Mass Accuracy and Mass
Resolution in TOF MS
October 2011
What will we discuss?
• What is mass resolution and mass accuracy
• Why and when they are important
• How to use resolution and mass accuracy for practical
analysis
Mass resolution
What is it?
Definition:
Mass resolution = (ion mass)/(mass peak width)
R = (m/z) / W1/2
Example:
W1/2 = Peak width
at half height
W1/2 = 0.5 Da.
R = 200 Da./0.5 Da.
R = 400
m/z = 200 Da.
Mass resolution
Older definition – peaks of equal heights
Definition:
Mass resolution = (ion mass)/(mass peak separation)
R = (m/z) / Dm
Dm ~ 2 W1/2
~ 1.0 Da.
From previous example:
R = 200 Da. / 1.0 Da.
R10% = 200
RW1/2 = 400
Valley Height
~10% of peak height
m/z = 200 Da.
Resolving power & mass accuracy
m/z = 613.964203
TOF, Q-TOF
SQ, TQ, IT
613.9642
613.9642
R = 614/0.68 = 903
R = 614/0.0423 = 14,522
W1/2 = 0.68 Da.
W1/2 = 0.0423 Da.
1 Da.
1 Da.
Why is resolution important?
Complex sample or matrix
Crude Oil
FTMS
334.3
100
320.2
348.3
306.2
294.2
362.3
80
376.3
280.2
390.3
274.2
60
404.3
250.2 264.2
256.1
416.3
40
432.4
236.1
446.4
460.4
20
474.4
222.1
488.4
208.1
502.4
228.2
518.5
528.5
0
200
250
300
350
400
Mass/Charge
6
450
500
550
600
Why is resolution important?
Complex sample or matrix – take a closer look
m/z 200 - 600 m/z = 250 - 325
Crude Oil
FTMS
334.3
100
320.2
348.3
306.2
294.2
362.3
80
376.3
280.2
390.3
274.2
60
404.3
250.2 264.2
256.1
416.3
40
432.4
236.1
446.4
460.4
20
474.4
222.1
488.4
208.1
502.4
228.2
518.5
528.5
0
200
250
300
350
400
Mass/Charge
7
450
500
550
600
Why is resolution important?
Complex sample or matrix – take a closer look
5X zoom  m/z 255 - 335
m/z = 290 - 300
Crude Oil
FTMS
100
320.2372
322.2528
306.2217
80
330.2216
316.2061
294.2217
332.2372
302.1904
280.2060
288.1748
290.1904
318.2217
296.2372
282.2216
274.1591278.1904
60
264.1747
268.2060
276.1748
304.2062
298.2529
310.2529
312.2685
324.2684
326.2842
284.2372
328.2061
270.1313
260.1434
262.1592
40
20
0
255
8
260
265
270
275
280
285
290
295
Mass/Charge
300
305
310
315
320
325
330
335
Why is resolution important?
Complex sample or matrix – take a closer look
40X zoom  m/z 290 - 300
Resolution  5000
Crude Oil
FTMS
100
295.3
80
292.2
293.2
290.2
294.3
297.3
291.2
299.3
60
296.3
298.3
40
20
0
290
292
294
296
Mass/Charge
9
298
300
Why is resolution important?
Complex sample or matrix – take a closer look
40X zoom  m/z 290 - 300
Resolution  237,000
Crude Oil
FTMS
m/z = 296.14 - 296.24
100
294.2219
292.1157
80
296.2374
290.1906
297.2249
60
298.2531
299.2406
295.2092
293.1934
40
291.1780
20
0
290
10
292
294
Mass/Charge
296
298
300
Why is resolution important?
Complex sample or matrix – take a closer look
4000X zoom  m/z 295.14 - 295.24
Resolution  237,000
Crude Oil
FTMS
65
296.2372
60
55
∆= 0.003 Da. = 3mDa
50
45
C19H22NS+ = 296.1468
40
296.1468
35
C21H30N+ = 296.2373
30
25
20
15
10
C22H18N+ = 296.1434
296.1438
296.2125
5
296.2011
296.2101
0
296.14
296.16
296.18
296.20
Mass/Charge
11
296.22
296.24
How much resolution is required?
• The resolution required to solve a given analytical problem
depends on the sample and matrix
• Sample preparation reduces potential chemical
interferences and therefore the resolution requirements
• Most applications require a resolution of 10,000
• For some sample/matrix combinations however, there may
never be enough resolution
How the mass determined - mass centroid
Definition: – “center of mass” of the peak = 1st moment
Aii
mi
Mass centroid = m =
mi Ai
Ai
Mass centroid
Why centroid and not apex?
m
Small changes in amplitude can change where the apex is located
Mass centroid
Why centroid and not apex?
m m’
Variation in mass measurement equals the mass spacing of data points
Centroid mass accuracy is greater than a single point measurement
Exact mass and accurate mass
• Accurate mass is the experimentally measured mass value
• Exact mass is the calculated mass based on adding up the
masses of each atom in the molecule
• Atomic mass of each element is determined relative to
Carbon having a mass of exactly 12.0000
• Mass defect is the difference between the mass of the
individual components of the nucleus alone, and the mass
of the bound nucleus
Mass error
• Mass error = (exact mass) – (accurate mass)
• Mass error in parts per million (ppm) =
(mass error)
(exact mass)
6
X 10
Accurate mass makes mass defect important
Element
Hydrogen
Symbol
H
2
D or H
Carbon
Oxygen
12
12
98.91
0
13
13.0034
1.1
0.0034
14
14
14.0031
99.6
0.0031
15
N
15
15.0001
0.4
0.0001
16
O
16
15.9949
99.76
-0.0051
17
17
16.9991
0.04
-0.0009
18
18
19
17.9992
18.9984
0.2
100
-0.0008
-0.0016
28
28
27.9769
92.2
-0.0231
29
29
28.9765
4.7
-0.0235
30
30
31
29.9738
30.9738
3.1
100
-0.0262
-0.0262
32
32
31.9721
95.02
-0.0279
33
33
32.9715
0.76
-0.0285
34
S
34
33.9679
4.22
-0.0321
35
Cl
35
34.9689
75.77
-0.0311
37
Cl
37
36.9659
24.23
-0.0341
79
Br
79
78.9183
50.5
-0.0817
81
81
127
80.9163
126.9045
49.5
100
-0.0837
-0.0955
13
C
N
Silicon
O
F
Si
Si
Phosphorus
Sulfur
Si
P
S
S
Chlorine
Bromine
Iodine
Br
I
2
Mass
Defect
0.0078
0.01
O
Fluorine
1
Exact
Mass
Abundance
1.0078
99.99
2.0141
12
C
Nitrogen
Integer
Mass
0.0141
Why is mass defect important?
Many defects are large enough to be measurable by TOF MS
Element
Hydrogen
Symbol
H
2
D or H
Carbon
Oxygen
Silicon
2.0141
0.01
0.0141
12
12
98.91
0
13
C
13
13.0034
1.1
0.0034
14
14
14.0031
99.6
0.0031
15
N
15
15.0001
0.4
0.0001
16
O
16
15.9949
99.76
-0.0051
17
17
16.9991
0.04
-0.0009
18
18
19
17.9992
18.9984
0.2
100
-0.0008
-0.0016
28
28
27.9769
92.2
-0.0231
29
29
28.9765
4.7
-0.0235
30
30
31
29.9738
30.9738
3.1
100
-0.0262
-0.0262
32
32
31.9721
95.02
-0.0279
33
33
32.9715
0.76
-0.0285
34
S
34
33.9679
4.22
-0.0321
35
Cl
35
34.9689
75.77
-0.0311
37
37
36.9659
24.23
-0.0341
N
O
F
Si
Si
Phosphorus
Sulfur
Si
P
S
S
Chlorine
Mass
Defect
0.0078
2
O
Fluorine
1
Exact
Mass
Abundance
1.0078
99.99
12
C
Nitrogen
Integer
Mass
Cl
One 1 H
+ 13 ppm
One 16 O
- 8 ppm
One 35 Cl
- 51 ppm
TOF capable of < 5 ppm measurement routinely
Many possible formulas with an MSD or IT
But only a few with TOF
Possible
formulas
for272
m/z =
Possible
Number ofchemical
Chemical Formulas
at m/z
C10F8 = 271.98667
# Possible Chemical Formulas
possible formulas
No. of
10000
QUADRUPOLE
mass
uncertainty
1000
100
10
TOF
1
0.1
1
10
100
mass
uncertainty, ppm
Mass
uncertainty
- ppm
# of Possible
ppm
amu
Formulas
1000
0.3
7657
368
0.1
4050
100
0.03
1223
37
0.01
466
10 0.003
120
4 0.001
43
1 0.0003
11
0.4 0.0001
5
0.1 0.00003
2
1000
Formulas made of:
C,H,N,O,F, & Cl
Isotopic abundances can further reduce the number of possible formulas
How is accurate mass used?
Extracted ion chromatograms - EIC
Chromatogram – EIC m1
Each point of EIC represents
the sum of ions in mass peak
within a specified mass range
m1-extracteded
Extraction window
m2
Mass peak
m/z
Extraction window
Wide extraction window measures more ions = more sensitivity
Chromatogram – EIC m1
m1-extracted
Extraction window
m2
Mass peak
m/z
Good results if m1 is well separated from m2 and comparable abundance
Why is resolution important?
Ions from m2 are added to m1 EIC thus shifting the m1 centroid
Chromatogram – EIC m1
What if m2 is not separated from
m1 or is in much larger abundance
at the same time?
m1-measured
m1-exact
m2
Extraction window
m1
m/z
Shifted centroid changes measured ion abundance and mass accuracy
Why is resolution important?
Wide extraction window will add in ions from adjacent mass
Chromatogram – EIC m1
What if m2 is not separated from
m1 or is in much larger abundance
at different times and no m1 is
present?
m2
m1-extracted
Extraction window
m/z
Ions from m2 are added into m1 EIC thus displaying a false m1 response
Mass centroid
Relative intensity affects the result
Profile view of mass peaks
Centroid
m/z
Centroid
m/z
m/z
Centroid
Centroid
m/z
Centroid view of mass peaks obscures this fact
How much resolution is enough?
Relative intensity affects the result
Flurenol methyl ester
m/z = 240.0781
Mass error = 1.7 ppm
240.1218
240.0785
Dimetilan
m/z = 240.1217
Mass error = 0.4 ppm
Dm = 0.043 Da.
~13,500 resolution FWHM
How much resolution is enough?
Intensity Ratio = 10:1
A resolution of 13,500 is not
enough - larger ratios require
more mass resolution, MS/MS
or better GC separation
240.0780
Flurenol methyl ester
m/z = 240.0781
Mass error = -0.4 ppm
Dimetilan
m/z = 240.1217
Mass error = -13.7 ppm
240.1184
Crude oil target analysis – target analysis
Problem: identify and quantify dibenzothiophene
Resolution 12,000
TIC
EIC 184.0341
Mass extraction window:
+/- 0.5 Da.
= +/- 2717 ppm
dibenzothiophene
Interferences
Crude oil target analysis – target analysis
Problem: identify and quantify dibenzothiophene
Resolution 12,000
TIC
EIC 184.0341
Mass extraction window:
+/- 0.00092 Da.
= +/- 5 ppm
dibenzothiophene
Crude oil target analysis
Problem: identify and quantify dibenzothiophene
Resolution 12,000
TIC
EIC 184.0341
Mass extraction window:
+/- 5 ppm
dibenzothiophene
Summary
• Mass accuracy (MA) is a valuable tool for target
quantification and confirmation
• High resolution (HR) ensures mass accuracy over sample
range – good test for adequate mass resolution
• How much resolution is required depends on sample,
matrix, and their relative abundance
• TOF mass accuracy & high resolution is an important tool
for solving complex analytical problems
Thank you for your attention