ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
2013
Plasma Chromatography of Alkyl Amines F. W.
Karasek" and S. H. Kim
Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G 1
Souji Rokushika Department of Chemistry, faculty of Science, Kyoto University, Kyoto 606, Japan
The plasma chromatography of primary, secondary, and tertiary alkylamines and related compounds has been studied. For each homologous series of amine, a simple relationship was found between reduced mobility at 150 O C and molecular weight. The isomers of C,H,,N show a reduced mobility order of hexylamine < di-n-propylamine < triethylamine. The reduced mobilities of lower molecular weight compounds depended on the drift tube temperature, while those of higher molecular weight compounds did not. The temperature dependence was shown by PCIMS to be due to ion-molecule clustering with H20 and N2 as well as dimer formation.
injection of the headspace vapor, while nonvolatile samples were introduced coated on a Pt wire. Mass identification was obtained with a Phemto-Chem MMS-160 Ion Mobility Spectrometer-Mass Spectrometer (PCP Inc., 2155 Indian Rd., West Palm Beach, Fla, 33409). Reagents. Linde high purity N2 (99.995%) was dried by Linde molecular sieve 13X and used for both carrier and drift gases. Methylamine and ethylamine were obtained from Dow Chemicals, n-amyl(penty1)amine and benzene from MCB Co. Other amines and pyridine were from Polyscience Analq-tical Standard Kit, 31A, 32A, and 81A. The other chemicals used were from J. T. Baker Chemical Co., reagent grade.
RESULTS AND DISCUSSION Plasma chromatography (PC)provides a sensitive analytical method for the study of organic compounds through their characteristic mobility spectra. T h e drift times or reduced ) a homologous series typically increase with mobilities ( K O of increasing molecular weight or ion mass (1-5), but also depend on the size or shape of the ions (6, 7). I n general, bulky ions require a longer drift time than compact ions of similar chemical structure. The mobility at atmospheric pressure will be controlled by the applied voltage and the probability of collisions between ion and drift gas. The most complete theoretical equation for ion mobility has been developed by Mason and Schamp ( 8 ) :
where r,n is a measure of the closest approach of the ion and neutral molecule in a binary collision, e is the ionic charge (esu), N is drift gas density (g/cm3), m the ion mass, M the drift gas mass, T the drift tube temperature, T and h are constants, and Q(' ')* is the collision integral to the first approximation (8,9). Experimental results are usually reported in terms of reduced mobility, KO:
where d = drift length (cm), t = drift time (s), E = electric field gradient (V/cm), T = temperature (K), and P = pressure (Torr). PC ions identities may be assigned with a coupled plasma chromatograph/mass spectrometer (PC/MS) (10-12). We report here a systematic investigation of the mobility spectra of alkylamines and related compounds, utilizing both PC and PC/MS techniques.
EXPERIMENTAL Mobility spectra were obtained with a Beta-VI plasma chromatograph (I). Operating conditions were as follows: carrier gas flow rate, 65 mL/min; drift gas flow rate, 350 mL/min; electric field gradient 142 V/cm except as noted in the figure captions; ion injection pulse and scan gating pulse, 0.2 ms. A 20- or 50-ms scan was recorded in 2 or 5 min. Volatile samples were introduced by 0003-2700/78/0350-2013$01 .OO/O
Positive ion mobility spectra were obtained for primary, secondary, and tertiary alkyl amines. For each series of alkyl amines, the reduced mobility KOdecreased with increasing molecular weight. Mobility spectra for the primary amines are shown in Figure 1, and reveal a prominent product ion peak with small adjacent peaks on each side. The higher KO peak increased in intensity with increasing drift tube temperature. Earlier studies (12) showed when ammonium reactant ion was employed instead of conventional water reactant ions, these small peaks disappeared and a single peak remained whose mobility coincided exactly with t h a t of the corresponding main peak in Figure 1. This peak was tentatively identified as the MH+ ion. This ion has been reported in atmospheric pressure ionization (API) mass spectrometry for n-heptylamine (13). Secondary and tertiary amines also gave relatively simple ion mobility spectra showing a single main ion peak accompanied by minor peaks a t lower KO. For each series of primary. secondary, and tertiary n-alkyl amines, the reduced mobility of the ion decreased with increasing molecular weight. However, any prediction of the value of KOmust take into account differences in molecular structure as well as molecular weight. For example, hexylamine, cli-n-propylamine, and triethylamine have the same molecular weight but different molecular structures. The reduced mobilities of these amines were 1.70, 1.88, and 1.95 cm2/V.s, respectively. These results can be explained in terms of Equation 1 by a smaller value of the collision integral R" "* for the more compact secondary and tertiary amines. In Figure 2. the reduced mobilities a t 150 "C for the compounds under investigation are plotted against t h e logarithm of the molecular weight. Each homologous series shows a linear relationship between KO and log MW with some exceptions. Methylamine, dimethylamine, and trimethylamine show a negative deviation from the linear relationship, and long chain primary n-alkyl amines show a positive deviation. Also plotted in Figure 2 are KOvalues for the [M-HI+ ions of normal alkanes from results reported previously ( 2 ) . On the basis of molecular shape and size, primary n-alkyl amines could be expected to yield reduced mobilities similar to those of the corresponding n-alkanes of similar molecular size and weight. For example, the protonated molecular ion of the primary amine CH3(CH2),NH1+could be expected to
'C1978 American Chemical
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2014
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
(CH,),NH,+ are in fact closest to those of the (M - H)+ ions of the n-alkane CH3(CH2),&H2+, indicating t h a t primary amines and normal alkanes differ considerably in collision cross section W I)*. For all four homologous series, the relationship between reduced mobility and molecular weight may be postulated as follows:
KO = A
PL 4
6
6
10
D R I F T TIME
REACTANT
1 2
14
IONS
16
18
(rns)
Figure 1. Mobility spectra of the primary alkyl amines. Abbreviations used in the figure are listed in Table I
appear a t a similar drift time to that of the (M - H)t ion of n-alkane, CH3(CH2),CH2+,which differs in mass by only 3 amu. However, the drift times of n-alkyl amines CH3-
1.4
1.6
1.8
-
(1.92 f 0.02) log MW
(3)
where A is a constant depending on the structure and nature of the homologous series. Values of A for primary, secondary, and tertiary amines and normal alkanes were calculated to be 5.55, 5.70, 5.78, and 5.75 cm*/V.s, respectively. T o study the effects of molecular structure on KO,various isomers of amines were examined. T h e drift time for npropylamine was greater than t h a t of is0 form, and namylamine appeared slower than is0 or tert forms. Similar results were obtained for di-n-propylamine and di-isopropylamine or di-n-butylamine and di-isobutylamine. Cyclic and heterocyclic compounds such as pyridine, benzene, or their derivatives gave larger KOvalues than the n-alkanes or n-alkylamines with similar molecular weights. However, no significant difference was observed in the reduced mobilities of isomers of xylene. Reduced mobilities for some isomers and derivatives are given in Table 1. T h e reduced mobilities for some compounds exhibit a temperature dependence. Reduced mobilities of alkyl amines and some aromatic compounds were therefore determined a t various temperatures ranging from room temperature to 240 "C. For smaller primary amines, from methylamine to hexylamine, KOincreased with temperature as shown in Figure 3. The slopes of the reduced mobility-temperature curve were largest below about 100 "C, decreased between 100 and 160 "C. and increased above 160 "C. The temperature dependence at lower temperature decreased with increasing molecular size and was not observed for amines larger than heptylamine. At higher temperatures, this effect was also ion mass dependent.
2.0 Log M o l e c u l a r Weight
2.2
2.4
2.6
Figure 2. Relationship of reduced mobility and molecular weight. Solid triangles are primary alkylarnines; solid squares, secondary alkylamines; solid circles, tertiary alkylamines; open circles, n-alkanes; open squares, methyl derivatives of benzene; a, aniline; b, benzylamine; c, toluene; d, xylene; e, mesitylene. Abbreviations for other compounds are listed in Table I
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
2015
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Table I. Reduced Mobility of Amines and Related Compounds compounds ( * ) " methylamine ( M ) ethylamine ( E ) n-propylamine ( P ) isopropylamine n-butylamine ( B ) sec-butylamine tert-butylamine n-pentylamine ( A ) tert-pen tylamine n-hexylamine (H,) n-heptylamine ( H p ) n-octylamine (0) n-decylamine ( D ) n-dodecylamine ( D d ) n-tetradecylamine (Td) dimethylamine (DM) diethylamine (DE) di-n-propylamine (DP) di-isopropylamine di-n-butylamine (DB) di-isobutylamine di-n-hexylamine (DH,) didodecylamine (DDd) trimethylamine ( T M ) triethylamine (TE ) tri-n-propylamine (TP) tri-n-butylamine (TB) tri-n-hexylamine (TH, ) benzylamine di benzylamine tribenzylamine pyridine ( p y ) 2,4-luti dine 2,4,6-collidine 4-tert-butylpyridine dicyclohex ylamine aniline cyclohexane benzene (B,) toluene ( T ) xylene (o,m,p) mesitylene
*
=
mol w t 31 45 59 59 73 73 73 87 87 101
115 129 157 185 21 3 45 73 101 101
129 129 185 353 59 101 143 185 269 107 197 28 7 79 107 121 135 181 93 84 78 92 106 120
r.educed mobility K,(cm '/V,s
temperature, "C
2.53 2.39 2.13 2.19 1.97 2.05 2.03 1.82 1.95 1.70 1.59 1.50 1.35 1.26 1.20 2.40 2.12 1.88
149 149 149 149 146 148 148 149 149 146 146 149 149 148 148 148 149 149 149 148 148 145 148 149 146 149 147 149 149 145 145 145 147 147 147 149 149 149 149 149 149 147
1.91
1.66 1.68 1.34 0.89 2.34 1.95 1.66 1.42 1.11
1.78 1.43 1.20 2.19 1.95 1.84 1.70 1.44 1.91 2.12 2.27 2.11 1.98 1.86
;n)/Kn(s5 )
1.07 1.04 1.04 1.04 1.03 1.04 1.03 1.02 1.01 1.00
1.07 1.04 1.01 1.01 1.00
1.05 1.00
1.04 1.00
0.99 1.05 1.00
0.99 0.98
abbreviations used in the figures.
Similar results were observed for secondary and tertiary amines. For methylamine, it was not possible to determine KObelow 100 "C because the product ion and reactant ion peaks overlapped. T h e product ion peak also shifted with time after the introduction of the sample, or sometimes produced more than one peak. T h e mobilities of ethylamine and dimethylamine also depended on the temperature. with these ions sharing a KOvalue throughout the temperature range examined. KOvalues of n-propylamine and diethylamine were also duplicated. Studies of nonpolar compounds show benzene exhibited a temperature dependence; no effects on the mobility for toluene were observed. The mobility of amines larger than n-heptylamine, di-n-butylamine, and triethylamine for each series of primary, secondary, and tertiary alkyl amines, respectively, exhibits no temperature dependence. The effects of temperature on mobility are tabulated in Table I as a ratio of K Ovalues a t 220 and 55 "C, K ~ 2 2 0 J K ~ s 5 1 . As mentioned previously, a t low temperatures, small compounds show lower KOvalues than expected. In particular, methylamine gave a complex mobility spectrum. In order to study the ionic species in the PC of methylamine, a combined PC/MS was employed. Figure 4 shows the ion mobility spectrum of methylamine as well as the reactant ions a t a temperature of 50 "C. T h e methylamine spectrum shows a complex pattern which includes at least two unresolved
Table 11. Ionic Species of Cluster Ions Identified by PC/MS in Methylamine Peaks at 50 " C Reactant Ions
product ion peaks and a shoulder at the left side of the reactant ion peak. Figure 5 shows the mass spectrum of the
ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
2016
1
M
50
150
100
200
V e
t
.------.-.
- ...-.
Figure 5. Mass spectra of reactant ions and methylamine product ions at 50 OC
produce ion-molecule clustering with water and/or nitrogen .I.-= DE
..
150
100
50
TEMPERATURE
2 50
200 (
'C)
Figure 3. The relation of temperature and mobility. See Table I for abbreviations
molecules. Identities and relative intensities of both reactant ions and product ions a t 50 "C are given in Table 11. At 150 "C, methylamine yielded a single peak in its ion mobility spectrum that was shown to be the MH+ ion at m / e 32. Above 150 "C,however, an unidentified minor peak was observed at mle 44, whose intensity increased with temperature. T h e mobilities of primary, secondary, and tertiary alkylamines, alkanes, and methyl derivatives of benzene have been studied. These results indicate that the temperature effects on mobility are due to the molecular size, shape, and polarity of the ionized compounds. No general equation for massmobility can be derived without considering structural effects. For series of compounds with similar chemical configuration and nature, excellent linearity can be obtained between mobility and the logarithm of the molecular weight or ion mass. On the other hand, two isomers of the same ion mass but different structures can be distinguished clearly by plasma chromatography.
LITERATURE CITED R E A C T A N
I 0
,
, 2
,
L
4
"
"
6
"
~
'
8 DRIFT
"
'
10
12
TIME
(ms)
14
16
18
Figure 4. Ion mobility spectrum for methylamine at 50 OC, taken by PCIMS
reactant ions (upper curve) and of the product ions of methylamine (lower curve) at 50 "C. A total of 16 ions including 7 reactant ions were produced in the m/e range of 10 to 200. Dimerization of methylamine to form MzH+ion is prevalent at this temperature, and both monomer and dimer ions
(1) F. W. Karasek, Anal. Chem.. 46, 710A (1974). (2) F. W. Karasek, D. W.Denney. and E. H. DeDecker, Anal. Chem., 46, 970 (1974). (3) F. W. Karasek and D. W. Denney, Anal. Chem., 46, 1312 (1974). (4) F. W. Karasek, A. Marian, and 0. S. Tatone, J . Chromatogr. Sci., 110, 295 (1975). (5) J. M. Preston, F. W. Karasek, and S. H. Kim, Anal. Chem., 49, 1746 ( 1977). (6) J. C. Tou and G. U. Boggs, Anal. Chem., 48, 1351 (1976). ( 7 ) T. W.Carr, d . Chromatogr. Sci., 15, 85 (1977). (8) E. A . Mason and H. W. Schamp, Jr.. Ann. Phys., 4, 233 (1958). (9) H. E. Rivercomb and W.E. Mason, Anal. Chem., 47, 970 (1975). ( l o ) D. I. Carroll, I. Dzidic, R. N. Stillwell, and E. C. Horning, Anal. Chem., 47, 1956 (1975). (11) F. W. Karasek. S. H. Kim, and H. H. Hill, Jr., Anal. Chem., 48, 1133 (1976). (12) S. H. Kim, F. W. Karasek, and S. Rokushika, Anal. Cbern., 50, 152 (1978). (13) 1. Dzidic, D. I. Carroll, T. N. Stillwell, and E. C. Horning, Anal. Chem., 48, 1763 (1976).
RECEIVED for review June 9:1978. Accepted August 29, 1978.
