Flame photometer instrumentation

An example of a modem instrument of this type is the Coming Model 410 flame photometer. This model can incorporate a lineariser module which provides a direct concentration read-out for a range of clinical specimens. Flame photometers are still widely used especially for the determination of alkali metals in body fluids, but are now being replaced in clinical laboratories by ion-selective electrode procedures (see Section 15.7). 

Although flame emission measurements can be made by using an atomic absorption spectrometer in the emission mode, the following account refers to the use of a simple flame photometer (the Coming Model 410 flame photometer). Before attempting to use the instrument read the instruction manual supplied by the manufacturers. 

At present, calcium and magnesium are estimated almost exclusively by atomic absorption (36). Present instrumentation permits the dilution of the specimen to approximately 1 - 100 for calcium and even higher for magnesium. For many instruments, the two elements are not read out simultaneously such as is practicable for sodium and potassium with the flame photometer. The lower limit of serum volime at present, for the practical assay for calciim and magnesiim in the laboratory of Neonatology, is approximately 10 ul The instruments are very readily automated, and it is not uncommon for results to be available at the rate of 240 per hour in the routine laboratory, where a typical atomic absorption instrument such as a Perkin-Elmer has been attached to an automatic feed system. 

The detailed design and construction of instruments used in the various branches of spectrometry are very different and at first sight there may seem to be little in common between an X-ray fluorescence analyser and a flame photometer. On closer examination, however, certain common features emerge and parallels between the functions of the various parts of the different instruments are observed. The basic functions of any spectrometer are threefold (Figure 7.4) ... 

This is achieved by using an atomic absorption spectrophotometer (or less accurately with a flame photometer). Some details could be instrument specific, so refer to the manufacturer s handbook, application data sheets, and obtain technical support if you lack experience in this area. Some general guidelines will be noted here. 

Its rapidity and detection limits, which are in the order of a few ppt (10-12) for many elements, make atomic emission one of the best techniques currently available for elemental analysis. These sophisticated instruments, however, are not intended to replace the flame photometers that are still used for many simple measurements. 

Photometric measurements performed in clinical laboratories use advanced chemical and biochemical methods and diverse instrumentation. Most analyses performed in clinical laboratories are based on spectrophotometric methods using photometric systems, such as absorption photometers, atomic absorption spectrophotometers and flame photometers. Typically, the result is expressed as mass concentration of analyte in solution (mg/dl), molar concentration (mmol/1), or catalytic concentration of enzyme activities in solution (U/l) [6],... 

The same algorithm was used to calibrate flame photometers and blood analyte analysers for Na, K and Ca determination. The results of calibrating such instruments are also presented in Table 1. Instrument 6 was an ion selective electrode analyser for Na/K/Cl with a 1.5% coefficient of variation at a 95% confidence interval. Finally, instruments 7-10 were flame photometers, validated against monoelement concentration CRMs [5] in accordance with legal metrological regulations. In... 

Flame photometry (see also p. 168) is almost exclusively used for the determination of alkali metals because of their low excitation potential (e.g. sodium 5.14eV and potassium 4.34 eV). This simplifies the instrumentation required and allows a cooler flame (air-propane, air-butane or air-natural gas) to be used in conjunction with a simpler spectrometer (interference filter). The use of an interference filter allows a large excess of light to be viewed by the detector. Thus, the expensive photomultiplier tube is not required and a cheaper detector can be used, e.g. a photodiode or photoemissive detector. The sample is introduced using a pneumatic nebulizer as described for FAAS (p. 172). Flame photometry is therefore a simple, robust and inexpensive technique for the determination of potassium (766.5 nm) or sodium (589.0nm) in clinical or environmental samples. The technique suffers from the same type of interferences as in FAAS. The operation of a flame photometer is described in Box 26.2. 

The early workers in this field built their own apparatus, often assembling these from suitable units available from instruments for other purposes. Emission flame photometers have been converted into atomic absorption spectrophotometers by appropriate attachments consisting of specific light sources, choppers, and lenses, a principle also employed by some manufacturers. 

Where only the determination of alkali metals is desired, a simple instrument can be devised in which selection of a resonance line can be obtained with the help of color filters. However, at present such instruments would not appear to have an appreciable advantage over flame emission methods, considering the sensitivity of the latter and the simplicity of presently available flame photometers. Appreciable advantages, on the other hand, are inherent in absorption over emission methods in the determination of the alkaline earths and magnesium. Since these metals have simple emission spectra, the use of filters, notably interference filters, would be feasible in instruments limited to the determination of these elements. 

To simplify these calculations, the capital cost of the instrument may be amortized over 5 or 6 years and maintenance costs ignored. The average daily cost can then be calculated and will be the same whether the instrument is used or not. Reagent costs are simple to calculate and are usually small in relation to other costs. Examples of labor and equipment costs of 5 commercial flame photometers, used to measure plasma sodium and potassium simultaneously, were given by Broughton and Dawson (B18). With small numbers of analyses, the least expensive instrument was the cheapest to run, but despite wide differences in capital outlay and labor requirements, the cost per analysis for the 5 instruments... 

The use of analytical atomic spectroscopy in clinical chemistry has developed rapidly over the last 20 years and there is now adequate knowledge and instrumentation available for the measurement of a wide range of elements (C12, H25, M4, W25) in concentrations as low as 1 ng/ml or amounts as small as 10" g. The cost of the instruments ranges from 100 ( 240) for the simplest flame photometer to 50,000 ( 120,000) for an advanced direct reading spectrometer with data handling facilities. 

A simple emission flame photometer is adequate for Na and K while a more selective emission/absorbance system is necessary for Ca, Mg, and trace metals. The range of trace metals which can be analyzed (e.g., Cu, Zn, Fe, As, Pb, Co, Mo, Se, Cd, Hg) with an instrument depends on the efficiency of atomization, excitation, and light collection, as well as the intensity and stability of the background. Owing to the difficulty of obtaining complete stability of baseline and sensitivity, frequent standardization of instruments is usually necessary. This can... 

The first commercially available flame photometer was introduced in the 1940s by the Perkin-Elmer Corporation. In 1948, Beckmann Instruments, Inc., introduced a flame attachment that could be used with their popular model D.U. spectrophotometer. By the late 1950s, instruments had been developed that used lithium as an internal standard to maximize precision. Autodilution features and microprocessor-controlled operations became widely used options in the 1970s. The most recent significant development was the introduction of cesium as the internal standard, by Instrumentation Laboratory, Inc. (Figs. 1-3). This development makes concurrent lithium determinations more practical. 

Fig. 1 Flame photometer using cesium as the internal standard. (Courtesy of Instrumentation Laboratory, Inc.)...

Fig. 2 Atomizer of IL 943 Flame Photometer a) sample orifice assembly b) air orifice c) gas tube assembly d) atomizer bowl drain e) atomizer thumb screws f) U-tube g) sample injection nozzle tubing h) ground fitting i) top atomizer assembly j) bottom atomizer assembly k) adjustment for aspiration rate setting. (Courtesy of Instrumentation Laboratory, Inc.)...

Fig. 3 Flame housing of IL 943 flame photometer a) sodium filter, 589 nm b) potassium filter, 776 run c) lithium filter, 670 nm d) cesium filter, 852 nm e) ignition detector f) burner assembly g) rubber gasket h) spark electrode i) ignition coil wire j) chimney. (Courtesy of Instrumentation Laboratory, Inc.)...

The analysis of clinical samples represents a typical application of flame photometry. Concentrations of sodium, potassium, and lithium in blood and urine are well within instrument working ranges. The specificity of the technique is a distinct advantage. Automated models of flame photometers, available during the past 25 years, are typically designed to serve the needs of the clinical chemist. Instrument calibration protocols are built into instruments to facilitate the timely analysis of sodium, potassium, and lithium in clinical samples. 

Many nonautomated flame photometers are now commercially available. The following study (MIO) was carried out on the E.E.L. instrument (Evans Electro-Selenium Ltd., Harlow, Essex, England), hut would apply correspondingly to any similar equipment. Some laboratories still use, in whole or in part, the scheme of operation shown diagrammatically in Fig. 8. By inspection of this figure it will be seen that, for the assay... 

A flame photometer is an instrument in which the intensity of the filtered radiation from the flame is measured with a photoelectric detector. The filter interposed between the flame and the detector, transmits only a strong line of the element. 

Undoubtedly, advances in ISE technology over the past IS years have had their greatest impact in the clinical chemistry laboratory. Indeed, flame photometers and atomic absorption instruments, for years the workhorses of most laboratories, have become obsolete as newer ISE-based instruments and methods now perform the task of determining electrolyte levels in blood and urine samples. Clearly, electrochemical measurement of fluid components is desirable, since in many cases no dilution or pretreatment of the sample is required before the actual measurement. Thus final test results are obtained more rapidly and economically. 

---