Thermo Scientific iCAP RQ ICP-MS: Typical limits of detection

TECHNICAL NOTE 43427

IntroductionInductively Coupled Plasma – Mass Spectrometry (ICP-MS) is known to be a powerful technique for the analysis of trace elements in many application areas such as environmental analysis, food safety testing and clinical analysis. The main reasons to choose ICP-MS are its low limits of detection combined with an extended linear dynamic range and the ability to analyze varying sample matrices without suffering from extensive matrix effects. The Thermo Scientific™ iCAP™ RQ ICP-MS has been designed to deliver these characteristics while also offering the greatest ease of use and the simplest method development.

In order to cover a broad range of different applications, the iCAP RQ ICP-MS is equipped with proprietary skimmer cone insert technology that allows the operator to easily optimize the characteristics of the instrument. The insert portfolio comprises a standard ‘High Matrix’ insert to cover most applications, a ‘High Sensitivity’ insert for ultra-trace analyses, and a ‘Robust’ insert where higher matrix tolerance is needed.

Detection limits are a set of key performance indicators detailing an instrument’s capabilities and are useful as an aid in determining that instrument’s suitability to a chosen task. They demonstrate the lowest level of analyte distinguishable from the background noise under optimal conditions and are typically determined several times for statistical accuracy.

AuthorTomoko Vincent

KeywordsBEC, interference removal, KED, LOD

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Table 1. Instrument configuration and operating parameters.

Parameter Value

Spraychamber Quartz cyclonic, cooled at 3 °C

NebulizerMicroMist borosilicate pumped at 400 µL·min-1

Injector 2.5 mm Quartz injector

Interface Ni sample cone and insert type skimmer

Plasma power 1550 W

Nebulizer gas 1.1 L·min-1

CRC gas He 4.5 mL·min-1

KED 3 V

Lens setting Auto tune method

MethodSample preparationBlank solutions and calibration standards at 1, 5 and 10 µg∙L-1 were prepared gravimetrically by adding the appropriate quantity of a multi-elemental stock solution (SPEX CertiPrep™) directly to a solution containing 2% m/m HNO3 (65% HNO3 Optima™ grade, Fisher Chemical) and 0.5% m/m HCl (32-35% HCl Optima™ grade, Fisher Chemical). All solutions were prepared in freshly rinsed vials. This acid matrix was selected to reflect a sample preparation procedure that is broadly appropriate to a wide variety of industries and applications.

Instrument configurationAn iCAP RQ ICP-MS was used for all tests. The instrument was equipped with a MicroMist borosilicate nebulizer (Glass Expansion, Australia), a Peltier-cooled, quartz spraychamber (operating at 3 °C), a 2.5 mm ID quartz injector and a demountable quartz torch. High sensitivity, High matrix and Robust interface skimmer cone inserts were used in this study.

Due to the relatively high concentration of HCl in the acid matrix used in this study (0.5% m/m), a high number of complex Cl-based polyatomic interferences were expected. Consequently, the system was optimized using KED interference correction for all the analytes in this study.

The instrument configuration and operation parameters are shown in Table 1.

In order to accurately detect and quantify an element of interest, it has to be made sure that the measured isotope can be detected free from spectral interferences. The most commonly observed interferences are so-called polyatomic interferences, which are formed through recombination of ions after leaving the plasma, or in the interface.

For effective interference removal in single quad ICP-MS instruments, collision/reaction cells (CRCs) are most commonly used. CRCs may be operated in different ways depending on the nature of the interference that has to be eliminated, however, the most comprehensive approach is the use of an inert gas, such as He, and the use of Kinetic Energy Discrimination (KED).

In KED, unwanted polyatomic interferences are filtered out based on the difference in collision cross-section sizes of the analyte and polyatomic interferences. In addition, the proprietary design of the Thermo Scientific QCell™ CRC adds an automatically configured low mass cut-off to the interference removal toolbox, which filters out unwanted precursor ions. These ions are then unable to recombine later in the QCell CRC, backgrounds are reduced further than KED alone and detection limits are improved.

Limits of DetectionDetection limits themselves can be defined as either the Instrument Detection Limit (IDL) or the Method Detection Limit (MDL), and are accompanied by the Background Equivalent Concentration (BEC). The IDL defines the lowest concentration of an analyte that can be detected under ideal conditions by the instrument, and is normally measured on a single element basis using a clean sample, e.g using ultrapure 2% nitric acid matrix.

The MDL, in comparison, determines the lowest level of analyte that can be detected in a sample matrix using the proposed method of analysis and takes into account sample preparation steps. This is usually determined on a multi-element basis by diluting standard solutions to concentration levels that can no longer be accurately read, or by adding a low concentration spike into a real sample. There are numerous procedures for determining and calculating the MDL, but most methods require numerous analytical runs, over several days, to ensure a realistic determination of instrument performance.

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ResultsAs described in the sample preparation section, each sample reagent solution contained a high amount of HCl (approximately 0.5 m/m %). Therefore a high amount of complex Cl based polyatomic interferences were created, leading to adverse effects on the detection of a variety of elements.

The effect of applying kinetic energy discrimination for interference removal is demonstrated with the measurement of As. Figures 1a and 1b show comparisons between the calibration curve results with STD mode and He KED mode on 75As. In STD mode, a high background of about 100 kcps is observed that compromises the attainable detection limit. When KED is applied, the background is reduced significantly. An examination of the two calibration curves produced confirms that He KED mode effectively eliminated interferences and dramatically improved the detection limit for As from 0.227 µg∙L-1 to 0.0035 µg∙L-1 and the BEC from 3.7 µg∙L-1 to 0.002 µg∙L-1.

Figure 1b. Calibration curve of 75As in He KED mode.

The same scenario can be seen for 51V and 52Cr, whereby the comprehensive He KED performance generated excellent calibration curve results and both analytes achieved LODs of less than 3 ng∙L-1 (Figures 2 and 3).

Figure 2. Calibration curve of 51V in He KED mode.

Figure 1a. Calibration curve of 75As in STD mode.

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Figure 3. Calibration curve of 52Cr in He KED mode.

Figure 4. Calibration curve of 7Li in He KED mode.

Figure 4 shows the calibration curve for 7Li with He KED mode, the high transmission flatapole design of the QCell CRC delivers good sensitivity even for low mass elements, such that low ng∙L-1 LODs can be achieved.

The analysis has demonstrated that using a single He KED measurement mode enables the iCAP RQ ICP-MS to achieve simple, high-sensitivity, multi-element sample analysis.

The typical performance of the iCAP RQ ICP-MS using STD and He KED mode are shown in Table 2, where detection limits were calculated using the raw intensity data from the standard and the blank, as per following equation:

IDL = 3SDblk xSTDconc

STDx – BLKx

Where:

• IDL is the instrument detection limit

• SDblk is the standard deviation of the intensities of the multiple blank measurements

• STDx is the mean signal for the standard

• BLKx is the mean signal for the blank

• STDconc is the concentration of the standard

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Measurement mode STDInsert 2.8 3.5 4.57Li 1.4 2.3 5.19Be 0.2 0.3 2.911B 9.0 12 1723Na 28 30 38124Mg 0.7 0.7 2.027Al 3.3 6.4 7.3

45Sc 5 6 1348Ti 0.3 0.7 1.851V 726 503 26252Cr 12 28 3255Mn 0.7 1.8 4.657Fe 349 501 59859Co 0.2 0.3 0.860Ni 2.0 2.2 4.763Cu 0.8 1.3 3.366Zn 2.9 3.7 5.671Ga 0.7 0.9 4.574Ge 1.2 1.4 3.775As 121 227 13878Se 181 241 25082Se 31 35 10585Rb 0.1 0.1 0.488Sr 0.1 0.1 0.189Y 0.02 0.03 0.0790Zr 0.06 0.1 0.493Nb 0.04 0.09 0.295Mo 0.3 0.5 2.7101Ru 0.1 0.2 0.3103Rh 0.04 0.05 0.1105Pd 0.2 0.4 0.8107Ag 0.1 0.2 0.3111Cd 0.2 0.3 0.4115In 0.02 0.05 0.4118Sn 5.6 6.9 7.4121Sb 0.4 1.0 2.0125Te 1.4 2.8 4.9133Cs 0.09 0.12 0.2138Ba 0.1 0.2 0.3139La 0.01 0.02 0.03140Ce 0.02 0.03 0.04141Pr 0.02 0.02 0.05146Nd 0.05 0.06 0.08147Sm 0.05 0.06 0.09153Eu 0.01 0.01 0.02157Gd 0.02 0.03 0.04159Tb 0.01 0.01 0.02163Dy 0.01 0.03 0.05165Ho 0.01 0.02 0.03166Er 0.01 0.02 0.03169Tm 0.01 0.01 0.02172Yb 0.02 0.03 0.04175Lu 0.01 0.02 0.03178Hf 0.03 0.05 0.11181Ta 0.1 0.2 0.3182W 0.5 1.0 1.4185Re 0.07 0.08 0.09193Ir 0.1 0.2 0.3195Pt 0.06 0.08 0.09197Au 0.2 0.3 0.8202Hg 3.9 4.5 11205Tl 0.04 0.04 0.5208Pb 0.07 0.1 0.2209Bi 0.08 0.1 0.2232Th 0.01 0.01 0.04238U 0.01 0.03 0.07

Measurement mode He KEDInsert 2.8 3.5 4.57Li 22 61 1839Be 15 56 16811B 121 123 25323Na 167 216 128924Mg 6.2 7.3 2227Al 39 42 11139K 425 794 181044Ca 20 47 9345Sc 1.0 1.6 3.248Ti 0.7 1.0 6.651V 2.5 3.0 1952Cr 2.1 2.5 1855Mn 0.9 1.2 3.256Fe 4.2 6.6 1859Co 0.8 1.5 6.160Ni 0.2 0.3 0.663Cu 1.1 2.1 4.666Zn 5.1 7.1 1671Ga 1.2 1.8 4.374Ge 0.7 1.0 2.075As 2.9 3.5 2278Se 24 51 118

85Rb 1.1 1.2 1.588Sr 0.1 0.2 0.989Y <0.01 0.01 0.0290Zr 0.2 0.3 0.493Nb 0.03 0.2 0.395Mo 0.5 0.6 0.7101Ru 0.1 0.1 0.2103Rh 0.06 0.09 0.4105Pd 0.3 0.4 2.9107Ag 0.7 0.8 0.9111Cd 0.1 0.2 0.3115In 0.2 0.2 0.4118Sn 11 12 16121Sb 1.9 2.1 3.7125Te <0.01 0.01 0.03133Cs 0.2 0.2 0.3138Ba 0.3 0.3 0.4139La 0.01 0.03 0.05140Ce 0.02 0.04 0.05141Pr 0.01 0.02 0.05146Nd <0.01 0.02 0.04147Sm 0.06 0.07 0.08153Eu <0.01 0.02 0.03157Gd <0.01 0.01 0.03159Tb <0.01 0.01 0.02163Dy 0.02 0.02 0.03165Ho <0.01 0.01 0.02166Er <0.01 0.01 0.02169Tm <0.01 0.01 0.02172Yb <0.01 0.01 0.02175Lu <0.01 0.01 0.02178Hf 0.04 0.07 0.2181Ta 0.1 0.3 0.7182W 0.7 0.9 1.4185Re 0.04 0.05 0.09193Ir 0.2 0.3 0.4195Pt 0.2 0.2 0.3197Au 0.3 0.4 0.9202Hg 3.3 4.8 9.2205Tl 0.3 0.4 0.5208Pb 0.2 0.3 0.4209Bi 0.1 0.3 0.4232Th 0.01 0.02 0.03238U 0.03 0.05 0.07

Table 2. Typical LODs achieved with the iCAP RQ ICP-MS in STD and He KED mode, using the three available skimmer cone inserts. All results are reported in ng·L-1.

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ConclusionThe iCAP RQ ICP-MS system with its integrated Thermo Scientific™ Qtegra™ ISDS Software demonstrated excellent performance to provide a significant improvement to analytical laboratory capabilities through simplicity, productivity and robustness. The flexible configuration delivers a complete solution for a wide range of trace elemental analysis in both research and routine applications.