Method and apparatus for the analytical determination of traces

In a method and apparatus for the analytical determination of trace compounds, portions of a sample disposed on a sample carrier are irradiated with light having an energy density of 1 to 100 MW/cm.sup.2 so that any organic trace compounds are removed from the irradiated sample portions, the removed trace compounds are collected by a capillary which is adjustably supported so as to be movable in close proximity to the irradiated sample portion and the organic trace compounds collected by the capillary are supplied to an analysis apparatus for detection.

BACKGROUND OF THE INVENTION 
The invention relates to a method and apparatus for the analytical 
determination of traces. 
In M. J. Cohen et al Journal of Chromatographic Science, 1970, 8, 330-337, 
an ion drift spectrometer (IMS) possibly coupled with a mass spectrometer 
is disclosed for use in a sensitive analysis method for certain organic 
compounds. It is also known from Baum, M. A., Etherton, R. L.; Hill, H. H. 
Jr; Anal. Chem. 1983, 55; 1761-1766, to use an IMS apparatus as a detector 
for a gas chromatograph (GC) wherein the capillary of the GC apparatus was 
connected to the IMS apparatus. 
Since with conventional IMS apparatus, no care has been taken to keep the 
volume and the reverse mixing of the sample chamber at a minimum, it is 
necessary with present apparatus to use substantially larger amounts of a 
substance as it would be necessary with substantially smaller probe 
chambers or with a capillary and a relatively long waiting time is needed 
for the substance to be again flushed out of the apparatus so that a new 
sample can be tested. 
An analysis indicating the location of the traces cannot be obtained with 
any of the apparatus. 
It is the object of the present invention to provide a method and apparatus 
for a trace analysis by which also the location of the traces can be 
determined. 
SUMMARY OF THE INVENTION 
In a method and apparatus for the analytical determination of trace 
compounds, portions of a sample disposed on a sample carrier are 
irradiated with light having an energy density of 1 to 100 MW/cm.sup.2 so 
that any organic trace compounds are removed from the irradiated sample 
portions, the removed trace compounds are collected by a capillary which 
is adjustably supported so as to be movable in close proximity to the 
irradiated sample portion and the organic trace compounds collected by the 
capillary are supplied to an analysis apparatus for detection. 
If the conventional sample chamber of an IMS apparatus which has a volume 
of about 10 ml is replaced by a smaller chamber of about 3.5 ml or by a 
0.5 m long quartz capillary with about 0.1 ml volume, the sample volume is 
substantially reduced which leads to a substantially smaller dilution and 
reverse mixing of the sample. "Reverse mixing" refers to mixing of volume 
elements in flow direction which is physically in its effects similar to 
diffusion. Since, as a result of the smaller reaction chamber, the 
substance volume to be analyzed is supplied to the ion molecule reactor in 
a substantially shorter period of time, the detection limit is lowered and 
the signals obtained are substantially narrower and higher. If instead of 
the relatively voluminous sample chamber used in connection with present 
apparatus a capillary is used the volume of the inlet system and the 
dilution as well as the reverse mixing of the substances to be analyzed 
are substantially reduced. In this way, narrow signals and small detection 
limits are obtained. It is however to be noted that possible adsorption of 
the substances being transported by the inner surface of the transport 
means may also lead to a kind of reverse mixing and should therefore be 
minimized. 
Since with the experimental detection methods, the substance volume which 
is disposed in the capillary could only be roughly estimated the increase 
of the sensitivity could not be accurately determined. However, in an 
apparatus using capillaries, the sensitivity is increased at least 10 to 
100 fold. 
Another advantage resides in the fact that the IMS apparatus can be 
miniaturized. The capillary also represents a sensitive "nose" for the 
surveillance of air with regard to noxious compounds. With multipoint 
measurements, it is possible with the use of capillaries to increase the 
interrogation rate.

DESCRIPTION OF A PREFERRED EMBODIMENT 
FIG. 1A shows the influence of the transport volume on the detector signal 
depending on time. On top, a transport line with an increased volume 3 
into which a gas sample 2 is introduced for measuring purposes is shown. 
Below, the detector signal obtained by the arrangement is shown. 
FIG. 1B shows on top an arrangement without an increased measuring volume. 
Below the detector signal obtained by the arrangement is shown. 
If a large transport volume 3 is used which permits a fast and almost 
complete reverse mixing the gas sample 2 introduced at a certain point in 
time is diluted. As a result, the detector signal over time starts at a 
relatively low value and then falls further with increasing dilution of 
the sample. 
If no increased volume is provided in the transport line as shown in FIG. 
1B, the detector shows no response until the sample passes by the 
detector, but then the detector signal shows a sharp peak. In this way, 
the detection limit is substantially reduced, that is, even small samples 
or small traces can be recognized. 
In the detecting arrangement as shown in FIG. 2, a laser is used as the 
light source 4. The laser beam 9 is coupled, by way of mirrors 5, into a 
microscope 6 and focused onto a sample 11 (not shown in FIG. 2, but in 
FIG. 3). The sample 11 is disposed in a desorption chamber 7. The 
described gas sample 2 is conducted to the measuring apparatus 8 by way of 
a capillary 1. 
For a location-dependent analysis of organic substances by means of an ion 
drift spectrometer, it is helpful to utilize a laser desorption chamber 
which shields the sample to be measured from the laboratory air since 
otherwise the measuring result can be adversely affected by contamination 
contained in the laboratory air. For this purpose, the desorption chamber 
7 as shown in FIG. 3 is provided. In order to avoid any contamination of 
the inert gas atmosphere by organic compounds no organic lubricants or 
sealing materials such as silicones or rubber are used in the design of 
the laser desorption chamber. Inspite of this, the sample can be 
positioned in the laser desorption chamber in any desired inert gas 
atmosphere under a microscope lens system in such a way that the organic 
compounds can be desorbed by a laser beam at room temperature on a surface 
area of about 50 .mu.m.sup.2. Also, the molecules released thereby can be 
removed from the laser desorption chamber 7 by way of a capillary 1 and 
can be supplied to an analysis apparatus such as an IMS apparatus. 
The laser beam 9 emerges from the microscope 6 at the bottom and passes 
through the window 12 and reaches the sample 11. The capillary 1 extends 
through a capillary penetration 10 and reaches up to a close vicinity of 
the desorption location. The capillary penetration consists of a ball of 
insulating material with a central bore in which the capillary is snugly 
received. The ball is sealed in the desorption chamber 7 by spherical 
grinding and a metal ring or Teflon ring in such a way that the ball 
remains freely movable. The capillary 1 consists of glass or quartz. The 
capability of the capillary 1 to collect the desorption sample depends 
essentially on its distance from the desorption location. Consequently, 
the front opening of the capillary should be as close as possible to the 
desorption location without interfering with the laser beam. The capillary 
diameter should be larger than the laser spot on the sample. A size ratio 
7 to 1 has been found to be advantageous. With excessively large size 
ratios (greater than 15), the detection sensitivity drops. If polarized 
substances are to be detected their adsorption in the capillary can be 
reduced by heating as well as by special deactivation materials such as 
silicon compounds. The heating of the capillaries is facilitated if the 
glass capillary is surrounded by a metal tube or by the use of a metal 
capillary which has a quartz-coated inner surface. In both cases, the 
metal layer is resistance-heated. The electric power connections are not 
shown in the drawings. 
The laser desorption chamber 7 is disposed on a microscope table which is 
adjustable in an x, y and z direction. The movement of the microscope 
table in x and y direction is transferred to a sample carrier 14 of brass 
in whose center a sample holder (not shown) is arranged which can be 
removed downwardly. As a result, a sample can be accurately positioned on 
the sample holder with respect to the top part of the desorption chamber 
(of Al) and the lens of the microscope with an accuracy of 1 .mu.m. The 
apparatus includes an aluminum cover which is bolted to a table by way of 
two retaining pins, a retaining plate and two legs. The microscope is also 
mounted on the table. As a result, the laser desorption chamber can be 
moved relative to the lens system only in z direction for adjusting the 
focus. 
In order to seal the interior space of the desorption chamber 7 against 
laboratory air the movable sample carrier 14 is pressed by two annular 
plates against the annular carrier support structure 15 consisting of V2A. 
The lower annular plate of the sample carrier 14 is removable and so 
mounted that the play of the polished plates is 10 to 20 .mu.m. Since 
lubricants could greatly affect the test results all tolerances must be so 
selected that appropriate slide properties can be achieved without the use 
of lubricants. 
Also, the tolerance fitting between the removable sample holder of V2A and 
the brass carrier does not use any lubricants. In order to be able to 
examine samples of different thickness the height of the sample carrier is 
adjustable. 
The laser beam coupled into the microscope enters the laser desorption 
chamber through a glass window and strikes the sample disposed on the 
sample carrier. With laser beam densities of 1-3 MW/cm.sup.2 any organic 
compounds present are desorbed from the sample surface by the energy of 
the laser beam in a generally non-destructive fashion. The window 12 is 
mounted into the top part with rubber or silicon or similar seals only by 
means of brass retainers having a circumferential thread. In this way, the 
chamber is not fully gas tight but, by way of the gas inlet 13, a pressure 
of about 4 mbar can be generated in the laser desorption chamber with a 
gas flow of 400 ml/mm whereby the inflow of laboratory air into the laser 
desorption chamber is prevented. As a result, the laser desorption 
procedure can be performed independently of the laboratory air by flushing 
the interior of the chamber with any desirable gas. Furthermore, the 
internal pressure is sufficient to carry the laser-desorbed substances, 
mixed with the gas present in the chamber, in a volume flow of about 4 
ml/min by way of a heated conduit to the IMS apparatus. Preferably, the 
flow through the capillary is as large, that is as fast, as possible. In 
this way, the time during which the gas sample 2 is in contact with the 
capillary is shortened. Also, the transport time for the sample through 
the capillary is reduced so that the subsequent location can sooner be 
irradiated and the desorption of the location can begin. 
The following advantages are achieved: 
IMS-spectra obtained by laser desorption with a locationresolution of about 
50 .mu.m.sup.2 provide much better information than arrangements without 
laser desorption chamber. 
The measurements are not falsified by lubricants in the laser desorption 
chamber. 
The measurements are not affected by the formation of gases from sealing 
materials such as rubber or silicones. 
The measurements are not affected by any laboratory air present at the 
desorption location. 
With the pressure difference between the laser desorption chamber and the 
IMS apparatus the desorbed substance can be forced into the IMS apparatus 
which, because of the not sealtight drift cell of the IMS apparatus, 
results in fewer contaminations in the IMS spectrum than could be obtained 
with suctioning at the gas outlet of the IMS apparatus. 
With the pressure difference between the laser desorption chamber and the 
IMS apparatus, the desorbed substance can be supplied under pressure to 
any measuring apparatus if this is advantageous. 
Laser desorption is possible in any gas for example an inert gas such as 
nitrogen or helium. 
The internal pressure and the volume flow to the IMS apparatus through the 
heated conduit are adjustable by control of the flow. 
A minimal distance between the lens system and the sample holder is 6.8 mm 
which is sufficient for a lens system with large operating distance and 
1000 fold enlargement. 
A combination of the location resolution by the laser desorption at room 
temperature and transport of the desorbed molecules at higher temperature 
which is needed for a high detection sensitivity becomes possible with the 
laser desorption chamber. 
It is advantageous if an ion drift spectrometer (IMS) or ion drift 
spectrometers with subsequent mass spectrographs (IMS/MS) as detectors are 
coupled with a unit which desorbs by laser with good location resolution 
and supplies the desorbed substances to an analysis apparatus. 
Laser desorption without location resolution has the following 
disadvantages: 
The location resolution obtained by the selection of a particular area is 
in the square millimeter to square centimeter range and consequently, is 
quite inadequate. Furthermore, such a method cannot detect contamination 
on an organic matrix such as plastic material since the gas volume 
generated by a plastic material, resin, cement etc. is so large that the 
contamination signal itself is no longer detectable. 
With the method and apparatus according to the invention a laser beam is 
applied to the surface to be analyzed which desorbs all the organic 
compounds in a surface area of about 50 .mu.m.sup.2. The molecules 
released in this way are carried by a carrier gas to the IMS apparatus 
which is a sensitive detector for certain organic substances and are 
detected thereby. It is possible to determine the composition at various 
parts of a surface to obtain an indication concerning the reason for the 
difference. In this way, it is for example possible to find explanations 
for the failure of electronic components (wafers). As a result, the 
manufacturing process can be optimized. 
To show the operability of the complete apparatus p-methoxylbenzoin acid 
was desorbed by an argon ion laser at .lambda.=488 nm with a laser power 
density of 2-3 MW/cm.sup.3. The neutral molecules vaporized in the process 
were captured by a transport means, in this case a heated capillary, and 
supplied to the IMS apparatus where the molecules were detected upon 
arrival. During the laser desorption, and for some minutes thereafter, an 
IMS spectrum was taken every seven seconds in order to determine the 
time-dependent signal intensity of the molecules arriving in the IMS 
apparatus. By measurements of p-methoxybenzoin acid with an IMS/MS 
apparatus, the signal with K.sub.0 =1.76 cm.sup.2 /(VS) observed during 
laser desorption could be clearly determined to be the reason for the IMS 
signal of the protonized molecule ion (MH) of p-methoxylbenzoinacid, which 
proves that this substance is not destroyed during the laser desorption 
procedure. 
A location resolution in the .mu.m.sup.2 range can be obtained therewith. 
The substances can be desorbed directly from the sample surface and 
supplied to the analysis apparatus without the use of a solvent as an 
extraction means. Solid samples can be measured without expensive 
preparation whereby the sampling speed can be substantially increased and 
costs can be reduced. For most carrier materials such as silicon wafers or 
metals the method is non-destructive. 
The matrix in which the substance to be analyzed is disposed can be masked 
out to a large degree so that only suspicious areas are irradiated by the 
laser whereas the surrounding areas remain cold. 
Many ways are known to transfer solid, liquid or gaseous samples into the 
separation column of a gas chromatograph. With surface examinations, a 
certain location resolution has been achieved in that material from 
certain selected areas was scraped off and extracted by a solvent and 
injected into the column, or the solvent was applied for extraction 
directly to predetermined surface areas and the extraction solution was 
then injected into a gas chromatograph. The location resolution obtainable 
with such methods is in the square millimeter to square centimeter range 
and is consequently relatively bad. Also, the procedure is dependent on 
how well the substances can be extracted with the solvents utilized. 
With the combination of location resolution, laser desorption and GC in 
accordance with the present invention, a laser beam is effective on the 
surface to be analyzed which desorbs all the organic compounds on a 
surface area of about 10-100 .mu.m.sup.2. The molecules released in this 
way are collected by a heated capillary which is placed closely adjacent 
the surface area being desorbed and are transported to the analysis 
apparatus. 
This provides for the following advantages: 
A location resolution in the .mu.m.sup.2 range is obtained. The substances 
can be desorbed directly from the sample surface and supplied to the 
analysis apparatus without the use of a solvent as an extraction means. 
The laser desorption is substantially faster than the extraction by a 
solvent which increases sampling speed and reduces costs. The matrix in 
which the substance to be analyzed is disposed can be masked out so that 
only suspicious locations are exposed to the laser beam whereas the 
surrounding matrix remains cold. Since the laser desorption will rarely 
lead to a fragmentation of organic molecules the libraries already present 
can still be used for the identification of a substance. 
Since the gas chromatograph technology (GC) is at a substantially higher 
state of development and is more widely used than ion drift spectrometers, 
a gas chromatograph (GC) is generally more suitable for commercial use. 
However, newest developments show that gas chromatographs (GC) have lower 
detection limits than ion drift spectrometers (IMS) and are therefore more 
suitable for use as highly sensitive detectors.