A P P L I C AT I O N N O T E
Gas Chromatography/
Mass Spectrometry
Author:
Dr. Adam Patkin, Ph.D.
PerkinElmer, Inc.
Shelton, CT
Cold EI GC/MS Enhancement
of High MW Hydrocarbon
Molecular Weight Information
Introduction
Electron Ionization Gas Chromatography/
Mass Spectrometry (EI GC/MS) is a
powerful and information-rich technique
for qualitative characterization and
quantitative analysis of the compounds in a mixture. One of its most valuable
functions is to provide the molecular weight of a compound. It is the single most
important peak in the mass spectrum, since it is most characteristic of the target
compound. Having a strong molecular ion in the spectrum helps the analyst reject
many false-positive library search matches. However, many compounds, including
long-chain and highly-branched hydrocarbons, do not yield a stable molecular ion
under EI conditions and it is small or even absent from their spectra.
This makes analyte identification more difficult, especially at low concentrations or
in complex matrices. Without the molecular ion, identification depends much more
heavily upon matching unknown compounds to the chromatographic retention time
of calibration standards.
Cold Electron Ionization GC/MS (Cold EI GC/MS) is an
enhancement of traditional EI available as an option on the
PerkinElmer AxION® iQT™ GC/MS/MS mass spectrometer.
Cold EI enhances the relative molecular ion abundance of
most compounds compared with conventional EI, while
retaining the EI fragmentation pattern for spectral library
searching. Its operation is described in the Discussion section.
Experimental and Results
Figure 1a shows the EI spectrum of n-decane (C10H22) found in
the NIST mass spectral library database¹, a commonly used tool
to match GC/MS spectra for compound identification. Note the
abundance of the molecular ion, indicated by the arrow. Figure
1b shows the Cold EI mass spectrum for the same compound,
and demonstrates significantly enhanced molecular ion intensity
relative to the other ions in the spectrum.
Figure 1a - n-Decane - EI (NIST)
Figure 1a - n-Decane - EI (NIST)
57
100
n-Decane - EI (NIST)
57
100
50
71
85
56
50
71
70
84
0
0
51
53
50
51
69
56 58
65 67
60
53
70
70
77 79
80
83
90
84
100
65 67
60
77 79
83
120
80
90
100
130
140
142
113
86
Figure 1b - n-Decane - Cold EI
70
n-Decane - Cold El
110
99
72
142
113
86
69
58
50
99
85
72
110
120
130
140
Figure 1b - n-Decane - Cold EI
71
100
85
57
71
100
85
142
57
84
98
142
50
70
113
84
98
50
56
55
0
53
56
50
55
113
70
69
72
58
59 61 63 65 67
60
73 75 77 79
70
69
53
67
73
61 63 65
75 77 79
Figure 1 a, b.0n-Decane (C10H2259
) EI
and Cold EI Spectra.
50
2
60
70
86
87 89 91 93 95 97
80
72
58
83
82
80
90
83
82
105 108
100
110
118 121 124 127 130
120
130
135 138
140
86
87 89 91 93 95 97
90
105 108
100
110
118 121 124 127 130
120
130
135 138
140
Figure 2a shows the increasing advantage of Cold EI as the
hydrocarbon molecular weight increases. Here the NIST spectrum
of n-dotriacontane (C32H66) demonstrates that the relative intensity
of a molecular ion in a long chain hydrocarbon can be quite small,
while Figure 2b shows
strong enhancement to this molecular ion
Figure 2a - n-Dotriacontane
- EIthe (NIST)
100
by Cold EI. The molecular weight of the compound is unambiguous.
Figure 2a - n-Dotriacontane - EI (NIST)
57
n-Dotriacontane - EI
100
57
71
85
50
71
85
50
99
113
0
0
60
80
60
80
100
99
127
141 155
169 183 197 211
225 239 253 267 281 295 309 323 337 351
365 379 393 407
421
120
140
160
180
113
127
141 155
169 183
200
220
240
260
280
300
320
340
360
380
120
140
160
180
420
197 211 225 239 253 267 281 295
309 323 337 351 365 379 393
407 421
Figure 2b - n-Dotriacontane - Cold EI
100
400
200
220
240
260
280
300
320
340
360
380
400
420
450
440
450
440
n-Dotriacontane - Cold EI
Figure 2b - n-Dotriacontane - Cold EI
450
100
450
100
50
50
57
0
71
99
113
85
60 71 80
57
85
197 211 225 239
127 141
253
352 366 380 394
155 169 183
266 280 295 309 322 337
408
422
99
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
113
197 211 225 239
127 141
253
352 366 380 394
155 169 183
266 280 295 309 322 337
408
422
) EI and Cold EI Spectra.
440
Figure 2 a, b. n-Dotriacontane (C32H66
0
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
3
Hydrocarbon mixtures such as crude oil may contain high
molecular weight compounds which may not be easily analyzed
by conventional GC/MS. Figure 3a displays the NIST spectrum of
n-tetrapentacontane (C54H110) with a small molecular ion, while
Figure 3b shows the much larger Cold EI-enhanced molecular ion.
57
100
n-Tetrapentacontane
- EI (NIST)
100
57 71
71
85
85
50
50
99
113
99
141
183
113
141
0
80
0
120
80
120
211
160 183200
211
160
200
239 267
295
323 351 379
421 446 474
504 531
560 588
240
280
320
360
400
440
480
520
239 267
295 323 351 379
421 446 474 504 531
560
240
560
280
320
360
400
440
480
520
616 644 672 700
600
560 588
640
680
729
720
616 644 672 700
600
640
680
729
720
758
760
758
760
n-Tetrapentacontane - Cold EI
758
100
758
100
50
50
71
57
97
71
0
0
113
155 183 211
239 266 294 322
364 392 420 447
489
80
120
113
160
200
240
280
320
155 183 211
239 266 294 322
360
400
440
364 392 420 447
480
489
520
560
600
532 559 588
80
120
360
480
520
57
97
160
200
240
280
Figure 3 a, b. n-Tetrapentacontane (C54H110) EI and Cold EI Spectra.
4
320
400
440
532 559
560
588
600
701
631
640
631
640
728
769
680 701720
728
760
680
760
720
769
easy. Its spectrum is not found in the NIST, Wiley², or ChemSpider³
Not available
in
mass spectral libraries, indicating the difficulty in obtaining
th
dition
NIS T 2014 or W iley
10 EI E
conventional
GC/MS
spectra ofor
such a low-volatility compound.
C hemS pider mas s s pectral databas es
Finally, Figure 4b shows the Cold EI spectrum of n-heptacontane
(C70H142). Although lower in relative intensity, it still shows a very
strong molecular ion enhancement, making compound identification
n-Heptacontane – EI (NIST)
Not available in
NIST 2014 or Wiley 10th Edition or
ChemSpider mass spectral databases
n-Heptacontane – Cold EI
71
100
57
97
50
982
0
50
100
150
200
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
Figure 4 a ,b. n-Heptacontane (C70H142) Cold EI Spectrum.
5
It is not only linear high molecular weight hydrocarbons which may
lack a significant molecular ion peak. The molecular ion abundance
for branched compounds drops even faster with molecular weight
then for linear compounds. Squalane (C30H62) is a triterpene
compound often used in cosmetics as a moisturizer and emollient
100
because of its low cost and resistance to oxidation. However, the
high degree of branching leads to an unstable molecular ion, with a
very low relative intensity in EI (0.2%) as shown in Figure 5a, but a
clearly identifiable Cold EI (20%) molecular ion, as shown in Figure 5b.
57
Squalane - EI (NIST)
100
57
71
71
50
85
113
99
50
141 155
113
0
0
239
169
197
99
60
80
60
80
100
100
0.2%
183
127
85
183
127
120
140
160
180
141 155
169
120
140
160
180
200
197
200
267
211 225
220
253
240
239
260
280
300
320
267
211 225
220
281 294 308 322
253
240
281 294 308 322
260
280
300
320
337
350
340
337
0.2%
365 378 392 406 420
360
350
340
380
400
420
365 378 392 406 420
360
380
400
420
Squalane - Cold EI
100
238
100
238
20%
50
50
113
0
20%
182
57
71
60
57
85
80
71
85
127 141 155
99
80
197 211
225
253
182
100
99
120
140
160
180
127 141 155
113
169
Figure 5 a, b. 0Squalane (C30H62) EI and Cold EI Spectra.
60
169
100
120
140
160
180
200
220
240
197 211
225
200
220
240
274
260
253
308
266
280
350
320
308
266
260
336
295
300
274
280
340
380
360
380
408
400
422
420
336
295
300
422
350
320
340
380
360
380
408
400
420
Discussion
Cold EI operates by expanding the GC effluent with a makeup gas
through a small nozzle. The associated supersonic expansion and
molecular beam formation lowers the vibrational and rotational
energies of the molecules in the GC stream, “cooling” them from
GC transferline temperature (up to 350 ºC or 623 K) to about 20 K
(-253 ºC). Excess carrier gas is skimmed off. The cooled analyte
molecules enter the ionization source in a supersonic molecular
beam where they are ionized by electrons, similar to a traditional
electron ionization source. However, because of the molecule’s
very low internal energy, the internal energies of the resulting
6
Cold EI ions are far lower than with conventional EI. Because of
the “cold” nature of these ions, fragmentation is substantially
reduced and molecular intensity enhanced for many analytes.
Generally, the larger the molecule, the greater the relative Cold EI
molecular ion enhancement.
The PerkinElmer Cold EI source also utilizes a “fly through”
design, to eliminate collisions between gas-phase molecules and
ions with the ion source walls, which might lead to additional
fragmentation or chromatographic peak tailing.
Figure 64 shows that, when analyzed by conventional EI, both the
relative and absolute molecular ion abundance of n-alkanes is
reduced by about 20% for each additional carbon in the chain.
However, in Cold EI the absolute molecular ion abundance is
roughly unchanged.
The absolute ion abundance enhancement thus exponentially
increases with carbon number as shown in Figure 74, up to a
factor of 2500 for C40H82.
To summarize, Cold EI substantially increases the molecular ion
intensity for both linear and branched hydrocarbons. As the
carbon number increases, the Cold EI enhancement effect is
exponentially larger. These features of Cold EI show that it is wellsuited to applications requiring reliable and routine characterization
of hydrocarbons and their isomers in complex mixtures.
References
1.NIST/EPA/NIH Mass Spectral Library, NIST Standard Reference
Database 1A, 2014.
Figure 6. Molecular Ion Abundance in EI.
2.Wiley Registry of Mass Spectral Data, 10th Edition, 2013.
3.http://www.chemspider.com.
4.Aviv Amirav, Uri Keshet, Tal Alon, Alexander B. Fialkov;
http://blog.avivanalytical.com/2012/09/how-much-ismolecular-ion-enhanced-in.html.
Figure 7. Cold EI Absolute Abundance Enhancement.
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