METHOD FOR IONIZING LASER PLUMES THROUGH ATMOSPHERIC PRESSURE CHEMICAL IONIZATION

METHODES D'IONISATION DE PANACHES LASER PAR IONISATION CHIMIQUE A PRESSION ATMOSPHERIQUE

English Abstract

Method for ionizing laser plumes using atmospheric pressure chemical
ionization (APCI) is disclosed.
A laser plumes is ionized by intercepting it with an ionized gas jet where
said ionized jet contains
dopant molecules that are previously ionized through atmospheric pressure
chemical ionization (APCI)
process.

Claims

Method for ionizing laser plumes through atmospheric pressure
chemical ionization
Claims:
1. A mass spectrometry system for analyzing molecular composition of a sample
in a condensed phase
comprising:
a pulsed laser capable of ablating the surface of said sample in a region
where atmospheric pressure
thus creating an ablation plume containing analyte molecules;
a system for delivering a gas jet where said gas jet contains dopant
molecules;
a metal object with a sharp tip placed in the vicinity of said gas jet;
a high voltage generator that is electrically connected to said metal object,
thus creating a corona
discharge at said sharp tip, where said corona discharge ionizes said dopant
molecules in said gas jet;
and where said gas jet intercepts said laser ablation plume whereby at least
one of subsequent direct or
indirect collisions between said ionized dopant molecules and said analyte
molecules results in
ionization of several said analyte molecules;
and where said ionized analyte molecules are directed towards a mass
spectrometer where said ionized
analyte molecules are analyzed providing information about molecular
composition of said sample.
2. A system described in Claim 1 where said system for delivering gas jet is a
capillary connected with
a tubing to a source of compressed gas.
3. A system described in Claim 2 where said source of compressed gas is a gas
cylinder or gas
compressor.
4. A system described in Claims 2 and 3 where said metal object with a sharp
tip has a form of a sharp
needle that is placed within said capillary.
5. A system described in Claims 1,2,3, and 4 where said gas contains air,
water vapour, N2, noble gas,
toluene, or a mixture of any of these gases.
6. A system described in Claims 1,2,3,4, and 5 where said high voltage source
is pulsed, modulated, or
constant.

7. A system described in Claim 1 where said system for delivering a gas jet
has a pulsed valve.

Description

CA 02937047 2016-07-25
Tide:
Method for ionizing laser plumes through atmospheric pressure
chemical ionization
Inventor:
Kresimir Franjic
1703-77 Huntley St.
Toronto ON M4Y 2P3
Canada
Background
Mass spectrometry is one of the most important diagnostic techniques for
determining molecular
composition of a sample. In case of samples in condensed phase, several
processing steps are required
for mass spectrometry analysis:
(a) desorption of sample molecules i.e. moving sample molecules from the
condensed into the gas
phase;
(b) ionization of the desorbed molecules;
(c) coupling such ionized sample molecules into a mass spectrometer.
In case of fragile molecules, such as majority of biological molecules, the
first two steps involving
desorption and ionization are critical since desorption and ionization
processes are generally energy
intensive; so it is easy to put excess energy into the sample molecules which
can cause their
modification or break-up. Recent advances in laser ablation techniques, which
desorb molecules
though creation of laser ablation plumes, make possible desorption with
imparting minimal excess
energy into desorbed sample molecules; however, the most optimal ionization
method of such desorbed
molecules is still to be determined.
One of the standard methods for ionizing gas phase molecules is atmospheric
pressure chemical
ionization (APCI) which consists of putting a sharp metal object under high
voltage in the vicinity of
sample molecules which are desorbed in a gas atmosphere. Such sharp metal
object creates corona
discharge which generates charges that are transferred to the sample molecules
either directly or
indirectly through interactions with molecules in the gas atmosphereof liquid
sample matrix molecules
(e.g. water or alcohol) that are desorbed simultaneously with the sample
molecules. APCI is a standard
ionization technique and its general features are known to the person having
ordinary skill in the art.
However, a standard APCI configuration is not an efficient method for ionizing
laser plumes since laser
plumes are highly localized in space and time while standard APCI corona
discharges are not.
For that reason, an APCI method that that delivers ions co-localized with gas
phase sample molecules
in time and space would be advantageous.
Description
Referring to FIGS. 1A-1D, a sample 10 in the condensed phase is deposited on a
sample holder 12.
A laser pulse 14 is directed at sample 10 where the laser pulse is absorbed
thus creating a superheated
portion of said sample. The superheated portion gets ablated creating a laser
plume 24 that is generally

CA 02937047 2016-07-25
an aerosol consisting of gas phase sample molecules, gas phase matrix
molecules, molecular
aggregates, small sample droplets, etc. A source 16 that creates a gas jet 26
is placed in the vicinity of
said sample where said gas jet contains dopant molecules. Close to the outlet
of said gas jet there is a
sharp metal object 18 that is in electrical contact with a high voltage source
19. Due to the high
voltage at the sharp tip of said sharp metal object, a corona discharge is
formed ionizing some of the
dopant molecules within said laser jet. Such ionized dopant molecules travel
with jet 26 which is
directed in a way to intercept laser plume 24. Some of these ionized dopant
molecules transfer their
charges to some of the sample molecules of interest within laser plume 24.
Such ionized sample
molecules form a cloud 28 which is directed towards a mass spectrometer 20
where these sample
molecules are analyzed.
Referring to FIG. 2A, the schematic shows an example of gas jet source 16
presented in Fig. 1. A
pressured gas source (for example a gas cylinder or a compressor) 34 delivers
a gas to a thin capillary
30 through a tubing 32. The gas passes through the capillary and exits at one
of its ends creating a gas
jet 26 corresponding to gas jet 26 from FIG. 1. Gas jet 26 is partially or
completely ionized by a
corona discharge created by sharp metal object 18 that is at high voltage
provided by high voltage
generator 19. High voltage generator 19 can provide constant high voltage or
it can be modulated or
pulsed in which case the frequency and phase of such modulation or pulsing
should be synchronized to
the ablative laser pulse in order to provide the optimal number of ions to the
laser plume.
FIG. 2B shows a possible variation of the system schematically presented in
FIG 2A. In this case,
sharp metal object 18 from FIG 2A has a form of a long needle 36 inserted
inside capillary 30 where
the sharp tip of needle 36 is positioned close to the capillary end where the
gas exists forming gas jet
26. Needle 36 is at high voltage provided by high voltage generator 19 thus
creates a corona discharge
that partially or completely ionizes gas jet 26.
FIG. 3A shows a simple schematic of mass spectrometer 20 from FIG 1. Mass
spectrometer 20 has an
orifice that is the entrance point for ions into mass spectrometer. Features
and design principles of
mass spectrometers and their orifices are known to the person having ordinary
skill in the art.
FIG. 3A shows a simple schematic of a system presented in FIG 3A with an added
feature 54 that
corresponds to a system that provides static or/and dynamic electromagnetic
fields that help guiding the
ion cloud 28 into mass spectrometer 50. Features and design principles of such
electromagnetic
systems are known to the person having ordinary skill in the art.
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Representative Drawing