Arrangement at a gas flow through cell for spectrophotometric analysis of chemical compounds

The present invention relates to a gas flow cell for spectrophotometric analysis of chemical compounds, consisting of a longish preferably cylindrical body (1) with a longitudinal central channel (2) which at the ends of the body (1) is limited by mutually parallel windows (3, 4) which are tightfitting against the channel (2) and transparent for the radiation used at the analysis. The channel (2) communicates at one end with connections (7, 8) for carrier gas and for injection of compounds and has at the other end an outlet (10) for the compounds and the carrier gas. Significant for the invention is that parallel with and at a distance from the mentioned channel (2) runs a second channel (11) serving for separation and connected between the connections (7, 8) and said end of the first mentioned channel (2). In this channel (11) there is a replaceable column tube (12) with filling of known composition.

The present invention relates to an arrangement at a gas flow cell for 
spectrophotometric analysis of chemical compounds in the gas phase 
consisting of a longitudinal preferably cylindrical body with a 
longitudinal central channel which at both ends of the body is limited by 
two mutually parallel windows which are tighfitting against the channel 
and transparent for the radiation used at the analysis, which channel 
communicates at one end with connections for a carrier gas and for 
injecting compounds and at the other end has an outlet for the compounds 
and the carrier gas. 
BACKGROUND OF THE INVENTION 
That part of analytical chemistry which is dealing with separation and 
detection of organic compounds in mixtures has ever since in the sixties 
been dominated by gas chromatography where the detection normally is 
obtained by an unspecific detector. More recently the technique has 
developed towards combinations of gas chromatography (GC) with qualitative 
analytical methods like mass spectroscopy (MS), IR-spectrophotometry (IR) 
or UV-spectrophotometry (UV). The advantage with these combinations is 
first of all that it gives possibilities for identification of compounds 
which are separated on the gas chromatograph. Further on it gives the 
possibility to achieve selective detection for example by selecting a 
wavelength on the UV- or IR-detector where only aromato compounds gives a 
response. 
DRAWBACKS OF KNOWN TECHNIQUE 
Gas chromatographs has in general a size and complexity which imply that 
the instrument must be permanently installed in one place. The injector, 
column and detector are further on individually heated and the 
possibilities for identification of an unknown compound is strongly 
restricted with conventionel instruments equipped with unspecific 
detectors. Additional drawbacks are lengthy start up times, occasionally 
relatively long analysis times and high instrument costs. 
Commersially available instrument combinations exist for GC/IR and for 
GC/MS but not for GC/UV. The commercial combination of instruments are 
very costly and have complicated connections between the gas chromatograph 
and the spectrophotometer which makes it considerably difficult for 
changing to other analytical techiques like IR-analysis of compounds in 
the liquid phase. The high costs and the complexity has led to that most 
analytical laboratories with a particularly quantitative direction will 
have restricted possibilities for identification of compounds. 
OBJECT OF THE INVENTION 
A primary object of the invention in question is to achieve a gas flow cell 
by initially described kind, by which the combinations GC/UV and GC/IR as 
well as enrichment of compounds can be carried out. 
This is obtained according to the primary characteristic of the invention 
in that there is a second channel serving for separation connected between 
the connections and one of the ends of the first mentioned channel, in 
which channel there is replaceable column with column filling of a kind 
known per se. 
In that the body, according to a special characteristic of the invention, 
is of an electrically leading polymeric material of a temperature 
self-regulating quality and has electric connections so that the body can 
be heated by connection to a voltage supply is obtained that through 
utilization of the body as a resistor element with temperature 
self-regulating characteristic depending on increasing resistance with 
increasing temperature the cell can be heated in a way that a good 
separation of the compounds is achieved.

DESCRIPTION OF AN EMBODIMENT 
On the drawing numeral 1 represents a longish preferably cylindrical body 
with a longitudinal central channel 2. The body 1 preferably has such 
dimensions that if fits in the cuvette housing of a spectrophotometer and 
owing to technical and constructive reasons is made in four parts 1a, 1b, 
1c and 1d which are kept together by screws 5 and, where so is necessary, 
are sealed with O-rings 6. Channel 2 is restricted at the both ends of the 
body 1 by mutually parallel windows 3,4 which are kept in place by the 
parts 1c and 1d and which are tightly connected to the channel 2. The 
windows 3, 4 are transparent for the radiation used at the analysis e.g. 
quartsglass at UV-detection and at IR-detection an IR-transparent material 
for example ZnSe. Channel 2 communicates partly with a connection channel 
7 for carrier gas and partly with a connection channel 8 for injection the 
mixtures of compounds which are going to be analysed. In the connection 
channel 8 there is a sealing injection membrane 9 through which injection 
can be carried out by a syringe. The opposite end of the channel 1 
commuincates with an outlet channel 10 for the compounds and the carrier 
gas. On the outside of the body there are connection tubes 7a and 10a 
which communicate with channels 7 and 10 respectively. 
According to the invention there is a channel 11 serving for separation 
and/or enrichment connected between the connections 7,8 in which there are 
a replaceable column tube with a composition known to a man skilled in the 
art. Channel 2 and 11 communicate with each other at the both ends turned 
against the window 3 at the left hand side in FIG. 2. The channel 11 with 
the column tube 12 and its filling serves for separation of different 
compounds in a sample as will be explained in the following. 
As is known there is as a rule a requirement for a defined temperature 
increase in order to separate different compounds in a sample. According 
to the present invention for this reason the body is therefore made of an 
electrically conducting material e.g. a carbon filled polymeric with 
self-regulating temperature characteristic and equipped with connections 
13, 14, 15 which can be connected to controlled voltage supply either by 
parallel connection through 13 and 14 resp 14 and 15 or in a series 
connection i.e. through 13 and 15 so that different kinds of heating of 
the body can be carried out. The channel 8 for the injection of the sample 
which is analysed goes through the connection 15. 
As can be seen the column tube 12 of the instrument is extremely short. In 
order to obtain comparable separation capacity as for conventionally 
packed GC-collumns the support material in the column tube 12 is of 
considerably smaller particle sizes. These support materials are 
commercially available as column filling materials for liquid 
chromatography. The liquid phase can be applied in a manner similar to 
what is usual for normal GC-supports. 
DESCRIPTION OF AN ANALYSIS EXAMPLE 
The gas flow cell is placed in a spectrophotometer and the connection tube 
7a for the carrier gas is connected to a gas supply and the electrical 
connections 13-15 to a voltage supply, not shown in the drawing. By 
applying a defined voltage over the poles 13-15, a defined temperature is 
achieved in the body 1. The sample is injected through the injection 
membrane 9 and is forced out in the space just in front of the separation 
column 12. Injection of samples can be carried out either in the gas phase 
or in the liquid phase. The compound in the sample is separated in a known 
way in the liquid phase. The compound in the sample is separated in a 
known way in the column. When the separated compounds reach the channel 2, 
they are registered by the spectrophotometer and the absorbance 
(proportional against concentration) and signal from the detector of the 
spectrophotometer can be recorded on a recorder or treated by an 
integrator in an usual manner. 
Selective detection of compound groups are obtained by the selection of a 
suitable wavelength on the spectrophotometer. 
At identification of compounds the analysis procedure is somewhat depending 
on the characteristics of the spectrophotometer. Advanced 
spectrophotometers having fast scanning possibilities can deliver a 
complete wavelength spectra while the compound is passing through the gas 
flow cell. When simpler and slower spectrophotometers are used the gas 
flow is stopped when the compound is in position in the gas cell and the 
spectra is registered from the captured compound whereupon the gas flow 
again is allowed to flow.