Anal. Chem. 1989, 61, 684-689
684
Ion Mobility Spectrometry of Aliphatic and Aromatic Amines Zeev Karpas Nuclear Research Center, Negeu, P.O. Box 9001, Beer Sheua, Israel 84190
The posltive Ion moblllty spectra of aliphatic and aromatk amines were measured. Reduced mobilltles In alr at 150, 200, and 250 O C were determined relatlve to that of lutldlne (2,4dknethylamlne) taken as 1.95 cm2/(V 8). TMS compound is hereby proposed as a standard reference compound for the mobillty scale In posltlve IMS studles. A comprehenslve reduced moblHty data base of more than 100 amlnes Is presented from these results and other published data. A number of trends in the moblUty of lsomers and Isobars are observed. The reduced mobility increases in the order primary < secondary < tertlary amlnes, llnear < branched, m a l < s e e ondary, aUphatlc < aromatic, and amlne < amide. Most interesting Is the result that the temperature dependence of the reduced moMlity Is posnlve for low molecular weight ions, is essentially lnvarlant for Intermediate mass (90-180 amu) Ions, and Is negatlve for heavy Ions.
The present study can serve as an aid to potential field applications of IMS techniques for monitoring amines and related compounds. As evident from a recent compilation of ion mobility data (4), surprisingly little work on amines has been published to date. Karasek et al. (5, 6) worked mainly on alkylamines, Hagen studied some isomers (7), while Lubman et al. (8-11) worked on more complex nitrogen bases, utilizing laser photoionization in addition to ionization by a 6 source. Lawrence (12,13) studied yet other types of nitrogen bases, those used in illicit and prescription drugs. A close study of the available data reveals that there are differences between reported measurements beyond those arising from different conditions of temperature, electric field, ionization method, and drift gas. Thus, large errors may arise from indiscriminate use of the basic equation for calculating the reduced mobility of an ion,
KO KO = ( b / E t ) ( P / 7 6 0 ) ( 2 7 3 / T )
INTRODUCTION Ion mobility spectrometry (IMS) is a technique in which ions are separated according to their mobility as they drift along an electric field through an atmospheric pressure drift gas. IMS has been used to differentiate between isomers and to derive structural information on quasi-molecular ions (1, 2), as the structure of the parent molecule is not usually significantly perturbed in the IMS source. The amines are unique as they can give rise to several types of isomers: linear, branched, and cyclic structures (common also to hydrocarbon compounds); positional isomers (like nand sec-butylamine); primary, secondary, and tertiary amines, and, finally, also aromatic positional isomers. In the latter, the nitrogen atom (or atoms) may be part of the aromatic system, like in methylpyridine (picoline), or attached to the ring like aniline, its isomeric form. When one considers isobaric compounds, in which the molecular weight is nominally similar (like ethylamine and formamide), in addition to isomeric compounds, the scope for comparative mobility studies becomes wider yet. Due to their high proton affinities, amines protonate readily in the IMS sowce, and there is usually little doubt with regard to the site of protonation, as the nitrogen electron lone pair is the favored site. The protonated molecule is in most cases the major ionic product, although in some cases protonated dimers or molecular or fragment ions are also formed. However, it is generally easy to distinguish the quasi-molecular ion from the fragment or cluster ions, due to the differences in their mass, which lead to significant mobility differences. IMS studies followed by mass spectrometric analysis (IMS/MS) confirm the above assumption that the protonated molecule is indeed the predominant ion in most amines. The widespread industrial uses of amines raises safety problems, as their toxicity imposes an allowable threshold limit value (TLV) in air of less than 10 ppm in most cases, and for some compounds, like p-anisidine for example, of 0.5 ppm (3). Therefore, monitoring the amine concentration in air by IMS requires knowledge and understanding of their properties, in particular their ion chemistry under the conditions of the IMS.
(1)
where T and P are the temperature and pressure in the cell, E is the electric field, b is the drift region length, and t is the experimentally determined drift time. It is therefore proposed here to measure reduced mobilities relative to some agreed upon reference compound. This standard should have a small temperature dependence, a large (preferably single) ion signal, and a reduced mobility value close to that of the organic compounds usually of interest. From a study of the reduced mobility data in ref 4 and work in this laboratory, use of lutidine (2,4-dimethylpyridine) is proposed as such a standard for the positive ion mobility scale. Furthermore, the carefully determined value of the reduced mobility of protonated lutidine could serve as an anchor point for the absolute mobility scale.
EXPERIMENTAL SECTION The work was carried out on a Phemto-Chem 100 ion mobility spectrometer made by PCP, Inc., equipped with the standard BSNi p ionization source. Further IMS/MS experiments, to confirm the identity of the major ions, were performed on the MMS-160 (PCP,Inc.). Both instruments were described previously (Chapter 1 of ref 7 ) . Typical experimental conditions for both instruments were an electrical field of 200 V/cm, cell temperature of 200 "C, carrier and drift gas (compressed air passed through a purifier) flow rates of 100 and 500 mL/min, respectively, and atmospheric pressure around 720 Torr at NRCN and 765 Torr in Florida. Measurements were made also (only at NRCN) at 150 and 250 "C with electric fields of 214 and 157 V/cm, respctively. In order to obtain sharp peaks, the gate pulse width was 0.1 ms and the sweep repetition period was 24 ms for most of the compounds. However, for the high molecular weight amines the long drift time of the ion of interest required longer sweep times (up to 40 ms) and the gate pulse width was increased to 0.2 ms to produce stronger signals. Sample introduction was by holding a syringe needle over the sample headspace for 5-10 s and then inserting the needle through the orifice in the inlet. Special care was taken to avoid overloading of the IMS. Data acquisitionwas by Computerscope (RC Electronits, Santa Barbara, CA) hardware and software connected to an IBM/PCXT. This allowed near instantaneous display and averaging of the mobility spectra on the computer screen. The acquisition rate
0003-2700/89/0361-0684$01.50/00 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989 I
I
I
I
'
I
I
665
V
VI
5- 2.4 \ N
E
2 t t -I E
s:
2.0 -.
&,y*:
1.6-
..
0
I.
I
0 0 W
2
1
W
IL)
1. 2
-
0,.
I
0.8.
C Y
0.19
1 4.92
I
I
7.92
I
I
I
10.92
TIME
1
I
13.92
(ins)
Figure 1. Positive ion mobility spectrum, averaged 391 times, in air, at 200 "C, of a mixture of ammonia (A), pyridine (P), 4-picoline (Pi), iutidine (L), and coHidine (C). The reduced mobility of the major ion from each compound is 3.02, 2.22, 2.07, 1.95, and 1.82 cm2/(V s), respectively. was high, so that acquiring lo00 spectra (with 24-ms sweep time), averaging them, and displaying them took 27 s. Thus, signal to noise ratios could readily be enhanced by increasing the number of spectra averaged without prohibitively increasing the sampling time. Samples were all commercially available and were used without any further purification.
RESULTS AND DISCUSSION 1. Calibrationof the Reduced Mobility Scale. In order to obtain meaningful and reproducible results, the reduced mobility scale has to be calibrated. Lutidine (2,4-dimethylpyridine) seemed to be a good choice as a standard reference compound for this purpose. It gives a single large distinct peak, corresponding to the protonated molecule a t 108 amu, in the mobility spectrum, and its relatively high proton affinity (227.3 kcal/mol (14)) will ensure that it can be used even in the presence of most other strong nitrogen bases. Lubman (8) showed that its reduced mobility has little dependence on the cell temperature and on the ionization method and gave a value of 1.91 cm2/(V s), in nitrogen at 175 and 220 "C,compared with Karasek et al. (6)who give a value of 1.95 cm2/(V s) a t 147 "C. In a later study, Lubman and Kronick (9) also reported a value of 1.95 cm2/(V s) for the reduced mobility of lutidine in nitrogen a t 220 "C for the photoionization method. From measurements in this laboratory and use of eq 1,a value of 1.95 cm2/(V s) was obtained for lutidine at 150,200, and 250 "C. We therefore use lutidine as our reference compound, and thu value as the anchor point for the absolute reduced mobility data reported here. A typical positive ion mobility spectrum, averaged 391 times, of a mixture of ammonia, pyridine, picoline (Cmethylpyridine), lutidine, and collidine (2,4,6-trimethylpyridine), is shown in Figure 1. The peaks represent the protonated molecules, and the effect of sequentially adding methyl groups to pyridine is evident. 2. The Mobility of Amines. Primary, secondary, tertiary, and quaternary aliphatic and aromatic amines, as well as amides and some heteroamines, were studied. The reduced mobility of the major ion from each compound, which was initially assumed to be (and subsequently confirmed by IMS/MS) the protonated molecule (with some noted exceptions) was determined relative to the reduced mobility of lutidine. The results are shown in Table I for mobility measurements at 150,200, and 250 "C,in air. Also shown are the available published data on the reduced mobility of amines. The table is arranged in the order of the ascending
0.21
0.23
REDUCED
MASS
0.25
Flgure 2. Mass-mobility Correlation curve of the major positive ion of amines in air at 200 "C, the reduced mobility as a function of the square root of the reduced mass of the ionldrift gas molecule. molecular weight of the compounds. Thus, one can compare our results with other published data, as well as use Table I as a comprehensive compilation of mobility measurements of amines. The main exceptions to the rule that the major ion formed by amines is the protonated compound and that the site of protonation is the nitrogen lone-pair electrons, are the following. All quaternary and some tertiary amines tend to fragment by loss of a stable leaving group and produce protonated amine fragments. Some amines, especially amides, tend to dimerize and produce protonated dimers (see below). Furthermore, the protonation site of amides is probably on the carbonyl oxygen and not on nitrogen (15). Phenylenediamines produce (M - H)+ ions through hydride loss, rather than the MH+ ion as the major ion. Anilines in general, have two mobility peaks that correspond to the protonated molecule, arising from two structural conformers-the nitrogen and carbon protonated species (16). The mass-mobility correlation can be seen in Figure 2, in which all the 'best" reduced mobility values (taken from this work at 200 "C,and where unavailable taken from the literature a t 200 "C or as close as possible to this temperature) are plotted as a function of the square root of the reduced mass of the drift gas/ion collision pair. This type of plot should yield a straight line if the mobility is governed by the reduced mass term, eq 2
(In,
K
= (3q/16N)(l/m
+ 1/M)"2(2a/kr)1'2(1/~~)
(2)
where q and m are the ion charge and mass, respectively, M and N are the drift gas mass and density, respectively, k is the Boltzmann constant, and T is the effective temperature. The deviations from the line indicate that the collisional cross section term, OD plays an important role in the mobility measurements. Naturally, a "better" fit (resemblence to a straight line) is obtained for a plot of the reduced mobility as a function of the logarithm of the ion mass, as has been suggested in other studies. However, this has no physical meaning and is also relatively insensitive to variations of the ion mass. A more detailed discussion on mass-mobility correlations can be found elsewhere (18). A number of trends are evident from analysis of the reduced mobility data in Table I. The general trends of the reduced mobility of isomeric amines are to increase in the order primary < secondary < tertiary, linear < branched, and also aliphatic < aromatic. All three trends can be rationalized in terms of more compact structures of the more mobile isomers. Thus, a tertiary amine will have three short chains on the nitrogen center, compared with the long chain its primary amine isomer will have. A branched amine, even one as large as triisooctylamine, has a higher mobility than its normal isomer. Also, it is interesting that the mobility of the heavy
686
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
Table I. Reduced Mobility of Aliphatic and Aromatic Amines
name of compound methylamine ethylamine formamide dimethylamine isopropylamine n-propylamine trimethylamine diaminoethane ethanolamine imidazole 1,2,4-triazole NJV-dimethylformamide diethylamine sec-butylamine isobutylamine n-butylamine tert-butylamine pyridine pyridazine pyrazine triazine N-methylimidazole N,N-dimethylacetamide morpholine n-pentylamine tert-pentylamine %picoline aniline aniline 4-picoline 2-picolhe 4-hydroxypyridine 3-hydroxypyridine cyclohexylamine triethylamine di-n-propylamine diisopropylamine n-hexylamine (dimethylamino)-1-propane NJV-dimethyl-l,2-propanediamine 3-cyanopyridine diethanolamine 2,4-lutidine 2-toluidine %toluidine benzylamine N-methylaniline %toluidine 1,2-phenylenediamine 1,3-phenylenediamine 1,4-phenylenediamine 2,6-dimethylpyrazine 2,5-dimethylpyrazine 2,3-dimethylpyrazine n-heptylamine hexamethylenediamine indoline 2-acetylpyridine 2,4,6-collidine N-ethylaniline (mono) N-ethylaniline (mono) N,N-dimethylaniline NJV-dimethylaniline 2-ethylaniline 2-ethylaniline 2,4-dimethylaniline 2,4-dimethylaniline 3-chloroaniline 2-chloroaniline 2-chloroaniline N,N-dimethylcyclohexylamine quinoline diisobutylamine
mol wt 31 45 45 45 59 59 59 60 61 68 69 73 73 73 73 73 73 79 80 80 81 82
87 87
150 "C
reduced mobility, (cm2/(VS) NRCN 200 "C 250 "C other work
2.04 2.01 1.95
2.65 2.38 2.45 2.46 2.20 2.14 2.36 2.25 2.23 2.29 2.21 2.21 2.16 2.06 2.02 1.98
2.28 2.25 2.33 2.26 2.23 2.16 2.08 2.04 2.01
2.18
2.21
2.24
2.15 2.06 1.96
2.19 2.09 2.00
2.21 2.1 2.01 1.85
2.25 2.24 2.16 2.12
2.21
2.17 2.25 2.17 2.15 2.11
87
87 93 93 93 93 93 95 95 99 101 101 101 101 101
102 104 105 107 107 107 107 107 107 108 108 108 108 108 io8 115 116 119
1.95
2.19
2.2
2.13," 2.16b 2.34"
2.08
2.06 2.03 1.84 1.94 1.88
2.06 2.06 1.83 1.95 1.87
2.08 2.08 1.84 1.95 1.87
1.95 1.96 1.86 1.92 1.95 1.83
1.94 1.94 1.91 1.91 1.95 1.83 1.95 1.78
1.95 1.86
1.96 1.87 1.82
1.93
1.72 1.94 1.94 1.93 1.94 1.95 1.84 1.8
2.12" 2.05" 1.97" 2.03a 2.19," 2.16' 2.07d 2.22d 1.95d
1.82"
1.95" 2.0d 1.89' 2.03d 2.04: 2.00' 2.03f
1.95," 2.0Oe 1.88," 1.95O 1.91," 1.93e 1.70," 1.95e
1.95," 1.91' 1.78" 1.91: 2.0P 1.91; 1.94e
1.96 1.88
1.89'
1.61
1.59"
1.77
121
1.81 1.74 1.72
127
1.87 1.78
127 127 129 129
1.85 1.67
1.92; 1.91d 1.91 1.82
1.79 1.84 1.81
1.87 1.74 1.84 1.72
1.84 1.86 1.79 1.91 1.73 1.82
1.66
1.91 1.81 1.8
2.21 2.16 2.06 2.02 1.98 2.03 2.21 2.07 2.22 1.95 2.19 2.09 2.00 1.85 1.95 2
1.93 2.07 2.07 2.03 2.06 2.06 1.83 1.95 1.87 1.92 1.72 1.94 1.94 1.91 1.91 1.95 1.83 1.95 1.78 1.95 1.92 1.96 1.87 1.82 1.87 1.90 1.89 1.61 1.77 1.92 1.91
1.84"
1.82
1.76,' 1.83d 1.68"
1.79 1.84 1.81 1.87 1.74 1.84 1.72 1.84 1.86 1.79 1.91 1.73 1.82 1.66
1.81 1.74 1.73 1.87 1.79 1.84 1.65
2.65 2.36 2.45 2.46 2.20 2.14 2.36 2.25 2.23 2.29 2.21
1.87' i.9d
1.91 1.83 1.82
121 127
2.400
2.08
1.76
2.53," 2.63* 2.39," 2.46b
2.23
1.93 2.07 2.07
121 121 121 121 121 121 121 121
2.41 2.39
"best value"
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989
667
Table I (Continued) reduced mobility, (cm2/(VS) name of compound di-n-butylamine isoquinoline n-octylamine quinoxaline cinnoline phthalazine 2-isopropylaniline benzyldimethylamine 4-tert-butylpyridine salicylamide n-nitroaniline o-nitroaniline hexamine 1-naphthylamine Tri-n-propylamine triethanolamine diethylaniline 2-phenylpyridine 4-phenylpyridine 2,2'-dipyridyl 2,4'-dipyridyl 2,3'-dipyridyl 4,4'-dipyridyl n-decylamine 4-amino-NJV-diethylaniline diphenylamine acridine dicyclohexylamine 2,4-dinitroaniline tri-n-butylamine di-n-hexylamine n-dodecylamine dibenzylamine bromoquinoline n-tetradecylamine n-hexadecylamine tri-n-hexylamine tribenzylamine tri-n-octylamine triisooctylamine didodecylamine tri-n-dodecylamine
mol w t
150 "C
NRCN 200 O C
250 O C
other work
"best value"
129 129 129 130 130 130 135 135 135 137 138 138 140 143 143 149 149 155 155 156 156 156 156 157 164 169 179 181 183 185 185 185 197 208 213 241 269 287 353 353 353 521
1.66
1.64
1.64
1.66' 1.89; 1.81f 1.5P 1.79; 1.811 1.82f 1.82f
1.64 1.85 1.51 1.80 1.82 1.82 1.66 1.71 1.70 1.81 1.78 1.78 1.68
1.51
.66
1.66
1.65
1.71
1.704
.8 .76 .73 1.71
1.74
1.81 1.78 1.78 1.68 1.74
1.81 1.77 1.79 1.66 1.70
1.74
1.6g 1.71 1.68
1.69 1.67
1.69 1.66
1.71
1.71
1.71
1.67c 1.66 1.66 1.7d 1.66 1.66 1.6d 1.35'
1.35 1.57 1.55
1.59 1.54
1.58 1.56 1.59
1.66 1.40
1.61 1.38
1.61 1.35
1.42
1.41
1.65
1.63
1.40 1.62
1.66 1.69 1.67 1.64 1.64 1.71
1.64 1.64 1.60 1.35 1.58 1.56 1.59 1.44 1.61 1.38 1.34 1.26 1.41 1.63 1.20
1.44' 1.42' 1.34O 1.26' 1.204 1.14
1.14 1.11
1.11' 1.204
0.89 0.96
0.88 0.93
0.85 0.91
0.69
0.66
0.64
1.20 0.88 0.93 0.89 0.66
0.89
OReference 6, 147 "C, N2. bReference 5, 207 "C, NP "Reference fReference 7, 230 OC, air.
9, 220 "C, NP. dReference 10, 200 O C , air. 'Reference 8, 220 OC, N2
(522 amu) protonated tri-n-dodecylamine could be measured with no difficulty. In order to demonstrate some of these trends, let us look, for example, at the compounds with a molecular weight of 73 amu: the primary butyl amines (normal, iso, tertiary, and secondary), the secondary amine (diethylamine), and the amide (N,N-dimethylformamide). We note that the amide has the highest mobility, then the secondary amine, and then the branched primary butylamines (secondary, iso, and tertiary) while the normal primary amine has the lowest mobility of this group. Similarly, the compounds with a molecular weight of 87 amu: tert-pentylamine and its normal isomer and morpholine and its isomer Nfl-dimethylacetamide. The normal amine has the lowest mobility, then the branched
other amines. Indeed, the quaternary amines examined in this work show reduced mobility values far above what would be expected from the mass-mobility correlation curve for an ion with the appropriate molecular formula. This its probably due to thermal dissociation of the compound in the heated IMS cell. The major product ion has a reduced mobility similar to that of the corresponding tertiary amine. In an IMS/MS study of quaternary ammonium hydroxides and halides this was readily verified. For example, two major ions
amine, and then the cyclic isobar, morpholine, and once more the amide has the highest mobility. The fact that amides are more mobile than their amine isobars could be due to the compactness of the carbonyl group relative to its isobaric ethylene moiety. Another difference could arise from the different protonation sites, nitrogen in the amines, and oxygen in the amides. Quaternary amines do not have a free electron pair on the nitrogen, so they are expected to differ fundamentally from
were formed from benzyltrimethylammonium hydroxide: one at 60 amu with a reduced mobility similar to that of protonated trimethylamine and one at 136 amu due to protonated benzyldimethylamine
C~HECHZN(CH~);
+
OH-
%
CsH&H2N(CH3)2H*
+
CH30H
(3) (CH3)aNH'
4- C ~ H ~ C H Z O H
The quaternary amines studied are not listed as such in Table I, as the major ionic species produced in the IMS were identified as the protonated tertiary amine fragments. For example, protonated triethylamine (anion of 102 amu), listed in Table I at 101 amu, was measured in triethylamine (101
666
ANALYTICAL CHEMISTRY, VOL. 61, NO. 7, APRIL 1, 1989 1
0
X
F 1.5
F0 1.2 W
0
1I
3 g1.1 E
ai
;I
X
x ab
X
, {.l,
’ X
* ,
,
H ;H :,*
E
.
30
al
2.2 . L -
F
0
N
E,
2.2-
M F.0
DA
1
t
5 5
0
2.0-
0
E 1.8LO
50
60
70
80
M
90
MASS OF RADICAL,R, ( a m u ) Figure 3. Ratio of the reduced mobility of primary to secondary (dots) and primary to tertiary (X) amines as a function of the radical mass (R). The radicals shown are ethyl (E), propyl (P), ethanol (ET), butyl (BU), phenyl (PH), cyclohexyl (CH), hexyl (H), and benzyl (B).
amu) and in tetraethylammonium hydroxide (147 amu). In Figure 3 the reduced mobility ratio of primary ( P = RNH2)to secondary (S = R2NH),PIS, and to tertiary ( T = R,N), PIT, amines is plotted as a function of the radical mass, R. Both ratios increased with R. This is contrary to what one would expect from reduced mass considerations alone (the mass ratios PIS and PIT should decrease as R increases), indicating again the importance of the collisional cross section term. This shows that the shape of the larger radicals affects the mobility of secondary and tertiary amines more than simple mass considerations would indicate. In addition, it can be seen from Figure 3 that the branched isomers have a higher mobility than the normal ones and that the aromatic amines have high mobilities relative to their mass. Compounds with heteroatom substituents, like chlorine, bromine, or a nitro group, have higher reduced mobilities than expected from the mass-mobility correlation curve. This is obviously due to the higher mass density in these compounds, namely, the small size of the heteroatoms relative to their mass, compared with that of hydrocarbons. 3. Temperature Dependence of Reduced Mobility. The reduced mobility of the amines was measured a t three different temperatures: 150, 200, and 250 O C . All values were normalized relative to the reduced mobility of 2,4-lutidine, taken as 1.95 cm2/(V s), and assumed to be constant in this temperature range. The trends of the temperature dependence of the reduced mobility values are interesting. The reduced mobility of the low molecular weight compounds increases with increasing temperature, by up to 2% for each 50 “C. The reduced mobility of the intermediate molecular weight compounds (90-180 amu) depends only slightly on the temperature and may increase or even decrease as the temperature is raised. The heavier compounds show a marked decrease in their reduced mobility as the temperature is increased. The behavior of low and intermediate molecular weight compounds (the reported values are for ions with less than 130 amu) was rationalized by Karasek et al. (6) in terms of the ion mass, shape, and polarity. Lubman (9) found no polarity effect; however, close examination of his results shows that the mobility of the ions from the heavier compounds, like fluorene (166 amu) and 1-bromo-2-chlorobenzene(191.5 amu), actually have a negative dependence on temperature. In a comprehensive treatise on the temperature dependence of ion mobilities, Parent and Bowers (19) examined the theories and experimental data on mobility measurements. Their conclusion was, that even for relatively simple ions (with some exceptions, like alkali ionIrare gas pairs), none of the theories can satisfactorily be used to rationalize the data. The results presented here may help in solving this problem, especially the heretofore unreported negative temperature dependence of the heavier ions. This matter is currently being further investigated in this laboratory.
T
“.o
OF
DA
0
&
D
l.L
D
1.2
T
I
0
I
100
I
I
200
I
9 300
ION MASS (amu) Figure 4. Reduced mobility, in air at 200 O C , of the protonated monomers and dimers of mono- (M), di- (D), and triethanolamlne (T), and formamide (F), N,Ndimethylformamlde (DF), and N,Ndimethylacetamide (DA), as a function of the ion mass.
4. Protonated Dimers. In some of the compounds, especially the ones containing oxygen (the amides and ethanolamines), an additional peak was observed in the mobility spectrum. This was assumed to be the protonated dimer, both on the basis of its drift time, which corresponds roughly to that expected from an ion of mass (2M + l)+, and because its intensity decreased with time (after sample injection) in the manner expected of a dimeric species. Subsequent IMS/MS studies confirmed this identification. If one assumes that the site of protonation in amides is on the oxygen (15), the proton-bridged conformation would probably be favored by the dimeric species. Similar results were found for other dimeric carbonyl ions like formaldehyde and formyl fluoride (20). In Figure 4 the reduced mobilities of the protonated monomers and dimers of mono-, di-, and triethanolamines and the amides (formamide, N,N-dimethylformamide, and N,Ndimethylacetamide), are plotted as a function of the ion mass. The protonated dimer of monoethanolamine and the monomer of triethanolamine have similar reduced mobilities, although the latter has a considerably higher mass (150 vs 123 amu). This is again evidence for the higher mobility of the more compact tertiary amines than that of the bulkier protonbridged dimer. 5. Precision of Reduced Mobility Values. As the “true” value of the reduced mobility is not really known, the precision rather than the accuracy may serve as a means of evaluating the experimental results. The relative standard deviation (RSD) which expresses the precision, is estimated to be better that 1.0%. This is after taking into consideration the width of the mobility peak a t half-height (less than 0.25 ms), its shape (symmetric), its position, and the reproducibility. While low mass ions with short drift times (about 10 ms) have sharper peaks, which could lead to an error of about 0.05 ms in their exact location, the larger ions with longer drift times are slightly broader, so that the relative error is similar. The symmetric peak shape allows for locating the position of the maximum to within one pixel (1/600 of the screen). The position of the peak on the IMS time scale depends to some extent on the convention used to determine the “zeron time of the gate opening pulse. Thus, while some workers use the center of this pulse as ”zero drift time”, others may refer to the initial rise or to the end of the gate pulse. In the present work the onset of the gate pulse was used as “zeron. Finally, the reproducibility of the reduced mobility values relative to the reference compound, lutidine, was better than 0.5%. Thus, if one considers that as the anchor point for the
Anal. Chem. 1989, 61,689-694
absolute reduced mobility scale, the values reported are within 1% of the given value. Agreement with available data is good in most cases, which indicates that this estimation of the error is reasonable. ACKNOWLEDGMENT I thank M. J. Cohen, R. F. Wernlund, and R. M. Stimac of PCP, Inc., for their help, and Z. Rappoport from Jerusalem for his cooperation. Registry No. Methylamine, 74-89-5; ethylamine, 75-04-7; formamide, 75-12-7; dimethylamine, 124-40-3; n-propylamine, 107-10-8; trimethylamine, 75-50-3; diaminoethane, 107-15-3; ethanolamine, 141-43-5; imidazole, 288-32-4; isopropylamine, 75-31-0;NJV-dimethylformamide, 68-12-2;diethylamine, 109-89-7; sec-butylamine, 13952-84-6;isobutylamine, 78-81-9; tert-butylamine, 75-64-9;pyridine, 110-86-1;pyridazine, 289-80-5; pyrazine, 290-37-9; 1,2,4-triazole,288-88-0; butylamine, 109-73-9;triazine, 290-87-9;N-methylimidazol, 616-47-7;N,N-dimethylacetamide, 127-19-5; morpholine, 110-91-8; n-pentylamine, 110-58-7; tertpentylamine, 594-39-8; 3-picoline, 108-99-6; aniline, 62-53-3; 4picoline, 108-89-4;2-picoline, 109-06-8;4-hydroxypyridine, 62664-2; 3-hydroxypyridine, 109-00-2; cyclohexylamine, 108-91-8; triethylamine, 121-44-8; dipropylamine, 142-84-7; diisopropylamine, 108-18-9; hexylamine, 111-26-2; (dimethylamino)-1119064propane, 926-63-6; N,N-dimethyl-1,2-propanediamine, 99-2; 3-cyanopyridine, 100-54-9;diethanolamine, 111-42-2;2,4lutidine, 108-47-4; 2-toluidine, 95-53-4; benzylamine, 100-46-9; N-methylaniline, 100-61-8;3-toluidine, 108-44-1;1,2-phenylenediamine, 95-54-5; 1,3-phenylenediamine, 108-45-2; 1,4phenylenediamine, 106-50-3; 2,6-dimethyipyrazine, 108-50-9; 2,5-dimethylpyrazine, 123-32-0;2,3-dimethylpyrazine, 5910-89-4; heptylamine, 111-68-2; hexamethylene diamine, 124-09-4;indoline, 496-15-1; 2-acetylpyridine, 1122-62-9; 2,4,6-collidine, 108-75-8; N-ethylaniline, 103-69-5; N,N-dimethylaniline, 121-69-7; 2ethylaniline, 578-54-1; 2,4-&methylaniline,95-68-1;3-chloroaniline, 108-42-9;2-chloroaniline, 95-51-2;N&-dimethylcyclohexylamine, 98-94-2; quinoline, 91-22-5; diisobutylamine, 110-96-3; dibutylamine, 111-92-2; isoquinoline, 119-65-3; octylamine, 111-86-4; quinoxaline, 91-19-0; cinnoline, 253-66-7; phthalazine, 253-52-1; 2-isopropylaniline, 643-28-7; benzyldimethylamine, 103-83-3; 4-tert-butylpyridine, 3978-81-2; salicylamide, 65-45-2; m-nitro-
689
aniline, 99-09-2; o-nitroaniline, 88-74-4; hexamine, 131-73-7; 1naphthylamine, 134-32-7;tripropylamine, 102-69-2;triethanolamine, 102-71-6; diethylaniline, 91-66-7; 2-phenylpyridine, 1008-89-5; 4-phenylpyridine, 939-23-1; 2,2’-dipyridyl,366-18-7; 2,4’-dipyridyl, 581-47-5; 2,3’-dipyridyl, 581-50-0; 4,4‘-dipyridyl, 553-26-4; decylamine, 2016-57-1; 4-amino-N,N-diethylaniline, 93-05-0; diphenylamine, 122-39-4; acridine, 260-94-6; dicyclohexylamine, 101-83-7;2,4-dinitroaniline,97-02-9; tributylamine, 102-82-9; dihexylamine, 143-16-8; dodecylamine, 124-22-1; dibenzylamine, 103-49-1; bromoquinoline, 119007-62-4; tetradecylamine, 2016-42-4;hexadecylamine, 143-27-1;trihexylamine, 102-86-3; tribenzylamine, 620-40-6; trioctylamine, 1116-76-3; triisooctylamine, 25549-16-0; didodecylamine, 3007-31-6; tridodecylamine, 102-87-4. LITERATURE C I T E D Karpas, 2.; Cohen, M. J.: Stimac, R. M.; Wernlund, R. F. Int. J . Mass Spectrom. Ion Processes 1988, 7 4 , 153. Karpas, 2.; Stirnac, R. M.; Rappoport, 2. Int. J. Mass Spectrom. Ion Processes 1988. 83, 163. Lenga, R. E. The Skma -AMrlch Library of Chemlcal Safety Data ; Sigma-Aldrich Corp.: Milwaukee. WI, 1985. Shumate, C.: St. Louis, R. H.; Hill, H. H., Jr. J. Chromatogr. 1088. 373, 141. Karasek, F. W.; Kim, S. H.; Rokushika, S. Anal. Chem. 1978, 50,
152. Karasek, F. W.; Kim, S. H.; Rokushika, S. Anal. Chem. 1978, 50,
2013.
Hagan, D. F. Plasma Chrometography: Carr, T . W.. Ed.: Plenum Press: New York, 1984; Chapter 4. Lubman, D. M.; Kronlck, M. N. Anal. Chem. 1083, 55, 867. Lubman, D. M. Anal. Chem. 1084, 56, 1298. Kolaitls, L.; Lubman, D. M. Anal. Chem. 1088, 58, 1993. Kolaitis, L.; Lubman, D. M. Anal. Chem. 1988, 58, 2137. Lawrence, A. H. Anal. Chem. 1088, 58, 1269. Lawrence, A. H. Forensic Scl. Int. 1087. 3 4 , 73. Lias, S. G.; Liebman. J. F.; Levln, R. D. J. Chem. Phys. Ref. Data 1984. 13, 695. Dewar, M. J. S.; Dieter, K. M. J. Am. Chem. SOC.1088, 708, 8075. Karpas, 2.; Berant, 2.; Stlmac, R. M. Sfruct. Chem., in press. Revercomb, H. E.: Mason, E. A. Anal. Chem. 1075. 4 7 , 970. Berant, 2.; Karpas, 2. J. Am. Chem. Soc., in press. Parent, D. C.; Bowers, M. T. Chem. Phys. 1981, 6 0 , 257. Hagler, A. T.; Karpas, 2.; Klein, F. S. J. Am. Chem. SOC.1979, 701,
2191.
RECEIVED for review February 2, 1988. Accepted December 7, 1988.
Photodissociation of Laser-Desorbed Ions as a Structure Determination Tool Lydia M. N u w a y s i r and C h a r l e s L. Wilkins* Department of Chemistry, University of California, Riverside, Riverside, California 92521
Laser desorptlon Fourler transform mass spectrometry (LD/ FTMS) of porphyrlns, metalloporphyrlns,and alkalolds Is used to lnvestlgate XeCl exclmer laser photodlssoclatlon of trapped Ions as an alternatlve to collision-induced dlssoclatlon for structure analysis purposes. I t Is shown that the presence of an approprlate metal enhances photodissoclatlon and that In stlu metal attachment durlng the laser desorption event may be a useful analytical strategy. Iron-, manganese-, and chromium-attached specles are examined In order to assess the effect of metal upon the propenslty for photodissoclatlon with 308-nm excitatlon.
Laser desorption has been used extensively in mass spectrometry for desorption and ionization of nonvolatile samples (1-5). I t is a relatively soft ionization technique and usually
yields high abundances of molecular ions or cation-attached molecular ions, allowing facile determinations of molecular weight. Laser desorption also results in fragment ions, depending upon the nature of the sample (6,7).However, more fragmentation often is required in order to allow inference of molecular structure. I t is therefore desirable, if laser desorption is used, to have available a means of readily dissociating ionic species to provide requisite structural information. Collision induced dissociation (CID) is the means most commonly employed for this purpose in both mass spectrometry/mass spectrometry (MS/MS) applications and for laser-desorbed species (8-1 1). Most such applications involve low mass ions ( m / z
