Trace elemental analysis of undiluted urine by ICP-MS 

Trace elemental analysis of undiluted urine by ICP-MS

By Glenn Woods
Agilent ICP-MS Applications Chemist

Inductively coupled plasma mass spectrometry (ICP-MS) is fast replacing graphite furnace atomic absorption spectroscopy (GFAAS) as the technique of choice for the trace elemental analysis of a variety of clinical and biological sample types. However, a number of challenges associated with the ICP-MS analysis still remain:

  • The high total dissolved solids (TDS) content of clinical samples can lead to signal suppression, signal drift, and the need to carry out frequent routine maintenance.
  • Several key elements, such as As, Cr, Cu, Se, V, and Zn, are compromised by argon- or matrix-based polyatomic interferences.
  • Clinical labs may require a high-throughput capability without compromising accuracy and ease of use.

A recent study evaluated the performance of the Agilent 7700x ICP-MS with Integrated Sample Introduction System for Discrete Sampling (ISIS-DS) for the routine analysis of human urine samples. The objective was to optimize both productivity and data quality, while minimizing sample preparation.

Save time and reagents with simplified sample preparation

Advances in ICP-MS technology allow you to reassess and simplify sample preparation procedures. For example, improved ICP-MS matrix tolerance enables you to eliminate the sample dilution step, which saves time and reagents and reduces sample contamination. Equally, developments in collision/reaction cell technology allow a single He mode method to be applied to almost any sample type, without requiring extensive method development for new or different sample matrices. To assess performance for the routine analysis of undiluted urine samples, we configured the 7700x ICP-MS, which includes the High Matrix Introduction (HMI) kit, for discrete sampling using the optional ISIS-DS accessory.

Elemental and
organic content

Concentration

Na

5000 mg/L

K

1200 mg/L

Ca

200 mg/L

Mg

70 mg/L

P

400 mg/L

Urea

15 g/L

Table 1. Salt and urea content of a typical human urine sample

Overcome high total dissolved solids

The usual recommended maximum TDS level for ICP-MS is 2000 ppm (0.2%). Urine samples are highly variable, but typically contain a much higher level of salt and organic constituents (Table 1), so you require a sampling strategy that allows the ICP-MS to measure such samples. These strategies may include:

  • Dilution to reduce the TDS to 2000 ppm or less, but this can lead to the introduction of contaminants from the diluent
  • Matrix elimination using techniques such as liquid-liquid extraction, solid phase extraction (SPE), chromatography, or hydride generation, but these are time-consuming and often need separate sample preparation
  • High Matrix Introduction (HMI), which performs a “dilution” using a dry, clean gas and allows the direct analysis of samples with much higher levels of TDS than can be tolerated by conventional ICP-MS
  • Discrete sampling (using ISIS-DS), which introduces a discrete “plug” of sample into a flowing carrier stream, to reduce the total sample load on the plasma and increase sample throughput

High sample throughput using ISIS-DS

The Agilent ISIS-DS consists of a six-port injection valve with a sample loop. In the “load” position, the sample fills the loop, with the excess being pumped directly to waste; at the same time a carrier flow is pumped to the ICP-MS nebulizer. When the valve switches, the sample is injected into the carrier steam and delivered to the nebulizer, giving a short-lived steady-state signal. The ICP-MS can make replicate multi-element measurements during the steady-state signal, and you can easily adjust the signal duration by changing the loop volume. ISIS-DS virtually eliminates uptake and rinse-out times, enabling much faster run times; because less sample matrix reaches the ICP-MS interface, it also reduces matrix loading, so minimizing signal drift and maintenance. A typical total run time is one to three minutes, depending on element list and loop size. This is two to three times faster than normal (continuous) sample introduction.

Figure 1. Normalized internal standard recoveries demonstrate stable performance throughout the run. (Enlarge image.)

Figure 2. Composite calibration plots from the start and end of the sample batch for 51V (top) and 238U (bottom) show excellent linearity and reproducibility. (Enlarge image.)

Direct analysis of human urine…

To test the 7700x ICP-MS system, we acidified 14 undiluted samples of human urine using 0.1% HNO3, and analyzed the samples repeatedly throughout the batch. We used the 7700x ICP-MS with Octopole Reaction System (ORS3) operated in helium collision mode (for interference removal), with HMI (for matrix tolerance) and ISIS-DS (for high sample throughput). In summary, we:

  • Analyzed 14 urine samples repeatedly to test repeatability and long-term stability
  • Measured 21 isotopes in each sample run (analytes and internal standards)
  • Analyzed each sample in only 1 minute 32 seconds
  • Ran calibration standards at the beginning and end of the sequence of 320 samples

…with stable and reproducible results

The results show that the 7700x ICP-MS maintained excellent performance despite the difficult sample matrix. The normalized internal standard (IS) recoveries (Figure 1) showed that there was no loss of sensitivity or signal drift, indicating no matrix deposition on the interface or lenses over the entire sequence.

The calibration curves likewise demonstrated consistent performance. We calibrated the 7700x ICP-MS before and after the sample sequence. The calibration plots shown in Figure 2 are composites of both standard sets with and without internal standard correction, for V (low mass analyte subject to interference) and U (high mass analyte). The linearity and reproducibility of the calibration curves demonstrate that the ORS3 in helium mode successfully removed the ClO interference on V at mass 51, and again confirm that there was no signal drift throughout the sequence.

In summary, the 7700x ICP-MS with HMI and ISIS-DS displayed excellent stability and reproducibility for the analysis of undiluted urine samples. HMI, which is standard on the 7700x ICP-MS, allowed direct determination of undiluted urine, while the optional ISIS-DS considerably enhanced sample throughput. If you need excellent performance for difficult sample matrices, view our interactive animation to learn more about the 7700x ICP-MS.

Agilent ICP-MS Journal

Are you interested in trace metals analysis and eager to keep up-to-date with the latest developments in the field of ICP-MS? Then take a closer look at Agilent’s dedicated ICP-MS Journal. Published four times a year and available as a PDF, you can view the latest issue of the ICP-MS Journal, plus all previous copies, by visiting the ICP-MS Journal Archive.

To receive a personal copy of the ICP-MS Journal direct to your inbox, please sign up here.

 
 
grey1
related_bottom_left clear clear clear
clear

Welcome To Agilent Technologies

To enhance your online experience and our partnership with you, please select your country from the lists below.This will ensure that you discover the most relevant information and services available, including local product news, events, and more.

Other Countries
Please select