|
Complete Article
Article written by Charles Torok , QC Chemist at APG
Internal standards are used for data correction for various samples during the run of a sample set. Slight variation in sample introduction, instrument response, and matrix from sample to sample can be corrected for, with the use of an internal standard.
Yttrium is often used as an internal standard in ICP-OES, but care must be taken with its use because of possible interferences. The Yttrium line at 242.220, which can be used as an internal standard for lower wavelength lines, can be interfered with by Iron. As seen in the attached figure, the Yttrium curve loses its Gaussian shape at high concentrations of Iron. Test samples were analyzed with Iron concentrations of 0ppm, 10ppm, 100ppm, 1,000ppm, and 10,000ppm. At 0ppm, 10ppm, and 100ppm the Yttrium 242.220 peak retains it Gaussian with very little to no distortion from Iron. At 1,000ppm a definite shoulder is present on the left side of the peak and a slightly elevated baseline on right. At 10,000ppm the shoulder has become a peak with almost the same amplitude as the Yttrium peak. In addition for the Fe 10,000ppm sample, the baseline to the right of the peak is extremely elevated. This value of 10,000ppm for Iron is not unheard of; especially for soils digests.
This interference, if not properly accounted for, can skew all the data linked to this line. If the baseline marker on the right is left on and where it is the values of the Yttrium peak, height/area will be depressed. This will cause the values for the analytes linked to this line to be inflated above their true values. Poor placement of the left baseline marker can have a similar effect. If the left baseline marker had been placed closer to the Yttrium peak during a calibration run in which this effect is not present, it could end up in the midst of the high concentration iron peak for a sample.
Though internal standards are useful for reducing sample-to-sample variation during a run, there is a danger of interference giving false data. This problem can be reduced through matrix matching or careful placement of baseline determination.
Figure: The Yttrium internal standard peak. Acquired using a Thermo Jarrell Ash IRIS Advantage using TEVA version 1.4.0.
|
 |
|
|
