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Geological Sciences 693 Spring 1997

Guidelines for Laboratory #2: Detection Limits, Background and Spectral Overlaps

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Your task in this laboratory is to explore the following:

C ICP-OES and ICP-MS detection limits in clean solutions for selected (Cd, Ca, Na,

Mn, Ba and Pb) elements

C Effect of integration time on detection limits

C Compare detection limits for selected elements by ICP-OES and ICP-MS

C Background in ICP-OES

C Continuum background

C effect of high concentrations of Al or Ca on background emission

C background correction

C off-peak background subtractionC subtraction using a blank

C multicomponent spectral fitting

C effect of continuum background on detection limits

C Spectral overlaps

C tables to predict spectral overlaps (Optima 3000 database, Lambda III

spectral line database, Boumans spectral overlap tables)

C effect of spectral overlaps and the standard deviation of the

background

C spectral overlap correction

C use of interelement correction factors

C multicomponent spectral fittingC effect of spectral overlaps on detection limits

C Background in ICP-MS

C Predicting spectral overlaps (isobaric overlaps, polyatomic ions) (MS View

software; ELAN 6000 software)

C ICP-MS spectral background with no solvent; ICP-MS spectral background

due to solvent

C Background origins and corrections

C isobaric overlaps

C molecular oxide ions

C polyatomic ions involving solvent and/or plasma species

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Page 2 of 7

Daily Instrument Performance Checks

ICP-OES. Run the diagnostic signals suggested by Mermet (to characterize the operation

of the ICP-OES instrument. The solution contains: 1 ppm Ba, Mg, Mn. Diagnostic information

from: Spectrochimica Acta 48B, 743 (1993).

Standard operating conditions are: 1300 W, 15 L/min, 0.3 L/min and 0.65 L/min for the outer,

intermediate and center gas flow rates, respectively, for end-on viewing of ICP emission and 1100

W, 15 L/min, 1.0 L/min and 1.0 L/min for the outer, intermediate and center gas flow rates,

respectively, if side-on viewing of the ICP is used. The sample uptake rate is 1.0 mL/min for the

cross flow nebulizer. Enter results of the diagnostic measurements into the instrument log book and

compare them to previous measurements. (We will continue to do this throughout the quarter and

then discuss some of the implications of these at the end of the quarter.)

ICP-MS. Optimize nebulizer gas flow rate, power and lens voltage to obtain the maximum

signal for a 10 ppb solution of Rh. (Use the Neb Power Lens Optimize.WRK workspace already set

up and the automatic optimization procedures available in the Optimize window.) The optimum

nebulizer gas flow rate should be in the 0.9 to 1.0 L/min range, the power in the 1000 to 1200 W

range and the lens voltage near 6 V. If the nebulizer gas flow rate or power optimum is outside of

this range there is likely a problem with the aerosol generation or torch alignment. If the optimum

lens voltage is greater than 7 or 7.5 V, the lens is likely coated with sample and needs to be cleaned

for optimum performance.

Run the diagnostic check solution (10 ppb Ba, Ca, Ce, Cu, Ge, Pb, Mg, Rh, Sc, Tb and Tlin 2% v/v nitric acid) (use the Daily Performance.WRK workspace). Checks the Mg, Rh and Ba

sensitivities (note these cover a wide mass range), the ratio of Ba to Ba signals, the ratio of CeO2+ + +

to Ce signals and the signal at mass 220 (should be very small). Check in the ELAN Software+

manual (Tuning and Optimization chapter) or using the on-line Help (Under Optimize the Elan, Daily

Optimization Procedures) to see if your experimental results are within specifications for a normally

operating instrument.

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Experimentally Measuring Detection Limits

Detection limits are often determined by measuring: (1) the sensitivity from a single standard

and a blank with appropriate concentrations for each element and (2) the standard deviation of the

blank. Caution: Rinse out of the standard completely to background levels may require long times.

A hint: Run a blank (3 replicates) and a single standard (3 replicates; concentration about 100

to 1000 times the expected detection limit) to establish a calibration curve (and therefore sensitivity).

Then make a sufficient number of replicate blank measurements (run as a sample) to obtain a standard

deviation with a reasonable uncertainty. The detection limit is 3 times the standard deviation of the

concentration of the blank as sample measurement. How many replicate blank measurements are

needed for a good estimate of the standard deviation?

I. Detection Limits in clean solutions

Determine detection limits for Cd, Ca, Na, Mn, Ba and Pb by ICP-OES and ICP-MS. (Note:

do not attempt to use mass 40 for measurement of Ca -- very high ion currents can damage the

ICP-MS detector).

Determine the effect of integration time (called sampling time on the Optima 3000) on the

detection limit. (Use 1 ms as the shortest integration time).

II. Background in ICP-OES

Emission at an analyte wavelength can be produced by sources other than the analyte.

Continuum emission exists at all wavelengths in ICP-OES (although it is more intense at higher

wavelengths). Emission from ion-electron recombination, wings of intense emission lines, partially

overlapping lines or direct spectral overlaps are possible. Spectral overlaps are common in ICP-OES

for complex samples because many elements emit at hundreds or thousands of different wavelengths.

Continuum background can be subtracted by measuring emission at two wavelengths, one on each

side of the analyte peak (called off-peak correction).

Two different approaches to corrections for spectral overlaps, interelement correction (on-

peak correction) and multicomponent spectral fitting methods (such as Multicomponent Spectral

Fitting available in the Optima software). The degradation of detection limits due to the presence of

a spectral overlap will depend on the extent of the overlap. The ability of the MSF technique to work

well will depend on the spectra within a subarray due to the analyte and interferant element(s) being

different from each other.

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A. Investigate the use of simultaneous off-peak background subtraction vs. sequential

blank subtraction for ICP-OES. Explore both manual selection of background and the defaultbackground subtraction points in the Optima 3000 software. Compare and assess your choices versus

those of the Optima software.

Observe the background from: a 2% v/v nitric acid blank, 5000 ppm Ca and 2% v/v nitric

acid, 1000 ppm Al with 2% v/v nitric acid. (10,000 ppm stock solutions of Ca and Al will be

provided. DO NOT USE THE COMMERCIAL STANDARD SOLUTIONS FOR CA AND AL.

Compare the background intensity (w/o subtraction) of the three solutions at:

588 nm (near the Na wavelength)

455 nm (near the Ba wavelength)

295 nm (near the Mn wavelength)226 nm (near the Cd line)

214 nm (near the Cd line)

Compare the precision of background subtraction of a blank with simultaneous off-peak

subtraction vs. subtraction of a blank (with no simultaneous blank subtraction).

Compare the detection limits for the most sensitive Cd, Ca, Na, Mn, Ba and Pb emission line

in a solution containing 5000 ppm Ca and a solution containing 1000 ppm Al to those obtained in a

clean solution. For these measurements you may assume that the sensitivity of each of the analyte

elements is constant in 2% v/v nitric acid, 1000 ppm Al and 5000 ppm Ca (This may not be strickly

true as we will see in Laboratory #3)..

B. Investigate the effect of spectral overlaps on detection limits in ICP-OES

1. On-peak subtraction (called interelement correction in the Optima software).

Before performing the experiment, predict the effect of the concentration of the interferant in the

sample on detection limits using literature detection limits, Boumans tables or Lambda III software,

and assuming a relative standard deviation of 1% for the signal due to the interferant.

For each case (separately), determine how the detection limit for the analyte changes with

concentration of the interferant.

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Page 5 of 7

Investigate the following:

Analyte Interferant Spectrometer

(Element, Wavelength) Element or Bandpass (nm)

Molecule

Sn 283.999 nm Cr 0.0086

Ba 413.066 Ce 0.0127

Nd 401.225 Ce 0.0123

Hg 253.652 Co 0.0077

V 310.23 OH 0.0095

b. Spectral overlap correction using Multicomponent Spectral Fitting for ICP-OES.

For the emission lines listed above, investigate the use of Multicomponent Spectral Fitting (MSF)

using the Optima 3000. Compare detection limits to those obtained using the InterElement

Correction (IEC) approach.

III. Background and spectral overlap correction in ICP-MS

In ICP-MS, their is no fundamental source of continuum m/z ions. Signals can be produced

from sources other than the analyte ion (fast neutrals, photons, ions not filtered out by the quadrupole

MS). Spectral overlaps can be a problem, particularly when the analyte is present at very low

concentrations. Overlaps can be due to plasma species, species produced from entrained air, isobaric

overlaps or oxides. It is possible, using alternative isotopes of the interferant to estimate the signal

due to the interferant at the analyte mass. Then it can be subtracted from the total signal measured

at the analyte mass, analogous to the on peak subtraction used in ICP-OES.

Analyte from previously analyzed samples can be deposited on the sampler, skimmer or within

the MS and then revaporized and ionized. Contamination and impurities in the reagents used for

sample preparation (acids, e.g.) can also produce a signal at the analyte mass to charge ratio. A blankprocesses in the same way as the sample can be used to subtract this contribution to the signal.

For each of the following (separately), determine how the detection limit for the analyte

changes with concentration of the interferant. Predict the effect of each (independently of doing the

experiment) using literature data and the assumption of 2% relative standard deviation of the

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Page 6 of 7

interferant signal. You may experimentally determine the signal produced at the mass of V due to

HCl as well as the Ba and Ti oxide/elemental ion signal ratios for use in your prediction.

Investigate the following:

Analyte Isotopes Interferant

V 51 Cl species from variable

concentrations of HCl

Cu 63, 65 Ti oxides

Eu 151, 153 Ba oxides

Ni 56, 58 Fe

Zn 64, 66 Ni

References

P. W. J. M. Boumans, Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission

Spectrometry, Volumes 1 and 2, Pergamon Press, 1984. (Includes listing of detection limits.)

P. W. J. M. Boumans, J. J. A. M. Vrakking,Spectral Interferences in Inductively Coupled PlasmaAtomic Emission Spectroscopy - I. A Theoretical and Experimental Study of the Effect of Spectral

Bandwidth on Selectivity, Limits of Determination and Detection Power, Spectrochimica Acta 40B,

1085 (1985).

E. Poussel and J. M. Mermet, Simple experiments for the control, the evaluation and the diagnosis

of inductively coupled plasma sequential systems, Spectrochimica Acta 48B, 743 (1993).

Inductively Coupled Plasmas in Analytical Atomic Spectrometry, A. Montaser and D. W. Golightly,

editors, VCH Publishers, New York, 1992.

P. W. J. M. Boumans and J. J. A. M. Vrakking, Detection Limit Including Selectivity as a Critierionfor Line Selection in Trace Analysis using Inductively Coupled Plasma Atomic Emission

Spectrometry (ICP-AES) - A Tutorial Treatment of a Fundamental Problem of AES, Spectrochim.

Acta 42B, 819 (1987).

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References (continued)

P. W. J. M. Boumans, J. C. Ivaldi and W. Slavin, Measuring Detection Limits in Inductively Coupled

Plasma Emission Spectrometry-II. Experimental Data and Their Interpretation, Spectrochim. Acta

46B, 641 (1991).

Y. Shao and G. Horlick, Recognition of Mass Spectral Interferences in Inductively Coupled Plasma-

Mass Spectrometry,Applied Spectroscopy 45, 143 (1991).

ELAN 6000 Operating Manual

Software

M.S. View, Windows based data base of spectral overlaps in ICP-MS, available on the computers

on the ELAN 6000 and Optima 3000.

Lambda III, DOS based spectral database and spectral simulation (can investigate effect of

spectrometer resolution). (Will be available on the computer on the Optima 3000 and another

computer in the lab.)

Wavelength tables accessible in the WinLab Optima 3000 software.

Some hints:

1. Use analyte concentrations that are on the order of 100 to 1000x the detection limit in a clean

sample.

2. Estimate the concentrations of interferants to use so that the interferant signal is about 1 and

10 times that of the analyte (in separate samples). (Do not use concentrations above 100 ppm

for ICP-MS and 1000 ppm for ICP-OES.) Limit the volume of samples you make to 100 mL

or less to conserve primary standards.

3. The "blank" should include the interferant.

4. Use 2% v/v HNO or HCl, for all solutions as appropriate.3

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