INDUCTIVELY COUPLED PLASMA – MASS MASS SPECTROMETRY (ICP-MS)
The ICP-MS instrument measures most of the elements in the periodic table. The elements shown in color in Figure 1 can be analyzed by ICP-MS with detection limits at or below the ppt range. Elements that are in white are either not measurable by ICP-MS (the upper right-hand side) or do not have naturally occurring isotopes. Most analyses performed on ICP-MS instrumentation are quantitative; however, it also can serve as an excellent semi-quantitative instrument. By using a semi-quantitative software package, an unknown sample can be analyzed for 80 elements in three minutes, providing semi-quantitative data that is typically within ±30% of the quantitative values. For reasons that often involve human health, knowing the isotopic composition of a sample can be highly important. Of the three techniques mentioned to this point, only ICPMS is used routinely for determining isotopic composition.
The primary reasons for the growing popularity of ICP-MS 1) Instrument detection limits are at or below the single part per trillion (ppt) level for much of the periodic table. 2) Analytical working range is nine orders of magnitude. 3) Productivity is unsurpassed by any other technique. 4) Isotopic analysis can be achieved readily.
The ICP-MS is operated only by appointed personnel. No users are allowed to operate the instrument or computer without the approval from the operator. The ICP-MS operator must be provided with (What you need to prepare): All solutions for ICP-MS analysis are prepared with 2% HNO3 in plastic containers. HCl is avoided as a sample.
Samples: All samples for analysis are in aqueous form. Unknown samples and standard solutions are free of solid particulates. Samples are filtered with a 0.45mm or smaller membrane filters. Solids in the samples are cause inaccurate readings and clog the nebulizer, resulting in longer running time, expensive repairs, replacements, and down time on ICPMS which to avoid.
Standard Solutions: The standard solutions are prepared by the users in the same matrix as the unknown samples. Prepare the standards carefully because it will affect the accuracy of the results. Inaccurate high standard solution and contaminated blank are result in inaccurate results for unknown samples. In addition, if result gives in an inaccurate quality control, it will fail more often and cause the whole process to take longer, which will cost more.
INSTRUMENT AND PROCEDURE OF USING ICP-MS
An ICP-MS consists of the following components:
1) Sample introduction system is compose of a nebulizer and spray chamber and provides the means of getting samples into the instrument. 2) ICP torch and RF coil is generates the argon plasma, which serves as the ion source of the ICP-MS. 3) Interface is links the atmospheric pressure ICP ion source to the high vacuum mass spectrometer. 4) Vacuum system is provides high vacuum for ion optics, quadrupole, and detector. 5) Collision/reaction cell is precedes the mass spectrometer and is used to remove interferences that can degrade the detection limits achieved. It is possible to have a cell that can be used both in the collision cell and reaction cell modes which is referred to as a universal cell. 6) Ion optics is guides the desiredions into the quadrupole while assuring the neutral species and photons are discarded from the ion beam. 7) Mass spectrometer is acts as a mass filter to sort ions by their mass-to-charge ratio (m/z) 8) Detector is counts individual ions exiting the quadrupole.
9) Data handling and system controller is controls all aspects of instrument control and data handling to obtain final concentration results.
Measurement by ICP-MS
MS working standard 1) 14ml Falcon tubes is set up in the ICP-MS auto sampler rack. 2) 10 mL of 2 % (v/v) nitric acid is added to each tube and label the tubes accordingly (i.e. MS 1, MS 2, MS 3, MS 4, MS 5 and MS 6). 3) The tube MS 1 serves as a calibration blank. 4) The MS working standard solutions is prepared in the rest 5 Falcon tubes from the inhouse OES working standard solutions and from the SPEC primary standard solutions (CLMS-2, CLMS-3 and CLMS-4). 5) MS6 contains 20 ppb of Rh (SPEC CLMS-3 contains 10 ppm of Rh already). 0.02 mL of 2 ppm Rh is spiked to MS1 – MS5 as the IRS. The nominal concentration is 4 ppb.
Preparing sample solutions for ICPMS 1) For “routine” samples, solution samples (10 mL) are added to 14-mL Falcon tubes. 2) 0.02 mL of 2 ppm Rh is spiked as the IRS. The nominal concentration is 4 ppb. 3) “None-routine” samples is prepared in some other methods, depending on the requested analyses, sample matrix, analyte concentrations, etc. For example, lowvolume or “over concentrated” samples are diluted before analysis. Turbid samples are left to stand overnight so that particles settle down to the bottom, or the samples are centrifugated so that particles are separated from the samples. 4) After a given amount of sample (weight or volume) is spiked with a given amount of rhodium (Rh), the concentration ratio of (analyte/Rh) is later used for quantification. Any further dilution does not change the concentration ratio.
The ICP-MS has a high maintenance expense, so we have to charge users for using it. However, since this is not a commercial lab instrument, the charge is minimal for our users. Currently, we are charging per time used and the minimum charge is 0.5 hour. After that the charge increment is 15 min. Users are charged for a minimum 30 min. warming up time plus the actual time used to run samples. When ICP-MS is sign up for using, a valid PTA number has to be provided online for running samples.
CONCLUSION As a conclusion, ICP-MS is an ideal choice for the laboratory that is seeking the
lowest possible detection limits and the highest level of productivity available. The technique is relatively free from interferences, and the interferences that do exist can often be reduced or removed through the use of a universal cell operating in either the collision mode or the reaction mode. Many laboratories find the ability to measure specific isotopes of an element invaluable. This guide is intended as a quick general overview of ICP-MS. For those interested in more details regarding the design and operation of ICP-MS instruments, additional details can be found in the scientific literature.