Mass Accuracy and Mass Resolution in TOF MS

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

W1/2 = Peak width

at half height

W1/2 = 0.5 Da.

Mass resolution What is it?

Definition: Mass resolution = (ion mass)/(mass peak width)

R = (m/z) / W1/2

m/z = 200 Da.

Example: R = 200 Da./0.5 Da.

R = 400

Mass resolution Older definition – peaks of equal heights

Definition: Mass resolution = (ion mass)/(mass peak separation)

R = (m/z) / Dm

Dm ~ 2 W1/2

Valley Height

~10% of peak height

m/z = 200 Da.

From previous example: R = 200 Da. / 1.0 Da.

R10% = 200

~ 1.0 Da.

RW1/2 = 400

Resolving power & mass accuracy m/z = 613.964203

R = 614/0.68 = 903

W1/2 = 0.68 Da.

SQ, TQ, IT

1 Da.

613.9642

TOF, Q-TOF

R = 614/0.0423 = 14,522

1 Da.

W1/2 = 0.0423 Da.

613.9642

6

0

20

40

60

80

100

200 250 300 350 400 450 500 550 600

334.3

320.2 348.3

306.2 362.3 294.2

376.3 280.2

390.3

274.2

404.3 264.2 250.2

256.1

416.3

432.4 236.1

446.4

460.4

474.4 222.1

488.4 502.4

518.5 228.2 528.5 208.1

Why is resolution important? Complex sample or matrix

Mass/Charge

Crude Oil

FTMS

7

0

20

40

60

80

100

200 250 300 350 400 450 500 550 600

334.3

320.2 348.3

306.2 362.3 294.2

376.3 280.2

390.3

274.2

404.3 264.2 250.2

256.1

416.3

432.4 236.1

446.4

460.4

474.4 222.1

488.4 502.4

518.5 228.2 528.5 208.1

Mass/Charge

Why is resolution important? Complex sample or matrix – take a closer look

m/z = 250 - 325 m/z 200 - 600

Crude Oil

FTMS

8

0

20

40

60

80

100

255 260 265 270 275 280 285 290 295 300 305 310 315 320 325 330 335

320.2372

322.2528 330.2216 306.2217

316.2061 294.2217

332.2372 302.1904

318.2217 280.2060 324.2684 288.1748

310.2529 296.2372 290.1904 304.2062 312.2685 282.2216 326.2842 278.1904 274.1591 298.2529

276.1748 284.2372 264.1747 268.2060 328.2061

270.1313

260.1434 262.1592

Mass/Charge

5X zoom m/z 255 - 335


m/z = 290 - 300 Crude Oil

FTMS

9

Mass/Charge

0

20

40

60

80

100

290 292 294 296 298 300

295.3

294.3 293.2 290.2 297.3

291.2 299.3

296.3 298.3

292.2


Crude Oil

FTMS

40X zoom m/z 290 - 300

Resolution 5000

10

0

20

40

60

80

100

290 292 294 296 298 300 Mass/Charge

294.2219 292.1157

296.2374 290.1906

297.2249 298.2531

299.2406 295.2092

293.1934

291.1780


Crude Oil

FTMS

40X zoom m/z 290 - 300

Resolution 237,000

m/z = 296.14 - 296.24

11

0

5

10

15

20

25

30

35

40

45

50

55

60

65

296.14 296.16 296.18 296.20 296.22 296.24

296.2372

296.1468

296.1438

296.2125

296.2011

C21H30N+ = 296.2373

C19H22NS+ = 296.1468

C22H18N+ = 296.1434

∆= 0.003 Da. = 3mDa

296.2101

Mass/Charge


4000X zoom m/z 295.14 - 295.24

Resolution 237,000

Crude Oil

FTMS

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

Ai

mi

Mass centroid = m = mi Ai

mi

mi

Ai

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) X 106

(exact mass)

Accurate mass makes mass defect important

Hydrogen H 1 1.0078 99.99 0.0078

D or 2H 2 2.0141 0.01 0.0141

Carbon12

C 12 12 98.91 013

C 13 13.0034 1.1 0.0034

Nitrogen14

N 14 14.0031 99.6 0.003115

N 15 15.0001 0.4 0.0001

Oxygen16

O 16 15.9949 99.76 -0.005117

O 17 16.9991 0.04 -0.000918

O 18 17.9992 0.2 -0.0008

Fluorine F 19 18.9984 100 -0.0016

Silicon28

Si 28 27.9769 92.2 -0.023129

Si 29 28.9765 4.7 -0.023530

Si 30 29.9738 3.1 -0.0262

Phosphorus P 31 30.9738 100 -0.0262

Sulfur32

S 32 31.9721 95.02 -0.027933

S 33 32.9715 0.76 -0.028534

S 34 33.9679 4.22 -0.0321

Chlorine35

Cl 35 34.9689 75.77 -0.031137

Cl 37 36.9659 24.23 -0.0341

Bromine79

Br 79 78.9183 50.5 -0.081781

Br 81 80.9163 49.5 -0.0837

Iodine I 127 126.9045 100 -0.0955

Mass

DefectElement Symbol Abundance

Integer

Mass

Exact

Mass

Why is mass defect important? Many defects are large enough to be measurable by TOF MS

One 16 O

- 8 ppm

One 35 Cl

- 51 ppm

One 1 H

+ 13 ppm Hydrogen H 1 1.0078 99.99 0.0078

D or 2H 2 2.0141 0.01 0.0141

Carbon12

C 12 12 98.91 013

C 13 13.0034 1.1 0.0034

Nitrogen14

N 14 14.0031 99.6 0.003115

N 15 15.0001 0.4 0.0001

Oxygen16

O 16 15.9949 99.76 -0.005117

O 17 16.9991 0.04 -0.000918

O 18 17.9992 0.2 -0.0008

Fluorine F 19 18.9984 100 -0.0016

Silicon28

Si 28 27.9769 92.2 -0.023129

Si 29 28.9765 4.7 -0.023530

Si 30 29.9738 3.1 -0.0262

Phosphorus P 31 30.9738 100 -0.0262

Sulfur32

S 32 31.9721 95.02 -0.027933

S 33 32.9715 0.76 -0.028534

S 34 33.9679 4.22 -0.0321

Chlorine35

Cl 35 34.9689 75.77 -0.031137

Cl 37 36.9659 24.23 -0.0341

Mass

DefectElement Symbol Abundance

Integer

Mass

Exact

Mass

TOF capable of < 5 ppm measurement routinely

Many possible formulas with an MSD or IT But only a few with TOF

1

10

100

1000

10000

0.1 1 10 100 1000

# P

oss

ible

Ch

emic

al F

orm

ula

s

mass uncertainty, ppm

Possible Number of Chemical Formulas at m/z 272

Formulas made of:

C,H,N,O,F, & Cl

mass uncertainty

ppm amu # of Possible

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

C10F8 = 271.98667 Possible chemical formulas for m/z =

Mass uncertainty - ppm

No

. o

f p

os

sib

le f

orm

ula

s

Isotopic abundances can further reduce the number of possible formulas

QUADRUPOLE

TOF

How is accurate mass used? Extracted ion chromatograms - EIC

Each point of EIC represents

the sum of ions in mass peak

within a specified mass range

Mass peak

m/z

m2

Chromatogram – EIC m1

m1-extracteded

Extraction window

Extraction window Wide extraction window measures more ions = more sensitivity

Mass peak


m/z

m2

Good results if m1 is well separated from m2 and comparable abundance

m1-extracted

Extraction window

Why is resolution important? Ions from m2 are added to m1 EIC thus shifting the m1 centroid

m1-measured

m/z

m2

What if m2 is not separated from

m1 or is in much larger abundance

at the same time?

Shifted centroid changes measured ion abundance and mass accuracy

Extraction window

m1-exact


m1-exact

m1

Why is resolution important? Wide extraction window will add in ions from adjacent mass

m/z

m2

What if m2 is not separated from

m1 or is in much larger abundance

at different times and no m1 is

present?

Ions from m2 are added into m1 EIC thus displaying a false m1 response


Extraction window

m1-extracted

Mass centroid Relative intensity affects the result

Centroid view of mass peaks obscures this fact

m/z m/z m/z m/z

Profile view of mass peaks

Centroid Centroid Centroid Centroid

240.1218

240.0785

Flurenol methyl ester

m/z = 240.0781

Mass error = 1.7 ppm

Dimetilan

m/z = 240.1217

Mass error = 0.4 ppm

~13,500 resolution FWHM

How much resolution is enough? Relative intensity affects the result

Dm = 0.043 Da.

240.1184

240.0780

Flurenol methyl ester

m/z = 240.0781

Mass error = -0.4 ppm

Dimetilan

m/z = 240.1217

Mass error = -13.7 ppm

A resolution of 13,500 is not

enough - larger ratios require

more mass resolution, MS/MS

or better GC separation

How much resolution is enough? Intensity Ratio = 10:1

Crude oil target analysis – target analysis

Problem: identify and quantify dibenzothiophene

Interferences

TIC

Mass extraction window:

+/- 0.5 Da.

= +/- 2717 ppm

EIC 184.0341 dibenzothiophene

Resolution 12,000


+/- 0.00092 Da.

= +/- 5 ppm

EIC 184.0341 dibenzothiophene

TIC

Crude oil target analysis – target analysis


Resolution 12,000

dibenzothiophene


+/- 5 ppm

EIC 184.0341

TIC

Crude oil target analysis


Resolution 12,000

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

Documents

Mass Accuracy and Mass Resolution in TOF MS