Problems of Sample Introduction

Therefore, if a large quantity of sample is introduced into the flame over a short period of lime, the flame temperature will fall, thus interfering with the basic processes leading to the formation and operation of the plasma. Consequendy introduction of samples into a plasma flame needs to be controlled, and there is a need for special sample-introduction techniques to deal with different kinds of samples. The major problem with introducing material other than argon into the plasma flame is that the additives can interfere with the process of electron formation, a basic factor in keeping the flame self-sustaining. If electrons are removed from the plasma by 

Flame temperature Flame dimensions (approx) Volume of 1.6 mL argon at 300K 

Heat content of flame Power output of flame (2 ms) Power input to flame 

Fundamentally, introduction of a gaseous sample is the easiest option for ICP/MS because all of the sample can be passed efficiently along the inlet tube and into the center of the flame. Unfortunately, gases are mainly confined to low-molecular-mass compounds, and many of the samples that need to be examined cannot be vaporized easily. Nevertheless, there are some key analyses that are carried out in this fashion the major one is the generation of volatile hydrides. Other methods for volatiles are discussed below. An important method of analysis uses lasers to vaporize nonvolatile samples such as bone or ceramics. With a laser, ablated (vaporized) sample material is swept into the plasma flame before it can condense out again. Similarly, electrically heated filaments or ovens are also used to volatilize solids, the vapor of which is then swept by argon makeup gas into the plasma torch. However, for convenience, the methods of introducing solid samples are discussed fully in Part C (Chapter 17). 

A number of elements form volatile hydrides, as shown in the table. Some elements form very unstable hydrides, and these have too transient an existence to exist long enough for analysis. Many elements do not form stable hydrides or do not form them at all. Some elements, such as sodium or calcium, form stable but very nonvolatile solid hydrides. The volatile hydrides listed in the fable are gaseous and sufficiently stable to allow analysis, particularly as the hydrides are swept into the plasma flame within a few seconds of being produced. In the flame, the hydrides are decomposed into ions of their constituent elements. 

Sample Inlets for Plasma Torches, Part A Gases 

A major advantage of this hydride approach lies in the separation of the remaining elements of the analyte solution from the element to be determined. Because the volatile hydrides are swept out of the analyte solution, the latter can be simply diverted to waste and not sent through the plasma flame Itself. Consequently potential interference from. sample-preparation constituents and by-products is reduced to very low levels. For example, a major interference for arsenic analysis arises from ions ArCE having m/z 75,77, which have the same integral m/z value as that of As+ ions themselves. Thus, any chlorides in the analyte solution (for example, from sea water) could produce serious interference in the accurate analysis of arsenic. The option of diverting the used analyte solution away from the plasma flame facilitates accurate, sensitive analysis of isotope concentrations. Inlet systems for generation of volatile hydrides can operate continuously or batchwise. 

---

Direct introduction of a sample into ICP produces information on only total element content. It is now recognised that information on the form of the element present, or trace element speciation, is important in various applications. One way of obtaining quantitative measurement of trace element speciation is to couple the separation power of chromatography to the ICP as a detector. Because most interesting trace metal speciation problems concern non-volatile or thermally unstable species, HPLC becomes the separation method of choice. HPLC as the separation technique requires introduction of a liquid sample into the ICP with the attendant problem of sample introduction. 

Micellar Electrokinetic Capillary Chromatography (MECC) is a relatively new technique in which the analytes are moved along in a capillary by micellar entrainment in an electric field. More commonly the technique is called Capillary Electrophoresis (CE). At present electrophoretic techniques in capillaries (packed and unpacked) receive a great amount of attention. First contributions from this laboratory show the possibilities which are characterized by speed, very high resolution and excellent inertness. Both alpha acids and iso-alpha acids have been separated completely and rapidly (38,39). If the problems of sample introduction and connected quantification can be solved, these techniques could well become very important for the brewery laboratories, even for routine analysis of bitter acids in hops and beer. 

Table 8.36 lists the main classical and newer approaches to solid sampling for elemental analysis. Little work on the introduction of solids into flames has been reported, because of problems of sample delivery and the relatively low source temperature. In arc and spark emission and in laser ablation as a sampling technique, the ablated sample material cannot be determined exactly. The limitations of arc or... 

The most commonly used technique of sample introduction is aspiration of the solution into the argon plasma flame. Because of the high temperatures in the flame, many of the problems associated with atomic absorption are eliminated. However, matrix effects such as significant differences in viscosity between sample and standard solutions can still have an effect. When needed, most of the techniques of sample introduction used in atomic absorption spectrophotometry can also be used for sample introduction in emission spectrophotometry [see the review articles " listed in the references]. 

Naturally, the overall successful quantitative analysis is a multifaceted problem of sample choice, quantitative recovery during extractions and purification steps, choice of internal standards, signal recording technology, etc. Most of these topics are beyond the scope of this chapter, as they are not deemed to be particularly characteristic of biochemical GC. However, sample treatment prior to its introduction into a gas chromatograph will be the subject of a later discussion. 

In conclusion, septum-type injection is the easiest and cheapest method of sample introduction. It can be used without problems up to 100 atm for the Single-septum type and up to ZOO atm for the double-septum construction. The injection of large volumes at high pressures, however, is limited because of the force needed to press the plunger against high pressures. Disadvantages of... 

A direct insertion probe is used for introduction of liquids with high boiling points and solids with sufficiently high vapor pressure. The sample is put into a glass capillary that fits into the tip of the probe shown in Fig. 9.5. The probe is inserted into the ionization source of the mass spectrometer and is heated electrically, vaporizing sample into the electron beam where ionization occurs. A problem with this type of sample introduction is that the mass spectrometer can be contaminated because of the volume of sample ionized. 

Most early work on the vibrational spectra of inorganic ions was carried out using Raman spectra, because of the problems of sample handling in the infra-red. However, a small amount of work was done using reflection spectra [ 1 ], and the development of the nujol mull technique stimulated many studies by Lecomte and his co-workers [2—5, 10]. The introduction of the pressed potassium bromide disc technique with the associated equipment for fine grinding has led to a stimulation of interest in this field. 

While operating a GC-MS instrument, the chemists routinely face diverse kinds of problems, which include lack of repeatability in analyte response, matrix effect, and so on. As a ready manual for the users of GC and GC-MS, this Handbook provides an answer to all such problems. The book has explained the fundamental concepts of sample preparation and provides an in-depth description of sample introduction to the GC system, column selection, and various advanced aspects, for example, 2D GC separations. The detection techniques described include the relatively simple detectors such as FID with a gradual transition to the complicated mass spectrometers, thus covering almost every single aspect of the GC and GC-MS technology with befitting explanatory examples. 

---