Solvent vapours, detection

Figure 2.12 Schematic representation of an on-line SPE-GC system consisting of three switching valves (VI-V3), two pumps (a solvent-delivery unit (SDU) pump and a syringe pump) and a GC system equipped with a solvent-vapour exit (SVE), an MS instrument detector, a retention gap, a retaining precolumn and an analytical column. Reprinted from Journal of Chromatography, AIIS, A. J. H. Eouter et al, Analysis of microcontaminants in aqueous samples hy fully automated on-line solid-phase extraction-gas chromatography-mass selective detection , pp. 67-83, copyright 1996, with permission from Elsevier Science.

The reactions were carried out in an Erlenmeyer flask, employing a conventional domestic microwave oven. To limit the presence of solvent vapours inside the cavity, the reaction mixtures were irradiated for 1 min and the deprotection reaction was subsequently monitored by thin layer chromatography (TLC). If starting material was present, additional cycles of 1 min were repeated until no traces of starting material could be detected by TLC. 

Selenium and tellurium The elements are present as selenite and tellurite in dilute nitric acid solution. The mixture is spotted upon paper and dried thoroughly in the air. The solvent is dry n-butyl alcohol containing 4 per cent (v/v) of dry methanol. The atmosphere in the separation vessel is saturated with respect to the solvent vapour and the relative humidity is also maintained at 50 per cent by means of a saturated solution of calcium nitrate. The solvent is allowed to diffuse 8-10 cm down the strip (c. 2 hours). After evaporation of the solvent, the strip is sprayed with 0 5m tin(II) chloride in dilute hydrochloric acid. The tellurium is indicated by a black band (RF 0 1) and the selenium as an orange band (RF 0 5). It is possible to detect 1-5 pg of Se in the presence of 1 mg of Te by this method (see also Fig. VI.5g). 

Colorimetric detection of solvent vapours using MIPs deposited on quartz crystals... 

Dickert and co-workers [18] have made an innovative advance by applying polyurethane-based MIPs to quartz crystals for the selective detection of solvent vapours (see also Section 20.2.5.8. and Chapter 21). Electrodes may be attached to quartz crystals to form quartz crystal microbalances (QCMs), wherein minute increases in the mass of the device (for example, upon adsorption of solvent vapour) result in a decrease of the resonant frequency of the crystal. For crystals... 

Imprinted polyurethanes as coatings for optrodes have been used for the detection of solvent vapours in air (see chapter 20). One per cent of substituted 3,3-diphenylphthalide as indicator has been intercalated within the polymer. The phthalide forms a highly coloured planar carbenium ion by interaction with an acidic component and a subsequent cleavage of the lactone ring - the unreacted phenolic groups in a polyurethane provide enough acidity for this reaction (Fig. 21.4). The incorporation of analytes reduces the acidity and the back-reaction... 

In this device the liquid sample is sprayed into a heated spray chamber, where the nebulizer gas transfers the aerosol through the membrane desolvator. An argon flow removes the solvent vapour from the exterior of the membrane. If compared to conventional pneumatic nebulizers, this system enhances analyte transport efficiency and limits solvent loading to the plasma. Oxide and hydride polyatomic ion interferences are significantly reduced, improving the detection limits by an order of magnitude. 

For ICP-OES-MS (inductively coupled plasma-optical emission spectroscopy-mass spectroscopy) work, the desolvator will remove oxide and hydride polyatomic ion interferences, i.e. ArO+ is reduced 100 fold, which allows for improved detection of Fe. The solvent loading reduction is caused by volatiles passing through the walls of a tubular microporous Teflon PTFE membrane. The argon gas removes the solvent vapour from the exterior of the membrane. Solvent-free analytes remain inside the membrane and are carried to the plasma for atomisation and excitation. 

Using the faculty of smell is a very crude method of detecting solvent vapour, but it is valuable to know for which solvents it is useless as a protection against harmful long-term exposure. These are the solvents which have an odour threshold higher than their TLV (e.g. chloroform). The odour threshold varies between individuals and tends to increase with length of exposure (i.e. one becomes used to a smell). It also can be affected by the presence of other solvents which can mask a smell. 

Dicker FL, Zenkel ME, Bulst W-E, Fischerauer G, Knauer U (1997) Fullerene/liquid crystal mixtures as QMB- and SAW-coatings - detection of diesel- and solvent-vapours. Fresenius J Anal Chem 357 27-31 Domingo-Garcia M, Lopez-Garzon FJ, Perez-Mendoza M (2000) Effect of some oxidation treatments on the textural characteristics and surface chemical nature of an activated carbon. J Colloid Interface Sci 222 233-240 Dumitrescu I, Unwin PR, Macpherson JV (2009) Electrochemistry at carbon nanotubes perspective and issues. Chem Commun (Camb) (45) 6886-6901... 

Except for the Corona model 117, two carefully matched thermistors, on which the solution and solvent drops are placed, are installed in a chamber saturated with solvent vapour. The temperature difference (T-Tq) is detected as a change in the resistance of the thermistors. A large factor governing the accuracy of the temperature measurement of the drops by the thermistors in VPO is the unmatched resistance of the thermistor bead-temperature relations between two thermistors. Even if an aged and well-matched pair of thermistors are chosen at a specific temperature, these do not always match at a different temperature. That is, even if the temperature of the surroundings fluctuates only to a small extent, the conventional Wheatstone bridge is kept in a balanced state only when [14]... 

EL. Dicker , P. Forth, P. Lieberzeit and M. Tortschanoff, Molecular imprinting in chemical sensing. Detection of aromatic and halogenated hydrocarbons as well as polar solvent vapour, Fresenius J. Anal. Chem., 360 (7-8) 759-762, 1998. 

Uhde and co-workers [34, 35] have studied the migration of 4,4 -thiobis-6-tert-butyl-w-cresol (Santonex R) from plastics utensils into sunflower seed oil. Sunflower seed oil that had been stored in vessels of polyethylene containing this antioxidant was diluted (3 5) with pentane and extracted with acetonitrile containing 5 % of water. The concentrated acetonitrile extract (or an ethanol solution of the residue on evaporation) was snbjected to thin-layer chromatography on Kieselgel with hexane-ethyl acetate (10 3) as solvent. To detect the antioxidant (down to 0.1 ppm) the plate was sprayed with 3,5-dichloro-p-benzoquinonechlorimine solution. To determine the antioxidant, the zone at Rf = 0.44 (located by means of iodine vapour) was removed and treated with fuming nitric acid sulfuric acid (1 1). The nitro-derivative of the antioxidant was determined in the product by polarography after the addition of urea and sodium acetate [35],... 

IMS can be used for chemical analysis of vapours from electronics packaging [287]. IMS-QMS has been used to analyse headspace vapours in sealed electronic packages [275,288] and to follow outgassing of polymers [287]. Various types of photoresist solvents, phtha-late plasticisers and other polymer additives, such as BHT, were detected. Other applications of IMS in semiconductor technology involve failure analysis control of the efficiency of cleaning and etching steps characterisation of process media and surveillance of the atmosphere of clean rooms. 

The vapour pressure osmometer method is more acceptable of all the methods involving measurement of colligative properties because of the sensitivity of the detector. For ideal solvent-solvents with a low heat of vaporisation, the differential thermistors of the VPO can detect differences in temperature of the order of 0.001°C this sensitivity determines the Molecular weight of the samples upto 20,000. 

TATP has such a high vapour pressure that it can probably be directly detected, whereas RDX has such a low vapour pressure that dogs alert on the bouquet of solvents used in its manufacture. Nitrate esters readily decompose to eliminate nitrogen dioxide (NO2). This can be a clue for canines and certainly is for chemiluminescence. 

The advantage of headspace mode is that only volatile components that will not contaminate the GC are injected. InvolatUes do not partition into the headspace and so never enter the injector. Effectively, the analyte is decoupled from the influence of the drug (but see the discussion on validation below). However, many analytes that are amenable to GC by direct injection are not sufficiently volatile to give a high-enough vapour pressure to be detected by conventional headspace injection. These semi-volatile components can sometimes be successfully analysed using a variant of the headspace technique known as total vaporisation headspace injection. In this instance, a few microlitres of the sample solution are injected into the headspace vial, which is then incubated at a temperature that vaporises the solvent completely into the headspace. 

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