Calibration Discussion

Ed Fortner, John Jayne

There are 3 fundamental calibrations for the

AMS

• Flow rate

• Ionization efficiency

• Particle size calibration

g

m3

mass

volume

From Mass Spectrometer

Ionization Efficiency calibration

From volumetric flow rate

Particle Mass Loading

g

m3

g

m3

mass

volume

mass

volume

From Mass Spectrometer

Ionization Efficiency calibration

From volumetric flow rate

Particle Mass Loading

Particle Size

pTOF to Dva

IE Calibration

• We have two approaches

– Single particle

– Total mass based method

Single Particle Based IE Calibration

Time

Sig

na

l

Single particle pulses

Particle threshold set

above single ion level

Single ions above electronic noise level

Time

Am

ps (

Co

ulo

mb

s/t

ime)

Average single ion pulse

Average single particle pulse = Ions per particle (IPP)

Ionization Efficiency = IPP/Molecules per Particle

• In principal single particle approach is

good.

– Avoids complications associated with DMA-

Q2s and lens transmission.

– Avoids errors that the CPC might introduce

However in practice there are details which if not followed WILL result in an inaccurate IE determination You need to have a low # conc of AN particles usually 200 or

less in order to ensure that you are only counting a single particle per extraction should have less than 5% hit rate

– Count at least 300 particles to get good counting statistics

– Selection of threshold is critical too low you count noise too high you bias toward higher signal particles

– Use as large a particle as practical to enhance IPP.

– Important to maximize time resolution to better resolve vaporization event. This means shorter pulser period. Pulser time effects IE. Need to correct.

More BFSP Considerations

– Get the RIE for NH4 using MS data. Don’t rely

on BFSP derived RIE for NH4

– Use a HEPA filter to ensure no false counts;

Also a tool to use for determining how low

threshold can go

– This is only a single point calibration

AMS DMA

CPC

Atomizer

Aerosol

Diluter

Setup for CPC-Mass Based IE Determination

By-pass

Input Mass = x Volume(size) x Number Instrument Response

filter

Plot Instrument Response vs Input Mass

Drier

Mixing Tube

Very Important! Very Important!!

Mass Based IE Determination

12

10

8

6

4

2

0

Mass r

eport

ed b

y V

1.5

ToF

DA

Q (

ug/m

3)

2520151050

Calculated/Input AN mass (based on CPC, ug/m3)

Dmob 300 nm

250 nm

intercept = 0.068 ± 0.041slope = 0.493 ± 0.003

Fast_IE_Cal.pxp

Slope is the IE correction factor

CPC-Mass Based IE Determination

Lens transmission limitation

Multiple charged diameters 12

10

8

6

4

2

0

Mass r

eport

ed b

y V

1.5

ToF

DA

Q (

ug/m

3)

2520151050

Calculated/Input AN mass (based on CPC, ug/m3)

Dmob 200 nm

250 nm

300 nm

400 nm

intercept = 0.068 ± 0.041slope = 0.493 ± 0.003

Fast_IE_Cal.pxp

Nit

rate

Sig

nal

Calculated Mass

Recommendations

• If Comparing BFSP IE to CPC based IE it is important to

consider differences in particle size between methods

BFSP optimum is 400 nm and CPC based IE optimum is

300 nm.

Do Both Calibrations

Velocity Calibration

12

8

4

0

Sig

na

l (b

its)

0.0060.0050.0040.0030.002

p-TOF (s)

NH4NO3 Dmob 100 nm 250 nm 450 nm 350 nm

d600/ship/U_Tokyo/velocity.pxp

5

6

7

8

9100

2

3

Ve

locity (

m/s

)

101

2 4 6 8

102

2 4 6 8

103

2 4 6 8

104

Aerodynamic Diameter (nm)

PSL, F=1.43 cc/s

NO3, F=1.41 cc/s

p_0 = 658 ± 0 fixedp_1 = 4.4553 ± 0.353p_2 = 0.43986 ± 0.00864p_3 = 16.449 ± 2.2

d600/ship/U_Tokyo/velocity.pxp

Particle Velocity Calibration

Daero = Dgeo * ρ* Shape Fac

Sample known size particles

and calculate a velocity…

Velocity = flight path / TOF

What’s wrong with this calibration?

Velocity calibration data should span maximum

measurable range

Extrapolation of fit beyond data points can introduce errors

Critical data points

Size Calibration Summary

• Use caution extrapolating velocity curve.

• Document lens pressure (flow) during a

calibration.

• If your lens pressure changes check if orifice is

clogged don’t take different sizes at different

flow rates.

• Watch for a lens pressure change when

connecting the AMS inlet to a sampling system.

Flow Rate Calibration

Flow Meter

Low pressure drop

flow meter

Gilibrator™ or

equivalent soap film

meter 10 Torr lens

pressure gauge

P

Aerodynamic Lens

Fine metering

valve

AMS Flow Rate Calibration Setup

Poiseuille equation relates volumetric

laminar flow (F) to a pressure drop

(ΔP) across the tube

Flow into

vacuum

Replace 100 um pin hole

with fine metering valve

SS-SS4.

Flow Calibration

Depends on Pressure and Temperature

3.0

2.5

2.0

1.5

1.0

0.5

0.0

Flo

w (

cc/s

)

2.52.01.51.00.50.0

Pressure (torr)

U. Tokyo Cal at ARI

NOAA Cal at ARI

NOAA Cal at Boulder

NOAA Scaled to 760 torr

Always document ambient pressure and temperature

100 µm leak

Flow Rate Calibration Summary

• In principal you only need to do this once as long as

you document ambient pressure and temperature

condition when you do the calibration (also assumes

you are always sampling air).

• What you are measuring/calibrating here is a

property of the aerodynamic lens via the Poiseuille

relation ship.

• When reporting µg m-3, always reference the T&P

conditions you use in the “m3”

• The flow rate calibration is independent of critical

orifice. Critical orifice is removed during calibration

Lens Alignment

• This is often not checked.

• Generally, this does not need to be adjusted but if its off and not checked your data may not be quantified correctly.

• Recommend checking this after shipping the AMS.

• Don’t be afraid to do this (Turn TPS off to be cautious)

Documentation

• Recommend save all of your cals in a cal

folder on the AMS computer.

• So when the next person comes along

they have a record of how this AMS is

running

END PRESENTATION

Converting your CPC number

concentration into an IE

• an_molec = numconc

* pi/6 * (300e-7)^3 *

1.72 * 0.8*1.35 / 80 *

6.022e23