Method and apparatus for sample introduction into a mass spectrometer for improving a sample analysis

There is provided a method for sample introduction into a mass spectrometer for performing sample analysis, including desorbing a sample by a laser beam and forming gaseous sample compounds, sweeping desorbed sample compounds with a carrier gas into a transfer line, transferring the sample compounds in the transfer line into a supersonic nozzle, expanding the sample compounds mixed with the carrier gas from the supersonic nozzle to form a supersonic free jet inside a vacuum chamber of a mass spectrometer, and ionizing and mass analyzing the sample compounds for the purpose of identification and/or quantification of the sample. An apparatus for carrying out the method is also provided.

BACKGROUND OF THE INVENTION 
The present invention relates to a method and apparatus for sample 
introduction into a mass spectrometer for performing sample analysis. 
DESCRIPTION OF THE PRIOR ART 
Mass spectrometry (MS) is a powerful tool for chemical analysis, combining 
excellent sensitivity and high level of molecular identification 
capability that in many cases enables sample identification. For the 
analysis of samples in complex matrices, a gas chromatograph (GC) is 
coupled to the MS to form a GC-MS that combines the capability of GC 
sample separation in time with the detection and identification 
capabilities of MS. Thus, GC-MS is considered to be the main analytical 
tool for chemical analysis. It is widely recognized that sample 
preparation constitutes the bottle-neck in the whole analysis and often 
requires several hours of expensive sample clean-up, extraction and 
concentration procedures in order to make it amenable for GC-MS analysis. 
A standard estimate for pesticide analysis in fruit and vegetables, or 
drug analysis in urine is about 2 hours for the preparation of a sample 
and about 30 minutes of the GC-MS analysis. The requirement for wet 
chemical or other sample preparation methods also eliminates spatial 
sample information in cases where the sample is unhomogenously deposited 
on a given surface or in the bulk. 
Laser desorption methods are of growing importance in combination with mass 
spectrometry, and in-vacuum laser desorption mass spectrometry methods are 
commercially available. Laser desorption of a sample placed in vacuum is 
known to be especially effective for the analysis of large bio-molecules. 
In some applications, the "in-vacuum" desorbed molecules are further swept 
and entrained in an expanding supersonic free jet where the supersonic 
nozzle source is close to the laser desorption focal point on the sample 
that is inside the vacuum chamber. 
When a given sample is placed inside a vacuum chamber, however, all the 
information concerning volatile organic matter is lost and the information 
on semi-volatile compounds is biased. In addition, the ability to use a GC 
is precluded. The use of focused or slightly defocused laser light for 
sample desorption and volatilization, seems to be the ideal tool to 
eliminate sample preparation and to retain the spatial sample position 
information. Moreover, the laser can also drill inside the bulk of a 
material and provide three-dimensional chemical information. Laser 
desorption in the open air or at a slightly higher inert atmosphere, is, 
however, confronted with problems of ineffective sample transfer to the 
mass spectrometer. In standard MS and GC-MS instruments, the column flow 
rate is limited to 1-2 ml/min due to limited pumping capacity of the MS 
pumps. Since the laser desorbed sample may expand into one milliliter 
volume or more, depending on the laser pulse energy, the sample transfer 
to the column may last more than one minute and volatile compounds can be 
poorly separated by the GC. In addition, the slow (typically 30 minutes) 
GC precludes the possibility of effective surface chemical mapping that 
could be realised only if a much faster GC-MS analysis could be achieved. 
A broad object of this invention is to provide a method and apparatus for 
enabling a much faster and more informative Laser Desorption (LD)-MS 
chemical analysis that will not be confronted with the limitations 
outlined above. One of the major aspects and advantages of the use of the 
LD-MS is its capability of sample injection at its natural condition, 
without sample preparation. This can be achieved by the combination of 
ambient or higher pressure laser desorption sampling with sample interface 
into the mass spectrometer through a supersonic expansion. This method can 
be further improved if the sample compound ionization is performed in the 
resulting supersonic molecular beam (SMB). Supersonic expansion occurs 
when a gas expands through a pinhole, typically 80-150 .mu.m diameter, 
into vacuum. The supersonic expansion is performed in a differentially 
pumped additional vacuum chamber and the relative concentration of the 
sample is highly enriched in the central line of the expansion. Thus, if 
this central portion of the expansion is skimmed and transferred to the 
mass spectrometer vacuum chamber, sample enrichment occurs and while most 
of the heavy sample compounds enter the MS chamber, the majority of the 
light carrier gas, such as hydrogen or helium, is differentially pumped. 
This known "jet separation", when coupled with laser desorption, provides 
two very important advantageous features: 
1. High carrier gas flow rate is permitted for superior transfer of laser 
desorbed sample into the transfer line or GC column, and 
2. The high carrier gas flow rate in the transfer line or GC column, 
enables very fast analysis either with, or without, GC separation. 
The supersonic expansion is also characterized by the supercooling of the 
intramolecular degrees of freedom and by the possible acceleration of the 
sample compounds that acquire hyperthermal kinetic energy (1-30 eV). These 
two additional features are very important for achieving a fast and 
informative LD-MS. The molecular hyperthermal kinetic energy enables 
vacuum background elimination based on differences in the ion energy of 
background ions and ions of molecules ionized in the supersonic molecular 
beam. Consequently, background ion filtration is achieved with simple 
electrostatic retarding or deflecting fields. Background ion filtration 
facilitates ultra fast ion source response time, since any molecule that 
scatters from a given wall would lose its directional kinetic energy and 
be filtered if ionized as thermal background. This feature also enables 
tail-free high temperature GC-MS to be achieved without ion source related 
limitations. It also exposes the genuine electron impact mass spectrum of 
the vibrationally cold sample compounds. These unique electron impact mass 
spectra are characterized by enhanced molecular ion peaks, by the total 
control of the degree of molecular ion dissociation through the reduction 
of the ionizing electron energy, by enhanced and clearer isomer mass 
spectral effects and, by additional isotopic and elemental information. In 
addition, the hyperthermal molecular kinetic energy enables another 
ionization method to be employed, namely, hyperthermal surface ionization 
(HSI). HSI is based on the large (orders of magnitude) increase in the 
surface ionization yield of organic compounds upon their hyperthermal 
surface scattering from a suitable solid surface in comparison with 
thermal surface ionization. Thus, HSI was found to be a very efficient 
ionization method with a tunable degree of ionization selectivity that 
favors the ionization of compounds with low ionization potential such as 
aromatic compounds and nitrogen containing drugs over aliphatic compounds. 
The combination of the unique features of SMB and its high flow rate 
capacity enables very fast GC-MS analysis to be achieved ranging from 1 
second to a few minutes. The very short residence time in the heated short 
transfer line or GC column, also largely reduces the thermal dissociation 
of thermally labile compounds. The ability to analyze fragile organic 
compounds is a very important additional benefit of the use of high flow 
rate supersonic expansion. As a result, the coupling of laser desorption 
injection with mass spectrometry through a supersonic expansion provides a 
new and very powerful tool for chemical analysis, characterized by the 
following desirable features: 
1. Very fast analysis is achieved; 
2. The fast analysis can be combined with fast GC separation; 
3. Effective and efficient sweeping of the laser desorbed species is 
performed followed by their efficient transfer into the MS ion source; 
4. Open air ambient laser desorption at a pressure of about 1 atmosphere 
can be achieved for easy and flexible sample handling; 
5. The laser desorption chamber can be held at low temperatures to retain 
the volatile organic compounds for this measurement; 
6. Any column can be used at any length and carrier gas flow rate for 
tailoring the optimal trade-off between GC resolution, sensitivity and 
analysis time; 
7. Effective flow programming can be employed due to the large flow rate 
tolerance, for optimal laser desorption injection combined with optimal GC 
resolution. Flow programming can also serve as an effective way of 
achieving fast GC of a mixture of compounds having a large boiling point 
range; 
8. Laser desorption microscopy chemical analysis can be achieved due to the 
fast analysis and the sample surface can be scanned for two dimensional 
chemical mapping; 
9. Very complex samples and matrices can be analyzed due to the enhanced 
selectivity of mass spectrometry in SMB; 
10. Relatively thermally labile compounds can be analyzed by the GC-MS with 
the supersonic expansion interface; 
11. Sample injection by laser desorption eliminates or substantially 
reduces the need for sample preparation; 
12. The open air or purged LD inlet enables LD injection of flowing liquid 
samples, and 
13. High frequency, repetitive fast sampling and analysis can be performed 
to continuously control process qualities. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is therefore provided a 
method for sample introduction into a mass spectrometer for performing 
sample analysis, comprising desorbing a sample by means of a laser beam 
and forming gaseous sample compounds, sweeping desorbed sample compounds 
with a carrier gas into a transfer line, transferring the sample compounds 
in said transfer line into a supersonic nozzle, expanding the sample 
compounds mixed with said carrier gas from the supersonic nozzle to form a 
supersonic free jet inside a vacuum chamber of a mass spectrometer, and 
ionizing and mass analyzing the sample compounds for the purpose of 
identification and/or quantification of said sample. 
The invention further provides an apparatus for sample introduction into a 
mass spectrometer for performing sample analysis, comprising a sample 
container arranged for positioning a sample to be analyzed therein for 
subsequent desorption by means of a laser beam directed thereon to form 
sample compounds, means for introducing a carrier gas in said container 
for sweeping desorbed sample compounds into a transfer line being in fluid 
communication at one end thereof, with said container and leading to a 
supersonic nozzle at the other end thereof, to enable a supersonic free 
jet of said desorbed sample compounds to be expanded into a vacuum chamber 
of a mass spectrometer. 
The invention will now be described in connection with certain preferred 
embodiments with reference to the following illustrative figures so that 
it may be more fully understood. 
With specific reference now to the figures in detail, it is stressed that 
the particulars shown are by way of example and for purposes of 
illustrative discussion of the preferred embodiments of the present 
invention only and are presented in the cause of providing what is 
believed to be the most useful and readily understood description of the 
principles and conceptual aspects of the invention. In this regard, no 
attempt is made to show structural details of the invention in more detail 
than is necessary for a fundamental understanding of the invention, the 
description taken with the drawings making apparent to those skilled in 
the art how the several forms of the invention may be embodied in practice 
.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 there is shown a schematic diagram of the laser desorption mass 
spectrometer apparatus having a sample introduction portion 2. Seen is a 
laser light beam 4 produced by a laser 6 focused by an optical system 8 on 
a sample 10 placed in the sample compartment 12. The laser beam desorbs 
the sample to form sample components which are further vaporized to form 
sample compounds. The compartment 12 is fitted with a gas inlet 14 for the 
introduction of a carrier gas, the flow of which is controlled by a valve 
16. A short column 18 serves as an outlet from the compartment 12 and 
advantageously, leads via a filter 20, to a standard GC column transfer 
line 22. The latter can also serve as a fast GC short column for fast GC 
separation by means of a temperature controlled oven 24. At the exit of 
the transfer line 22, the sample compounds and carrier gas are optionally 
mixed with a make-up gas provided via control valve 26 to be expanded into 
a vacuum chamber 28 through a supersonic nozzle 30, forming a supersonic 
free jet. The central portion of the supersonic free jet is then further 
collimated by a skimmer 34 and transferred in the form of a molecular beam 
through a differential pumping chamber 36 into the mass spectrometer's 
main vacuum chamber 38. The supersonic molecular beam is, in turn, ionized 
by an electron ionization ion source 40 and the ions are deflected by an 
ion mirror 42, at an angle of substantially 90.degree., into a mass 
analyzer 44 constituted by a quadruple mass analyzer, to be detected by an 
ion detector 46. Advantageously, the ionization of the sample compounds 
can also be carried out by a laser. The resulting signals are processed 
and displaced by microcomputer 48. A suitable surface 50 can be provided 
above the surface of the ion mirror 42 and is positioned in the SMB 
trajectory for HSI. 
In FIG. 2 there is illustrated a more detailed schematic diagram of the 
laser desorption inlet portion 2. The sample 10 is introduced on the 
sample support 52 beneath a window 54 formed in the upper wall of 
compartment 12. The laser 6 emits a light beam 4 that is focused and 
guided by the optical system 8 onto the sample 10, which, during 
operation, can be viewed by a microscope 56, with or without a video 
monitor 58. The laser 6 may advantageously be a pulsed laser operating in 
a high frequency periodic fashion. The desorption may also be performed by 
several laser pulses transmitted at a controlled repetition and time for 
total desorption. During desorption, adsorbing reagent may optionally be 
added. The laser desorbed sample compounds are swept by the carrier gas, 
the flow rate of which is controlled by valve 16, into the introduction 
short capillary column 18. The sample and carrier gas are transferred 
through the dust and particle heated filter 20 into the heated GC 
separation or transfer line 22. The laser desorption compartment 12 and 
outlet line 18 are thermally insulated from the heated transfer line 22 
and the contact area thereinbetween can be sealed by a seal 60. 
Alternatively, a carrier gas protective purge flow can be provided. The 
flow rate of the sample compounds and carrier gas from the line 22 can be 
controlled by a make up gas through the control valve 26 (FIG. 1). The 
sample introduction portion 2 may advantageously be thermally insulated by 
a suitable support 62. 
While the GC and sample introduction portion 2 described above are 
"home-made" apparatus, it is understood that a standard commercially 
available GC can also be coupled to the laser desorption introduction 
portion 2, following a similar approach. 
The sample analysis may be performed in a MS--MS or MS.sup.n system. The 
laser desorption can be achieved by means of sample vaporization, sample 
ablation, or by means of sample blasting into small dust particles, 
techniques. When the last-mentioned technique is used, the dust particles 
are, in turn, further thermally vaporized inside the heated transfer line 
or GC. 
In FIG. 3 there are shown chromatograpms of ultra fast laser desorption 
GC-MS trace emerging from laser desorption of a synthetic mixture of a) 
anthracene, b) lidocaine, c) pyrene and d) 9,10-dichloroanthracene placed 
on a glass surface. A train of 20 pluses of XeCl Excimer laser was used 
for desorption, with pulse energy of 3 mJ each. It is shown that with half 
a meter short capillary column (0.53 mm ID), these compounds are vaporized 
and separated in time and the computer reconstructed chromatograms provide 
clean and quantitatively time-integrated peaks for each compound. Note 
should be made of the short GC time of under 20 seconds. 
In FIG. 4 there is illustrated the LD-GC-MS of methylparathion desorbed 
from the surface of liquid water. A large drop of water spiked with the 
pesticide was placed on the concaved sample holder. Five laser desorption 
events are shown, where each pesticide peak appears 2.5 seconds after the 
laser pulse. It is shown that each laser train of pulses depeleted about 
50% of the pesticide on the water surface. After a waiting period of 25 
seconds, the water surface concentration was partially recovered. The most 
important aspect shown in FIG. 4 is the demonstrated capability of 
analyzing an organic compound in a volatile liquid solution. This 
application cannot be performed by any of the known "in-vacuum" laser 
desorption methods. 
In FIG. 5 the determination of relative caffeine content in decaffeinated 
coffee is shown. Instant coffee powder was used as is without any sample 
treatment. A certain brand of coffee powder was studied. For achieving 
better precision, five consecutive laser desorption pulses were applied, 
and the five results were averaged. It is shown that considering the gain 
increase by a factor of 25 with the lower trace, the relative content of 
caffeine in the decaffeinated coffee is close to 2% of that in the regular 
coffee, exemplifying the use of LD-GC-MS for the analysis of organic 
matter in powders. 
FIG. 6 illustrates the analysis of lidocaine drug spiked in mouse blood 
with the LD-GC-MS. A single ion monitoring trace at m/z 86 was used with a 
hyperthermal surface ionization ion source. In spite of the complexity of 
the blood matrix, the lidocaine peak is clearly observed and can be 
analyzed at 1 ppm level in blood in under 10 seconds, without sample 
preparation. A single drop of blood was used and each laser desorption 
injection evaporated an area of 10.sup.-4 cm.sup.2 containing about 1 
microgram of coagulated blood. 
These applications uniquely demonstrate the effectiveness and analytical 
power according to the method of the present invention. Other examples of 
studied analysis include traces of lead as tetraethyllead from evaporated 
car gasoline, aldicarb and methylparathion pesticides from an orange leaf, 
caffeine drug from dry urine, cleaning process of a stainless steel 
surface from dioctylphthalate oil, plastic polymer composition, etc. 
It will be evident to those skilled in the art that the invention is not 
limited to the details of the foregoing illustrated embodiments and that 
the present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.