ICP-MS Measurement

Quantitative Analysis Measurement Techniques

What follows are some of the more popular techniques used for quantitative analysis using ICP-MS.

External Calibration using Calibration

This is the calibration technique that is most popular. Many analysts use this approach for matrices that are known and can be matched. The use of internal standards is helpful in accounting for drift. The choice of the internal standard / isotope mass combination is reasonably well understood. Finally, the use of spike recoveries on a split portion of the sample allows the analyst to determine if space charge effects are significant. The following are some brief notes on this topic:

• Know or learn about the sample composition. A semi-quantitative analysis using a scanning approach for the entire mass range allows the analyst to predict interferences and select internal standards and analyte isotopic masses.
• Perform interference check analysis. Prepare for the variations in the matrix and analyte composition and determine if corrections that have been built into the procedure are capable of providing the required accuracy.
• Use internal standards to help correct for drift.

Follow these basic guide lines for internal standard selection:

1. Avoid M²⁺ interferences
2. Avoid MO and other molecular interferences
3. Any naturally occurring internal standard element in your sample must be insignificant in comparison to the amount added
4. Use internal standard elements as close as possible to the masses of the analyte elements
5. Make sure the matrix doesn't react with the internal standard to alter (lower) it's concentration
6. Common internal standard elements are ⁶Li, Be, Sc, Ga, Ge, Y, Rh, In, Cs, Pr, Tb, Ho, Re, Bi, and Th - note many are monoisotopic.

• Use peak hopping rather than scanning for the final analysis.

Peak hopping will save time and this capability is one of the major advantages of low-resolution systems.

• At the beginning of the analytical day, optimize the instrument using 'optimization' standards.

I prefer to use a 10 ppb combination of Mg, U, Ce, and Rh. In addition, I like to optimize the instrument to obtain ¹⁴⁰CeO / ¹⁴⁰Ce and 140Ce⁺² / ¹⁴⁰Ce currents of < 0.5% relative. I routinely obtain a 'time scan' of ²⁴Mg, ³⁶Ar, ⁷⁰Ce⁺², ¹⁰³Rh, ¹⁴⁰Ce, ¹⁵⁶CeO, ²³⁰BKG, and ²³⁸U at the beginning of each analytical day. These scans are saved and accompany the following analytical data. Torch alignment, sample argon (nebulizer) flow, and ion optics settings are the parameters I most often change (in the order listed) in the optimization process .

• Make sure the introduction system is clean.

I prefer using glass concentric nebulizers and cyclonic spray chambers. I use dilute nitric acid for cleaning. It is often advantageous to change the entire introduction system, sipper to torch, each analytical day. In addition, the sample interface cones need to be rotated each analytical day with cleaned cones. Cleaning the cones in a 1% solution of nitric acid using an ultrasonic bath for 1-2 minutes is typically all that is required. Carefully dry the cones in a drying oven before reusing.

• Periodically check the performance of the ICP-MS during the analytical 'run.'

I prefer to split the sample and spike half of the sample with a known low ppb addition of an assortment of analytes ranging from Mg to U. After confirming the calibration by analyzing the standards, I like to use an analysis sequence of blank, sample, and sample + spike. The spike recovery allows me to determine if space charge effects from the matrix element(s) have significantly lowered the analyte signal.

• Know the stability of your standard.

For guidance, consult our Part-Per-Billion Stability Study.

Standard Additions

This approach is common with ICP-OES but it may give the analyst a false sense of security when using ICP-MS. It is a concern that this technique has earned such a 'good reputation' in view of the fact that it does not guarantee anything except a perfect matrix match. ICP-MS has many more potentially serious problems than matrix matching. The same interference issues discussed above must come into consideration if you choose to use standard additions. For example, if you have a molecular MO interference before the addition and do not use an alternate mass or perform a correction, you will still obtain a false high result. Spend the time to learn about the matrix and identify potential interference issues. After you reach a high level of confidence in the identification and correction for and/or elimination of interferences, then the standard additions approach is a convenient way to 'match' a complicated matrix.

Isotope Dilution

The technique of isotope dilution ICP mass spectrometry (ID-ICP-MS) provides the analyst with the possibility of using a primary (definitive) analytical method for the determination of trace metals in a variety of sample types. Examples of primary analytical methods are isotope dilution mass spectrometry (IDMS), ID-ICP-MS, gravimetry, titrimetry, coulometry, differential scanning calorimetry and nuclear magnetic resonance spectroscopy. ID-ICP-MS is of particular interest to the Reference Material producer of materials for trace metals content. Unfortunately, there are several of the elements that are monoisotopic (⁹Be, ²³Na, ²⁷Al, ⁴⁵Sc, ⁵⁵Mn, ⁷⁵As, ⁸⁹Y, ¹⁰³Rh, ¹²⁷I, ¹³³Cs, ¹⁴¹Pr, ¹⁵⁹Tb, ¹⁶⁵Ho, ¹⁶⁹Tm, ¹⁹⁷Au, and ²³²Th), making ID-ICP-MS useless for these elements. Our laboratory has been studying ID-ICP-MS along with the execution of accurate isotopic abundance ratio measurements (another possible primary method) and will publish these studies in the months to come.

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