ICP-MS: When Sensitivity Does Matter - - Spectroscopy
 Home   Mass Spectrometry   ICP-MS   Infrared   FT-IR   UV-Vis   Raman   NMR   X-Ray   Fluorescence  
Issue Archive
Special Issues
The Application Notebook
Current Issue
Submission Guidelines
Digital Edition
Subscribe to the Digital Edition
The Wavelength
Subcribe to The Wavelength
Subscribe to the MS E-news
Market Profiles
Information for Authors
Advertiser services
Contact Us
Atomic Perspectives
Chemometrics in Spectroscopy
Focus on Quality
Laser and Optics Interface
Mass Spectrometry Forum
The Baseline
Molecular Spectroscopy Workbench

ICP-MS: When Sensitivity Does Matter

Special Issues
pp. s36-s43

It makes intuitive sense — the higher the sensitivity of an inductively coupled plasma–mass spectrometry (ICP-MS) system, the lower the detection limit. But there are many factors that affect the detection limit for a given isotope in a given sample. These factors include sensitivity, background noise, and interferences.

In many applications in environmental management, semiconductor manufacture, and clinical research, some important elements are subject to spectral interferences. For the accurate determination of these elements, some form of interference management — collision reaction interface, collision cell, or reaction cell — often is employed.

With the recent focus on interference management techniques, less attention has been paid to one of the major benefits of inductively coupled plasma–mass spectrometry (ICP-MS) — a benefit that has spurred its growth over the past two decades. We are referring to sensitivity, the ratio of net signal to concentration. The importance of sensitivity should not be forgotten. Many elements of interest in many samples are not subject to significant spectral interference, and for such elements, the quantitation limit is set primarily by sensitivity.

Signal Noise, Sensitivity, and Detection Limits in ICP-MS

Detection limits for ICP-MS are quoted most often as "3 sigma" detection limits. These detection limits are derived using the following equation:

detection limit = (3 σbl)/sensitivity

where the standard deviation of the blank (σbl) is expressed in counts per second, and the sensitivity is expressed in counts per second (cps) per unit concentration (ng/L).

This equation clearly shows the direct dependence of detection limits on sensitivity. In practice, the standard deviation of the blank also is affected by the sensitivity. This arises because part of the blank signal arises from traces of the element of interest. We go to a lot of trouble to avoid such contamination, but its complete elimination is not possible.

The total variation in background noise (σbl) is a combination of both source flicker noise (caused by instabilities in the operation of the nebulizer, spraychamber, and plasma, σsf) and Poisson or counting statistics noise (caused by the random nature of the arrival of ions at the detector, σcs) (2,3). Source flicker noise can be modeled as being a constant fraction of the total ion count rate, typically 0.5%. The counting statistics noise can be modeled as the square root of the average number of counts during a measurement:

σcs = √(average number of counts)

Practically, there is always some elemental contamination present in the analytical solutions. Assuming a constant low blank contamination, counting statistics (and hence, detection limits) on the blank count rate can be improved by increasing the sensitivity.

Table I: Theoretical calculations showing the effect of sensitivity on detection limits
Table I shows the theoretical relationship between sensitivity, the two sources of background noise, and the resulting detection limit. For this example, it is assumed that there is a continuous background of 10 cps, a real blank signal arising from contamination of the blank with 1 ng/L of the element of interest, and that the integration time is 1 s. These results clearly show that counting statistics noise becomes more dominant as a proportion of the total noise as the count rate drops, and that the best detection limit is found when the sensitivity is greatest.

Rate This Article
Your original vote has been tallied and is included in the ratings results.
View our top pages
Average rating for this page is: 3.83
Headlines from LCGC North America and Chromatography Online
LCGC TV Library
New Carbon-Based Phases for 2D LC
Emerging Trends in Pharmaceutical Analysis
Waters EU - Combining Mass and UV Spectral Data with Empower 3 Software to Streamline Peak Tracking and Coelution Detection
Pharma Focus: Where pharmaceutical analysis is heading
Source: Special Issues,
Click here