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. . . . . .. . . . . . . . One of the oldest adages in analytical chemistry is that the analysis can only be as good as the sample. By analogy, in atomic spectroscopy the analysis can only be as good as the sample introduction. The sample introduction process conditions the sample as it passes to the flame or plasma, and so largely determines the accuracy of the analysis. With this in mind, it might he thought that sample introduction systems would have occupied a central role in research and development in atomic spectroscopy over the past decade. However, even a cursory comparison of instruments produced in that time shows that sample introduction systems have not shared in the advances made by optical and electronic components. As a direct consequence, there has been relatively little progress over the past 10-15 years in some critical benchmarks. For example, there have been no dramatic improvements either in detection limits or in measurement precision. Additionally, the future growth of “speciation” measurements using atomic spectroscopy may be curtailed. In speciation studies, the sample introduction system acts as the interface between the chromatograph and the spectrometer. Developments 786A
*
in the interface may hold the key to the success or failure of the whole approach. How, then, has this strong divergence between the excellent optical and electronic components of most modern atomic spectrometers, and the rather inadequate sample introduction systems, become so strong? Furthermore, does there appear to be any hope that significant advances in sample introduction techniques can be expected in the next few years? We helieve that the key to the second question is contained in the answer to the first. Only by identifying the causes of the present blockage in the research pipeline can we hope for significant advances. The problems of sample introduc...
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strwwntal techniqi
ANALYTICAL CHEMISTRY, VOL. 56. NO. 7. JUNE 1984
,
tion have been clear for many years to most workers in the field. What has been less obvious are suitable means to overcome them. Primary weaknesses have resulted both from lack of unified theory and from deficiencies in existing models of critical processes. Without such models, it is very difficult either to optimize existing systems or to move into new areas. The current state of the art is probably the limit of empirical development, and major advances will come only when a sound framework of fundamental knowledge is in place. Without this structure, it will be very difficult even to define clear goals for future research. In contrast to the rather negative picture painted above, there has been a strong move in the past few years to develop improved sample introduction procedures. A comparison of papers published in the various areas of atomic spectroscopy, between 1979 and 1983, bears evidence to this (Figure 1).A steady increase in publications that may he considered as about “sample introduction” is seen. In addition to broadening the range of sample introduction techniques available, these developments may also finally lead to the much-needed improve0003-2700/84/035 1-786A$01.50/0
0 1984 American Chemical Society
Remrt Richard F. Browner School of Chemistry Georgia Institute of Technology Atlanta, Cia. 30332
Andrew W. Boorn Sciex 55 Glen Cameron Road, #202 Thornhill. Ontario L3T tP2, Canada
The Achilles' Heel of Atomic Spectroscopy?
menta in some of the analytical benchmarks mentioned earlier. In the remainder of this REPORT we will attempt to describe some of the major problems that remain to be confronted in sample introduction. Additionally, current understanding of basic mechanisms will be reviewed. The sample introduction routes we will consider are those most widely used a t present: liquid aerosol, vapor, and dry aerosol (electrothermal) (Figure 2). In this month's INSTRUMENTATION (page 875 A), some recent developments in techniques for sample introduction will be described.
Deflnlng Goals Stated simply, the goals of sample introduction are the following: the reproducible transfer of a representative portion of sample material to the atomizer cell, with high efficiency and with no adverse interference effects. Unfortunately, in certain circumstances, several of these criteria are mutually contradictory. As a further .complication, the criteria for optimum sample introduction in, for example, flame atomic absorption spectroscopy (FAAS) differ markedly from those for inductively coupled plasma (ICP) emission spectroscopy. Another prob-
lem to be considered is that as ICP spectroscopists are drawn into studies with ICP-mass spectroscopy interfacing, the sample introduction criteria of this new technique may possibly differ from those for optical emission spec-
troscopy. And this is occurring a: a time when we have barely scraped the surface of establishing acceptable criteria for the emission technique! As a redeeming feature of this situation, there is reason to believe that there-
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Figure 1. Publicationtrends in atomic spectroscopy, 1979-1983 ANALYTICAL CHEMISTRY, VOL. 56. NO. 7 , JUNE 1984
787A
\I
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&Plasma
Figure 2. Sample introduction routes for atomic spectroscopy
quirements of ICP/MS interfacing actually may he a little less stringent than those of optical emission spectroscopy. For example, height profiles should he less important with the MS interface, because of the different sampling mode. Nevertheless, all current knowledge from the emission spectroscopy field will require reevaluation for mass spectroscopy applications.
Liquid Sample Introduction One of the most attractive aspects of liquid sample introduction is its relative simplicity and reliability. This, allied with the fact that a sample dissolution step is often necessary to provide suitable sampling statistics, is probably the reason for the overwhelming use of liquid sample introduction in all branches of atomic spectroscopy. Nevertheless, an ICP spectroscopist observing 99%of a hard won analyte solution passing to waste, or having to wait 60 s between the introduction of successive samples to the spectrometer, is aware of the need for improvement. Table I indicates the relative wastefulness of pneumatic nebulizers used for liquid sample introduction. In atomic absorption spectroscopy, no higher than 10%efficiency can be expected. In ICP spectroscopy, a typical value is 1%. The most direct approach to overcoming poor sample transport efficiency (defined as the percentage of analyte mass reaching the atomizer, compared to that aspirated) is to allow more of the pneumatically generated aerosol to pass directly to the flame or plasma. Indeed, some early atomic ahsorption spectrometers used a total consumption burner placed directly beneath the laminar flame slot as a means of high-efficiency sample introduction. However, users of these instruments will vouch for the poor precision and high vaporization interfer788A
ences that they experienced. In flame atomic absorption spectroscopy, the guiding principle is that any improvement in transport efficiency must not he achieved by introducing large aerosol drops to the flame. For a solution of concentration C (pg/mL), contained in a drop of initial diameter do (pm), the dry salt crystal of density p (g/cm3), formed after solvent loss will have diameter d, (pm), given by:
For example, a 2-pm-diameter drop of 100 pg/mL sodium chloride solution will dry to a particle of approximately 0.07 pm diameter. A 10-pm-diameter drop of the same solution will give a 0.36-pm-diameter particle. When nonvolatile matrices are present in high concentration, the vaporization rate of the analyte will be inhibited. In lower temperature flames, such as air-acetylene, this may lead to severe vaporization interferences of the calcium-phosphate variety.
Experimental Sample introduction Benchmarks While transport efficiency, E ” , is a commonly used criterion for evaluating nebulizers and spray chambers, it does not readily lend itself to comparison of different systems. For example,. the transport efficiency of a system often can he increased simply by reducing solution uptake rate to the nebulizer with a smaller diameter uptake tube. However, this does not result in any net increase in analyte mass transport to the atomizer, or in any signal increase. It has therefore recently been suggested that a more useful criterion for assessing nehulizer and spray chamber performance is the W-parameter ( I ) . W,,, is a mass
ANALYTICAL CHEMISTRY, VOL. 56. NO. 7. JUNE 1984
transport term that describes the total analyte mass transport rate to the atomizer. This may he further suhdivided into components W , and W,, which are the mass transport rates of useful and excess analyte, respectively. Here, “useful” analyte is considered to refer to analyte contained in drops sufficiently small to produce useful analytical signal in the atomizer. The “excess” term refers to analyte contained in drops too large to produce useful signal in the atomizer. Although there is no sharp dividing line between these two terms, the concept is felt to have considerable practical use. It provides an indication of what drop size the atomizer is able to accommodate, while giving rise to interference-free analytical signals. For example, in FAAS using the premixed air-acetylene flame, the transition from useful to excess aerosol, for the removal of the classic phosphate-calcium interference, occurs in the region of three to five pm. The diameter of the aerosol drop, which contributes less than 1%to the analytical signal, is designated dmOx.Although numerical data for d,,, are presently sparse, this parameter should also be a useful benchmark for comparing atomizer capabilities. Lack of transferability of interference and detection limit data between laboratories has been a lingering prohlem in atomic spectroscopy. A major source for the discrepancies has been unrealized variations in drop size distributions of aerosols generated hy different instruments. In the past this has led to the unfortunate situation where lack of data correlation has been taken as an indication of had technique. With more objective benchmarks, it should he possible to avoid this type of unproductive conflict. There is currently no reliable sample transport data base, even for parameters such as E.. This is a consequence of the widespread historical use of indirect methods for measuring
Table 1. Transport Efficiency Data for Pneumatic Nebulizers a e. % Auxlllary Gas flow, umin 0 8.8
System
AA-no burner head AA-with burner head ICP-wncentric #I
iCP-concentric iCP-aossflow *A
#2b
5.1
7.4
5.3 6.6 0.6 1.4 1.5 -
~ ~ ~ osolUtiM. u s ICP concentric nebulizers $I and #2 are differem examples of the same model.
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
789A
Ultrahigh Resolution Chromatography /
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Flgure 3. Aerosols from pneumatic nebulizers (top) AA nebulizer (bonan) ICP nebulizer
transport properties. Indirect methods are generally unreliable, and those wishing to carry out 6, or W measurements are referred to recent publications for better procedures (2,3). The essential point to note is that except where transport efficiencies are exceptionally high, i.e., 20% or greater, the simple arithmetic of calculation ensures that data obtained by indirect waste collection will have a much higher error than data obtained by direct aerosol collection. Errors with ICP systems are especially high, because of the generally low (0.5-2.0%) transport efficiencies common to them. With indirect procedures, it is easy to obtain e, values with a 500% positive error (2).
Aerosol Generation and Transport Processes The limitations of pneumatic nebulizers are clear from looking at Figure 3. Aerosols produced from atomic absorption and ICP nebulizers can be seen to have both extremely wide drop size ranges, and very turbulent gas flow patterns. The photographs were taken with the nebulizers operating in free air, and the presence of a spray 7SOA
chamber modifies the flow patterns considerably. Nevertheless, a comparable range of drop sizes, and equally turbulent gas and aerosol flow patterns, will still exist in the more constrained environment. The Sauter mean diameter of the a e r m l produced by pneumatic nebulizers (e.g., the diameter of the drop whose volume-to-surface-arearatio is the mean of the distribution) has traditionally been described by the equation due to Nukiyama and Tanasawa. The relationship, derived from their c h i c series of papers (4), is:
I
Expands th. field of chromatography by recordingthese recant advancms in ulttahtgh raaoluiion chromatography. Reporis the latest developments in capillary GLC, HPLC. and SFC.Coven topics w c h as interactions bet w w n raiaHva peak. relaUve retention, and abwiut. mtentkm; the sftact of column dmmeier reduction in capillary chromatography:and the UM of a small volume t h m l con. ductinty detector.
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CONTENTS Overview Munlple Pathways to Unrahlgh Remlution Chromatography Slatlslcal Model 01 Component Ovedap by ComputerGeneratsdChromatograms* Ultrahigh Resolution GC * lnteractonsBetween Relawe Peak, Relalava Retention. and Ab. soIu1e Retention Redductm of Column Diameler m Capwary GC Small Wume Thermal Conductmy Delenor for Capillary GC * Advances in Liquui Chromalogaphk SeIectivny * Narrow Bore Micropartrcte Packed Column High Pertormance GC * Micm High PerfamanceGC &plication to Separation d Complex Mixlures * Hqh Rerolutton SFC * CO,Based SFC Super Rasolutm in Chmmatographyby Numerical Wconw(ullon Role ot Ultrahigh Resolutm Cnromatographyin Chsm1-1 lndustw eIIS9a.m
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where d. (Nm) is the Sauter mean diameter, V, the velocity difference hetween gas and liquid flows (ds), u, the surface tension of the liquid (dynl em), p , the liquid density (g/cmS), 7, the liquid viscosity (poise), and Q, and Qg.the volume flow rates of liquid and gas, respectively (em3/s). The limitations of this equation are well known, and it is doubtful that it
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
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Figure 4. Aerosol modifying processes
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is an accurate description of aerosols generated hy all pneumatic nebulizers. Nevertheless, it has been shown to have considerable value, a t least for predicting trends for aerosol generation in atomic spectroscopy. The important thing to note is that this equation refers to the aerosol emerging from the nebulizer, not to the aerosol reaching the atomizer. The aerosol modifying processes that convert the primary aerosol produced by the nehulizer to the tertiary aerosol arriving a t the atomizer are caused by devices such as spray chambers, impact heads, impaction surfaces, etc., placed in the aerosol path (Figure 4) (I). Aerosol modifiers may act in a variety of ways, hut typically remove large drops from the stream, and allow only drops of less than a certain size to pass. Impact beads may also act to generate small drops by shattering larger drops from the primary aerosol. This results in production of the secondary aerosol. The effectiveness of these aerosol modifiers in the removal of drops is
I CIRCLE 2 ON READER SERVICE CAR0
792A * ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
Figure 5. Drop size distribution curves
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generally expressed as the cutoff diameter of the spray chamber, d,. This refers to the drop size, on a drop size distribution plot, where the ordinate mass transport term is reduced to 50% of its peak value hy the action of the spray chamber (Figure 5). The transition from primary to tertiary aerosol is accompanied by a large reduction in mean drop size of the aerosol. The mass fraction of aerosol contained in the larger drops is substantial, and its loss accounts for the high rate of wastage found with pneumatic nebulization, as essentially all the large drops pass to waste.
Solvent Vapor lnteractlons with Flames and Plasmas When more aerosol is introduced into a flame or plasma, not only the mass transport rate of analyte to the plasma increases, hut also the mass transport rate of the accompanying solvent. This may have a significant effect in lowering the temperature of atomic absorption flames, hut its influence is observed most dramatically with the ICP, where high solvent loadings may actually extinguish the plasma discharge. This becomes particularly noticeable when volatile organic solvents are used. Rapid aerosol evaporation with volatile solvents results in a higher mass transport rate of analyte to the plasma, hut also a corresponding higher mass transport rate of solvent (5). Additionally, the solvent vapor coming from aerosol evaporation also passes to the plasma. Several publications have shown that the effect of organic solvent on the plasma is generally to reduce its excitation properties significantly (6, 7). This may result in a substantial net reduction of analytical signal, in spite of the increase in analyte mass transport rate to the plasma. Some selected mass transport data for several solvents are shown in Table 11, and signal data in Table 111. It is possible to overcome the negative effect of increased solvent transport to the plasma by one of two means: Either oxygen may he added to the plasma to oxidize the organic species present (8),or excess solvent vapor may he removed before aerosol
'able II. Transport Efficiency
lata for Various Solventsa
Xylenes Nlbobenzene Water a
794,.
IC"
-
29.1 43.9 72.7
.rDBsflow nebulizer.
14 15 1.5
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:hlwoform MlBK
Xylenes
0.4 2.8 3.0
0.1 0.3 0.8
2.2 3.2 3.2
1.1 1.8 2.5
*Measured at 15-mm obsewation height,with 1.0Mlmin nebuiizerflow, l2lIminplasma gas "^w, l.M/min auxiliary gas flow, and 1.75.kW rf pawr-
introduction to the plasma, with either a cooled spray chamber ( 9 )or a condenser (7). The case of organic solvent introduction is a severe one. Nevertheless, even aqueous solutions exert a major effect on plasma excitation properties. Recent studies have indicated that, for example, a doubling in water aerosol transport rate to an ICP, for a fixed analyte mass transport rate, can result in up to a hundredfold reduction in emission intensity (IO). Taken one step further, the high water loading from an ultrasonic nebulizer can even extinguish a plasma. Consequently, in order to obtain improved detection limits using ultrasonic nehulization, the solvent loading must he greatly reduced. With ultrasonic nehulization, it is possible to increase W (the analyte mass transport rate) to the plasma by about 10 times (2). With (partial) aerosol desolvation, the signal improvement also can he close to this value. The whole area of solvent-plasma, vapor-plasma, and aerosol-plasma interaction is in need of thorough investigation. Without a better understanding of fundamentals, it will he very difficult to make any significant improvements in liquid aerosol introduction for the ICP. The optimization of low flow mini- and microtorches (11,12) will also require parallel studies, as undoubtedly the conditions for optimum sample introduction will vary for the different combinations of gas flow and power loading. These studies are also relevant to electrothermal sample introduction, as discussed helow.
Electrothermal Vaporization Electrothermal devices show a great deal of promise when used as sample introduction devices for the ICP (13, 14). Transport mechanisms with electrothermal vaporization (ETV) are much simpler than those involving liquid sampling. The two mechanisms that can inhibit transport of material between the vaporizer and the plasma are first, the formation of nonvolatile carbides on the furnace walls (e.g., with silicon, vanadium, etc.) and second, the loss of very volatile materials
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7. JUNE 1984
on the walls of the tubing used for transport between the vaporizer and the plasma. For example, volatile elements such as cadmium and zinc may he readily lost in the transport process. With ETV, the criterion for optimum sample transport becomes the production of highly dispersed, particulate material that may be transported readily in the gas stream between the vaporizer and the plasma. This is in marked contrast to the situation in atomic absorption or emission spectroscopy, where efficient atomization in the furnace is essential for accurate analysis. With the ICP, the desired species are not atomic but molecular, and furthermore preferably condensed molecular species. Consequently it is desirable that there should he rapid cooling of the vapor after it leaves the vaporizer surface. This will assist the rapid formation of highly dispersed nuclei, with suhsequent growth by condensation, to give dry aerosol in an easily transportable form. The efficient transport of less volatile or carhide-forming species has been shown to he capable of improvement by the addition of volatilizing gases, such as freons, to the argon carrier stream (15).This approach is similar to the carrier distillation techniques well known to spectrographers. While the transport mechanisms associated with electrothermal vaporization are much simpler than they are for liquid sample introduction, the uncertainty about optimum plasma excitation conditions still remains to he clarified, as mentioned earlier.
Vapor Introduction Vapor introduction techniques are well established in AAS for stable, hydride-forming species (e.g., arsenic, antimony, selenium, tellurium, germanium, tin, bismuth, and lead) and also for mercury vapor introduction. These techniques have been applied recently to sample introduction with the ICP, with considerable success (16,17). Additionally, a more specialized approach, which involves the production of volatile metal chelates from a wide range of transition elements, has been used successfullywith the inductively coupled plasma (18).
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Vapor introduction offers several advantages over conventional liquid sample introduction: It gives a transport efficiency approaching 100%and it offers the possibility of preconcentration. Disadvantages result from significant interelement interferences from other hydride-forming elements and the adverse effect of hydrogen in the argon carrier gas on plasma stability. Hydrogen acts to quench the discharge, and this can cause a significant reduction in excitation temperature. In AAS, hydrogen generated by the sodium borohydride is typically used in a hydrogen diffusion flame, which is a convenient thermal reservoir for hydride atomization. Hydride generation sample introduction, therefore, appears to be another instance where the ICP is significantly less robust in response to changes in its operating environment than the combustion flame.
Liquid Chromatography and Flow Injection Interfacing There are two prime areas where sample introduction procedures hold the key to the success or failure of an entire analytical approach. These are liquid chromatography-ICP (LC/ICP) and flow injection-ICP (FI/ICP) interfacing. “Speciation” has been a highly considered development area for atomic spectroscopy for many years (19). However, in spite of some very successful applications, there is relatively little widespread use of chromatography in atomic spectroscopy. Gas chromatography, coupled with FAA or ICP spectrometers, appears to offer few practical problems. However, the number of environmentally or biomedically important organometallic species capable of gas chromatographic separation is relatively few. The main use of this technique has been for volatile species, such as alkyllead compounds (20). Liquid chromatography, either using ion exchange, normal phase, or reversed-phase packings, is necessary for the separation of most organometallic species (21,22). The basic criterion for a successful LC/ICP or LC/AA interface is efficient analyte mass transport (23).A typical band eluting from a liquid chromatographic column may contain only nanograms of the species of interest, as passage through the chromatographic system will have caused substantial dilution from the initial injected analyte concentration. Additional band dispersion and distortion also may occur between the column and the nebulizer, unless the nebulizer is positioned close to the column end (23). Passage of the eluting band through a conventional ICP pneumatic nebu-
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
lizer-spray chamber system will result in the loss of approximately 99% of the analyte, resulting in a net analyte transport to the plasma in the picogram range. At this level, system response is usually inadequate for practical analysis. It may in fact be inferior to more conventional, though less selective UV or fluorescence detectors. LC/ICP interfacing is an instance where current limitations of nebulizers, often acceptable for conventional sample introduction, become unacceptable. The very specific limitations of analyte amount and liquid flow rate, imposed on the nebulizer and spray chamber by the chromatographic separation, create serious problems. Again, the obvious means to improve the interface-by allowing more aerosol to reach the plasma-is subject to the same restrictions discussed previously for continuous sample introduction. Nevertheless, some success with this approach has been reported recently, using a micro-LC system (24). In terms of transport mechanisms, a plug of sample injected into a flowing stream behaves identically to the same sample introduced continuously over an extended time period. Consequently, although a transient peak appears at the atomizer, as far as the nebulizer is concerned the process is that of continuous stream introduction. Therefore, no improvement in transport efficiency is obtained, by contrast to the introduction of discrete microsamples into pneumatic nebulizers (25).Without some major improvements in interface performance, it is unlikely that LC/ICP interfacing will achieve widespread use. Speciation studies may then remain restricted to the few volatile compounds suitable for gas chromatographic separation. Flow injection has great potential in atomic spectroscopy. It offers the potential for microsample handling, combined with greatly enhanced sample throughput (26).As such, it could have a dramatic influence on liquid sample introduction in the next few years, and may even become the liquid sample introduction technique of choice for the ICP (27).Flow injection also has similar attractions in FAAS (28).At present, the response problems of FI/ICP interfacing are identical to those of LC/ICP interfacing, as in essence the FI experiment is the LC experiment without prior column separation.
Summary Is sample introduction, then, really the Achilles’ heel of atomic spectroscopy? The answer is probably a qualified “yes.” “Yes,” because there is still a lack of fundamental knowledge in many critical areas. “Qualified,” because nonetheless some significant ad-
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