Joseph W. Waggonera, Lisa S. Milsteina, Mikhail Belkina, Karen L. Suttona, Joseph A. Caruso*a and Harry B. Fanninb
aDepartment of Chemistry, University of Cincinnati, Cincinnati, OH 45221-0172, USA
bDepartment of Chemistry, Murray State University, Murray, KY 42071, USA
First published on UnassignedUnassigned7th January 2000
A low power, reduced pressure helium inductively coupled plasma source for mass spectrometry with capillary gas chromatography was applied to the detection of the organobromine species bromobenzene, 1-bromoheptane and benzyl bromide and ethylated forms of the ionic tri-substituted organotin species tributyltin chloride (TBuSnCl) and tripropyltin chloride (TPrSnCl). Total elemental information obtained with plasma source ionization was augmented with a capability to take fragment spectra that provides structural information about gas chromatographic eluents. Limits of detection for bromobenzene, 1-bromoheptane and benzyl bromide were 11, 6 and 4 pg, respectively. Molecular ions for all three organobromine compounds were obtained, as well as some characteristic alkyl chain fragments for 1-bromoheptane, resembling an electron impact mass spectrum. The ethyl, propyl and butyl substituents of ethylated TBuSnCl and TPrSnCl were clearly identified as fragments in the mass spectra. Chromatographic retention time and mass spectral matches with ethylated organotin standards were obtained for the TBuSnCl component of the certified reference material NIES-11.
Conventional plasma source mass spectrometry, that is, a high power (>1000 W), atmospheric-pressure argon plasma (ICP), has provided enhanced sensitivity and element selectivity for organometallic analyses when compared with electron ionization mass spectrometry (EI-MS); however, the high power sources provide no molecular information. Many speciation studies thus require complementary techniques for qualitative and quantitative determinations. The new generation of plasma sources for mass spectrometry provide complementary quantitative and qualitative information with the same instrument.
In addition to demonstrating the analytical capabilities of experimental plasma sources, much research has focused on making such sources more cost effective, and thus potentially a viable addition to analytical methods. Jerrell and co-workers12 reported the use of a novel low power, reduced pressure (LP/RP) helium ICP generator with optical emission spectrometry (OES) as a detector for gas chromatography in the analysis of organohalogens. This low cost, “in-house" manufactured plasma generator, based on a design introduced by May and May for optogalvanic spectrometry,13 was assembled from readily available parts for less than 100.
A recent publication from this research group2 described the development of Jerrell's LP/RP-ICP source for elemental mass spectrometry where tetra-substituted organotin compounds were separated by gas chromatography and detected at low picogram limits of detection with a quadrupole mass spectrometer. In addition, characteristic mass spectra, resembling those of EI-MS, were obtained with this system.
The focus of this paper is to demonstrate the versatility of the aforementioned GC-LP/RP-He-ICP-MS system,2 by extending the method to (1) volatile organobromine analytes as a representative of organohalogen speciation and (2) a certified reference material (CRM) containing tri-substituted organotin species. The ability to detect organohalogens at low levels with this experimental plasma system could be applicable to the analysis of halogenated pesticides and herbicides. Tributyltin has been a contaminant of concern for some time due to its widespread use as an anti-fouling agent in boat bottom paints and the subsequent detrimental effect on marine organisms. Tributyltin can be fatal to marine life at low part-per-billion levels.
The choice of organobromine analytes and derivatized organotin compounds establishes a comparative basis with previously reported LP/RP-ICP-MS instrumentation and methods. Castillano and co-workers11 utilized an LP/RP-ICP source with mass spectrometry for the detection of the same organobromine compounds. This source was obtained by modifying the impedance matching network of the conventional ICP torchbox. However, a stable plasma could not be achieved below 90 W rf power with the Henry-type rf generator and the fragmentation was too extensive to obtain molecular spectra. O'Connor and co-workers5,8 modified both the plasma generator and matching network of a commercial ICP system which allowed for stable operation of a helium plasma down to 5 W rf power. On interfacing with a customized mass spectrometer, molecular spectra were obtained for both organometallics and organohalogens.
The gas chromatographic analysis of tri-substituted organotin species and CRMs utilizing various pre-column derivatization techniques with EI-MS or chemical ionization (CI)-MS14–16 and ICP-MS15,17–20 detection has been established. There has been little work reported involving analyses of CRMs with LP/RP plasma sources for application or method validation purposes.1,8 For this ionization source, the detection of tributyltin with structural information is the first application involving a real sample.
A PlasmaQuad II STE quadrupole ICP mass spectrometer (Fisons, Winsford, Cheshire, UK) was used after removing and disabling the conventional torch box and generator. The first stage rotary pump of the instrument was used to maintain reduced pressure in the plasma torch cell and the expansion stage of the mass spectrometer. The plasma torch was a Pyrex™ tube, 1/4′′ od, 3/16′′ id, and 3.5′′ in length, and was sealed to a customized sampler (1 mm orifice) with epoxy resin. A schematic diagram and detailed description of the GC-LP/RP-He-ICP-MS interface is available elsewhere.2
The organotin standards were ethylated in a similar manner. Single component and mixtures of 100 µl amounts of 100 mg ml−1 TPhSnCl, TBuSnCl and TPrSnCl standards were pipetted into a separatiory funnel along with 2 ml of a 1% w/w solution of NaBEt4, 5 ml of sodium acetate buffer solution (pH 5.0) and 1 ml of isooctane. The mixture was shaken for 5 min and the organic layer was extracted, dried with Drierite™, diluted with isooctane to obtain a 10 mg kg−1 standard and filtered through nylon syringe filters.
a1 bar = 105 Pa. | |
---|---|
Incident rf power | 12–15 W |
Plasma gas flow | 625–790 ml min−1 |
Expansion stage pressure (operating) | 1.0–1.4 mbara |
Intermediate stage pressure | 2.0 × 10−4 mbara |
Analyzer stage pressure | 2.6–3.6 × 10−6 mbara |
Residual cell pressure (expansion stage pressure) | 6.8 × 10−2 mbara |
GC carrier gas flow | 8.0–11.0 ml min−1 |
GC transfer line temperature | 270–300°C |
Fig. 1 GC-LP/RP-He-ICP mass spectra of organobromine species; 26 ng injection (as Br); TRA data acquisition mode. (a) Bromobenzene. (b) 1-Bromoheptane. (c) Benzyl bromide. (d) EI mass spectra for 1-bromoheptane.23 |
Fragment m/z | Fragment assignment | Compound |
---|---|---|
aMe = methyl; Eth = ethyl; Pr = propyl; Bu = butyl. bMolecular ion. cPossible ethylated degradation product of TPrSnCl. dPossible recombination products. | ||
Organobromine compounds— | ||
79 | 79Br+ | All |
93 | CH2–79Br+ | 1-Bromoheptane |
107 | (CH2)2–79Br+ | 1-Bromoheptane |
121 | (CH2)3–79Br+ | 1-Bromoheptane |
135 | (CH2)4–79Br+ | 1-Bromoheptane |
150 | (CH2)5–79BrH+ | 1-Bromoheptane |
156 | (C6H5)–79Br+b | Bromobenzene |
171 | (C6H5)–CH3–79Br+b | Benzyl bromide |
178 | (CH2)7–79Br+b | 1-Bromoheptane |
Organotin compounds— | ||
120 | 120Sn+ | All |
135 | (Me)–120Sn+ | All |
149 | (Eth)–120Sn+ | All |
163 | (Pr)–120Sn+ | TPrSnCl derivative |
177 | (Bu)–120Sn+ | TBuSnCl derivative |
178 | (Eth)2–120Sn+ c | TPrSnCl derivative |
193 | (Eth)2–Me–120Sn+d | All |
207 | (Eth)3–120Sn+ d | All |
235 | (Eth)–(Pr)2–120Sn+ | TPrSnCl derivative |
The molecular ions obtained with this source were less intense than that reported by O'Connor and co-workers8 for dibromobenzene, in which, by adding isobutane to the plasma gas, a limit of detection (LOD) of 229 pg was achieved with SIM at the molecular ion mass-to-charge ratio (m/z 236) and 76 pg with SIM at the elemental mass (m/z 81). In comparison, the LODs for organobromine compounds using the source described in this paper were 4–11 pg obtained at the elemental mass (m/z 79) and calibration could not be achieved with SIM at any of the molecular ion masses. While the source utilized by O'Connor and co-workers operated at a slightly lower rf power, it was proposed that the isobutane reduced the ionization energy in the plasma to the point where molecular ions could be obtained in high abundance.5
A representative SIM chromatogram for the organobromine separation is shown in Fig. 2. Retention times were reproducible with relative standard deviations (%RSD) less than 0.6%. Substantial peak tailing was observed and the baseline progressively decreased during the separation. These problems are most likely a memory effect attributable to the choice of stationary phase (DB-1701) and column bleed. High levels of bromine were left on the column from prior use and each successive injection and were flushed off by the solvent with each subsequent injection. Quantitative figures of merit were calculated after first subtracting the background area from each peak. The background area was determined at the front of the peak by integrating over a time interval equivalent to the width of each analyte peak.
Fig. 2 GC-LP/RP-He-ICP-MS chromatogram, SIM data acquisition mode, m/z 79; 26 pg injection (as Br). (1) Bromobenzene, (2) 1-bromoheptane and (3) benzyl bromide. |
The figures of merit for the detection of the species (as Br) are compiled in Table 3. The LODs (3σ/slope) were limited by the elevated baseline. Bromobenzene has the largest peak height of the three analytes, but the poorest LOD, resulting from the substantial background area subtracted and the high standard deviation of the background. The figures of merit achieved with this experimental set-up (Table 3) are comparable to those of a previous GC-LP/RP-He-ICP-MS effort from Castillano and co-workers,11 in which LODs of 16 and 7.6 pg were achieved for bromobenzene and benzyl bromide, respectively, utilizing a 100 W plasma source. This source provides comparable sensitivity for elemental detection at a much lower rf power, therefore allowing for molecular spectra capability. The combination of low picogram LOD, retention time matches and mass spectra with molecular ions, could allow for low level detection and identification of these organobromine species in an unknown sample.
Bromobenzene | 1-Bromoheptane | Benzyl bromide | |
---|---|---|---|
aIntensity values background-corrected. bCalculations based on peak integrals. cLOD calculated as 3σ/slope of calibration graph.dBased on five replicate injections. | |||
Linear range/decades | 2.5 | 2.5 | 2.5 |
Slope/counts pg−1a,b | 152 | 199 | 151 |
R2 value | 0.9994 | 0.9997 | 0.9994 |
Log-log slope | 0.9448 | 0.9565 | 0.9517 |
Limit of detectiona,b,c/pg | 11 | 6.4 | 4.2 |
RSDd (%) | 3 | 5 | 7 |
Fig. 3 GC-LP/RP-He-ICP-MS chromatograms for ethylated organotin standards and NIES-11 CRM; SIM data acquisition mode, m/z 120; A,B unidentified. (a) TBuSnCl and TPrSnCl standard mix, 1.6 ng (as tin). (b) TPrSnCl spiked (1.6 ng as tin) NIES-11 CRM. |
The multiple peaks obtained for the ethylated TPrSnCl spike and other unidentified tin peaks may be attributed to the formation of degradation products at various steps of the sample preparation and analysis or from isomeric impurity in the starting reagents. Two reproducible peaks were initially attributed to TPrSnCl from retention time matching of the spiked ethylated fish sample with a single component ethylated TPrSnCl standard. Two unidentified yet reproducible small peaks (A and B in Fig. 3) were also observed in the TPrSnCl standard chromatogram and were initially considered to be minor degradation products or possibly just contaminants. However, much more intense peaks with matching retention times were observed in the NIES-11 CRM chromatogram [Fig. 3(b)]. From this observation, it appears that there could be some enhanced degradation of the TPrSnCl derivative due to the microwave extraction process similar to that reported by Rodriguez and co-workers22 for diphenyltin chloride (DPhSnCl). A more thorough study will be required to confirm this possibility. NIES report 1.3 ± 0.1 mg kg−1 tributyltin (as chloride) for NIES-11 and do not report specifically for tripropyltin. They indicate 2.4 ± 0.1 mg kg−1 as total tin for the sample.
Triphenyltin chloride (TPhSnCl) was not detected as a standard or identified as a component in NIES-11 CRM, possibly due to incomplete ethylation or severe degradation. The chromatogram for a single component ethylated TPhSnCl standard (not shown) consisted of several small and irreproducible peaks. The ethylation process in some instances may have been slowed by degradation of the NaBEt4.
Table 4 summarizes the semi-quantitative data obtained for the spiked NIES-11 CRM. The TPrSnCl spike did give a preliminary indication of the ethylation/microwave extraction efficiency by comparison of the ethylated TPrSnCl peak areas in NIES-11 CRM and the standard. The amount of the TPrSnCl detected in the NIES-11 CRM was roughly half of what was expected, suggesting that ethylation and/or extraction was incomplete. Initially, the TPrSnCl spike was intended as an internal standard to provide a correction factor for quantitative analysis, compensating for incomplete ethylation. The relative contributions of ethylation efficiency and microwave extraction efficiency to the amount of compound detected has not been determined. Further characterization of the derivatization process and the apparent degradation of TPrSnCl will be necessary to perform a fully quantitative analysis for TBuSnCl.
TPrSnCl (ethyl derivative) | ||||
---|---|---|---|---|
1st peak | 2nd peak | Sum (average) | TBuSnCl (ethyl derivative) | |
aIntegrated raw counts. bBased on three injections of a single ethylated mixed standard. cBased on three extractions of NIES-11 CRM. dBased on proportion between the peak areas of the sample component and a 10 mg kg−1 standard. eSum of two peaks attributed to TPrSn. fNIES report 1.3 ± 0.1 mg kg−1 as chloride for TBuSnCl for NIES-11 and 2.4 ± 0.1 mg kg−1 as total tin for the sample. Tripropyltin is not reported. | ||||
Standard peak areaa | 38603 | 95073 | 133676 | 141578 |
RSDbfor standard peak area (%) | 9 | 4 | (7) | 20 |
Sample peak areaa | 27207 | 59633 | 86840 | 25441 |
RSDc for sample peak area (%) | 16 | 12 | (14) | 8 |
Semi-quantitative concentration/mg kg−1 (as tin)d,e,f | 6.4 | 1.8 |
The semi-quantitative values reported in Table 4 for TBuSnCl and the TPrSnCl spike in NIES-11 CRM are based on a proportion between the CRM sample peak area and that of a 10 mg kg−1 standard. The high semi-quantitative value reported relative to the certified value may be attributed to differences in the ethylation procedure between the standard and the sample. The sample (NIES-11 CRM) was exposed to microwave energy after addition of the ethylation reagent. The TBuSnCl and TPrSnCl standards were not exposed to microwave energy during ethylation.
Fig. 4 GC-LP/RP-He-ICP mass spectra for ethylated organotin compounds, TRA data acquisition mode. (a) TBuSnCl standard; 16 ng injection (as tin). (b) TBuSnCl in NIES-11 CRM. (c) TPrSnCl standard, peak 2; 16 ng injection (as tin). (d) TPrSnCl standard, peak 1; 16 ng injection (as tin). (e) TPrSnCl spike in NIES-11 CRM, peak 2. |
Mass spectra of derivatized compounds are difficult to interpret because the chemical form of the target analyte has been changed; however, there are some characteristic fragments that can help differentiate between compounds with different organic substituents. These fragments and possible fragment assignments are summarized in Table 2. The ethylation of these compounds is confirmed by the presence of a fragment at m/z 149. For ethylated TBuSnCl, a characteristic structural fragment is identified at m/z 177. For ethylated TPrSnCl, propyl fragments are identified at m/z 163 and 235. Some fragments in these spectra can be attributed to significant memory effects from previous injections. Other unexplained fragments may be attributed to excessive fragmentation and recombination in the plasma, in particular, the fragments at m/z 193 and 207.
The fragment assignments for m/z 193 and 207 as recombinations of ethyl group fragments are tentative, but sensible, since each analyte studied was ethylated. Such ethyl recombination products for TBuSn are atypical of EI-MS.14 While the similarities between LP/RP-He-ICP and EI mass spectra are at times striking, it is suggested by O'Connor and co-workers that there are numerous processes occurring simultaneously that contribute to the observed mass fragmentation.5
Molecular mass spectra were obtained for the unidentified peaks (labeled A,B) in Fig. 3(a) and (b), but no discernible structural fragments were observed that could help identify these components as specific alkyl or aryl tin species. The fact that these unknown components in the NIES-11 CRM fish sample are clearly tin compounds (from SIM at m/z 120) that lack structural characteristics may suggest severe degradation resulting from metabolic processes.
Comparison of the mass spectra in Fig. 4(c) and (d) for the two peaks attributed to TPrSnCl in Fig. 3(a) and (b) illustrates the value of augmenting chromatographic information with structural information in elemental speciation studies. One possible explanation for the appearance of two peaks with propyl substituents may be ethylation of a degradation product, possibly dipropyltin chloride, which would explain the higher occurrence of ethyl fragments relative to propyl fragments in the mass spectrum of the first peak. It seems reasonable that a compound with a higher degree of ethylation would exhibit more intense ethyl fragments. In addition, there is a possible fragment at m/z 178 in Fig. 4(c) and (d) which could represent the diethyl fragment ion.
Total element information at picogram levels of detection and extensive molecular information, including molecular ions, have been obtained for organobromine species. A previously reported study for tetra-substituted organotin compounds with this source has been extended to the less volatile ionic organotin compounds. The ethylated form of tributyltin chloride was identified in NIES-11 CRM by chromatographic retention time and mass spectral matches. The mass spectral capabilities also provide information on possible degradation products. The semi-quantitative result for TBuSnCl in NIES-11 CRM indicates that the detection of TBuSnCl may be quantifiable at low mg kg−1 levels with the system described after improvement of the derivatization and extraction technique.
Future work will focus on coupling the LP/RP-ICP-MS with low-flow liquid separation techniques, which would eliminate the need for derivatization in most cases and expand the capabilities of the instrumentation to many more speciation studies.
This journal is © The Royal Society of Chemistry 2000 |