CA2937047A1

CA2937047A1 - Method for ionizing laser plumes through atmospheric pressure chemical ionization

Info

Publication number: CA2937047A1
Application number: CA2937047A
Authority: CA
Inventors: Kresimir Franjic
Original assignee: Individual
Current assignee: Individual
Priority date: 2016-07-25
Filing date: 2016-07-25
Publication date: 2018-01-25

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.

Description

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 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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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.