a) Electron impact ionization and (b) CH, chemical ionization.

CF3CH20CH2CF, indicates [M + H ] + as the base peak. In the case of CH,CH,CH,CH,OCF,CF,H, no quasi-molecular ion can be detected. As described here, the molecular weight

84

T Isernurcr et al. /Journal

ofFluorine

can be easily established from the strongest peak corresponding to [M-F] + for PFCs and HFCs. However, CI spectra of HFEs depend on their structures, so that a general rule cannot be defined for HFEs.

Chemistry

I00 1

80 (1996) RI-85

‘9 [/C,F,H,l+

(a>

3.2. Positive CI MS using reagent gases of the non-acidbase reaction type Reagent gases described in the previous section aims at an ionization of a sample molecule by protonation. In this section, CF,, HFC134a (CFJFH,) and SF, are employed as reagent gases, expecting some interactions between fluorine atoms in the sample and the reagent gas. Included also is a charge exchange type reagent gas, NO, because of its low first ionization energy (9.2 eV) [ lo]. NO has been employed previously as a reagent gas [ 1l-151. CI mass spectra using CF, or HFC134a are similar to those using CH,, though they are not shown in Table 1. However, a little more fragmentation occurs for PFCs and HFCs compared with the CH4 CI. When SF6 is used, fragmentation proceeds so extensively that sensitivity is reduced. Accordingly, this type of reagent gas has no advantages for the CI reactions of such fluorocompounds. NO CI spectra are similar to EI spectra except for generating a [M + NO] + peak in several HFCs and HFEs as shown in Table 1, by which the molecular weight of the sample can be established. EI, CH4 CI, and NO CI mass spectra of CF3CF2CF2CF2CH2CH3 (MW248) are shown in Fig. 2. The CH4 CI mass spectrum Fig. 2(b) shows three peaks around the molecular weight. It is not easy to assign each peak to [M+H] +, [M-F] +, and [M-F,H] + etc., because their intervals are 20 amu (atomic mass unit) from each other, which corresponds to the molecular weight of HF. In such a case, the NO CI spectrum is helpful to assign them, detecting [M + NO] + as shown in Fig. 2(c). For some HFEs, for example CH,CH1CH2CH,0CF2CF2H, no quasi-molecular ion is detected by CH, CI as mentioned before, while [M + NO] + can successfully be detected using NO CI. As a result, NO CI has proved to be useful for the molecularweight estimation of a sample in following cases: 1. A quasi-molecular ion is not detected by EI and even by CH, CI. 2. When assigning a quasi-molecular ion is difficult, because there are several peaks around the molecular weight. 3.3. Mechanism of [M-F]+

generation in positive Cl

In many cases, [M-F] + is prominently generated as a base peak in positive CI of PFCs or HFCs using CH4, CF,, or HFCl34a as reagent gases. Four mechanisms for [M-F] + generation are suggested as follows: A: [M + H] + generation due to protonation followed by HF elimination, B: Fluoride (F-) abstraction in analogy with a proton abstraction in the case of hydrocarbons by CH4 CI,

50

0

I00

200

m/z

300

Fig. 2. Mass spectra of CF,CF,CF,CF,CH2CH, by (a) Electron impact ionization, (b) CH, chemical ionization and (c) NO chemical ionization.

C: [M] + generation due to charge exchange followed by fragmentation, and D: Electron impact (including EI for which electron energy is lowered by the collision with reagent gas). Though it is difficult to distinguish them experimentally, those reaction mechanisms are examined by discussion of the mass spectra obtained using various reagent gases as follows. First, a [M-F] + peak is generated efficiently even with a reagent gas containing no proton, such as CF4. This fact indicates that the mechanism A does not apply in this case. Sensitive detection of an [M-F] + peak needs an appropriate difference between the recombination energy and the first ionization energy in charge exchange CI (mechanism C) . The recombination energy is 13.3 eV for [ CH,] +, a typical reactant ion of methane [ 161. The other reactant ions have similar values, while the first ionization energy of typical HFCs is more than 14 eV. Therefore, it is hard to understand the soft (less fragmentation) ionization shown by CH4, CF,, or HFC 134a. When NO is used as a reagent gas, the fragmentary pattern is similar to the EI spectra for HFCs and HFEs as mentioned before. It means that this fragmentation is due not to charge exchange but to EI (mechanism D) , because the recombination energy of NO is only 9.2 eV. In the case of CH4, it is unlikely that mechanism D causes [M] + generation followed

T. Isemurcc et al. /Journal

of Fluorine

by fragmentation to [M-F] ’ , because CH, CI spectra are very different from EI spectra though there should not be too much difference between electron energy distributions in the reagent gases NO and CH4. Consequently, [M-F] + is assumed to be generated by mechanism B. The [M-F] * generation mechanism between CF$F,CF&F,CH,CH:, and [ C,H,] +, one of the reactant ions of CH4, is illustrated as: CF,CF,CF,CF,CH,CH,

+ [ C,H,] + -+ [ C4F,CH2CH3] + + C,H,F

This reaction is endothermic when the F- affinity of the reactant ion is greater than that of [M-F] +. The generation efficiency of [M-F] + should depend on the difference between those F- affinities. 3.4. Negative

Cl MS

Negative EI MS has several limitations, for example, generation of worthless low mass ions such as F ~ and Cl ~, and sensitivities several orders of magnitude lower than positive EI [ 11. Negative CI MS, however, is useful in some fields, such as selective and sensitive detection of ions taking advantage of halogen electronegativity. In this section, some PFCs, HFCs, and HFEs are examined in negative CI mode with the use of various reagent gases including fluorocompounds. Negative CI reaction has two categories: electron capture (EC) and ion-molecule reaction. Though further details of these reactions are not discussed here, CH,, i-C,Hlo, NH,, HFCl34a, and SF, are used as EC type reagent gases, and CHCl, is used as an ion-molecular reaction type reagent gas. Representative results are shown for i-C4HI0 and CHCl, in Table 2. PFCs show efficient [M] ~ peaks by using an EC type reagent gas. But in the cases of HFCs and HFEs, quasimolecular ions do not appear in the spectra and some insignificant ions are insensitively detected. HFEs give an ether bond cleavage to form [R-O] -. However, in the case of ion-molecular type reactions, Clgenerated from CHCl, adds to specific sample molecules as a reagent ion [ 17,181. Some HFCs and HFEs having -CHFgroups provide an [M + Cl] - ion. This ion peak is intense and stable enough to give no detectable fragment ion. Cll may electrostatically add to a 6+ site such as a -CHF- group. However, for molecules having no St site, CHC13 CI is not useful, showing no ion or only insensitive ions generated by EC.

Chemistry

80 (1996) 81-85

85

4. Conclusion CI mass spectrometry is applied to HFCs and HFEs whose importance will increase still more in the future. At first, CH4 should be used as a reagent gas for soft ionization of PFCs, HFCs, and HFEs under the conditions described in the experimental section. When a satisfactory result is not obtained as a result of ambiguous peak assignment or the lack of quasimolecular ions, two alternatives can be tried. NO is used as a reagent gas for [M + NO] + formation. The other uses CHCl, in the negative mode aiming at [M + Cl] - formation for samples which may have a -CHF- site. Furthermore, the effective generation of [M-F] + for HFCs and HFEs by CH4 positive CI is attributed to fluoride abstraction from a sample molecule by a reactant ion. Therefore, the generation efficiency of [M-F] + depends on the difference in fluoride affinity between [M-F] + and the reactant ion.

References [I] J.R. Chapman, Pructical Organic Mass Specwometry, Wiley, Manchester, UK, 2nd. edn., 1993. [2] T.M. Trainor and P. Vouros. Anal. Chem.. 59 (1987) 601. [3] S. Shang-Zhi and A.M. Duffield, J. Chrom., 284 (1984) 157. [4] S. Daishima, Y. Iida and F. Kanda, J. Trace and Microprobe Techniques., 7 ( 1989) 87. [5] S.K. Huang, K.A. Despot, A. Sarkahian and T.W. Bier], Bio. Environ. Mass Spectrom.,

19 ( 1990) 202.

[6] K. Mervett, A.B. Young and A.G. Harrison, Org. Mass Specrrom., 2R (1993) 1124. [ 71 J.L. Franklin and S.D. Dey, Inr. .I. MussSpecrrom. Ion Phys., I4 ( 1974) 311. [ 81 SM. Spyrou, S.R. Hunter and L.G. Christophorou, J. Chem. Phys., R3 (1985) 641. [9] J. Hrus& D. Schroder. T. Weiske and H. Schwarz, J. Am. Chem. Sot., 115 (1993) 2015. [ 101 K. Hiraoka, Mass Spectroscopy, 28 (1980) 185. [ 111 D.F. Hunt and J.F. Ryan, J. C. S. Chem. Comm., (1972) 620. [ 121 B.L. Jelus, B. Munson and C. Fenselau, Biomed. Mass Spectrom., I ( 1974) 96. [ 131 D.F. Hunt, C.N. McEwen and T.M. Harvey, Anal. Chem., 47 (1975) 1730. [ 141 D.F. Hunt and T.M. Harvey, Anal. Chem., 47 (1975) 1965. [ 151 D.F. Hunt and T.M. Harvey, Anal. Chem.. 47 ( 1975) 2136. [ 161 F.H. Field and J.L. Franklin, Electron Impact Phenomena and rhe Properties of Gaseow Ions., Academic Press, New York, 1957. [ 171 A.K. Bose, H. Fujiwara and B.N. Pramanik, Tetrahedron Lerr., 42 (1979) 4017. [ 181 H.P. Tannenbaum, J.D. Roberts and R.C. Dougherty,AnaI. Chem., 47 (1975) 49.