5 The Liquid Skin Pen
5.4 An experiment to determine the efficiency of elemental extraction using the LSP
As detailed in Chapter 1, there is little information regarding the elemental content of human skin as a result of different types of stress placed on the body. The objective of this experiment was to investigate if the LSP was able to extract one drop of 10 µg.ml-1 Mn from a nitrile surface using a sample solution of 0.1M EDTA. Furthermore, should extraction of Mn be successful what amount of recovery could be achieved in the primary sample and subsequent wash samples. H0: there was no difference in the amount of Mn recovered from a nitrile glove using the LSP, between surfaces that has been spiked with Mn (Sample ID: B) compared to one that has not (Sample ID: A). i.e. H0: [A] = [B].
5.4.1 Experimental Outline
One drop of 10 µg.mL-1 Mn was weighed and added onto the surface of a fresh (out of the box), unused nitrile glove (Barber Health Care Limited, Leyburn, UK) using a 20 µg pipette (Gilson Inc., Middleton, USA). The drop was allowed to sit on the surface and dry for 30 minutes prior to sample collection. The first sample (sample B) was collected from the target area followed by three subsequent wash samples (B1, B2 and B3). All samples were collected from the exact same spiked target area of the glove. Samples were stored in a 20°C freezer immediately following collection.
Figure 5.5. Experimental procedure for LSP recovery testing
5.4.1.1 Sample Analysis
All samples were analysed on the subsequent day of collection and were allowed to thaw at room temperature for 30 minutes after which they were homogenised using a vortex mixer (Scientific Industries Inc., NY, USA). For all samples, 1 ml of collected sample was diluted
SAMPLE ANALYSIS SAMPLE COLLECTION
Spot target area with 1 drop of SRM Sample target area with LSP Dilute samples with 10ml 2% HNO3 Re-sample target area (x3) Allow spot To dry (30 mins) PRE SAMPLING SRM 1000 → 100 → 10 µg.ml-1 Make up 0.1M EDTA (Di-Na) Collect LSP Pen Blank Connect vials to LSP Storage at -20° C Thaw all samples ICP-MS
Analysis Data Output
Conc Recovered
with 10 ml 2% HNO3 in 15 ml fresh, unused, polypropylene centrifuge tubes (Elkay Ltd, Basingstoke, UK).
Elemental detection of Mn was performed using an ICP-MS (as described in Chapter 1 section 1.3.5). A 10 µg.mL-1 Mn sample was also analysed in order to semi-quantify sample concentrations. Three replicates were taken for each sample analysed. Elemental concentrations in the sample were semi quantified by using the ratio of the intensity obtained for the known Mn concentration against intensity obtained for all samples. Additionally, relative Mn recovery was determined from each of the collected samples. 5.4.2 Results and Discussion
A total of 8 samples were successfully analysed on the same day. Samples obtained from the LSP included a LSP solvent blank, one Mn sample and three Mn wash samples. Also analysed were two solvent blanks, which consisted of DI H2O and 2% HNO3, and a 10 µg.mL-1 Mn sample.
Table 5.3. Concentration of samples (µ), blank subtracted samples (µ (BS)), relative standard deviation (RSD) and recovery of Mn in samples obtained using the LSP. n = 3
Sample Concentration ( ng.mL -1 ) % µ µ (BS) RSD Recovery A 2.71 9.4 B 7.24 4.53 6.1 41.8 B1 4.18 1.47 4.3 13.5 B2 3.44 0.73 5.6 6.7 B3 3.06 0.35 6.2 3.2 C 0.34 9.4 D 0.32 10.3
The LSP successfully extracted the Mn sample from the target surface (Table 5.3). 2.71 ng.mL-1 of Mn was extracted using the LSP from the unspiked target surface and used as the baseline value. However, once a drop of the Mn sample was spiked on the target surface a concentration of 7.24 ng.mL-1 Mn was obtained. Subsequent washes exhibited a decrease in the amount of Mn extracted from the surface with 4.18, 3.44 and 3.06 ng.mL-1 found in washes 1, 2 and 3, respectively. The amount of Mn attained in the solvent blanks was less than the amount obtained from the blank. Figure 5.6 illustrates the reduction in the concentration of Mn following each wash. In the first sample, following the surface spike with the Mn (B), the highest concentration of Mn is acquired when compared to subsequent samples where the Mn concentration gradually decreases from wash 1 (B1) to wash 2 (B2) and finally wash 3 (B3). This data suggests that although the LSP is able to extract a
significant amount of Mn in the first sample on the target surface, it does not achieve 100% recovery, as proven by the Mn concentrations found in the wash solutions. By removing the amount of Mn found in the blank sample from the spiked sample as well as subsequent wash samples, the Mn concentration achieved are 4.53, 1.47, 0.73 and 0.35 ng.mL-1 for the initial spiked sample, wash sample 1, 2 and 3, respectively.
5.4.2.1 Recovery of Mn using the LSP
The theoretical concentration of Mn expected in the final sample analysed by ICP-MS can be calculated using a predicted 100% recovery. There were three transfer phases for Mn, each step diluting the sample analyte more than the previous phase. The three phases are; step 1, where a known amount of Mn sample was added to the target surface; step 2, a known amount of sampling solvent was used to sample the target surface; and step 3, dilution of the collected LSP sample with a matrix suitable for ICP-MS analysis.
Theoretical Concentration Step 1
An average mass of one drop of Mn used was 0.0179 g (σ = 0.0001, n = 3) with a measured concentration 10.0087 µg.mL-1. Therefore, an estimated 17.9 mg Mn was added onto the target surface of an unused, fresh latex glove.
Step 2
Using the LSP, 1.5 ml EDTA was sampled and collected over the target area containing Mn resulting in the first dilution.
(Equation 5.1)
Where C1 = Initial concentration V1 = Initial volume C2 = Final concentration V2 = Final volume
Using equation 5.1, if C1 = 10.0087 µg.mL-1, V1 = 0.0179 and V2 = 1.5 then C2 = 119.2 µg. L-1. Step 3
1 ml of sample collected was diluted with 10 ml 2% HNO3, in a 15 ml polypropylene tube, prior to ICP-MS introduction. Re-arranging equation 5.1, the theoretical 100% recovery concentration (C2) can be calculated. C2 = 0.01083 µg.mL-1, when C1 = 0.1192 µg.mL-1, V1 = 1ml and V2 = 11 ml.
1
1
3
Figure 5.6. Mean concentration ([i]aq/ng.mL-1) of Mn (σ = 5%) from samples collected using the LSP. Bars = Concentrations (ng.mL-1) Line = Blank subtracted concentrations (ng.mL-1). Insert: Bars = Blank subtracted concentration (ng.mL-1), Line = Recovery
0.0
1.6
3.2
4.8
6.4
8.0
A
B
B1
B2
B3
C
D
[i
]
aq/
µ
g.
L
-1Sample
(Equation 5.2)
Therefore, the theoretical concentration of 100% recovery would be 10.83 µg.mL-1. Table 5.3 shows the calculated relative recovery of Mn for each sample, obtained using Equation 5.2. The initial sample contained 41.8% (4.53 ng.mL-1) of the original Mn sample whereas recoveries of 13.5 (1.47 ng.mL-1), 6.7 (0.73 ng.mL-1) and 3.2% (0.35 ng.mL-1) were obtained for the subsequent washes, 1, 2 and 3, respectively. This is illustrated in Figure 5.6.
H0 was rejected as a difference in the amount of Mn recovered from a nitrile glove, using the LSP, between a spiked surface and blank surface (H0: B ≠ Blank) was discovered. Although it
was proved that the LSP can be used to detect a sample of Mn on the surface of a nitrile glove, multiple washes are needed to extract as much Mn as possible. However, it is worth noting that the first sample taken with the LSP did achieve the highest recovery.
Issues encountered included sample uptake stalling from the delivery vial. This was solved by replacing the cap and septum on both the delivery vials and re-placing them into the vial holders. It was important to ensure that vials were inserted into the vial holders vertically in order to avoid bending the hypodermic needles at both the delivery and collection vial sites of the LSP.