Direct Insertion of Samples

Various techniques have been employed to introduce solid samples directly into plasma. These techniques may be divided into two groups (1) direct insertion of samples into the plasma, and (2) methods that convert solid samples into an aerosol or a vapour, which is then transported into the plasma. The latter techniques include electrical spark and arc ablation, electrothermal vaporization, and laser ablation. 

1 Direct Insertion of Samples. In direct insertion the sample is introduced on a probe directly into the plasma. The sample is usually ground into a 

By using automatic position devices, such as stepper motors, DC motors, electrically operated car aerial motors, a combination of pneumatic actuators and motor-driven screw assemblies, and pneumatic elevators, the positions of the sample probe (intermediate and final positions) and duration at each 

Many of the operating parameters normally optimized for ICP-AES with solution nebulization, are also important in direct sample analyses. These include observation height, RF power, gas flow rates, integration time, and wavelength. In direct sample insertion, sample insertion heights for drying, ashing, and vaporization, rate and duration of insertion, and sample size must also be optimized. 

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In practice, direct insertion of samples requires a somewhat more elaborate arrangement than might be supposed. The sample must be placed on an electrode before insertion into the plasma flame. However, this sample support material is not an electrode in the usual meaning of the term since no electrical current flows through it. Heating of the electrode is done by the plasma flame. The electrode or probe should have small thermal mass so it heats rapidly, and it must be stable at the high temperatures reached in the plasma flame. For these reasons, the sort of materials used... 

Junk, G.A. Svec, H.J. A Vacuum Lock for the Direct Insertion of Samples into a Mass Spectrometer. Anal. Chem. 1965, S7, 1629-1630. 

Electrothermal vaporization (ETV) in addition to its features for the analysis of microsamples, in ICP-MS has the additional advantage of introducing a dry analyte vapor into the plasma. Hence, it has been found to be useful for elements for which the detection limits are high as a result of spectral interferences with cluster ions. In the case of 56 Fe, which is subject to interference by 40ArO+, Park et al. [529] showed that the detection limit could be improved considerably by ETV. For similar reasons the direct insertion of samples in ICP-MS leads to the highest absolute power of detection (detection limits in the pg range and lower [530, 531]). 

Another method devised for direct sampling of solids involves direct insertion of the sample into the plasma. In this procedure the sample is delivered through the central tube of the torch. The sample may be premixed with graphite powder. 

The coupling of a GLC column with the sample inlet system of a mass spectrometer is relatively easy, as the effluents are already in gaseous form. The main problem is the relatively high pressure at which these effluents reach the spectrometer and the excess of carrier gas in the stream. Several experimental devices now allow separation of the sample from the carrier gas, either by an effusion process or with the help of a thin, semi-permeable membrane222,353. The use of capillary columns permits direct insertion of the GLC effluent into the ion source without overtaxing the pumping capacity of the mass spectrometer 311 3 5 5 >3 5 6. 

The mass spectra of atenolol and compound were obtained by direct insertion of the sample into CEC 21-Ho B mass spectrometer. Characteristics of these mass spectra are summarized in Tables 5. and 6, and Pig. 5 The ion source temperature was 15o°C and 2oo-25o°C, respectively, and the ionizing electron beam energy was 7o eV. 

All mass spectrometers share common features. (See Figure 1.2) Some sort of chromatography usually accomplishes introduction of the sample into the mass spectrometer, although many instruments also allow for direct insertion of the sample into the ionization chamber. All mass spectrometers have methods for ionizing the sample and for separating the ions on the basis of mlz. These methods are discussed in detail below. Once separated, the ions must be detected and quantified. A typical ion collector consists of collimating slits that direct only one set of ions at a time into the collector, where they are detected and amplified by an electron multiplier. The method of ion detection is dependent to some extent on the method of ion separation. 

The sample must be introduced into the ionization source so that vacuum inside the instrument remains unchanged. Samples are often introduced without compromising the vacuum using direct infusion or direct insertion methods. For direct infusion, a capillary is employed to introduce the sample as a gas or a solution. For direct insertion, the sample is placed on a probe, a plate or a target that is then inserted into the source through a vacuum interlock. For the sources that work at atmospheric pressure and are known as atmospheric pressure ionization (API) sources, introduction of the sample is easy because the complicated procedure for sample introduction into the high vacuum of the mass spectrometer is removed. 

Many of the remarks made in the previous section concerning fibres can be applied to the analysis of plastics. Some polymers are soluble in organic solvents and samples may be prepared for direct aspiration into a flame in this way, e.g. MIBK is a suitable solvent for polyesters, polystyrene, polysiloxanes, cellulose acetate and butyrate dimethyl formamide for polyacrylonitrile, dimethyl acetamide for polycarbonates and polyvinyl chloride cyclohexanone for polyvinyl chloride and polyvinyl acetate formic acid for polyamides and methanol for polyethers. These organic solutions may alternatively be injected into a graphite furnace. Otherwise, polymers may be wet or dry ashed and the resultant ash dissolved in acid. An approach which is attracting increasing interest is the direct insertion of solid samples into a graphite furnace. 

The papers dealing with the GC/MS detection of 10 different opiates in hair are listed in Table 7. The first report was published in 1984 about codeine detection in animal hair after administration of the drug. One paper is included with direct insertion of the probe and Cl. As heroin samples always contain codeine as an impurity, this substance also can be detected in cases of heroin abuse. Morphine is a metabolite of codeine and can be detected when codeine is abused. The quantitation of both drugs allows differentiation between codeine and heroin abuse. The detection of heroin or 6-acetylmorphine opens the possibility to prove directly the abuse of heroin.2 2" Here also, the more lipophilic 6-acetylmorphine exceeds the morphine in most samples. 

Two particularly important considerations in ultrasonic slurry GF-AAS are that method development and optimization can be based on conventional liquid sample protocols and that, frequently, only limited changes are needed to facilitate direct insertion of solids. The recommended overall procedure for ultrasonic slurry GF-AAS determinations [101] involves six steps that are conducted in the following sequence ... 

The second batch of extract was fractionated into pentane-soluble and pentane-insoluble components 31% of the methylene chloride extract was soluble in pentane. This pentane-soluble fraction was then analyzed by HRMS by directly inserting the sample into the ion source. 

Wyeth Lot No. F-665901) was obtained- -- - by direct insertion of the sample into an MS-902 double focusing mass spectrometer. Results are summarized as a bar graph (Figure U). The ion source temperature was ll 0°C. and the ionizing electron beam energy was 70 eV. Data were compiled by Kuhlman and Shrader- - with the aid of an on-line PDP 8 Digital Computer. 

The mass spectrum of meprobamate (N.F. reference standard) was obtained by direct insertion of the sample into the MS-9 double focusing, high resolution mass spectrometer . The sample was run at 100°C. and 1.8 x 10 ° torr. The high resolution data was compiled and tabulated with the aid of the PDP-8 Digital Computer. The data is given in Table I and Figure 3. 

The mass spectrum of triamcinolone acetonide was obtained from Squibb House Standard 45885-008 by direct insertion of a sample into an MS-9 double focusing mass spectrometer. Intensities were measured from the low resolution mass spectrum. Results are summarized as a bar graph (Fig. 3). The high resolution mass spec-... 

More typical applications of mass spectrometry, such as GC/FTMS, direct insertion probe sampling and gas/vapor analysis have also been demonstrated as routine using this instrument. 

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