Injection headspace techniques

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. 

The United States Pharmacopeia (USP) test (467) describes three different approaches to measuring organic volatile impurities in pharmaceuticals. Method I uses a wide-bore coated open tubular column (G-27, 5% phenyl-95 % methylpolysiloxane) with a silica guard column deactivated with phe-nylmethyl siloxane and a flame-ionization detector. The samples are dissolved in water and about 1 p is injected. Limits are set for benzene, chloroform, 1,4-dioxane, methylene chloride, and trichloroethylene. Methods V and VI are nearly identical to method I except for varying the chromatographic conditions. For the measurement of methylene chloride in coated tablets, the headspace techniques described above are recommended. 

Volatile substances are generally liquids of a variety of chemical types. Gas-liquid chromatography is the simplest approach for simultaneous separation and quantitation in many cases. The simple alcohols can be measured by injecting a diluted body fluid directly onto the column of the chromatograph. A more common approach is to use the headspace technique, as is done for gases, after incubating the specimen at an elevated temperature. 

Direct sample introduction without any derivatization is used as an alternative to derivatization and is most commonly applied to VFA. GC introduction techniques include direct aqueous and solvent injections, ° " ° headspace, and more recently solid-phase microextraction (SPME) Specially designed capillary GC columns with polar phases are typically followed... 

This special injection technique has advantages over all other methods of analyzing volatile substances with respect to separating power, sensitivity, ease of handling, and automation possibilities, and the headspace technique is therefore becoming more and more widely used. 

The term linearity is used today to indicate that the split ratio is identical for all sample components. This is the basic precondition for ensuring that the small amount of sample material analyzed by the column has the same composition as the sample in the injector. However, it does not mean that the true split flow must be the same as the pre-set split flow. A complementary concept is the idea of discrimination, which is the opposite of linearity. Discrimination is not a very important effect in the headspace technique as the sample is already in vapor form, but it is considerably more significant in liquid injection [11]. 

The SPME sampling device can be immersed directly into an aqueous sample such as groundwater for a finite period of time, then withdrawn, the fiber and rod retracted back into the needle, brought to the hot-injection port of a gas chromatograph, and then inserted into the septum, the fiber and rod extended into the injection port for a finite period of time in which thermal desorption is accomplished. For analysis of aqueous samples for VOCs, the fiber is inserted into the headspace, sampled, retracted, and then injected directly into the injection port. For solids, wastewater, sludge, etc. this headspace technique is appropriate provided that analytes can partition... 

Several others techniques dealing with the injection problems have been developed. Among them the solid-phase microextraction method (SPME) and the full evaporation technique must be mentioned. According to Camarasu, the SPME technique seems to be very promising for RS determination in pharmaceuticals, with much better sensitivity than the static headspace technique. 

Current official GC methods are described in USP XXIII under chapter 467 Organic volatile impurities . Four methods (I, IV, V, VI) are mentioned. Methods I, V and VI are based on direct injection. They are suitable for water-soluble drugs and V for water insoluble drugs. Method IV describes the static headspace technique and is used for water soluble drugs. Method VI is very general and refers to the individual monograph which describes the chromatographic conditions (injection, column, conditions) which should be used. The main characteristics of these four methods are summarized in Table 16.2.2. 

Clinical samples of urine, blood, expired air, and tissue have been examined using headspace sampling approaches. Thus, chlorinated organic compounds, methanol, acetone, methyl ethyl ketone, and phenols have been determined in urine. Volatile substances in urine have also been used as a guide to acute poisoning, and the determination of stimulants in urine has been proposed as screening test for field use. The determination of the concentration of blood alcohol is the most well-known application of headspace techniques to biological samples. Blood has also been examined for cyanide, methyl sulfide, and formaldehyde levels, the last as a measure of methanol intoxication. The headspace approach for blood samples overcomes the difficulties associated with the alternative direct injection of two-phase samples. 

Milk, being a two-phase substance, also creates some analytical difficulties for direct injection methods and therefore headspace techniques have been used to overcome these and to determine the odor components that have a bearing upon milk quality in processing and storage. 

The SPME device not only combines extraction and concentration but also directly transfers the absorbed compounds into a GC injector. These features of HS-SPME provide major advantages over previous headspace techniques. Coupling to GC, GC-MS (including ion-trap), split/splitless and on-column injection or desorption of the analytes in an SPME-HPLC interface have been described. A significant difference in sensitivity between direct and headspace sampling can occur only for very volatile analytes. HS-SPME introduces some selectivity into the extraction technique as only analytes with sufficient vapour pressure at room temperature are detected. An obvious drawback of HS-SPME is that semi- and non-volatiles will not be present in detectable amounts in the headspace. In combination with GC this is actually advantageous and enables faster equilibration than sampling from liquid [992]. 

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