Authors: C. Derrick Quarles, Jr. *, Patrick Sullivan, M. Paul Field, Hwan Kim, and Daniel R. Wiederin

Transcription

1 Elemental Scientific prepfast IC: Inline Autodilution Method to Eliminate Species Interconversion for LC-ICPMS Based Applications Authors: C. Derrick Quarles, Jr. *, Patrick Sullivan, M. Paul Field, Hwan Kim, and Daniel R. Wiederin Use of an Inline Autodilution Method to Eliminate Species Interconversion for LC-ICPMS with Elemental Scientific's prepfast IC Introduction Trace element analysis is vital for industrial, government, and academic communities. Knowing the total concentration of an element in a given sample is important, but it does not tell the whole story. The chemical species (e.g., As (III), As (V), or AsCH 5 O 3 ) plays an important part in understanding how it can affect the environment, its bioavailability, or its toxicity. For example, inorganic arsenic (As (III)) is more toxic than organic arsenic (monomethylarsonic acid), whereas organic mercury (methylmercury) is more toxic than its inorganic form (Hg (II)). 1 The most common technique for trace metals analysis is inductively coupled plasma (ICP) coupled with either an optical emission spectrometer (-OES) or mass spectrometer (-MS). However, to determine the species of an element the chemical forms need to be selectively separated and the most common technique for this has become liquid chromatography (LC). Combining the two techniques, LC- ICPMS has become a very powerful and important tool for speciation of clinical, biological, food, pharmaceutical, and environmental based samples. 2 Typically, trace elemental analysis measurements are done by acidifying the sample (e.g., 2% HNO 3 ) and introducing it into an ICPMS (or ICP-OES). When determining the chemical species in a given sample, the sample integrity is paramount, and thus the sample must be kept in its native form. Species interconversion (the process of a chemical species changing form) becomes more likely during sample preparation as it is handled and exposed to external conditions. Yet most analytical columns used for chromatography have a low mass loading capacity which require diluted samples and/or small sample volumes (µl s). In order to address this issue we at Elemental Scientific (ESI) have developed an all-inclusive autosampler instrument, prepfast IC, which can be used for both total concentration analysis and speciation analysis. One of the key features of this instrument is that the samples are autodiluted inline, eliminating the need for sample preparation thus keeping the sample integrity intact. The other added benefit is that the analyst has the option to use the instrument for totals or speciation depending on the need at that given time, without having to attach multiple pieces of equipment to the frontend of the ICPMS. The prepfast IC is completely automated giving the user two instruments in one. The following study will be focused on arsenic species in urine in order to demonstrate the capabilities of the prepfast IC. This study will look at the precision and robustness for a syringe driven LC method, the advantages of autocalibration and autodilution of samples versus manual preparation, and figures of merit. Elemental Scientific 7277 World Communications Drive Omaha, NE Tel: sales@icpms.com 1

2 Instrumentation In many laboratories adding an additional LC-ICPMS system may be too costly, thus ESI has developed and introduced the prepfast IC that is capable of providing total elemental analysis and elemental speciation within the same system. This system allows the user to seamlessly switch between total metal analysis and speciation without having to change any hardware, solutions, or samples. In addition no sample preparation is needed as standards and samples can be autocalibrated/autodiluted inline. Load column with sample Eluent 1 separates AsB, DMA, and As (III) Eluent 2 removes MMA immediately Auto switching between speciation and total metals Autocalibrate - Total metals - Speciation Autodilute - Total metals - Speciation Ion chromatography Gradient elution syringe pump Automated tuning Syringe loading - Viscous samples - Allows for all sample types Completely metal free system Eluent 2 separates MMA and As (V) Cl is retained onto column and eluted during wash step Figure 2. Illustration of the chromatographic separation of AsB, DMA, As (III), MMA, and As (V) as they elute off of the anion exchange column. Figure 1. Chemical structure of each arsenic species analyzed in this method. 2

3 Column Chemistry An anion exchange column is used for separating 5 species of arsenic (Fig. 1) in urine, aresenobetaine (AsB), dimethylarsinic acid (DMA), arsenite (As (III)), monomethylarsonic acid (MMA), and arsenate (As (V)). The separation involves a two-step elution gradient that varies in ph and ionic strength (Fig. 2). AsB, DMA, and As (III) elute in step 1 and MMA and As (V) elute in step 2. The Cl matrix, which can act as an interference and potentially cause high backgrounds, is eluted during the wash step using the FAST valve, ensuring that it never reaches the nebulizer. An ideal method allows for the user to have a range of sample dilutions, therefore dilution factors from 0X 100X were evaluated. All sample dilutions were performed using the inline autodilution feature. After the sample is diluted, it is injected directly onto the column; this minimizes the time that the sample can undergo any species interconversions. Figure 3 shows the chromatogram for 30X, 50X, and 100X urine samples that were spiked with 10 ppb arsenic species. Dilution factors from 0X 20X displayed varying retention times, suggesting that amount of sample (~ 23 µl of sample injected) was beyond the loading capacity of this anion exchange column. Inline autodilution factors of 30X up to 100X urine samples displayed the exact same retention times. Table 2 displays the retention times for each species using different dilution factors and different columns. A very small variation was seen for As (III) retention times (± 2.7 s) when changing columns, while all other species were ~ 1 s or less in peak variation. To test the reproducibility of the column and the syringe driven chromatography, 20 urine samples spiked with 5 ppb arsenic species were analyzed. In between each spiked urine sample a urine blank sample was analyzed to check for carryover from sample to sample. Table 1 displays the precision for each arsenic species tested during this experiment. The blank had no noticeable carryover and the background changed very little from sample to sample, 1.2 %RSD. The 5 ppb spiked samples showed good precision with the highest variation of 2.6 %RSD for AsB and all other species 1.4 %RSD. Autodilution of 10 ppb Spiked Urine Samples Table 1. Reproducibility of the column with a syringe driven chromatography method. % RSD (n=20) AsB DMA As (III) MMA As (V) Blank ppb Spiked Urine Alternating urine blank and spiked urine samples. 40 samples in total (20 blank and 20 samples). Figure ppb spiked urine samples analyzed at a 30X, 50X, and 100X dilution factor using the inline autodilution feature. Table 2. Effects of autodilution and changing of columns on the retention time for each arsenic species. Retention Times (s) Dilution Factor AsB DMA As (III) MMA As (V) Column A 30X ± ± ± ± ± X ± ± ± ± ± X ± ± ± ± ± 0.6 Average All ± ± ± ± ± 0.5 Column B 30X ± ± ± ± ± X ± ± ± ± ± X ± ± ± ± ± 0.9 Average All ± ± ± ± ± 0.3 Column A & B All ± ± ± ± ± 1.2 3

4 Manual Dilution vs Inline Autodilution Autocalibration of Arsenic Species A comparison of manual dilution calibrations to inline autocalibrations showed similar chromatograms (Fig. 4) and resulted in almost identical sensitivity. The calibration curves ranging from 0 25 ppb for these two methods were used to evaluate the effects of time until analysis for a manual sample preparation method compared to the inline autodilution method. Preliminary tests with spikes of 1 ppb arsenic suggested up to a 43 % conversion of As (III) to As (V) after 24 h. Urine samples (from same urine pool) were spiked with 10 ppb of each arsenic species, individually. The urine samples were analyzed directly after the spiking process (t = 0 h) and at 1 h, 3 h, 6 h, 12 h, and 24 h after spiking. The overlay of the time study is displayed for the manual dilution method (Fig. 5) and the inline autodilution method (Fig. 6). The As (III) showed no conversion to As (V) at t = 0 h for manual and inline dilution methods. However, at t = 6 h the manual dilution showed a detectable peak at both the As (III) and As (V) elution times. The measured concentration for the complete 24 h study can be found in Table 3. The average recovery for the manual dilutions was found to be 4 11 % less than that of the autodilution method for AsB, DMA, MMA, and As (V). The poor recovery for As (III) in the manual dilution mode is due to the species interconversion to As (V). Figure 4. Chromatograms for the manual dilution calibration (top) and inline autocalibration (bottom). Table 3. Manual dilution vs inline autodilution. Measured Concentration (ppb) % Recovery Manual Dilution Time (h) AsB DMA As (III) MMA As (V) AsB DMA As(III) MMA As (V) Measured Concentration (ppb) % Recovery Inline Autodilution Time (h) AsB DMA As (III) MMA As (V) AsB DMA As(III) MMA As (V)

5 Manual Dilution Inline Autodilution Figure 5. Manual dilutions for each arsenic species at time = 0, 1, 3, 6, 12 and 24 h. Individual arsenic species spiked into urine at a concentration of 10 ppb. Figure 6. Inline autodilutions for each arsenic species at time = 0, 1, 3, 6, 12 and 24 h. Individual arsenic species spiked into urine at a concentration of 10 ppb. 5

6 Table 4. The amount of As (III) that converted to As (V) over a 24 h time period for manual dilution compared to inline autodilution sample preparation. Manual Dilution Inline Autodilution 10 ppb As (III) Spiked Urine Samples Time (h) As (III) % As (V) % As (III) % = (x h As (III) / 0 h As (III)) * 100 As (V) % = (x h As (V) / 0 h As (III)) * 100 As (III) + As (V) = As (III) % + As (V) % As (III) + As (V) ppb As (III) Spiked Urine Samples Time (h) As (III) % As (V) % As (III) + As (V) Table 5. Limits of detection (LOD) and limits of quantification (LOQ) determined over 9 different analytical runs for each arsenic species. Stats are based on LODs and LOQs from 9 different studies during the method development/evaluation. AsB DMA As (III) MMA As (V) LOD (ppt) LOQ (ppt) LOD = (3 x σblank)/m; LOQ = (10 x σblank)/m The % recovery decreased for As (III) during the manual dilution experiments. This was expected since there was a noticeable species interconversion from As (III) to As (V). The interconversion was noticeable in these urine samples after 6 h from sample preparation. Assuming that the measured sample at t = 0 h is 100%, Table 4 shows the % conversion of As (III) to As (V) over the 24 h study as compared to t = 0 h. After 24 h, there was 21.4 % As (V) converted from As (III) in the manual dilution mode as compared to 1.3 % converted in the inline autodilution method. Species Interconversion Figure 7. Effect of time on manual sample preparation vs inline autodilution. The limits of detection (LOD) and limits of quantification (LOQ) were determined for 9 different analytical runs. The values listed in Table 5 are on par or better than what is found in the published literature. 3,4 The 9 analytical runs were collected by different users over a 1 month time period on a NexION 350d ICPMS (in KED mode) that is shared for multiple users within the facility for multiple types of applications. The aforementioned spiked samples used in the species interconversion study represented good accuracy/recovery, but were prepared in house. To test the method accuracy for samples not prepared in house, NIST SRM 2669 arsenic species in frozen urine was used. Table 6 displays the accuracy results for NIST SRM The measured values are compared to the NIST certified values and the minimum and maximum reported values from the SRM certification process. 5 6

7 Table 6. Accuracy to NIST SRM 2669 (arsenic species in frozen urine) levels 1 and 2. Measured values from 3 separate days (n = 3), AC = Arsenocholine, TMAO = trimethylarsine oxide. ^ AsB + TMAO = 3.37 ppb (2.44 ppb (min) 4.86 ppb Conclusions All Methods Combined (ppb) Measured Values (ppb) NIST Level 1 Certified Value (ppb) Min Max Avg ± 1 SD AsB 12.4 ± ± 1.0 DMA 3.47 ± ± 0.35 As (III) 1.47 ± ± 0.37 MMA 1.87 ± ± 0.17 As (V) 2.41 ± ± 0.53 Total 22.2 ± All Methods Combined (ppb) Measured Values (ppb) NIST Level 2 Certified Value (ppb) Min Max Avg ± 1 SD AsB 1.43 ± ± 0.54^ DMA 25.3 ± ± 3.6 As (III) 5.03 ± ± 0.24 MMA 7.18 ± ± 0.79 As (V) 6.16 ± ± 0.85 AC 3.74 ± n/a TMAO 1.94 ± n/a Total 50.7 ± The data reported here suggest that a syringe driven chromatography system is fully capable of reliable and reproducible results. There were no matrix effects seen for urine that was diluted from X, giving the user a range of sample dilutions from which to choose. The manual dilutions revealed that the longer a sample is diluted and sits out before analysis, the more likely it is to undergo species interconversion. This study resulted in up to 21% conversion of As (III) to As (V). The inline autodilution option allows for longer time to analysis for samples to sit on the deck of an autosampler with little to no species interconversion. References 1. Wang, T., Liquid chromatography-inductively coupled plasma-mass spectrometry (LC-ICP-MS), J. Liq. Chrom. & Related Tech., 2007, 30, Montes-Bayon, M., DeNicola, K., Caruso, J.A., Liquid chromatography-inductively coupled plasma-mass spectrometry, J. Chrom. A, 2003, 1000, Carioni, V.M.O., McElroy, J.A., Guthrie, J.M., Ngwenyama, R.A., Brockman, J.D., Fast and reliable method for As speciation in urine samples containing low levels of As by LC-ICP-MS: Focus on epidemiological studies, Talanta, 2017, 165, Xie, R., Johnson, W., Spayd, S., Hall, G.S., Buckley, B., Arsenic speciation analysis of human urine using ion exchange chromatography coupled to inductively coupled plasma mass spectrometry, Anal. Chim. Acta, 2006, 578, Yu, L.L., Verdon, C.P., Davis, W.C., Turk, G.C., Caldwell, K.L., Jones, R.L., Buckley, B., Xie, R., A human urine standard reference material for accurate assessment of arsenic exposure, Anal. Methods, 2011, 3,

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