Visual Bullet Test This is a simple test that can often be performed on a daily basis. In this test, the user introduces a solution containing 1000 mg/L or more of an element whose atom emission produces a well defined "bullet" in the center of the ICP discharge. The mere presence of the bullet indicates that sample aerosol is reaching the plasmawhile the vertical position of the bullet in the discharge is an indicator of the gas flow and RF power settings being used. Sodium and yttrium are often good elements to use for this test. Signal Intensity The number of emission counts for a given concentration of an element is often used as a quick instrument diagnostic. It is usually more useful for trend analysis than as an absolute indicator of performance since the emission counts may vary somewhat from day today. BEC The background equivalent concentration (BEC) is an indicator of relative sensitivity for an emission line. A BEC of a higher than normal value often indicates problems with the efficiency of the sample introduction system, although it can actually be due to a number of causes. Precision The short-term precision obtained for several (5-10) measurements for a strong emission line is often used as an indicator of the noise associated with sample introduction. The precision is usually expressed as % CV (coefficient of variation) or % RSD. The precision for an argon emission line is sometimes used as a diagnostic for the RF generator. Detection Limits Detection limits are often used as the ultimate tests of instrument performance since they combine measurement of sensitivity with measurement of noise. Detection limits by themselves, however, are not very good diagnostic indicators. If the measured detection limits are not within their expected ranges, there could be several different causes. Detection limits are best used for diagnostic purposes when combined with a series of other, more specific tests. Ion/Atom Ratio Many ICP users employ a ratio of the emission intensity of an ion line to the emission intensity of an atom line as a diagnostic indicator of the relative excitation conditions in the plasma. Because such ratios are usually sensitive to RF power, inner or nebulizer flow and viewing height, they can be used to ensure that an equivalent combination of these parameters is used for an analysis. Running an analysis under a specific set of conditions can be vital when predetermined factors, such as IEC coefficients or stored calibration curves, are used in an analysis. Wavelength/Peak Alignment Because all dispersive UV/visible spectrometers are subject to some degree of drifting, it is important to be sure that the spectrometer is calibrated properly in terms of wavelength before an analysis. For some instruments, this is done automatically by the instrument software at the beginning of an analysis. Other instruments, mainly polychromator-based instruments with refractor plate spectrum shifters, may require manual adjustment of the wavelength drive mechanism.
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