PRACTICAL GUIDE: CHROMIUM ISOTOPE RATIO MEASUREMENTS IN GROUND WATER INVESTIGATIONS
The Cr isotope approach is still being developed and there are some aspects that we do not understand well. However, we have made rapid progress on the central aspects of the method and believe that 53Cr/52Cr measurements can be very valuable in practical environmental work.
TWO MAJOR GOALS:
1) Distinguishing sources: Some people ask if Cr isotopes can be used to distinguish Cr contamination from different sources. The simple answer at this point is “maybe.” So far, it appears that Cr isotope ratios of various industrial Cr supplies vary little. However, chemical reactions at the contaminant source can alter these ratios, and perhaps different sources end up isotopically distinct. There is evidence from a few studies suggesting that natural background Cr derived from rock weathering may be isotopically distinct from anthropogenic Cr contamination.
2) Detecting Cr(VI) reduction: We have worked extensively to develop Cr isotope ratios as indicators of Cr(VI) reduction, which converts toxic, mobile Cr(VI) to less toxic, immobile Cr(III). As reduction proceeds, 53Cr/52Cr increases systematically. The amount of increase depends on the reduction mechanism, though. In our laboratory experiments we found that 50% reduction led to increases between 1.7 and 2.9 per mil. Estimates of the amount of Cr(VI) removed by reduction can be obtained, but carry considerable uncertainty.
MEASUREMENT METHODS AND PRECISION:
We use a specialized mass spectrometer (known as an MC-ICP-MS) to measure 53Cr/52Cr ratios. This instrument is specially designed to measure isotope ratios, and as far as we know, no commercial environmental lab has one. Only a handful of university labs that can measure Cr isotope ratios well. Measurements must carefully avoid or account for major interferences; without sufficient experience and care, one can obtain precise but very inaccurate results.
Cr must be separated from other elements in ground water samples prior to analysis. This is done by ion exchange, and is done with care to avoid contamination during the separation procedures. The whole process takes several hours of lab time per batch of samples.
The current method for water samples is designed to measure only dissolved Cr(VI). Measurement of solid samples is more complex and must be discussed in detail.
Precision is currently about 0.13 per mil for most samples.
SAMPLE REQUIREMENTS:
The method requires about one microgram of Cr. For example:
at 1 mg/L … 1 mL is needed
at 0.1 mg/L … 10 mL
at 0.01 mg/L … 100 mL
Analyses can be done with somewhat smaller amounts of Cr, but precision will be degraded.
SAMPLE PRESERVATION, SHIPPING, HOLDING TIMES AND STORAGE:
Filtered, NOT acidified, and refrigerated works best.
Using this approach, we have found no measureable Cr isotope change in many water samples stored for months or years. However, there is the potential for reduction of Cr(VI) during sample shipping and storage if reducing agents are present in samples. We can address this by checking for decreases in Cr(VI) over time. In most cases we have experienced so far, keeping holding times very short (e.g., <24 hours) is not necessary. See below information on turnaround times.
TURNAROUND TIMES:
We are primarily a research laboratory. We do a limited number of analyses for people outside the university to promote the use of stable isotope methods in practical situations. Accordingly, we are not structured to perform analyses with short-turnaround times. Typically, we hope to have several weeks’ time to analyze samples. Shorter turnaround times can be negotiated in some cases.
COSTS:
The cost is currently a few hundred dollars per sample. We charge only for the running costs of the mass spectrometer and the labor needed to prepare and analyze the samples.
QUALITY CONTROL MEASURES:
— Standard solutions are analyzed periodically during analysis sessions.
— Standard solutions are processed along with samples and analyzed.
— Blank solutions are processed along with samples to ensure that effects of contamination from laboratory air and reagents are negligible.
— Some samples are prepared and analyzed in duplicate.
— Replicate analyses of prepared samples are carried out.
CERTIFICATION:
As this is a new method, no EPA method or certification exists.
This material is based upon work supported by the National Science Foundation under Grant No. 02-29079. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.