SUPPORTING INFORMATION
Direct analysis of samples under ambient condition by high-voltage-assisted laser
desorption ionization mass spectrometry in both positive and negative ion mode
Xinxin Ren, Jia Liu, Chengsen Zhang and Hai Luo*
Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular
Engineering, Peking University, Beijing 100871, China
*Correspondence to: H. Luo, Beijing National Laboratory for Molecular Sciences, College of
Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
E-mail: hluo@pku.edu.cn
Table of Contents
1. Signal
responses
on
laser
energy
and
target
high
voltage
in
negative
HALDI-MS.......................................................................................................................S2
2. ESI-MS analysis of cytochrome c.....................................................................................S3
3. Analytical figures of merit of HALDI-MS.......................................................................S4
4. Analysis of small drug molecules by both HALDI-MS and LDSPI-MS in negative ion
mode……………………………………………………………………………………..S6
5. The concentration effect on the formation of ODN clusters in HALDI and LDSPI
processes............................................................................................................................S7
S1
1. Signal responses on laser energy and target high voltage in negative HALDI-MS
We also investigated the effects of laser energy and target high voltage (HV) on the signal
responses of HALDI-MS in negative ion mode using a 1 × 10–4 M ibuprofen (IBP) solution
as the test sample. The obtained results (see Fig. S1) were similar to those obtained by
HALDI-MS in positive ion mode by using cytochrome c as the analyte (see Fig. 2 in the main
document). The total ion chromatogram (TIC) shows that the ion signals appeared only when
both the laser irradiation and the HV were on (Fig. S1(a)). Figure S1(b) shows the
corresponding mass spectrum in which deprotonated monomer and dimer ions of IBP at m/z
205 and 411 were observed with good signal-to-noise. Figures S1(c) and S1(d) are the signal
responses on the laser energy and the target HV, respectively, which were also in agreement
with the trends shown in positive HALDI-MS (see Figs. 2(c) and 2(d) in the main document).
Figure S1. (a) Total ion chromatogram (TIC) of 1 × 10–4 M ibuprofen (IBP) solution
obtained by continuous HALDI-MS scan in negative ion mode while the laser power or the
target high voltage (HV) is switched on/off; (b) HALDI-MS spectrum of 1×10-4 M IBP
solution obtained with both the laser power and the target HV on; (c) signal response on the
applied target HV when the laser energy is set at 1.1 mJ/pulse; (d) signal response on the
applied laser energy when the target HV is set at -5.5 kV.
S2
2. ESI-MS analysis of cytochrome c
The ESI-MS spectrum (Fig. S2) of 1 × 10–5 M cytochrome c solution with 0.1% formic acid
displays multiply charged ions from [M + 7H]7+ to [M + 17H]17+, which is similar to the
spectrum obtained by HALDI-MS (see Fig. 2(b) in the main document), although the highest
charge state observed in the ESI-MS spectrum was lower than that in the HALDI-MS
spectrum.
Figure S2. ESI-MS spectrum obtained from 1 × 10–5 M cytochrome c in methanol/water (1:1,
v/v) with 0.1% formic acid. The main experimental parameters are as follows: flow rate 1.0
μL/min, emitter tip-to-orifice distance 7 mm, spray voltage 5.5 kV, and no auxiliary gas.
S3
3. Analytical figures of merit of HALDI-MS
The analytical figures of merit of HALDI-MS were investigated using rhodamine B (RB) as
the test analyte. Figure S3 shows the HALDI-MS spectrum of 1 × 10–8 M RB. The peak
intensities of [RB – Cl]+ at m/z 443 relative to that of the base peak at m/z 279 which is a
background signal were measured over a 3-day period (6 measurements per day). The
intra-day relative standard deviations (RSDs) obtained on these three days were 11%, 10%,
and 10%, while the inter-day RSD was 8%. Such good reproducibility of HALDI-MS
secured the reliability of the results obtained in its applications. In addition, HALDI-MS had
a quantitation dynamic range of over three orders of magnitude (e.g. from 1 × 10–8 M to 1 ×
10–5 M, as shown in the inset) without the use of an internal standard. Higher RB
concentrations caused deviation from the linearity, probably due to signal saturation. The
limit of detection (LOD) (S/N = 3) of HALDI-MS for RB was 2 × 10–9 M.
Since 100 µL of the sample solution is deposited each time for the detection, we calculated
that the amount of RB on the sample target was 200 fmol at the LOD. In comparison, the
LODs of ESI-MS and LDSPI-MS for RB were 1 × 10–9 M and 2 × 10–9 M, respectively.
From these results, we can see that the sensitivity of HALDI-MS is similar to that of
LDSPI-MS, and slightly lower than that of ESI-MS. Note that the experimental parameters
used in HALDI-MS and LDSPI-MS are the same as described in the Experimental section of
the main document, while the main experimental parameters used in ESI-MS are: flow rate
2.0 µL/min, emitter tip-to-orifice distance 7 mm, spray voltage 5.5 kV, and no auxiliary gas.
S4
Figure S3. HALDI-MS spectrum of 1 × 10–8 M rhodamine B (RB). The signal of [RB – Cl]+
ion is at m/z 443. The peaks at m/z 279 and 432 are background signals. The inset shows the
signal intensity of RB as a function of its concentration on a log-log scale. The linear
regression equation is log(intensity) = 9.7 + 0.7 log(CRB) and the correlation coefficient R is
0.97.
S5
4. Analysis of small drug molecules by both HALDI-MS and LDSPI-MS in negative ion
mode
Ibuprofen (IBP) and flurbiprofen (FBP) were analyzed in negative ion mode by both
HALDI-MS (see Figs. S4(a) and S4(b)) and LDSPI-MS (Figs S4(c) and S4(d)). The
monomer and dimer ions of IBP (m/z 205, 411) and FBP (m/z 243, 487) were observed in the
HALDI-MS spectra with good signal-to-noise (S/N). The LDSPI-MS spectra showed the
monomer ions (m/z 205 and 243) with large background signals but no dimer ions of the drug
molecules. The limits of detection (LODs) (S/N >3) of both IBP and FBP by HALDI-MS
were about 3 times lower than in LDSPI-MS (~2.5 × 10–6 M by HALDI-MS vs. ~1.0 × 10–5
M by LDSPI-MS). These results demonstrate that HALDI-MS can produce analyte signals
with better S/N and more cluster ions than LDSPI-MS in negative ion mode.
Figure S4. The top panels show the negative HALDI-MS spectra of 1 × 10–4 M (a) ibuprofen
(IBP) and (b) flurbiprofen (FBP). The bottom panels show the negative LDSPI-MS spectra of
1 × 10–4 M (c) IBP and (d) FBP. IBP and FBP are dissolved in methanol/water (1:1, v/v)
containing 1 × 10–4 M NH4HCO3. The spray solution of LDSPI-MS is methanol/water (1:1,
v/v).
S6
5. The concentration effect on the formation of ODN clusters in HALDI and LDSPI
processes
If the HALDI-MS spectra obtained from the ODN solutions of 1 × 10–4 M (Fig. 4(b) in the
main document), 5 × 10–5 M (Fig. S5(a)), and 1 × 10–5 M (Fig. S5(b)) are compared, it is
found that the relative intensity of the ODN dimer signal decreased with decreasing ODN
sample concentration. When the sample concentration was lowered to 1 × 10–5 M (Fig. S5(b)),
the dimer ion signal actually disappeared. Similarly, in LDSPI-MS, the ODN dimer ions
could also be generated from the more concentrated sample solutions (e.g., 5 × 10–4 M ODN
in Fig. S5(c) shown below vs. 1 × 10–4 M ODN in Fig. 4(d) shown in the main document).
Figure S5. Negative HALDI-MS spectra obtained from (a) 5 × 10–5 M and (b) 1 × 10-5 M of
the 12-mer (5'-CCCCTTAACCCC-3') ODN in methanol/water (50:50). (c) Negative
LDSPI-MS spectrum obtained from 5×10-4 M of the 12-mer ODN solution. The spray
solution for LDSPI-MS is methanol/water (1:1, v/v).
S7