Chromium by ICPMS-DRC

Chromium Isotopes

While chromium (Cr) has four naturally occurring isotopes, only the 52 and 53 isotopes do not suffer from interferences from other elements. Even so, samples containing carbon (organic or inorganic) or chloride can produce results biased high for ⁵²Cr and ⁵³Cr due to molecular

ion interferences produced in the plasma, ⁴⁰Ar¹²C and ³⁷Cl¹⁶O (see Table 1). Sample digestion may reduce carbon levels but does not necessarily eliminate the ArC interference. And while theoretically one may be able to correct for the ³⁷Cl¹⁶O interference using ³⁵Cl¹⁶O, corrections for these polyatomic interferences are unreliable.

Table 1. Isotopes and Interferences for Chromium

Element or	% Relative Abundance of Isotope at Mass
Interference	50	51	52	53	54
Titanium	5.4
Vanadium	0.25	99.8
Chromium	4.35		83.8	9.5	2.37
Iron					5.8
ArC			98.5	1.1
ClO		75.8		24.2

Masses normally used to quantify elements are in BOLD.

ICPMS Cell Technology

Removal of interfering species in a reactionor collision cell situated between the argon plasma torch and the analyzer quadrupole of the mass spectrometer is a reliable way to improve the accuracy and detection limit for Cr. A dynamic reaction cell or "DRC" (using ammonia as reaction gas and a reaction-cell quadrupole producing a low-mass cutoff) practically eliminates ArC⁺ and ClO⁺ ions produced in the plasma while allowing Cr⁺ to pass.

To demonstrate this, both a blank and a known reference sample (USGS T-143, Cr = 37.0 + 2.6 ug/L) were analyzed under standard conditions (without DRC) and under DRC conditions. Methanol (MeOH, 1% by volume in the samples) was added to the blank and T-143 to produce the dissolved carbon interference ⁴⁰Ar¹²C for mass 52 (Table 2). Hydrochloric acid (HCl, 1% by volume) was also added to produce the ³⁷Cl¹⁶O for mass 53 (Table 3).

Table 2. The Effect of Carbon on Cr Determinations at Mass 52

	Chromium,ug/L^*
Sample	True Value	without DRC	with DRC
Blank with 1% MeOH	0	170	ND<0.2
USGS T-143 with 1% MeOH	37.0	209	39

Table 3. The Effect of Chloride on Cr Determinations at Mass 53

	Chromium,ug/L^*
Sample	True Value	without DRC	with DRC
Blank with 1% HCl	0	90	ND<0.2
USGS T-143 with 1% HCl	37.0	131	38

* The concentration of total Cr is reported based on the total Cr in the standard but calculated for each individual isotopic mass assuming natural abundance of the isotopes. All quantitations were performed with an internal standard under DRC conditions.

Conclusion

While 1% solutions of HCl and MeOH create interferences equivalent to approximately 100 ug/L in the blank and T-143, the results demonstrate that the DRC effectively eliminates the interference for both isotopes, allowing accurate measurement of Cr at both masses.

When two or more isotopes give results that agree, this is a good indication that they are free from interferences. Comparing the data in the Tables 2 and 3 for mass 52 and 53, data for both the blank and T-143 without DRC gave results for the two masses that differed by almost X2. But with DRC, both masses gave results for the blank and T-143 that agree within experimental error.

This method has been successfully applied to drinking water samples without digestion, digested bicarbonate impinger solutions, and digested soluble fiber.

Additional ICPMS publications by WCAS:

Metals Analysis by ICPMS, ICPMS vs.GFAA, ICPMS Metals Screens, Lead (Pb) by Isotope Dilution ICPMS, and "Environmental Analysis Using ICP-MS", Environmental Laboratory, August/September 1993, pg. 44-49. "ICP-MS for the Analysis of Metals on Membrane Filters", American Industrial Hygiene Association Journal, 48(2), pg. 977-979, (1987).

For Cr(VI) speciation click on Cr(VI).

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