Spectroscopy arc and spark emission

In most modern laboratories, arc and spark emission is limited to the analysis of solid samples, because liquids are much more easily analyzed by ICP or DCP emission spectroscopy. 

Arc and spark emission spectroscopy is widely used for the qualitative, semiquanti-tative, and quantitative determination of elements in geological samples, metals, alloys, ceramics, glasses, and other solid samples. Quantitative analysis of more than 70 elements 

Bulk solid samples must be machined or polished into the correct shape, and this must be done without contaminating the surface from the grinding material or altering the surface composition by polishing out a soft phase, for example. The surface must be cleaned of any lubricant or oil used in the machining process before analysis. 

Advances in understanding of the spark and arc sources and improvement in electronics have led to redesign of the sources and spectrometers, so that in modem instmments, stmcture-induced interelement effects can be minimized by proper choice of excitation and measurement conditions. This has reduced the need for a common matrix and has permitted, in some cases, the use of one global cahbration curve for multiple materials. However, if an interelement effect does exist, correction factors can be calculated to compensate for the effect. The text by Thomsen has a detailed explanation of how this is done. 

If there is no suitable analytical hne free of spectral overlap, corrections can be made for the interfering element, but only if the overlapping element s intensity is less than 10% of the analyte intensity. Less than 1 % is even better, if possible. Corrections cannot be made if the overlapping line is one of the major elements in the sample (e.g., overlap from an iron line on Cr in a steel sample cannot be corrected mathematically). 

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Previous experience in arc and spark emission spectroscopy has revealed numerous spectral overlap problems. Wavelength tables exist that tabulate spectral emission lines and relative intensities for the purpose of facilitating wavelength selection. Although the spectral interference information available from arc and spark spectroscopy is extremely useful, the information is not sufficient to avoid all ICP spectral interferences. ICP spectra differ from arc and spark emission spectra because the line intensities are not directly comparable. As of yet, there is no atlas of ICP emission line intensity data, that would facilitate line selection based upon element concentrations, intensity ratios and spectral band pass. This is indeed unfortunate because the ICP instrumentation is now capable of precise and easily duplicated intensity measurements. 

In arc- and spark-emission spectroscopy, one of the critical aspects of quantitative analysis is the need to match the standard as closely as possible to the sample. Dilution of sample and standards by a common matrix in DC-arc methods somewhat reduces the dependence upon exact matches. Gordon and Chapman devised a common matrix-dilution technique for DC-arc analysis which is almost totally independent of the forms of the sample and the standard [4]. 

The relative accuracy and precision obtained by arc and spark emission spectroscopy is commonly about 5%, but may be as poor as 20-30%. Arc emission is much more prone to matrix effects than spark emission due to the lower temperature of the discharge. Both arc and spark excitation may require matrix matching of sample and standards for accurate analyses, and usually require the use of an internal standard. 

Arc and spark emission spectroscopy is widely used for the qualitative, semiquantitative, and quantitative determination of elements in geological samples, metals, alloys, ceramics, glasses, and other solid samples. Quantitative analysis of more than 70 elements at concentration levels as low as 10-100 ppb can be achieved with no sample dissolution. The determination of critical nonmetal elements in metal alloys, such as oxygen, hydrogen, nitrogen, and carbon, can be performed simultaneously with the metal elements in the alloys. 

Applications of Arc and Spark Emission Spectroscopy 7.2.3.1 Qualitative Analysis... 

Spectroscopic methods for the deterrnination of impurities in niobium include the older arc and spark emission procedures (53) along with newer inductively coupled plasma source optical emission methods (54). Some work has been done using inductively coupled mass spectroscopy to determine impurities in niobium (55,56). X-ray fluorescence analysis, a widely used method for niobium analysis, is used for routine work by niobium concentrates producers (57,58). Paying careful attention to matrix effects, precision and accuracy of x-ray fluorescence analyses are at least equal to those of the gravimetric and ion-exchange methods. 

Arc and spark source spectroscopies were ihc first instrumental methods to become widely used lor analysis. These techniques, which began to replace the chis-sical gravimetric and volumetric methods for elemental analysis in the 1920s, were based on excitation of emission spectra of elements with electric arcs or high-voltage sparks. These spectra permitted the qualitative... 

Atomic fluorescence is the most recent development in analytical atomic spectroscopy thus it has not had time to be evaluated as well as other techniques. Further developments in this field with respect to optimizing sources and sample cells, together with improvements in instrumental parameters and development of readily available commercial instrumentation, should lead to this technique serving in the area of analytical spectral methods to supplement the already well-established arc and spark emission, flame emission, and atomic absorption spectroscopy. 

Automatic Atomic Emission Spectroscopy, 2nd edn, Slickers, K., Briihlsche Universitatsdmckerei, Giessen, 1993. A very useful practical guide to arc and spark methods in the metallurgical industry. 

The major disadvantage of arc/spark emission spectroscopy is the instability of the excitation source. This problem can be virtually eliminated by the use of a plasma torch. The most common commercially available method uses an inductively coupled plasma (ICP), which is also called RF plasma, to excite the sample (13-19). The resulting spectrometers (Fig. 4) can simultaneously measure up to 60 elements with high sensitivity and an extraordinarily wide linear dynamic range. 

Classical methods for analysis of manganese have been the periodate method in air, and the permanganate method in water (Saric 1986). Nowadays, among the solid-state analytical methods available, neutron activation analysis (NAA) is the most reliable to determine manganese in biological and environmental materials. This method of choice combines both high specificity, sensitivity and reproducibility for very low concentrations of manganese, whereas X-ray fluorescence (XRF) spectroscopy showed standardization problems and arc/ spark emission spectroscopy suffered from electrode contamination (Chiswell and Johnson 1994). 

There are different techniques in atomic emission spectroscopy that are based upon the types of excitation and detection used. Under this heading arc and spark excitation and photographic and multiphotometric detection will be discussed. Flame photometry although by principle belonging to this group will be discussed together with atomic absorption spectrometry. 

This chapter deals with optical atomic, emission spectrometry (AES). Generally, the atomizers listed in Table 8-1 not only convert the component of samples to atoms or elementary ions but, in the process, excite a fraction of these species to higher electronic stales.. 4, the excited species rapidly relax back to lower states, ultraviolet and visible line spectra arise that are useful for qualitative ant quantitative elemental analysis. Plasma sources have become, the most important and most widely used sources for AES. These devices, including the popular inductively coupled plasma source, are discussedfirst in this chapter. Then, emission spectroscopy based on electric arc and electric spark atomization and excitation is described. Historically, arc and spark sources were quite important in emission spectrometry, and they still have important applications for the determination of some metallic elements. Finally several miscellaneous atomic emission source.s, including jlanies, glow discharges, and lasers are presented. 

Inductively Coupled Plasma (ICP) Discharge. The arc and spark sources date to the early development of emission spectroscopy in the mid-1800s the inductively coupled plasma (ICP) discharge is a relatively recent development, and is perhaps the most promising emission spectroscopic source today. Commercial ICP systems became available only in 1974, but research on this source has been going on since the early 1960s [7]. 

Emission spectroscopy is widely used for both qualitative and quantitative analysis. The high sensitivity and the possible simultaneous excitation of as many as 72 elements, notably metals and metalloids, makes emission spectroscopy especially suited for rapid survey analysis of the elemental content in small samples at the level of 10 /ug/g or less. With control over excitation conditions to maintain constant and reliable atomization and excitation, the spectral line intensities can be used for quantitatively determining concentrations. An analytical curve must be constructed with known standards, and often the ratio of analyte intensity to the intensity of a second element contained in, or added to, the sample (the internal-standard method) is used to improve the precision of quantitative analyses. Preparation of standards for arc and spark techniques requires considerable care to match chemical and physical forms to the sample this is not commonly required for ICP discharge. 

Emission spectroscopy is an important quantitative technique widely used in many industrial and research laboratories. In order to achieve an absolute concentration error of less than 10%, sample preparation and handling, experimental variables, and operating parameters must be strictly controlled. With conventional arc and spark procedures, absolute errors of l-57o can be achieved. The development of a routine spectrometric analysis may take months, but once the method is optimized, high-quality quantitative results are obtained rapidly and routinely for large numbers of similar samples. 

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